Period Of Rifting Research Articles

Uniform processes of melt differentiation in the central Izu Bonin volcanic arc (NW Pacific)

Abstract The intra-oceanic Izu Bonin arc (NW Pacific) has produced a bimodal spectrum of melts with maxima in the basaltic andesitic ( c . 53–54 wt% SiO 2 ) and rhyolitic range ( c . 70–72 wt% SiO 2 ) since arc inception c . 48–49 million years ago. Composition of phenocrysts and accessory minerals from 21 contemporaneous fallout tephras from ODP Site 782A confirm the bimodality and uniformity of the erupted melts. The basaltic andesite melts equilibrated with calcic plagioclase ( c . An 70–95 ), high-Mg# clino- and orthopyroxene and low-Ti titanomagnetite. Dacitic and rhyolitic melts crystallized sodic plagioclase ( c . An 40–60 ), low-Mg# clino- and orthopyroxene, apatite, Ti-rich titanomagnetite in addition to occasional ilmenite and amphibole. The Izu melts are inferred to crystallize at oxygen fugacities between c . 0 to +2.5 log 10 units relative to FMQ, at temperatures between c . 775° and 1100 °C and at pressures between c . 300 and c . 1100 MPa, corresponding to c . 5–35 km lithospheric depth. The compositional uniformity of the tephra layers, which are spaced on average 230 ± 380 ka apart, suggest uniform processes of differentiation since at least c . 42 Ma ago. The tephra record shows no indication of periodic or progressive crustal growth that might correlate with the alternate periods of arc formation, arc rifting or backarc spreading, or would suggest an increasingly efficient ‘crustal filter’ with time. The tephra data tentatively conform to a model where crust grows steadily through intrusions of mafic and evolved melt body batches whereby buoyancy controls the level of solidification. While the tephra compositions demonstrate the uniformity of the processes of melt formation and differentiation through time, the data do not permit the differentiation processes themselves to be constrained. These may comprise fractional crystallization, crustal fusion, fusion of non-peridotitic sub-crustal lithologies, or any combination of these processes.

Late Devonian and Triassic basalts from the southern continental margin of the East European Platform, tracers of a single heterogeneous lithospheric mantle source

In Late Devonian and Early-to-Late Triassic times, the southern continental margin of the Eastern European Platform was the site of a basaltic volcanism in the Donbas and Fore-Caucasus areas respectively. Both volcanic piles rest unconformably upon Paleoproterozoic and Late Paleozoic units respectively, and emplaced during continental rifting periods some 600 km away from expected locations of active oceanic subduction zones.This paper reports a comparative geochemical study of the basaltic rocks, and views them as the best tracers of the involved mantle below the Eastern European Platform. The Late Devonian alkaline basic rocks differ from the calc-alkaline Triassic basic rocks by their higher alkali-silica ratio, their higher TiO2, K2O, P2O5 and FeO contents, their higher trace element contents, a higher degree of fractionation between the most and the least incompatible elements and the absence of Ta-Nb negative anomalies. These general features, clearly distinct from those of partial melting and fractional crystallization, are due to mantle source effects. With similar Nd and Sr isotopic signatures indicating mantle-crust mixing, both suites would originate from the melting of a same but heterogeneous continental mantle lithosphere (refertilized depleted mantle). Accordingly the Nd model ages, the youngest major event associated with mantle metasomatism occurred during Early Neoproterozoic times (∼650Ma).

Geological evolution, regional perspectives and hydrocarbon potential of the northwest Phu Khanh Basin, offshore Central Vietnam

Seismic stratigraphic and structural analyses of the northwest Phu Khanh Basin, offshore Central Vietnam, based on 2-D seismic data, indicate that the initial rifting began during the latest Cretaceous? or Palaeogene controlled by left-lateral transtension along the East Vietnam Boundary Fault Zone (EVBFZ) and northwest–southeast directed extension east of the EVBFZ. Rifting stopped due to transpression during middle Oligocene times but resumed by left-lateral transtension during the Late Oligocene. Thick sequences of lacustrine and alluvial sediments were deposited during the Palaeogene rift periods. The Late Oligocene rifting ended due to inversion, triggered by right-lateral wrenching near the Palaeogene–Neogene boundary. Following the onset of this inversion regional uplift and volcanism took place in the southern half of the study area and contemporaneous subsidence and transgression took place farther north, leading to widespread carbonate deposition. As the right-lateral wrenching decreased during the early Neogene, thermal subsidence and siliciclastic sedimentation became dominant, resulting in the buildup and southward propagation of the shelf slope. Sediment accumulation and subsidence rates increased after the Middle Miocene times due to eastward tilting of Central Vietnam and the adjacent offshore area. Potential direct hydrocarbon indicators (DHIs) in the Phu Khanh Basin include common amplitude anomalies, gas chimney-like features and seafloor gas seeps. In addition, oil seeps are found at Dam Thi Nai, immediately landward of the basin. Geochemical analyses of the oil seeps indicate the existence of at least two early to peak mature source rocks. Maturation modelling, combined with the seismic analysis, suggests the likely presence of oil kitchens 40–50 km downdip in the basin, and a fairly simple oil migration route. The basin is probably charged from lacustrine syn-rift mudstones, humic coals and fore-reef marls. Potential reservoirs are turbidite, lowstand delta, shelf and coastal sandstones, as well as platform and reef carbonates and fractured basement. Various structural and stratigraphic traps formed during the Palaeogene and early Neogene, prior to the main stage of petroleum generation.

