Footwall Rotation in a Regional Detachment Fault System: Evidence for Horizontal‐Axis Rotational Flow in the Miocene Searchlight Pluton, NV

Abstract

The Miocene Searchlight pluton, exposed in the Colorado River Extensional Corridor of southern Nevada, tilted up to 90° on its side in the footwall of the east‐directed Dupont Mountain detachment fault system. Rapid extension and rotation occurred immediately after the ca. 17–16‐Ma emplacement of this 10 × 10‐km granite‐monzogranite body. To constrain the mechanism and timing of rapid footwall exhumation, we conducted detailed field, microstructural, electron backscatter diffraction, and zircon (U‐Th)/He (ZHe) and Ar/Ar hornblende thermochronological analyses. Steeply dipping fabrics across this pluton formed over a range of temperature conditions, from magmatic to high‐ to low‐temperature subsolidus strain, and display distributed eastside‐up shear. ZHe cooling ages are consistent with constraints from tilted volcanic strata and crosscutting dikes that suggest initial rapid rotation (~75°/Myr) at 16.2–15.7 Ma followed by more modest exhumation rates until ca. 13 Ma. Our observations are used to test tilting models for the Searchlight pluton, including rigid‐body rotation, antithetic imbrication, or flow‐like rotation. Available observations are most consistent with a flow‐like tilting mechanism. We present scaling analyses that highlight how footwall tilting‐dominated extension more effectively cools the upper crust than pure‐shear extension because the hottest deep materials exhumed rapidly toward the cooler surface. This extensional mechanism efficiently cools the upper crust, causing a negative feedback whereby the rapidly cooled crust becomes strong enough to halt further fast simple‐shear extension. This may explain why rapid extension was transient and further extension is mostly accommodated by high‐angle low‐offset magnitude normal faults that developed in a colder stronger crust.