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Subduction zones and their associated slabs are the main drivers of plate tectonics and mantle flow, but how these zones initiate remains enigmatic. In the Scotia Sea region, subduction started in the Late Cretaceous/Early Cenozoic in a pristine ocean basin setting devoid of other subduction/collision zones. How this subduction zone initiated remains intensely debated, as exemplified by the variability of published plate tectonic reconstructions. Despite such variability, several works argue for a subduction initiation mechanism, in which a South America-Antarctica relative plate motion change, in combination with a particular plate boundary geometry in the western Weddell Sea, caused convergence across a transform plate boundary segment that subsequently evolved into a subduction zone. Here we discuss this kinematic model of subduction initiation, and, following geometric and kinematic arguments, highlight several unsolved issues that call for alternative explanations. Furthermore, we present new tectonic reconstructions of the Scotia region involving a simpler middle-Late Cretaceous plate boundary configuration, which avoid the geometric and kinematic problems of earlier reconstructions and that call for a new mechanism of subduction initiation. We refer to this mechanism as Subduction Invasion Polarity Switch (SIPS), which involves a long-lived and wide subduction zone (South American-Antarctic subduction zone) with lower mantle slab penetration, which imposes major horizontal trench-normal compressive deviatoric stresses on the overriding plate. This plate consists of a narrow continental lithospheric (land) bridge at the trench (Cretaceous-Early Cenozoic Antarctica-South America land bridge) with oceanic lithosphere behind it (Weddell Sea-Atlantic Ocean). The stresses cause shortening and thrusting at the continent-ocean boundary in the backarc region of the overriding plate, forcing oceanic lithosphere under continental lithosphere, starting the subduction initiation process, and eventually leading to a new, self-sustaining, subduction zone (Scotia subduction zone) with an opposite polarity compared to the long-lived subduction zone. The model thus involves invasion of a new subduction zone into a pristine ocean basin (Atlantic Ocean), with the primary driver being a long-lived subduction zone in another ocean basin. To test the physical viability of the SIPS model, we have conducted numerical geodynamic simulations of buoyancy-driven subduction. Numerical results demonstrate that the SIPS model is viable, with compressive stresses in the overriding plate resulting from strong trenchward basal drag induced by subduction-driven whole-mantle poloidal return flow and compression at the subduction zone plate boundary. Subduction initiation starts in the overriding plate after ∼100 Myr of long-lived subduction, eventually resulting in the formation of a new, opposite-dipping, subduction zone. This new subduction zone develops at the continent-ocean boundary for models without and with a pre-imposed weak zone. We further propose that the SIPS model might explain subduction initiation elsewhere, including the New Caledonia subduction zone in the Southwest Pacific, the Lesser Antilles-Puerto Rico subduction zone in the Caribbean region, and the subduction zones that consumed the Rocas Verdes and Arperos backarc basins in South America and Central America, respectively. We further postulate that active backarc shortening in the Japan Sea, with eastward under-thrusting of Japan Sea oceanic lithosphere below the Japan arc, represents an early stage of SIPS.

The geotectonic setting and emplacement style of ophiolites are very significant for an understanding of the geological evolution of any region. In this context, it is essential to know the true tectonic relationship between ophiolites and adjacent units as accurately as possible. This study focuses on the Balkan sector, where Mesozoic convergence/collision processes left behind several NNW–SSE stretching ophiolite belts. The easternmost belt, known as the East Vardar Zone (EVZ), differs from other Balkan ophiolites (i.e. West Vardar) in having pronounced supra-subduction affinity as well as in lacking large peridotitic masses, metamorphic sole assemblages, and clear evidence of a westward obduction. This study was aimed at unraveling the true structural position of the EVZ ophiolites and the surrounding units. We applied 2D modeling of gravity and magnetic data and pseudo 3D modeling of the two largest exposures of the East Vardar Zone (EVZ) ophiolites. The methodology included: qualitative (remote sensing and field observation data as well as mathematical transformations of geophysical data), and quantitative analysis (2D modeling of both, aeromagnetic and gravity data along six cross-sections and the creation of pseudo 3D models). The obtained results invariably suggested that the EVZ ophiolites are dipping to the east beneath the Serbo-Macedonian Massif (SMM). The buried ophiolite bodies display steep contacts in shallow levels, but the contacts become much less steep with depth. The models are considered robust for the first 2–3 km, however, there are indications that the EVZ ophiolites exist at much greater depths, as well. Our geophysical–geological model implies that the mode of emplacement of the EVZ ophiolites was related to accretion/underthrusting and it was strikingly different from the emplacement of the other Balkan ophiolites that were uniformly obducted toward the west. This scenario is supported by the independent evidence provided by mantle xenolith studies and studies of lamproitic lavas, which indicated that at least parts of the mantle underneath the Tethyan Mesozoic suture are compositionally more similar to oceanic supra-subduction mantle.

The purpose of this paper is the development of a new Bouguer anomaly map for the broader Adriatic Sea area to be used as geo-scientific tool which will answer questions mainly regarding the deep and intermediate depth structure of the Adriatic and its surrounding areas. As a first step, a consistent, high-resolution and accuracy free-air gravity anomaly database is created using all the available terrestrial and satellite altimetry data sources. The least-squares collocation (LSC) method is employed to validate the different gravity data sets through an appropriate covariance analysis and following the remove–compute–restore procedure (RCR) for the detailed study of the gravity spectrum. The finally generated gravity database has a resolution of a 0.0333° (∼3.2 km) in both latitude and longitude, while its external and internal accuracy is estimated to about ±5 mGal and ±0.5 mGal, respectively. Based on the derived free-air gravity database a new Bouguer gravity anomaly database is determined at the same resolution for the wider region of the Adriatic Sea. Comparison with existing results confirms the reliability of the new Bouguer map for the study area, mainly due to the combination of both land and sea data. The derived Bouguer anomaly map can support different kind of geophysical investigations of both local and regional structure of the study region. As an example three density models are constructed along selected cross sections extracted from the Bouguer anomaly database, in order to obtain structural information for the crustal–upper mantle structure of the region and draw relevant conclusions. The obtained results show that the Bouguer map can provide important information for the lower-crustal/upper-mantle depth range but also helps to constrain the 2D geometry of the shallow geological units, in very good agreement with the observed surface geology and the current understanding of the geotectonic setting of the area.