The Transition from an Active to a Passive Margin (SW End of the South Shetland Trench, Antarctic Peninsula)

Tectonophysics 366 (2003) 55–81 www.elsevier.com/locate/tecto

The transition from an active to a passive margin (SW end of the South Shetland Trench, Antarctic Peninsula)

Antonio Jabaloya,*, Juań-Carlos Balanya´ b,c, Antonio Barnolasd, Jesu´s Galindo-Zaldı´vara, F. Javier Hernańdez-Molinae, Andre´s Maldonadoc, Jose´-Miguel Martıńez-Martıńeza,c, Jose´ Rodrı´guez-Fernańdezc, Carlos Sanz de Galdeanoc, Luis Somozad, Emma Surinãchf, Juań Toma´sVa´zquezg

a Department Geodina´mica, University Granada, 18071 Granada, Spain b Department de Ciencias Experimentales, University Pablo de Olavide, Sevilla, Spain c Instituto Andaluz de Ciencias de la Tierra, C.S.I.C.-University, Granada, Spain d Instituto Geolo´gico y Minero de Espan˜a, Madrid, Spain e Department Geociencias Marinas y O. D. Territorio, University Vigo, Vigo, Pontevedra, Spain f Departament de Geodinamica i Geofı´sica, Universitat de Barcelona, Barcelona, Spain g Facultad de Ciencias del Mar, Universidad de Ca´diz, Puerto Real, Ca´diz, 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

1. Introduction

* Corresponding author. Fax: +34-958-248527. Trenches associated with oceanic subduction zones E-mail address: [email protected] (A. Jabaloy). usually end in triple junctions or against plate boun-

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. A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 57 daries—normally another subduction zone or a trans- SW segments of the Phoenix–Antarctic spreading form fault. However, in several areas of the world, axis separated by NW-SE transform faults collided there are oceanic trenches that show a transition with the trench. After the collision, the continental toward a passive margin or the abyssal plain in broad margin lithosphere and the oceanic lithosphere to the bands of deformation. An example of these trenches is north of the spreading axis came into contact, which the northern end of the East Luzon Trough, which is led to the end of subduction and the disappearance of the prolongation of the Philippine Trench towards the the trench bathymetry (Herron and Tucholke, 1976; north, located at the eastern end of the Philippine Barker, 1982; Larter and Barker, 1991). The collisions Islands (Hayes and Lewis, 1984). Another example is progressively migrated northeastward and the differ- near the Antarctic Peninsula, where an active small ent sectors of the active margin evolved towards a trench known as the South Shetland Trench (Maldo- passive style following the subduction of the segments nado et al., 1994) coexists with inactive subduction of the spreading axis. zones in the oceanic lithosphere of the southwestern The last two collisions took place to the north of sector of the Antarctic Peninsula (Gohl et al., 1997). the North Anvers Fracture Zone and involved two Although these trenches are known to terminate segments of the Phoenix–Antarctic spreading axis laterally, the literature contains no description of the separated by the C Fracture Zone (Larter and Barker, morphology and structure associated with such termi- 1991) (Fig. 1). The segment located between the nations. North Anvers and the C fracture zones collided The main aim of the present paper is the analysis of obliquely after the formation in the oceanic crust of the structure and stratigraphy of the SW end of the anomaly chron C4n (Fig. 1A). The segment located South Shetland Trench in the Antarctic Peninsula in between the C and Hero fracture zones also underwent order to determine how and why this trench terminates an oblique collision, after anomaly C3An (Fig. 1A). (Fig. 1). A second objective is to study the transition During the anomaly chron C2An, activity in the from an active to a passive margin and how the Phoenix–Antarctic spreading axis ended and the deformation along the transition zone between the Phoenix Plate became part of the Antarctic Plate two is accommodated. Most continental passive mar- (Livermore et al., 2000). Meanwhile, activity in the gins result from a previous rifting phase that produces trench continued, though at present convergence rates a continental break-up stage followed by a spreading are slow (Maldonado et al., 1994; Kim et al., 1995). stage. However, the western margin of the Antarctic Aldaya and Maldonado (1996) defined the South Peninsula evolved from an active margin, progres- Shetland Block as an independent fragment of the sively decreasing in length since the Jurassic as results Antarctic Plate (Fig. 1B). Its movement is slightly of trench-ridge collisions, towards a passive-type different from this plate, which is now migrating margin. northwestward from the Antarctic Peninsula due to spreading in the Bransfield Strait Rift. This block constitutes the upper plate of the South Shetland 2. Geological setting Trench. The subduction has generated a Cainozoic accre- The South Shetland Trench is a slightly arched NE- tionary prism in the forearc with a lobular front. In SW trench located in the northwestern sector of the contrast, the trench region comprises a relatively small Antarctic Peninsula (Maldonado et al., 1994).Itis area that is probably experiencing tectonic erosion and associated with a small island arc represented by the where the subduction has originated the Hespe´rides South Shetland Islands. The Bransfield Strait, a nar- forearc basin (Maldonado et al., 1994). This forearc row NE-SW basin with depths greater than 2000 m, basin records a subsidence history in the middle separates these islands from the Antarctic Peninsula continental slope, including the migration of the depo- (Barker and Austin, 1998) (Fig. 1A and B). centers toward the continent. This trench is the last remnant of a once extensive The continental shelf contains progradational trench that occupied the Pacific margin of the Ant- sequences produced mainly by the action of ice sheets arctic Peninsula. During the Cainozoic, several NE- during the last glacial maximum (Larter and Barker, 58 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81

