www.elsevier.com/locate/tecto

Tectonophysics 366 (2003) 55–81

The transition from an active to a passive margin

(SW end of the South Shetland Trench, Antarctic Peninsula)

Antonio Jabaloya,*, Juan-Carlos Balanyab,c, Antonio Barnolasd,Jesus Galindo-Zaldıvara, F. Javier Hernandez-Molinae, Andres Maldonadoc,

Jose-Miguel Martınez-Martıneza,c, Jose Rodrıguez-Fernandezc,Carlos Sanz de Galdeanoc, Luis Somozad, Emma Surinachf, Juan Tomas Vazquezg

aDepartment Geodinamica, University Granada, 18071 Granada, SpainbDepartment de Ciencias Experimentales, University Pablo de Olavide, Sevilla, Spain

c Instituto Andaluz de Ciencias de la Tierra, C.S.I.C.-University, Granada, Spaind Instituto Geologico y Minero de Espana, Madrid, Spain

eDepartment Geociencias Marinas y O. D. Territorio, University Vigo, Vigo, Pontevedra, SpainfDepartament de Geodinamica i Geofısica, Universitat de Barcelona, Barcelona, Spain

gFacultad de Ciencias del Mar, Universidad de Cadiz, Puerto Real, Cadiz, Spain

Received 12 December 2001; accepted 12 February 2003

Abstract

The lateral ending of the South Shetland Trench is analysed on the basis of swath bathymetry and multichannel seismic

profiles in order to establish the tectonic and stratigraphic features of the transition from an northeastward active to a

southwestward passive margin style. This trench is associated with a lithospheric-scale thrust accommodating the internal

deformation in the Antarctic Plate and its lateral end represents the tip-line of this thrust. The evolutionary model deduced from

the structures and the stratigraphic record includes a first stage with a compressional deformation, predating the end of the

subduction in the southwestern part of the study area that produced reverse faults in the oceanic crust during the Tortonian. The

second stage occurred during the Messinian and includes distributed compressional deformation around the tip-line of the basal

detachment, originating a high at the base of the slope and the collapse of the now inactive accretionary prism of the passive

margin. The initial subduction of the high at the base of the slope induced the deformation of the accretionary prism and the

formation of another high in the shelf—the Shelf Transition High. The third stage, from the Early Pliocene to the present-day,

includes the active compressional deformation of the shelf and the base-of-slope around the tip-line of the basal detachment,

while extensional deformations are active in the outer swell of the trench.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Subduction zone; Active margin; Trench termination; Tip-line; Antarctic Peninsula

0040-1951/03/$ - see front matter D 2003 Elsevier Science B.V. All right

doi:10.1016/S0040-1951(03)00060-X

* Corresponding author. Fax: +34-958-248527.

E-mail address: jabaloy@ugr.es (A. Jabaloy).

1. Introduction

Trenches associated with oceanic subduction zones

usually end in triple junctions or against plate boun-

s reserved.

Fig. 1. (A) Geological setting of the Antarctic Peninsula and the Drake Passage; the rectangle indicates the study area. The location of the magnetic anomalies and transform faults

south of the Shackleton Fracture Zone are from Larter and Barker (1991). (B) Distribution of the plates around chron C2An, when the Phoenix–Antarctic spreading axis became

inactive.

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A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 57

daries—normally another subduction zone or a trans-

form fault. However, in several areas of the world,

there are oceanic trenches that show a transition

toward a passive margin or the abyssal plain in broad

bands of deformation. An example of these trenches is

the northern end of the East Luzon Trough, which is

the prolongation of the Philippine Trench towards the

north, located at the eastern end of the Philippine

Islands (Hayes and Lewis, 1984). Another example is

near the Antarctic Peninsula, where an active small

trench known as the South Shetland Trench (Maldo-

nado et al., 1994) coexists with inactive subduction

zones in the oceanic lithosphere of the southwestern

sector of the Antarctic Peninsula (Gohl et al., 1997).

Although these trenches are known to terminate

laterally, the literature contains no description of the

morphology and structure associated with such termi-

nations.

The main aim of the present paper is the analysis of

the structure and stratigraphy of the SW end of the

South Shetland Trench in the Antarctic Peninsula in

order to determine how and why this trench terminates

(Fig. 1). A second objective is to study the transition

from an active to a passive margin and how the

deformation along the transition zone between the

two is accommodated. Most continental passive mar-

gins result from a previous rifting phase that produces

a continental break-up stage followed by a spreading

stage. However, the western margin of the Antarctic

Peninsula evolved from an active margin, progres-

sively decreasing in length since the Jurassic as results

of trench-ridge collisions, towards a passive-type

margin.

2. Geological setting

The South Shetland Trench is a slightly arched NE-

SW trench located in the northwestern sector of the

Antarctic Peninsula (Maldonado et al., 1994). It is

associated with a small island arc represented by the

South Shetland Islands. The Bransfield Strait, a nar-

row NE-SW basin with depths greater than 2000 m,

separates these islands from the Antarctic Peninsula

(Barker and Austin, 1998) (Fig. 1A and B).

