T230 Tectonic Development of the Northern North American Cordillera

Wednesday, 12 October 2011
8:00 AM - 12:00 PM
Room 200DE

PROVENANCE OF JURASSIC TO CRETACEOUS FLYSCH ALONG THE TALKEETNA-CHILCHITNA FAULT SYSTEM HIGHLIGHTS THE NORTHERN BOUNDARY OF THE WRANGELLIA COMPOSITE TERRANE, ALASKA

Paper number: T230-6
Program: Technical Sessions
Time: 9:20 AM – 9:35 AM

Authors

Abstract

The Talkeetna and Chilchitna faults are segments of a 500-km-long high-angle fault system (TCFS) running from the Clearwater Mts. to the Lake Clark region. Provenance data for Jurassic to Cretaceous flysch on either side of the TCFS suggest that the flysch units are dissimilar. Flysch north of the TCFS overlies terranes that were part of the paleo-Alaskan continental margin (Farwell and Yukon-Tanana), whereas flysch south of the TCFS overlies the allochthonous Wrangellia composite terrane (WCT). Paleocurrent indicators north of the TCFS show transport was to the south, opposite of paleocurrents south of the TCFS. Lithic wacke north of the TCFS contains abundant sedimentary rock fragments, metamorphic rock fragments, and quartz grains; point counts suggest derivation from a recycled orogen. In contrast, feldspathic wacke south of the TCFS contains abundant plagioclase grains and volcanic rock fragments; point counts suggest derivation from a volcanic arc. Clasts in conglomerate north of the TCFS match terranes to the north, whereas clasts south of the TCFS match the WCT.

Detrital zircon ages also differentiate the flysch on either side of the TCFS. Samples north of the TCFS contain on average 54% Mesozoic, 14% Paleozoic, and 32% Precambrian zircons, reflecting derivation from terranes of continental affinity. In comparison, samples south of the TCFS contain on average 94% Mesozoic, 1% Paleozoic, and 5% Precambrian zircons; consistent with the abundant Mesozoic magmatic arc rocks of the WCT. In particular, age distributions south of the TCFS contain a dominant Middle Jurassic to Early Cretaceous population matching the Late Jurassic to Early Cretaceous Chitina arc and Middle to Late Jurassic plutons in the WCT, whereas flysch north of the TCFS does not contain this population.

These provenance data suggest that flysch overlying the WCT is distinct from the flysch overlying the paleo-Alaskan margin. Paleomagnetic data from late Mesozoic and early Tertiary rocks of the WCT suggest it was located far south relative to rocks north of the TCFS. In conjunction with available geophysical models, these data suggest that the TCFS is a through-going crustal-scale feature and that the TCFS represents the northern boundary of the WCT.