Plate Tectonic Setting Research Articles

Late Miocene movement within the Himalayan Main Central Thrust shear zone, Sikkim, north‐east India

AbstractIn the Sikkim region of north‐east India, the Main Central Thrust (MCT) juxtaposes high‐grade gneisses of the Greater Himalayan Crystallines over lower‐grade slates, phyllites and schists of the Lesser Himalaya Formation. Inverted metamorphism characterizes rocks that immediately underlie the thrust, and the large‐scale South Tibet Detachment System (STDS) bounds the northern side of the Greater Himalayan Crystallines. In situ Th–Pb monazite ages indicate that the MCT shear zone in the Sikkim region was active at c. 22, 14–15 and 12–10 Ma, whereas zircon and monazite ages from a slightly deformed horizon of a High Himalayan leucogranite within the STDS suggest normal slip activity at c. 17 and 14–15 Ma. Although average monazite ages decrease towards structurally lower levels of the MCT shear zone, individual results do not follow a progressive younging pattern. Lesser Himalaya sample KBP1062A records monazite crystallization from 11.5 ± 0.2 to 12.2 ± 0.1 Ma and peak conditions of 610 ± 25 °C and 7.5 ± 0.5 kbar, whereas, in the MCT shear zone rock CHG14103, monazite crystallized from 13.8 ± 0.5 to 11.9 ± 0.3 Ma at lower grade conditions of 525 ± 25 °C and 6 ± 1 kbar. The P–T–t results indicate that the shear zone experienced a complicated slip history, and have implications for the understanding of mid‐crustal extrusion and the role of out‐of‐sequence thrusts in convergent plate tectonic settings.

Andean deformation and rift inversion, eastern edge of Cordillera Oriental (Guateque–Medina area), Colombia

In the Guateque–Medina area, Paleozoic basement and Mesozoic rift basins have been uplifted and exhumed during the Andean orogeny (12 Ma to present). Surface exposures and subsurface data constrain the deformation style and the rift geometry. We have mapped a regional transect and restored a cross section. We have also reconciled existing stratigraphic data, from cordillera, foothills and foreland basin, and have added new data of our own. In Early Cretaceous shales, there is evidence for fault-controlled sedimentation. A brecciated evaporitic layer, which is locally emerald bearing, has acted as a regional detachment. The underlying basement, composed of Paleozoic sediments, crops out as the Quetame Massif. It was uplifted during the Andean orogeny on a series of high-angle reverse faults. The main SE-verging Tesalia fault has resulted from Andean reactivation of an Early Cretaceous normal fault, which bounded a half-rift. A series of NW-verging back-thrusts may have resulted from Andean reactivation of Paleozoic faults. Between the back-thrusts and the Tesalia fault is a basement pop-up. It may be part of a flower structure, because components of right-lateral slip have been identified. These are attributed to eastward subduction of the Nazca plate beneath South America. In general, the style and timing of Andean deformation in the Guateque–Medina area are compatible with the plate tectonic setting of the northern Andes.

Geochemistry of mafic dykes from Perhentian and Redang islands: an example of petrogenesis of the younger (dolerite) dykes from the Eastern Belt of Peninsular Malaysia

Mesozoic mafic dykes in the Perhentian islands and their surrounding area can be divided into two types, based on their field occurrence, i.e. the older and younger dykes. The older dykes are synplutonic to their felsic host and the younger dykes post date their felsic host. The younger dykes, which are more abundant, are mainly doleritic in composition and are similar to those found throughout the Eastern Belt of Peninsular Malaysia. They are made up of plagioclase, clinopyroxene, amphibole, iron ore and chlorite. The silica content of the dykes is between 47.17 to 53.7% and can be classify as basalt, trachybasalt and basaltic trachyandesite. Geochemical study shows that the younger dykes formed in a continental within plate tectonic setting.

