Instability of a lithospheric step beneath western North Island, New Zealand

Abstract

Lithospheres of different thicknesses are often juxtaposed by movement on a continental-transform boundary. Such a boundary with a step change in densities may trigger a gravitational instability as lateral pressure gradients are created where normal mantle lithosphere terminates against less dense asthenospheric mantle. Here we show, for plausible values of the lithospheric viscosity, a mechanism by which the thicker mantle lithosphere will drip off into the lower density asthenosphere. As the mantle deforms it also progressively thickens and then thins the overlying crust, creating a topographic wave that migrates in concert with the removal of mantle lithosphere. Within western North Island, New Zealand, geophysical data defi ne a sharp lithospheric step across the Taranaki-Ruapehu line, and geological observations provide a history of uplift and subsidence that has propagated southward in the past 12 m.y. The rate of observed north to south migration of the wave (~30 mm/yr), its wavelength (~250 km), and amplitude (~±1 km) are compatible with it being caused by progressive removal of mantle lithosphere, if the viscosity of the uppermost lithospheric mantle is ~5 × 10 20 Pa·s, providing one of the clearest examples yet of this fundamental geological process.

The burning heart — The Proterozoic geology and geological evolution of the west Musgrave Region, central Australia

“…In addition, such movement could have juxtaposed lithosphere of contrasting thermal and physical properties. Gorczky et al (2012), Gessner et al (2013) and Stern et al (2013) have highlighted the role that lithospheric inhomogeneities play in localising lithospheric gravitational instabilities that may lead to major asthenospheric upwellings. In the case of the Musgrave Province, the juxtaposition of the Musgrave thermal anomaly against colder cratonic lithosphere might not only have triggered complete destabilisation of any remaining or regrown lithospheric mantle on the Musgrave Province side of the suture, but the models of Stern et al (2013) suggest that the colder cratonic lithosphere to the west might have also been removed.…”

Magmatic expressions of continental lithosphere removal

“…In contrast, delamination should exhibit an asymmetric surface expression (Figure ). This is consistent with observations from the North Island of New Zealand, where a lateral migration of surface subsidence/uplift and high‐K magmatism have been proposed to be related to lithosphere delamination [ Kear , ; Stern et al ., ]. High‐K magmatism is typically interpreted to indicate crustal melting induced by rapid heating following lithosphere removal [e.g., Farmer et al ., ]; other studies argue that low‐degree melts from the upper mantle can produce this signal [e.g., Putirka and Busby , ].…”

“…It should be noted that there are intermediate styles of lithosphere removal. For example, at a sharp transition in lithosphere thickness, edge‐driven convection may cause asymmetric viscous removal of the thicker lithosphere [e.g., van Wijk et al ., ; Stern et al ., ].…”

Cenozoic uplift of the Central Andes in northern Chile and Bolivia—reconciling paleoaltimetry with the geological evolution

“…This is followed by rapid detachment of the gravitationally unstable lithospheric root, although recent dynamical models suggest that this is only likely to occur if there is an abrupt lateral change in lithospheric thickness (Stern et al 2013). If detachment does occur, the overlying lithosphere could undergo uplift of several kilometres, and this could be on a timescale of only a few million years or less (Molnar and Garzione 2007;Stern et al 2013).…”

“…The largest density contrast here is between the crust and mantle, and so crustal shortening and thickening, driven by stresses that come from the interaction of the tectonic plates, is a powerful mechanism of surface uplift-the uplift will correlate closely with the amount and timing of crustal shortening, on time scales of millions to tens of millions of years. In marked contrast, numerical and theoretical models show that in certain cases an instability in the higher density and fluid-like lower part of the lithosphere can grow slowly during long-term lithospheric shortening, then reach a critical stage where it rapidly detaches and sinks into the underlying hotter and lower density asthenosphere, leaving behind a thinned lithospheric mantle; isostatic rebound in the overlying crust could result in several kilometres of surface uplift, depending on the thickness of the mantle root-here, uplift post-dates shortening, and may occur on very short time scales (Ͻ Ͻ5 Ma) (Houseman et al 1981;Molnar et al 1993;Molnar and Garzione 2007;Stern et al 2013).…”