1989; Larter and Cunningham, 1993; Bart and Ander- described in this shelf. A NNW-SSE anticline char- son, 1995). The location of the shelf break is related to acterises the outer shelf near Smith Island, while to the the sediment supply and, in several sectors, the shelf SW, a NE-SW basement high called the Mid Shelf progrades above the forearc basin (Maldonado et al., High occupies the middle continental shelf (Larter and 1994). A number of compressive structures have been Barker, 1991). Volcanic bodies of Upper Pleistocene–

Fig. 2. Location chart of the ANTPAC 97/98 cruise with B/O Hespe´rides track lines in the study area. Thick track lines show the location of the MCS profiles in Figs. 4–8, while thin track lines correspond to profiles not shown in this work. Dashed lines correspond to transit lines of the cruise. The thin lines represent the bathymetry of the area derived from the Shipboard Scientific Party (1999), modified with our own data. Isolines are every 500 m except in the shelf, where an additional isoline at 250 m depth is marked. Circles represent the location of the sites of ODP Leg 178 within the study area. A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 59

Holocene alkaline basalts are present in the shelf a trench and an accretionary prism (Figs. 2, 3 and 6). (Hole and Larter, 1993); these authors propose a The two segments are separated by two bathymetric magmatic origin associated with a slab window below highs where the trench ends. We refer to the first one, the Antarctic Peninsula, after the collision of the located in the shelf break at 63j15VS, 64j15VW, as the spreading axis with the trench. ‘‘Shelf Transition High’’. The second one is located at thebaseoftheslopeat63j00VS, 64j30VW, and separates the continental rise from the trench; we call 3. Data acquisition it the ‘‘Base-of-Slope Transition High’’. In both segments of the margin, the continental Our data were obtained aboard the Spanish vessel shelf is predominantly subhorizontal, although several B/O Hespe´rides during the ANTPAC 97/98 cruise in important depressions and swells trending NE-SW are the Antarctic summer of 1997/1998. During this observed (Fig. 2). The shelf basins show a maximum cruise, approximately 1200 km of swath bathymetry depth of 750 m, whereas the bathymetric highs of the data along profiles and some 900 km of multichannel inner shelf are about 150 m. The shelf break is usually reflection seismic profiles were obtained at the SW located between 350 and 450 m, though in the Shelf end of the South Shetland Trench. Nine of these Transition High, with NE-SW elongation, it is only at profiles are orthogonal to the continental margin a depth of about 275 m (Fig. 2). (SHSM-01a, SHSM-03, SHSM-04, SHSM-05, The continental slope in the passive margin can SHSM-06, SHSM-08, SHSM-09, SHSM-10 and be divided into two adjacent sectors. The NE slope SHSM-13)andthreerunparalleltothemargin is characterised by a staircase geometry influenced (SHSM-1b, SHSM-07 and SHSM-11) (Fig. 2).In by the structure of the basement (Figs. 3 and 4), and addition, we gathered two profiles oblique to the is less subsident and progradational than the south- margin trend (SHSM-02 and SHSM-12). western sector. The southwestern slope is a homo- The multichannel reflection seismic profiles were geneous steep slope (Fig. 4). The continental slope obtained with a tuned array of five BOLT air in the transition sector between the active and the guns, having a total volume of 22.14 l and a streamer passive margin shows the greatest dip, with 31j of with a total length of 2.4 km and 96 channels. The mean slope. The continental slope in the active shot interval was 50 m and pressure was 140 atm. margin is characterised by a reduced accretionary Data were recorded with a DFS V digital system, a prism, with a mean dip around 15j. An incipient sampling record interval of 2 ms and 10 s record mid-slope forearc depression develops over this length. We processed the data with a standard se- accretionary prism. The upper slope features scarps quence, including migration using a DISCO/FOCUS with 300 m of relief. system. In the passive margin, there is a narrow base-of- The swath bathymetry data were obtained with a slope that is gradually transitional towards the slightly SIMRAD EM 12 system and post-processed with undulated continental rise at approximately 3000 m NEPTUNE software at the Instituto de Ciencias del water depth, where the C Fracture Zone has almost no Mar of Barcelona (Spain). They were visualized at the associated relief (Fig. 2). In the transition between the Oregon State University (USA) using the comercial active and the passive margin, the oceanic basin floor software FLEDERMAUS. 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 4. Physiography and major morpho-sedimentary Transition High (Figs. 2, 3 and 5). These troughs features merge with the South Shetland Trench to the northeast, but they disappear southwestward. Seaward, the The continental margin varies from an active mar- basin plain is dissected by several E-W or ENE-WSW gin in the South Shetland forearc to a passive style in elongated highs with elevations of 200–300 m above the southwestern segment (Figs. 2 and 3). The active the surrounding relief. In the active margin, the lower margin segment is characterised by the development of slope ends at 4400 m water depth in the floor of the 60 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81

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. near-horizontal trench fill deposits (Figs. 2 and 6). shows the presence of others scarps trending NNW- The deposits in the trench are dissected by ENE- SSE to NW-SE. The elongated NW-SE relief associ- WSW to NE-SW scarps, which developed a staircase ated with the Hero Fracture Zone dissects the base of bathymetry in several places within the trench. The the slope, which is at a water depth of about 3900 m GLORIA image (Maldonado et al., 1994, their Fig. 2) (Fig. 2). A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 pp. 61–62

