Structural Analysis of Boat Mountain Area in Rocky Mountain Foreland, Southern Madison Range, Montana

Jeanette M. Sablock

Detailed mapping, together with stereographic analysis of over 400 field measurements, has been used to determine the geological structure of the Boat Mountain area in southern Madison Range, Gallatin County, Montana. The principal structure is a north-plunging, north-northwest-trending, overturned and thrusted synform, termed the Bear Creek syncline. Thrusting consists of a duplex of roof and sole thrusts enclosing an imbricate stack of horses in the southwest part of Boat Mountain. Precambrian to Cretaceous (Kootenai Formation) strata are involved in the deformation, which is interpreted as Laramide and of a "Foothills Family" type, involving several separate pulses of deformation. Stereographic determinations of maximum principal stress directions have shown that an in tial northeast-directed stress formed and overturned the synform, caused movement on the out-of-syncline sole thrust, and imbricated the overturned limb. A later, easterly directed stress moved the back-limb roof thrust over the already folded and thrust-faulted rocks of Boat Mountain. Thrusting was succeeded by sinistrally directed tear (or strike-slip) faulting along a northeast-striking fault at the southern end of Boat Mountain. Listric normal faulting on Laramide thrust-fault planes is interpreted as a response to Tertiary extension. Recent normal faulting, on steep-dipping, east-west-striking fault planes and continuing to the present, is interpreted as a response to Yellowstone doming.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.