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Our focus during the past year has been on the completion of manuscripts for Tectonics and Geological Society of America Bulletin (both in review). The results of laboratory work are reported in two abstracts presented at the annual meeting of the Geological Society of America in 2009. Byrdie Renik defended her Ph.D. dissertation in September, 2009, and is now a research geologist at ExxonMobil Upstream Research Company in Houston, Texas.

Fish Lake Valley-Northern Death Valley-Furnace Creek fault zone (B. Renik et al., Tectonics)
The Fish Lake Valley-Northern Death Valley-Furnace Creek fault zone, a ~250-km-long, predominantly right-lateral structure in eastern California and western Nevada, is a key component both in displacement budgets along the Eastern California Shear Zone, and in tectonic reconstructions of the Death Valley area and central Basin and Range Province. Our manuscript presents a new synthesis of constraints on dextral offset.

Pre-extensional contractile structures and igneous rocks are found to be the markers that best constrain total displacement at their respective localities (<50 km of offset). Miocene and younger igneous and sedimentary rocks constrain the partitioning of displacement over time. Most motion along the full length of the fault zone is interpreted to be ~13-10 Ma and younger. Average displacement rates over both the full history of the fault zone and shorter time intervals are generally between 3 and 5 mm/yr in the middle section of the fault zone, and drop off toward its tips.

The results bear directly on palinspastic reconstruction of the region. The offset markers imply ~68 ± 14 km of combined extension and strike slip between the Cottonwood Mountains and the Resting Spring-Nopah Range, with post-~15 Ma offset totaling ~60 ± 14 km. This in turn suggests that a previous interpretation of ~104 ± 7 km of displacement between those ranges, based on an alluvial fan interpretation of the middle Miocene Eagle Mountain Formation, is an overestimate by a factor of ~50%.

Sheephead fault zone (B. Renik et al., Geological Society of America Bulletin)
The Sheephead fault is best exposed at Sheephead Pass, where it separates a pervasively broken block of middle Miocene volcanic rocks to the north from comparatively intact Paleoproterozoic gneiss to the south. The collection of fault planes suggests a bookshelf-style, dextral structure, with a component of down-to-the-north, normal motion. The structure was active mainly in late Miocene and early Pliocene time, with offset estimated as between a few kilometers and ~18.5 ± 8.5 km. The latter figure is based upon a kinematic assessment of rotating bookshelf faults.

The Sheephead fault contributes both to plate-boundary dextral shear, and to the partitioning of extension and magmatic dilation. The inferred motion sense, age, and displacement are consistent with the hypothesis of pull-apart deformation between the Sheephead and Northern Death Valley-Furnace Creek fault zone, but this is an incomplete description of the Neogene development of the area, which includes Basin and Range extension. Our data are consistent with modest rather than extreme extension across Death Valley, and inconsistent with the rolling hinge interpretation for the Amargosa fault, despite the significance of this example for the development of the rolling hinge model.

⁴⁰Ar/³⁹Ar geochronology at Sheephead Mountain, California
(M.M. Tremblay et al., GSA Abstracts with Programs, 41(7), 137)
⁴⁰Ar/³⁹Ar geochronology of volcanic flows and tuffs at Sheephead Mountain provides new constraints on the timing of faulting and tilting of blocks in the southeastern Black Mountains on the east flank of Death Valley, California. Provisional ages for the kilometer-thick succession range from ~10.5 Ma to 9.48 ± 0.05 Ma, indicating that much of the eastward tilting and hence faulting took place after ~9.5 Ma. Thickening of the Rhodes Tuff (10.34 ± 0.05 Ma; 10.48 ± 0.05 Ma) and an underlying porphyritic dacite (~10.5 Ma) towards the right-lateral Sheephead fault, which bounds Sheephead Mountain to the south, suggests that faulting initiated during the emplacement of those units (at ~10.5-10.3 Ma). All but one of the quoted ages are based on isochrons calculated from single-step laser fusions of 9 individual biotite crystals. The age of the dacite (stepheating isochron for feldspar) is currently least well constrained. Movement on the Sheephead fault is inferred to have been co-ordinated with normal faulting, and to have ceased in the early Pliocene (by ~3 Ma or earlier), based on the lack of deformation of Lake Tecopa sediments.

Lead isotope constraints on a tectonic reconstruction of southern Nevada
(L. Neiswanger et al., GSA Abstracts with Programs, 41(7), 663)
New Pb isotope data from clasts of 1.45 Ga rapakivi granite in Miocene avalanche breccia at Frenchman Mountain in southern Nevada are remarkably similar to data obtained from their long-accepted source at Gold Butte, 60 km to the east. Samples were crushed and sieved, and feldspar picked from the non-magnetic fraction (210-750 μm). Pb isotope composition was measured by MC-ICP-MS (Multi-Collector Inductively Coupled Plasma Mass Spectrometry), with precisions of 0.03% for both ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb. Average Pb concentrations in feldspar are high (50 ppm), while U and Th are excluded from the crystal lattice. Thus feldspar records the initial Pb isotope composition of the host.

Our results are surprising for three reasons. 1) Extension-related tilting at Frenchman Mountain (from 12-9 Ma) postdates rapid exhumation of Gold Butte (the assumed footwall; 17-14 Ma). 2) New ⁴⁰Ar/³⁹Ar dating of clasts of two-mica granite from the Miocene at Frenchman Mountain indicates exhumation prior to 1.1 Ga (muscovite and biotite ages). The previously assumed source of the clasts at Gold Butte is of Cretaceous age (68-66 Ma). 3) A recent re-evaluation of the crustal structure in a nearby transect from the Meadow Valley Mountains, Nevada to the Beaver Dam Mountains, Utah implies only ~20 km of extension (C.D. Walker, Ph.D., 2008), compared with the generally accepted figure of 54 ± 10 km. Two interpretations of the new data are suggested. 1) The rapakivi granite clasts were derived from Gold Butte, consistent with 65 ± 15 km of WSW tectonic transport of Frenchman Mountain. 2) Regional Pb isotope variations in southern Nevada are sufficiently small that sources other than Gold Butte are possible.