Hot Mantle Research Articles

Development of volcanic passive margins: Three‐dimensional laboratory models

Continental breakup above an anomalously hot mantle may lead to the development of volcanic margins. Volcanic margins are characterized by (1) thick seaward dipping lava flow sequences, (2) central intrusive complexes associated with dyke swarms parallel to the coast, and (3) high seismic velocity bodies in the lower crust attributable to magma underplating. A conceptual model for volcanic margins development has recently been proposed based on onshore studies of the Greenland margins and the British Tertiary Igneous Province. It is proposed that the long‐lived central intrusions are genetically linked to underlying persistent zones of mantle fusion. These localized melting domains (or soft spots), equivalent to small mantle diapirs, may locally soften the extending continental lithosphere. The low‐viscosity diapirs would (1) localize tectonic strain and (2) feed the volcanic margin with magma. Thus such soft spots can control the along‐strike magmatic and tectonic segmentation of volcanic margins. Recent geophysical investigations appear to show that the along‐strike structure of volcanic passive margins is compatible with such a segmentation process. Here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub‐Moho mantle may have a mechanical effect on continental breakup. Four‐layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle. The soft spots are simulated by low‐viscosity silicone putty emplaced within the brittle material. At the scale of the entire breakup zone, the soft spots display an oceanic‐type strength profile defining low‐strength zones where continental breakup is initiated. The rift orientation and segmentation are strongly controlled by the distribution of the low‐viscosity heterogeneities, rather than by the direction of regional extension. The experiments are compared with the geometry and segmentation of the onshore part of the Greenland volcanic margins.

Slabs in the lower mantle and their modulation of plume formation

Numerical mantle convection models indicate that subducting slabs can reach the core‐mantle boundary (CMB) for a wide range of assumed material properties and plate tectonic histories. An increase in lower mantle viscosity, a phase transition at 660 km depth, depth‐dependent thermal expansivity, and depth‐dependent thermal diffusivity do not preclude model slabs from reaching the CMB. We find that ancient slabs could be associated with lateral temperature anomalies ∼500°C cooler than ambient mantle. Plausible increases of thermal conductivity with depth will not cause slabs to diffuse away. Regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the CMB, even for radially simple mantle structures. The observation from tomography showing only a few continuous slab‐like features from the surface to the CMB may be a result of complex plate kinematics, not mantle layering. There are important consequences of deeply penetrating slabs. Our models show that plumes preferentially develop on the edge of slabs. In areas on the CMB free of slabs, plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin. However, in areas beneath slabs, the basal thermal boundary layer would be thicker and plume formation infrequent. Beneath slabs, a substantial amount of hot mantle can be trapped over long periods of time, leading to “mega‐plume” formation. We predict that patches of low seismic velocity may be found beneath large‐scale high seismic velocity structures at the core‐mantle boundary. We find that the location, buoyancy, and geochemistry of mega‐plumes will differ from those plumes forming at the edge of slabs. Various geophysical and geochemical implications of this finding are discussed.

Formation of volcanic rifted margins: Are temperature anomalies required?

A major controversy for understanding volcanic margin formation is whether a thermal anomaly in the mantle is required to generate the characteristic thick igneous crust. We use a fluid‐dynamical model of mantle flow that includes feedback from melting on the physical properties of the mantle to study the temporal evolution of volcanic margins during continental breakup. We test how weak the mantle must be to induce small‐scale convection and thus enhance melt production during breakup. We find that the mantle reference viscosity required to generate a breakup instability is sufficiently low that an unrealistic time dependence develops in the subsequent oceanic spreading system. The viscosity increase associated with dehydration melting can stabilize the system, but only at the expense of eliminating the breakup instability. An exhaustible reservoir of hot mantle resolves this inconsistency. Melt productivity increases through a deepening of the mantle solidus, while a concurrent increase in buoyancy and a decrease in viscosity in the melting region promote an initial transient phase of small‐scale convection.

