Diapiric Uprise Research Articles

Sublithospheric diamond ages and the supercontinent cycle

Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths1,2 and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO3 inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability.

Modelling the long‐term deformation of the sedimentary substrate of Mt. Etna volcano (Italy)

AbstractThis study investigates in detail the deformation events that have affected the sedimentary successions forming the substrate of Mt. Etna volcano (Italy). Based on the geometric reconstruction of a buried sedimentary marker, we have been able to identify and quantify the effects of three different mechanisms of deformation that have affected the area in the last 600 ka. Numerical results from Finite Element Method (FEM) applied to model viscoelastic deformation suggest the occurrence of a crustal doming process originating at the mantle‐crust transition (~16 km). We propose that the source of deformation is related to the diapiric uprise of hydrothermal material originating in altered ocean‐like crust and its emplacement at a shallower level in the crust. This process has great relevance in the volcanic system and should be considered for the full assessment of its origin and evolution.

Evidence for subaqueously resedimented sulphate evaporites on Mars

This article documents the clastic nature of sulphate evaporite beds in the Tithonium Chasma located in the Valles Marineris region of Mars. These beds form a stratified succession characterised by very thick interbedded channel-fill breccia bodies. We infer that the bouldery channel-fills were deposited by voluminous mass-flow processes occurring in a relatively deep subaqueous environment. The redeposition of the coarse-grained evaporite would have responded to phases of high denudation rates in rapidly uplifting hinterlands. Tectonic activity also caused the diapiric uprise and exhumation of evaporite diapirs within the Valles Marineris chasmata, where the apparently young and well development karstic landforms probably formed during the late Amazonian age. These new data strongly suggest the deposition of both primary and resedimented evaporites in a marginal basin area, which effectively restricted ocean access through the proposed “proto-Valles Marineris Strait”. The associated ocean may be the “Ocean Borealis” of Late Noachian–Early Hesperian age.

The Neoproterozoic Quatipuru ophiolite and the Araguaia fold belt, central-northern Brazil, compared with correlatives in NW Africa

Abstract The Araguaia Belt, the northern branch of Neoproterozoic Tocantins tectonic province, developed during West Gondwana amalgamation as a result of collision between the Amazon and West African and/or São Francisco/Congo cratons. The external zone of the belt consists of ophiolitic slices and fragments, sedimentary rocks derived from magmatic arc sources, volcanic rocks, and part of a passive continental margin with low-grade metamorphic rocks, while the internal zone corresponds to a pile of low- to medium-grade metasedimentary rocks. The largest and best preserved ophiolitic bodies occur in the southern part of the belt, where the Quatipuru and Morro do Agostinho ophiolites are composed predominantly of mantle peridotites (mainly residual harzburgite) representing the base of the Moho transition zone. They contain chromitite pods, dunitic lensoid bodies and a suite of mafic–ultramafic dykes and/or sills resulting from partial melting, magma impregnation and diapiric up-rise. A Sm–Nd isochron age of 757±49 Ma indicates oceanic crust formation during the Early Neoproterozoic. NW African correlatives of the Araguaia Belt, the Mauritanide–Bassaride–Rokelide belt, show similarities with respect to lithostratigraphic units, the ages of basement and supracrustal rocks, the presence of Neoproterozoic ophiolitic slices and fragments, suture zones characterized by high gravity anomalies and centrifugal tectonic vergence. We conclude that these belts were probably formed around the same Neoproterozoic ocean or several small coeval oceans.

