Deep-seated Processes Research Articles

Plate tectonics and mantle plumes as a basis of deep-seated Earth’s tectonic activity for the last 2 Ga

The onset of the modern tectonic style that combines plate-related and plume-related mechanisms has been discussed. Such a tectonic style could have started on the Earth when all layers of our planet had formed. Analysis of available geological data shows that the solid inner core crystallized by 2.7 Ga. Typical geologic complexes appeared on the continents as a result of plate tectonics processes at that time. The layer D″ that accumulates lithospheric slabs, which do not remain at the upper-lower mantle interface but can go down to the core-mantle boundary, was finally produced by 2 Ga. At pressures and temperatures of the D″ layer perovskite, a principal mineral of the lower mantle transforms into postperovskite phase. The isotope data suggest the existence of mantle (EM-I and EM-II) domains, being the sources for oceanic island basalts (OIB) and the depleted upper mantle (asthenosphere), the source for mid-ocean ridge basalts (MORB), from at least 2 Ga. It is accepted that the recent tectonic style started on the Earth at that period. Deep-seated processes suggest the involvement of all Earth’s layers. The asthenosphere–lithosphere interaction is responsible for different types of large surface structures that develop as fold mountains, oceanic spaces, and subduction zones. The descending lithospheric slabs (cold mantle material) and ascending mantle material in hot mantle provinces or the so-called low shear velocity provinces are responsible for the lower mantle convection. The plume is produced in the layer D″, which accumulates the descending lithosphere slabs as well as light elements released from the outer liquid core, which are oxidized in this layer, thus resulting in thermochemical plume formation. At the same time the molten ferric iron penetrates the core. So, we see the interaction of all Earth’s layers. The article also considers the evolutionary history of the Siberian continent over 700 Myr as early as the breakdown of Rodinia and the formation of mountain folds and rifting structures and the associated metallogeny.

Signs of the impact of the Earth’s core on the planet’s population

When investigating the rhythms of the Earth's electromagnetic noise and seismicity, as well as numerous calls for ambulance, cases of baby births and people death, the authors have found that such rhythms have diurnal, seasonal and annual variations and they are principal for human being's life. The analysis of both main regularities and single peculiarities of diurnal and annual rhythms in the living and non-living nature has led us to assumption that the deep-seated processes relating. to the eccentric rotation of the Earth's core and shell could be a powerful conductor of the life and the death on the Earth. The results obtained in our study not only confirm the existence of deep-seated waves generated by the Earth's core but also make us sure that such constantly circulating waves produce a certain impact on a human being's health, birth and death and even "orchestrate" suicides.

Mixing in mantle magma reservoirs prior to and during the 2011-2012 eruption at El Hierro, Canary Islands

Understanding magmatic systems feeding volcanoes is critical for accurate interpretation of monitoring data and, ultimately, eruption forecasting. Following 3 months of precursory unrest, the first historical eruption at El Hierro, Canary Islands, took place ∼2 km offshore from October 2011 to March 2012. Our detailed petrological analysis of lava samples reveals that at least two distinct magmas initially supplied from reservoirs in the mantle underwent hybridization at 15–25 km depth, i.e., also largely within the upper mantle beneath El Hierro. Diffusion chronometry applied to zoned olivine crystals indicates that magma mixing began during the period of preeruptive seismicity and continued for weeks after the eruption onset. Our data also capture a magma stagnation level at 10–15 km depth in the lower crust, consistent with lateral propagation of an intrusion over substantial distances before rapid magma transit to the seafloor. The remarkable spatial and temporal correlation of petrological and geophysical data at El Hierro suggests that the observed seismicity records magma mixing and forceful intrusion as well as subsequent reservoir dynamics. These results demonstrate that eruptions at El Hierro are controlled principally by deep-seated processes, with little influence from shallow crustal levels, and have important implications for monitoring of renewed unrest at long-dormant volcanoes.

