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Till, A.B., 1994

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Till, A.B., 1994, The nature of the involvement of lower crust in contractional mountain building, as deduced from metamorphic, structural, argon thermochronometric, and detrital mineral data from the Brooks Range, Alaska: University of Washington, Seattle, Ph.D. dissertation, 251 p.


Blueschist-facies metamorphism accompanied development of an antiformal duplex in the internal zone of the western Brooks Range. Deformation was concentrated on a system of discrete ductile shear zones that are mappable as thrust systems. The map pattern of the duplex (the Nanielik antiform) closely resembles map patterns of duplexes deformed in much shallower settings. Conditions of epidote-blueschist-facies metamorphism, calculated to have been 350-400°C at pressures of 6-9 kb, indicate that development of the duplex occurred at a depth of 18-25 km. Blueschist-facies metamorphism and construction of the Nanielik antiform were synchronous with deformation in the shallow fold and thrust belt. The two areas were undoubtedly mechanically linked. 40Ar/39Ar ages from white mica in rocks of the Nanielik antiform indicate that the duplex developed at about 120 Ma and was remobilized on its southern boundary at ~108 Ma. At those times, the basal detachment of the orogen must have reached depths of 18-25 km. Detrital metamorphic minerals found in the foreland basin of the Brooks Range were derived from Jurassic blueschist-facies rocks that now sit structurally above the Nanielik antiform. The metamorphic minerals are contained in sedimentary rocks at least as old as middle Albian in age, and may have arrived earlier. The source of the minerals is the blueschist-facies schist belt, the structurally highest, more internal of two structurally and metamorphically distinct belts that occupy the metamorphic core of the Brooks Range. The blueschist facies rocks of the schist belt were exhumed between Late Jurassic and middle Albian time, during the major pulse of contractional deformation in the Brooks Range orogeny. The processes by which the blueschist-facies rocks were uplifted and denuded must have operated during contractional tectonism.

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