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. 63–64

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. 65–68

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 69

5. Sedimentary sequences ces MS-4 to MS-1 can be correlated with sequences M3 to M1 of Rebesco et al. (1997), which were also The sedimentary record in the continental margin studied at Site 1101 of Leg 178 (located west of the presents two major seismic units that show different study area at 3509 m water depth in the continental acoustic responses and different stacking patterns in rise of the margin) (Shipboard Scientific Party, 1999). the various domains of the margin. 5.2. Shelf and slope 5.1. Base of the slope The Lower Unit also has a low to intermediate At the base of the slope, the Lower Unit covers the reflective acoustic response with a local transparent entire study area except in front of the Shelf Transition configuration (profiles SHSM-03 and SHSM-09; High, where only the upper major unit appears. The Figs. 4 and 6). An irregular high-amplitude reflector Lower Unit has a weak reflective response, with a marks the top of this unit and has the geometry of a locally transparent configuration (profiles SHSM-1b, major discontinuity, which can be traced to the MR SHSM-02, SHSM-03 and SHSM-08; Figs. 4, 6, 7 and discontinuity of the base of the slope (Figs. 4, 6, 7 and 8). The top of the Lower Unit is a major discontinuity 8, Table 1). There are two discontinuities inside the marked by a high-amplitude reflector designated as Lower Unit separating reflectors identified by an the Middle Reflector (MR) Discontinuity (Figs. 4, 6, 7 aggradational to divergent stacking pattern. These and 8, Table 1). Although there are no direct data from discontinuities serve to separate three depositional drillings, the sediments most likely consist of pelagic sequences called MS-7, MS-6 and MS-5, the MR deposits. The existence of two minor erosional dis- discontinuity conforming the top of sequence MS-5. continuities allows us to distinguish three depositional The internal reflector of each depositional sequence sequences in this Lower Unit: MS-7, MS-6 and MS-5, tends to on-lap the discontinuity at its base. from bottom to top. Depositional sequence MS-7 The Upper Unit in the shelf and slope is charac- occurs only on the oceanic crust southwest of the C terised by discontinuous reflectors of intermediate to Fracture Zone (profiles SHSM-02 and SHSM-03; high amplitude (profiles SHSM-03, SHSM-06 and Figs. 4 and 8), where magnetic anomalies Chron SHSM-09; Figs. 4–6). Three high-amplitude reflec- C5n to Chron C3An are identified (Larter and Barker, tors (UR3, UR2 and UR1), corresponding to the three 1991). This sequence represents the first sedimentary unconformities in the shelf, allow us to differentiate record above the igneous oceanic crust. Depositional four upper sequences, MS-4 to MS-1 (profiles SHSM- sequences MS-6 and MS-5 directly cap the igneous 03, SHSM-06 and SHSM-09; Figs. 4–6). These rocks of layer 2 (profile SHSM-08; Fig. 6), which correlate to the sequences described above for the show anomaly Chron C3An (Larter and Barker, base of the slope. In relation to the seismic stacking 1991). These two sequences include wedge-shaped pattern of every sequence, the MS-4 sequence is deposits with a high reflective acoustic response and a aggradational to progradational, while the MS-3 chaotic internal structure that may represent synsedi- sequence is progradational. In turn, sequence MS-2 mentary olistostromic rocks. is an aggradational to progradational one and the last The Upper Unit has a very reflective acoustic sequence (MS-1) shows an aggradational pattern. response characterised by intermediate- to high-ampli- Whereas we have identified four depositional tude reflectors that are laterally continuous. They can sequences in this area, Larter and Barker (1991) and be observed throughout the base of the slope and in the Shipboard Scientific