High-frequency ambient noise tomography of southeast Australia: New constraints on Tasmania's tectonic past

Abstract

The island of Tasmania, at the southeast tip of Australia, is an ideal natural laboratory for ambient noise tomography, as the surrounding oceans provide an energetic and relatively even distribution of noise sources. We extract Rayleigh wave dispersion curves from the continuous records of 104 stations with ∼15 km separation. Unlike most passive experiments of this type, which observe very little coherent noise below a 5 s period, we clearly detect energy at periods as short as 1 s, thanks largely to the close proximity of oceanic microseisms on all sides. The main structural elements of the eastern and northern Tasmanian crust are revealed by inverting the dispersion curves (between 1 and 12 s period) for both group and phase velocity maps. Of particular significance is a pronounced band of low velocity, observed across all periods, that underlies the Tamar River Valley and continues south until dissipating in southeast Tasmania. Together with evidence from combined active source and teleseismic tomography and heat flow data, we interpret this region as a diffuse zone of strong deformation associated with the mid‐Paleozoic accretion of oceanic crust along the eastern margin of Proterozoic Tasmania, which has important implications for the evolution of the Tasman Orogen of eastern Australia. In the northwest, a narrower low‐velocity anomaly is seen in the vicinity of the Arthur Lineament, which may be attributed to local sediments and strong deformation and folding associated with the final phases of the Tyennan Orogeny.

On the use of sensitivity tests in seismic tomography

“…This code is best suited to the inversion of surface wave traveltimes for group or phase velocity maps at a specified period, and has been most frequently used for ambient noise tomography (e.g. Saygin & Kennett 2010;Stankiewicz et al 2010;Young et al 2011). Although the code is 2-D and assumes that the high frequency assumption is valid, the implications of our results should largely hold for any class of linear or iterative nonlinear tomography.…”

Crustal surface wave velocity structure of the east Albany-Fraser Orogen, Western Australia, from ambient noise recordings

“…For determining phase velocities, we relied on a semi-automated approach (modified from Arroucau et al 2010;Young et al 2011) that picks phase velocity dispersion curves from plots of phase velocity against period (after Yao et al 2006). We first carefully handpicked an average dispersion curve, based on which a corridor of 'sensible' velocities was established.…”

“…Group velocities were determined using frequency-time analysis (FTAN, e.g. Levshin et al 1992) and a phase-matched filtering approach (Levshin & Ritzwoller 2001;Arroucau et al 2010;Young et al 2011) on the envelope of the symmetric component (averaged causal and acausal parts) of the empirical Green's functions. Only cross-correlograms between stations with a spatial separation larger than 3 wavelengths were used for the retrieval of dispersion curves (e.g.…”

Beneath Bass Strait: Linking Tasmania and Mainland Australia using Ambient Seismic Noise

“…Subsequently, detailed images of the lithosphere structure in various parts of the world have been obtained by exploiting this class of data (Shapiro & Campillo, 2005;Yang et al, 2007;Bensen et al, 2009;Arroucau et al, 2010;Young et al, 2011;Saygin & Kennett, 2010, 2012. It turns out that long term cross-correlation of the background noise recorded at two stations produces an estimate of the Rayleigh surface wave (from the vertical component or Love wave from the horizontals) packet (known as the empirical the Green's function) that is equivalent to the signal that would arrive at one station if the source waveform were a delta function located at the other station.…”

“…One of the main difficulties in reconciling mainland Australian and Tasmanian tectonics is the lack of Precambrian exposure in the Lachlan Orogen (Victoria), which contrasts with the West Tasmania Terrane that exhibits numerous outcrops of Proterozoic rocks (Elliot and Gray, 2002), apparently excluding any tectonic affinity between them. Furthermore, the West Tasmania Terrane differs significantly from the East Tasmania Terrane in that the latter does not contain any evidence of Precambrian rocks and no evidence of a Proterozoic continental basement has been reported, either in outcrop nor inferred from geophysical surveys (Williams, 1989;Reed, 2001;Young et al, 2011). Perhaps most significantly, the presence of Bass Strait and the Mesozoic and Cainozoic sedimentary and volcanic sequences that mask the older terranes, makes the link between Tasmania and southeast mainland Australia even harder to decipher.…”