Stress history of folding and cleavage development, Baraboo syncline, Wisconsin

Abstract

Analysis of quartz subfabric elements was undertaken in specimens of Precambrian Baraboo Quartzite collected around the doubly plunging, asymmetric Baraboo syncline. Principal stress axes determined from quartz deformation lamellae in specimens from the hinge zone of the fold are σ 1 (greatest compressive principal stress) and σ 2 in the plane of the bedding and σ 3 (least compressive principal stress) normal to bedding. In the closures, σ 1 is essentially perpendicular to the hinge line. On the limbs of the fold, σ 1 is mostly parallel to the hinge line of the fold and σ 3 is nearly vertical. At the eastern end of the northern limb where the fold is tightest, σ 3 is again vertical, but σ 1 is horizontal north-south and nearly perpendicular to vertical east-striking bedding and to the hinge line. Gliding flow features in minerals of most folded layered rocks have reflected early layer-parallel compression (σ 1 ) oriented perpendicular to the hinge line of the fold. Prior to our new data from the Baraboo syncline only macrofractures, tension gash bands, and (in one case) twin lamellae in calcite have shown σ 1 parallel to hinge lines of folds or perpendicular to bedding. The two latter stress configurations are theoretically characteristic of stresses associated with a later stage in the history of folding. Hence, the detrital quartz grains on the limbs of the Baraboo syncline apparently contain deformation lamellae produced mainly by later stage stresses rather than earlier in the fold history, when σ 1 was parallel to bedding and perpendicular to the hinge line. The closely spaced cracks forming the cleavage in the quartzite possibly were controlled in orientation by relaxation following the early layer-parallel compression.