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Igneous Petrology and the Nature of Magmas
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
The asthenosphere is mostly solid but is partially melted in some places, notably beneath mid-ocean ridges where the lithosphere is very thin and the asthenosphere is only a few kilometers below the ocean floor. In other parts of the oceans, the asthenosphere may be beneath 100 kilometers of lithosphere, and in continental regions it is generally deeper, often as deep as 200 kilometers. Earthquake waves pass through the asthenosphere more slowly than through the lithosphere, in large part due to the presence of some melt, and the upper part of the asthenosphere is often called the low-velocity zone (LVZ). Because the asthenosphere is partially melted in some places and at or near its melting temperature in other places, it is less rigid than the lithosphere. In fact, the apparently solid material of the asthenosphere acts in some ways like a liquid; it convects (flows) at rates as fast as centimeters per year. As the asthenosphere moves, it carries the more rigid lithosphere above it, in part providing the mechanism for plate tectonics (described in Chapter 2).
Tectonic subsidence and uplift within Canterbury Basin, South Island, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2023
Katherine Dvorak, Michelle Kominz, Martin Crundwell
One possible hypothesis that can explain the uplift event in this region is delamination of the lower lithosphere of the Pacific plate beneath the Canterbury Basin, resulting in a total uplift of 1070 ± 155 m. Remembering that the thermal subsidence requires thickening of the lithosphere to 95-110 km (e.g. McKenzie 1978; Kominz et al. 2011), the presence of thin lower lithosphere beneath this region of the South Island is consistent with delamination of the base of the lower lithosphere (Figure 10). Rayleigh wave dispersion was analysed by Ball et al. (2016) to generate an S-wave velocity model of the upper 140 km beneath Zealandia. Their results show the presence of a low-velocity zone beneath the Canterbury Basin at a depth greater than 60 km. This low-velocity zone has been interpreted as the asthenosphere, which, they suggest, is present at this shallow depth due to the loss of a portion of the mantle lithosphere. A thinned lithosphere is also a weakened lithosphere, and Ji et al.’s (2020) gravity analysis, which interprets the effective elastic thickness as 15 km beneath the Canterbury Basin, is also consistent with lithospheric mantle delamination.
Geochemical patterns of late Cenozoic intraplate basaltic volcanism in northern New Zealand and their relationship to the behaviour of the mantle
Published in New Zealand Journal of Geology and Geophysics, 2021
Ian E. M. Smith, Shane J. Cronin
In terms of their chemical compositions the Northland and Auckland Province volcano fields tell different stories. In the north, compositions are alkali basalt through tholeiitic basalt to transitional basalt and intermediate compositions; alkalic and peralkalic rhyolite are spatially associated with the basaltic rocks (Ashcroft 1986). Although compositions compatible with derivation from upper mantle sources are part of the association, there is petrographic and chemical evidence for fractionation and magma mixing processes at crustal levels (Coote and Shane 2018; Coote et al. 2018; Shane and Coote 2018) and this is consistent with tomographic evidence for a low velocity zone within the crust beneath Northland (Horspool et al. 2006). The chemical compositions of relatively primitive samples indicate that mantle sources of the Northland fields were relatively shallow.