THE SEDIMENTARY AND MAGMATIC RECORD OF THE MESOZOIC-CENOZOIC TRANSITION ALONG THE NORTHWESTERN CORDILLERA

The Mesozoic to Cenozoic transition along the southern convergent margin of Alaska is defined by fundamental changes in sedimentary basin types, structural styles, and magmatism. In south-central Alaska, the main focus of this talk, this transition is marked by at least four major tectonic stages. Stage I includes mainly submarine fan deposition in Late Jurassic to Early Cretaceous retroarc foreland and remnant ocean basins located between the colliding Wrangellia composite terrane (WCT) and Mesozoic continental margin rocks. The final stages of this collisional event marked a change from an oceanic arc system to a Late Cretaceous continental arc system which marks the beginning of Stage II. Establishment of this continental arc system resulted in the development of an extensive retroarc foreland basin system. New radiometric ages from bentonite layers and maximum depositional ages based on U-Pb detrital zircon ages indicate nonmarine and shallow marine deposition from ~ 75 – 65 Ma. Stage III is characterized by a change from Cretaceous deposition in the retroarc foreland basin to localized Paleocene-Eocene deposition in nonmarine strike-slip basins, inboard magmatism, and exhumation. During this stage, volcanic strata of the Cantwell Formation (60-55 Ma) and co-magmatic McKinley sequence granitic plutons were erupted and intruded along the former Mesozoic suture zone between the WCT and the former continental margin rocks. Magmatism was coincident with and in some cases related to Paleocene-Eocene shallow subduction of a spreading ridge. Ridge subduction in the forearc region resulted in surface uplift, unconformity development, and a change from marine to nonmarine environments. Stage IV is defined by the Oligocene to Recent collision of the Yakutat terrane, an oceanic plateau, with the southern margin of Alaska. This collision has prompted growth of the largest coastal mountain range on Earth, exhumation of older forearc basinal strata, and renewed uplift of the Alaska Range. Surface uplift of the central Alaska Range during the late Oligocene to Recent is supported by: detrital zircon geochronology, the spatial pattern of regional basin development, and thermochronologic cooling ages that show the onset of rapid uplift across parts of the central Alaska Range region and Mesozoic basin inversion at ~25 Ma.