The Louisville Ridge-Tonga Trench collision: Implications for subduction zone dynamics

Lead Research Organisation: Durham University
Department Name: Earth Sciences

Abstract

The plate tectonics paradigm provides the fundamental model for the destruction of oceanic lithosphere at subduction zones. But the dynamics of subduction zones are also responsible for the construction of arc lithosphere whose features include some of the largest and most active volcanoes on Earth and the majority of large earthquakes. The arcuate system of island arcs and deep sea trenches that comprise the SW Pacific is amongst the most structurally complex, geologically active section of the global subduction system and has the highest concentration of volcanic, seismic and associated tsunami hazard on Earth. Understanding the dynamics of this system is complicated by the diversity in the age, morphology and tectonic setting of the material that is entering the subduction zone, and yet it is the influence of this material which is a major factor in determining the architecture and composition of the entire trench, island arc, and back-arc system. Between ~5S-35S in the SW Pacific, the Tonga-Kermadec Trench subduction system has a deep, linear topographic depression at which Cretaceous Pacific oceanic crust is subducting beneath the Indo-Australian plate. However, at ~25S the Tonga Trench intersects with the Louisville Ridge, a linear chain of seamounts that runs obliquely to and is being subducted at the fastest rate of plate convergence on Earth (~80 mm/yr). Subduction of this ridge locally deforms the trench, and the point of collision is progressively moving north-to-south at ~118 mm/yr due to the oblique subduction geometry. The Tonga system can thus be divided into three parts. To the south of 27S, normal oceanic lithosphere of the Pacific plate is being subducted. Between 26S-25S, the thickened crust and seamount chain of the Louisville Ridge is entering the subduction zone with seamounts either being subducted intact or decapitated and subsumed into the overriding plate. North of 24S, the Louisville Ridge has been subducted and its long-term effect on the overriding plate is preserved in the wake of the point of collision. Two disparate phenomena appear spatially related to this intersection. Firstly, the current site of ridge subduction is characterised by a pronounced shallowing of the trench and extensive deformation and uplift of the arc; and secondly, a quiescent gap in the shallow seismicity is observed at the point of collision. The unusual tectonic setting of the intersection of the Louisville Ridge and the Tonga Trench therefore makes it a unique location in which to study the relationships between subduction input, subduction zone behaviour, and system architecture and dynamics. This study will provide unique models of crustal structure throughout the collision zone and obtain the necessary direct observations to parameterise and constrain numerical modelling of the thermo-mechanically coupled visco-plastic-elastic response of the lithosphere and the distribution of deformation within the subducting and overriding plates. The observations and measurements on which this study is based will be made during an expedition to the collision zone by a research ship. State-of-the-art equipment will be used to determine the structure of the Earth's crust and uppermost mantle to depths of ~40km sub-seabed using sound waves, recording these signals with instruments deployed onto the seabed in water depths of up to 6000m and towed behind the ship itself. Our target is a 500x300km region of the Tonga island arc-trench system that extends from the outer rise where flexural bending stresses are deforming the subducting plate, across the trench at the point of ridge collision and into the island arc. The resulting images of the crust, uppermost mantle and seabed will allow us to determine how the crust was constructed, modified and deformed, and how the plate boundary system is evolving over time in response to the subduction of significant plate topography.

Publications

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Funnell M (2013) Structure and deformation of the Tonga-Kermadec subduction system in the Louisville Ridge pre-collision zone in Postgraduate Research in Marine and Earth Sciences

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Funnell M (2014) Crustal Structure and Flexural Characteristics of the Louisville Ridge and Tonga-Kermadec Subduction System in British Geophysical Association Postgraduate Research in Progress Meeting

 
Description Images of crustal structure of the overriding Australasian and downgoing Pacific plates have been determined using both the acquired seismic refraction and reflection data. The crustal structure of the chain of seamounts (Louisville Chain) - both surface and sub-crustal - has resolved the question of what happens to seamounts when they are subducted.
Exploitation Route The project's results have led to a new understanding of dynamics at plate boundaries and the development and evolution, eventually, of new continental lithosphere. The stress regime related to the subduction of large-scale plate topography is also revealed and its consequences for earthquake initiation.
Sectors Education,Environment,Other

 
Description Publications in journals and presentations at conferences BBC News bulletins Outreach activities in local schools Academic presentations Academic publications Science outreach and awareness
First Year Of Impact 2011
Sector Education,Environment,Other
 
Description Imaging magma chambers and plutons: geophysical constraints and thermal modelling 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Imaging magma chambers and plutons: geophysical constraints and thermal modelling

Stimulated questions from scientific audience
Year(s) Of Engagement Activity 2012
 
Description Missing Earthquakes in the South West Pacific - The Louisville Gap 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Engagement of school children with Earth Sciences results and current hot topics in the Earth Sciences

Enthusiasm of school children to understand and study the Earth.
Year(s) Of Engagement Activity 2015
 
Description Monowai - Rise and fall of underwater volcano revealed 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Monowai - Rise and fall of underwater volcano revealed

Stimulated questions from schools audience
Year(s) Of Engagement Activity 2012
 
Description Monowai - Rise and fall of underwater volcano revealed 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Monowai - Rise and fall of underwater volcano revealed

Stimulated questions from scientific audience
Year(s) Of Engagement Activity 2012
 
Description Monowai - Rise and fall of underwater volcano revealed 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Monowai - Rise and fall of underwater volcano revealed

Resulted in interest expressed by school children and teachers
Year(s) Of Engagement Activity 2012
 
Description OBS'ing at Sea 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Blog on operations and activities of the Ocean-Bottom Instrumentation Facility and the instrument activities undertaken as part of grant service deliveries.
Year(s) Of Engagement Activity 2015,2016,2017,2018
URL http://obsatsea.wordpress.com/
 
Description Seamount subduction in the South Pacific 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Seamount subduction in the South Pacific

Stimulated questions from scientific audience
Year(s) Of Engagement Activity 2012
 
Description The Earth beneath our oceans - how do we listen to see what's there? 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Outreach talk as part of the Northern Ireland Science Week.
Year(s) Of Engagement Activity 2018
 
Description Understanding the Earth's large-scale processes - Imaging the deepest depths of the oceans and beyond 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Understanding the Earth's large-scale processes -

Imaging the deepest depths of the oceans and beyond

Stimulated questions from public audience
Year(s) Of Engagement Activity 2012