Lithosphere Removal in the Central Andes: Reconciling Seismic Images and Elevation History

• Author / Creator

  Henderson, Olivia A.

• The central Andes (18° - 26°S) is an active Cordilleran orogen, formed through the subduction of the Nazca oceanic plate beneath the South American continent. Despite significant tectonic crustal shortening and thickening (60 -70 km), deep seismic imaging indicates a thin to absent mantle lithosphere beneath the central Andean plateau (Altiplano-Puna plateau). In order to explain this as well as regional geological observations (e.g. widespread mafic volcanism, and along-strike timing variations in uplift across the plateau), lithosphere removal has been proposed to have occurred within the central Andean system within the last 20 Ma. Conditions within Cordilleran systems, such as the central Andes, can produce density inversions (i.e. roots) within the plateau mantle lithosphere, including: changing pressure-temperature conditions induced by subduction, crustal tectonic shortening, and cyclical magmatic processes. The presence of gravitationally unstable mantle lithosphere drives its removal. Motivated by these observations, the present study uses thermal-mechanically coupled geodynamic models in order to study gravitational removal of single and multiple dense roots within an active subduction zone setting, representative of the central Andes. Both lithosphere removal via large-scale coherent delamination and localized Rayleigh-Taylor (RT)-type instabilities are investigated. Emphasis is placed on i) the dynamics and timing of removal events, ii) the topographic response to lithosphere removal, and iii) the predicted seismic velocity structures generated as lithosphere removal proceeds. In general, subduction-induced flow aids the removal process, perturbing the lithosphere and entraining the foundering material to remove it from the system. For the modelled lithospheric structure, delamination causes mantle lithosphere to coherently peel away from the overlying crust towards the subducting oceanic slab. The numerical modelling experiments show this delamination occurs within 5 – 12 m.y., resulting in 0.5 – 0.7 km isostatic uplift, with removal proceeding faster for decreased root density and increased mantle lithosphere strength (i.e. dehydrated conditions). In comparison, RT-type instabilities of greater density and decreased strength are more readily destabilized. Mantle lithosphere is locally removed within 2 – 12 m.y., and the root’s down-going trajectory is largely dependent on its relative position to the trench and the rate at which is descends. Note that lithosphere removal of a single RT instability results in local uplift of 0.5 km, whereas destabilization of multiple roots positioned across the extent of the plateau causes regional uplift of 1 – 1.2 km. This study shows that present-day lithosphere structure across the Altiplano-Puna plateau can be reproduced through episodes of gravitationally driven lithosphere removal. Overall, for both removal mechanisms (i.e. delamination and Rayleigh-Taylor-type instabilities), predicted seismic velocity structures are able to reconcile previously identified upper mantle velocity anomalies to a certain degree, supporting the idea that low velocity zones beneath the central Andean plateau represent high temperature areas of thinned mantle lithosphere. However, regardless of the manner of removal, the resulting surface uplift is limited to ~1 km, suggesting the modern Altiplano-Puna plateau topography is likely not solely linked to removal-induced isostatic and dynamic elevation changes.

• Subjects / Keywords

  • Surface expression
  • Lithosphere dynamics
  • Altiplano
  • Numerical model
  • Uplift
  • Delamination
  • Seismic velocity structure
  • Central Andes
  • Rayleigh-Taylor instabilities
  • Gravitational instability
  • Lithosphere removal
  • Subduction zone
  • Puna

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3WS8J10Z

• 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.