GEOLOGIC MAP OF THE ELEVENMILE CANYON CALDERA, STILLWATER RANGE AND CLAN ALPINE AND DESATOYA MOUNTAINS, WEST-CENTRAL NEVADA

A compilation of new and published geologic mapping documents the Elevenmile Canyon caldera and nearby igneous rocks that record a major episode of the mid-Tertiary ignimbrite flareup in west-central Nevada. An early cycle of caldera formation took place ca. 28-30 Ma with formation of the Deep Canyon, Job Canyon, and Campbell Creek calderas. After an ~3 Ma hiatus, a second cycle included the Grover Canyon (25.4 Ma) and Poco Canyon (25.3 Ma) calderas, culminating with the giant Elevenmile Canyon caldera (25.1 Ma). After post-caldera extension, the Elevenmile Canyon caldera is about 40 km N-S by 60 km E-W; intracaldera tuff is up to 4 km thick with a minimum volume of 2500 km³. Voluminous rhyolite lavas erupted throughout the 25 Ma caldera cycle, including immediately before and after the tuff of Elevenmile Canyon; minor intermediate lavas also erupted post-Elevenmile in the Clan Alpine Mountains. Where Miocene faulting in the Stillwater Range unroofed the Oligocene crustal section to depths of ~9 km, the upper crust consists entirely of Cenozoic igneous rocks, including large granitic plutons close in age to the Elevenmile and Poco Canyon calderas; Oligocene granites are also exposed at Chalk Mountain. A prominent, WNW-trending, ~23-21 Ma silicic dike swarm cuts the tuff of Elevenmile Canyon and older rocks; local alteration by these dikes formed several small epithermal Au-Ag deposits including Wonder. Volcanic activity continued with eruption of the Miocene West Gate lavas and finally formation of the ca. 19 Ma Fairview Peak caldera. Oligocene and Miocene igneous rocks are cut and tilted by Miocene and younger normal faults; tilting and strain increasing westward from minor in the Desatoya Mountains to extreme in the Stillwater Range, where the entire Oligocene crustal section is tilted nearly on-end. Sedimentary basin fill and low-temperature cooling ages indicate this episode of extension took place in the early to middle Miocene, accompanied by eruption of mafic to intermediate lavas. This extensional faulting was followed by late Miocene to Pliocene high-angle faulting that formed the modern Dixie and Edwards Creek Valleys. Quaternary deposits in the modern valleys record extensional tectonics, growth and retreat of large Pleistocene pluvial lakes, and episodic alluvial deposition.