Series Of Belts Research Articles

Source of the Dalradian Supergroup constrained by U–Pb dating of detrital zircon and implications for the East Laurentian margin

Detrital zircons in siliciclastic units of the Dalradian Supergroup yield U–Pb ages ranging from 3.2 to 0.5 Ga. Detrital zircons from the sub-Grampian Group basement and the Grampian Group are predominantly of Palaeoproterozoic and Mesoproterozoic ages with Archaean grains absent or rare. In contrast, the overlying Appin, Argyll and Southern Highland groups contain a significant contribution of Archaean detrital zircon grains, some of which locally preserve evidence for late Palaeoproterozoic overgrowths dated at c . 1.8 Ga. In addition, on concordia plots Archaean grains are slightly discordant with a lower intercept at c . 1.8 Ga suggesting they were affected by a tectonothermal event at this time. Late Palaeoproterozoic and Mesoproterozoic grains also show evidence for overprinting by a tectonothermal event around 1.0–0.9 Ga. These tectonothermal events occurred in the source region before accumulation of the siliciclastic detritus. The analysed samples contained no definitive evidence for having been affected by a late Neoproterozoic (Knoydartian) event or of containing detritus derived from a source showing evidence for this event. The overall age range of detritus, combined with sparse palaeocurrent data, is consistent with derivation from the Laurentian foreland, especially the Labrador–Greenland region. Archaean detritus overlaps with that of Archaean cratons, notably the Superior, whereas Palaeoproterozoic detritus corresponds to the timing of suturing of Archaean cratons by a series of orogenic belts (Ketilidian–Makkovik, New Quebec, Nagssugtoqidian, Torngat belts). Mesoproterozoic detritus is consistent with derivation from the Grenville Orogen. The presence of a series of detrital age peaks in the late Palaeoproterozoic and early Mesoproterozoic (1.8–1.5 Ga), the paucity of mid-Mesoproterozoic detritus (1.4–1.2 Ga), and evidence for a tectonothermal event between 1.0 and 0.9 Ga is typical of the geological history of the Labrador–Greenland region of Laurentia.

Late Silurian bimodal volcanism of southwestern New Brunswick, Canada: Products of continental extension

Silurian–Devonian bimodal volcanism—a common feature of the northern Appalachians of the northeastern United States and eastern Canada—forms major belts of igneous rocks including the Coastal volcanic belt of New England and New Brunswick. The Coastal volcanic belt is one of the largest bimodal volcanic provinces in the world. Controversy remains regarding the tectonic setting of this volcanism; suggestions range from volcanic arc, to extensional continental rift, transpressional basin, backarc, or plume-modified subduction zone. The mapped Passamaquoddy Bay volcanic sequence of the Coastal volcanic belt of southwestern New Brunswick has a minimum thickness of 4 km and comprises 63 lithologic units that are divided into four distinct cycles of mafic-felsic volcanism. A preliminary U-Pb zircon date indicates an age of 423 Ma, similar to the age of the Cranberry Island bimodal volcanic series of the Coastal volcanic belt in Maine. Our analysis of 115 volcanic rocks for major and trace element concentrations indicates that the sequence is subalkalic and bimodal (basalt-rhyolite) with within-plate tectonic affinities. Most mafic rocks range in composition from basalt to basaltic andesite. The Mg# [defined as 100 × MgO/(MgO + FeO), where FeO = FeO T × 0.9] of the basalts ranges from 30 to 70, and there is a trend toward more primitive compositions upward in the section. The light rare earth element abundances of the basalts are enriched to levels of 30–130 times chondritic values. The flows are enriched in incompatible elements with respect to primitive mantle and show distinct Nb, Hf, and Sr anomalies. We interpret the basalts to be mantle melts modified by crustal contamination and mantle metasomatism from a previous subduction event. Most of the chemical variation can be explained by replenishment and fractionation of small magma chambers. The rhyolitic rocks are crustal melts, modified by crystal fractionation. Within each of the rhyolitic cycles there is an upward trend toward more evolved compositions as a result of an evolving magma chamber and magma-chamber zonation. We suggest that the rocks were erupted in an extensional environment and that there was a significant zone of continental extension during the Late Silurian in the northern Appalachians. However, controversy remains regarding the precise nature of the tectonic setting and the duration and extent of this volcanism.

