T25 Deconstructing Damage: Holistic Perspectives on the Spatiotemporal Evolution of Brittle Fault Zones

Sunday, 25 September 2016
8:00 AM - 12:00 PM
Room 203

THE DEVELOPMENT OF INNER DAMAGE ZONES AROUND FAULTS AND THEIR RELATIONSHIP TO SEISMIC ATTRIBUTES IN FAULTS:  A VIRTUOUS FEEDBACK AMONGST SEISMICITY, FLUID FLOW, HEAT, ALTERATION, AND DEFORMATION

Paper number: T25-1
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

We quantify hydrological, mechanical, and seismological properties in fault zones from the San Andreas Fault system, in borehole and outcrop, to constrain the behavior and properties of fault zones over a 5-7 km depth range in diverse crystalline and sedimentary host rocks. In the Mecca Hills, dextral strike-slip faults juxtapose the Orocopia Schist and crystalline gneiss against Plio-Pleistocene sedimentary rocks. These faults exhibit significant hydrothermal alteration across several meters in their inner damage zones. Neoformed corrensite and palygoskite and significant changes in whole-rock chemistry indicate that hydrothermal fluid flow and crystal growth accompanied faulting. Damage zones are strongly asymmetric, with thicker zones and high degrees of alteration on the sedimentary rock side of the faults. Rocks from 3.8 km deep in the Cajon Pass drill hole are altered, sheared, and hydrothermally indurated laumontite ± clay-rich foliated cataclasites. Franciscan rocks from the SAFOD drill hole from a low-velocity zone at 3.9 km deep, exhibit alteration to saponite, calcite veins, high degrees of chemical alteration, and interstitial hydrocarbons. The concentrated nature of the hydrothermal alteration in the inner damage zone and on principle slip surfaces suggests that seismic energy may be a source of heat for these chemical reactions.

We invert borehole-based Vp/Vs from the Cajon Pass and SAFOD boreholes to estimate that the elastic strength of damaged rocks is reduced at least 50% from host rock values; permeabilites may vary by 10-1000x, and seismic attenuation factor, Q is reduced by 10-1000x. The boundaries between inner and outer damage zones can be sharp. Reduction in Q is due to the growth of phyllosilicates, the presence of compliant minerals such as zeolites, dynamically generated pore pressure, and the anisotropy of fault fabric. We suggest that increased alteration decreases Q in fault zones. The reduction in Q by 1-3 orders of magnitude will impact the energy distribution and reverberations within a fault zone. Field and core-based analyses are able to assess physical parameters at higher resolution than most geophysical methods, and thus can help quantify syn- and post-slip processes and inform high-resolution modeling of dynamic rupture processes and attenuation along active faults.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-286326

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