SPACE GEODETIC STUDIES OF OROGENESIS AND FLAT SUBDUCTION: THE PRECORDILLERA AND SIERRAS PAMPEANAS OF WESTERN ARGENTINA

Great mountain belts are built, in large part, by contraction occurring at their margins. Usually, crust in a 'thin-skinned' wedge is thickened and accreted to the core of the mountains in two principal ways: (1) by advection of continental crust from the craton into the orogenic root, and (2) by telescoping of overriding rock layers. Another type of contractional orogenesis, such as in the Laramide ranges of Western North America, occurs when the entire crust appears to fail. This less prosaic 'thick-skinned' deformation is usually correlated with flat subduction and the Argentine Sierras Pampeanas are often cited as a modern analogue to the Laramides. One expects to need a very powerful vice to compress a craton so strongly that the entire crust fails and competing mechanisms comprising end-loading or lithospheric basal traction have been proposed. In the Andes, as with the Laramides, the apparent contemporaneity of both thin- and thick-skinned shortening at least over geological timescales, creates a further mechanical conundrum: would not the shortening in the thin-skinned region alleviate the significant compression needed to form the thick-skinned ranges?

For the past decade we have used GPS geodesy to study active orogenesis associated with flat subduction in the Argentine Precordillera and Sierras Pampeanas. We find that the horizontal velocity field can be reproduced to within ~2mm with a model where the Andes act as an coherent block overriding the South American craton at geological timescales. The results imply that the main plate boundary is fully locked, and the decollement underlying the Precordillera is continuously creeping along most of its length, although it is probably locked at shallow depths. Strain rates at the Precordillera thrust front (3-4x10-7/yr) are nearly an order of magnitude greater than across the Sierras Pampeanas. Moreover, the entire Sierras Pampeanas field can be explained as an ephemeral elastic effect from a fully-locked Nazca plate boundary. Both regions, however, have experienced earthquakes greater than M 5.5 in the past 150 years. We examine these results in the context of the proposed models and suggest future research directions aimed at resolving the mechanical conundrum associated with contemporaneaous Precordillera and Sierras Pampeanas deformation.