Party (1999) distinguish three the trench (profiles SHSM1b, SHSM-02, SHSM-03, sequences in the shelf (S3, S2 and S1 from bottom to SHSM-06 and SHSM-08; Figs. 4–8). Several internal top, separated by two major discontinuities). Our discontinuities marked by regional high-amplitude sequences MS-1 and MS-2 directly correlate with reflectors that we have termed UR3, UR2 and UR1 the sequence S1 of these authors. Sequences MS-3 (UR: Upper Reflector), from bottom to top, help and MS-4 could respectively correlate with the S2 and define four depositional sequences in this Upper Unit: S3 sequences of Larter and Barker (1991) and the MS-4 to MS-1, likewise from bottom to top. Sequen- Shipboard Scientific Party (1999) (Table 1), while our 70 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 71

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

Table 1 Depositional sequences in the continental margin of the study area and cronostratigraphical approach Depositional Chron Age Sequences of Sequences of sequences Later and Barker Rebesco et al. (1991) (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 their central part and a NE-SW trend at their ends the tops of sequences S2 and S3 previously identified (Fig. 9). In the seismic profiles, this fabric can be (Table 1). The results of sites 1100, 1102, and 1103 related with faults (profiles SHSM-03 and SHSM-06; from Leg 178 indicate that all four sequences from Figs. 4 and 5). There are normal and reverse faults, MS-4 to MS-1 have a glacial origin in the shelf dipping towards the south or the north (profiles (Shipboard Scientific Party, 1999). 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 6. Structure and are capped by sequence MS-6 (profiles SHSM-03 and SHSM-06; Figs. 4 and 5). 6.1. Base of the slope In addition, active reverse faults cut the entire depositional cover (profile SHSM-1b, Fig. 7). The The oceanic basement of the former Phoenix Plate, most important of these active reverse faults is located northeast of the Hero Fracture Zone, has a regular close to the trench end; we call it the Transition High fabric made up of NE-SW parallel lineaments with a Reverse Fault (THRF). The trace of the THRF in the lateral continuity of up to 30 km in length (Fig. 9),as sea bottom is an E-W scarp that is slightly concave interpreted from the GLORIA images by Maldonado towards the south, with a clear relief in the swath et al. (1994, their Fig. 2). In our seismic profiles, these bathymetry (Figs. 2 and 9). This THRF separates two lineaments correspond to the scarps of normal faults. blocks where the top of the oceanic basement lies at Where these faults displace the most recent reflectors different depths, the downthrown block being the of the depositional sequences they have a clear north block near the trench (profile SHSM-1b, Fig. bathymetric expression, but in other locations they 7A and B). This reverse fault intersects the main are capped by the MS-3, MS-2 and MS-1 depositional detachment fault at the base of the accretionary prism sequences (profiles SHSM-06 and SHSM-08; Figs. 5 and clearly deforms all the depositional sequences and 6). These faults were probably generated as a (profile SHSM-1b, Fig. 7). response to the flexure of the lithosphere near the trench (NW end of profile ANT-92-2, Maldonado et 6.2. Accretionary prism al., 1994). Southwest of the confluence of the trench with the The accretionary prism is 15-25 km wide in the Hero Fracture Zone, the oceanic crust shows a sig- profiles perpendicular to the margin. The front is moidal fabric in the GLORIA image (Maldonado et active in the NE sector, as shown by the large detach- al., 1994) defined by lineaments with an E-W trend in ment fault at the base of the prism that crops out at the A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 73 Fig. 8. Multichannel seismic profile SHSM-02 in the oceanic crust and line-drawing interpretation. 74 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81

