Normal Faults of the Lake Champlain Region

Abstract

Between the compressional structures of the Green Mountains of Vermont and the dome-like Adirondack mass lie the Lake Champlain lowlands characterized by normal faults, moderate dips, and a scarcity of folds. The normal faults belong mainly to two systems, one having a northeast-southwest strike and one a north-south strike. It appears that both systems were formed simultaneously, and structural evidence seems to show that the normal faulting occurred in Ordovician time, before the Green Mountain overthrusting. These faults cause the pre-Cambrian surface at the base of the Paleozoic series to descend to the east by steps. A study of the stretching involved in the faulting shows that there were tensional forces acting downward toward the east. The distribution and pattern of the faults indicate a primary relation to the Green Mountains and a secondary relation to the Adirondacks, as though due to shear strains between the sinking geosyncline and the up-standing Adirondacks. The normal faults appear to have been formed during the geosynclinal stage, and although conceivably due to unknown magmatic movements, appear to have been formed by the sagging of the geosyncline under the weight of accumulating sediments or to tensional forces which cause the geosynclines.