High-rate flexure of the East Greenland volcanic margin: constraints from 40Ar/39Ar dating of basaltic dykes
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Continental Margin
Passive margin
Continental Margin
Convergent boundary
Obduction
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We present the results of 44 new heat flow stations which were taken in 1984 and 1989 in profiles across the Goban Spur and Galicia Bank continental margins (NW Atlantic Ocean). Simple extensional models indicate that the heat flow across both these Early Cretaceous rifted margins should increase from values of 45–50 mW/m 2 over oceanic crust to 65–80 mW/m 2 on the continents. The rate of this increase should help to constrain the mechanism (simple versus pure shear) by which the upper, more radiogenic continental crust is thinned. Measurements across Goban Spur increase from values of 40–45 mW/m 2 over oceanic crust to 50–55 mW/m 2 near the continental shelf. They follow the predicted trend for pure‐shear rifting, but only if the value of upper crustal radiogenic heating is low (1–2 μW/m 3 ). Otherwise, they would require the upper crust to thin more rapidly than the total crustal thickness, as with a lower plate, simple‐shear margin. Measurements across Galicia Bank show a very different pattern, with similar values over oceanic crust but much lower values (30–35 mW/m 2 ) nearer land. This is difficult to reconcile with any simple, single rifting event but is more compatible with an origin as a pure‐shear or lower plate rather than upper plate margin. We also note that oceanic values of heat flow require asthenospheric temperatures 100°C lower than normal for both margins. This indicates that the triple junction in existance between these margin segments during the breakup of Iberia, Europe, and North America was not the site of a major mantle plume.
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Metamorphic core complex
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Leg 149 of the Ocean Drilling Program drilled a transect of basement holes across the ocean‐continent transition (OCT) of the rifted continental margin off the west coast of Portugal. The principal objective was to sample a series of basement highs beneath the Iberia Abyssal Plain, which were thought to span the transition from the oldest oceanic crust to a peridotite ridge separating oceanic from extended continental crusts to a broad region of highly‐extended continental crust. Drilling demonstrated that the late rift‐stage exposures of peridotite at the seafloor are more laterally extensive than previously thought; peridotite was found on two highs 19 km apart. From the region expected to be extended continental crust, we cored 57 m of metagabbro basement whose continental or oceanic affinities are not yet clear. This rock may be Hercynian in age, and therefore part of the continental crust that was rifted to form this margin, or it may have been emplaced, uplifted, and exposed during rifting some 125 m.y.a. At our most landward site, Tithonian sediments, deposited about 20 m.y. before the onset of seafloor spreading on this segment of margin, are interpreted to overlie extended continental crust. The results of the leg will shed new light on the mechanisms by which upper mantle and perhaps lower crustal rocks are exposed at the seafloor during the late stages of continental breakup.
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Peridotite
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Continental Margin
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Rifted margins are created as a result of stretching and breakup of continental lithosphere that eventually leads to oceanic spreading and formation of a new oceanic basin. A cornerstone for understanding what processes control the final transition to seafloor spreading is the nature of the continent-ocean transition (COT). We reprocessed multichannel seismic profiles and use available gravity data to study the structure and variability of the COT along the Northwest subbasin (NWSB) of the South China Sea. We have interpreted the seismic images to discern continental from oceanic domains. The continental-crust domain is characterized by tilted fault blocks generally overlain by thick syn-rift sedimentary units, and underlain by fairly continuous Moho reflections typically at 8–10 s twtt. The thickness of the continental crust changes greatly across the basin, from ~20 to 25 km under the shelf and uppermost slope, to ~9–6 km under the lower slope. The oceanic-crust domain is characterized by a highly reflective top of basement, little faulting, no syntectonic strata and fairly constant thickness (over tens to hundreds of km) of typically 6 km, but ranging from 4 to 8 km. The COT is imaged as a ~5–10 km wide zone where oceanic-type features directly abut or lap on continental-type structures. The South China margin continental crust is cut by abundant normal faults. Seismic profiles show an along-strike variation in the tectonic structure of the continental margin. The NE-most lines display ~20–40 km wide segments of intense faulting under the slope and associated continental-crust thinning, giving way to a narrow COT and oceanic crust. Towards the SW, faulting and thinning of the continental crust occurs across a ~100–110 km wide segment with a narrow COT and abutting oceanic crust. We interpret this 3D structural variability and the narrow COT as a consequence of the abrupt termination of continental rifting tectonics by the NE to SW propagation of a spreading centre. We suggest that breakup occurred abruptly by spreading centre propagation rather than by thinning during continental rifting. We propose a kinematic evolution for the oceanic domain of the NWSB consisting of a southward spreading centre propagation followed by a first narrow ridge jump to the north, and then a younger larger jump to the SE, to abandon the NWSB and create the East subbasin of the South China Sea.
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Seafloor Spreading
Oceanic basin
Basement
Convergent boundary
Passive margin
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Abstract During extension, the continental lithosphere thins and breaks up, forming either wide or narrow rifts depending on the thermo-mechanical state of the extending lithosphere. Wide continental rifts, which can reach 1,000 km across, have been extensively studied in the North American Cordillera and in the Aegean domain. Yet, the evolutionary process from wide continental rift to continental breakup remains enigmatic due to the lack of seismically resolvable data on the distal passive margin and an absence of onshore natural exposures. Here, we show that Eocene extension across the northern margin of the South China Sea records the transition between a wide continental rift and highly extended (<15 km) continental margin. On the basis of high-resolution seismic data, we document the presence of dome structures, a corrugated and grooved detachment fault, and subdetachment deformation involving crustal-scale nappe folds and magmatic intrusions, which are coeval with supradetachment basins. The thermal and mechanical weakening of this broad continental domain allowed for the formation of metamorphic core complexes, boudinage of the upper crust and exhumation of middle/lower crust through detachment faulting. The structural architecture of the northern South China Sea continental margin is strikingly similar to the broad continental rifts in the North American Cordillera and in the Aegean domain, and reflects the transition from wide rift to continental breakup.
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Detachment fault
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The south Atlantic passive margin formed during the Late Jurassic - Aptian due to rifting of South America and West Africa. The passive margin in offshore Gabon depicts excellent examples of structures in magma-poor passive margins. This chapter presents a map of West African passive margin near Gabon. It highlights the upper continental crust (cc), the lower continental crust (lcc), the continental upper lithospheric mantle (m), and the top of the continental basement (b). The map present in the chapter also highlights the sedimentary layers (s1-s3), the proto-oceanic crust (poc), the magmatic rock-dominated rock suites 1 and 2 (mrsl and mrs2), and the syn-rift strata (r). The chapter also presents an uninterpreted dip-oriented reflection seismic profile through the rifted continental margin of Gabon. The breakup process was delivered in the crust first-mantle second (magma-poor) scenario.
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Basement
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