Tensional Regime Research Articles

Impact of arc-continent collision on the conditions of burial and exhumation of UHP/LT rocks: experimental and numerical modelling

A 2-D physical and finite-element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere. The experimental technique allowed us to model the whole subduction/collision process from oceanic subduction to deep subduction of the continental crust. With the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process. The results obtained by both techniques are coherent and mutually complementary. They show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction. The conditions for such a failure (the weakening of this plate) are prepared during oceanic subduction. The weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates, and due to back-arc spreading. In oceanic subduction zones with a compressional regime (no back-arc opening, thick and strong back-arc lithosphere), the weakest zone is volcanic arc area. When weakening becomes sufficient during subduction, the lithosphere fails in this area. The failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore-arc. Almost half of the presently active subduction zones are characterised by a tensional subduction regime with back-arc spreading. In such subduction zones, the weakest place is not the volcanic arc but the back-arc spreading centre. When a subduction regime changes from tensional to compressional, failure occurs in the vicinity of the extinct spreading centre. This process can occur during oceanic subduction again along either a trench-vergent or trenchward-dipping fault, but the formation of a trench-verging fault is most likely. In this latter case, which is a principal subject of our study, the failure is followed by partial subduction of the arc plate. Complete subduction occurs during arc-continent collision (subduction of the continental margin) when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle. The arc itself can be subducted completely or be partially or completely scraped-off and accreted. A deeply subducted material (including continental margin) is preserved at relatively low temperatures between the lithospheric mantle and the “cold” subducted arc plate to about 150-km depth. Subduction of the arc plate is a major phenomenon, which affects all processes associated with continental subduction from deep burial and HP/LT metamorphism to exhumation of subducted material. Does this process occur in nature? Future investigations will allow us to answer this question. In this paper, we analyse the conditions of emplacement of a very young oceanic lithosphere (Samail ophiolite) on the continental crust in Oman in the late Cretaceous and argue that this lithosphere formed in a back-arc basin. It reached and overthrust the Arabian continent after complete subduction of the arc plate.

Post Jotnian basic Intrusions in the Fennoscandian Shield, and the break up of Baltica from Laurentia: a palaeomagnetic and AMS study

A palaeomagnetic and anisotropy of magnetic susceptibility (AMS) study has been performed on dolerite sills of the Central Scandinavian Dolerite Group (CSDG) in the Fennoscandian Shield. The dolerites occur in four previously known complexes in central Sweden and Finland and from the results of this palaeomagnetic study another complex has been identified in northern Sweden. These complexes cover an area of at least 100 000 km 2 and the palaeomagnetic data suggest a small difference in time between the intrusion of the dolerites. The measurements of anisotropy of magnetic susceptibility reveal a magnetic fabric with almost horizontal foliation planes and lineations that indicate fairly uniform ca NW or SE directed magma flows. The dolerites of the CSDG are geochemically rather uniform and have compositions typical of mantle derived melts formed in continental tensional settings. In a palaeomagnetic reconstruction of Baltica versus Laurentia at ca 1.27 Ga the two continents were joined, with NE Greenland attached to NW Baltica. AMS data from a few dolerites and a basalt in NE Greenland indicate magma flow directions that in the tectonic reconstruction are more or less parallel to the flow of the dolerites in Sweden. This may suggest a common magma source located at the reconstructed contact between Baltica and Laurentia. Both the dolerites in Greenland and those in Sweden are of tholeitic composition indicating an intraplate origin, which supports the interpretation of joined continents at that time. The tensional regime, that is reflected by the huge sill complexes, is in our interpretation related to the break up of Baltica from Laurentia at ca 1.27 Ga ago.

Influence of nonlinearity and double elasticity on flexure of rock beams — I. Technical theory

