Pulverized Tejon Lookout Granite: Attempts at Placing Constraints on the Processes

We have described and analyzed pulverized Tejon Lookout granite recovered from several transects of the western segment of the Garlock fault on Tejon Ranch in southern California. Observations and data collected at this location are compared to a sampled transect of the San Andreas fault at Tejon Pass previously studied by Wilson et al. (2005), also exposing the Tejon Lookout granite. The purpose of this study is to characterize the physical and chemical properties of the pervasively pulverized leucocratic rocks at multiple locations and to hopefully place constraints on the processes producing them. To accomplish this we performed particle size analysis with the use of both laser particle analyzer and pipette methodology; major and trace chemistry analyses determined by XRF; clay mineralogy determined by XRD; and we evaluated fabric and texture through the study of thin sections.

Recovered samples met the field criteria of pulverization developed by Dor et al., 2006 - that is, the individual 1-2 mm-sized crystals can be recognized in the field but the granite (including quartz and feldspar) can be mashed with ones fingers and exhibits the texture of toothpaste. All samples were analyzed on a Horiba LA930 Laser Particle Analyzer in an attempt to reproduce the earlier results of Wilson et al. (2005) with similar methodology. We also utilized the classic pipette methodology to ensure complete discrimination of particle sizes. Our PSD analysis shows that the dominant particle size falls in the 31-125 micron range, much coarser than previously reported by Wilson et al. (2005), with >90% of the total sample falling in the >31 micron size range. We can reproduce the previously documented results by allowing the samples to circulate for long periods of time at slow circulation speeds in the laser particle size analyzer, during which time the coarse fraction settles out, thereby leaving only the fine fraction for detection. However, subsequent increase in the circulation speed leads to a complete recovery of the original PSD. Our XRF and XRD analyses provide evidence of the lack of major weathering products and their inability to skew the PSD results in a significant way. Dor et al. (2007) and Stillings et al. (2007) document evidence that support theoretical predictions and previous inferences of pulverization occurring in the upper few kilometers, especially along faults of the southern San Andreas system. Geophysical observations of Lewis et al. (2005, 2007) provide evidence that low velocity fault- parallel layers, which are likely made of pulverized or highly damaged material, are dominant in the upper few kilometers of the crust. Their asymmetric position with respect to the slipping zone, in agreement with asymmetric patterns of small scale mapped rock damage (Dor et al., 2006), suggest that pulverized rocks are likely the product of a preferred rupture direction during dynamic slip. Our results combined with the above mentioned works imply that pulverized fault zone rocks at multiple locations are much less damaged than suggested in previous studies.