U-PB AND HF ISOTOPE DETRITAL ZIRCON GEOCHRONOLOGY OF THE NUTZOTIN MOUNTAINS SEQUENCE, SOUTH-CENTRAL ALASKA: IMPLICATIONS FOR THE CONFIGURATION OF THE MESOZOIC CONVERGENT MARGIN, NW CORDILLERA

The Nutzotin Mountains Sequence (NMS) is a 3-km-thick, Jurassic-Cretaceous marine sedimentary and volcaniclastic sequence located between oceanic rocks of the accreted Wrangellia Composite terrane (WCT) and continental rocks of the Ancestral North American margin. Determining the timing of deposition, and provenance of the NMS have important implications for ongoing debates on the Mesozoic configuration of the continental margin. To better address these questions, we present new U-Pb and Hf isotope detrital zircon analyses from sandstone samples as well as new field data from the NMS.

All analyzed samples from the NMS exhibit a unimodal peak age, with maximum depositional ages younging up-section (155 Ma, 154 Ma, 145 Ma, 144 Ma, 144 Ma, and 133 Ma). Our samples yield predominantly Mesozoic U-Pb zircon ages, with subordinate Paleozoic and Precambrian ages (<1% each). Hf_(t) values range from -0.5 to +11, indicating that zircons were sourced from intermediate and juvenile magmas. The U-Pb and Hf zircon data suggest a WCT affinity for the NMS, indicating that the WCT was not proximal enough to the continental margin for the NMS to receive significant continental detritus by 133 Ma. Previously reported U-Pb detrital zircon ages from nonmarine strata stratigraphically above the NMS indicate that continental detritus was being supplied to the basin by ~117 Ma (Koepp et al., 2017). Combined with published zircon ages from Mesozoic strata in the Alaska forearc basin and accretionary prism, our data indicate that the NMS and forearc basin record the progressive exhumation of the Talkeetna (201-153 Ma), Chitina (155-140 Ma), and Chisana (140-115 Ma) arcs, which were all built upon the WCT. Synthesizing regional detrital zircon datasets suggest that the NMS was deposited in a deep marine, back-arc basin relative to the coeval arc(s). Our findings provide new insights for ongoing debates about the tectonic configuration of the Mesozoic convergent margin; different published interpretations include oblique closure of an east-dipping subduction zone, a west-dipping subduction zone inboard of the WCT, westward subduction beneath a ribbon continent, and a sinistral continental margin transform. These different interpretations have wide-ranging ramifications for the growth of the North American continent.