FROM ISLAND ARC TO CONTINENT: TRANSECTING WEST-CENTRAL IDAHO

From west to east, this unbroken 50-km transect spans Hells Canyon of the Snake River, the northern Seven Devils Mtns., and lower Salmon River Canyon at the latitude of Riggins, Idaho [~45.3N]. Through strip-map and structural section, fabric element relationships across the mid- to Late Cretaceous Salmon River suture zone are highlighted in [1] accreted oceanic rocks of the Blue Mountains province, [2] Laurentian continental margin rocks, and [3] plutons emplaced into this crustal-scale convergent boundary. In the west, Permo-Triassic arc volcanogenic rocks of the Wallowa terrane are deformed in upright, symmetric, gentle-to-open NNE- and E-trending folds which form extensive dip slopes [western foothills of Seven Devils, Granite Ck. drainage]. These broad wavelength structures are cut by systematic, NW-striking cross joints of regional extent. In central portions, greenschist facies tectonites overlie massive basaltic-andesite flows across the ENE-dipping post-metamorphic [?] Heavens Gate fault. East of the fault, strong linear-planar fabric deforms pebble-to-boulder, clast-supported metaconglomerate. Long-to-short strain axes [X:Z] of volcanic and carbonate clasts locally exceed 10:1, where X defines a moderately ESE-plunging stretching lineation [L₁] on the regional synmetamorphic foliation [S₁]. High strain greenschist+amphibolite LS tectonites are traced >25 km eastward—across Rapid River and Pollock Mountain thrust plates—into the Salmon River Canyon before steepening to vertical at the arc-craton boundary [Sr_i-0.706 isopleth]. Superimposed on early penetrative elements are thin [<5 m-wide, mesoscale], shallow ENE-dipping mylonitic shears [S₂] which are best developed in the ~105 Ma Crevice pluton. Late-stage, NW-striking spaced/crenulation cleavage [S₃] is axial-planar to the SE-plunging Riggins synform and Lake Creek antiform. Across the suture zone, structures are remarkably symmetrical; i.e., from triaxially-deformed lithic clasts in the west to rotated garnet porphyroblasts, feldspar porphyroclasts, and boudinaged pegmatite veins in the east. Combined structural/geochronological studies across this transect indicate the laterally continuous, L₁-S₁ fabric elements formed over a progressive flattening strain history that post-dates 136 Ma [Gray et al. 2012].