Extensional origin of ductile fabrics in the Schist Belt, Central Brooks Range, Alaska—II. Microstructural and petrofabric evidence
A regional system of S-dipping faults is exposed in the Florence and Fall Creeks area of the south-central Brooks Range. This fault system has previously been mapped as the 'root zone' of a N-vergent fold-and-thrust belt of Jurassic-Cretaceous age, although individual faults juxtapose lower grade on higher grade metamorphic rocks suggesting apparent extensional geometries. The structurally highest of these faults places rocks of the oceanic Paleozoic-Mesozoic Angayucham terrane, together with unconformably overlying Cretaceous clastic rocks, on Devonian metagreywacke and phyllite. This metagreywacke-phyllite (MP) unit in turn structurally overlies Devonian (?) and older basement rocks of the Brooks Range Schist Belt along a S-dipping structural contact previously mapped as the Florence Creek fault. The Schist Belt and MP units are both characterized by a regionally developed, S-dipping greenschist facies foliation that displays a pronounced N-S-trending, down-dip elongation lineation which earlier workers regarded as being associated with N-vergent thrusting. While only one foliation (S_d) is recognized in the MP unit, in the underlying Schist Belt this foliation (S_2) overprints an earlier blueschist facies mineral assemblage and associated foliation (S_1).
© 1994 Published by Elsevier Ltd. Received 6 May 1992; accepted in revised form 13 September 1993. Fieldwork for this study was funded by a grant from ARCO Alaska to Miller, and made possible by helicopter and logistical support from the Trans-Alaska Crustal Transect Geologic Studies Project of the U.S. Geological Survey headed by T. E. Moore and W. J. Nokleberg during the summers of 1989 and 1990. R. D. Law particularly wishes to thank Richard Gottschalk for generously supplying unpublished data, Dianna Solie of the Alaska Division of Geological and Geophysical Surveys for her hospitality during fieldwork and Delpfine Welch for advice on computer programs. Laboratory work and preparation of data for publication was supported by National Science Foundation Grants EAR-9018929 to R. D. Law and EAR-9018922 to E. L. Miller. Critical reviews by J. Gilotti, R. R. Gottschalk, J. W. Handschy, D. Solie and S. F. Wojtal substantially improved the manuscript and are gratefully acknowledged.