Showing papers on "Terrane published in 1988"

The tectonic evolution of the Tibetan Plateau

Abstract: 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.

Tectonic history of subduction zones inferred from retrograde blueschist P-T paths

Abstract: Many Phanerozoic convergent plate junctions are marked by discontinuous blueschist belts, reflecting relatively high-pressure (P) prograde trajectories. Common blueschist paragneisses, such as those of the western Alps, exhibit widespread overprinting by greenschist and/or epidote-amphibolite facies assemblages. For this type of high-P belt, retrograde metamorphism involved fairly rapid, nearly isothermal decompression; some terranes underwent continued heating during early stages of pressure release. Uplift probably occurred as a consequence of the entrance of an island arc, oceanic plateau, or segment of continental crust into the subduction zone (collision), resulting in marked deceleration or cessation of underflow and buoyant, approximately adiabatic rise of the stranded, recrystallized subduction complex. Other high-P belts, such as the Franciscan of western California, preserve metamorphic aragonite and lack a low-P overprint; retrogression approximately retraced the prograde P-T (temperature) path, or for early formed high-grade blocks, occurred at even higher P/T ratios. Parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration. Upward motion took place as tectonically imbricated slices, as laminar return flow in melange zones, and perhaps partly a lateral spreading/extension of the underplated accretionary prism. Retrograde P-T trajectories of high-Pmore » belts therefore provide important constraints on the tectonic evolution of convergent plate junctions.« less

Early Proterozoic Assembly of Tectonostratigraphic Terranes in Southwestern North America

Abstract: A 500 km wide early Proterozoic orogenic belt in Arizona and adjacent areas is divided into lithotectonic blocks by northeast- and north-trending shear zones Structural and U-Pb zircon studies suggest that these blocks may have experienced different tectonic histories prior to their juxtaposition by thrust and strike-slip movements on shear zones A northwestern province, here called the Yavapai Province, is composed of at least five tectonic blocks and was assembled at about 1700 Ma A southeastern province, here called the Mazatzal Province, is composed of at least three blocks and was assembled and juxtaposed with the northwestern province during the Mazatzal orogeny, between 1695 and 1630 Ma The diversity of seemingly incompatible 1700 Ma tectonic regimes now in close proximity to each other, and the absence of systematic cross-strike changes in age and character of deformation, plutonism, and metamorphism, suggest a model whereby the orogenic belt developed, and North America grew, by assembly of d

Paleogene history of the Kula plate: Offshore evidence and onshore implications

Abstract: Paleocene to middle Eocene magnetic anomalies were mapped over oceanic crust that accreted at the Kula-Pacific spreading center and is now obliquely entering the western Aleutian Trench between 179°E and 168°E. The strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56-55 Ma, when north-south spreading veered to northwest-southeast (310°-130°). Kula-Pacific spreading ceased in 43 Ma. A 75-km-long section of the fossil Kula Rift axis has avoided subduction, although it now intersects the trench axis (almost orthogonally) near 171.5°E. A narrow remnant of the former Kula plate, northwest of this fossil spreading center, is bounded by a fossil Kula-Pacific transform with a high transverse ridge alongside a sediment-filled transform valley. Anomalies on this remnant show that Eocene Kula-Pacific spreading was highly asymmetric (2:1). The 56-55 Ma change in Kula plate rotation inferred from the change in spreading direction coincided with birth of the Aleutian subduction zone, and was probably a consequence of the resulting change in slab-pull stresses on the oceanic lithosphere. The change in direction of Kula-North American motion is a plausible explanation for the detachment of continental terranes from the Pacific Northwest and their migration around the Gulf of Alaska, and for the early Eocene demise of Alaska Range are volcanism. The cessation of Kula-Pacific spreading coincides with a major change in Pacific-plate rotation, and the subsequent direction of convergence of the Pacific plate with the Aleutian arc was similar to the 55-43 Ma direction of Kula-plate convergence.

