3-D tomographic imaging of the Southeast Australian crust : new insight into the evolution of the east Gondwana margin
Date
2014
Authors
Pilia, Simone
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Abstract
Seismic tomography is arguably the most powerful geophysical method for imaging the Earth's interior. From its origins in the early 1970s, rapid growth in computing power, increased availability of high quality digital data, and ongoing technique development have all conspired to propel it to a stage where images of unprecedented detail are now produced on a regular basis. In this thesis, cutting edge seismic tomography methods will be used to image 3-D crustal structure in southeast Australia, with the aim of better understanding the tectonic evolution of the former east Gondwana margin. A key objective of the first strand of this thesis is to use data from an array of 24 seismometers to image the crust beneath the Flinders Ranges by using Local Earthquake Tomography. A subset of P- and S-wave traveltimes is inverted to jointly recover earthquake hypocenters, P-wave velocity structure and Vp/Vs anomalies to improve our understanding of crustal structure, rheology, and the mechanism responsible for the localised intraplate deformation that characterises this area. Clusters of seismicity are observed within distinct low velocity regions (i.e., between the Archean-Mesoproterozoic Gawler Craton and the Palaeo-Mesoproterozoic Curnamona Province and along a major sequence of N-S trending Ross-Delamerian thrust faults). I postulate that pre-existing mechanical weaknesses in the lithosphere, principally due to structure and composition, exert first-order control on the distribution of seismicity in the Flinders Ranges. In the second strand of my research, ambient noise data from an array of 24 seismometers is used to produce a 3-D crustal shear wave velocity model of Bass Strait, the key to understanding the missing link between Tasmania and mainland Australia. I apply a transdimensional, hierarchical Bayesian inversion approach to construct a 3-D shear wave velocity model of the area. This allows the entire crust beneath Bass Strait to be imaged in high detail and elucidates the geometry and position of key crustal features. A key feature of the 3-D tomography model is a distinct mid-lower crustal NW-SE high velocity zone that extends from northwestern Tasmania to south-central Victoria, confirming a Proterozoic geological connection that has not previously been established in the deep crust. A recent study that uses 3-D geodynamic modelling has controversially suggested that the entrainment of exotic continental fragments may exert first-order control on continental growth via subduction accretion. Evidence from kinematic modelling suggests that the pre-Carboniferous Tasmanides in southeastern Australia may have been subjected to this process. In the final strand of my research, I present the first high-resolution crustal 3-D shear velocity model of southeastern Australia by applying the same inversion approach that was used for the Bass Strait datasets to seismic noise recorded by the WOMBAT seismic array. The main elements of the proposed continental accretionary model are fully imaged here for the first time, including the remains of the ingested continental fragment and the tectonic escape of the back arc region. To date, no other geophysical study has revealed structures in the deep crust related to the entrainment of continental fragments at former convergent accretionary margins.
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