Environment of Deformation, Monkton Quartzite, Shelburne Bay, Western Vermont

Abstract

A 700-m 2 outcrop of Monkton Quartzite, located 275 m above the Champlain thrust and 760 m south of the Shelburne Bay cross fault, contains two generations of fractures, two generations of wrench faults, and en echelon fractures in sinistral and dextral arrays. Feather fractures are associated with some of the faults. Microscopic structures include planes of hematitic inclusions, recrystallized quartz veins with prograde chlorite, unfilled fractures, and quartz deformation lamellae. All large- and small-scale structures were produced by two relatively simple stress fields that were orthogonally superimposed so that the directions of σ 1 and σ 3 were interchanged from the first to the second generation. Quartz deformation lamellae developed only during the first event. Evidence based on the small angle of shear failure from the deduced position of σ 1 , experimental compression tests on the Monkton Quartzite, the angle between the c axis and the pole to deformation lamellae ( c ∧⊥dl) in deformed quartz, and the assemblage quartz-hematite-chlorite suggest that deformation at Shelburne Access Area developed in a shallow crustal environment (less than 1,830 m) as a result of a long period of stress in a large area; this stress system underwent local modification toward the end of the period. Structures of the first generation probably formed under decreasing lithostatic pressure and increasing rates of strain. The first-generation wrench faults are correlated with the Shelburne Bay cross fault, which offsets the north-trending trace of the Champlain thrust. The inferred direction of σ 1 is remarkably similar for all these structures, regardless of scale. The wrench faults are thought to be part of the Acadian orogeny of Middle to Late Devonian age.