Origin and assembly of south-east Asian continental terranes

Abstract: Abstract The pre-Tertiary continental core of south-east Asia is a composite of at least four tectonostratigraphic terranes; Sibumasu (Shan States of Burma, north-west Thailand, Peninsular Burma and Thailand, Western Malaya, and north-western Sumatra), Indo-China (east Thailand, Laos, Vietnam south-west of the Song Ma-Song Da zone and Kapuchea), East Malaya (including south-east Sumatra and Natuna) and south-west Borneo. These terranes are separated from each other by sutures, and are now joined to South China along the Song Ma-Song Da zone. Stratigraphic, palaeontological, and palaeomagnetic evidence suggests that all of these crustal blocks probably had their origin on the north-eastern margin of Gondwanaland where they formed part of a complex continental margin. The Indo-China and East Malaya terranes both have typical late Palaeozoic Cathaysian floras, which indicate that they were close to South China in the Permian. Stratigraphic studies in northeast Vietnam suggest that Indo-China and South China may well have sutured as early as the late Devonian to early Carboniferous. Carboniferous and Permian faunas and floras of these blocks do not appear to be related to those of north-eastern Gondwanaland, indicating that they rifted away at some earlier time. The Sibumasu block remained on the north-western Australian margin of Gondwanaland until the late Lower Permian, and was in continuity with the Tibetan Lhasa and Changtang blocks (south of the Langcangjiang Fracture Zone) and the proto-Kreios microcontinent further to the west. Late Carboniferous and Lower Permian sediments of the Sibumasu, Lhasa, and Changtang blocks, and the Iranian and Afghan portions of proto-Kreios include extensive glacial-marine deposits. Early Permian shallow-marine faunas were of cold-water type until a major change occurred in late Lower Permian times, with Middle Permian faunas becoming typically warm-water Tethyan in aspect. It seems most likely that Sibumasu (still attached to the Lhasa and Changtang blocks and proto-Kreios) rifted from Gondwanaland in the late Lower Permian. Amalgamation of the south-east Asian blocks was probably completed by the end of the Triassic. This is indicated by terrestrial vertebrate biogeography and palaeomagnetism. It is still a matter of debate whether Indo-China sutured to South China in the Devonian-Carboniferous along the Song Ma line, or in the late Triassic along the Song Da zone. Evidence seems stronger for a late Triassic suturing of Sibumasu to the other ‘Cathaysian’ southeast Asian blocks, although a Permian closure of ‘palaeo-Tethys’ has recently been proposed for the Shan-Thai-Indo-China region. The south-west Borneo block probably rifted from the margin of Indo-China in the Cretaceous, and travelled south to its present position during the opening of the proto-South China Sea.

Volcanic Rocks of the 1985 Tibet Geotraverse: Lhasa to Golmud

Abstract: Volcanic rocks encountered during the Tibet Geotraverse have been studied in the field, in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment. Rocks from a total of eleven distinct volcanic provinces were studied in this way. They provide evidence for: an active continental margin or post-collision province of probable Devonian/early Carboniferous age in the northern Kunlun mountains; an active continental margin of late Carboniferous age in the southern Lhasa Terrane; Permian continental rifts in the central Qiangtang and central Kunlun Terranes; Triassic volcanic arcs in the southern Lhasa and northern Qiangtang Terranes; a Triassic active continental margin dyke swarm in the northern Kunlun mountains; a Jurassic post-collision or back-arc rifting province in the southern Qiangtang Terrane; a Jurassic island arc in the northern Lhasa Terrane; a Cretaceous post-collision province in the northern Lhasa Terrane possibly extending into the southern Qiangtang Terrane; and a Palaeogene active continental margin in the southern Lhasa Terrane. An Oligocene trachyte plug in the northern Qiangtang Terrane was the only evidence encountered during the Geotraverse of volcanism post-dating the Palaeogene India--Eurasia collision. However, the composition of this plug, coupled with new and published analyses from Miocene volcanics in the southern Lhasa terrane and from the Pliocene-Recent volcanic province of northwest Tibet, places important constraints on models for post-collision underplating of Tibet by continental lithosphere: any underplating is likely to have been (a) much later than the start of collision, (b) directed beneath Tibet from the north as well as the south, and (c) limited in extent.