As a rule, solids display nonlinearity during loading in the relation between strains and stresses. Deviations from Hooke’s linear constitutive law were also registered in the range of initial, small loads both in uniaxial compression and tension of crystalline rocks. Nonlinearity of strain in rocks is manifested primarily in the stress dependency of tangent or secant elasticity modulus and Poisson’s ratio and is caused by closure, initiation, propagation and linkup of pre-existing and new microcracks, frictional sliding along cracks, growth of dislocations, etc. Many experimenters and standardization procedures assume that the dependence of the strain on the applied stress is linear and for practical calculations only two elasticity constants are used: the tangent or secant elasticity modulus at 50% of the failure load in compression and Poisson’s ratio at the same stress level. Apart from nonlinearity many rock types and concretes have quite different stress–strain relations in tension and compression. Yet direct tensile testing is seldom performed because of its many inherent difficulties. Such unrecognized double elasticity and nonlinearity of rocks can invalidate a stress analysis, and in addition, produce a meaningless overestimate (or underestimate) of tensile strength based upon the modulus of rupture derived from a bending test. In Part I of the present study, it is shown that both double elasticity and nonlinearity have a profound effect on flexural strength of rocks as predicted by application of fundamental continuum damage mechanics relations and an appropriate technical theory. The proposed theory is validated in Part II of this work, in which an appropriate back-analysis procedure is suggested for the characterization of the mechanical properties of Dionysos marble in the uniaxial tension and compression regime from properly designed three-point bending tests.

A preliminary study of stress-assisted fluid penetration in ceramic bricks

Solid bricks manufactured using clays from Thessaloniki area have been analysed by X-ray powder diffraction and their mineralogical composition showed the presence of chlorite, muscovite, quartz, and albite. The compressive strength values of dried and wetted bricks were determined and a decrease of about 25% was observed for water saturated bricks. Using mechanical extensiometers, stress strain diagrams and Poisson ratios were measured. In order to model the liquid diffusion through the brick and the chemomechanical damage due to the coupled action of liquid presence and applied loads, the front of a penetrating solute has been mathematically expressed by using Aifantis'stress-assisted diffusion theory. Pre-loaded bending specimens were sprayed with a liquid. After a certain time the specimens were loaded until fracture. Different phenomenological constants of the stress-assisted diffusion theory have been assumed for the compression and tension regime and a satisfactory agreement between theoretical predictions and experimental data has been obtained.

The Agnano–Monte Spina eruption (4100 years BP) in the restless Campi Flegrei caldera (Italy)

The Agnano–Monte Spina tephra (AMST), dated at 4100 years BP by 40 Ar/ 39 Ar and 14 C AMS techniques, is the product of the highest-magnitude eruption in the Campi Flegrei caldera (CFc) during its last epoch of activity (4800–3800 years BP). The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two main pumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the vent area. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flows overtopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit is subdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological characteristics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun during the course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, and generated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali–trachytic composition, whereas later-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that two isotopically distinct magmas, one alkali–trachytic and the other trachytic, were tapped and partially mixed during the eruption. The small volume (1.2 km 3 DRE) of erupted magma and the structural position of the vent suggest that the eruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.

Collective membrane motions of high and low amplitude, studied by dynamic light scattering and micro-interferometry.

Undulations of lipid bilayers were experimentally studied for the two limiting cases of high and weak lateral tension using two well established model systems: freely suspended planar lipid bilayers, so-called black lipid membranes (BLM) for high-tension studies and large unilamellar vesicles (LUV) for measurements at weak tension. This variation in tension results in changes of undulation amplitudes from several hundred nm (LUV) down to 1 nm (BLM), thus requiring different physical methods for their detection. We have employed microinterferometric techniques (RICM) for studying the regime of weak tension and dynamic light scattering (DLS) for that of high tension. The dedicated DLS set-up allowed the measurements of undulations over a wide wave vector range of 250 < q/cm-1 < 35,000 cm-1. This enabled the observation of collective membrane modes in two regimes, the oscillating one at low q and the overdamped regime at high q. The transition between both regimes at the bifurcation point is rather abrupt and depends on the lateral tension of the bilayer, as is demonstrated by comparing the dispersion curves of pure lipid and of lipid-cholestrol BLMs over the same q-range. The DLS measurements allowed a critical test of a hydrodynamic theory of the dispersion behaviour of membrane collective modes under tension. The DLS measurements are compared with RICM results of undulatory excitations of giant vesicles weakly adhering to substrates in the 10(-6)-2.5 x 10(-7) m wavelength regime and at low frequencies (0.1-25 Hz). Experimental evidence for the strong decrease in the relaxation rate by the hydrodynamic coupling of the membrane with the wall is established.