New insights into the lithosphere beneath the Superior Province from Rayleigh wave dispersion and receiver function analysis

SummaryWe present new models of shear wave velocity structure of the lithosphere and upper mantle beneath northern and eastern Ontario and surrounding regions. The study area is dominated by the Archean Superior Province, with Proterozoic orogenic belts to the south and southeast. Over the course of ∼3 Ga, the region has been shaped by accretionary and orogenic events, periods of rifting and the influence of a number of mantle hotspots.New data from the broad-band POLARIS/FedNor seismic network, along with permanent stations of the Canadian National Seismograph Network, are used for a seismic study of the crust and upper mantle beneath the region. This article concentrates primarily on a study of the upper mantle, carried out using two-station phase velocity analysis of teleseismic Rayleigh waves. Dispersion curves were measured for 100 two-station paths across the region, of which 30 were analysed to give 1-D path-averaged models of shear wave velocity structure. Dispersion measurements yielded phase velocity data for periods from ∼25 to ∼170 s, allowing us to resolve shear wave velocities to a depth of ∼300 km. The dispersion curves indicate an upper-mantle structure broadly characteristic of continental shield regions, but there are significant variations in the properties of the data across the province.In the central and western Superior Province, inferred lithospheric thicknesses vary between approximately 140 and 200 km. In general, the models are characterized by a well-developed ‘lid’ of high-velocity mantle underlain by a zone of reduced velocity. The highest lid velocities are modelled along a path whose azimuth corresponds to the fast direction of anisotropy resolved through SKS splitting analysis. The far northeast of Ontario has the thickest (∼220–240 km) lithospheric lid. Eastern Ontario has a complex structure that gives rise to a large variation in the structures modelled from the two-station paths. Apparent lithospheric thicknesses range from ∼100 to ∼220 km. This large variation is interpreted to arise from highly heterogeneous mantle structure and/or anisotropy across the Abitibi Greenstone Belt, the Southern Province, and the Proterozoic Grenville Province.Receiver function analysis shows variations in Moho depth and bulk Poisson's Ratio in the Superior Province crust. The crustal thickness varies from ∼34 to ∼44 km through most of the province, with a zone of anomalously thick crust (48 km) in the Kapuskasing Structural Zone region. Measurements of bulk Poisson's Ratio indicate that the crust is highly felsic in some parts of the province, but includes a significant mafic component in parts of the western Superior and eastern Ontario, regions that have been affected by uplift or rifting.The new results, together with information from other recent studies of shear wave splitting and body-wave tomography, show that the Superior craton is far from a homogeneous entity. Rather it is divisible, on the basis of thickness, anisotropy and velocity structure of the crust and lithosphere, into distinct subdomains that reflect the complex tectonic history of this region. The large range of inferred thicknesses of the crust (34–48 km) and lithosphere (100–240 km) within the Superior craton represents a significantly greater variability than has been previously observed for Archean lithosphere.

Sandstone petrofacies in the northwestern sector of the Iberian Basin

During the most active rifting stages in the northwestern sector of the Iberian Basin (Cameros Basin and Aragonese Branch of the Iberian Range), thick sequences of continental clastic deposits were generated. Sandstone records from Rift cycle 1 (Permo-Triassic) and Rift cycle 2 (Late Jurassic-Early Cretaceous) show similarities in composition. Based on the most recent data, this paper describes sandstone petrofacies developed during both rifting periods. Six petrofacies can be distinguished: two associated with Rift cycle 1 (PT-1 and PT-2) and four with Rift cycle 2 (JC-1 to JC-4). All six petrofacies can be classifi ed as sedimentoclastic or plutoniclastic. Sedimentoclastic petrofacies developed during early rifting stages either through the recycling of pre-rift sediments or signifi cant palaeogeographical changes. These facies comprise a thin succession (<100 m) of clastic deposits with mature quartzose and quartzolithic sandstones containing sedimentary and metasedimentary rock fragments. Carbonate diagenesis is more common than clay mineral diagenesis. Sedimentoclastic petrofacies have been identifi ed in Rift cycle 1 (Saxonian facies, PT-1) and Rift cycle 2 (JC-1 and JC-3; Tithonian and Valanginian, respectively). In the absence of the pre-rift sedimentary cover, metasedimentoclastic petrofacies sometimes develop as a product of the erosion of the low- to medium-grade metamorphic substratum (Petrofacies JC-2, Tithonian-Berriasian). Plutoniclastic petrofacies were generated during periods of high tectonic activity and accompanied by substantial denudation and the erosion of plutonites. Forming thick stratigraphic successions (1000 to 4000 m), these feldspar-rich petrofacies show a rigid framework and clay mineral diagenesis. In Rift cycle 1, plutoniclastic petrofacies (PT-2) are associated with the Buntsandstein. This type of petrofacies also developed in Rift cycle 2 in the Cameros Basin (JC-4) from DS-5 to DS-8 (Hauterivian-Early Albian), and represents the main basin fi ll interval. Sedimentoclastic and plutoniclastic petrofacies can be grouped into three pairs of basic petrofacies. Each pair represents a ‘provenance cycle’ that records a complete clastic cycle within a rifting period. Petrofacies PT-1 and PT-2 represent the ‘provenance cycle’ during Rift-1. In the Cameros Basin, two provenance cycles may be discerned during Rift cycle 2, related both to the Tithonian-Berriasian and the Valanginian-Early Albian megasequences. Tectonics is the main factor controlling petrofacies. Other factors (e.g., maturation during transport, local supply) may modulate the compositional signatures of the petrofacies yet their main character persists and even outlines he hierarchy of the main bounding surfaces between depositional sequences in the intracontinental Iberian Rift Basin.

