Timing of Tectonic Juxtaposition of the Bourne and Greenhorn Subterranes, Baker Terrane, NE Oregon

The contact between the Bourne and Greenhorn subterranes of the composite Baker terrane is a fundamental lithotectonic boundary in the Blue Mountains province, NE Oregon. This boundary juxtaposes late Paleozoic through Early Jurassic(?) Elkhorn Ridge Argillite and associated mélange of the Bourne subterrane against serpentinite-matrix mélange of the Greenhorn subterrane. In the Desolation Butte area, the boundary between the two subterranes is characterized by numerous brittle faults, intense brecciation, and extensive networks of cataclastic shear zones. At the contact between the two subterranes, an intense greenschist-facies shear zone separates serpentinite, metavolcanic, and metaplutonic rocks of the Greenhorn subterrane from intensely brecciated Elkhorn Ridge Argillite of the Bourne subterrane. The orientation of this fault zone ranges from approximately northwest-southeast in the Desolation Butte area, to nearly east-west in adjoining areas. Fault surfaces dip at ~50-80° to the south-southeast and scattered minor brittle-fault features suggest top-to-the-northwest. In the footwall of the fault zone, Elkhorn Ridge Argillite is intensely brecciated up to 2-3 km from the subterrane contact. A suite of undeformed diorite, quartz diorite, and tonalite of the Gold Bug pluton intrudes this fault zone, indicating that major brittle deformation ended prior to the emplacement of this plutonic sequence.

U-Pb zircon ages from the Gold Bug pluton coupled with Rb-Sr and Sm-Nd isotopic data from metasedimentary rocks, provide an important timing constraint on the juxtaposition of the subterranes. New SHRIMP data from a biotite granodiorite and a biotite-hornblende diorite from the Gold Bug pluton yielded weighted average 206Pb/238U ages of 146.3 ± 1.5 Ma and 153.9 ± 1.3 Ma, respectively. These data suggest that imbrication and subterrane juxtaposition occurred prior to Late Jurassic pluton emplacement, and the tectonic boundary between the Bourne and Greenhorn subterranes may be related to arc-arc collision of the Olds Ferry and Wallowa island-arc terranes.