The Variscan post-collisional volcanism in Late Carboniferous–Permian sequences of Ligurian Alps, Southern Alps and Sardinia (Italy): a synthesis

In the considered wide sector of the West-Mediterranean southern Europe, the collisional phase of the Variscan orogeny during Late Carboniferous and Permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental, alluvial to lacustrine basins originated by wrench- to normal-fault systems. The first volcanic cycle (generally Late Carboniferous-Early Permian in age) is represented by early calc-alkaline andesites and rhyolites, in variable amounts, and by following large volume of rhyolites, and by dacites. Both andesites and rhyolites show K-normal and high-K calc-alkaline character. The origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle–crust interface telescoped within a thickened crust. The melting is considered as the consequence of thermal re-equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt; the ascent of liquids occurred in a (trans-)tensional regime. The second magmatic cycle is represented by alkaline magmatism, and exhibits typical anorogenic features consistent with a rifting regime. This event was no more related with the collapse of the Variscan belt, but rather to the post-Variscan global re-organization of plates that evolved during Late Triassic times to the neo-Tethyan rifting. In both cycles, important differences in timing, areal distribution and outpoured volumes arise.

Subduction in the Indo‐Burma Region: Is it still active?

The Indo‐Burma region (which includes longitude 92°–96°E, latitude 20°–26°N) is a subduction zone where the Indian plate underthrusts the South‐Eastern Asian plate. But the nature of subduction is complex. I examine here the distribution of the P, T and B axes of 37 earthquakes (the time period 1977–1995, depths 0–153 km and magnitudes mb 4.8–6.8) with respect to the geometry of the Wadati‐Benioif Zone. The analysis shows that the T axes are clustered close to the down‐dip direction of the subducting slab implying a predominant down‐dip tensional stress regime within the slab, which is typical of intermediate depth seismicity of active subduction zones. These results suggest that the subduction in the Indo‐Burma region is possibly active.

The Citronen Fjord massive sulphide deposit, Peary Land, North Greenland: discovery, stratigraphy, mineralization and structural setting

The Citronen Fjord massive sulphide deposit in the Lower Palaeozoic of North Greenland is the world's most northerly base metal mineralization. Since discovery in 1993, it has been intensively investigated by geological and geophysical surveys, and by drilling. The deposit is generally flat lying with a thickness up to 50 m; it extends from outcrop level to depths of 300 m.&#x0D; Three main stratiform sulphide sheets occur within a 200 m thick stratigraphic sequence; these are composed of massive and bedded pyrite with variable amounts of sphalerite and minor galena. The proven mineralization is continuous over a strike length of at least 3 km with a maximum width of 500 m; an additional 5 km of mineralization along the same trend is suggested by geological mapping and gravity surveys. The total tonnage of sulphides is estimated to exceed 350 million tons. The overall base metal resource is estimated at 20 million tons of 7 per cent zinc, with a higher grade core of 7 million tons containing 9 per cent zinc and 1 per cent lead.&#x0D; The Citronen Fjord deposit is located at the eastern end of the Palaeozoic Franklinian Basin that extends through the Arctic Islands of Canada and across northern Greenland. Its discovery is an example of a successful exploration strategy based on regional evaluation, sparse but telltale surface mineralization observations and low-cost logistics: a skidoo-sledge expedition. The stratiform mineralization is hosted in the dark argillaceous rocks of the Amundsen Land Group of latest Ordovician to Early Silurian age that comprises a starved basin sequence of cherts and shales with siltstones and mudstones, punctuated by carbonate debris flow conglomerates derived from the nearby southern carbonate shelf. The Lower Palaeozoic strata at Citronen Fjord are part of the southern margin of the North Greenland Fold Belt characterized by southerly-facing folds and thrust faults.&#x0D; A new geological map of the Citronen Fjord area is presented featuring Cambrian, Ordovician and Silurian strata, with two north-south cross-sections illustrating the main structure. Twelve informally-named lithostratigraphic units are recognized and comments given on correlation to the regional stratigraphy. Tectonic contacts separate Lower Cambrian strata from the Ordovician-Silurian part of the succession. It is concluded that the Citronen Fjord stratigraphy could be of local development in a sub-basin controlled by syn-genetic faults.&#x0D; The lead-zinc deposit is interpreted to be of sedimentary-exhalative origin formed by the precipitation of sulphides from metal-bearing fluids introduced onto the sea-floor through underlying fractures. The significant components of this deposition model include the existence of a tensional tectonic regime, deep-seated fractures and a restricted sub-basin morphology. Massive to dendritic-textured pyrite is interpreted to represent vent-facies deposition while the bedded sulphides are taken to be the corresponding distal facies. The precise tectonic control of the fractures is debatable, as is the role of the so-called Navarana Fjord Escarpment - a palaeo-topographic feature marking the junction between shelf and trough that is assumed to lie immediately to the south of the Citronen Fjord.

