Abstract

The Toquima caldera complex, located in the Toquima Range of central Nevada, consists of three overlapping to nested calderas. The Moores Creek caldera is the largest (∼30 by 20 km); it formed ∼27.2 Ma in response to eruption of the high-silica rhyolite tuff of Moores Creek. Because of recurrent volcanic activity and subsequent basin-range faulting, only the northern segment of the Moores Creek caldera is preserved; its eastern and western margins are downfaulted below valley fill, and its southern part was obscured by collapse of the Mount Jefferson caldera. Eruption of the tuff of Mount Jefferson resulted in collapse of the 18- by 20-km Mount Jefferson caldera ∼26.5 Ma. The ash-flow tuff exposed at Round Mountain is a silicic outflow-facies equivalent of the compositionally zoned (76–67 wt % SiO2) intracaldera tuff of Mount Jefferson. Pyroclastic eruptive activity in the complex concluded ∼23.6 Ma with formation of the comparatively small 8- by 10-km Trail Canyon caldera. With time, caldera size diminished, and the focus of volcanic activity shifted progressively southeastward. The southeastward migration of volcanism in the complex approximately parallels regional northwest-striking faults, suggesting fundamental structural control in the rise and eruption of magma.

Arcuate faults and small, aphyric to porphyritic plugs outline the structural margins of the calderas. Some plugs, on the basis of gradational textural changes from vitroclastic in the tuff to flow-layered and porphyro-aphanitic in the plug, appear to represent lava-choked, ash-flow-tuff, feeder vents. Caldera collapse breccias are locally well exposed in all three calderas.

Caldera resurgence is not strongly developed in the Toquima caldera complex. The Mount Jefferson massif, which dominates the complex at an elevation of 3,640 m, is an artifact of young basin-range block faulting. The limited development of resurgence in the Mount Jefferson caldera is attributed to ponding of the bulk of erupted material within the caldera.

Intermediate to mafic volcanic rocks, premonitory or coeval to silicic ash-flow tuff, are volumetrically minor over central Nevada and are lacking in the Toquima caldera complex. The less silicic magma apparently became lodged in the middle to lower crust below an extensive zone of more silicic, less dense magma.

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