Thermal and tectonic evolution of the Falkland Islands: implications for hydrocarbon exploration in the adjacent offshore region

Apatite fission track and vitrinite reflectance data combine to suggest that the Falkland Islands experienced three discrete episodes of heating and cooling during the Phanerozoic, each of which can be related to major changes in the plate tectonic setting of the islands. Combined with published data from the adjacent offshore areas, the results provide unique insights into the behaviour of the region during the phases of convergent and divergent tectonic movements that have affected the region since the Permian. Initial cooling of the Falkland Islands in the Late Permian was initially restricted to West Falkland, and was the result of differential uplift and erosion of West Falkland relative to East Falkland, coinciding with D2 Gondwanian deformation. Subsequent Early Jurassic cooling of both East and West Falkland was associated with plume-related thermal uplift preceding the separation of East and West Gondwana and the development of the Falkland Plateau Basin. The Falkland Islands experienced renewed heating during the Mesozoic, probably due to increased burial, suggesting that the offshore basins originally extended onto the present onshore region. Late Cretaceous/Early Tertiary cooling of the Falkland Islands was contemporaneous with the development of a transpressional plate boundary along the southern margin of the Falkland Plateau resulting in erosion of the islands that were situated on a flexural forebulge.

Petrogenesis of metaluminous A-type rhyolites from the St Francois Mountains, Missouri and the Mesoproterozoic evolution of the southern Laurentian margin

Predominantly acid volcanic and intrusive rocks of the approximately 1.48 Ga Eastern Granite–Rhyolite Province are exposed only in the St Francois Mountains inlier, Missouri. Combined Nd isotopic, major and trace element analyses are presented for the predominantly acid volcanic rocks of the St Francois Mountains. They are metaluminous to mildly peraluminous, A-type compositions with positive initial ε Nd values of 3.1–5.0. They are interpreted to be the result of crystal fractionation of partially melted calc-alkaline igneous rocks formed at a recently active subduction zone. The likelihood of a juvenile, calc-alkaline, subduction-generated protolith for the St Francois Mountains rhyolites leads to a hypothesis in which dacitic parental magmas resulted from partial melting in a continental ‘back-arc’ setting. Basaltic magmatism due to decompression melting of the mantle resulted from lithospheric extension and thinning and provided the heat source for melting in the middle or lower crust. Penecontemporaneous magmatism elsewhere in Laurentia included subduction related calc-alkaline rocks in the Pinware Terrane of Labrador; some felsic igneous rocks of uncertain plate tectonic setting within the Grenville Province also have 1.5–1.6 Ga T DM ages. These are interpreted as magmatic products of approximately 1.5 Ga northward dipping subduction beneath much of the length of the southern margin of Laurentia. The Eastern Granite Rhyolite Province is consequently seen as an integral inboard expression of this subduction related igneous activity. Petrogenetic models involving a mantle plume or the development of a continental rift are not required to explain this granite–rhyolite terrane and evidence against their involvement is discussed.

The Ancenis Terrane: an exotic duplex in the Hercynian belt of Armorica, western France

The Ancenis Terrane, bordering the Loire, France, consists of Precambrian Brioverian basement and three Paleozoic sedimentary sequences: marine Ordovician to Devonian shelf sediments; Lower Carboniferous synorogenic intermontane sediments (the ‘Culm’); and coal-bearing Namurian deposits. The terrane is completely bound by faults, and its geology is in marked contrast with that of its neighbours. New data and observations along the terrane's southern boundary show that the Ordovician–Devonian sequence occupies a transpressional shear zone, and can be described as a tectonic mélange. The Culm outcrop, previously interpreted as a sinistral pull-apart structure, is reinterpreted as a Lower Carboniferous dextral pull-apart, and it is shown that Carboniferous transpression and thrusting remodelled the pull-apart basin into its present ‘mica-fish’ shape. With its remarkable concentric structure, and within a plate tectonic setting of mega dextral displacements along the northern boundary of Gondwana, the Ancenis Terrane can be interpreted as an exotic duplex.

