Assuming a typical K+ concentration of 100–200ppm for sedimentary brines, a present-day geothermal gradient of 35☌/km and a borehole temperature Adopting available kinetic models for the crystallization of I–S in burial sedimentary environments and the current borehole depths and thermal structure, theĬonditions and timing of I–S growth can be evaluated. However, the effect of rock deformation in the matrix appears only minor, with weak flattening fabrics defined largely by kinked and fractured mica grains. The matrix of all mudrock samples show extensive quartz and feldspar (both plagioclase and K-feldspar) dissolution associated with the crystallization of pore-filling clay minerals.

The most smectite-rich mixed-layered assemblage with the highest water content occurs in the actively deforming creep zone at ca. 2–5% smectite layers is the dominant phase beneath

20–25% smectite layers is one of the dominant authigenic clay species across the San Andreas Fault zone (sampled at 3,066 and 3,436m measured depths/MD), whereas an authigenic illite with ca. The characteristics of authigenic illite–smectite (I–S) and chlorite–smectite (C–S) mixed-layer mineral clays indicate a deep diagenetic origin. A detailed mineralogical study is presented of the matrix of mudrocks sampled from spot coring at three key locations along the San Andreas Fault Observatory at depth (SAFOD) drill hole.

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