Rift-transform junction in North Iceland: rigid blocks and narrow accommodation zones revealed by GPS 1997-1999-2002

SUMMARY The current tectonics of North Iceland are characterized by rifting episodes along fissure swarms and by ML 6‐7 earthquakes in the transform zone between the northern rift zone and the Kolbeinsey Ridge north of Iceland. The last rifting period (1975‐1984) was associated with an average opening of 5 m along the Krafla fissure swarm. Post-rifting deformation has been revealed by GPS investigations in Northern Iceland. A GPS network was occupied in 1997, 1999 and 2002 to quantify present-day displacements and their variation with time, on

Hydrocarbon generation modelling in a multiple rifted and volcanic basin: a case study in the Foinaven Sub-basin, Faroe–Shetland Basin, UK Atlantic margin

Synopsis Hydrocarbon generation modelling in the Faroe–Shetland Basin (FSB) is enigmatic in that the depth of burial suggests that the main Middle and Upper Jurassic source rocks are overmature for hydrocarbon generation. However, the reservoired hydrocarbons comprise largely oil with only minor amounts of oil-associated gases, not the high maturity gas expected from the maturation modelling. The modelling also indicates that oil generation largely preceded both the deposition of the Paleocene reservoirs and the time of trap formation. This mismatch in timing has been reconciled by the use of holding tank models where the oil is stored in deeply buried Cretaceous reservoirs, before remigration after the formation of the Paleocene reservoirs and traps. The thermal histories used for this modelling are generally calibrated against geochemical parameters, such as vitrinite reflectance, but contain a number of flaws when the physical relationships between force, energy and work are evaluated. Foremost amongst the problems is the use of constant heat flows during periods of rifting, since it is impossible for a basin to be rifted under such conditions. A thermal history that incorporates elevated heat flows during each period of rifting and volcanic activity has been derived from the tectonic history of the FSB, and was checked by calibration to measured vitrinite reflectance data in wells around the margins of the Foinaven Sub-basin. The vitrinite reflectance predictions were obtained by using the PresRo ® model, in which the vitrinite reflectance is retarded by overpressures that developed during the rapid Paleocene burial in the FSB. The PresRo ® model predicts late oil window maturities for the Jurassic source rocks in a pseudowell in the Foinaven Sub-basin kitchen. The results from the maturation modelling were then integrated with the fluid inclusion and hydrocarbon analyses from Westray area and Judd High wells. Three phases of hydrocarbon charging into the Foinaven Field can be identified. The first involves early and mid-mature oils that charged earlier Cretaceous sandstones during the Late Cretaceous, while the Paleocene reservoired hydrocarbons consist of two separate phases of oil charged during the Tertiary with an intermediate period of biodegradation.

New Oligocene–early Miocene microflora from the southwestern Turkana Basin: Palaeoenvironmental implications in the northern Kenya Rift

We report five Oligocene–early Miocene pollen assemblages from the Loperot-1 exploration well drilled in the semi-desert Lokichar Basin (latitude 02° 21′ 46.15″ N, longitude 35° 52′ 23.47″ E, ground elevation 615 m above MSL), near Lake Turkana in northern Kenya. They represent the second oldest plant microfossils so far recovered from East Africa and add significantly to the Paleogene–Neogene tropical African fossil plant record. The Loperot pollen indicate a mosaic environment of semi-deciduous forest and humid woodland whose floristic composition presents strong affinities with the vegetation occurring today in the Guinea-Congolia/Zambezia phytogeographical transition zone, with a rainfall more than 1000 mm/year and a well defined dry season. The weak representation of Poaceae and herbaceous taxa characteristic of grassland, dry bushland or savanna and the abundance of shade tolerant plants such as ferns all point to a vegetation composed to a variety of communities with closed forest formations predominant. The lack of typical temperate mountains elements, mainly Podocarpus and Juniperus today widespread on the East African highlands, indicates that the geography of the region was different from that of today. The plateaux or uplands adjacent to the Lokichar basin were probably still not high enough during this period of early rifting in East Africa to support the temperate coniferous forests characteristic of the Plio-Pleistocene.

