Crustal structure of the Rhinegraben area

Numerous ge ological and geophysical investigations within the past decades have shown that the Rhinegraben is the most pronounced segment of an extended continental rift system in Europe. The structure of the upper and lower crust is significantly different from the structure of the adjacent “normal” continental crust.

Two crustal cross-sections across the central and southern part of the Rhinegraben have been constructed based on a new evaluation of seismic refraction and reflection measurements. The most striking features of the structure derived are the existence of a well-developed velocity reversal in the upper crust and of a characteristic cushion-like layer with a compressional velocity of 7.6–7.7 km/sec in the lower crust above a normal mantle with 8.2 km/sec. Immediately below the sialic low-velocity zone in the middle part of the crust, an intermediate layer with lamellar structure and of presumably basic composition could be mapped.

It is interesting to note that the asymmetry of the sedimentary fill in the central Rhinegraben seems to extend down deeper into the upper crust as indicated by the focal depths of earthquakes. The top of the rift “cushion” shows a marked relief which has no obvious relation to the crustal structure above it or the visible rift at the surface.     

New D.S.S.-data on the crustal structure of the baltic and Ukrainian Shields

Recently completed investigations of the crustal structure on ancient shields of the East European platform carried out with the method of “deep seismic sounding” (D.S.S.) have drastically changed the previous notions about the deep structure of shields in general. In the upper crust, in the so-called “granitic” layer, complex anticlinal and synclinal structures as well as numerous faults, thrusts, etc., have been identified. A flattening of steeply dipping seismic interfaces with depth is observed. The crustal thickness in different tectonic zones ranges from 30 to 60 km. It is shown that the M-structure correlates with the sub-surface tectonics in the Ukrainian Shield.     

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.     

Lower lithospheric structure beneath the Trans-European Suture Zone from POLONAISE''97 seismic profiles

The large-scale POLONAISE'97 seismic experiment investigated the velocity structure of the lithosphere in the Trans-European Suture Zone (TESZ) region between the Precambrian East European Craton (EEC) and Palaeozoic Platform (PP). In the area of the Polish Basin, the P-wave velocity is very low (Vp <6.1 km/s) down to depths of 15–20 km, and the consolidated basement (Vp5.7–5.8 km/s) is 5–12 km deep. The thickness of the crust is 30 km beneath the Palaeozoic Platform, 40–45 km beneath the TESZ, and 40–50 km beneath the EEC. The compressional wave velocity of the sub-Moho mantle is >8.25 km/s in the Palaeozoic Platform and 8.1 km/s in the Precambrian Platform. Good quality record sections were obtained to the longest offsets of about 600 km from the shot points, with clear first arrivals and later phases of waves reflected/refracted in the lower lithosphere. Two-dimensional interpretation of the reversed system of travel times constrains a series of reflectors in the depth range of 50–90 km. A seismic reflector appears as a general feature at around 10 km depth below Moho in the area, independent of the actual depth to the Moho and sub-Moho seismic velocity. “Ringing reflections” are explained by relatively small-scale heterogeneities beneath the depth interval from 90 to 110 km. Qualitative interpretation of the observed wave field shows a differentiation of the reflectivity in the lower lithosphere. The seismic reflectivity of the uppermost mantle is stronger beneath the Palaeozoic Platform and TESZ than the East European Platform. The deepest interpreted seismic reflector with zone of high reflectivity may mark a change in upper mantle structure from an upper zone characterised by seismic scatterers of small vertical dimension to a lower zone with vertically larger seismic scatterers, possible caused by inclusions of partial melt.     

The deep-seated crustal structure in the western part of the Black Sea and adjacent areas: Seismic reflection measurement

In recent years the northwestern Black Sea has been investigated by a great number of geophysical methods. Charts of the M discontinuity and (isopachous) charts of the “granitic”, the “basaltic”, the Paleozoic, the Jurassic-Triassic, the Upper and Lower Cretaceous and the Eocene layers were plotted based on the results of the combined data of these investigations together with associated drilling data. The data for different velocity levels confirms the concept of layered-block structure of the crust, where large blocks are divided by deep faults penetrating to the upper mantle. Sedimentation within each block is continuous while reverse fault zones, dividing the East European Platform with a crustal thickness of more than 40 km and the Scythian Platform with a crust of about 30 km thick, and the latter from the Black Sea depression with crust of about 20 km, are discontinuous. Therefore, one can speak of a continuous-discontinuous nature of the sedimentation.

