The Ionian zone is a classic thin-skinned linear fold and thrust belt forming a part of the external Hellenides, in westernmost Greece. The region has been a focus of intensive paleomagnetic investigation since the early 1980s, and it is now generally believed to have undergone a multiphase clockwise vertical-axis rotation of 40°–60° since the Miocene, although the timings are disputed, and spatial variations within this trend have been largely ignored thus far. We present data from thirty new paleomagnetic sites and a reappraisal of previous results from the Ionian zone in an attempt to construct a unified model for the tectonic evolution of the Ionian zone. We find that the clockwise rotations may be due, at least partially, to rotation during thrust sheet emplacement, with evidence of a forelandward decrease in rotation. However, superimposed on this pattern of thin-skinned rotations we observe post-Pliocene rotations that affect multiple thrust sheets in a consistent manner. These are interpreted to result from regional tectonics associated with, for example, the Kefallonia fault zone at the western termination of the Hellenic arc and from deformation in the transition zone between Anatolian westward extrusion and southern Aegean extension. Overall, the result is a pattern of thin-skinned, westward-decreasing clockwise rotations distorted by superimposed thick-skinned rotations resulting from the complex interplay of plate motions in the eastern Mediterranean.

In the Aegean area, distinct fault patterns with their associated stress regimes are evidenced along a curved convergent plate boundary. In this article, we analyze and review late collisional and extensional structures in five structural regions along the External Hellenides orogenic belt in order to define, primarily, the evolution of onshore basins and, secondarily, the evolution of off-shore basins and the role of the inherited structures within the present geotectonic framework. We also evaluate how these structures act on seismicity as well as present-day motion and magmatism in the Aegean area. Northwestern Greece, which corresponds to our Region I, represents an area of active continental collision in which a previously overthickened crust collapsed mainly parallel to the structural grain of the orogen. At present, the most active structures in this region are the northwest-trending thrusts and the northeast-trending normal faults. Strong coupling and the transmission of horizontal forces from the collision front appear to explain the deformation within this region. Central Greece (Region II) displays a mixed type of contractional-extensional deformation. Mesozoic inherited transverse structures are reactivated as WNW-trending faults and appear to accommodate most of the active north-south-trending present-day extension. Deformation in this area appears to be controlled both by rollback of the subducting slab and by the lateral extrusion of the Anatolia plate. The areas that spread along the more curved parts of the Hellenic arc (Regions III and V) well emphasize the control exerted by pre-existing northeast-trending structures as well as their long-lived activity during the evolution of the arc. Present deformation within these regions possibly reflects the oblique convergence process, which is occurring in both areas in different degrees. Finally, analysis of deformation patterns in the central part of the Hellenic arc and the Aegean Sea (Region IV) suggests that almost all pre-existing structures have remained active until the present and accommodate extensional deformation and rapid motion through a nonorthogonal fault system that crosscuts a thin and thermally weakened continental crust. Deformation north of our Region IV is accommodated primarily by the North Aegean trough. Basin-bounding faults within the North Aegean trough are dominantly characterized by apparently normal components as well as strike-slip components. Plate boundary conditions, as well as pre-existing structures with a favorable orientation relative to the current crustal motion, control and variably affect the present-day deformation along the Hellenic arc and trench system.

Narrow, discontinuous bands of high-grade subophiolitic metamorphic rocks, comprising predominantly amphibolite facies metabasites with rare metasediments, are observed at the contact between the complexes and subjacent mélanges of the Mesohellenic Ophiolite exposed in northwestern Greece. Both conventional and pseudosection thermobarometry have been used to yield estimated peak pressure-temperature (P-T) conditions of these tectonic sheets. Toward the leading edge of the ophiolite, subophiolitic rocks of the Vourinos Complex record peak metamorphic temperatures of 770 ± 100 °C. Pressures of 4 ± 1 kbar beneath the Vourinos are estimated on the basis of hornblende composition and are similar to the expected pressures from ophiolitic overburden. Beneath the exposed Dramala Complex, at the trailing edge of the ophiolitic body southwest of the Vourinos, estimated temperatures reached 800 ± 40 °C and 12.00 ± 1.27 kbar at the top of an apparent inverted metamorphic gradient imposed by discrete phases of accretion. High pressure assemblages beneath ophiolitic bodies imply exhumation relative to the overlying ophiolite. Estimated homologous temperatures in the upper plate are similar to those inferred for channeled exhumation during continental collision. Mineral assemblages lower in the Dramala sole indicate reduced temperatures and peak pressures. Similar pressures obtained within lower temperature sole rocks beneath Vourinos and Pindos suggest that a shallowly dipping thrust may have been responsible for obduction. Peak temperatures and pressures are in agreement with those estimated for secondary thrust propagation beneath a proto-arc after subduction in an intra-oceanic setting.

