Investigation of Overthrust Faults by Seismic Methods
In the great mountain belts the earth's crust has been severely buckled and fractured, apparently mainly by horizontal compressive forces. Of the various types of folds and faults that developed in the course of this deformation the enormous overthrust faults are among the most important and interesting. Along these horizontal or gently sloping fractures huge slabs of the crust ranging from thousands of feet to miles in thickness have been thrust forward for distances of miles and sometimes for many tens of miles. The length of these faults, traced along the surface, is from tens to hundreds of miles. Their extension backward and downward into the crust is presumably of comparable dimensions, but in view of the limited thickness of that part of the crust which has suffered mountain-making deformation, it is unlikely that the depth to which they reach exceeds a few tens of miles. On our own continent the Rocky Mountains of the northern United States and of Canada have been thrust many miles eastward over the margin of the Great Plains along such overthrust faults. In the southern Appalachians each of the several slabs shown in any cross section has ridden westward over its neighbor. In the Alps and in the Highlands of Scotland overthrusts on a tremendous scale have been recognized.