Evans, James P.

Janecke, Susanne U.

Prante, Mitchell R.

English

DigitalCommons@USU

Utah State University DigitalCommons@USU Geosciences Presentations Geosciences 2012 Preservation of the Seismic Cycle in a Continental Low-Angle Normal Fault: West Salton Detachment Fault, USA Mitchell R. Prante Utah State University Susanne U. Janecke Utah State University James P. Evans Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/geology_pres  Part of the Geology Commons Recommended Citation Prante, M.R., Janecke S.U., and Evans J.P., 2012, Preservation of the seismic cycle in a continental low-angle normal fault: West Salton detachment fault, USA: Presented at annual meeting of the Southern California Earthquake Center, Palm Springs, CA. This Poster is brought to you for free and open access by the Geosciences at DigitalCommons@USU. It has been accepted for inclusion in Geosciences Presentations by an authorized administrator of DigitalCommons@USU. For more information, please contact digitalcommons@usu.edu. Preservation of the seismic cycle from a continental low-angle normal fault: West Salton detachment fault, USAShear StressTimeMelt Melt1mm~1500 ˚C170-200 ˚CCataclasis UndeformedReworked Pseudotachylyte Increased pore uid pressureMitchell Prante, Susanne Janecke, James Evans, Utah State University, Logan UT5 cmQuenched MarginGeologic map of the western Salton Trough, CA (Dorsey et al., 2011), Field area (red box); CCF—Coyote Creek fault; FCM—Fish Creek Mountains; FCMF—Fish Creek Mountains fault; GM—Granite Mountain; PR—Pinyon Ridge; RRC—Red Rock Canyon; SFF—San Felipe fault; SMG—Split Mountain Gorge; TBM—Tierra Blanca Mountains; VM—Vallecito Mountains; YR—Yaqui Ridge; WP—Whale Peak1mmU.CtSpecial thanks to:James Evans Susanne JaneckeAdolph Yonkee (WSU) Southern California Earthquake Center Geological Society of AmericaAnadarko PetroleumUSU Graduate Student Senate Anza Borrego State Park1mmCt 1Ct 2Table 2:Reported occurrences of melt-related pseudotachylyte from low-angle normal faults and estimated depth of formation (Modified from Collettini, 2011). LANF Mean dip Pseudotachylyte depth References Whipple low-angle normal fault 10-15° Poorly constrained Lister and Davis, 1989; Reynolds and Lister, 1987; Wang, 1997 South Mountains detachment fault 10-20° ~11 km Davis et al., 1986; Goodwin, 1999; Goodwin et al., 1998; Reynolds et al., 1998 Chemehuevi-Sacramento detachment fault  <20° 6-10 km John, 1987  Alaşehir detachment fault 10-30° Poorly constrained Isık et al., 2003 Otago schist (Tucker Hill) 10-30° 6-12 km Barker et al., 2010  Nordfjord-Sogn detachment fault <30° Seismogenic zone (~6-12 km) Braathen et al., 2004 Eidsfjord detachment fault  25-30 ° Seismogenic zone (~6-12 km) Steltenpohl et al., 2011  Xiaoqinling detachment fault  25-50° Poorly constrained Zhang et al.,  1998  West Salton detachment fault  <30° 4-7 km (poorly constrained) Kairouz et al., 2003; Schultejann, 1984; Steely,  2006; Janecke et al., 2008; Janecke and others unpublished data  QalQoacxcgKsmKlpTcKgmlCohesive fault breccia and cataclasiteChlorite fault breccia Quaternary alluvium Quaternary older alluvium Pliocene Canebrake ConglomerateCretaceous  ‘La Posta-type’ tonalite Cretaceous psammitic schist and undierentiated metasedimentary rocksCretaceous granitic and protomylonite ^ Sample location PseudotachylyteX X’Sample TraverseGeologic map of the Yaqui Ridge area (Modied from Steely et al., 2009), > 1m thick accumulations of tectonic pseudotachylyte crop out in both the hanging wall and footwall of the WSDF in the hinge zone of the Yaqui Ridge anticline (black stars). Type I < 