COMPLEX DEFORMATION PATTERNS CAUSED BY PROTOLITH HETEROGENEITY: GEM LAKE SHEAR ZONE, EAST-CENTRAL SIERRA NEVADA

The Cretaceous Gem Lake shear zone of the east-central Sierra Nevada affected a complexly layered, previously deformed stratigraphy of metavolcanic and minor metasedimentary rock in the region of Gem Lake. We used quantitative and qualitative measures of deformation to study how this mechanically heterogeneous protolith was affected by the shear zone. The different rock types affected by the shear zone are deformed heterogeneously and recognizing an idealized strain gradient is consequently difficult. From the margin to the center of the shear zone, a general increase in the amount of bulk finite strain is apparent but superimposed on this trend is a more complex pattern of localized high strain regions. These smaller zones of relatively high strain generally occur within distinct packages of rock whose composition is different from that of the surrounding rock. Outside the previously mapped extent of the shear zone we observe several zones of strain localization with a lineation whose orientation is more like that seen in the main shear zone than the lineation outside the zone. We hypothesize that these strain localization zones were particularly weak regions within a generally stronger package of rock and that these weak regions were affected by shear zone deformation while the stronger rock was not. We believe the complex deformation patterns in the Gem Lake area reflect the superposition of a finite strain gradient on a mechanically heterogeneous package of rock. Deformation localizes in weak rock, although this effect is more pronounced in the main body of the shear zone than outside it. Defining a discrete shear zone boundary is therefore difficult and may be a misleading practice in some cases.