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 the MCS profiles in Figs. 4–8, while thin track lines correspond to profiles not shown in this work. 76 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 prism toe (profile SHSM-08; Figs. 6 and 9). This is evident in the upper slope of the active margin. The prism cannot be recognised in the area of the Shelf thickness and width of the basin decrease towards the Transition High (profile SHSM-06, Fig. 5). However, SW and cannot be recognised in profile SHSM-06 a relict accretionary prism appears in the passive (Figs. 5 and 9). Very open folds trending NNW-SSE margin segment (profile SHSM-03, Fig. 4), super- to N-S deform the forearc deposits. These folds affect posed over sequence MS-7. The rocks of sequences all the depositional sequences with the exception of MS-6 and MS-5 are intercalated with olistostromic the upper part of sequence MS-1. deposits at the front of the accretionary prism. How- A basin with synform geometry, buried under more ever, these deposits show no evidence of being recent deposits, can be seen in the middle slope of the affected by the subduction. Sequences MS-4 to MS- passive margin over the relict accretionary prism. The 1 capped the accretionary prism (profile SHSM-03, basin is associated with a slope break and probably Fig. 4). represents a relict forearc basin (profile SHSM-03, The transition between the active and the passive Fig. 4). segments is marked by a concave inflexion of the The shelf is deformed by a set of very open trace of the lobular front of the accretionary prism, synforms and antiforms (Fig. 9). The free-air gravity which advances landward partially surrounding the anomalies in the shelf approximately coincide with Base-of-Slope Transition High. The Shelf Transition the location of these folds, and the elongation of the High is just in front of the concave inflexion and, in anomalies indicates they have a NE-SW trend (Figs. 9 profile SHSM-06 (Fig. 5), appears as a raised block and 10). The antiforms usually reveal outcrops of the bounded by normal faults. However, the dips of the basement, which are eroded, yet locally buried below reflectors of the upper depositional sequences MS-4 to a very thin sedimentary cover. The synforms are MS-01 also indicate that it is an antiform. Southwest normally filled by thin depositional sequences and of this concave inflexion, the relict accretionary prism usually correspond to ancient glacial troughs. In the develops another seaward convex inflexion (Fig. 9). area between Smith Island and the Shelf Transition Top layers 1 and 2 of the oceanic crust and the High, these NE-SW folds interfere with the NNW- basal detachment are clearly seen over a distance of SSE to N-S trending folds. Only in one case, a small 10–20 km below the accretionary prism (profile half-graben associated with a normal fault is observed SHSM-08; Fig. 6). The detachment dip is around in the shelf (profile SHSM-08-09; Figs. 6 and 9). This 2–4j, assuming Vp = 1.8 km/s in the rocks of the normal fault dips with a NW component. 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 7. Discussion detachment ramp grades to a detachment flat geometry. Only the upper part of the trench filling (0.1–0.3 s In this area of transition between an active and a TWT) is accreted into the prism, while most of the passive margin, different tectonic and depositional trench deposits (around 0.7 s TWT) are subducted. processes were at work, controlling the margin growth Although the internal structure of the accretionary patterns. We will first discuss the possible age of the prism is not apparent, several faults dipping towards depositional sequences and then the structure of the the SE can be seen to produce a reverse offset of the area and its relationship with the sedimentary pro- reflectors. In the middle continental slope, several cesses. extensional faults induce tilting of the reflectors in the hanging wall (profile SHSM-09; Fig. 6). They are 7.1. Age of the sequences: a chronological approach responsible for the scarps observed in the upper slope. Depositional sequences MS-7, MS-6 and MS-5 6.3. Slope and shelf settled diachronously directly on top of layer 2 of the oceanic crust, producing the magnetic anomalies The southwestward extension of the Hespe´rides ranging from Chron C5n to Chron C3An (according Forearc Basin, identified by Maldonado et al. (1994), 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 part remains unknown. The MS-1 and MS-2 sequen- Zone