Chronological link between deep-seated processes in magma chambers and eruptions: Permo-Carboniferous magmatism in the core of Pangaea (Southern Pyrenees)

In the Southern Pyrenees there are Upper Carboniferous–Lower Permian sedimentary basins with a significant volume of volcanic material derived from explosive eruptions (rhyolitic ignimbrites and andesitic flows). These basins are spatially associated with granodiorites and dacitic dykes emplaced in Variscan basement rocks. U–Pb SHRIMP dating of zircons extracted from three granodiorites, an andesitic flow, a dacitic dyke and six ignimbrites, revealed that magmatism occurred over an extended period of thirty eight million years, from ca. 304Ma to ca. 266Ma (Upper Carboniferous–Middle Permian). A scattering of zircon ages in each sample shows that the history of melt crystallization was complex, with more than one zircon-forming event in each magma chamber. The prolonged crystallization history was transferred to the product of the eruptions. A chronological link between the deep-seated magma chambers and processes in eruptions was identified on the basis of four overlapping intervals at: ca. 309–307Ma (Upper Carboniferous), ca. 304–296Ma (Upper Carboniferous–Lower Permian), ca. 294–282Ma (Lower Permian), and ca. 276Ma (Lower Permian). The variation of zircon U/Th ratios exposes a tendency for an increase in mafic sources as crystallization advances in the Permian. Zircons probably crystallized from melt phases related to both a felsic-intermediate metaluminous source from ca. 310–293Ma (mostly 0.1<Th/U<0.6) to ca. 289–273Ma (especially in the range 0.6<Th/U<1) and a mafic source (mostly 1.2<Th/U<1) at ca. 266–265Ma. U–Pb zircon ages from volcanic and plutonic rocks of the Southern Pyrenees are consistent with the ages of the post-Variscan magmatism of Iberia associated with orocline generation and subduction of the Paleotethys Ocean (ca. 304–283Ma), and in addition reveal a later magmatic event at ca. 276–266Ma (Lower–Middle Permian). The location of the Iberian orocline in the core of Pangaea and near the western end of the subduction zone of the Paleotethys Ocean leads to the hypothesis that this later magmatic activity of the Southern Pyrenees could provide the missing link between the Variscan and Cimmerian cycles that acted sequentially in Permo-Carboniferous times.

Petrogenesis and Geological Implications of the Oligocene Chongmuda-Mingze Adakite-Like Intrusions and Their Mafic Enclaves, Southern Tibet

The Oligocene to the Miocene was a critical period for the growth of the Tibetan Plateau. This growth is commonly considered to have been controlled by deep geodynamic processes. The ultrapotassic and adakite-like igneous rocks that developed during this period offer constraints on these deep-seated processes in southern Tibet. Whole-rock geochemistry, U-Pb zircon geochronology, and in situ zircon Hf isotopes have been determined for the mafic enclaves and host adakite-like granitoids in the Oligocene Chongmuda-Mingze intrusive complex of southern Tibet. The host rocks, including granodiorite and monzogranite, are mainly high-K and calc-alkaline in composition. Their whole-rock geochemistry (low MgO, Ni, and Cr contents; negative εNd(t) values [−2.5 to −3.4]; and low 87Sr/86Sr(i) values [0.7061–0.7066]) and in situ zircon εHf(t) values (0.6–6.1) indicate that they were derived by partial melting of a juvenile lower crust, implying that the southern Tibetan crust was already thickened to up to 50 km before ∼30 Ma. Mafic enclaves show typical igneous textures, acicular apatites, backveining structures, quenched margins, and crystallization ages identical to those of the host granites, indicating that they are of magmatic origin. The mafic enclaves have high-K to shoshonitic metaluminous compositions and are strongly enriched in large-ion lithophile elements and light rare earth elements, are depleted in high-field-strength elements, have negative εNd(t) values (−2.6 to −4.9), have relatively high 87Sr/86Sr(i) values (0.7060–0.7072), and have low zircon εHf(t) values (2.3–5.5), indicating that they were derived from a relatively enriched lithospheric mantle source. Our new data, together with previously published work, lead us to suggest that deep geodynamic processes below the southern Tibetan region during the Oligocene to the Miocene were characterized by convective removal of the lower lithosphere. Upwelling of the asthenosphere induced by the delamination of the southern Tibetan lithospheric root could have supplied heat to induce anatexis of the residual lithosphere of southern Tibet, generating the adakite-like rocks and related mafic enclaves there.