An inverse relationship in thicknesses of the “granitic” and sedimentary layers has been established. In places of intensive sedimentation the thickness of the “granitic” layer is less than that within the stable unsagging blocks. On the whole the greater the thickness of “basaltic” layer, the greater is the crustal thickness.

The relationship between the main geological structures of the area should be sought in the nature of structure of these “granitic” and “basaltic” layers.     

Active faults, seismicity and recent fracturing in the lithosphere of the Baikal rift system

A map of major active faults has been constructed for the Baikal rift system (BRS). Recent active faults are identified using seismological data. The BRS seismicity of the past 40 years is statistically analyzed. Areas of a “stable” concentration of epicenters are revealed. On this basis, a zone of recent fracturing of the lithosphere is identified and its relation to active and developing faults of the BRS is analyzed. The zone of the lithosphere fracturing is a major tectonic structure, which controls both the recent seismic process and the reactivation of ancient faults. It is demonstrated that the available seismological data can provide a basis for a detailed classification of faults by degree of their tectonic activity. Regularities in the distribution of strong earthquakes along the zone of the recent fracturing of the lithosphere are established, as well as regularities in the distribution of strong and weak seismic events relative to transform and other faults. The degree of the fault reactivation is determined by their spatial closeness to the axial zone of the recent rupturing of the lithosphere.     

Structure of the crust and upper mantle beneath the central european rift system

The crustal structure of the central European rift system has been investigated by seismic methods with varying success. Only a few investigations deal with the upper-mantle structure. Beneath the Rhinegraben the Moho is elevated, with a minimum depth of 25 km. Below the flanks it is a first-order discontinuity, while within the graben it is replaced by a transition zone with the strongest velocity gradient at 20–22 km depth. An anomalously high velocity of up to 8.6 km/s seems to exist within the underlying upper mantle at 40–50 km depth. A similar structure is also found beneath the Limagnegraben and the young volcanic zones within the Massif Central of France, but the velocity within the upper mantle at 40–50 km depth seems to be slightly lower. Here, the total crustal thickness reaches only 25 km. The crystalline crust becomes extremely thin beneath the southern Rhônegraben, where the sediments reach a thickness of about 10 km while the Moho is found at 24 km depth. The pronounced crustal thinning does not continue along the entire graben system. North of the Rhinegraben in particular the typical graben structure is interrupted by the Rhenohercynian zone with a “normal” West-European crust of 30 km thickness evident beneath the north-trending Hessische Senke. A single-ended profile again indicates a graben-like crustal structure west of the Leinegraben north of the Rhenohercynian zone. No details are available for the North German Plain where the central European rift system disappears beneath a sedimentary sequence of more than 10 km thickness.     

On certain regularities of crustal structure associated with the major geologic features of Southeastern Europe

The analysis of numerous seismic studies from various geological provinces has demonstrated that variations in crustal thickness depend primarily on the thickness of the “basaltic” layer. In some areas two M discontinuities can be found — the present one and an ancient one. The lower crust, formed in Proterozoic time is apparently still preserved. Roots exist under the former Proterozoic orogens, in spite of the complete denudation of the orogenic mountains. Younger (Paleozoic-Mesozoic) subsurface structures are not so clearly pronounced in the crustal structure. More active reconstruction of the crust seems to have taken place in the course of Alpine orogenesis.     

Provenance of the Neoproterozoic Maricá Formation (Sul-rio-grandense Shield, Southern Brazil): Petrographic and Sm–Nd isotopic constraints