This trench is the last remnant of a once extensive

trench that occupied the Pacific margin of the Ant-

arctic Peninsula. During the Cainozoic, several NE-

SW segments of the Phoenix–Antarctic spreading

axis separated by NW-SE transform faults collided

with the trench. After the collision, the continental

margin lithosphere and the oceanic lithosphere to the

north of the spreading axis came into contact, which

led to the end of subduction and the disappearance of

the trench bathymetry (Herron and Tucholke, 1976;

Barker, 1982; Larter and Barker, 1991). The collisions

progressively migrated northeastward and the differ-

ent sectors of the active margin evolved towards a

passive style following the subduction of the segments

of the spreading axis.

The last two collisions took place to the north of

the North Anvers Fracture Zone and involved two

segments of the Phoenix–Antarctic spreading axis

separated by the C Fracture Zone (Larter and Barker,

1991) (Fig. 1). The segment located between the

North Anvers and the C fracture zones collided

obliquely after the formation in the oceanic crust of

anomaly chron C4n (Fig. 1A). The segment located

between the C and Hero fracture zones also underwent

an oblique collision, after anomaly C3An (Fig. 1A).

During the anomaly chron C2An, activity in the

Phoenix–Antarctic spreading axis ended and the

Phoenix Plate became part of the Antarctic Plate

(Livermore et al., 2000). Meanwhile, activity in the

trench continued, though at present convergence rates

are slow (Maldonado et al., 1994; Kim et al., 1995).

Aldaya and Maldonado (1996) defined the South

Shetland Block as an independent fragment of the

Antarctic Plate (Fig. 1B). Its movement is slightly

different from this plate, which is now migrating

northwestward from the Antarctic Peninsula due to

spreading in the Bransfield Strait Rift. This block

constitutes the upper plate of the South Shetland

Trench.

The subduction has generated a Cainozoic accre-

tionary prism in the forearc with a lobular front. In

contrast, the trench region comprises a relatively small

area that is probably experiencing tectonic erosion and

where the subduction has originated the Hesperides

forearc basin (Maldonado et al., 1994). This forearc

basin records a subsidence history in the middle

continental slope, including the migration of the depo-

centers toward the continent.

The continental shelf contains progradational

sequences produced mainly by the action of ice sheets

during the last glacial maximum (Larter and Barker,

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–8158

1989; Larter and Cunningham, 1993; Bart and Ander-

son, 1995). The location of the shelf break is related to

the sediment supply and, in several sectors, the shelf

progrades above the forearc basin (Maldonado et al.,

1994). A number of compressive structures have been

Fig. 2. Location chart of the ANTPAC 97/98 cruise with B/O Hesperides tr

MCS profiles in Figs. 4–8, while thin track lines correspond to profiles no

cruise. The thin lines represent the bathymetry of the area derived from t

Isolines are every 500 m except in the shelf, where an additional isoline at

ODP Leg 178 within the study area.

described in this shelf. A NNW-SSE anticline char-

acterises the outer shelf near Smith Island, while to the

SW, a NE-SW basement high called the Mid Shelf

High occupies the middle continental shelf (Larter and

Barker, 1991). Volcanic bodies of Upper Pleistocene–

ack lines in the study area. Thick track lines show the location of the

t shown in this work. Dashed lines correspond to transit lines of the

he Shipboard Scientific Party (1999), modified with our own data.

250 m depth is marked. Circles represent the location of the sites of

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 59

Holocene alkaline basalts are present in the shelf

(Hole and Larter, 1993); these authors propose a

magmatic origin associated with a slab window below

the Antarctic Peninsula, after the collision of the

spreading axis with the trench.

3. Data acquisition

Our data were obtained aboard the Spanish vessel

B/O Hesperides during the ANTPAC 97/98 cruise in

the Antarctic summer of 1997/1998. During this

cruise, approximately 1200 km of swath bathymetry

data along profiles and some 900 km of multichannel

reflection seismic profiles were obtained at the SW

end of the South Shetland Trench. Nine of these

profiles are orthogonal to the continental margin

(SHSM-01a, SHSM-03, SHSM-04, SHSM-05,

SHSM-06, SHSM-08, SHSM-09, SHSM-10 and

SHSM-13) and three run parallel to the margin

(SHSM-1b, SHSM-07 and SHSM-11) (Fig. 2). In

addition, we gathered two profiles oblique to the

margin trend (SHSM-02 and SHSM-12).

The multichannel reflection seismic profiles were

obtained with a tuned array of five BOLT air

guns, having a total volume of 22.14 l and a streamer

with a total length of 2.4 km and 96 channels. The

shot interval was 50 m and pressure was 140 atm.

Data were recorded with a DFS V digital system, a

sampling record interval of 2 ms and 10 s record

length. We processed the data with a standard se-

quence, including migration using a DISCO/FOCUS

system.

The swath bathymetry data were obtained with a

SIMRAD EM 12 system and post-processed with

NEPTUNE software at the Instituto de Ciencias del

Mar of Barcelona (Spain). They were visualized at the

Oregon State University (USA) using the comercial

software FLEDERMAUS.

