The Accretionary Complex of Southernmost Chile from the Analysis of Multichannel Seismic Data

The structural setting of the southernmost Chilean margin is characterised by convergence processes between the Scotia and Antarctic plates. Subduction is roughly orthogonal to the North (50 °S-53°S) while it becomes progressively more oblique moving to the South. The continental margin is a terraced forearc with a prominent subduction complex formed by a Cenozoic accretionary wedge, a wide forearc basin, and a Paleozoic to Mesozoic continental backstop. The structural and sedimentary record of subduction varies along the accretionary complex both in cross section, and along strike. Main structural vergence is directed towards the ocean but, in some areas, thrust sheets have opposite direction of movement. The width of the accretionary wedge varies from 15 to 40 km: where the wedge is wider the deformation rate is low and the forearc basin shows its maximum extension. Structural variations are directly related to the depth of the décollement level: where the wedge is wider the décollement is deeper and the thickness of the accreted section is maximum. Variations in structural style within the accretionary wedge, depth of the décollement, and geometry of the subduction complex, suggest the existence of areas with different tectonic evolution. The nature of their boundaries is not clear because of the lack of a high-resolution bathymetric data. We suggest that they may be ascribed either to neo-tectonic lineaments caused by rotating crustal blocks or to inheritance of structures that are present on the incoming oceanic crust or within the backstop as a response to convergence and transpression of a single more stable block. We also investigated the area close to the western end of the Strait of Magellan in order to study the structural setting of the plate boundary between Scotia and South America plates and its interaction with the Chile trench. The area close to the triple junction between Antarctic, Scotia and South America is thus imaged for the first time. Between the accretionary wedge and the continental shelf, a wide area of deformation is imaged, and this is the site of a number of faults that displace the seafloor and the uppermost sedimentary cover of about 1000 m. interaction between these two structures; Herron et al. (1977) assumed that the Shackleton Fracture Zone splays at its northern end into a series of northwest stepping faults that follow the continental margin. A gradual change from normal subduction to transpressive and mainly transcurrent motions is likely too. South of 52°S, the continental margin is a terraced forearc, according to the terminology of Seely and Dickinson (1977). Along the trench, a 20 Ma old oceanic crust, belonging to the Antarctic plate, subducts below the Scotia plate at a rate of about 10-20 mm/yr (DeMets et al., 1990) while the slip vector between the plates is oriented roughly E/NE. The oceanic crust gets older moving away from the trench as the mid oceanic ridge, which has emplaced this crust (Chile Ridge), was subducted about 10-14 Ma (Cande and Leslie, 1986). Since then the Chile triple junction moved northward at the modern site of ridge consumption (Taitao Peninsula at about 46 °S, Fig. 1). Subduction of a mid-oceanic ridge is a catastrophic event, which should have completely modified the geodynamic and structural scenario of this region. Before ridge consumption, subduction involved South America and Nazca plates at a rate of about 90 mm/yr (Cande and Leslie, 1986, DeMets et al., 1990). After the ridge consumption INTRODUCTION AND PREVIOUS WORK The geodynamic setting of the southernmost Chilean margin close to the Scotia sea is driven by relative movements between three main plates: Antarctic, Scotia and South America (Fig. 1). The boundary between Antarctic and Scotia plates is located along the Chile Trench and the Shackleton fracture zone, while the Scotia/ South America plate boundary corresponds to the Magellan Fault. The triple junction between these plates is interpreted to be a diffuse area of deformation located west of the Strait of Magellan (Forsyth, 1975, Cunningham, 1993), on the projection of the Magellan fault to the Chile trench. North of this area, normal subduction involves Antarctic and South America plates, while south of the inferred tripl junction, convergence between Antarctic and Scotia plates becomes progressively more oblique. The obliquity of subduction is mainly related to the sharp bend in the strike of the continental margin at these latitudes (from 52 °S to 57°S) which has been interpreted, on the basis of paleomagnetic and structural data, as a product of a counterclockwise tectonic rotation (Cunningham, 1993). At about 57°S, the Chile Trench passes into the Shackleton Fracture Zone. Little is known about the Corresponding author (alina.polonia@igm.bo.cnr.it) © Terra Antartica Publication 88 A. Polonia et al. subduction resumed in the present regime of slow plate convergence. Moreover, this event should have disrupted and eroded a huge amount of material of the overriding plate, as reported by many authors in the actual triple junction area (Forsyth and Nelson, 1985, Cande and Leslie, 1986, Behrmann et al., 1994 among others). The present day accretionary wedge is interpreted to be post late Miocene in age (Polonia et al., 1999) built up when normal subduction resumed after ridge consumption. The Paleozoic and early Mesozoic record of plate convergence is confined within the backstop. On the basis of indirect geophysical observations (Polonia et al., 1997), the backstop has been interpreted to be constituted partly by the Jurassic Patagonian Batholith, and partly by the PaleoMesozoic accretionary wedge. They actually constitutes part of the continental basement of southern South America (Dalziel, 1982, Forsyth, 1982, Davidson et al., 1987, Grunow et al., 1992). Oblique convergence between plates, transcurrent motion, and tectonic rotation, make the geodynamic setting more complex than that of the central Andes. Contraction and uplift in the trench occurs at the same time that rifting is active on land. Diraison et al. (1997) pointed out that the Magellan strait forms part of a Neogene rift system perpendicular to the orogen. All major depressions onland are interpreted as rifts or half-graben developed during Neogene and still active today. Seismicity along the margin is low. The slow Fig. 1 Regional structural map, showing the main tectonic features of the studied area. The Antarctic, Scotia and South American plates meet together at the triple junction indicated by the black dot at about 51°S, located at the seaward projection of the Magellan Fault. The Chile triple junction, at the Taitao Peninsula, where the Chile ridge is being subducted below the Chile trench, is also indicated. Modified from Barker (1982). The slip vector is indicated by the black arrows. Fig. 2 Satellite derived gravity map (Sandwell and Smith, 1997) with the location of the OGS-EXPLORA seismic profiles. Thick black lines represent the multichannel seismic profiles described in