| The Talkeetna fault is located along the southern margin of the Mesozoic suture zone that juxtaposes an allochthonous island arc, the Wrangellia composite terrane, with the Mesozoic continental margin, the Yukon-Tanana terrane, in south-central Alaska. A discontinuous belt of Mesozoic siliciclastic strata were deposited in the Alaska Range suture zone, an ∼150km wide zone of Mesozoic and Cenozoic basins that record the closing of an ocean basin and the collision of exotic terranes. These strata lie along almost the entire length of the margin of the Northern Cordillera, recording the history of the closing of this ocean basin. New geochronologic, structural, and mapping data from the Clearwater Mountains of south-central Alaska demonstrate that the Talkeetna fault is not a major thrust fault, but likely represents a normal fault overprinting a relict, deep-seated, strike slip terrane boundary. LA-ICPMS analysis of zircons from 5 detrital and 2 igneous samples collected from the Clearwater Mountains reveal a unique distribution of age peaks. Four detrital samples from fine-grained strata contain almost exclusively Mesozoic age zircons with single major peaks of ∼150 Ma that represent the maximum depositional age of the strata. These strata contain none of the younger zircon signatures commonly found elsewhere in the Alaska Range suture zone. A detrital sample from a structurally unconformable boulder conglomerate juxtaposed against fine-grained siliciclastic strata contains Mesozoic, Paleozoic, and Precambrian grains with a maximum depositional age of ∼85 Ma. Zircon ages from igneous rocks within the fine-grained siliciclastic strata, an intrusive gabbro (∼142 Ma) and an interlayered volcanic flow (∼149 Ma), provide additional constraints on the depositional age of the basin. Collectively, the U-Pb geochronologic data show that the sediments of the Clearwater Mountains were deposited in a location far from the continental margin with material shed from outboard terranes. With no potential sources in the immediate vicinity able to provide detritus of this age to the strata of the Clearwater Mountains, it is clear that these rocks must have been translated along the margin on a strike slip fault. |
