The tectonic significance of the Early Cretaceous forearc-metamorphic assemblage in south-central Alaska based on detrital zircon U-Pb dating of sedimentary protoliths

Abstract

A complex array of faulted arc rocks and variably metamorphosed forearc accretionary complex rocks form a mappable arc-forearc boundary in southern Alaska known as the Border Ranges fault (BRF). We use detrital U-Pb zircon dating of metasedimentary rocks within the Knik River terrane in the western Chugach Mountains to show that a belt of Early Cretaceous amphibolite-facies metamorphic rocks along the BRF was formed when older mélange rocks of the Chugach accretionary complex were reworked in a sinistral-oblique thrust reactivation of the BRF during a period of forearc plutonism. The metamorphic subterrane of the Knik River terrane has a maximum depositional age of 156.5 ± 1.5 Ma and a detrital zircon age spectrum that is indistinguishable from the Potter Creek assemblage of the Chugach accretionary complex, supporting correlation of these units. These ages contrast strongly with new and existing data that show Triassic to Earliest Jurassic detrital zircon ages from metamorphic screens in the plutonic subterrane of the Knik River terrane, a fragmented Early Jurassic plutonic assemblage generally interpreted as the basement of the Peninsular terrane. Based on these findings, we propose new terminology for the Knik River terrane. We propose the terms: (1) “Carpenter Creek metamorphic complex” for the Early Cretaceous “metamorphic subterrane”; (2) “western Chugach trondhjemite suite” for the Early Cretaceous forearc plutons within the belt; (3) “Friday Creek assemblage” for a transitional mélange unit that contains blocks of the Carpenter Creek complex in a chert-argillite matrix; and (4) “Knik River metamorphic complex” in reference to metamorphic rocks engulfed by Early Jurassic plutons of the Peninsular terrane that represent the roots of the Talkeetna arc). The correlation of the Carpenter Creek metamorphic complex with the Chugach mélange indicates that the trace of the Border Ranges fault lies ~1–5 km north of the map trace shown on geologic maps, although like other segments of the Border Ranges fault, this boundary is blurred by local complexities within the Border Ranges fault system. Ductile deformation of the mélange is sufficiently intense that few vestiges of its original mélange fabric exist, suggesting the scarcity of rocks described as mélange in the cores of many orogens may result from misidentification of rocks that have been intensely overprinted by younger, ductile deformation.