The Great Alaskan Terrane Wreck: reconciliation of paleomagnetic and geological data in the northern Cordillera

Paleomagnetic studies place much of the Cordilleran orogen ≥3000 km south, relative to autochthonous North America at 85 Ma. This conflicts with geological studies which have been interpreted to indicate minor displacement. The inability to reconcile these interpretations has precluded understanding the tectonic development of the Cordillera, and has cast doubt on global paleogeographic reconstructions which rely upon paleomagnetic studies. I demonstrate that Alaska and Yukon are divisible into a series of geological belts, including the northerly east–west-trending Arctic Alaska, the central southwest-trending Ruby, the southerly east-trending Dillinger and the southeast-trending Yukon–Tanana belts. Each belt is characterized by four regularly arranged rock sequences; (1) a Paleozoic continental margin strata, (2) a Devonian–Mississippian arc assemblage, (3) an ophiolite, and (4) an Early to mid-Cretaceous arc. These correlations greatly simplify the terrane nomenclature of the northern Cordillera and provide a basis for a new palinspastic reconstruction of the Cordillera. Continuity of the belts can be demonstrated through connecting oroclines, and facing a structural vergence directions vary in a consistent and predictable fashion indicating that the belts previously formed part of a linear ribbon continent. This ribbon continent, referred to here as SAYBIA, originally extended south from eastern Siberia and was ∼8000 km long. Northward translation in response to coupling with the Kula plate was accommodated by buckling (oroclinal orogeny) of SAYBIA, much like a derailing train, with scissored ‘cars’ forming much of Alaska. Oroclinal orogeny makes consistent interpretations of available geological and paleomagnetic data in the Cordillera, provides a means for the rapid construction of continent, and may have been an important process involved in the construction of ancient continental nuclei.

Stable Continental Region Earthquakes in South China

— This paper reviews some remarkable characteristics of earthquakes in a Stable Continental Region (SCR) of the South China Block (SCB). The kernel of the SCB is the Yangtze platform solidified in late Proterozoic time, with continental growth to the southeast by a series of fold belts in Paleozoic time. The facts that the deviatoric stress is low, the orientations of the major tectonic features in the SCB are substantially normal to the maximum horizontal principal stress, and a relatively uniform crust, seem to be the major reasons for lack of significant seismicity in most regions of the SCB. Earthquakes in this region are mainly associated with three seismic zones: (1) the Southeast China Coast seismic zone related to Guangdong-Fujian coastal folding belt (associated with Eurasia-Philippine Sea plate collision); (2) the Southern Yellow Sea seismic zone associated with continental shelf rifts and basins; and (3) the Downstream Yangtze River seismic zone spatially coinciding with Tertiary rifts and basin development. All three seismic zones are close to one or two major economic and population centers in the SCB so that they pose significant seismic hazards. Earthquake focal mechanisms in the SCB are consistent with strike-slip to normal faulting stress regimes. Because of the global and national economic significance of the SCB and its dense population, the seismic hazard of the region is of outstanding importance. Comparing the SCB with another less developed region, a pending earthquake with the same size and tectonic setting would cause substantially more severe social and economic losses in the SCB. This paper also compiles an inventory of historic moderate to great earthquakes in the SCB; most of the data are not widely available in English literature.

