Origin and significance of clay‐coated fractures in mudrock fragments of the SAFOD borehole (Parkfield, California)

Abstract

[1] The clay mineralogy and texture of rock fragments from the SAFOD borehole at 3067 m and 3436 m measured depth (MD) was investigated by electron microscopy (SEM, TEM) and X-ray-diffraction (XRD). The washed and ultrasonically cleaned samples show slickenfiber striations and thin films of Ca-K bearing smectite that are formed on polished fault surfaces, along freshly opened fractures and within adjacent mineralized veins. The cation composition and hydration behavior of these films differ from the Namontmorillonite of the fresh bentonite drilling mud, although there is more similarity with circulated mud recovered from 3479 m MD. We propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep, probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the San Andreas Fault.

Constraints on mineralization, fluid‐rock interaction, and mass transfer during faulting at 2–3 km depth from the SAFOD drill hole

“…Whereas the precipitation of C-S may be triggered by the transportation of Mg and Fe from the wall rock into the fault rocks, K, and Ca are leading elements for precipitation of I-S minerals as thin films on fracture surfaces. The reduced amount of fluid migration is likely related to the higher amount of thin film coatings containing clays with greater amounts of interlayered smectite that precipitate on fracture surfaces [Schleicher et al, 2006b] and not in the wall rock. Stress-enhanced dissolution at grain contacts is therefore proposed as the main mechanism for mineral transformation in these fault rocks, based on the localization of neomineralization on slip surfaces.…”

“…basis of TEM observations some very small veins are also filled with smectitic minerals [Schleicher et al, 2006b]. …”

A large mantle water source for the northern San Andreas fault system: a ghost of subduction past

“…Second, secondary hydrous minerals were observed in the drill core that penetrated the active segment of the fault, indicating that aqueous fluids were present in the zone at some time in the recent past. Some hydrous minerals, such as talc (Moore and Rymer 2007) and clay minerals (Schleicher et al 2006;Schleicher et al 2010) are weak shear-zone materials and may be responsible for fault creep behavior. Finally, our model does not preclude the possibility that fluid flow through the crust from the shrinking serpentinite mantle wedge could be episodic as affected by tectonic and seismic activities and that our estimates of long-term average water discharge rates do not reflect the possibility of transient fluid transport.…”

An Experimental Study of the Formation of Talc through CaMg(CO3)2–SiO2–H2O Interaction at 100–200°C and Vapor-Saturation Pressures

“…Therefore, faults are likely to be weakened due to high temperatures in the deep crust, but not in the cool shallow crust. The pervasive distribution of clay minerals along faults has also been thought to weaken faults [33][34][35][91][92][93][94], because layered clay minerals exhibit much lower frictional coefficients than other minerals [95]. For example, talc discovered along the San Andreas fault zone is responsible for helping in aiding slippage along the fault [35,95].…”

“…Geologically pervasive silica-rich hydrothermal fluids derived from sources, such as deep formation brines or magma intrusions (e.g., [26][27][28]), can react with dolomite to form talc in carbonate reservoirs, influencing the reservoirs' physical properties [29][30][31]. In addition, talc has 2 Geofluids a low friction coefficient, so its formation along faults in carbonate rocks can promote the stable creep of a fault, releasing accumulated elastic strain and preventing strong earthquakes [32][33][34][35]. Therefore, thorough investigation of the CaMg(CO 3 ) 2 -SiO 2 -H 2 O interaction can help elucidate many geological processes.…”