Vertical crustal movements on the continents as a reflection of deep-seated processes in the earth’s crust and mantle: Geological effects

The large volume of studies conducted makes it possible to assert that the recent geological history of the earth is characterized by rapid vertical crustal movements, which resulted in the formation of vast elevated areas and deep depressions. The existing geological concepts do not explain accurately the nature of these processes. The author of this paper uses basic science to explain them, as well as to forecast future tectonic movements and to develop a way of prospecting for mineral deposits.

Coupling Effects on Gold Mineralization of Deep and Shallow Structures in the Northwestern Jiaodong Peninsula, Eastern China

For understanding the possible deep-seated processes and geodynamic constrains on gold mineralization, comprehensive physicochemical and geochemical studies of gold mineralization have been undertaken within the paleo-lithosphere framework during the metallogenic epoch from the northwestern part of the Jiaodong Peninsula in this paper. A general image of the paleo-crust has been remained although it has been superimposed and reformed by post-metallogenic tectonic movements. The gold ore deposits occur usually in local uplifts and gradient belts featuring a turn from steep to gentle in granite-metamorphic contact zones, relative uplifts of gradient zones of the Curier isothermal interfaces, depressions of the Moho discontinuity and areas where depth contours are cut by isotherms perpendicularly. Gold mineralization and lithogenesis are characterized by high temperature, low pressure and high strength of thermal flux. The depth of mineralization ranges from 0.8 to 4.5 km. The depth of the top interface of the granitic complex in the metallogenic epoch is about 3 km. There is a low-velocity layer (LVL) at the bottom of the upper crust with a depth close to 19.5 km, which may be a detachment belt in the crust. The appearance of the LVL indicates the existence of paleo-hyperthermal fluid or relics of molten magma chambers, which reflects partial melting within the crust during the diagenetic and metallogenic epochs and the superposition effects of strike-slip shearing of the Tanlu fault zone. The subsidence of the Moho is probably attributed to the coupling process of the NW-SE continental collision between North China and the Yangtze Block and the strike-slip movement of the Tanlu fault accompanied with underplating of mantle magma in the northwestern part of the Jiaodong Peninsula. The underplating of mantle magma may result in partial melting and make granite magma transfer upwards. This is favorable for the migration of metallogenic materials from deep to shallow to be enriched to form deposits. Coupling interactions between the strike-slip of the Tanlu fault, the underplating of mantle magma, partial melting within the crust, and hyperthermal fluid, etc. may be the important factors controlling the gold mineralization and spatial structures in the metallogenic system.

A tectonic pressure pulse and increased geodynamic activity recorded from the long-term monitoring of faults in Europe

