Gravity Analysis of the Tendaho Graben, Afar Depression_ethiopia

GRAVITY ANALYSIS OF THE TENDAHO GRABEN, AFAR DEPRESSION,

ETHIOPIA

A Masters Thesis

Presented to

The Graduate College of

Missouri State University

In Partial Fulfillment

Of the Requirements for the Degree

Master of Science, Geospatial Sciences in Geography and Geology

By

Benjamin Weide

August 2011

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ETHIOPIA

Geography, Geology and Planning

Missouri State University, August 2011

Master of Science

Benjamin Weide

ABSTRACT

The Tendaho graben (TG) is located at the intersection of the East African, Red Sea and Gulf of Aden rifts. The TG lies within the southern portion of the Red Sea propagator, which is the landward extension of the Red Sea spreading center. The region has experienced extensional forces resulting in thinned crust and formation of oceanic crust. To investigate the crustal structure of the TG, previous detailed gravity, magnetic, and seismic surveys were completed for geothermal resources. I performed a detailed gravity and magnetic survey within the graben to aid in determining the crustal structure. Bouguer gravity anomaly maps of the Afar indicate a regional gravity maximum in comparison to the Ethiopian and Somalian plateau, which is related to crustal thinning. Additionally, smaller wavelength gravity maxima occur over both the Red Sea and Gulf of Aden propagators. Wavelength filtered gravity maps show that the maximum of the Red Sea propagator terminates in the south-central portion of the TG and does not intersect the Gulf of Aden propagator. Four models, which were constrained by magnetic modeling and drill hole data, indicate that the graben is 1.6 km thick with a 6-8 km wide high density zone correlating to recent volcanic activity. The data suggest the basin evolved from being tectonically controlled and transitioned to being magmatically controlled.

KEYWORDS: Afar Depression, Ethiopia, gravity analysis, Tendaho Graben, Bouguer

This abstract is approved as to form and content

______________________________________ Dr. Kevin L. Mickus Chairperson, Advistory Committee Missouri State University

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ETHIOPIA

By

Benjamin Weide

A Masters Thesis

Submitted to the Graduate College Of Missouri State University

In Partial Fulfillment of the Requirements For the Degree of Master of Science, Geospatial Sciences in Geography and Geology

August 2011

Approved:

Dr. Kevin Mickus

Dr. Kevin Evans

Dr. Jun Luo

_______________________________________ Pawan Kohol, Interim, Graduate College Dean

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ACKNOWLEDGEMENTS

I would like to thank my graduate advisor, Dr. Kevin Mickus, for giving me the

opportunity to work and travel under him at Missouri State University. Dr. Mickus has

given me the encouragement and patience while completing my studies. Also, I would

like to thank my other committee members, Dr. Kevin Evans and Dr. Jun Luo for their

input and advice. I would also like to thank Statoil-hydro for funding my fieldwork and

my research assistantships. I would also like to thank the GGP department at Missouri

State University and all of the students for taking me into their academic family. Finally,

I would like to thank my parents Christopher and Catherine Weide for all of their support

during my academic life.

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TABLE OF CONTENTS

INTRODUCTION ...............................................................................................................1

REGIONAL GEOLOGY AND TECTONIC BACKGROUND .........................................3 East Africa Rift ........................................................................................................3 Crustal Structure of the EAR ...................................................................................6 Timing of the Development of the EAR ................................................................11 Main Ethiopian Rift ...............................................................................................12

STUDY AREA .................................................................................................................15 Geology of the Tendaho Graben ............................................................................15

GEOPHYSICAL INVESTIGATIONS ..............................................................................21 Main Ethiopian Rift and the Afar Depression .......................................................21 Seismic Studies ..........................................................................................21 Gravity and Magnetic Studies ................................................................................28 Tendaho Graben Geophysical Studies ...................................................................34

GRAVITY DATA ANALYSIS .........................................................................................37 Gravity Data Interpretation ...................................................................................41 Residual Gravity Anomalies .................................................................................51 Gravity Data Modeling ........................................................................................53

MAGNETIC DATA ANALYSIS ......................................................................................63 Discussion ..............................................................................................................70

CONCLUSIONS................................................................................................................75

REFERENCES ..................................................................................................................76

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LIST OF FIGURES

Figure 1. Map of the East African Rift (EAR) system. .....................................................4

Figure 2. Topographic relief map of the Afar Depression showing the major structures and microplates .................................................................................................5

Figure 3. Bouguer gravity anomaly map of the central portion of the East African rift .......................................................................................................7

Figure 4. Regional gravity model across the Equator from 25 degrees E to 41 degrees E ......................................................................................................9

Figure 5. Tectonic map of the MER showing the major tectonic features .......................13

Figure 6. Geological and structural map of the Tendaho Graben .....................................15

Figure 7. Geologic map and model of the Tendaho graben. .............................................17

Figure 8. Stratigraphic columns of two drill holes in the TG associated with the Tendaho Geothermal Project. .........................................................................19 Figure 9. Seismic refraction model along profile illustrating a crustal model of Afar region. ............................................................................................................22

Figure 10. Seismic refraction model along lines 1 and 2 of the EAGLE project .............23

Figure 11. A velocity slice at 10 km depth of a 3D seismic tomographic model ..............25

Figure 12. Tomographic model from the multimode surface wave study at depths of 75, 100, and 125 km ..............................................................................................26

Figure 13. Shear wave splitting results showing two slices of the crust at 50 and 300 km. ..........................................................................................................27

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Figure 14. Forward model and predicted Bouguer gravity anomaly of a profile along the MER and southern Afar ..................................................................................30

Figure 15. Bouguer gravity anomaly map of the MER ....................................................31

Figure 16. Band-pass filtered gravity anomaly map where wavelengths between 25 and 250 km were passed. ..............................................................................32 Figure 17. Model and Bouguer gravity anomaly along a profile along the MER and southern Afar ..................................................................................................33 Figure 18. Gravity stations (+) in the study area including Afar, Djibouti, the Red Sea, the Gulf of Aden and the edges of the Ethiopian and Somalian Plateaux. .....38

Figure 19. Complete Bouguer gravity anomaly map of the study area including the Afar Depression. ......................................................................................42

Figure 20. Bouguer gravity anomaly map of the Tendaho Graben ..................................43 Figure 21. Bouguer gravity anomaly map of the Tendaho graben ...................................44 Figure 22. Filtered gravity anomaly map of the Afar region. ...........................................46

Figure 23. Filtered gravity anomaly map of the Tendaho Graben ....................................47

Figure 24. Location of the profiles used for the gravity modeling of the TG ...................54

Figure 25. Two and one-half dimensional model along Profile A ...................................55

Figure 26. Two and one-half dimensional model along Profile B ....................................56

Figure 27. Two and one-half dimensional model along Profile C that runs along the graben axis .....................................................................................................57

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Figure 28. Two and one-half dimensional model along Profile D gravity anomaly due to the model and the black line is the observed gravity anomaly. .........58

Figure 29. Total field magnetic anomaly map of the TG. ................................................64

Figure 30. Vertical derivative magnetic anomaly map of the TG .....................................66

Figure 31. Depth estimation map of the magnetic bodies within the TG determined using Euler deconvolution. ...........................................................................68

Figure 32. Two and one-half dimensional model of magnetic data ..................................69

Figure 33. Illustration of the evolution of the Tendaho Graben .......................................74

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INTRODUCTION

The East African Rift System (EAR) is an intra-continental ridge system that has

developed along narrow zones of thinned lithosphere due to intrusions from the

asthenosphere. The system runs along the eastern edge of Africa and is joined together

by series of grabens linked by transform, transfer, and accommodation zones (Chorowitz,

2005). The northern portion of the EAR system is made up of the Main Ethiopian Rift

(MER) and the Afar Depression. The MER is similar to the structure of the EAR system

to the south; however, it is influenced by the Afar mantle plume (Chorowicz, 2005). The

Afar depression is the most developed and thinned section of the rift (Gao et al., 2010).

Further thinning is caused by the conversion of three rift systems in this region.

The Afar depression represents one of the few places on Earth to study the

transition from continental rifting to oceanic spreading and how continental rifts develop

into oceanic ridges (Mickus et al., 2007). Within the Afar depression there are several

grabens, with the Tendaho Graben (TG) being the largest and most important. This work

emphasizes the structural configuration and origin of the TG, which is located near the

center of the Afar Depression and has been cited to be the location of the triple junction

(Acocella et al., 2008). The southern portion of the Red Sea propagator which is the

landward extension of the Red Sea rift system meets the MER in this southern portion of

the TG and the landward extension of the Gulf of Aden propagator lies to the east of the

TG. Given the lack of subsurface drilling and that the Stratoid basalts are the only

exposed lithologies, geophysical analyzes are the only techniques that can be used to

determine the structural configuration and thus the geologic history of this region. A

recent detailed gravity survey across the TG provided supporting evidence that

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significant high-density material from the asthenosphere occurs along the central axis of

the graben (Mickus et al., 2010). In addition, geochronologic studies on the surficial

basalts (Barberi et al., 1975; Lahitte et al., 2001; Kidane et al., 2003) found that the

basalts became younger toward the center of the graben. The gravity data analysis is

supported by magnetic data that confirms that dyking in the TG which is responsible for

the transition of continental spreading into oceanic seafloor spreading (Bridges et al.,

2010).

This study will complement the existing gravity and magnetic analyzes (Mickus,

2010) by producing a series of Bouguer gravity anomaly maps residual gravity anomaly

maps using wavelength filtering. To further investigate the crustal structure, four two-

dimensional gravity models will be constructed across and parallel to the graben axis in

order to constrain the geometry of the TG. Initial modeling constrained by magnetic

modeling and drill hole data, indicates that the graben is 1.6 km thick with a 10 km wide

high density zone correlating to recent volcanic activity. These models will be analyzed

to better understand the mechanics of the current extension forces within the Afar

Depression.

