Showing papers on "Continental margin published in 1988"

Formation and evolution of strike‐slip faults, rifts, and basins during the India‐Asia Collision: An experimental approach

Abstract: The processes which have governed the formation and evolution of large Tertiary strike-slip faults during the penetration of India into eastern Asia are investigated by means of plane strain indentation experiments on layered plasticine models. The steady state deformation of plasticine obeys a power creep flow law (e˙=C(T)σn). The stress exponent (n) is between 6 and 9 at 25°C. Uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening, a process observed for strain rates ranging from 3.5×10−5 to 3.6×10−3 s−1. Fault or shear zones form after only 7–10 % bulk strain. Subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine. A series of nine plane strain indentation experiments shows the influence of boundary conditions, as well as that of the internal structure of the plasticine model on the faulting sequence. The ubiquity of strain softening in experimental deformation of a variety of rocks, as well as the widespread occurrence of shear zones in nature suggest that long-term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain. The deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision-induced strikeslip faulting in Asia, particularly to total offsets and rates of movements on the faults. The experiments simulate the evolution of the western ends of the strike-slip faults, which have probably been analogous to trench-fault-fault triple junctions. The experiments also illustrate mechanisms for the formation of extensional basins, such as the South China Sea, North China Basin, and Andaman Sea, near active continental margins. The basins, which appear to absorb terminal offsets along major strike-slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts. The existence of distinct phases of strike-slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous, non-steady state deformation of the continental lithosphere.

Continental tectonics in the aftermath of plate tectonics

Abstract: It is shown that the basic tenet of plate tectonics, rigid-body movements of large plates of lithosphere, fails to apply to continental interiors. There, buoyant continental crust can detach from the underlying mantle to form mountain ranges and broad zones of diffuse tectonic activity. The role of crustal blocks and of the detachment of crustal fragments in this process is discussed. Future areas of investigation are addressed.

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.

Age and Significance of Sequence Boundaries on Jurassic and Early Cretaceous Rifted Continental Margins

Abstract: The age and significance of sequence boundaries on Jurassic to Early Cretaceous rifted continental margins in three ocean basins have been documented. The margins are the Santos basin in the South Atlantic, the Grand Banks in the North Atlantic, and the Beaufort Sea in the Arctic Ocean. Large industry data bases were used for the interpretation of each area. Megasequence boundaries separate the major phases of basin evolution, for example syn-rift and post-rift. Boundaries developed with an average periodicity of 49 m.y. Sequence boundaries define the component parts of each megasequence and developed with a modal periodicity of 10-15 m.y. Out of 27 total boundary ages, most (16) are developed on just one margin. Only two possible age ranges overlap on all three margins. ighty percent of the megasequence boundaries and 50% of the sequence boundaries show a direct causal connection with coeval faulting and/or folding. The rest of the boundaries appear as unstructured surfaces separating transgressive and/or regressive sedimentary wedges and are interpreted to result from changes in the rate of basin subsidence, sediment input, and long-term eustatic sea level. These data do not support theories advocating synchronous worldwide boundary development resulting from periodic, short-term falls in global eustatic sea level. Only in like basins of the same age, with identical subsidence and sediment input rates, are boundaries likely to develop synchronously. Hence, the concept of global synchroneity of sequence boundary development may well be an illusion creat d by the similarity in age of the majority of basins studied. As a result of this study, it seems wise discard the global approach to basin analysis.

Late Cenozoic environments in the Barents Sea

Abstract: We examine the paleoceanography and paleogeography of the Barents Sea during the late Cenozoic glacial regime by seismostratigraphy and core analysis. During the smaller glaciations which probably dominated the first period of the glaciogenic regime (approximately 2.5–0.8 Ma?), glaciomarine processes with sediment influx from the adjacent mainland prevailed. The Barents Sea has been covered by grounded ice sheets several (five to 10) times during the late Cenozoic. We speculate that these glaciations occurred during the last 0.8 Ma. During these larger glaciations the margin of the ice sheet expanded to the shelf edge, and the continental shelf break and slope acted as a depocenter and prograded rapidly. Frequent sediment gravity flows accumulated on the lower continental slope. During interglacials, the Barents Sea was converted to a starved continental margin. Most of the Barents Sea was glaciated during the maximum of the last glaciation (19–16 ka). A two-stepped deglaciation, 16 to 13 ka and 13 to 10 ka, occurred. Incipient Atlantic water reached the western areas about 13 ka and totally replaced the Arctic water by 10 ka. These paleoceanographic events were delayed in the eastern Barents Sea.