BIFROST project: 3-D crustal deformation rates derived from GPS confirm postglacial rebound in Fennoscandia

Since autumn 1993 the BIFROST project has provided daily GPS solutions of geodetic positions from a network of more than 40 stations covering a large area of the Baltic shield. This area is expected to show large vertical motion due to glacial isostatic rebound following the deglaciation at the end of the Pleistocene. This paper will discuss the inference of three-dimensional rates of crustal motion at the GPS stations with respect to (1) a plate-fixed average for the horizontal components; (2) a geocentric reference in order to infer absolute changes of sea level from vertical crustal motion and models of geoidal rebound. We show that the horizontal strain rate pattern is largely dominated by unilateral extension and not exhibiting horizontal shear to an important extent. In regard to the vertical motion a crucial issue is the stability of the geocentre in the GPS frame. We show results from an Empirical Orthogonal Function analysis that attenuates regionally correlated noise. In all components our observations suggest reasonably close agreement with forward computions on the basis of postglacial isostatic adjustment. A dominant tectonic signal would lead to a certain fraction of the batch of baselines to exhibit shortening. A tectonic process leading to a similar pattern of horizontal motion as expected from postglacial rebound can safely be dismissed in the context of the currently accepted plate tectonic setting. Thus, our baseline rate comparison will be a critical first order test of the prevailing style of deformation.

Search for a deep-mantle component in mafic lavas using a Nb–Y–Zr plot

Concentrations of trace elements with bulk distribution coefficients < 1 have been utilized to discriminate between mafic magmas erupted in different tectonic settings. Herein, the Nb–Y–Zr contents of mafic lavas will be utilized to pinpoint derivation from a deep-mantle source. Magmas derived from the Icelandic plume exhibit ΔNb > 0, where ΔNb = log (Nb/Y) + 1.74 – 1.92 log (Zr/Y) (Fitton et al. 1997), whereas melts derived from depleted sections of the mantle and the crust show ΔNb < 0. Regardless of the nature of the melting event, it was suggested the ΔNb value of magmas were essentially identical to that of the source material. Theoretical modeling of a garnet lherzolite source suggests very small partial melts may show elevated ΔNb values, yielding a spurious plumelike signature. For mafic rocks from different areas, with hypothesized hot spot derivation, ΔNb is calculated to search for a deep-mantle signature (ΔNb > 0). The signature of the Réunion hot spot, from ~65–0 Ma, is seen in the Deccan Traps, India, lavas from the Mascarene Plateau, and Réunion Island. The signature of the Kerguélen plume is detected in rocks from Kerguélen Island and the Ninetyeast and Broken Ridges, Indian Ocean, a few lavas from the Rajmahal province, India, but none in the Bunbury Basalt, Australia. Some rocks from continental flood basalt provinces show a hot spot component, in agreement with conclusions based on 3He/4He studies, others show ΔNb < 0, due to considerable subcontinental contamination. For lavas from Heard Island and Grand Comore, Indian Ocean, the technique pinpoints those showing a plume component. In an area of plate convergence in the Pacific Ocean, lavas with very low (<1 ppm) Nb contents, serve as a critical test for this method. Only rocks close to the Samoan hot spot show ΔNb > 0, whereas the others show the Nb depletion typical of arc-related material. In conjunction with trace-element and isotopic discriminants, this appears to be a useful tool in identifying a deep-mantle component in mafic lavas from a variety of plate tectonic settings.

Graptolites as constraints on models of sedimentation across Iapetus: a review

The use of graptolites (specifically, of the planktonic graptoloids) in sedimentary facies analysis is discussed and illustrated, with particular reference to the margins of the former Iapetus Ocean in the UK. By far the most important use stems from the high time resolution afforded by graptolite biostratigraphy, particularly in the Silurian, where biozones/subzones average under half a million years in duration. Increased resolution may be obtained where biostratigraphy is combined with the event stratigraphy provided by repeated oscillations between anoxic and oxygenated sea floors, which yielded laminated (‘graptolite shale’) and burrowed (‘barren beds’) hemipelagites respectively. The resolution thus afforded has allowed the elucidation of detailed sedimentary architecture; examples are drawn from the Llandovery turbidites of the Welsh Basin and the late Ordovician to early Silurian accretionary prism of the Southern Uplands of Scotland. Use of this biozonation as a temporal framework has also allowed an interpretation of the plate tectonic setting of individual basins via subsidence analysis and the sequence stratigraphic interpretation of the basin-fills. The use of graptolites as indicators of broader palaeogeographic and palaeoceanographic settings shows promise but, in practice, is still hindered by incomplete understanding of the various factors that controlled the distribution of graptoloids within the early Palaeozoic seas. Finally, current-transported graptolites, may be used, cautiously, as palaeocurrent indicators.