Analogue modeling of overlapping spreading centers: insights into their propagation and coalescence

The propagation and segmentation of mid-ocean ridges is studied using centrifuged analogue models built with non-linear materials. The deformation of the brittle-ductile model is controlled by diapiric uprise of buoyant analogous asthenospheric material induced by a centrifugal body force. This linear upwelling laterally stretches the model mantle that, in turn, induces failure in the upper layer simulating the brittle crust. Arrays of fractures initiate within zones of high stress concentration above the diapir. Fractures propagate laterally in a direction perpendicular to the maximum tensile (the minimum principal) stress. Secondary tension cracks initiate in the vicinity of parent fracture tips. Through-going fractures that crosscut the model surface develop by short fractures propagating toward each other and coalescing in different types of patterns. The overlap, overstep and inclination of fractures developed at the initial stage of extension control their subsequent growth and coalescence. Non-overlapping sub-parallel fractures propagate along nearly straight paths and coalesce to produce a single planar fracture. If overlapping fractures are parallel, they propagate towards each other along curved paths that enclose an intervening elliptical core of intact material. Fracture curvature in this case results from crack–crack interaction and is similar to that of overlapping spreading centers (OSCs) observed along mid-ocean ridges. Overlapping non-parallel fractures tend to coalesce by one of their tips propagating sub-parallel to the spreading direction toward the other fracture. Such offsets can serve as models for the development of the orthogonal ridge-transform fault patterns common along mid-ocean ridges.

Neogene evolution of the Carpatho-Pannonian region: an interplay of subduction and back-arc diapiric uprise in the mantle

Neogene evolution of the Carpatho-Pannonian region: an interplay of subduction and back-arc diapiric uprise in the mantle

Fe-Ti deposits in Rogaland anorthosites (South Norway): geochemical characteristics and problems of interpretation

The Rogaland anorthosite province (S. Norway) contains numerous Fe-Ti oxide deposits, including the second most important ilmenite deposit in the world, the Tellnes deposit. The largest deposits are located in the Ana-Sira anorthosite massif. Others occur in the Haland-Helleren anorthosite massif, particularly along the deformed contact with the Egersund-Ogna massif, where they were previously considered formed by metasomatic processes. All deposits are now regarded as magmatic. The structure, mineralogy and geochemistry of 11 selected Fe-Ti deposits (Tellnes, Storgangen, Blafjell, Laksedal, Kydlandsvatn, Kagnuden, Rodemyr, Hestnes, Eigeroy, Svanes, and Jerneld) are discussed in light of recent models proposed for the origin of Rogaland anorthosites and related rocks. Massif-type anorthosites result from the diapiric uprise of a plagioclase crystal mush which crystallized along a large P–T interval. Except for Tellnes, which is related to a post-deformation dyke, the Fe-Ti deposits in anorthosite massifs have been deformed by this movement during and after their crystallization. The differentiation process of the jotunitic parental magma has built up cumulates in the Bjerkreim-Sokndal layered intrusion and liquids in the Tellnes dyke and other jotunitic intrusions. Ilmenite is a liquidus mineral immediately after plagioclase in the sequence of crystallization of these jotunites, its interstitial character in the rocks resulting from subsolidus recrystallization. Ilmenite can thus accumulate early in the evolution of jotunitic magmas. This feature, together with high contents in Cr, V, Mg and Ni, links the Jerneld, Blafjell and Svanes deposits (type␣1) to the early evolution of a jotunitic magma. In the Bjerkreim-Sokndal intrusion, magnetite can appear with ilmenite at the very beginning of the sequence of crystallization, but normally crystallizes after orthopyroxene and before clinopyroxene and apatite. The early appearance of magnetite is a characteristic feature of type 2 deposits (Tellnes, Storgangen, Kydlandsvatn, Rodemyr I) and suggests conditions similar to the early magnetite cumulates in the Bjerkreim-Sokndal intrusion. Evidence of layering further favours gravity-controlled sorting processes to concentrate the oxides. Large-scale subsolidus segregation of the oxides due to high-temperature deformation can further concentrate these minerals in silicate-absent meter-sized masses. Type 3 deposits (Rodemyr II, Kagnuden, Hestnes and Eigeroy) could be derived from the more evolved stages of differentiation, as indicated by high REE in apatite, high Ti and Zn in magnetite and relatively low Cr, V, Mg, Ni contents in both oxides. The Cr content in both oxide minerals is however higher than in the equivalent cumulates of the Bjerkreim-Sokndal intrusion. Although immiscibility as the mechanism of enrichment leading to silicate-absent oxide-apatite veins, as in Hestnes and Eigeroy, cannot be precluded, there is no direct evidence in the veins, nor has any structural or geochemical evidence of immiscibility ever been found in jotunite dykes and Fe-Ti-P-rich rocks. Further investigations on the influence of subsolidus exchange of elements between the two oxides are needed to improve the use of trace elements as differentiation indexes.