Isotopic evidence for Palaeoproterozoic accretion in the basement of the East European Craton

The Precambrian basement in the western part of the East European Craton (EEC) consists of a series of arcuate tectonic belts metamorphosed in amphibolite and granulite facies. These are separated from each other by crustal discontinuities generally trending NNE–SSW. In conjunction with previously published Sm–Nd and U–Pb isotopic data, the results reported now show that the basement between the Baltic and Ukrainian Shields is Palaeoproterozoic in age. It was formed between 2.1 and 1.8Ga ago.Igneous rocks from the various arcuate belts generally have positive initial ϵNd values between +1 and +3. Thus they are essentially juvenile and contain little material derived from older continental crust. Most metasedimentary rocks, in contrast, contain a distinct component of older crustal materials. This is demonstrated by initial ϵNd values ranging from −1 to −3 and by the results of U–Pb multigrain and single grain (SIMS) analyses of detrital zircons.The oldest magmatic activity in the region occurred 2.1–2.0Ga ago in the southeast, along the margin of the Archaean protocontinent of Sarmatia. Two subsequent episodes of volcanism in the tectonic belts farther west have been dated at ca. 2.0 and 1.87Ga using the U–Pb zircon approach. The Proterozoic volcanic rocks are thus younging towards the northwest, away from Sarmatia. The high-grade metamorphism, that has been dated employing U–Pb in monazite and zircon, is also younging from the southeast to the northwest. Its ages range from 1.95 to less than 1.79Ga.The available data are explained best by accretionary plate tectonics. These involved repeated episodes of generation of juvenile continental crust along and outboard of the margin of the dominantly Archaean Craton in Sarmatia. An Andean-type of geodynamic setting between 2.1 and 2.0Ga ago was succeeded by the formation of several island arcs and accretion of these onto the older continent between 2.0 and 1.8Ga. Subsequent compression of the newly formed crust lasted until ca. 1.7Ga.

Primary colonisation of mudflat estuaries by Spartina maritima (Curtis) Fernald in Northwest Spain: vegetation structure and sediment accretion

Spartina maritima in Northwest Spain follows a colonisation pattern similar to that described from other latitudes and other Spartina coloniser species. After the establishment of vegetative fragments on the mudflat, rhizomes grow centrifugally forming rounded patches that act like sediment traps, becoming dome-shaped due to the accumulation of sediment towards the centre of the patch. Patch size has been marked as one of the main determinants of the accretion due to the presence of Spartina. We propose a model to relate patch size and sediment accretion at the first stages of colonisation, which seems to be logarithmic rather than linear as had been previously suggested. Other factors such as distance to water exchange source and elevation prior to Spartina colonisation have proved to be important accretion determinants. Shoot size within patches increased from the edge towards the centre, while shoot density showed a complex distribution characterised by a series of concentric belts of different densities, with the lowest density at the centre of the patch. Despite, the higher elevation and the lower Spartina density at the centre of the patches, no other species have been found colonising those spots.

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/Lapm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Ybn=1–8), high Th/Nbpm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO2, Ta, P2O5, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Ybn tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Ybn of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs.

Wall rock alteration, Atud gold mine, Eastern Desert, Egypt: processes and PTXCO 2 conditions of metasomatism

The Atud gold mine, central Eastern Desert of Egypt, is located in an intrusive metagabbro-diorite complex that abutts the conglomerate-greywacke-slate series of the Pan-African Belt in Egypt. Gold-bearing quartz veins occur as fracture filling in the Neoproterozoic dioritic rocks and along their contacts with the metagabbro. Gold mineralisation is associated with discrete metasomatic alteration zones around shear zones and quartz-carbonate vein arrays. Textures indicate that the zoned wall rock alteration and associated Au mineralisation postdate regional metamorphism. Alteration zones delimited by mapping alteration isograds encompass individual veins and merge to form the mine-scale alteration. The alteration zones seem to have been formed through metasomatism of CO 2, H 2O, K and lesser Na, as determined by mass balance calculations on whole rock chemical data. Alteration minerals are dependent on the original rock composition, with actinolite and epidote characteristic of the unaltered host rock. Boundaries between the individual zones can be mapped as alteration isograds. In order of decreasing alteration or increasing distance from a vein or shear zone, these isograds in the diorite separate the zones: (1) chlorite and calcite; (2) albite and ankerite; and (3) albite, muscovite and kaolinite. Zone 1 contains evidence of the transition from titanite to rutile and calcite. The formation of albite is due to CO 2 metasomatism in an Al-rich host rock; its coexistence with muscovite is significant as it defines the K + Na + ratio of the hydrothermal fluid associated with the Au mineralisation. Using K + Na + , Na + H + and CO 2, the path of alteration was modelled corresponding to observations of texture and paragenesis. Thermodynamic calculations also suggest that the isograds represent the degree of carbonate, potassic and sodic metasomatism by reaction of the Au-bearing hydrothermal fluid with the unaltered metamorphic rocks, and display the importance of original rock composition on the mineralogy of the alteration zones.