Continental Heat-Flow Density

Abstract: Although on the geological time and space scales the geothermal regime of the Earth is, strictly speaking, both a transient and three dimensional phenomenon, on the global scale by far the most important component of heat transfer is radial. Furthermore, over times of the order of the thermal time constant for the crust, that is a few hundred thousand to a few million years, we consider the thermal regime to be quasi-steady state.

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.

Triassic-Jurassic rifting : continental breakup and the origin of the Atlantic Ocean and passive margins

Abstract: PART A. I. Pangaean plate in time and space. 1. Variscan-Alleghanian Orogen (N. Rast). 2. Triassic-Jurassic plate migrations and paleogeographic reconstructions in the Atlantic domain (R. van der Voo). II. Basins: North American and African plates. 3. Triassic-Jurassic rifting and opening of the Atlantic: An overview (W. Manspeizer). Offshore. 4. Evolution of rift basins on the continental margin off southern New England (D.R. Hutchinson, K.D. Klitgord). 5. Early Mesozoic rift basins and the development of the United States middle Atlantic continental margin (R.N. Benson, R.G. Doyle). 6. Extensional tectonics, structural styles and stratigraphy of the Mesozoic Grand Banks of Newfoundland (A.J. Tankard, H.J. Welsink). 7. Biostratigraphy of the COST G-2 well (Georges Bank): A record of Late Triassic synrift evaporite deposition Liassic doming and mid-Jurassic to Miocene postrift marine sedimentation (H.L. Cousminer, W.E. Steinkraus). Onshore. 8. Paleontology and paleoecology of the Newark Supergroup (Early Mesozoic, eastern North America) (P.E. Olsen). 9. Sedimentology of braided-river deposits in Upper Triassic Wolfville redbeds, southern shore of Cobequid Bay, Nova Scotia, Canada (J.F. Hubert, M.F. Forlenza).10. Massive mudstones in basin analysis and paleoclimate interpretation of the Newark Supergroup (J.P. Smoot, P.E. Olsen). 11. Mesozoic tectogenesis: Development and deformation of `Newark' rift zones in the Appalachians (with special emphasis on the Hartford Basin, Connecticut) (J. Zeilinga de Boer, A.E. Clifford). 12. A foreland-type fold and related structures in the Newark rift basin (M. Lucas et al). 13. Petrology of Mesozoic sandstones in the Newark basin, central New Jersey and adjacent New York (M.E. Oshchudlak, J.F. Hubert). 14. Structure and hydrocarbon potential of the Gettysburg basin, Pennsylvania and Maryland (S. Root). 15. Late Triassic and Early Jurassic lacustrine sedimentation in the Culpeper basin, Virginia (P.J.W. Gore). 16. A gravity and magnetic study of the Triassic Richmond basin, Virginia (K.L. Mickus et al). 17. Late Triassic depositional history of the Richmond and Taylorsville basins, eastern Virginia (R. Ressetar, G.K. Taylor). 18. A kinematic model for the evolution of Richmond Triassic basin, Virginia (R. Venkatakrishnan, R. Lutz). 19. Early Mesozoic faults of the northern Gulf Coastal Plain in the context of opening of the Atlantic Ocean (W.A. Thomas). 20. Triassic sedimentation and rifting in the High Atlas (Morocco) (J. Beauchamp). 21. A multiple releasing and restraining stepover model for the Jurassic strike-slip basin of the Central High Atlas (Morocco) (E. Laville). PART B. Evaporite basins. 22. Evaporite deposits of the North Atlantic rift (W.T. Holser et al). Igneous activity. 23. Chemical and physical constraints on petrogenesis and emplacement of ENA olivine diabase magma types (D. Whittington). 24. Eastern North American quartz tholeites: Geochemistry and petrology (J.H. Puffer, A.R. Philpotts).

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.

The continent-ocean boundary at the rifted margin off eastern Canada: New results from deep seismic reflection studies

Abstract: Seismic reflection data were collected and processed to 20 s two-way travel time along four lines which cross the rifted continent-ocean boundary off the Grand Banks region of eastern Canada specifically to examine the origin, age, and nature of this fundamental boundary. This represents the first regional study of its kind. The most important result is the presence of landward dipping reflectors near the foot of the continental slope. These occur where oceanic crust appears to terminate against the continent. We suggest that the dipping reflectors mark the continent-ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary. Sedimentary basins lie just landward of the continent-ocean boundary. Their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting, and this may be an important stage leading to continental breakup. Rift basins formed further landward on the Grand Banks do not exhibit this thinning. Other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent-ocean boundary. Also, the oceanic crust exhibits a complexity of reflections, some of which may be due to compositional zonation during magmatic crystallization. Finally, our results may have important consequences for continental predrift reconstruction, as oceanic crust appears to extend farther landward in the Newfoundland Basin than some recent studies have indicated.