The geology, SHRIMP U–Pb geochronology and metallogenic significance of the Ankisatra-Besakay District, Andriamena belt, northern Madagascar

The Ankisatra-Besakay District (A-BD), located about 200 km north of Antananarivo and 75 km east of Maevatanana in central-northern Madagascar, hosts two historical mines, the Ankisatra Pb–Zn ± Au and Besakay Pb–Ag mines. These shear-hosted en echelon quartz veins at Besakay and deformed tensional quartz veins at Ankisatra produced a total of 4446 t of lead and 156 t of zinc in the early 1940s. In addition, there is Pb–Zn–Cu mineralisation in both quartz-feldspar leucosome veins/bands and metasomatised granulite-facies mafic orthogneiss, Cu–Zn and associated Fe–Mn mineralisation in magnetite–pyrite enderbite breccia, and Cu–Zn mineralisation in retrograde shear zones in granulite-facies paragneiss in the A-BD. The country rocks in the A-BD consist of amphibolite to granulite-facies mafic and granitoid orthogneisses and paragneisses with horizons of silicate-facies BIF. The paragneissic rocks in the district tectonically overlie the biotite–granitoid hornfels and sub-volcanic mangeritic rocks, which are separated from amphibolite-facies alkali-feldspar granitoid and enderbitic rocks by a major structure. The A-BD is structurally characterised by: (1) E–W-trending tensional fractures, quartz veins and dolerite dykes; (2) buckling-related axial-planar fractures; and (3) N–S, NE–SW and NNE–SSW trending dextral strike-slip shear zones, dolerite sills and quartz veins in transpressional extensional zones. Uranium–Pb SHRIMP II geochronology of zircon constrains the peak of magmatic, metamorphic, deformational and metasomatic events in the A-BD. An important constraint is whether hosting terranes contain signatures of the ca. 1690–1590 Ma critical age window for world-class BHT Pb–Zn–Ag deposits elsewhere in the world. At least two magmatic events are recorded from the A-BD. An early magmatic event is recorded by a 2725 ± 12 Ma single xenocrystic magmatic zircon in the >2676 ± 6 Ma precursor to the granulite-facies mafic orthogneiss. A ca. 2503–2460 Ma event is recorded by a 2465 ± 6 Ma minimum age of magmatism for the precursor to metasomatised granulite-facies mafic orthogneiss and 2483 ± 20 Ma for the precursor to biotite–granitoid hornfels. Zircons extracted from both the metasomatised and unaltered granulite-facies mafic orthogneisses record peak metamorphic ages of 2465 ± 12 and 2390 ± 10 Ma, probably representing compressional deformation, partial melting, and associated local magmatic events within the ca. 2475–2380 Ma period. Inherited zircons from the quartzo-feldspathic granulite-facies paragneisses return ages of protolithic supracrustal rocks ranging from ca. 2870 to 1700 Ma. A widespread period of rifting, anatectic magmatism, basic-ultrabasic and mangerite magmatism, and related granulite-facies metamorphism occurred between ca. 820 and 780 Ma. The possible exhalative units (silicate-facies BIF and metasomatic, garnet–quartz–plagioclase rock) are of late-Archaean to early-Palaeoproterozoic, rather than Mesoproterozoic age. The terrane lacks the critical evolution age window of ca. 1770–1550 Ma, characteristic of well-documented BHT Provinces in the Broken Hill Block and Mt. Isa Block, Australia and ca. 1959–1135 Ma from the Bushmanland Ore District, South Africa. This suggests that either such an event did not occur in the crust now forming the A-BD or that the equivalent supracrustal rocks containing these age signatures were eroded during Proterozoic times. It is less likely that the intense 780–820 Ma event destroyed evidence for their prior existence. The galenas from the Ankisatra and Besakay deposits have signatures characteristic of small-scale mineralised systems which derived most of their lead from local crustal rocks older than ca. >2.7 Ga. They are thus atypical of BHT deposits and associated vein-style mineralisation from well-endowed terranes. It is concluded that there are neither direct signs nor indirect temporal signals of giant stratiform/stratabound BHT Pb–Zn–Ag mineralisation, nor clear evidence for the presence of characteristic transitional sequences and alteration styles associated with BHT mineralisation in the A-BD, thus downgrading its prospectivity.

