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ARENAL VOLCANO, COSTA RICA. OIL ON CANVAS BY LYNN JADAMEC Constructing the Costa Rica-Nicaragua subduction zone in 3D Could you briefly describe your research project and its main objectives? I am constructing regional, 3D, geodynamic, numerical models of the Central American plate boundary, with a special focus on the Costa RicaNicaragua region. Here, the Cocos plate subducts beneath the Caribbean plate and descends into the Earth’s mantle. Just to the south, the Nazca plate is also subducting, but observational indicate the subducting Cocos and Nazca plates are discontinuous. This gap in the subducting plates could serve as a conduit for the underlying mantle to flow through. The numerical models examine the sensitivity of mantle flow rates and plate-mantle coupling to the viscosity structure, melt and water content in the mantle wedge, and gaps in the subducted plate. This will allow us to test the recent hypothesis of rapid mantle flow in subduction zones. Why have you chosen to study the Costa Rica-Nicaragua subduction zone in particular? subducting plates changes along the length of the subduction zone. This inclination influences the position of volcanoes, the location of mountains, and the direction in which the mantle underneath the plate moves. Whereas two-dimensional models are limited to one slice of a 3D earth, 3D models allow for motion of material into and out of a cross section plane or map slice. Therefore, in a 3D model I can include a subducting plate of finite lateral extent, which allows the mantle to flow around the edges of the downgoing plate. Where this happens, the mantle flow rates are very fast. This flow pattern cannot occur in a 2D model which can only represent a tectonic plate of infinite length, and hence with no edges. Have you experienced any problems while constructing these models? What are the advantages of using 3D numerical models of subduction zones rather than traditional 2D? Observational and experimental constraints indicate it is becoming necessary to incorporate more geologic realism into numerical models of subduction. This includes modelling the plate boundaries in three-dimensions and allowing for large viscosity variations between the tectonic plates and the underlying mantle. However, smoothly representing this geometric complexity on a model grid and incorporating the large viscosity gradients into the viscous flow simulations pose computational challenges. This is an active area of research in both the geophysical and computational science communities. To tackle part of this problem, I wrote a C/C++ code during my PhD to map complex 3D subduction geometries smoothly onto a model grid. An additional problem that arises from using large 3D simulations is the massive output that then needs to be analysed. Three-dimensional immersive virtual reality is one approach I use to analyse and explore the complex and spatially variable model output. The margins of tectonic plates are inherently three-dimensional with variations in physical properties and geometry along the length of the boundary as well as perpendicular to the boundary. For example, the inclination of One of your aims is to gain insight into the underlying physics that governs mantle flow in subduction zones. Have you been successful and what is the importance of this understanding? The Costa Rica-Nicaragua subduction zone is a region that can uniquely address how the mantle flow rates in subduction zones depend on the geometry of the subducting plate and the variations in melt and water content within the mantle wedge. In addition, this subduction zone can be used to study the mechanisms for the locally independent motion between the plates and underlying mantle at subduction zones. The Costa Rica-Nicaragua region is particularly attractive from a numerical modeling standpoint because there has been enough geologic, geophysical, and geochemical data collected to constrain competing hypotheses tested by the numerical geodynamic models. DR MARGARETE JADAMEC Dr Margarete Jadamec is applying pioneering 3D modelling techniques to a Central American plate boundary, which, if successful, could transform the way we understand the movemen movement of the mantle in subduction zones Subduction zones are a type of convergent plate boundary, where one plate descends beneath the other into the underlying less viscous mantle. It has previously been assumed that as the downgoing plate descends into the mantle, the rate of motion of the plate after it has