DETRITAL ZIRCON RECORD OF THE EXHUMATION OF THE HIGHEST MOUNTAIN BELT IN NORTH AMERICA: CENTRAL ALASKA RANGE, TANANA BASIN, ALASKA

U-Pb detrital zircon geochronology from the Neogene Tanana foreland basin, located along the northern flank of the Alaska Range, documents a transition from northern foreland sources (Precambrian to Mesozoic metamorphic and igneous rocks ) to southern hinterland sources (Mesozoic to Cenozoic igneous rocks and sedimentary strata). Detrital zircon samples were collected from a continuous measured section of the basin strata which consists of ~800 m of the Miocene Usibelli Group (2 samples) and ~1200 m of the Pliocene Nenana Gravel (2 samples). The lower Miocene sample from the Usibelli Group has a dominant Precambrian source (Pc-Pz-Mz-Cz – 42/28/23/7%) with age peaks at 1.8-2.0 Ga and 2.6 Ga indicative of the foreland source region (Yukon composite terrane, 1.7-2.0 Ga and 2.5-2.8 Ga). The middle Miocene Usibelli Group sample, in contrast, has dominant age peaks at 57-65 Ma, 158-166 Ma, and 184-210 Ma (Pc-Pz-Mz-Cz – 8/13/53/26%), all of which can be correlated to hinterland plutonic sources (Upper Cretaceous McKinley sequence and the Jurassic-Triassic Talkeetna arc). The lower Pliocene Nenana Gravel sample has similar age peaks to the middle Miocene Usibelli Group sample (Pc-Pz-Mz-Cz – 6/4/51/39%) with peak ages at 65-77 Ma and 190-205 Ma, whereas the upper Nenana Gravel sample (Pc-Pz-Mz – 16/9/75%) has a dominant peak from 97-105 Ma in addition to the Late Cretaceous, Jurassic and Triassic peaks.

The changes in provenance documented in the detrital zircon geochronology are coupled with timing of Neogene foreland basin development along the northern margin of the central Alaska Range: (1) The lower Miocene sedimentary strata represent the initiation of foreland basin deposition and were sourced from northern foreland sources. (2) Continued development of the thrust belt along the southern margin of the basin exhumed hinterland Mesozoic igneous sources and resulted in deposition in the foredeep part of the basin. (3) By Pliocene time, deposition of the Nenana Gravel occurred in the wedge-top part of the foreland basin and is characterized by hinterland Mesozoic primary sources and secondary recycled grains derived from Cenozoic strata incorporated and exhumed in the Neogene thrust belt.