4. Physiography and major morpho-sedimentary

features

The continental margin varies from an active mar-

gin in the South Shetland forearc to a passive style in

the southwestern segment (Figs. 2 and 3). The active

margin segment is characterised by the development of

a trench and an accretionary prism (Figs. 2, 3 and 6).

The two segments are separated by two bathymetric

highs where the trench ends. We refer to the first one,

located in the shelf break at 63j15VS, 64j15VW, as the

‘‘Shelf Transition High’’. The second one is located at

the base of the slope at 63j00VS, 64j30VW, and

separates the continental rise from the trench; we call

it the ‘‘Base-of-Slope Transition High’’.

In both segments of the margin, the continental

shelf is predominantly subhorizontal, although several

important depressions and swells trending NE-SW are

observed (Fig. 2). The shelf basins show a maximum

depth of 750 m, whereas the bathymetric highs of the

inner shelf are about 150 m. The shelf break is usually

located between 350 and 450 m, though in the Shelf

Transition High, with NE-SW elongation, it is only at

a depth of about 275 m (Fig. 2).

The continental slope in the passive margin can

be divided into two adjacent sectors. The NE slope

is characterised by a staircase geometry influenced

by the structure of the basement (Figs. 3 and 4), and

is less subsident and progradational than the south-

western sector. The southwestern slope is a homo-

geneous steep slope (Fig. 4). The continental slope

in the transition sector between the active and the

passive margin shows the greatest dip, with 31j of

mean slope. The continental slope in the active

margin is characterised by a reduced accretionary

prism, with a mean dip around 15j. An incipient

mid-slope forearc depression develops over this

accretionary prism. The upper slope features scarps

with 300 m of relief.

In the passive margin, there is a narrow base-of-

slope that is gradually transitional towards the slightly

undulated continental rise at approximately 3000 m

water depth, where the C Fracture Zone has almost no

associated relief (Fig. 2). In the transition between the

active and the passive margin, the oceanic basin floor

has depths between 2900 and 3650 m and the sea

floor has an irregular physiography, with two NE-SW

troughs separated by the elongated Base-of-Slope

Transition High (Figs. 2, 3 and 5). These troughs

merge with the South Shetland Trench to the north-

east, but they disappear southwestward. Seaward, the

basin plain is dissected by several E-W or ENE-WSW

elongated highs with elevations of 200–300 m above

the surrounding relief. In the active margin, the lower

slope ends at 4400 m water depth in the floor of the

Fig. 3. Shaded relief images (Fledermaus) oblique views of the SW end of the South Shetland Trench. Top: view from the W towards the E.

Bottom: view from the SW towards the NE. Length of area is about 130 km, the arrow indicating north.

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–8160

near-horizontal trench fill deposits (Figs. 2 and 6).

The deposits in the trench are dissected by ENE-

WSW to NE-SW scarps, which developed a staircase

bathymetry in several places within the trench. The

GLORIA image (Maldonado et al., 1994, their Fig. 2)

shows the presence of others scarps trending NNW-

SSE to NW-SE. The elongated NW-SE relief associ-

ated with the Hero Fracture Zone dissects the base of

the slope, which is at a water depth of about 3900 m

(Fig. 2).

Fig. 4. Multichannel seismic profile SHSM-03 in the passive margin and line-drawing interpretation.

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 pp. 61–62

Fig. 5. Multichannel seismic profile SHSM-06 in the transition zone and line-drawing interpretation.

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 pp. 63–64

Fig. 6. Multichannel seismic profiles SHSM-08 and SHSM-09 in the active margin and line-drawing interpretation.

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 pp. 65–68

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 69

5. Sedimentary sequences

The sedimentary record in the continental margin

presents two major seismic units that show different

acoustic responses and different stacking patterns in

the various domains of the margin.

5.1. Base of the slope

At the base of the slope, the Lower Unit covers the

entire study area except in front of the Shelf Transition

High, where only the upper major unit appears. The

Lower Unit has a weak reflective response, with a

locally transparent configuration (profiles SHSM-1b,

SHSM-02, SHSM-03 and SHSM-08; Figs. 4, 6, 7 and

8). The top of the Lower Unit is a major discontinuity

marked by a high-amplitude reflector designated as

the Middle Reflector (MR) Discontinuity (Figs. 4, 6, 7

and 8, Table 1). Although there are no direct data from

drillings, the sediments most likely consist of pelagic

deposits. The existence of two minor erosional dis-

continuities allows us to distinguish three depositional

sequences in this Lower Unit: MS-7, MS-6 and MS-5,

from bottom to top. Depositional sequence MS-7

occurs only on the oceanic crust southwest of the C

Fracture Zone (profiles SHSM-02 and SHSM-03;

Figs. 4 and 8), where magnetic anomalies Chron

C5n to Chron C3An are identified (Larter and Barker,

1991). This sequence represents the first sedimentary

record above the igneous oceanic crust. Depositional

sequences MS-6 and MS-5 directly cap the igneous

rocks of layer 2 (profile SHSM-08; Fig. 6), which

show anomaly Chron C3An (Larter and Barker,

1991). These two sequences include wedge-shaped

deposits with a high reflective acoustic response and a

chaotic internal structure that may represent synsedi-

mentary olistostromic rocks.