Granitoid Series and Mineralization in the Circum‐Pacific Phanerozoic Granitic Belts

Abstract: Throughout the Phanerozoic Era, magnetite‐series and ilmenite‐series granitoids are heterogeneously distributed on both sides of the Pacific Ocean. Ilmenite‐series granitoids are dominant in the western Pacific rim, while magnetite‐series granitoids prediminate in the eastern Pacific orogenic belt. Regional distribution patterns of the two series of paired belt are also recognized differently, implying different tectonic and geochemical settings on both sides of the Pacific Ocean. The differences reflect on the predominance of Sn‐W deposits in the western Pacific but sulfide‐forming mineral commodities in the eastern Pacific rims. To determine redox state of granitoids is the first step in mineral exploration of granitoid affinity.

Metamorphic peridotite and rock series of ophiolite belt in Mt. Ailao, Yunnan Province

The metamophic peridotite of ophiolite belt in Mt. Ailao is mmposed of lhenolite and harzburgite. The former shows the charateristics of primary pyrolite and the latter shows those of deleted (relict)pyrolite. By partial melting of Ihedite, two primary magmas: tholeiitic magma and picrite-basalt magma are formed. The former evoluted into gabbmdiabase-pyroxenic besalt rock series and show the characteristics of MORB; while the latter evoluted into gabbro-diorite-albite basalt-picrite basalt one, and show the characteristics of para-MORB.

Constrained gravity models and structural evolution of the Ashanti Belt, southwest Ghana

The Palaeoproterozoic terrane of southwest Ghana comprises a sequence of folded sedimentary/volcaniclastic rocks which separate a series of northeast trending volcanic belts. While the stratigraphy and structure of the belts are similar, the Ashanti Belt is characterised by a more tectonised northwest margin along which most of the major epigenetic Au deposits in Ghana are located. Early models of the tectonic evolution of this belt, which generally invoked two phases of deformation, are inconsistent with more recent structural and isotopic evidence (Eisenlohr and Hirdes, 1992). Modelling of regional gravity data by Hastings (1982) acknowledged the tectonic significance of the belt margins but lacked the benefit of constraints provided by more detailed mapping. This paper re-examines existing gravity data over the Ashanti Belt and presents constrained cross-sectional models which honour the mapped geology along the length of the belt and are consistent with the concept of a single, continuous deformational event. A synoptic scenario for the large-scale structural evolution of the belt is proposed which has implications for the location and timing of Au mineralisation.

Regional depositional setting and pore network systems of the El Garia Formation (Metlaoui Group, Lower Eocene), offshore Tunisia