Crust and upper mantle structure beneath western China from P wave travel time tomography

Three‐dimensional velocity images of the crust and upper mantle beneath western China are obtained through seismic tomography by using P wave arrival times of local/regional and teleseismic events recorded in Chinese and Kyrgyzstan seismic networks. At shallow depths, most mountain belts and tectonically elevated areas are imaged as higher velocities, relative to adjacent sediment‐filled basins and foreland depressions with lower velocities. At mid‐lower crust depths, the Kyrgyz Tian Shan is underlain by lower velocity material and a distinct low‐velocity zone in the south‐north direction is observed between the eastern and western parts of the Chinese Tian Shan. The Moho of the southern Tarim basin has a greater depth over 50 km in front of the western Kun Lun, compared to the northern and western Tarim basin. Lateral heterogeneity within the upper mantle is apparent at depths from 71 to 571 km. The uppermost mantle beneath the Kyrgyz Tian Shan, Altay, Qi Lian, and eastern Kun Lun is characterized by low‐velocity material that is probably related to the upwelling of hot mantle. The lithospheric thickness varies from 120 to 170 km beneath the Chinese Tian Shan and northern Tibetan Plateau to 200–250 km beneath the Tarim basin. Deep coupling pattern of the Chinese Tian Shan with adjacent provinces can be classified into the embedding, detaching and sinking, and underthrusting of lithospheric roots. In addition, the lithosphere of the northern edge of the Tibetan Plateau seems to be thickened, in contrast to the central part of the northern plateau. North dipping low velocities exist in the upper mantle between the Tibetan Plateau and provinces to its north, which might indicate mantle flow from the Tibetan Plateau toward surrounding areas, but it has been blocked by the thick Tarim lithosphere.

Hot fingers in the mantle wedge: new insights into magma genesis in subduction zones

Quaternary volcanoes in the Northeast Japan arc can be grouped into 10 volcanic clusters striking transverse to the arc; these have an average width of 50 km, and are separated by parallel gaps 30–75 km wide. This clustering of volcanic centres, topographic profiles, low-velocity regions in the mantle wedge and local negative Bouguer gravity anomalies along the Japan Sea side of the volcanic arc are closely correlated. All these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined, 50 km wide fingers. Each of the 10 fingers recognised extends from deep mantle (>150 km) below the back-arc region towards the shallower mantle (∼50 km) beneath the volcanic front. Quaternary volcanoes are built immediately above the hot mantle fingers. The volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust, accompanied by Quaternary volcanic activity at the surface. Although volcanic activity is rare along the Japan Sea coast, tomographic results show that hot, low-velocity mantle fingers exist within the mantle wedge. The negative Bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers; these have not yet been erupted, but have accumulated at the Moho discontinuity.

Mineralogical and geochemical constraints for the origin and evolution of magmas in Sierra Chichinautzin, Central Mexican Volcanic Belt

