No abstract

No abstract

The Salmon River suture zone in west-central Idaho is a steep ocean-continent plate boundary separating Paleozoic-Mesozoic island-arc terranes and the ancestral western Laurentian margin that characterizes much of the central North American Cordillera. In the Riggins region, the most complete record of arc-continent collision and subsequent modification of the accretionary boundary is exposed because of the lower abundance of Cretaceous plutonism as compared to exposures of the boundary regionally along strike, and the deep degree of erosion along the Salmon River Canyon. Using recent mapping, microtectonic analysis, geochronological data, and structural models, this field trip explores the time-transgressive structures currently exposed across the Salmon River suture zone from the eastern foothills of the Seven Devils Mountains into the Salmon River Canyon. The Salmon River suture zone contains a Late Jurassic to Early Cretaceous, west-vergent thrust belt that is overprinted along its eastern extent by the Late Cretaceous, transpressional western Idaho shear zone and Late Cretaceous(?) and Cenozoic ductile-brittle extensional structures. A distinct amalgamation of metavol-canogenic and metasedimentary rocks characterizes the thrust belt and includes the (1) northeastern Wallowa terrane, (2) western Salmon River belt, formally grouped with the Wallowa terrane, and (3) eastern Salmon River belt, known locally as the Riggins Group and Pollock Mountain Amphibolite. The western Idaho shear zone overprints the easternmost rocks and structures associated with the eastern Salmon River belt. It also contains heterogeneous units of tonalite, trondhjemite, and grano-diorite orthogneiss, as well as individual tonalite, granodiorite, and granite plutons that display a gradation and partitioning of deformation and strain internally within the shear zone. East of the magmatic injection zone located along the arc-continent boundary, Laurentian continental metasedimentary rocks and tonalite and granodio-rite plutons occupy the eastern portions of both the shear zone and larger suture zone. Geochronologic data, obtained largely from metaplutonic rocks in the McCall region south of the Riggins region, provide the temporal resolution to constrain current tectonic models proposed for Salmon River suture zone evolution.

The western Idaho shear zone (WISZ) is a Late Cretaceous, mid-crustal exposure of intense shear localized in the Cordillera of western North America. This shear zone is characterized by transpressional fabrics, i.e., downdip stretching lineations and vertical foliations. Folded and boudinaged late-stage dikes indicate a dextral sense of shear. The vorticity-normal section is identified by examining the three-dimensional shape preferred orientation of feldspar populations and the intragranular lattice rotation in quartz grains in deformed quartzites. The short axes of the shape preferred orientation ellipsoid gather on a plane perpendicular to the vorticity vector. In western Idaho this plane dips gently to the west, suggesting a vertical vorticity vector. Similarly, sample-scale crystallographic vorticity axis analysis of quartzite tectonites provides an independent assessment of vorticity and also indicates a subvertical vorticity vector. Constraints on the magnitude of vorticity are provided by field fabrics and porphyroclasts with strain shadows. Together these data indicate that the McCall segment of the WISZ displays dextral transpression with a vertical vorticity vector and an angle of oblique convergence ≥60°. North and south of McCall, movement is coeval on the Owyhee segment of the WISZ and the Ahsahka shear zone. Together, the kinematics of these shear zones are consistent with northeast-southwest-directed convergence. Plate motion in this orientation acting on a curved plate boundary could have produced pure shear-dominated transpression in the Owyhee (a = 40°) and McCall (a = 60°) segments of the WISZ, while causing reverse-sense shearing (a = 90°) in the Ahsahka shear zone.

No abstract