Thinning Of Continental Crust Research Articles

Geochemistry of the Early Paleozoic Baiyin Volcanic Rocks (NW China): Implications for the Tectonic Evolution of the North Qilian Orogenic Belt

Abstract The Qilian Mountains in NW China comprise the North Qilian Orogenic Belt, Central Qilian Block, and South Qilian Orogenic Belt. The North Qilian Orogenic Belt consists of the Northern and Southern terranes separated by a volcanic rock belt. This belt is composed mainly of felsic and mafic volcanic rocks. Volcanic rocks in the Baiyin area of the eastern part of the belt include rhyolites, rhyodacite, andesitic basalts, and basalts. New zircon U‐Pb isotopic data yield a crystallization age of ca. 445 Ma for the rhyolite, 30 m.yr. younger than the associated basalts. The mafic volcanic rocks are relatively enriched in Th, Sr, and light rare earth element with (La/Yb)N ratios ranging from 4.2 to 5.6 and LaN ranging from 40 to 49, and depleted in high field strength elements, with high Th/Nb ratios (0.9–1.3). These features together with their eNd(T) values (−1.4 to +3.1) are consistent with a subduction‐related origin, most likely in a mature island arc or an arc built on thin continental crust in an...

The crustal structure and regional development of the Irish Atlantic margin region

During the past decade, a suite of wide-angle seismic reflection/refraction profiles has been shot in the Porcupine, Rockall and Hatton basins, as well as across the Hatton continental margin. Integration of the wide-angle seismic data with normal-incidence reflection profiles and with gravity and magnetic data reveals a clear picture of the regional crustal and upper mantle structure, and of the large-scale sedimentary geometry of the Late Palaeozoic–Cenozoic basins in the area. The region contains a set of large sedimentary basins resting upon variably thinned continental crust. The crust beneath the basin-bounding structural highs is typically around 25km thick, with the crust beneath the basins being as thin as 6km. The Porcupine Basin contains up to 10 km of sediments. Up to 7km of sedimentary strata are indicated in the Rockall Basin, with 4km of sediment in the Hatton Basin. Several seismically distinct layers, with P-wave velocities of 1.8–5.3 kms −1 , were identified above the basement in the sedimentary basins. A significant variability in thickness of the sedimentary layers is seen on the wide-angle data. These reflect the presence of a number of older Mesozoic rift sedimentary basins. The wide-angle data reveal marked topographic irregularity at the base of the sedimentary succession in the Rockall Basin, interpreted as the result of extensive multi-phase Mesozoic rifting. The Moho beneath the Rockall Basin shows a clear asymmetry, with a steeper gradient along the eastern margin of the basin. The continent–ocean boundary west of the Hatton High has a significant underplated and volcanic component, while that at the southern margin of the Rockall Basin shows evidence of a greater transpressive component. The nature of the lower crust appears to exert a controlling influence on the location of crustal separation. In areas where thinning is accommodated by upper and middle crustal attenuation, the strength of the upper crust (and underlying brittle mantle) has served to preserve the integrity of the continental crust and inhibit seafloor spreading. However, where the lower crust is thinned significantly, as beneath the Hatton continental margin, crustal separation has taken place even though the upper and middle crust remains relatively thick.

Provenance history of a Late Triassic‐Jurassic Gondwana margin forearc basin, Murihiku Terrane, North Island, New Zealand: Petrographic and geochemical constraints

