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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

This was the final year of research supported by this grant, operating under a time extension.  Nevertheless another undergraduate student, Joshua Vance, was supported in the completion of a senior thesis and new research results were obtained.   Joshua has now gone on to graduate school in volcanology.  In total this grant has supported or partially supported seven undergraduate students, three of whom completed undergraduate theses and went on to graduate school.  In addition to the undergraduate theses, four abstracts with undergraduate co-authors were presented at meetings of the Geological Society of America, and three publications are planned or in preparation.The work of the past year represents a shift from previous years’ work.  We focused on testing a recently proposed model for Mid-Cenozoic paleogeography of the northeastern Great Basin that envisions an Altiplano-type highland deeply incised by eastward-draining paleovalleys that acted as distributary channels for major Middle to Late Eocene ignimbrite flows.  By conducting new geologic mapping in the southern part of the Jarbidge Wilderness Area in northeastern Nevada we were able to support this hypothesis by delineating a deeply incised canyon system filled with Middle Eocene ignimbrite.  The Jarbidge Wilderness is thickly blanketed by Early Middle Miocene Jarbidge Rhyolite, but in the southern part of the area erosion exposes an underlying terrain of stacked thrust sheets with far-traveled deep marine rocks thrust over Mississippian to Triassic rocks which are in turn thrust over Ordovician shelf strata.  Twisting through this terrain is a discontinuous linear belt of distinctive pink ignimbrite ~30 km long, 1 - 3 km wide and up to >300 m thick that we correlate with a similar sequence bracketed between 41.3 Ma and 42.7 Ma in the Dead Horse tuff in Copper Basin 10 km to the west.  This ignimbrite appears to be confined to a previously undescribed paleovalley that must have eroded through the upper thrust sheet (at least 500-600 m thick) before incising 300 m into the underlying thrust sheet.  Further supporting this interpretation, detailed mapping of the basal contact of this unit near the headwaters of Camp Creek revealed that a single massive cooling unit of the ignimbrite is underlain by a previously unrecognized sequence of sedimentary breccia.  Taken together, these results delineate a Late Eocene paleovalley system at least 900 m deep consistent with the inference of a deeply incised highland that may well have had the potential energy needed to drive regional extension of the hinterland of the Sevier orogenic belt.  Thus these exposures preserve a valuable snapshot of Eocene paleogeography at a critical juncture synchronous with the ignimbrite flare-up and the transition from Contraction to extension at the end of Laramide time.

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