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Dinklage, W.S., 1998

Extension of a convergent orogen: Structural evolution of the high-pressure/low-temperature schist belt, Brooks Range, Alaska

Bibliographic Reference

Dinklage, W.S., 1998, Extension of a convergent orogen: Structural evolution of the high-pressure/low-temperature schist belt, Brooks Range, Alaska: University of California, Santa Barbara, Ph.D. dissertation, 255 p.


Metamorphic rocks in the core of the Brooks Range orogen contain fabrics recording plate convergence (D1) followed by crustal extension (D2). The D1 fabric in blueschist/greenschist facies rocks was overgrown by porphyroblastic albite. A low greenschist-facies, south-dipping extensional cleavage and south-dipping normal-sense shear zones (S2) overprinted the D1 fabric and the albite porphyroblasts. The dominant S1 foliation in epidote-amphibolite facies rocks is overgrown by albite porphyroblasts with syn-extensional rims and oligoclase trails. Extension in the epidote-amphibolite facies rocks occurred at peak metamorphism. Spatial trends in geochronologic data and large-scale map relations indicate collapse of steeply dipping layering within the epidote-amphibolite facies rocks. 40Ar/39Ar cooling ages on muscovite and hornblende indicate that extension was Aptian, consistent with stratigraphic evidence for the cessation of thrusting in the foreland and for the onset of rapid sedimentation in both the foreland and the hinterland. Porphyroblast-matrix microstructures in the epidote-amphibolite facies rocks reveal the mechanism and magnitude of extension. D2 occurred by collapse of steeply dipping layering, south-directed shear parallel to S2, and north-directed reactivation of S1. Albite and garnet porphyroblasts rotated in the same direction as layering during collapse but by a lesser amount. The porphyroblasts did not rotate by shear parallel to S1 or S2. Estimates of stretching factors of 2 to 4 are made from the obliquity between inclusion trails and matrix foliation. This amount of extension integrated to the top of the crust could have exhumed the epidote-amphibolite facies rocks half to three-quarters of the way to the surface by tectonic processes alone. Quartz petrofabrics record evidence of both D1 and D2 strain. Fabrics from blueschist/greenschist facies rocks record a slight north-directed component of D1 shear and variable overprinting by D2. Fabrics from the epidote-amphibolite facies rocks record north-directed reactivation of S1 by D2. Inclusion trail-matrix obliquity in porphyroblasts is used to establish that the matrix foliation was reactivated during D2 at strains high enough to reset petrofabrics.

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