3D Balancing of Oblique Backlimb-Tightening Anticlines on the Northeastern Flank of

the Front Range Arch, Colorado

By

Corey M. Fortezzo and Eric A. Erslev

Colorado State University, Fort Collins, CO

Steve Holdaway

Colorado State University, now with ChevronTexaco

Backlimb tightening zones (BLTs) are important, often petroleum-bearing areas of fold and fault shortening on the backlimbs of asymmetrical basement arches. BLTs develop due to crowding above areas where regional detachments curve upward into ramp geometries.

The NE Front Range is similar to BLTs on the northeast flank of the Wind River Arch and southeast flank of the Bighorn arch in it’s location on the backlimb of the Front Range arch and the dominance of out-of-the-basin reverse faults. The NE Front Range differs, however, in that its N-S-trend is oblique to the NNW-SSE-trends of its fault propagation fold structures. In addition, the repeated occurrence of smaller N-S anticlines in the footwalls of larger NNW-SSE folds suggests folding was controlled by 3D geometric limitations.

Extensive minor fault analyses show slip and compression directions oriented perpendicular to the NNW-SSW trends of major structures. Minor strike-slip faults are twice as numerous as thrust faults, indicating extension parallel to the fold axes. 2D balancing of regional cross-sections shows slip linkage between the en echelon major structures. More detailed sections of individual structures show out-of the basin slip and shortening consistent with the abundant minor faults. 3D balancing will try to explain the existence of the N-S-trending footwall folds.

The overall geometry of structures on the northeastern flank of the Front Range suggests a BLT zone where regional, ENE-WSW-directed slip occurred on an oblique, N-S striking ramp. The interplay between these structural elements generated numerous anticlines with 3D closure and both past and future petroleum potential.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming