New methods for estimating displacements of thrust faults affecting Atlantic‐type shelf sequences: With an application to the Champlain Thrust, Vermont

Large thrust‐related displacements occur in many orogenic belts. Existing methods for estimating the displacements across such faults require knowledge of the sub‐surface geometry of the fault. Where appropriate well and seismic reflection data are not available, standard structural techniques of projecting stratigraphic separations down the dip of the fault yield only minimum estimates of displacement. Since many fold‐thrust belts involve sedimentary sequences developed along ancient Atlantic‐type margin continental shelves, the prethrusting cross‐sectional shape of such sequences may be used to estimate displacements across large thrust faults affecting these sequences. The characteristic shape of such shelf sequences is that of a triangular, seaward thickening wedge. Simple trigonometric relationships utilizing various data, including stratigraphic thickness for a given time‐stratigraphic interval in transported and untransported sections, the position of the prethrusting depositional hinge line, the time span in millions of years of the chosen time‐stratigraphic interval, or assumed dimensions and rates of an average Atlantic‐type margin, are used to derive estimates of displacements. Some limitations of these methods are discussed qualitatively; a rigorous test is not, however, attempted now. Two of the methods are applied to the Champlain Thrust of Vermont. Previous estimates suggest much less the 30 km of east‐west displacement of this fault. The displacement across this thrust estimated by using methods proposed here suggest somewhat greater than 80 km of east‐west displacement. This amount of displacement requires that the entire Green Mountain Anticlinorium, including Grenville basement, is detached and constitutes a relatively thin basement‐cored thrust sheet geometrically similar to the Blue Ridge Thrust sheet of the Southern Appalachians.