HR: 1340h
AN: T13C-0491    [Abstracts]
TI: The Mint River Fault: an Extensional Detachment in the York Mountains, Seward Peninsula, Alaska
AU: * Toro, J
EM: jtoro@wvu.edu
AF: West Virginia University, 425 White Hall, Morgantown, WV 26506
AU: Burnette, L
EM: lburnett@unm.edu
AF: Earth and Planetary Sciences, UNM, Albuquerque, NM 87131
AU: Amato, J
EM: amato@nmsu.edu
AF: Geological Sciences, NMSU, Las Cruces, NM 88003
AU: Repetski, J
EM: jrepetski@usgs.gov
AF: USGS, 12201 Sunrise Dr., Reston, VA 20192
AU: Gehrels, G
EM: ggehrels@geo.arizona.edu
AF: Dept. of Geosciences, Univeristy of Arizona, Tucson, AZ 85721
AB: The role of crustal extension in the origin of the gneiss domes of the Bering Strait region of Alaska and Russia has been debated for over a decade. Alternative models for gneiss dome formation include 1) thermal re-equilibration after crustal thickening by arc collision (Lieberman, 1988; Patrick and Evans, 1989); 2) extensional collapse of the crust during with mid-Cretaceous magmatism (Miller et al., 1992; Amato et al., 1994) and 3) thermally-induced diapiric rise of the high-grade rocks (Calvert et al., 1999). One major difference with the classic metamorphic core complexes of the Basin and Range is that, because of deep exhumation, evidence for mid-Cretaceous supra-crustal extension has not been widely documented in the Bering Strait region. In the York Mountains, the one area of the Seward Peninsula where unmetamorphosed rocks are preserved, the structure was originally described as a thrust belt (Sainsbury, 1969). New detailed mapping, structural analysis, 40Ar/39Ar thermochronology, and conodont biostratigraphy carried out in the York Mountains 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 in the upper plate. The upper plate is cut by three major normal faults, the largest of which has about 4 km of down-to-the-south slip. These faults also tilt the Early Paleozoic carbonate succession. A younger-on-older relationship across one of the mayor faults is documented by conodont biostratigraphy demonstrating that these are not thrusts, as was previously believed. Stress inversion, based on minor brittle faults in the upper plate, indicates a direction of extension of 194, which is consistent with the strike of major normal faults. 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 latest 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. The 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 does suggest that greenschist facies metamorphism took place in the Late Mesozoic. 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 so far, it appears to be compatible with the rise of the Kigluiak dome, which culminated at about 90 Ma (Amato et al., 2001).
DE: 1100 GEOCHRONOLOGY
DE: 8010 Fractures and faults
DE: 8109 Continental tectonics: extensional (0905)
DE: 9315 Arctic region (0718, 4207)
SC: Tectonophysics [T]
MN: Fall Meeting 2005