Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 2-5
Presentation Time: 9:25 AM


REGAN, Sean, Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775-9702, MARSH, Jeffrey, Laurentian Univ., Laurentian University, 935 Ramsey Lake Rd, WGMC, 8th floor (lab, SUDBURY, AK P3E 2C6, WALDIEN, Trevor, Geology and Geological Engineering, South Dakota School of Mines and Technology, 501 E. St. Joseph St., Rapid City, SD 57701, HOLLAND, Mark, Life Earth and Environmental Sciences, West Texas A&M University, 2403 Russell Long Blvd, Canyon, TX 79015 and SUHEY, Jane, Geophysical Institute, University of Alaska Fairbanks, 900 Yukon Dr, Fairbanks, AK 99775-9702

The formation of inverted metamorphic field gradients is persistently debated in the structure/tectonics and metamorphic petrology communities. Interpretations range from a continuum model wherein telescoped paleoisotherms are transposed by non-coaxial flow within a thick ductile shear zone to a discrete model wherein discrete thrusts catalyze advective heating and thermal diffusion from the hanging wall into structurally lower levels. Inverted metamorphism along the 6 km thick south-vergent Valdez Creek shear zone (VCsz) in the western Clearwater Mountains marks the collisional suture between the insular and intermontane belts of the northern North American Cordillera. Textures in the VCsz vary systematically from gneissic at the structural top through schist, phyllite, and discrete and variably mineralized m-scale thrusts at the structural base. We present in-situ monazite, apatite, and rutile geo/thermo chronology, microstructural analysis, and thermobarometry from four samples taken along a transect through the shear zone.

The new data reveal: 1) peak pressures of ~1.0-0.7 GPa and temperatures of 600-650°C from the structurally highest sample; 2) decreasing age of syn-deformational monazite growth structurally downward through the inverted metamorphic gradient from 64-56 Ma; and 3) consistent thermochronology indicating cooling through 500-400°C by ca. 50 Ma of all four samples. The data support a scenario wherein the VCsz formed by non-coaxial flow that progressively incorporated footwall material during prograde metamorphism and rapid uplift of high-grade gneisses in the hangingwall. Based on the metallogenic history, structural evolution, plutonic history, tectonic role of the VCsz, and new strike-slip restorations of Alaska, we correlate the VCsz with the western metamorphic belt of the Coast shear zone, thereby creating a >1750 km long zone of inverted metamorphism along the largest accretionary suture preserved in the Phanerozoic record of North America.