The Murihiku Terrane in the North Island was a forearc basin adjacent to a volcanic arc along the eastern margin of Gondwana during the Mesozoic. The rocks that infill the basin are mainly volcaniclastic sandstones and mudstones, often turbiditic, with sparse shellbeds, rhyolitic tuffs, carbonaceous sandstones, plant beds, concretionary horizons, and rare thick granitoid‐rich conglomerates. Petrographic studies of the rock fragments in the sandstones show that andesites are the dominant lithic type, but there is a wide range of other lithologies, including dacites, rhyolites, ignimbrites, granitoids, quartzofeldspathic mica schists, rare amphibolites, and reworked mudstones and sandstones. The sandstones are texturally and mineralogically immature and suggest deposition relatively close to a source of high relief, undergoing physical rather than chemical weathering in cool‐ to cold‐temperate conditions. Geochemical analyses of 67 whole‐rock volcaniclastic sandstones and siltstones indicate that they were derived from an active and dissected volcanic arc in a convergent margin setting built upon relatively thin continental crust. Modal petrographic data and whole‐rock geochemistry both confirm that there were systematic variations with time in the composition of clastic material being supplied to the basin. From the Late Triassic to Middle Jurassic, there was a decrease in silicic volcanic material, plutonics, and metamorphics, and an increase in the supply of andesitic detritus. This was followed in the Late Jurassic by a broader range of volcanic detritus, varying from basaltic andesite to rhyolite, which may have been caused by progressive extension of the volcanic arc and thinning of the crust, a precursor to the breakup of Gondwana in the Early‐Middle Cretaceous. Comparison with the Southland segment of the Murihiku Terrane in the South Island suggests that there were significant along‐arc source variations, with relatively less silicic but greater andesitic and continental crust contributions in the North Island than in Southland. This may be analogous to the modern Taupo‐Kermadec arc where there is a south—north along‐arc transition from a continental to an oceanic arc.

Regional crustal thickness and precipitation in young mountain chains.

Crustal thickness is related to climate through precipitation-induced erosion. Along the Andes, the highest mountains and thickest crust (approximately 70 km) occur at 25 degrees south, a region of low precipitation. Westerly winds warm passing over the Atacama Desert; precipitation is modest in the High Andes and eastward over the Altiplano. Severe aridity, hence low erosion rates, helps to account for the elevated volcanogenic contractional arc and high, internally draining plateau in its rain shadow. Weak erosion along the north-central arc provides scant amounts of sediment to the Chile-Peru Trench, starving the subduction channel. Subcrustal removal might be expected to reduce the crustal thickness, but is not a factor at 25 degrees south. The thickness of the gravitationally compensated continental crust cannot reflect underplating and/or partial fusion of sediments, but must be caused chiefly by volcanism-plutonism and contraction. Contrasting climate typifies the terrain at 45 degrees south where moisture-laden westerly winds encounter a cool margin, bringing abundant precipitation. The alpine landscape is of lower average elevation compared with the north-central Andes and is supported by thinner continental crust (approximately 35 km). Intense erosion supplies voluminous clastic debris to the offshore trench, and vast quantities are subducted. However, the southern Andean crust is only about half as thick as that at 25 degrees south, suggesting that erosion, not subcrustal sediment accretion or anatexis, is partly responsible for the thickness of the mountain belt. The Himalayas plus Tibetan Plateau, the Sierra Nevada plus Colorado Plateau, and the Japanese Islands exhibit analogous relationships between crustal thickness and climate.

Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3–4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to <1.3 km and exhibits an unusual, highly reflective layering. We propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep-marine sheet flows to form this layering. Subsequent seafloor spreading formed normal, ∼6-km-thick oceanic crust. This interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow-spreading system was established.

Crustal structure of the ocean‐continent transition at Flemish Cap: Seismic refraction results

We conducted a seismic refraction experiment across Flemish Cap and into the deep basin east of Newfoundland, Canada, and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots. The continental crust at Flemish Cap is 30 km thick and is divided into three layers with P wave velocities of 6.0–6.7 km/s. Across the southeast Flemish Cap margin, the continental crust thins over a 90‐km‐wide zone to only 1.2 km. The ocean‐continent boundary is near the base of Flemish Cap and is marked by a fault between thinned continental crust and 3‐km‐thick crust with velocities of 4.7–7.0 km/s interpreted as crust from magma‐starved oceanic accretion. This thin crust continues seaward for 55 km and thins locally to ∼1.5 km. Below a sediment cover (1.9–3.1 km/s), oceanic layer 2 (4.7–4.9 km/s) is ∼1.5 km thick, while layer 3 (6.9 km/s) seems to disappear in the thinnest segment of the oceanic crust. At the seawardmost end of the line the crust thickens to ∼6 km. Mantle with velocities of 7.6–8.0 km/s underlies both the thin continental and thin oceanic crust in an 80‐km‐wide zone. A gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth. Normal mantle velocities of 8.0 km/s are observed ∼6 km below basement. There are major differences compared to the conjugate Galicia Bank margin, which has a wide zone of extended continental crust, more faulting, and prominent detachment faults. Crust formed by seafloor spreading appears symmetric, however, with 30‐km‐wide zones of oceanic crust accreted on both margins beginning about 4.5 m.y. before formation of magnetic anomaly M0 (∼118 Ma).