The Sierra Chichinautzin Volcanic Field is a key area for understanding the origin of the controversial Mexican Volcanic Belt (MVB). This is due to its recent volcanic activity, which is located at the front of the Central MVB. Although a direct genetic relationship between the volcanism of the MVB and the subduction of the Cocos plate is accepted by most authors, geological, geophysical and geochemical key features of the MVB depart from those of typical magmatic arcs. This fact has promoted a debate on the nature of the complex mantle beneath the MVB: (1) slab-induced convection in the mantle wedge that causes advection of asthenospheric mantle and (2) anomalously hot mantle related to the eastward channeling of plume-related material. This is a petrological study of the Sierra Chichinautzin volcanism, using mineralogical data from selected representative samples, together with their geochemistry, to refine the existing models of magmatic processes. The integrated petrological analyses have been used to define four main groups of rocks in Sierra Chichinautzin: (1) Ol-mafic rocks, (2) Opx-intermediate rocks, (3) Opx-felsic rocks, and (4) Qtz-intermediate/felsic rocks. Moreover, two different types of mantle-derived primitive mafic magmas have been described based on their mineralogical and geochemical features: (1) OIB-like mafic magmas (presence of olivine+plagioclase phenocrysts; higher incompatible element contents with no HFSE/LILE negative anomalies), and (2) mafic magmas derived from a metasomatized mantle (no plagioclase phenocrysts and more forsteritic olivine; HFSE/LILE negative anomalies). We prove that the diversity of magmas (basalts to dacites) in Sierra Chichinautzin is un-related to fractional crystallization processes from the mafic magmas. Rather, magma mixing phenomena between mafic magmas and two different crustal-derived felsic magma types (Opx-dacite and Qtz+Bt+Amp+Pl+Cpx+Opx-dacite) explain the textural, mineralogical, and geochemical signatures of most of the Sierra Chichinautzin rocks. There is a strong genetic relationship between the mafic and felsic end members of each mixing trend. The relatively anhydrous OIB-like mafic magmas promote partial melting of a basaltic lower crust under low water fugacity conditions, generating Opx-bearing felsic rocks. On the contrary, hydrous mafic magmas derived from a metasomatized mantle, promote basaltic crustal partial melting under high water fugacities, yielding magmas with a complex hydrous phenocryst assemblage. Several important geodynamical implications for the origin of the MVB magmatism can be extracted from this petrological study: (1) the existence of two different primitive mafic magmas implies the presence of an heterogeneous mantle source beneath Sierra Chichinautzin; (2) present data do not offer enough evidence to constrain the nature of the metasomatizing fluids beneath Sierra Chichinautzin; and (3) the negative Nb anomaly of most of the Sierra Chichinautzin rocks (andesites and dacites) is due to magma mixing, and therefore is not a signature of the role of fluids from the subducting Cocos plate. These geochemical features, together with the geophysical and tectonic scenario of the Sierra Chichinautzin, indicate that volcanism in this area is mainly related to the upwelling of an anomalously hot OIB-like mantle, and cast further doubts on a significant role of the subducting Cocos plate in the origin of this magmatism.

Episodic Volcanism and Hot Mantle: Implications for Volcanic Hazard Studies at the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada

Episodic Volcanism and Hot Mantle: Implications for Volcanic Hazard Studies at the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada

The emplacement of peridotites and associated oceanic rocks from the Lizard Complex, southwest England

Upper mantle peridotites and associated oceanic rocks from the Lizard Complex, southwest England, preserve evidence for a multistage geological history. Steeply dipping pre-emplacement fabrics record high-temperature (900–1100°C) shearing and exhumation of the mantle peridotites apparently formed during localized NE–SW rifting in a pull-apart basin setting (c. 400–390 Ma). Associated oceanic rocks (Landewednack amphibolites) preserve a pre-emplacement prograde brown amphibole-bearing metamorphic assemblage and steeply dipping fabric thought to have formed as the newly formed oceanic crust was juxtaposed with newly exhumed hot mantle peridotite during NE–SW rifting. In both the peridotites and Landewednack amphibolites, steep pre-emplacement structures are cross-cut by low-angle mylonitic fabrics thought to have formed during the initial phases of emplacement of mantle over crustal rocks in a partially intra-oceanic setting (c. 390–375 Ma). The fabrics in peridotites and amphibolites exhibit retrograde mineral assemblages (c. 500–800°C), with the amphibolites preserving two superimposed assemblages, green amphibole + titanite and colourless magnesio-hornblende, respectively, that are thought to record progressive down-temperature deformation during thrusting. Emplacement-related structures in both the basal peridotites and amphibolites consistently dip at low to moderate angles NW, with down-dip lineations and kinematic indicators showing consistent top-to-the-NW senses of shear. Syn-emplacement magmatism is recorded by intrusions of foliated Kennack Gneiss. Anastomosing serpentine-filled faults mark many existing low-angle contacts between the peridotites and Landewednack amphibolites and appear to represent the final, lowest-temperature (< 250°C) stages of emplacement (c. 370 Ma). This study shows that ‘dynamothermal aureoles’ in ophiolites may preserve evidence for tectonothermal events that pre-date thrust emplacement.