Isotope geochemistry of the 1985 Tibet Geotraverse, Lhasa to Golmud

Abstract: Geochronological data from the Golmud —Lhasa section across the Tibetan Plateau indicate progressively younger periods of magmatism from north to south associated with successively younger ocean closures. Pre -collision Eocene magmatism (50—4 0 Ma) exposed along the southern margin of the Lhasa Terrane in the Gangdise Belt resulted from anatexis of mid -Proterozoic crust (~ 1000 Ma) at depths greater than 10 km, but at higher crustal levels subduction-related intrusions were predominantly mantle-derived with ~ 30 % crustal assimilation . Intrusions from the northern Lhasa Terrane are early Cretaceous in age (130 —110 Ma). These form a bimodal suite comprised of two-mica granites derived from anatex is of Mid -Proterozoic crust and of biotite -hornblende granodiorites from about 60 % crustal assimilation by mantle magmas above a post-collision subduction zone. They place a minimum constraint on collision between the Lhasa and Qiangtang Terranes of 130 Ma . Granite magmatism from the Kunlun Mountains is late P ermian -early Jurassic in age (260—190 Ma). The Kunlun batholith represents reworked mid-Proterozoic crust (1400 —1000 Ma) at an active continental margin from 260 —2 4 0 M a . Post-tectonic granites were emplaced in a post-collision setting (200 -190M a). Collision between the Qiangtang and Kunlun Terranes is dated as end -Triassic. Nd model ages of sediments from across the plateau record up lift and erosion of young source regions throughout the Phanerozoic confirming that the Tibetan Plateau is the site of multiple continental collision through time. Phanerozoic magmagenesis throughout the plateau requires considerable crustal reworking and limited crustal growth which suggests thickened continental crust in the region may predate the most recent Eocene collision.

Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia

Abstract: Many granites have compositional features that directly reflect the composition of their source rocks. Since most granites come from the deeper parts of the Earth's crust, their study provides information about the nature of parts of that deep crust. Granites and related volcanic rocks are abundant and widely distributed in the Palaeozoic Lachlan Fold Belt of southeastern Australia. These granites show patterns of regional variation in which sharp discontinuities occur between provinces which internally are of a rather constant character. Such a discontinuity has long been recognized at the I‐S line and the extent of that line can now be defined more fully. Breaks of this type are thought to correspond to sharp changes in the composition of the deep crust that correspond to unexposed or basement terranes. Nine such basement terranes can be recognized in the Lachlan Fold Belt. The character of these basement terranes appears to be different from that of the terranes recognized in the Mesozoic‐Cainozoic Cor...

Cross section of an accretionary wedge: Barbados Ridge complex

Abstract: Many major geological terranes are interpreted as accretionary complexes, and there are several speculative models for their structure and mode of formation. The seismic reflection section across the Barbados Ridge complex at lat 16°12′N presented here shows, for the first time, the entire cross-sectional shape of a large accretionary wedge and its forearc basin. Atlantic oceanic crust underlies 122 km of the wedge and then passes beneath the crust of the forearc of the Caribbean plate, where it can be traced 15 km farther; it dips landward at 9°. The forearc basement dips seaward to meet the ocean crust. The maximum thickness of the wedge is about 10 km. A layer of sediments, 1 km thick, is drawn beneath the accretionary wedge on the surface of the oceanic crust, with little disturbance, for a distance of 70 km, and some sediments still appear to adhere to the ocean crust to where it passes beneath the forearc basement. It is not clear whether sediment is subducted deeper, but it appears probable. The principal resistance to landward motion of the accretionary wedge is provided by the weight of up to 6 km of forearc-basin sediments on the seaward-dipping forearc basement. Both the forearc sediments and the basement have been deformed as a consequence of the horizontal compression produced by the subduction of ocean crust.

Proterozoic and Phanerozoic basement terranes of Mexico from Nd isotopic studies

Abstract: Nd isotopic data were collected on Precambrian crystalline rocks exposed in northern, eastern, and southern Mexico, as well as from lower crustal xenoliths from central Mexico, in order to constrain the age and character of the Mexican basement. The data indicate that basement belonging to the Grenville (1.0 Ga) tectonothermal event extends from Los Filtros, in Chihuahua, northern Mexico, to Oaxaca, in southern Mexico. These rocks all have average Nd crustal residence times (TDM ages) in the range 1.60 to 1.35 Ga. We infer that this results from mixing average 1.9 Ga or older recycled continental crust with 70% to 90% newly derived mantle-crustal material during the Grenville orogeny. To the west of the Precambrian, the basement contains large amounts of Phanerozoic (probably Paleozoic) crust, identified from lower crustal xenoliths with TDM ages less than 1.0 Ga. The crust represented by these xenoliths may have been emplaced as suspect terranes in Mesozoic Cordilleran events. Alternatively, the apparent Paleozoic crust that underlies parts of central Mexico may connect to the Paleozoic metamorphic Acatlan complex in southern Mexico, and together they would constitute a continuation of the Appalachian-Caledonian orogenic belt through Mexico. Our data do not preclude either of these two models.