Magmatic characteristics of the Paleocene Cerro Nevazón region and other Late Cretaceous to Early Tertiary calc-alkaline subvolcanic to plutonic units in the Neuquén Andes, Argentina

Voluminous Late Cretaceous–Early Tertiary calc-alkaline magmatism in the northwest part of the Neuquén province, Argentina, is characterized by a NS-trending belt of olivine basalt-augitic andesite volcanic and amphibole-rich diorite subvolcanic facies that were grouped in the Neuquén-Mendoza volcanic province. Of samples associated with skarn mineralization in the Cerro Nevazón area, two were dated by K/Ar in amphibole and one by Ar/Ar in plagioclase; together, they yield ages between 60.1±1.6 and 56.0±1.7 Ma. The igneous rocks contain normally zoned plagioclase (64–74%), amphibole (magnesiohastingsite and edenite; 15–20%), biotite (5–10%), quartz (4–18%), minor enstatite–ferrosilite (Wo 2.5–3En 71–73Fs 24–25), and augite (Wo 42–43En 42–43Fs 14–15), with accessory magnetite, minor ilmenite, apatite, sphene, and traces of zircon. The fractionation of calcic plagioclase+olivine+pyroxene±magnetite±ilmenite is responsible for the evolution of the parental magmas, which originated in approximately 10% melting of a mantle source contaminated by fluids from the oceanic subducted lithosphere, with sparse or no residual garnet. The gabbro rare earth element patterns are similar to those of the Antuco volcano (37°25′S) and other Quaternary basaltic rocks from the southern volcanic zone (CSVZ) of the Andes (37–41°5′S) emplaced in a relatively thin continental crust (≈30 km). Although the Cerro Nevazón granitoids share some geochemical signatures with the Paleogene Caicayén igneous units, also located in the northwestern region of Neuquén, they have different evolutionary styles, as expressed by the lack of amphibole fractionation and less Cs enrichment in the Caicayén rocks. With similar SiO 2 contents, the Upper Cretaceous–Paleocene Campana Mahuida subvolcanic rocks from the same region are richer in incompatible trace elements (K, Rb, Sr, Ba, Nb, La, Ce, Yb, Th, and U) than either the Nevazón or Caicayén units, which suggests that their parental magmas were contaminated with crustal material. The low Fe 2O 3/FeO ratios in the Nevazón igneous rocks, combined with the presence of ilmenite in most rocks and the low Mg/(Mg+Fe +2) ratios in mafic minerals, suggest that the Nevazón magmas evolved under more reduced conditions than did the Caicayén or Campana Mahuida magmas.

Deep seismic experiments investigate crustal development at continental margin

Important advances have been made in recent years regarding our understanding of the processes involved during continental break‐up. Much of this knowledge has arisen from deep seismic profiles of the Atlantic margins. The continent‐ocean transition (COT) at many locations along the North Atlantic margin consists of a zone of ‘transitional’ crust that lies between thinned continental crust and unequivocal oceanic crust. The origin of this transitional crust is poorly understood, and yet it is likely to provide key information regarding the break‐up process.

