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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Subduction zone Water and Metamorphism: a Modelling and Imaging Study

Alternative title: Vann og Metamorfose i Subduksjonsoner: Modellering og Seismisk Avbildning

Awarded: NOK 7.2 mill.

Subduction zones are regions of the Earth where two tectonic plates collide and one of the two plates plunges (subducts) beneath the other one. These regions represent the main entry point of water into the Earth's interior. The water is carried down by the subducting plate, and is progressively released as the plate plunges into the mantle and is subjected to increasing pressure and temperature conditions, causing the rocks to transform through a process called metamorphism. This release of water is associated with formidable geological phenomena such as earthquakes and volcanoes. Accurately mapping water transport and distribution in subduction zones is thus essential for improving our understanding of these phenomena and assessing the associated risks. Though we have learned a great deal about the role of water in subduction zones over the past decades, much is still unknown about precisely where dehydration reactions occur and how they relate to subduction processes that cause earthquakes and volcanic activity. Of particular interest is intraslab seismicity - earthquakes that occur within the subducted slab and pose significant risks to human life and infrastructure. Project SWaMMIS proposes to establish a fully integrated link between high-resolution seismic images and numerical models of subduction zones to better understand the role of water and metamorphism in the generation of intraslab earthquakes. This research endeavour will aid in understanding the triggering mechanisms of damaging subduction earthquakes. SWaMMIS brings an exceptional depth of expertise in seismology, geodynamics and petrology to the problem. We employ an innovative multidisciplinary approach that combines state-of-the-art seismic imaging technology, novel modelling tools, and important observational constraints from high-pressure Norwegian rocks. A global catalogue of high-resolution seismic images of subduction zones has been built, with special focus on the Hellenic subduction zone as a reference system. A reanalysis of seismic data confirms that continental subduction is occurring in northern Greece and finds that some intraslab earthquakes occur in this part of the system. We also found new evidence for mantle-wedge seismicity that could be due to the interaction between slab-derived fluids and rocks in the cold portion of the mantle wedge beneath the Peloponnesus. In parallel, our efforts have been focusing on the development of seismic, geodynamic and petrological modelling of this and other subduction zones worldwide. A novel thermo-mechanical modelling strategy allows us to identify the paths taken by fluids in subduction systems, and petrological studies are giving us new insight into how minerals from high-pressure rocks breakdown to form sulfides. We can now put these pieces together and, with constraints from high-pressure rocks from Holsnøy, will be able to estimate how much rock is typically processed by earthquakes in subduction zones.

Achieved: -Catalogue of high-resolution seismic images of subduction zones, with focus on western Greece -Most comprehensive seismic velocity model and earthquake catalogue for western Greece -Found the existence of unusual mantle-wedge earthquakes -New tools to compute synthetic seismic waves through complex subduction zone models -New thermo-petrological modelling framework that simulates two-phase flow processes in subduction zones -Models found two main fluid paths in subduction zones, one towards the arc and one towards the trench -Synthesis of seismic and thermo-petrological models: new theory explains how earthquakes in the slab and mantle wedge are generated by fluids that travel updip in the slab and sometimes escape through localized vents at the interface Expected: -Six more publications -New seismic station deployment: Bergen Arcs Seismic Experiment (BASE) -Coordination of an ITN proposal: Chemical and Rheological Intraslab Processes (CRISP)

Subducting plates transport water into the Earth's interior. The release of this water through metamorphism is associated with formidable geological phenomena such as earthquakes and volcanoes. Accurately mapping water transport and distribution in subduc tion zones is thus essential for improving our understanding of these phenomena and assessing the associated risks. Though the role of water in subduction zones has become clearer over the past decades, much is still unknown about precisely where dehydr ation reactions occur and how they relate to subduction processes that cause earthquakes and volcanic activity. Of particular interest is intraslab seismicity - earthquakes that occur within the subducted slab and pose significant risks to human life and infrastructure. Are these earthquakes related mainly to the embrittlement of dehydrating rocks, or to fluids that circulate through (and therefore weaken) discrete fractures that dissect otherwise dry rocks? We propose to establish a groundbreaking multi -disciplinary programme in subduction zone studies at the Universities of Bergen and Oslo, in collaboration with an extended network of key international partners. The central goal of this programme is to establish a fully integrated link between high-res olution seismic images and thermal-petrological models of subduction zones to elucidate the role of water in the generation of intraslab earthquakes. This link will rely on constraints from surface geological observations of high-pressure and ultrahigh-pr essure rocks from western Norway. The unique integrated framework will combine novel imaging and modelling approaches exploiting the high-quality data sets and high-performance computing facilities that have only become available in recent years. We expec t this activity to yield exceptional new insight into the role of water in determining the seismic structure, dynamic behaviour and causes of intraslab seismicity in active subduction zones worldwide.

Publications from Cristin

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FRINATEK-Fri prosj.st. mat.,naturv.,tek