The Citronen Fjord massive sulphide deposit, Peary Land, North Greenland: discovery, stratigraphy, mineralization and structural setting

The Citronen Fjord massive sulphide deposit in the Lower Palaeozoic of North Greenland is the world's most northerly base metal mineralization. Since discovery in 1993, it has been intensively investigated by geological and geophysical surveys, and by drilling. The deposit is generally flat lying with a thickness up to 50 m; it extends from outcrop level to depths of 300 m.&#x0D; Three main stratiform sulphide sheets occur within a 200 m thick stratigraphic sequence; these are composed of massive and bedded pyrite with variable amounts of sphalerite and minor galena. The proven mineralization is continuous over a strike length of at least 3 km with a maximum width of 500 m; an additional 5 km of mineralization along the same trend is suggested by geological mapping and gravity surveys. The total tonnage of sulphides is estimated to exceed 350 million tons. The overall base metal resource is estimated at 20 million tons of 7 per cent zinc, with a higher grade core of 7 million tons containing 9 per cent zinc and 1 per cent lead.&#x0D; The Citronen Fjord deposit is located at the eastern end of the Palaeozoic Franklinian Basin that extends through the Arctic Islands of Canada and across northern Greenland. Its discovery is an example of a successful exploration strategy based on regional evaluation, sparse but telltale surface mineralization observations and low-cost logistics: a skidoo-sledge expedition. The stratiform mineralization is hosted in the dark argillaceous rocks of the Amundsen Land Group of latest Ordovician to Early Silurian age that comprises a starved basin sequence of cherts and shales with siltstones and mudstones, punctuated by carbonate debris flow conglomerates derived from the nearby southern carbonate shelf. The Lower Palaeozoic strata at Citronen Fjord are part of the southern margin of the North Greenland Fold Belt characterized by southerly-facing folds and thrust faults.&#x0D; A new geological map of the Citronen Fjord area is presented featuring Cambrian, Ordovician and Silurian strata, with two north-south cross-sections illustrating the main structure. Twelve informally-named lithostratigraphic units are recognized and comments given on correlation to the regional stratigraphy. Tectonic contacts separate Lower Cambrian strata from the Ordovician-Silurian part of the succession. It is concluded that the Citronen Fjord stratigraphy could be of local development in a sub-basin controlled by syn-genetic faults.&#x0D; The lead-zinc deposit is interpreted to be of sedimentary-exhalative origin formed by the precipitation of sulphides from metal-bearing fluids introduced onto the sea-floor through underlying fractures. The significant components of this deposition model include the existence of a tensional tectonic regime, deep-seated fractures and a restricted sub-basin morphology. Massive to dendritic-textured pyrite is interpreted to represent vent-facies deposition while the bedded sulphides are taken to be the corresponding distal facies. The precise tectonic control of the fractures is debatable, as is the role of the so-called Navarana Fjord Escarpment - a palaeo-topographic feature marking the junction between shelf and trough that is assumed to lie immediately to the south of the Citronen Fjord.

Axial summit trough of the East Pacific Rice 9°–10°N: Geological characteristics and evolution of the axial zone on fast spreading mid‐ocean ridge

