Dissertation/Thesis Abstract

Paleomagnetic Analysis of Vertical Axis Rotation Along the Tascotal Mesa Fault Zone in Far West Texas
by Helesic, Jacob Billie, M.S., , 2020, 107; 28086031
Abstract (Summary)

The Tascotal Mesa fault (TMF) is accommodating crustal deformation in dextral strike-slip faulting, synthetic faults propagating from the main strand and synmagmatic extension observed in thickness differences across the fault. The paleomagnetic data presented demonstrate that strain was partitioned and accommodated by vertical-axis rotation of fault-bounded structural blocks in the Alazan Basin. The 70 km TMF is at the eastern margin of the Basin and Range extensional province and functions as a transfer zone within the Rio Grande rift in west Texas. The Oligocene Rawls Formation and Miocene Closed Canyon Formation along the Alazan Basin allow for the study of the TMF’s role in transferring strain across the region through two episodes of late Oligocene and early Miocene tectonism. Quantifiable evidence of 1 km dextral offset is documented by the 35.4 Ma offset rhyolite body at the eastern terminus of the Alazan Basin. Samples collected from stratigraphic sequences of the Oligocene Rawls Formation reveal a time transgressive pattern of clockwise vertical-axis rotation. The oldest Sauceda Member yields a rotation (R) and flattening (F) of (R=48.8° ± 10.8; F=-20.1° ± 9.8), (R=48.0° ± 15.1; F=-3.0° ± 10.2) and (R=29.0° ± 12.9 F=-6.0° ± 9.4), the Rancho Viejo tuff yields (R=8.8° ± 3.8 F=1.5° ± 2.7) and (R=9.5° ± 3.8 F=2.0° ± 3.0) and the youngest Alazan Member yields (R=13.7° ± 11.53 F=31.2° ± 9.45) (R=28.1° ±12.1 F=9.4° ±7.8). Following a 2 Ma hiatus of volcanic activity, the Closed Canyon Formation basalt flows erupted contemporaneously with early Basin and Range extension providing a second episode of early Miocene activity expressed in the Alazan Basin (R=28.1° ±12.1 F=9.4° ±7.8). The documented block rotations are accompanied by anisotropy of magnetic susceptibility (AMS) data of the Rancho Viejo Tuff Member and the interbedded tuffs within the Sauceda Member of the Rawls Formation. These data indicate a dominate northeast transport direction of the Rancho Viejo tuff while the Sauceda interbedded tuffs reveal two transport directions. The two transport directions reveal a potential tectonic uplift reversal within the Sauceda Member deposition or an additional fissure source for the interbedded tuffs. Magnetic polarity reversals and groupings of paleomagnetic directional data help refine geochronological constraints within the magnetic stratigraphy through the Oligocene and Miocene. Quantitative paleomagnetic data reveal a tectonically active Tascotal Mesa fault zone, specifically along the Alazan Basin, through the Oligocene and Miocene.

Indexing (document details)
Advisor: Urbanczyk, Kevin M.
Commitee: Rohr, David M., Kelsch, Jessica M., Petronis, Michael S., Dickerson, Patricia W.
Department: Geology
Subjects: Geology, Continental Dynamics, Geophysics
Keywords: Paleomagnetic, Paleomagnetism, Tascotal, Vertical axis
Publication Number: 28086031
ISBN: 9798664788495
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