Andean-age structure of Eastern Cordillera (Province of La Paz, Bolivia)

Abstract: A Moho root beneath the Bolivian Andes, 40 km deep, is consistent with 230 km of overlap of Neogene age on a single, trenchward dipping transcrustal thrust fault with 10° of finite ramp cutoff (Main Andean Thrust (MAT)). Only 10% of the Andean crustal volume is ascribable to magmatic addition. The MAT is located within South American crust of full thickness. It intersects the basement top 450 km inland from the Neogene crustal margin. It is not a collision suture as shown by persistent pre-Neogene facies continuity. Thrusting is not accompanied by terrane accretion. The present bilaterally symmetrical thrust belt responds to elastic line loading and to Coulomb rheology. In the hanging wall of the MAT, a deep high-stress wedge base builds a steep critical slope. In the footwall, the foredeep response is fast subcritical growth by progradation and blind thrusting on a low-stress decollement. Interaction is maintained by out-of-sequence renewal of movement on the MAT.

Recent developments in Carboniferous geology: a critical review with implications for the British Isles and N.W. Europe

Abstract: Significant increases in our understanding of Carboniferous geological and geographical processes, including plate tectonics, palaeomagnetism, climatology and sea level changes have occurred in recent years. Further advances will increasingly depend on the accurate determination of radiometric ages for the boundaries of the major Carboniferous stratigraphie subdivisions. The recent 39 Ar/ 40 Ar dating of sanidines from European Silesian tonsteins holds out great hopes that structural, igneous and metamorphic events dated by radiometric methods can be better correlated with stratigraphie events defined by goniatite zonation. Palaeomagnetic and tectonic studies in the European Hercynides have established that the Upper Palaeozoic geological evolution of the British Isles took place to the north (present coordinates) of an active micro-plate collision zone along the Galician-Brittany-Massif Central line. Lithospheric stretching of the British/Irish Hercynian ‘foreland’ in the Lower Carboniferous was followed by a belt of north-migrating crustal shortening which disrupted the thermal sag phase of extensional subsidence in northern Britain from Westphalian C times onwards. Backstripped subsidence curves for north British Carboniferous basins indicate that subsidence may have occurred in response to lithospheric thinning of up to 50%. The proposal that there was crustal extension and limited seafloor spreading between Greenland and Scotto-Scandinavia along the Rockall/Faroes line during Carboniferous times is discussed and it is suggested that strike-slip tectonics, known to have been active in Maritime Canada may have played a more important role. Radiometric studies of detrital zircons reveal that the nature of the sourcelands for the huge amounts of Carboniferous detritus in the northern British Isles changed little during the course of the period. They were dominated by outcrops of post-Archaean sediments, minor Archaean basement and abundant Caledonian granitoids with little evidence for Proterozoic crustal growth in the hinterlands. A combination of Mid-Carboniferous climatic change, to a more humid regime, and granite/gneiss terrane unroofing, substantially explains the flushing-out of huge amounts of feldspathic detritus in the Namurian. This Carboniferous climatic change itself must have been influenced by the growth of the E-W Hercynian mountain chain and the accompanying fusion of Gondwanaland with Pangea. Some palaeomagnetic evidence also exists for latitudinal shift at this time. Perhaps the most important influence was the early Namurian expansion of the great Gondwanan ice centre. Waxing and waning of this on a Milankovich time scale dominated Silesian sea level changes and facies evolution. Many late Dinantian and Silesian ‘minor’ sedimentary cycles are probably of glacio-eustatic origins, but there seems little evidence that supposed Dinantian and Namurian mesothemic cycles have such an origin. There are, in fact, increasing doubts as to the actual existence of these particular cycles.

Late Archaean terrane accretion in the Godthåb region, southern West Greenland

Abstract: Tectono-stratigraphic terranes—fault-bounded blocks of the Earth's crust characterized by a geological history distinct from that of adjacent terranes1—are now widely recognized in Phanerozoic and Proterozoic orogenic belts, and their present configurations are thought to result from plate-tectonic processes similar to those in operation today2–4. There has been much debate about whether such processes operated in Archaean times5,6. Here we present new data from the Archaean high-grade gneiss complex of the Godthab (now Nuuk) region of southern West Greenland, which show that discrete tectonic units recognized in the south of the region7 can be more widely identified. At least four terranes are recognized, each of which evolved separately until their juxtaposition in the late Archaean. This suggests that at least some Archaean high-grade gneiss complexes may resemble more recent orogenic belts, formed by plate-tectonic processes.