Stratigraphy and Tectonic Evolution of Nefertiti Corona on Venus

This paper deals with some aspects of the formation and development of coronae, specific large circular structures on Venus. The origin of coronae is commonly associated with the effect of rising and subsequently relaxing hot mantle plumes (diapirs) on the surface layers of the lithosphere. A detailed photogeologic study of one of such structures, Nefertiti corona, which is undertaken in this paper, is based on an analysis of Magellanradar images. A sequence of geologic formations revealed in the territory under investigation made it possible, in combination with an analysis of tectonic structures, to develop a step-by-step scenario of the evolution of this structure. It was established that Nefertiti has gone the entire cycle of corona evolution—from the formation of a radially fractured rising dome (“nova”) to the mature corona. At the final stage of evolution of this corona, traces of its rejuvenation and the origin of a new system of radial fracturing in the central part of the corona are observed. Our observations of Nefertiti corona are compared to theoretical (numerically solved) and tectonophysical models of corona formation, which were described by some other researchers. The inferred evolution of Nefertiti agrees with commonly accepted geologic models of corona evolution.

A mantle melting profile across the Basin and Range, SW USA

The major and trace element composition of late Cenozoic basalts (0–10 Ma) across the Basin and Range province (B&R) preserve a clear signal of mantle melting depth variations. FeO, Fe8.0, and Tb/Yb increase, whereas Si8.0 and Al8.0 decrease, from west to east across the B&R along a profile at 36°–37°N. These variations are qualitatively consistent with shallower melting beneath the Western Great Basin (WGB) than in the central B&R. In order to quantify the depth range and percent of decompression melting, we invert primary Na2O and FeO contents of basalts using a melting model based on the partitioning of FeO and MgO in olivine and Na2O in clinopyroxene. An independent inversion, using the rare earth elements (REE), corroborates the melting depths obtained from the major element model and places most of the melting beneath the central B&R in the garnet‐peridotite stability field. We find that the shape of the melting region across the B&R closely mimics the shape of the mantle lithosphere, as inferred from geological and geophysical observations. Melting across the study area occurs largely within the asthenosphere and generally stops at the base of the mantle lithosphere. In the WGB, melting paths are shallow, from 75 to 50 km, and in some cases extend almost to the base of the crust. These melting paths are consistent with adiabatic melting in normal‐temperature asthenosphere, beneath an extensively thinned (or absent) mantle lithosphere. Shallow melting is consistent with geobarometry and isotopic compositions of local mantle xenoliths. Lithospheric thinning was caused by thermal erosion during Mesozoic subduction and/or simple shear or foundering during Cenozoic extension. In contrast, melting beneath the central B&R occurs beneath thick mantle lithosphere and requires mantle potential temperatures 200°C hotter than normal (melting paths from 140 to 100 km). The excess temperature beneath the central B&R is consistent with active upwelling of hot mantle in this region.

Mantle insulation beneath the West African craton during the Precambrian-Cambrian transition

At the time of the Precambrian-Cambrian transition, the West African craton underwent widespread magmatism, hydrothermal activity, and thermal rejuvenation. This tectonothermal event gave rise to an anorogenic ‘‘ring of fire’’ along the rim of this craton, following the Pan-African‐Brasiliano belt that was reactivated by extension and transtension. The thermal phenomena were due to the progressive peripheral release of mantle heat that had built up beneath this craton because of strong insulating conditions. The West African craton at the Precambrian-Cambrian transition can thus be envisioned in terms of a gigantic pressure-cooker with a thick blanketing lithospheric lid. These insulation processes triggered an unusually hot mantle that was channeled by edge-driven convection toward the peri‐West African craton extensional corridors and released through magmatic pressure-relief valves. Massive ice melting and outgassing of volcanic CO2 gave rise to a planet-scale sea-level rise, a greenhouse effect, and the end of the icehouse snowball Earth. These processes played an important role in the Phanerozoic explosion of life on Earth.

Mantle plumes and sequence stratigraphy; Late Maastrichtian- Early Paleocene of West Greenland

The sedimentary history of the upper Maastrichtian–Paleocene succession underneath the extensive Paleocene flood basalts in central West Greenland supports models for the generation of flood basalt provinces in response to rising, hot mantle plumes. The rise of the North Atlantic mantle plume was associated with deposition of at least three sedimentary sequences; each associated with incision of submarine canyons and valleys. Relative sea-level changes were caused by plumerelated tectonics and generation of sequence boundaries was in general associated with catastrophic sedimentation and very rapid development of sequences. As such the late Maastrichtian–early Paleocene sequences record a spectacular and significant but rare geological event.

