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

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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