descended into the mantle is similar to the rate of motion of plate on the Earth’s surface. However, the subducting plate in fact pivots as it steepens into the underling mantle, causing its hinge to weaken and allowing its speed to vary. The 3D models from my PhD work show that by including a slab edge, the return flow around the edge localises rapid flow in the mantle. By using a non-Newtonian viscosity, which seismic and experimental data indicate is more appropriate for the upper mantle, the maximum velocities of the mantle can be on the order of 90 cm/yr, which is about 10 times the rate of the surface plates. I am now testing this hypothesis on the Costa Rica-Nicaragua subduction zone. What other key findings have you made during this research? Three-dimensional regional models of the Costa Rica-Nicaragua region may shed light on the role of mantle upwelling along the edges of subducting plates in contributing to adakitic volcanism. The links between adakitic geochemical signatures preserved in volcanoes near the edges of subduction zones and upwelling associated with toroidal flow at slab edges is not well understood. I have worked on this question recently in collaboration with geologist Dr Patricia Durance at Monash University in Australia, in studying the slab edge at the eastern New Hebrides trench and in supervising an undergraduate honours thesis on this topic. The preliminary work implies that upwelling associated with the Cocos-Nazca spreading centre as well as upwelling associated with toroidal flow around the slab gap between the Cocos and Nazca plates could be important for interpreting geochemical signatures in subduction zones. WWW.RESEARCHMEDIA.EU 61 DR MARGARETE JADAMEC Understanding subduction zones using 3D constructs High-resolution three-dimensional regional numerical models of the Costa Rica-Nicaragua Subduction Zone by the Department of Geosciences at Brown University, Rhode Island, are set to revolutionise how we understand the Earth’s interactions WHEN ONE TECTONIC plate clashes with another, the less dense tectonic plate rides over the denser one, driving it into the viscous 2,900 km layer of superheated rock that sits under the Earth’s crust, known as the Earth’s mantle. As well as creating new land, not to mention causing potentially devastating earthquakes, tsunamis and volcanoes, the subduction of the Earth’s lithosphere (the crust plus the uppermost layer of the mantle) impacts the underlying mantle as well, inducing complex flow patterns in the mantle as the lithosphere descends. Until the last decade, most models of subduction were either two-dimensional or were threedimensional but with a simplified geometry or viscosity structure. It was believed that the rate of the mantle in the subduction zone corresponded to the rate of travel of the surface plate, on the order of just 1-15 cm per year, and that this rate was relatively uniform in the mantle at subduction zones in both space and time. JOINING PIONEERS One of the pioneers of three-dimensional numerical modelling of subduction at the Department of Geosciences at Rhode Island’s Brown University, Dr Margarete Jadamec is conducting a research project that looks into the role of rheology and water in rapid mantle flow; taking Costa RicaNicaragua Subduction Zone as a case study. As her geodynamic modelling approach relies heavily on seismic data, she is collaborating with seismologist Dr Karen Fischer who has worked extensively in imaging the structure of the Costa Rica-Nicaragua subduction zone and in measuring seismic anisotropy. Jadamec is also working with two other leading academics in geological sciences, Dr Marc Parmentier and Dr Greg Hirth, who study the effect of variable water and melt in the mantle. The combined expertise could lead to real advances in our understanding of subduction zone processes. “The seismic and geochemical data suggest a localisation of water and melt beneath the Nicaragua section of the subduction zone. Both water and melt affect the viscosity of 62 INTERNATIONAL INNOVATION FIGURE 1. Tectonic setting of Central America with hypothesised mantle flow field (right). Slab data (White, yellow lines) from Gudmundsson and Sambridge, 1998; Syracuse and Abers, 2006. Spreading centres (red) from Muller, et al 2008. olivine, in that raising the water and melt content will decrease the viscosity, which in turn could affect the local flow dynamics,” Jadamec explains. “The flow dynamics produced by the 3D numerical models can then be tested against observations of seismic anisotropy from