NEWLY RECOGNIZED CHIEF JOSEPH METAMORPHIC CORE COMPLEX AND YOUNGER STRUCTURES IN THE NORTH-CENTRAL BEAVERHEAD MOUNTAINS, IDAHO AND MONTANA--EXPANDING THE TEMPORAL AND SPATIAL LIMITS OF POST-CONTRACTIONAL EXTENSION IN THE NORTHERN ROCKY MOUNTAINS

The Beaverhead Mountains mark the boundary between southwest Montana and adjacent Idaho from the Snake River Plain north to Lost Trail Pass. The rocks in the north-central part of this range are uniquely partitioned. On the east, Proterozoic quartzites are over 5 km thick, weakly deformed, and are vertical to slightly overturned; top is to the east. On the west, Proterozoic argillite and siltite, also over 5 km thick, are now metamorphic tectonites whose foliation dips steeply west. These two packages of rocks are separated from each other by the vertical, 200-m-thick, mylonitic Beaverhead Divide-Miner Lake fault zone. The fault zone grades westward into metamorphic tectonites; on the east, mylonite is in sharp contact with quartzite strata. The north-central Beaverhead Mountains represent a continuous 10-km-thick crustal section, now lying on its side, that records at least three periods of major crustal extension. (1) The mylonitic Beaverhead Divide-Miner Lake fault zone and the penetratively deformed rocks are footwall to the newly recognized and herein named Chief Joseph metamorphic core complex that is centered on the metamorphic-plutonic terrane in the northern Beaverheads and originally described by Desmarais in 1983. The quartzites are structurally above the mylonite and represent the detached, unmetamorphosed carapace of the core complex. The mylonitic rocks of the footwall deform 78 Ma plutons and are cut by 58 Ma plutons; thus this newly recognized core complex is older than the middle Eocene Bitterroot and Anaconda metamorphic core complexes directly north of the Beaverheads. (2) The up-turned crustal section of this metamorphic core complex was rotated to its present position along a younger, huge west-dipping listric normal fault system originating in the adjacent, 5-km-deep Big Hole Valley on the east. (3) The westernmost slopes of this part of the Beaverheads are also cut by low-angle detachment faults that formed during 45-40 Ma Challis volcanism. The three extensional events—Late Cretaceous-early Tertiary brittle-ductile, S-SE-directed crustal extension; middle Eocene, down-to-the-west listric normal faulting; and low-angle, west-directed late Eocene detachment faulting—record at least 25 Ma of Late Cretaceous-early Tertiary low-angle normal faulting in this part of Montana and Idaho.