Abstract The Metlaoui Group was deposited on a broad ramp that deepened to the northeast into the Tethyan Sea. The ramp contains a series of broad facies belts. The inner-ramp facies belt is composed of three facies tracts: (1) Faid sabkha (most landward) characterised by skeletal-poor dolomudstones and wackestones and interbedded evaporites, (2) Ain Merhotta restricted shallow-lagoon facies composed of sparse skeletal wackestones, packstones, and cross-bedded gastropod grainstones; (3) updip El Garia high-energy shoal complex (most seaward) composed of red algal-discocyclinid grainstones and packstones which are locally cross-bedded. The El Garia carbonates in the mid-ramp setting are predominantly composed of large, flat nummulites. Dominant textures are burrowed, mud-poor packstones and poorly sorted or bimodal grainstones. The accumulation was located below fair-weather wave base and above storm wave base, at a water depth estimated to have ranged from 30 to 60 m (middle neritic). The deep-water (deep neritic), outer-ramp Ousselat Member consists of coarse silt- to medium sand-sized nummulithoclastic debris that grades from coarser to finer debris down the ramp. The Bou Dabbous Formation is composed of basinal foraminiferal mudstones, wackestone and packstones deposited by suspension and less commonly by gravity-flow deposition in upper bathyal water depths. Reservoir quality is variable, with most lime packstones and grainstones having moderate to high porosity (related to intraparticle and microporosity), but only poor to fair permeability (generally less than 10 mD). The higher-quality reservoirs have preserved interparticle porosity with permeabilities ranging from tens of millidarcys to several darcys. Permeable nummulitic packstones and grainstones are favoured by the following factors: (1) low abundance of lime mud, (2) low abundance of nummulithoclastic debris, (3) low abundance of echinoderm fragments, (4) moderate sorting, (5) minor precipitation of late burial cements, (6) dolomitization.

庄内堆積盆地東部における伏在断層の成長に伴う活しゅう曲の変形過程

Detailed morphotectonic and geological research shows the the Quaternary surface deformation processes associated with the thrusting in the eastern Shonai basin, northeast Japan.The Neogene and Quaternary structure in the eastern Shonai basin is dominated by a series of major north-south trending folds and thrust belts which include the Aosawa trend, the Sakata trend, and the Oishi trend from east (mountain-side) to the west (basin central-side). The Aosawa trend, which bounded the Shonai basin and the Dewa Hills, began to move in the late Pliocene. The Sakata trend and the Oishi trend, which are located at the eastern margin of the Shonai alluvial lowland, began to move in the Early Pleistocene and the Middle Pleistocene, respectively with the frontal migration of thrusting. These structures are interpreted to be fault propagation folds on a east dipping detachment fault.On the Aosawa trend, the late Pleistocene terraces are tilted with little faulting overlying the late Pliocene to middle Quaternary reverse faults. On the Sakata trend, the late middle Pleistocene deformation is similar to the underlying Neogene structure, which is characterized by asymmetrical folds with thrusts. On the Oishi trend, the late Quaternary terraces deformed as asymmetrical folding with a narrow flexure zone on the western (thrust tip side) limb overlying symmetrical early to middle Quaternary folds. These morphological figures show the general deformation processes of each structure in the eastern Shonai fold and thrust zone as follows.·In the early stage, the tip of the concealed thrust elongates to the surface, so the figurefor surface deformation changes from symmetrical folding to asymmetrical folding with a narrow flexure zone on the thrust tip side limb.·In the middle stage, the fault tip reaches near the surface, with continuing uniformdeformation composed of asymmetrical folding with surface faulting.·In the later stage, the thrusting on the fault near the surface is decreased, and gentletilting becomes to predominate on the surface.A simple dislocation model suggests that the elongation rate of the fault tip of the thrust underlying the Oishi trend is about 1 mm/yr in the late Pleistocene. This rate is similar to its vertical displacement rate.

