CENTRAL IDAHO THRUST BELT

Central Idaho, between the Montana foreland fold and thrust belt and the Salmon River suture, generally is thought to be underlain by parautochthonous Mesoproterozoic or older rocks metamorphosed and intruded by the Idaho batholith. Recent mapping substantiates prior conceptual predictions of continuity of a northwest-trending fold and thrust belt across this void in the hinterland (Skipp, 1987, Geology) prior to Cretaceous and Tertiary magmatism.

Eight major thrust faults in three thrust-slab systems of a newly mapped central Idaho thrust belt are identified by juxtaposition of plates containing fundamentally different Proterozoic strata, Mesoproterozoic to Paleozoic facies, and (or) Proterozoic metamorphic and deformational histories. Most of the thrust faults were reactivated by major normal faults and the slabs have undergone considerable subsequent tectonic collapse. Parallelism among the thrust slabs and the metamorphic and sedimentary facies belts shows probable control of both slabs and belts by multiply reactivated Proterozoic structures.

The Brushy Gulch-Red River thrust system is traced continuously across the center of the thrust belt for 350 km. Its hangingwall contains Mesoproterozoic (probable Lemhi Group) rocks at upper amphibolite migmatite grade in northwestern and central portions, that were subjected to intense Mesoproterozoic orogenic events, and Lemhi Group at lower greenschist facies in the east. Exposed rocks at the base of the slab are 1380 Ma amphibolite and megacrystic granite and granite gneiss that outline the trace of this structure and are only known in this thrust slab. Regionally, the Mesoproterozoic metamorphic and associated magmatic rocks are structurally sandwiched between lower grade but stratigraphically equivalent Mesoproterozoic sedimentary rocks. Higher grade rocks are northwest of the trace of the northeast-trending Great Falls tectonic zone, indicating that zone exerted control on the extent and intensity of Cretaceous and earlier orogenic events.

Exposure of metamorphic and magmatic rocks on the Brushy Gulch-Red River thrust plate coincides with the zone previously called the pre-Belt Salmon River arch. Instead, this zone provides evidence of a 1380-Ma orogenic belt exposed by a major Cretaceous thrust slab.