The nature and morphological characteristics of axial summit troughs on fast (∼90–130 mm/yr−1 full spreading rate) and superfast spreading (&gt;130 mm/yr−1) mid‐ocean ridge crests reflect the time‐integrated effects of long‐term magmatic cycles, short‐term volcanic episodicity, and the tensional stress regime imposed on young ocean crust. Two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid‐ocean ridge crests. (1) Narrow axial troughs, &lt;∼200 m wide and &lt;∼15 m deep, are termed axial summit collapse troughs and are interpreted to be generated by shallow collapse of lava flow surfaces over zones of primary eruptive fissures and subsurface lava conduits (such as channels and tubes). Axial summit collapse trough position, physical characteristics, and associations with hydrothermal venting on the ridge crest strongly suggest a primary volcanic/magmatic origin for the trough. (2) Wider, fault‐bounded axial troughs (&gt;∼300–2000 m wide and ∼30–100 m deep) on the East Pacific Rise crest are classified as axial summit graben. The dimensions of axial summit graben, as well as the morphological and structural character of their walls and floors, suggest a primary tectonic origin. An axial summit graben may contain a nested axial summit collapse trough, implying that processes responsible for these endemic features may be linked. Near‐bottom, side‐looking sonar and observational data collected using the towed vehicle Argo I and submersible Alvin have been used to characterize the axial summit trough of the fast spreading East Pacific Rise between 9° and 10°N. A four‐stage model is presented for the evolution of this axial summit collapse trough, as well as for other well‐studied portions of the East Pacific Rise crest from 21°N to ∼20°S. We propose that the transition from a narrow, surface collapse‐dominated axial trough to a broader, fault‐bounded graben is controlled by the relative importance of diking, volcanism, hydrothermal cooling, and tectonism along a ridge segment over time periods &lt;104 years.

An experimental and theoretical investigation of phonons and lattice instabilities in metastable decompressed perovskite

We report detailed Raman and IR spectroscopic measurements for the decompressed high-pressure perovskite phase of . The appearance of a first-order Raman spectrum and slight splittings in the infrared bands suggest that the symmetry of the recovered metastable perovskite phase is lowered from Pm3m. This interpretation is fully supported by first-principles LDA calculations using the LAPW method, which indicate a small tetragonal distortion. The static lattice energy is lowered by 3.3 meV (per formula unit) by allowing rotational relaxation of the octahedra. The calculations permit a reliable assignment of the zone centre phonon modes of perovskite. The calculated pressure dependence of the ferroic IR-active modes is in excellent agreement with our measured data and reveals an incipient soft-mode behaviour in the tension regime. Further calculations of the unit as a function of octahedral volume reveal instabilities to local off-centre displacements as the octahedra are expanded. This provides a detailed understanding of the large dielectric constant for this non-transition-metal-containing decompressed perovskite phase.

Evidence on Mixing of Mantle and Crustal Derived Magmas in Bodrum (Mugla) Area Volcanic Rocks, Southwest Turkey

The study area is located at Turgutreis-OrtakentYalikavak-Akyarlar areas, the west of Bodrum (Mugla) in the Southwest Turkey where calcalkaline, alkaline and locally shoshonotic volcanism were effective after the end of Middle Miocene. These volcanics has been attributed to continental rifting due to tensional regime (e.g. Ercan et al., 1984). Tensional tectonics in the region affected thickened and partially melted continental crust, and caused the mixing of crustal calcalkalineand mantle derived alkaline magmas. Two volcanic cycles were effective in the Bodrum Peninsula during Middle-Upper Miocene times. First cycle volcanites are tuffagglomerate, high-K calcalkaline andesite-daciterhyodacite-rhyolite suite developed at the end of Middle Miocene. This cycle ceased with shoshonitic latite and trachyandesite, and second cycle developed with shoshonitic basalt, trachybasalt and trachyte as small outcrops and dykes in Upper Miocene (Ercan et al., 1984). The volcanites studied, namely andesite, latite, dacite, rhyodacite and rhyolite (Fig. 1), show generally porphyric and sometimes felsitic, hyalopilitic, microlitic, pilotaxitic textures. Phenoand microphenocrystals are plagioclase, biotite, hornblende, quartz, augite and sanidine. Groundmass is made up generally by feldspar microlites and/or glass, and sometimes alteration products. Plagioclases are subhedral laths and rarely resorbed and fragmented crystals. Some of them commonly display sieve texture with mottled extinction or patchy zoning, and inclusions of pale grown glass, sometimes with an overgrowth rim which may be separated from the cores by a dark grey zone of 'clouded' plagioclase charged with micron-sized melt inclusions. Plagioclases also show discontinuous and sometimes oscillatory zoning. Hornblendes are subhedral to anhedral, but often 'ghost like' crystals, pseudomorphed by opaques, pyroxene, feldspars and quartz or simply rimmed by opaques. Biotites are subhedral to anhedral with common reaction rims or breakdown products, or pseudomorphed by opaques. Breakdown of biotite forms opaques or a granular intergrowth of opx + ilmenite + magnetite + K-feldspar. These reaction products of hornblend and biotite were caused by dehydration above the hornblend and biotite stability limit as result of magma mixing (e.g. Nixon, 1988). The rocks are generally high-K calcalkaline in compos i t ion , having 4 7 % K20 (Fig. 1), 15.8-17.8% AlaO3, 26.3-47.1 Mg-number, low Ni, Cr, Co and high La, Ce contents. Furthermore low Na20/K20, Nb, TiO2 (<1%) and high LILE contents resembles volcanites developed at active continental margins. Geochemical variations show that the rocks are cogenetic and related to each other by fractional crystallisation, assimilation and magma mixing processes during the evolution of crustal magma chambers. Major and trace element variations can be explanied by fractionating phenocrystal phases. Low Y and high La/Y may imply that