This paper presents the results of a long-term monitoring study undertaken during the period 2002–2007. The study recorded the displacement of various widely-distributed tectonic structures along the generally aseismic Bohemian Massif (Czech Republic) and specifically along the Sudeten Marginal Fault Zone. The derived results were compared with data recorded over the same time period in the Upper Rhine Graben (Germany), Central Apennines (Italy), and the Gulf of Corinth (Greece). It is clear that a tectonic pressure pulse initiated a period of increased geodynamic activity across central and southern Europe during the course of our monitoring. Within the Bohemian Massif, the pressure pulse was first recorded in mid 2003 and the ensuing period of increased geodynamic activity lasted for nearly three years. This period is associated with compression along the Sudeten Marginal Fault Zone. The southern flank was repeatedly downthrust beneath the northern flank, which caused the latter to continually uplift and subside. The period of increased geodynamic activity was terminated by two strong local earthquakes. Consequently, it is considered that this period induced earthquake activity within the Bohemian Massif. When taken together, the presented data provides robust evidence for the reconfiguration of stress and strain relationships within central and southern Europe. Whilst it is possible that tectonic activity in the Alps produced a north-verging pressure pulse that affected central Europe (e.g. in the Bohemian Massif and Upper Rhine Graben), it cannot account for the pressure pulse recognised to the south (e.g. in the Central Apennines and the Gulf of Corinth). Due to the predominance of vertical movements, it is suggested that this reconfiguration results from deep seated processes within the lithosphere–asthenosphere. However, the influence of horizontal movements transmitted from the compressional zone between Africa and Europe cannot be discounted.

Geodynamic setting of recent volcanism in North Eurasia

A GIS layout of the map of recent volcanism in North Eurasia is used to estimate the geodynamic setting of this volcanism. The fields of recent volcanic activity surround the Russian and Siberian platforms—the largest ancient tectonic blocks of Eurasia—from the arctic part of North Eurasia to the Russian Northeast and Far East and then via Central Asia to the Caucasus and West Europe. Asymmetry in the spatial distribution of recent volcanics of North Eurasia is emphasized by compositional variations and corresponding geodynamic settings. Recent volcanic rocks in the arctic part of North Eurasia comprise the within-plate alkaline and subalkaline basic rocks on the islands of the Arctic Ocean and tholeiitic basalts of the mid-ocean Gakkel Ridge. The southern, eastern, and western volcanic fields are characterized by a combination of within-plate alkaline and subalkaline basic rocks, including carbonatites in Afghanistan, and island-arc or collision basalt-andesite-rhyolite associations. The spatial distribution of recent volcanism is controlled by the thermal state of the mantle beneath North Eurasia. The enormous mass of the oceanic lithosphere was subducted during the formation of the Pangea supercontinent primarily beneath Eurasia (cold superplume) and cooled its mantle, having retained the North Pangea supercontinent almost unchanged for 200 Ma. Volcanic activity was related to the development of various shallow-seated geodynamic settings and deep-seated within-plate processes. Within-plate volcanism in eastern and southern North Eurasia is controlled, as a rule, by upper mantle plumes, which appeared in zones of convergence of lithospheric plates in connection with ascending hot flows compensating submergence of cold lithospheric slabs. After the breakdown of Pangea, which affected the northern hemisphere of the Earth insignificantly, marine basins with oceanic crust started to form in the Cretaceous and Cenozoic in response to the subsequent breakdown of the supercontinent in the northern hemisphere. In our opinion, the young Arctic Ocean that arose before the growth of the Gakkel Ridge and, probably, the oceanic portion of the Amerasia Basin should be regarded as a typical intracontinental basin within the supercontinent [48]. Most likely, this basin was formed under the effect of mantle plumes in the course of their propagation (expansion, after Yu.M. Pushcharovsky) to the north of the Central Atlantic, including an inferred plume of the North Pole (HALIP).

Deep Fault Drilling Project—Alpine Fault, New Zealand

The Alpine Fault, South Island, New Zealand, constitutes a globally significant natural laboratory for research into how active plate-bounding continental faults work and, in particular, how rocks exposed at the surface today relate to deep-seated processes of tectonic deformation, seismogenesis, and mineralization. The along-strike homogeneity of the hanging wall, rapid rate of dextral-reverse slip on an inclined fault plane, and relatively shallow depths to mechanical and chemical transitions make the Alpine Fault and the broader South Island plate boundary an important international site for multi-disciplinary research and a realistic target for an ambitious long-term program of scientific drilling investigations.

