STRUCTURAL INVESTIGATIONS OF FAULTED BISHOP TUFF: ANALOG FOR NON-WELDED PAINTBRUSH TUFF, YUCCA MOUNTAIN

A key technical issue regarding fluid flow at Yucca Mountain is the patterns and rates of fluid flow in unsaturated fractured and faulted tuffs. We examine the structure and mineralization of fractured and faulted, non-welded to densely welded Bishop Tuff. This 764 Ka ignimbrite sequence is exposed in the Volcanic Tableland, California, and is a structural and lithologic analog to the non-welded portion of the Paintbrush Tuff at Yucca Mountain. We examine faults with cm to 17 m slip. The faults with ~8 m slip exhibit zones of rough, unmineralized fractures 10 m wide in the densely welded phase; moderately welded sequences have fracture zones ~5 m wide. In the non-welded, pumice-rich, basal surge deposits, the faults are typically very smooth surfaces mm thick, coated with silica and calcite with the overall fault zone 5-10 cm thick. Fracture networks in the densely welded phase are composites of orthorhombic cooling joints with superposed north-striking tectonic joints. Joint sets appear in non-welded phases near faults and in places where bending or rocks occurred near the larger faults. Most fractures in the non-welded sequence are N to NNE striking, and range in length from 10’s cm to 5-10 meters and are either isolated or tip out to sub parallel joints. All joints in the non- to partly-welded phases are coated by silica and calcite. Microstructural analyses reveal that open fractures exist in even the highest (>50%) porosity sequence, commonly as transgranular fractures. Comminution to micron-sized grains along with mineralization creates the smooth walled fault surfaces. These results support a "funnel" shape model for the faults, in which the open fractures in the densely welded sequence narrow to the slip surface. Such a structure might serve to collect fluids higher in the section and focus flow from the surface to the unsaturated portion of the sequence.