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,*, Jua´n-Carlos Balanya´ b,c, Antonio Barnolasd, Jesu´s Galindo-Zaldı´vara, F. Javier Herna´ndez-Molinae, Andre´s Maldonadoc, Jose´-Miguel Martı´nez-Martı´neza,c, Jose´ Rodrı´guez-Ferna´ndezc, Carlos Sanz de Galdeanoc, Luis Somozad, Emma Surin˜achf, Jua´n 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- 0040-1951/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0040-1951(03)00060-X 56 A. Jabaloy et al. / Tectonophysics 366 (2003) 55–81 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.