Crustal complexity in the Lachlan Orogen revealed from teleseismic receiver functions

Abstract

There is an ongoing debate about the tectonic evolution of southeast Australia, particularly about the causes and nature of its accretion to a much older Precambrian core to the west. Seismic imaging of the crust can provide useful clues to address this issue. Seismic tomography imaging is a powerful tool often employed to map elastic properties of the Earth's lithosphere, but in most cases does not constrain well the depth of discontinuities such as the Mohorovi ci c (Moho). In this study, an alternative imaging technique known as receiver function (RF) has been employed for seismic stations near Canberra in the Lachlan Orogen to investigate: (i) the shear-wave-velocity profile in the crust and uppermost mantle, (ii) variations in the Moho depth beneath the Lachlan Orogen, and (iii) the nature of the transition between the crust and mantle. A number of styles of RF analyses were conducted: H-K stacking to obtain the best compressional-shear velocity (V P /V S ) ratio and crustal thickness; nonlinear inversion for the shear-wave-velocity structure and inversion of the observed variations in RFs with backazimuth to investigate potential dipping of the crustal layers and anisotropy. The thick crust (up to 48 km) and the mostly intermediate nature of the crustÀmantle transition in the Lachlan Orogen could be due to the presence of underplating at the base of the crust, and possibly to the existing thick piles of Ordovician mafic rocks present in the mid and lower crust. Results from numerical modelling of RFs at three seismic stations (CAN, CNB and YNG) suggest that the observed variations with back-azimuth could be related to a complex structure beneath these stations with the likelihood of both a dipping Moho and crustal anisotropy. Our analysis reveals crustal thickening to the west beneath CAN station which could be due to slab convergence. The crustal thickening may also be related to the broad Macquarie volcanic arc, which is rooted to the Moho. The crustal anisotropy may arise from a strong N-S structural trend in the eastern Lachlan Orogen and to the preferred crystallographic orientation of seismically anisotropic minerals in the lower and middle crust related to the paleo-Pacific plate convergence.

Crustal and mantle structure beneath the Terre Adelie Craton, East Antarctica: insights from receiver function and seismic anisotropy measurements

“…Because of the non-uniqueness of the RF solution (Ammon et al 1990), the NA method combines a Monte Carlo search technique and the properties of the Voronoi geometry in parameter space to find an ensemble of the best-fitting models and performs a global optimization. Fontaine et al (2013b) showed good agreement between Moho depths and results from seismic reflection profiles. Here, we applied the same non-linear inversion method to match the observed radial RFs.…”

“…The TAC is inferred to have been connected to the Gawler Craton (Oliver & Fanning 1997;Peucat et al 1999;Fanning et al 2002;Di Vincenzo et al 2007). The Moho depth was estimated in the Gawler Craton at about 40 km by Collins (1991), Clitheroe et al (2000) and between 40 and 45 km by Fontaine et al (2013b). Moreover, all the authors highlight evidences of magmatic underplating beneath the Gawler Craton.…”

“…We represented the crust and uppermost mantle structures by a six-layers, 1-D seismic velocity model ( Table 2). Parameters of this model were proposed by Shibutani et al (1996) and used by Fontaine et al (2013b) for the Gawler Craton in southern Australia. We tested four-layers, five-layers and a different six-layers model but we obtained more realistic S-wave velocities with the Australian six-layers model, as shown in Fig.…”

Crustal and uppermost mantle structure variation beneath La Réunion hotspot track

“…However, the goal of this study and particularly with the present limited backazimuthal coverage is not to constrain dipping effect or anisotropic effect on the RFs (e.g. Peng & Humphreys 1997;Savage 1998;Fontaine et al 2013b). Instead, we choose to concentrate on stacking similar radial RFs for a limited range of backazimuths and ray parameters in order to estimate the depths of first-order seismic discontinuities present beneath the station (e.g.…”

Crustal structure of southeast Australia from teleseismic receiver functions

“…Earlier RF studies in southeast Australia (Shibutani et al, 1996;Clitheroe et al, 2000;Fontaine et al, 2013a,b) suggested the presence of complex lateral velocity variations in the mid-lower crust that probably reflect the interaction of igneous underplating, associated thinning of the lithosphere, recent hotspot volcanism and uplift. Furthermore, the intermediate to high crustal V p /V s ratio of 1.70-1.78 in this region (Fontaine et al, 2013a), relative to ak135 continental crust where V p /V s is ∼1.68, may indicate a mafic composition that includes mafic granulite rocks, granite-gneiss and biotite gneiss. Body-and surface-wave tomography (Fishwick and Rawlinson, 2012;Rawlinson et al, 2015) reveal P and S wave velocity anomalies in the uppermost mantle beneath Bass Strait and the Lachlan Fold Belt.…”