Impact of bending-related faulting on the seismic properties of the incoming oceanic lithosphere offshore of Nicaragua.

Ivandic, Monika (2008) Impact of bending-related faulting on the seismic properties of the incoming oceanic lithosphere offshore of Nicaragua. Open Access (PhD/ Doctoral thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 109 pp.

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Abstract

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.

Document Type: Thesis (PhD/ Doctoral thesis)
Thesis Advisor: Grevemeyer, Ingo and Rabbel, Wolfgang
Keywords: Subduktio zones,seismics,bending-related faulting,fluids
Research affiliation: OceanRep > SFB 574
Kiel University
OceanRep > SFB 574 > A5
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Refereed: No
Projects: SFB574
Expeditions/Models/Experiments:
Date Deposited: 16 Jan 2009 12:04
Last Modified: 23 Sep 2019 22:12
URI: http://oceanrep.geomar.de/id/eprint/3321

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