Trying to understand Alpine sediments—before 1950

Most Alpine geologists, before and after the 1893–1902 breakthrough of the nappe theory, paid surprisingly little attention to sedimentological phenomena. They were preoccupied with stratigraphic and especially tectonic problems. The Alpine concept of “geosyncline” differed in several ways from the original one. In 1894, Marcel Bertrand suggested a fourfold division of “complexes” (informal lithostratigraphic assemblages) among the sediments of the Alps and of older mountain chains. For the Alps, his classification may be expanded to comprise: I, Late Variscan detritals; II, pre-rift platform sediments; III, syn-rift deposits; IV, post-rift platform and basin sediments; V, starved deep-sea formations on oceanic or thinned continental crust; VI, Bündnerschiefer or Schistes lustrés; VII, Flysch; and VIII, Molasse. The last named is not discussed here. On the European (Helvetic) margin, the processes giving rise to shallowing-upward sequences of condensed glauconite beds, shales and limestones (complex IV) were discussed for a long time. The work of Arnold Heim is particularly instructive. Gustav Steinmann was one of the first to recognize the deep-sea origin of certain rocks, especially radiolarian cherts. The controversy on the bathymetry of the Alpine seas during the oceanic and post-oceanic stages (complexes V–VII) went on until about 1950. Since 1916, the seas and islands around eastern Indonesia were proposed as analogues to the Late Mesozoic paleogeographic pattern of the Alps. Two 18th century scientists deserve a special mention: Moritz Anton Kappeler (M.A. Cappellerius, 1767) provided an actualistic interpretation of ripple marks on steep-standing sandstones, and Johann Jacob Scheuchzer (1718) understood the importance of graded bedding, which he attributed to the action of the Deluge.

On tectonic problems of the Okinawa Trough

The Okinawa Trough is a very active tectonic zone at the margin of the Northwest Pacific and is typical of back-are-rifting at the young stage of tectonic evolution. Many scientists from Japan, China, Germany, France, the U.S.A. and Russia have done a lot of geologic and geophysical investigations there. It is well known that the Okinawa Trough is an active back-are rift with extremely high heat flow, very strong hydrothermal circulation, strong volcanic and magmatic activity, frequent earthquakes, rapid subsidence and rifting, well-developed fault and central graben. But up to now, there are still some important tectonic problems about the Okinawa Trough that require clarification on some aspects such as the type of its crust, its forming time, its tectonic evolution, the distribution of its central grabens, the relationship between its high heat flow and tectonic activity. Based on the data obtained from seismic survey, geomagnetic and gravity measurements, submarine sampling and heat flow measurements in the last 15 years, the author discusses the following tectonic problems about the Okinawa Trough: (1) If the Okinawa Trough develops oceanic crust or not. (2) Is the South Okinawa Trough tectonically more active than the North Okinawa Trough with shallower water and few investigation data on it. (3) The formation time of the Okinawa Trough and its tectonic evolution. The Okinawa Trough has a very thin continental crust. Up to now, there is no evidence of oceanic crust in the Okinawa Trough. The North, Middle and South Okinawa Trough are all very strongly active areas. From 6 Ma B. P., the Okinawa Trough began to form. Since 2 Ma, the Okinawa Trough has been very active.

Anomalous melt production after continental break-up in the southern Iberia Abyssal Plain

Abstract Recent geophysical work and Ocean Drilling Program drilling in the southern Iberia Abyssal Plain have indicated that, in a transition zone up to 170 km wide between thinned continental crust and oceanic crust, the basement consists of serpentinized peridotite mantle with sparse mafic intrusive or extrusive rocks. There is no evidence for the addition of significant magmatic material to the stretched continental crust landward of this zone during the last phase of rifting, whereas seaward of this zone, where the half-spreading rate is about 10 mm a −1 , the crust rapidly reaches a thickness of c. 6 km, which is normal for Atlantic oceanic crust. Models of melt generation during pure shear, finite-duration continental rifting can successfully reproduce the observed absence of significant syn-rift magmatism on, within and beneath the thinned continental crust if the rifting episode is longer than 10–20 Ma. However, for normal mantle potential temperatures, such models predict significant melt generation in the transition zone seaward of the thinned continental crust even for rift durations longer than 20 Ma. Restricted melting beneath the transition zone might be explained partly by lateral heat loss to the adjacent continental lithosphere, by anomalously low mantle potential temperatures at break-up time, or by depth-dependent stretching such that the observed infinite stretching factor for the crust is not representative of the lithosphere as a whole. An additional mechanism for restricted melt production involves a transitional state between the end of continental extension and the onset of steady-state sea-floor spreading, during which mantle upwelling is less focused than at normal oceanic spreading centres.

