The Heart Mountain break-away fault was the last of the four phases of the Heart Mountain detachment fault to be discovered and is the only fault of this type yet recognized and described. The descriptive term, “break-away fault,” was introduced in 1960 for the steep surface along which the rocks of the Heart Mountain decollement fault mass separated from the stationary block on the west.
The break-away fault, located near the northeast entrance to Yellowstone National Park, has a known linear extent of 37 km. The northern one-third trends north, and the remainder trends southeast and south. Rocks on its western side are Paleozoic carbonate rocks and Eocene volcanic rocks; the volcanic rocks are pre–Heart Mountain fault in age. The rocks on its eastern side are younger and consist of Eocene volcanic rocks which were deposited against and over the fault. Thus the break-away fault is unusual in that one side is a fault surface and the other side is a depositional surface, and in that respect is similar to parts of the bedding-plane phase of the Heart Mountain detachment fault.
Slickensides and brecciation in the Madison Limestone adjoining the break-away indicate horizontally directed compressive shear. Movement was probably right lateral, and then laterally southeastward away from the fault at an angle of about 45°.
To the north, in Wolverine Pass and in the valley of Soda Butte Creek, the break-away fault extends down to a bed in the basal part of the Bighorn Dolomite where it joins or terminates at the bedding plane phase of the Heart Mountain detachment fault; movement of upper-plate rocks away from the break-away fault increases from about 0.25 km at the northernmost trace of the fault to more than 1.5 km on the south side of Soda Butte Creek valley. Several kilometres southeastward, in Cache Creek valley, the Heart Mountain break-away fault terminates downward in volcanic rocks of the undivided Lamar River and Cathedral Cliffs Formations and joins a low-angle fault in those rocks. This low-angle fault probably steps downward to the east where it reaches the Heart Mountain detachment fault surface. Three kilometres farther to the southeast, the break-away fault is presumably represented by a low-angle fault in these volcanic rocks that probably also steps down to the northeast beneath the volcanic cover to join the Heart Mountain detachment fault. Erosion in this area probably has removed rocks that contained the high-angle part of the fault.
The south end of the break-away fault is overlapped and concealed by the Wapiti Formation. It presumably continues southward, but it probably does not join a normal fault near Black Mountain, although Voight (1974) has suggested that it does.