Sedimentary record of transpressional tectonics and ridge subduction in the Tertiary Matanuska Valley-Talkeetna Mountains forearc basin, southern Alaska

Abstract

The Chickaloon, Arkose Ridge, Wishbone, and Tsadaka Formations consist of more than 2800 m of Paleocene-Oligocene sedimentary and volcanic strata that are the products of sedimentation, volcanism, and faulting in the Matanuska Valley-Talkeetna Mountains forearc basin. These deposits provide a record of early Tertiary tectonic processes that formed the southern Alaska convergent margin. The northern margin of the forearc basin is characterized by nonmarine sandstone, conglomerate, and minor mudstone that interfinger with volcanic strata. On the basis of lithofacies, paleocurrent, and compositional data, the northern basin margin deposits are interpreted to represent southward prograding alluvial-fluvial systems. New 4 0 Ar/ 3 9 Ar ages from detrital feldspars in volcaniclastic sandstone and from igneous clasts in conglomerate suggest that these deposits were derived from Middle Jurassic and Paleocene-Eocene volcanic arc-related rocks. Stratigraphic and structural data from the northern basin margin adjacent to the Castle Mountain fault document syndepositional faulting that produced footwall growth synclines in the forearc basin. Paleocene-Oligocene strata exposed along the southern margin of the forearc basin are characterized by nonmarine sedimentary deposits that lack volcanic strata. Lithofacies, paleocurrent, and compositional data from these deposits are interpreted as recording northward prograding alluvial-fluvial systems that were derived from metavolcanic and metasedimentary source terranes of the accretionary prism. Both northern and southern basin-margin deposits merge into basin-axis deposits characterized by thick sections of carbonaceous mudstone and coal, and minor channelized sandstone. Basin-axis strata are interpreted as products of high-sinuosity fluvial and lacustrine environments that drained southwestward into the ancestral Cook Inlet basin. Unlike most previously studied ancient forearc basins, the Matanuska Valley-Talkeetna Mountains basin contains a fairly complete stratigraphic record of nonmarine sedimentation and volcanism. These deposits record multiple episodes of transpressional deformation that may be related to northward translation of the forearc basin along the continental margin, oroclinal bending of Alaska, and/or subduction of a spreading ridge. To evaluate the record of ridge subduction in Paleocene-Oligocene forearc basin deposits, two reconstructions are presented. In one reconstruction, the forearc basin and accretionary prism were translated northward as a single block with most displacement accommodated along inboard dextral strike-slip faults such as the Castle Mountain and Denali fault systems. In this reconstruction, ridge subduction beneath the forearc basin would have occurred at ca. 54-50 Ma, coeval with basinward progradation of coarse-grained deposystems, and with syndepositional displacement and growth-syncline development along the Castle Mountain fault. In the second reconstruction, in addition to displacement on inboard strike-slip faults, significant northward displacement was accommodated along the Border Ranges and Hanagita faults. These fault systems separated the forearc basin from the accretionary prism. In this reconstruction, the forearc basin and accretionary prism were translated separately and have different displacement histories; ridge subduction beneath the forearc basin would have occurred at ca. 61-58 Ma. The sedimentary record of ridge subduction in this reconstruction is represented by a basinwide unconformity and/or deposition of relatively fine-grained deposits in the forearc basin. We prefer the first reconstruction, but until additional high-resolution geochronological data are available, and the displacement histories of major fault systems are better known, both reconstructions are feasible.