The Structure of the 1985 Tibet Geotraverse, Lhasa to Golmud

Abstract: The structures of Tibet were generated during the accretion on to the Asian plate, firstly of the Qiangtang Terrane during the Triassic, then the Lhasa Terrane during the Jurassic--Cretaceous and finally the Indian continent during the Palaeogene. The southern Kunlun mountains show intense deformation associated with the accretion of deep water sediments on to an active plate margin. The deformation was essentially by footwall propagation of thrusts, though there was pronounced out-of-sequence thrusting with the deformation of basins above the main thrust zone, and the back steepening and backthrusting of earlier structures. The Jinsha Suture probably represents the southern edge of this zone. The Banggong Suture between the Qiangtang and Lhasa Terranes is characterized by pre-collisional ophiolite obduction for over 100 km to the south across the Lhasa Terrane, plus local intense intracratonic deformation of parts of the Lhasa Terrane. However, for this collision there is now very little evidence for intense deformation along the line of the suture and the Qiangtang Terrane itself remained only weakly deformed throughout. Post--Middle Cretaceous, pre-Tertiary deformation of the Lhasa region produced upright- to north-verging folds which decrease in intensity northwards. They may have been formed at the margin of the Gangdise batholith, or they may have originated from early collisional phases along the line of the Indus--Zangbo Suture. However this deformation is approximately synchronous with the more intense deformation of the Xigatse flysch on the accretionary prism and is therefore probably subduction-related, predating collision. Tertiary deformation is relatively widespread across Tibet, producing SSE-directed thrusts across the Fenghuo Shan region of the Qiangtang Terrane and across the northern part of the Lhasa Terrane. Several hundred kilometres shortening can be estimated to have occurred during this deformation, probably reworking older Mesozoic structures. However this shortening is insufficient to provide all of that estimated from palaeomagnetic work or from a study of displacement rates of the Indian plate, and much of the displacement of India into Asia during the Tertiary must be taken up on strike-slip faults in Tibet or on thrusts and strike-slip faults in central Asia north of the Tibetan Plateau. The Tertiary shortening cannot account for all the thickening of the Tibetan crust.

Crustal structure of the Grenville front and adjacent terranes

Abstract: Under the auspices of the Great Lakes International Multidisciplinary Program on Crustal Evolution, approximately 320 km of deep seismic reflection data were collected in Lake Huron along a profile that extends east from the Manitoulin terrane across the Grenville front to the interior of the Grenville orogen. The Manitoulin terrane is characterized by a series of gently east-dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer. Imaged by strong reflections at the western end of a spectacular band of moderately east-dipping reflections, the Grenville front clearly truncates Manitoulin terrane structures to the west. These data are interpreted in terms of a speculative two-stage model involving (1) creation of a major decollement during northward collision of an allochthonous terrane with the southern Superior cratonic margin (1.83-1.89 Ga; Penokean orogeny) and (2) northwest-directed stacking of microterranes at the southeastern margin of the craton, followed by crust-penetrating ductile imbrication under high-pressure-high-temperature conditions leading to the ramping of deeply buried rocks to the near surface (1.0-1.3 Ga; Grenvillian orogeny).

Single zircon dating constraining the maximum age of the Barberton Greenstone Belt, southern Africa

Abstract: One of the central issues in early Precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high-grade gneiss terranes. The crucial question, in most cases, is which of the two is older. We report zircon ages, using the new single grain evaporation technique, from a metaquartzite and a felsic volcanic flow of the Onverwacht Group in the Barberton greenstone belt (BGB), South Africa and Swaziland, that constrain the age of this basal part of the greenstone sequence between 3451 ± 15 and 3438 ± 6 Ma. This is almost 100 Ma younger than previous age estimates assigned to the Onverwacht and agrees well with recent Sm-Nd, Pb-Pb and zircon ion-microprobe dating. Tonalitic orthogneisses of the Ancient Gneiss Complex (AGC) adjacent to the BGB have ages up to 3644 ± 4 Ma and are thus demonstrably older than the greenstones; also, they cannot be derived from them by partial melting as has been proposed previously. This age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the Onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the BGB and suggest that continental crust was either below or nearby.

