Magnetotelluric constraints on the role of fluids in convergent plate boundaries

• Author / Creator

  Rippe, Dennis

• Convergent plate boundaries have played an important role in the growth and assembly of the modern continents. To obtain a better understanding of the tectonic processes in these boundaries, it is necessary to constrain the rheology of the crust and upper mantle in these regions. Magnetotelluric studies measure electrical resistivities and provide an excellent tool to determine the fluid content and thermal structure. These are key parameters in determining the rheology. Magnetotelluric studies were used to investigate two types of convergent plate boundaries: (i) the Cascadia subduction zone as an example for active subduction of an oceanic plate beneath a continent and (ii) the Indian-Eurasian collision as an example for a modern continent-continent collision.

  Geodynamic models requiring vigorous convection of a low viscosity mantle have been successful in explaining high mantle temperatures in the backarc of the Cascadia subduction zone. Geochemical calculations indicate that high temperatures and water content can significantly reduce mantle viscosity. Long-period magnetotelluric data of the Cascadia subduction zone are used to determine the electrical resistivity structure and to constrain the backarc mantle rheology, specifically in terms of the amount of fluids. Non-uniqueness in the interpretation is reduced by using constraints from seismic tomography and geochemical calculations. At shallow mantle depths, water contents of 500-1000 ppm and melt fractions of 0.5-2.5% are required, which can reduce mantle viscosity and allow for vigorous mantle convection.

  In the India-Eurasia continental collision, models invoking flow in a weak lower crustal layer have been successful in explaining a number of geological and geophysical observations associated with the evolution of the Tibetan plateau. Geophysical observations indicate that the weak lower crustal layer may be the result of partial melting and/or aqueous fluids. Laboratory studies predict an order reduction in strength for melt fractions of 5-10%. By relating these laboratory studies to magnetotelluric observations it is possible to estimate the flow parameters associated with the channel flow model. Magnetotelluric studies require conductances of up to 20,000 S beneath the Tibetan Plateau, suggesting flow velocities of 0.02-4.5 cm/a. These flow velocities support the idea that channel flow could occur beneath parts of Tibet.

• Subjects / Keywords

  • Magnetotellurics
  • Continent-continent collision
  • Rheology
  • Canadian Cordillera
  • Channel flow
  • Subduction zone
  • Tibetan Plateau
  • Dehydration melting

• Graduation date

  Fall 2012

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3P94P

• 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.