ELSEVIER Earth and Planetary Science Letters I5 I ( 1997) 13-3 I

EPSL

The East ridge system 28%32OS East Pacific rise: Implications for overlapping spreading center development

Fernando Martinez * , Richard N. Hey, Paul D. Johnson

Received 8 July 1996: revised 24 March 1997: accepted 30 April 1997

Abstract

We report here on geophysical data from the East ridge and surrounding areas of the large-offset overlapping spreading centers (OSCs) that accommodate Pacific-Nazca opening between 28.5” and 32% The East ridge overlaps and is offset from the West ridge system by - 120 km, forming the largest known pair of OSCs. In this area spreading rates reach the fastest currently active on Earth of - 149 mm/yr. Although the East ridge is composed of 4 morphologically defined segments separated by 3 small OSCs, other geophysical characteristics imply I upwelling segment. All the active ridge segments in this area (including the propagating tips of the East and West ridges) form relative topographic highs with

respect to the flanking sea floor; however, identified abandoned ridge tips form deeps. We interpret these data in terms of a model in which the propagating segment represents an overshoot of a surficial rupture of the brittle lithospheric layer, only

partially coupled to the diverging flow of a more broadly distributed ductile deformation zone (DDZ). surrounding the steady-state ridges and crossing the offset between the OSCs. The topographic high of the propagating segment may be maintained primarily by along-axis melt migration from the stable spreading segments rather than by direct upwelling from beneath the ridge. The large overlapping ridges are inherently unstable and continued extension causes the overlapping axes to become offset from the stably spreading segments, cut off from the supply of melt, and replaced by a new set. The failed rift tips, for a period of time, overlie the broad DDZ and preferentially undergo continued extension and subsidence. The

DDZ surrounding the ridge axes may be very broad in this area because of the very fast spreading rate, creating a very thin lithosphere susceptible to perturbation by relatively small mantle heterogeneities advected near the ridge axis, leading to the formation of the smaller OSCs observed. 0 1997 Elsevier Science B.V.

Kewvrds; sea-floor spreading; East Pacific Rise; plate tectonics; plate boundaries

1. Introduction

Pacific-Nazca separation near 29% occurs at a rate of - 149 km/Ma. Since about 1.5-2.0 Ma the

* Corresponding author. Tel.: + I 808 956 6881. Fax: + I 808 956 3188. E-mail: martinez@soest.hawaii.edu

kinematics of this area have involved the overall southward propagation of large-offset overlapping spreading centers (OSCs) (Fig. 1) [ 1.21. Previously, the plate boundary here was a single spreading sys- tem between the Easter [3.4] and Juan Femandez [51 microplates. The former ridge system terminated at its northern end against what may have been the

worlds fastest-slipping transform fault [I], which then

OOIZ-821X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.

PI/ SOOIZ-821X(97)00095-2

14 F. Martinez et al. /Earth and PlanetaT Science Letters 1.51 (1997) 13-31

may have also formed the southern boundary of the Easter Microplate. For unclear reasons, but possibly

related to hotspot activity near Easter Island [6], ridge propagation initiated on a spreading system

near the transform, replacing it by the large-offset

osc. We refer to the current pair of ridge systems in

this area as the West and East ridges. Each is

composed of smaller, morphologically defined, non- transform offset segments: W l-W4 and El-E4 re-

spectively (Fig. 2). The current spreading systems may be an initial stage in the formation of a mi-

croplate [1,7]. The sea floor spreading centers over- lap and are separated by approximately 120 km, giving them an overlap to separation ratio of 1: 1,

similar to the surrounding microplates, rather than

the 3: 1 typical of overlapping spreading centers [8,9]. Further description of the regional geology and kine-

matics can be found in Hey et al. [I], Koronaga and Hey [2], and Johnson [lo].

Nearly complete bathymetric, gravity and magnet- its coverage of the East ridge allows a comparison

of geophysical signatures between its recently propa- gated northern segment, which overlaps with the

West ridge, and its southern segments which appear to be more stably spreading since at least anomaly 2 [l]. Data coverage is not as complete for the West ridge and bad weather during that part of the survey significantly degraded its partial gravity coverage. In

addition, the West ridge system appears to be anoma-

lous, with spreading accommodated on multiple, short, overlapping segments with large offsets and overlap configurations that do not conform to the

usual inward curvatures typically observed elsewhere on OSCs of the EPR [S,ll,12]. Some of these fea-

tures may be hotspot influenced [6]: in particular the very large inflation of some of the segments [l] and the overall southward migration of the ridge system. Because of these complications and incomplete cov- erage we restrict this study to the East ridge system.

Although the terms “OSC” and “propagating” spreading centers have been previously defined based on various criteria (e.g. [13]), here we adopt a sim- plified usage for these terms based on geometric considerations that we wish to emphasize rather than mechanisms for these related features. We use “OSC” to emphasize the overlapping configuration of spreading centers whether or not one is dominant

{-’ AN7

PACIFIC j-1” PLATE $

Wilkes Transform

Garrett Transform

% NAZCA I

PLATE

142 km/Myr )i

-?b i

PW iipw 1 PW

Fig. 1. Location map showing the study area, regional plate

boundaries, tectonic rates, and ship track of the Gloria survey

(modified from [ 11).

or whether they have a preferred migration direction. We use “propagating” to emphasize the along-axis extension of a spreading center into pre-existing lithosphere or the overall migration in a preferred direction of pairs of OSCs.