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REGIONAL GEOLOGY AND TECTONIC BACKGROUND

East Africa Rift

The East African Rift (EAR) is an active continental rift system that trends north

to south on the eastern edge of Africa (e.g., Mohr, 1983; Rosendahl, 1987; Braile et al.,

1995; Chorowitz, 2005) (Figure 1). The EAR splits the African plate into two subplates:

the Nubian on the eastern portion and Somalian plate on the west. The north section of

the rift begins at the Afar triple junction in Ethiopia and at this point the Arabian,

Somalian, and Nubian plates are being rifted apart. However, the Afar region is made up

of several microplates (Acton et al., 1991), so there is no single point for the location of

the triple junction (Figure 2). The Afar Depression marks the transition from continental

crust (southern part) to oceanic crust (northern part) (Hayward and Ebinger, 1996). From

the triple junction, the Main Ethiopian Rift (MER) trends south through the Ethiopian

dome and bifurcates into the Eastern and Western branches of the rift system (Figure 1).

The EAR splits at the sutured zones surrounding the strong, thick crust of the Tanzanian

craton. The eastern and western branches rejoin on the southern side of the Tanzanian

craton, and merge into the Malawi rift. This portion of rift trends southeast towards the

coast of Mozambique and is thought to terminate there but the lack of data in

Mozambique makes this observation difficult. The most characteristic feature of the rift

system a as a whole is the narrow zones of thinned continental with intrusions originating

from the asthenosphere. The rift system has been divided into five stages based on

tectonics.

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Figure 1. Map of the East African Rift (EAR) system. Figures on the right, show the crustal structure at different locations of the EAR.

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Figure 2. Topographic relief map of the Afar Depression showing the major structures and microplates. 1= Danakil Rift; 2= Red Sea Trend. 3=Erta Ale Volcano. 4=Alyata Volcano. 5=Tat Ale Volcano. 6= Manda Harraro - Gobaad Rift. 7= Awsa Plain. 8=Dobi Graben 9= Tendaho Gobaat Discontinuity. 10= Asal - Manda Inakir Rift. 11= Gulf of Aden Trend (Beyene and Abdelsalam, 2005).

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Crustal Structure of the EAR

The EAR has all five rift forming stages present making it a unique system

(Chorowicz, 2005). The first stage is characterized by increased earthquake activity and

a large distribution of strike slip faults. In this stage, there are no true grabens being

formed yet. The first stage is found in the youngest part of the rift which is located near

the coast of Mozambique. The second stage is identified by divergent movements of the

crust, frequent earthquakes, and intrusions which result in a negative Bouguer gravity

anomaly value (Figure 3). This stage is represented by the Malawi rift which is the

southern part of the western rift. The third stage, called the typical rift stage, is

characterized by a well defined rift valley and full graben structures. The portion of the

EAR near Lake Tanganyika is representative of the third stage. The fourth stage is

represented by a large negative regional gravity anomaly and a positive gravity anomaly

in the axis of the rift due the flux on magmatic material (Figure 3). This stage is found in

the Kenyan Rift and the central portion of the MER. The final stage is the formation of

oceanic crust. The transition between the fourth and fifth stage is likely where oceanic

crust first begins forming. The final stage of rifting is found in the Afar region, on the

northernmost portion of the MER (Chorowitz, 2005).

Voluminous volcanism and uplift started prior to the main rifting phases,

suggesting a mantle plume influence on the Tertiary deformation in East Africa (Ebinger

and Sleep, 1998). Different plume hypothesis have been suggested (Ebinger and Sleep,

1998; Rogers et al., 2000; Yirgu et al., 2006), with recent models indicating the existence

of deep super plume originating at the core-mantle boundary beneath southern Africa,

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Figure 3. Bouguer gravity anomaly map of the central portion of the East African rift (Simiyu and Keller, 1997). Legend shows gravity values and contour levels.

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rising in a north-northeastward direction toward eastern Africa, and feeding multiple

plume stems in the upper mantle. However, the existence of this whole-mantle feature

and its possible connection with Tertiary rifting are highly debated (Corti, 2009). The

Afar mantle plume has developed the northern portion of the rift system by thermal uplift

and extensions due to dyking. The beginning of the track is dated around 30 million

years ago when the continental flood basalts erupted in the Ethiopia. However, Ebinger

and Sleep (1998) have suggested that magmatism throughout most of eastern Africa can

be related to a single deep mantle plume that initiated basalt volcanism in southern

Ethiopia about 45 million years ago. The most common theory states the Afar plume has

migrated under the current day Tanzanian craton (Rogers et al., 2000). The upward

movement puts stress on the ancient suture zones, creating the eastern and western

branches of the rift system.

The East African plateau is associated with a broad negative gravity anomaly with

amplitude of -150 plus or minus 20 mGal (Figure 3). The long wavelength gravity

anomaly is a result of lithospheric heating and thinning (Simiyu and Keller, 1997) (Figure

4). The broad Bouguer gravity low correlates negatively with topography (Ebinger,

1989). The regional gravity away from the rift system is -50 mGal, with an average

lithosphere thickness of 100 km (Brown and Girlder, 1980). The area west of Lake

Tanganyika has Bouguer gravity values of -80 mGal with the gravity values decrease

heading eastward towards the rift system. As the two arms of the rift (Figure 3) split

around the Tanzanian craton, large gravity gradients are found. This is due to the

topography and boundaries between the suture zones (Simiyu and Keller, 1997) (Figure

4). The Tanzanian craton region is characterized by low gradients and Bouguer gravity

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Figure 4. Regional gravity model across the equator from 25 ° degrees E to 41° degrees E showing the lithospheric structure of the EAR as it splits around the Tanzanian Craton (Simiyu and Keller, 1997).

values of -150 plus or minus 20 mGal. The Bouguer gravity increases eastward heading

towards the Indian Ocean, due to crustal thinning.

The area between the Ethiopia and Kenya dome has an average Bouguer gravity

value of -70 mGal, which has a large amplitude, regional positive anomaly (Figure 3).

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The high value is related to rifting episodes and very thin crust (Simiyu and Keller, 1997)

as shown by gravity modeling (Figure 4). The maximum crustal attenuation occurs

beneath the Afar depression, indicating that the Afar depression undergoes an intense

fragmentation of the crust resulting from faulting and magmatic activity (Simiyu and

Keller, 1997). However, our computed crustal thickness in the Afar depression falls

within an upper bound of depths compared to other tectonically active rift zones (Simiyu

and Keller, 1997). This can be explained in terms of crustal accretion resulting from an

impact of the Afar mantle plume since approximately 30 Ma ago (Hoffmann et al., 1997).

The residual gravity anomalies obtained using low-pass filtering reveals a significant

density contrast between the northern and southern sectors of the rift (Simiyu and Keller,

1997). The northern part of the rift is characterized by regular patterns of positive gravity

anomalies, which can be interpreted in terms of a zone of crustal thinning through which

relatively dense materials have intruded the overlying crust. In contrast, south of the

MER, the gravity anomalies are characterized by random patterns and low amplitudes.

The along-rift-axis variation in gravity anomalies implies that the style of crustal

deformation changed progressively, beginning with regionally distributed crustal

deformation, such as the one we observe within the more juvenile and wider southern

segment of the rift, to localized deformation within the active and narrow rift zones of the

northern sector of the Ethiopian Rift (Tessema and Antoine, 2004).

Several geophysical methods (broadband seismic, seismic refraction, earthquake

analysis and gravity) have been used to determine crustal thickness in the rift system.

Modeling of earthquake data have shown the Moho discontinuity is between 30-35 km, in

the western rift (Bram et. al, , 1975). Microearthquake data were collected over the

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Tanzanian craton and analysis of this data shows a crustal thickness of 35 plus or minus 5

km. Gravity data have shown an average of 35 km crustal thickness and slightly thicker

crust at suture zones in the central rift system (Tesha et al., 1997).

The crustal structure of the majority of the rift system is similar based on seismic

analyzes but there are significant differences (Figure 1). The Afar depression region is

the oldest and most rifted portion of the EAR. The crustal structure is a combination of

oceanic and continental crust, making difficulties in determining the crustal thickness but

the thickness is the least of the entire EAR. Seismic refraction studies have estimated

thickness of 38km in the Ethiopian plateau and thicknesses between 16-26 km in the Afar

depression (Berckhemer, et al, 1975). More recently, thicknesses have been estimated at

26 km in the Afar, and decreasing to 14 km towards the intersection of the Red Sea and

Gulf Aden (Makris and Ginzburg, 1987). These studies have shown that the lithosphere

is thinned and is believed to be underlain by true oceanic crust. These thicknesses are

shallow compared to those of the Kenya Rift (Figure 4), which is approximately 40 km

(Simiyu and Keller, 1997).

Timing of the Development of the EAR

The initial stages of the development of the EAR occurred around 30 million

years ago in the Afar plateau (Hoffman, et al., 1997) as the Afar plume released large

basalt flows in this area (Hoffman, 1997). This volcanism coincides with converging

grabens near Lake Tana forming a triple junction, which further weakened the

lithosphere. Rifting in the Gulf of Aden began between 29.9-28.7 million years ago, and

the southern Red Sea at 27.5-23 million years ago (Chorowitz, 2005). Around 20 million

years ago, volcanism began in northern Kenya further developing the rift system to the

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south. The MER began major rifting episodes at 11 million years ago. This rifting

merged the Kenyan and Afar rift system creating much of the eastern branch of the EAR

(Chorowitz, 2005).

The first development of the western branch of the EAR was in Virunga (Congo),

with lavas dated at 12.6 million years ago (Chorowitz, 2005). Lake Albert in Uganda

experienced its first major rifting episode at 8 million years ago while Lake Tanganyika

experienced rifting between 12-9 million years ago. The last rifting period began

between 5 and 6million years ago in the Lake Malawi area (Chorowicz, 2005).

The EAR has propagated on a north- south trend for the last 30 million years.

