Eastern Australia’s Tasmanides are a series of ancient mountain chains—now mostly eroded and underlying the country’s flat outback—that records the long tectonic history of Gondwana’s eastern margin, where convergence has forged five separate mountain belts, all of which display large-scale, map view curvatures called oroclines. Although such bends appear to form in response to rotation around a vertical axis, the underlying processes associated with their development in this region are still poorly understood.
Now Abdullah and Rosenbaum are harnessing newly available data to investigate the origins of a large-scale orocline located at the transition between the southern Tasmanides’ Delamerian Orogen and the Thomson Orogen in the northern Tasmanides. Through their analysis of borehole data, 2-D seismic reflection profiles, and gravity and magnetic potential field data, the team documented that the northern Tasmanides are underlain by thin crust that is separated from the southern Tasmanides by a distinctive, east–west trending shear zone. The researchers also mapped a second shear zone that divides the northern Tasmanides from the abutting North Australian Craton.
Between the two shear zones lies the Thomson Orogen, whose crust is riddled with a series of basins the researchers attribute to Devonian back-arc extension that occurred in response to the retreat of a subduction zone trench along Gondwana’s eastern margin. Because this extension coincided with alternating episodes of compression and extension in the southern Tasmanides, these findings suggest the two regions experienced very different styles of tectonism during this period.
According to the authors, these different styles are most likely due to variations in the rate at which the trench retreated or advanced. In combination with slab tearing and its associated magmatism, they argue, these processes segmented the crust and ultimately resulted in the orocline’s formation. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2018JB015724, 2018)
—Terri Cook, Freelance Writer