CACHE CREEK OROCLINAL ENTRAPMENT: TRIBULATIONS AND TRIALS FOR A TEENAGE HYPOTHESIS

The Tectonic Assemblage Map of the Canadian Cordillera (Wheeler and McFeely, 1991) was a seminal contribution arising from the Decade of North American Geology. A fundamental feature of this map is the enigmatic arrangement of assemblages in the central Cordillera: exotic, ophiolitic, blueschist-bearing oceanic crust along a central axis (Cache Creek terrane, CCT), enveloped to the north, east and west by less exotic Paleozoic and Mesozoic arc terranes (Stikine and Quesnel, ST-QN), enveloped in turn by pericratonic, continental margin terranes (Kootenay, Yukon-Tanana). An explanation of this arrangement of assemblages was proposed in 1991 (presented in full by Mihalynuk et al., 1994) that invoked oroclinal bending of an arc-subduction complex. Geological events that could be predicted by this oroclinal hypothesis include crustal thickening and rapid exhumation of the orocline hinge, where, in 2003, >2.8 GPa eclogite clasts were found in Early Jurassic strata. Oroclinal geometry is mimicked by the intrusion-related metallogenic belt of QN-ST, where recent age dating from the Minto deposit provides a deformed orocline hinge zone linkage between the two Triassic-Jurassic porphyry belts. A potential flaw in the hypothesis, the possible non-contemporaneity of Tethyan strata upon which the exotic nature of CCT is based, has been resolved with discovery of additional families and ages of exotic fauna. However, the model has not yet been fully tested. A robust geodynamic explanation for the orocline is lacking, linkages between ST and outboard terranes have only been hinted at, and reconciliation of geology and tectonics with that in the USA, beyond the southern limits of ST and QN, is incomplete. Definitive paleomagnetic tests still need to be conducted; although they will likely be plagued by difficulties of attaining primary remnant magnetization in Mesozoic and older rocks as well as the large variable rotations found in other oroclines. Polarity of the Triassic Stikine arc needs to be established to distinguish between oroclinal closure and large-scale transcurrent duplication. Such defining studies and tests, as well as linkages to older and younger events, will dictate whether or not the oroclinal entrapment hypothesis survives its reckless teenage years.