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Unique Layered Cataclasites Below the West Salton Detachment Fault, Southern California GEOSPHERE; V

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Unique Layered Cataclasites Below the West Salton Detachment Fault, Southern California GEOSPHERE; V Research Paper GEOSPHERE A rock record of paleoseismic cycling: Unique layered cataclasites below the West Salton detachment fault, southern California GEOSPHERE; v. 14, no. 1 Gary J. Axen1, Jane Selverstone2, and Amy Luther1,* 1Department of Earth & Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA doi:10.1130/GES01499.1 2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA 16 figures ABSTRACT as products of seismogenic slip. In contrast, clay-rich gouge and upper-plate CORRESPONDENCE: Gary .Axen@nmt .edu breccias formed at depth ≤2–3 km. The footwall fault core is clay poor and thus Only relatively rare fault rocks record paleoseismicity (e.g., pseudotachy- apparently passed through the uppermost crust with little overprint. CITATION: Axen, G.J., Selverstone, J., and Luther, lyte), though most fault slip probably accumulates during earthquakes. We The layered cataclasites and detachment fault core display only minor A., 2018, A rock record of paleoseismic cycling: Unique layered cataclasites below the West Salton describe two fault-rock assemblages, made of common fault-rock types, and chemical alteration, and the layers record limited shear strain relative to fault- detachment fault, southern California: Geosphere, argue that fault-rock assemblages can provide criteria for inferring seismogenic core rocks. Thus, the layered cataclasites may be good natural analogs for ex- v. 14, no. 1, p. 187–214, doi: 10 .1130 /GES01499.1. slip on exhumed faults that are more common than individual fault-rock types. perimentally formed fault rocks. Both assemblages formed from intermediate Such assemblages allow direct study of fault-slip products formed under vari- plutonic protoliths, similar to “average” mid-crustal rocks, and from the same Science Editor: Shanaka de Silva ous crustal conditions that are not accessible on active faults. One assemblage, lithologies as cut by the active San Jacinto and Elsinore faults. Thus, these of layered cataclasites, is preserved several meters below the detachment. It two assemblages provide accessible, paleoseismically formed analogs to fault Received 20 January 2017 is rare in the rock record and is best interpreted as recording parts of multiple rocks presumably present at seismogenic depth along active faults. Revision received 24 August 2017 Accepted 16 October 2017 seismic cycles. A second assemblage is the two-part detachment fault core: Published online 22 November 2017 inner fault-core ultracataclasite layers separated by principal slip surfaces and INTRODUCTION outer fault-core cataclasites. We hypothesize that this common assemblage forms mainly during seismogenic slip. We interpret a third assemblage of clay Fault rocks formed in and above the brittle-plastic (or frictional-viscous) gouge and breccia as recording slip and alteration above the seismogenic zone. transition are of particular interest for understanding fault and earthquake pro- Detailed structural analysis of the layered cataclasites shows that they cesses (e.g., reviews by Cowan, 1999 and Rowe and Griffith, 2015): fault-zone formed and were deformed episodically, most consistent with multiple evolution (e.g., Scholz, 1987; Chester et al., 1993; Scholz et al., 1993; Ben Zion seismic cycles that probably were defined by West Salton detachment fault and Sammis, 2003), fault stress states and strength, energy budget and con- (WSDF) main shocks. Cataclasite layers formed sequentially at the expense stitutive relations (e.g., Axen and Selverstone 1994; Chester and Chester, 1998; of overlying quartz diorite, while older, deeper layers were abandoned, folded, Marone, 1998; Reches and Dewers, 2005; Collettini et al., 2009; Axen et al., and faulted. Upward growth of the stack of layers probably reflects both strain 2015), shear sense (e.g., Petit, 1987), and fault-related fluid flow, fluid pressure, weakening of the overlying rocks and strain hardening of the layered cata- and alteration (e.g., Sibson et al., 1988; Sibson, 1989; Caine et al., 1996; Smith clasites. Each layer formed in two stages: (1) Lithologically heterogeneous et al., 2008; Haines and van der Pluijm, 2012; Selverstone et al., 2012). and/or cataclastically foliated, fault-bounded precursor slivers above the top The factors controlling whether fault slip accumulates seismically versus OLD G layer record grain-size reduction by cataclasis and cataclastic foliation devel- aseismically are not fully understood: temperature, friction properties, fluid opment and probably record inter-seismic shear. (2) One or more slip event(s), effects, and rock type(s) form a partial