Triassic – Jurassic rifting and opening of the Atlantic: An overview

Abstract: Events leading to the breakup of the Pangean plate and evolution of the Atlantic passive margins are recorded in the rock record of more than 40 offshore and onshore Late Triassic – Early Jurassic synrift basins that formed on the Variscan – Alleghanian orogen. The record shows that rifting took place along low-angle detachment faults, giving rise to half-grabens along a conjugate set of lower and upper plate margins that are noteably asymmetric. The American plate was marked by a broad belt of marginal plateaus with many northeast-trending detrital basins that were linked to eachother by transfer faults and displaced by cross faults. The Moroccan plate, on the other hand, was marked by few broadly subsiding evaporite basins. Typically each half-graben on the American plate was bordered by a hinged margin and one major basin-bounding fault, which delineated the surface trace of synthetic or antithetic listric faults on a seaward-dipping detachment zone. The American plate (during the Late Triassic) was dominated by high relief with high-altitude fluvial-lacustrine basins along the western part of the orogen, and by low-relief sea-level evaporite basins proximal to the future spreading axis. During detachment faulting, in the Late Triassic – Early Jurassic, the lower plate must have been uplifted isostatically into a broad central arch that migrated seaward, as the load of the overlying upper plate continued to be reduced by erosion and listric faulting. This had the consequence of elevating Late Triassic marine strata that lay near the proto-Atlantic axis. During the Lias, these marine basins were eroded and their strata reworked and transported landward toward the onshore basins of Morocco and North America. The topographic reversal is thought to reflect the easterly migration of upwelling asthenosphere, in response to tectonic thinning along the newly forming margin. It was a time of major crustal thinning with development of the postrift unconformity (COST G-2 cores), and adiabatic decompression on the upwelling asthenosphere. Whereas the earliest melts yielded off-axis alkaline-rich volcanics (as in Morocco), subsequent melts, which were derived from later partial melt derivatives, were tholeiitic (as in the Palisades). As the upwelling asthenosphere migrated eastward in response to tectonic thinning, the ‘abandoned’ rift-stage crust cooled and subsided, thereby ushering in the drifting phase of the margin. The Moroccan plate, by contrast, was a broad region of low relief throughout most of the Triassic and Liassic. It was distinguished by few detrital basins, and almost all of these occurred along the South Atlas fracture zone, as Triassic strike-slip basins in the High Atlas. Except for the offshore Essaouira basin, which is a seaward extension of the High Atlas Argana basin, the Moroccan margin (unlike the American) consists of few documented Triassic – Liassic rift basins. Triassic rifting of the Middle Atlas (e.g. at Bab-Bou-Idir and Berkane) broke the orogen into the Oranian and Moroccan mesetas, and is manifested by a thick carbonate sequence. The majority of the intraplate basins of North African occur on the mesetas, and are nonrift; typically they contain nonclastic, marine and fresh water evaporites of Liassic and younger strata that formed in broad, shallow, drift-type basins on a generally subsiding terrane of low relief, near the very end of synrift time.

Sedimentology, palaeoecology and palaeoenvironmental evolution of the 1985 Lhasa to Golmud Geotraverse

Abstract: Vertical and horizontal measurements of almost 30 km of sections were made along the Geotraverse route at 113 localities ranging in age from Ordovician to Tertiary. Over 280 palaeocurrent measurements were taken and 200 thin sections were studied. Ordovician strata occur only in the Kunlun Terrane, w here thick metamorphosed sequences of elastics and carbonates occur. These are tentatively interpreted as platform margin and slope deposits. During Carboniferous times in the Kunlun Terrane transgressive late-Dinantian marine limestones with tropical to subtropical Eurasian reefoidal faunas overlie fluviatile redbeds derived from an unroofed orogenic belt. The Lhasa Terrane contains shelf basin elastics with low -diversity faunas succeeded by thick late Carboniferous/early Permian glacio-marine mixtites deposited by iceberg meltout. Permian carbonate ramp and shelf facies with reefoidal developments occur over both the Lhasa and Qiangtang Terranes, with coal-bearing elastics and fluviatile redbeds also occurring in the latter.

Precise U–Pb zircon dates from the Avalon Terrane in Newfoundland

Abstract: The following new U–Pb dates are provided for zircons from volcanic and intrusive rocks of the Avalon Terrane of Newfoundland: Burin Group ophiolite (Wandsworth pegmatitic gabbro), ; Marystown Grou...

