Present-day stress field and tectonic inversion in the Pannonian basin

Abstract

This paper presents a latest compilation of data on the present-day stress pattern in the Pannonian basin, and its tectonic environment, the Alpine–Dinaric orogens. Extensional formation of the basin system commenced in the early Miocene, whereas its structural reactivation, in the form of gradual basin inversion, has been taking place since Pliocene to recent times. Reconstructed compression and associated horizontal contraction are mainly governed by the convergence between Adria and its buffer, the Alpine belt of orogens. The resulting contemporaneous stress field exhibits important lateral variation resulting in a complex pattern of ongoing tectonic activity. In the Friuli zone of the Southern Alps, where thrust faulting prevails, compression is orthogonal to the strike of the mountain belt. More to the southeast, intense contraction is combined with active strike–slip faulting constituting the dextral Dinaric transpressional corridor. Stresses are transferred far from Adria into the Pannonian basin, and the dominant style of deformation gradually changes from pure contraction through transpression to strike–slip faulting. The importance of late-stage inversion in the Pannonian basin is interpreted in a more general context of structural reactivation of back-arc basins where the sources of compression driving basin inversion are also identified and discussed. The state of recent stress and deformation in the Pannonian basin, particularly in its western and southern part, is governed by the complex interaction of plate boundary and intra-plate forces. The counterclockwise rotation and north-northeast-directed indentation of the Adriatic microplate appears to be of key importance as the dominant source of compression (“Adria-push”). Intra-plate stress sources, such as buoyancy forces associated with an elevated topography, and crustal as well as lithospheric inhomogeneities can also play essential, yet rather local role.

Introduction

The World Stress Map Project (WSMP) has been running for two decades, providing excellent service for both the academic and industrial community. As present-day stress field governs ongoing deformation in the lithosphere, the reconstruction of the stress pattern helps the understanding and quantification of active tectonic processes. First, the concept of regionally uniform stress orientations and magnitudes was established, and the first-order stress provinces were defined on a global (c.f. Zoback, 1992) as well as European (c.f. Müller et al., 1992) scale. Parallel with the significant increase in the number of stress indicators, the spatial resolution of the European stress map has improved considerably. It allowed the recognition of short-scale lateral changes of stress regimes and the vertical variation of stress directions (e.g. Rebaï et al., 1992, Müller et al., 1997, Jarosiński, 1998, Mariucci et al., 2002, Sperner et al., 2003, Kastrup et al., 2004, Jarosiński, 2005). To explain the profound changes in the recent stress field, a link had to be found between forces acting at plate boundaries or arising in intra-plate setting, and the resulting deformation pattern. Efforts resulted in a number of modelling studies (e.g. Gölke and Coblentz, 1996, Meijer and Wortel, 1997, Muñoz Martin et al., 1998, Ragg et al., 1999, Bada et al., 2001, Heidbach and Drewes, 2003, Heidbach, 2005, Jarosiński et al., 2006) establishing a reliable neotectonic framework for interpreting the available stress data.

The WSMP data base (see latest data release in Reinecker et al., 2005) has clearly shown that Europe represents a region of complicated contemporaneous stress pattern (Fig. 1). Particular complexities exist in the broad bend of Europe–Africa collision. The Mediterranean system of subduction and collision zones, and related back-arc basins within the convergence zone between the African (Nubian) and Eurasian plates represent a good example of short-scale stress perturbations. The changes of stress directions occur over a distance that is comparable to or even smaller than the thickness of the lithosphere. In the central Mediterranean, where the Adriatic microplate is indenting the Alpine–Dinaric orogen, active compression or, where convergence is oblique, transpression takes place at the edges of Adria. On the other hand, behind subduction zones lithospheric extension occurs in the Tyrrhenian, Aegean and Alboran back-arc domains (Fig. 1).

Due to the availability of good quality stress data, the Pannonian basin system is an area suitable to study the short-scale variation of the modern stress field, which is manifested in the changes of both the orientation of the stress axis and the tectonic regimes. Unlike other Mediterranean back-arc basins, the Intra-Carpathian area is characterised mainly by strike–slip and thrust faulting stress regimes (Fig. 1). The Pannonian basin has reached an advanced stage of evolution, and its structural inversion has been taking place for the last few million years. A major change in the paleostress fields from extension, governing Miocene basin formation, to compression, controlling Pliocene to Quaternary neotectonic deformation, was recognised (c.f. Horváth and Cloetingh, 1996, Fodor et al., 1999, Bada et al., 2001, Bada et al., in press, Fodor et al., 2005, Horváth et al., 2006). The concept of ongoing basin inversion was confirmed already by the first regional compilation of stress indicators for the area of the Pannonian basin and surrounding orogens (Gerner et al., 1999).

The last few years has witnessed a substantial increase in the number of stress data for the Pannonian region. A high-sensitivity seismic monitoring network, installed in the central part of the basin system in 1995 (Tóth et al., 2002a, Tóth et al., 2005a), provides about a dozen of new focal mechanisms yearly. The network has been operating for more than a decade that permits the reconstruction of stress regimes at an unprecedented spatial resolution allowing the definition of different stress provinces. In addition, data exchange between academia and industry led to new borehole breakout analyses providing further insights in the inhomogeneities of horizontal stress directions. These data represent powerful means to constrain inversion dynamics of sedimentary basins, and to better understand active tectonic processes in the Pannonian region.

Having a stress database with ca. 450 data entries and a novel model of basin inversion developed for the Pannonian basin system (Bada et al., in press, Cloetingh et al., 2006, Horváth et al., 2006), the purpose of this contribution is twofold. Firstly, the latest edition of the Pannonian stress database is presented showing important lateral variation of both the maximum horizontal stress directions and the reconstructed stress regimes. Secondly, the present-day stress field is discussed in the light of the neotectonics of the region, whereas basin inversion is analysed in the context of stress propagation in the upper, elasto-brittle part of the Pannonian lithosphere. The focus of the study is on the Pannonian basin and its vicinity in the west and south, which are the Eastern and Southern Alps and the Dinarides, respectively (see insert box in Fig. 1). This region includes tectonic units with several common or strongly interrelated elements in their Miocene to present-day structural evolution. Thus, it seems useful first to overview the neotectonic habitat of the study area. This is followed by the presentation of the stress data. The paper concludes with a discussion on the principal features of basin inversion and active tectonics in the Pannonian basin, and the origin of the reconstructed present-day stress field.