Antarctic seismicity and neotectonics

A map of Antarctic intraplate earthquakes, 1900-99, is presented including records from the Global Seismic Network and recordings made at Antarctic seismic observatories and temporary stations. The results show a low but significant level of seismicity through the Transantarctic Mountains and across George V Land to Adelie Land. They also suggest some seismic deformation is taking place in the Antarctic Peninsula, Weddell Sea, East Antarctic coast, and Ross Sea. No globally recorded events have occurred in Marie Byrd Land to date. Local recordings across the continent show that a denser distribution of recording stations is needed before drawing conclusions from areas of apparent Antarctic aseismicity. This review is one of the first contributions to the SCAR-approved ANTEC initiative, which aims to co-ordinate work on neotectonic deformation across the continent

Insights into the structure and dynamics of the upper mantle beneath Bass Strait, southeast Australia, using shear wave splitting

© 2019 Elsevier B.V. We investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in Bass Strait and the surrounding region of southeast Australia. Our dataset includes ∼366 individual splitting measurements from SKS and SKKS phases. The pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current N-S absolute plate motion. In the eastern Lachlan Fold Belt, fast shear waves are polarized parallel to the structural trend (∼N25E). Further south, fast shear wave polarization directions trend on average N25–75E from the Western Tasmania Terrane through Bass Strait to southern Victoria, which is consistent with the presence of an exotic Precambrian microcontinent in this region as previously postulated. Stations located on and around the Neogene-Quaternary Newer Volcanics Province in southern Victoria display sizeable delay times (∼2.7 s). These values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. In the Eastern Tasmania Terrane, nearly all observed fast directions are approximately NW-SE. Although part of our data set strongly favours anisotropy originating from “fabric” frozen in the lithospheric mantle, a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. We suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring, and so variations in anisotropy related to mantle flow may be expected. Alternatively, the pattern of fast polarisation orientations observed around Bass Strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered Cosgrove volcanic track. However, it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs. asthenospheric upper mantle due to poor backazimuthal coverage of the data

Structure of the crust and upper mantle beneath Bass Strait, southeast Australia, from teleseismic body wave tomography

© 2019 Elsevier B.V. We present new constraints on the lithospheric velocity structure of Bass Strait and the adjoining landmasses of mainland Australia and Tasmania in order to better constrain their geological and tectonic relationship. This is achieved by performing teleseismic tomography using data from fifteen deployments of WOMBAT and BASS transportable arrays, which span southeastern Australia. The starting model for the teleseismic tomography includes crustal velocity structure constrained by surface waves extracted from ambient seismic noise data and a Moho surface and broad-scale variations in 3-D upper mantle velocity structure from the Australian seismological reference Earth model (AuSREM). As a consequence, we produce a model with a high level of detail in both the crust and upper mantle. Our new results strengthen the argument for a low velocity upper mantle anomaly that extends down to ~150 km depth directly beneath the Newer Volcanics Province in Victoria, which is likely related to recent intra-plate volcanism. Beneath Bass Strait, which is thought to host the entrained VanDieland microcontinent, upper mantle velocities are low relative to those typically found beneath Precambrian continental crust; it is possible that failed rifting in Bass Strait during the Cretaceous, opening of the Tasman Sea, extension of VanDieland during Rodinian break-up and recent plume activity in the past 5 Ma may have altered the seismic character of this region. The data nevertheless suggest: (1) the velocity structure of the VanDieland microcontinent lacks continuity within its lithosphere; (2) the Moyston Fault defines an area of strong velocity transition at the boundary between the Cambrian Delamerian Orogen and the Cambrian-Carboniferous Lachlan Orogen; and (3) there is a rapid decrease in mantle velocity inboard of the east coast of Australia, which is consistent with substantial thinning of the lithosphere towards the passive margin