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The Bidahochi Basin hypothesis: Hopi Buttes volcanics and paleolake marks location of local, small-scale lithosphere foundering and asthenosphere upwelling beneath the Colorado Plateau
  • John He,
  • Paul Kapp
John He
University of Arizona

Corresponding Author:johnhe@email.arizona.edu

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Paul Kapp
Univ Arizona
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Spatial correspondence of topographic features, upper mantle structure, and locations of heightened late Miocene exhumation rates has led to the proposal that lithosphere-asthenosphere interaction plays a central role in driving the uplift and incision of the Colorado Plateau. These observations have generally been based on long wavelength features (> c. 250 km) indicative of large scale removal of the Colorado Plateau mantle lithosphere and subsequent asthenosphere upwelling, and has led to the hypothesis that convective removal may be responsible for much of the 2-3 km of Colorado Plateau surface uplift. However, except one currently active downwelling that can be seismically imaged, there is little evidence of the individual occurrences of lithosphere foundering that contributed to the bulk removal of lithosphere. We propose that the lacustrine lower member of the Bidahochi formation in the Navajo Nation may be evidence of such an occurrence. Fine grained lacustrine sedimentation from 16-14 Ma was associated with rapid sediment accumulation, followed by a condensed section deposited from 14-8 Ma, and finally mantle-derived late Miocene to Pliocene magmatism (Hopi Buttes volcanic field) from 8-5 Ma. The Pliocene Hopi Buttes volcanic rocks likely erupted into an ephemeral lake or playa, and are generally nephelinites with up to 10% MgO, originating from high-T decompression melting from as deep as 70 km. Juvenile epsilon Nd values up to +4 indicates at least a partial melt component from an isotopically depleted mantle source. The most plausible explanation for the spatial coincidence and temporal sequence of lacustrine deposition, slowed sedimentation, high-Mg, mantle-derived magmatism is that these are all associated with 1.) descent of lithosphere and associated subsidence, 2.) subsequent lithosphere removal, rising asthenosphere, and local uplift (on the order of tens or hundreds meters), 3.) magmatism derived from asthenosphere upwelling. Alternative hypotheses for the local subsidence at the Bidahochi Basin include deposition in the regional syncline associated with the adjacent Defiance uplift, but this is less likely, given that apatite thermochronology dates indicate that Defiance uplift was rapidly exhumed at c. 55 Ma, far before active lacustrine deposition in the Bidahochi Basin.