Effectiveness of Road Ripping in Restoring Infiltration Capacity of Forest Roads

AbstractMany forest roads are being closed as a step in watershed restoration. Ripping roads with subsoilers or rock rippers is a common practice to increase the infiltration capacity of roads before closure. When considering the effectiveness of ripping for reducing runoff and erosion and the potential reduction in slope stability by saturating road fills, it is important to know how ripping changes the infiltration capacity of forest roads. Hydrographs from simulated rainfall on 1 × 1 m plots were analyzed to find the saturated hydraulic conductivity, an indicator of infiltration capacity. I examined saturated hydraulic conductivity for three treatments on two different soils. One road was built in a soil derived from the metamorphic belt series geology of northern Idaho, a soil noted for its high rock fragment content. The second road was built in a sandy soil derived from decomposed granitics of the Idaho batholith. On each soil, five plots were installed on a road before ripping, and nine plots were installed on the same road segment following ripping, four covered with a heavy straw mulch and five without. Three half‐hour rainfall events with intensities near 90 mm/hr were simulated on each plot. Results show that ripping increases hydraulic conductivities enough to reduce risk of runoff but does not restore the natural hydraulic conductivity of a forested slope. The unripped road surfaces had hydraulic conductivities in the range of 0–4 mm/hr, whereas ripped roads were in the range of 20–40 mm/hr after the second event. Surface sealing and tilled soil subsidence processes are important in reducing the hydraulic conductivity of the soils with repeated wetting. Subsidence appears to be important on the granitic soil, whereas surface sealing was more important on the belt series soil.

Occurrence of Arsenic-contaminatedGroundwater in Alluvial Aquifers from Delta Plains, Eastern India: Options for Safe Drinking Water Supply

Arsenic contamination in groundwater used for drinking purposes has been envisaged as a problem of global concern. Exploitation of groundwater contaminated with arsenic within the delta plains in West Bengal has caused adverse health effects among the population within a span of 8-10 years. The sources of arsenic in natural water are a function of the local geology, hydrology and geochemical characteristics of the aquifers. The retention and mobility of different arsenic species are sensitive to varying redox conditions. The delta plains in West Bengal are characterized by a series of meander belts formed by the fluvial processes comprising different cycles of complete or truncated fining upward sequences (sand-silt-clay). The arseniferous groundwater belts are mainly located in the upper delta plain and in abandoned meander channels. Mineralogical investigations have established that arsenic in the silty clay as well as in the sandy layers occurs as coatings on mineral grains. Clayey sediments intercalated with sandy aquifers at depths between 20 and 80 m are reported as a major source of arsenic in groundwater.Integrated knowledge on geological, hydrologicaland geochemical characteristics of the multi-level aquifer system of the upper delta plain is therefore necessary in predicting the origin, occurrence and mobility of arsenic in groundwater in West Bengal. This would also provide a basis for developing suitable low-cost techniques for safe drinking water supply in the region.

Sedimentary sequences, seismofacies and evolution of depositional systems of the Oligo/Miocene Lower Freshwater Molasse Group, Switzerland

ABSTRACTMagnetostratigraphic chronologies, together with sedimentological, petrological, seismic and borehole data derived from the Oligo/Miocene Lower Freshwater Molasse Group of the North Alpine foreland basin enable a detailed reconstruction of alluvial architecture in relation to Alpine orogenic events. Six depositional systems are recorded in the Lower Freshwater Molasse Group. The bajada depositional system comprises 200–400‐m‐thick successions of ribbon channel conglomerates and overbank fines including mud‐ and debris‐flows which were derived from the Alpine border chain. The alluvial megafan depositional system is made up of massive pebble‐to‐cobble conglomerates up to 3 km thick which reveal a fan‐shaped geometry. This depositional environment grades downcurrent into the conglomerate channel belt depositional system, which comprises an ≈2‐km‐thick alternation of channel conglomerates and overbank fines. The sandstone channel belt depositional system is bordered by the 100–400‐m‐thick overbank fines assigned to the floodplain depositional system. At the feather edge of the basin, 50–400‐m‐thick lacustrine sediments in both clastic and carbonate facies represent the lacustrine depositional system.The spatial and temporal arrangement of these depositional systems was controlled by the geometrical evolution of the Molasse Basin. During periods of enhanced sediment supply and during phases of stable sliding of the entire wedge, >2000‐m‐thick coarsening‐and thickening‐upward megasequences comprising the conglomerate channel belt, alluvial megafan and bajada depositional systems were deposited in a narrow wedge‐shaped basin. In the distal reaches of the basin, however, no sedimentary trend developed, and the basin fill comprises a <500‐m‐thick series of sandstone meander belt, floodplain and lacustrine depositional systems. During phases of accretion at the toe of the wedge, the basin widened, and prograding systems of multistorey channel sandstones extended from the thrust front to the distal reaches of the basin.The rearrangement of the depositional systems as a function of changing orogenic conditions created discordances, which are expressed seismically by onlap and erosion of beds delimiting sedimentary sequences. Whereas stable sliding of the wedge succeeded by accretion at the toe of the wedge is recorded in the proximal Lower Freshwater Molasse by a coarsening‐and thickening‐upward megasequence followed by erosion, the opposite trend developed in the distal reaches of the Molasse. Here, fine‐grained sandstones and mudstones were deposited during periods of stable sliding, whereas phases of accretion caused a coarsening‐ and thickening‐up megasequence to form.