The Upper Unit has a very reflective acoustic

response characterised by intermediate- to high-ampli-

tude reflectors that are laterally continuous. They can

be observed throughout the base of the slope and in

the trench (profiles SHSM1b, SHSM-02, SHSM-03,

SHSM-06 and SHSM-08; Figs. 4–8). Several internal

discontinuities marked by regional high-amplitude

reflectors that we have termed UR3, UR2 and UR1

(UR: Upper Reflector), from bottom to top, help

define four depositional sequences in this Upper Unit:

MS-4 to MS-1, likewise from bottom to top. Sequen-

ces MS-4 to MS-1 can be correlated with sequences

M3 to M1 of Rebesco et al. (1997), which were also

studied at Site 1101 of Leg 178 (located west of the

study area at 3509 m water depth in the continental

rise of the margin) (Shipboard Scientific Party, 1999).

5.2. Shelf and slope

The Lower Unit also has a low to intermediate

reflective acoustic response with a local transparent

configuration (profiles SHSM-03 and SHSM-09;

Figs. 4 and 6). An irregular high-amplitude reflector

marks the top of this unit and has the geometry of a

major discontinuity, which can be traced to the MR

discontinuity of the base of the slope (Figs. 4, 6, 7 and

8, Table 1). There are two discontinuities inside the

Lower Unit separating reflectors identified by an

aggradational to divergent stacking pattern. These

discontinuities serve to separate three depositional

sequences called MS-7, MS-6 and MS-5, the MR

discontinuity conforming the top of sequence MS-5.

The internal reflector of each depositional sequence

tends to on-lap the discontinuity at its base.

The Upper Unit in the shelf and slope is charac-

terised by discontinuous reflectors of intermediate to

high amplitude (profiles SHSM-03, SHSM-06 and

SHSM-09; Figs. 4–6). Three high-amplitude reflec-

tors (UR3, UR2 and UR1), corresponding to the three

unconformities in the shelf, allow us to differentiate

four upper sequences, MS-4 to MS-1 (profiles SHSM-

03, SHSM-06 and SHSM-09; Figs. 4–6). These

correlate to the sequences described above for the

base of the slope. In relation to the seismic stacking

pattern of every sequence, the MS-4 sequence is

aggradational to progradational, while the MS-3

sequence is progradational. In turn, sequence MS-2

is an aggradational to progradational one and the last

sequence (MS-1) shows an aggradational pattern.

Whereas we have identified four depositional

sequences in this area, Larter and Barker (1991) and

the Shipboard Scientific Party (1999) distinguish three

sequences in the shelf (S3, S2 and S1 from bottom to

top, separated by two major discontinuities). Our

sequences MS-1 and MS-2 directly correlate with

the sequence S1 of these authors. Sequences MS-3

and MS-4 could respectively correlate with the S2 and

S3 sequences of Larter and Barker (1991) and the

Shipboard Scientific Party (1999) (Table 1), while our

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Fig. 7. (A) Multichannel seismic profile SHSM-01b in the oceanic crust and line-drawing interpretation. (B) Window of the same profile in the

area of the Base-of-Slope Transition High.

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Table 1

Depositional sequences in the continental margin of the study area and cronostratigraphical approach

Depositional

sequences

Chron Age Sequences of

Later and Barker

(1991)

Sequences of

Rebesco et al.

(1997)

Upper Unit MS-1 C1n Middle Pleistocene–Holocene S1 M1

UR1 Discontinuity

MS-2 C2r.2r Late Pliocene–Middle Pleistocene S1 M2

UR2 Discontinuity

MS-3 C2n (uncertain data age) Pliocene S2 M3

UR3 Discontinuity

MS-4 Early Pliocene S3 M4

Lower Unit MR-Discontinuity (Late Messinian–Early Pliocene)

MS-5 C3An Messinian S3 ¿?

MS-6 S4 ¿?

MS-7 C4n Tortonian–Early Messinian S5 ¿?

reflectors UR2 and UR3 are respective equivalents of

the tops of sequences S2 and S3 previously identified

(Table 1). The results of sites 1100, 1102, and 1103

from Leg 178 indicate that all four sequences from

MS-4 to MS-1 have a glacial origin in the shelf

(Shipboard Scientific Party, 1999).

6. Structure

6.1. Base of the slope

The oceanic basement of the former Phoenix Plate,

northeast of the Hero Fracture Zone, has a regular

fabric made up of NE-SW parallel lineaments with a

lateral continuity of up to 30 km in length (Fig. 9), as

interpreted from the GLORIA images by Maldonado

et al. (1994, their Fig. 2). In our seismic profiles, these

lineaments correspond to the scarps of normal faults.

Where these faults displace the most recent reflectors

of the depositional sequences they have a clear

bathymetric expression, but in other locations they

are capped by the MS-3, MS-2 and MS-1 depositional

sequences (profiles SHSM-06 and SHSM-08; Figs. 5

and 6). These faults were probably generated as a

response to the flexure of the lithosphere near the

trench (NW end of profile ANT-92-2, Maldonado et

al., 1994).

