A fundamental problem in continental tectonics centers on how orogenesis initiates at noncollisional convergent margins. Field studies and the results of mechanical modeling show that the strength and rheology of the pre-orogenic overriding plate is a crucial factor that controls structural variability in these settings. In the Andes, the archetypal noncollisional orogen, a leading cause of pre-orogenic weakening involved the formation and destruction of large extensional basins during the Mesozoic. However, in most areas, the effects of the modern Andean orogen has obscured the geologic record of how these the transition from extension to contraction controlled the initial rise of the Cordillera from below sea level. Even fewer areas preserve a record of how the deep crust evolved through these changing tectonic regimes. The objective of this project is to determine how the high temperatures, thin crust, and rheological stratifications that result from intraplate extension affected the thermal and structural evolution of the deep crust during subsequent shortening. This observational approach is possible because the Darwin Complex in southernmost Chile preserves a nearly complete record of the thermal and structural evolution of the middle crust during the formation and destruction of a large extensional basin. Preliminary results indicate that basin closure in the southernmost Andes led to the large-scale underthrusting and/or partial subduction of continental lithosphere in a noncollisional, intraplate setting. The project utilizes structural geology, metamorphic petrology, U-Pb geochronology, and thermochronology to determine the ages of deformation as well as the nature of the tectonic and thermal evolution of Darwin Complex.
This project is an intensive geological exploration of an important, yet poorly understood, region in southernmost South America. Here, crust that had been stretched in one tectonic event subsequently experienced contraction during a mountain building episode. As such, this region affords examination of how mechanical properties established in one tectonic cycle influence the mechanical behavior in later events. Results of this study should shed light on how narrow orogenic belts characterized by thick skinned tectonics are formed and how deeply buried continental rocks are subducted and exhumed. This project also involves a substantial collaborations among U.S., Chilean, and Australian institutions.