2. Data acquisition and processing

Data presented here are primarily from the 1993 Gloria expedition [l] on the R/V Melville. The ship

F. Martinez et al. /Earth and Planetary Science Letters 151 (1997) 13-31 15

27”s

29‘5

PAClNC

NAZCA

I

32”s JUAN FERNANDEZ -mm- ---m-,

115”W 114”W 113”W 112”W 111”W 11O”W 109”W

Fig. 2. Tectonic lineation, teleseismic earthquake focal mechanisms, magnetic isochrons, and plate boundaries in the region of the

overlapping spreading centers from the Gloria survey (modified from [l]). FR = failed rift); IPF = inner pseudofault; OPF = outer

pseudofault; OZ = overlap zone southern boundary; FZ = fracture zone; EOPF = Easter microplate outer pseudofault; SWR = Easter

microplate southwest rift; dashed line = a graben thought to be a failed rift of the West rift; beach balls (earthquake focal mechanisms in

area of overlap) from [SO]. others are Harvard centroid moment tensor solutions. Shaded area delimits the Ahu volcanic field. Additional

description in text.

track of the Gloria expedition is shown in Fig. 1. A

summary diagram of the principal tectonic and mag- netic features of the area following Hey et al. [I] is shown in Fig. 2. The Gloria tracks were oriented primarily to acquire sidescan acoustic imagery and were run ENE-WSW, oblique to the predominant

sea floor fabric. In the more complex ridge overlap area a complementary set of WNW-ESE tracks were also run. Axis crossing tracks were approximately spaced at the GLORI-B acoustic imagery swath width for this survey of 28 km, and a track was also run along the East ridge axes. The GLORY-B bathymetric

16 F. Martinez et al./Earth and Planetary Science Letters I51 (1997) 13-31

1li'W

B

112-w 28%

29’S

30’S

-31s

- 32’S

Fig. 3. East ridge study area showing the distribution of (A) the

gravity and (B) magnetics shipboard data used in this study and

the segments of the East ridge (bold lines).

swath width was somewhat narrower, between 20 and 24 km. Together with archive geophysical data

(Fig. 31, the survey provides near complete along-axis coverage of the East ridge.

2.1. Bathymetry

The bathymetry-capable towed GLORI-B [14] sidescan sonar and hull-mounted SeaBeam 2000 multibeam systems were jointly operated on R/V Melville. After initial processing of the GLORI-B phase data and merging with navigation at the Insti- tute of Oceanographic Science (10s) as described in [14], the spatially registered data were further pro-

cessed and merged with SeaBeam 2ooO data at the University of Hawaii following the procedures dis- cussed in [15,10]. Only a general description of these

data and their processing will be given here as they pertain to this study. The final data set consists of the

SeaBeam 2000 swath superimposed on the GLORI-B data gridded at 0.003”. Gaps were filled in by inter-

polation using a minimum curvature surface-fitting

routine [16] and an offset-adjusted bathymetric pre- diction derived from satellite altimetry and ship data south of 30”s [17]. Because the axial profiles de-

scribed in this study have SeaBeam 2000 bathymetry within a swath 3.46 X water depth and most also have surrounding GLORI-B bathymetry out to about

20 km, the gravity and magnetic reductions are not sensitive to the method of interpolation of the far-field bathymetry. The bathymetric grid of the East rift

study area is shown in Fig. 4A as a color shaded

relief surface.

2.2. Cross-axis area

The cross-axis area and shape of the spreading center, also referred to as “inflation”, has been used

as an indicator of ridge hydrothermal vigor [ 181 as well as of the magmatic “robustness” of the spread- ing system and predictor for the occurrence of an axial magma chamber [8,19]. Following the method

described in Scheirer and Macdonald [19], with the

differences noted below, we calculate the cross-axis area along the digitized ridge axes. Depth values

within a 16 X 1 km box centered on the ridge with its long axis oriented parallel to the spreading direc-

tion ( = 104”) are projected onto a line parallel to the direction of spreading. This is different from the procedure in Scheirer and Macdonald [19] where the

measurements are made perpendicular to the axis, but due to significant curvature of the El segment this method avoids attributing an element of flanking sea floor to two different positions on the axis. The area is calculated as a simple trapezoid using the height of the projected points relative to a datum given by the average depth of 0.5 Ma sea floor flanking the axis. The 0.5 Ma profiles determined by interpolation from the Brunhes/Matuyama boundary (0.78 Ma) are shown in Fig. 4A. In the present study the sea floor flanking the ridge segments is not all generated on those segments, due to ridge propaga-

F. Martinez et al. /Earth and Planetary Science Letters 151 (1997) 13-31 17

tion into pre-existing sea floor. Therefore we use only the sea floor flanking the central segments of the East Ridge (E2 and E3) where there is SeaBeam 2000 and GLORI-B control to determine the average

0.5 Ma depths. These segments appear to have a relatively stable history of spreading, based on a relatively continuous sequence of flanking magnetic

isochrons identified out to at least anomaly 2 [1,2,5], although possible local axis migration [20] or small-

offset propagation events may have occurred.

Anomalous depths related to a small seamount on the western 0.5 Ma profile (Fig. 4A) have not been

included in the average. The calculated areas are shown in Fig. 4C.

600 kg/m3 between crust and mantle. The bathy-

metrically predicted gravity, observed shipboard free-air gravity, Bouguer and MBA profiles sampled

along the ridge axis are shown in Fig. 4D.

2.4. Mugnetics

The region considered in the inflation calculation

near the propagating tip of segment El crosses its pseudofaults and therefore partly measures sea floor

not formed on that segment. The significance of the calculation here may therefore be different from that

in Scheirer and Macdonald 1191, who suggest that the area may in part measure volcanic production from a

particular element of ridge in addition to the topo- graphic effects caused by sub-axial zone buoyancy.

The area calculated here nevertheless has a useful physical significance in terms of quantifying the

distribution of mass surrounding the ridge axis, a property which is also reflected in the gravity. as discussed below.

Total field magnetic anomaly data from the Glo- ria survey were compiled with archive data from the

National Geophysical Data Center (NGDC) and the

University of Hawaii. Data locations are shown in

Fig. 3B. Using a 3-D implementation [22.23] of a Fourier inversion method, sea floor magnetization

intensities were calculated from the gridded bathymetry and magnetic field resampled onto a

630 X 630 grid with a spacing of approximately I km. Wavelengths longer than 1000 km and shorter than 6 km were cut and between 500 and 12 km

were passed. The source layer was assumed to be 1 km thick with its top as the sea floor. The anomaly and magnetization intensity values were sampled

along the ridge axis and are shown in Fig. 4E.

3. Results

2.3. Grmig

The compiled gravity data shown in Fig. 3A were

cross-over adjusted by having their average discrep-

ancy with the data from the Gloria survey removed. The data were then median filtered and gridded in

1 X 1 minute cells. The ship tracks generally lie very close to the picked position of the ridge axis. the largest separations of about 4.5 km occurring in a small data gap on El.

