DUCTILE DEFORMATION ZONES AND MYLONITES - MECHANICAL PROCESSES INVOLVED IN DEFORMATION OF CRYSTALLINE BASEMENT ROCKS

Ductile deformation zones (DDZ) are narrow zones of extreme but continuous deformation and mylonite development, typical of deformed crystalline basement rocks in orogenic belts. Insights into the formation of DDZ at low greenschist grades of metamorphism are provided by a study of quartzo-fedlspathic rocks of the Blue Ridge basement complex in northern Virginia. A new petrographic technique is suggested for the simultaneous measurement of grain-size, surface/volume ratio, and relative volumes of different minerals in a rock.