The Ophiolites of the Tibetan Geotraverses, Lhasa to Golmud (1985) and Lhasa to Kathmandu (1986)

Abstract: Ophiolite belts are found in Tibet along the Zangbo, Banggong and Jinsha River Sutures and in the Anyemaqen mountains, the eastern extension of the Kunlun mountains. Where studied, the Zangbo Suture ophiolites are characterized by: apparently thin crustal sequences (3-3.5 km); an abundance of sills and dykes throughout the crustal and uppermost mantle sequences; common intraoceanic melanges and unconformities; and an N-MORB petrological and geochemical composition. The ophiolites probably formed within the main neo-Tethyan ocean and the unusual features may be due to proximity to ridge-transform intersections, rather than to genesis at very slow-spreading ridges as the current consensus suggests. The Banggong Suture ophiolites have a supra-subduction zone petrological and geochemical composition -- although at least one locality in the Ado Massif shows MORB characteristics. However, it is also apparent that the dykes and lavas show a regional chemical zonation, from boninites and primitive island arc tholeiites in the south of the ophiolite belt, through normal island arc tholeiites in the central belt to island arc tholeiites transitional to N-MORB in the north. The ophiolites could represent fragments of a fore-arc, island arc, back-arc complex developed above a Jurassic, northward-dipping subduction zone and emplaced in several stages during convergence of the Lhasa and Qiangtang terranes. The ophiolites of the Jinsha River Suture have a N-MORB composition where analysed, but more information is needed for a proper characterization. The Anyemaqen ophiolites, where studied, have a within-plate tholeiite composition and may have originated at a passive margin: it is not, however, certain whether true oceanic lithosphere, as opposed to strongly attenuated continental lithosphere, existed in this region.

Proterozoic Lithospheric Evolution

Abstract: Recent years have seen significant new results on the evolution of Proterozoic terranes, and many of these resulted from new techniques, new concepts, and increasing multi-disciplinary research. In 1981, Working Group (WG) 3 of the International Lithosphere Program (ILP) was established. Its principal tasks were clarification of the origin and evolution of the Proterozoic rocks and of the magmatic, metamorphic, deformational processes involved in their evolution; elucidation of the basic patterns of the structure and composition of the Proterozoic lithosphere and their implicatin for the geochemical differentiation of mantle and crust and the growth of continents; and characterization of Proterozoic tectonic regimes and evaluation of the role of plate tectonics in Proterozoic orogenesis and lithospheric evolution.

The deep structure of northern England and the Iapetus Suture zone from BIRPS deep seismic reflection profiles

Abstract: The NEC deep seismic reflection profile images structures in the lower crust which represent the Iapetus Suture zone beneath southern Scotland and northern England. On the basis of reflectivity characteristics of the lower crust and reflection Moho we recognize four different crustal zones which correspond to four crustal/terrane types: Midland Valley (zone A), a sub-continental subduction complex (zone B ), Lake District (zone C ) and Midland Platform (zone D). The junctions between each of these zones are interpreted as being tectonic. A and B originate from the northern continental margin of the Iapetus Ocean and are separated from C and D (which are derived from a southern continent) by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust. The structure responsible for these reflections does not displace the reflection Moho. North of the AB/CD junction, the base of zone B (and base of the reflective crust) is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km. These reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust. At their northerly limit the deepest of the two parallel reflectors transects the Moho to a depth of 6-7 km into the uppermost mantle. We suggest that their termination delimits the northerly extent of a decollement or shear zone at the present continental Moho. The shear zone was active during the late stages of Caledonian collision when mantle, originally from beneath the southern continent, underthrust old and newly created crust of the collision zone.

Plutonic rocks of the 1985 Tibet Geotraverse, Lhasa to Golmud

Abstract: Two large east-trending granitic batholiths are exposed on the plateau of Central Tibet. In the southern Lhasa Terrane , north of the Zangbo Suture, the Gangdise Belt is a calc-alkaline composite batholith dominated by monzodiorites, tonalites, granodiorites and monzo granites. Trace elements indicate that strongly fractionated melts were emplaced at an active continental margin; deeper crustal levels of the batholith are exposed in the crustally -derived Nyainqentanglha orthogneiss. A long the northern edge o f the plateau , a syn-tectonic calcic to calc-alkaline suite of tonalites, granodiorites and monzo granites forms the Kunlun batholith with post-tectonic granites emplaced to the south. The Kunlun intrusions are derived from anatexis of a garnet-bearing source at in termediate crustal depths above an active or recently active continental margin . Between these two batholiths, a bimodal suite of metaluminous tonalite -grano-diorite and peraluminous two-mica granite is exposed in the northern Lhasa Terrane, indicative of melting both in the upper crust and at deeper levels in the crust or upper mantle. This association suggests a post-collision setting.