170 ˚C Type II 170-200 ˚C Type III > 200 ˚C Type IV > 200 ˚C~20 μmCalcite twin morphology as a function of deformation temperature (modied from Ferrill et al., 2004). Based calcite twin morphology from cross-cutting veins, late stage deformation (post-pseduotachylyte formation) occurred at temperatures between 170 and 200 °C (Type II twins). Micro-fracture texture with highly angular and matrix supported fragments in calcite veins is consistent with elevated uid pressures (greater than lithostatic pressure). Cataclastic fault rocks in the fault zone of the WSDF include fault breccia, cataclasite, and ultracataclasite. Multiple generations of cataclasite (Ct 1, Ct 2) and reworked ultracataclasite (U. Ct.) have been observed. ProtolithUndeformed La Posta pluton, composed of plagioclase, k-feldspar, quartz, biotite, hornblende, myrmekite, allanite, ± calcite, ± chlorite, with some undulose extinction and sub-grain development in quartz.1mmXPL500 μmSphPPLXPL2 mm Quenched MarginPPLPPLPPL200 μmDuctile ow structurePPLTable 1: Summary of field and laboratory observations that are consistent with fault-related pseudotachylyte interpretation for black fault-rock (Magloughlin and Spray, 1992; Lin, 2008; Kirkpatrick et al., 2009). Feature Outcrop/hand sample Optical Microscopy  Scanning Electron Microscopy  Aphanitic matrix  X X  Injection vein morphology  X X X Quenched margins  X  Flow structures  X X X Quartz and Feldspar clasts  X X Sherulites/Microlites   X  Embayed clasts   X    XXXXXWSDFPt(B)(B)PtS N5 cm1.0 mFault Breccia Damage ZoneCoreCt/fault brecciaPSS200 μmQtzQtzPlBioPl200 μmSphSphPPLQtz200 μmBioPseudotachylyte fragments in cataclasite, is consistent with multiple deformation events and brittle reworking of pseudotachylyte STSFEFColorado R.Gila R.S.D.U.S.AMexicoPresent Day 116° 114°32°30°Pacic PlateNorth AmericanPlate32°30°100 km 100 kmS.D.Colorado R.Gila R.ca. 5.3 MaWSDFRegional tectonic map of the northern Gulf Extensional Province and eld area (blue box); B) reconstruction for 5.3 Ma, restoring 350 km of dextral oset on the San Andreas Fault System (modied from Dorsey et al., 2007). EF Elsinore fault, SAF San Andreas fault, SF San Felipe fault, ST Salton Trough, SD San Diego, WSDF West Salton detachment fault.(A) Field photograph of southwest limb of the Yaqui Ridge anticline and WSDF in Quartz Vein Wash, with fault breccia and thick accumulation of pseudotachylyte (Pt) in the hanging wall, and cataclasite (Ct) and fault breccia in the footwall. (B) Detailed portion of Pt outcrop in A (blue box) with interpreted pseudotachylyte injection veins cross-cutting white fault breccia.N5 mFault CoreUn-altered Protolith Cataclasite and UltracataclasitePseudotachylyteFault BrecciaFractures Principal Slip SurfaceSchematic block diagram of West Salton detachment fault zone at Yaqui ridge where both the hanging wall and footwall of the detachment are composed of crystalline rock illustrating key fault zone components and structural relationships.- Tectonic pseudotachylyte (rapidly quenched frictional melt formed during earthquakes) has been documented along faults from a variety of protoliths and tectonic settings (Sibson, 1975; Sibson and Toy; 2006; Lin, 2008; Kirckpatrick et al., 2009; and many others).