GEOLOGIC MAPPING AND GEOCHRONOLOGY OF THE HEAVENS GATE 7.5-MINUTE QUADRANGLE: HOW LONG DOES IT TAKE TO ACCRETE AN ISLAND ARC TO A CONTINENT?

The Blue Mountain province of western Idaho and eastern Oregon is composed of a mélange of geologic terranes that represent Permian and Triassic island arcs that collided with North America in the Mesozoic, resulting in westward growth of the continent. Separating these accreted rocks from North America are the mid- crustal metamorphic rocks of the Salmon River suture zone. Containing units and features associated with the accreted island arc terranes and suture zone is the Heavens Gate 7.5-minute quadrangle, located in Idaho county, Idaho. Within the quadrangle the Salmon River suture zone is divided into structural blocks by a series of N-S trending, east dipping thrust faults, the Morrison Ridge, Rapid River, and Pollock Mountain thrust faults (west to east). Formal units mapped within the quadrangle include the Hunsaker Creek and Wild Sheep Creek Formations (Seven Devils Group), the Morrison Ridge Formation, Lucille Slate, the Lightning Creek, Fiddle Creek, and Squaw Creek Schists (Riggins Group), and the Imnaha Basalt. Informal units mapped include tonalitic and quartz diorite plutons, quaternary deposits, and the Pollock Mountain Amphibolite and Cold Springs Orthogneiss and migmatite. Rocks of the Seven Devils Group, part of the Wallowa oceanic island arc, are folded into a north plunging anticline within the central portion of the map, with folding bracketed by zircon geochronology at 140-130 Ma. The anticline is cut by the Morrison Ridge thrust fault, emplacing the Martin Bridge Formation and Lucille Slate above the Seven Devils Group. Structurally above these units lies the Riggins Group, exposed east of the Rapid River and above the Rapid River thrust fault. The highest structural sheet contains the Pollock Mountain Amphibolite and Cold Springs orthogneiss. Zircon geochronology of volcanic, deformed and undeformed plutonic, and metamorphic rocks were used to determine that thrust fault development in the Salmon River suture zone occurred out of sequence with nearly synchronous activation along the Morrison Ridge thrust to the west (pre-123 Ma) and the Pollock Mountain thrust to the east (117 Ma). The approximately 109 Ma. Rapid River thrust was the final thrust fault to develop in the region.