Extensional origin of ductile fabrics in the Schist Belt, Central Brooks Range, Alaska—I. Geologic and structural studies

Abstract

Blueschist-facies rocks are locally preserved in the internal zone of the Brooks Range orogen, where a high-strain foliation ( S 2) and down-dip stretching lineation occur for hundreds of kilometers along strike of the normal fault-bounded, southern edge of the orogen. This locally mylonitic fabric is inferred to represent an XY finite strain ratio of >30. The foliation formed during greenschist-facies overprinting of the blueschists and has been interpreted in different ways by geologists who disagree about the mechanism by which the high- P rocks were exhumed. Metamorphic fabrics and structures were studied in detail in the south-central Brooks Range, where the Florence Creek fault separates a hangingwall of weakly metamorphosed sedimentary rocks from high- P rocks of the Brooks Range Schist Belt. This fault may have in part pre-dated the D 2 overprint, as mylonitic fabrics are imprinted across both sides of the structure. The mylonites occur near the top of a ductile shear zone >8 km thick, and contain oblique-grain shape fabrics, shear bands and asymmetric quartz petrofabrics indicative of top-down-to-the-south (normal) shear. D 2 fabrics in quartzose schists are mylonitic and asymmetric at high structural levels of the footwall but are increasingly granoblastic and symmetric at deeper levels, suggesting increasing operation of temperature-sensitive recovery processes and a larger irrotational component of deformation with structural depth. D 2 strain decreases markedly about 10 km north of the fault zone, so that the earlier foliation is no longer transposed into S 2, and relict M 1 minerals (garnet, chloritoid) are preserved. Our data suggest that high-strain tectonites along the southern margin of the Schist Belt did not form during contractional deformation in the Brooks Range orogen, but during mid-Cretaceous crustal extension, which was superimposed on previously thickened continental crust of the northernmost Cordillera. Temperatures may have increased during the later stages of extension in the southern Brooks Range and adjacent Yukon-Koyukuk basin region in concert with mid- to Late Cretaceous magmatism, allowing deformation and metamorphism of earlier-formed normal faults. Subsequent folding in the latest Cretaceous or Early Tertiary resulted in further exhumation of the schists and modified the geometry of the faults by large-scale folding.