Model for evolution of rocky mountains east of Idaho Batholith

Abstract

Studies of thick, synorogenic, continental conglomerates in south-western Montana indicate that by Mid-Cretaceous time erosion of the roof of the Cretaceous Idaho Batholith had caused extensive exposure of Precambrian quartzites in a high mountainous terrain occupying part of the former geosyncline in central Idaho. Evidently, emplacement of the batholith had been accompanied by rapid regional uplift, creating a large tumor or undation. The gravity potential provided in this manner represents at least one known source of energy for the contemporary and subsequent orogenic processes in the northern Rocky Mountains. The Late Cretaceous to Paleocene folds and thrusts east of the batholith may be seen as the result of compressive shortening compensating for extension in the domed roof due to down-flank gravitative spreading. Compressive stress was propagated toward the edge of the cratonic shelf as earlier thrusts were folded and locked and newer thrusts formed farther east, finally involving the conglomerates. Local uplifts gave rise to epidermal gliding sheets. On a smaller scale, the Boulder Batholith in Montana played a somewhat similar role in causing westward directed folds and thrusts. The undation model for the northern Rockies accounts well for the radially directed tectonics around the northeast corner of the Idaho Batholith, for the dominant westward transport on the Pacific side of the Cordilleran belt, and for the recent seismic data indicating that the Sial in this belt not only lacks a root, but is actually thinner than normal by 5–20 km. The alternative model fitting the field data would require longdistance centrifugal overthrusting of the solidified batholithic core itself, or centripetal underthrusting of the craton and ocean basin in eastward, south-ward, and westward directions beneath this core. This alternative is difficult to accept from a mechanical point of view and also would lead to a thickening of the Sial, in contradiction to the seismic data.