Evidence for a synchronous circum‐Iberian subsidence event and its relation to the African‐Iberian plate convergence in the Late Cretaceous

Subsidence curves from Mesozoic sedimentary basins at the southern Iberian margin (Betic Cordilleras) display pronounced changes in subsidence rates around 85 Ma (chron 34, Late Cretaceous, Santonian to earliest Campanian). The subsidence events correlate with changes in the bulk and clay mineral composition in these basins, as well as with an Eoalpine high‐pressure metamorphic event in the western Mediterranean region. The synchroneity with subsidence events observed in basins around the Iberian microplate suggests a causal relationship with the regional plate tectonic setting. We propose that the circum‐Iberian subsidence event was largely controlled by the convergence and incipient collision of the Iberian microplate with Africa.

Evolution of Mesozoic Sandstone Compositions, Southern Junggar, Northern Tarim, and Western Turpan Basins, Northwest China: A Detrital Record of the Ancestral Tian Shan

ABSTRACT Sandstone compositional data can be a powerful tool in the interpretation of tectonic and climatologic influences on sedimentary basin fill, in addition to yielding important information about porosity in sandstone petroleum reservoirs. In order to explore these relationships, a modified Gazzi-Dickinson point-counting technique was used to analyze the composition of 143 Mesozoic sandstone samples from the southern Junggar, northern Tarim, and western Turpan basins of northwestern China. Results indicate that a Mesozoic, ancestral version of the Tian Shan physiographically separated the Junggar and Tarim basins and provided sand of very different composition to each basin. Mesozoic sandstone from the northern Tarim basin is diverse in composition and lithic-rich (Qm41F14L45; Qp42Lvm26Lsm32), is locally micaceous, and contains common radiolarian-chert grains and few dense accessory minerals. Inferred source rocks include upper Paleozoic alkali granite and metamorphic complexes, thick Silurian bedded-chert sequences, and lower Paleozoic strata of a passive continental margin. In contrast, sandstone from the southern Junggar and western Turpan basins is uniformly volcanic-rich (Qm21F21Lt58; Qp13Lvm68Lsm19), and contains abundant dense accessory minerals and only local radiolarian chert and mica. Inferred principal source rocks are Devonian-Carboniferous andesitic arc volcanics. The effect of sampling scale on sandstone composition outweighs that of plate-tectonic setting. Samples were derived mostly from medium- and coarse-grained fluvial systems that likely drained only portions of the ancestral Tian Shan and hence preserve local source-rock signatures, rather than an integrated compositional signal that can be directly compared to plate-tectonic petrofacies models. In addition, though Mesozoic basins of western China were most akin to broken foreland basins, Mesozoic sandstone is considerably more compositionally diverse and lithic-rich than that of modern or ancient broken foreland basins because of the variety of accreted terranes constituting the ancestral Tian Shan. Temporal changes in sandstone composition are consistent with episodes of Mesozoic deformation in the Tian Shan. Each deformational episode increased physiographic relief of the ancestral range, produced renewed downcutting and erosion of source rocks, and resulted in the deposition of compositionally very immature sandstone in adjacent basins. Although a regional early Mesozoic megamonsoon and an Early Cretaceous rain shadow cast across the northern Tarim basin are interpreted from regional facies and paleontologic data, neither paleoclimatic phenomenon appears to have significantly modified sandstone composition in the study area. Calculations of intergranular volume (% porosity + % cement) indicate that porosity in sandstone from the Tarim and Junggar basin depocenters was reduced principally by burial compaction and that the rate of porosity reduction was highest for lithic-rich samples.

