Thermal architecture of the Salmon River suture zone, Idaho, USA: Implications for the structural evolution of a ductile accretionary complex during arc-continent collision

Abstract

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 to quantify the thermal architecture of the Salmon River suture zone in west-central Idaho, USA, which records the Cretaceous collision of the Wallowa island arc terrane with North America. We integrate this thermal architecture with published structural interpretations, geochronology, 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). These ductile thrust sheets are separated by 100–500-m-thick intervals of inverted temperatures that surround the mapped positions of 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 ca. 144–105 Ma collision-related deformation. Accretion of ductile thrust sheets began at ~30–35 km depths and completed at depths of ~10–20 km. 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 top-to-the-west displacement was accommodated within 100–500-m-thick, high-strain, thrust-sense ductile shear zones.