Plate re-organization in the western Mediterranean: lithospheric causes and topographic consequences (TopoMed)
Final Report Abstract
Seismological deployment in the Alboran Sea and the Gulf of Cadiz provided for the first time seismological data recorded in the offshore areas, indicating that seismicity is controlled by the thermal structure to the west and east of the Gibraltar arc. Higher heat flow to the east restricted seismicity to shallow depth of 2 to 15 km, in turn decreasing the potential size of future earthquakes. Deformation in the Gulf of Cadiz occurred in old Jurassic lithosphere, providing major seismicity within the mantle. Thus, mantle heat flow controls seismicity pattern. In the Alboran Sea micro-earthquakes occurs along a NNE-SSW trending band of seismicity, rather than defining a well-defined plate boundary fault. The 20 to 40 km wide band of seismicity mimics the Trans-Alboran shear zone, stretching from the Morrocan city of Al-Hoceima to Adra on the Spanish coast and the Betic shear zone. Earthquakes often correlate well with submarine faults imaged in high-resolution bathymetry and seismic reflection data. Interestingly, a number of faults known to be historically active, did not show any seismic activity during the monitoring, suggesting that faults are either temporary inactive, creeping or being locked. High heat flow and observed P-wave seismic velocities and Vp/Vs ratios of 1.7 to 1.8 indicate rather normal mantle, suggesting that thermal structure controls rather low mantle velocities in the central part of the Alboran Sea. Earthquakes in the Gulf of Cadiz occurred preferentially in a band roughly paralleling the Algarve continental margin. Seismicity and stress pattern are similar to features observed along the passive margin of the eastern US and northern Brazil, suggesting a common forcing. Events along the US east coast are interpreted to be related to the re-activation of rifting related structures. We therefore suggest that seismicity along the Algarve margin is cause by re-activation of pre-existing margin parallel faults rather than being a developing plate boundary fault. Data of both deployments have been used by project partners, providing tomographic images of the Western Mediterranean at a much high resolution, suggesting (i) that the Trans-Alboran shear zone might be controlled by structures associated with Miocene arc volcanism, and (ii) that a seismic anomaly of fast velocities below the West Alboran Sea is caused by a subducting slab. The idea of a slab is supported by mantle flow pattern derived from seismic anisotropy. Most of the proposed goals were reached during the joint efforts. Thus, (i) the Trans-Alboran Shear Zone seems to be governed by terrain boundaries, (ii) mantle structure suggests that mantle velocity is controlled by thermal effect rather than serpentinization (with a high Vp/Vs ration of >2), (iii) a subducting slab was deduced from tomography and mantle flow pattern. Interestingly, deformation expressed by seismicity could be linked to a number of faults imaged seismic and bathymetric data. However, some large-scale tectonic structures like to Alboran Ridge perhaps reflect past deformation and faults know to be active were seismically quiet.
Publications
- Report on the cruises RV POSEIDON POS389 & POS393 & RV Maria S. Merian MSM15/5 - TOPO-MED: Topographic, structural and seismotectonic consequences of plate re-organization in the Gulf of Cadiz and Alboran Sea, IFM-GEOMAR Report, 45, 52 pp, Kiel, 2011
Grevemeyer, I. (ed.)
- Seismicity and active tectonics in the Alboran Sea, Western Mediterranean: constraints from an offshore-onshore seismological network and swath bathymetry data, J. Geophys. Res., Solid Earth, Volume 120, Issue 12, December 2015, Pages 8348-8365
Grevemeyer, I., E. Gràcia, A. Villaseñor, W. Leuchters, A.B. Watts
(See online at https://doi.org/10.1002/2015JB012073) - Mantle earthquakes beneath the South Iberia continental margin and Gulf of Cadiz – constraints from an onshore-offshore seismological network- Earth Planet. Journal of Geodynamics, Volume 99, September 2016, Pages 39-50
Grevemeyer, I., L. Matias, and S. Silva
(See online at https://doi.org/10.1016/j.jog.2016.06.001)