MAPPING THE PLATE TECTONIC RECONSTRUCTION OF SOUTHERN AND SOUTHEASTERN AUSTRALIA AND IMPLICATIONS FOR PETROLEUM SYSTEMS

Southern Australian breakup history is divisible into three phases. The first phase began with Callovian (c.159–165 Ma) rifting in the western Bight Basin. During the Tithonian (c.142–146 Ma), rifting extended eastwards into the Duntroon, Otway and Gippsland Basins. By the Valanginian (c.130–135 Ma), ocean crust formed between India and western Australia. Structural style in the western Bight changed to thermal subsidence. However, fluvio-lacustrine rift sedimentation continued in Duntroon, Otway and Gippsland until the Barremian (c.115–123 Ma) when these basins also changed to thermal subsidence. The diachronous progression of basin fill types produces a progressive shift in ages of potential source, seal and reservoir intervals along the margin.The second phase began during the Cenomanian (c.92–97.5 Ma) with uplift in eastern Australia, stress reorganisation and divergence of basin development. The Otway, Sorell and Great South Basins formed in a transtensional regime. These tectonics resulted in trap generation through faulting, inversion and wrenching. During the Santonian, oceanic spreading began in the southern Tasman Sea (c.85 Ma). Slow extension caused thinning of continental crust in the Bight and Otway Basins and subsidence into deeper water. Ocean crust formed south of the Bight Basin in the Early Campanian (c.83 Ma) and also started extending up the eastern Australian coast.The third stage in development was caused by Eocene changes to fast spreading in the Southern Ocean (c.44 Ma), final separation of Australia and Antarctica, and cessation of Tasman Sea spreading. These events caused collapse of continental margins and widespread marine transgression. The resultant loading, maturation and marine seal deposition are critical to petroleum prospectivity in the Gippsland Basin.

Evidence for crustal extension and inversion in eastern Tasmania, Australia, during the Neoproterozoic and Early Palaeozoic

The island of Tasmania in southeast Australia consists of a number of stratotectonic elements. The relationships between these elements are largely obscured by younger cover of the Tasmania Basin, which contains extensive dolerite sills that limit the ability of potential field techniques to map basement. Therefore the development of a robust tectonic model for Tasmania has been inhibited. To assist in the development of a tectonic model, a deep seismic reflection program undertaken offshore around the entire island was designed to map the large-scale structures of Tasmania at depth. The airgun seismic energy was also recorded at a number of seismographs deployed across the island, allowing low resolution 3D tomographic imaging. Short reflection profiles were recorded onshore across structures which could not be imaged by the offshore profiling. This paper focuses on eastern Tasmania. In the seismic sections, the Proterozoic basement in the southeast is mostly featureless, except for large rotated blocks with weakly reflective boundary faults, indicating extension of the Tyennan Element by block faulting. The deposition of the sedimentary succession of the Adamsfield–Jubilee Element was related to this extensional event. In the northeast, a reflective lower crust is interpreted to represent thrust slices of previously highly extended continental crust and possibly fragments of oceanic crust. The Early Palaeozoic sedimentary succession of the Northeast Tasmania Element formed across the inverted margin. The apparently complex geology of eastern Tasmania therefore fits into an extensional model where continental extension eventually led to the formation of very thin continental crust and possibly oceanic crust to the east. The extension was probably related to Late Neoproterozoic extension recorded elsewhere in Australia. The region was subsequently shortened, probably in a northeast–southwest direction, with most shortening accommodated in the seismically reflective, probably oceanic part of the crust, and little or no shortening in the block-faulted, and extended continental crust.

Magnetic evidence for slow seafloor spreading during the formation of the Newfoundland and Iberian margins