The propagation rate is estimated to be around 5 centimeters per year (Kampunzu, et al.,

1998). The system is continuing to propagate in Mozambique and possibly offshore.

The evolution of the EAR has been piece wise, with small area rifting and later

connecting to the system as a whole.

Main Ethiopian Rift

The MER extends in a NE-SW to N-S direction from the Afar depression, at the

Red Sea-Gulf of Aden junction, southwards to the Turkana depression in Kenya. The rift

valley of the MER separates the elevated Ethiopian and Somalian plateaus (Figure 5).

The MER development is related to the roughly E-W motion between Nubia and Somalia

plates (Corti, 2009). Rifting in the MER occurred during the Tertiary around 30 million

years ago based on dating of the large flood basalts that cover the areas around the rift.

These basalts and continuing rifting from a possible plume system created the Ethiopian

and Somalian plateaus to the east and west of the rift.

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Figure 5. Tectonic map of the MER showing the major tectonic features including the MER, Ethiopian Plateau and Somalian Plateau.

14

Continental rifting during the Miocene and Pliocene in the MER was

characterized by displacement along boundary faults, subsidence within the rift valley,

and widespread magmatic activity (Chorowicz, 2005). Initially, the rift was dominated

by magmatism and transitioned into tectonic extension during the Pleistocene (Corti,

2009). The northern portion of the MER demonstrated the transition through riftward

narrowing of volcanic activity (Corti, 2009). Extensional deformation has localized in

the Wonji magmatic segments since ~3 mya, in an oblique direction of the Miocene

boundary fault (Beutel, et al, 2010).

The northern Ethiopian plateau is composed of tholeiitic and basaltic lavas (Mohr

and Zanettin, 1988), which erupted 30 mya over a span of ~1 my (Hoffman et al., 1997).

The basalts cover an area of ~210,000 km², creating a sequence 2000 m in thickness in

the central portion of the MER. However, in the northern and southern segments of the

MER, the basalts are only 500m in thickness. On the eastern boundary of the MER,

rhyolites have been found in this sequence, characterizing the start of continental rifting,

(Ayalew et al., 2006) similar to the sequences found along the TG boundary faults.

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STUDY AREA

Geology of the Tendaho Graben

The Tendaho Graben (TG) is located in the central Afar and is the largest basin in

the area (Acocella et al., 2008). On average, the basin is 50 kilometers in width and

trends in a NW-SE direction for 100 kilometers. The TG is a combination of the Manda

Hararo rift (Figure 6) in the northwest, the Tendaho rift and the Goba Ad half-graben to

the southeast (Abbate et al., 1995).

Figure 6. Geological and structural map of the Tendaho Graben (adapted from Acocella, 2010).

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The MER joins the TG near the Dama Ale volcano and eventually terminates

structurally just to the southwest of the TG (Figure 6). The bounding faults of the TG are

the Logya fault on the west and the Gamare fault on the east. The basin is tectonically

bounded by the East Central Block to the east and the escarpment that is the edge of the

MER to the west. The East Central Block is made up of several microplates that are

separated by deep graben structures (Acton et al., 1991). The East Central Block has

been rotated clockwise about ten degrees in the past 2 Ma because of the dextral shear

between the Gulf of Aden propagator and the Red Sea propagator. However, the East

Central Block has ceased rotation because of rifting within the TG (Acton et al., 2000).

The main lithologies within and surrounding the TG are composed of Stratoid

basalts that become younger in age towards the axis of the graben (Kidane et al., 2003;

Lahitte et al., 2001). The Stratoid basalts have been dated around 30 Ma with the

opening of the rift and large flood basalt episodes (Abbate et al., 1995). They make up

the lower unit of the TG, with the deepest units being around 1500 meters in depth.

There are two types of basalts that compose the Stratoid basalts within the TG, one that

is enriched in light rare earth elements (LREE) and another that is depleted in LREE

(Barrat et al., 2003). The cause of the two basalts is likely a result of a LREE depleted

component of the Afar plume. However, isotopic ratios have shown that both the

Tertiary and Quaternary basalts have a similar mantle source (Barrat et al., 2003).

Additionally, along the edges occur Pleistocene rhyolites (Figure 7). However, the floor

of the basin is composed of younger volcanic and lacustrine deposits (Lahitte et al.,

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Figure 7. Geologic map and model of the Tendaho graben (Aquater, 1996). The A to A’ profile is shown below.

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2001). The lacustrine sediments have been estimated to have a maximum thickness of

1600 meters based on drill data in the southern sections of the TG (Aquater, 1996).

Additionally, there are several basalt flows encountered in several drill holes (Figure 8).

The relief from the flanks to the bottom of the basin is only several hundred meters but

the greater thickness of the lacustrine sediments suggests there is some type of structural

control of the basin’s depth and this will be explored below in my gravity analysis. The

sediment filling is topped by recent volcanoes deposits from the two volcanoes in the

basin, Kurub and Damal Ale (Figure 6).

The lava flows from the Kurub volcano have been dated as recent as 0.3 mya.

However, the youngest volcanic in the TG are found along the Mando Hararo rift and

been dated up to 0.3 mya. Absolute dating of the volcanics in the TG has suggested that

the basin started to open about 1.8 mya (Acton et al., 1991).

The Mando Hararo rift, which is one of the main structural features within the

Afar depression, has open fissures and elongate blocks along the rift axis. The vertical

throws of faults controlling the formation of the rift have been estimated to be around ten

meters (Abbate et al., 1995). This is similar to those found on the graben flanks, which

suggests that the region is influenced by some type of deep mechanism. The closely

spaced throws on the faults suggest that there is a thin brittle layer over a shallow soft

layer (Abbate et al., 1995). As a result of having a soft, possibly warm layer, there is

abundant geothermal potential in the area. Northwest striking faults near the Kurub

volcano are seen by aligned fumaroles, small hot springs and hydrothermal deposits. To

the southwest, there is continued evidence of past and present geothermal activity.

Tectonic studies in the region have shown that most faults have a dip slip

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Figure 8. Stratigraphic columns of two drill holes in the TG associated with the Tendaho Geothermal Project (Battistelli et al., 2002). Both wells drill into the Stratoid basalts.

20

motion with an absence of a strike slip component that can be seen from slickenslides.

However, there has been some strike slip evidence from offset by siliceous veins near the

TG, which contradicts the dip slip motion found in nearby faults (Abbate et al., 1995).

In the Gum’ Atmali area, dextral shear is seen in the extensional fractures. The

northeastern flank of the rift has a discontinuous tectonic regime, that transitions into

block-faulted structures within the main basin within the TG. Also, slickenslides near

Serdo on the eastern edge of the TG indicate a dip slip movement. However, the Serdo

Earthquake of 1969 had a focal mechanism showing dextral shear from the northeast.

The variety of focal mechanisms in the TG indicates a complex seismic regime caused by

the Afar plume and the surrounding rifting related propagators.

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GEOPHYSICAL INVESTIGATIONS

Main Ethiopian Rift and the Afar Depression

Seismic Studies. There have been several geophysical investigations of the MER

and the Afar Depression in order to determine the crustal and upper mantle structure of

the MER and the Afar Depression. These include seismic refraction (e.g., Berckhemer,

1975; MacKenzie et al., 2006), broadband seismic (Walker et al., 2004, Kendall et al.,

2005, and Gao et al., 2010), gravity (e.g., Makris et al., 1970; Searle and Gouin, 1972;

Mahatsente et al., 1999, 2000; Jentzsch et al., 2000) and magnetic (Girdler, 1970;

Courtillot, 1980; Lemma et al., 2010). The first seismic refraction study in the Afar was

completed in 1972 by Berckhemer et al., (1975) who collected of five deep refraction

profiles that were between 120 and 250 km long. The results show that the crust beneath

the Ethiopian Plateau is consistent of continental crust with a thickness of 38 km (Figure

9). The velocity of the mantle was found to be between 7.3-7.6 km/s. The crustal

thickness in the Afar ranges from 26 km in the south near Asaita to 16 km in the north

near Assab, Eritrea. The majority of the crust has a velocity of 6.6-6.8 km. Berckhemer

et al. (1975), Herbert and Langston (1985), and Makris and Ginzberg (1987) all found

that the velocity of the crust in the upper 5 km is approximately 6.0 km/s in the Afar

Depression. However, the results have a low resolution due to the wide spacing of the

seismometers.

The most complete seismic refraction study was completed by the EAGLE

(Ethiopia-Afar Grand Lithospheric Experiment) project (Maguire et al., 2006). The

profiles consisted of a one rift-axial that extended from the central MER to the Afar

Depression and one cross-rift profile in the central MER. Additionally, a 2D array

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Figure 9. Seismic refraction model (dashed and solid lines) along profile 4 of Berckhemer et al. (1975). This model illustrates a crustal model of Afar region. The numbers are velocities in km/sec.

23

of seismic recorders were placed in order to determine the 3D velocity structure of the

volcanically active region in the central MER. Figure 10 shows that the velocity of the

mid and upper crust ranges from 6.1 km/s beneath the rift flanks to 6.6 km/s along the

axis of the rift (Maguire et al., 2006). Also, the thickness of crust varies from 40 km in

the southern portion of the MER to 26 km beneath the Afar Depression. The large

change in thickness suggests the transition of continental rifting to oceanic spreading

(Maguire et al., 2006). Additionally, a high velocity lower crustal layer was found under

the Ethiopian Plateau implying magmatic underplating.

Figure 10. Seismic refraction model along lines 1 and 2 of the EAGLE project (Maguire et al., 2006). Line 1 runs perpendicular to the MER, while Line 2 runs along the axis. Also, shows crustal thickness variations along the rift.

24

Keranen et al. (2004) used the 3D seismic station coverage within the central

MER and seismic tomography to construct a 3D velocity structure of the upper crust

(Figure 11). They found that high velocity zones underlie each of the tectonically active

magmatic segments that represent solidified intruded igneous material formed during

volcanic events. They implied that these magmatic segments are the zones of future

oceanic crustal regions.