This paper reports the integrated application of petrographic and Sm–Nd isotopic analyses for studying the provenance of the Neoproterozoic Maricá Formation, southern Brazil. This unit encompasses sedimentary rocks of fluvial and marine affiliations. In the lower fluvial succession, sandstones plot in the “craton interior” and “transitional continental” fields of the QFL diagram. Chemical weathering probably caused the decrease of the ¹⁴⁷Sm/¹⁴⁴Nd ratios to 0.0826 and 0.0960, consequently lowering originally > 2.0 Ga T_DM ages to 1.76 and 1.81 Ga. ¹⁴³Nd/¹⁴⁴Nd ratios are also low (0.511521 to 0.511633), corresponding to negative ε_Nd present-day values (− 21.8 and − 19.6). In the intermediate marine succession, sandstones plot in the “dissected arc” field, reflecting the input of andesitic clasts. Siltstones and shales reveal low ¹⁴³Nd/¹⁴⁴Nd ratios (0.511429 to 0.511710), ε_Nd values of − 18.1 and − 23.6, and T_DM ages of 2.16 and 2.37 Ga. Sandstones of the upper fluvial succession have “dissected arc” and “recycled orogen” provenance. ¹⁴³Nd/¹⁴⁴Nd isotopic ratios are also relatively low, from 0.511487 to 0.511560, corresponding to ε_Nd values of − 22.4 and − 21.0 and T_DM of 2.07 Ga. A uniform granite–gneissic basement block of Paleoproterozoic age, with subordinate volcanic rocks, is suggested as the main sediment source of the Maricá Formation.     

Early Ordovician continental break-up in Variscan Europe: Nd---Sr isotope and trace element evidence from bimodal igneous associations of the Southern Massif Central, France

The high-grade Marvejols Group and the low-grade Albigeois-Cévennes sedimentary sequence contain bimodal igneous rocks of Early Ordovician age which are representative of a widespread thermal event in the European Variscides. Comparison of their Nd---Sr isotope and trace-element characteristics provides additional evidence for their origin in an ensialic extensional setting. As an alternative to the back-arc model proposed by previous authors, we propose that these associations record a continental break-up episode unrelated to contemporaneous subduction. In this model, the widespread Early Paleozoic bimodal magmatism marks the birth of a Mid-European oceanic arm. We propose that the break-up was controlled by both transtensional processes and mantle-plume activity.

High grade associations, such as the Marvejols Group, and low-grade bimodal associations, such as the Albigeois-Cévennes, might be regarded as representing opposite rifted passive margins. The “northern”, Marvejols-type margin was involved in a Late Ordovician-Silurian subduction-like process, with HP-HT metamorphism. In contrast, the “southern” Albigeois/Cévennes margin remained largely unaffected, possibly as part of the overriding plate of the subduction zone.

“Pseudo-calc-alkaline” signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break-up episodes. In this case, negative Nb anomalies are produced by the addition of crustal components enriched in Th and LREE relative to Nb. This inferred alternative origin of Nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data.     

Paragenesis of “box-work geodes”, Tampa Bay, Florida

An unusual suite of silicified rocks was excavated during a recent harbour-deepening project in Tampa Bay, Florida. These rocks, which we have termed “box-work geodes”, are composed of convoluted, intersecting silica walls enclosing cavities which are either voids or filled with relatively pure monoclinic palygorskite. The “box-work geodes” are interpreted as having formed in shallow lagoonal environments, similar to the Coorong Lagoon of South Australia. Synaeresis of syngenetic palygorskite was followed by opal deposition and case hardening of the material. Subsequent chemical deposition of chalcedony, megacrystalline quartz, barite, and calcite on the void facing walls indicates an open chemical system.

The existence of opal saturated lagoons, as inferred from the “box-work geodes”, suggests that much of the replacement chert, porcelanite, and silicified fossils in the Tertiary deposits of peninsular Florida formed in the shallow subsurface. Subsequent weathering of carbonates and clays not encapsulated in the box works has resulted in formation of a green montmorillonite residual clay bed.     

Cumulate-like textures and chemical relationships in the Bressanone (Brixen) Granodiorite (Eastern Alps, Italy)— A new genetic approach

The shallow level pluton of Bressanone is a Late Hercynian multiple intrusion into the South Alpine basement of the Eastern Alps. Most of this complex is composed of anatectic granodiorites and granites intruded in separate stocks 282 ± 14 Ma ago; gabbros and leucogranites occur in smaller quantities. The chronological intrusion sequence is: layered gabbro, granodiorites and granites, two-mica cordierite leucogranite and fayalite leucogranites.

The granodiorites and granites may contain hornblende or garnet. The hornblende and garnet rocks differ both in chemistry and (⁸⁷Sr/⁸⁶Sr)_i ratio, and may be identified as “I-type” and “S-type”, respectively, according to the Chappell-White classification.

Textural and chemical patterns show that the granites may be linked to the granodiorites by cumulate-like processes. The granodiorite → granite transition, attributed to filter pressing, expresses an increase in the liquid/xenolith ratio in a magma where the liquid fraction was a minimum melt and the solid fraction was restitic material.     

