Laramide Front Range Tectonics

Nesse, William D. ¹ (1) University of Northern Colorado, Greeley, CO

Detailed E-W structural cross sections across the Colorado Front Range between 38^o 30’ and 41^o 00’ N latitude at 7.5’ intervals show that from 41^oN to ~40^oN (near Boulder) the east flank of the Front Range is bounded by a large east-dipping monocline cut at intervals by relatively small E- to NE-dipping, high-angle reverse faults and fault-propagation folds. From Boulder south, the monocline is discontinuously modified by east-directed thrusts with net slips less than ~3 km. The west flank is bounded by major west-directed thrusts with ~10-15 km of slip and large-scale, often overturned folds. Additional thrusts are found in the North, Middle, and South Park basins and the Laramie River valley.

These observations indicate that the Laramide Front Range was produced by dominantly west-directed thrusting associated with shortening the crust beneath the range. Tectonic uplift relative to the Denver Basin exceeds 6 km. West-dipping thrusts along the east flank are interpreted as backthrusts. Tectonic models that involve vertical uplift on faults whose dip increases with depth on both flanks of the range are not compatible with the geologic data, nor are models that treat the range as a large flower structure on a wrench fault system.

It is suggested that localization of the Front Range and related Laramide uplifts in the foreland basin was a result of late Cretaceous - early Tertiary foundering of the flat-subducted Farallon Plate beneath the foreland basin well east of the Sevier fold/thrust belt and related tectonic activity along the active margin of the continent.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana