Paper No. 0
Presentation Time: 9:30 AM
LATE CRETACEOUS TERMINATION OF DEFORMATION IN THE EAST SIERRAN THRUST SYSTEM, EAST-CENTRAL CALIFORNIA
The East Sierran thrust system (ESTS) of Mesozoic age records several episodes of arc-normal contractional deformation that continued episodically until sometime after ~140 Ma. Owing to a paucity of Late Cretaceous/early Tertiary rocks, dating the termination of widespread ductile contractional deformation within the ESTS has been problematic, but clues from the Late Cretaceous history of the adjacent Sierra Nevada may be relevant. There, cleavage reflecting ~arc-normal contraction in roof pendants as well as the ~arc-parallel right-lateral Sierran Crest ductile shear system developed in response to post-90 Ma right-oblique plate convergence stresses. Widespread transpressional deformation of both kinds apparently waned after ~80 Ma, giving way to development of a predominantly brittle conjugate strike-slip fracture system reflecting a small amount of ~N50°E-directed contraction active during the interval ~83 Ma-79 Ma.
Evidence from the Inyo Mountains is consistent with a similar Late Cretaceous tectonic scenario there as well. Moderately widespread conjugate strike-slip faults with orientations and slip senses similar to those in the adjacent Sierra accommodate a small amount of ~N45°E-oriented contraction. These faults cut all ductile contractional structures that reflect arc-normal shortening oriented ~N75°E. Indirect evidence suggests that regional stress patterns favoring development of these older structures persisted until at least 83-85 Ma before the geologic environment changed to one favoring brittle conjugate faulting accommodating ~N45°E-directed contraction. This changed environment may reflect changes in rock ductility and/or in regional stress patterns, both perhaps triggered by initiation of low-angle subduction that is inferred to have begun ~80 Ma.