Thermochronological constraints on the timing and magnitude of Miocene and Pliocene extension in the central Wassuk Range, western Nevada


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Thermochronological constraints on the timing and magnitude of Miocene and Pliocene extension in the central Wassuk Range, western Nevada
  Thermochronological constraints on the timing and magnitudeof Miocene and Pliocene extension in the central Wassuk Range, western Nevada Daniel F. Stockli, 1 Benjamin E. Surpless, and Trevor A. Dumitru Department of Geological and Environmental Sciences, Stanford University, Stanford, California, USA Kenneth A. Farley Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA Received 23 April 2001; revised 5 February 2002; accepted 27 February 2002; published 8 August 2002 [ 1 ]  Apatite fission track and (U-Th)/He thermochro-nological data provide new constraints on the timingof faulting and exhumation of the Wassuk Range,western Nevada, where east dipping normal faultshave accommodated large-magnitude ENE-WSWoriented extension. Extensional deformation hasresulted in the exhumation of structurally coherent fault blocks that expose sections of preextensionalmostly granitic upper crust in the Grey Hills andcentral Wassuk Range. These fault blocks displaywestward tilts of    60   and expose preextensional paleodepths of up to  8.5 km, based on the structuralreconstruction of tilted preextensional Tertiaryandesite flows that unconformably overlie Mesozoic basement rocks. Apatite fission track and (U-Th)/Hethermochronological data from the fault blocksconstrain the onset of rapid footwall exhumation at   15 Ma. Fission track modeling results indicate rapidfault block exhumation occurred between   15 and12 Ma, which is in agreement with Miocene volcanicrocks that bracket the tilting history. In addition,fission track and (U-Th)/He data suggest reducedrates of cooling following major extension, as wellas renewed cooling related to active, high-anglefaulting along the present-day range front starting at   4 Ma. Thermochronological data from structurallyrestored fault blocks indicate a preextensionalMiocene geothermal gradient of 27   ± 5  C/km. Thethermochronological constraints on the timing of extensional faulting and the eruptive history in theWassuk Range imply a model for extension wherecrustal heating and volcanism precede the onset of rapid large magnitude extension, and wheresynextensional magmatism is suppressed during thehighest rates of extension.  I   NDEX   T   ERMS  :  8109Tectonophysics: Continental tectonics—extensional (0905); 1035Geochemistry: Geochronology; 8015 Structural Geology: Localcrustal structure; 9604 Information Related to Geologic Time:Cenozoic;  K   EYWORDS  :  Basin and Range, Walker Lane Belt,Wassuk Range, extension, thermochronology 1. Introduction [ 2 ] In many localities throughout the Basin and Range province, large-magnitude extension has exhumed andexposed many kilometers thick structural sections of the preextensional upper crust. Where structurally coherent,these tilted fault blocks provide an opportunity to studythe structural and thermal history of the upper crust,especially at the onset of major extension [e.g.,  Foster et al. , 1990, 1991;  Fitzgerald et al. , 1991;  Gans et al. , 1991;  Howard and Foster  , 1996;  Miller et al. , 1999;  Foster and  John , 1999;  Stockli et al. , 2000]. Several of these citedstudies demonstrated geothermal gradients at the onset of extension in the Basin and Range province that vary fromvalues as low as   15  C/km [  Howard and Foster  , 1996; Stockli , 1999;  Stockli et al. , 2000] to as high as 50   ± 20  C/ km [  Foster et al. , 1991]. These variations in the thermalstructure of the crust have been hypothesized to correlatewith the presence or absence of synextensional magmatismor with distance from metamorphic core complexes[  Howard and Foster  , 1996], but the exact role of heat incrustal weakening and the initiation of extension remains anintriguing question that is not yet answered.