FAULT ROCK AND HYDROTHERMAL “PIPES” ASSOCIATED WITH THE ABSAROKA THRUST SYSTEM, WESTERN WYOMING

The Absaroka thrust system is a major tectonic element of the Sevier orogen, with a multi-stage emplacement history from the Santonian to the early Paleocene. The Absaroka thrust is well-exposed in the northern Salt River Range of western Wyoming where it forms the roof thrust of a large, ramp-top duplex fault zone. Mapping and subsurface relationships in this area demonstrate that the Absaroka is out-of-sequence, truncating footwall structures of the duplex. Exposures of the Absaroka thrust contact (juxtaposing Cambrian or Mississippian carbonates over the Jurassic Twin Creek Formation) reveal: 1) a sharp, striated fault contact that truncates earlier breccia, 2) a well-defined core zone of crushed rock with anastomosing shear surfaces (kakirite), and 3) an extensive damage zone in the hanging wall and a less extensive damage zone in the footwall. Breccia fragments in the damage zone were cemented with coarse, syn-tectonic calcite prior to final emplacement of the Absaroka allochthon. Imbricate-fan thrust faults in the hanging wall of the Absaroka show a similar pattern of fault zone deformation consistent with a minimum of two stages of emplacement; early calcite-cemented fault breccia is invariably cross-cut by late-stage, sharp/discrete fault surfaces. In addition, the Absaroka thrust and associated imbricates are overprinted at many outcrops by low-temperature, advective hydrothermal diagenesis, involving coarse breccia with disseminated silica/sulfide mineralization, euhedral saddle dolomite, cauliform/spherulite chert and mini-herkimer quartz, bitumen, and box-work fracturing. Hydrothermal breccia zones (“pipes”) display a variety of geometric attitudes in that they both follow and cross-cut fault surfaces, branch-out parallel to stratigraphy in places, and are controlled by regional fracture sets in other places. Collectively, hydrothermal alteration is interpreted to be a post-kinematic over-print on the Absaroka allochthon driven by Eocene and even present-day high crustal heat flow; warm CO₂-rich brine migrated up-dip to the east through the allochthon and possibly from basement fractures beneath the allochthon, thus over-printing and complicating the interpretation of primary fault zone fabrics.