A 40Ar / 39Ar and U / Pb isotopic study of the Ilímaussaq complex, South Greenland: Implications for the 40K decay constant and for the duration of magmatic activity in a peralkaline complex

Magmatism in the Gardar Province, South Greenland, is related to two main rifting events at 1280 Ma and 1180 to 1140 Ma. Little is known about the duration of the magmatic activity in a specific complex. The Ilímaussaq intrusion belongs to the second period of rifting and comprises an extraordinary diversity of granitic and syenitic rock types, which intruded and fractionated in three successive magmatic events. As the intrusion contains some of the most evolved, incompatible element-rich rocks on Earth, it was chosen for a detailed geochronological study to quantify the duration of melt production, intrusion, fractionation and cooling. Amphiboles, which are abundant in all rock types, including pegmatites and late magmatic veins, were dated using the 40Ar / 39Ar technique. Since the solidus temperature of the most evolved melts is below the closure temperature of amphibole, and no later heating event occurred, the 40Ar / 39Ar ages reflect the magmatic crystallisation and can be used to determine the duration of igneous differentiation. The 40Ar / 39Ar plateau ages range between 1142.6 ± 2.2 Ma and 1152.3 ± 3.7 Ma using the Steiger and Jäger [Steiger, R.H. and Jäger, E., (1977). Subcommision on Geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362.] 40K decay constant. These ages are younger than an U–Pb age of 1160 ± 5 Ma on baddeleyite from the first magma batch. Our results indicate that the 40K decay constant of Steiger and Jäger may be too high; a lambda similar to that proposed by Kwon et al. [Kwon, J., Min, K., Bickel, P. and Renne, P.R., (2002). Statistical methods for jointly estimating decay constant of 40K and age of a dating standard. Math. Geol. 34, 457–474.] is required to make the 40Ar / 39Ar match the U–Pb age.

“Sour gas” hydrothermal jarosite: ancient to modern acid-sulfate mineralization in the southern Rio Grande Rift

As many as 29 mining districts along the Rio Grande Rift in southern New Mexico contain Rio Grande Rift-type (RGR) deposits consisting of fluorite–barite±sulfide–jarosite, and additional RGR deposits occur to the south in the Basin and Range province near Chihuahua, Mexico. Jarosite occurs in many of these deposits as a late-stage hydrothermal mineral coprecipitated with fluorite, or in veinlets that crosscut barite. In these deposits, many of which are limestone-hosted, jarosite is followed by natrojarosite and is nested within silicified or argillized wallrock and a sequence of fluorite–barite±sulfide and late hematite–gypsum. These deposits range in age from ∼10 to 0.4 Ma on the basis of 40Ar/ 39Ar dating of jarosite. There is a crude north–south distribution of ages, with older deposits concentrated toward the south. Recent deposits also occur in the south, but are confined to the central axis of the rift and are associated with modern geothermal systems. The duration of hydrothermal jarosite mineralization in one of the deposits was approximately 1.0 my. Most Δ 18O SO 4–OH values indicate that jarosite precipitated between 80 and 240 °C, which is consistent with the range of filling temperatures of fluid inclusions in late fluorite throughout the rift, and in jarosite (180 °C) from Peña Blanca, Chihuahua, Mexico. These temperatures, along with mineral occurrence, require that the jarosite have had a hydrothermal origin in a shallow steam-heated environment wherein the low pH necessary for the precipitation of jarosite was achieved by the oxidation of H 2S derived from deeper hydrothermal fluids. The jarosite also has high trace-element contents (notably As and F), and the jarosite parental fluids have calculated isotopic signatures similar to those of modern geothermal waters along the southern rift; isotopic values range from those typical of meteoric water to those of deep brine that has been shown to form from the dissolution of Permian evaporite by deeply circulating meteoric water. Jarosite δ 34S values range from −24‰ to 5‰, overlapping the values for barite and gypsum at the high end of the range and for sulfides at the low end. Most δ 34S values for barite are 10.6‰ to 13.1‰, and many δ 34S values for gypsum range from 13.1‰ to 13.9‰ indicating that a component of aqueous sulfate was derived from Permian evaporites ( δ 34S=12±2‰). The requisite H 2SO 4 for jarosite formation was derived from oxidation of H 2S which was likely largely sour gas derived from the thermochemical reduction of Permian sulfate. The low δ 34S values for the precursor H 2S probably resulted from exchange deeper in the basin with the more abundant Permian SO 4 2− at ∼150 to 200 °C. Jarosite formed at shallow levels after the pH buffering capacity of the host rock (typically limestone) was neutralized by precipitation of earlier minerals. Some limestone-hosted deposits contain caves that may have been caused by the low pH of the deep basin fluids due to the addition of deep-seated HF and other magmatic gases during periods of renewed rifting. Caves in other deposits may be due to sulfuric acid speleogenesis as a result of H 2S incursion into oxygenated groundwaters. The isotopic data in these “sour gas” jarosite occurrences encode a record of episodic tectonic or hydrologic processes that have operated in the rift over the last 10 my.

