REACTIVATED MARTIN BRIDGE\WILD SHEEP CREEK BOUNDARY ACROSS THE EASTERN WALL OF RAPID RIVER CANYON; RIGGINS, IDAHO

The north-to northeast-striking, 25+ km-wide Salmon River suture zone (SRSZ) of western Idaho straddles accreted late Paleozoic to Mesozoic island-arc rocks of the Blue Mountains province and western Laurentia. Across the western SRSZ and eastern wall of lower Rapid River canyon, near Riggins, Idaho, tectonized marble of the Late Triassic Martin Bridge (MB) Formation overlies Middle to Late Triassic volcanogenic rocks of the Wild Sheep Creek (WSC) Formation. This younging-upward age relationship in the eastern Wallowa terrane raises the question of whether the local WSC\MB contact (~N10E 40SE), interpreted by most as a west-vergent thrust fault, is tectonic or preserves an unconformity in the island arc assemblage. Asymmetric mesoscopic folds observed in WSC rocks below and truncated by the contact show southeast vergence. In this area, the Early Cretaceous Fish Hatchery Stock (~130 Ma; U-Pb zircon) intrudes the WSC and contains low-angle slip surfaces possibly coeval with movement on the WSC\MB boundary. Field relations support dip-slip reactivation of a depositional contact originally separating units on the flanks of a submarine volcano. However, additional work is needed before confirming east-side up ‘thrusting’ (e.g., Aliberti, 1988) or normal motion (a new model considered here). If east-side up displacement, then shortening may relate to contraction in the Rapid River thrust system (basal structure?). Alternatively, normal slip along the unconformity involves collapse of this island arc-continent collisional orogen (SRSZ).