LATE MESOZOIC TERRANE ACCRETION IN THE NORTHWESTERN US CORDILLERA: IMPLICATIONS FOR COEVAL ‘NON-COLLISIONAL’ OROGENESIS

Integrated structural, geochronological, and metamorphic data provide a basis for synthesizing long-lived ~east-west shortening across the Cordilleran orogen. In the Riggins region of west-central Idaho, the 25+ km-wide, north- to northeast-striking Salmon River suture zone [SRSZ; as defined by Gray, 2013] records latest Jurassic [ca. 145 Ma] to early Late Cretaceous [ca. 92 Ma] contractional tectonism. Although aspects of structural style, degree of strain, and kinematics of deformation differ, temporally overlapping structures are found in the Sevier fold-thrust belt [SFTB] of southeastern Idaho, westernmost Wyoming, and central Utah. With minor exception, high-strain synmetamorphic fabric and related faults [Pollock Mountain, Rapid River, Morrison, Heavens Gate] in the SRSZ are attributed to collision of the Blue Mountains province with western Laurentia. In contrast, age-equivalent structures of the SFTB are explained by late Mesozoic ocean-continent plate convergence and non-collisional orogenesis. Given the coeval nature of contraction in the SRSZ and SFTB, initial collision of the Blue Mountains block coincided with, and possibly initiated, early thrusting in the SFTB [~44-46ºN]. Continued ~east-west contraction across the arc—continent boundary [Cordilleran hinterland] and retroarc region east of the Idaho batholith resulted in >300 km of cumulative shortening. In this scenario, Jura-Cretaceous terrane accretion in the hinterland drove ~east-vergent deformation in upper-crustal sedimentary and volcanic rocks of the SFTB. Accordingly, west- to northwest-vergent contractional structures in the SRSZ [mid-crustal metamorphic rocks] evolved together with eastward-propagating structures of the Sevier belt. An alternative to non-collisional orogenesis, this model implies a collision-related origin and evolution for part of the SFTB.