METAMORPHOSED MCHUGH COMPLEX IN THE BORDER RANGES FAULT SYSTEM: DETRITAL ZIRCON DATING OF THE KNIK RIVER TERRANE ASSEMBLAGE

Detrital zircon data from the Knik River terrane, an Early Cretaceous metamorphic and plutonic assemblage along the Border Ranges fault (BRF) in southern Alaska, indicate the metamorphic assemblage is a medium to high-grade metamorphic equivalent of the older, Jurassic part of the McHugh complex, the mélange subterrane of the Chugach terrane. The Border Ranges fault system of the Alaskan-Aleutian arc was formed as the megathrust of an oceanic arc system in the Early Mesozoic that was subject to reactivation into the late Cenozoic. The sense of the motion of this fault system is not clear due to these reactivations deforming evidence of the original activity. The Knik River terrane assemblage lies along the BRF in the western Chugach Mountains near Anchorage, Alaska and has a known history of Early Cretaceous metamorphism and plutonism. Because the Knik River terrane lacks clear evidence of Early Jurassic plutons characteristics of the adjacent Peninsular terrane and was more metamorphosed than the adjacent McHugh complex, it had the potential to be a distinct assemblage along the BRF. The terrane boundary between the McHugh complex and the Knik River terrane suggested both assemblages shared an Early Cretaceous deformation history with syntectonic Early Cretaceous plutons, implying that the Knik River terrane may be metamorphosed McHugh (Pavlis, et al, 1998, Tectonics). In order to test this hypothesis, samples were taken of this assemblage for analysis of the detrital zircon signatures for comparison to adjacent assemblages. Our new analyses indicate a correlation to the McHugh Complex but raises the question about the origin of the ~120-125 Ma metamorphism and plutonism. The metamorphism appears to be directly related to the heat of plutons and followed an apparent period of subduction erosion on the margin, supporting the thought of ridge subduction as the cause of this fault system. Recognition of the correlation of the Knik River terrane with the McHugh complex also indicates that the trace of the Border Ranges fault lies well north of its normal map trace in the western Chugach Mountains, and that the Wolverine and Eklutna ultramafic bodies may be large blocks in the McHugh complex.