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

摘要: A regional system of S-dipping faults is exposed in the Florence and Fall Creeks area south-central Brooks Range. This fault has previously been mapped as ·root zone’ a N-vergent fold-and-thrust belt Jurassic-Cretaceous age, although individual juxtapose lower grade on higher metamorphic rocks suggesting apparent extensional geometries. The structurally highest these places oceanic Paleozoic-Mesozoic Angayucham terrane, together with unconformably overlying Cretaceous clastic rocks, Devonian metagreywacke phyllite. metagreywacke-phyllite (MP) unit turn overlies (?) older basement Range Schist Belt along structural contact Creek fault. MP units are both characterized by regionally developed, greenschist facies foliation that displays pronounced N-S-trending, down-dip elongation lineation which earlier workers regarded being associated thrusting. While only one (Sd) recognized unit, underlying this (S2) overprints an blueschist mineral assemblage (S1). Mylonites strong asymmetric crystal fabrics well developed quartz stringers located at less than 250 m beneath These single- cross-girdle c-axis indicate top down-to-the-south shear sense. sense confirmed XZ sections presence elongate, dynamically recrystallized grains oblique to Sd − S2 mylonitic foliation. Quartzose quartzo-feldspathic lithologies become coarser grained deeper levels Belt, greater proportion equant outline increasing importance temperature-sensitive recovery processes. level tectonites display strongly grain shape alignments for least further 7.5 km fault, asymmetry still indicating top-down-to-the-south Of 35 quartz-rich samples selected detailed microstructural fabric analysis, two contain top-up-to-the-north sense. This pervasive inconsistent tectonite related thrust faulting, but accords model exhumation high-P Belt. Extension-related high strain persist thickness 7.7 Fault, may be situated over large-scale (at 8.0 thick) zone dipping towards south. How far penetrative deformation can traced strike remains unclear.