MID-CRUSTAL DOMAINS IN THE NORTHERN ROCKY MOUNTAINS FROM LONG-WAVELENGTH ISOSTATIC RESIDUAL GRAVITY AND AEROMAGNETIC DATA, MONTANA AND IDAHO

Gridded, wavelength-filtered potential field data provide constraints on constrasting rock densities and magnetic susceptibilities of regional-scale rock bodies in the upper 10-15 km of the crust of the northern Rocky Mountains. The potential field signature reflects the character of the crystalline basement underlying the Great Plains and areas extending 50-100 km W of the Cordilleran thrust front. Further W of the thrust front, the mid-crustal signature reflects features above the thrust decollement that have been transported 100-150 km E of their original crustal underpinnings. Under 1-3 km of Phanerozoic strata in the Great Plains between Canada and the Bighorn Mountains, the geophysical data define 5 NE-trending Archean basement domains developed from slices of the Medicine Hat (N) and Wyoming (S) provinces juxtaposed at 1.8-Ga. Isotopic and seismic data indicate 1.8-Ga LIL enrichment of the lithospheric mantle and underplating of 10-25 km of mafic rocks occurred beneath all these domains. Pervasive NE-trending magnetic and gravity highs in this region may represent 1.8 Ga mafic dike-like bodies. The SW flanks of the Archean domains were affected by extension orthogonal to the NE grain at 1.5 Ga, and their different responses to stretching had important control on subsequent crustal evolution. The 18-km-thick Belt basin is localized SW of the Medicine Hat province, perhaps utilizing its Archean SW-dipping compressional fabric (from seismic data) to accommodate Belt-age extension. A prominent NNW gravity ridge under the central Belt basin reflects a Cordilleran thrust ramp anticline of lower Belt strata with abundant mafic sills. The syn-depositional fault-bounded S end of the Belt-Yellowjacket-Lemhi basin roughly tracks one of the NE-trending geophysical domain boundaries; NW-trending Middle Proterozoic dikes and grabens cut exposed Archean rocks further S. The SE-most of the 5 Archean domains continues SW along the trend of the E Snake River Plain, suggesting that pre-existing crustal weaknesses localized the Neogene Yellowstone “hotspot” trend. It is less clear if the ancestry of Cretaceous and Tertiary WNW trends cutting the thrust belt (e.g. Lewis & Clark zone, Trans-Idaho discontinuity further S) is truly Precambrian and whether that ancestry resides above or below the thrust decollement.