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

Maurizio Battaglia, Mark H. Murray and Roland Bürgmann

Introduction

In this study we use surface velocities recorded by GPS measurements and block modeling to investigate the active deformation of the Adriatic region, a component of the zone of distributed deformation between the African and Eurasian plates. The region includes the relatively stable Adriatic area (Po Valley, Adriatic Sea and Apulia), surrounded on the eastern, northern and western margins by a mountain belt which includes the Albanides, the Dinarides, the Alps and the Apennines. The southern margin of the Adriatic region, representing the boundary with the African plate, is still undefined. This study was prompted by a need to resolve the uncertainty surrounding the tectonic representation of this area, alternatively viewed as a promontory of North Africa or as a microplate within the Africa-Eurasia plate boundary.

The absence or low level of seismic activity in the Adriatic Sea indicates that its behavior is that of a relatively rigid plate within a deforming region. The bulk motion of the plate can be described as rigid rotation about an Euler pole in North Italy. Fault plane solutions suggest that the motions at the boundaries of the Adriatic area may not reflect directly Africa-Eurasia convergence. Some authors question the existence of an independent Adriatic plate, suggesting that a counterclockwise rotation of the Adriatic block, driven by impingement of the Africa plate against the Calabrian Arc, may explain the major tectonic events in the region, such as the opening of the Tyrrhenian basin, the evolution of the Appennic-Maghrebian chain, the extension in the northern Ionian basin, and the shortening process along the Alps-Dinarides-Ellenides. On order to test the competing tectonic models proposed, we plan to develop a block model of regional deformation. This approach incorporates the secular velocities from GPS, fault geometry estimates and elastic-strain accumulation, making possible to determine how different tectonic hypothesis are compatible with geodetic data.

Figure 31.1: GPS horizontal velocities and their 95% confidence ellipses in a Eurasia-fixed reference frame for the period 1999-2002. Black arrows: velocities from this study. Gray arrows: publicly available velocities (McCluski et al., 2000). Dark gray arrows: velocities by E. Serpelloni (INGV, personal communication)
\begin{figure*}\begin{center}
\epsfig{file=battaglia03_1_1.eps, width=16cm}\end{center}\end{figure*}

Deformation Velocities

We employ publicly available GPS observations made at 30 stations of the European Reference Permanent Network (EUREF) and the Italian Space Agency (ASI) continuous GPS networks to estimate deformation in the Adriatic region (Figure 31.1). We analyze the data using the GAMIT/GLOBK software in a three step approach described by (McCluski et al, 2000). To improve the realization of a stable reference frame for the velocity solution, additional sites from the International GPS Service (IGS) and EUREF networks are included through the publicly available global regional loosely constrained solutions performed by the Scripps Orbit and Permanent Array Center (SOPAC). Given the small velocities (from 2 to 5 mm/yr) recorded along the Italian peninsula, the choice of the appropriate definition of a stable Eurasian frame of reference may be critical.

The velocities shown in Figure 1 are referenced to the stable Eurasian frame, based on 15 stable sites in Europe and Asia (McCluski et al., 2000). All together, our solution includes data spanning 4 years from 138 stations, including 45 in the Mediterranean area. We incorporate 50 additional sites from publicly available solutions (McCluski et al., 2000) to resolve the deformation in the Eastern Mediterranean and Caucasus. To better assess the real uncertainties of the GPS solutions, we scale the covariances of the daily and monthly average to be consistent with the internal residual scatter of their combinations (i.e., chi-square statistics are approximately 1). These scalings do not compensate for systematic reference frame biases, possible non-Gaussian errors or possible correlations between solutions. Monthly combinations of daily solutions, which tests suggest have white-noise characteristics, provide sufficient observations to obtain robust chi-square statistics on the residuals about the linear trends to properly weight the velocities. We compute velocity solutions using the monthly combinations, and scale the formal errors by the square root of chi-square of the solution. We allow a random walk of 1 mm/yr to take into account possible monument instability.

The active deformation in the Adriatic region (Figure 31.1) is highly variable with velocities decreasing from south ($\sim $5 mm/yr) to north ($\sim $2 mm/yr). Preliminary motion estimates (1999-2002) for stations located on the northern edge of the African plate (RABT, LAMP, NOTO, MATR, HELW and MEST) show a north-westward motion (N 23$\pm$2 W) at 6$\pm$1 mm/yr. Sites in Corsica (AJAC) and North-western Italy (ELBA, GENO, TORI, NOVA, UNPG) show no significant deformation, while the stations on the Italian peninsula close to the Adriatic sea (VOLT, VENE, CAME, TREM, ANGE, MATE, SPEC) are characterized by a north-eastward motion (N 29$\pm$3 E) at 5$\pm$1 mm/yr. Stations located on the eastern edge of the Adriatic Sea (GSR1, DUBR, SRJV) move in the same direction (N 23$\pm$7 E) at a somewhat slower rate (3$\pm$1 mm/yr). The northward displacement (N 3$\pm$4 E at 3$\pm$1 mm/yr) of sites in the southern Italian peninsula (TGRC, VLUC) may reflect a transition between the African plate and the Adriatic region.

Acknowledgements

This work has been partially funded by the Istituto Nazionale di Oceanografia e Geofisica Sperimentale (OGS), Udine, Italy.

References

McCluski et al., Global Positioning Constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus, J. Geophys. Res.,105, 5695, 2000.

Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
Questions or comments? Send e-mail: www@seismo.berkeley.edu
© 2004, The Regents of the University of California