Southwest of the confluence of the trench with the

Hero Fracture Zone, the oceanic crust shows a sig-

moidal fabric in the GLORIA image (Maldonado et

al., 1994) defined by lineaments with an E-W trend in

their central part and a NE-SW trend at their ends

(Fig. 9). In the seismic profiles, this fabric can be

related with faults (profiles SHSM-03 and SHSM-06;

Figs. 4 and 5). There are normal and reverse faults,

dipping towards the south or the north (profiles

SHSM-1b, SHSM-03, SHSM-06 and SHSM-07,

SHSM-08, SHSM-09; Figs. 4, 5, 6, 7 and 9). Most

of the faults affect only depositional sequence MS-7

and are capped by sequence MS-6 (profiles SHSM-03

and SHSM-06; Figs. 4 and 5).

In addition, active reverse faults cut the entire

depositional cover (profile SHSM-1b, Fig. 7). The

most important of these active reverse faults is located

close to the trench end; we call it the Transition High

Reverse Fault (THRF). The trace of the THRF in the

sea bottom is an E-W scarp that is slightly concave

towards the south, with a clear relief in the swath

bathymetry (Figs. 2 and 9). This THRF separates two

blocks where the top of the oceanic basement lies at

different depths, the downthrown block being the

north block near the trench (profile SHSM-1b, Fig.

7A and B). This reverse fault intersects the main

detachment fault at the base of the accretionary prism

and clearly deforms all the depositional sequences

(profile SHSM-1b, Fig. 7).

6.2. Accretionary prism

The accretionary prism is 15-25 km wide in the

profiles perpendicular to the margin. The front is

active in the NE sector, as shown by the large detach-

ment fault at the base of the prism that crops out at the

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Fig. 8. Multichannel seismic profile SHSM-02 in the oceanic crust and line-drawing interpretation.

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Fig. 9. Tectonic map of the study area deduced from the seismic profiles. The lower rectangle includes a tectonic sketch of the area. Fine lines

are the interpretation of GLORIA image from Tomlinson et al. (1992) and Maldonado et al. (1994).

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 75

Fig. 10. Map of the GEOSAT free-air gravity anomalies from Sandwell and Smith (1997), isolines every 10 mGals. Stars represent earthquake

epicentres. Thick track lines show the location of theMCSprofiles in Figs. 4–8, while thin track lines correspond to profiles not shown in this work.

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–8176

prism toe (profile SHSM-08; Figs. 6 and 9). This

prism cannot be recognised in the area of the Shelf

Transition High (profile SHSM-06, Fig. 5). However,

a relict accretionary prism appears in the passive

margin segment (profile SHSM-03, Fig. 4), super-

posed over sequence MS-7. The rocks of sequences

MS-6 and MS-5 are intercalated with olistostromic

deposits at the front of the accretionary prism. How-

ever, these deposits show no evidence of being

affected by the subduction. Sequences MS-4 to MS-

1 capped the accretionary prism (profile SHSM-03,

Fig. 4).

The transition between the active and the passive

segments is marked by a concave inflexion of the

trace of the lobular front of the accretionary prism,

which advances landward partially surrounding the

Base-of-Slope Transition High. The Shelf Transition

High is just in front of the concave inflexion and, in

profile SHSM-06 (Fig. 5), appears as a raised block

bounded by normal faults. However, the dips of the

reflectors of the upper depositional sequences MS-4 to

MS-01 also indicate that it is an antiform. Southwest

of this concave inflexion, the relict accretionary prism

develops another seaward convex inflexion (Fig. 9).

Top layers 1 and 2 of the oceanic crust and the

basal detachment are clearly seen over a distance of

10–20 km below the accretionary prism (profile

SHSM-08; Fig. 6). The detachment dip is around

2–4j, assuming Vp = 1.8 km/s in the rocks of the

prism and trench. Its frontal part has a footwall ramp

geometry, cutting the reflectors of the trench upward

with a very low angle. Landward, however, the

detachment ramp grades to a detachment flat geome-

try. Only the upper part of the trench filling (0.1–0.3 s

TWT) is accreted into the prism, while most of the

trench deposits (around 0.7 s TWT) are subducted.

Although the internal structure of the accretionary

prism is not apparent, several faults dipping towards

the SE can be seen to produce a reverse offset of the

reflectors. In the middle continental slope, several

extensional faults induce tilting of the reflectors in

the hanging wall (profile SHSM-09; Fig. 6). They are

responsible for the scarps observed in the upper slope.

6.3. Slope and shelf

The southwestward extension of the Hesperides

Forearc Basin, identified by Maldonado et al. (1994),

is evident in the upper slope of the active margin. The

thickness and width of the basin decrease towards the

SW and cannot be recognised in profile SHSM-06

(Figs. 5 and 9). Very open folds trending NNW-SSE

to N-S deform the forearc deposits. These folds affect

all the depositional sequences with the exception of

the upper part of sequence MS-1.

A basin with synform geometry, buried under more

recent deposits, can be seen in the middle slope of the

passive margin over the relict accretionary prism. The

basin is associated with a slope break and probably

represents a relict forearc basin (profile SHSM-03,

Fig. 4).

