THERMAL ARCHITECTURE OF THE SALMON RIVER SUTURE ZONE AT THE LATITUDE OF RIGGINS, IDAHO: IMPLICATIONS FOR THE STRUCTURAL EVOLUTION OF A DUCTILE ACCRETIONARY COMPLEX DURING ARC-CONTINENT COLLISION

Documenting the tectono-thermal evolution of the exhumed ductile portions of orogenic systems is critical for interpreting orogen dynamics. Here, we utilize Raman spectroscopy of carbonaceous material thermometry from 32 metasedimentary samples to quantify the thermal architecture of the Salmon River suture zone in west-central Idaho, which records the Early Cretaceous collision of the Wallowa island arc terrane with North America. We integrate this thermal architecture with published structural interpretations, geochronology, thermochronometry, and pressure-temperature-time histories to interpret the evolution of deformation during arc-continent collision in this portion of the North America Cordillera. Mean peak temperatures within four, ~1-3 km-thick, penetratively deformed thrust sheets in the western part of the suture zone decrease moving structurally downward from 652 ± 28 °C (Pollock Mountain thrust sheet), to 577 ± 30 °C (Rapid River thrust sheet), to 426 ± 32 °C (Morrison Ridge thrust sheet), to 358 ± 18 °C (Heavens Gate thrust sheet), and these ductile thrust sheets are separated by ~100-500 m-thick intervals of inverted temperatures that surround the mapped positions of top-to-the-west thrust faults. We interpret the western part of the suture zone as a ductile accretionary complex that records the progressive underplating and top-to-the-west translation of ductile thrust sheets that were derived from the Wallowa terrane during east-directed, Alpine-type subduction of the leading portion of this terrane between ~144-105 Ma. We interpret the stepwise decrease in peak temperatures moving structurally downward and westward as the consequence of in-sequence accretion of ductile thrust sheets at progressively shallower depths with time, which started at ~30-35 km depths (Pollock Mountain thrust sheet) and completed at depths likely as shallow as ~10-20 km (Heavens Gate thrust sheet). Rocks at all structural levels in the suture zone exhibit distributed ductile fabrics, but the inverted thermal gradients that surround the mapped positions of thrust faults suggest that the majority of differential top-to-the-west displacement was accommodated within relatively discrete (~100-500 m-thick), high-strain, thrust-sense ductile shear zones.