Flood and Shield Basalts from Ethiopia: Magmas from the African Superswell

The Ethiopian plateau is made up of several distinct volcanic centres of different ages and magmatic affinities. In the NE, a thick sequence of 30 Ma flood basalts is overlain by the 30 Ma Simien shield volcano. The flood basalts and most of this shield volcano, except for a thin veneer of alkali basalt, are tholeiitic. In the centre of the province, a far thinner sequence of flood basalt is overlain by the 22 Ma Choke and Guguftu shield volcanoes. Like the underlying flood basalts, these shields are composed of alkaline lavas. A third type of magma, which also erupted at 30 Ma, is more magnesian, alkaline and strongly enriched in incompatible trace elements. Eruption of this magma was confined to the NE of the province, a region where the lava flows are steeply tilted as a result of deformation contemporaneous with their emplacement. Younger shields (e.g. Mt Guna, 10·7 Ma) are composed of Si-undersaturated lavas. The three main types of magma have very different major and trace element characteristics ranging from compositions low in incompatible elements in the tholeiites [e.g. 10 ppm La at 7 wt % MgO (=La7), La/Yb = 4·2], moderate in the alkali basalts (La7 = 24, La/Yb = 9·2), and very high in the magnesian alkaline magmas (La7 = 43, La/Yb = 17). Although their Nd and Sr isotope compositions are similar, Pb isotopic compositions vary considerably; 206Pb/204Pb varies in the range of ∼17·9–18·6 in the tholeiites and ∼19·0–19·6 in the 22 Ma shields. A conventional model of melting in a mantle plume, or series of plumes, cannot explain the synchronous eruption of incompatible-element-poor tholeiites and incompatible-element-rich alkali lavas, the large range of Pb isotope compositions and the broad transition from tholeiitic to alkali magmatism during a period of continental rifting. The lithospheric mantle played only a passive role in the volcanism and does not represent a major source of magma. The mantle source of the Ethiopian volcanism can be compared with the broad region of mantle upwelling in the South Pacific that gave rise to the volcanic islands of French Polynesia. Melting in large hotter-than-average parts of the Ethiopian superswell produced the flood basalts; melting in small compositionally distinct regions produced the magmas that fed the shield volcanoes.

A sequence analyzed from the basin to the platform : the Middle Oxfordian calcareous succession in southeastern France

Abstract Middle Oxfordian sedimentation is very homogeneous in southeastern France. It is characterized by a specific alternation of marls and fine-grained limestones (G. transversarium Zone). This work shows that sets of calcareous beds allow accurate stratigraphic correlations in various paleogeographic areas, from the shallow Jura platform to the deep Dauphinois basin. Following a rifting period, this sedimentation illustrates a doming period with decreasing subsidence and water depth. This corresponds to the establishment of a wide marine area more favourable to carbonate sedimentation. Considering sequence stratigraphy, some slight variations occur in the sediment record and lead to interesting comparisons between the platform and the basin. The geochemical evolution, mainly the manganese content, seems to be dependant of the activity of the Ligurian Tethys oceanic ridge. This fact shows that sedimentation in southeastern France could be significantly controlled by tectono-eustasy.

Polyphase evolution of the East Gobi basin: sedimentary and structural records of Mesozoic–Cenozoic intraplate deformation in Mongolia

AbstractMapping and correlation of 2D seismic reflection data define the overall subsurface structure of the East Gobi basin (EGB), and reflect Jurassic–Cretaceous intracontinental rift evolution through deposition of at least five distinct stratigraphic sequences. Three major northeast–southwest‐trending fault zones divide the basin, including the North Zuunbayan (NZB) fault zone, a major strike‐slip fault separating the Unegt and Zuunbayan subbasins. The left‐lateral NZB fault cuts and deforms post‐rift strata, implying some post‐middle‐Cretaceous movement. This fault likely also had an earlier history, based on its apparent role as a basin‐bounding normal or transtensional fault controlling deposition of the Jurassic–Cretaceous synrift sequence, in addition to radiometric data suggesting a Late Triassic (206–209 Ma) age of deformation at the Tavan Har locality. Deposits of the Unegt subbasin record an early history of basin subsidence beginning ∼155 Ma, with deposition of the Upper Jurassic Sharilyn and Lower Cretaceous Tsagantsav Formations (synrift sequences 1–3). Continued Lower Cretaceous synrift deposition is best recorded by thick deposits of the Zuunbayan Formation in the Zuunbayan subbasin, including newly defined synrift sequences 4–5. Geohistory modelling supports an extensional origin for the EGB, and preliminary thermal maturation studies suggest that a history of variable, moderately high heat flow characterized the Jurassic–Cretaceous rift period. These models predict early to peak oil window conditions for Type 1 or Type 2 kerogen source units in the Upper Tsagantsav/Lower Zuunbayan Formations (Synrift Sequences 3–4). Higher levels of maturity could be generated from distal depocentres with greater overburden accumulation, and this could also account for the observed difference in maturity between oil samples from the Tsagan Els and Zuunbayan fields.

