Paleomagnetic and 40Ar/39Ar geochronologic data bearing on the web.gps. 2009-01-08¢  Department of Earth

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  • For permission to copy, contact editing@geosociety.org q 2002 Geological Society of America1108

    GSA Bulletin; September 2002; v. 114; no. 9; p. 1108–1130; 12 figures; 4 tables; Data Repository item 2002112.

    Paleomagnetic and 40Ar/39Ar geochronologic data bearing on the structural evolution of the Silver Peak extensional complex,

    west-central Nevada

    Michael S. Petronis* John W. Geissman Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116, USA

    John S. Oldow Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA

    William C. McIntosh New Mexico Geochronological Research Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA

    ABSTRACT

    The Silver Peak extensional complex, lo- cated in the Silver Peak Range of west- central Nevada, is a displacement-transfer system linking the Furnace Creek–Fish Lake Valley fault system and transcurrent faults of the central Walker Lane. Late Neogene, northwest-directed motion of an upper plate, composed of lower Paleozoic sedimentary rocks and late Tertiary volca- nic and volcaniclastic strata, exhumed a lower-plate assemblage of metamorphic tectonites with Proterozoic and Mesozoic protoliths.

    Paleomagnetic investigation of Miocene– Pliocene pyroclastic and sedimentary rocks of the upper plate and Miocene mafic dikes in the lower plate reveals modest horizontal- axis tilting (northwest-side-up) and vertical- axis rotation (clockwise) within the exten- sional complex. Eight to ten samples from each of 123 sites were demagnetized; 95 sites yielded interpretable results. Dual- polarity results from one population of mafic dikes in the lower-plate assemblage indicate moderate, northwest-side-up tilting (decli- nation D 5 3298, inclination I 5 378, a95 5 4.38, number N 5 30 sites; in situ) (a95 5 the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction). Some dikes yield exclusively normal-polarity re-

    *E-mail: mspetro@unm.edu.

    sults that are interpreted to indicate modest clockwise vertical-axis rotation (D 5 0218, I 5 578, a95 5 4.38, N 5 19 sites; in situ) concurrent with uplift of the lower-plate rocks, and nine sites yield magnetization di- rections that are north-directed with posi- tive inclinations of moderate steepness, sim- ilar to an expected Miocene field. Late Miocene pyroclastic rocks in the upper plate yield normal-polarity magnetizations suggestive of moderate, clockwise, vertical- axis rotation (D 5 0328, I 5 538, a95 5 8.88, N 5 10 sites). The apparent clockwise ro- tation is unlikely to result from incomplete sampling of the geomagnetic field, because the overall dispersion of the VGP (virtual geomagnetic pole) positions is high for the latitude of the site location. Middle Miocene sedimentary rocks probably were remag- netized shortly after deposition. Of eight 40Ar/39Ar determinations from mafic dikes in the lower plate, five groundmass concen- trates yield saddle-shaped age spectra, and one separate provided a plateau date of low confidence. Isochron analysis reveals that all six groundmass concentrates contain ex- cess Ar. If rapid cooling and Ar retention below ;250 8C are assumed, the preferred age estimate for mafic intrusions is provid- ed by isochron dates and suggests emplace- ment between 12 and 10.5 Ma. The 40Ar/ 39Ar age-spectrum data are consistent with existing fission-track cooling and K-Ar iso- topic age information from lower-plate gra- nitic rocks and indicate rapid cooling of the

    lower-plate assemblage from well above 300 8C to 100 8C between 13 and 5 Ma. Rapid cooling may explain the overall distribution of paleomagnetic results from lower-plate intrusions such that the earliest acquired magnetizations reflect both northwest-side- up tilt and clockwise rotation and the youn- ger magnetizations reflect northwest-side- up tilt. Overall, the paleomagnetic data from the Silver Peak extensional complex are interpreted to suggest that vertical-axis rotation of crustal-scale blocks, associated with displacement transfer in the central Walker Lane, may play an integral part in accommodating strain within a continental displacement-transfer system.

    Keywords: ash-flow tuffs, extensional tec- tonics, geochronology, metamorphic core complexes, paleomagnetism, Walker Lane.