C over the oldest oceanic crust of the area, with ces (equivalent to sequence S1) can be assumed to be anomalies from Chron C5n to Chron C4n. However, Pleistocene to latest Pliocene in age (Shipboard Sci- sequences MS-6 and MS-5 directly overlie the igne- entific Party, 1999). ous rocks in the active margin northeast of Fracture Zone C, where we find the younger oceanic crust with 7.2. Active margin anomaly Chron C3An. In accordance with the ages proposed by Berggren et al. (1995), sequence MS-7, At present, active subduction in the South Shetland deposited prior to Chron C3n and after Chron C4n, Trench is a subject of debate. Barker and Burrell must be Late Tortonian to Early Messinian in age. (1977), the GRAPE Team (1990) and more recently Sequences MS-6 and MS-5 can be assigned to the Kuminuma (1995) suggest the subduction is now Messinian (Table 1). inactive. Barker (1982) and Pelayo and Wiens (1989) At the base of the slope, the data from Site 1101 of indicate that there is no Wadati-Benioff zone associ- Leg 178 indicate that the Upper Unit, characterised by ated with a subducting slab, yet on the basis of the strong reflectivity in the seismic profiles, probably has earthquake focal mechanisms, they conclude that sub- an age between Late Pliocene and Quaternary (Ship- duction is still active. Iba´n˜ez et al. (1997) describe the board Scientific Party, 1999). These site data allow the existence of intermediate earthquakes (50 < h < 150 base of sequence MS-1 to be dated at the base of Chron km) below the western South Shetland Islands, sug- C1n, giving Middle Pleistocene to Holocene ages for gesting the existence of a subducting slab. The litho- this unit (Table 1). The base of sequence MS-2 can be sphere velocity model obtained by Grad et al. (1993) dated at the top of Chron C2r.2r, which indicates an based on refraction seismic experiments favours this Early Pleistocene to Late Pliocene age. Moreover, the interpretation. In this model, the authors obtain a top of sequence MS-3 covers Chron C2r.2r up to the slab of oceanic lithosphere with a dip of 25j subduct- base of Chron C2An.1n, indicating a Late Pliocene ing below the continental crust of the South Shet- age. These ages are also suggested by the MCS profile land Trench. Neither the earthquakes nor the velocity presented by Rebesco et al. (in press) between Site model confirm the existence of the slab below 80–130 1096 and the study area, allowing a correlation km. between the depositional sequences and the ODP drill The seismic reflection studies carried out by Larter Site. On this basis we attribute an Early Pliocene age to (1991), Maldonado et al. (1994) and Kim et al. (1995) sequence MS-4. Discontinuity MR, marking the tran- show that the most recent trench deposits are sition from the Upper (sequences MS-4 to MS-1) to the deformed at the accretionary prism, below the basal Lower Unit (sequences MS-7 to MS-5), may have a detachment. These data agree with our observations in Late Messinian to Early Pliocene age. Depositional the NE active segment of the study area, where a sequences MS-7, MS-6 and MS-5 are equivalent to small accretionary prism overthrusts the deposits of sequences S5, S4 and S3 of Larter and Barker (1991) the trench, whereas most of the trench fill sediments in the Antarctic Pacific Margin. are subducted (Fig. 6). The subduction of the trench Data are scarce in the shelf, but agree with the ages deposits has been observed in other parts of the trench determined for the base of the slope. It is also possible as well (Maldonado et al., 1994), suggesting the to correlate the sequences determined in the base of existence of tectonic erosion. These authors estimate the slope with the sequences of the slope and the shelf, the convergence rates at around 4–6 cm/year from 30 because they are laterally continuous. In any case, the to 6.7 Ma, when they underwent a sharp decrease. The top of sequence MS-4, marked by reflector UR3, has present-day convergence rates are approximately 1–2 been dated at 4.5-4.6 Ma by the Shipboard Scientific cm/year. Party (1999). We can thus propose an Early Pliocene age for the MS-4 sequence as well (Table 1). 7.3. Passive margin and shelf Sequence MS-3, equivalent to sequence S2 of the Shipboard Scientific Party (1999), has been dated at The relict accretionary prism lies over sequence its base as Early Pliocene, while the age of its upper MS-7 in the passive margin. Sequences MS-6 and 78 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81

