A POSSIBLE LINKAGE BETWEEN SHEAR ZONES IN THE 97 MA SOLIDIER LAKE PLUTON AND LATE CRETACEOUS TRANSPRESSIONAL SUBDUCTION IN THE CONTINENTAL ARC

The 97 Ma granodioritic Soldier Lake pluton (SLP) emplaced into a ~175 Ma Mesozoic meta-volcanic and meta-sedimentary package of the Virginia Canyon area in the central Sierra Nevada. Mineralogy and microstructures in host rocks suggest the existence of amphibolite grade metamorphism and ductile deformation. Steep lineations developed in both pluton and host rocks and xenoliths found in the pluton show that ductile downward flow and stoping took place during emplacement.

Field mapping reveals that magmatic fabrics defined by the alignment of euhedral plagioclase and biotite in the pluton, and metamorphic fabrics defined mostly by micas in the host rocks strike consistently NW cutting across contacts. Interpreted as a syn-tectonic pluton, these coupled fabrics indicate a NE-SW contractional tectonic setting in this arc during SLP emplacement.

Several meter-scale sinistral shear zones were found in the SLP striking E-W cutting the magmatic and subsolidus fabrics in the SLP and offsetting magmatic veins from the adjacent 86 Ma Cathedral Peak pluton (CPP), indicating that these formed after the CPP was emplaced. A microstructural study shows that the quartz and biotite foliation planes anastomose around the K-feldspar relics indicating ~450 C for the shearing temperature. The shearing at these medium to low temperatures in the pluton occurred after the crystallization of the SLP, because of its later age and relative low temperature.

One interpretation for the shearing in the SLP is that the shear zones could be coupled to the dextral strike-slip faults in the host rocks. Their orientations are likely coupled to the dextral transpressional subduction of the Late Cretaceous continental arc. An alternative explanation is that the “L” shape boundary of the CPP imposed inhomogeneous strain on the SLP when the CPP was emplaced at 86 Ma and reheated the SLP to the temperature allowing subsolidus shearing to occur. Further study on the ages of faulting in host rocks and shearing in the pluton should be helpful to confirm the different relationships.