Some characteristics of the orogenic belts in Qinghai-Tibet plateau

The Qinghai-Tibet plateau can be divided into a northern and a southern part, with the Kunlun central thrust forming the boundary. The nothern orogenic belts developed on the rigid basement, starting with rifting and geosynclines in the preorogenic stage, followed by collision to form mountains, and postorogenic deformation, mainly producing overthrusting and overlapping structures. The southern counterparts developed on a flexible basement, characterized by strong activity, and polycyclic opening and closing, with well defined oceanic basins and widespread terrigenous deposits on both flanks in the preorogenic stage, and complicated deformation in the postorogenic stage with a large number of strike-slip faults besides overthrusting and overlapping structures. The overall tectonic framework is a series of unique orogenic belts in the Qinghai-Tibet plateau. There are numerous orogenic belts developed in the Qinghai-Tibet plateau, such as the Himalayas, Gangdise, Tanggula, Hoh Xil, Kunlun and Qilian mountains. These belts provide valuable information on the evolution of the lithosphere. Therefore, it is important to summarize the main characteristics of the orogenic belts.

Petrology, geochemistry and geodynamic setting of Proterozoic igneous suites of the Orós fold belt (Borborema Province, Northeast Brazil)

The Orós belt is a metamorphosed and deformed supracrustal sequence whose deposition started at ca. 1.8 Ga. The volcanic rocks form an essentially bimodal association with a predominance of felsic volcanics. Mafic volcanics show geochemical and Nd isotopic differences which point to separate origins for each type. The mafic rocks are either chemically similar to EMORBs or are transalkaline types, enriched in LILE and LREE and relatively depleted in HFSE but with different isotopic signatures. One mafic type is associated with dominant andesites in a suite which could have evolved by an AFC process involving a Transamazonian-age source rock which later produced some of the overlying felsic volcanics. The rhyolites have variable geochemical signatures, all typical of anatectic products derived from continental crustal rocks. Sedimentary rocks are dominated by pelitic types of different provenances, accompanied by minor arenites and lesser carbonate rocks and calc-silicates. The depositional environment of the supracrustal sequence was continental, and a number of lines of evidence suggest that a rift environment probably developed during relaxation following the Transamazonian orogeny. The most abundant igneous rocks are felsic plutonics, emplaced nearly 100 Ma after the volcanic activity, but whose geochemical signature is similar to that of most of the rhyolites. These are cut by more alkaline anorogenic intrusives. The volcano-sedimentary sequence was intruded at ca. 0.9 Ga by a mafic-ultramafic sill. All deformation and metamorphism occurred during the Brasiliano orogeny, which was accompanied by the intrusion of syn-tectonic granites. The Orós belt, therefore, represents an intracontinental ensialic late Paleoproterozoic volcano-sedimentary basin initially related to strain relaxation of the crust. With subsequent collapse and development of faults, the anorogenic alkaline granites intruded at ca. 1.7 Ga. Based on the data available on this province, the Orós belt forms a part of a series of supracrustal belts of different ages, which were consolidated and welded in the Brasiliano orogeny. Within the Borborema Province, supracrustal sequences with ages of ≈ 2.0-1.9 Ga, ≈ 1.8-1.7 Ga (Orós), ≈ 1.1-1.0 Ga, and ≈ 0.6 Ga are presently known. All were aglutinated or amalgamated to neighboring blocks during the Brasiliano orogenic cycle.

