Regional tectonics, crustal deformation and contrasting batholith orientations in southeastern Australia: why is Late Devonian central Victoria different?

In most of southeastern Australia, Paleozoic granitic plutons are elongated parallel to fold-axis surface traces, foliations, trends of main lithological contacts and most major faults, but there is a particularly glaring exception—the central part of the state of Victoria. Here, Late Devonian plutons are elongated approximately orthogonal to the structural grain in their wall rocks. Evidence from Late Devonian granitic rocks and their high-grade metamorphic xenoliths implies that during the Middle Devonian east–west Tabberabberan compression, the mainly Proterozoic Selwyn Block, which forms the crustal basement, was at relatively low metamorphic grades and non-rigid. Thus, the wedge-shaped Selwyn Block would have responded by mainly southward-directed extrusion and thinning, preventing the whole crust from overthickening. The detachment zone at the base of the Paleozoic cover protected the upper crust from intense effects of the compression, with strain partitioned into the warm and extruding Selwyn Block. This contrasts with the more rigid behaviour of the deep crust in the Stawell and Bendigo zones, to the west, where crustal thickening was significant during both the Benambran and Tabberabberan orogenies. With the cessation of compression, essentially the entire crust retained residual stress. In the latest Devonian east–west extension, the Selwyn Block relaxed this residual stress, accompanied by heating and local partial melting of the deep crust, caused by intrusion of mantle-derived magmas. During the Late Devonian extensional epoch, the presence of partial melt would have promoted east–west flow in the Selwyn Block, reducing Tabberabberan surface uplift. Meanwhile, the much cooler and still brittle upper crust responded partly through the formation of east–west-orientated joint systems that guided granitic magma ascent and pluton emplacement. This evolution would explain the formation of discordant upper-crustal granitic batholiths with distinct east–west elongations as well as the preservation of relatively thin, subaerially deposited, volcanic successions.