The shelf is deformed by a set of very open

synforms and antiforms (Fig. 9). The free-air gravity

anomalies in the shelf approximately coincide with

the location of these folds, and the elongation of the

anomalies indicates they have a NE-SW trend (Figs. 9

and 10). The antiforms usually reveal outcrops of the

basement, which are eroded, yet locally buried below

a very thin sedimentary cover. The synforms are

normally filled by thin depositional sequences and

usually correspond to ancient glacial troughs. In the

area between Smith Island and the Shelf Transition

High, these NE-SW folds interfere with the NNW-

SSE to N-S trending folds. Only in one case, a small

half-graben associated with a normal fault is observed

in the shelf (profile SHSM-08-09; Figs. 6 and 9). This

normal fault dips with a NW component.

7. Discussion

In this area of transition between an active and a

passive margin, different tectonic and depositional

processes were at work, controlling the margin growth

patterns. We will first discuss the possible age of the

depositional sequences and then the structure of the

area and its relationship with the sedimentary pro-

cesses.

7.1. Age of the sequences: a chronological approach

Depositional sequences MS-7, MS-6 and MS-5

settled diachronously directly on top of layer 2 of

the oceanic crust, producing the magnetic anomalies

ranging from Chron C5n to Chron C3An (according

to the magnetic data published by Larter and Barker,

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 77

1991). Sequence MS-7 lies southwest of Fracture

Zone C over the oldest oceanic crust of the area, with

anomalies from Chron C5n to Chron C4n. However,

sequences MS-6 and MS-5 directly overlie the igne-

ous rocks in the active margin northeast of Fracture

Zone C, where we find the younger oceanic crust with

anomaly Chron C3An. In accordance with the ages

proposed by Berggren et al. (1995), sequence MS-7,

deposited prior to Chron C3n and after Chron C4n,

must be Late Tortonian to Early Messinian in age.

Sequences MS-6 and MS-5 can be assigned to the

Messinian (Table 1).

At the base of the slope, the data from Site 1101 of

Leg 178 indicate that the Upper Unit, characterised by

strong reflectivity in the seismic profiles, probably has

an age between Late Pliocene and Quaternary (Ship-

board Scientific Party, 1999). These site data allow the

base of sequence MS-1 to be dated at the base of Chron

C1n, giving Middle Pleistocene to Holocene ages for

this unit (Table 1). The base of sequence MS-2 can be

dated at the top of Chron C2r.2r, which indicates an

Early Pleistocene to Late Pliocene age. Moreover, the

top of sequence MS-3 covers Chron C2r.2r up to the

base of Chron C2An.1n, indicating a Late Pliocene

age. These ages are also suggested by the MCS profile

presented by Rebesco et al. (in press) between Site

1096 and the study area, allowing a correlation

between the depositional sequences and the ODP drill

Site. On this basis we attribute an Early Pliocene age to

sequence MS-4. Discontinuity MR, marking the tran-

sition from the Upper (sequences MS-4 to MS-1) to the

Lower Unit (sequences MS-7 to MS-5), may have a

Late Messinian to Early Pliocene age. Depositional

sequences MS-7, MS-6 and MS-5 are equivalent to

sequences S5, S4 and S3 of Larter and Barker (1991)

in the Antarctic Pacific Margin.

Data are scarce in the shelf, but agree with the ages

determined for the base of the slope. It is also possible

to correlate the sequences determined in the base of

the slope with the sequences of the slope and the shelf,

because they are laterally continuous. In any case, the

top of sequence MS-4, marked by reflector UR3, has

been dated at 4.5-4.6 Ma by the Shipboard Scientific

Party (1999). We can thus propose an Early Pliocene

age for the MS-4 sequence as well (Table 1).

Sequence MS-3, equivalent to sequence S2 of the

Shipboard Scientific Party (1999), has been dated at

its base as Early Pliocene, while the age of its upper

part remains unknown. The MS-1 and MS-2 sequen-

ces (equivalent to sequence S1) can be assumed to be

Pleistocene to latest Pliocene in age (Shipboard Sci-

entific Party, 1999).

7.2. Active margin

At present, active subduction in the South Shetland

Trench is a subject of debate. Barker and Burrell

(1977), the GRAPE Team (1990) and more recently

Kuminuma (1995) suggest the subduction is now

inactive. Barker (1982) and Pelayo and Wiens (1989)

indicate that there is no Wadati-Benioff zone associ-

ated with a subducting slab, yet on the basis of the

earthquake focal mechanisms, they conclude that sub-

duction is still active. Ibanez et al. (1997) describe the

existence of intermediate earthquakes (50 < h < 150

km) below the western South Shetland Islands, sug-

gesting the existence of a subducting slab. The litho-

sphere velocity model obtained by Grad et al. (1993)

based on refraction seismic experiments favours this

interpretation. In this model, the authors obtain a

slab of oceanic lithosphere with a dip of 25j subduct-

ing below the continental crust of the South Shet-

land Trench. Neither the earthquakes nor the velocity

model confirm the existence of the slab below 80–130

km.

