THE SECRET LIVES OF MOUNTAIN BELTS REVEALED BY U-PB DATING OF DETRITAL ZIRCON SUITES: THE LATE JURASSIC-EARLY CRETACEOUS BROADER BROOKS RANGE-VERKOYANSK OROGEN, ARCTIC ALASKA AND NE RUSSIA

The Brooks Range thrust belt is a classic N-vergent orogen with a telescoped S-facing continental margin sequence and structurally overlying oceanic and ultramafic allochthons. However, the driver or collider for orogenesis is missing, having rifted southward away from the orogen and subsequently buried by deep water basinal successions shortly after thrust faulting. These same relationships characterize a much broader sector of the Arctic for an along strike distance of more than 3000 km including the northern paleo-Pacific margin of NE Russia, where continental margin fold -thrust belts and syn-orogenic sedimentary rocks shed towards the exterior of the belt are coeval based on fossil control and cross-cutting relationships.

Detrital zircon studies from syn-orogenic sedimentary rocks along this belt show remarkable similarities and reveal the age range of the now missing arc system involved in orogenesis. Youngest zircons range from ~ 140-180 Ma, indicating that shortening between a Jurassic arc and the continental margin ultimately drove this orogenic event. The next older group of zircons ranges from ~ 250-320 Ma on the west to 200-320 on the east, with most of the population between 230 and 320 Ma. The colliding arc in the Russian sector included crystalline basement dated at ~1.8-2.0 Ga. In the Bering Strait region and Alaska, older components are more variable, represented by Siluro-Ordovician (420-480 Ma), Neoproterozoic (550-750 Ma) and 1-2 Ga and older zircons.

This data set suggests that it might be possible to locate and determine which terranes of the paleo-Pacific margins of NE Russia and the Cordillera could represent the remaining fragments of this arc terrane. In the inner zone of the Verkoyansk fold belt, upper Paleozoic and Mesozoic arc volcanic sequences have been identified. Similarly, in Alaska, the Jurassic Talkeetna arc was likely built upon an upper Paleozoic arc within the greater Wrangelia-Alexander Terrane.