- Pseudotachylyte is the only convincing evidence for ancient seismicity from fault zones exhumed from the seismogenic zone (e.g. Cowan, 1999). -Identication of pseudotachylyte along exhumed LANFs is an eective test of the hypothesis that LANFs are seismically active in the brittle crust. Importance of pseudotachylyte from LANFsPseudotachylyte vein with numerous spherulites (Sph), and a reduction in grain size interpreted to represent a quenched margin.Hand sample of pseudotachylyte bearing fault breccia in the hanging wall of the WSDF, apparent reduction in grain size approaching the margin of the vein, is interpreted to represent a quenched margin.Asymmetric fold in very ne-grained (< 1 μm), black fault rock from WSDF footwall, these features are interpreted to represent ductile ow in a viscous melt (pseudotachylyte). (A) WSDF, present geometry (B) Santa Rosa shear zone (at Yaqui Ridge), present geometry (C)(D)Evidence for low-angle slipSph(Steely et al., 2009) (Janecke et al., 2008)5 km10 km15 km Brittle-Plastic Transition 150˚C300˚CMetamorphismUn-MetamorphosedSubgreenschistGreenschistAmphiboliteFault RocksFault gouge and brecciaCataclasiteProtomyloniteMyloniteSouth Mountain Otago Schist Whipple /Chemehuevi Xiaoqinling Alasehir Nordord-Sogn/Eidsord  WSDF ^ ^^^^^^^^^^^^^^^^^^^^^ ^^^^^^#KlpQalKgmlKgmlQoQoaQoaQalcxKlpKlpQoaQoaQoacgQoaKgmlcgQoaTcKlpQoaKlpTcKgmlKsmQoaQoaQoacgcgQoaQoacxQoaQoacgcxcgKlpQoaKsmKsmcgKlpKlpKgmlKsmQoaKsmcxKsmKlpQoaKlpA’ABB’116°17'0"W116°17'30"W116°18'0"W33°8'30"N33°8'0"N33°7'30"N$1KilometersField locality and tectonic setting Fault zone modelConclusionsSEM-BSE images of tectonic pseudo-tachylyte from the hanging wall of the WSDF, with numerous plagio-clase (Pl) and biotite (Bio) microlites. Microlites are very ne-grained (< 100 μm), tabular and acicular crys-tals with highly variable morphol-ogy and composition. Microlites are typically interpreted to represent primary crystals formed during rapid (< 1 minute) cooling of a fric-tional melt (Lin, 2008).  1) Previous work along the WSDF docu-ments slip at a low-angle (Schultejann, 1984; Axen and Fletcher, 1998; Steely et al., 2009; Dorsey et al., 2012)2) Well preserved tectonic pseudotachylyte (frictional melt) in the footwall and hanging wall of the WSDF is consistent with seismic-ity along a LANF (Kairouz et al., 2003; Ja-necke et al., 2008).3) Pervasive brittle deformation associated with the WSDF and reworking pseudotachy-lyte suggests that melt formed in the brittle crust. 4) Thick accumulations of pseudotachylyte, multiple generations of cataclastic rocks and brittle reworking of pseudotachylyte is con-sistent with multiple seismic events along the detachment. 5) These conclusions have important impli-cations regarding the seismic potential of LANFs and fault mechanics.  (A) Structural data from the WSDF at Yaqui Ridge anticline, illustrates local sub-parallelism with Santa Rosa shear zone (B). Based on regional and local structural relationships the WSDF is inter-preted to have reactivated the low-angle, thrust-sense Santa Rosa shear zone (C, D). 1) Sub-parallelism between the WSDF and Cretaceous Santa Rosa shear zone (thrust), suggests reactivation of low-angle thrust during extension. 2)  WSDF cross-cuts Tertiary conglomerate in the hanging wall, at a low-angle (< 30˚).Summary cross-section of LANF with expected fault rock and metamorphic grade (geothermal gradient=30 °C), and reported occurrences of pseudotachylyte from LANFs with estimated depth of formation (Table 2). Approximate nucleation depths of several > Mw 6.0 earthquakes that have been interpreted to have nucleated along LANFs  (Abers et al., 1997).

Preservation of the Seismic Cycle in a Continental Low-Angle Normal Fault: West Salton Detachment Fault, USA

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Preservation of the Seismic Cycle in a Continental Low-Angle Normal Fault: West Salton Detachment Fault, USA

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