Focussed time‐dependent martian volcanism from chemical differentiation coupled with variable thermal conductivity

Recently the Orbiter Camera on the Mars Global Surveyor imaged on some lava flows. The sparse cratering and thence young age of these flows indicates that volcanic activity on Mars is long‐lived and perhaps pulsating. These pulses beg for an explanation, not yet provided by one‐dimensional convection models simulating the thermal and compositional evolution of the Martian interior. We have, for the first time, simulated the thermochemical evolution of Mars with a two‐dimensional model for the entire planetary history. Our results show that variable thermal conductivity k(T, P) can maintain sublithospheric “hot spots” over time scales of several hundred Ma and is essential in the focussing of hot mantle upwellings at almost the same location. This scenario can explain the persistent pulsating and focussed volcanism in the Tharsis region.

Ilha dos Búzios, Litoral Norte do Estado de São Paulo: aspectos geológicos e petrográficos

Cretaceous syenitic rock-types intruded into Precambrian charnockites characterize the geology of Búzios island (7.5 km²) in area off the northern coast of the State of São Paulo, SE Brazil. Both lithologic units are cut by a large number of dykes, mostly NE-trending and quite variable in composition. The syenites may include charnockitic and mafic rock xenoliths and also contain quartz-filled, irregular miarolitic cavities. The alkaline rocks, corresponding to approximately 90% of the whole island, are dominantly coarse-grained and range in composition from alkali feldspar syenites to quartz-alkali feldspar syenites. Locally, fine-grained syenitic types are also found. Dykes are widespread over the island and can be placed into the felsic group, which includes lithologies varying from phonolites to trachytes to rhyolites (microgranites), or into the mafic-ultramafic group, the latter mainly consisting of diabases, microdiorites and lamprophyres. Alkali feldspar (micromesoperthite with slight dominance of albititic phases) is the most abundant mineral in syenitic rocks. Additional components are clinopyroxene (diopside-augite ranging to aegirine-augite), commonly replaced by amphibole/biotite, and opaques (magnetite, ilmenite), with apatite, sphene and zircon as accessories. The felsic dykes may contain other feldspars (sanidine and albite) either as micro- to phenocrysts or as groundmass phases; feldspathoids (nepheline, sodalite) are also found in some microsyenites. The mafic-ultramafic dykes, in particular the lamprophyres, are mainly characterized by a primary mineral assemblage consisting of olivine, clinopyroxene (titanian augite) and amphibole (kaersutite), besides a groundmass bearing glassy material and analcite. Also to be noted in the lamprophyres is the presence of small grains or aggregates of carbonate ocelli as primary phases. Dykes showing a more basaltic composition have zoned plagioclase (andesine-labradorite), interstitial alkali feldspar and, as main ferromagnesian minerals, clinopyroxene (diopside-augite), amphibole, biotite and opaques. In spite of the petrographic and chemical differences, the alkaline bodies forming the islands of Vitória, Búzios, São Sebastião and Monte de Trigo, all found off the northern coast of the State of São Paulo, seem to be part of magmatic events related to the same geologic setting. They probably represent contemporaneous intrusions derived from an anomalously hot mantle and are closely associated with the Mesozoic tectonic evolution of Southern Brazil.

Trace element and isotopic (Sr, Nd, Pb, O) arguments for a mid-crustal origin of Pan-African garnet-bearing S-type granites from the Damara orogen (Namibia)

