Magmatic Diapir Research Articles

Tectonic Structure, Classification, and Evolution of Arachnoids on Venus: Preliminary Results

An analysis has been done of the topography and geologic structure of arachnoids—specific radial/concentric volcannic-tectonic structures on the surface of Venus. A representative sample (53 arachnoids) from 265 structures of this type, which are listed in the catalog of volcanic structures of the surface of Venus (Crumpler and Aubele, 2000), has been studied. The overwhelming majority of arachnoids are shown to be depressions that are commonly outlined by concentric extensional structures. Following Head et al. (1992) and Aittola and Kostama (2001), the assumption is confirmed and substantiated that arachnoids are formed by gravitational relaxation of small magmatic diapirs. Several types of arachnoids are identified on the basis of an analysis of structural patterns characteristic of such structures. It is also shown that the formation of different types of arachnoids depends on the depth of the magmatic diapir under the surface, on the thickness and reologic properties of the structures superposed on the evolving magmatic diapir, and on the character of regional stress fields that arise in the process of formation of such structures. The conclusion is drawn that most of the arachnoids were formed due to the gravitational relaxation of magmatic diapirs within the brittle part of the lithosphere, and some of them appeared as a result of the gravitational relaxation of radially fractured centers—novae. It is also shown that arachnoids are long-lived and multistep structures. At least some of them began to evolve before the formation of regional plains with wrinkle ridges, and their development ended after this event.

Seismic tomography of the Dead Sea region: thinned crust, anomalous velocities and possible magmatic diapirism

Abstract Analysis of first arrivals of P waves from 113 earthquakes in the Dead Sea region and the calculation of a tomographic model indicates anomalous distribution of seismic velocities in the crust and the upper mantle. The seismic tomography model was applied to the lower and intermediate crust and the uppermost mantle there, at depths greater than the detection limits of seismic reflection surveys. At these depths the inversion shows two deep layers, at depths of 10–22 km and 22–32 km. The deeper layer shows average seismic velocity of 7.7 km s −1 , and the shallower one 6.5 km s −1 . The model suggests therefore that the Moho under the central Dead Sea is found at depth of 22 km, and the seismic velocity in the upper lithospheric mantle is anomalously low. The velocity of the layer overlying the Moho suggests a modelled layer of composite mineralogy, but high velocity anomalies were encountered in the 10–22 km layer under the boundary faults of the Dead Sea Rift, and a low velocity anomaly under the central Dead Sea is plausible. The probable interpretation of these anomalies is that the mantle under the central Dead Sea is shallow and anomalously hot, and that the lower and intermediate crust under the axial zone of the central and northern Dead Sea is also anomalously hot. Furthermore, it seems probable that magmatic diapirs ascend along the boundary faults of the rift into the intermediate crust. Structural similarity of the upper mantle and the lower crust between the Dead Sea and the northern Red Sea suggests an analogous tectonic regime for these two regions.

In defense of magmatic diapirs

Diapirism as a crustal magma ascent mechanism has been recently criticized. We contend that this reflects an overly simplistic view that diapirs must resemble modeled hot-Stokes diapirs and the perception that magma ascent in dikes is a more problem-free mechanism for the construction of plutons. We describe four Cordilleran plutons that have characteristics much more compatible with diapirs than dike-fed chambers. These plutons were emplaced at depths ranging from ∼10 to 30 km and record different parts of diapiric ascent paths. Most ascended during complex visco-elastic flow of host rock during regional deformation, have narrow structural aureoles indicating power-law behavior of host rock, and were constructed of multiple batches of magma, attributes enabling them to ascend greater distances than single hot-Stokes diapirs. Some features of these plutons are not typically attributed to diapirs, and thus we introduce the term visco-elastic diapir for bodies consisting of one or more batches of magma rising together, with length to width ratios <100, surrounded by host rock deforming by brittle and ductile processes, and for which ascent is driven by buoyancy plus regional stress. We conclude that diapirism remains a valid magma ascent mechanism for the crust.