A recent tomographic study by Debayle et al. (2005) shows seismic shear wave

velocity variations within the upper mantle of the MER and Afar region (Figure 12). The

model was constructed from 9000 Rayleigh waveforms recorded at 250 stations from the

area’s earthquakes. The model shows diffuse low velocity zones beneath the Afar

depression and the Ethiopian plateaus.

In addition to seismic tomography studies, shear wave splitting studies in the

MER and the Afar Depression has shown that seismic anisotropy may be due to

Precambrian sutures or vertical magmatic dikes (Barruol and Hoffmann, 1999; Barruol

and Ben Ismail, 2001; Gashawbeza et al., 2004; Walker et al., 2004; Kendall et al., 2005).

However, Gao et al. (2010) conducted a study that analyzed teleseismic data for shear

wave splitting using all the available broadband seismic data recorded in the Afar

Depression, MER, and Ethiopian Plateau. Their study used over 450 measurements that

showed insignificant azimuthal variations over the Ethiopian Plateau and in the MER.

These values showed MER parallel fast directions with a single layer of anisotropy

(Figure 13). The values of the 150 stations in the Afar Depression showed an azimuthal

dependence of splitting parameters with a π/2 periodicity which can be explained by a

two layers of anisotropy (Gao et al., 2010). The top layer is characterized

25

Figure 11. A velocity slice at 10 km depth of a 3D seismic tomographic model (Keranen et al., 2004). The red areas represent high velocity zones under the magmatic segments. Gray dotted lines represent the boundaries of the MER, solid grey lines represent the boundaries of the magmatic segments and diamonds represent volcanic centers. NMER-Northern Main Ethiopian Rift, FD-Fantale magmatic segment, BMS-Bosetti magmatic segment, GTMS- Gademsa–Tullu Moye magmatic segment.

26

Figure 12. Tomographic model from the multimode surface wave study (Debayle et al., 2005). Map shows the 3D models at depths of 75, 100, and 125 km.

27

Figure 13. Shear wave splitting results showing two slices of the crust at 50 and 300 km (Gao et al., 2010). Both maps show a general NE directed flow of anisotropy in the asthenosphere. The blue dotted line represents the outline of the Main Ethiopia Rift and the violet line is the outline of the Red Sea. Red areas represent regions with lower shear wave velocities.

28

by a relatively small (0.65 s) splitting delay time and a WNW fast direction that can be

attributed to magmatic dikes within the lithosphere, and the lower layer has a larger (2.0

s) delay time and a NE fast direction (Gao et al., 2010). The study predicted a depth of

300 km for the source of anisotropy and a NE directed flow based on spatial coherency of

the splitting (Figure 13).

Gravity and Magnetic Studies

The gravity fields of the MER and Afar Depression have been studied since the

1960s. The first investigations by Gouin and Mohr (1964) and Mohr and Rogers (1966)

were undertaken to find the general crustal structure of Ethiopia. The first gravity survey

in the Afar Depression was completed by Makris et al. (1975), who recorded 4000

gravity stations and produced a Bouguer gravity anomaly map of the region. They

showed that the lowest Bouguer gravity anomaly of approximately -260 mGal was

located along the escarpment bounding the Afar Depression. While, the largest Bouguer

gravity anomaly was found along the Red Sea off the coast of Eritrea with a value of

approximately 15 mGal. The Bouguer gravity field is roughly correlated with the

topography and morphology of the Afar Depression. Other early studies such as Makris

et al., (1970), Makris et al. (1972), and Searle and Gouin (1972) determined that the MER

and the northern Afar region correlated to a regional Bouguer gravity minimum that is

related to crustal attenuation, while the southern Afar region is less attenuated and may

contain continental crust. Mahatsente et al. (1999, 2000) and Jentzsch et al. (2000)

produced a Bouguer gravity anomaly map by merging all know datasets and further

adding stations in the Afar region. These three studies performed 3D inversion of the

gravity data to determine the crustal thickness and density variations within the crust. All

29

their results showed a zone of thinning along the MER that ranged from 31 km in the

central rift to around 50 km along the rift margin. Mahatsente et al. (2000) concluded

that the MER is underlain by continental crust due to the horizontal and vertical sizes of

the intrusions along the MER.

Tiberi et al. (2005) performed a 3D inversion and forward modeling of the

regional gravity field to investigate the lateral variations of the crust and mantle. A large

merged dataset was used for the study combining data from Hayward and Ebinger

(1996) and the Ethiopian Geological Survey data. The modeling and inversion revealed a

thinned crust of 23 km at the triple junction zone. Crustal variation is from 33km at the

southern portion of the profile to 24 km beneath the southern portion of the Afar

depression. Also, significant underplating is seen beneath collapsed caldera sequences in

the MER (Figure 14).

The EAGLE project also completed a gravity survey and analysis linked to a

regional seismic survey in 2003 (Mickus et al., 2007). Mickus et al. (2007) found that

the Bouguer gravity field is controlled by a regional gravity anomaly that increases in

magnitude from the MER towards the southern Afar region (Figure 15). The Bouguer

gravity map shows regional lows over the Ethiopian and Somalian plateau. Also, a high

gravity is seen over the MER.

However, the residual gravity anomaly shows gravity maxima correlating to the

magmatic segments within the MER (Figure 16). Modeling based on the gravity data

show that the lower crust in denser because of mafic material from extension, but the

mantle is less dense due to a hotter thermal regime (Mickus et al., 2007) (Figure 17).

30

Figure 14. Forward model and predicted Bouguer gravity anomaly of a profile along the MER and southern Afar (Tiberi et al., 2005). The letters on the surface of the model indicate calderas or volcanoes along the transect. U represents the assumed region of crustal underplating. The dotted line in the model is the Moho depth determined from the gravity inversion.

31

Figure 15. Bouguer gravity anomaly map of the MER after the study (Mickus et al., 2007). Contour interval is 10 mGal. (+)-Gravity stations. Also shown are the magmatic segments and border faults shown in bolded lines.

32

Figure 16. Band-pass filtered gravity anomaly map where wavelengths between 25 and 250 km were passed. Contour interval is 10 mGal. Gravity stations are shown by +. The magmatic segments and border faults are shown as bolded lines.

33

Figure 17. Model and Bouguer gravity anomaly along a profile along the MER and southern Afar (Mickus et al., 2007). Aluto, Doseti, Kone and Fantale are volcanoes within magmatic centers. The numbers are the densities of the different bodies.

34

The first magnetic survey in the Afar Depression was completed by the US

Magnet Project in 1966. The survey consisted of 8 profiles that trended NW-SE and 1

profile that ran perpendicular through the other 8 profiles. The spacing between profiles

was approximately 90 km with the flight height increasing towards the southwest. The

survey found that long wavelength anomalies are dominant over the Afar Depression.

However, the survey did not cover the entire TG. This led Girdler (1970) to complete a

survey with 75 lines and 15 tie lines. Based on these flights, linear magnetic anomalies

were found in and near the TG, which is consistent to rifting processes. The first ground

magnetic survey was completed in relation to the Tendaho Geothermal project (Aquater,

1996). At each gravity station in the Aquater project, a magnetic reading was taken as

well. However, this survey was designed to run parallel and perpendicular to the strike of

the TG, unlike the Girdler (1970) survey. The results of the profiles showed the same

conclusions found by Girdler, that there are linear magnetic structures in the TG. Values

ranged from 400 nT to -600nT, which is similar to values found in other oceanic

spreading centers.

Tendaho Graben Geophysical Studies

There have been limited geophysical investigations within the Tendaho Graben

and most that have been performed have been related to geothermal exploration. The

first detailed investigation was a gravity survey (between Tendaho, Serdo and Asayita)

that was conducted by Searle and Gouin (1972) in an area that covered 1000 km². They

created a Bouguer gravity anomaly map using an average crustal density of 2.67 gm/cm3.

The main feature of this map was a decrease in Bouguer gravity field from -40 mGal in

the north to -55 mGal along the Awash River in the south, that is mainly caused by in the

35

increase in sediment thickness in the Awash Basin. A few short wavelength Bouguer

gravity anomalies were found along the southern edge of the surveyed area. A relative

positive Bouguer gravity anomaly was located near Tendaho village and a negative

Bouguer gravity anomaly near the Dubti is caused due to active faulting.

The Italian company Aquater performed the first high density gravity survey of

the Tendaho graben for the Tendaho geoelectric project (Aquater, 1996). The data set for

their survey was more complete than the surveys conducted by Makris et al. (1970, 1972,

and 1975) because of the use of off road vehicles. The survey area was centered on the

Dubti geothermal field situated in the middle of the TG. Bouguer gravity values ranged

from -65 mGal along the flanks of the graben and -45 mGal along the axis. A gravity

maximum trends along the axis of the Mando Hararo Rift (Figure 14) is probably caused

by dyking within the rift.

A detailed gravity survey by Lemma and Hailu (2006) indicated that was long

wavelength Bouguer gravity anomaly in the Ayerobera geothermal area and Bouguer

gravity minimum over the Dubti plantation area. These contrasting Bouguer gravity

anomalies were interpreted to indicate the presence of an ENE- trending regional crustal

discontinuity. The Bouguer gravity maximum and magnetic minima in the Ayrobera and

Gebelaytu plains may indicate the presence of intrusive body or is associated with

deposition of hydrothermal minerals on the volcanic rocks (Lemma and Hailu, 2006).

Aquater (1996b) has also collected magnetic data in conjunction with their gravity

data collection and produced a total field magnetic map that shows a general NW-SE

magnetic anomaly. The anomaly extends from northwest to southeast following the

36

Tendaho graben axis with minor anomaly interruptions attributed to near surface in

homogeneities. This data is discussed in the magnetic anomaly section.