Direct measurement of timing: Underpinning a reliable petroleum system model for the Mid-Continent rift system

The Amoco 1 Eischeid well (Sec. 6, T83N, R35W, Carroll County, Iowa) was the first significant well drilled along the flank of the Iowa horst, providing an opportunity to evaluate the stratigraphic and structural history of the enigmatic Mid-Continent rift system of the United States. New thermal-history data obtained from cutting and core samples show that three paleothermal events affected the drilled section at the Amoco 1 Eischeid well. Results show that three significant cooling events occurred sometime between 300 and 200, 70 and 50, and 35 and 10 Ma. The first of these events was associated with a high paleogeothermal gradient (35C/km), demonstrably greater than the present-day value (16C/km). These new results provide direct evidence of a significantly higher paleogeothermal gradient (and by inference, higher heat flow) during the Paleozoic. In addition, results show that maximum maturity of the rich Precambrian source, the Nonesuch Shale (equivalent), was reached during the 300–200-Ma event and not 600–700 m.y. earlier as previously proposed. Previous work has been significantly influenced by the view that the craton was relatively stable or behaved uniformly throughout the Phanerozoic. Previous models impose a Proterozoic time of generation and entrapment for any Precambrian source intervals. With recognition of significant Phanerozoic heating events, the opportunity for later generation emerges, thus reducing the preservation time and increasing the probability of entrapment and preservation needed for exploration success in the region. Furthermore, these events mapped at the Mid-Continent rift system may be of great regional extent and correlative in time to other well-known events along the North American plate margins (e.g., Alleghanian, Laramide, Ancestral Rockies orogenies), thus offering insights to new possible exploration targets within the rift, perhaps indicative of deep-seated processes in the mantle beneath the North American craton during the Phanerozoic.

Imaging of palpable masses in the head and neck: a practical approach using high resolution ultrasound

Head and neck lumps are common and a significant proportion are malignant.Quick Early Diagnosis is best achieved in the setting of a “One Stop Clinic”.A relevant clinical history is frequently critical to narrowing the differential diagnosis or establishing the significance of radiological findings.Ultrasound±fine needle aspiration cytology (FNAc) will make the diagnosis in the majority of cases.Distinguishing normal structures from lymphadenopathy and non-nodal pathologies is an important step in differential diagnosis.Additional cross-sectional imaging may be required to characterize deep seated disease, markedly inflammatory or infiltrative processes, and stage malignancy. High-resolution ultrasound is now the imaging modality of choice for the assessment of the vast majority of palpable swellings in the head and neck. In experienced hands, it is frequently the only imaging technique required for diagnosis, as it can be performed rapidly, safely and permits accurate tissue sampling. A systematic approa...

The protracted Permo-Triassic crisis and multi-episode extinction around the Permian–Triassic boundary

The Permo-Triassic crisis was a major turning point in geological history. Following the end-Guadalupian extinction phase, the Palaeozoic biota underwent a steady decline through the Lopingian (Late Permian), resulting in their decimation at the level that is adopted as the Permian–Triassic boundary (PTB). This trend coincided with the greatest Phanerozoic regression. The extinction at the end of the Guadalupian and that marking the end of the Permian are therefore related. The subsequent recovery of the biota occupied the whole of the Early Triassic. Several phases of perturbations in δ 13C carb occurred through a similar period, from the late Wuchiapingian to the end of the Early Triassic. Therefore, the Permian–Triassic crisis was protracted, and spanned Late Permian and Early Triassic time. The extinction associated with the PTB occurred in two episodes, the main act with a prelude and the epilogue. The prelude commenced prior to beds 25 and 26 at Meishan and coincided with the end-Permian regression. The main act itself happened in beds 25 and 26 at Meishan. The epilogue occurred in the late Griesbachian and coincided with the second volcanogenic layer (bed 28) at Meishan. The temporal distribution of these episodes constrains the interpretation of mechanisms responsible for the greatest Phanerozoic mass extinction, particularly the significance of a postulated bolide impact that to our view may have occurred about 50,000 Myr after the prelude. The prolonged and multi-phase nature of the Permo-Triassic crisis favours the mechanisms of the Earth's intrinsic evolution rather than extraterrestrial catastrophe. The most significant regression in the Phanerozoic, the palaeomagnetic disturbance of the Permo-Triassic Mixed Superchron, widespread extensive volcanism, and other events, may all be related, through deep-seated processes that occurred during the integration of Pangea. These combined processes could be responsible for the profound changes in marine, terrestrial and atmospheric environments that resulted in the end-Permian mass extinction. Bolide impact is possible but is neither an adequate nor a necessary explanation for these changes.

