DETRITAL GEOCHRONOLOGIC RECORD OF JURASSIC OCEANIC ARC GENESIS, ARC-CONTINENT COLLISION, AND CRETACEOUS CONSTRUCTION OF A CONTINENTAL ARC IN A FOREARC BASIN, MATANUSKA VALLEY-TALKEETNA MOUNTAINS, ALASKA

Forearc basin strata provide a rich archive of tectonic processes along active convergent margins. The Mesozoic Matanuska Valley-Talkeetna Mountains (MVTM) forearc basin in southern Alaska contains one of the best-preserved records of an intraoceanic arc-continent collision. This study uses the U-Pb ages of detrital zircons in the MVTM forearc basin strata to document convergent margin processes during precollisional, collisional, and postcollisional stages. The precollisional stage is recorded in the Lower to Upper Jurassic Talkeetna, Tuxedni, and Naknek Formations, which exhibit unimodal detrital zircon ages of 175, 167, and 151 Ma, respectively. Integration of these detrital ages with published geochronologic ages from adjacent oceanic arc plutons shows that the youngest population of detrital zircon ages preserved in forearc strata approximates the age of progressively younger igneous rocks that were emplaced into the arc edifice, eroded, and transported into the forearc basin. The onset of the collisional stage in the MVTM basin is marked by an upsection increase in coarse-grained clastic detritus in the Upper Jurassic Naknek Fm., an angular unconformity between the Naknek Fm. and overlying Cretaceous Matanuska Fm., and a magmatic hiatus documented by a paucity of 145-100 Ma detrital zircon grains in the sampled Mesozoic sandstone. The postcollisional stage, recorded in the Cretaceous Matanuska Fm., is characterized by a broader distribution of detrital zircon ages, including a dominant Late Cretaceous mode (ca. 95–75 Ma) sourced from a newly established continental arc, a subordinate Jurassic population reflecting continued erosion of the accreted, exhumed oceanic arc, and sparse Paleozoic-Precambrian ages transported from retroarc continental sources. Recent studies reveal similar broadening of detrital zircon ages in the adjacent Chugach accretionary prism from Jurassic to Cretaceous time. Analyzing the Mesozoic detrital zircon record of the MVTM forearc basin strata documents a protracted record of the growth of an intraoceanic arc-forearc basin system, arc collision, and development of a continental arc-forearc basin system. Our findings provide a template to evaluate links between magmatic arc-forearc basin systems and collisional processes along convergent plate boundaries.