The Heart Mountain Thrust

2.5 meters above the base of the Bighorn Dolomite a bedding-plane fault developed covering an area 96km long and 24km wide, causing rocks to slide eastward. The sliding rocks caused a high-angle breakaway zone to develop on the northwest margin of the detatchment, and the rocks moved along a transgressive fault zone (15km long and 3km wide), cutting upsection and distributing upper plate blocks up to 50km across the ground surface! The heart mountain blocks (50) were distributed over a total area of 3400km squared.
One of the most fascinating things about the blocks is that they are not klippes (erosional remnants from a thrust sheet). If one were to add all the heart mt fault blocks they would fit almost perfectly into the place of origin. Therefore, the blocks were emplaced as seperate bodies. The question of how the blocks were transported such a great distance is still under debate. Here are some of the major theories:

• Catastrophic Emplacement: The area from which the blocks originally rested is part of a volcanic center, or hot spot, located below the continental crust. As the plates continuously move (due to tectonics), the spot changes with time, thus allowing the area to experience seperate stages of volcanism. In one of these stages there was a violent eruption (Huckleberry Ridge Eruption in Yellowstone, 2ma), one which makes the Mt. Saint Helens eruption look tiny. In fact, it was to this eruption that paleontologists attribute the death of most of the life living in the vicinity. The eruption was so great that it is said (by some) to have been responsible for literally blasting the fault blocks into thier present locations. This would mean that the blocks would be traveling at an enormous rate of speed and actually 'hovered' over the ground until finally coming to rest in thier present locations. Some have suggested that the blocks traveled along cushions of volcanic gas.
  

  Another catastrophic theory is that of a violent earthquake, one which could have shaken the area so much that the heart mountain fault blocks were literally ripped out of place and tossed into there present locations. However, the physics for this theory does not seem to match up with what the Earth is capable of withstanding, for it seems the Earth would literally fall apart at an earthquake of this proportion!
  
  

• Gradual Emplacement: Another theory is the Landslide. Some suggest that the growth of a strato-volcano (the Ancestral Sunlight Volcano) caused the overlying rock layers to slide off, thus transporting heart mt fault blocks to different locations. The rates that have been estimated for this emplacement are roughly 1 cm per year.

The dikes of the region contain plagioclase, horneblende, and augite and are therefore andesitic. The size and dominance of crystals in these rock bodies vary from outcrop to outcrop. This is due to the presence of a series of dikes, each occuring at different times, the latest ones cutting into the earlier. Dikes cap certain hills in the mapping region, or are repeated continiously down and along a saddle between two peaks. The latter occurance can be interpretted as a set of radial dikes, each of which came from the same magma chamber, which in this case would be that which was located below the Ancestral Sunlight Volcano.

Rocks containing volcanic clasts in a muddy, volcanic matrix are remnants of lahars. Not as common as dikes in this region, lahars are alligned with dikes on some peaks and along saddles in the mapping area. The clast size within lahars vary from flow to flow, and in some places seem to grade from small to larger stratigraphically upward. This represents the presence of seperate lahars occuring each after the previous.

Eocene Volcanism occured elsewhere, outside of the mapping area, in close approximity with the Ancestral Sunlight Volcano. This is evident due to the presence of radiating dikes, which typically occur around a volcanic center. The radial dikes found in the southwestern portion of the mapping area are now located too far from a volcanic center to have originated in the mapped area. Therefore, the thrusting occured after the formation of dikes and lahars (volcanics).

The mere presence of Madison Limestone (Mississippian) resting on younger, Chugwater Fm (Triassic) and Park City - Dinwoody Fms. (Permian) is evidence of the thrust. In the southwest, the volcanics are intermingled with Madison Limestone, resting on Park City - Dinwoody Fms. This is evidence that the volcanics formed elsewhere since they appear to have been transported along with older rock, as a Heart mt fault block.

The simplest evidence that volcanics occured elsewhere, before the thrusting, is on the top of Hill 8038T where an andesitic dike and volcanic lahar are resting unconformably on Triassic Chugwater Fm. Also, the dike ends abruptly at the Chugwater Fm, obviously not rooted in it.