Ductile-brittle transition structures in the basal shear zone of a rhyolite lava flow, eastern Australia

Structural and textural evidence indicates that the basal breccia of the Tertiary Minyon Falls Rhyolite of Tweed Volcano, eastern Australia, formed mainly by fragmentation within the basal shear zone of the lava flow. Cooling at the flow base led to lava behaviour passing through the ductile-brittle transition while subjected to progressive deformation. The intact lavas of the basal zone record intense ductile shearing in multiple folds, rotated phenocrysts, strong alignment of crystallites and micro-folding of these crystal alignments. Minor brittle structures, including faults on fold limbs and microfaults, occur in the intact lava immediately above the basal breccia. These structures represent incipient brecciation and complete brecciation has occurred by intense faulting of previously intact lava. Brittle deformation migrated upward into overlying intact lava as cooling occurred. This process is distinctly different from overriding and incorporation of surface breccias which is commonly invoked as the primary process of basal breccia formation.