Three-Dimensional Thermal Structure of Subduction Zones

• Author / Creator

  Rosas Bonilla, Juan C

• The temperature distribution controls many processes occurring at subduction zones, such as slab dehydration and metamorphism, mantle wedge melting and the formation of arc volcanoes, and the rupture width of megathrust earthquakes. The thermal structure of the subducting oceanic plate (slab) is primarily controlled by conduction and advection of heat due to its downward motion. In the mantle wedge, heat is transported by the solid-state flow of the mantle driven by the coupling with the slab. The resulting flow pattern depends on the geometry of the subduction zone and in the rheology employed for the mantle. A common approach for the study of the thermal structure of subduction zones consists in creating two-dimensional (2D) models in which a specific geometry and velocity is assigned to the slab and upper plate. The mantle wedge flows dynamically in a 2D corner flow driven by viscous coupling with the slab. Whereas this approach has been very successful and applied to a wide range of real subduction zones, the last decade has seen a surge of three-dimensional (3D) subduction models. 3D effects are important in subduction zones in which the along-strike geometry exhibits significant changes (i.e changes in the dip, trench curvature, or proximity to slab edges). As the driving force for mantle wedge flow is the pressure gradient induced by the viscous drag of the slab, the presence of along-strike changes in the structure of a subduction zone adds a trench-parallel component that can induce along-strike mantle wedge flow. This 3D flow geometry results in a thermal structure that differs from that obtained by traditional 2D models. In this thesis, I present 3D thermal models of several sections of the subduction zone along the Middle America Trench (MAT). The models employ a kinematic slab and a dynamic mantle wedge that flows in response to the drag imposed by the downgoing plate. Along-strike changes in the age of the plate are incorporated through a 2D oceanic boundary condition. As the thermal state of the slab is also affected by hydrothermal circulation in the uppermost oceanic crust, the effects of along-strike changes in the efficiency of hydrothermal circulation are studied through 2D and 3D models. For the mantle wedge flow, model results show that the mantle flows along-strike from steep-dip regions to shallow-dip regions in the presence of changes in the angle of subduction. The magnitude and geometry of the flow depends on the rheology employed, with non-Newtonian, dislocation creep viscosities giving faster flow velocities and higher temperatures for the wedge with respect to an isoviscous rheology. For the MAT, the thermal change in the wedge due to along-strike flow is larger than 70◦C, which may be important for magma-generation depending on the amount of water the mantle holds. The direction and orientation of the flow can also be compared to seismic anisotropy studies. Theoretical studies show that the flow direction and the a-axis of olivine minerals are aligned. Along the MAT, several studies have shown an anisotropy pattern in which the a-axis have a trench-parallel component. In central Mexico, this orientation has been proposed to be due to a slab tear that cuts the Cocos plate into two sections, which allows asthenospheric mantle to flow into the mantle wedge and disrupt the corner flow. For this thesis, instead of incorporating this tear, a continuous slab geometry is employed in the models of Mexico. The mantle wedge flow obtained form these models is shown to be mostly aligned with the anisotropy pattern, which suggests the Cocos plate is probably not fragmented in central Mexico. This result is also in agreement with geochemical studies of the volcanic arc of Mexico, in which no particular signal that would suggest a broken slab is found. For the case of Central America, seismic anisotropy observations also reveal an along-strike flow component. Modeling results for this area shows that the along-strike flow caused by slab dip variations has a southeastward direction, which is opposite to the northwestward direction inferred by arc geochemical studies. This suggests the along-strike flow in this region is induced by a different mechanism, most likely a combination of slab rollback and proximity to a slab edge near Panama.

• Subjects / Keywords

  • Thermal structure
  • Mantle flow
  • Subduction zones

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3XW4842S

• 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.