TABULAR FRACTURE CLUSTERS, SIERRA NEVADA BATHOLITH, CALIFORNIA

We present evidence for a new type of fracture system - tabular fracture clusters (TFCs) – that occur in the Cathedral Peak granodiorite, Sierra Nevada batholith, California. TFCs range in length from ~3 m to ~100 m, and in width from ~0.15 m to ~0.40 m, and are expressed topographically as recessed, highly eroded gullies. TFCs can be distinguished in the field as having ≥4 fractures per 0.1 m, and are commonly associated with the presence of an orange alteration rind. TFCs are opening-mode structures, as no shear offset is observed at any scale. Microscopic observations indicate that individual fractures within TFCs consist of micro-breccia, with angular clasts of host rock situated in a fine-grained matrix. XRD/SEM analyses indicate that the fine grained material filling TFC fractures is either: 1) A combination of zeolite and quartz; or 2) Aphanitic composition resembling high-silica melt. Further, microstructural and XRF analysis indicates that white mica is present in the TFCs, but is absent in the host rock.

TFCs are observed exclusively in the Cathedral Peak granodiorite, preferentially occurring adjacent to the younger Johnson granite porphyry. In general, the orientation of the TFCs is 020-040, with a sub-vertical dip. However, immediately adjacent to the Cathedral Peak/Johnson granite contact in particular places, a second, high-angle orientation (090-110, sub-vertical dip) set of TFCs occur. TFCs belonging to the EW-oriented, second set are generally limited by the better-developed, 020-040 oriented TFCs, although locally the two sets are mutually cross-cutting. Areas where both sets are observed also contain a high density of 020-040 oriented TFCs. The absence of TFCs in the Johnson granite porphyry and the increasing density toward the Cathedral Peak / Johnson granite contact suggest that volatile release associated with intrusion of the Johnson granite porphyry may have formed these structures.