There is considerable debate concerning the nature and origin of the thin crust within the ocean–continent transition (OCT) zones of many passive non-volcanic continental margins, located between thinned continental and true oceanic crust. This crust is usually found to be underlain by upper mantle material of 7.2–7.4 km/s velocity at shallow depths (1–2 km). It has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading. One of the examples of occurrence of such a crust are the conjugate margins of Newfoundland and Iberia. Here we present an interpretation of magnetic data from these regions to show that their OCT zones are underlain by crustal material formed by slow seafloor spreading (6.7 mm/yr) soon after Iberia separated from the Grand Banks of Newfoundland in the late Jurassic. Similarities in the magnetic anomalies and velocity distributions from these regions with those from the Sohm Abyssal Plain, a region lying immediately south of the Newfoundland Basin and formed by seafloor spreading at a similar rate of spreading, give further support to such an interpretation. The idea that these regions were formed by unroofing of upper mantle during rifting of Iberia from Newfoundland may be likely but the presence of weak magnetic anomalies in these regions, which bear all the characteristics of seafloor spreading anomalies, makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading. The similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the North Atlantic and that of the Labrador Sea suggest that this OCT velocity structure may be the norm rather than the exception across those passive non-volcanic margins where the initial seafloor spreading was slow. Furthermore, the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges.

Local and regional variations in Central American arc lavas controlled by variations in subducted sediment input

The sedimentary section (at DSDP Site 495) on the subducting Cocos Plate has large stratigraphic changes in incompatible elements and element ratios, the result of early carbonate deposition followed by late hemipelagic deposition. Lavas from Central America define both local and regional geochemical trends that reflect the strong influence of the two Cocos Plate sediment units. Element ratios with large stratigraphic variations on the Cocos Plate (e.g. Ba/Th, U/La) define local variations within individual volcanic centers in Central America, indicating that marine stratigraphy controls some geochemical characteristics of the lavas. These local trends can be explained by changing the proportions of hemipelagic sediment input into the magma generation process. These local trends are observed in all the segments of the arc, regardless of the intensity of the slab signature. Regional variations are most clearly seen in element ratios that are nearly constant through the Cocos Plate sediment stratigraphy (e.g. Ba/La, U/Th), suggesting that regional variations reflect differences in the intensity of the flux from the subducting slab. The slab signal is strongest in Nicaragua and along the volcanic front. The signal decreases to the northwest and southeast of Nicaragua and toward the back arc. The large slab signature in the lavas from western Nicaragua occurs in the area with the thinnest continental crust and steepest dip of the slab. The mass flux of incompatible elements into the system is easily estimated, except for elements, like Pb, that have high and variable abundances in the basaltic oceanic crust section. The mass flux of elements out of the system depends on eruption rates, which are variable along the arc and only approximately known. Comparison of input and output fluxes for five different segments of the arc reveals that some elements (K, B, Cs, and Rb) are very efficiently delivered to the volcanoes from the subducted slab. Other elements (Sr, Ba, and U) are returned to the surface with moderate efficiency, whereas some elements (REEs) may come mostly from the mantle wedge with minor slab contribution. The relative order of recycling efficiencies of incompatible elements implies that a hydrous fluid dominates the transfer of material from the slab to mantle.

Oligocene to Middle Miocene evolution of the central Catalan margin (northwestern Mediterranean)

The Catalan margin belongs to the northern sector of the margin that separates the extremely thinned continental crust of the Valencia trough from the undeformed or thickened crust of the Iberian microplate. The integration of the available onshore and offshore data (mapping, stratigraphic sections, seismic profiles and well data) enables the present structure of the central Catalan margin to be identified as a major northwest-directed, thick-skinned thrust sheet compartmentalised in a system of NE–SW oriented horsts and grabens. The geometry and depositional features of the Tertiary successions denote that this structure resulted from two successive evolutionary episodes. The first episode was compressional and gave rise to the emplacement of the Catalan major thrust sheet; the second was extensional, linked to the opening of the Valencia trough. The compressional ( pre-extensional) episode is recorded by Lower to lower Upper Oligocene deposits sedimented in a piggyback basin that developed in the present-day offshore. The pre-rift signature of these sedimentary successions is clear (they show constant thickness, are cut by all the normal faults, etc.) and their facies distribution indicates a mostly southwestward spreading of the sediments. The extensional episode includes the syn-rift and post-rift stages. The syn-rift stage resulted in the present horst-and-graben structure of the Catalan margin and it is recorded by the uppermost Oligocene–lower Burdigalian (Lower Miocene) sediments restricted to the graben troughs. The post-rift stage started in the late Langhian and has continued until the present. It is characterised by the attenuation of the tectonic activity, the sediment spreading over the horsts (most of them were completely overlapped during this stage) and no sediment thickness changes near the faults. The upper Burdigalian and lower Langhian sequences record that the transition between the syn-rift and post-rift stages was not sharp. These units show intermediate features since they concealed some minor faults and overlapped horsts, but were still affected by most of the major normal faults. The onshore and offshore tectono-sedimentary and palaeogeographic evolution show that: (1) the northwestern Mediterranean basin existed during the early to middle Oligocene as an extensional basin in the Gulf of Lions and as a contractional piggyback basin in the northeastern Valencia trough; (2) the extension started in the early Oligocene in southern France and migrated progressively southwestward, affecting the northeast Valencia trough during the latest Oligocene–Aquitanian.

