Download Constructing the Costa Rica-Nicaragua

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.
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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
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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.
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