Magma in forearcs: implication for ophiolite generation

Forearc areas (“non-volcanic” arcs) of contemporary island arcs at convergent plate boundaries contain magmatic rocks. Geological evidence, seismic profiles, heat flow data, density considerations and petrological and geochemical arguments suggest that a forearc tholeiitic association (FAT) (containing high-Mg calc-alkaline andesites) is present in “non-volcanic” arcs at some stage of island-arc development. The fractionated, as well as primitive magma, is unable to penetrate low-density sediments and underplates thick piles of unconsolidated accreting rocks. The underplating causes upwelling. The occurrence of magma in forearcs provides an alternative interpretation for the tectonic setting of some ophiolitic masses. Rather than “ocean-ridge formation” and later “obduction” it offers an autochthonous (island-arc bound and geologically-substantiated) interpretation for the ophiolite suite.     

The Oslo Rift: P-T relations and lithospheric structure

Extrusion temperatures for basaltic lavas in the Permo-Carboniferous Oslo Rift, estimated from whole rock major element compositions, are estimated to be 1270 to 1340°C. This means that magmatism during the Oslo rifting event was not associated with a large temperature anomaly in the underlying upper mantle. Partial melting is believed to be caused by a combination of crustal extension, a weak temperature anomaly in the underlying asthenosphere, and/or high fluid-contents in the mantle source region (“wet-spot”). Petrological and gcochemical data imply that large masses of cumulate rocks were deposited in the deep crust during the Oslo rifting event. The densities and seismic velocities (V_p) of these cumulate rocks are estimated to be 2.8–3.5 g/cm³ and 7.5–8.0 km/s. A rough estimate suggests that cumulus minerals alone account for a net transfer of at least 2 × 10¹⁷ kg of magmatic material from the mantle into the deep crust. In addition comes material representing

1. (a) cumulate minerals corresponding to eroded magmatic surface and subsurface rocks

2. (b) intercumulus material, and

3. (c) magmas crystallized to completion in the deep crust.

Estimates based exclusively on geophysical data tend to underestimate the true transfer of mass into the lower crust as gabbroic cumulate rocks, and melts crystallizing to completion in the lower crust have densities and seismic velocities similar to those of lower crustal wallrocks.     

Structure of the earth''s crust on the territory of the U.S.S.R.

The compilation of statistical data for 269 seismic crustal sections (total length: 81,000 km) which are available in the U.S.S.R. has shown that the preliminary conclusions drawn on relations between the elevation of the surface relief and Bouguer anomalies on one hand and crustal thickness (depth to the M-discontinuity) on the other hand are not fulfilled for the continental part of the U.S.S.R. The level of isostatic compensation has been found to be much deeper than the base of the earth's crust due to density inhomogeneities of the crust and upper mantle down to a depth of 150 km.

The results of seismic investigations have revealed a great diversity of relations between shallow geological and deep crustal structures:

Changes in the relief of the M-discontinuity have been found within the ancient platforms which are conformable with the Precambrian structures and which can exceed 20 km. In the North Caspian syneclise, extended areas devoid of the “granitic” layer have been discovered for the first time in continents. The crust was found to be thicker in the syneclises and anteclises of the Turanian EpiHercynian plate. In the West Siberian platforms these relations are reversed to a great extent.

Substantial differences in crustal structure and thickness were found in the crust of the Palaeo zoides and Mesozoides. Regions of substantial neotectonic activity in the Tien-Shan Palaeozoides do not greatly differ in crustal thickness if compared to the Kazakhstan Palaeozoides which were little active in Cenozoic time. The same is true for the South Siberian Palaeozoides.

The Alpides of the southern areas in the U.S.S.R. display a sharply differing surface relief and a strongly varying crustal structure. Mountains with roots (Greater Caucasus, Crimea) and without roots (Kopet-Dagh, Lesser Caucasus) were found there.

The Cenozoides of the Far East are characterized by a rugged topography of the M-discontinuity, a thinner crust and a less-pronounced “granitic” layer. A relatively small thickness of the crust was discovered in the Baikal rift zone.

The effective thickness of the magnetized domains of the crust as well as other calculations show that the temperature at the depth of the M-discontinuity (i.e., at depths of 40–50 km) is not higher than 300–400° C for most parts of the U.S.S.R.     

