CONSTRAINTS ON LATE CRETACEOUS KINEMATICS IN THE SIERRA NEVADA, CALIFORNIA FROM STRAIN ANALYSIS AND TRANSPRESSIONAL MODELING

The east-central Sierra Nevada of California contains abundant metamorphosed wallrock pendants relative to other parts of the Mesozoic volcanic arc region. These metamorphic rocks provide a more complete record of regional deformation history than the surrounding igneous rocks. Focusing on the relatively recent Late Cretaceous deformation history of the wallrocks, we use finite strain observations and forward modeling to study heterogeneous deformation and place constraints on variations in regional strain fields. Dating of metamorphic minerals indicates that the entire east-central Sierra Nevada region was deforming in the Late Cretaceous but that deformation was particularly intense in the Gem Lake shear zone, an ~1 km wide, NNW-striking region of high strain with steeply dipping mylonitic cleavage and a moderately to steeply pitching mineral stretching lineation. Strain outside the shear zone is different in orientation and lower in magnitude than that observed inside the zone.

Using finite strain observed outside the shear zone as an initial condition and assuming transpressional kinematics, we mathematically add more finite strain to simulate localization of deformation inside the zone. By varying the angle of oblique convergence, shear zone dip, and total amount of shortening in the shear zone, we calculate strain increments to add to observed strain outside the shear zone. Permissible models reproduce the strain observed inside the shear zone. Comparing permissible results to numerical models of finite strain variation in heterogeneous deformation, we estimate that the angle of oblique convergence inside the zone was ~15±10º (more wrenching than contraction) and that regional shortening was modest (20 to 60%). This estimated angle of oblique convergence is considerably different from the regional angle of~70±10º predicted by tectonic reconstructions. Our results imply therefore that regional deformation was moderately strike-slip partitioned during the Late Cretaceous.