PT Unknown
AU Ivandic, M
TI Impact of bending related faultingon the seismic properties of the incoming oceanic lithosphereoffshore of Nicaragua
PY 2008
PU Christian-Albrechts-Universität zu Kiel
WP https://macau.uni-kiel.de/receive/diss_mods_00002897
LA en
DE Subduktio zones; seismics; bending-related faulting; fluids
AB The subduction of H2O is inherently related to the hydrogeology of the oceanic
lithosphere entering the trench. Water transported within the subducting oceanic
plate affects a number of processes, such as intraslab earthquakes and arc magmatism.
Bending related faulting in the subducting lithospheres may intensify hydrothermal
flow through aged crust and provide pathways for seawater to reach lower crustal and
upper mantle depths. A number of seismic wide-angle reflection and refraction experiments
were conducted offshore of Nicaragua to investigate the impact of bending
related normal faulting on the seismic properties of the oceanic lithosphere prior to subduction.
Based on the reflectivity pattern of multi-channel seismic reflection (MCS)
data collected offshore of Nicaragua it has been suggested that bending-related faulting
facilitates hydration and serpentinization of the incoming oceanic plate. First seismic
wide-angle and refraction data were collected along the profile p50 which extends from
the region well seaward of the outer rise, not yet affected by subduction, into the trench
northwest of the Nicoya Peninsula, where multibeam bathymetric data show prominent
normal faults on the seaward trench slope. A tomographic joint inversion of the
seismic refraction and wide-angle reflection data yields a decrease in P-wave velocities
in the crust and uppermost mantle as the plate approaches the trench. Seaward of the
outer rise velocities are typical for »24 Myr old oceanic lithosphere. In the near-trench
region, however, crustal velocities are reduced by 0.2-0.5 km/s compared to normal
mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6-7.8 km/s
and hence 5-7% lower than the typical velocity of mantle peridotite. These systematic
changes in P-wave velocity indicate an evolutionary process in the subducting slab
consistent with percolation of seawater through the faulted and fractured lithosphere
and serpentinization of mantle peridotites. Two other profiles, located northwest of the
profile p50, are parallel to the trench axis. This geometry was chosen to reveal if serpentinization
is a common process in the subducting Cocos plate offshore of Nicaragua
and not just a local feature. Tomographic inversion of both data sets indeed has shown
that seismic velocities are profoundly reduced along the entire lenght of the profiles,
both in the crust and uppermost mantle. Upper crustal velocities are not higher than
3.8-4.0 km/s, and the upper mantle is characterized by strong alteration that has
i
ii
caused a reduction of the seismic velocities by 8-10%. The anomalous behaviour is
more profound here than in the profile p50, but this observation is most likely due to
the geometry of the lines, i.e. fault density is higher along a line parallel to the trench
than along one which lies perpendicular. The modeling of the impact of water-filled
microcracks on the elastic properties of rocks shows that they could significantly influence
the seismic behavior of fractured media. The modeling of the S-wave velocities of
the profile p50 has revealed that velocities in the crust are more profoundly reduced
than in the P-wave structure. This is an indication that fracture porosity is extensively
developed and, thus, has a significant impact on the seismic properties of the crust.
One can assume a similar scenario for the upper mantle within a few kilometers just
below the Moho. As both processes, hydration and fracturing, are related to each
other, it is difficult to separate their effects on seismic properties. Thus, an estimate of
12-17% serpentinization in the uppermost 3-4 km of the mantle is just an upper limit
of hydration, i.e. if the reduced velocities we observe in our velocity models were due
solely to hydration.
PI Kiel
ER