Great Falls Lineament, Idaho and Montana: ABSTRACT

Abstract

The name Falls is given to a northeast-trending zone of diverse geologic features that can be traced northeastward from the Idaho batholith in the Cordilleran miogeocline of the United States, across thrust belt structures and basement rocks of west-central and southwestern Montana, through the cratonic rocks of central Montana, and into southwesternmost Saskatchewan, Canada. The zone is well represented in east-central Idaho and west-central Montana where geologic mapping has outlined northeast-trending, high-angle faults and shear zones that: (1) extend more than 150 km (93 mi) from near Salmon, Idaho, northeastward toward Anaconda, Montana; (2) define a nearly continuous zone of faulting that shows recurrent movement from middle Proterozoic to Holocen time; (3) controlled the intrusion and orientation of some Late Cretaceous to early Tertiary batholithic rocks and early Tertiary dike swarms; and (4) controlled the uplift and orientation of the Anaconda-Pintlar Range. Recurrent movement along these faults and their strong structural control over igneous intrusions in this region suggest that northeast-trending faults represent a fundamental tectonic feature of the region. Figure Geologic features that are similar to those mapped in the Salmon-Anaconda region are present to the southwest and the northeast. In central Idaho, these structures include numerous northeast-trending faults and pronounced topographic lineaments that cut across the southern part of the Idaho batholith, and a northeast alignment of Tertiary igneous rocks that cut the Idaho batholith and adjacent rocks. East and southeast of the Anaconda-Pintlar Range, subparallel, high-angle faults and topographic lineaments are present in the Highland, Pioneer, Ruby, and Tobacco Root Mountains. High-angle faults may have in part controlled the orientation of the northeast-elongate Boulder batholith. Northeast-trending structures are not easily traced across the thrust belt of western Montana or across he Lewis and Clark line. In the central Montana plains, northeast of the disturbed belt, however, a broad zone of colinear, northeast-trending structures is present, and includes: parallel, buried basement highs that in part controlled depositional patterns of some Paleozoic and Mesozoic sedimentary rocks; major physiographic features, such as the remarkably straight, 175-km (109-mi) long segment of the Missouri River, and equally long, buried river channels in southwestern Saskatchewan; a northeasterly alignment of highly differentiated igneous rocks and a belt of ultrabasic intrusions and related diatremes; End_Page 1350------------------------------ and a well-defined pattern of northeast-trending gravity and aeromagnetic anomalies underlying this part of central Montana and southwesternmost Saskatchewan. Taken together, all these geologic features define a broad, northeast-trending zone at least 150 to 200 km (93 to 125 mi) wide and more than 1,000 km (620 mi) long. The zone is approximately colinear but not demonstrably continuous with the well-exposed boundary in eastern Saskatchewan and Manitoba between the Archean Superior and the Proterozoic Churchill provinces of the Canadian Shield. This boundary is also characterized by: high-angle faults, shear zones, and topographic lineaments; pronounced linear gravity and magnetic anomalies; igneous intrusions; and fault controlled depositional patterns and mineralization. That the Great Falls lineament is controlled by a similar Precambrian boundary between the Archean Wyoming province of southwestern Montana and early Proterozoic terrane to the north is speculative; however, the geologic features found along the Great Falls lineament share many common characteristics with features present along the Archean-Proterozoic boundary in Canada. End_of_Article - Last_Page 1351------------