Tectonic features of the Precambrian Belt Basin and their influence on post-Belt structures

Abstract

The Belt basin represents a slowly sinking reentrant on the North American craton that began to form about 1,500 m.y. ago and persisted for more than 600 m.y. This sinking block somewhat resembles an aulacogen, but the basin is not a true grabenlike trough extending into the craton at a plate separation. The sinking block was at times almost triangular in shape; a central platform was bounded by the North American craton and associated narrow troughs on the south and northeast, and on the northwest by the Cordilleran miogeocline that extended past several reentrants along the North American craton. Following the end of Belt sedimentation and the onset of the East Kootenay orogeny about 850 m.y. ago, the Belt basin has been subjected to a variety of stresses. Within the basin the strain appears to reflect the inhomogeneities of the platform and troughs formed in Belt time. During and after the East Kootenay orogeny the central platform acted as a somewhat rigid block and contains gentle folds and vertical block faults; the southern trough contains a series of tear faults and tight folds of the Lewis and Clark line; the northeastern trough and old cratonic edge are the site of the Montana disturbed belt; the intersection of the two troughs forms an embayment that contains the Boulder batholith and related volcanics; and the old miogeocline on the northwest is the site of the Kootenay arc mobile belt, which contains gneiss domes and thrusts that rode eastward up over the platform block.