Geochronological data, major and trace element abundances, Nd and Sr isotope ratios, δ 18O whole rock values and Pb isotope ratios from leached feldspars are presented for garnet-bearing granites (locality at Oetmoed and outcrop 10 km north of Omaruru) from the Damara Belt (Namibia). For the granites from outcrop 10 km N′ Omaruru, reversely discordant U–Pb monazite data give 207Pb/ 235U ages of 511±2 Ma and 517±2 Ma, similar to previously published estimates for the time of regional high grade metamorphism in the Central Zone. Based on textural and compositional variations, garnets from these granites are inferred to be refractory residues from partial melting in the deep crust. Because P– T estimates from these xenocrystic garnets are significantly higher (800°C/9–10 kbar) than regional estimates (700°C/5 kbar), the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites. Sm–Nd garnet–whole rock ages are between ∼500 and ∼490 Ma indicating the age of extraction of the granites from their deep crustal sources. For the granites from Oetmoed, both Sm–Nd and Pb–Pb ages obtained on igneous garnets range from ∼500 to ∼490 Ma. These ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of ∼520 Ma for these granites based on Rb/Sr whole rock age determinations. Major and trace element compositions indicate that the granites are moderately to strongly peraluminous S-type granites. High initial 87Sr/ 86Sr ratios (>0.716), high δ 18O values of >13.8‰, negative initial ε Nd values between −4 and −7 and evolved Pb isotope ratios indicate formation of the granites by anatexis of mid-crustal rocks similar to the exposed metapelites into which they intruded. The large range of Pb isotope ratios and the lack of correlation between Pb isotope ratios and Nd and Sr isotope ratios indicate heterogeneity of the involved crustal rocks. Evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely. The crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat-producing elements. Very limited fluxing of volatiles from underthrust low- to medium-grade metasedimentary rocks may have also been a factor in promoting partial melting. Furthermore, delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high-T regions. The presence of coeval A-type granites at Oetmoed that have been derived at least in part from a mantle source supports this model.

The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA

Abstract Exposures in the Lake Superior region, and associated geophysical evidence, show that a 2000 km-long rift system developed within the North American craton ∽1109–1087 Ma, the age span of most of the volcanic rocks. This system is characterized by immense volumes of mafic igneous rocks, mostly subaerial plateau basalts, generated in two major pulses largely by a hot mantle plume. A new ocean basin was nearly formed before rifting ceased, perhaps due to the remote effect of the Grenville continental collision to the east. Broad sagging/subsidence, combined with a system of axial half-grabens separated along the length of the rift by accommodation zones, provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post-rift clastic sediments, both along the rift axis and in basins flanking a central, post-volcanic horst. Pre-rift mature, quartzose sandstones imply little or no uplift prior to the onset of rift volcanism. Early post-rift red-bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings; the exception is one gray to black carbon-bearing lacustrine(?) unit. This early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red-bed, fluvial sediments with an extra-basinal provenance.

Seismic tomography beneath the orogenic belts and adjacent basins of northwestern China

Three-dimensional velocity images of the crust and upper mantle beneath orogenic belts and adjacent basins of the northwestern continent of China are reconstructed by seismic tomography, based on arrival data of P wave recorded in seismic networks in Xinjiang, Qinghai, Gansu of China and Kyrgyzstan. The velocity images of upper crust demonstrate the tectonic framework on the ground surface. High velocities are observed beneath orogenic belts, and low velocities are observed in the basins and depressions that are obviously related to unconsolidated sediments. The velocity image in mid-crust maintains the above features, and in addition low velocities appear in some earthquake regions and a low velocity boundary separates the western Tianshan Mts. from eastern Tianshan Mts. The orogenic belts and the northern Tibetan plateau have a Moho depth over 50 km, whereas the depths of the Moho in basins and depressions are smaller than 50 km. The velocity images of upper mantle clearly reveal the colliding relationship and location of deep boundaries of the continental blocks in northwestern China, indicating a weakness of the upper mantle structure of orogenic belts. The top depth of upper mantle asthenosphere varies from place to place. It seems shallower under the northern Tibetan plateau, Altay and Qilian Mts., and deeper under the Tarim and Tianshan regions. Hot mantle probably rose to the bottom of some orogenic belts along tectonic boundaries when continental blocks collided to each other. Therefore their dynamic features are closely correlated to the formation and evolution of orogenic belts in northwestern China.

The formation of the West European Rift; a new model as exemplified by the Massif Central area

Abstract In this paper, we use mainly field data from the Massif Central area, which have been presented in a companion paper [Michon and Merle, 2001], to discuss the origin and the evolution of the West European Rift system. It is shown that the tectonic event in the Tertiary is two-stage. The overall geological evolution reveal a tectonic paradox as the first stage strongly suggests passive rifting, whereas the second stage displays the first stage of active rifting. In the north, crustal thinning, graben formation and sedimentation at sea level without volcanism during the Lower Oligocene, followed by scattered volcanism in a thinned area during Upper Oligocene and Lower Miocene, represent the classical evolution of a rift resulting from extensional stresses within the lithosphere (i.e. passive rifting). In the south, thinning of the lithospheric mantle associated with doming and volcanism in the Upper Miocene, together with the lack of crustal thinning, may be easily interpreted in terms of the first stage of active rifting due to the ascent of a mantle plume. This active rifting process would have been inhibited before stretching of the crust, as asthenospheric rise associated with uplift and volcanism are the only tectonic events observed. The diachronism of these two events is emphasized by two clearly distinct orientations of crustal thinning in the north and mantle lithospheric thinning in the south. To understand this tectonic paradox, a new model is discussed taking into account the Tertiary evolution of the Alpine chain. It is shown that the formation of a deep lithospheric root may have important mechanical consequences on the adjacent lithosphere. The downward gravitational force acting on the descending slab may induce coeval extension in the surrounding lithosphere. This could trigger graben formation and laguno-marine sedimentation at sea level followed by volcanism as expected for passive rifting. Concurrently, the descending lithospheric flow induces a flow pattern in the asthenosphere which can bring up hot mantle to the base of the adjacent lithosphere. Slow thermal erosion of the base of the lithosphere may lead to a late-stage volcanism and uplift as expected for active rifting.

Thermal evolution of the lithosphere beneath the western Pannonian Basin: evidence from deep-seated xenoliths

A series of recently and previously collected peridotite xenolith samples from the western Pannonian Basin has intensively been studied using electron microprobe techniques and a thermo-barometric approach in order to obtain relevant information about the thermal evolution of the lithosphere. Most of the xenoliths have chemically homogeneous mineral phases reflecting ambient equilibrium P–T conditions. The results indicate hot and thinned lithosphere in agreement with heat flow, seismic and magnetotelluric properties of the area. A strong relationship has been observed between deformation and temperatures of peridotites, in as much as coarse-grained protogranular and poikilitic xenoliths have high temperatures (up to 1175°C), whereas fine-grained equigranular and mosaic xenoliths have low temperatures (800–900°C). This picture suggests that diapiric uplift of hot mantle material into a cooler uppermost mantle has probably taken place.

Cenozoic and Cretaceous transient uplift in the Porcupine Basin and its relationship to a mantle plume

Abstract The Mesozoic and Cenozoic history of the Porcupine Basin may be broadly summarized as a Jurassic synrift phase, followed by Cretaceous and Cenozoic post-rift subsidence. Two periods, Early Cretaceous and Early Eocene times, do not fit the simple pattern of post-rift subsidence and are characterized by increased sedimentation. We recognize distinctive sedimentological responses to the basin flanks being either exposed or submerged, and infer that transient regional uplift caused the Early Eocene event. Modelling subsidence histories of wells and of the Porcupine Bank allows quantification of the magnitude and timing of anomalous uplift and subsidence. Transient uplift of 300–600 m occurred at the Paleocene-Eocene boundary, followed by subsidence of 500–800 m after Early Eocene time, over a period with a minimum length of 25 Ma and a maximum of 55 Ma. Renewed rifting is unlikely to be responsible for the Paleogene subsidence because it cannot account for the preceding uplift, and significant normal faults of Paleogene age are absent. A Paleogene uplift-subsidence cycle has also been noted in the basins surrounding Scotland and along Hatton continental margin. One way to explain regional subsidence between Eocene time and the present is that the European plate moved off the topographic swell above the Iceland plume following continental separation between Greenland and Europe in Early Eocene time. Another possibility is that an anomalously hot layer c. 50 km thick was emplaced beneath the entire region just before the onset of sea-floor spreading in Early Eocene time and was then dissipated by convection following continental separation. A Cretaceous transient uplift-subsidence cycle that shares many similarities with the Paleogene cycle is also recognized. Immediately following Late Jurassic rifting, 200–700 m transient uplift occurred in Early Cretaceous time, followed by 0–500 m subsidence coeval with the onset of sea-floor spreading at the Goban Spur margin. The Cretaceous uplift-subsidence cycle might also be caused by anomalously hot mantle.