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

Abstract

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. 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 1 0 intersection lineation suggest recumbent folding occurred during east-west to northwest-southeast shortening. Firstgeneration 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°). 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.

Timing of Proterozoic metamorphism in the southern Curnamona Province: implications for tectonic models and continental reconstructions ∗

“…We speculate that the Olarian Orogeny at ca 1610 -1585 Ma was a response to convergence along the eastern margin of the North Australian Craton. The Isan and Ewamin Orogenies also record evidence of intermittent shortening along the eastern margin of the North Australia Craton in the Mt Isa and Georgetown Inliers between ca 1600 and 1580 Ma (Betts et al 2000;Giles & Nutman 2002). Clearly, such widespread compressional tectonics and high-grade metamorphism along the eastern margin of the North Australia Craton during this time interval was a response to major tectonic processes.…”

Sequential development of a mid-crustal fold-thrust complex: evidence from the Mitakoodi Culminationin the eastern Mt Isa Inlier, Australia

“…The style and characteristics of deformation during the Isan Orogeny are variable between the Eastern and Western Fold Belts (O'Dea et al 1997b). The Eastern Fold Belt has an architecture that is characteristic of many fold and thrust belts (MacCready et al 1998;Betts et al 2000), whereas the Western Fold Belt has been strongly influenced by the underlying extensional fault architecture (O'Dea & Lister 1995;Lister et al 1999;Betts 2001;Betts et al 2004). …”

“…However, the Duck Creek and Bulonga Thrusts are relatively small as they appear to be minor structures formed during internal deformation within an individual thrust sheet. The magnitude of displacement across the Overhang Shear Zone is unconstrained, although we infer that it may have been relatively large (greater than tens of kilometres) given that: (i) it juxtaposed two different tectonostratigraphic sequences of differing ages; (ii) it was associated with the development of large F 1 recumbent folds in the hangingwall (Betts et al 2000); and (iii) hangingwall and footwall successions to the north and east of the Mitakoodi Culmination have different metamorphic grades.…”

“…For example, the thermal histories of the Australian Proterozoic orogens are dominated by high-temperature, low-pressure metamorphism (Collins & Vernon 1991;Stü we & Ehlers 1997;Rubenach & Barker 1998), which occurs relatively early in the orogenic history, and may pre-date maximum crustal thickening (Page & Sun 1998;Giles & Nutman 2002, 2003Hand & Rubatto 2002). This crustal thickening has involved thin-skinned lateral crustal translations (MacCready et al 1998;Betts et al 2000;Forbes et al 2004;Giles et al 2004).…”

“…Thus, the evolution of the Mitakoodi Culmination experienced a change in deformational style from thin-skinned thrusting (D 1 and D 2 ) to thick-skinned folding and faulting (D 3 ) (MacCready et al 1998;Betts et al 2000).…”