Exhumation and Displacement along Extensional Detachments in the Colorado River Extensional Corridor

Michael Prior, Colorado State University
John Singleton, Colorado State University
Daniel Stockli, The University of Texas at Austin
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Extensive (U-Th)/He data collected from metamorphic core complexes across the Colorado River extensional corridor over the last ~15 years, in concert with hanging-wall stratigraphy and geochronology, provides constraints on initial geometries, displacement magnitudes, and the timing of successive breakaway fault development along low-angle normal faults that supports a rolling hinge model. We focus on examples from the Eagle Eye detachment and the Buckskin detachment.

Zircon (U-Th)/He ages (ZHe) from 31 samples along a ~55 km extension-parallel transect in the Harquahala Mountains preserve a complete profile of preextensional, partially reset, and completely reset cooling ages along the Eagle Eye detachment. An inflection point in this profile ~34 km SW of the present detachment fault records the minimum fault displacement and initial fault slip at ~22-20 Ma. Exhumation of the zircon partial retention zone requires ~8-12 km of displacement along a 60°-35° dipping detachment, in addition to the ~34 km derived from the spatial extent of reset ZHe ages, resulting in total displacement of ~42-46 km. This estimate agrees with a lithologic correlation between plutonic footwall rocks in the Little Harquahala Mountains and preserved clasts within Miocene breccia ~45 km away, at the NE end of the Harcuvar Mountains. Restoration of apatite and zircon (U-Th)/He samples to a paleodepth at ~21 Ma is best fit by a 34 ± 9° fault dip in the upper ~7 km of the crust, supporting detachment slip at initially low angles.

Further to the NW, ZHe ages from Buckskin Mountains record initial extension along the Buckskin detachment system at ~21-20 Ma, whereas synextensional hanging-wall conglomerates dominated by mylonitic clasts record initiation of successive breakaway faults. Cessation of tilting in the westernmost Buckskin Mountains, and deposition of a coarse conglomerate entirely comprised of mylonitized Swansea Plutonic Suite clasts that have apatite (U-Th)/He ages of 16.8 ± 1.0 Ma, is interpreted to record footwall surface exposure along a secondary breakaway at ~16.5 Ma. In the Lincoln Ranch basin to the NE, the Sandtrap Conglomerate has a maximum depositional age of ~14.5 Ma based on U-Pb dating of underlying tuffs, and is also dominated by mylonitic clasts. We interpret that the Sandtrap Conglomerate records footwall surface exposure along a tertiary breakaway at ~14.5 Ma. A linear relationship between timing of initial and successive breakaways, and distance from the current location of the Buckskin detachment, indicates a "fault propagation" rate of ~5 km/Myr. The propagation rate overlaps with slip rate estimates of ~2-6 km/Myrs suggesting that a rolling hinge style progression of fault initiation kept pace with fault slip.

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