Fig. 4 summarizes the East ridge bathymetry and axial depth, cross-axis area, gravity, and magnetic variations. Except for segment El, the East ridge

differs significantly from the EPR north of the Easter Microplate mainly in having shallower axial depths

(- 2350 ml on the 3 southern segments (over a combined length of over 200 km). Other character- istics, such as cross-axis area, MBA amplitudes and

gradient, and sea floor magnetization values, are

within the range observed elsewhere on the EPR.

3. I. Axiul depths

Using the bathymetric grid the predicted gravity The southern 3 segments of the East ridge, E2-E4, was calculated using a Fourier method 1211. A den- exhibit shallow and relatively uniform axial depths sity contrast of 1700 kg/m3 with respect to sea near 2350 + 50 m over a distance greater than 200 water was assumed for the bathymetry. The values of km. These segments are similar to some parts of the the predicted gravity were subtracted from the ob- southern Pacific-Nazca [ 12,241 and Pacific- served gravity to produce the Bouguer anomaly pro- Antarctic [25] EPR which have relatively flat axial files. The “mantle Bouguer anomaly” (MBA) was profiles and are unlike the “humped” profiles, which similarly calculated assuming a Moho 6 km from the are more typical of the Pacific-Cocos EPR [8]. The bathymetry and using an assumed density contrast of axial depths of these segments, however. are dis-

18 F. Martinez et al/Earth and Planetary Science Letters I51 (1997) 13-31

tinctly shallower than the Pacific-Nazca EPR, which

regionally shallows from depths greater than 2900 m near the equator to = 2600 m near 17S”S [12]. The southernmost segment of the East ridge, F4, overlaps

by about 30 km and is offset by _ 10 km from another ridge system to the west, which comprises

part of the western boundary of the Juan Femandez

microplate. East of E4, ridges and valleys associated with a diffuse deformation zone extend from the

Endeavor Deep area (Fig. 1) of the northern bound-

ary of the Juan Femandez microplate [5] to nearly intersect the E4 axis at a high angle, implying that a

significant part of the E4 segment is within the

diffuse northern boundary zone of the Juan Feman- dez microplate. Despite the proximity of these defor-

mation zones and boundaries of the Juan Femandez

microplate, axial depths on E4 only begin to deepen within 10 km of the southern end of the segment by

about 100 m. Smaller deepenings of < 50 m in axial

depths can be seen at the terminations of E3. A somewhat greater deepening occurs at the northern end of segment E2, to about 2490 m, comparable to the southern end of E4. Cross-trending chains of

small seamounts extend at high angles outward from near the East ridge at various locations. Although these represent prominent off-axis disturbances to

the average sea floor depth, they do not appear to originate at the ridge and do not significantly influ-

ence the axial depth at their projected intersection

with the ridge. In contrast to the near-constant axial depths of

E2-E4, the propagating segment El exhibits system-

atically deeper axial depths (> 2490 ml that are almost everywhere greater than on the rest of the East ridge. The minimum depth on El occurs near to E2 and where it matches its depth, possibly resulting from a shared magma chamber, as has been sug- gested for small-offset overlapping spreading centers

elsewhere i&11,26]. North of this point the axial depths of El become progressively deeper (average gradient of about 2 m/km) and are more variable

than on E2-E4, finally reaching a depth of 2800 m near its northern tip. Although this ridge tip depth is

very near the 0.5 Ma average depth of the flanks of E2 and E3, it is still shallower than the depths of the

sea floor flanking the tip (Fig. 4A) and thus does not form a depression as observed at intermediate

spreading rate propagating centers [27]. The greater flanking depths of the sea floor here represent trans- ferred lithosphere (due to overall West ridge propa-

gation) and sea floor of the overlap region.

3.2. Area

The axial area calculated for the East ridge is shown in Fig. 4C. The area is greatest near 31”53’S,

where it approaches a value of 6 km’. To the south, the area of E4 decreases very rapidly to a minimum

of about 1 km’. This large decrease in cross-axis area may be partly influenced by the overlap with the Juan Femandez western ridge axis and by the intersection of the East ridge axis with the deforma-

tion zone associated with the microplate’s northern boundary. However, as noted, above these distur-

bances do not influence the axial depths. To the north the area decreases progressively, but with small

irregular variations, to near zero near the northern tip of El. A large local lowering of the cross axis area

occurs near the El-E2 segment overlap. The low

values exceeding the regional trend here appear to be partly due to a deep overlap of the El-E2 OSC (Fig. 4A Fig. 5A). Although the El ridge axis continues to form a positive topographic high with respect to the flanking sea floor as far as its northern tip, near zero

and even negative inflation calculated near the tip results from partial inclusion of the old pre-existing

Fig. 4. Geophysical axial variations along the East ridge. (A) Color shaded relief bathymetry map of the East ridge showing the location of

the segment axes and the 0.5 Ma isochrons. Map is in linearly spaced latitude and longitude units rather than a projection to facilitate

comparison with values plotted against linear latitude. Color scale indicates depth values. (B) Axial depth profile measured from the gridded

SeaBeam 2000 data along the profile locations shown in A. Also plotted are the depths along the 0.5 Ma profiles from the ridge flanks. The

average depth from these profiles (excluding the western seamount) were used to determine the base level for the area calculation. (C) Cross

axis area calculated following the technique described in [19] using cross axis depth measured from a 1 X 16 km box parallel to the

spreading direction and centered along points on the ridge axis. (D) Profiles of the axial calculated gravity from bathymetry assuming a

density contrast of 1700 kg/m3 (red), observed free-air gravity (blue), Bouguer anomaly (black), and mantle Bouguer anomaly (dotted). (E)

Total field magnetic anomaly and sea floor magnetization measured along the ridge axes.

F. Martinez et al./Earth and Planetary Science L.etters 151 (1997) 13-31

0

s g” F StR 0

8 P

19

0

c Oa

E

f 9

20 F. Martinez et al. /Earth and Planetary Science Letters 181 (1997) 13-31

m

a

F. Mariine: et al. /Earth and Planetan, Science Letters 151 (1997) 13-31 21

sea floor, as described above within the area calcula-

tion box (Fig. 6).