list. Exhumed fault zones allow direct, interpreted as seismogenic, transformed precursor slivers into layers of homo- detailed, three-dimensional observations of natural fault-zone products devel- geneous, random-fabric cataclasite. Many layers are overprinted by weakly oped at depth and thus yield key data for comparison to conceptual models of OPEN ACCESS developed foliation, probably of postseismic origin. faulting, which are based largely upon results of laboratory experiments, theo- The detachment fault core formed at ≥2.3–4 km depth, in the upper seis- retical considerations, indirect seismic observations, and/or modeling (e.g., mogenic zone. Pseudotachylyte along the detachment nearby also records Rice, 1992; Marone, 1998; Sibson, 1998; many others). multiple seismogenic WSDF slip events. Similarities between the cataclas- Mature faults in brittle upper-crustal rocks display a wide variety of rock tic layers and detachment fault-core rocks support the interpretation of both types and textures, including (1) random-fabric cataclastic rocks; (2) phyllosili- cate-rich material, commonly foliated, either smeared along the fault from a This paper is published under the terms of the *Present address: Department of Geology and Geophysics, Louisiana State University, Baton wall rock or grown in situ as alteration products; (3) minor to significant vein CC‑BY‑NC license. Rouge, Louisiana 70803, USA material; (4) frictional melt products and other less common materials inter- © 2017 The Authors GEOSPHERE | Volume 14 | Number 1 Axen et al. | Layered paleoseismic fault rocks Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/1/187/4035303/187.pdf 187 by guest on 28 September 2021 Research Paper preted to record paleoseismicity (e.g., amorphous silicates) (see Rowe and these active faults. At the study site, these plutonic rocks lack preexisting me- Griffith, 2015, and references therein). chanical anisotropy (e.g., Cretaceous mylonitic fabrics; Simpson, 1984), simpli- We describe two fault-rock assemblages and interpret them as records of fying mechanical studies, and are little altered; therefore, chemical processes paleoseismicity. Such assemblages probably are more common than the few probably did not dominate fault-rock genesis. Post-WSDF deformation was individual rock types known to form during earthquakes, but few are pres- relatively minor and localized near the younger dextral faults. ently described (e.g., Smith et al., 2008; Rowe et al., 2011). Pseudotachylyte, formed by frictional melting along faults during fast slip, is the main, natural WEST SALTON DETACHMENT FAULT fault rock generally agreed to record paleoseismicity (Cowan, 1999, and ref- erences therein), but it is not particularly common. Others exist but are even The WSDF is a low-angle normal (detachment) fault that forms the bound- more rare and are difficult to identify in natural faults (Rowe and Griffith, 2015, ary between the Peninsular Ranges (footwall) and the Salton Trough (upper and references therein). plate) in southern California (Fig. 1; Axen and Fletcher, 1998), defining the We focus on an assemblage of uncommonly well-layered cataclasites northern segment of the Gulf of California rift system (Axen, 1995). It was ac- found a few meters below the West Salton detachment fault (WSDF), a late tive from ca. 5–8 Ma to ca. 1 Ma and accommodated ~10 km of slip (Axen and Neogene–Quaternary low-angle normal fault (Axen and Fletcher, 1998). Us- Fletcher, 1998; Lutz et al., 2006; Dorsey et al., 2007, 2012; Shirvell et al., 2009). ing textural analysis and structural arguments, we conclude that the layered The WSDF accommodated ~east-west extension within the southern San An- cataclasites formed, and were deformed, sequentially during several seismic dreas plate-boundary fault system (Axen and Fletcher, 1998; Axen et al., 1998). cycles. For several reasons, we infer that the two-part fault core at the top of The stratigraphic record shows that the dextral San Jacinto and Elsinore fault the WSDF footwall also probably records seismic activity. zones dismembered the WSDF ~1.1–1.2 m.y. ago and indicates ~100–200 k.y. of Low-angle normal faults exhume their footwalls directly, yielding abundant temporal overlap of slip on the WSDF and these dextral faults (Lutz et al., 2006; exposures of fault rocks that were processed in crustal levels ranging from the Janecke et al., 2010; Dorsey et al., 2012). brittle-plastic transition to the near surface (e.g., John, 1987; Axen and Selver- WSDF slip was mainly top-ESE (~110° azimuth), parallel to ESE-plunging stone, 1994; Axen et al., 2001; Cowan et al., 2003; Numelin et al., 2007; Smith corrugations of the fault surface (Fig. 1B; Axen and Fletcher, 1998; Kairouz, et al., 2008; many others). Many are young (Neogene or Quaternary) and thus 2005; Steely et al., 2009; Dorsey
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