[ 3 ] Along the western margin of the northern Basin andRange province at the latitude of Lake Tahoe (Figure 1), thecentral Wassuk Range area exposes several essentiallyintact, tilted fault blocks composed almost entirely of quartzmonzonite of the Mesozoic Sierran magmatic arc [  Dilles ,1992;  Dilles and Gans , 1995;  Surpless , 1999] (Figure 2).These fault blocks expose up to   8.5 km of the preexten-sional upper crust as determined by measuring down-sectionfrom a preserved preextensional Tertiary basal unconformitywhich is tilted   60   westward [ Surpless , 1999]. Apatitefission track and (U-Th)/He thermochronological data from preextensional paleodepth transects across two major blocksin the central Wassuk Range area allow us to assess thetiming of footwall cooling related to extensional deforma- TECTONICS, VOL. 21, NO. 4, 1028, 10.1029/2001TC001295, 2002 1  Now at Geology Department, University of Kansas, Lawrence, Kansas,USA.Copyright 2002 by the American Geophysical Union.0278-7407/02/2001TC001295 10  -  1  tion and to study the thermal structure of the upper crust immediately prior to and during extension. These thermo-chronological methods are particularly powerful when usedtogether and when integrated over a wide range of preexten-sional paleodepths, as they provide time-temperature histor-ies for individual samples over the temperature range 40   – 110  C. The integrated structural and thermochronologicalapproach also provides important insights into the temporalrelationship between extensional faulting and volcanism andinto the issue of whether extensional faulting in the centralWassuk Range area was continuous or episodic in naturethrough time. 2. Geological Setting [ 4 ] Tertiary extensional faulting along the western mar-gin of the Basin and Range province at the latitude of LakeTahoe is characterized by a series of tilted fault block  bounded by east dipping normal faults [  Moore and Arch-bold  , 1969;  Proffett  , 1977;  Stewart et al. , 1984;  Surpless ,1999] (Figure 1). The Wassuk Range trends approximately N-S and is bound on its eastern flank by a major east dipping, high-angle normal fault. The central Wassuk Range consists of Jurassic and Cretaceous intrusive rock and Triassic and metavolcanic rocks thought to be correla-tive with metamorphic rocks exposed in the Singatse Rangeto the west [  Bingler  , 1978;  Proffett and Dilles , 1984; Surpless , 1999]. The Mesozoic granitic rocks represent  portions of the former Sierran magmatic arc, dissected byBasin and Range faulting and extension. An unconformity probably developed subsequent to crustal thickening duringthe late stages of Sierran magmatism as a result of LateCretaceous-early Tertiary uplift and erosion [e.g.,  Wernicke ,1992] (Figures 2 and 3). This Late Cretaceous-early Ter-tiary unconformity is overlain by the Oligocene MickeyPass and Singatse rhyolite ash flow tuffs in the Wassuk Range and Singatse Range. These thick Oligocene rhyolitetuffs are thought to be associated with a major southwardsweep of intermediate to silicic volcanism in the northernBasin and Range prior to extension [  Proffett and Proffett  ,1976;  Stewart  , 1980;  Christiansen et al. , 1992;  Glazner and  Bartley , 1984]. The extrusion of the Lincoln Flat andesite at   15 Ma also predates the onset of extensional faulting inthe central Wassuk Range [ Surpless , 1999]. During exten-sional faulting as evidenced by progressive tilting of volcanic units, only minor, localized basaltic andesite flowswere erupted and sedimentation of the early Wassuk Grouptook place [  Dilles , 1992;  Surpless , 1999]. These Tertiarysedimentary and volcanic deposits and their stratal orienta-tions bracket the timing of fault motion, the magnitude of fault block tilting, and the total displacements along indi-vidual normal faults. 3. Structural Geology of The CentralWassuk Range Area [ 5 ] The structural analysis presented here is based ondetailed geologic mapping and compilation by  Surpless [1999], which includes previous geologic mapping by  McIn-tyre  [1990]. The stratigraphy and structural style of faulting Figure 1.  