Abstract The well-known Cenozoic Aegean extensional regime, initiated at c. 25 Ma, thinned the crust so that most of it now lies submerged. North of the western continuation of the North Anatolian Fault the Aegean extensional regime is present in central and southern Bulgaria, northern Greece, Former Yugoslavian Republic (FYR) of Macedonia and eastern Albania. Here the system is exposed on land and offers an opportunity to reconstruct the extensional evolution of the system. The southern Balkan peninsula forms the northern part of the Aegean extensional system; deformation is not as great as in the Aegean, but reconstruction of this part of the extensional regime will provide important constraints on its dynamics. Following a period of arc-normal extension associated with Late Eocene-Late Oligocene magmatism, major lithospheric extension appears have begun between 26 and 21 Ma in northern Greece, involving east-northeast-west-southwest extension east of Mount Olympos, on the Island of Thasos and near Kavala. This period of extension may have been accompanied by a short period of coeval compression north of the arc during Early Miocene time or perhaps a little earlier in the Thrace Basin of northwestern Turkey. Northeast-southwest directed Middle-Late Miocene extension appears to have developed obliquely to the older magmatic arc and migrated northward into southwestern Bulgaria in the Sandanski Graben (and perhaps also into the Mesta and Padesh Grabens) by 16.3–13.6 Ma, and in the Blagoevgrad and Djerman Grabens by c. 9 Ma. Extension in south-western Bulgaria was reorganized by c. 5 Ma and in northern Greece extension on the Strymon Valley detachment fault ended by c. 3.5 Ma, but extension continued on new fault systems. From limited structural and stratigraphic data, it is speculated here that related extension may have also occurred during this time in FYR Macedonia and eastern Albania. This northeast-southwest extension is interpreted to be related to trench roll-back along the northern part of the subduction boundary in the western Hellenides. North-south extension along east-west striking faults in central Bulgaria began only after extension was well underway in northern Greece and the Sandanski Graben of south-western Bulgaria. Within the Sofia Graben, the Sub-Balkan grabens, and grabens to their east, north-south extension began at c. 9 Ma, and may have begun about the same time in the Plovdiv, Zagore and Tundja Grabens of the northern Thracian Basin: north-south extension has continued to the present in these grabens. The cause of the north-south extension is unclear and may be related to trench roll-back along the central part of the subduction zone in the Hellenides, or more local causes of clockwise and counterclockwise rotation of the western Hellenides and western Turkey, respectively. By Late Pliocene time a major erosion surface, the sub-Quaternary surface, was developed over a large area of central Bulgaria creating a major unconformity that marks the beginning of Quaternary deposition in the basinal areas. Many large and small graben-bounding faults in west-central Bulgaria displace this erosion surface and demonstrate the widespread extent of Quaternary north-south extension. North-south extension extended westward, with probably decreasing magnitude, across the older northwest trending graben of southwest Bulgaria (the Simitli and Djerman Grabens) and into eastern FYR Macedonia. During latest Pliocene(?) and Quaternary time, northern Greece developed a complex pattern of northeast-southwest extension associated with northeast to east-west striking right-lateral faults forming transfer faults between more local extensional areas. This system of faults overprinted the older northwest trending extensional faults, such as the Strymon Detachment, and may be related to the propagation of the right-lateral North Anatolian Fault into the north Aegean Sea and formation of parallel faults to its north. These two different tectonic regimes extend into FYR Macedonia, where a third regime of east-west extension in western FYR Macedonia and eastern Albania is present, and where extension may represent the continuation of the east-west extensional regime initiated in Middle-Late Miocene time. Active deformation determined from seismicity and Global Positioning System studies suggest northern Greece, and perhaps southwest Bulgaria and FYR Macedonia, is dominated by north-south extension. This pattern of deformation must have developed as recently as perhaps Late Quaternary time. Except for mountains near the Adriatic Sea all of the mountainous topography in the southern Balkan region may be the result of Miocene-Recent extension.