Wednesday, May 16th, 2018

Wednesday Seminar

4:10 PM, 55 Roessler
Tea and cookies at 3:45 in the aviary - (2110 EPS)

“Is flat subduction a consequence of rheologically strong lithosphere?”

      – by Dr. Ravi Kanda, Utah State University

Subduction of oceanic plates into Earth's mantle most commonly occurs with slab dip angle gradually increasing to depths of 150 km, below which slabs dip ~30-70 degrees into the mantle transition zone. Occasionally, slab dip 'flattens' in the upper 150 km, attaining near horizontal contact with the overriding plate before diving steeply into the mantle. Slab flattening occurs in < 10% of global subduction zones, that too at ocean-continent margins, and has been attributed to one or more of the following factors: large (> 10 cm/yr) plate convergence rates; increased buoyancy of the subducting plate due to oceanic plateaux, ridges, or young lithospheric age; low viscosity within the mantle wedge; and overriding (continental) plates with thick cratonic roots. Here, we show that slab dynamics owing to realistic, laboratory-based viscoplastic rheological laws and driven by far-field plate forcing differs from that postulated in many previous modeling studies. As in previous studies, we find that one of the necessary and sufficient conditions for slab flattening is the advance of the overriding plate due to far-field 'ridge-push'. However, we find that the other key requirement is long-term resistance to slab penetration into the lower mantle (~660 km depth), likely caused by encountering piles of older slab material and/or the endothermic Spinel-Perovskite phase transition. For a slab in forced retreat, this 'tip-resistance' causes its dip to shallow in the uppermost mantle, restricting flow into - and cooling - the mantle wedge. The resulting slab wedge viscosity increase then exerts sufficient suction on the slab to overcome its bending resistance, allowing flattening beneath the overriding plate. We find that this delicate interaction between deep resistance to subduction and near-surface rheological evolution of the subducted slab and mantle-wedge controls the initiation, duration, and extent of flat-slabs. Our modeled slab profiles are consistent with present day slab geometry inferred from seismic tomography at most of the World's flat subduction zones, as well as their subduction history. Our models also simulate free-surface topographic evolution modulated by 1D erosion/sedimentation. Free-surface expression includes broad arc/backarc uplifts and/or shallow cratonic basins; and orogenic belts; and consistent with the cessation of arc volcanism as the slab shallows. Our results provide new insights into the feedbacks between mantle and lithospheric deformation that we hope will inspire more sophisticated 3D regional data-assimilation modeling studies using such realistic rheological laws.

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