The seismic reflection studies carried out by Larter

(1991), Maldonado et al. (1994) and Kim et al. (1995)

show that the most recent trench deposits are

deformed at the accretionary prism, below the basal

detachment. These data agree with our observations in

the NE active segment of the study area, where a

small accretionary prism overthrusts the deposits of

the trench, whereas most of the trench fill sediments

are subducted (Fig. 6). The subduction of the trench

deposits has been observed in other parts of the trench

as well (Maldonado et al., 1994), suggesting the

existence of tectonic erosion. These authors estimate

the convergence rates at around 4–6 cm/year from 30

to 6.7 Ma, when they underwent a sharp decrease. The

present-day convergence rates are approximately 1–2

cm/year.

7.3. Passive margin and shelf

The relict accretionary prism lies over sequence

MS-7 in the passive margin. Sequences MS-6 and

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–8178

MS-5 are not subducted and are intercalated with

olistostromes derived from the accretionary prism.

These features indicate a latest Tortonian age for the

end of the subduction in this area, which is compat-

ible with the end of subduction following the ridge-

trench collision after chron C4n. Sequences MS-4 to

MS-1 capped the olistostromes and the relict accre-

tionary prism. The reverse faults that deformed layer

2 of the oceanic crust (profiles SHSM-03 and

SHSM-06; Figs. 4 and 5) were coetaneous with the

deposition of sequence MS-7, indicating the exis-

tence of compressive deformation during the Torto-

nian, prior to the end of subduction. The accretionary

prism has a front with convex inflexion that suggests

the collapse of the prism and the seaward advance

of its front (Fig. 9). Collapse is also implied by

the presence of the olistostromes in the front and

their relationship with sequences MS-6 and MS-5

suggests a Messinian age for the collapse of the

accretionary prism. There are no major deformations

in the slope or the continental rise. After the de-

position of units MS-5, sedimentary processes do-

minated during the recent stages of the margin

development.

The most important structures in the shelf are the

NE-SW folds. Single-channel seismic surveys of

previous works revealed only the antiform located in

the middle shelf (Kimura, 1982; Anderson et al.,

1990; Larter and Barker, 1991), termed the ‘‘Midshelf

High’’ (Larter and Barker, 1991). New MCS data

indicate that these folds, which are now growing,

deform the entire shelf. The folds have the same

characteristics in both the active and passive margins,

indicating that the whole shelf is undergoing NW-SE

compression.

In the sector between Smith Island and the Shelf

Transition High, the NNW-SSE to N-S folds devel-

oped practically coetaneously with the aforemen-

tioned NE-SW ones. The two-fold systems interfere,

suggesting that this sector has undergone constric-

tional deformation with area reduction.

7.4. Transition between the passive and the active

margins

The tectonic map obtained from the interpretation

of the seismic profiles shows that the South Shetland

Trench continues 50 km southwest of the Hero

Fracture Zone (Fig. 9), as previously reported by

Tomlinson et al. (1992) and Maldonado et al.

(1994). We located the end of the trench at the

Base-of-Slope Transition High. Southwestward from

the end of the trench, the basal detachment of the

accretionary prism splits into two active major reverse

faults (the THRF and the basal detachment itself),

whereas the trench splits into two small troughs. The

two major reverse faults end toward the SW and are

replaced by E-W active reverse faults. This means that

the area of accommodation of the shortening is greater

than the bifurcation area of the main detachment fault.

Southwest of the Base-of-Slope Transition High, all

the compressive structures are inactive, indicating the

end of the compressional tectonics.

The upper plate of the subduction is the South

Shetland Block (part of the Antarctic Plate), while the

lower one is the oceanic crust of the same Antarctic

Plate. The two elements join in the transition area

between the active and the passive margins. There is

no evidence of a transform fault separating these

elements, as has been proposed in previous models.

The trench is acting as an internal subduction zone

that accommodates shortening inside the Antarctic

Plate and the study region represents the outcrop area

of the tip-line of this subduction zone.

The Shelf Transition High is an antiform fold

bounded by normal faults. This fold may be respon-

sible for the loss of continuity in the Hesperides

Forearc Basin, which develops two perisynclinal

terminations on either side of the high. The main

deformation phase produced highly deformed depo-

sitional sequences MS-7, MS-6 and MS-5, whereas

the internal reflectors of sequences MS-4 to MS-1

developed progressive unconformities on the conti-

nental side of the high. This suggests that the high

was generated in the Late Messinian–Early Pliocene

and has been growing continuously to the present

time.

The concave major inflexion in the lobular front of

the accretionary prism is associated with the presence

of the Base-of-Slope and the Shelf Transition Highs,

the discontinuity of the forearc basin and the defor-

mation of the accretionary prism. Such an association

of structures is strongly reminiscent of the structures

produced by the subduction of a seamount (Domi-

nguez et al., 1998, 2000). However, as we have

pointed out above, there is no evidence of a high in

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 79

the oceanic crust except for the Base-of-Slope Tran-

sition High, which is due to the active reverse faults

originating from the split of the basal detachment. We

propose that this inflexion may be associated with the

tendency to subduction of the Base-of-Slope Transi-

tion High, which is the deformed area that accom-

modates the end of the trench.