3.3. Graoity

Axial gravity variations of the East ridge are

shown in Fig. 4D. As expected, the predicted 3-D gravity calculated assuming constant-density bathymetry closely follows the form of the axial area

variations since both calculations reflect the inte- grated near-axis volume. The predicted gravity varia- tion ranges over nearly 30 mGals. Total variations in

axial free-air gravity are generally less than 10 mGals, however. implying that to a large extent axial regions

with large inflation are inversely compensated by lower densities. This effect is reflected in the Bouguer

and MBA gravity which varies overall in an inverse way with the axial area. The MBA variation is a

somewhat amplified version of the Bouguer variation because the mantle topography is assumed to exactly follow the sea floor -6 km, removing an additional gravity effect similar to that of the bathymetry but

with a longer wavelength and smaller amplitude. The overall Bouguer and MBA variations are similar.

forming a single broad minimum skewed toward the southern part of the East ridge. Average along-axis MBA gradients 1281 for segments E2-E4 are about

0. I 1 mGal/km. similar to and within the small range of values generally found for fast-spreading ridges

[28,291. Along segment El the Bouguer and MBA

variations show a continuous northward increase with a somewhat larger average axial MBA gradient of 0. I5 mGal/km.

East ridge axial magnetic field variations and sea floor magnetization values are shown in Fig. 4E. Total-field magnetic anomalies have distinctly larger

( > 500 nT) maximum values near the southern and northern ends of the El segment and the northern E2 segment. Correspondingly, sea floor magnetization

values are also distinctly higher here, reaching values over 30 A/m compared to more typical values near 15 A/m over E3 and E4 and somewhat higher values of near 18 A/m over the central part of E 1. Klaus et al. [7] show a high amplitude dipolar anomaly near the northern end of El. High magneti-

zation values have been associated with propagating

ridge tips and axial discontinuities elsewhere [23.30,31], where they are believed to reflect higher FeTi concentrations resulting from enhanced mag-

matic fractionation [32,33], suggesting along-axis flow as one mechanism for their formation 134,351. The observation of the magnetization highs ap-

proaching the tips of the El propagating segment

suggests that these closed tips may enhance fraction- ation by forming stagnation points in the magmatic

flow relative to the central portion of the segment.

3.5. Small-@et OSCs of the East ridge

The small overlapping spreading centers that de-

fine the third-order segmentation of the East Ridge have offsets with minimum separations of the axes

of 1.5-3 km (Fig. 5). In the case of E2-E3 and

E3-E4 overlaps. the area between ridge crests forms

a relatively gentle sag of less than y 20 m between the slightly higher axial ridges (Fig. 5B,C). In the E 1 -E2 overlap (Fig. 5A) the topographically defined ridge tips surround relative lows with relief of up to

- 400 m or more with respect to the ridge crests. The maximum depth of the lows, however, is com-

parable to the depth of the tlanking sea floor. To the north of these lows the El and E2 segments ap- proach a common depth near 2460 m and merge in a

broad shallow area with depths less than 2500 m. The El axis to the north is everywhere deeper than

in this region. The inflated region at this overlap is

broader than to the north and south. giving rise to a local maximum in the calculated cross-sectional area.

Across all of the non-transform offsets of the East ridge. a shallow inflated zone continuously spans the separation of the bathymetrically defined ridge axis.

Thus, as suggested for other small offset OSCs on the EPR. there may be a continuous magmatic con- duit spanning these offsets. In fact. cross-sectional area values across all of these offsets predict a 90% likelihood of an observable axial magma chamber

reflector using the criteria of Scheirer and Macdon- ald [ 191.

An axial magmatic conduit may be less continu- ous at the El-E2 offset compared to the others, however. This is suggested by the abruptly deeper and narrower El axis compared to E2 and the mor- phology of the flanks of the area near the offset. which exhibit large irregular lobate features to the

22 F. Martinez et al/Earth and Planetary Science Letters I51 (1997) 13-31

north and east of the overlap between 30”02’S and 3O”l I’S, suggesting volcanic outpourings (Fig. 5A).

Also, the southern El ridge tip from about 30”09’S to 30”14’S forms a distinct topographic ridge sepa-

rate from the inflated area, suggesting any magmatic flow into this ridge tip would be isolated in a closed

end conduit. These features suggest a “constriction” in the along axis magmatic continuity between El

and E2 and may explain the magnetic field and magnetization high that exists near this offset by a

local increase in the volcanic layer thickness and

enhanced magmatic fractionation in a closed-ended

conduit.

4. Discussion

4.1. Nature of segmentation of the East ridge

Although the East Ridge has been subdivided into

4 third-order segments based on the small non-trans-

form offsets of its axis (Fig. 5) other geophysical characteristics indicate its deeper magmatic and up-

welling patterns define one segment. Bouguer and MBA anomalies (Fig. 4D) show a

single broad minimum associated with the East ridge.

This gravity minimum is skewed to the southern part of the East ridge but along-axis gravity gradients are similar to other fast spreading ridges having an axial high [29]. The single gravity minimum is compatible

with a single center of upwelling underlying the East ridge and argues against separate upwelling centers

beneath each morphologic segment. A similar pattern of upwelling can be inferred

from the form of the cross-axis area variation of the East ridge (Fig. 4C). Modeling of gravity and topog- raphy of the axial high at fast spreading mid-ocean ridges indicates the lithosphere at the axis is weak

[36,37]. Low seismic velocities generally observed beneath the axial regions of fast spreading centers [38,39] suggest hot rock and a small component of partial melt underlying the axial zone. At fast spread- ing rates the axial high is generally interpreted as reflecting isostatic uplift caused by this buoyant zone [19,36,37]. Although variations in volcanic layer thickness may contribute to axial topography, as indicated by split “bow-form” [40] volcanic con- structional highs that persist off axis at intermediate

spreading rates, at fast spreading ridges observations

that the axial high generally disappears off-axis indi- cate that it is a dynamic feature at these rates [41]. The larger cross-axis area associated with the south- em East ridge is thus compatible with greater up-

welling or more partial melt at depth beneath the southern ridge, creating greater buoyancy which de-

creases northward, in particular beneath the El seg- ment.

