Crustal deformation induced by mantle dynamics: insights from models of gravitational lithosphere removal

Abstract

S U M M A R YMantle-based stresses have been proposed to explain the occurrence of deformation in the interior regions of continental plates, far from the effects of plate boundary processes. We examine how the gravitational removal of a dense mantle lithosphere root may induce deformation of the overlying crust. Simplified numerical models and a theoretical analysis are used to investigate the physical mechanisms for deformation and assess the surface expression of removal. Three behaviours are identified: (1) where the entire crust is strong, stresses from the downwelling mantle are efficiently transferred through the crust. There is little crustal deformation and removal is accompanied by surface subsidence and a negative free-air gravity anomaly. Surface uplift and increased free-air gravity occur after the dense root detaches. (2) If the mid-crust is weak, the dense root creates a lateral pressure gradient in the crust that drives Poiseuille flow in the weak layer. This induces crustal thickening, surface uplift and a minor free-air gravity anomaly above the root. (3) If the lower crust is weak, deformation occurs through pressure-driven Poiseuille flow and Couette flow due to basal shear. This can overthicken the crust, producing a topographic high and a negative free-air gravity anomaly above the root. In the latter two cases, surface uplift occurs prior to the removal of the mantle stress. The modeling results predict that syn-removal uplift will occur if the crustal viscosity is less than ∼10 21 Pa s, corresponding to temperatures greater than ∼400-500 • C for a dry and felsic or wet and mafic composition, and ∼900 • C for a dry and mafic composition. If crustal temperatures are lower than this, lithosphere removal is marked by the formation of a basin. These results can explain the variety of surface expressions observed above areas of downwelling mantle. In addition, observations of the surface deflection may provide a way to constrain the vertical rheological structure of the crust.

Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

“…On the basis of the spatial coherence of our measurements, we hypothesize that the azimuthal anisotropy in NE Oregon results from the lattice preferred orientation (LPO) of anisotropic minerals with the subhorizontal flow of the mid-lower crust (23,24). Recent numerical studies show that stresses transmitted upward from the underlying mantle can induce large amounts of intraplate deformation through Poiseuille and Couette flow due to lateral pressure variations and basal shear (25). This style of deformation requires the lithospheric rocks to have low viscous strength to form a channeled ductile flow system in the mid-lower crust that decouples the upper crustal and upper mantle stress fields.…”

“…We also impose a small-scale load, with moderate stress magnitude, to the predicted Moho traction in the Wallowas area to account for the foundering of the mountain’s pluton root. This last step is taken because this event is a rather short-lived transient phenomenon that is not captured by the seismic tomography and may also contribute to the observed anisotropy; the destabilization and subsequent removal of the root would cause the weak mid-lower crust around the Wallowas to flow toward the vacated root region ( 25 ). Moho traction calculations made without incorporating the localized Wallowa load reveal that the first-order vertical stress distribution is not significantly altered and are presented in fig.…”

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).…”

“…Lithospheric foundering beneath the southern Puna plateau is hypothesized based on volcanic geochemistry (Ducea et al, 2013;Kay et al, 1994Kay et al, , 1999Murray et al, 2015), as well as geophysical interpretations of thinned mantle lithosphere beneath the plateau (Heit et al, 2014;Tassara & Echaurren, 2012) and foundered material sinking in the asthenosphere (Beck et al, 2015;Calixto et al, 2013;Scire et al, 2015). Numerical modeling of foundering processes suggests that two modes of foundering are dominant, dripping (Figure 9e) and delamination (Figure 9f), each of which may be able to cause diverse changes to crustal deformation depending on the relative viscosities, thicknesses, and densities of various components of the lithosphere (Göǧüş et al, 2017;Göğüş & Pysklywec, 2008;Krystopowicz & Currie, 2013;Molnar, 2015;Neil & Houseman, 1999;Wang & Currie, 2017;Wang et al, 2015). Both modes of foundering have been proposed for the southern Puna (Beck et al, 2015;DeCelles et al, 2015;Ducea et al, 2013;Murray et al, 2015;Schoenbohm & Carrapa, 2015).…”

Diverse Styles of Lithospheric Dripping: Synthesizing Gravitational Instability Models, Continental Tectonics, and Geologic Observations

“…Post-detachment rebound is negligible (<50 m) for the strongest crusts (η′ ≥ 100 Elkins-Tanton, 2007) but is significant for crusts with more moderate strengths (η′ ∼ 10, H. . Rebound is also increased by weakening the middle-lower crust, which allows for some lower-crustal thickening during drip growth (e.g., Göğüş & Pysklywec, 2008;Göğüş et al, 2017;Kaus & Becker, 2007;H. Wang & Currie, 2017;.…”