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1 LGSR - Laboratoire GéoSciences Réunion 2 Research School of Earth Sciences, Australian National University, Canberra

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 Mohorovičić 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 VP-VS ratio and crustal thickness; non-linear inversion for the shear wave velocity structure and inversion of the observed variations in RFs with back-azimuth to investigate potential dipping of the crustal layers and anisotropy.The thick crust up to 48 km and the mostly intermediate nature of the crust-mantle 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 receiver functions at 3 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 palaeo-Pacific plate convergence.

Keywords : crustal thickness Moho seismic anisotropy Lachlan Orogen receiver function seismic structure crustal complexity





Autor: Fabrice R. Fontaine - Hrvoje Tkalcic - Brian L. N. Kennett -

Fuente: https://hal.archives-ouvertes.fr/



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