Inversion of an Oligocene volcano-tectonic basin and uplifting of its superimposed Miocene magmatic arc in the Chilean Central Andes: first seismic and gravity evidences

A new structural model is presented for the Cenozoic part of the main range east of Santiago, a chain made up of thick Miocene ignimbrites and andesitic lavas overlying primitive Oligocene volcanoclastics deposited on a thin continental crust. Because the latter (Coya-Machalı́ Formation) locally show tight folding close to the margins of the Miocene volcanics, they had previously been described as deformed prior to construction of the arc. Our field observations, analysed along two profiles (at 33° and 34°15′S), however, support the conclusion hinted at by ongoing geochronological and geochemical studies: deformation of both formations is synchronous and related to Late Miocene inversion of the Coya-Machalı́ Intra-Arc Basin. Uplift of the rigid arc rocks and part of its volcanoclastic basement took place mainly along two northward shallowing boundary structures that seem to share a lower decollement: the eastern Fierro out-of-sequence thrust and a series of western back-thrusts that resemble triangle zones. Seismic data obtained from El Teniente porphyry copper mine (34°S) rockbursts and two gravimetric profiles at the western end of the southern profile are compatible with a decollement located at that locality, 2.3 km under the surface.

Structure of a Volcanic Continental Margin Derived from Ocean-bottom Seismographic Data: The Northern Vøring Margin, off Norway

—This paper presents a crustal model derived from an Ocean Bottom Seismograph (OBS) study along the northern Voring margin off Norway. The profile was acquired to map the crustal structure in the northernmost part of the Voring Basin, and to link crustal models of the Lofoten and central Voring Basin obtained by previous OBS studies. The Voring margin, as well as the Lofoten margin to the north, was created by continental breakup between Norway and Greenland in late Paleocene-early Eocene. The rifting and continental breakup process were accompanied by intense extrusive and intrusive magmatic activities. The OBS data provide the whole crustal structure along the northern Voring margin, in the area where the deep crustal structure cannot be resolved by conventional multichannel reflection data due to sill intrusions in the sedimentary sequence. The shallow part of the crustal model is characterized by up to 10 km thick sediments, a sequence of flood basalts and sill intrusions. The P-wave velocities in the flood basalts and sill intrusions are estimated to 5.0 km/s and 4.7–5.8 km/s, respectively. The model indicates an abrupt thickening of the upper crystalline crust from approx.3 km in the NE, to about 10 km towards the SE, with velocities of 6.0–6.2 km/s. The lower crustal velocities are not well resolved due to lack of clear refraction arrivals from the lower crust. However, the observed amplitude versus offsets are best explained by a model with a change in lower crustal velocities from 6.8 to 7.2 km/s beneath the Bivrost lineament. The modelling infers the presence of a lower crustal reflector beneath the lineament, which represents the landward continuation of the Bivrost lineament. Reflection arrivals from the Moho reveal a Moho depth of 23 km in the middle of the profile and 18– 20 km in the northeastern part of the profile. A 370 km long crustal section from the central part of the Voring Basin to the Lofoten margin, obtained by the results of this study and previous OBS studies, shows a simple thinned continental crust on the Lofoten margin, and a high velocity lower crust underlying an upper crust of varying thickness in the Voring Basin. The transition between these structures is situated beneath the Bivrost lineament in the lower crust, and beneath the basement high about 40 km south of the lineament in the upper crust.