Boninite-like rocks from the Palaeoproterozoic greenstone belt of Bogoin, Central African Republic: geochemistry and petrogenesis

Abstract The upper pillowed amphibolites of Bogoin (UPAB) are part of the supracrustal series of the Bogoin-Boali greenstone belt (western Central African Republic). They are underlain by Al-depleted komatiites, back-arc tholeiites and arc-related greywackes; they are interbedded with undepleted komatiites and overlain by banded iron formations (BIFs). PbPb and SmNd isotopic data yield an imprecise isochron age of about 2.3 Ga, which is believed to represent the time of greenstone deposition. No magmatic minerals are preserved in the UPAB, but pseudomorphs after olivine, Ca-poor and Ca-rich pyroxenes are present. The secondary paragenesis is related to upper greenschist facies metamorphism. Immobile element geochemistry suggests that UPAB are not siliceous high-magnesium basalts but are very similar to modern boninites. They are depleted in P and Ti, enriched in Sr and Zr and show high Zr/Sm, Sr/Sm, Al2O3/TiO2 and low Ti/V and Ti/Sc values. The REE patterns are U-shaped or LREE-depleted/HREE-enriched. The overprint of a magma mixing with the interbedded komatiites on a partial melting event can explain most of the differences noted between UPAB and modern boninites. UPAB genesis is explained by mixing depleted and enriched components. Sm/Nd isotopic data and major element contents suggest participation of a trondhjemite-like melt rather than sediments as the enriched component. The depleted component can be produced by remelting of a peridotitic residue following extraction of tholeiites similar to the tholeiites of the lower unit. The ascending diapir that yielded the upper komatiites may have furnished the necessary heat input to the remelting event. The presence of back-arc tholeiites, arc-related greywackes and boninite-like UPAB argues strongly for a compressive margin plate boundary setting for the Bogoin greenstone belt.

Depositional Setting of the Upper Jurassic Hith Anhydrite of the Arabian Gulf: An Analog to Holocene Evaporites of the United Arab Emirates and Lake MacLeod of Western Australia

The Upper Jurassic Hith Anhydrite is a major hydrocarbon seal in the Arabian Gulf region. Outcrops, core samples from the subsurface, and the literature indicate that the Hith Formation is composed mainly of anhydrite. In most locations where a section of the Hith Formation has been measured, this unit contains less than 20% carbonate, much of which is in the form of thin laminations. This lack of carbonate, locally thick layers of salt, and the predominance of anhydrite favor a playa for the setting in which this sediment was accumulated. In fact, much of the Hith has the sedimentary characteristics of the Holocene Lake MacLeod playa of Western Australia, which is dominated by layers of gypsum and halite (what little carbonate that occurs is found in layers at the base o the section). Locally the Hith appears to have accumulated in a sabkha setting, particularly toward central Abu Dhabi where it pinches out into shallow-water, and peritidal carbonate. This sabkha setting is indicated by the interbedded relationship of the Hith anhydrites with these carbonates and the local predominance of horizontally flattened nodules and enterolithic layers of anhydrite. These latter features match some of the characteristic fabrics found in the Holocene coastal sabkhas of the United Arab Emirates. As with the local occurrences in the Hith, the Holocene sabkhas are dominated by carbonates and are divisible into a series of lateral facies belts. These are also expressed as equivalent vertical layers. Traced from seaward to landward, or from the base of the vertical sequence upward these facies are characterized by (1) algal mat, (2) a layer of a gypsum crystal mush (3) active anhydrite replacement of gypsum (4) anhydrite with no gypsum mush, and (5) recycled eolianite and storm-washover sediments.