An electrical survey by Oluma et al. (1996) consisted of Schlumberger array

soundings. The electrical resistivity data were collected at three well sites (TD1, TD2,

TD4). They created an electrical map based on a current electrode separation of 500m

that indicated a prominent low resistivity anomaly with deeper parts in the SE section of

the survey area. The result of electrical resistivity map for current electrode spacing of

1000 m indicated a narrow NE elongation of the low resistivity zone. This low electrical

resistivity anomaly is along dominant regional trend, i.e., NW and NE, and is interpreted

to along major structures controlling the flow of hot geothermal fluids.

37

GRAVITY DATA ANALYSIS

The majority of the gravity data for this study (Figure 18) were obtained from the

Italian geothermal company, Aquater, which collected detailed gravity data for

geothermal exploration in the Tendaho graben. Additional gravity data were obtained

from the National Geospatial and Imaging Agency (NGA), the National Geophysical

Data Center (marine data) the Ethiopian Geological Survey, and a survey by Kevin

Mickus who collected data in 2008. The above data were combined with gravity points

taken by the author during the field season of 2009, which collected 81 closely spaced

data points. The above data sets were merged together to create a master set of nearly

24,000 gravity values that were used for the analysis of the graben and the surrounding

region including the Afar Depression, the Red Sea and the edges of the Ethiopian and

Somalian Plateaux (Figure 18).

The most important gravity data for this study were the data from the Aquater

survey which had data spacing between a ¼ and ½ km, while the data obtained by the

author and those obtained in 2008, were spaced between ½ and 1 km depending on the

targeted structures in the study area. The remaining data have spacing between 1 and 20

km, which makes for detailed analyzes difficult outside the region of the southern

Tendaho Graben.

The new data were acquired during December 2009 using a Lacoste and Romberg

model G gravity meter and a Topcon GB-1000 dual frequency GPS unit. At each station,

the field data collected included: 1) multiple observed gravity readings to increase

accuracy; 2) geographic location of the station including latitude, longitude and elevation,

38

Figure 18. Gravity stations (+) in the study area including Afar, Djibouti, the Red Sea, the Gulf of Aden and the edges of the Ethiopian and Somalian Plateaux. The bolded lines represent country boundaries.

39

3) date and time; and 4) terrain variations surrounding the station. A local base station

was established at the kitchen within the Aquater Geothermal camp to correct for

instrumental drift in the town of Semera. A regional absolute gravity base station is on

the southern edge of Semera and was used to tie our data into the 1971 International

Gravity Standardization Net. This was done in order to merge our data into the

previously collected data. The Semera absolute gravity base station was established by

Cindy Ebinger who tied it to the absolute gravity station at the Geophysical Observatory

at the University of Addis Ababa.

Gravity meters produce data in meter units and are not a direct measurement of

the gravitational acceleration. To determine the gravity value in mGals, the meter

reading is multiplied by an instrumental calibration factor. The instrumental calibration

factor depends on the type of gravity meter as each meter has its own set of meter

constants. The corrected gravity data must be tied to an absolute gravity base station, to

make the data universal and thus can be merged with other gravity data. This is done by

comparing the gravity values at your local base station versus an absolute base station.

The difference is either subtracted or added to the rest of the gravity points in the data set.

Since the metal springs in the gravity meter are not truly elastic, they begin to creep over

time, resulting in an apparent change in gravity, when there is really not any change in

the gravity reading. To correct for this instrumental drift, the difference between

successive measurements at the base station are plotted on a graph versus time to produce

a drift curve showing the change of gravity during a given time period.

The latitude, longitude and ellipsoidal elevation of each gravity station were

obtained using a dual frequency GPS unit. At each gravity station, GPS data were

40

collected for 15 minutes using static data collection. These data were then post-processed

to remove atmospheric effects using a GPS base station at Serdo maintained by Eric

Calais from Purdue University. The resulting horizontal and vertical resolution varied

between 0.5 and 2 meters. The ellipsoidal elevations were transformed to geoidal

elevations using a geoid of Ethiopia provided by NGA.

The observed gravity values are corrected by subtracting the difference in the

values at a particular time on the drift curve compared to the initial value taken at a local

base station. Large instrumental drift curves (e.g., 1 mGal/hr) are unusual, so a minimal

drift (0.05 mGal/hr) is commonly expected for a day in the field. Using the International

Gravity formula (Morelli, 1976), the gravity data are corrected for the Earth’s shape and

rotation with is mainly a function of latitude. Since the Earth is an oblate spheroid that

bulges at the equator and is not a true sphere, lower latitudes have higher gravity

readings. The Free-air correction is used to correct for a reduction in the magnitude of

gravity with an increase of the height above the geoid. Therefore, the Free-air correction

takes the difference between gravity at sea level and the particular height of your gravity

point. This correction is the reason for the use of a dual frequency GPS system in order

to collect accurate elevation data.

The Bouguer correction is used to account for rock mass between the gravity

station and sea level. An infinite slab of rock with a thickness of the elevation about the

geoid, is assigned a density, typically 2.67 g/cm^3. However, the density value can

change if the area is highly influenced by denser or lighter rocks.

Terrain corrections were calculated using 1 km digital elevation models (DEM)

and the method of Nagy (1966). The resultant terrain corrections varied between 0.0 and

41

22 mGals for the Tendaho graben and surrounding regions. These values were added to

the Bouguer gravity anomaly values in order to obtain complete Bouguer gravity

anomalies. These values were used in all subsequent interpretations. The complete

Bouguer gravity values were gridded using the minimum curvature technique (Briggs,

1976) with gridding intervals of 2 km for regional gravity maps +0.5 kim for RG maps.

Then the resultant grids were contoured at an interval of 10 mGals in order to produce a

complete Bouguer gravity anomaly map (Figures 19, 20, and 21).

Gravity Data Interpretation

The main end-product of gravity data reduction is the complete Bouguer gravity

anomaly, which should correlate only with lateral variations in density within the Earth

and which are of most interest to applied geophysicists and geologists (Reynolds, 1997).

The variation of the Bouguer gravity anomalies should reflect the lateral variation in the

density such that a high density feature in a lower density medium should give rise to a

positive Bouguer gravity anomaly. Conversely, a low density feature in a higher density

medium should result in a negative Bouguer gravity anomaly (Reynolds, 1997). There

may be a gentle trend in the Bouguer gravity field, reflecting a long wavelength gravity

anomaly attritutable to deep-seated crustal features; this is known as a regional anomaly.

Shorter wavelength anomalies arising from shallower geological features may be

superimposed on the regional gravity anomaly, and it is these anomalies that are often to

be isolated for further analysis (Reynolds, 1997). A separation between the regional

gravity anomaly and the Bouguer gravity anomaly will create a residual gravity anomaly,

which is usually the anomaly that one is interested in interpreting.

42

Figure 19. Complete Bouguer gravity anomaly map of the study area including the Afar Depression. The contour interval is 10 mGal. Bolded numbers represent anomalies discussed in the text.

43

Figure 20. Bouguer gravity anomaly map of the Tendaho Graben. Contour interval is 10 mGals. Bolded numbers represent anomalies discussed in the text.

44

Figure 21. Bouguer gravity anomaly map of the Tendaho graben. + are the locations of gravity stations. Contour interval is 10 mGal. Bolded numbers represent anomalies discussed in the text.

45

To interpret Bouguer gravity data, anomaly maps are produced that show the

variations between gravity values. Bouguer gravity anomalies can be enhanced to show

small scale gravitational differences through techniques such as wavelength filtering

(Peeples et al., 1986), polynomial trend surfaces (Lance, 1982), edge enhancements, and

isostatic residual anomalies. These techniques are used to qualitatively interpret both the

regional and residual Bouguer gravity anomalies. However, these techniques are based

on mathematical interpretation of the data, and anomalies may be created that are not

related to actual density contrasts (Ulrych, 1968).

A simple technique in interpreting gravity data is comparing the Bouguer gravity

anomaly map to a geological map. For example, the geological map of the TG (Figure 7)

shows a strong correlation to the Bouguer gravity anomaly map (Figure 20). For

example, young volcanic in the graben produces a high Bouguer gravity value (anomaly

1) while, the flanks of the graben correspond to a low Bouguer gravity value.

Additionally, high-pass wavelength filtering of the Bouguer gravity data, which removes

the long wavelength anomalies, can be used to enhance shallow features in the basin.

The regional gravity anomaly map of the Afar Depression was filtered at wavelengths

between 70 to 2500 km (Figure 22). However, the regional map of the TG was filtered

between 20 to 1000 km because of the smaller scale anomalies (Figure 23).

As a first step in my analysis, a Bouguer gravity anomaly map was constructed

for the entire Afar depression using a Bouguer reduction density of 2.670 g/cm3 and sea

level as a datum (Figure 19). The newly acquired gravity data have allowed this map to

show classic rift features better than previous maps (Mahatsente et al., (1999, 2000),

46

Figure 22. Filtered gravity anomaly map of the Afar region. Wavelengths were passed at 70 to 2500 km. Contour interval is 10 mGal. Bolded numbers represent anomalies discussed in the text. Bolded lines represent country boundaries.

47

Figure 23. Filtered gravity anomaly map of the Tendaho Graben. Wavelengths were passed at 20 to 1000 km. Contour interval is 10 mGal. Bolded numbers represent anomalies discussed in the text. Bolded line is the Ethiopia-Djibouti border.

48

Jentzsch et al. (2000) and Tiberi et al. (2005). However, the white areas are due to lack

of data in Somalia and the Arabian Peninsula.

The areas bounding the Afar depression are associated with Bouguer gravity

values that average approximately -220 mGal (anomalies 1 and 2) which separate the

anomalies within the Afar depression and the broad minima related to the Ethiopian

escarpment. The Ethiopian plateau and Somalian plateau are indicated by relatively

gravity minima (anomalies 1 and 2) on the western and southern portion of the map,

respectively. The low amplitude Bouguer gravity values are due to significantly thicker

crust than in the Afar depression as shown by several seismic studies (Berckhemer, 1975;

MacKenzie et al., 2005; Maguire et al., 2006). The steep Bouguer gravity gradients

transition the Ethiopian and Somalian plateaus into the Afar depression. The relatively

high amplitude Bouguer gravity anomalies are due to thinning of the crust as shown by

seismic refraction (Berckhemer, 1975; MacKenzie et al., 2006) and tomographic studies

(Keraren et al., 2004; Debayle et al., 2005).

