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

The southern Chilean convergent margin south of the Strait of Magellan, bet ween 52 and 57 degrees S, is undergoing a style of subduction where obliqui ty 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-s tack 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 fro m sonobuoys has been used to further constrain the depth-converted section. 2D gravity modelling has supported the deduced crustal structure. Four mai n domains are imaged, which, from SW to NE, are: (1) the oceanic domain, wh ere the Chilean trench is buried with a elastic sedimentary wedge reaching maximum thickness of 4km, lying on a 7-8-km-thick crystalline oceanic crust that gently dips landward (3-4 degrees); (2) the accretionary prism domain , about 40 km wide, of highly deformed sediments with a clear Bottom Simula ting Reflector (BSR); (3) the forearc basin domain, a 25-km-wide basin (nea rly undeformed) with a maximum sediment thickness of 4.5 km, which is borde red along its seaward edge by a zone of outer-are structural highs and the accretionary prism forming the so-called 'Fuegian terrace'; and (4) the con tinental domain which consists of an 8 degrees dip continental slope and th e continental shelf. Seismic reflection data along the continental shelf re veal 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 corre sponding sonobuoy are not conclusive enough to confirm such a hypothesis, b ut the brightness of these reflectors suggests the presence of a 10-km-thic k reflective lower crust between 14 and 24 km depth. At greater depths (bet ween 11 and 14 s twt), there is a thin package of bright landward-dipping r eflectors, which may indicate the top of the subducting slab as deduced fro m gravity data. Furthermore, gravity modelling shows that the slab subducts at a very low angle of about 7-8 degrees. In spite of the lack of seismici ty, our results favour the conclusion that subduction still occurs in this region of the Chilean margin or has recently ceased. (C) 2000 Elsevier Scie nce B.V. All rights reserved.