Gravity anomalies and segmentation of the continental margin offshore West Africa

The free-air ‘edge effect’ anomaly at passive continental margins provides constraints on the thermal and mechanical properties of the lithosphere in extensional regions. Like many margins, the edge effect ‘high’ offshore Gabon, West Africa is located at the shelf break in the region of maximum sediment thickness. Gabon differs, however, from other margins in that the edge effect ‘low’ is displaced from over the continental rise to the slope. The origin of the low has been investigated by backstripping the sediments at the margin. We have used the backstrip to calculate the amount of crustal thinning, assuming Airy isostasy. Such a model implies zero strength during rifting. Alternatively, this amount of crustal thinning can be explained by a strong margin and a depth of necking that exactly balances the upward and downward forces that act on the crust during rifting. We have developed a method, which takes into account variations in the strength during rifting and the depth of necking, to compute the thinning directly from the sediment backstrip. The backstrip and the crustal thinning have been used to calculate the rifting gravity anomaly. Differences between the backstrip and the present-day water depth give the geometry of the sediment load and hence, the contribution to the gaavity anomaly of sedimentation. By comparing the combined rifting and sedimentation anomaly to the observed free-air gravity anomaly, we have been able to constrain the flexural rigidity (and equivalent elastic thickness, T e ) of the lithosphere during and after rifting. The best fitting model is one in which the ‘low’ is caused by a 125 km wide zone of relatively thin (i.e. ∼15 km) continental crust which was weak (i.e. 0 < T e < 10k ) during rifting and has remained so since then. We speculate that the weak zone extends for 350–400 km along-strike of the Gabon margin and that passive margins may be highly segmented as regards their long-term strength.

A mantle-derived bimodal suite in the Hercynian Belt: Nd isotope and trace element evidence for a subduction-related rift origin of the Late Devonian Br�venne metavolcanics, Massif Central (France)

In the Brevenne Series (NE Massif Central), a low-grade bimodal association of metabasalts and metarhyolites is exposed, together with intrusive trondhjemite bodies. Zircon U-Pb dating constrains their magmatic emplacement at 366 ± 5 Ma and 358 ± 1 Ma, respectively. The metabasalts are characterized by a distinct enrichment in incompatible elements (e.g. Th and LREE) and positive ɛNdi (from +5 to +8). Combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable cause of the LILE (large ion lithophile element)-enrichment. Rather, a mixing process between a component similar to mid ocean ridge basalts and an enriched end-member with ɛNdi > +5 is suggested. An enriched-mantle source of ocean island basalt affinity is precluded by the relative depletion of high field strength elements, especially Nb which shows negative anomalies in chondrite-normalized patterns. On the contrary, a subduction-related origin for the LILE enrichment would be more consistent. It may be inferred that arc-like melts [enriched in Th and LREE (light rare earth elements) and depleted in Nb, with ɛNdi > +5] were produced through partial melting of a depleted-mantle source, to which a small amount of crustally derived component had been added. The metarhyolites are enriched in LILE, and have a close genetic relationship with the metabasalts, as evidenced by their high ɛNdi (from +4.7 to +6.8). Although the chemical evidence remains ambiguous, it is suggested that fractional crystallization, accompanied by subordinate assimilation, is the petrogenetic process most consistent with the data. The trondhjemites are isotopically distinct from the metarhyolites. Their ɛNdi values (from −1.0 to +2.2) reflect an important contribution of continental crust to their genesis, and disprove their inferred cogenetism with the felsic volcanics. A review of modern environments in which such bimodal suites are exposed, shows that settings involving incipient rifting of a volcanic arc fringing a continental margin, or built upon young, thin continental crust might provide suitable analogues. Geodynamic reconstructions are complicated by subsequent tectonic events which disrupted the initial patterns, and by Mesozoic-Cenozoic sedimentary cover. However, this subduction-related magmatism enlarges the growing body of evidence for southward subduction processes until the Late Devonian during the evolution of the northern flank of the European Variscides. As a general implication, it is suggested that the combined use of the Sm-Nd system with incompatible elements relatively resistant during alteration and low-grade metamorphism (REE, Th, Zr, Nb) may provide diagnostic criteria for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts.