40Ar/39Ar Thermochronology of the Malton Gneiss, Malton Complex, Southeastern British Columbia.
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The Malton complex, located at the northern tip of the Shuswap complex in southeastern British Columbia, is one of several structural culminations in which Precambrian basement is exposed from beneath Neoproterozoic cover sediments. The complexes preserve structure and fabric related to compressional stresses from Middle Jurassic to Late Cretaceous - early Paleocene time which are overprinted by extensional features of mid-Paleocene to Miocene age. Two high angle west-side-down normal faults, the North Thompson-Albreda and Rocky Mountain faults, bound the Malton Gneiss of the Malton complex and define the northern tip of the Shuswap complex. 40Ar/39Ar analyses of amphibole and muscovite obtained along two transects oriented perpendicular the Rocky Mountain Fault were conducted in order to constrain the cooling history of the Malton Gneiss. Only one amphibole analysis was successful, yielding a cooling age of 116 ± 3.4 Ma in the hanging wall of the North Thompson-Albreda Fault. Muscovite analyses were generally more successful and yielded cooling ages of 62 ± 0.4 Ma in the hanging walls and 60 ± 0.4 in the footwalls of the North Thompson-Albreda and Rocky Mountain faults. Temperature vs. time plots, combining already existing AFT data with the results of this study, estimate low temperature cooling rates of ca. 6 0C/Ma and ca. 22 0C/Ma for the Bulldog Gneiss (located in the footwall of the Rocky Mountain Fault) and the western Malton Gneiss respectively. A high temperature cooling rate of ca. 3 0C/Ma is estimated for the hanging wall of the North Thompson-Albreda fault. The relatively faster cooling rate of the Malton Gneiss in comparison to the Bulldog Gniess is likely the combined result of differential vertical offset on the North Thompson-Albreda Fault, differential erosion in Paleocene- Oligocene(?) time, and cooling by fluid flow along the fault. Cooling ages are 10-20 Ma less and cooling rates markedly slower than those described for the southern complexes of the Shuswap. The results of this study conclude an older cooling age, a slower cooling rate, and different exhumation mechanisms exist for the Malton Gneiss in comparison to the southern complexes of the Shuswap. Pages: 123 Supervisor: Rebecca Jamieson