AN EXPOSED CROSS SECTION THROUGH A LATE PALEOZOIC–MESOZOIC ACCRETIONARY COMPLEX: THE SOUTHERN MARGIN OF THE BAKER TERRANE, BLUE MOUNTAINS PROVINCE, NORTHEAST OREGON

Reverse-sense displacement along the Connor Creek fault (CCf) provides a tilted cross section through a late Paleozoic(?)–Early Jurassic accretionary complex, the composite Baker terrane of the Blue Mountains Province of northeast Oregon. Fault-bounded tectonostratigraphic units in the hanging wall of the CCf record processes characteristic of a subduction-related mélange: accretion, sediment subduction, subduction erosion, and tectonic underplating. These units are, from northwest to southeast, the Elkhorn Ridge Argillite (ERA), Blue Spring Gulch plutonic complex, Burnt River Schist (BRS), and serpentinite-matrix mélange (interpreted as part of the Greenhorn subterrane).

Contrasting depositional ages and deformational histories indicate tectonic juxtaposition of these units. The maximum depositional age of the ERA is Early Jurassic (early Toarcian), based on radiolarian fauna in thin-bedded chert, exposed in the Elkhorn Mountains west of Baker, Oregon. The Blue Spring Gulch complex, a deformed greenschist-facies composite arc-plutonic complex, has an igneous crystallization age of Middle Triassic. The BRS has a maximum depositional age of Early Jurassic, indicated by detrital zircon analyses. In other parts of the Baker terrane, serpentinite-matrix mélange has a multi-stage developmental history that likely spanned from the late Paleozoic to Late Triassic time.

Findings from recent geologic mapping indicate that the ERA–Blue Spring Gulch contact is a block mélange zone containing blocks of the ERA, Blue Spring Gulch metaplutonic rocks, and scarce exotic rocks such as amphibolitic rocks. The Blue Spring Gulch complex is interpreted as a fault-bounded tectonic slab. Serpentinite-matrix mélange is concentrated along the CCf, where it includes rocks of oceanic affinity (i.e., ophiolitic mélange). Serpentinite-matrix mélange is in fault contact or intrusive contact (e.g., diapiric in nature) with the rocks of the BRS.

Cenozoic faults commonly reactivate and disrupt Mesozoic structural boundaries. Normal-sense faults striking east–west, northeast–southwest, and northwest–southeast have overprinted Mesozoic structures, yielding a re-entrant of the CCf (i.e., Clarks Creek re-entrant) in the Bridgeport area, and exhumed the block mélange zone which separates the BRS and ERA.