Original paper

The geological significance of mixed-layered clay minerals in the San Andreas Fault Observatory at depth (SAFOD) drillhole in Parkfield, California

Schleicher, Anja M.; War, Laurence N.; van der Pluijm, Ben A.


A detailed mineralogical study of fine-grained mudrocks sampled from spot coring at three key locations along the San Andreas Fault Observatory at depth (SAFOD) drill hole (at down-hole depths of 3066 m, 3436 m and 3992 m) reveal abundant authigenic illite-smectite (I-S) and chlorite-smectite (C-S) mixed-layer mineral clays characteristic of deep diagenetic conditions. A randomly ordered (RO-R3)I-S mineral with ca. 20 to 25 %smectite layers is the dominant clay species across the San Andreas fault zone (sampled at 3066 and 3436 m measured depths/MD), whereas an ordered (>R3) I-S with ca. 2-5 %smectite layers is the dominant phase beneath the fault zone (sampled at 3992 m MD). Adopting available kinetic models for the crystallization of I-S in burial sedimentary environments, and the current borehole depths and thermal structure, the conditions and timing ofI-S growth can be evaluated. Assuming a typical K concentration of 100 - 200 ppm for sedimentary brines, a present day geothermal gradient of 35 °C/km and a borehole temperature of ca. 112°C for the sampled depths, most of the I-S minerals can be predicted to have formed over the last 4 to II Ma and are probably still in equilibriumwith circulating fluids.