Comment on “New evidence of magmatic diapirs in the intermediate crust under the Dead Sea, Israel” by Nitzan Rabinowitz, Jean Steinberg, and Yossi Mart

Comment on “New evidence of magmatic diapirs in the intermediate crust under the Dead Sea, Israel” by Nitzan Rabinowitz, Jean Steinberg, and Yossi Mart

Reply: [to “Comment on “New evidence of magmatic diapirs in the intermediate crust under the Dead Sea, Israel’”

Reply: [to “Comment on “New evidence of magmatic diapirs in the intermediate crust under the Dead Sea, Israel’”

Morphotectonics, Volcanism and Hydrothermal Activity on the East Pacific Rise between 21°12′ S and 22°40′ S

The morphotectonic setting of the East Pacific Rise (EPR) between21°12′ and 22°40′ S and its recent and past hydrothermalactivity were the focus of the Russian R/V Geolog Fersman’s expeditionin 1987–1988. The EPR axial zone in the study area is comprised of three segmentsseparated by overlapping spreading centers (OSCs) near 21°44′ and22°08′ S. The northern segment is the shallowest of three and hasa distinct massive axial ridge, trapeziodal in cross-section, toppedby a very wide flat summit surface and cut by a well-developedcentral graben. These features testify to intense magmatism and to avoluminous crustal magmatic chamber underlying the whole segment.Fine-scale segmentation is most clearly revealed in the structure ofthe central graben within which several 4th-order segments can bedistinguished. This scale of segmentation is also reflected on flanks of theaxis by variations in the character and intensity of faulting. According to structural and petrologic data, the magmatism is mostintense in the central part of the segment which is probably locateddirectly over a magmatic diapir supplying the melt to the whole segment.Magma migration at the subcrustal level from the center towards the ends ofthe segment with discrete injection into the crustal magmatic chamber ispresumed. The central segment is broken into two morphologically distinct partsseparated by a deval. In the subsided northern part, the wide summit of theaxial ridge is cut by a well-developed, intensely fractured axialgraben. In the southern part, the axial ridge is relatively elevated, butnarrow with an ephemeral graben along its crest. The character and intensityof faulting on the axial flanks are also considerably different in thenorthern and southern parts of the segment. Thus, the magmatic supply tothese two parts is thought to originate from two different sources. If so,then at present the magma chamber underlying the southern part of thesegment is probably at the stage of replenishment, while in the north it isat the stage of deep cooling. The southern segment is structurally similar to the central one. Howeverthere is considerably less intensive magmatic activity in this region,especially south of 22°30′ S where the axial ridge is narrow, andtriangular in cross-section. Both OSCs studied are marked by abrupt narrowing and sharp subsidence ofthe tips of axial ridges within the northern limbs. The southern OSC limbsare morphologically similar to normal sections of axial ridges. In bothcases the flanks are structurally and morphologically disrupted adjacent tothe OSCs and oblique structures can be traced far southward of the OSCflanks. Due to the spatial position of oblique structures on the the flanksit is presumed that the OSC near 22°07′ S is migrating northward.The 21°44′ S OSC zone has apparently undergone small spatialoscillations. In spite of the small amplitude of lateral displacement, thiszone is marked by prominent bathymetric anomalies. Numerous massive sulfide deposits were discovered atop the axial ridgealong the entire length of the uplifted and hydrothermally active northernsegment. Ore metal concentrations in near-bottom waters are maximumover the southern part of the northern segment, while maximum concentrationsof the same metals in surficial sediments are confined to the central partof the same segment. We surmise that there has been a recentalong-axis shift of the zone of maximum hydrothermal activity fromthe middle of the segment to its present position in the southern part ofthe segment. Considering sedimentation rates, the age of this shift can beapproximately estimated to be 5 to 10 thousand years before the present. The relatively Mg-enriched basalts of the middle part of thenorthern segment represent a tike of a more primitive pattern, while therelatively Fe-rich rocks of its southern part probably reflect alarge degree of fractionation at shallow crustal levels. Considering thistrend, in addition to morphotectonic data we presume that subaxial magmaflow from the middle to the southern part of the segment is responsible forthe along-axis shift of hydrothermal activity. In the central segment of the study area, massive sulfides have only beendiscovered south of the 21°55′ S deval, where the axial ridgeshoals and where the existence of a subjacent magma chamber is presumed. The very weak manifestations of recent volcanism within the southernsegment explain the absence of hydrothermal activity and sulfide depositswithin this segment.

New evidence of magmatic diapirs in the intermediate crust under the Dead Sea, Israel

Simultaneous inversion of P wave arrivals of 262 local earthquakes, recorded in the region of the Dead Sea, was carried out to model the velocity stratification in the underlying crust and uppermost mantle. The resulting tomographic model delineates several large magmatic domes, rising from the lower crust, at depth of approximately 20 km, and ascending more than 12 km into the overlying intermediate crust. Commonly, the domal ascent took place along the boundary faults of the Dead Sea rift. The domes are elliptical in shape, measuring nearly 5 × 15 km in diameter, and are spaced about 20 km apart. Earthquake foci are abundant around the diapirs but rare within them. The occurrence of these magmatic diapirs under the Dead Sea implies anomalously high temperatures with metastable stratification in the lower crust and suggests magmatic upwelling.