High amplitude Bouguer gravity gradients and a relative gravity maximum occur

along the MER found to the southeast of the Afar triangle (anomaly 3). This anomaly

continues south to the central MER (Mickus et al., 2007) and represents thinner crust

and/or igneous intrusions in the upper crust. The MER Bouguer gravity values ranges

from -150 to -100 mGal.

Within the Red Sea and Gulf of Aden regions, the gravity field is characterized by

regional Free-air gravity maxima with an average value of 10 mGal. These areas are

indicated by orange and red colors on the gravity anomaly map (anomalies 4 and 5,

Figure 19). These anomalies are due to a combination of crustal thinning within the rift

49

margin and topographic highs associated with the rifting. The extension of the Gulf of

Aden rift onto land can be seen at anomaly 6, which is associated with the Tadjoura rift in

Djibouti (Manighetti et al., 1998). The rift is associated with a gravity minimum due to

the rift valley and extends into Djibouti until label 7 where gravity maxima within

Djibouti overprint the rift anomaly. This implies the major rifting features extend only a

few kilometers into Djibouti and does not meet at one point with the Red Sea propagator.

The majority of the Afar depression is characterized by average Bouguer gravity

anomalies of approximately -75 mGal (Figure 19) and is a regional gravity maximum

compared to the Ethiopian and Somalian Plateaux. However, there are several maxima

where the Red Sea (anomaly 8) and Gulf of Aden (anomaly 7) propagators come on land.

Bouguer gravity values over the propagators range from -40 to -10 mGal, while the Red

Sea propagator is associated with relatively higher values than the Gulf of Aden

propagator. The maxima are probably caused by thinner crust caused by the onset of

oceanic spreading. The rifting has not created large scale grabens that produce gravity

minima as seen in the Red Sea. These two maxima do not intersect which indicates that

Afar triple junction does not meet at one point but is dispersed throughout the region

between the two ends of the gravity maxima. The other prominent gravity anomaly in

Afar is the region bounded by the Red Sea propagator and the Red Sea. Here, the

Danakil block is associated with a gravity minimum (anomaly 9) with an average

Bouguer gravity anomaly of -45 mGal which is due to thick Precambrian crust

juxtaposed next to highly thinned oceanic crust (Eagles et al., 2002) and is a region that

has so far escaped the rifting events in the rest of Afar.

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In order to analyze the gravity anomalies associated with the Tendaho graben

which lies between 40 – 43 degrees E and 10 -13 degrees N., a smaller scale Bouguer

gravity anomaly map was created (Figure 20). This map shows that the TG is associated

with a relative gravity maximum (anomaly 1) that trends NW-SE in the center of the

graben (Figure 20). This high, which I will show in the modeling section, is probably

caused the denser mafic material intruding from mantle in the rift center. The TG is

bounded by a broad Bouguer gravity minimum (anomaly 2) on the western flanks due to

thicker crust along the graben escarpment. While, a Bouguer gravity maximum (anomaly

3) is along the eastern flanks of the graben may be caused by thinned crust or additional

dense mafic in a detailed gravity survey was completed within the southern section of the

TG near Semera in order to evaluate the geothermal potential of the region and the

structure of the graben. Figure 21 shows that this section of the TG is associated with a

series of short wavelength maxima superimposed on a Bouguer gravity minimum. The

largest amplitude maxima are associated with two volcanoes, Karub (anomaly 1) and

Damal (anomaly 2). Longer wavelength and relative positive Bouguer gravity anomalies

(anomaly 3) are found in the northwestern portion of the TG, which are caused by

intrusion of mafic material. The low amplitude Bouguer gravity anomalies (anomaly 4)

within the Beyhaile plain are due increases in thickness of the graben-fill sediments

(mostly lacustrine) and to the attenuation of the mafic dyking seen to the north. The

highest amplitude Bouguer gravity maximum (anomaly 5) on the western portion of the

Beyhaile plain is associated with hydrothermal minerals and activity (Lemma et al.,

2010).

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The short wavelength Bouguer maxima and minima (anomaly 6, Figure 21) near

Serdo are due to rhyolites (minima) and basalts (maxima) in the extrusive complexes

along the margins of the graben. The area near the Dubti plantation is associated with

lower amplitude Bouguer gravity anomalies (anomaly 7) because of the intrusive and

thicker low density sediments above thin layer of basalts erupted from Karub. The

thickness of sediments increases to towards the southeast as this can be seen in the

decrease in the Bouguer gravity values.

Residual Gravity Anomalies

The main objective of gravity anomaly enhancement is to produce a map with

anomalies that are related to features of interest, which are called residual gravity

anomalies. By creating residual gravity anomalies by removing (or enhancing)

wavelengths certain sizes by wavelength, anomalies on the Bouguer gravity anomaly map

are seen much clearer on the filtered or residual gravity anomaly map. There are no set

combinations of wavelengths that define a residual gravity anomaly map so commonly a

number of filtered gravity maps are constructed. Then, the user will choose which map

best illustrates the features they are interested in interpreting.

At first glance, most filtered gravity anomaly maps are similar to the Bouguer

gravity anomaly map with certain features (usually shorter wavelength anomalies)

enhanced and more easily interpretable. As can be seen in Figure 22 where wavelengths

between 70 to 2500 km were passed (or enhanced), the Ethiopian and Somalian plateaus

(anomalies 1 and 2) are seen clearly as relative minima on the western and southeastern

portions of the map. Also, the general shape of the gravity anomaly due to the crustal

thinning in the Afar depression is similar in shape to the Bouguer gravity anomaly map

52

(Figure 19). However, the MER is enhanced and showing a short wavelength regional

high trending in a SW-NE direction (anomaly 3). This anomaly has been shown in the

MER to the south by Mickus et al. (2007) who interprets this anomaly to shallow

intrusion of mafic material into the upper crust related to the magmatic segments

(Ebinger and Casey, 2001). The shorter wavelength anomalies due to the shallow,

small-scale structures (e.g., grabens, dyking) have been removed by the choice of

wavelengths.

Both the Gulf of Aden and Red Sea propagators are seen more clearly by regional

highs (anomalies 4 and 5, respectively) on Figure 22. The Gulf of Aden propagator can

be traced coming onto to land near Djibouti and trending due west until reaching the

central Afar. However, this anomaly is not continuous and does not coincide with the

Tadjoura Rift and the gravity anomalies (minima related to rift basins) seen on the

Bouguer gravity anomaly map (Figure 19) suggesting that anomaly 4 may be due to other

mafic intrusions or that the Gulf of Aden propagator is wider than the surface expression

of the Tadjoura Rift. While, the Red Sea propagator comes onto land trending N-S and

switching to a NW-SE trend at approximately 13.7 N, which is in a similar direction to

structures found in the Tendaho Graben (Beyene and Abdelsalam, 2005). Though the

Afar depression is the location of the Afar triple junction caused by three rift systems, the

filtered gravity anomaly map shows clearly that there is not one intersection point.

Figure 23 is a filtered gravity anomaly map where wavelengths between 20 and

1000 km are passed and emphasizes the gravity field due to the TG. The Tendaho

Graben is clearly seen by the relative high on the western portion of the map due the

mafic dyking within the TG (anomaly 1). Also, the MER is clearly seen trending NE and

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intersecting the TG on the southwestern portion of the map (anomaly 2). The two

relative gravity minima (anomalies 3 and 4) are seen more clearly than on the Bouguer

gravity anomaly map (Figure 19). The gravity maximum separating these two regions is

enhanced and corresponds with the geothermal activity at Ayrobera.

Gravity Data Modeling

The above discussion of the gravity anomaly maps, showed that the anomalies are

caused by a variety of sources related to the formation of the Afar depression and TG.

By only qualitatively interpreted the sources of these anomaly maps, one may

misinterpret the location and geometry of the sources. To make the interpretation of the

gravity data more quanitative, three gravity models were constructed in the southern

sections of the TG where the most data are abundant and some constraints (e.g.,

drillholes) are available. The three profiles (A, B, and C on Figure 24) were selected

across the TG, two running perpendicular across the basin (profiles A and B (Figure 25

and Figure 26), and the other one (profile C (Figure 27) trending along the axis of the

TG. The other profile (profile D, Figure 28) was modeled by Mickus et al. (2010) and

follows the main road across the TG and contains the most detailed gravity data.

The models were determined using the GMSYS software which computes the

gravitational attraction of multiple two and one-half dimensional polygonally-shaped

bodies with finite strike lengths. The gravitational attractions due to each body were

calculated using the gravity station elevations and the gravitational attraction due to all

the bodies were added to together to determine the total gravitational attraction due to

entire model. This calculated gravity anomaly was then compared to the observed

complete Bouguer gravity anomaly data. When the difference between the observed

54

Figure 24. Location of the profiles used for the gravity modeling of the TG. Each profile is labeled with corresponding letter (A,B,C, and D). The colors and contour interval are the same as Figure 21.

55

Figure 25. Two and one-half dimensional model along Profile A. The solid line is the calculated gravity anomaly due to the model and the squares are the observed gravity anomalies. Bodies include: Lacustrine sediments-light green, young basalts-dark green, Stratoid basalts-blue, basement-purple, mafic intrusions-red. These bodies are the same for profiles B and C.

56

Figure 26. Two and one-half dimensional model along Profile B. The solid line is the calculated gravity anomaly due to the model and the squares are the observed gravity anomalies.

57

Figure 27. Two and one-half dimensional model along Profile C that runs along the graben axis. The solid line is the calculated gravity anomaly due to the model and the squares are the observed gravity anomalies.

