REEXAMINING THE SOURCE OF SAFOD CORE RECOVERED FROM ADJACENT TO THE SOUTHWEST DEFORMING ZONE

The presently active trace of the San Andreas Fault (SAF) near Parkfield, California lies within the North American Plate, and the main drillhole (Phases 1 and 2) of the San Andreas Fault Observatory at Depth (SAFOD) crossed the entire SAF and terminated in Lake Cretaceous Great Valley Sequence siltstones. All of the sedimentary rock units in the spot cores subsequently recovered from within and adjacent to the actively deforming traces at depth during Phase 3 (Hole G) are also generally considered to be part of the Great Valley Sequence, a forearc-basin deposit that received detritus from the exhumed Sierra Nevada batholith. However, a possible exception may be the foliated siltstone-shale located outside the SAF damage zone immediately adjacent to the southwest deforming zone at the southwest end of Hole G (3186.8-3193.8 m), whose detrital and diagenetic characteristics differ substantially from the other sedimentary rocks in the Hole G core. This rock unit consisted principally of a reddish-colored, faintly bedded, fine-grained siltstone with some centimeter-thick lenses of sand-sized detritus. Textures of polycrystalline clasts containing quartz + K-feldspar + plagioclase ± biotite suggest derivation from an aphanitic volcanic-rock source, and the clasts are an important detrital component of the siltstone. The matrix of the coarse-grained lenses is a mosaic of fine-grained quartz crystals, and portions of the red siltstones contain dense networks of quartz-filled veinlets, the products of interaction with migrating silica-rich groundwaters. In consequence, silica concentrations obtained for this unit by X-ray fluorescence techniques typically exceed 70 wt% SiO₂. Evidence of similar silica enrichment is lacking in the other Hole G sedimentary rocks, all of which contain ≤60 wt% SiO₂. The differences in detrital character and silica enrichment suggest that this foliated siltstone-shale is a fault sliver derived from a rock formation other than the Great Valley Sequence, and future studies will be aimed at identifying its origin and age. Possible detrital sources associated with felsic volcanic rocks include the Mid-Tertiary volcanic centers of central California, which formed above a slab window and were subsequently disrupted and offset considerable distances by San Andreas transform faulting.