Numerical Modelling of Melting Processes and Induced Diapirism In the Lower Crust

SUMMARY The processes of partial melting and magmatic diapirism within the lower crust are evaluated using a numerical underplating model. Fully molten basalt (T= 1200°C) is emplaced at the Moho beneath a solid granite (T= 750OC) in order that a melt front grows into the granite. If diapirism does not occur, this melt front in the granite reaches a minimal depth in the crust before (like in the molten basalt) crystallization takes place. The density contrast between the partially molten granite layer and the overlying solid granite can lead to a Rayleigh-Taylor instability (RTI) which results in diapiric rise of the partially molten granite. Assuming a binary eutectic system for both the granite and the underplating basalt and a temperature- and stress-dependent rheology for the granite, we numerically solve the governing equations and find (a) that diapirism occurs only within a certain but possibly realistic range of parameters, and (b) that if diapirs occur, they do not rise to levels shallower than 15 or perhaps 12km. The growth rate depends on the degree of melting and the thickness of the partially molten layer, as well as the viscosity of the solid and the partially molten granite. From a comparison of the growth rate with the velocity of a Stefan front it is possible to predict whether a melt front will become unstable and result in diapiric ascent or whether a partially molten layer is created, which remains at depth. We carry out such a comparison using our thermodynamically and thermomechanically consistent model of melting and diapirism.

Magmatically induced metamorphism and deformation in the Kigluaik gneiss dome, Seward Peninsula, Alaska

Field observations and U‐Pb isotopic data from plutonic and high‐grade metamorphic rocks within the Kigluaik gneiss dome on the Seward Peninsula, Alaska, document a Late Cretaceous age of peak metamorphism and shed light on the relationship between fundamentally mantle‐derived magmatism and the development of the gneiss dome. The dome consists of upper‐amphibolite‐facies to granulite‐facies metasedimentary rocks mantling the granitoid Kigluaik pluton. The main deformational fabric in the dome is a well‐defined, moderately dipping foliation and compositional layering that contains a pervasive, east‐west trending mineral‐elongation lineation defined by sillimanite and hornblende. Leucosomal segregations in migmatite are boudinaged and isoclinally folded, and late‐stage tension gashes are filled with melt, demonstrating that most deformation occurred at peak metamorphic temperatures. The large pluton in the core of the dome consists of a granitic cap overlying a mafic to intermediate root. Mafic pillows with crenulate margins and spectacular magma mingling textures indicate that the two magmas were coeval. The Kigluaik pluton is largely undeformed and discordant, although some dikes possess a weak deformational fabric. The lack of quench textures at the margins of the pluton and very limited alteration in adjacent wall rock suggest that the pluton was emplaced while country rocks were still at high temperatures. U‐Pb analyses of three fractions of zircon from a highly strained and concordant garnet‐bearing granite orthogneiss yield an intrusive age of 105 Ma; therefore significant deformation in the dome must have occurred after 105 Ma. Several U‐Pb analyses of monazite from metapelite and pegmatite (derived from partial melting of metapelite) yield an age of 91 ± 1 Ma for high‐temperature metamorphism and deformation. U‐Pb analysis of eight fractions of zircon from the Kigluaik pluton shows that it crystallized at 92 ± 2 Ma. Thus there is both field and geochronologic evidence for coeval magmatism, metamorphism, and deformation at about 92 Ma, which may represent the latest stages of a more protracted tectonic event. We present a model for gneiss dome development wherein a large, silicic to intermediate magmatic diapir with its mantling gneisses ascended from ∼35 km to ∼15 km during an event associated with crustal extension in northern Alaska. We emphasize the close link between plutonism and gneiss dome development and the fundamentally mafic character of this plutonism.

