STRUCTURAL MAPPING AND EBSD OF FOLDED CHERT BEDS IN THE PALEOARCHEAN WARRAWOONA GREENSTONE BELT, EAST PILBARA CRATON

We present detailed structural mapping and quartz microstructural data from a set of 200-300 m scale folds in the Warrawoona Greenstone Belt (WGB) in the East Pilbara craton, Western Australia. The folds are part of a multiphase deformation associated with the emplacement of the adjacent Mt Edgar granitic complex, a structural dome that emplaced through and deformed the greenstone belt between 3.5 and 3.2 Ga. These new data provide new kinematic insights into the development of structural granitic domes in the early Earth. Structural data collected in the field include chert bedding, foliations, lineations, fold hinges and axial planes, axial planar crenulation cleavages, and fault/slickenline orientations. We observe several important relationships between these structures. A large dextral fault offsets the folded sequence and separates symmetrical folds to the west from dextral asymmetric folds to the east. Steeply plunging stretching lineations in chert mylonites are hinge-parallel in the west, but wrap around fold hinges in the east, implying that the lineation predates the asymmetric folding. Mylonitic chert samples and a fault breccia were analyzed using Electron Backscatter Diffraction (EBSD) for microstructural analysis. Large area EBSD maps characterize the crystallographic preferred orientation (CPO), crystallographic vorticity, misorientation, and grain size analyses (and hence stress using for paleopiezometers). Pole figures of quartz CPO calculated from the chert mylonites show coaxial, plane-strain crystallographic preferred orientation patterns in both the west and east ends of the mapped area. Ongoing analysis of the microstructures of the fault breccia will constrain the relationship between discrete faulting and folding accommodated by solid state deformation.