THE MINT RIVER FAULT: AN EXTENSIONAL DETACHMENT IN THE YORK MOUNTAINS, SEWARD PENINSULA, ALASKA

The origin of the gneiss domes of the Bering Strait region has long been debated because, unlike the core complexes of the Basin and Range, evidence for supra-crustal extension has not been widely documented. The York Mountains of the Seward Pen. were originally described as a thrust belt (Sainsbury, 1969). New detailed mapping, 40Ar/39Ar data, and conodont ages show that the Mint River Fault, which is the basal detachment of the supposed thrust belt, is actually a low angle extensional detachment fault. This fault separates polydeformed low greenschist grade rocks in the lower plate from unmetamorphosed Lower Ordovician to Silurian carbonates. The upper plate is cut by three major normal faults. A younger-on-older relationship across one these faults is demonstrated by conodont ages. Stress inversion, based on brittle faults in the upper plate, indicates a direction of extension of 194. No direct evidence of bedding-plane thrusting could be documented, although minor folds do exist, probably formed during shortening that predated extensional deformation. The lower plate is composed of Proterozoic to early Cambrian lithic quartzites and slates intruded by 540±5 Ma gabbros (LA-ICPMS U/Pb in zircon), overlain by Lower Ordovician metacarbonates. The structure of the lower plate is characterized by at least two generations of folds. Kinematics of the earliest event are obscure, as the F1 fold axes are widely scattered and fold vergence is inconsistent. The D2 event resulted in vertical flattening and the dominant sub-horizontal metamorphic fabric. Lower plate slates yielded disturbed 40Ar/39Ar spectra with predominantly mid-Jurassic to mid-Cretaceous age steps. Although the quality of these data does not allow unambiguous interpretation, it suggests Late Mesozoic greenschist facies metamorphism. Extensional deformation must have occurred after metamorphism and prior to intrusion of the Brooks Mountain granite, which cuts the Mint River Fault, and is dated at 77.8±1 Ma by 40Ar/39Ar in biotite. Basaltic dikes in the upper plate, which are subparallel to the mayor normal faults, yielded 40Ar/39Ar ages of 77 to 78 Ma. Although the timing of the onset of extension in the York Mountains is poorly constrained, it appears to be compatible with the rise of the Kigluiak dome at about 90 Ma.