THE KINEMATIC AND TEMPORAL HISTORIES OF DEFORMATION IN THE   SALMON RIVER SUTURE ZONE AND WESTERN IDAHO SHEAR ZONE, IDAHO, USA: INSIGHTS INTO THE EVOLUTION OF AN ISLAND ARC-CONTINENT BOUNDARY

Andrea Elaine Richardson

The western half of the Jurassic-Paleogene North American Cordilleran orogen was constructed by the progressive accretion of island arc terranes. Therefore, investigating the timing and deformation histories of individual accretion events is critical for understanding the tectonic evolution of the Cordilleran mountain belt. This dissertation presents investigations into the kinematic and timing histories of two structural systems of the Cordillera in west-central Idaho: the Salmon River suture zone, which is a west-directed ductile fold-thrust system that accommodated deformation associated with the east-directed collision of the Wallowa island arc terrane with the western edge of the North American continent, and the western Idaho shear zone, which is a north-striking, subvertical, dextral-transpressional ductile shear zone that overprinted the eastern portion of the Salmon River suture zone and accommodated northward displacement of the terranes that lay to its west. Chapter 2 is an investigation into the contribution of penetrative ductile deformation to the strain field and large-scale mass transfer in the mid-crustal portions of orogens through the case study sites of Salmon River suture zone and western Idaho shear zone. This chapter presents new kinematic observations and finite and minimum strain measurements of rocks across these structural systems. The thrust sheets in the Salmon River suture zone experienced large amounts of foliation-normal shortening and lineation-parallel stretching that added an estimated 27 km of cumulative thrust displacement to the 34 km of cumulative thrust displacement on the four major thrust faults of the Salmon River suture, and the rocks in the western Idaho shear zone experienced a minimum of 65% subvertical stretching during transpressional shearing. In Chapter 3, new constraints on the timing of ductile shearing in the western Idaho shear zone are presented, using new U-Pb dating of zircon of pre-, syn-, and post-kinematic dikes within the shear zone. The results constrain shearing to have finished by ~85 Ma, which is contemporaneous with the timing of shearing found in published studies in other parts of the western Idaho shear zone. Chapter 4 is a study of the timing and duration of deformation in the Salmon River suture zone, as well as the exhumation and cooling history of the Salmon River suture zone and western Idaho shear zone. In order to constrain the cooling history of the individual zones of theses systems, time-temperature paths were produced using new and published multi-technique thermochronometry (40Ar/39Ar hornblende, muscovite, biotite; zircon and apatite fission-track, zircon and apatite (U-Th)/He) in conjunction with published thermobarometry and thermometry. The resulting time-temperature paths indicate a forward-breaking sequence of ductile thrusting in the Salmon River suture zone between ~124-105 Ma, which is associated with the accretion of the Wallowa terrane to North America, and the paths for the western Idaho shear zone show rapid cooling over ~90-85 Ma, which is consistent with the late stage of ductile shearing constrained in chapter 3.

THE KINEMATIC AND TEMPORAL HISTORIES OF DEFORMATION IN THE SALMON RIVER SUTURE ZONE AND WESTERN IDAHO SHEAR ZONE, IDAHO, USA: INSIGHTS INTO THE EVOLUTION OF AN ISLAND ARC-CONTINENT BOUNDARY

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