PRECAMBRIAN BASEMENT GEOLOGIC MAP OF MONTANA—AN INTERPRETATION OF AEROMAGNETIC ANOMALY MAP

Newly compiled aeromagnetic anomaly data of Montana, in conjunction with the known geologic framework of basement rocks, have been combined to produce a new interpretive geologic basement map of Montana. Crystalline basement rocks compose the basement, but are exposed only in the cores of mountain ranges in southwestern Montana. Principal features deduced from the map are: (1) A prominent northeast-trending, 200-km-wide zone of spaced negative anomalies extending more than 700 km from southwestern Montana’s Beaverhead Mountains to the Canadian border reflects suturing of the Archean Mexican Hat block against the Archean Wyoming province along the Paleoproterozoic Trans-Montana orogen (new name) at ca. 1.9-1.8 Ga; (2) North- northwest-trending magnetic lows in northeastern Montana reflect the 1.9-1.8 Ga Trans-Hudson orogen which truncates the older Trans-Montana zone; and (3) Subtle northwest- and west-trending negative anomalies in central and western Montana represent the northernmost segment of brittle-ductile transcurrent faults of the newly recognized Mesoproterozoic Trans-Rocky Mountain fault system. Structures developed in the Proterozoic provided zones of crustal weakness reactivated during younger Proterozoic and Phanerozoic igneous and tectonic activity. For example, the Trans-Montana zone guided basement involved thrust faulting in southwestern Montana during the Sevier orogeny. The Boulder batholith and associated ore deposits and the linear belt of alkaline intrusions to the northeast were localized along a zone of weakness between the Missouri River suture and the Dillon shear zone of the Trans-Montana orogen. The northwest-trending faults of Trans-Rocky Mountain system outline depocenters for sedimentary rocks in the Belt basin. This fault system provided zones of weakness that guided Laramide uplifts during basement crustal shortening. Northwest-trending zones have been locally reactivated during Neogene basin-range extension.