PROTEROZOIC BASINS AND OROGENIC BELTS OF CENTRAL IDAHO

Poorly studied metamorphosed strata exposed on two northwest-trending central Idaho thrust slabs document three Proterozoic depositional basins deformed by two Proterozoic orogenic events.

Mesoproterozoic metasandstones and siltites of the Lemhi Group, exposed on the regional Brushy Gulch-Red River thrust slab, increase in metamorphic grade from greenschist in the southeast to migmatite in the northwest. In the Beaverhead Mountains on the east, white mica in axial planar cleavage of large-scale overturned folds is dated by 40Ar/39Ar at about 1.36 Ga. Near Salmon 1.38 Ga megacrystic granites cut cleaved greenschist strata and near Elk City cut approximately 1.4 Ga migmatitic strata (bracketed by 1.41 Ga detrital zircons and 1.38 Ga crosscutting granite). These coeval metamorphic and magmatic events may be equivalent to the East Kootenay orogeny of British Columbia.

Erosion of the 1.38 Ga orogenic belt resulted in micaceous feldspathic sandstones and conglomerates containing detrital zircon populations that are dominated by 1.38 Ga and older grains. These were deposited on the granitic source rocks in a second basin of poorly known extent in east-central Idaho.

Evidence of a second orogeny in the belt is suggested by possible 1.2-1.0 Ga polymetallic mineralization in the southeast (Gillerman et al., 2002, GSA abs) and amphibolite facies metamorphism in the northwest (Sha et al., 2004, GSA abs). It is further substantiated by quartzite near Lowell, dominated by 1.2 to 1.0 Ga detrital zircons presumably from a local source. This orogeny is time equivalent to low grade metamorphism in Belt-Purcell strata to the north and to the eastern U.S. Grenville orogeny.

40Ar/39Ar data from the Beaverheads further suggest < 800 Ma Neoproterozoic reheating that is time equivalent with the Gunbarrel igneous event (Harlan et al., 2004, Geology) and Goat River orogeny of B.C. Detrital zircons as young as 760 Ma in the quartzite near Lowell document onset of Windermere sedimentation in a third depositional basin.

This central Idaho belt was tectonically active, including formation of depositional basins, orogenic belts, and erosional cycles, from 1.4 Ga through the Neoproterozoic. This is in strong contrast to 1.47-1.40 Ga relatively continuous sedimentation and post-depositional tectonic stability in the Belt basin to the north.