THE CONTRIBUTION OF DUCTILE STRETCHING TO EXHUMATION AND MASS TRANSFER IN FOLD-THRUST AND TRANSPRESSIONAL SYSTEMS: AN INVESTIGATION OF STRAIN AND KINEMATICS IN THE SALMON RIVER SUTURE ZONE AND WESTERN IDAHO SHEAR ZONE

Penetrative ductile strain can make a significant contribution to the 3D strain field in contractional structural systems, making it vital for understanding the deformation processes that operate during orogenesis. Here, we investigate ductile strain and kinematics in two Cordilleran structural systems in north-central Idaho: the Salmon River suture zone (SRSZ), a W-vergent ductile fold-thrust system that accommodated shortening associated with terrane collision between ~144-105 Ma, and the N-striking, subvertical western Idaho shear zone (WISZ), which accommodated dextral-transpressional shearing between ~105-86 Ma. We collected finite strain data from stretched clasts in three penetratively deformed thrust sheets in the SRSZ, which define 61-85% average thrust-parallel stretching and 35-51% average thrust-normal shortening. Thrust-parallel stretching fed >27 km of cumulative displacement to the up-dip portion of the fold-thrust system, which is comparable to the 29 km of cumulative thrust displacement estimated at down-dip levels. Thus, the SRSZ demonstrates that distributed, thrust-parallel stretching can be as significant a component of the strain field as discrete thrust displacement. Thrust-parallel stretching elevated rocks in individual thrust sheets as much as 5.4 km structurally higher than compared to a scenario with no stretching, and related thrust-normal thinning removed 7.6 km of total overburden, demonstrating the large contribution of ductile strain to exhumation. To the east in the WISZ, we documented dextral kinematics in lineation-normal view planes. We measured boudinaged and folded granitic dikes to estimate late-stage (~91-86 Ma) bulk strain in the WISZ, which yielded 65% minimum subvertical, lineation-parallel stretching and 50% minimum E-W shortening. Using published barometry and thermochronometry, we estimate that subvertical stretching in the WISZ accommodated >9-10 km of exhumation relative to the Idaho batholith to the east between ~91-86 Ma, which is consistent with the dominant top-down-to-E kinematics that we observed in lineation-parallel view planes in the eastern WISZ. Therefore, the WISZ demonstrates that distributed lineation-parallel stretching is a 1^st-order process for accommodating exhumation in transpressional settings.