15:45 〜 16:00
[SIT20-08] 3-D mapping of seismic velocity and discontinuities in the continental upper mantle: A review for Australia
キーワード：上部マントル、大陸、LAB、MLD、表面波、レシーバー関数
The unique geological setting and complicated tectonic history of Australia since Archean have attracted significant attention from seismologists over the decades. The SKIPPY project in Australia in the 1990s pioneered continent-wide seismic imaging using transportable broadband seismic instruments, which has been followed by numerous successive projects in Australia and in many continental regions, such as the USArray.
The Australian continent comprises three major cratonic blocks, amalgamated about 1.3 Ga, forming its stable western and central regions, while the eastern area is composed of younger Phanerozoic basement associated with the ancient subduction in the east of Gondwana before the separation of Australia from Antarctica. The Tasman Line separates the stable cratonic regions in the western 2/3 of Australia and much younger eastern Australia. Since about 40 Ma, Australia has been moving northward at around 7 cm/year.
In the past decade, we have worked on unraveling the detailed seismic structure of the Australian lithosphere and asthenosphere via (1) tomographic imaging with enhanced horizontal and vertical resolution using multimode surface waves and (2) the detection of seismic discontinuities from the joint Bayesian analysis of body-wave receiver functions and surface wave dispersions. Combining the variety of results from surface wave tomography and the latest receiver function analysis, we could obtain clear seismological images of the continental upper mantle beneath fast-drifting Australia.
The comprehensive Australian seismic model, which combines surface-wave and body-wave information, enables us to assess the large-scale shear velocity structure accompanied by multiple seismic discontinuities, such as the Mid-Lithospheric Discontinuities (MLDs), Lithosphere-Asthenosphere Boundary (LAB), and X-discontinuities (X-Ds) in the depth of the asthenosphere below LAB.
Our combined images of 3-D maps of seismic velocity, anisotropy, and discontinuities have clarified the realistic makeup of the Australian upper mantle, characterized by the vertical changes in the seismic velocity and anisotropic properties across each discontinuity. The Australian LAB is characterized by the increasing radial anisotropy (SH>SV) below the LAB. In contrast, such characteristic anisotropy tends to faint below the X-Ds (around 200-300 km depth), suggesting that X-Ds may represent the base of the asthenosphere, where lateral shear flow between LAB and X-Ds may control the fast drifting speed of the Australian continent via basal drag.
Multiple MLDs have been found in the many regions of the cratonic lithosphere at around 60 km and 90 km depths. The MLDs are generally characterized by negative velocity jumps in Archean cratonic blocks in western and southern Australia, while those in northern Australia indicate positive jumps. In some regions, the other deeper MLD below 100 km can also be found beneath the cratonic region of central Australia, which is characterized by thicker lithosphere. The formation process of MLD is still under debate. Still, the latest images suggest its strong relation with seismic anisotropy, which may have influenced the seismologically detected MLD using the receiver functions, indicating the possibility that they may represent the apparent discontinuities associated with the vertical changes in seismic anisotropy.
The latest seismological images of the Australian upper mantle have successfully clarified the spatial distributions of upper mantle discontinuities and their relationship with vertical changes in seismic velocity and anisotropy. Our combined approach, incorporating surface waves and receiver functions for 3-D imaging of the continental lithosphere, would further our knowledge of stable continents and help unravel the formation and evolution process of long-lived cratonic lithosphere.
The Australian continent comprises three major cratonic blocks, amalgamated about 1.3 Ga, forming its stable western and central regions, while the eastern area is composed of younger Phanerozoic basement associated with the ancient subduction in the east of Gondwana before the separation of Australia from Antarctica. The Tasman Line separates the stable cratonic regions in the western 2/3 of Australia and much younger eastern Australia. Since about 40 Ma, Australia has been moving northward at around 7 cm/year.
In the past decade, we have worked on unraveling the detailed seismic structure of the Australian lithosphere and asthenosphere via (1) tomographic imaging with enhanced horizontal and vertical resolution using multimode surface waves and (2) the detection of seismic discontinuities from the joint Bayesian analysis of body-wave receiver functions and surface wave dispersions. Combining the variety of results from surface wave tomography and the latest receiver function analysis, we could obtain clear seismological images of the continental upper mantle beneath fast-drifting Australia.
The comprehensive Australian seismic model, which combines surface-wave and body-wave information, enables us to assess the large-scale shear velocity structure accompanied by multiple seismic discontinuities, such as the Mid-Lithospheric Discontinuities (MLDs), Lithosphere-Asthenosphere Boundary (LAB), and X-discontinuities (X-Ds) in the depth of the asthenosphere below LAB.
Our combined images of 3-D maps of seismic velocity, anisotropy, and discontinuities have clarified the realistic makeup of the Australian upper mantle, characterized by the vertical changes in the seismic velocity and anisotropic properties across each discontinuity. The Australian LAB is characterized by the increasing radial anisotropy (SH>SV) below the LAB. In contrast, such characteristic anisotropy tends to faint below the X-Ds (around 200-300 km depth), suggesting that X-Ds may represent the base of the asthenosphere, where lateral shear flow between LAB and X-Ds may control the fast drifting speed of the Australian continent via basal drag.
Multiple MLDs have been found in the many regions of the cratonic lithosphere at around 60 km and 90 km depths. The MLDs are generally characterized by negative velocity jumps in Archean cratonic blocks in western and southern Australia, while those in northern Australia indicate positive jumps. In some regions, the other deeper MLD below 100 km can also be found beneath the cratonic region of central Australia, which is characterized by thicker lithosphere. The formation process of MLD is still under debate. Still, the latest images suggest its strong relation with seismic anisotropy, which may have influenced the seismologically detected MLD using the receiver functions, indicating the possibility that they may represent the apparent discontinuities associated with the vertical changes in seismic anisotropy.
The latest seismological images of the Australian upper mantle have successfully clarified the spatial distributions of upper mantle discontinuities and their relationship with vertical changes in seismic velocity and anisotropy. Our combined approach, incorporating surface waves and receiver functions for 3-D imaging of the continental lithosphere, would further our knowledge of stable continents and help unravel the formation and evolution process of long-lived cratonic lithosphere.