The Ljugaren granite massif, Dalarna, central Sweden

Abstract The circular, 13 km wide, c. 1700 Ma old Ljugaren granite massif intrudes and deforms 1890 to 1850 Ma old gneissose rocks and 1790 to 1740 Ma old late-orogenic granite sheets of the Svecofennian Orogenic Province. It is satellitic to the 1840 to 1680 Ma old Transscandinavian Granite-Porphyry Belt, and is partly overlain unconformably by Ordovician-Silurian sediments which are preserved around the Siljan Ring meteorite impact structure. Aeromagnetic and structural geological studies show that regional country-rock structure swings into concordance with the massif's margins 1–3 km from the contact. Gravity measurements indicate that the intrusion is shaped like an asymmetric funnel. Internally, the Ljugaren granite shows a weak, inwardly dipping, planar magmatic fabric concordant to its margins. Evidence to date indicates that the massif was emplaced into ductile crust as a magmatic diapir after the culmination of the Svecofennian orogeny c. 1700 Ma ago.

Internal circulation in a buoyant two-fluid Newtonian sphere: implications for composed magmatic diapirs

Igneous plutons frequently show chemical zoning. The most commonly documented zoning is with the lighter, more silicic, rocks in the centre of the body and the denser, more basic, rocks in the external zone (normal zoning). Less commonly, some plutons show reverse zoning so that the more basic rocks occupy the centre. Widespread evidence shows that zoning in many plutons is the result of interaction between basic and silicic melts. This work studies, by means of finite difference numerical models, pluton zoning which is due to internal circulation in diapirs comprising two magmas of different composition. Diapirs are modelled here as buoyant isothermal spheres composed of two Newtonian fluids rising through a Newtonian ambient fluid. Ratios of viscosities and densities of the two fluids were varied and the results demonstrated two different styles of internal circulation in rising spheres. The first style, termed “coupled circulation”, is characterised by continuous overturning of both the fluids in a single cell, evolving through both normal and reverse compositional zoning. The overturns stir the fluids and enhance both magma mingling and mixing. Coupled circulation develops in spheres comprising fluids of similar densities and viscosities. As these properties become increasingly different the internal circulation tends to decouple. “Decoupled circulation”, is characterised by circulation of the fluids in two separate cells. Decoupling stops the overturns between the two magmas so that the diapir preserves a reverse zoning throughout its rise, with the denser fluid occupying the central zone. There is less possibility of magma mingling in diapirs undergoing decoupled circulation. Thus, pairs of magmas of similar properties, such as andesite and rhyolite, are most likely to develop coupled circulation leading to both normal and reverse zoning in diapirs; whereas magmas of very different properties, such as basalt and rhyolite, are most likely to decouple resulting in reverse zonation. The models indicate that reverse zoning would be the most common internal pluton geometry if zoning were controlled by internal circulation alone. Model diapirs which rise along channels of warm, low viscosity wall-rock (hot Stokes' models) or low viscosity shear zones show an increased tendency towards coupled circulation and more intense mechanical stirring of the magmas.

Structural tests of diapir hypotheses in Archean crust of Ontario

Detailed structural maps of two granitoid complexes in the Wabigoon Subprovince are used to test three diapir hypotheses advanced in earlier papers. The gneiss masses of, and individual domes within, the complexes fail the test for solid-state diapirism. The gneiss domes also fail the test for tensile bending caused by hypothetical magmatic diapirs in the subsurface. An oval pluton located near the best-exposed gneiss dome proves to be a synformal sheet rather than a funnel-shaped magmatic diapir. This pluton could be a syenite–diorite phacolith emplaced into a concordant zone of dilation during the late upright folding of the gneiss mass about horizontal axes. Earlier tight folds were probably recumbent and south verging and, like the gneissosity, generated in a ductile shear regime with subhorizontal glide planes. These observations have important implications for Archean tectonics, especially the relative horizontal displacement of large greenstone masses (potentially allochthonous greenstone belts).

The thermal history of ascending magma diapirs and the thermal and physical evolution of magmatic conduits