this region, thus allowing for competing hypotheses to be tested.” and therefore contain the observed geometric complexity and variable thermal and viscosity structure. With this approach, the numerical modelling results can be compared to geologic and geophysical observations from the region. WRITING CODE Computing power is crucial to ensure realism in 3D geological constructs. The geometric complexity and large viscosity variations require massive parallel computing and models are run on the NSF supported Extreme Science and Engineering Discovery Environment (XSEDE) supercomputers. FIGURE 2. M Jadamec examining a subductiontransform plate boundary in the KeckCAVES immersive visualisation facility. Models from Jadamec and Billen, 2010, 2012. Photo by Oliver Kreylos. DATA CONSTRUCTIONS The Costa Rica-Nicaragua subduction zone was chosen because of the extensive geologic, geophysical and geochemical data already available for numerical modelling, in particular that obtained during the National Science Foundation (NSF) MARGINS TUCAN experiment, yet no threedimensional numerical models incorporating these data have yet taken place: “Constraints from isotope geochemistry indicate rapid mantle flow in the mantle wedge beneath Costa Rica-Nicaragua, which goes against the predictions from most Newtonian models of subduction,” notes Jadamec. Her previous 3D regional models of the subduction zone between the Pacific and North American plates in Alaska, constructed using a nonNewtonian rheology, indicate that rapid flow may be a common phenomenon in the mantle close to subduction zones. These models were designed to be representative of a specific plate boundary In addition, the 3D immersive virtual reality facilities at the Center for Computation and Visualization at Brown University and at the KeckCAVES at the University of California at Davis help to visualise and explore gigabytes of data: “To construct the numerical representation of the Cocos-northern Nazca subduction system, I use a programme called SlabGenerator, which I wrote during my PhD,” explains Jadamec. “SlabGenerator generates the spatially variable 3D plate boundary interface, slab thermal field, and over-riding plate thermal field specific to the Cocos-northern Nazca plate boundary configuration. The 3D models are then visualised using the open source software 3DVisualizer and ShowEarthModel, written by Dr Oliver Kreylos of the KeckCAVES.” PARADIGM LOST Through using this technique Jadamec has overturned old assumptions on fixing the rate of the slab, the piece of the subducting plate that has entered the mantle, to the surface plate motion which places a limit on the rate of the flow of the mantle. She has revealed that as the subducting plate pivots as it steepens into the underlying mantle, it causes the mantle speed to vary and this rate depends strongly on the viscosity structure of the mantle and slab: “Recent rock experiments and geodynamic models indicate the deformation of olivine rich mantle is sensitive to the local strain rate, which can lead to a non-linear weakening of the resistance to the subducting slab”. upwelling associated with toroidal flow around the slab gap between the Cocos and Nazca plates could be important for interpreting geochemical signatures in subduction zones”. VIRTUAL REALITY FIGURE 3. Toroidal flow in mantle around slab edge in Alaska subduction zone. Models from Jadamec and Billen, 2010. By incorporating this strain-rate dependent viscosity into 3D geodynamic models of Alaska, she found that faster velocities are predicted for the subducted portion of the plate and the surrounding mantle, with values of 90 cm per-year in the mantle, which is much faster than previously expected. She is now testing this hypothesis at the Costa Rica-Nicaragua subduction zone. TRENCH PARALLEL FLOW This study will provide insight into an apparent trench parallel flow beneath Costa Rica and Nicaragua. This work suggests the mantle is decoupled from the surface plates in velocity as well as in direction, as Jadamec explains: “A realistic rheology for olivine, the dominant constituent of the upper mantle, can explain both observations of seismic anisotropy and the decoupling of mantle flow from surface motion in terms of magnitude and direction”. However, this theory has only recently been incorporated into 3D models of subduction. “Modelling a subduction zone with 3D variations of water and melt in the mantle wedge could provide significant insights into how these parameters contribute to rapid mantle flow and plate-mantle