Cranial neuronavigation in neurosurgery: assessment of usefulness in relation to type and site of pathology in 284 patients.

Neuronavigation improves intraoperative topographical orientation in neurosurgery. We wanted to better define the practical value of this technique in relation to the pathology operated on and the types of cranial surgery that profit the most from it. Usefulness, interactive use and probably preventive effect of neuronavigation in cranial neurosurgery were assessed in a consecutive series of 284 patients on the basis of questionnaires with two- or five-point scale ratings by different neurosurgeons. Neuronavigation was most helpful in tumors of the hemispheres (particularly the central area) not visible at the cortical surface or resembling normal white matter, and in endoscopic procedures within small ventricles or cysts with non-translucent walls or when vision was blurred by cloudy CSF. In the same pathologies and surgical procedures, the device was interactively used, taking advantage of the specific possibilities of interactive image-guided neurosurgery. A probably preventive effect of neuronavigation was noted in operations in eloquent areas; highest scores were given for intraaxial tumors of the central region. The subjective assessments of usefulness, interactive use or preventive effect were not dependent on the involvement of the neurosurgeons in this study. We recommend this technique in resecting tumors in eloquent areas of the cortex or white matter, in approaching deep-seated processes not visible at the cortical surface, in defining borders of tumors resembling normal brain tissue, and in guiding endoscopes where ventricles are small or vision is blurred. This recommendation applies to any neurosurgeon familiar with the technique and managing neurosurgical cases requiring precise topographical orientation where normal anatomic landmarks are missing.

Neuro-navigation in the central area: impact on different surgical steps related to the location and various pathological processes

The neurosurgical treatment of space occupying processes in the central area bears a relatively high risk of either postoperative neurological deficits ("radical approach") or of residual tumor ("conservative approach"). Therefore, special techniques of intraoperative topographic orientation (image-guided surgery) play an important role here. The possible impact of neuronavigation on different neurosurgical steps (craniotomy, corticotomy, localization of the process, definition of borders of resection) was studied in relation to the site of pathology (extraaxial, intraaxial/superficial, intraaxial/deep) in 46 patients harbouring space occupying lesions of the central area. In intraaxial pathologies, additional electrophysiological monitoring was done. It could be shown, that in cases of deep seated processes, neuronavigation had the greatest impact on craniotomy, corticotomy and localization of the process, whereas the borders of resection were defined predominantly on the basis of differences in colour or consistency. In extraaxial pathologies, neuronavigation was of significance only for craniotomy; in intraaxial processes visible at the surface, it had an impact on craniotomy and--in a few cases--on definition of resection borders. In neurosurgery of intraaxial pathologies of the central area (particularly those not visible at the surface), the use of neuronavigation (or another method of intraoperative localization) in combination with neurophysiologic monitoring is strongly recommended.

Experiences with Cranial Neuronavigation in Pediatric Neurosurgery

Frameless, infrared-based neuronavigation (Surgical Microscope Navigator^® and Surgical Tool Navigator^®) was applied to intracranial neurosurgery in 22 children and infants (age range 7 months to 16 years). Predominant diagnoses were tumor and hydrocephalus (including multiloculated forms); the surgical procedures were open microneurosurgery in 10 cases and neuroendoscopy in 12 patients. There were no principal technical problems even in very young infants. Surgical and clinical results were satisfying in all cases. Neuronavigation proved to be particularly helpful in targeting deep-seated processes and in guiding neuroendoscopy in cases of multiloculated hydrocephalus or small ventricles.

