KINEMATICS AND P-T CONDITIONS OF BRITTLE DEFORMATION IN AN ANCIENT ACCRETIONARY PRISM SETTING: SAN JUAN ISLANDS, NW WASHINGTON

Fault-bounded slices of Paleozoic to Mesozoic bedrock in the San Juan Islands record a controversial structural history. Workers disagree on orogen-normal vs. translational kinematics during emplacement, the relative timing of faulting and metamorphism, and PT conditions during faulting. High Pressure/Low Temperature fabric formed at ~ 18 km and < 150 – 200° C (Brandon et al., 1988). Recent Ar/Ar geochronology suggests HP/LT fabric formed in the Late Jurassic or Early Cretaceous (Brown and Lapen, 2003; Lamb and Schermer, 2003), preceding major faulting at 100-84 Ma (Brandon et al., 1988). Meter-scale faults, extension veins and shear veins that crosscut shear fabric and pressure solution fabric exist throughout the Lopez Structural Complex (LSC), a 2.5 km thick fault zone composed of slices from adjacent San Juan terranes and exotic lenses. The oldest faults are mainly W-SW vergent thrusts subparallel to but cross-cutting foliation. Next, subvertical extension veins followed by normal faults suggest ~vertical shortening and NW/SE extension, with a smaller population suggesting NE/SW extension. Younger structures, including en echelon vein sets and strike-slip shear veins and faults, are variable in trend and shear sense, but most are consistent with NW/SE extension and may be coeval with some normal faults. Many strike-slip faults reactivate older fault surfaces and foliation. The presence of comparable structures in terranes of the eastern San Juan Islands (Lamb and Schermer, 2003) as well as adjacent to the LSC indicates extensional and strike-slip deformation occurred after terrane amalgamation.

Quartz and carbonate veins are abundant in rocks of the LSC. Aragonite and prehnite-bearing veins are associated with brittle thrust, normal, and strike-slip structures. Most of the latest strike-slip faults contain only calcite. Aragonite in post-fabric structures suggests uplift of the LSC at >15 km depth occurred by brittle extension in an accretionary wedge environment. Strike-slip faults without veins may represent younger deformation at shallower levels.