THREE-DIMENSIONAL PURE SHEAR DURING TRANSPRESSION: THE EFFECT OF CRETACEOUS PLUTONISM ON REGIONAL STRAIN FIELDS IN THE SIERRA NEVADA BATHOLITH, CALIFORNIA

Previous fieldwork along the Eastern margin of the Cretaceous Tuolumne Intrusive Suite (TIS), Sierra Nevada Batholith, California and theoretical strain modeling propose that lineations in wrench-dominated zones of transpression are oriented either horizontal or vertical, depending on the angle of convergence and the ratio of finite strain axes. The northern Sierra Crest shear zone system (from South to North: the Rosy Finch, Gem Lake, and Cascade shear zones) appears to be a particularly interesting setting, because the existence of both, shallowly- and steeply plunging lineations can be documented along strike in a structurally continuous shear zone. However, we suggest that strain fields in arcs are more complicated and theoretical models of transpression should include additional strain variation.

Our recent field work in the Saddlebag Lake pendant (Cascade shear zone) indicates that stretching lineations in the Triassic to Jurassic host rocks are sub-vertical but rotate to shallower orientations approaching the pluton contact. Three-dimensional strain measurements show that strain in the host rocks is constrictional but the contact zone is characterized by pure flattening. Kinematic indicators at the contact are highly variable and neither consistent with dextral nor with sinistral shear. Pure flattening strain at the contact is further documented by frequent occurrence of aplites showing chocolate-tablet boudinage, indicating stretching in two mutually perpendicular directions (horizontal and vertical).

Our results indicate that dominantly dextral shear in the Cascade Lake shear zone due to regional simple shear deformation is locally overprinted by pure flattening strain associated with a pure shear component during emplacement-related deformation of the TIS. This interpretation has important implications regarding both, emplacement models of the TIS and models of regional transpression. The TIS generally shows features characteristic for diapiric nesting of successive intrusive pulses. However, the last strain increments recorded in the host rocks indicate that lateral pure shear deformation occurred at the emplacement level. Hence, we argue that shear zones in arcs can be more complicated than previously recognized and display both, emplacement-related and regional strain.