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 t he Tertiary Minyon Falls Rhyolite of Tweed Volcano, eastern Australia, formed mainly by fragmentation within the basal shear zone of the lav a flow. Cooling at the flow base led to lava behaviour passing through the ductile-brittle transition while subjected to progressive deforma tion. The intact lavas of the basal zone record intense ductile sheari ng in multiple folds, rotated phenocrysts, strong alignment of crystal lites and micro-folding of these crystal alignments. Minor brittle str uctures, including faults on fold limbs and microfaults, occur in the intact lava immediately above the basal breccia. These structures repr esent incipient brecciation and complete brecciation has occurred by i ntense faulting of previously intact lava. Brittle deformation migrate d upward into overlying intact lava as cooling occurred. This process is distinctly different from overriding and incorporation of surface b reccias which is commonly invoked as the primary process of basal brec cia formation.