Deformational style of the Castlemaine area, Bendigo‐Ballarat Zone: Implications for evolution of crustal structure in central Victoria

Publisher Website
Google Scholar
Abstract
The Early Ordovician quartz‐wacke turbidite sequence (Castlemaine Supergroup) in the Castlemaine area of central Victoria has been folded about upright to steeply west‐dipping axial surfaces producing close to tight folds with long subplanar limbs and narrow hinge zones. Fold wavelengths range from 150 m to 500 m. The regional enveloping surface is gently north‐dipping. Fold growth has been accompanied by the development of a mesoscopically penetrative cleavage, particularly in pelitic to semi‐pelitic units. Convergent fanning solution cleavage is pronounced in arenite units in fold hinge zones. Folding has been associated with the development of steeply to moderately west‐dipping and east‐dipping reverse faults. Many of the smaller faults are accommodation structures formed during tightening of major chevron‐style folds. However, major west‐dipping reverse faults cut through folds and are responsible for nearly 2 km of vertical displacement over a strike‐normal section of 9 km. Gold mineralization and associated high fluid fluxes have been associated with the development of some reverse fault zones. Folding, together with associated cleavage development and contraction faulting, has resulted in at least 65% east‐west shortening of the relatively thin blanket of Ordovician sediments. Fault and fold asymmetry indicate eastwards structural vergence. The structural style, tectonic setting and metamorphic history of the Castlemaine Supergroup supports the concept that regional mid‐Devonian deformation in this part of the Lachlan Fold Belt has involved thin‐skinned deformation processes. Major decollement or thrust surfaces are likely to be present near the base of the Ordovician sequence, and also at deeper structural levels. The Mt William‐Mt Ida‐McIvor Fault Zone at the eastern margin of the Bendigo‐Ballarat Zone is interpreted as part of a linked system of west‐dipping imbricate reverse faults that splay off subsurface detachment zones beneath the Bendigo‐Ballarat Zone. The large fluid fluxes associated with gold mineralization, and the widespread emplacement of post‐tectonic granites, indicate extensive lower crustal metamorphism late during mid‐Devonian regional deformation. Although displacement above detachment zones may have controlled structural development in the upper crust, the late tectonic lower crustal metamorphism and melting indicate that convergent deformation initially may have thickened the entire lithosphere, and resulted in convective thinning or detachment of thickened mantle lithosphere and emplacement of hot asthenosphere near the base of the crust late during crustal shortening.