THE CONNOR CREEK FAULT: A LATE MESOZOIC TERRANE BOUNDARY IN THE BLUE MOUNTAINS PROVINCE, NORTHEAST OREGON

The Connor Creek fault is a reverse-sense tectonic boundary of Late Jurassic to Early Cretaceous age in the Blue Mountains province of NE Oregon. For ~200 km along strike, the Connor Creek fault and inferred equivalent boundaries emplace oceanic rocks of the Baker terrane structurally above forearc-basin rocks of the Izee terrane. Middle Jurassic (Callovian) ammonites in the Weatherby Formation of the footwall block provide a maximum age for the fault-displacement history, whereas Early Cretaceous (~124 Ma) quartz dioritic plutons, which intrude the fault zone, provide an upper age limit of displacement along the Connor Creek fault. Where the Connor Creek fault crosses the Snake River near its type locality, the fault strikes 060º and dips ~70º NW. In many exposures of the fault zone, serpentinite-matrix mélange forms the hanging-wall block immediately adjacent to low-grade metasedimentary rocks of the footwall block. Penetrative deformation in the Weatherby Formation commonly transposed original bedding, especially in proximity to the Connor Creek fault.

Southeast of Baker, Oregon, the hanging-wall block of the Connor Creek fault progressively exposes deeper structural levels of the Baker terrane accretionary complex. Approaching the fault zone from the north, four distinct fault-bounded subterranes are exposed: chert-argillite broken formation (chiefly Elkhorn Ridge Argillite of the Bourne subterrane), deformed mafic to silicic (meta)igneous rocks (Blue Spring Gulch plutonic complex), polyphase-deformed, greenschist-facies, phyllitic metasedimentary and associated mafic metavolcanic rocks (Burnt River Schist subterrane), and serpentinite-matrix mélange (Greenhorn subterrane). Miocene normal faulting striking northeast-southwest, north-south, northwest-southeast, and east-west overprinted the original trace of the Connor Creek fault and consequently created the Clarks Creek re-entrant near Bridgeport, Oregon.

The Connor Creek fault and equivalent boundaries form a regional tectonic feature, which can be traced from east of the Snake River to the John Day inlier, forming the southern boundary of the composite Baker terrane. This tectonic boundary is a major late Mesozoic terrane boundary that exposes a tilted cross section across the composite Baker accretionary terrane.