A SEARCH FOR THE ILLUSIVE QUATERNARY TECTONIC HISTORY OF WESTERN PATAGONIA USING MULTIPLE APERTURE /FREQUENCY ACOUSTIC PROFILING

For 50 years geologist have noted ‘remarkable’ geomorphologic features in the Southern Andes which appear on geologic maps as Quaternary faults. Despite the landscape-defining nature of these ‘faults’, little evidence exists to justify interpretations. The pervasive rain forest, recent glaciation and extensive Holocene volcanism combine to camouflage this record on land. In the last 4-5 years we have employed a portable digital multiple aperture and frequency acoustic profiling system to image the upper few hundred meters of deposits lying within lake and near shore environments. Some 300 kilometers of profiles have been obtained within diverse bathymetric and depositional environments. With a central frequency of 400 Hz, the system commonly reveals moderate scale (10-40 meter) acoustic stratigraphic sequences with internal heterolithic bedding definable down to a 2-3 meter scale. Evidence for Quaternary /Holocene faulting has been discovered in most of the environments thus far surveyed along the arc-parallel Liqune-Ofqui Fault, as well as in other areas away from the main fault zone. Movements of these faults in association with large earthquake events is of some interest in areas such as Valdivia, a city devasted by tectonic ground disturbance during the 1960 Chilean Earthquake.