SPATIAL AND TEMPORAL OVERLAP OF STRUCTURAL STYLES IN THE HIGHLAND MOUNTAINS, MONTANA, USA: AN EXAMPLE OF COLLABORATION BETWEEN THE MONTANA BUREAU OF MINES AND GEOLOGY AND USGS EDMAP PROGRAM PARTICIPANTS

The fold-thrust belt of the North American Cordillera is commonly delineated into the thin-skinned Sevier fold-thrust belt and the Laramide province of basement-involved deformation. However, spatial overlap between these domains occurs within the Mesozoic–Paleogene Idaho-Montana fold-thrust belt (44°N–46°N, 112ºW–114°W). During mapping of the Butte South 30’ x 60’ quadrangle, Montana Bureau of Mines and Geology (MBMG) mappers highlighted areas of structural complexity likely associated with this overlap in the southwest portion of the Highland Mountains, located ~20 km south of Butte, Montana. There, their interpretations partially differed from those of previous work by the USGS. With support from the MBMG and USGS EDMAP program, we revisited these areas and completed a 1:24,000 scale geologic map of the encompassing eastern Melrose and western Wickiup Creek 7.5’ quadrangles.

Five groups of structures were recognized within the map area: (1) a series of SE-striking bedding sub-parallel faults involving Paleozoic rocks, with variable stratigraphic separation along-strike; (2) a faulted zone at the contact between the Cambrian Flathead Sandstone and metamorphic basement rock; (3) a series of NW-striking and steeply dipping faults involving basement rock; (4) the E-W striking Camp Creek Fault, carrying Belt Supergroup rocks in its hanging-wall; and (5) Cenozoic normal faults bounding the western range front. Mutually cross-cutting relationships and fault-fold interactions indicate spatial and temporal overlap of group (1–3) faults. New geo/thermochronologic data place timing constraints on key structures. A pluton cross-cutting a group (1) fault yielded a weighted mean zircon U-Pb age of 74.4 +0.4/-0.7 Ma. Apatite and zircon [U-Th(-Sm)]/He data indicate cooling of basement rock between ~90 Ma and ~55 Ma. These results suggest basement-involved deformation preceded and overlapped with out-of-sequence thin-skinned thrusting in the study area, and the latter terminated prior to Maastrichtian time. Our interpretations partially reconcile those of previous work by USGS and MBMG geologists. This project demonstrates the effectiveness of the EDMAP program in fostering collaboration between federal, state, and academic geologists and their students—the next generation of geologic mappers.