STRUCTURAL EVOLUTION AND HIGH-PRESSURE LOW-TEMPERATURE METAMORPHISM OF THE SAN JUAN THRUST SYSTEM, ORCAS ISLAND, WA

The San Juan Thrust System (SJTS) represents the western elements of the Northwest Cascade thrust system and preserves evidence for Late Cretaceous Cordilleran margin tectonics. While the structural and metamorphic history of this stack of thrust sheets containing Paleozoic to Cretaceous oceanic and island arc rocks is well studied, the kinematics of deformation phases and their relationship to accretionary wedge HP-LT metamorphism remain unsolved. Detailed field mapping on Orcas Island, WA, kinematic analysis of structures, and P-T constraints from vein mineralogy reveal the P-T-D evolution of the Orcas and Rosario thrusts, two of the major thrusts in the SJTS. A widespread gently to moderately dipping S1 flattening fabric (D1) is subparallel to and cut by a localized S2 fabric found in meters-thick brittle-ductile shear zones that bound the nappes (D2). Post-cleavage brittle structures (D3) are widespread, and include cm- to m-scale slip on normal, strike-slip, and thrust faults that have mutual cross-cutting relationships. Maximum temperatures of ~250˚C and pressures of ~6 kbar are indicated by the presence of aragonite within D1-D3 veins, and by the presence of a minor crystal-plastic component of deformation in quartz in D2 shear zones.

These results are interpreted as deformation in an accretionary wedge setting with; (1) D1 terrane accretion and fabric formation somewhere to the south along the continental margin, (2) NW-directed translation along the margin where terranes were assembled into their current nappe stack along D2 shear zones, and (3) subhorizontal extension in all directions and vertical thinning along brittle D3 structures. This model supports previous interpretations of a NW-dominated thrust system, but indicates that assembly of the nappe stack and later D3 brittle deformation also took place in an accretionary wedge setting. Furthermore, previous models need to be modified to account for D3 extension after terrane assembly.