8. Structure of Large-Displacement, Strike-Slip Fault Zones in the Brittle Continental Crust

Abstract

Characterizing the structure of fault zones is necessary to understand the mechanical, fluid flow and geophysical properties of the lithosphere. This paper provides a detailed characterization of two large-displacement, strike-slip fault zones of the San Andreas system in southern California, the Punchbowl and North Branch San Gabriel faults. The faults cut crystalline and well-lithified sedimentary rocks, and consist of broad zones of fractured and faulted rock (damage zone) containing one, or more, narrow, tabular zones of highly sheared rock (fault core). Subsidiary faults and fractures of the damage zone formed in response to stress cycling associated with slip. The fault core is composed of very fine-grained, altered fault-rocks that reflect high shear strain, extreme comminution, and enhanced fluid-rock reactions. The characteristic and relatively ordered structure of these large-displacement faults is consistent with progressive damage accumulation over the lifetime of the fault. Layers of ultracataclasite containing mesoscopic slip surfaces within fault cores record extreme localization of slip at the macroscopic and mesoscopic scale. Progressive accumulation of ultracataclasite from abrasive wear along a slip surface throughout faulting history can explain the particle size distribution, layered structure, and sharp boundaries of the ultracataclasite layer. The longevity and relative stability of the slip surface is evidenced by the accumulation of progressively younger ultracataclasite towards the slip surface. Although not a common feature, the incorporation of slivers of wall rock into the ultracataclasite layer document occasional branching of the slip surface within fault cores. The damage-zone and fault-core characterization may be used to describe the geophysical, mechanical, and fluid-flow properties of brittle fault zones in crystalline and well-lithified sedimentary rocks of the continental crust. fault zones is necessary to understand the mechanical, fluid flow, and geophysical properties of the lithosphere.Our current understanding of the mesoscopic structure of large-displacement, strike-slip fault zones primarily results from study of uplifted and exhumed faults (e.g., Little 1995;Chester and Logan 1986; Flinn 1977) and faults exposed in mines and drill core (e.g., Ohtani et al. 2000;Wallace and Morris 1986). Early petrographic study of mylonites and cataclasites initiated considerable research into the classification and origin of fault rocks (for review, see Snoke and Tullis 1998). The most commonly used fault-rock classification schemes are based on particle size, fabric and cohesion. For example, in Sibson's 1977 classification, mylonites and cataclasites both are distinguished by primary cohesion and tectonic grain size reduction, but mylonites have a well-developed foliation with some evidence of syntectonic recovery and recrystallization, whereas cataclasites have random fabrics and microstructures indicative of cataclasis. The breccia-gouge series is distinguished by lack of primar...

Off‐fault plasticity and earthquake rupture dynamics: 1. Dry materials or neglect of fluid pressure changes

“…In many faults this typical structure, as shown in Figure 1, is not symmetric about the fault [Chester et al, 2004], as might especially be the case when the principal fault surface separates different host rocks [Wibberley and Shimamoto, 2003;Dor et al, 2006]. Dor et al [2006] and Shi and Ben-Zion [2006] suggest that asymmetries in damage are associated with variations in lithology of the wall rock, in that a contrast in lithology across a fault may cause dynamic rupture events to have a preferred direction.…”

“…[2] Geologic observations of the structure surrounding mature, large-displacement faults show that the typical fault zone consists of an inner core composed of finely granulated material bordered by a gouge layer that grades into fractured-damaged wall rock [Chester and Logan, 1986;Chester et al, 1993;Caine et al, 1996;Biegel and Sammis, 2004;Chester et al, 2004]. In many faults this typical structure, as shown in Figure 1, is not symmetric about the fault [Chester et al, 2004], as might especially be the case when the principal fault surface separates different host rocks [Wibberley and Shimamoto, 2003;Dor et al, 2006].…”

Off‐fault plasticity favors the arrest of dynamic ruptures on strength heterogeneity: Two‐dimensional cases

“…[2] A plastic mechanical behavior is expected from the damaged medium that is likely to encompass the faults up to several tens of meters [e.g., Chester et al, 2004;Dor et al, 2006]. Off-fault cracking has been also identified as a possible mechanism to explain slip profiles linear trends that show off from natural earthquakes [Manighetti et al, 2004].…”

Application of rate and state friction formalism and flash melting to thin permanent slip zones of major faults

“…[2] Observations of exhumed faults indicate that kilometers of slip occurred along a ∼1 mm wide called here permanent slip zone (PSZ) (rather than "surface" to emphasize its finite width) [Chester et al, 1993[Chester et al, , 2004Chester and Chester, 1998;Schulz and Evans, 1998;Chester et al, 2005]. A "core" of highly strained rock surrounds the PSZ ( Figure 1); it is sometimes as narrow as ∼10 mm.…”