ABSTRACT: Heart Mountain Fault: Evidence of a Thin-Skinned Thrust Salient, Park County, Wyoming

CLAREY, TIMOTHY L., Chevron U.S.A. Inc., New Orleans, LA

Remnants of the Heart Mountain fault are spread over 1300 sq. miles (3400 sq. kilometers) in northwest Wyoming, adjacent to and east of Yellowstone National Park. Gravity is commonly accepted as the driving mechanism for fault emplacement. Individual fault blocks were presumably scattered along a 65 mi (105 km) slope, which dipped just a few degrees to the southeast.

A new interpretation of the Heart Mountain fault is proposed, whereby the fault is viewed as a single component of a multitiered, thin-skinned thrust system. This model ties the Heart Mountain fault, the Reef Creek fault, and the South Fork fault into one comprehensive deformational sequence. All three faults exhibit thin-skinned structural geometries, similar to the Wyoming-Utah-Idaho thrust belt. Individual faults either follow bedding-plane surfaces, cut up section in the direction of tectonic transport, or form backthrusts. The identification of tear faults and triangle zones further support the reinterpretation. Thrust sheet emplacement probably occurred during the early to middle Eocene, starting with the Reef Creek fault, then the Heart Mountain fault, and finally, the South Fo k fault. This age relationship of the respective faults is both consistent with observed field relations and thin-skinned thrust theory.

Large-scale gravity sliding is not necessary in this model. All observed structural geometries can be interpreted as developing under an east- or southeast-directed compressional regime. Subsequent normal faulting and gravity readjustments were probably local phenomena, compared to the magnitude of the thin-skinned thrust system.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)