SP 2002 BULLETIN Paper
Paleomagnetic and 40Ar/39Ar Geochronoloic Data Bearing on the Structural Evolution of the Silver Peak Range, West-Central Nevada
Michael S. Petronis and John W. Geissman
Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131-1116, John S. Oldow Department of Geology and Engineering, University of Idaho, Moscow, ID 83844, William C. McIntosh New Mexico Technical Institute, New Mexico Geochronological Research Laboratory, Socorro, NM 87801.
Accepted to the GSA Bulletin
The Silver Peak Range, west-central Nevada, has recently been reinterpreted as a major transfer link between the Furnace Creek-Fish Lake Valley and Walker Lane fault zones (Oldow et al., 1994). Late Neogene, northwest-directed extension in the Silver Peak Range resulted in the exhumation of the Mineral Ridge metamorphic core complex along the Mineral Ridge-Lone Mountain detachment fault with inferred northwest-side-up tilting of the footwall. Paleomagnetic investigations of Tertiary pyroclastic, sedimentary, and, until recently, undated mafic dikes and sills in the Silver Peak Range were conducted to constrain the magnitude of horizontal-axis tilting and vertical-axis rotation associated with dextral shear and detachment faulting related to displacement transfer in the region. In addition, we have obtained some high-quality 40Ar/39Ar age spectrum data that is consistent with existing fission track and K/Ar isotopic age data. We report paleomagnetic results from 123 sites in the Silver Peak Range and eight 40Ar/39Ar dates from mafic intrusions from the footwall of the Mineral Ridge metamorphic core complex. Eight to ten oriented samples from 123 sites have been demagnetized with 106 sites yielding interpretable results; 69 sites in mid-Miocene mafic dikes and sills, 24 sites in upper Miocene to Pliocene pyroclastic rocks, and 13 in mid-Miocene lacustrine sedimentary rocks. The dual polarity results from some mafic intrusions are interpreted to indicate moderate northwest-side-up tilting of the Mineral Ridge footwall rocks (D = 329°, I = 37°, a95 = 4.3°, n = 30), and some results, all normal polarity, indicate a modest degree of clockwise vertical-axis rotation (D = 021°, I = 57°, a95 = 4.3°, n = 19) concurrent to shortly after uplift of the footwall rocks. Upper Miocene pyroclastic rocks yield single polarity data that also are interpreted to indicate a moderate degree of clockwise vertical-axis rotation (D = 032°, I = 53°, a95 = 8.8°, n = 10), although this data set may be biased by incomplete sampling of the geomagnetic field. Tertiary sedimentary rocks, unfortunately, appear to have been remagnetized during of shortly after extension. Five groundmass concentrates of the eight mafic intrusion samples submitted for 40Ar/39Ar dating yield saddle-shape age spectra and one separate yielded a moderately stable plateau age but of low confidence. Isochron analysis reveals that all six groundmass concentrates contained excess Ar, and the preferred cooling estimates of the intrusions are the isochron dates. Overall, the 40Ar/39Ar isochron values suggest mafic dike and sill emplacement between 13 to about 10.5 Ma. These data support the hypothesis that the intrusions were emplaced into an actively deforming footwall concurrent with rapid exhumation of the footwall rocks and may explain the overall dispersion of the paleomagnetic data from these rocks. If the vertical-axis rotation of the Silver Peak Range proves to be valid, as at least to a limited degree substantiated here, this will be the first documented case of vertical-axis rotation associated with displacement transfer in the central Walker Lane Belt.
DV 2002 JGR Paper
Assessing Vertical-Axis Rotations In Large-Magnitude Extensional Settings: A Transect Across The Death Valley Extended Terrane, California
Michael S. Petronis and John W. Geissman
, Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico, Daniel K. Holm Department of Geology, Kent State University, Kent, Ohio, Brian Wernicke and Edwin Schauble , Division of Geological and Planetary Sciences, CalTech, Pasadena, California
January 2002 Journal of Geophysical Research
Models recently proposed for Neogene crustal deformation within the central Death Valley extended terrane, southeastern California, differ markedly in their estimates of the degree of true crustal thinning and on the relative importance of shear translations. Documentation and realistic interpretation of vertical-axis rotations of range-scale crustal blocks (or parts thereof) are critical when attempting to reconstruct the kinematic history of this highly-extended region. To better define the magnitude, areal extent, and timing of vertical-axis rotation in parts of this area, paleomagnetic data were obtained from appropriate rocks along a roughly east-west traverse parallel to about 36°N latitude. Sites were established in ca. 7 Ma volcanic sequences (Greenwater Canyon and Brown's Peak) and the ca. 10 Ma Chocolate Sundae Mountain granite in the Greenwater Range, ca. 8.5 Ma and 5 - 4 Ma basalts on the east flank of the Black Mountains, the 10.5 Ma Little Chief stock and 11 (?) Ma basalts in the eastern Panamint Mountains, and Paleozoic Poginop Group carbonate strata in the central Panamint Mountains.
At the site level most materials yield readily interpretable paleomagnetic data. Group mean directions, after moderate structural corrections (where appropriate), are interpreted to indicate no major vertical-axis rotation of the Greenwater Range (D = 359°, I = 46°), no post-5 Ma rotation of the eastern Black Mountains (D = 006°, I = 61°), and no post-10 Ma rotation of the Panamint Range (D = 181°, I = - 51°). Data from the Little Chief stock and adjacent (remagnetized) Paleozoic carbonates are consistent with moderate east-side-down tilting of the Panamint Mountains. Paleomagnetic data (and published paleocurrent data) suggest that the Panamint Mountains shared none of the 7 Ma to recent clockwise rotation of the Black Mountains crystalline core as proposed in recent transtensional models.
Download the complete PDF published in January 2002 JGR

SP 1999 AGU Abstract
40Ar/39Ar Age Determinations on Mafic Dikes and Sills in the Silver Peak Range, west-central Nevada and Their Bearing on Paleomagnetic data
PETRONIS, Michael S.,
Exxon Exploration Company, 233 Benmar Pl., Houston, TX 77386, michael.s.petronis@exxon.sprint.com, GEISSMAN, John W., Dept. of Earth & Planetary Sciences, Univ. of New Mexico, Albuquerque, NM 87108, and OLDOW, John S., Dept. of Geology and Engineering, Univ. of Idaho, Moscow, Id 83844, MCINTOSH, William, C., New Mexico Technical Institute, New Mexico Geochronological Research Laboratory, Socorro, NM 87801.
Late Neogene, NW-directed extension in the Silver Peak Range (SPR) resulted in the exhumation of the Mineral Ridge metamorphic core complex with inferred NW-side-up tilting of the footwall. Until recently, high-quality geochronology was lacking for rocks in the region the thermal evolution of footwall rocks and timing of uplift were provided by limited zircon and apatite fission track (Kohler et al., 1994) and K/Ar data (Albers and Stewart, 1972; Edwards and McLaughlin, 1972). Although undated at the time, a total of 76 paleomagnetic sampling sites in mafic dikes and sills in footwall rocks of the Mineral Ridge metamorphic core complex place limits on the magnitude of extensional deformation in the region (Petronis et al., 1998). New 40Ar/39Ar dates on mafic intrusions from the Mineral Ridge area of the SPR allow a reinterpretation of the paleomagnetic data.
Eight samples of least-altered intrusions with different structural attitudes, bulk composition, and demagnetization behavior were dated at the New Mexico Geochronological Research Laboratory. Microprobe analysis revealed minor to moderate chloritic alteration of hornblende. For two samples, hornblende separates were used; the remaining six samples were analyzed as groundmass concentrates. Five of the groundmass concentrates yield saddle-shaped age spectra. Isochron analyses reveal that all six groundmass concentrates contained excess Ar with 40Ar/36Ar ratios ranging from 300 to 399. The inferred cooling ages of the intrusions are approximated by the isochron dates in the case of excess Ar. Isochron dates are: SP55, 11.61 +/- 0.44; SP69, 11.56 +/- 0.24; SP75, 11.45 +/- 0.22; SP92, 10.52 +/- 0.21; SP96, 13.05 +/- 0.34; and SP112, 10.86 +/- 0.32 Ma. The hornblende separates yield complex age spectra that are the result of alteration and recoil and do not yield reliable cooling ages. Overall, the 40Ar/39Ar data suggest mafic dike and sill emplacement between 13 and 11 Ma, possibly at very shallow crustal levels. Emplacement was followed by rapid exhumation of the footwall rocks by 11.0 +/- 0.7 Ma (Kohler et al., 1994).
Petronis et al. (1998) proposed a complex, multi-stage emplacement history to explain the orientation of the intrusions and the paleomagnetic data. In light of the new 40Ar/39Ar age determinations, we infer that remanence acquisition most likely occurred in the middle Miocene. The distribution of in situ site mean data may reflect dike emplacement into the actively deforming footwall rocks during rapid uplift associated with extension. Nonetheless, the overall distribution of the data preclude either substantial tilting or vertical-axis rotation of the footwall rocks of the Mineral Ridge metamorphic core complex.
