FRACTURES IN THE CRETACEOUS PLUTONS OF LITTLE LAKES VALLEY, EASTERN SIERRA NEVADA: COOLING STRUCTURES OR THE RESULT OF REGIONAL TECTONIC STRESS?

The glacial trough of Little Lakes Valley in the eastern Sierra Nevada near Bishop, California contains prominent north-striking fractures which parallel local drainage trends. These fractures served as conduits for hydrothermal fluids and are characterized by epidote filling and a bleached zone in the host granodiorite that is typically a few centimeters thick. The fractures, occurring primarily in three Cretaceous plutons, were examined to see if they are dextral faults related to Late Cretaceous intra-batholithic transpression, (e.g., Rosy Finch shear zone), or to post-Cretaceous dextral faulting along the eastern margin of the Sierra Nevada.

Field examination reveals no evidence of significant strike-slip deformation, with the exception of a previously mapped sinistral fault with no more than 50 m of displacement. Instead, the fractures are dominated by dilation. Occurrences of centimeter-scale displacement are rare and statistically negligible. Six transects at different locations and in different plutons revealed only joints, none of which have measurable offset, despite an abundance of markers in the form of aplite dikes, mafic enclaves, and primary igneous layering. About 4% of the joints display short cracks crossing the main fracture in an en echelon configuration, which indicates that the joints were beginning to respond to shear but failed to develop measurable slip.

The joints are probably the result of regional tectonic stresses. They commonly cross plutonic contacts without deflection, which seems to preclude an origin by thermal stresses during cooling. Because the Cretaceous Rock Creek Gabbro and Mono Creek Granite both display field evidence of magma mingling with the Round Valley Peak Granodiorite in some areas, a thermal origin for the joints cannot be completely discounted until better ages are obtained for these plutons. Single grain U-Pb analyses of zircons by TIMS are currently in progress.