Digital shaded relief map of the western margin of the Basin and Range province with major mountain ranges labeled. Heavy solid lines indicate major present-day, east dipping, range-boundingnormal faults. The white box outlines the map area of Figure 2. The Wassuk and Carson range-boundingfaults are the most seismically active today and display components of both normal and dextral motion. 10  -  2 STOCKLI ET AL.: MIO-PLIOCENE EXTENSION, CENTRAL WASSUK RANGE, NEVADA  476877 .  Normal fault (ball on hanging wall)Foliation measurementBedding measurementDike with dip measurement(Tts) Oligocene silicic ash flow tuffs(Kqm) Cretaceous quartz monzonite(Kqmp) Cretaceous quartz monzonite porphyry(Jhqm) Jurassic hornblende quartz monzonite(Trmv) Triassic metavolcanic rocks(Tertiary basal unconformity)(Qa)(Tws)(Tba)Quaternary alluviumTertiary basaltic andesites (7 Ma)Tertiary Wassuk Group(Tlf) Miocene Lincoln Flat andesite (15 Ma)unconf.unconf.unconf.Sample location11.4b kilometers1 20 3 N 118˚ 52' 30"119˚ 00' E    a   s   t    W    a   l    k   e   r     R   i    v   e   r    38˚ 52' 30"118˚ 45'118˚ 45'38˚45'00" Walker  Lake . .  ? . .. . . .  . ........ ..... ? ... ...   ......... . ............ ................ . .... Tlf  Tlf  TwsTlf TbaTtsTlf Tlf Tlf TtsTwsTwsTwsTlf Tlf TtsTlf KqmJhqmTlf TrmvTtsJhqmKqmpTrmvTrmvTws .. .  TrmvTrmvTtsTws Trmv TtsTrmvTtsJhqmTlf  Tlf  Tlf Tlf TtsQaQaKqmKqmTbaTba Tba Qa Tts Jhqm Tba TbaTba A'A"'A" 6354636350 626168126262 6160606265618686265826138122561356072 59 60686268 5537 353531476882738177307575 747783 WASSUKBLOCK GREY HILLSBLOCKBUCK BRUSHSPRING BLOCK GH-4 A GH-3GH-2GH-1Jhqm11.011.1a11.311.411.511.211.111.1b11.3a11.4a11.2a11.2b11.4b11.4c11.5a11.5b11.611.5c .. 68 QaKqm . TbaTbaTbaTws Figure 2.  Simplified geologic map of the central Wassuk Range and Grey Hills area, consisting of three major tilted fault  blocks: the Grey Hill block, the Buck Brush Spring block, and the Wassuk block (modified after   Surpless  [1999]). A–A 0 and A 00  –A outline traces of cross sections shown in Figure 3.  S T  O C KL I  E T AL  . :  MI   O-P L I   O C E  NE E XT E  N S I   O N , C E  NT RAL  WA S  S  UKRA N GE  , NE  VADA 1   0   -  3   in the central Wassuk Range map area are similar to that documented in the Singatse Range to the west [  Proffett  ,1977;  Proffett and Dilles , 1984;  Dilles and Gans , 1995](Figure 1). In the study area, most normal faults strike northto NNWand have accommodated large-magnitude extensionin a   WSW-ENE direction. These normal faults causedwestward tilting of fault blocks about a N-NNW strikinghorizontal axis, as defined by the westward tilting of Oligo-cene to Miocene volcanic rocks. The Tertiary stratigraphicsection is structurally repeated multiple times across the maparea, defining the Grey Hills, Buck Brush Spring, andWassuk fault blocks from east to west (Figures 2 and 3).The seismically active fault zone along the modern rangefront strikes    NNW and accommodates oblique dextralfaulting that appears to be associated with right-lateralshearing within the Walker Lane belt [ Oldow , 1992;  Stewart  ,1992; Surpless ,1999].ExtensionaldeformationinthecentralWassuk Range is characterized entirely by brittle faulting,with no evidence for mylonitization of any footwall rocks.Extensive cataclastic deformation is best exposed along theactive range front fault system of the Wassuk Range. 3.1. Faulting History of the Central Wassuk Range [ 6 ] Large-magnitude crustal extension in the centralWassuk Range area was accommodated along east dip- ping normal faults that initiated at high angles. These first generation faults have been progressively tilted westwardduring footwall exhumation and subsequently further rotated to very shallow to subhorizontal attitudes byyounger generations of east dipping normal faults[ Surpless , 1999] (Figure 3). Although these early normalfaults are generally poorly exposed in the map area, theyclearly cut and tilt both the preextensional OligoceneMickey Pass and Singatse silicic ash flow tuffs and theMiocene