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Theory and practices involved in depth and source localization of anisotropy
Published in Rajib Biswas, Recent Developments in Using Seismic Waves as a Probe for Subsurface Investigations, 2023
In case of the Shillong plateau, the case study satisfied all the conditions required for the central depth calculation and found with the minima of the variation factor (Fv) at a depth of around 100 km. The exact minimum is at 94 km, and 102 km for grid sizes 0.36 and 0.40, respectively, with a fixed central difference of 0.11°. Also, XKS splitting parameters plotted at the ray-piercing point of 100 km depth and positively showed maximum coherency (Figure 3.3) than all other depths. Devi et al. (2011) suggests the approximate depth of the lithosphere-asthenosphere boundary (LAB) at around 100 km from the converted phases receiver function analysis. So the carried study (Mohanty et al., 2021) successfully defends the fact that the LAB must be the source of this high strength anisotropy at this particular depth range of 100 km, which is established from the spatial coherency model. The shear wave splitting (SWS) results of this region show a fast polarization direction (FPD) towards the NE-SW direction coinciding with the absolute plate motion (APM) of the Indian plate. Overall, the source localization of anisotropy at a depth of 100 km is suggestive of the fact that, the asthenospheric drag at the base of the lithosphere (LAB region) is the major source of anisotropy which governs the present-day deformation patterns beneath the plateau mass and is responsible for the present tectonics.
The Norfolk Ridge seamounts: Eocene–Miocene volcanoes near Zealandia’s rifted continental margin
Published in Australian Journal of Earth Sciences, 2021
N. Mortimer, M. Patriat, P. B. Gans, A. Agranier, G. Chazot, J. Collot, M. P. Crundwell, P. M. J. Durance, H. J. Campbell, S. Etienne
Several north–south alignments of volcanoes are present in the northern Zealandia region (Figure 1). Tasmantid and Lord Howe seamount chains are confidently identified as age-progressive intraplate volcanoes, with a deep mantle hotspot-type origin (e.g. Seton et al., 2019). Three Kings-Loyalty Ridge is thought to be an Eocene–Miocene subduction-related volcanic arc (e.g. Mortimer et al., 2007). We have investigated the origin of eight large seamount volcanoes that form a ∼670 km-long line along the western flank of the Norfolk Ridge. Ages of dredged rocks from four sites (and volcaniclastic rocks drilled in U1507) range from 33 to 21 Ma and compositions range from alkali basalt to subalkaline basalt to shoshonitic trachyandesite. Despite the limited sampling, and poor quality of volcanic rocks recovered, we show that the Norfolk Ridge volcanoes probably represent neither an age-progressive hotspot track nor a simple subduction-related volcanic arc. We combine earlier petrological and tectonic models to explain the location of the Norfolk Ridge seamounts by mantle melting near a step in the lithosphere–asthenosphere boundary created during and after the Eocene foundering of the New Caledonia Trough. The variable age and chemistry result from shear-induced melting of heterogeneous mantle, possibly with a subducted slab being involved in the petrogenesis of some of the lavas.
The geochemistry and petrogenesis of Carnley Volcano, Auckland Islands, SW Pacific
Published in New Zealand Journal of Geology and Geophysics, 2018
John A. Gamble, Chris J. Adams, Paul A. Morris, Richard J. Wysoczanski, Monica Handler, Christian Timm
Lithospheric dimensions for the Campbell Plateau are reported by Davy 2006, 2008; Grobys et al. 2008, 2009; Pysklywec et al. 2010; Ball et al. 2016. Significantly, Grobys et al. (2009) conclude that continental crust thins from around 33 km in the southern South Island, to 21 km beneath the Great South Basin, thickening again to ∼27 km under the central Campbell Plateau. Moreover, three dimensional modelling by Pysklywec et al. (2010) suggest appreciable and abrupt thickness variations across the lithospheric mantle of southern Zealandia and this is further supported by results of the South Island Geophysical Transect (SIGHT) project (Davey et al. 2007; Okaya et al. 2007) that show thickened lithosphere to the SW of the South Island and fractures penetrating deep crust into lithospheric mantle. We therefore suggest that the lithosphere–asthenosphere boundary beneath southern Zealandia is likely to be uneven and locally stepped.
Implications of upper-mantle seismicity for deformation in the continental collision zone beneath the Alpine Fault, South Island, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2018
Carolin M. Boese, Tim A. Stern, Konstantinos Michailos, John Townend, Calum J. Chamberlain
Upper mantle earthquakes elsewhere are typically located in areas of thick lithosphere or strong gradients in lithospheric thickness (Sloan and Jackson 2012, and references therein). Recent 3-D estimates of the lithosphere-asthenosphere boundary (LAB) by Hua et al. (2018) show pronounced thickness changes throughout the South Island. Perpendicular to the central Alpine Fault, the LAB changes from thick on the Australian side (90–100 km but up to 130 km) to thin on the Pacific side (70–80 km). Along the strike of the Alpine Fault, the transition from thick lithosphere to thin lithosphere in the north occurs in the vicinity of Hokitika (west of the northern cluster of upper mantle earthquakes), and in the south near Marty River (between the southern cluster of upper mantle earthquakes and the Dart Cluster). The central cluster of upper mantle earthquakes is located along the eastern transition in lithospheric thickness.