SP 1998 AGU Abstract
Paleomagnetic Data Bearing on Vertical Axis-Rotation Associated with a Simple-Shear Transfer System in the Silver Peak Range, West-Central Nevada p>
PETRONIS, Michael S., GEISSMAN, John W.,
Dept of Earth & Planetary Sciences, Univ. of New Mexico, Albuquerque, NM 87108, zar1kog1@ev1.net, and OLDOW, John S., Dept. of Geology and Engineering, Univ. of Idaho, Moscow, Id 83844.
Models proposed for large-scale continental crustal extension must consider the role and importance of vertical-axis rotation of range-scale crustal blocks associated with simple-shear transfer systems. Identifying and proper interpretation of vertical-axis rotations are critical when attempting to understand the kinematic history of a highly-extended region. The Silver Peak Range (SPR), Nevada, has recently been reinterpreted as a major transfer link between the Furnace Creek-Fish Lake Valley (FC) and Walker Lane Belt (WLB) fault zones (Oldow et al., 1994). Late Neogene, NW-directed extension in the SPR resulted in the exhumation of the Mineral Ridge metamorphic core complex with inferred NW-side-up tilting of the footwall. NE-trending, left-oblique faults bounding the SPR with strikes at high-angle to the FC-WLB probably have facilitated displacement transfer across the region since the mid-Miocene. We have attempted to quantify horizontal-axis tilting and vertical-axis rotation from critical parts of the SPR with paleomagnetic data from appropriate rocks that we infer to have spanned much of the period of extension in the area. Sites were established in Tertiary (?) mafic intrusive rocks (as dikes and sills) (Td), upper Miocene to Pliocene pyroclastic rocks (Tv), and mid Miocene lacustrine sedimentary rocks (Ts3). Eight to ten oriented samples from 123 sites across the SPR have been demagnetized with 106 sites yielding interpretable results; 69 in Td, 24 in Tv, and 13 in Ts3. After removal of a recent viscous overprint (by 10-25 mT and 100-300) most samples yield one well-defined magnetization that is well-grouped at the site level. Td yield two statistically distinct magnetization directions (1) NW-declination, moderate positive inclination (329, 37, a95=4.3, k=37.6, n=30 sites (25N, 5R)) and (2) NNE-declination, moderate positive inclination (021, 57, a95=4.3, k=51.1, n=22 sites (all N)). The remaining 17 sites yield directions similar to the present day field. Tv yield two statistically distinct magnetization directions (1) NE-declination, moderate positive inclination (032, 53, a95=8.84, k=22.9, n=13 sites (all N)) and (2) NW-declination, moderate positive inclination (327, 49, a95=13.9, k=31.3, n=5 sites (3N,2R)). Ts3 yield both normal and reversed polarity directions, which after simple structural corrections indicate that Ts3 sediments were remagnetized during or after deformation in the area. NW-declination, moderate positive inclination data from Td are consistent with NW-side-up tilting of the footwall during NW-directed extension. Assuming these are younger magnetizations, the NE-declination, moderate positive inclination results are interpreted to suggest modest clockwise rotation of parts or all of the SPR following NW-directed extension. The SPR data suggest that vertical-axis rotations, as well as titling, of range-scale crustal blocks may be an intergral part of a simple-shear transfer system.
SP 1997 GSA Abstract
Paleomagnetism and structural history of the Silver Peak Range, west-central Nevada
PETRONIS, Michael S., GEISSMAN, John W.,
Dept. of Earth & Planetary Sciences, Univ. of New Mexico, Albuquerque, NM 87108, zar1kof1@ev1.net, and OLDOW, John S., Dept. of Geology and Engineering, Univ. of Idaho, Moscow, Id 83844
The Silver Peak Range (SPR), west-central Nevada, has recently been reinterpreted (Oldow et al., 1994) as a major accommodation zone at the northern terminus of the NW-trending Furnace Creek-Death Valley fault zone (FCDVFZ). Late Miocene extension resulted in the exhumation of the Mineral Ridge metamorphic core complex with inferred NW-side-up tilting of the footwall. The NE-striking Emigrant Peak fault on the NW and the Clayton Ridge fault on the SE margin of the range are active and probably facilitated displacement transfer between the FCDVFZ and Walker Lane Belt. To better understand the structural development of the SPR, paleomagnetic data have been obtained from Tertiary (?) lower plate mafic dikes (Td), Miocene silicic volcanic rocks (Mv), and Miocene-Pliocene lacustrine strata (Mlk). These data can be used to asses if the Emigrant Peak-Clayton Ridge faults are Riedel shears (R') off the FCDVFZ. If so, significant clockwise vertical-axis rotation of the range should be expressed as a paleomagnetic declination anomaly. In the absence of evidence of vertical-axis rotation, these faults may be interpreted as normal faults roughly perpendicular to the maximum extension direction.
