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The Thermal Structure of Subduction Zones and Backarcs

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The Thermal Structure of Subduction Zones and Backarcs The Thermal Structure of Subduction Zones and Backarcs by Claire A. Currie Ph.D. Dissertation School of Earth and Ocean Sciences University of Victoria University of Victoria The Thermal Structure of Subduction Zones and Backarcs Claire A. Currie B.Sc., University of Western Ontario, 1999 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in the School of Earth and Ocean Sciences O Claire Currie, 2004 University of Victoria All rights reserved. This dissertation may not be reproduced in whole or in part, by photocopying or other means, without the permission of the author. Supervisor: Dr. Roy D. Hyndman ABSTRACT Temperature plays a dominant role in many subduction zone processes, including arc magma generation and the distribution of earthquakes. In this thesis, observational constraints on forearc and backarc thermal structure are integrated with numerical models to better understand the thermal consequences of subduction. Forearc thermal models, applied as an example to the Mexico subduction zone, indicate that the forearc is cool, as expected due to the cool subducting slab. The brittle part of this subduction fault extends to depths >30 km where, even though very weak, the fault may generate small but non- negligible frictional heating. The rupture extent of Mexican megathrust earthquakes is consistent with proposed seismogenic limits of 100 and 350•‹C. A remarkable feature of subduction zones is that, although the subducting plate cools the over-riding crust and produces low temperatures in the forearc, the arc and backarc regions are consistently extremely hot, as indicated by arc volcanism, surface heat flow, seismic velocities, T,, and other observations. At the Cascadia subduction zone, backarc temperatures are -1200•‹C at 50-60 krn depth with little variation for 500 km east to the craton. Thermal constraints from other subduction zone backarcs, including those that have had no recent extension, show that they are similarly hot for 100's-1000's of krn behind the arc. Local sources of heat (e.g., radiogenic heat production, frictional heating) appear to be small, and mantle flow is invoked to carry heat into the backarc. Thermal-mechanical numerical models for slab-driven comer flow give flow that is strongly focussed into the wedge comer below the arc but low mantle temperatures further into the backarc, inconsistent with observations. It is concluded that slab-driven flow is insufficient to satisfy the heat budget at a subduction zone. Geodetic and geological constraints indicate extremely low mantle viscosities in several backarcs (<1019 Pa s), suggesting that thermal buoyancy may be the primary driving force for flow. High temperatures and hydration of the mantle wedge by the subducting slab may reduce the viscosity, allowing vigourous thermal convection that rapidly carries heat upward from depth into the subduction zone. TABLE OF CONTENTS page.. Abstract ............................. .............................................................. 11 Table of contents ............................................................................... iv List of tables ........................................................................................ ix List of figures ....................................................................................... x ... Acknowledgements ..................................................................................................... xm CHAPTER 1 :Introduction 1.1 Motivation for study ............................................................................ 1 1.2 Thermal studies of subduction zones ........................................................ 2 1.3 Thesis objectives ............................................................................... 6 1.4 Outline of thesis ................................................................................7 CHAPTER 2: Thermal models of the Mexico subduction zone 2.1 Introduction ..................................................................................... 9 2.2 The megathrust seismogenic zone ......................................................... 10 2.2.1 Temperature limits on the seismogenic zone ................................. 10 2.2.2 Alternative downdip limit: Serpentinized forerarc mantle wedge .......... 11 2.3 Modelling the Mexico subduction zone thermal structure ............................. 12 2.3.1 Modelling approach .............................................................. 12 2.3.2 Oceanic geotherm ............................................................. 15 2.3.3 Oceanic plate geometry ........................................................... 17 2.3.4 Continental crust thickness ...................................................... 18 2.3.5 Convergence rate ................................................................. 19 2.3.6 Thermal parameters ............................................................. 20 2.3.7 Parameter sensitivity analysis ................................................... 21 2.3.8 Frictional heating .................................................................. 28 2.3.9 Hydrothermal circulation in the incoming oceanic crust .................... 33 2.4 Surface heat flow observations ............................................................. 33 2.5 Past megathrust earthquakes ................................................................ 35 2.5.1 Jalisco .............................................................................. 35 2.5.2 Michoacan .......................................................................... 37 2.5.3 Guerrero ............................................................................ 37 2.5.4 Oaxaca .............................................................................. 38 2.6 Mexico seismogenic zone .................................................................. 38 2.6.1 Comparison of rupture areas with the Moho intersection ................... 38 2.6.2 Comparison of rupture areas with the thermal models ...................... 39 2.7 Conclusions ................................................................................... 43 CHAPTER 3: Backarc Thermal Structure I .The Cascadia subduction zone 3.1 Introduction ................................................................................... 45 3.2 Constraints from arc volcanics .............................................................. 46 3.3 Observational techniques for constraining backarc temperatures ..................... 48 3.3.1 Indirect methods ....................................................................... 49 3.3.2 Direct methods ......................................................................................... 50 3.4 Cascadia subduction zone ........................................................................................ 51 3.5 General observations ................................................................................................ 53 3.6 Surface heat flow and heat generation observations ................................................ 54 3.6.1 Surface heat flow ...................................................................................... 54 3.6.2 Near-surface radiogenic heat production .................................................. 58 3.6.3 Calculation of geotherms .......................................................................... 61 3.6.4 Cascadia backarc geotherm ....................................................................... 67 3.7 Additional thermal constraints ................................................................................. 68 3.7.1 Upper mantle seismic velocities ............................................................... 68 3.7.2 Upper mantle xenoliths ............................................................................. 70 3.7.3 Thermal isostasy ...................................................................................... 70 3.7.4 Base of the lithosphere .............................................................................. 71 3.7.5 Summary of backarc thermal structure ..................................................... 71 3.8 Eastern limit of the Cascadia backarc ...................................................................... 73 3.9 Discussion ................................................................................................................ 77 CHAPTER 4: Backarc Thermal Structure I1 .A Global Survey 4.1 Introduction .............................................................................................................. 79 4.2 Non-extensional backarcs ........................................................................................ 81 4.2.1 MexicoICentral America ........................................................................... 81 4.2.2 South America ......................................................................................... 83 4.2.3 Alaska and the eastern Aleutians .............................................................. 90 4.2.4 Kamchatka ............................................................................................... 92 4.2.5 Sunda ......................................................................................................... 95 4.2.6 Summary ................................................................................................... 97 Southern Europe and Asia .....................................................................................
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