Geology and tectonic evolution of Palaeoproterozoic basins of the eastern Capricorn Orogen, Western Australia

Abstract The Palaeoproterozoic Yerrida, Bryah, Padbury, and Earaheedy Basins extend for about 700 km along the northern margin of the Yilgarn Craton and are part of the Capricorn Orogen. The basins developed between ca. 2.2 and 1.8 Ga, recording periods of sedimentation, volcanism, rifting, accretion and passive-margin tectonism. The oldest is the 2.17 Ga Yerrida Basin, which began as an intracontinental sag within which siliciclastics and evaporites accumulated. At about 2.0 Ga convergence of the Yilgarn Craton and the Glenburgh terrane closed an oceanic arm, resulting in the accretion of oceanic crust and plateaux onto the Yilgarn margin and the western side of the Yerrida sag basin. This accretion produced the Narracoota Formation, which together with the clastic and chemical (banded iron-formation) sedimentation that followed, forms the Bryah Basin. Continuing collision resulted in the establishment, around 1.96 Ga, of the Padbury foreland basin, developed on top of the Bryah Basin rocks. The ca. 1.83 Ga convergence and collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny), resulted in deformation of the Bryah–Padbury Basins and orogenic uplift with coarse clastic sedimentation accompanied by the eruption of flood basalts into the Yerrida Basin. To the east of the collision zone, and possibly postdating collision, a passive margin was present. Sedimentation along it consisted of clastic and chemical sediments (granular iron-formation), which form the Earaheedy Basin. During the ca. 1.79 Ga Yapungku Orogeny, due to the convergence of the North Australian and Western Australian cratons, the northern margin of the Earaheedy Basin was deformed (Stanley Fold Belt). This last deformation event concludes the Palaeoproterozoic history of the eastern Capricorn Orogen basins.

Neoproterozoic A-type granitoids of the central and southern Appalachians: intraplate magmatism associated with episodic rifting of the Rodinian supercontinent

Emplacement of compositionally distinctive granitic plutons accompanied two pulses (765–680 and 620–550 Ma) of crustal extension that affected the Rodinian craton at the present location of the central Appalachians during the Neoproterozoic. The dominantly metaluminous plutons display mineralogical and geochemical characteristics of A-type granites including high FeOt/MgO ratios, high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu. These dike-like, sheet complexes occur throughout the Blue Ridge province of Virginia and North Carolina, and were emplaced at shallow levels in continental crust during active extension, forming locally multiple-intrusive plutons elongated perpendicular to the axis of extension. New U–Pb zircon ages obtained from the Polly Wright Cove (706±4 Ma) and Suck Mountain (680±4 Ma) plutons indicate that metaluminous magmas continued to be replenished near the end of the first pulse of rifting. The Suck Mountain body is presently the youngest known igneous body associated with earlier rifting. U–Pb zircon ages for the Pound Ridge Granite Gneiss (562±5 Ma) and Yonkers Gneiss (563±2 Ma) in the Manhattan prong of southeastern New York constitute the first evidence of plutonic felsic activity associated with the later period of rifting in the U.S. Appalachians, and suggest that similar melt-generation processes were operative during both intervals of crustal extension. Fractionation processes involving primary minerals were responsible for much of the compositional variation within individual plutons. Compositions of mapped lithologic units in a subset of plutons studied in detail define overlapping data arrays, indicating that, throughout the province, similar petrologic processes operated locally on magmas that became successively more chemically evolved. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with results of melting experiments and indicates that metaluminous granitoids of the supersuite likely were derived through melting of lower crustal sources. Mildly peralkaline rocks of the Robertson River batholith and Irish Creek pluton may be derived from more chemically primitive sources similar in composition to ocean–island basalts. Blue Ridge granitoids define a plutonic episode that occurred during an unsuccessful pulse of crustal extension which predated opening of Iapetus by more than 100 million years. Granitoid gneisses in New York were emplaced during an extension-related, dominantly mafic magmatic episode that ultimately led to development of Iapetus.

Early Palaeozoic near-surface deformation in the Neptune Range, Antarctica: implications for the Ross and Gondwanian orogenies

The Neptune Range of the Pensacola Mountains, East Antarctica, exposes a record of Early Palaeozoic to Early Mesozoic polyphase deformation along the former East Antarctic margin of Gondwana that is unique within the 3500 km length of the Transantarctic Mountains. The earliest of these orogenic events is the polyphase Early Palaeozoic Ross orogeny. Following two phases of intense pre-late Mid-Cambrian deformation (D 1 and D 2 ), a succession of volcanic and sedimentary rocks (sequences 2 and 3) were deposited in late Mid- to Late Cambrian time during a transient period of back-arc rifting. New stratigraphical and structural relationships suggest that the Cambrian succession was deformed during latest Cambrian time while at or near the palaeosurface in a foreland basin setting (D 3 ). D 3 deformation is characterized by thrust faulting and folding without the development of a widespread coeval cleavage, with evidence for syntectonic deposition of the basal Neptune Group (sequence 4). The main regional cleavage developed through the Palaeozoic cover rocks (sequences 2–5) is also developed in rare Permo-Carboniferous clastic dykes that cut down through the stratigraphy from the base of the Gale Mudstone. The dyke–cleavage relationship, together with observations of the conglomerate cobbles from throughout the stratigraphical succession, suggests that the most intense deformation within the Cambrian sequences is Permo-Triassic (Gondwanian orogeny) rather than latest Cambrian (late Ross orogeny) as previously concluded. Our data corroborate recent suggestions that the main phase of Ross orogenic deformation is pre-late Mid-Cambrian and not Late Cambrian or Early Ordovician.

