Particle size distributions, microstructures and chemistry of fault rocks in a shallow borehole adjacent to the San Andreas Fault near Little Rock, CA
Abstract
In mapping the distribution of fractured crystalline rocks along the Mojave section of the San Andreas Fault (SAF), Dor et al. (2006) found that almost all rocks within 50 to 200 m from the fault are pulverized to some degree. In an effort to characterize the role of surface weathering, and quantify the mesoscale and microscale deformation of these rocks as a function of depth in the shallow subsurface environment, we have collected a nearly continuous, 42 meter-deep core from the pulverized adjacent to the main strand of the SAF near Little Rock, California. The Little Rock site is characterized by extensive outcrops of granitic rock displaying varying degrees of damage up to a few hundreds of meters from the fault’s primary active strand. The cored section primarily is composed of pulverized granites and granodiorites, and is cut by numerous mesoscopic secondary shears. Medium to coarse silt- and fine sand-size particles dominate throughout the cored section; very few micron-scale particles are observed and only minor amounts of clay weathering products are present. Mapping on optical and SEM images of core samples at various depths and magnifications defines the distribution of two main fault rock types, pulverized zones displaying primarily opening-mode fractures, and cataclastic fault zones. The pulverized regions display large host-rock crystals that are fractured to produce angular particles often ranging from 10-100 microns in diameter. The fractured parts display optical continuity and a high density of fluid inclusion trails suggesting episodes of fracture healing. The cataclastic zones are characterized by smaller (0.5-10 microns) and more rounded grains, a greater clay content, and sometimes show repeated stages of calcite cementation and shear. The distribution of pulverized particles and cataclastic zones indicate multiple fracture-healing cycles to produce an outcrop that reduces to powder when dug out with a hammer. Most samples analyzed to date suggest that cataclastic grain size reduction and shear at the microscale are significant processes in the formation of this pulverized zone. Currently, we are quantifying the particle size distribution and aerial extent of representative cataclastic zones in this region.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.T54A..01W
- Keywords:
-
- 8010 STRUCTURAL GEOLOGY / Fractures and faults;
- 8111 TECTONOPHYSICS / Continental tectonics: strike-slip and transform