Fault block kinematics at a releasing stepover of the Eastern California shear zone: Partitioning of rotation style in and around the Coso geothermal area and nascent metamorphic core complex
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Journal Article: Fault block kinematics at a releasing stepover of the Eastern California shear zone: Partitioning of rotation style in and around the Coso geothermal area and nascent metamorphic core complex
Abstract
Pliocene lavas and sediments of Wild Horse Mesa in the Coso Range, CA exhibit clockwise vertical-axis rotation of fault-bounded blocks. This indicates localization of one strand of the Eastern California shear zone/Walker Lane Belt within a large-scale, transtensional, dextral, releasing stepover. We measured rotations paleomagnetically relative to two different reference frames. At two localities we averaged secular variation through sedimentary sections to reveal rotation or its absence relative to paleogeographic north. Where sediments are lacking we used areally-extensive lava flows from individual cooling units or short eruptive episodes to measure the relative rotation of localities by comparing their paleomagnetic remanence directions to one another. At the western edge of Wild Horse Mesa the fanglomerate member of the Coso Formation (c.a. 3 Ma) exhibits between 8.4° ± 7.8° and 26.2° ± 9.0° (two endmember models of a continuum) absolute clockwise rotation. Within Wild Horse Mesa, 3–3.5 Ma lavas at 5 different localities exhibit about 12.0° ± 4.6° (weighted mean) clockwise rotation relative to the margins of the area, a result statistically indistinguishable from the absolute rotation. Hence the segment of the Eastern California shear zone passing through Wild Horse Mesa has caused vertical axis rotation of fault-bounded blocks as part of the overall dextral shear strain. The magnitude of block rotation at Wild Horse Mesa suggests that rotation has accommodated: 1) 1.5 km of dextral shear along an azimuth of about north 30° west since ca. 3 Ma between the area's bounding faults and 2) 2 km of extension perpendicular to the Coso Wash normal fault during this same period. This corresponds to 13–25% extension across the mesa. In contrast to Wild Horse Mesa, the opposite (western) side of the trace of the Coso Wash normal fault hosts the Coso geothermal area and what Monastero et al. (F.C. Monastero, A.M. Katzenstein, J.S. Miller, J.R. Unruh, M.C. Adams, K. Richards-Dinger, The Coso geothermal field: a nascent metamorphic core complex, Geol. Soc. Amer. Bull. 117 (2005) 1534-1553.) characterize as a nascent metamorphic core complex. Consistent with upper plate disruption above a detachment, surface rocks (i.e. the upper plate of the detachment system) at the Coso geothermal area are tilted westward. However they appear to exhibit no detectable rotation. Thus, the style of block rotation may be partitioned: with clockwise vertical-axis rotation dominating in the Wild Horse Mesa and horizontal axis rotation (tilting) in the geothermal area.
- Authors
- Pluhar, C.J.; Coe, R.S. ; Lewis, J.C.; Monastero, F.C. ; Glen and J.M.G
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Citation
Pluhar, C.J.; Coe, R.S. ; Lewis, J.C.; Monastero, F.C. ; Glen, J.M.G . 1/1/2006. Fault block kinematics at a releasing stepover of the Eastern California shear zone: Partitioning of rotation style in and around the Coso geothermal area and nascent metamorphic core complex. Earth and Planetary Science Letters.
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