Abstract: The Transition from East-Directed Thrusting to West-Directed Backthrusting along the Eastern Front Range, Colorado

Bjorn W. Selvig, Eric A. Erslev

Compressional, basement-cored foreland arches expose a variety of structural geometries on their flanks. In the southeastern margin of the Front Range in Colorado, the west-dipping Golden fault system thrusts Precambrian crystalline rocks over the sedimentary strata of the Denver Basin. Further north, however, a broader zone of spaced, northeast-dipping thrusts are directed toward the range and appear to be backthrusts off a blind detachment in the basement.

The transfer zone between these distinct structural responses to Laramide shortening occurs between Golden and Boulder. Fault slip analyses of 2150 minor faults indicate synchronous east-directed thrusting and west-directed backthrusting. A recent seismic profile through the range front and the Rocky Flats nuclear facility shows two major, west-dipping thrusts bounding a rotated wedge of Phanerozoic strata. Cross-section balancing and minor faulting indicates that slip on the blind lower thrust is dissipated on west-directed backthrusts parallelling the east-dipping strata of the thrust wedge. To the north, the east-directed thrusts diverge and the intervening, more moderately tilted panel is broken by oblique, basement-involved backthrusts.

The eastern Front Range and many other arch margins can be viewed as basin-directed shear zones of variable surface width. Where the shear zones narrow, basin-directed thrusts create substantial basement overhangs and backthrusting is largely limited to the sedimentary strata. Where the shear zones widen, spaced, basement-involved backthrusts within the shear zones rotate strata "domino-style" to form the repeated hogbacks characteristic of many arch margins.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994