Fluid inclusions in upper mantle xenoliths from Northern Patagonia, Argentina: Evidence for an upper mantle diapir

Three generations of fluid inclusions can be recognized in upper mantle xenoliths from alkali basalts of the Somoncura Massif, Northern Patagonia, Argentina. The first (“early”, “primary”) one consists of dense CO2 inclusions which were trapped in the mantle-crust boundary zone (22–36 km minimum trapping depth). Their co-genetic relationship with silicate melt inclusions enables us to constrain their minimum trapping temperature at 1200°C, indicating a high temperature event in a cooler environment. The “late” (“pseudosecondary” and “secondary”) generations of fluid inclusions were classified in accordance with their homogenization temperature to liquid CO2 (L1) and vapor CO2 (L2) phase. The minimum trapping depth for the first of the late inclusions (L1) is about 16 km. In spite of the uncertainties related to this value, L1 inclusions indicate that the upper mantle rocks, of which samples were delivered by the basalts, had some residence time in the middle crust where they experienced a metasomatic event. The fact that this event did not destroy the earlier inclusions, places severe constraints on its duration. The second late inclusions (L2) are low-pressure CO2 inclusions with a minimum trapping depth of only 2 km, presumably a shallow magma chamber of the host basalts. The succession of fluid inclusions strongly points toward a fairly fast uprising upper mantle underneath Northern Patagonia. The petrology and mineral chemistry of the peridotitic xenoliths support this view. Extensive partial melting and loss of these melts is indicated by the preponderance of harzburgites in the upper mantle underneath Northern Patagonia, a fairly unusual feature for a continental upper mantle. That depletion event as well as several metasomatic events — including those which left traces of fluid inclusions — are possibly related to a high-speed diapiric uprise of the upper mantle in this area. The path can be traced from the garnet peridotite stability field into the middle crust, a journey which must have been unusually fast. Differences in rock, mineral, and fluid inclusion properties between geographic locations suggest a diffuse and differential type of diapirism. Future studies will hopefully help to map the full extent and the highs and lows of this diapir and elucidate questions related to its origin and future.

Metamorphic facies zonation in the Ronda peridotites: spectroscopic results from field and GER imaging spectrometer data

Abstract The Ronda peridotite massif and bordering marbles (Málaga province, southern Spain) are studied in Geophysical Environmental Research 63-channel Imaging Spectrometer data (GERIS). Reflectance spectra of weathered rock and soil samples form the basis for selecting endmembers for spectral unmixing of the GERIS data and mineral mapping by means of spectral angle mapping. Six spectral endmembers model the continuum reflectance of each pixel: three soil types (a peridotite soil, a limestone soil, and a spectrally quenched soil), green vegetation (and grass), senescent vegetation, and decayed vegetation. The spatial distribution of the soil endmembers shows that the spectrally quenched soil is restricted to an area near to the mylonitized contact zone between the marbles and the peridotites. Mineral mapping from GERIS data confirms the known metamorphic zonation of the Ronda peridotite body into a garnet-, spinel-, and a plagioclase-lherzolite facies related to the diapiric uprise of the ultramafics from the asthenosphere into the crust. A prograde series of minerals is mapped within the marbles grading from tremolite through diopside to fosterite near the peridotite contact. These mineral assemblages form typically along an isothermal decompression path related to contact metamorphism.

