Abstract

Paleomagnetic investigations of Tertiary dikes and Ordovician carbonate rocks at Bare Mountain and in the surrounding region of southwestern Nevada were conducted to further constrain possible rotations related to Walker Lane dextral shear and Basin and Range detachment faulting. Demagnetization experiments revealed two distinct remanence components in the Ordovician carbonate rocks: (1) a dual polarity Permian-Triassic magnetization carried by magnetite that was acquired after Paleozoic or Mesozoic folding but prior to Tertiary tilting; and (2) a dual polarity Miocene magnetization carried by hematite that was acquired after folding and tilting. The Tertiary dikes also contain a dual polarity magnetization carried by magnetite that can be interpreted either as a primary thermoremanent magnetization or a secondary chemical remanent magnetization acquired shortly after dike intrusion. Overall, mean directions for all three magnetizations roughly coincide with corresponding reference directions for Nevada. The largest declination discordance of 16° ± 7° (clockwise) comes from comparison of the Permian-Triassic magnetite magnetization to the Late Permian reference direction for Nevada. Thus, although structural and geothermal data, including zircon fission-track ages, indicate that northern Bare Mountain was tilted 20°–40° northeastward about a horizontal axis during early Tertiary time, we find no evidence for large-scale (i.e., >30°) vertical-axis rotation. On the basis of these results, we suggest that Bare Mountain forms an autochthonous or parautochthonous footwall within the Basin and Range and Walker Lane, in a region where many of the surrounding upper crustal rocks are detached and significantly rotated about vertical and steeply inclined axes. In addition, the nature and timing of footwall uplift indicated by our paleomagnetic results do not support tectonic models that reconstruct Bare Mountain and the Striped Hills as adjacent segments of a continuous fold and thrust belt system. Finally, we suggest that the postfolding Permian–Triassic magnetization at Bare Mountain coupled with Paleozoic zircon fission-track ages indicate that the Meiklejohn Peak thrust predates Jurassic-Cretaceous (Sevier) deformation and may have instead resulted from a Permian or earlier contractional event.

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