HR: 1340h
AN: T13C-0482    [Abstracts]
TI: Differential Strain Rate as a Mechanism for the Formation of Detachment-Fault Corrugations: A Case Study From Western Norway
AU: * Johnston, S M
EM: johnston@umail.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences, Bldg 526, Santa Barbara, CA 93106 United States
AU: Hacker, B
EM: hacker@geol.ucsb.edu
AF: University of California, Santa Barbara, Department of Geological Sciences, Bldg 526, Santa Barbara, CA 93106 United States
AB: Transport-parallel corrugations on normal-sense detachment faults are described in extensional tectonic settings at the range through orogen scale. The mechanisms by which these corrugations form have implications for the original shape of the detachment fault, pre-existing basement structures, the formation and architecture of supra-detachment basins, and the overall tectonic setting of the orogenic event. Western Norway provides an ideal setting to investigate the formation of detachment-fault corrugations. There, the normal-sense Nordfjord-Sogn Detachment Zone--thought to be one of the primary structures responsible for exhuming the high- and ultrahigh-pressure rocks of the Western Gneiss Complex--is corrugated by a series of extension-parallel folds that deform footwall rocks, hanging-wall rocks, and muscovite chrontours. The largest of these corrugations displays a minimum fold amplitude of tectonostratigraphic layers of 5 km, and exposes muscovite cooling ages in the core of the antiform that are 10--15 Myr younger than the immediately adjacent synformal structures. Here, we suggest that the limbs of these corrugations represent transfer faults/shear zones between regions of differential exhumation that were localized by the density-driven ascent of the ultrahigh-pressure Western Gneiss Complex. The viability of this hypothesis was tested through a series of 2-D kinematic/finite-difference thermal models. Subducted lithosphere with an initially cold geotherm was allowed to exhume via a combination of pure-shear thinning and simple-shear removal of upper crust along a broad detachment zone, followed by conductive relaxation. The models indicate that even modest differences in exhumation rate (less than even 20 percent) create fold amplitudes within the tectonostratigraphy of up to 7 km, and produce 2--5 km offsets of the 400° isotherm (muscovite closure to Ar). These results are consistent with outcrop patterns and 10--15 Myr differences in muscovite cooling ages observed in Western Norway. This work has important implications for the geology of Western Norway, suggesting that the corrugations on the Nordfjord Sogn Detachment Zone may be the result of differential exhumation rather than orogen-scale constriction and transtension, and indicates that transfer faults/shear zones should be considered a viable mechanism for corrugated detachments in extensional settings worldwide.
DE: 1140 Thermochronology
DE: 3654 Ultra-high pressure metamorphism
DE: 8020 Mechanics, theory, and modeling
DE: 8109 Continental tectonics: extensional (0905)
DE: 8111 Continental tectonics: strike-slip and transform
SC: Tectonophysics [T]
MN: Fall Meeting 2005