Crustal structure of the southernmost Chilean margin from seismic and gravity data

Abstract

The southern Chilean convergent margin south of the Strait of Magellan, between 52 and 57°S, is undergoing a style of subduction where obliquity plays a significant role. Seismic reflection and gravity data collected during 1988 by LDEO (RC2902) across the study area image crustal structures from oceanic crust (seaward of the trench) to the continental shelf. Pre-stack and post-stack depth migration have been applied to MCS profile RC2902-790, from which we have obtained a depth section, with true geometries and an associated accurate velocity model. Additional velocity information from sonobuoys has been used to further constrain the depth-converted section. 2D gravity modelling has supported the deduced crustal structure. Four main domains are imaged, which, from SW to NE, are: (1) the oceanic domain, where the Chilean trench is buried with a clastic sedimentary wedge reaching maximum thickness of 4 km, lying on a 7–8-km-thick crystalline oceanic crust that gently dips landward (3–4°); (2) the accretionary prism domain, about 40 km wide, of highly deformed sediments with a clear Bottom Simulating Reflector (BSR); (3) the forearc basin domain, a 25-km-wide basin (nearly undeformed) with a maximum sediment thickness of 4.5 km, which is bordered along its seaward edge by a zone of outer-arc structural highs and the accretionary prism forming the so-called ‘Fuegian terrace’; and (4) the continental domain which consists of an 8° dip continental slope and the continental shelf. Seismic reflection data along the continental shelf reveal that between 5 and 8 s twt, there is package of reflections that could mark the presence of a high reflective lower crust. Results from the corresponding sonobuoy are not conclusive enough to confirm such a hypothesis, but the brightness of these reflectors suggests the presence of a 10-km-thick reflective lower crust between 14 and 24 km depth. At greater depths (between 11 and 14 s twt), there is a thin package of bright landward-dipping reflectors, which may indicate the top of the subducting slab as deduced from gravity data. Furthermore, gravity modelling shows that the slab subducts at a very low angle of about 7–8°. In spite of the lack of seismicity, our results favour the conclusion that subduction still occurs in this region of the Chilean margin or has recently ceased.