The uranium ore from Mina Fe (Salamanca, Spain) as a natural analogue of processes in a spent fuel repository

In the frame of the ENRESA natural analogue programme, the uranium ore from the “Mina Fe” (Salamanca, Spain) has been studied as a natural analogue of radioactive spent fuel behaviour. This uranium mine is hosted in highly fractured schistose rocks, a geological setting that has not been envisaged in the Spanish options for radioactive waste burial. However, some analogies with the processes that might be involved in the evolution of these geological repositories suggested this investigation.

The pitchblende–pyrite–carbonate paragenesis has been studied “in situ” as natural analogue of the nuclear spent fuel behaviour under extremely oxidative dissolution conditions. Similarly, secondary Fe oxyhydroxides and clay minerals have also been considered as relevant analogue materials for the retention of uranium and other analogous trace metals. A multidisciplinary characterisation of the site has been performed in order to study these processes.

Though the intense mining activities in the site hindered precise determination of the original hydrogeological and hydrochemical features of the investigated zone (Boa fault zone), the mineralogy and geochemistry of fracture fillings, mineralisation and associated clayey materials have allowed the geochemical evolution of the system to be established. Three geochemical zones have been clearly differentiated: (i) the oxidised zone, from the surface to approximately 20 m depth, (ii) the redox transition zone, from 20 to 50 m depth, and (iii) the reduced zone, located below the transition zone.

The oxidised zone is characterised by the presence of the typical mineral association resulting from the strong acid conditions caused by the total oxidation of pyrite and other sulphides. The total oxidation, dissolution and leaching of U(IV), as uranyl–sulphate aqueous complexes, prevailed in this oxidised zone. The redox transition zone is characterised by the coexistence of the primary uranium paragenesis, oxidised minerals, as well as numerous secondary solid phases as a result of the physico-chemical changes in the environment. The optimal physico-chemical conditions for the coffinitisation of pitchblende and the co-precipitation of Fe(III)–U(VI) took place in this zone. In the reduced zone, where the primary uranium paragenesis is present, we currently find the necessary physico-chemical conditions to stabilise pitchblende, pyrite and carbonates.

The physico-chemical conditions of the oxidised zone are not relevant to disposal conditions. In the transition zone, two main geochemical processes take place: (i) the coffinitisation of pitchblende, which may be an important process for the stability of spent fuel in reducing conditions, and (ii) the co-precipitation of the Fe(III) and U(VI) as oxyhydroxides, another relevant mechanism for the retention of uranium. The physico-chemical conditions that prevail below 50 m depth should be sufficient to stabilise a spent nuclear fuel repository, in the same way as they have been able to preserve the 34-Ma-old uranium deposit of the Mina Fe.     

Palaeozoic structural development along the Tornquist Zone, Kattegat area, Denmark

The interpretation of newly released commercial 2D reflection seismic data in the Kattegat area, Denmark, has provided us with a better understanding of the Palaeozoic tectonic processes along the Tornquist Fault Zone. A Base Palaeozoic time structure map, a Lower Palaeozoic TWT isopach map, a “true” Lower Palaeozoic TWT isopach map, an Upper Carboniferous/Lower Permian syn-rift TWT isopach map, a Top pre-Zechstein time structure map and a Zechstein combined TWT isopach and Palaeogeography map have been generated. The uniform Lower Palaeozoic sequence thickness in the Kattegat, both inside and outside the Tornquist Zone indicates only minor lateral movements if any, whereas the extensive Upper Silurian sequence, increasing in thickness to the north, indicates a relatively fast regional subsidence. The Base Palaeozoic time structure map and the Late Palaeozoic syn-rift isopach map show a clear Late Palaeozoic extension in the area. The syn-rift isopach map, in combination with the time-equivalent opening of the Skagerrak graben at right angles to the Tornquist Zone in the Kattegat, indicates that this extensional tectonic event had a dextral slip component. Measurements on internal extensional faults in the Tornquist Zone, give a minimum right-lateral displacement of 10.4 km. The footwall blocks were deeply eroded during the Early Permian rifting, and at Zechstein times the area became a peneplane. The Tornquist Zone was later exposed to several tectonic phases, where dextral slip played a role, indicated by the “push up” and “pull down” structures caused by restraining and releasing bends of the Børglum Fault. The dextral displacement along the Børglum Fault since the beginning of the Permian is in the order of 5–7 km based on the displacement of a Lower Palaeozoic local depocentre. Early Permian depocentres and faults, which gives a total amount of right-lateral displacement since the Early Palaeozoic in the order of 15–20 km. The continuously repeated tectonic episodes along the Tornquist Zone throughout most of the Phanerozoic, show that the zone was easily reactivated, implying deep-seated basement faults. The Tornquist Zone can be seen as a “buffer zone”, between continental blocks, whenever changes in the regional stress field are induced.     

