The Adriatic Region: An Independent Microplate within the Africa-Eurasia Collision Zone

Maurizio Battaglia, Mark H. Murray, Enrico Serpelloni (INGV) and Roland Bürgmann

Introduction

The tectonics of the Mediterranean is shaped by deformation related to the collision between the Nubia (Africa), Eurasia, and Anatolia plates. In this study, we use block modeling of surface velocities recorded by GPS measurements (Battaglia et al., 2004) to investigate the present-day deformation of the Adriatic (Figure 23.1).

Figure 23.1: Location of the segments (solid lines) and blocks used to model the Adriatic region. [N Ad] North Adria, [S Ad] South Adria. [G] Gargano-Dubrovnik fault zone; [K] Kefallinia fault zone; [A] Apulia escarpment. GPS velocities and their 95% confidence ellipses. The grey dots indicate the location of the shallow seismicity from 1975 to 2000 (M $>$ 3.5).
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The tectonics of the Adriatic is not well constrained and remains controversial. Given the lack of significant seismic activity along the southern margin of the Adriatic Sea, the boundary with the Nubia plate, if it exists, is not well defined. Geomagnetic data averaged over several Myr and Sn shear wave propagation observations suggest that Nubia extends as a promontory into the Adriatic region (Mele, 2001), whereas historic geodetic and seismic evidence suggest that the Adriatic is an independent microplate (Adria) within the Nubia-Eurasia plate boundary zone (Nocquet and Calais, 2003). Oldow et al. (2002) propose that Adria is divided by the Gargano-Dubrovnik fault into two blocks. Northwestern Adria has little or no motion relative to Europe and is part of the Alpine collage of southern Europe. Southeastern Adria is moving together with Nubia and is continuous from Sicily to Apulia. Other studies suggest that the Adriatic is an area of distributed deformation (Nocquet et al, 2001).

To test different tectonic models for the Adriatic region, we develop a block model of regional deformation (Figure 23.1). This approach incorporates secular velocity and fault geometry estimates, as well as elastic strain accumulation. With this model we can assess whether different hypotheses are compatible with geodetic data, estimates of fault slip rates and locking depths, areas of rigid block rotation, and regions of anomalous strain accumulation (Meade et al., 2002; Battaglia et al., 2004).

GPS Measurements of Deformation

We use publicly available observations made at 30 continuous GPS stations of the European Reference Permanent Network (EUREF) and Italian Space Agency networks to estimate deformation in the Adriatic region (Figure 23.1). To improve the realization of a stable reference frame for the velocity solution , additional sites from the International GPS Service and EUREF networks are included as loosely constrained solutions provided by the Scripps Orbit and Permanenent Array Center. Our solution includes data spanning 4 years from 102 stations, including 50 in the Mediterranean area. We incorporate velocities from 38 episodic GPS (EGPS) sites from McClusky et al. (2000) and 10 EGSP sites from Serpelloni et al. (2001) to better constrain defor mation in the Eastern Mediterranean (Aegean and Anatolian plates) and southern Adriatic regions.

Block Model

Our block model of the Adriatic includes the interaction between the Eurasia, Nubia, Adria, Anatolia, Aegea, and Arabia plates. The plate boundaries are based on the description of the tectonic settings of the Mediterranean after van Dijk and Scheepers (1995), and seismicity distribution in the in the Mediterranean basin (International Seismological Center, 2001).

Plate boundary strain is determined from single continuous faults along the Calabrian coast, the Apennines, the Alps, the Dinarides, and the Hellenic Arc (Figure 23.1). This approach provides a first-order kinematic description in areas with more broadly distributed deformation, where the station distribution is insufficient for detailed study.

We evaluate several possible representations of Adria and the Adria-Nubia margin: (1) the Adriatic is a region of continuous deformation within the Eurasian plate; (2) Northwestern Adria is part of the Alpine collage of southern Europe with the southern boundary with Nubia being the Gargano-Dubrovnik fault]; (3) Nubia extends as a promontory into the Adriatic region; (4) Adria is divided from Nubia by the Gargano-Dubrovnik fault; (5) Adria is divided from Nubia and Aegea by the Apulia Escarpment and the Kefallinia fault; (6) Adria consists of two blocks separated by the Gargano-Dubrovnik fault in the middle and divided from Nubia and Aegea by the Apulia Escarpment and the Kefallinia fault.

Figure 23.2: Observed and modeled GPS velocities for the single block (AP) and the two blocks (GDAP) model of Adria.
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Results

The geodetic data and the models presented here (Figure 23.2) indicate that the Adriatic block is neither part of the Eurasia nor the Nubia plate. Geodetic data show that the Nubia plate is moving NW with respect to Eurasia with a velocity of 6 mm/yr, while the Adriatic microplate moves NE at a rate of 4-5 mm/yr (McClusky et al., 2000). Our results show that independent microplate models of Adria offer a better fit to GPS velocities than models considering Adria as continuous with the Nubia or Eurasia plate. Geodetic data alone cannot discriminate between a single block (AP) or a two block (GDAP) description of Adria (Figure 23.2), but the GDAP model predicts boundary slip rates that are in better agreement with observations from previous studies. Modeling results suggest that a possible location of the southern Adriatic/Nubia boundary could be the Apulia Escarpment lineament.

Acknowledgements

We acknowledge the GEODAF data archive of the Italian Space Agency (ASI) for providing GPS data. B. Meade (MIT) kindly allowed the use of his block model code. This work was supported by the Berkeley Seismological Laboratory and the Istituto Nazionale di Oceanografia e Geofisica Sperimentale (CRS-INOGS).

References

Battaglia M., Murray M.H., Serpelloni E., and R. Bürgmann. The Adriatic region: an independent microplate within the Africa-Eurasia collision zone. Geophys.Res. Lett., 31(9), L09605, 10.1029/2004GL019723, 2004.

McClusky, S., et al., Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus, J. Geophys. Res., 105, 5695-5719, 2000.

Meade, B.J. et al., Estimates of seismic potential in the Marmara Sea region from block models of secular deformation constrained by Global Positioning System measurements, Bull. Seism. Soc. Am., 92, 208-215, 2002.

Mele, G., The Adriatic lithosphere is a promontory of the Africa Plate; evidence of a continuous mantle lid in the Ionian Sea from efficient Sn propagation, Geophys. Res. Lett., 28, 431-434, 2001.

Nocquet, J.M., and E. Calais, Crustal velocity field of Western Europe from permanent GPS array solutions, 1996-2001, Geophys. J. Int., 154, 72-88, 2003.

Nocquet, J.M., E. Calais, Z. Altamini, P. Sillard, and C. Boucher, Intraplate deformation in Western Europe deduced from an analysis of the International Terrestrial Reference Frame 1997 (ITRF97) velocity field, J. Geophys. Res., 106, 11,239-11,257, 2001.

Oldow, J.S. et al., Active fragmentation of Adria, the north Africa promontory, central Mediterranean orogen, Geology, 30; 779-782, 2002.

Serpelloni, E., et al., Combination of permanent and non-permanent GPS networks for the evaluation of the strain-rate field in the central Mediterranean area. Boll. Geof. Teor. Appl., 43, 195-219, 2002.

van Dijk, J.P., and P.J.J. Scheepers, Neotectonic rotations in the Calabrian Arc; implications for a Pliocene-Recent geodynamic scenario for the Central Mediterranean, Earth-Science Rev., 39, 207-246, 1995.

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