Tabular fracture clusters: Dynamic fracturing produced by volatile expulsion, Sierra Nevada Batholith, California

An undocumented type of fracture system – Tabular Fracture Clusters (TFCs) – occurs in the Cathedral Peak granodiorite, Sierra Nevada Batholith, CA. TFCs are linear zones of sub-parallel, densely spaced fractures, approximately 4–40 cm wide and 3–100 m long. TFCs occur in highest density adjacent to the Johnson granite porphyry, which intruded the Cathedral Peak granodiorite. Individual fractures in TFCs exhibit only opening-mode displacement. Microstructural analysis indicates that TFC fractures contain micro-breccia of angular clasts of host rock. Fine-grained zeolite and quartz are observed along many of the fractures within TFCs, all of which are absent in the host rock. The characteristics of TFCs suggest that they differ from previously documented fracture systems in the Sierra Nevada Batholith. The dense spacing, nature of the fractures, and association of TFCs with miarolitic cavities provide compelling evidence that TFCs formed in response to volatile overpressure from the adjacent Johnson granite porphyry. We attribute the formation of TFCs to dynamic fracturing, based on the clustered nature of the extension fractures and the geometric similarities of TFCs to other dynamic fracturing arrays.