Abstract: The formation of Supergiants in the Central and South Caspian Basins: Structural Evolution and Plate Tectonic Setting

Abstract: The formation of Supergiants in the Central and South Caspian Basins: Structural Evolution and Plate Tectonic Setting

Interplay of intraplate tectonics and surface processes in the Sierra de Guadarrama (central Spain) assessed by apatite fission track analysis

Apatite fission track analysis is used as a tectonic tool to unravel the evolution of the Sierra de Guadarrama, an mountain range in central Spain, and the far-field effects of the Alpine plate tectonics, expressed by reactivation of NE-SW trending lineaments in the Hercynian basement. 18 basement samples were analysed, and 4 sediments of Mesozoic and Tertiary age. Thermal histories were modelled for most samples and conversion to resultant amounts of denudation and rock uplift was possible for the Tertiary history, because of constraints on the paleo-topography and -elevation in Upper Cretaceous to Paleocene times. Accelerated cooling (up to 100 °C in 5 Ma) occurred around 100 Ma in the entire Sierra de Guadarrama. In the northern part, this cooling was preceded by reheating of Lower Triassic sediments up to 110 °C, suggesting sedimentation of about 3 km of, now eroded, Upper Triassic to Jurassic. The period of greatest erosion occurred in the Pliocene and Quaternary and affected almost the entire Sierra de Guadarrama. It was preceded by a Middle-Miocene cooling event that correlates with the beginning of the neo-tectonic setting of central Spain. The greatest Tertiary rock uplift occurred in the central part of the Sierra de Guadarrama: 5.9 ± 11.6 km. The Pliocene to recent event constitutes most of the Tertiary denudation. It is accommodated by active NE-SW trending reverse faults, and attended by about 3.2 km of denudation. These data fit as far-field effects in the plate tectonic setting of ongoing NW-SE oriented convergence between the European and African plate.

Ile de Groix: retrogression and structural developments in an extensional régime

The Paleozoic metamorphic rocks of Ile de Groix, France, record a retrogressive metamorphism from eclogite facies through blueschist to greenschist facies, produced during progressive uplift and exhumation, and accompanied throughout by high extensional strain. Extension was accompanied by formation of a generally flat-lying foliation, the result of either pure shear or conjugate simple shearing, with top to the northwest and southeast shear senses equally important. The foliation is either wrapped around pods of relatively competent and/or higher grade rock, elongate in the direction of stretching, or folded on axes parallel to the stretching direction; these structures represent the effects of the intermediate strain axis which transiently and locally fluctuated between being one of extension or shortening. Large-scale open folds of foliation may represent a wrapping of foliation about two giant pods which define the entire island. We suggest the so-called lawsonite pseudomorphs might possibly be altered calcium-bearing plagioclase which indicates a simpler path of retrogression than would lawsonite. The plate tectonic setting of Groix might be solved by applying the concept of suspect tectonostratigraphic terranes to Gondwanan France and Iberia, and invoking large-scale displacements.

Growth of granite–greenstone terranes at convergent margins, and stabilization of Archean cratons

Archean granite–greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may have been underplated beneath these orogens, forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere. Back-stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. Eventual collision of these juvenile orogens with other continental blocks formed anatectic granites, then thickened the crust beyond its ability to support its own mass, which initiated gravitational collapse and decompressional release of syn- to late-tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs, and aided in the cratonization of the granite–greenstone terranes. Deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid- and upper-crust, forming ore deposits and plutons. Most preserved granite–greenstone terranes have been tectonically stable since the Archean, and form the cratonic interiors of many continents.

Aeromagnetic legacy of early Paleozoic subduction along the Pacific margin of Gondwana

Comparison of the aeromagnetic signatures and geology of southeastern Australia and northern Victoria Land, Antarctica, with similar data from ancient subduction zones in California and Japan, provides a framework for reinterpretation of the plate tectonic setting of the Pacific margin of early Paleozoic Gondwana. In our model, the plutons in the Glenelg (southeastern Australia) and Wilson (northern Victoria Land) zones formed the roots of continental-margin magmatic arcs. Eastward shifting of arc magmatism resulted in the Stavely (southeastern Australia) and Bowers (northern Victoria Land) volcanic eruptions onto oceanic forearc crust. The turbidites in the Stawell (southeastern Australia) and Robertson Bay (northern Victoria Land zones) shed from the Glenelg and Wilson zones, respectively, were deposited along the trench and onto the subducting oceanic plate. The margin was subsequently truncated by thrust faults and uplifted during the Delamerian and Ross orogenies, leading to the present-day aeromagnetic signatures.