The Jeremoabo transpressional transfer fault, Recôncavo–Tucano Rift, NE Brazil

The Jeremoabo fault, located in the Early Cretaceous Recôncavo–Tucano Rift, northeastern Brazil, represents an example of transfer fault. A field-based structural analysis along 20 km of exposures revealed an E–W-trending and steeply dipping reverse-sinistral fault that resulted from transpressional reactivation of a pre-existent weak zone. The fault is associated with a 5-km-wide footwall deformation zone, in which sandstone layers are folded and locally overturned becoming parallel to the fault. The deformation zone contains a pervasive system of NE- and NNW-trending conjugate Riedel shear fractures of outcrop to kilometer scale. A paleostress analysis based on these Riedel fractures indicated σ 1 and σ 3 trending, respectively, 016° and 108°, and σ 2 subvertical, consistent with transpression and local switch between σ 1 and σ 2 relative to the regional paleostress field. The Jeremoabo fault links smaller rift border faults and has sense of slip consistent with the mapped offset of the basin border. Its transfer function is attested by linkage with a major dextral transfer fault and a transfer zone during the rifting period. By acting together these structures accommodated the change in asymmetry between the two major half-grabens of the rift.

Salinity variation of formation water and diagenesis reaction in abnormal pressure environments

An understanding to the chemistry of formation waters in sedimentary basins is important for many geological processes, such as the fluid-rock interaction, the migrating paths of fluid and the entrapment mechanisms of hydrocarbon. This paper deals with the salinity variation of formation water and diagenesis reaction in the abnormally pressured system. The Shiwu depression of the Songliao basin and the Yinggehai basin are selected for case studies. The studies indicate that there is a distinct difference in the chemistry of subsurface water between hydropressured and abnormally pressured systems. The Shiwu depression of the Songliao basin is composed of terrigeneous clastics in fluvial-lacustrine environments, which contain brackish water with salinity ranging from less than 1000 mg/L to 12000 mg/L. Water composition varies with depth and hydrochemical regions. In the underpressured strata deposited during the rifting period, the formation water is characterized by CaCl2 type water with high salinity. NaHCO3-dominanted water with lower salinity occurs at the hydropressured strata deposited during the post-rifting period. In this halite-free basin, brackish water may be attributed to the condensation of meteoric water and water-rock interaction. In the deeply buried underpressured water, a predominated diagenesis reaction resulting in enrichment of Ca and Cl and reduction of Na may be related to the albitization of plagioclase following the basinal fluid line (BFL). The Yinggehai basin constitutes clastic deposits in nearshore, neritic, shelf environments and contains brackish and saline water with salinity ranging from less than 15000 mg/L to 50000 mg/L. Pore water in these marine deposits must have originated from seawater. In the overpressured system, however, the formation water has much lower salinity and ion concentrations (except bicarbonate and carbonate) than normal seawater. The low salinity indicates that interstitial connate seawater is diluted by the water released from the transformation of smectite to illite. High bicarbonate and carbonate concentrations may contribute to mineral dehydration and kerogen-cracking reactions. Furthermore, low salinity water also occurs near and above the top of the geopressured zone in the diapiric structures, which may indicate that pore water with normal seawater salinity has been mixed by fresher saline water from deeper portions of the overpressured stratigraphic section.

Regional variation of formation water chemistry and diagenesis reaction in underpressured system: example from Shiwu depression of Songliao basin, NE China

Geochemistry of formation waters in the Shiwu depression of the Songliao basin, northeast China, shows a distinct variation in different pressured systems. The study area is composed of terrigeneous clastics in fluvial-lacustrine environments, which contain formation water with salinity ranging from less than 1000 to 12,000 mg/l. Water composition varies with depth and hydrochemical regions. In the underpressured strata deposited during the rifting period, formation water is characterized by water type of CaCl 2 with higher salinity. NaHCO 3-dominanted water with lower salinity occurs in the normally pressured strata deposited during the post-rifting period. In the transition area between the normally pressured and underpressured zones, total dissolved solid (TDS) content ranges from 1000 to 7600 mg/1 and increases with depth. In this halite-free basin, brackish water may be attributed to the evaporation of formation water and water-rock interaction. In the deep buried underpressured water, a predominated diagenesis reaction resulting in Ca and Cl enrichment and reduction of Na may be related to albitization of plagioclase following the basinal fluid line (BFL). The results of this study indicate that salinity variation of formation water and diagenesis reaction is closely related to hydrochemical environments within different pressured systems.