Comparative geochemistry of West African kimberlites: Evidence for a micaceous kimberlite endmember of sublithospheric origin

A suite of largely unaltered, aphanitic, mica-bearing hypabyssal kimberlites from the Koidu kimberlite complex of the West African Craton have been investigated to determine their geochemical affinity relative to Group I (nonmicaceous) and Group II (micaceous) kimberlites of southern Africa. Comparison is made with altered kimberlites from Liberia, other West African and global kimberlites. Based on major element oxides, the Koidu kimberlites, though mica-bearing, show closest compositional similarity with the Group IA kimberlites of southern Africa. Based on major and trace elements, the Koidu kimberlites show an unusual geochemical signature. This signature is similar to that of the distinctive, micaceous Aries kimberlite of northwest Australia, and includes high Nb U (most samples > 46), Ce Sr(>0.4) , Ta Hf(>2) , and Nb Zr(>1) ratios and low P 2O 5/Ce ∗10 4(<27), Ba Rb(<32) , and U Th(<0.2) ratios compared with Group I kimberlites. Koidu kimberlites can be readily discriminated from Group II kimberlites by their higher Ti K(>0.4) and Mb La(>1) ratios and lower Ba Nb(<10) and Pb Ce(<0.06) ratios. The compositions of Liberian kimberlites are leached of mobile incompatible elements, but least affected samples show affinity to Group I. Guinea kimberlites appear to be of two types: one having affinity with Group IA and the other, micaceous variety, having affinity with the Aries kimberlite. Kimberlites with an Aries geochemical signature appear to exist on some other cratons, e.g., the Kundelungu kimberlites (Zaire) and two mica-bearing Group I kimberlites (S. Africa). The Koidu kimberlites exhibit compositionally-dependent isotopic heterogeneity though initial ϵ Nd and ϵ Sr values are broadly asthenospheric (i.e., near bulk earth) similar to Group I and Aries. A compositional spectrum appears to exist between nonmicaceous Group I kimberlites through mica-bearing Koidu kimberlites to extreme endmembers of the Aries type. This spectrum can be modelled as partial melts of heterogeneous peridotite sources which incorporate a potassic, high-Nb source component. The component may represent a fluoro-K-richterite-bearing peridotite residue derived by melt extraction from subduction-zone metasomatized peridotites. Such materials may have been trapped, together with former oceanic lithosphere, in the Transition Zone of the mantle. In response to lower mantle upwelling, diapiric uprise accompanying reduced volatile degassing of the kimberlite source may occur. Because of differences in oxidation potential across the 400 km discontinuity, reactions in the ascending diapir will lead to redox melting and ultimately segregation of the kimberlite melt at the base of the thermal boundary layer (P ~ 13 GPa) separating the subcratonic lithosphere from the convective mantle.

The interaction and geometries of diapiric uprise centres along mid-ocean ridges — evidence from mantle fabric studies of ophiolite complexes

Abstract Detailed structural mapping and thin-section fabric studies of oceanic upper mantle sections exposed within 3 ophiolite complexes have shown that in all of the ophiolites studied two co-existing high-temperature mantle fabric orientations can be identified. The fabrics are related both in time and space in that one fabric is ductile shear-folded by a later one to varying degrees. Olivine and orthopyroxene crystal slip-system and fabric orientation studies show that all the fabrics identified are high-temperature ‘primary’ asthenospheric flow fabrics and that simple shearing is the main flow process. The fabrics observed are at varying angles to the orientation of the Moho plane, with the slip directions varying both along strike with the Moho as well as vertically down from the Moho plane. The identified flow fabrics are considered to be indicative of mantle flow directions ‘frozen’ into the mantle as it cooled from being ductile asthenosphere to lithosphere. Each fabric is interpreted as originating from a different localized mantle diapiric uprise centre. The centres interact with each other both in space and time, the observable ‘frozen’ fabrics indicating the closeness and ‘strength’ of individual diapirs relative to the position of the data measurement point.

