Gravitational instability of mantle lithosphere and core complexes

Abstract

For a wide range of viscosity structures, convergent and downward flow of the mantle lithosphere during the growth of gravitational instability induces not only thickening of overlying crust but also concurrent horizontal extension in the upper crust. Such extension, if it occurred in the Earth, would include normal faulting of the upper crust above a region of horizontal shortening in the lower crust and uppermost mantle. Convergent flow in the lower crust would also create shear stress on horizontal planes and localized upward flow of the lower crust. These features-extension of upper crust and exhumation of strained lower crust-characterize metamorphic core complexes exposed in regions of normal to thick continental crust. Thus, convergent flow and downwelling mantle lithosphere might contribute to the development of core complexes, at least in some settings. If horizontal shortening and crustal thickening at depth do occur simultaneously with normal faulting at the surface of the Earth today, evidence of this process does not seem obvious, but perhaps it has occurred concurrently with widespread regional crustal extension in places like the Basin and Range Province, Tibet, the Pamir, or the Aegean. If such mantle flow does participate in the development of core complexes, a weak lower crust might not be a prerequisite for their formation.

Late Cenozoic Extensional Formation of the Antofalla Depression, Southern Puna Plateau, Argentina: An Effect of Lithospheric Foundering?

“…Several studies show Rayleigh‐Taylor instabilities (“drips”) that grow until dense mantle lithosphere and, in some cases, eclogitized lower crust eventually detach and sink into the asthenosphere (Göğüş & Pysklywec, 2008; Göǧüş et al., 2017; Molnar, 2015; Neil & Houseman, 1999; Wang et al., 2015; Wang & Currie, 2017). Numerical models suggest that the growth of lithospheric drips takes several million years (Wang et al., 2015).…”

“…Numerical models suggest that the growth of lithospheric drips takes several million years (Wang et al., 2015). If the crust is weak, drips induce crustal flow into the area above the drip (Molnar, 2015; Neil & Houseman, 1999; Wang et al., 2015), causing surface uplift and upper crustal extension (Figure 9e; Molnar, 2015; Wang et al., 2015), with coeval upper crustal contraction some distance away (Molnar, 2015). Models also indicate post‐detachment isostatic uplift and gravitational collapse of thickened crust overlying the drip location (Neil & Houseman, 1999; Wang et al., 2015).…”

Imaging continental breakup using teleseismic body waves: The Woodlark Rift, Papua New Guinea

“…Such large thermal (and, hence, density) contrast implies the system may be gravitationally unstable. Mareschal [1983] proposed that rapid zones of extension such as this one require lithospheric delamination, a process which has been implicated in the development of metamorphic core complexes [Molnar, 2015;Lachenbruch et al, 1994] and which could contribute to rapid UHP exhumation during rifting Ellis et al, 2011]. Abers and Roecker [1991] postulated that the intermediate depth seismicity beneath the Papuan Peninsula north of 8.58 S could be explained by lithospheric convective instability (drip) just as well as any proposed subduction, suggesting that volcanism on the DEI represents a region where a continental lithospheric root has already detached ( Figure 11c); this interpretation is similar to that suggested for intermediate depth earthquakes and fast mantle structures in the Carpathians [Ren et al, 2012].…”

The Anatomy and Origin of a Synconvergent Grenvillian-Age Metamorphic Core Complex, Chottanagpur Gneiss Complex, Eastern India

“…Long et al [31] demonstrate that regions of upper crustal thickening directly control the spatial location of synorogenic extension. Additionally, vertical partitioning of strain in the crust during convergence can localize extension in the upper crust and contraction in the ductile lower crust (e.g., [1,7,[32][33][34][35][36][37]).…”