    INTRODUCTION

    Middle to late Tertiary extension in the cen- tral Basin and Range province is manifested in north-northeast–trending mountain ranges and intervening valleys bounded by high- angle faults (e.g., Wernicke, 1988, 1992). Along the western margin of the Great Basin, near its boundary with the Sierra Nevada, the physiographic expression of the tectonic prov- ince changes and is characterized by arcuate- shaped ranges underlying the northwest-trending Walker Lane (Locke et al., 1940). The Walker Lane region has long been recognized as a

  • Geological Society of America Bulletin, September 2002 1109

    STRUCTURAL EVOLUTION OF THE SILVER PEAK EXTENSIONAL COMPLEX

    zone of diffuse intracrustal shear (Turner, 1902), and Gianella and Callaghan (1934) at- tributed deformation to displacement within a complex fault zone, the character of which is similar to that of the San Andreas fault sys- tem. The Walker Lane is seismically active (Wallace, 1984; Rogers et al., 1991) and ex- hibits a complex array of coeval extensional and transcurrent faults (Stewart, 1988; Oldow, 1992) that are kinematically linked to the Death Valley and Eastern California shear zones to the south (Dokka and Travis, 1990) and to the central Nevada seismic belt (Wal- lace, 1984) to the northeast. Combined, the active belts of deformation form part of a dif- fuse continental boundary that accommodates up to 25% of the relative movement between the Pacific and North American plates (Savage et al., 1990; Dixon et al., 1995; Dixon et al., 2000; Miller et al., 2001; Hearn and Hum- phries, 1998; Bennett et al., 1999; Thatcher et al., 1999; Oldow et al., 2001).

    The Walker Lane forms a boundary zone separating the central Basin and Range from the Sierra Nevada, which is moving northwest at 12–14 mm/yr relative to stable North Amer- ica (Argus and Gordon, 1991; Dixon et al., 2000). East of the southern Sierra Nevada, the active belt of deformation is narrow and trends north-northwest. Displacement is accommo- dated by kinematically linked faults bounded on the east and west by the Furnace Creek and Owens Valley fault systems, respectively (Fig. 1A). At the latitude of the central Sierra Ne- vada, the belt of active deformation broadens and bifurcates. From south to north, the Fur- nace Creek and Owens Valley fault systems transfer much of their displacement to the east, to northwest-trending transcurrent faults of the central Walker Lane (Fig. 1A). Farther north, as much as 6 mm/yr of contemporary motion is transferred from the Walker Lane to north-northeast–trending extensional faults of the central Nevada seismic belt (Oldow et al., 2001). Displacement transfer from the Fur- nace Creek and Owens Valley fault systems is marked by seismically active east-northeast– trending transcurrent and extensional faults (Hardyman and Oldow, 1991; Oldow, 1992; Oldow et al., 2001) forming an east-northeast– trending belt .50 km wide and 80 km long (Fig. 1A). The east-northeast–trending faults are part of a large-scale, right step in the dom- inantly northwest-trending dextral fault sys- tem (Oldow, 1992).

    Prior to the 3 Ma initiation of the Owens Valley fault system, displacement transfer was restricted to structures linking the Furnace Creek fault and transcurrent structures of the Walker Lane. The Furnace Creek fault system

    stretches 250 km from eastern California to a terminus in northern Fish Lake Valley east of the White Mountains (Fig. 1A); displacement along this fault system, the site of 50 km of right-lateral strike-slip, steps east to the central Walker Lane by way of east-northeast–trend- ing faults that merge into a shallow northwest- dipping extensional detachment (Oldow, 1992; Oldow et al., 1994). Motion on the de- tachment fragmented an upper plate of supra- crustal rocks and exhumed lower-plate rocks of the Silver Peak–Lone Mountain extensional complex (Oldow, 1992). The upper plate is composed of Paleozoic carbonate and clastic rocks overlain by preextensional Tertiary vol- canic rocks and upper Miocene to Pliocene(?) synorogenic sedimentary and volcanic rocks (Stewart and Diamond, 1990; Prestia and Ol- dow, 2000). The upper-plate assemblage rests on a shallow-dipping detachment-fault system that is folded in doubly plunging, northwest- tending folds with half wavelengths of 5–10 km and amplitudes of 1–1.5 km. Beneath the deformed detachment, the lower-plate assem- blage is composed of metamorphic tectonites recrystallized at amphibolite facies and ex- posed as turtleback structures originally rec- ognized by Kirsch (1968). Metasedimentary rocks have Proterozoic protoliths and were in- truded by late Mesozoic syntectonic granitic rocks (Kohler et al., 1993). On the basis of fission-track analysis of zircon and apatite, in a range of lower-plate rocks, the lower-plate assemblage cooled below 220 8C and 100 8C at 11 and 5 Ma, respectively (Oldow et al., 1994).

    Displacement-transfer systems are well- established features in extensional and strike- slip tectonic regimes (e.g., Duebendorfer and Wallin, 1991), and many are associated with vertical-axis rotation of structural blocks (Syl- vester, 1988). Large-magnitude block rota- tions are increasingly recognized as an inte- gral part of extensional deformation in many tectonic settings (Hudson and Geissman, 1987; Hagstrum and Gans, 1989; Hudson and Geissman, 1991; Snow, 1992; Holm e