Abstract The European Alps, the prototype collisional orogen and playground of geologists from all over the world, have been studied by generations of Earth scientists. The density of data is probably matched by no other mountain chain. Still, the Alpine chain is far from being over-studied, since many fundamental questions have not yet found a satisfactory and generally accepted answer, e.g. the formation of the Western Alpine arc. In recent years however, tectonic research on the Alpine mountain chains has made dramatic progress due to new findings (e.g. coesite), new methods (e.g. GPS), and new—or newly considered—concepts (e.g. subduction roll-back). Our picture of the Alpine orogeny has changed completely. Extremely important for Alpine research, the opening of borders between western and eastern parts of Europe has opened new perspectives: seen from the east, the Alps are the result of the junction of the Dinarides and the Carpathians. Parts of the Alpine evolution, e.g. Jurassic tectonics in the Northern Calcareous Alps, can only be understood in the context of processes in the Internal Dinarides and Internal Carpathians. The exchange of information and ideas between Alpine, Carpathian, Pannonian and Dinaride Earth scientists—in which Stefan Schmid played and still plays a most important role—has been fruitful for all sides. The present volume on the Alps, Carpathians and Dinarides (Fig. 1 ) includes articles that are related to key aspects of the tectonic evolution of these mountain chains. These articles are examples of the Alpine approach to orogeny, which combines careful fieldwork with a broad

The closing of the Tethys Ocean and continent-continent collision along the Alpine-Himalayan chain ultimately produced large Himalayan-type mountain belts and large plateaux, such as Tibet. Earlier stages in the collision process, however, can be seen in the Oman Mountains of eastern Arabia and the Zagros Mountains of SW Iran. In Oman, a large, intact ophiolite was emplaced onto a Mesozoic passive continental margin, largely by thin-skinned thrust processes, prior to continental collision. The ophiolite and a granulite-amphibolite-greenschist facies inverted metamorphic sole were formed in a subduction zone setting during the early stages of emplacement. Eclogites were formed by the attempted subduction of the continental margin, and its rapid expulsion back up the same subduction zone, during later stages of the orogeny. The early stages of continental collision are best seen in the Zagros Mountains where thick-skinned thrusting and simple folding has resulted in a relatively small amount of crustal shortening (50–70 km) with almost no metamorphic or magmatic consequences. Burial metamorphism may be occurring presently at deep levels of the internal zone and the Turkish-Iranian Plateau where the crust is thicker, but this remains unexposed at the surface. The collision of the Indian plate with Asia since ca. 50 Ma resulted in formation of the Himalaya along the north margin of India, and the Karakoram–Hindu Kush Mountains along the south Asian margin. Together with renewed uplift and crustal thickening of the Tibetan Plateau, this was arguably the largest continental collision in the last 450 m.y. of Earth history. The Himalayan-type orogeny involved large amounts of crustal shortening (∼500–1000 km), early ultrahigh-pressure (UHP) coesite-eclogite facies metamorphism, peak Barrovian facies kyanite and sillimanite metamorphism, and mid-crustal anatexis resulting in garnet, tourmaline, muscovite-bearing migmatites, and leucogranites. Processes involved in the construction of the Tibetan Plateau include crustal shortening and doubling the thickness of the crust to 65–90 km. High-pressure (HP) eclogite and high-temperature/high-pressure (HT-HP) granulite metamorphism may be occurring at depth today in the lower crust beneath Tibet. Widespread ultrapotassic volcanism across Tibet indicates the presence of a hot subcontinental mantle, which was progressively shifted northwards as the cold, Indian lithosphere underthrust southern Tibet. Whereas Tibet shows mainly upper crustal sedimentary and volcanic rocks at the present surface, the Karakoram Range, along strike to the west, shows mostly deep crustal high-grade metamorphic rocks, multiple granite intrusions, and over 60 m.y. of high-temperature metamorphism. This paper reviews the salient geological features of Oman-, Zagros-, Himalayan-, Tibetan-, and Karakoram-type orogenic belts. These features can be used in studies of older orogenic belts to give indications of their tectonic origins.

