Patched removal of the mantle lithosphere under orogens: A systematic numerical study

Delamination or convective thinning could cause large-scale and complete removal of the mantle lithosphere under orogens. However, geological and geophysical observations suggest that patched removal of the mantle lithosphere has occurred in some orogens, such as the northeastern Tibetan Plateau, the central Tianshan, and the central Andes. Dislocation-creep-induced strain localization cannot promote effective removal of the mantle lithosphere to the Moho on a small-scale. Recent rheological studies propose that dislocation-accommodated grain boundary sliding (DisGBS) may dominate upper mantle deformation. DisGBS could make the lower lithospheric mantle rheologically weaker than dry olivine. With 2-D high-resolution thermo-mechanical modeling, we systematically investigated the conditions for the initiation of small-scale lithospheric thinning under orogens and explored the minimum range of removal of the mantle lithosphere. The numerical results indicate that classic convective drip cannot effectively thin the mantle lithosphere to the Moho on a small-scale. In contrast, small-scale thinning can be induced by lithospheric heterogeneity with DisGBS and plasticity. The rheological heterogeneity can be verified by magmatism and metasomatism under the central Andes and orogens between terranes under the northeastern Tibetan Plateau or in Tianshan.