If, over millions of years, the positions of island arc volcanoes remain fixed relative to their source regions, magma bodies must repeatedly traverse a path through the lithospheric mantle. The repeated passage of such bodies thermally and chemically modifies the adjacent mantle. For diapirs with radii of 3–6 km, ascending at 4 × 10−8 to 2 × 10−7 m/s, 60,000 to ∼900,000 years are required to establish a magmatic conduit to crustal depths. During this period, wall-rock temperatures increase 200–400°C below 60 km and 400–750°C between 60 and 20 km. The solidification of large volumes of magma within the mantle also provides a mechanism for the introduction of kilometer-scale chemical and isotopic heterogeneities into the lithospheric mantle. Because the crystallization sequences of magmas ascending at different stages of conduit development vary, conduit thermal and physical evolution also influences magmatic differentiation. These differences in crystallization sequence result from the different P·T trajectories followed by individual magmatic diapirs. Crystal fractionation of successive diapirs, with or without concurrent wall-rock assimilation can, therefore, generate different liquid lines of descent. As successive magma bodies chemically react with their wall rock and solidify at various depths, the mantle also changes in composition from peridotite toward basalt. Depending on the stage of conduit development, assimilation of wall rock could, therefore, yield variable magmatic differentiation trends. For successive magma bodies, the degree of contamination may decline as the conduit and its margin evolve physically. Lithospheric conduit evolution has several important ramifications for geochemical modelling. First, since magma ascent from source regions to crustal depths is unlikely to be isochemical, radiogenic and incompatible-element ratios may not always reflect characteristics of the magma source region. Second, the earliest stage of arc magmatism, e.g. submarine volcanism, is unlikely to reflect the processes of arc magma generation. Rather, this phase of volcanism records the interaction between ascending magma and unmodified mantle. The characteristics of primary arc magmas are likely to be understood only through examining more highly developed volcanic centers supplied by mature magmatic conduits.

Tectonic and magmatic processes in the Eastern Branch of the East African Rift and implications for magmatically active continental rifts

Magmatism has played a major role in the development of the Eastern Branch of the East African Rift System. Geological, geochemical and geophysical constraints indicate that magmatism and tectonism vary significantly with depth and along strike. In the uppermost crust, faulting, fissuring, block rotation and volcanic construction have been widely documented. However, in the middle to lower crust magmatic intrusion dominates. In some places, the volume of intrusive material generated per kilometer of rift axis equals or exceeds the cross-sectional area created by tectonic extension, thereby maintaining or even increasing the crustal thickness. Elsewhere, magmatism is much less voluminous, and tectonic extension may result in thinning of the crust. Basic and ultrabasic igneous bodies clustered in the crust and uppermost mantle beneath the rift valley probably account for the observed positive axial gravity anomalies and high compressional wave velocities centered there. Incompatible trace-elementratios of Quaternary volcanos suggest that individual magmatic diapirs have intruded the crust and upper mantle of the rift. The diapirs may be fed by a vertical, wedge-like body of asthenosphere with a concentration of partial melt along its apex. In Turkana, where seismic reflection data are available, diapirs corresponding to Quaternary volcanos are located beneath the centers of individual half-graben rift basins. A similar relation between rift basins and Quaternary volcanic centers is inferred for the entire Eastern Branch. The magnitude of extension across the rift units is probably insufficient to cause mantle upwelling and extensive partial melting. Thus, the geometry of rifting at the surface must be controlled by that of the mantle diapirism and related magmatism, not vice versa. The implied style of rifting has important implications for the partitioning of extensional strain through the lithosphere and the constitution of the crust beneath the rift. Aulacogens and rifted continental margins, such as the southern Red Sea and the Norwegian-E. Greenland margins, that formed from magmatically active rifts, may have passed through a similar stage of evolution.

Structure of Hood Lake syenite — an undeformed Archean Pluton in the western Superior Province, southern Canadian Shield

Abstract The Hood Lake syenite is a late Archean pluton that has escaped significant tectonic deformation. The intrusive contact dips inward in most localities, which provides some constraint of its three-dimensional geometry. The pluton has a concordant planar fabric of perthite megacrysts, and could have grown by mature diapirism (‘mushrooming’) as well as by in-situ ballooning. Only magmatic diapirism seems to be capable of explaining the heart-like map pattern of the pluton and its location at the junction of two vertical zones of weakness, a regional fault and a ductile shear zone.

An investigation of the relationship between the geology of indonesian sedimentary basins and heat flow density

Geothermal data, collected from 929 wells drilled by oil companies in 20 Tertiary basins of Indonesia, have been related to the geology and tectonics of the area. It is found that the thermal conductivity increases with the depth of burial and degree of compaction in both transgressive and regressive sediments but decreases with increases in impurities and heterogeneous grain sizes. The temperature gradient is controlled mainly by the depth and temperature of a heat source beneath a basin. Very high heat-flow densities (> 125 mW m −2) found in central Sumatra, South Sumatra, Salawati Basin and Bintuni Basin may be caused by relatively shallow magmatic diapirism.

