DETRITAL ZIRCON PROVENANCE ANALYSIS OF CRETACEOUS–OLIGOCENE SEDIMENTARY STRATA FROM THE MATANUSKA VALLEY-TALKEETNA MOUNTAINS FOREARC BASIN, SOUTHERN ALASKA

Geochronological analysis of detrital zircon populations from the Matanuska forearc basin provides improved constraints on the tectonic evolution of the southern Alaska convergent margin. U-Pb geochronologic analyses of detrital zircon grains from 18 Cretaceous–Oligocene samples constrain surface exhumation of adjacent magmatic arc plutons and accretionary prism strata. Detrital ages also permit recognition of previously undocumented phases of arc magmatism and long-distance transport of detritus from inboard accreted terranes.

The detrital-zircon age spectra are dominated by Late Cretaceous–Paleocene (65% of all grains) and Jurassic–Early Cretaceous (26%) populations. Minor populations include Eocene (2%), Penn–Permian (4%), and Precambrian (1.5%) ages. These age distributions confirm that Jurassic–Paleocene arc plutons exposed along the northern basin margin were a primary source terrane. Late Cretaceous–Paleocene plutons represent a contemporaneous continental arc that was rapidly exhumed. Jurassic plutons correspond to an oceanic arc that accreted prior to deposition. A broad distribution of Early Cretaceous detrital zircon ages suggests erosion of plutons that have been completely exhumed or have not yet been recognized in the study area. Sparse Penn–Permian and Precambrian grains indicate contributions from the accreted Wrangellia and Yukon-Tanana terranes, which crop >30–300 km north of the arc plutons. All samples lack detrital age peaks <56 Ma, consistent with prior studies that indicate Paleocene cessation of arc magmatism.

Variations in detrital-zircon age populations constrain the paleogeographic evolution of the forearc basin. Campanian-Maastrichtian marine strata yield populations that reflect derivation exclusively from the arcward margin of the basin. Detrital ages from Paleocene–Oligocene nonmarine strata record subaerial uplift of marine accretionary prism deposits and development of a two-sided basin with axial drainage. Northern P-O samples yield nearly unimodal age distributions derived from geographically restricted catchments that drained arc plutons. Southern P-O samples have broader, more variable age populations consistent with erosion of accretionary prism strata and spatially restricted Early Jurassic plutons.