We present new detailed analyses of samples of pulverized Tejon Lookout granite collected from sections adjacent to the San Andreas and Garlock faults in southern California. The Tejon Lookout granite is pulverized in all exposures within about 100 m from both faults. Chemical analyses indicate no or little weathering in the collected samples, although XRD analysis shows the presence of smectite, illite, and minor kaolinite in the clay-size fraction. Weathering products may dominate in the less than 1 micron fraction. The average grain size in all samples of pulverized Tejon Lookout granite ranges between 26 and 208 microns (silt to fine sand), with the particle size distribution in part a function of proximity to the primary slip zone. The San Andreas fault samples that we studied are generally finer grained than those collected from adjacent to the Garlock fault. The particle size distribution for each studied sample from both faults follows a pseudo-power law with a continuously changing exponent, which suggests that pulverization is not simply a consequence of direct shear. The average particle size that we determined for our samples is considerably coarser than reported in previous investigations, which we attribute to possible measurement errors in the prior work. Our data and observations suggest that dynamic fracturing in the wall rock of the San Andreas and Garlock faults only accounts for about 1% or less of the earthquake energy budget.

The Cuyamaca‐Laguna Mountain shear zone (CLMSZ) lies along the axis of the Peninsular Ranges batholith, separating it into an eastern and western plutonic zones. The shear zone involves Triassic‐Jurassic and Early Cretaceous plutonic units which intruded the Triassic Julian Schist and transects the eastern edge of a cryptic lithospheric boundary, separating oceanic crust on the west from continental crust on the east. The Julian Schist crops out on either side of the cryptic lithospheric boundary and is interpreted to represent an overlap sequence. This structural/stratigraphic relationship indicates that the contrasting lithospheric types must have been juxtaposed prior to approximately the Triassic time, and as a result, the CLMSZ probably developed in an intra‐arc setting. At least two periods of deformation produced the polygenetic CLMSZ. Structures that formed during D1 include S1 and L1. In Triassic‐Jurassic and Early Cretaceous orthogneisses, S1, a pervasive NW striking and NE dipping mylonitic gneissosity, obliterates nearly all traces of an older magmatic fabric. L1 plunges steeply to the NE, lies within the plane of S1, and is locally a well‐developed stretching lineation. D1 structures can be traced from the ∼115 Ma Oriflamme Canyon protomylonite into the adjacent Julian Schist and are represented by a well developed S‐C mylonitic structures indicative of NE–SW contraction. D1 structures in the Oriflamme Canyon protomylonite and in the ∼118 Ma Pine Valley granodiorite developed while these plutons were incompletely solidified. Hence D1 probably occurred between ∼118 and ∼115 Ma and had culminated in the 105 My emplacement of the Las Bancas tonalite. Normal convergence, ∼125 to 115 Ma, between the North American and Farallon plates is coincident with D1 and the syntectonic emplacement of the Pine Valley granodiorite and the Oriflamme Canyon protomylonite. This relationship suggests that the mechanically weak, thermally and melt‐softened cryptic lithospheric interface between oceanic and continental lithosphere may have yielded during the normal convergence event, resulting in the concentration of strain into the CLMSZ during arc magmatism. Such a conclusion underscores the possibility of using intra‐arc structures to deduce convergence patterns, as elegantly argued in several recent papers. A >12‐km long normal sense shear zone transects D1 structures and formed during D2. Mesoscopic structure associated with the normal sense shear zone includes S2, L2, and C2. D2 structures are the record of NE–SW extension between ∼105‐ and ∼94 Ma. They may be related to the vertical loading of the CLMSZ by the hanging wall block of the westward verging Santa Rosa and Borrego Springs mylonite belts or they may represent an early, local response to magmatically and structurally overthickened, gravitationally unstable crust. In the latter interpretation, D2 structures are the harbingers of Tertiary‐aged, gravity‐driven collapse of the SW Cordilleran margin.

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S U M M A R YWe present results on the composition, structure and particle size distribution (PSD) of pulverized and damaged granitic rocks in a 42-m-deep core adjacent to the San Andreas Fault near Littlerock, 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 analyser methods, including a novel calibration method. The mean particle size for the majority of samples falls between 50 and 470 μm. 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 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 the core reflects multiple mechanisms of comminution. The zones of pulverization that lack significant weathering likely result from repeating episodes of dynamic dilation and contraction.