John F. Dewey

Plate Tectonics and the Evolution of the Alpine System

It is contended that the Late Triassic to present-day gross evolution of the Alpine system in the Mediterranean region has been the result of activity along an evolving network of accreting, transform, and subducting plate boundaries between the large stable cratons of Europe and Africa. A refined assembly of the outlines of the continents around the North and central Atlantic, before the initial dispersion of Gondwanaland in Early Jurassic times, is presented. By considering geologic facies, structural fabric, and paleomagnetic criteria, the smaller continental fragments now found within the Alpine system are restored to their proposed initial positions relative to each other in the reconstruction offered. The motion of the major plate of Africa relative to Europe, commencing with the initial continental fragmentation, is documented by analysis of the sea-floor spreading history of the Atlantic Ocean, with the assumption that plate accretion there has occurred between torsionally rigid lithospheric plates. By the computerized fitting of well-defined and well-dated key pairs of symmetric magnetic anomaly lineations back together by a series of finite rotations, the relative position of North America to both Europe and Africa has been determined for the following times: 180 m.y. (Toarcian Stage, Early Jurassic); 148 m.y. (Kimmeridgian Stage, Late Jurassic); 80 m.y. (Santonian Stage, Late Cretaceous); 63 m.y. (Danian Stage, Paleocene); 53 m.y. (Ypresian Stage, Eocene); and 9 m.y. (Tortonian Stage, Miocene). From these positions, a series of rotation poles presumed to describe the stepwise motion of Africa relative to Europe were computed. The motions of the smaller intervening microplates have been inferred from the style of tectonic deformation on their borders, and these motions have been constrained to satisfy both changes in paleo-latitude with time and progressive rotations relative to the large macroplates that can be deduced from paleomagnetic measurements. The evolution of Tethys does not involve a single simple plate boundary between Europe and Africa, as has been envisioned previously, but, instead, a constantly evolving mosaic of subsiding continental margins, migrating mid-oceanic ridges, transform faults, trenches, island arcs, and marginal seas (back-arc basins). The periods of passive-continental margin development are recognized by a transgressive facies of platform carbonate rocks and thick prisms of continental-rise type sedimentation; accreting ridges by ultramafic rocks, gabbro, pillow basalt, deep-sea pelagic ooze, and abyssal red clay of the ophiolite suite; trenches by a migrating series of progressively younger linear flysch troughs whose immature mineral composition reflects nearby andesitic and metamorphic source terrains; the arcs themselves by calc-alkaline volcanism and the intrusion of silicic to intermediate plutons; the polarities of these arcs by the direction of overthrust nappe sheets and gradients in the ratio of potash to silica in the extrusives; their orientation by paired belts of high T and P and high P-T metamorphics; and finally the spreading back-arc basins by outpourings of basaltic magmas and evidence of flipping Benioff planes. A compilation of eight phases or chapters in Atlantic spreading history are outlined, which are based on the recognition of discrete differences and (or) relative motion between the continents bordering the Atlantic. All of these changes are reflected in the Tethys by reorganizations of the intervening plate boundaries and, we believe, are most explicitly recorded in the deformational history of the subducting zones. A montage of geometrically assembled plate-boundary interpretations are pictorially displayed as time-lapse frames of the evolving Alpine system. The montage begins with the Late Triassic (pre-Atlantic) setting of the Tethys 1 Ocean and extends to the present through nine phases of Tethyan history. Each phase is recognized on the basis of the age of intrusion and extrusion of basic lavas in ophiolite complexes, which mark the creation of new oceanic areas by both axial accretion in rift valleys of mid-oceanic ridges between rigid plates or by a more uncertain type of spreading in basins behind active island arcs. All the schemes presented are best estimates of the gross geometrical arrangements at discrete time intervals and should be treated as merely educated guesses. Despite the fact that we only have rigorous constraints for the relative positions of the nondeformed forelands of Europe and Africa, our models nevertheless imply that the motions of the larger plates will, by and large, dictate the general behavior of the smaller microplates through the particular styles of deformation set up along the adjoining plate boundaries. The Tethys 1 Ocean, located between Africa and Europe in Triassic times, has been almost entirely swallowed up in subduction zones of the Major Caucasus Mountains along its former northern margin and in similar zones of the Pontides and Minor Caucasus along its southern margin. The only remnants of Tethys 1 are the areas of oceanic crust in the Black and South Caspian Seas. There is considerable evidence to suggest that the Tethys 1 Ocean had an actively spreading ridge. Some tens of millions of years prior to the opening of the central North Atlantic, a branch of this ridge system entered into the Vardar Zone of eastern Greece and broke off fragments of northeast North Africa to initiate the development of the present-day Ionian and Levantine Basins of the eastern Mediterranean. Additional fragments (the Moroccan and Oranaise Meseta) were ruptured from northwest Africa following its separation from North America. The intervening Jurassic Atlas, seaway developed along an accreting plate boundary extending from the eastern Tethys to the crest of the embryonic Mid-Atlantic Ridge where it formed a migrating triple junction whose trace, we believe, follows the trend of the New England seamount chain. The western Mediterranean basins of the Alboran, Balearic, and Tyrrhenian Seas are very much younger, being initially opened in the early Miocene as a string of back-arc marginal seas behind the developing Apennine, Tel Atlas, and Rif suture zone that today marks the sites of subduction of Jurassic and Lower Cretaceous oceanic crust. The contemporary Alpine system displays a spectrum of stages in the building of mountain belts. Embryonic nappes within the Mediterranean Ridge in proximity to melange zones of the inner wall of the Hellenic Trench are, perhaps, signs of the initial deformation of sedimentary passengers on oceanic crust arriving at a subduction zone. Total closure of an ocean followed by the partial consumption of a passive continental margin leads to events such as the tectonic emplacement of crystalline basement nappes of the European “chaine calcaire” onto northwest Africa. Arc-continent collisions of this type which have then been succeeded by total destruction of marginal back-arc basins are recognizable in the Hellenides and Pontides. There are, as well, collisions that have not involved the disappearance of large oceanic areas; these are most apparent in the particular tectonic style of the Pyrenees and High Atlas Mountains.