Extension controls Quaternary tectonics, geomorphology and sedimentation of the N-Appennies foothills and adjacent Po Plain (Italy)

In the middle Pleistocene, thrusting in the N-Apennine fold-and-thrust belt came to an end and foredeep sedimentation in the adjacent Po Plain basin ceased. A new tectono-sedimentary regime was installed. Extensional faults were activated in the foothills and are limited to the northeast by a major, SW-dipping normal fault, the Bologna fault for which a vertical displacement of >1000 m is estimated. The region northeast of the fault, i.e. its footwall, was progressively tilted towards the Po Plain. The SW sectors of the tilting region were progressively uplifted above sea level, subjected to erosion thereby feeding a deltaic system which prograded towards the northeast in the subsiding regions. Compressional structures without major deformation are recorded in the zone of maximum curvature. All these features are explained by the onset of extensional deformation along the Bologna fault and consequent flexural unloading of its footwall. The evidence, therefore, shows that the transition region between the N Apennine foothills and Po Plain has been under a tensional stress regime since the middle Pleistocene.

The alkaline Meidob volcanic field (Late Cenozoic, northwest Sudan)

The Meidob volcanic field (MVF) forms part of the Darfur Volcanic Province and developed from 7 Ma to 5 ka as indicated by K/Ar, thermoluminescence and 14C ages. It is situated in an uplifted high of the Pan-African basement, which consists of greenstones, high-grade gneisses and granites, and which is covered by Cretaceous sandstone. The MVF basaltic lavas, which originated from more than 300 scoria cones, formed a lava plateau of 50×100 km and up to 400 m thickness in the time between 7 and < 0.3 Ma. Young phonolitic mesa flows, together with rare trachyticbenmoreitic lava flows, trachytic pumice fallout deposits, ignimbrites and maars, form the central part of the field. The total amount of volcanic rocks is between 1400 and 1800 km 3, with 98 vol.% being basaltic rocks, which results in an integrated magma output rate of ∼ 0.0002 km 3 a −1. A combination of age data of the lavas with erosional features yields uplift rates for the Darfur Dome of ∼30 m Ma- 1 in the MVF area. Magma was generated by 3–5% melting of predominantly asthenospheric mantle with a HIMU contribution. Fractionation of olivine, pyroxene, An-poor plagioclaseanorthoclase, magnetite and apatite leads to a differentiation from basanite to phonolite. Assimilation of crustal rocks near the top of the phonolitic upper crustal magma chambers - facilitated by volatile enrichment - produced magmas which gave way to benmoreitic and trachytic lavas, as well as to trachytic ignimbrites and pumice fallout deposits. Ultramafic cumulate xenoliths indicate the existence of major magma reservoirs at the crust-mantle boundary during MVF activity. Magma ascent occurred in a tensional regime, which changed its orientation at around 1 Ma. Early during MVF development, west-east and subordinately northeastsouthwest trending lineaments were active whereas volcanic activity younger than 1 Ma took place along northwest-southeast and northeast-southwest trending systems. The Central African Fault Zone, a transcontinental, lithospheric shear zone, played an important role for the rise of magmas in the Darfur Dome.

