EXPLORING THE GEODYNAMICS OF CONTINENTAL CRUSTAL THINNING AND EXTENSION USING NUMERICAL MODELS AND GEOLOGICAL OBSERVATIONS

Numerical experiments relate the distribution and style of deformation in rifted margins to thermomechanical localization processes. Four end-member margins categorized by their width and symmetry result by varying initial lithospheric strength, crustal thicknesses, and extension rates. In the experiments, the stretching mode can be very short or protracted, and the thinning or the exhumation modes can be incomplete or absent. When thinning profiles are extracted we find that the upper to lower crust extension discrepancy is partly solved by polyphase sequential faulting and detachment faulting. Moreover, depth-dependent stretching appears to dominate at specific stages of rift evolution because crustal and mantle thinning distributions are not always spatially coincident, and at times are not even balanced by an equal magnitude of thinning in two-dimensions. The rheological mechanisms for these patterns are expressed in the rock record, as illustrated with field examples from the Death Valley region and the Mauleon basin in the Western Pyrenees. In both areas, migmatitic fabrics characterize the mid- to lower crustal rocks, and extension-related, retrograde fabrics overprint the peak-metamorphic ones. Yet, these areas show contrasting retrograde assemblages attributed to different melt-segregation histories resulting in changes in the bulk composition and thus crustal strength. The Death Valley region typifies a weak-crust extensional province with widespread extensional ductile shear zones and little mantle response. In contrast, the Mauleon basin typifies stronger crust with few extensional brittle shear zones and mantle exhumation. As predicted by the models, in both cases the relative strength of the mantle to crust originates in the orogenic histories of the regions.