THE LEMHI ARCH OF EAST-CENTRAL IDAHO: A STRANDED FAULT BLOCK WITHIN THE WESTERN LAURENTIAN RIFT MARGIN

In northern Utah and southeastern Idaho, the transition to thermally driven subsidence during final Laurentian rifting occurred in Middle Cambrian time. However, the abrupt along-strike transition northwestward across the Snake River Plain to the Lemhi Arch, where Middle Ordovician sandstone unconformably overlies tilted Mesoproterozoic Belt-Purcell Supergroup-equivalent quartzite, requires a major change in the geometry and timing of rifting in east-central Idaho. Recent work demonstrates that: 1) on its northwestern margin, the Lemhi Arch forms the footwall of a major southwest-dipping Neoproterozoic to Ordovician normal fault; 2) exhumation of the Lemhi Arch was ongoing during Late Cambrian time, as indicated by alkalic Big Creek-Beaverhead plutons intruded into the fault’s footwall and rapidly eroded into the Upper Cambrian Worm Creek Member of the St. Charles Formation in southeastern Idaho; 3) southwest of the Lemhi Arch, near Clayton in central Idaho, a thin Neoproterozoic (Ediacaran) to Middle Cambrian stratigraphic record of rifting is overlain by nearly 4 km of primarily clastic Ordovician sediment; attenuated country rocks in the Idaho batholith documented by others near Stibnite and Stanley, Idaho suggest that similar rocks are distributed over a broad region west of the Lemhi Arch; and 4) the Lemhi Arch approximately parallels the ~1.4 Ga Lemhi subbasin that is underlain, at least locally, by dominantly mafic basement formed during intrusion of a ~1.38 Ga magmatic complex. Taken together, these results suggest that the Lemhi Arch is a rotated fault block stranded within the miogeocline as the southwestern margin of the Belt-Purcell basin was temporarily reactivated during breakup of Rodinia. Mafic magmatism concomitant with major Mesoproterozoic extension may have previously strengthened the lower crust in this region, predisposed the region of the Lemhi Arch to remain intact during extension, and promoted formation of a fundamentally different Laurentian rift margin than to the southeast. Furthermore, rifting in Idaho north of the Snake River Plain persisted into Late Cambrian and Early Ordovician time, which is ~50 Ma later than along-strike to the southeast.