Description
Pulverized rocks have been identified in meteorite impacts and occur along crustal scale faults primarily in igneous and metamorphic rocks (Dor et al., 2006; Mitchell et al., 2011; Aben et al. 2016). In addition, they occur to a lesser extent in both clastic and carbonate sedimentary rocks (Dor et al., 2009; Aben et al., 2017). They also have been identified in meteorite impacts (Key and Schultz, 2011). In the field, a rock is considered pulverized if all the crystals in a hand sample yield a powdery rock–flour texture when pressed by hand (Dor et al., 2006). Under the microscope this unique texture resembles a jig-saw puzzle, and is characterized by sub-grain fracturing of wallrock to the micron to tens of microns scale with little to no grain rotation or evidence of shear. Along the Clark segment of the San Jacinto Fault, pulverized rocks occur in 10-15 m wide damage zones. In such settings, pulverized rocks occur dominantly as leucocratic igneous material within high grade paragneisses (Peppard et al., 2018). In contrast, pulverized rocks have not been identified along other segments of the San Jacinto fault zone, as for example the Buck Ridge or Coyote Creek faults. Hence, I undertook a reconnaissance study aimed at assessing the presence or absence of pulverized rocks along the Buck Ridge fault. The Buck Ridge fault lies north of the Clark strand, and merges with the Clark NE of Anza (Figure 1). It transects high-grade metamorphic rocks of the pre-mid Cretaceous Burnt Valley Complex and Pleistocene alluvial sediments of the Bautista Formation. This paper will document: (1) the characteristic properties of alluvial sandstones of the Bautista Formation outside the Buck Ridge fault zone, (2) how those properties are modified within the fault zone, outside the Buck Ridge fault zone, (2) how those properties are modified within the fault zone, pulverization process. The results of work reported here suggest that pulverization, at the grain scale, is restricted to a broad ~25 meter thick zone of damaged rock along the Buck Ridge fault, and to the growing body of data supporting the idea that pulverization is another key signature of seismogenically active faults.