An expanded view of Jurassic orogenesis in the western United States Cordillera: Middle Jurassic (pre-Nevadan) regional metamorphism and thrust faulting within an active arc environment, Klamath Mountains, California

Abstract: Basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic-ultramafic intrusive complexes delineate a Middle Jurassic arc terrane within the Klamath Mountain province of northern California. Exposures of the supracrustal volcaniclastic rocks are restricted to a single fault-bounded terrane, but the deeper level intrusive complexes were emplaced into most, if not all, the pre-Late Jurassic terranes of the Klamath Mountain region. The pre-Late Jurassic terranes thus constitute the basement of the Middle Jurassic arc. U-Pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus K-Ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of ∼177-159 Ma. The active arc and its basement were imbricated by a compressive deformational event, the signature of which included thrust faulting, isoclinal folding, and regional metamorphism. Several diverse lines of evidence, including K-Ar dating of metamorphic rocks, crosscutting relations of dated intrusive complexes to thrust faults, and U-Pb dating of synmetamorphic intrusive complexes, establish a distinctly pre-Nevadan Middle Jurassic age (ongoing at ∼169 Ma and over by at least 161 Ma) for this compressive deformational episode. "Outboard" and structurally beneath the Middle Jurassic arc and its basement are several terranes that collectively comprise the western Jurassic belt. These terranes were deformed and regionally metamorphosed during the Late Jurassic Nevadan orogeny, which occurred within the time interval of ∼157-150 Ma, as Upper Jurassic plutons with 150- to 142-m.y.-old zircon ages have contact aureoles that overprint the Nevadan fabric, and the ∼157-m.y.-old Rogue Formation was deformed in the Nevadan event. The Middle and Late Jurassic compressive deformational events were thus distinct and separated by as much as 15-20 m.y. The relation between Middle and Late Jurassic magmatism and deformation suggests that the Klamath Mountain province records the evolution of a considerably long-lived arc system that evolved above an east-dipping subduction zone. In addition, we suggest that this are system may represent an oceanic continuation of the long-recognized early Mesozoic arc terrane of the western U.S. Cordillera.

Pennsylvanian pluton stitching of Wrangellia and the Alexander terrane, Wrangell Mountains, Alaska

Abstract: A quartz monzonite-syenite-alkali granite plutonic complex in eastern Alaska crosscuts the contact of the Alexander terrane and Wrangellia and intrudes the basement rocks of both terranes. Zircon U-Pb data indicate an intrusion age of 309 {plus minus} 5 Ma (Middle Pennsylvanian) for the pluton, and {sup 40}K-{sup 40}Ar age for hornblende separates indicate cooling to about 450 C during Middle Pennsylvanian-Early Permian time. The new field relations and age data demonstrate the Wrangellia and the Alexander terrane were contiguous during the Middle Pennsylvanian. This conclusion provides an important new constraint on paleogeographic reconstructions of the northwest Cordillera, and necessitates reassessment of stratigraphic and paleomagnetic data that were cited as evidence that the terranes evolved separately until the late Mesozoic.

Tectonostratigraphic Terranes of the Circum-Pacific Region

Abstract: Have you always wondered where the Tujunga, Baldy, and Cortez terranes might be located today, let alone during the Cretaceous or early Tertiary? This book may provide the answer, because in a little less than 600 pages for $32, which includes a marvelously produced color map of the entire Circum-Pacific region, one can read almost everything one wants to know about Earth's “ring of fire” and its displaced or suspect terranes. The printing, proofreading, illustrations, and references are all of the highest caliber, and the book is handsomely produced indeed. In page-by-page reading, I found maybe five typographical errors, but I will spare you the details. The contents of the book are divided into five parts, comprising principles or applications of terrane analysis and four unequally long parts on the four quadrants of the Pacific coasts. The northeast quadrant includes Alaska, the Canadian Cordillera, the U.S. coastal and Rocky Mountain belts, and Mexico; the northwest includes Kamchatka, northeast Asia, China, Japan, Taiwan, and the Philippines; the southwest section has articles on Australia, Malaya, Indonesia, New Zealand, and Antarctica; and the southeast comprises the Andes from Colombia to southern Chile. The book offers introductory text for beginning students of terrane analysis, as well as plenty of useful details and data for the expert who needs a handy reference volume. Subject matter or emphasis ranges from hydrocarbon generation in marginal basins, biogeography, paleomagnetism, geochronology, and structural and metamorphic aspects to stratigraphy and shows how the entire discipline of geological sciences is contributing to terrane analysis. There is literally something here for everyone in solid Earth science.