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Characterization of Pulverized Granitoids in a Shallow Core along the San Andreas Fault, Littlerock, CA

Neta Wechsler, Emily E. Allen, Thomas K. Rockwell, Gary H. Girty, Judith S. Chester, & Yehuda Ben-Zion

Published 2011, SCEC Contribution #1458

We present results on the composition, structure, and particle size distribution (PSD) of pulverized and damaged granitic rocks in a 42 meter-deep core adjacent to the San Andreas Fault near Little Rock, CA. The cored section is composed of pulverized granites and granodiorites, and is cut by numerous mesoscopic secondary shears. The analysis employs XRD, XRF, thin sections and semi-automated particle size analyzer methods, including a novel calibration method. The mean particle size for the majority of samples falls between 50 and 470 microns. The PSDs can be fitted by a power-law, with D-values ranging between 2.5 and 3.1, as well as by a superposition of two Gaussians. Fracture surface energy calculations based on the observed particle distributions provide very low values, indicating that the part of the total earthquake energy budget expended for breaking or shattering rocks is small. Shear deformation is likely to dominate near the secondary faults. The most pronounced fault-related alteration occurs along the secondary shears, and is a function of both composition and depth. The alteration to clay appears to be the result of fluid-rock interaction and brittle deformation under low temperature conditions, rather than of surface-related weathering. The particle size reduction noted in this region reflects multiple mechanisms of comminution. The zones of pulverization that lack significant weathering likely result from repeating episodes of dynamic dilation and contraction.

Wechsler, N., Allen, E. E., Rockwell, T. K., Girty, G. H., Chester, J. S., & Ben-Zion, Y. (2011). Characterization of Pulverized Granitoids in a Shallow Core along the San Andreas Fault, Littlerock, CA. Geophysical Journal International, 186, 401-417. doi: 10.1111/j.1365-246X.2011.05059.x.