Foliation patterns in large gneiss bodies of the Archaean Wabigoon Subprovince, southern Canadian shield

Two areas of gneissic tonalite-granodiorite are being studied in the western Wabigoon Subprovince, the Rainy Lake gneiss body and the southwestern Irene-Eltrut Lakes gneiss body. The Rainy Lake gneiss body is an aggregate of large oval structures outlined by foliation trajectories. The body is dominated by the Ash Bay dome whose leuco-gneiss core is characterized by highly foliated rocks and surrounded by a zone of mafic gneiss rich of greenstone relics. This oval dome seems to have distorted the adjacent oval structures while growing to its present size. Of the various hypotheses of doming considered herein, only two can be eliminated from further consideration (1) large-scale buckling of the gneiss-greenstone interface and (2) bending of thrust sheets while moving over rigid ramps. Diapirism sensu lato remains an attractive mechanism for the growth of the oval structures under consideration. Whether the tonality-granodiorite became a gneiss while rising actively or deforming passively, as the wall rock of magmatic diapirs, remains to be determined.

Low-pressure metamorphism and anatexis of Carolina Slate Belt Phyllites in the contact aureole of the Lilesville Pluton, North Carolina, USA

In the contact aureole of the Lilesville granite and comagmatic Pee Dee gabbro, N.C., greenschist-facies phyllites of the Carolina slate belt have been overprinted by a series of metamorphic reactions producing opx-bearing hornfelses and migmatitic gneisses. In the exterior aureole the slate belt assemblage (chl+ms+ep+ab+qz) gives way to the continuous reaction assemblages (chl+bt+cd+ ms+ab±ep+qz), (bt+cd+ms+An8–29+qz), (bt+cd+ kf+ms+pl+qz), (bt+cd+als±ms+kf+pl+qz), and (bt+cd+ga+kf+pl+qz), from lowest to highest grade. The interior aureole, interpreted as part of the floor of the granite, bears the continuous and discontinuous reaction assemblages (bt+cd+als+kf+pl+qz),(bt+cd+kf+ pl+qz), and, near the gabbro, (bt+cd+ga+opx+kf+ pl+qz). The leucosomes of the migmatitic interior aureole are predominantly trondhjemites with the assemblage (An35–45+qz±bt±cd±kf). Restites in the migmatitic interior aureole contain the AFM assemblages (bt), (bt + cd), (bt+cd+als), and (bt+cd+ga), plus kf, An40–50, and qz. Contact metamorphism was isobaric at 4.0–5.1 or 2.0–3.5 kb depending on choice of aluminosilicate triple point; temperatures reached 650° C in the migmatitic interior aureole and approached 750° C near the gabbro; \({\text{P}}_{{\text{H}}_{\text{2}} {\text{O}}}\) was less than 0.8 in the migmatites, and was lower in the interior aureole and in the high grade exterior aureole. Partial melting in the migmatitic interior aureole took place during dynamothermal metamorphism caused by the magmatic diapir. Incipient melting occurred by the reaction bt+cd+kf+pl+qz+w = liquid. The melt was H2O-undersaturated and coefficients of the reactants were weighted heavily toward the felsic minerals; the proportion of felsic minerals in the leucosomes was controlled in part by modal abundance of kf, pl, and qz available for melting. The incorporation of K into biotite by subsolidus reactions, coupled with the high thermal stability and low solubility of biotite in a felsic melt, are responsible for the trondhjemitic composition of the early anatectic liquids.

Pressure-temperature conditions in the Wadi Kid metamorphic complex: Implications for the pan-african event in SE Sinai

Geothermobarometry using continuous equilibria in pelitic rocks of the central and northeastern Wadi Kid areas, Sinai, yields pressure estimates of 320 MPa (3.2 kbar) and temperature estimates of 565° C and >620° C for staurolite-andalusite zone assemblages and anatectic rocks, respectively. This corresponds to geothermal gradients of 47° C/km and ≧50° C/km. TheseP-T conditions are shown to be compatible with burial under a volcanic cover. The principal heat source for high-grade metamorphism and anatexis is thought to be rising magmatic diapirs. Uplift was relatively slow and the final stages of heating and early stages of cooling probably occurred isobarically. The data imply a high heat flow regime for the Pan-African event in Sinai.

Magma generated structures and their subsequent development in the Late Archaean evolution of Northern Buksefjorden, Southern West Greenland

The Inugssugssuaq nappe is made up of a membrane of anorthositic rocks sheathed by grey gneisses of Nuk age (2.800 Ma) and with a core of highly xenolithic Nuk grey gneiss. The structure was generated by magmatic diapirism followed by tectonic activity, but both stages probably resulted from thermally induced gravitational instability. Relations with geometrically similar structures in the area are discussed.