Extensional origin of ductile fabrics in the Schist Belt, Central Brooks Range, Alaska—II. Microstructural and petrofabric evidence

Abstract

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). Mylonites with strong asymmetric crystal fabrics are well developed in quartz stringers in the MP unit and in Schist Belt rocks located at less than 250 m beneath the Florence Creek fault. These asymmetric single- and cross-girdle c-axis fabrics indicate a top down-to-the-south shear sense. This shear sense is confirmed in XZ sections by the presence of elongate, dynamically recrystallized quartz grains oblique to the S d − S 2 mylonitic foliation. Quartzose and quartzo-feldspathic lithologies become coarser grained in the deeper levels of the Schist Belt, and a greater proportion of the quartz grains are equant in outline suggesting the increasing importance of temperature-sensitive recovery processes. These deeper level quartz tectonites display less strongly asymmetric cross-girdle c-axis fabrics and oblique grain shape alignments for at least a further 7.5 km beneath the fault, the asymmetry still indicating a top-down-to-the-south shear sense. Of the 35 quartz-rich samples selected for detailed microstructural and crystal fabric analysis, only two contain c-axis fabrics and oblique grain shape alignments indicating a top-up-to-the-north shear sense. This pervasive top-down-to-the-south shear sense is inconsistent with tectonite fabrics in the Schist Belt being related to N-vergent thrust faulting, but accords well with an extensional model for exhumation of high- P rocks of the Schist Belt. Extension-related high strain fabrics persist for a structural thickness of at least 7.7 km beneath the Florence Creek Fault, indicating that the Florence and Fall Creeks area may be situated over a large-scale (at least 8.0 km thick) zone of extensional shear dipping towards the south. How far this zone of penetrative deformation can be traced along strike remains unclear.