4:30 PM - 5:00 PM
[S21-2-01] Continental growth in eastern Australia: Insights from the mantle lithosphere
invited
The Tasmanides of eastern Australia is a complex series of Palaeozoic to early Mesozoic orogens which is juxtaposed against the eastern margin of the Precambrian shield region of central and western Australia. The subduction-accretion process was accommodated by retreat of the proto-Pacific plate along the eastern margin of Gondwana, which incorporated both Australia and Antarctica. A large region of the Tasmanides and its transition to Precambrian terrane in the west has been gradually covered by a transportable seismic array called WOMBAT, which has been in operation for the last two decades and has involved the deployment of over 750 stations.
Here, the latest body wave tomography results for the upper mantle will be presented, which reveal a number of distinctive features underlying the Palaeozoic terrane. The dominant long-wavelength signature is a gradual decrease in (P-wave) velocity from near the Proterozoic-Palaeozoic transition zone to the continental margins in the south and east. However, a pronounced N-S oriented “fragment" of high velocity mantle material, not present in lower resolution regional and global tomography models, is located some 200-300 km inboard of the east coast, which acts to form a low velocity embayment beneath the Lachlan Orogen, the southernmost fold belt of eastern mainland Australia. By using seismic wavespeed variations to infer changes in lithospheric thickness, we find that the Lachlan Orogen is likely underlain by thin lithosphere, with the high velocity “fragment" to the east forming a salient of thick lithosphere that attaches to the cratonic interior of Australia to the north. These results support the idea that the core of the Lachlan Orogen is a large orocline that formed due to differential roll-back of the proto-Pacific margin of east Gondwana, which had the effect of introducing, via continental transform faulting, older and thicker lithosphere from the north into a region dominated by accreted oceanic lithosphere.
Here, the latest body wave tomography results for the upper mantle will be presented, which reveal a number of distinctive features underlying the Palaeozoic terrane. The dominant long-wavelength signature is a gradual decrease in (P-wave) velocity from near the Proterozoic-Palaeozoic transition zone to the continental margins in the south and east. However, a pronounced N-S oriented “fragment" of high velocity mantle material, not present in lower resolution regional and global tomography models, is located some 200-300 km inboard of the east coast, which acts to form a low velocity embayment beneath the Lachlan Orogen, the southernmost fold belt of eastern mainland Australia. By using seismic wavespeed variations to infer changes in lithospheric thickness, we find that the Lachlan Orogen is likely underlain by thin lithosphere, with the high velocity “fragment" to the east forming a salient of thick lithosphere that attaches to the cratonic interior of Australia to the north. These results support the idea that the core of the Lachlan Orogen is a large orocline that formed due to differential roll-back of the proto-Pacific margin of east Gondwana, which had the effect of introducing, via continental transform faulting, older and thicker lithosphere from the north into a region dominated by accreted oceanic lithosphere.