EVALUATING BULK CRUSTAL RHEOLOGY AND FORCE BALANCE IN A CRUSTAL CONTRACTION-EXTENSION CYCLE

Understanding crustal behavior during progressive mountain building and subsequent orogenic collapse can provide insights into bulk crustal rheology and force balance. The North American Cordillera, western US, provides a relatively well-constrained framework for this topic. From Middle-Late Jurassic to early-middle Cenozoic, the orogen at the approximate latitude of central Nevada experienced a prolonged phase (~100-150 Myr) of E-W crustal contraction due to the subduction along the western margin of the North America. Contractional strain affected an initially thin crust (~30 km thick) and resulted in 40-50% crustal shortening leading to an orogenic plateau of ~50-60 km crustal thickness by the Late Cretaceous-early Cenozoic. Contraction was followed by the Basin and Range extension, which started on a regional scale across Nevada at ~18 Ma , causing 40-50% E-W crustal extension and stretching of the crust back to ~35 km thickness. The two distinct contractional and extensional events have apparently similar strain magnitudes but operated over different timescales: contraction took ~100 Myr and extension ~10 Myr. Thus, for the same amount of shortening or stretching strain, the stretching occurred approximately 10 times faster than the shortening. Assuming crust deforms viscously according to a simple non-Newtonian quartzite flow law, we computed the tradeoff relationship between the changes of the bulk crustal temperature and the effective stress imposed on the orogen boundaries in order to match the observed strain rate ratio between regional contraction and extension. We suggest that a thermally weakened crust prior to Basin and Range extension can explain this strain rate difference assuming similar effective stress during contraction and extension. We hypothesize that the magmatism during the Late Cretaceous-middle Cenozoic Laramide event plays a crucial role in thermally weakening the crust, and the gravitational potential energy built-up during the contractional event could drive the extension in a warmed and weakened crust. We are evaluating more comprehensive models to test this hypothesis.