MS-5 are not subducted and are intercalated with Fracture Zone (Fig. 9), as previously reported by olistostromes derived from the accretionary prism. Tomlinson et al. (1992) and Maldonado et al. These features indicate a latest Tortonian age for the (1994). We located the end of the trench at the end of the subduction in this area, which is compat- Base-of-Slope Transition High. Southwestward from ible with the end of subduction following the ridge- the end of the trench, the basal detachment of the trench collision after chron C4n. Sequences MS-4 to accretionary prism splits into two active major reverse MS-1 capped the olistostromes and the relict accre- faults (the THRF and the basal detachment itself), tionary prism. The reverse faults that deformed layer whereas the trench splits into two small troughs. The 2 of the oceanic crust (profiles SHSM-03 and two major reverse faults end toward the SW and are SHSM-06; Figs. 4 and 5) were coetaneous with the replaced by E-W active reverse faults. This means that deposition of sequence MS-7, indicating the exis- the area of accommodation of the shortening is greater tence of compressive deformation during the Torto- than the bifurcation area of the main detachment fault. nian, prior to the end of subduction. The accretionary Southwest of the Base-of-Slope Transition High, all prism has a front with convex inflexion that suggests the compressive structures are inactive, indicating the the collapse of the prism and the seaward advance end of the compressional tectonics. of its front (Fig. 9). Collapse is also implied by The upper plate of the subduction is the South the presence of the olistostromes in the front and Shetland Block (part of the Antarctic Plate), while the their relationship with sequences MS-6 and MS-5 lower one is the oceanic crust of the same Antarctic suggests a Messinian age for the collapse of the Plate. The two elements join in the transition area accretionary prism. There are no major deformations between the active and the passive margins. There is in the slope or the continental rise. After the de- no evidence of a transform fault separating these position of units MS-5, sedimentary processes do- elements, as has been proposed in previous models. minated during the recent stages of the margin The trench is acting as an internal subduction zone development. that accommodates shortening inside the Antarctic The most important structures in the shelf are the Plate and the study region represents the outcrop area NE-SW folds. Single-channel seismic surveys of of the tip-line of this subduction zone. previous works revealed only the antiform located in The Shelf Transition High is an antiform fold the middle shelf (Kimura, 1982; Anderson et al., bounded by normal faults. This fold may be respon- 1990; Larter and Barker, 1991), termed the ‘‘Midshelf sible for the loss of continuity in the Hespe´rides High’’ (Larter and Barker, 1991). New MCS data Forearc Basin, which develops two perisynclinal indicate that these folds, which are now growing, terminations on either side of the high. The main deform the entire shelf. The folds have the same deformation phase produced highly deformed depo- characteristics in both the active and passive margins, sitional sequences MS-7, MS-6 and MS-5, whereas indicating that the whole shelf is undergoing NW-SE the internal reflectors of sequences MS-4 to MS-1 compression. developed progressive unconformities on the conti- In the sector between Smith Island and the Shelf nental side of the high. This suggests that the high Transition High, the NNW-SSE to N-S folds devel- was generated in the Late Messinian–Early Pliocene oped practically coetaneously with the aforemen- and has been growing continuously to the present tioned NE-SW ones. The two-fold systems interfere, time. suggesting that this sector has undergone constric- The concave major inflexion in the lobular front of tional deformation with area reduction. the accretionary prism is associated with the presence of the Base-of-Slope and the Shelf Transition Highs, 7.4. Transition between the passive and the active the discontinuity of the forearc basin and the defor- margins mation of the accretionary prism. Such an association of structures is strongly reminiscent of the structures The tectonic map obtained from the interpretation produced by the subduction of a seamount (Domi- of the seismic profiles shows that the South Shetland nguez et al., 1998, 2000). However, as we have Trench continues 50 km southwest