58

Figure 28. Two and one-half dimensional model along Profile D. Densities of the bodies in g/cc are labeled on the model. After Mickus et al. (2010) Green-Basement, Brown-Stratoid, Yellow-Lacustrine Deposits, Gray-Rhyolite, Orange- Mafic body, Pink- Hot Material, Red and Violet- Post Stratoid Basalts. Red line is the calculated gravity anomaly due to the model and the black line is the observed gravity anomaly.

59

complete Bouguer gravity and calculated gravity data was less than 10 percent, the match

was considered sufficient.

One problem with contructing a gravity model in most locations and in the TG

particularly, is the lack of constraints. Subsurface constraints may include drillhole data

(depths to geologic units and densities), surface geological mapping, seismic reflection

and/or refraction models, and electromagnetic models. Surface geological studies and

maps indicates that the basin walls are composed entirely of the Afar Stratoid Series

(Abbate et al., 1995). While the basin is filled with lacustrine, alluvial deposits, and

younger basalt flows (Lahitte et al., 2001). The only major subsurface constraints come

from a project by Aquater which provided limited drillhole data in the TG where four

geothermal wells were drilled withint the TG. Three of the four wells were placed inside

of the Dubti Cotton Plantation. Three of the wells were drilled into the Afar Stratoids,

while the other well was a shallow test well. There was no density information of the

major rock units and the starting densities were based on average densities for each rock

type (Telford et al., 1988). Additionally, the starting densities were based on a gravity

model (profile D) by Mickus et al. (2010). The starting densities were varied by 10

percent during the modeling process. Additionally, based on geological mapping and the

drill hole data, five major layers were indentified including lacustrine sediments (light

green), young volcanics (dark green), Afar Stratoid basalts (light blue), hot dense mantle

material (red) and the basement (dark blue).

The final models (Figures 25 through 28) show one possible solution to the

oberved complete Bouguer gravity anomalies. These models are reasonable based on the

60

limited constraints and geometries, thicknesses, and depth are reasonable based on the

limited geological information (Lahitte et al., 2001, Abbate et al., 1995). The densities of

layers in g/cc are: mafic body (2.88); Afar Stratoid basalts (2.75); post Stratoid basalts

(2.75); lacustrine/alluvial sediments (2.2); basement (2.72). The basement of the TG was

given a density of 2.72 g/cc because the density of the basement rocks (either

Precambrian or Mesozoic rock units) are not known. The deepest drill hole only

penetrated the Afar Stratoid basalts in the TG. The gravity models show several layers

with a varying densities from 2.20-2.88 g/cc. The models assume that the anomalies are

caused by upper crustal density variations as the profiles are too short to be affected by

lower crustal and/or mantle density variations. The models represent one possible

solution based on geophysical constraints. However, more constraints will provide a

more accurate solution.

The Afar Stratoid basalts, which are the most common exposed lithology within

the TG, are located beneath the lacustrine sediments and along the edges of the TG and

are modelled directly above the basement lithologies which are of an unknown type.

Their thickness ranges from 500-1000 meters which is acceptable due to the flood basalts

that covered the area nearly 30 Mya. Also, the thickness I used are based on exposures in

nearby grabens (e.g., Dobi) which have thicker exposures of the basalts. Even though the

exposed Strataoid basalts are nearly horizontal in most regions, I modeled them with

some structure (e.g, model A and B, Figures 25 and 26). These structures were needed

into order to fit the observed gravity anomalies. The structures were caused by the

extensional faulting that formed the TG and resulted in the Stratoid basalts being

displaced deeper within the graben. The post Stratoid basalts are given the same density

61

value as the Afar Stratoid basalts. However, they are represented as a separate unit in my

models because their age corresponds to the opening of the TG.

The TG is mainly overlain by a layer composed primarily of lacustrine and

aeolian deposits that range from 1500-2000 meters thick and this thickness agrees with

the few deep drill holes in the study area. However, Figures 25 and 26 indicate that the

basin thickens towards the south as the amplitude of the gravity minima increases from

the modeled region. The low denstiy layer results in a large density contrast with the

surrounding Stratoid basalts producing gravity minimum over most of the TG.

The Tendaho graben has been modeled as having a high denstiy mafic body near

its center surrounded by lower density basalts, volcanics, and lacustrine deposits (Figure

25 through 28). This body was first modeled by Mickus et al. (2010) as shown in Model

D (Figure 28). The body is thought to represent mafic dikes originating from the mantle

and Mickus et al. (2010) showed additionally that there may be a lower density portion of

the dike that is caused by high heat flow values. These two bodies are nonunique and

were originally modeled in conjunction with magnetic data modeling (Mickus et al.,

2010). I did not put the lower density body in my mafic dyking as the gravity data did

not require such a body and there was no outside constraints that required such a body.

Models A and B (Figures 25 and 26) show that the high density mafic body is

approximately 6-8 km wide and has its top 1.5 km below the surface. This model may

represent deep-seated mafic dyking, which results in the abundance of geothermal

activity and could be the source of the recent volcanic activity. This assumption is

supported by the gravity maxima over the rift axis on the filtered gravity anomaly map

(Figure 23) and drillhole data that indicated 1.5 km thickness of the top unit. The two

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dense bodies in profile A-A’ are associated with the central mafic body. Each body is

less than 250 meters wide; however, its top is only 1 km from the surface. These bodies

coincide with the Dubti fault, as well as hot springs and thermal anomalies. Model C

(Figure 27) indicates that the mafic dyking is continuous along the axis of the southern

portion of the TG. However due to the lack of gravity data, I could not determine how

far to north or south, this high density body continued.

Forward modelling (Figure 14) by Mickus et al., (2010) corresponds to profile D

(Figure 28). This model was used the starting models for the densities and basin structure

of models A-C. Additionally, this model was constrained by magnetic modeling (Mickus

et al., 2010) which necessitated the insertion of a lower density region within the mafic

dyking region. Additionally, the thin, horizontal basaltic bodies within the lacustrine

sediments where not required by the gravity data but were required by the magnetic data

modeling. Additionally, drill holes in the region did encounter several thin basaltic layers

within the lacustrine sediments. The gravity high on this model is again interpreted as a

mafic body, though wider on profile D. Rhyolites which outcrop along the border of the

TG are seen bounding the model with a density of 2.35 g/cc, which do not occur on either

profile A or B. The small scale thickness variations of the lacustrine sediments are

thought to be caused by extensional faulting related to the formation of the TG.

63

MAGNETIC DATA ANALYSIS

The magnetic data were obtained from Aquater who collected total-field magnetic

data in 1980. A total of 2042 stations were measured with a proton precession

magnetometer. The data was input into the Oasis Montaj program and gridded using

minimum curvature. The grid size was 10 km and the contour interval was 15 nT. Once

the data was gridded, Oasis Montaj generated an anomaly map of the magnetic data. The

map is useful in showing structures within the basin.

The magnetic total field anomaly map (Figure 29) shows clearly the structure of

the rift axis. The problem with interpreting magnetic data within the TG is that it is

completely surrounded by the Stratoid basalts and the Afar is located near the magnetic

equator. Magnetic anomalies are usually caused by variations in magnetite in a rock and

this can be extremely useful in delineating basement structures beneath sedimentary

basins (e.g., Mickus et al., 1988). However, when the basement is basalt and there are

basalt outcrops within the basin, the anomalies are usually caused by variations of

magnetite within the basalts and may not provide a lot of information about structures in

the region. Additionally, since the TG is near the magnetic equator, the data are difficult

to interpret since positive magnetic anomalies are caused by a lack of magnetite and

rocks with a higher magnetite content cause negative anomalies. This is the opposite of

what occurs in northern latitude. Despite these limitations, I will show that the magnetic

data are useful in determining tectonic information of the TG.

The TG is associated with a large amplitude minimum that trends NW-SE and is

bounded by relative magnetic highs (Figure 29). As mentioned above, typically in the

northern hemisphere, the rift axis would be represented by a magnetic maximum,

64

Figure 29. Total field magnetic anomaly map of the TG. Bolded numbers represent anomalies discussed in the text. Blank regions are areas with no data. Units are gammas.

however, the TG is located at lower latitudes and the resulting inducing field creates a

magnetic minimum. I interpret this minimum to be caused by normal polarity basaltic

material recently formed (Bridges et al., 2010). Conversely, the linear magnetic maxima

(anomalies 1 and 2, Figure 29) that are parallel to the magnetic minimum are interpreted

to be caused by reverse polarity segments. The segments consist of older basalts

(Bridges et al., 2010). The highest amplitude magnetic minimum is seen from the

65

volcanic center Damal (anomaly 3) and is probably caused by a thicker unit of magnetite-

rich igneous rocks.

There are a number of techniques that can be used to interpret and enhance

magnetic and are in general the same as for gravity data. Because I am interpreting the

anomalies in the magnetic data to be mainly caused by variations in the polarity of the

basaltic rocks, I will not apply wavelength filtering of the data. However, one method

that is commonly used in magnetic data interpretation is three-dimensional Euler

deconvolution (Reynolds, 1997). This technique uses the gradients of the magnetic field

in three-dimensions to differentiate the horizontal boundaries of magnetic bodies and

depth to source. Euler’s equation relates the Earth’s magnetic field and it gradient

components (dX, dY, and dZ) to the degree of homogeneity. Homogeneity for geological

purposes is expressed as a structural index ranging from 0-3, with each value representing

a different geological environment or geometry of the source body. I used a structural

index of 1 for the TG since a structural index of 1 represents vertical dikes and based on

the gravity modeling, there is widespread mafic dyking in the TG. The main advantage

to Euler deconvolution is the no geological constraints or model is assumed prior to

processing. This is useful in the case of the TG, because few geophysical studies and

constraints are available for the region.