A teleseismic and petrological study of the crust and upper mantle beneath the geothermal anomaly Urach/SW-Germany

Teleseismic P-wave travel time residuals recorded by a mobile, shortperiod network are interpreted using the composition of crustal and upper mantle xenoliths. A three-dimensional inversion of the P residuals revealed a small-scale anomalous domain with a velocity reduction of 3% in the crust and a broad low-velocity anomaly of 4% beneath the Moho down to a depth of 50 km. Xenoliths of the Miocene Urach volcanic field (“UVF”, 1300 km 2; 355 eruptive centres, mostly diatremes) comprise phlogopite-bearing, clinopyroxene-rich mantle xenoliths, evidence for large ion lithophile element (LIL) metasomatism in an originally depleted harzburgitic mantle, reflecting chemical modification of the upper mantle beneath the UVF. The metasomatism caused partial melting in the spinel peridotite stability field, indicated by glasses in xenoliths, and a diapiric uprise of the partially molten metasomatized mantle. The observed reduction of the P-wave velocity of 4% in the lithospheric mantle can be explained by a local enrichment of phlogopite accompanied by increased temperatures. The crustal low-velocity body coincides very well with the postulated volume of an ancient intracrustal magma chamber or system of chambers required by the composition of xenoliths, the magnetization of the diatremes and the spatial distribution of the volcanic eruption centres.

Structural and metamorphic evolution of the Gabon Orogenic Belt: Collision tectonics in the lower proterozoic?

Abstract The Archaean granite-gneiss terrain of central Gabon comprises medium- to high-grade gneisses metamorphosed at 3.15 Ga, forming the Monts Cristal and underlying the Mitzic region, intruded by and enclosed as enclaves in granitoids 3.0–2.6 Ga old that form the extensive Chaillu and North Gabon blocks. These rocks are unconformably overlain in the east of Gabon by Early Proterozoic Francevillian sedimentary rocks, between 2.3 and 2.0 Ga old, forming the Franceville and Booue Basins. The Francevillian is overthrust on its west side by the Ogooue Metamorphics, themselves considered to be of Early Proterozoic age. These rocks crop out in a broad synclinorium consisting essentially of paragneisses, with orthogneisses appearing locally at the base. The overall tectonic setting is discussed in terms of the evolution of the various domains from west to east. 1. (1) On the west flank of the synclinorium is a migmatitic-granulitic dome, the Abamie Dome, in which structures diverge away from the core. Metamorphism is of high-temperature, intermediate- to low-pressure type. 2. (2) On the east flank of the Ogooue synclinorium the medium-grade Ogooue Metamorphics (sillimanite-biotite, kyanite-staurolite, staurolite-muscovite-biotite) have been thrust eastwards on to the low-grade (kyanite-sericite) Francevillian. 3. (3) The Francevillian is parautochthonous, being widely detached from its Archaean basement owing to the effects of the same eastward-directed tangential tectonics. 4. (4) Locally, slices of Archaean basement have been tectonically incorporated. The structural cross-section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts. The suture marking the collision is interpreted to be located in the Abamie migmatite dome, where the nappes are rooted. The Gabon Orogenic Belt is correlated with the collision belts of about the same age on the eastern margin of the Congo craton and with the Trans-Amazonian Orogenic Belt of the Guyana Shield.