Kinematics of the western Mediterranean

Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.

The Tectonic Evolution of the Tibetan Plateau

The Tibetan Plateau, between the Kunlun Shan and the Himalayas, consists of terranes accreted successively to Eurasia. The northernmost, the Songban Ganzi Terrane, was accreted to the Kunlun (Tarim-North China Terrane) along the Kunlun-Qinling Suture during the late Permian. The Qiangtang Terrane accreted to the Songban-Ganzi along the Jinsha Suture during the late Triassic or earliest Jurassic, the Lhasa Terrane to the Qiangtang along the Banggong Suture during the late Jurassic and, finally, Peninsular India to the Lhasa Terrane along the Zangbo Suture during the Middle Eocene. The Kunlun Shan, Qiangtang and Lhasa Terranes are all underlain by Precambrian continental crust at least a billion years old. The Qiangtang and Lhasa Terranes came from Gondwanaland. Substantial southward ophiolite obduction occurred across the Lhasa Terrane from the Banggong Suture in the late Jurassic and from the Zangbo Suture in the latest Cretaceous-earliest Palaeocene. Palaeomagnetic data suggest successive wide Palaeotethyan oceans during the late Palaeozoic and early Mesozoic and a Neotethys which was at least 6000 km wide during the mid-Cretaceous. Thickening of the Tibetan crust to almost double the normal thickness occurred by northward-migrating north-south shortening and vertical stretching during the mid-Eocene to earliest Miocene indentation of Asia by India; Neogene strata are almost flat-lying and rest unconformably upon Palaeogene or older strata. Since the early Miocene, the northward motion of India has been accommodated principally by north south shortening both north and south of Tibet. From early Pliocene to the Present, the Tibetan Plateau has risen by about two kilometres and has suffered east-west extension. Little, if any, of the India Eurasia convergence has been accommodated by eastward lateral extrusion.

Tibetan, Variscan, and Precambrian Basement Reactivation: Products of Continental Collision

Extensive terranes of basement reactivation are interpreted as resulting from crustal thickening following continental collision. It is suggested that terranes, such as the Grenville Province and much of the Variscan orogenic belt in Europe, have their modern analog in the Tibetan Plateau. The Tibetan Plateau is underlain by a continental crust between 60 and 80 km thick and is characterized by extensive high-potash Neogene vulcanism. Following T. H. Greens arguments that partial melting of a dioritic lower crust may yield potassic granitic liquids and refractory anorthositic residues, we consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust. At high structural levels, silicic-potassic ignimbrites are extruded in intermontane basin-horst terranes, with subjacent granite plutons. At deeper levels, a dry refractory lower crust consisting of pyroxene granulites and anor-thosites is generated.

Aegean and surrounding regions: Complex multiplate and continuum tectonics in a convergent zone

The tectonics of the Aegean region involves complex slip patterns across the boundaries of several microplates that segment the end of the Anatolian plate, which is moving in a westward direction from the Bitlis zone, an intracontinental suture zone, to consume oceanic lithosphere in the eastern Mediterranean. The segmentation of the western end of the Anatolian plate into scholles with adjacent zones of grabens, strike-slip, and thrust semicontinuum tectonics results from “locking’ across the two North Anatolian transform strands where they change orientation at the western end of the Sea of Marmara. This fast lateral motion of buoyant continental slivers is a transient phase of the early stages of continental collision resulting from the irregularity of colliding margins. It is, however, a tectonic phase that leaves a fundamental signature on the convergent zone by imprinting a complex widespread series of structures that mask, and make difficult the interpretation of, earlier structures.

Plume-Generated Triple Junctions: Key Indicators in Applying Plate Tectonics to Old Rocks

Continental lithosphere-especially where stationary with respect to mantle plumes-is marked by plume-generated uplifts typically crested by volcanoes that rupture in three rifts at angles of about 120° to each other, perhaps because this configuration requires the least work. It is proposed that since the plate tectonic regime began, about

The Cambrian-Silurian tectonic evolution of the northern Appalachians and British Caledonides: history of a complex, west and southwest Pacific-type segment of Iapetus

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A plate-kinematic framework for models of Caribbean evolution

years B.P., divergent plate motion has commonly begun at axial dikes emplaced in rifts formed in this way. A normal course of events is that two of the rifts meeting at a junction to open by plate accretion while the third rift becomes inactive as a failed arm. The evolution of 45 selected junctions, with ages ranging back to