Blind orogen: Integrated appraisal of multiple episodes of Mesoproterozoic deformation and reworking in the Fowler Domain, western Gawler Craton, Australia

Abstract

The Fowler Domain in the western Gawler Craton in southern Australia is a poorly exposed region that can only presented be explored using a combination of information from drilling and potential field geophysics. Regional maps of the Total Magnetic Intensity (TMI) field of the Fowler Domain highlight an anastomosing system of terrain-scale shear zones that bound four crustal-scale tectonic blocks: from west to east, the Colona, Barton, Central and Nundroo blocks. Integrated thermobarometry and electron microprobe chemical dating of metamorphic monazites from drillholes in the Fowler Domain suggest that the Colona Block in the west underwent two mid-crustal amphibolite grade metamorphic events at ca. 1643 and 1600 Ma. The younger age corresponds to the timing of regional high-grade metamorphism in the Barton Block. Together, the age data suggest that the western Fowler Domain underwent a major tectonothermal event at ca. 1600 Ma. In contrast, regional lower crustal metamorphism in the Nundroo Block, which forms the eastern Fowler Domain, occurred at ca. 1545 Ma. In both the Barton and Nundroo Blocks, petrological relationships, mineral zoning, and pressure-temperature (P-T) modelling suggest the terrains cooled in the mid- to lower crust, rather than undergoing exhumation immediately following peak metamorphism. Age data from the geophysically defined shear zone systems that bound the blocks suggest that exhumation of these lower crustal domains occurred between ca. 1470 and 1450 Ma and was associated with transpressional reactivation of the terrain during the Coorabie Orogeny. A key finding of this study is that the tectonic evolution of the crustal blocks in the Fowler Domain was not in concert until at least 1500 Ma. Thus, the evolution of the individual blocks is unlikely to be related to the macroscopic character of the terrain defined by the regional-scale shear zone systems, which are one of the youngest tectonic imprints on the Fowler Domain. Coincident gravity and magnetic forward modelling of significant bounding faults suggests the shear zones form a steeply dipping transpressional array, consistent with the observed metamorphic field character of the different blocks. This study provides a demonstration of the integration of geophysical and petrological approaches to investigate the time-integrated tectonic evolution of poorly exposed basement terrains. © 2008 Elsevier B.V. All rights reserved.