Paleomagnetic data bearing on the evolution of the Walker Lane Belt transfer zone from mid-Miocene to present : an investigation of the inferred southern and eastern boundaries

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Grow, Jack
paleomagnetism; Walker Lane Belt; Nevada; transfer zone
thesis / dissertation description
The Walker Lane Belt (WLB) transfer zone, which initiated in the mid-Miocene, presently links the Eastern California Shear Zone in the south to the Central Nevada Seismic Belt and northern WLB to the northeast and north, respectively. The boundaries of the transfer system are clear on the northern and western margins but the extent of the system to the south and east is only inferred. The extent of deformation and development of the WLB transfer zone since the mid-Miocene was examined through a paleomagnetic study of rocks collected at 135 sites, including Neogene volcanic rocks and ashflow tuffs near the inferred southern and eastern boundaries. Results from 32 sites within the inferred southern boundary show a mean declination (D), inclination (I), and ±95 of D = 025.8°, I = 60.6°, ±95 = 4.3°, respectively. This is discordant from the expected mid-Miocene direction of D = 358.8°, I = 58.3°, ±95 = 5.0°, indicating about 27 degrees of clockwise vertical axis rotation. Areas previously thought to lie outside the southern boundary where 22 sites were sampled show about 50 degrees of clockwise vertical axis rotation (D = 041.2°, I = 55.5°, ±95 = 5.8°). One locality near the eastern boundary (southern San Antonio Range) of the transfer zone show about 23 degrees of clockwise vertical-axis rotation (D = 021.5°, I = 53.9°, ±95 = 5.3°, N = 21 sites). Another locality located to the east of the San Antonio Range (i.e., Thunder Mountain) indicates no appreciable rotation (D = 355.8°, I = 64.4°, ±95 = 11.1°, N = 5 sites). Overall, the available paleomagnetic data suggest that the southern and eastern extent of the area affected by modest magnitude clockwise vertical-axis rotation, presumably associated with WLB transfer zone development, was larger than previously expected during the mid-Miocene to mid-Pliocene. Paleomagnetic data also suggest modifications to a testable forward model of the WLB transfer zone. Based on previous paleomagnetic, structural, and geodetic studies of the area, these paleomagnetic data support a transition from more diffuse to localized deformation (forming the Mina Deflection) at about 3 Ma.