Download Effect of wedge geometry and structural heterogeneity on

XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
S9_003
Effect of wedge geometry and structural heterogeneity on
subduction erosion processes: insights from 2D sandbox
analogue experiments
Albert, F.1, Kukowski, N.1
(1) Department 3 Geodynamik, GeoForschungsZentrum Potsdam, Telegrafenberg, 14473
Potsdam, Germany
[email protected]
At convergent margins, which are lacking an accretionary wedge, material transfer is
thought to be controlled by subduction erosion. This process works by removing material
at the front of the margin (frontal erosion) or along of the underside of the continental
plate (basal erosion). Several margins such as Costa Rica, Mexico, Tonga, Guatemala,
Peru, Chile and Japan (among others) are subject to this tectonic erosion [1]. The process
of subduction erosion has been discussed in relatively few studies, and most of these
studies addressed the concepts of the subduction channel [2; 3], trench retreat [4],
subsidence of the middle slope [5], and potential bathymetric expressions of subduction
erosion.
In order to get insights in the processes related to subduction erosion and in particular
about the roles of material heterogeneities in the overriding plate, surface slope, as well
as the position and size of bodies of different strength in the forearc, 2D sand box
analogue experiments have been performed. The apparatus used for this study consists of
a 100-cm-long and 10-cm-wide glass box containing a sand structure representing the
crustal wedge (scaling factor of about 105). This wedge is located above a high basal
friction interface, which is represented by a conveyor belt with a surface consisting of
sand paper that is moved in order to simulate the subducting plate. Three basic
configurations were tested: A) Sand lenses surrounded by thin and weak glass beads
layers located inside a wedge of pure sand representing overpressured shear zones or a
fractured zone. Variable lengths, heights, positions and number of lenses were
considered. B) A single lens embedded in a wedge of pure sand, composed of either pure
glass beads (representing a weakened zone in a forearc), of mortar, or of a flour-sand
mixture (representing a stronger material, i.e. a mafic intrusive body). The lens was
variable in length, height and position. C) Pure sand wedge shaped bodies with different
slopes and heights.
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XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
The results of our experiments show that the tip of the wedge retreats faster if the wedge
includes heterogeneities. In the case where this heterogeneity was a flour-sand mixture,
the rate of the retreat is even higher compared to the other cases. This behavior was also
observed in cases in which the initial surface slope was lower.
After an initial deformation phase, a quasi steady state phase was characterized by a slope
angle of α= 35-40˚. However, in the group of experiments with a single big lens-shaped
heterogeneity composed of glass beads, the slope of the wedge started to decrease after
1.5 meters of convergence (about 150 km scaled to nature).
The structural style of the wedge seems to be strongly influenced by the presence,
position and composition of the heterogeneities. When the latter was composed of glass
beads or of sand and was directly placed on the conveyor belt, a reverse fault developed
in the rear part of the wedge and basal accretion occurred mainly between the sandpaper
and the heterogeneity. The mechanically weak material accumulates most of the
deformation and transmits the strain towards the rear part. In cases with an initially low
surface slope, reverse faulting was favored in the lower slope. A lower initial slope angle
implies less material and therefore a smaller normal force permitting an easier
accumulation of accreted material which translates into a faster tip retreat. On the other
hand, when the heterogeneity was composed of a mixture of flour and sand, deformation
was concentrated close to the toe of the wedge during initial stages of convergence and
was evenly distributed over the entire wedge afterwards. The stronger material initially
acted as a backstop concentrating the strain in the front. Subsequently, only uplift was
observed.
The first results of our study suggest that at convergent margins undergoing subduction
erosion, the steepness of the continental slope as well as the presence and position of a
weaker or stronger region within the continental wedge strongly influence deformation
and velocity of the tip retreat. The position of a weak zone determines the relative
location of both, faulting and basal accretion. A shallowly inclined continental slope
favors the development of compressional uplift in the lower slope. The presence of a
strong body within the forearc wedge would accelerate the tip retreat.
Our experiments are suitable to provide valuable information about the influence
of different forearc features, likely inherited from the long-term (~107 yr) geological
history of a margin (e.g. the Central Andes), on the process of subduction erosion. Such
features are namely, the presence of intrusives, which usually contain strong mafic roots,
or alternatively weaker zones like overpressured layers, or hydrothermally altered
material. Further work focusing on simulating forearc heterogeneities induced by local
geological conditions at specific convergent margins will be performed to validate our
modeling approach.
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XII Congreso Geológico Chileno
Santiago, 22-26 Noviembre, 2009
Referencias
[1] Clift, P., Vannucchi, P. (2004) Control on tectonic accretion versus erosion in
subduction zones: implication for the origin and recycling of the continental crust. Review
of Geophysics, vol. 42, RG2001, doi:10.1029/2003RG000127.
[2] Cloos, M., Shreve, R. (1988) Subduction channel model of prism accretion, mélange
formation, sediment subduction, and subduction erosion at convergent plate margins: 2.
Implications and discussion: Pure and Applied Geophysics, vol. 128, 501-545
[3] Lohrman, J., Kukowski, N., Oncken, O., Sick, Ch., Sobiesiak, M., Rietbrock, A.
(2006) Subduction Channel Evolution in Brittle Fore-Arc Wedges — a Combined Study
with Scaled Sandbox Experiments, Seismological and Reflection Seismic Data and
Geological Field Evidence. M. Strecker & P. Wigger (eds.), The Andes - active
subduction orogeny. Frontiers in Earth Sciences, 1, Springer Verlag, Heidelberg, 237262.
[4] von Huene, R., Culotta, R.C. (1989) Tectonic erosion at the front of the Japan Trench
convergent margin, in Cadet, J.P., and Uyeda, S., eds., Subduction zones: The Kaiko
Project: Tectonophysics, vol. 160, 75-90
[5] von Huene, R., Lallemand, S. (1990) Tectonic erosion along the Japan and Peru
convergent margins. Bulletin of the Geological Society of America vol. 102, 704–720.
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