Although variations in upwelling along the East

ridge can be inferred from the gravity and cross-axis

area, the flat axial depth profile for segments E2-E4 (Fig. 4B) suggest magmatic flow at shallow levels

eliminates depth variations, as proposed for other fast spreading ridge systems with near constant axial depth [24,36]. The comparatively small OSCs (Fig. 5) which appear as secondary features superimposed

on the broad and continuous axial high of the E2-E4 segments, may not impede efficient flow across these third-order segments. Increasing magnetization inten-

sities toward the northern tip of segment E2 and generally high magnetizations of segment El, espe-

cially approaching its tips, may reflect along-axis flow and increasing fractionation of magma from the more robust southern segments of the East ridge, in a

mechanism similar to that described by Batiza and

Niu [34] but crossing the third-order segments. The larger offset of the El-E2 OSC may decrease the efficiency of along-axis flow across this offset, as

suggested by the deeper axis of the El segment immediately north of the OSC (although this may also result from the fact that its northern end has

recently propagated). However, the indication from the gravity pattern of decreasing upwelling from beneath, and its high magnetization suggest that the

highly inflated southern East ridge segments may be feeding magma to the El segment. A further discus- sion of the magma source for the El segment based on our OSC model is presented below.

The magmatically robust E2-E4 segments of the East ridge indicate that small-offset (l-3 km) OSCs can form when there is an apparent abundance of magma (inferred from the high cross-axis area and flat axial depth profiles) throughout all the linked segments. In addition, the morphologic segmentation defined by the small OSCs of the East ridge does not correlate with the deeper magmatic and upwelling patterns inferred from the axial gravity and cross-

F. Martinez et al. / Earth and Planetary Science Letters 151 (1997) 13-31 23

sectional area variations. These observations argue against the development of the small offset OSCs requiring magmatic pulses which travel along axis

and are offset at the segment ends [81. They suggest that the small-offset OSCs of the East ridge are a manifestation of the upper thin lithospheric layer

rather than a reflection of deep segment-scale mantle upwelling and magmatic processes. This type of OSC is probably ephemeral and may arise due to

local perturbations of the near-axis lithosphere, caused by small upwelling mantle heterogeneities as

described below. There is no evidence from the flanking sea floor fabric that the small East ridge

OSCs have been stable (that is, recurring in the same area). Fixed and slowly migrating OSCs on the EPR

which do correlate with the ends of magmatic seg- ments, on the other hand, suggest that the origin of those features is different and likely related to the

deeper magmatic and upwelling pattern of the ridge systems [8].

Thus the entire East ridge appears to be underlain

by one upwelling center (as inferred from its Bouguer and MBA gravity pattern and cross-axis area varia-

tion). At shallower levels the ridge may develop

morphologic and magmatic “sub-segments” that re- flect local and ephemeral perturbations to the litho-

spheric strength that offset the spreading center. The El segment is significantly different in other ways from the southern East ridge segments, however. Its

large offset and overlap with the West ridge suggest a major portion of its length is unstable, This is supported by the identification of pseudofaults SUJ-

rounding this axis, indicating it has recently propa-

gated to its current position Cl]. As described in a later section, we infer that the ridge axes in the large

overlap area are offset and partly decoupled from the

center of divergence of a surrounding broad litho- spheric ductile deformation zone (DDZ) at depth which promotes the development of unstable propa- gating segments at spreading center offsets.

4.2. Differences from prer?ious ridge propagation models

The observation that the El propagating ridge tip is higher than the flanking sea floor suggests signifi- cant differences from models of slower spreading propagating ridges. Neither East nor West propagat- ing ridges ends in a depression, as observed, for

example, at the intermediate rate 95S”W Galapagos

propagator [42]. The relative axial high extending to

the ridge tip appears to be a common feature of ridge segment terminations on the fast-spreading EPR. At the 95S”W Galapagos propagating ridge the axial tip depression is explained as a dynamic feature caused

by the viscous resistance of asthenosphere flowing in

a narrow crack-like conduit [27,43]. No aspect of that model. however, predicts an axial high. As discussed above, gravity and axial cross section vari-

ations predict a marked decrease in the upwelling from directly beneath this segment compared to the

southern East ridge segments, so the high is unlikely to be caused by sub-axial buoyancy. The relative

axial high near the northern tip of propagating seg- ment El can be explained as a volcanic construc-

tional feature created by along-axis flow of magma

which has filled in and overflowed the initial rupture in the brittle lithosphere. This is similar to the model

proposed by Macdonald et al. [441 for the distal ends of fast spreading ridges at discontinuities which are interpreted as volcanic constructional features where

apparently entire rift tips are rafted off as highs onto

the flanking sea floor. Although some features in this

area have also been interpreted as abandoned rift tip highs [I], a significant difference here is the common occurrence of small elongate deeps in the position

and orientation expected for abandoned ridge tips (Fig. 6). We suggest that, in this area. once the

propagating ridge tip is truncated by a subsequent propagation event (self-decapitation), its magma sup- ply is cut off but the now abandoned tip is still within the ductile deformation zone, and experiences

continued rifting and subsidence, forming the deeps

flanking the new ridge axis.

4.3. Modei of OSC tectonics

Models of how OSCs form have emphasized dif-

ferent physical and kinematic mechanisms. Rea [45] proposed that, if one of the faults bounding the ridge axis intersects the magma chamber. it could capture the magmatic flow and locally shift the location of the spreading center to that of the axis-bounding fault. Macdonald et al. [8] presented a model in which OSCs occur at the distal ends of magmatic segments of the ridge when along-axis magmatic pulses fail to meet head on. Lonsdale [25] pJOpOSed that changes in direction of sea floor spreading may

24 F. Martinez et al./Earth and Planetary Science Letters 151 11997) 13-31

cause the ridge to break up into small en-echelon segments which form non-transform offsets when

adjacent ridge axes are offset less than a critical distance (which depends on spreading rate) required

to form a transform. Naar and Hey [46] proposed that

there exists a speed limit for the stability of trans-

form faults. In their model, transforms become un- stable at slip rates faster than N 145 km/my and

ridge axis offsets are accommodated by overlapping

spreading centers. Central to models of OSC mechanics is non-rigid

behavior of the lithosphere. Rea [20] noted that at the

extreme rates that characterize spreading at 31”S, a broad area 230 km wide surrounding the ridge axis

has the same age as the 31 km wide axial valley at the Mid-Atlantic ridge at 36.5”N. He argues that the

thin lithosphere surrounding the ridge at 3 1’S thus may not have sufficient strength to preserve a con-

stant ridge axis configuration, leading to curved

magnetic isochrons observed here. Similarly, Mac-

donald et al. [8] propose that although the develop- ment of OSCs has a first-order spreading rate depen- dence, a more fundamental consideration is the