Is average continental crust generated at subduction zones

Abstract: Many crustal estimates suggest that the continents are approximately andesitic in composition However, in recent subduction related environments, andesites are thought to originate by intracrustal differentiation from basaltic parental magmas The net flux from mantle to crust along destructive plate margins is therefore basaltic with low Rb/Sr, and is not readily reconciled with the development of intermediate, high Rb/Sr continental crust Either the continental crust is considerably more mafic than andesite, or the mechanism of crust formation has changed with time, such that a large proportion of the continental crust was formed by processes unlike those active in recent subduction zones It is significant that there is evidence from Archean rocks to support the latter hypothesis

Thrust tectonics of the Dibba zone and the structural evolution of the Arabian continental margin along the Musandam mountains (Oman and United Arab Emirates)

Abstract: In the northern Oman Mountains several major thrust sheets of Tethyan oceanic rocks (Dibba zone) and a 12km+ thick complete ophiolite sequence have been thrust westward onto the depressed Arabian continental margin (Musandam zone) during the Turonian to Campanian (late Cretaceous). Early thrusting was dominantly a downward foreland-propagating sequence as the higher earlier thrust sheets are folded around structurally lower and later culminations. Breached roof thrusts, where imbricate thrusts do not always converge with a major roof thrust but in places actually penetrate up into the overlying duplex, are common at outcrop scale affecting adjacent thrust sheets. Late stage ‘breakback’ or ‘leap-frog’ thrusts cut through the whole pile of duplexes on to previously higher, earlier dulexes. Late thrusting in the Musandam zone affects the deeper, previously autochthonous Permian–Cenomanian shelf carbonate sequence and probably also the pre-Permian basement, thrusting these rocks at least 15 km over previously allochthonous Hawasina rocks. Late stage oblique dorsal culmination collapse along the SE margin of the Musandam mountains has downfaulted Dibba zone rocks up to 2.5 km to the SE along a hinged listric normal fault. Strain is transferred from the NE–SW-trending late Cretaceous Dibba zone to the N–S-striking mid-Tertiary Musandam culmination at a major pivot point or transfer zone SW of the shelf carbonate culmination.

THE NORTH AND NORTH-WEST SPANISH CONTINENTAL MARGIN: A REVIEW < 1 l

Abstract: The west Galicia margin (Spain) provides a model for passive continental margin evolution. During the Meso­ zoic, the continental crust of the margin experienced severa! episodes of extension. The main stage was during the Berriasian-late Aptian interval. The stretching of the lithosphere resulted in a) the thinning of the continental crust, testified by superficial extensional structures (normal faults, tilted fault blocks) and rapid subsidence, and b) the emplacement of mantle rocks (peridotite) at the continental rift axis. The crustal thinning and the final emplacement of peridotite onto the seafloor are explained by uniform, normal, simple shear of the continental lithosphere, following asthenosphere diapirism. The sedimentary response to the rifting process was the rapid de­ position of coarse detrital sediments in the half grabens of the rift. The pre-rift sediments are platform carbonates, and the post-rift sediments are distal turbidites followed by pelagic sediments. To the south of the Bay of Biscay, the north Galicia and Asturias margin evolved as the Galicia passive margin until latest Cretaceous time. During the Paleocene-Eocene interval, it was converted into an active margin as a result of the Eurasian-lberian plate convergence and related southward subduction of the European plate beneath the lberian plate. A marginal trench and a tectonic accretionary prism developed at the plate boundary and the margin was shortened, deformed and eroded, becoming a subduction-related fold belt. Today it is the westward prolongation of the Pyrenean and Cantabric collisional chain. Therefore models accounting for the emerged part of the Pyrenees must also be constrained by offshore data.

Post-breakup stratigraphy of the Kaikoura Synthem (Cretaceous-Cenozoic), continental margin, southeastern New Zealand

Abstract: The stratigraphy and structure of the Kaikoura Synthem (Cretaceous—Cenozoic) is synthesised from available seismic records and well reports on the continental margin of southeastern South Island. Four seismic sequences are distinguished, corresponding to lithostratigraphic groups that can also be recognised nearby on land: Matakea Group (Cretaceous), Onekakara Group (late Cretaceous—Oligocene), Kekenodon Group (Late Oligocene—Miocene), and Otakou Group (Miocene—Recent). These sequences achieve maximum thickness within three sedimentary basins underlying the continental shelf (Canterbury and Foveaux Basins) and slope (Great South Basin). The Great South Basin is separated from the Canterbury and Foveaux Basins across the WaipounamouFaultSystem, anortheasterly oriented zone of faults which controlled rifting and Matakea Group sedimentation along the western edge of a Cretaceous aulacogen now manifest as the Bounty Trough. The Canterbury andFoveaux Basins contain transgressive sequences (Onekakara Group) tha...