decoupling,” states Jadamec. VOLCANIC ACTIVITY Three-dimensional models of the Costa RicaNicaragua region may shed light on the role of mantle upwelling along the edges of subducting plates in contributing to adakitic volcanism. Currently, the links between adakitic geochemical signatures preserved in volcanoes near the edges of subduction zones and upwelling associated with toroidal flow at slab edges are not well understood, underlines Jadamec: “The preliminary work implies that upwelling associated with the Cocos-Nazca spreading centre as well as One spin-off to blazing a three-dimensional trail is the expansion from conventional twodimensional desktop software to 3D virtual reality in the learning experience of undergraduate and graduates at Brown University. Because students are now faced with manipulating massive geologic and geophysical datasets, as well as output from parallel numerical simulations, they are finding traditional methods incapable of adequately representing this threedimensionality, complex behaviour, and range in scales of data. So Jadamec has turned to 3D visualisation and 3D Virtual Reality to address the gap in the communication and conceptualisation of information in the earth sciences: “I am using 3D immersive visualisation to communicate the inherent three-dimensionality in plate tectonics and mantle convection. By showing students seismic data that images the whole earth structure in 3D, we can better demonstrate the relative dimensions of the lithosphere, mantle, and core, for example, and the variability within each of these components of the Earth,” she states. ART AND SCIENCE Far from being a purely science-based academic, Jadamec jumped at the chance to work with artist and Tides Foundation Lambent Fellow Lynn Jadamec, who passionately believes in engaging people living in earthquake prone regions with the science behind disaster through art. Together they defined the ‘Living with Plate Boundaries’ project, where scientist and artist travel to plate boundary zones: “Lynn paints the geomorphic expressions of the plate boundaries, the people, and the landscape, and I construct 3D models of the plate boundary and a description of the tectonic hazards,” elucidates Jadamec. Initial work on the San Andreas transform boundary and Costa Rica subduction zone were shown in an exhibit at the Monash Science Center in Australia. “This is a long-term project, initially funded by grants from the Tides Foundation and Sugarman Foundation awarded to Lynn,” explains Jadamec. After the work on Costa Rica is completed, we aim to take this project to other plate boundary types, such as to an active rifting region like Iceland or the East African Rift Valley. INTELLIGENCE ROLE OF RHEOLOGY AND WATER IN RAPID MANTLE FLOW: 3D NUMERICAL MODELS OF THE COSTA RICANICARAGUA SUBDUCTION ZONE OBJECTIVES • To investigate the controls of non-linear rheology, slab shape, and water and melt content in the mantle wedge on the velocity of the mantle in subduction zones. • To study this process, observationally-based 3D numerical models of the Cocos-northern Nazca slab gap are being constructed, with a special focus on the Costa Rica-Nicaragua portion of the subduction zone. KEY COLLABORATORS Dr Karen Fischer, Professor of Geological Sciences, Brown University • Dr Marc Parmentier, Professor of Geological Sciences, Brown University • Dr Greg Hirth, Professor of Geological Sciences, Brown University FUNDING National Science Foundation – contract no. 1049545 CONTACT Dr Margarete Jadamec Project leader Department of Geological Sciences Brown University 324 Brook Street Box 1846 Providence, RI 02912 USA T +1 773 332 0044 E [email protected] MARGARETE JADAMEC received her BS in Geology and Geophysics from the University of Connecticut in 1999, a MS in Geology from the University of Alaska in 2003, and a PhD in Geodynamics from the University of California, Davis in 2009. During her PhD she developed high-resolution plate boundary scale numerical models of the subductiontransform margin in southern Alaska. These models show mantle flow velocities may be up to 90 cm/yr in subduction zones with short slabs undergoing slab steeping, calling into question the paradigm of 2D corner flow models and 3D models that use a simple Newtonian rheology. FIGURE 4. Constraints on lithosphere thickness in geodynamic models from seafloor ages with imposed upper plate age (left) and from seismic velocities (centre and right). Data re-processed from Bird, 2003; Muller, et al. 2008; Lekic and Romanowicz 2011. WWW.RESEARCHMEDIA.EU 63