Timing of Basalt Magmatism in the Ruby and Blacktail Mountains: Implications for the Sedimentologic, Geomorphic, and Tectonic Evolution of SW Montana

Document Type

Grant Proposal

Date Accepted

Summer 2009

Project Description/Abstract

Rocks in the Blacktail and Ruby mountains record evidence of tectonic and geomorphic processes that have shaped this region over the last 50 million years (Ma). Development and growth of these ranges is the subject of ongoing debate. Several workers have suggested this region was characterized by low-lying topography with sediment deposition in a broad, quiescent basin, with mountainous topography forming very recently (~ 4 Ma.). Our work suggests an alternative explanation in which high-standing topography existed as far back as 30-40 Ma with sediment deposition in tectonically active basins.

The origin, nature, and timing of basalt magmatism throughout this region are poorly constrained. This proposal is designed to apply high-precision 40Ar/39Ar geochronology to date basalts within the sedimentary sequence. These ages will provide important constraints on; 1) timing of basalt volcanism, 2) establish age brackets for sedimentary deposits, and 3) elucidate the geomorphic evolution in response to regional tectonism.

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