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Understanding the 3D structure of the Gilmore Fault Zone through geophysical modelling: implications for Lachlan tectonic reconstructions

This study aims to clarify the tectonic evolution of the Lachlan Orogen by modelling the subsurface morphology of the Gilmore Fault Zone (GFZ). The GFZ marks a distinct geophysical contrast between (high gravity, low magnetic intensity) high-grade metamorphic rocks found in the Wagga metamorphic belt (WMB), to the west, and the (low gravity, uniformly high magnetic intensity) low-grade volcanic rocks found in the Macquarie Arc and Silurian rift basins to the east. Understanding the structure of this fault at depth should provide constraints on existing models for the tectonic evolution of the Lachlan Orogen. Subsurface structure around the GFZ in the vicinity of Barmedman has been inverted by iterative 2.5D potential-field modelling of gravity and magnetics, constrained by pre-existing reflection seismic profiles, potential-field interpretations by previous workers, and physical properties data collected on representative lithologies. Preliminary findings show that the surface structure mapped as the Gilmore Fault is an east-dipping, shallow thrust fault, and doesn’t correspond to the major crustal ‘suture’ envisaged in regional tectonic studies. It is a secondary antithetic structure off the main west-dipping, crustal penetrating fault that separates the Macquarie arc and the WMB. Another, steeper west-dipping fault cuts these structures and separates the WMB from the Silurian Tumut Trough. This larger structure defines the regionally extensive tectonic feature of the GFZ. The trace of the mappable Gilmore Fault (as opposed to the GFZ) is curved, and terminates abruptly to the north, indicating the Gilmore Fault is the base of a series of thrust flakes imposed on the pre-existing main fault in the GFZ. West-dipping crustal-penetrating thrust faults east of the GFZ are indicative of successive collision, accretion and extension events. Thus far the modelled structure of the GFZ is not consistent with the terrane accretion model but is consistent with the accretionary orogen and orocline model.