Lincoln Flat andesite flows. Where exposed,these faults are characterized by fault zone silicificationand alteration of volcanic rocks adjacent to fault surfaces.The lack of significant westward tilting during the timespan between deposition of Oligocene rhyolite tuffs andMiocene andesites (Figures 2, 3, and 4) implies that Tertiary extensional faulting postdates the extrusion of the Miocene Lincoln Flat andesite dated at    15.0–14.8Ma [  Dilles and Gans , 1995;  Surpless , 1999]. The LincolnFlat andesite represents the youngest preextensional unit and defines a preextensional paleohorizontal referencedatum (Figures 2 and 3).[ 7 ] During extensional faulting no volcanism is docu-mented in the central Wassuk Range with the exception of asmall synextensional basaltic andesite flow exposed in thewestern Grey Hills. This flow, dated at 14.39 ± 0.04 Ma,unconformably overlies the Lincoln Flat andesite and dipsonly   35   to the west [ Surpless , 1999], suggesting   25  westward tilt during the first    0.4–0.6 m.y. after the onset of extension in the central Wassuk Range. After extrusion of this synextensional flow, no more volcanic rocks wereextruded until the deposition of basaltic andesite flows of the late Miocene Wassuk Group dated at 8.63 ± 1.22 Ma[  McIntyre , 1990;  Dilles , 1992;  Surpless , 1999].[ 8 ] A younger generation of east dipping normal faultsappears to have controlled the sedimentation of earlyWassuk Group strata in small, asymmetric half grabens Figure 3.  Simplified cross sections of the central Wassuk Range (modified after   Surpless  [1999]). Fault  blocks are tilted to the west    60   and expose a cross-sectional view of the upper crust.Thermochronological samples were collected from a range of paleodepths in the Grey Hills and Wassuk  blocks. Samples were not collected from the Buck Brush Spring block because of extensive hydrothermalalteration of the Jurassic quartz monzonite. 10  -  4 STOCKLI ET AL.: MIO-PLIOCENE EXTENSION, CENTRAL WASSUK RANGE, NEVADA  [  McIntyre , 1990;  Surpless , 1999]. Motion along these faultsvaried from dip-slip to dextral oblique slip and is welldocumented in the northern part of the central Wassuk Range, where dextral oblique motion has resulted in theformation of a rhomboidal pull-apart structure within theWassuk Group sediments (Figure 2).[ 9 ] Volumetrically minor basaltic andesite flows wereerupted following deposition of the Wassuk Group [ Surp-less , 1999] (Figures 2 and 3). The oldest of these basalticandesite flows, dated at 6.9 Ma, are now tilted up to  12   tothe west [  McIntyre , 1990] (Figure 3). Similar basaltic ande-site dikes within the Wassuk block dip steeply to the east (78   ± 5  ), providing additional evidence for   12   westwardfault block rotations, assuming vertical dike emplacement (Figures 2 and 3). The episode of tilting beginning after   6.9Ma is ascribed to the active, high-angle normal fault systemthat bounds the east side of the Wassuk Range [ Surpless ,1999]. Modern seismicity and fault scarp morphology Figure 4.  The structural reconstruction of the central Wassuk Range area prior to the onset of large-magnitude extensional faulting at    15 Ma is based on the youngest preextensional stratigraphic marker unit, the Miocene Lincoln Flat andesite (Tlf). Lincoln Flat andesite flows indicate a total post-15 Ma west tilt of   60  . The total extension derived from this palinspastic restoration is >200% [ Surpless , 1999] Thedocumented cumulative magnitude of westward fault block tilting from 15 Ma to the present is consistent with paleomagnetic data from the Wassuk fault block, which indicate at least 60   of westward tilting(J. Oldow and J. Geissman, personal communication, 1997). The preextensional paleodepth estimatesderived from structural reconstruction of the pre-15 Ma geometry of the fault blocks permit the proper interpretation of the thermochronological data. Unit symbols given in Figure 2. STOCKLI ET AL.: MIO-PLIOCENE EXTENSION, CENTRAL WASSUK RANGE, NEVADA  10  -  5
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