SURFACE TOPOGRAPHY, CRUSTAL DEFORMATION AND MAGMATISM INDUCED BY LITHOSPHERIC REMOVAL

Localized crustal deformation, surface subsidence/uplift and magmatism have been observed in the interior of North America and Asia. In some cases, these events can not be directly linked with plate boundary tectonics, and in places, the region of deformation is underlain by abnormally thin mantle lithosphere at present-day. More puzzling, the surface magmas have an upper plate composition but were melted at sub-lithospheric depths. Gravitational removal of dense lithosphere has been postulated as a mechanism to explain these observations. However, it is still unclear why the surface expression of lithosphere removal varies between different regions. For example, inferred lithosphere removal beneath the southern Sierra Nevada (USA) was accompanied by surface subsidence, crustal contraction and eruption of magma derived from lithosphere and asthenosphere sources. In contrast, in central Tibet, volcanism has been linked to melting of foundering lithosphere but little surface deformation has occurred.

Here, we use 2D thermal-mechanical numerical models to investigate the range of surface expressions and magmatism associated with lithospheric removal. We find that topography and deformation are primarily controlled by lithosphere structure. In regions of strong crust (i.e., thin and/or cool), lithosphere removal induces surface subsidence and crustal shortening, followed by uplift and extension. Conversely, little surface deflection or uplift occurs if the crust is weak (i.e., thick and/or warm). For all crustal strengths, magmas can be generated by both conductive heating of fusible (e.g., pyroxenite rich) lithosphere as it founders, as well as decompression melting of the upwelling asthenosphere. Transport of magmas to the surface may depend on crustal strength. If the near-surface crust is sufficient weak, thrust faults may develop above the foundering lithosphere, providing channels for magma migration. Our results indicate the surface deformation and magmatism in the Sierra Nevada is consistent with lithosphere foundering below a strong crust but with weak near-surface rocks, whereas magmatism in the absence of deformation in Tibet may be due to lithosphere removal beneath a weak crust.