Dike emplacement, footwall rotation, and the transition from magmatic to tectonic extension in the Whipple Mountains metamorphic core complex, southeastern California

Abstract The Chambers Well dike swarm and associated plutonic/volcanic rocks in the western footwall of the Whipple Detachment Fault (WDF) provide key insight into the evolution of this metamorphic core complex. New structural and geochronologic data suggest that the western 12–15 km of exposed footwall is steeply tilted to the SW, providing a cross‐sectional view of the upper crust, from the Miocene erosion surface to the top of the coeval mylonitization. Ages and compositions of dikes are indistinguishable from adjacent thick volcanic successions. Several kilometers of early Miocene extension (~20.5 to 19.0 Ma) were accommodated by magmatic accretion but transitioned to rapid extensional faulting and tilting at 19.0–18.5 Ma. The subhorizontal WDF in this area initiated as a northeast dipping high‐angle (50–60°) normal fault that breached the surface locally, not in a breakaway tens of kilometers to the west. Large‐scale tilting and differential uplift of the western footwall was in part coeval with mylonitization and dike emplacement and was accomplished by block rotation in the hanging wall of additional normal faults, isostatic uplift, and flow of lower crust from beneath less extended regions to the west. The WDF is likely a composite surface with a western segment that had ceased moving by ~18.5 Ma, cut by successively younger and steeper fault(s) to the east. Perhaps, the most important difference between seismogenic high‐angle normal faults and low‐angle “detachment faults” characteristic of metamorphic core complexes is one of magnitude and rate of total accumulated slip, not of initial failure conditions.