After the fall: Lithospheric Structure After Thinning via Gravitational Instability

Lithospheric foundering (commonly referred to as delamination) has been inferred from increases in crustal heat flow in specific regions, rapid regional uplift, and from the appearance of signature high-potassium magmas. Seismic studies also support foundering in specific areas where "drips" appear to be seen as cool material beneath the lithospheric boundary. Seismic imaging can only be used to see material in the process of foundering, however. Using numerical experiments approximating the rheology and density of continental crust, lithosphere, and underlying asthenosphere this study investigates the consequences of foundering via lithospheric gravitational instabilities, with an aim to describe possible resulting lithospheric structures that might be imaged seismically. Here removal of the lithosphere (mantle lithosphere with or without portions of the crust) is hypothesized to occur in a ductile manner, first by flowing horizontally to create a growing boundary perturbation and then sinking vertically as a gravitational instability. This mechanism requires no specific structural weakness beyond a dense region in the lithosphere that is gravitationally unstable with respect to the underlying mantle and that possesses a rheology conducive to flow. Experimental starting conditions include a lithosphere with flat bottom and top boundaries and a region of warm, dense material injected into the lithosphere at the axis of symmetry of these axisymmetric models. These numerical experiments indicate that lithospheric removal from a lithosphere 100 km thick may allow asthenospheric material to rise and to melt adiabatically, but only in a shallow annulus around the sinking material. Following the sinking of the gravitationally unstable material, experiments with a range of conditions all display a near-horizontal lower lithospheric boundary within 3 to 7 Myr, with the exception of a long-lived cool cusp of material hanging down into the asthenosphere at the axis of symmetry of the gravitational instability. In some cases, the lithosphere retains portions of high density material that was too cool to flow into the instability; such structures, if they exist in nature, may be visible in seismic imaging. The structure of the lithosphere after foundering is complete has been hypothesized to include a dome in the lower lithosphere. Topographic uplift caused by the replacement of the dense lithospheric region with hot buoyant asthenosphere is commonly cited evidence for lithospheric delamination. A profound uplift, however, is not seen in these experiments. No dome structures were created in the lower lithosphere; in fact, a slight thickening occurs at the axis of symmetry where the remnants of the drip remain as a cusp. A shallow annulus of higher topography can persist around the axis of symmetry for between 1 and 10 Myr. Topographic uplift is caused by removal of dense material and return to isostatic equilibrium. Following foundering, therefore, these experiments indicate that the lithosphere-asthenosphere boundary would be largely flat, and that thinning would be spread over a wide region rather than isolated at the point of instability. If lithospheric domes are to be created through thinning processes, then different mechanisms must be called upon, perhaps including brittle delamination or significant lateral heterogeneity.