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

Extension of a convergent orogen: structural evolution of the high-pressure/low-temperature schist belt, Brooks Range, Alaska
Dinklage, W. S.
University of California, Santa Barbara
255 p.
Geophysical Institute Library: THESIS NOT UAF
Metamorphic rocks in the core of the Brooks Range orogen contain fabrics recording plate convergence (D$\sb1$) followed by crustal extension (D$\sb2$). The D$\sb1$ 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 (S$\sb2$) overprinted the D$\sb1$ fabric and the albite porphyroblasts. The dominant S$\sb1$ foliation in epidote-amphibolite facies rocks is overgrown by albite porphyroblasts with syn-extensional rims and oligoclase tails. 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. $\sp{40}$Ar/$\sp{39}$Ar 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. D$\sb2$ occurred by collapse of steeply dipping layering, south-directed shear parallel to S$\sb2$, and north-directed reactivation of S$\sb1.$ 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 S$\sb1$ or S$\sb2.$ 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 D$\sb1$ and D$\sb2$ strain. Fabrics from blueschist/greenschist facies rocks record a slight north-directed component of D$\sb1$ shear and variable overprinting by D$\sb2.$ Fabrics from the epidote-amphibolite facies rocks record north-directed reactivation of S$\sb1$ by D$\sb2.$ Inclusion trail-matrix obliquity in porphyroblasts is used to establish that the matrix foliation was reactivated during D$\sb2$ at strains high enough to reset petrofabrics.
Ph.D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)