Anthracitization of coals in the South Wales coalfield

Investigations by the Institute of Geological Sciences on potential geothermal water sites in the coalfield of South Wales have been reported. They are of considerable interest and revive the controversy about the nature of the heat-source responsible for the anthracitization of some coals in this classic area. In the absence of exposed post-Carboniferous igneous rocks in the coalfield, general opinion has favoured heat generation by friction along faults or “disturbances”. We re-examine the evidence in the light of more recent geophysical and geochemical investigations which suggest the possibility of igneous intrusions at a depth of about 3.5 km or shallower. The likely age of such intrusions is also considered. High seismic velocity basement rocks rise to only 2.5 km beneath the surface of the Carboniferous in the northwestern anthracitized zone of the coalfield and we suggest that anthracitization in South Wales is not the result of depth of burial nor of frictional heat but of proximity to a magmatic heat source associated with crystalline rocks of the basement.     

The crust-mantle boundary beneath cratons and craton margins: a transect across the south-west margin of the Kaapvaal craton

“Lower-crustal suite” xenoliths occur in “on-craton” and “off-craton” kimberlites located across the south-western margin of the Kaapvaal craton, southern Africa.

Rock types include mafic granulite (plagioclase-bearing assemblages), eclogite (plagioclase-absent assemblages with omphacitic clinopyroxene) and garnet pyroxenite (“orthopyroxene-bearing eclogite”). The mafic granulites are subdivided into three groups: garnet granulites (cpx + grt + plag + qtz); two pyroxene garnet granulites (cpx + opx + grt + plag); kyanite granulites (cpx + grt + ky + plag + qtz). Reaction microstructures preserved in many of the granulite xenoliths involve the breakdown of plagioclase by a combination of reactions: (1) cpx + plag → grt + qtz; (2) plag → grt + ky + qtz; (3) plag → cpx (jd-rich) + qtz. Compositional zoning in minerals associated with these reactions records the continuous transition from granulite facies mineral assemblages and pressure (P) — temperature (T) conditions to those of eclogite facies.

Two distinct P-T arrays are produced: (1) “off-craton” granulites away from the craton margin define a trend from 680 °C, 7.5 kbar to 850 °C, 12 kbar; (2) granulite xenoliths from kimberlites near the craton margin and “on-craton” granulites produce a trend with similar geothermal gradient but displaced to lower T by ˜ 100 °C. Both P-T fields define higher geothermal gradients than the model steady state conductive continental geotherm (40 mWm²) and are not consistent with the paleogeotherm constructed from mantle-derived garnet peridotite xenoliths.

A model involving intrusion of basic magmas around the crust/mantle boundary followed by isobaric cooling is proposed to explain the thermal history of the lower crust beneath the craton margin. The model is consistent with the thermal evolution of the exposed Namaqua-Natal mobile belt low-pressure granulites and the addition of material from the mantle during the Namaqua thermal event (c. 1150 Ma). The xenolith P-T arrays are not interpreted as representing paleogeotherms at the time of entrainment in the host kimberlite. They most likely record P-T conditions “frozen-in” during various stages of the tectonic juxtaposition of the Namaqua Mobile Belt with the Kaapvaal craton.     

准噶尔盆地滴北凸起早石炭世沉积充填特征及油气地质意义

利用钻井、测井、地震、古生物及地化分析资料,研究了准噶尔盆地滴北凸起早石炭世构造环境、沉积充填特征及其油气地质意义。结果表明,滴北凸起早石炭世发育半地堑式伸展断陷,发育海相复理石建造并发育海底火山喷发岩,具备烃源岩的发育环境。泉5井最新的地化分析表明,该区烃源岩发育且成熟度高,为好较好烃源岩;这个结果改写了滴北凸起石炭系烃源岩成熟度低、无勘探潜力的认识。依据克拉美丽气田“近源隆控”的成熟勘探模式,提出滴北凸起发育“源储一体”和“近源隆控”2种模式,明确了有利勘探区带。