Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590-1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first-generation tight to isoclinal folds and an associated axial-planar slaty cleavage. Preservation of recumbent first-generation folds in the hinge zones of second-generation folds, and the approximately northeast-southwest orientation of restored L-0(1) intersection lineation suggest recumbent folding occurred during east-west to northwest-southeast shortening. First- generation folds are refolded by north-south-oriented upright non-cylindrical tight to isoclinal second-generation folds. A differentiated axial-planar cleavage to the second-generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second-generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (> 45 degrees). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late-orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin-skinned crustal shortening during the development of first-generation structures to thick-skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.