TESTING THE ENTRAPMENT OF THE CACHE CREEK TERRANE IN BRITISH COLUMBIA AND YUKON

The distribution of Quesnellia, Stikinia, Yukon-Tanana (YTT) and Cache Creek (CCT) terranes has lead to several hypotheses regarding the tectonic emplacement of the demonstrably exotic CCT. The hook-shaped Sr isopleths and distribution of Stikinia, YTT and Quesnellia were used to support oroclinal entrapment of the CCT. Testing the orocline hypothesis directly is difficult. Polarity of subduction under the Triassic Stikinia and its rotation are poorly constrained due to sparse geochronological, geochemical and paleomagnetic data. Herein, the underlying assumptions, plate velocities and subduction rates of the oroclinal entrapment model are evaluated. In this model, the Late Triassic to Early Jurassic Stikinia and Quesnellia segments form linear limbs that rotate about an oroclinal hinge. During the closure of the orocline, absolute velocity of the limbs increases from the hinge to the limbs, requiring different rates of subduction and hence highly variably nature of magmatism along strike that contrasts the observed geology. The model also requires decreasing subduction rates over time and, as the orocline closes, predominantly oblique convergence along the limbs and a progressively decreasing driving force for the closure. Placement of the hinge along the northwestern-most extent of Stikinia requires velocities along the limb tips that exceed typical modern upper plate velocities; however, length of limbs, and hence maximum plate velocities, may be underestimated by this hinge because Triassic arc cumulates (e.g., Pyroxene Mountain suite, Yukon) and arc plutons (Taylor Mountain batholith, Alaska) occur outside of the defined terrane boundaries. Constraining the rotational motion to Stikinia alone, as suggested by the extant models, greatly compounds the velocity problem as it requires doubling of the absolute plate velocity along the Stikinia segment. The previously proposed modern analogues, such as the Molucca Sea and Banda Arc, do not adequately explain the observed relationships in BC and Yukon. However, a combination of Kurile-Aleutian-Emperor like configuration for Stikinia-Quesnellia-CCT in the Triassic followed by strike-slip and thrust faulting in the Jurassic may explain the present distribution terranes and the apparent entrapment of CCT.