Initial Publication Date: June 14, 2024

The important role of ductile strain for accommodating mass transfer and exhumation in contractional orogens: Case studies from the Salmon River suture zone fold-thrust system and the transpressional western Idaho shear zone

Andrea Richardson, Washington State University
Sean Long, Washington State University
William Barba, Washington State University
Matthew McKay, Missouri State University

Abstract

Quantifying the magnitude of penetrative strain accommodated by ductile shearing is fundamental for understanding the 3D strain field of orogenic systems. Here, we investigate ductile strain and kinematics across two Cordilleran structural systems in north-central Idaho: the Salmon River suture zone (SRSZ), which is a top-to-west ductile fold-thrust system that accommodated shortening associated with terrane collision between ca. 144-105 Ma, and the western Idaho shear zone (WISZ), which is a north-striking, subvertical shear zone that accommodated dextral-transpressional shearing between ca. 105-86 Ma. We performed the Rf-phi method on stretched sand- to pebble-sized clasts to measure finite strain in three penetratively deformed thrust sheets in the SRSZ. Moving structurally upward (eastward), the Heavens Gate, Morrison Ridge, and Rapid River thrust sheets record 61%, 85%, and 63% average lineation-parallel extension and 35%, 46%, and 51% average foliation-normal shortening, respectively. Thrust-parallel stretching fed >27 km of cumulative displacement to the up-dip portion of the fold-thrust system, which is comparable to the 34 km of total thrust displacement that we estimate at down-dip levels. The SRSZ therefore demonstrates that distributed, thrust-parallel stretching can contribute as significantly to the strain field as discrete thrust displacement. Thrust-parallel stretching translated rocks in these thrust sheets to as much as 5.4 km shallower depths than they would have under a scenario with no stretching, and thrust-normal thinning removed 7.6 km of total overburden above rocks at the base of the SRSZ. These observations demonstrate the significant contribution that ductile strain can make to exhumation. In the WISZ, we measured stretched (boudinaged) and folded granitic dikes to estimate late-stage (ca. 91-86 Ma) ductile strain. Our measurements define 65% minimum subvertical, lineation-parallel stretching and 50% minimum subhorizontal, east-west-directed shortening. We documented dextral kinematics in lineation-normal view planes across the width of the WISZ, and dominant top-down-to-east kinematics in foliation-normal, lineation-parallel view planes in the eastern part of the WISZ. 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 ca. 91-86 Ma, which is consistent with our observation of dominant top-down-to-east kinematics in the eastern WISZ. The WISZ therefore demonstrates the significant role that distributed lineation-parallel stretching can play in accommodating exhumation of deep-seated rocks in transpressional settings. The SRSZ and WISZ both demonstrate the 1st-order importance of distributed ductile stretching for accommodating mass transfer and exhumation in fold-thrust and transpressional systems.

Session

Convergent and transpressional orogens