strength of the lithosphere, which is strongly affected

by temperature. They cite the large-offset neovol- canic zones on Iceland as a possible analogue of OSCs developed at slow spreading rates where the

lithosphere is weak, due to the effect of the Icelandic

hot spot. We propose a qualitative tectonic model derived

from the geophysical observations from the East

ridge that may also apply to smaller offset OSCs (Figs. 7 and 8). The model incorporates some of the

features of the previous models mentioned above. Following Rea [20], we suggest that, at the Earth’s fastest spreading rates that are predicted for the EPR

between the Easter and Juan Femandez microplates, the lithosphere is extremely thin and weak over a broad area surrounding the ridge axes. A “plate boundary zone” of deformation surrounding spread- ing centers has been defined as “the region in which the newly created lithosphere is undergoing active faulting and tectonic deformation before becoming part of a relatively aseismic plate” [47]. This defini- tion is based on observations of tectonic faulting and seismicity from the brittle lithosphere near the sea floor and includes processes such as tectonism asso- ciated with the “unbending” of the dynamically

maintained axial valley at slow spreading centers, a process which is fundamentally a manifestation of

the flexural lithospheric strength there. In contrast, we suggest that at fast spreading centers the weak ductile lithospheric layer is susceptible to stretching

and may be partially decoupled from the brittle

lithosphere, grading with depth into a flow field similar to that envisioned by theoretical models [48]

of the pattern of viscous flow driven by passive plate separation (Figs. 7 and 8). We refer to this proposed

zone of lithospheric deformation surrounding the ridge axes as the ductile deformation zone (DDZ)

(Fig. 8). Passive flow models indicate broad zones of deformation at depth surrounding ridge axes and

their offsets (transforms). We suggest that with in- creasing spreading rates deformation of the progres- sively thinner and weaker carapace of the lithosphere begins to resemble the viscous flow pattern at depth.

Although current models of sub-axial mantle flow

are strongly debated (e.g. Wilson [49]), and therefore

the above conceptual model is highly speculative in terms of any specific pattern of deformation, there is

general consensus that overlapping spreading centers require distributed deformation. The existence of the large offset overlapping spreading centers near 29”s

and their recorded evolution over N 1.5-2 Ma in this area [ 1,2] therefore provides empirical evidence

of broadly distributed deformation. Further, studies of earthquakes in the large overlap area between East

and West ridges [50] indicate that active faulting is distributed across the 120 km overlap zone and is not

concentrated along transform or other narrow bound-

aries (see also Fig. 2). The focal mechanisms are

consistent with bookshelf faulting t.501 within the overlap zone, which is a form of distributed shear

deformation observed between propagating and dy- ing ridges [51,52]. Thus this evidence for broadly

distributed (over a 120 km offset) brittle deformation suggests the deeper mantle should also be deforming over at least as broad an area, since it is unreason- able to assume the deeper ductile deformation should be more localized than that of the upper brittle layer. Other evidence for a DDZ at depth surrounding the spreading centers and OSC offset includes our inter- pretation of the failed rift tip deeps, which we pro- pose underwent extension off-axis, causing the origi- nal ridges which formed the rift tips to stretch and subside.

F. Martinez et al. / Earth and Planetary Science Letters IS1 (1997) 13-31 2.5

We suggest that the deep viscous mantle only “feels” the effect of the broad divergence of the Pacific and Nazca plates in the overlap area rather than the individual episodic ridge propagation events.

A simple assumption is that the divergent flow at

depth in the vicinity of the OSC spans the offset in a continuous way. A similar mantle flow pattern has been proposed for explaining the oval deep that is

28’S

‘S

‘S

113-w 112’W 111-w

28’S

112-w 111-w

meter 1OOOm

-2800 -2800 -1399

Fig. 6. Gridded Sea Beam 2000 and Glori-B bathymetry in the area of the large-offset overlapping ridge system shown as a surface

illuminated from the northeast with dotted contours at even 200 m intervals. Profiles show depth variations sampled from near the center of the NE-SW oriented Sea Beam 2000 swaths relative to a depth of 2817 m (the average 0.5 Ma depth) indicated by the profile line. Gray

scale and annotated bar indicate depth scale for the map and profiles, respectively. Opposing arrows show locations of elongate deeps. The

NNR-SSE orientation of the deeps is similar to that of the curving ridge tips of the large-offset overlapping spreading centers but is

discordant to the more N-S spreading-normal sea floor fabric elsewhere, supporting the interpretation that the deeps represent failed ridge

tips that have undergone extension and subsidence.

26 F. Martinez et al./ Earth and Planetary Science Letters 151 (1997) 13-31

frequently found at smaller offset OSCs [26]. For simplicity, it is shown in Figs. 7 and 8 as smoothly and symmetrically spanning the offset between the ridge crests of the OSCs (ignoring complications such as the relative motion of the ridge systems as a whole with respect to the deeper mantle, e.g. Stein et al. [53]). This flow pattern also predicts a broad shear underlying the OSC and is consistent with the observed earthquake focal mechanisms and inferred broadly distributed bookshelf faulting in this area

[50]. In the context of this model El represents a fissure in the surficial brittle layer which has over- shot the end of the steady-state ridges (E2-E4) and is overlying a diffuse extensional zone. The reason it does not follow the center of deeper divergence across the offset and thereby continuously link the two ridge systems may be that the propagation kine- matics within thin brittle lithosphere follow the me- chanics of crack growth [9] and that the curvature of the center of divergent flow is too high for a brittle

F. Martinez et al./ Earth and Planetup Science Letters 151 (1997) 13-31 21

fracture to follow. It may also be that the deep upwelling becomes “defocussed” in the overlap area as a consequence of the overlap itself and of the rapidly changing location of the propagating ridges. In either case, the model implies a strong decoupling of the surficial brittle failure of the lithosphere from

the deeper pattern of upwelling and lithospheric de- formation in the area of the OSCs. Observed offsets

in magma chamber reflectors from the spreading

center axes at the OSCs near 9”N have been pro- posed as evidence for such decoupling [54]. The surficial fissure representing El is initially aligned

with the steady-state ridge segments (E2-E4) and taps magma from these segments by along-axis flow, since it does not overlie a well developed steady-state

axial upwelling zone (Fig. 7). If the center of diver- gent flow which crosses the OSC offset continues to

generate melt (which is probably unlikely for large offsets but probable for small offsets), El may also

receive magma by lateral flow, as suggested by Kent et al. [54]. This model may be an alternative to the

lag in the response of the mantle to the propagation of the ridge, as suggested by Chen et al. [55], in that the center of the upwelling pattern at an OSC may

already be in place but simply offset (and likely defocussed) from the unstable propagating fractures

rather than lagging behind them.