Control of the production of individual fusicoccins at different dissolved oxygen concentrations

The regulatory effect of different concentrations of dissolved oxygen on the production of fusicoccins by the fungus Fusicoccum amygdali Del. was studied. The maximum output of total fusicoccins was obtained by using a profiled dissolved oxygen tension (DOT) regime, in which the DOT was maintained at 15–20% during the biomass growth phase and at 5–8% during the fusicoccins production phase. In comparison with the profiled regime, the maintenance of DOT at 15–20% during the whole fermentation shortened the fusicoccins production phase. The fermentation performance at a low DOT (5–8%) inhibited both the accumulation of biomass and the production of fusicoccins. At high DOT (40–50%), an accelerated accumulation of the biomass with an expressed autolysis of mycelia took place, and the production of fusicoccins was lowered. The qualitative composition of individual fusicoccins varied substantially at different DOTs. Fusicoccins, A, C, D, J, H, 16-O-demethyl-J, detretpentenylfusicoccin and some minor fusicoccin metabolites were found in the fermentation broth using the method of liquid secondary ion mass spectrometry. It was established that the profiled DOT regime (15–20% to 5–8%) provided both the maximum concentration of fusicoccins and an enhanced accumulation of the main metabolite – fusicoccin A (FC A). The performance of the fermentation at a DOT of 15–20% decreased the content of FC A by 2–6% in comparison with the profiled DOT regime, and increased the content of fusicoccin C to 14–20% of the total fusicoccins. Fermentation at DOT of 5–8% was characterized by the highest content of the precursors of FC A, the less oxidized fusicoccins H and J, the contents of which were in range 7–12% and 16–17% of total fusicoccins, respectively.

The Co-Existence of The Syn-Colg and Post-Colg Plutons in The Yıldızeli Area (W-Sivas)

The Upper Cretaceous-Paleocene plutons in the Yıldızeli region, emplaced in the crustal metamorphics of the Central Anatolian Crystalline Complex and covered by Lower Tertiary units, are referred to as Yücebaca and Davulalan Plutons. Davulalan Pluton is also intruded into Yücebaba Pluton. Yücebaba Pluton consist of granitic rocks with a K-feld+plg+qz+bi+sph+ap+ep+al+op mineralogical composition. Davulalan Pluton is composed of syenitic and quartz-syenitic rocks and, less frequently, of monzonitic and monzo-gabbroic dykes. Their mineralogical compositions are K-feld+per+qz+sph, K-feld+plg+bi+qz+sph+ap+ep+op, plg+K-feld+hst+di+sph+ap+op, and plg+K-feld+hst+ae-aug+op, respectively. Mineralogical and major-element geochemical data show that Yücebaca Pluton is S-type and peraluminous in character, whereas Davulalan Pluton is alkaline and has an alumino-cafemic composition. Major and trace-element data reveal the characters of Yücebaba and Davulalan to be syn-collisional and post-collisional, respectively. When the mineralogical-petrographical and geochemical characteristics are considered together with the space-time relations in the regional geological setting, the following petrogenetic models can be suggested for these plutons: Yücebaca Pluton is thought to have originated from anatexis in the thickened crust during collision, whereas Davulalan Pluton is assumed to have originated from a magma source which was produced by the partial melting of Upper Mantle material as a result of crustal attenuation during the tensional regime immediately after crustal thickening.

Stress transmission across an active plate boundary: an example from southern Mexico

Paleostress tensors calculated from inversion of fault-slip data at 60 stations between Zihuatanejo and Tehuantepec in southern Mexico were separated into four distinct groups and assigned to time intervals between 150 Ma and Recent. The change in orientation of the maximum horizontal stress closely traces the change in direction of convergence between the North American and the Farallon/Cocos plates for the same time interval. Depending on the angle between the convergence direction and the plate margin, both thrust fault and strike-slip stress regimes were active. Based on this comparison, we infer that the stress distribution in the continental crust of southern Mexico has been controlled by interplate stress transmission. Latest Cretaceous-Early Tertiary mylonites at the northern edge of the Xolapa complex portray left-lateral oblique crustal extension and oceanward migration of the upper plate. The orientation of extension at mid-crustal level grossly correspond to the orientation of the minimum compressive stress at upper crustal level. During the last 20 Ma a tensional stress regime as a response to surface uplift has been dominant in southern Mexico.