New and recycled intermediate to felsic crust in island arcs and continental margins is generated by magmatism surmounting convergent plate junctions. Paired Pacific-type orogens develop adjacent sites of long-lived subduction of oceanic lithosphere. They consist of (1) an outboard accretionary prism deposited in and continentward from the oceanic trench, the filling of which is derived mainly from (2) an inboard calc-alkaline volcanic-plutonic arc. The trench assemblage includes arc-sourced clastic mélanges, minor but widespread deepwater cherts and carbonates, and tectonically disaggregated ophiolites. These relatively incompetent sections recrystallize under high-pressure (HP) conditions and decouple from the descending oceanic plate at depths of ∼15–50 km. A massive, slightly older to coeval andesitic-granodioritic arc dominates the landward belt where new sialic crust is added from I-type magmas, and preexisting continental materials are recycled as S-type melts; high temperature (HT) characterizes local-regional metamorphism of the wall rocks. In contrast, Alpine-type orogens result from the underflow of an oceanic plate that transports island arcs, microcontinents, and/or continental salients into the subduction zone. Reflecting lithospheric coherence, the sialic crust may be carried down as much as 90–140 km, becomes thermally softened, and decouples from the sinking plate. Metamorphism of deeply subducted parts of Alpine belts ranges from high pressure to ultrahigh pressure (UHP), and is not paired with a HT calc-alkaline arc. In both types of subduction complex, outboard thrust faults dip landward and fold vergence is seaward, reflecting the polarity of underflow (and rollback) of the oceanic lithosphere. Antithetic thrusting typifies some contractional continental realms. Propelled by buoyancy, allochthonous nappes conduct relatively low-density HP and UHP sections to midcrustal levels. At convergent syntaxes (plate-margin cusps) of overthickened arcs, tectonic aneurysms may produce domical uplifts, further exhuming UHP units surfaceward. The regional crustal thickness of an active mountain belt partly reflects climate as well as the main convergent-transform plate processes producing the orogen. The volcanogenic Chilean Cordillera parallels the seaward convergent plate junction and the eastward-sinking Nazca plate. The highest ranges and thickest continental crust (∼70 km) occur in the compressed north-central Andes at ∼25°S, a region of extremely low rain- and snowfall. Aridity and low erosion rates help account for the high-standing calc-alkaline volcanic-plutonic contractional arc and the elevated, internally drained Altiplano downwind. Thus erosional degradation is weakly developed. A very different climate typifies the orogen at ∼45°S, where abundant moisture-laden westerlies bring abundant precipitation to the Chilean margin. The rugged mountain belt is of lower elevation compared with the north-central Andes and is supported by a crust of only moderate thickness (35–40 km). Vigorous erosion supplies voluminous detritus to the offshore Chile-Peru Trench as well as eastward, so a plateau is lacking in the lee of the arc. The Himalayas and Tibetan Plateau, the Sierra Nevada and Colorado Plateau, the Japanese island arc, and New Zealand exhibit somewhat similar relationships between crustal thickness and precipitation-linked erosion.

The Hellenides constitute an integral part of the Alpine orogenic system in southeast Europe. Despite the recognition of several subparallel zones, which are interpreted as terranes separated by ophiolitic sutures (e.g., Pindos and Vardar sutures), the classical view of an orogen with a foreland fold-and-thrust belt, a central crystalline zone, and a rather undeformed hinterland is still under discussion. This paper concentrates on basement terranes of exotic provenance in two of the internal zones of the Hellenides that support the interpretation of the Hellenides as an accretionary orogen formed by amalgamation of crustal segments during the subduction of Tethyan oceanic basins. The oldest of these terranes, the Florina terrane in the Pelagonian zone, is composed of Neoproterozoic arc-related orthogneisses. Two other exotic terranes occur east of the Vardar zone within the Serbo-Macedonian Massif. The Pirgadikia terrane is a microterrane in the southern Chalkidiki Peninsula that consists of Pan-African mylonitic orthogneisses with volcanic arc–related trace-element geochemistry and Sr isotopic composition. The Vertiskos terrane occupies the northwestern part of the Serbo-Macedonian Massif and is primarily composed of coarse-grained, volcanic arc–related peraluminous orthogneisses of Silurian age. These terranes are exotic in relation to the other parts of the Hellenides. The provenance of the late Proterozoic Pan-African Florina and Pirgadikia terranes is assumed to be Gondwanan, whereas the Silurian Vertiskos terrane may have been part of the so-called Hun ter-rane, which formed at the northern active continental margin of Gondwana in the early Paleozoic.