The configuration of the axes in this model are

predicted to change as opening progresses. Although sea floor spreading on the axes of the OSC will cause them to separate, an additional component of separation may occur as a consequence of stretching

of the overlap area due to the divergent flow of the

DDZ (see also Chen and Morgan [26]). For the symmetric case shown (Fig. 7A), the ductile defor-

mation zone centered between the OSC has a net

outward flow. With time its partial coupling with the brittle plate will tend to drag the limbs of the OSC outward, stretching the crust between the ridges, and

disconnecting them from the narrow, along-axis,

steady-state magma source. As described above, the abandoned axis is still within the DDZ, however, and continues to undergo differential stretching for a

significant distance (Fig. 7B). The abandoned ridge preferentially takes up extension, probably because it

represents a discontinuity and is hotter and weaker

than the immediately surrounding sea floor. The region beyond the end of the steady-state ridge con-

tinually overlies a zone undergoing differential stretching at depth and this may facilitate a new

fissure to nucleate and overshoot the end of the

steady-state ridge (Fig. 7B). The asymmetric pattern of ductile deformation at the offset ends of the steady-state ridges may thus promote the repeated

Fig. 7. Schematic diagram illustrating elements in a model for the formation of OSCs. Ridge axes on the surficial brittle lithospheric layer

are shown as bold parallel and curving lines and failed rifts are shown as dashed lines. The brittle layer overlies and is only partially coupled

to a lower ductile lithospheric layer. The ductile deformation in this lower layer is a continuum, indicated as a gradient from solid gray

(uniform motion) to white (zero motion) and by arrows with changing length. Brittle deformation of the upper layer occurs on discrete

fractures forming the ridge axes (double lines) and within the overlap zone as distributed bookshelf faulting (not shown). (A) In fast sea

floor spreading the ductile deformation zone is broad and is roughly centered surrounding the steady-state portion of the ridges (straight

parallel lines) and continuously crosses the offset between the steady-state ridges. The brittle layer, however. accommodates the offset by

episodically propagating discrete fractures (curved converging lines) from near the steady state parts of the ridge. Differential extension in

the ductile layer ahead of the steady-state segments may facilitate propagation, but crack growth dynamics [9] of the very rapidly

propagating rifts in the brittle layer may determine their geometry. The propagating sections overlie a ductile region with net outward

translation, and with time are swept outward. This outward translation will deflect them from alignment with the steady-state segments and

cut off their magma supply that is primarily by along-axis flow from these segments, although some off-axis flow may migrate laterally

toward the ridge axis from the center of divergence (white area that crosses the offset). (B) New rifts propagate from the steady state

segments, completely cutting off the dying segments. Now off axis, the abandoned rifts (dashed lines) continue to overlie the ductile

deformation zone and undergo additional amagmatic differential extension and subsidence forming deeps. (Cl At smaller offsets the center

of diverging ductile deformation links in a diagonal zone across the overlap basin inducing a greater component of vertical advection in the

mantle, which may be responsible for off-axis melt generation beneath the overlap zone and significant stretching of the brittle layer creating

the overlap basin (e.g. [26,54]). (D) At slow spreading rates the ductile deformation zone surrounding the ridge axes and transform are

narrow because of a greater (spatial) rate of lithospheric thickening, due to a larger proportion of heat loss at a given distance from the axes.

The thicker lithospheric plates undergoing little differential extension beyond the ridge axes inhibit propagation and favor the formation of

discrete ridge-transform intersections.

28 F. Martinez et al. /Earth and Planetary Science Letters 151 (1997) 13-31

Fig. 8. Block diagram showing principal elements of the model of

OSC tectonics. The upper layer represents the brittle lithosphere

which overlies a lower ductile lithospheric layer, both of which

are temperature dependent and thicken with age. Overlapping

spreading centers are formed as fractures in the brittle layer

following crack propagation mechanics [9]. Near the spreading

centers the brittle layer is only partly coupled to a lower litho-

spheric layer within a zone that forms the DDZ (gray area

projected onto top surface and shown on the frontal section as a

gray region within the ductile lithospheric layer), which deforms

in a ductile fashion grading into the viscous flow of the underly-

ing mantle (shown as dashed flow lines). The center of divergence

(shown as dashed line projected onto the top surface) of this

deeper flow is aligned along the axis of the ‘steady state’ parts of

the ridge, but separates from the axes of the OSCs as it continu-

ously crosses the offset between them. The OSC axes are thus

largely decoupled from the deeper flow in the overlap area. The

outward flow from the center of divergence causes the overlap-

ping ridges to become progressively displaced outward and even-

tually replaced by new, more favorably oriented spreading centers.

The abandoned ridge tips which overlie the DDZ may experience

continued extension and subside forming deeps (crescent shapes).

OSCs may originate as a result of mantle heterogeneities (stippled

ovals) which become entrained in one limb of the mantle flow,

melt, and locally weaken the lithosphere on one side of the

spreading center causing the axis to shift.

abandonment of the overlapping ridges and their replacement by new, more favorably located, frac- tures. Although these are shown as propagating on the inside of the failed rifts in Fig. 7, variations due to crack growth mechanics [9] and overall migration of the plates with respect to the mantle [53] may explain the variations observed. This is a significant difference from previous models of OX and du- elling ridge axis behavior, which viewed them as responses to axial magmatic pulses [56].

The variation in width of the DDZ with spreading

rate may explain why OSCs in general are not found at slow spreading rates. At slow spreading centers

the width of the DDZ surrounding the ridge axes and transform domains is expected to be much narrower

because the cooling lithosphere thickens at a shorter distance from the axes (Fig. 7D). Although the vis-

cous asthenospheric flow at depth may still be broad, even a relatively short overshoot of the ridge axis beyond the steady-state ridge segment would place it within strong lithosphere not undergoing significant

differential stretching at depth. These overshoots are therefore not favored. Increasing temperatures,

caused either by faster spreading or hotspot activity, will increase the width of the DDZ surrounding the

ridge axes. Such thermal events might, therefore, trigger the transformation from stable ridge-trans-

form intersections to ridge propagation or the forma- tion of OSCs. Similarly, a waning hotspot influence

or a slowdown in spreading may promote a large- offset OSC to become a ridge-transform boundary

or possibly a spinning rigid microplate.