Geochemistry of metabasites from the Nevado-Filabride complex, betic cordilleras, Spain: Relics of a dismembered ophiolitic sequence

Major- and trace-element distribution, including REE data, in the metabasites of the Nevado-Filabride Complex (Betic Cordilleras, Spain) suggests that the whole mafic-ultramafic association represents a dismembered ophiolitic sequence. All the analyzed samples have been affected by sea-floor alteration. Yet only the fine-grained rocks have suffered some significant mobilization of the LREE. Life the other Western Mediterranean ophiolites, this sequence is related to the opening of the Liguro-Piemontese basin during Mesozoic times. However, the metabasites from Betic Cordilleras differ from the ophiolitic basalts by higher values of the more incompatible over less incompatible element ratios. The entire variation of these ratios in the Liguro-Piemontese basin is related to a dynamic melting process coupled with a diapiric uprise of the mantle source from the stability field of garnet peridotites. possibly up to the spinel-plagioclase facies transition. In this process, the ophiolites of the Betic Cordilleras would record the deepest stages and the lowest degrees of melting. This feature is related to the peculiar location of these ophiolites at the western end of the Liguro-Piemontese basin.

Deep subduction and mantle heterogeneities

The globe-encircling isotopic anomaly centred on latitude 30°S defined from the geochemistry of mid-ocean ridge and particularly intraplate basalts has been previously shown to be correlative with degree-2 features of the Earth's geoid. The same region appears to show a correlation with zones of low seismic velocities, perhaps indicative of heated rocks, which persist to at least the transition zone between the upper and lower mantle. That the basalts in this anomalous region are derived by diapiric uprise from a mantle reservoir of old deeply subducted materials is favourably demonstrated by olivine and clinopyroxene-controlled partial melting trends on geochemical discrimination diagrams.

Geochemistry of the quaternary volcanic rocks of the northeast Japan arc

Major and trace-element data are presented for a series of lavas from 17 volcanic centres in the NE Japan arc. These represent a transect of the Quaternary arc from its volcanic front (type A volcanoes) in the east, across its central zone (type B) to its western margin (type C). Rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase, mafic silicates and titanomagnetite. For comparative purposes trace-element data for each volcano are presented on a 55 wt.% SiO 2 normalised basis. The volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements (Rb, Sr, Ba, K) and depletion in high field strength elements (Zr, Nb, Hf, Ta). Using these data and derived primary magma compositions (Tatsumi et al., 1983), mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt.% SiO 2 liquids. These data together with published distribution coefficients allow the calculation of trace-element abundances in the primary magmas. Reciprocal trace-element plots indicate that irrespective of position with respect to the volcanic front, the elements, La, Ba, K, Sr, Nb, Y and Zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source. However, those melts close to the volcanic front (type A) show anomalous Rb, Th and Pb values suggesting that their mantle source was enriched in these elements. Further calculations suggest that the HFS depletion is not due to retention by residual mineral phases but is a feature of the source. Thus the data suggest: (a) that the source of the arc volcanism was homogeneous; and (b) that the characteristic chemical features were an inherent feature of the source. Since Zr/Nb ratios in the source melts are close to those found in N-type MORB it is suggested that a major component of arc-volcanic source rocks was a depleted mantle of the type which has given melts of N-type MORB composition. LIL elements added to such a depleted source are derived by the loss of silica-rich aqueous fluids from the descending (subducting) slab. Such losses occur within the first 100 km and are fixed in the overlying mantle wedge. This contaminated material is transported down by drag-induced convection, producing a homogeneous, but metasomatised mantle within the melting zone. Diapiric uprise produced by small-scale melting, produces further melting, with the major control of melt chemistry being the degree of partial melting. Superimposed on this chemistry is a localised addition of Pb, Rb and Th in the source of those volcanoes nearest the volcanic front.

