Magma-facilitated transpressional strain partitioning within the Sawtooth metamorphic complex, Idaho: A zone accommodating Cretaceous orogen-parallel translation in the Idaho batholith

Abstract

Structural data from metasedimentary rocks and geochronologic data from intrusive rocks in the Idaho batholith provide evidence for the relationship between deformation and magmatism in the northern U.S. Cordillera. The Sawtooth metamorphic complex (SMC), Idaho, is exposed as an inlier in the central Idaho batholith and contains strongly deformed metasedimentary and intrusive rocks. Geologic mapping reveals north-south-striking, alternating contraction- and shear-dominated domains across strike. The contraction-dominated domains consist of centimeter- to tens of meter-scale, shallowly to steeply plunging upright folds with subhorizontal lineations. The shear-dominated domains are characterized by highly strained subvertical foliations, subhorizontal lineations, and syntectonic intrusive sheets. Pervasive S-C structures, winged porphyroclasts, and asymmetric folds indicate dextral strike-slip shearing. The fabrics in the two types of domains are structurally compatible and are interpreted to be broadly synchronous. This work suggests that the SMC structures represent a wrench-dominated transpressional zone, in which the regional strain partitioned into the contraction- and shear-dominated domains and the partitioning was facilitated by emplacement of syntectonic magmas. Zircon U-Pb data of the syntectonic intrusions indicate that the SMC transpressional deformation occurred mainly between ca. 95-92Ma and ca. 84Ma and had ended by ca. 77Ma. The transpressional deformation in the SMC and the western Idaho shear zone (WISZ) were kinematically compatible and partially coeval. This suggests that the SMC and WISZ represent a regional transpression system and that crustal deformation inboard of the continental margin may have contributed to the northward orogen-parallel translation of accreted terranes during the Late Cretaceous.