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40130-B8
Thermochronological Evolution and Timing of Extension in the Hinterland of the Sevier Orogenic Belt, Northeastern Nevada

Allen J. McGrew, University of Dayton

In recent decades northeastern Nevada (especially Elko County) has emerged as one of the richest gold-producing provinces in the world, with the principle phase of gold mineralization in this area dating from Eocene time. However, the paleogeography and regional tectonic setting of northeastern Nevada during this crucial time period remains poorly understood. The results of this study, currently in preparation for publication, shed light on this important problem by documenting that significant regional extension initiated concurrently with a widespread volcanic episode during Late Eocene time.

As reported in an abstract submitted for the 2007 Geological Society of America Annual Meeting, newly acquired U-Th/He and ⁴⁰Ar/³⁹Ar radiometric age data confirm previous years' results indicating that the Copper Mountains region in northernmost Elko County, Nevada records the earliest onset of extension in Elko County, closely following the initiation of major regional ash-flow volcanism at approximately 45 Ma. In addition, these results unfold the subsequent evolution of that extension, documenting a second pulse of extension in the same area in Oligocene time (esp. from ~32.5 to 29.5 Ma). However, the early extension appears to be localized to the Copper Mountains and the Ruby Mountains-East Humboldt Range metamorphic core complex as most other deeply exhumed terrains and well-dated normal faults in northeastern Nevada did not become active until later in Cenozoic time. In particular, new U-Th/He and ⁴⁰Ar/³⁹Ar age data from the Pilot and northern Toano Ranges in eastern Elko County indicate that contrary to the inferences of some earlier workers, significant exhumation and presumably extension in that area did not begin until Miocene time.

In addition to the above results, field work involving an undergraduate coworker during the summer of 2007 provides new insights into the paleogeographic evolution of northeastern Nevada during the crucial Middle to Late Eocene time frame. During this field work, a linear belt of inferred Eocene ignimbrite deposits that had never before been mapped in detail were delineated through the southern part of the Jarbidge Wilderness Area in northernmost Elko County. Following a new interpretation for the Eocene paleogeography of northeastern Nevada (Henry, in press), we hypothesized that this belt of ignimbrites filled a deep, eastward-draining paleovalley incised into an Eocene highland analogous to the modern-day Andean Altiplano. Our new mapping of this area documents that the ignimbrites bury previously unrecognized stream gravels, and the large-scale geometry leaves little alternative to the buried valley interpretation. The impressive depth of the buried valley system implies that it was incised a kilometer or more into the underlying pre-Eocene bedrock, eroding through an overlying thrust sheet of western facies rocks down into the subjacent parautochthon. The impressive depth of this paleovalley system provides a minimum estimate on the paleoelevation at the onset of regional volcanism in Middle Eocene time. Visual correlation of the ignimbrite with previously mapped units in Copper Basin several kilometers to the west suggests an age of ~42.5 Ma for the ignimbrite unit, just before the accumulation of the Copper Basin lacustrine deposits.

Taken all together, these results support a new regional synthesis for Eocene tectonics and paleogeography in northernmost Nevada. By Middle Eocene time, northern Elko County was a high-standing plateau (“the Nevadaplano”) externally drained by >1 km deep river valleys flowing eastward into the Wasatch Basin. With the onset of voluminous ash-flow volcanism at approximately 42-45 Ma and the initiation of extension and basin subsidence on the Dead Horse fault soon thereafter, this drainage system was blocked, forming Dead Horse Lake in the Copper Basin area. After a period of quiescence, renewed extension during Oligocene time resulted in rejuvenation of the Copper Mountains and progradation of the Meadow Fork alluvial fan sequence over the older Dead Horse Formation lake deposits. This older extensional system was subsequently buried by Miocene volcanic rocks and dissected by younger, more regionally distributed normal fault systems.

To date, this grant has supported six undergraduate students, leading to two senior theses (with a third expected in the Spring of 2008) and four presentations at regional and national meetings of the Geological Society of America (two with undergraduate co-authors) and an undergraduate presentation at the Ohio Academy of Sciences. The next year should see an additional presentation at a regional meeting of the Geological Society of America and the submission of a major synthesis targeted for publication in the Geological Society of America Bulletin.

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