Reassessment of the north-western border zone of the Palaeoproterozoic Ketilidian orogen, South Greenland

As part of ongoing research into the plate tectonic setting of the Palaeoproterozoic Ketilidian orogen led by the Geological Survey of Greenland and Denmark, four geologists from Denmark and the U.K. re-examined parts of the north-western border zone in July–August 1997. The field work was generously supported by the Danish Natural Science Research Council and the Carlsberg Foundation. One team studied the Proterozoic (Ketilidian) sedimentary and volcanic rocks and the regional structure, working from six inland camps along the variably deformed Archaean–Proterozoic unconformity between Midternæs and Qoornoq and on Arsuk Ø (Fig. 1). A second team investigated the plutonic and kinematic evolution of the Kobberminebugt area at the northwestern margin of the Julianehåb batholith (Fig. 1); the latter forms the central part of the Ketilidian orogen (Chadwick & Garde 1996). In addition, samples of volcanic and granitic rocks were collected for geochemical studies and dating of depositional and tectonic events. The first systematic study of the Ketilidian orogen took place in the 1960s and was largely concentrated in its western and southern parts (Allaart 1976). Essential new data from the central and eastern parts of the orogen were acquired during the Survey’s SUPRASYD project (1992–1996; e.g. Garde & Schønwandt 1994, 1995; Garde et al. 1997; Stendal et al. 1997), which was initiated with the aim of assessing the potential for mineral resources in supracrustal sequences (Nielsen et al. 1993). In the course of the SUPRASYD project a new plate-tectonic model for the entire orogen was also published (Chadwick & Garde 1996), in which the orogen is viewed as the result of oblique convergence between the Archaean craton of southern Greenland and a supposed oceanic plate located south of the present orogen, which was subducted towards the north. Chadwick & Garde (1996) also suggested a new division of the Ketilidian orogen into a ‘Border Zone’ adjacent to the Archaean craton, the ‘Julianehåb batholith’ (formerly the Julianehåb granite) in the central part of the orogen, and the ‘Psammite Zone’ and ‘Pelite Zone’ to the south-east, which largely consist of deformed and metamorphosed erosion products derived from the evolving batholith. The north-western border zone and the Ketilidian supracrustal sequences were mapped in the 1960s by Harry & Oen Ing Soen (1964), Watterson (1965), Bondesen (1970), Higgins (1970), Muller (1974), Berthelsen & Henriksen (1975) and Pulvertaft (1977). It was shown that an Archaean gneiss complex (part of the Archaean craton of southern West Greenland) and Palaeoproterozoic basic igneous rocks (the so-called ‘MD’ (metadolerite) dyke swarms and related intrusions), and the unconformably overlying Ketilidian supracrustal rocks are progressively affected by Ketilidian deformation and metamorphism towards the Julianehåb batholith in the south and south-east. Boundary relationships were reviewed by Henriksen (1969). Where the Ketilidian supracrustal rocks are best preserved at Midternæs and Grænseland, Bondesen (1970) and Higgins (1970) divided them into the Vallen Group, which largely consists of sedimentary rocks (Table 1), and the overlying Sortis Group, in which basic pillow lavas and related doleritic to gabbroic sills predominate. Supracrustal rocks of presumed Palaeoproterozoic age further south have previously been referred to as the Qipisarqo and Ilordleq Groups (Berthelsen & NoeNygaard 1965; Allaart et al. 1969). Based on data collected in the 1960s, the earliest plate-tectonic interpretation of the Ketilidian orogen included a prominent suture in Kobberminebugt (Windley 1991). However, other critical aspects of Windley’s model were not substantiated during the Survey’s studies in 1992–1996 (Chadwick et al. 1994; Chadwick & Garde 1996), and a re-evaluation of the north-west border zone was therefore a natural focus of subsequent investigations.

