UNRAVELING THE INTERRELATIONS BETWEEN MAGMATISM AND EXTENSION: THE SAGA OF THE PRE- TO SYNEXTENSIONAL SEARCHLIGHT MAGMATIC SYSTEM, NORTHERN COLORADO RIVER EXTENSIONAL CORRIDOR, SOUTHERN NEVADA

The Searchlight magmatic system (SMS) is a large Miocene plutonic-volcanic complex stretching across the Colorado River extensional corridor in S. Nevada and NW Arizona. It includes 1) the 10-km-thick ~17.7-15.8 Ma Searchlight pluton (SLP), 2) ~18-16 Ma stratovolcano with intermediate domes and flanking flows and sills, 3) ~16 Ma rhyolite domes and flows, and 4) N-S and E-W dike swarms. The SMS is tilted on end (70-90°) in the S. Eldorado Mountains and exposed in a 15 km thick cross section that includes a Miocene brittle-ductile transition. Two decades of careful research by Calvin Miller and his graduate students have constrained the evolution and anatomy of this thick section of magmatic crust and thus contributed significantly to elucidating the interrelations between magmatism and extension.

The SMS evolved through an abrupt pre- to synextensional transition ~16 Ma. A relatively isotropic early Miocene stress field provided a favorable environment for construction of new magmatic crust, allowing for development of long-lived, highly evolved magma chambers, subhorizontal intrusions, magma mixing, and intermediate volcanism. As extension began, bimodal volcanism became dominant, with felsic volcanism peaking then ending ~0.5 Ma later. Vertical attenuation of mushy plutons and proliferating ~N-S fractures-faults induced rapid evacuation of magma in mostly N-S dike swarms. The rapidly extending crust limited residence time of new mafic magmas, which stymied crustal melting and magma mixing, curtailed intermediate to felsic magmatism, and favored mafic volcanism. However, magmatism locally influenced the stress field, as some E-W dikes and subhorizontal sheets slightly postdate the onset of E-W extension. The pre-extensional lower SLP has a steep, W-dipping magmatic foliation overprinted by a subsolidus coaxial to mylonitic fabric. The fabric restores to subhorizontal and implies intrusion as subhorizontal sheets followed by synextensional vertical attenuation at the onset of extension. An E-dipping normal fault system dissected the SMS, with faults initiating at steep dips and rotating to gentle dips during block tilting. The Dupont Mt detachment fault continued to accommodate slip at a gentle dip, as it may have tapped into a relatively shallow subhorizontal brittle-ductile transition at depth.