MAGMA-FACILITATED TRANSPRESSIONAL DEFORMATION WITHIN THE IDAHO BATHOLITH (SAWTOOTH METAMORPHIC COMPLEX, IDAHO): A ZONE ACCOMMODATING CRETACEOUS OROGEN-PARALLEL TRANSLATION

Detailed structural analysis of metasedimentary rocks and geochronologic data of intrusive rocks within the Atlanta lobe of the Idaho batholith provide insight to the relationship between transpression and magmatism in the northern U.S. Cordillera. The Sawtooth metamorphic complex (SMC) contains strongly deformed medium to high-grade paragneisses and intrusive rocks that preserve syntectonic magmatic foliations. Geologic mapping of the SMC reveals alternating contraction- and shear-dominated domains across strike. The contraction-dominated domains are north-south striking and consist of centimeter- to several hundred meter-scale, shallowly to steeply plunging upright folds and associated thrusts. The shear-dominated domains are characterized by highly strained north-south striking, subvertical foliations and subhorizontal lineations. Pervasive S-C structures, winged porphyroclasts, and asymmetrical folds on the movement plane consistently indicate a dextral sense of shearing. Syntectonic intrusive sheets (centimeter- to meter-scale) are common in the shear-dominated domains. The folds, thrusts, and sheared fabrics (including the magmatic foliations) in the two types of domains are structurally compatible and kinematically linked, and are, therefore, interpreted to be broadly synchronous. Consequently, structures within the SMC represent a wrench-dominated transpressional zone, in which the regional strain partitions into the contraction- and shear-dominated domains facilitated by emplacement of syntectonic magmas and lithologies with different strengths. U-Pb dating of zircons from the syntectonic intrusive sheets indicates that deformation of the SMC transpressional zone occurred mainly between ca. 91-87 Ma and ca. 80 Ma, and ended by ca. 73 Ma. This deformational history is also reflected in the ages of metamorphic rims on detrital zircons of the SMC metapsammites. The SMC transpressional zone is kinematically compatible and was coeval with deformation of the western Idaho shear zone. This suggests that the SMC and the western Idaho shear zone were parts of a regional transpressional 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.