Intracontinental Deformation Caused by Gravitational Lithosphere Removal

• Author / Creator

  Wang,Huilin

• Gravitational removal of the dense lower lithosphere is proposed to be a fundamental process in continental tectonics. This has been used to explain seismological observations of an abnormally thin lithosphere beneath some regions and evidence for detached lithospheric materials at 100-200 km depth. In addition, geochemical arguments suggest that a significant portion of the lower lithosphere may have been recycled into the deeper mantle. The removal process should significantly affect the overlying crust, causing transient uplift/subsidence, crustal contraction/extension and pulses of volcanism. As removal can occur in continental plate interiors, it may provide an explanation of areas of anomalous intraplate deformation that can not be readily linked to tectonic processes. This thesis uses two-dimensional thermal-mechanical numerical models to explore the surface deflection and magmatism induced by gravitational lithosphere removal. Removal is widely believed to be associated with surface subsidence and widespread asthenospheric magmatism. However, the models show that the surface expression depends strongly on the thermal and rheological structure of the lithosphere. If the crust is weak, the descending dense lithosphere induces lateral crustal flow, leading to crustal thickening and surface uplift. Crustal flow in a mid-crustal channel will smooth the surface subsidence caused by the dense lithosphere; crustal flow in a lower-crust channel can cause the surface to invert to become a topographic high. Magmatism caused by lithosphere removal depends on the removal style and the initial thermal structure of lithosphere. During a Rayleigh-Taylor instability (drip), three types of magmas are found: (1) for a hot lithosphere (e.g., back arc), the foundering lithosphere can melt as it is descends and the asthenosphere can undergo decompression melting as it upwells to replace the removed lithosphere; (2) for a warm lithosphere (e.g., average Phanerozoic lithosphere), only asthenospheric melt is predicted; (3) for a cold and thick lithosphere (e.g., craton), no magmas are generated during removal. If removal occurs through delamination, the dense mantle lithosphere rapidly peels along the Moho and sinks into the deep mantle before it can melt. However, significant decompression melting of the asthenosphere may occur as it upwells to the base of crust. Delamination is associated with an asymmetric surface signature, where crustal deformation and magmatism migrate with the detachment hinge. In a Rayleigh-Taylor instability, the deformation and magmatism are symmetric. Observational data from a number of regions (e.g., southern Sierra Nevada in North America, Puna plateau in the central Andes, and Tibet in western China) are consistent with the numerical models, suggesting that intracontinental deformation and magmatism in these areas are related to lithosphere removal.

• Subjects / Keywords

  • Continental deformation
  • Lithosphere removal

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3707WV71

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.