U-PB DATING OF PLUTONS ALONG THE TRANSFER ZONE BETWEEN THE BITTERROOT AND PRIEST RIVER METAMORPHIC CORE COMPLEXES

Eocene extensional strain was transferred from the southern end of the Priest River complex to the northern end of the Bitterroot core complex through a crustal-scale transform system. The northwestern part, the St. Joe fault, and the southeastern part, the Benton Creek-Kelly Forks fault system, are joined by transfer or stepover systems that include the down-to-the-west White Rock fault and the more complicated Boehls Butte core complex. Small elongate granitic intrusions, common along the relatively diffuse western part of the Benton Creek fault, consist of biotite granite and subordinate biotite granodiorite. These typically have a mylonitic fabric with a well-developed WNW-ESE subhorizontal mineral lineation and dextral kinematic indicators. Zircon from a lineated granite body along the fault zone yielded a weighted mean 206Pb/238U SHRIMP age of 48.5 ± 0.4 Ma indicating that strike-slip deformation on the Benton-Kelly Forks fault system was occurring at this time. To the southeast, the Beaver Creek pluton intruded into and north of the Benton Creek fault. The 46.4 ± 0.5 Ma (based on concordant TIMS zircon analyses) Beaver Creek pluton is an equant body of biotite granite and granodiorite that is much less deformed than the lineated granite bodies, indicating that deformation on the Benton-Kelly Forks fault system had waned by this time. These ages compare favorably with 48-46 Ma extension documented in the Bitterroot complex (Hodges and Applegate, 1993; Geology, v. 21). To the southeast the Kelly Forks fault is traced as far as Cayuse Creek where it appears to be intruded by young plutons, which, although undated, are probably as young or younger than the Beaver Creek body.