Continental subduction and exhumation of HP rocks in Paleozoic orogenic belts: Uralides and Variscides

Most of the Phanerozoic orogenic belts exhibit HP to UHP metamorphism. First discovered in the Alps and the Norwegian Caledonides, ultra-high pressure rocks are now described in various Paleozoic belts including the Kazakhstan Caledonides, the Uralides, and the Variscides. In these orogenic belts both oceanic and continental, crustal and mantle rocks underwent HP to UHP metamorphism in a variable range of temperatures. The PT conditions require subduction of these rocks to depths sometimes, in the case of continental crust, exceeding 100 km. In the Uralides as in the Variscides, the HP metamorphism occurred very early, 100 to 110 m.y. before the end of the orogeny. In the Uralides, there is a linear HP belt west of the main ophiolitic suture. The HP metamorphism (15-18 kbar/500-700°C) developed in supracrustal continental rocks outcropping beneath an ophiolitic nappe. This is the result of the eastward subduction of the East European continental margin beneath a Uralian oceanic lithosphere and an island arc. The island arc is well preserved and the volcanites underwent only low-grade metamorphism. The Variscides exhibit two kinds of HP rocks: (1) the lower allochthonous units, beneath ophiolitic nappes, are characterized by HP to UHP/LT metamorphism (12-25 kbar/400-700°C), developed in the supracrustal rocks of a thinned continental margin, (2) the upper unit includes oceanic rocks and arc crust and mantle with a HP/MT granulitic metamorphism (10-15 kbar/700-900°C). For the northern Iberian and Massif Central sections, this duality may be explained by the following model: HP/UHP rocks of the lower allochthonous units may be interpreted as parts of a thinned passive margin of the Gondwana supercontinent, deeply subducted (between 400 and 380 Ma) beneath an oceanic and island-arc lithosphere, part of a Galicia-Massif Central ocean. The plate tectonic setting is similar to that of the Urals and Oman. Granulitic rocks of the upper unit may represent the deep crust and mantle of an island arc, itself subducted at about the same time beneath the conjugate thinned passive margin (Avalon) of the Galicia-Massif Central ocean.

The structural response to evolving plate kinematics during transpression: evolution of the Lebanese restraining bend of the Dead Sea Transform

Abstract Structural evolution along continental transform faults may be related to fault zone geometry and to regional variations in plate kinematics. Using a case study of the Lebanese sector of the Dead Sea Transform, the finite geometry of transpression at a restraining bend can be shown to have evolved in time. Relative structural chronologies, calibrated against dated landscape features such as lava-covered palaeosurfaces, coastal erosion surfaces and their incised drainage basins, are used to establish the timing of displacement activity on the major transcurrent faults. The early part of the region’s structural history, up to late Miocene times, was controlled by the geometry of the through-going Yammouneh Fault. Transpression on this right-trending left-lateral structure was accommodated by strike-slip and distributed crustal shortening represented by the initial uplift of Mount Lebanon. For the past 6 Ma the principal active strand of the transform has been the Roum Fault. For much of this period it is presumed to have been a through-going fault which accommodated about 30 km left-lateral displacement. During the Quaternary, the fault zone has become strongly segmented. Although the location of active transcurrent faulting has migrated during the history of the transform, the major site of crustal shortening, the Mount Lebanon-Jabel Barouk structure, has remained broadly fixed. However, the rates of amplification of this structure and the coastal flexure appear to have varied. Continuing uplift of Mount Lebanon and local Plio-Quaternary folding suggest that the offshore continuation of the Roum Fault contains a rightwards, transpressive bend. We relate this multistage history for the Lebanese sector of the transform to an evolving plate tectonic setting: the rotation pole for relative plate motion between Africa and Arabia has migrated through time and the triple junction between the transform and the Tethyan destructive plate margin to the north has moved from onshore SE Turkey to now lie in the NE Mediterranean. Our case study illustrates the transient and evolving nature of deformation in continental restraining bends.

The Dharwar craton, southern India, and its Late Archaean plate tectonic setting: current interpretations and controversies

In spite of detailed geological investigations of the Dharwar craton since the 1890s, its principal lithological units, structure and chronology remain contentious. Important new work on lithostratigraphy, basin development, structure, geochemistry and geochronology has led to wide-ranging speculation on the Late Archaean plate tectonic setting. Much of the speculation is based on uniformitarian models which contrast with a recent proposal that the evolution of the craton was controlled by gravity-driven processes with no crustal shortening.