Potentially Pulverized Granites along the Garlock Fault: An analysis into their Physical and Chemical Properties

We collected samples of pulverized granitic rock along three trench exposures across the Garlock fault, Tejon Ranch, CA. Our primary purpose is to assess the roles of mechanical fragmentation versus chemical weathering in the reduction of grain size displayed by samples collected during this study. In each transect, granitic rocks are finer-grained immediately adjacent to the active trace of the fault. Feldspar and quartz grains range from 1-2 mm in size, and material along the fault is so powdered that it was difficult to collect large, intact samples. Quartz and feldspar grains smear to powder when pressed between the fingers. Laboratory work initiated during this study includes bulk and grain density, porosity, mineral XRD, major and trace element chemistry, thin section, and SEM. Resulting data will resolve whether chemical weathering has played a significant role in the observed reduction of grain size. To date, we have completed major and trace element analyses of 11 samples. Resulting Chemical Index of Alteration (CIA) values range from 46-53, and are like values displayed by unweathered granite in the Peninsular Ranges. This result indicates that the finely-powdered granitic material is not distinguishable from fresh, unweathered granite, and that there is no indication of significant chemical weathering. Initial particle size distribution analysis shows grain sizes <0.2 m. These preliminary observations are consistent with the idea that collected samples are derived solely from mechanical (dynamic) fragmentation during slip along the Garlock fault.