Eight to ten oriented samples from each of 54 sites across the SPR have been demagnetized; 17 in Td, 24 in Mv, and 13 in Mlk. After removal of a recent viscous overprint (by 10-25mT or 100-300) most samples yield one well-defined magnetization, of dual polarity, that is well-grouped at the site level. Comparison with the Miocene expected direction (normal: 354, +54) reveals the following inferred rotations/tilts: Td yield two distinct magnetizations: (1) NW-declination, moderate positive inclination (328.4, +26.0, a95= 5.9, N = 9 sites) and (2) NE-declination, moderate positive inclination (029.7, +49.9, a95= 17.4, N = 5 sites). The NW, moderate positive inclination magnetization is consistent with moderate SE-side-down tilting of the range during NW-directed extension. Mv and Mlk consistently yield NE-declination, moderate positive inclination data (031.4, +54, a95= 8.3, N = 19 sites), suggesting a modest clockwise, vertical-axis rotation of at least parts of the range. We tentatively interpret the declination discordance as consistent with left-lateral slip along the Emigrant Peak-Clayton Ridge faults.
DV 1997 AGU Abstract
Paleomagnetic Data Bearing on Vertical-Axis Rotation Within the Central Death Valley Extended Terrane
M S Petronis
(Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131; zar1kof1@ev1.net); D K Holm (Department of Geology, Kent State University, Kent, OH 44242); J W Geissman (Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131); E Schauble, B P Wernicke (Both at: Div. of Geological and Planetary Sciences, California Inst. of Technology, Pasadena, CA 91125
Models recently proposed for Neogene crustal deformation within the central Death Valley extended terrane differ markedly in their estimates of the degree of true crustal thinning and on the relative importance of shear translations. Documentation and proper interpretation of vertical-axis rotations of range-scale crustal blocks (or parts thereof) are critical when attempting to reconstruct the kinematic history of this highly-extended region. To better define the magnitude, areal extent, and timing of vertical-axis rotation in parts of this area, paleomagnetic data were obtained from appropriate rocks along a roughly east-west traverse parallel to 36(N latitude. Sites were established in 7 Ma volcanic sequences (Greenwater Canyon, GC and Browns Peak, BP) and the 10 Ma Chocolate Sundae Mountain granite (CSMG) in the Greenwater Range (GR), ~ 8 Ma and 5-4 Ma basalts on the east flank of the Black Mountains (BM), the 11.8 Ma Little Chief (LC) stock in the Panamint Mountains (PM), 11 Ma basalts on the east flank of the PM, and lower Paleozoic carbonate strata in the central PM. Eight to ten independent samples from each of over than 65 sites across the central Death Valley region have been demagnetized. Typically, after removal of a recent viscous overprint (by 10-25 mT or 100 to 300(C) most samples yield one well-defined magnetization that is well grouped at the site level. After moderate structural corrections (where possible) the following results have been obtained:
Locality D I a95 k N(Sites)
---------------------------------------------------------------------------------------------
GC 359 +46 8.0 30.6 12 (7N, 5R)
BP 352 +52 12.8 23.1 7 (4N, 3R)
CSMG (in situ) 332 +36 10.4 22.7 10 (8N, 2R)
8 Ma BM basalts 038 +61 14.0 41.8 4 (3N, 1R)
4-5 Ma BM basalts 006 +62 4.0 93.2 15 (9N, 6R)
LC stock (in situ) 155 -48 6.5 51.3 10 (all R)
11 Ma PM basalts 157 -27 7.9 43.1 9 (all R)
Plz carb (in situ) 303 +29 11.2 36.8 6 (all N)
Plz carb (corrected) 028 +51 20.3 11.8
These data are interpreted to indicate no major vertical-axis rotation of the GR, no post 5 Ma rotation of the BM, and no post 10 Ma rotation of the PM. Data from the Little Chief stock and (remagnetized) Paleozoic carbonates are consistent with moderate east-side-down tilting of the PM. Paleomagnetic data (and published paleocurrent data; Snow & Prave, 1994, Tectonics) suggest that the PM shared none of the 10 to 5 Ma rotation of the BM crystalline core, which is consistent with dextral drag along the southern Death Valley Fault zone, not range-scale block rotations as proposed by recent transtensional models (Serpa & Pavlis, 1996).
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