Another process that may have contributed to the

major inflexion is the rollback process proposed for

this trench (Larter, 1991; Maldonado et al., 1994).

The advance of the front of the prism towards the

NW—while the outcrop of the tip-line remained in

place—could have originated a rotation of the front,

producing an arched pattern for the prism. In this

model, extension in the back-arc region that is

accommodated by the Bransfield Strait rift could

not continue southwestward from the end of the

trench. This is supported by the observation that the

shelf in the study area is under compression and the

extensional deformations are reduced to a single

normal fault SE of Smith Island that disappears

toward the SW.

7.5. Oceanic crust

The lineaments observed in the oceanic crust run

parallel to the trench NE of the Hero Fracture Zone,

and have a sigmoidal pattern between the C and the

Hero Fracture Zones (Fig. 9). Most of these linea-

ments are normal faults that cut layer 1 of the oceanic

crust, although there are also several normal faults

capped by the most recent sediments. Northeast of the

Hero Fracture Zone, these faults are interpreted as

normal faults produced by extension of the subducting

slab in the outer swell (Maldonado et al., 1994; Kim et

al., 1995). In the sector between the C and the Hero

Fracture Zones, the sigmoidal pattern of the linea-

ments could indicate that the outer swell is separating

from the trench and is located farther towards the

ocean, where it disappears. In this region, the outer

swell also becomes wider than in the NE sector of the

area and the normal faults cut even the flat bottom of

the trench. The sigmoidal pattern of the structures

could also be favoured by an originally sigmoidal

pattern of the fabric of the oceanic rocks suggested by

the oblique orientation of the C3An and C4An mag-

netic anomalies of the oceanic crust in this area (Larter

and Barker, 1991).

7.6. Tectonic model

The model of the evolution in the area can be

summarised in several stages. The first stage, in the

Tortonian, corresponds to the ridge-trench collision

after anomaly chron C4n, which preceded the end of

subduction in the present-day passive margin. A

compressive deformation event deformed the oceanic

crust simultaneously with the deposition of sequence

MS-7. This stage finished in the latest Tortonian–

Early Messinian, when subduction ended in the pas-

sive margin. The tip-line of the intraplate subduction

moved to its present-day location and marks the onset

of the second tectonic stage during the Messinian.

During this second stage, the accretionary prism

collapsed in the passive margin sector. The internal

deformation in the area of the tip-line, mainly by

reverse faults, generated the Base-of-Slope Transition

High in the oceanic crust. The second stage ended in

the latest Messinian–Early Pliocene with the conver-

gence of this high against the accretionary prism,

thereby producing its concave inflexion and internal

deformation, as well as the loss of continuity of the

forearc basin. Sequences MS-6 and MS-5 were depos-

ited during this stage The third stage, from the Early

Pliocene to the present-day, corresponds to the devel-

opment of compressive deformation in the shelf and in

the base of the slope near the Base-of-Slope Transition

High, while extensional deformation occurs in the fore

bulge of the subduction footwall. During this time, the

deposition of sequences MS-4 to MS-1 originated the

progradation of the passive margin, while these

sequences are deformed in the active margin.

8. Conclusions

The structural model discussed in this paper stands

as one of the few well-documented case studies of the

tectonic evolution of the lateral end of a subduction

zone, and of the transition from an active to a passive

margin style. The lateral end of subduction is accom-

panied by the decrease of the slip of the basal detach-

ment, which ends in a tip-line. Around the tip-line lies

an area with distributed compressional deformation,

both in the oceanic crust of the subducting plate and

the accretionary prism of the overriding plate. The

evolution of the area includes a first stage when a

A. Jabaloy et al. / Tectonophysics 366 (2003) 55–8180

compressional deformation preceding the end of the

subduction produced reverse faults in the oceanic

crust. This first stage ended when subduction ceased

southwest of the study area in the latest Tortonian.

The second stage involves the development of dis-

tributed compressional deformation around the tip-

line of the basal detachment of the accretionary prism.

The deformation gave rise to a high in the oceanic

crust at the base of the slope. The subduction of this

high produced the deformation of the accretionary

prism and the formation of another high in the shelf

(the Shelf Transition High) at the end of the Messinian

or the beginning of the Pliocene. The third stage

began in the Early Pliocene and continues at present.

It entails the active deformation, in a compressional

setting, of the shelf and the base of the slope around

the tip-line of the basal detachment, while extensional

deformations are active in the outer swell of the

trench.

Acknowledgements

We would like to thank the Commander, officers

and crew of the B/O Hesperides for their help during

the ANTPAC 97-98 cruise. We are grateful for the

help of the U.G.B.O. technicians participating in the

cruise. The expert help of Emilia Litcheva and Javier

Maldonado, who processed the MCS and the swath

bathymetry, is appreciated. We also acknowledge the

work of Jean Sanders in reviewing the English text

and A. Carbo for the release of the Lanzada software.

The Spanish ‘‘Comision Interministerial de Ciencia y

Tecnologıa (CICYT)’’ provided financial support

through research project ANT99-0817.

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