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Evolving Dynamic Subsidence in the Late Cretaceous of North America from Geodynamic Inverse Models

Liu, Lijun 1; Spasojevic, Sonja 1; Gurnis, Michael 1
1 Seismo Lab, Caltech, Pasadena, CA.

Using an adjoint mantle convection model that assimilates global seismic tomography and plate motions, we reconstruct the subduction of the Farallon plate beneath North America back to 100 Ma. The associated surface dynamic topographies allow reproduction of paleoshorelines, sediment isopachs and tectonic borehole subsidence, whose spatial and temporal evolution constrains the depth dependence of mantle viscosity and buoyancy.

In our preferred model that satisfies stratigraphy, the Farallon slab was flat lying in the Late Cretaceous, where both its shape and location match long-standing geological reconstructions of tectonic compression (Laramide, basement-cutting faults). Specifically, the flat slab formed north of restored Baja California at about 96 Ma, migrated to west of the Colorado Plateau at about 92 Ma, and stopped its eastward translation at 84 Ma when the subduction front of the Farallon slab halted in eastern Colorado.

The recovered evolution of the flat slab is consistent with recent study of dynamic subsidence over the Rocky Mountain sedimentary basins by Liu and Nummedal (this session). They find little dynamic topography prior to 91 Ma, corresponding to our model before flat slab formation, and that the subsidence depocenters migrated from Utah to Colorado during 91-80 Ma, as predicted with a regional dynamic topography low traversing the Uinta basin in the west to the Denver-Julesburg basin to the east when the flat slab under thrust this region.

The predicted dynamic topography over the Colorado Plateau suggests that the plateau subsided by ~1 km in Late Cretaceous associated with the flat slab below, and started to uplift around 60 Ma due to the demise of flat lying subduction. The plateau reached its present elevation no later than 40 Ma. This is consistent with paleo-botanical studies suggesting high elevations in the Eocene and a recent thermochronology study indicating initial formation of the Grand Canyon in Paleocene.

Since the geodynamic models can be tuned globally with existing vertical motion data (borehole tectonic subsidence, thermo-chronological constraints on rock uplift) and geophysical data (present residual topography, geoid and especially seismic tomography), while fully assimilating plate motions, the method offers the ability to “predict” basin subsidence in frontier areas.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009