4.4. Initiation of OSCs

Although the large offset East and West ridges appear to have originated from an inherited large

offset when a transform fault was eliminated [l], in

our model smaller OSCs can also originate without ridge jumps [45], misalignment of axial magmatic pulses [8], or changes in direction of sea floor

spreading 1121. We suggest the very thin and weak lithosphere surrounding fast spreading centers is sus- ceptible to perturbation by relatively small mantle anomalies, which may asymmetrically affect only one side of the ridge (Fig. 8). Thus, if the local

rheology of the lithosphere surrounding the axis is affected, a change in the pattern of deformation will result. Such a model was previously suggested by Rea 1451, however, he did not favor it because he believed it was unlikely that small local thermal anomalies could exist in the mantle and because he believe the small ridge offsets occurred at character- istic distances of N 10 km, which he thought would be unlikely for mantle anomalies to create consis- tently. Subsequent surveys have shown, however, that a full continuum exists in the size of ridge axis

I;. Marti’nez et nl. / Earth and Planetarv Scieme Letters 151 f 1997) 13-31 29

offsets. There is also evidence that local composi- tional anomalies do exist in the mantle (rather than thermal anomalies) and can form small chains of

seamounts or isolated seamounts preferentially on only one flank of the ridge axis [49,57]. We propose that small mantle compositional heterogeneities can be captured by the sub-axial flow and be swept into

the DDZ from one side or the other of the ridge (Fig. 8). Such an off-axis anomaly entrained in the flow

may primarily remain within one advective cell and thus primarily affect one side of the diverging plates.

The anomaly need not necessarily form a seamount or seamount chain. In fact, melt from a near-axis anomaly may be channelled to the axis, thereby not producing a distinct volcanic edifice and still provide

a sufficientiy asymmetric perturbation to affect the

near-axis rheology and offset the spreading center. This may be another explanation for the occurrence

of magma chamber reflectors that are not centered beneath the ridge axis (e.g. [58]). Such an asymmet- ric distribution of melt (and the heat that it would

advect to shallow levels) would cause the weakest point in the neovolcanic zone to migrate toward the

melt anomaly and thus offset the spreading center. in a localized version of the mechanism proposed by Hayes [59] to explain asymmetric spreading. With

varying offset and magnitude, such a mechanism

could account for a range of axial offsets from devals to second order OSCs, or possibly larger offsets. Thus a change in the strength of already thin

and weak lithosphere surrounding fast spreading cen-

ters may occur locally and asymmetrically, as a result of mantle melt anomalies and an initially

symmetric pattern of ductile extension may be con- verted to an asymmetric one offsetting the spreading center. The random arrival of mantle heterogeneities to either side of the axis can thus explain the varying

left and right offsets of OSCs on the EPR. On a larger scale, the arrival of anomalously hot

asthenosphere from the Easter hotspot to near the transform that previously existed near the southern

end of the Easter microplate may have sufficiently weakened the lithosphere and broadened the DDZ to

trigger the propagation of the West ridge, eliminate the transform, and form the large offset OSC. Con-

tinued flow of hotspot material along the West ridge may be responsible for its overall southward propa- gation.

5. Conclusions

The East ridge forms part of a large offset (120 km) overlapping spreading system with the West ridge. The ridge is composed of four morphologic third-order segments, El -E4. defined by small-off-

set, overlapping spreading centers. Its northern seg-

ment, E 1. has recently propagated and overlaps most of its length with the West ridge system. The gravity

and cross-axis area variations, however, indicate

deeper magmatic and upwelling patterns compatible with one segment with mantle upwelling focussed

beneath the southern East ridge. Melt generation is probably also concentrated below this area with transport to the other segments occurring at shallow

levels by along axis flow across small offset OSCs

leading to the nearly constant axial depths of E2-E4. Melt transport to the propagating segment El, how-

ever. is probably even more dependent on along-axis

flow. as indicated by the Bouguer and MBA highs that characterize this segment and its low cross axis

area. but may be constricted at the larger offset El-E2 OSC. leading to its abruptly deeper axial depths. This suggests that, at the East ridge, the

segmentation of the brittle upper lithosphere is at least partly decoupled from the deeper magmatic and

upwelling segmentation. We propose a model in

which a deeper lithospheric ductile deformation zone (DDZ) underlies and is partially decoupled from the upper brittle lithosphere. The center of the diverging

flow in this deeper layer underlies the steady-state parts of the ridge axes and continuously crosses the

offset between the large overlapping spreading sys- tems. The propagating and overlapping ridge seg- ments. however, represent overshoots of a brittle fracture in the upper lithospheric layer, which sepa-

rate and become largely decoupled from the center of divergence in the lower layer of ductile deforma- tion. With time. the propagating segments are pro-

gressively displaced from alignment with the

steady-state segments and a new fracture develops, capturing along-axis magma flow from the steady-

state segments. The abandoned ridge segment contin- ues to undergo extension while it overlies the broad DDZ and subsides. forming a deep. At fast spreading centers, where the lithosphere is already thin and weak. mantle compositional heterogeneities may be swept into the axial region. melt, and locally affect

30 F. Martkez et al/Earth and Planetary Science Letters I51 (1997) 13-31

the rheology causing the spreading center to offset

and form an OSC. We suggest that the temperature- dependent width of the DDZ surrounding ridge axes

relative to their offset can control whether OSCs or ridge-transform intersections form. Two important

controls on the temperature are spreading rate and

hotspot influence. This latter effect may have con- verted a stable ridge transform intersection into the

world’s largest OSC in this area at 1.5-2 Ma.

Acknowledgements

We thank Brian Taylor and Garret Ito for insight-

ful discussions and David Naar and two anonymous reviewers for their valuable criticisms. This work

was supported by NSF grant OCE-9529737 to RNH. SOEST contribution No. 4474, HIGP contribution

No. 936. [CL]

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