GEOLOGIC MAPPING AND GEOCHRONOLOGY OF THE HEAVENS GATE 7.5’ QUADRANGLE, IDAHO COUNTY, IDAHO, WESTERN IDAHO: MESOZOIC DEFORMATION IN THE SALMON RIVER SUTURE ZONE

The Blue Mountain province of western Idaho and eastern Oregon is comprised of a mélange of geologic terranes and rock units that collided with North America in the Mesozoic, resulting in westward growth of the continent. The associated deformation and metamorphism in the suture and shear zones record the mountain building processes. In western Idaho, volcaniclastic and sedimentary sequences were intensely deformed and buried to mid-crustal conditions blurring the boundaries between the accreted terranes and the pre-collisional continental margin. Geologic mapping of the Heavens Gate 7.5’ quadrangle was conducted to investigate structural relationships between thrust sheets within the accreted terranes and the potential protoliths and metamorphic units of the Salmon River Suture Zone (SRSZ).

The western portion of the Heavens Gate quadrangle contains east dipping metasedimentary and metavolcanic rocks of the Seven Devils Group. The overlying lower greenschist facies rocks of the Lucille Slate and Martin Bridge Formation are juxtaposed onto the Seven Devils Group along the NE-SW trending thrust fault known as the Morrison Ridge fault. Structurally above the Lucille and Martin Bridge units lies the Riggins Group, that is exposed east of the Rapid River and above the N-S trending Rapid River thrust fault. The highest structural thrust sheet contains rocks of the Pollock Mountain Amphibolite and Cold Springs orthogneiss. Post-accretion related rocks in the quadrangle include Columbia River Basalt Group (CRBG) which is present on ridges in the south eastern sections of the quadrangle, and colluvium landslide deposits.

Age estimates for thrust faulting will be presented from U-Pb zircon geochronology of plutons that are structurally dissected by the thrust faults. U-Pb detrital zircon ages from midcrustal metamorphic rocks will provide insight into the sediment sources for the rocks prior to metamorphism, which may yield information into the timing of terranes collision and sediment routing during collisional accretion.