Title: Bending-related faulting and mantle serpentinization at the Nicaraguan subduction zone
Abstract: At a convergent margin large amounts of structurally bound water are carried into
the Earth’s interior and - as the subducting plate descends and the temperature
rises - are driven off to some extent into the mantle wedge, where they are thought
to trigger intermediate-depth earthquakes in the Wadati-Benioff zone and melting
under volcanic arcs. However, a largely uncertain fraction outlasts sub-arc fluid
release and hence enters the deeper mantle, which leads to a connection between
the oceans and the Earth’s deep water cycle. Thus, a detailed knowledge of the water
budget of a subduction zone is not only important to understand arc volcanism, but
as well to comprehend the chemical development of the Earth’s mantle. For this
purpose, profound information about the amount of water that is subducted along
with the oceanic plate is indispensable.
The present thesis uses geophysical methods to determine the degree of hydration
of the Cocos Plate offshore Nicaragua, which is subducted beneath the Caribbean
Plate.
In general it was assumed that structured water is transported into the slab
in sediments and the upper crust only, though in recent years growing evidence
suggested that lower crust and upper mantle might contain capacious amounts of
fluids as well, since the bending of the incoming oceanic plate leads to a reactivation
or creation of normal faults (bend-faults), which are visible in batrymetric data
and have been inferred to cut deep enough into the plate to provide a pathway
for seawater to penetrate into the lithosphere, changing ”dry” peridotites to ”wet”
serpentinites, which contain up to 13% of water. Such a mechanism could transport
much more fluids into the earth’s interior than any other considered possibility.
However, the cutting depth of these bend-faults and hence the depth that seawater
could penetrate into the mantle was not well-defined, for one reason since focal depth
of earthquakes associated with the bend-faults were poorly known. Yet previous
studies assumed cutting depths such that serpentinization is firstly restricted by its
thermal limit of 600± C.
This study uses openly accessible, global broadband data of earthquakes offshore
Central America as well as an unique dataset from a local long-period earthquake
monitoring network offshore Nicaragua, to determine typical focal depths off earthquakes
at the trench-outer rise and further relates these focal depths to the cutting
depths of bend-faults. In addition, a full 3d-tomographic inversion that consistently
integrates seismic airgun blasts and local as well as regional seismicity, could show
reduced seismic mantle velocities at the outer rise and nearby the deep sea trench
with an evolutionary trend towards it. Best explained is this by a fractured and
ii
partly serpentinized lithosphere. The use of regional sources (i.e. earthquakes in
distances of ¸200 km from the seismic network) in the tomographic inversion process
made it possible, for the first time, to reflect the entire brittle lithosphere. In
a second approach, relative arrival times of large earthquakes that occurred during
the deployment of the seismic network were investigated. Again, it could be shown
that seismic mantle velocities decrease in accordance with the onset of bend-faults
in the bathymetry.
But not only seismic velocities decrease nearby the trench, the average moment
magnitude of outer rise earthquakes does as well, though the number of events
increases significantly. We explain this a weakened lithosphere and hence a reduced
yield strain, which again suggests an occurrence of serpentinite.
However, tomographic images suggest that the area of reduced seismic velocities
and in turn possible serpentinization does not reach the cutting depth of bend-faults
nor the depth of the 600± C isotherm. Focal mechanisms of several earthquakes were
determined via moment tensor inversion and forward modelling respectively and it
could be shown that where seismic velocities are reduced only tensional ruptures
occur, which allow for water infiltration, meanwhile the area beneath is dominated by
compressional rupture behaviour, which presents a barrier for seawater. This result
does not only confirm and enlarge flexure models of subducting plates [Chapple
and Forsyth, 1979; Christensen and Ruff, 1988], but also establishes a coherent
connection between stress distribution in the incoming plate and penetration depth
of seawater and is the first study in this vein.
Publication Year: 2008
Publication Date: 2008-05-19
Language: en
Type: dissertation
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Cited By Count: 1
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