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4 ESC2010 6-10 September 2010, Montpellier, France - Keynotes
Geodynamics of the Alboran Sea
We discuss mantle structure and dynamics under the Gibraltar/Alboran Sea region. That region has been much stu- died, and a wealth of information is available about its crust. Nevertheless, there is no consensus yet on the type of model that best explains the geodynamic situation and evolution in the region. Figure 1 shows the two models that are most frequently invoked to describe the geodynamic situation of the region. These are, on one hand, the subduc- tion-rollback (SR) model, and on the other, continental delamination or convective removal (CDCR).
a) Oceanic model (retreating subduction)
b) Continental model (convective removal)
Figure 1 (redrawn from Calvert et al., 2000): The two types of geodynamic models that we test, a) the retreating subduction model of Loner- gan and White (1997) and Gutscher et al (2002), and b) the convective removal model of Platt and Vissers (1989).
We attempt to direct test the geodynamic models, using adapted seismological techniques. The new constraints are based on observations of P-wave dispersion that are indicative of anomalous (layered) structure along the propaga- tion path, and also seismic anisotropy, since it is capable of constraining the geometry of mantle flow, which is also rather indicative of the kind of geodynamic environment. We propose that distinguishing the two types of models requires resolving two questions, 1) what is the nature of
5 ESC2010 6-10 September 2010, Montpellier, France - Keynotes
that high-velocity anomaly, e.g., is it composed of oceanic or of continental lithospheric material?, and 2) what is the shape of the anomaly? It appears that up to recently, neither of these two questions has been well-resolved. We will thus propose techniques that are designed to directly address these questions.
The first technique that we use is based on observations of dispersed P-wavetrains that are expected for certain ray directions. Figure 2 shows waveforms from two teleseismic events observed at two stations, MTE in Portugal and CEU in Ceuta. The latter station has a fortunate location, in that it is positioned along the continuation of the high-velo- city anomaly in tomographic models (see for example Gutscher et al., 2002). That station is thus at the ideal location for looking for the described dispersion phenomena. On the other hand, the station in Portugal (MTE) serves as a reference station, so that we can assure that any observed dispersion is really due to the anomalous upper mantle under the Alboran Sea, and not due to effects at larger distance or due to the earthquake source.
Figure 2: Dispersion observations. Waveforms of P-waves arriving at two stations a) from the West (earthquake in Columbia, 15.11.2004, Magnitude 7.2, lat. 4.69 Thursday, 11 February 2010Thursday, 11 February 2010deg, long -77.5 deg, distance 72.8 deg) and b) from the West (earthquake in Crete, 17.3.2004, magnitude 6, lat. 34.59 deg, long. 32.32 deg, distance 23.45 deg). Each seismogram is shown filtered in a series of filter bands (from the top: < 0.5 Hz, 0.05 – 0.5, 0.5 – 1.5, 1.5 – 2.5, 2.5 – 3.5, and 3.5 – 4.5 Hz), and corresponding smoothed envelo- pes. Amplitudes are given by numbers, and ticks give time intervals of 2 second. The observed dispersion is illustrated by a dashed line (after Bokelmann and Maufroy, 2007).
Figure 2c shows P-waves arriving from the East, for an earthquake occurring in Crete. The seismograms were filtered in a series of frequency bands. Waveforms are complicated, and smoothed envelopes are useful for tracing the arrival time across the different frequencies. The envelopes show an interesting difference between the two stations. While all frequencies arrive at about the same time at the reference station MTE, there is a clear dispersion at station CEU. High frequencies arrive up to about one second after the low-frequency arrivals. This effect has been observed in numerical modelling, and also in observations from known subduction zones around the Pacific (Abers and Sarker, 1996; Abers 2005). The P-waves arriving from the West, in Figure 2a, do not show such a difference between the two stations. In fact, there is hardly any frequency dependence of arrival times at the two stations, somewhat similar to the arrival from the east at MTE. The anomalous behaviour that requires explanation is the Eastern arrival at CEU that is passing through the upper mantle under the Alboran Sea. We have studied a larger number of events, and we frequently find the dispersion behaviour for rays arriving at Ceuta from eastern direction, but never from Western direction, and neither do we find dispersion for the reference station in Portugal. This observation is consistent with a presence of a subducted slab under the Alboran Sea (to the east of Gibraltar).
7 ESC2010 6-10 September 2010, Montpellier, France - Keynotes
References
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