In performing an Euler deconvolution, gradient components in the x, y, and z

direction are determined. These components are useful in determining the lateral

boundaries of magnetic bodies with the most useful of the components being the vertical

derivative (dZ) (Reynolds, 1997). Figure 30 shows the vertical gradient and the

approximate edges of the magnetic bodies. The higher gradients represent the boundaries

66

Figure 30. Vertical derivative magnetic anomaly map of the TG. Numbers represent anomalies discussed in the text. Blank areas represent regions without data. Units are gammas/km.

of the magnetic bodies. The main features are the small magnetic bodies (e.g., anomalies

1 and 2) within the rift axis and are probably caused by variations in magnetite within the

Stratoid basalts. These anomalies represent the edges of active dyking along the rift axis.

The linear magnetic anomalies related to polarity changes are also seen as in Figure 30

67

(anomalies 3 and 4). This dyking is supported by the previous gravity modeling (e.g.,

Figure 26).

The Euler deconvolution method also provides depth estimation to the top of the

magnetic bodies and can be used to obtain a general idea about the depth of sources.

However, some of the depths that are estimated may not be valid because of geologic

constraints. Since the gravity modeling (Figures 25 through 27) and drillhole constraints

showed that most depths of the lacustrine sediments are less than 2km, depths greater

than 3 km were eliminated. However, there were less than 10% of the total depths

greater than 3 km. Figure 31 shows the depths determining using Euler deconvolution.

Most of the depths are between 1 and 2 km, which agree with the gravity models.

Additionally, the location of the depths, which are calculated at gradient maxima, can

give an idea of structural trends in a region. Figure 31 shows a general NW-SE trend,

which is expected given the magnetic analysis shown above. Other trends are probably

caused by magnetite variations within the basalts and are not related to structures.

Depths less than 1-2 km are probably interbedded basalts within the lacustrine deposits.

Magnetic data can be modeled in a similar fashion as is done with gravity data. I

did not perform such modeling as Bridges et al. (2010) constructed a series of magnetic

models across the TG. Figure 32 shows the final forward model that is similar to the

gravity model D (Figure 28) along the main road, has a ~10 km wide region along the

axis of the graben exhibits normal polarity (Figure 32) causing a magnetic minimum.

68

Figure 31. Depth estimation map of the magnetic bodies within the TG determined using Euler deconvolution. The symbols indicate different depths to the top of magnetic bodies. The sources are dikes (structure index 1).

69

Figure 32. Two and one-half dimensional model of magnetic data. Densities of the bodies in g/cc are labeled on the models. After Bridges et al. (2010) Green-Basement, Brown-Stratoid, Yellow-Lacustrine Deposits, Orange- Mafic body, Pink- Hot Material, Red and Violet- Post Stratoid Basalts. Red line is the calculated gravity anomaly due to the model and the black line is the observed gravity anomaly.

70

Within the minimum is a short wavelength maximum that is modeled by a region of high

flow lowering the effective magnetic susceptibility of the magnetite within the basalts.

To each side of the minimum are larger shoulders of dominantly reverse polarity units

causing magnetic maxima, similar as those seen on the magnetic anomaly map (Figure

29). A Bouguer gravity maximum coincides exactly with the magnetic minimum (Figure

29). The gravity maximum and magnetic minimum are likely due to the active dyking

and/or concentrated high heat flow along the current ridge axis. The magnetic minimum,

that follows the ridge axis, exhibits a narrow 2- 3 km zone of slightly elevated magnetic

values indicative of higher crustal heat flow as mentioned above.

Discussion

The above discussion of the Bouguer gravity and magnetic anomaly maps and

models show that they mainly reflect Pleistocene tectonic regime and volcanic events that

formed the Afar depression and the TG. The initial stages of the development of the

EAR occurred around 30 million years ago in the Afar plateau (Hoffman, 1997) as the

Afar plume released large basalt flows in this area (Hoffman, 1997). The TG began to

open 1.8 Mya with eruptive fissures that emplace the Stratoid basalts (Acton et al., 2000).

When the basin first formed, it opened at a rate of 1.6 cm/ yr and the once surface

Stratoid basalts became the walls and basement of the TG. The basin was mainly formed

by extensional forces as evidenced by with normal faults now parallel to walls of the TG

and later these walls experienced a clockwise rotation (Beyene and Abdelsalam, 2005)

due to the interaction with the Red Sea and Gulf of Aden propagators. Throughout the

Pleistocene, the formation of the basin transitioned from tectonically controlled to

magnetically controlled. However, further post Stratoid basalt eruptions occurred and

71

were interbedded within the basin fill. These events can be evidenced by my analysis of

the gravity and magnetic fields within the TG.

Bouguer gravity anomaly maps (Figures 29, 30, and 31) of the Afar Depression

indicate a regional gravity maximum in comparison to the Ethiopian and Somalian

plateaux. The maximum is due to a highly thinned crust with a thickness of 26 km

compared to nearly 40 km thickness of the plateaux confirmed by seismic data (Maguire

et al., 2006). This difference in thickness is due magmatic underplating beneath the

Ethiopian plateau.

The Bouguer gravity wavelength filtered maps (Figures 22 and 23) which

represent density bodies due to crustal features indicate a deeper, denser structure

throughout the center of the TG, which corresponds clearly to the Red Sea Propagator.

However, the Gulf of Aden propagator does not terminate in the TG proving there is no

single point that can be called the Afar triple junction. The gravity maxima in the TG are

caused by the high density basalts released from the dyking and volcanic sequences that

date back to the opening of the TG. The geochemistry of the basalts shows that there are

two types of basalts in the TG, one associated with the Afar plume and the other

associated with oceanic seafloor spreading. Initially, the basin was influenced by the

Afar plume and basalts of reverse polarity were formed within the basin. The basalts

show a change in polarity around 0.6 Mya from reverse to normal polarity. The magnetic

anomaly maps (Figures 29 and 30) suggest a 6-8 wide body of normal polarity bounded

by reversed polarity lithologies confirming the transition from continental rift to seafloor

spreading.

72

Modeling of the gravity data indicates that the basin thickness ranges from 1500-

2000 meters, thickening towards the southeast of the basin. The basin is bounded by

normal faults that produce a graben. Additionally the extensional nature of the basin’s

formation can be seen on the top of the Stratoid basalts where small scale highs and lows

are interpreted as normal faults. A 6-8 wide mafic body associated with high heat flow

lies underneath the basin fill. The high heat flow and low density layer above it provides

a good medium for geothermal activity which is present in the TG. This mafic zone is

composed of high magnetic susceptibility bodies with a center of higher heat flow as

confirmed by magnetic mapping and modeling. Geochronological data combined with

the gravity and magnetic modeling indicates that the basin opened nearly 1.8 Mya with a

spreading rate of 1.6 cm/yr (Acton et al., 1991). Recent data suggests that the graben

spreading rate is slowing (Bridges et al., 2010) and that the basin is transitioning from

being tectonically controlled to magmatically controlled.

Several models have been suggested proposed for the evolution of the TG (e.g.,

Thurmond et al., 2007; Acocella et al., 2008). Common to all models is the placement of

the Stratoid and post Stratoid basalts. However, both Acocella (2008) and Thurmond

(2007) disagree with the placement of the rhyolites along the border faults. Our proposed

model suggests that rhyolites were deposited after the rift initiation of the TG rather than

prior to the basin opening. This new work has provided more constraints and can explain

the evolution more accurately. Figure 33 shows a new model on the formation of the TG.

The Stratoid basalts were emplaced between 2.0-1.8 Mya (model A, Figure 25). Based

on spreading rates derived from geochronologic data of the basalts, the TG opened

around 1.8 Mya (Model B, Figure 26). The ryholites were emplaced around 1.3 Mya

73

along the escarpment faults (Model D, Figure 28). During its evolution, the basin

deepened and post stratoid basalts were deposited on the young basin floor. The data

indicate that the mafic dyking is continuous along the axis of the southern portion of the

TG (Model C, Figure 27). This dyking led to the young volcanic emplaced along the

basin floor during the remainder of the Pleistocene. The period continued with high

deposition of Aeolian and lacustrine deposits interbedding with the young volcanics,

which is confirmed by drill data (Aquater, 1996). At 0.8 Mya, accommodation of

extension in the TG moved from the margins towards the center of the graben (Figure

29). At this time, the basalts in the TG were dominated by reversed polarity. However,

the polarity transitioned to normal polarity and confirms the normal polarized mafic body

at the basin axis. The present TG width of ~50 km is the result of both brittle

deformation and magma emplacement. Dyking and geothermal activity is concentrated

in the center of the graben following the Dubti fault as well as the Ayrobeara Geothermal

field.

74

Figure 33. Illustration of the evolution of the Tendaho Graben (After Bridges et al., 2010). Brown- Afar Stratoid basalts, Red- Young volcanics, Yellow- Lacustrine Deposits, Purple- Post Stratoid basalts, Tan- Rhyolites.

75

CONCLUSIONS

An analysis of the gravity and magnetic field of the Afar depression and the

Tendaho Graben revealed the crustal structure of the region. The Tendaho graben is the

largest basin within the Afar Depression and is located at triple junction of the East

African, Red Sea and Gulf of Aden rifts. The region has undergone extensional forces

and that has produced highly thinned crust and evidenced by seismic and gravity

analyzes. Gravity data analyzed by a variety of anomaly maps and two-dimensional

modeling supported by magnetic, geochemical, and geochronological data clearly show

that the extension of the Red Sea propagator created a 6-8 km wide mafic body within the

TG. Magnetic maps and modeling correspond with the mafic body at the rift axis and

indicate that the body has normal polarity. However, dominantly reversed Stratoid

basalts along the margins show a change in polarity that is dated at 0.8 Mya. The mafic

body is understood to be the initial stage of magnetic striping that occurs in oceanic crust

and thus indicates that oceanic spreading is occurring within a continental setting. The

basin geometry supported by gravity data suggests that the TG has transitioned or is

transitioning from a continental rift into an oceanic spreading center.

76

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Gravity Analysis of the Tendaho Graben, Afar Depression_ethiopia

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