Stratigraphy, facies and synsedimentary tectonics in the Jurassic Rosso Ammonitico Veronese (Altopiano di Asiago, NE Italy)

The red, pelagic limestones of the Rosso Ammonitico Veronese (Upper Bajocian-Tithonian) of the Altopiano di Asiago area can be subdivided into eight facies. They differ from each other in structure (bedding style, presence and type of nodularity) and texture (nature of components, grain-vs mud-support, compactional features). Several discontinuities could be recognized, based on sedimentological or biostratigraphic evidence. In the context of a drowned platform, where sediments essentially consist of skeletal remains of both planktonic and benthic organism, the different facies are interpreted as reflecting a varying influence of currents on the winnowing of micrite and on triggering early cementation. Early cementation in turn, controlled the patterns of bioturbation and the response of sediments to later compaction and pressure-dissolution. At times, microbial mats, of unidentified nature, were important in trapping and binding sediment, giving rise to early lithified nodules and layers of stromatolitic aspect. The Rosso Ammonitico Veronese can be subdivided into three units: lower Rosso Ammonitico (RAI: Upper Bajocian-Lower Callovian), middle Rosso Ammonitico (RAM: Upper Callovian-Middle Oxfordian), and upper Rosso Ammonitico (RAS: Lower Kimmeridgian-Tithonian). Frequent ammonite moulds allow the precise dating of the base and top of each unit, and the documentation of facies heteropies and hiatusses in the more fossiliferous RAS. The lower unit (RAM) is massive and essentially nodular; the middle unit (RAM) is well bedded, non-nodular, and cherty; the upper unit (RAS) is richer in clay and typically nodular. The RAI and the RAS are present everywhere, though significant facies and thickness changes affect particularly the RAS; the RAM is much more variable, ranging from 0 to 10 metres. These variations, that may be gradual or abrupt, are inter-preted as the result of Middle-Late Callovian block-faulting which generated an irregular sea floor topography where the swells were more exposed to currents that continuously removed sediments, inducing long-lasting periods of nondeposition. Sediments preferentially accumulated in the adjacent lows. A confirmation of this hypothesis is provided by evidence of synsedimentary tectonics, described for the first time in the Rosso Ammonitico Veronese. Neptunian dykes, both vertical and horizontal, are developed at the top of the RAI and are filled with laminated sediments or collapse breccias. Glides of metre-size blocks and slumps are present at the top of the RAI and at the base of the RAM, respectively. Cm-thick layers of mud supported breccias are intercalated in the upper part of the RAI and within the RAM: they are interpreted as seismites. All these features document a tensional regime that generated fractures in more or less lithified sediments and failure along steep fault scarps or gently dipping slopes of tilted fault blocks. Recognition of this Callovian-Oxfordian tectonic activity shows that, after the Bajocian drowning, the Trento Plateau did not simply experience a uniform and general foundering: a small-scale block-faulting was still active and affected the pattern of facies distribution.

Space and Time Relations of Three Plutonic Phases in the Eastern Pontides, Turkey

Granitoid plutons located between the towns of Dereli and Sebinkarahisar are differentiated into three groups, representing the products of three distinct phases; each of these phases is subdivided into several subunits. The first plutonic phase, intruding into the Santonian-Campanian volcanics, is composed of the Maastrichtian Tamdere quartz monzonite, Surmen granodiorite, and Aksu biotite monzogranite. It appears to have formed as differentiation products of a single hybrid magma source. The subunits of the second plutonic phase consist of the Paleocene Gökcebel syenite and Sebinkarahisar quartz syenite, which are intruded into the Aksu biotitemonzogranite to the north and the Santonian-Campanian volcanics to the south. The third plutonic phase consists solely of the Eocene-age Yücedere diorite/gabbro, which intrudes the subunits of the first plutonic phase. The first and second plutonic phases include microgranular mafic magmatic enclaves and display special microscopic textures, the former indicating magma mingling and the latter magma mixing. The first plutonic phase has cafemic (CAFEM)/calc-alkaline (CALK), I-type and arc-related magma characteristics. The second plutonic phase is derived from a magma exhibiting cafemic (CAFEM)/alkaline (silica oversaturated alkaline, ALKOS), I/A-type, and postcollisional characteristics. The third phase is a typical product of a cafemic (CAFEM)/low-K tholeiitic, M-type, postcollisional mafic magma source. Within a regional geologic context, the first plutonic phase is suggested as part of the well-known Eastern Pontide arc magmatism, induced by northward subduction of the northern branch of Neotethys beneath the Eurasian plate during Late Cretaceous time. The products of the second plutonic phase may have been formed from an alkaline magma generated in relation to the tensional regime after crustal thickening caused by the Anatolide-Pontide collision. The third plutonic phase appears to have orginated from a mantle-derived magma generated by the partial melting of upwelled mantle material during an advanced stage of extension.