of the Hero 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- 7.6. Tectonic model sition High, which is due to the active reverse faults originating from the split of the basal detachment. We The model of the evolution in the area can be propose that this inflexion may be associated with the summarised in several stages. The first stage, in the tendency to subduction of the Base-of-Slope Transi- Tortonian, corresponds to the ridge-trench collision tion High, which is the deformed area that accom- after anomaly chron C4n, which preceded the end of modates the end of the trench. subduction in the present-day passive margin. A Another process that may have contributed to the compressive deformation event deformed the oceanic major inflexion is the rollback process proposed for crust simultaneously with the deposition of sequence this trench (Larter, 1991; Maldonado et al., 1994). MS-7. This stage finished in the latest Tortonian– The advance of the front of the prism towards the Early Messinian, when subduction ended in the pas- NW—while the outcrop of the tip-line remained in sive margin. The tip-line of the intraplate subduction place—could have originated a rotation of the front, moved to its present-day location and marks the onset producing an arched pattern for the prism. In this of the second tectonic stage during the Messinian. model, extension in the back-arc region that is During this second stage, the accretionary prism accommodated by the Bransfield Strait rift could collapsed in the passive margin sector. The internal not continue southwestward from the end of the deformation in the area of the tip-line, mainly by trench. This is supported by the observation that the reverse faults, generated the Base-of-Slope Transition shelf in the study area is under compression and the High in the oceanic crust. The second stage ended in extensional deformations are reduced to a single the latest Messinian–Early Pliocene with the conver- normal fault SE of Smith Island that disappears gence of this high against the accretionary prism, toward the SW. thereby producing its concave inflexion and internal deformation, as well as the loss of continuity of the 7.5. Oceanic crust forearc basin. Sequences MS-6 and MS-5 were deposited during this stage The third stage, from the Early The lineaments observed in the oceanic crust run Pliocene to the present-day, corresponds to the devel- parallel to the trench NE of the Hero Fracture Zone, opment of compressive deformation in the shelf and in and have a sigmoidal pattern between the C and the the base of the slope near the Base-of-Slope Transition Hero Fracture Zones (Fig. 9). Most of these linea- High, while extensional deformation occurs in the fore ments are normal faults that cut layer 1 of the oceanic bulge of the subduction footwall. During this time, the crust, although there are also several normal faults deposition of sequences MS-4 to MS-1 originated the capped by the most recent sediments. Northeast of the progradation of the passive margin, while these Hero Fracture Zone, these faults are interpreted as sequences are deformed in the active margin. 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 8. Conclusions Fracture Zones, the sigmoidal pattern of the lineaments could indicate that the outer swell is separating The structural model discussed in this paper stands from the trench and is located farther towards the as one of the few well-documented case studies of the ocean, where it disappears. In this region, the outer tectonic evolution of the lateral end of a subduction swell also becomes wider than in the NE sector of the zone, and of the transition from an active to a passive area and the normal faults cut even the flat bottom of margin style. The lateral end of subduction is accom- the trench. The sigmoidal pattern of the structures panied by the decrease of the slip of the basal detach- could also be favoured by an originally sigmoidal ment, which ends in a tip-line. Around the tip-line lies pattern of the fabric of the oceanic rocks suggested by an area with distributed compressional deformation, the oblique orientation of the C3An and C4An mag- both in the oceanic crust of the subducting plate and netic anomalies of the oceanic crust in this area (Larter the accretionary prism of the overriding plate. The and Barker, 1991). evolution of the area includes a first stage when a 80 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81

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