TIMING RELATIONS BETWEEN THE SOUTH FORK AND HEART MOUNTAIN FAULT SYSTEMS WITH IMPLICATIONS FOR EMPLACEMENT, WYOMING, USA

The age relationship between the South Fork Fault (SFF) and the Heart Mountain Fault (HMF) has been in question for 90 years. Physical overlap between the two fault systems is largely obscured by later Eocene-age, Absaroka volcanic deposits and by Holocene erosion. Published geologic maps provide insufficient data to discern the order of faulting. Recent identification of the SFF break-away, and an associated denuded zone, has implied that the SFF was the younger system, but the question has remained unresolved.

New field work has concentrated along a small intersection of the two fault systems, at a location where the Castle Tear Fault (CTF) strikes NW-SE along the South Fork Shoshone River valley. Seismic data and well control have demonstrated that the CTF is a high-angle tear in the SFF system, paralleling transport and detaching in the Jurassic. The discovery of Jurassic-age outcrops in the North Fork Shoshone River valley further suggests that the CTF extends north to the SFF break-away. A relatively small, Paleozoic-age carbonate block, derived from the HMF, sits astride the CTF southwest of Sheep Mountain. Cross-sections using orientation data on the east side of the CTF show folding within the carbonate block identical to the underlying, north-dipping ramp fold in the SFF system. Whereas, cross-sections demonstrate that the carbonate block west of the CTF is relatively unfolded or folded dipping westward, again matching the sedimentary rocks beneath in the SFF system. Kinematic data and field relations in the overlying HMF carbonate block suggest “pushing” from below along the east edge of the carbonate block, folding the HMF, as the CTF developed during transport of the SFF system. These newly identified cross-cutting relations, and the earlier discovery of a SFF break-away and a denuded zone, indicate that the SFF is younger than the HMF. Catastrophic loading during emplacement of HMF carbonate blocks, and associated volcanics, may have served to initiate subsequent movement on the SFF system, trapping water in a near frictionless anhydrite-water slurry within the Jurassic Gypsum Spring Formation. Rapid movement along this boundary caused spreading of several large carbonate blocks of the HMF system to the southeast.