TWO STAGE EMPLACEMENT OF THE SOUTH FORK – HEART MOUNTAIN FAULT SYSTEM, NW WYOMING

The Heart Mountain detachment fault in NW Wyoming, the slide surface of the largest known subaerial rockslide, formed ca. 48-50 Ma as an ~1300 km² portion of a large volcanic edifice collapsed. Displacement along a bedding surface in the underlying Ordovician dolomite substrate ramped up into Eocene strata in the direction of sliding. The sliding mass spread and thinned, ultimately advancing more than 30 km into basins to the east and south and covering >3500 km². We propose the displacement occurred as two distinct events: early, slow (cms/yr) gravitative spreading (~10 km displacement) was followed by catastrophic collapse (up to several 10’s of km displacement). During the first phase, slip ramped up to the south to feed displacement to the South Fork fault, a small scale (~10 km shortening) fold/thrust belt exposed along the South Fork of the Shoshone River but not previously recognized as part of the Heart Mountain fault system. Subsequent catastrophic collapse was probably initiated by a cataclysmic eruptive event at the Crandall or New World center. During this entire time, volcanic and volcaniclastic rocks of the Absaroka Supergroup were accumulating.

In support of our assertion that the South Fork thrust roots northwestward into the stratigraphically lower, more proximal, and higher-elevation Heart Mountain bedding-plane detachment, we present a cross-section that is compatible with available surface geology and well data. Several phases of emplacement of allochthons at different levels (and different rates?) are strongly implied. Stratigraphic and structural relationships within the volcanic sequence are not well enough mapped or dated to precisely constrain the sequence or number of major structural events related to South Fork and Heart Mountain faulting. However, evidence that the terminal catastrophic event was later than ~48.5 Ma comes from new Ar/Ar ages of 48.34 and 48.59 Ma from rootless plutons in the upper plate. Study of this remarkable history of collapse may allow estimation of the risk of catastrophic failure of modern slowly spreading volcanoes.