Geographical distribution of 3He/4He ratios in Japan: Implications for arc tectonics and incipient magmatism

The 3He/4He and 4He/20Ne ratios of 115 natural gas samples, including various types of gases from volcanoes, hot springs, mineral springs, water wells, petroleum fields, and natural gas fields, were measured using a mass spectrometer. The observed 3He/4He and 4He/20Ne ratios range from 7.47×10−8 to 9.65×10 −6 and from 0.26 to 1100, respectively. The 3He/4He ratios reflect well the geotectonic structure of the Japanese Islands. In northeastern (NE) Japan there is a clear geographical difference in the 3He/4He ratios between the frontal arc (forearc) and volcanic arc (back arc) regions. Lower ratios were found in the trench side region and higher ratios in the back arc side. This result suggests that the mantle‐derived helium in the volcanic arc region is associated with the diapiric uprise of a magma. Lower 3He/4He ratios in the frontal arc region may be due to radiogenic He produced by radioactive decay of U and Th in the crustal and sedimentary rocks. In southwestern (SW) Japan there is no clear geographical contrast in the 3He/4He ratios. Some samples in the frontal arc region show quite high 3He/4He ratios. The tendency of the 3He/4He ratios agrees with the distribution of terrestrial heat flow data and reflects geotectonic structures different from NE Japan. The high 3He/4He ratios observed in the frontal arc region in SW Japan may be indicative of renewed or incipient magmatism due to a descending young and warm slab.

Dynamic models of continental rift valley systems

In experimental scale models simulating the major continental rift valley systems rifting is caused by diapiric uprise of viscous material induced by a centrifugal body force; this force plays the same role as gravity in nature. The scale models show that bending alone as a mechanism of rifting is unrealistic. Rather, the rift structures form as a result of the combined effect of gentle bending and lateral stretching of the crust. The models also show that graben structures could form at any stage of development of an ascending and laterally spreading diapiric body depending upon the density and viscosity contrast of the body and its surroundings and the strength of the model crust. The close resemblance in structure of the models with natural rift systems suggests a close similarity of dynamics and driving forces.

Plate and intraplate processes of Hercynian Europe during the late paleozoic

It is speculated that until Late Carboniferous time the region of Hercynian Europe was occupied by an elongated island arc system underlain by a segment of continental crust. In the Upper Carboniferous, two subduction zones are assumed to have extended from the north and south beneath Hercynian Europe. An extensive zone of hot, partially molten upper mantle lay above and between these, and diapiric uprise of portions of this material led to separation of mafic magmas, widespread partial melting in the lower and middle crust, high temperature-low pressure metamorphism in crustal rocks, and regional uplift and extension of the crust, as indicated by intermontane troughs and their associated volcanic rocks. In Visean to Westphalian time Hercynian Europe collided with both the large neighbouring plates North America-Europe and Africa. During these diachronous collisions and owing to reduced rigidity of the relatively hot island arc crust, the irregular continental margins of the larger and thicker continental plates induced oroclinal bending of Hercynian Europe. After the collision processes had been terminated, processes of upper mantle activity continued, causing further crustal uplift and even, enhanced crustal extension for several tens of million years into the Lower Permian. Decline of the upper mantle activity beneath Hercynian Europe is indicated by crustal subsidence and formation of a peneplain in Permian time followed by the Upper Permian transgression of both the Zechstein sea and the Tethys sea which mark the end of the Hercynian geodynamic cycle.

A rolling mill effect in asthenosphere beneath oceanic spreading centers

From structural studies in seventeen ophiolite massifs, information has been drawn about the activity of the asthenosphere beneath oceanic spreading centers. This information, together with geophysical data pertaining to oceanic ridges, has been integrated into a numerical model. It is inferred that for a fast-spreading ridge (< 5 cm/yr) a local diapiric uprise of melted peridodite is superimposed to the main circulation driven by the lithosphere drifting. As a result, the upward flow splits, at a depth of 35 km, into a 10–20 km wide axial jet and a horizontal flow diverging away from the ridge. The small diapir channels upward most of the partially molten peridotites. Magmatic activity in the crust is thus constrained to an area of similar horizontal extension, in agreement with ophiolitic and oceanic data. Also in agreement with findings in ophiolites, the meeting at a few tens of kilometers away from the ridge of the two diverging mantle circulations, creates a change in shear sense of the flow. From there on, the horizontal diverging flow is compatible with lithosphere drift.