LATE CENOZOIC MOVEMENT ASSOCIATED WITH THE ARC-PARALLEL LIQUINE-OFQUI FAULT ZONE AND THE CHILE TRIPLE JUNCTION DOCUMENTED BY ACOUSTIC PROFILING OF SHALLOW MARINE AND LACUSTRINE DEPOSITS OF SOUTHERN CHILE

Erosion, thick vegetation and recent volcanism obscure the tectonic history of the Liquine-Ofqui Fault Zone (LOFZ) leading to multiple interpretations of its late Mesozoic and Cenozoic movement. Since 1998 we have conducted a multiple aperture/frequency acoustic profiling program to image the upper few hundred meters of strata within shallow marine and lacustrine environments at several sites along the LOFZ. Investigations center on shallow marine deposits at the south end (Bahia San Quintin), on estuarine and lacustrine deposits in the central region (Estuario Reloncavi and Lago Todo Los Santos), and on estuarine deposits at the north end (Estuario Valdivia) of the LOFZ. 300 km of single channel seismic lines have been obtained that reveal 10-40 m thick sedimentary sequences containing packages of strata 2-4 m thick. All four sites show syndepositional extensional and compressional movements that cut up to and displace the sediment-water interface. Offshore faulting can be correlated locally to onshore fault scarps, e.g. the Rio San Tadeo and Rio Nevado fault escarpments on the Isthmus of Ofqui where acoustic data combined with regional geology suggest at least 15-20 m of differential vertical movement cutting across Quaternary strata, and ~2 m of subsidence in the last few hundred years evidenced by drowned stands of cedar trees. Geologic and geophysical evidence now strongly support the LOFZ being an active, lithospheric scale, right-slip dislocation separating the forearc Chiloe block from the main South American plate, superimposed by upper crustal transtensional and transpressional accommodation features that are well developed along its leading (northern) and trailing (southern) ends. Correlations of the LOFZ with Quaternary arc volcanism, hydrothermal activity and interplate seismicity (i.e. rupture zone for the 1960 Chilean Earthquake) point to the Chiloe block and LOFZ having a dynamical role in the first order plate interactions near the Chile Triple Junction.