CONTINENTAL RIFT BASIN ASYMMETRY AND THE INFLUENCE OF MECHANICAL PROPERTIES OF UPPER CRUSTAL ROCKS

Continental rift basins around the world display a relationship between major bounding faults and spatial variations in upper-crustal rock strength. Cross-sectional rift basin geometries are compared to finite element geodynamic model results to determine the role of bulk mechanical properties on rift basin shape. Some major rift basin faults form at the boundary between pre-rift, thick volcanic rock units and laterally adjacent Precambrian intrusive and metamorphic rock. Examples include the San Luis Basin of the Rio Grande Rift (RGR) and the Shire Rift of the East African Rift system where the major basin-bounding fault dips toward softer, volcanic units. Published rock mechanics data suggests large differences in elastic stiffness and yield strength for extrusive and intrusive igneous rocks; yet, mechanical property values for rocks have wide ranges, and the uncertainty in rock strength values increases when extrapolating deformation to kilometer spatial scales and million-year time scales. The geodynamic numerical model, ASPECT, is used with visco-plastic rheology and a Drucker-Prager yield criterion to calculate rift basin shapes. Model results show that basin-bounding normal faults with greater offset tend to localize in zones of stronger upper-crustal rock, whereas, given the same tectonic extension, shallow-dipping basin-bounding faults with less offset localize in zones of softer rock. Basin shape and shear strain magnitudes in these models is strongly controlled by internal angle of friction and rock unit thickness. The models successfully reproduce the shape of the San Luis Basin when the model included pre-rift rock suites representative of field-mapped rocks. Predictions of rheological and material yielding behavior for some settings are less intuitive without numerical model results. For example, the major basin-bounding fault of the Upper Arkansas River Valley of the RGR divides Precambrian metamorphic rocks and much younger, silicic granites. Geodynamic model results from this study suggest that the granite must be stronger than adjacent metamorphic rocks. Our results demonstrate the significant influence of pre-rift rock mechanical properties on rift basin geometries, thus providing useful insights that could aid the understanding of basin evolution of rifts and rifted margins.