RECOGNITION OF MESOPROTEROZOIC DUCTILE SHEAR ALONG THE BLACK FAULT IN SOUTHEASTERN MISSOURI

Regionally, the Black fault is one of a set of NW-SE-striking faults that bound the outcrop area of the St. Francis igneous terrane. Like other large faults in that regional set, deformation on the Black fault was polyphase and ultimately produced sinistral oblique-slip displacement. Cohesive cataclasites with phyllitic cleavage are well developed along the Black fault in Mesoproterozoic rhyolite east of Patterson, MO. At this locality, splaying on a southwest-vergent, transpressional half-flower structure produced a thrust duplex system. Cataclastic flow in narrow zones of high strain transformed rhyolite to foliated cataclasite with S-C fabric indicating that ductile shear preceded brittle reactivation.

Petrographic examination of the cataclasites shows phenocryst grain refinement by fracture, rotation, and sliding. This deformation produced pressure shadows on quartz and pyrite, kinked and curved twins in feldspar, and pressure bands and lamellae in quartz. Quartz clasts show no evidence of plasticity. Many quartz clasts, however, developed sigma tails in pressure shadows via pressure solution, migration, and redeposition - processes that tend to annihilate deformation fabric. The original glassy matrix behaved ductilely and developed penetrative S-C phyllosilicate (chlorite-white mica) foliations and cleavage. The foliated cataclasites are interpreted as products of hydration and shear under lower greenschist facies conditions (i.e., T ~ 300C).

Ductile deformation and greenschist mineralogy require a high thermal regime and imply that the Black fault was active soon after emplacement of the volcanic rocks (i.e., ~1.48 Ga). Similar relations occur on the Ironton fault. Integration of observations indicates that favorably oriented Mesoproterozoic basement shear zones were reactivated by later superimposed stress fields.