Mountain building and earth rotation

SUMMARY Whereas the present-day true polar wander and the secular non-tidal acceleration of the Earth have usually been attributed to postglacial rebound, it has recently been suggested that non-glacially induced vertical tectonic movements taking place under non-isostatic conditions can also be effective in changing the Earth's rotation. We present a case study in which we analyse the effects of some simple uplift histories of the Himalayas and the Tibetan Plateau on the rotational axis and on the second-degree zonal harmonic of the geoid, for time-scales of up to a few million years. We first assume a permanent amount of overcompensation, which is consistent with observed geoid anomalies over the Himalayas, and then we model by means of the normal-mode techniques, the viscous relaxation in the mantle, with the elastic lithosphere supporting elastically 2 per cent of isostatic disequilibrium. In our normal-mode analysis, the Earth is divided into five layers: an effectively elastic lithosphere, a viscoelastic shallow upper mantle, transition zone and lower mantle characterized by the Maxwell rheology and an inviscid core. The readjustment of the equatorial bulge due to viscous flow in the mantle is taken into account in our studies by solving the linearized Liouville equations for the conservation of angular momentum, via the Love numbers formalism. Polar wander is sensitive to the rate of relaxation of the modes M1 and M2 due to the discontinuities between the three mantle layers, positioned at 420 and 670 kilometres depth. The rate of readjustment is sensitive to the viscosity of the transition zone whenever the lower mantle/shallow upper mantle viscosity ratio is small. The highest present-day velocity of polar wander due to Himalayan and Tibetan Plateau uplift is estimated to be 1° Myr−1 for an isoviscous mantle that has the same magnitude of the observed value, reduced to 0.1° Myr−1 for a factor 50 viscosity increase in the lower mantle. These numbers are about the same as those found from postglacial rebound that occurs on the short time-scale of a thousand years instead of the million years of our analysis, but represent upper bounds for mountain building, obtained only in the case in which a permanent deviation from isostasy of at least 2 per cent is assumed. In general, the proposed mechanism is less efficient in driving long-term rotation instabilities than deep-seated processes characterized by the same time-scale of a million years such as subduction; polar-wander velocity is extremely sensitive to the depth of the uncompensated anomalous root of the topography for the models in which full mantle relaxation is allowed.

Stresses in the lithosphere and sedimentary basin formation

Seven fundamental questions on the relationship of lithospheric stresses and sedimentary basin origin and evolution are posed and elaborated. These are: 1. (1) How is the evolution of sedimentary basins tied to lithospheric dynamics? 2. (2) What are the sources of stress that lead to formation of sedimentary basins? 3. (3) How does the interaction between internally and externally applied stresses affect the formation and evolution of sedimentary basins? 4. (4) How do space and time variations of rheology and pore pressure affect stress transmission, strain, vertical motions, and stratigraphy during basin evolution? 5. (5) What factors influence spatial and temporal variations of rheology in, and around, different kinds of sedimentary basins? 6. (6) What is the role of magmatism in basin formation and what deep-seated processes does it reflect? 7. (7) What is the relationship between basin evolution, thermal history, and fluid flow? These fundamental questions can be investigated by developing fully-dynamic models of processes at the basin and sub-basin scale. Models must incorporate or honour basin history and plate reconstructions; the intregration of structural and sedimentological data; direct observations of the physical state; and the synthesis of geological and geophysical data, including tectono/dynamic stratigraphy, petrological and geochemical data, and geophysical data (gravity, magnetics, heat flow, deep and shallow seismics). A number of type basins — in passive margin, foreland, rift, and strike-slip tectonic settings — suitable for modelling studies elucidating the fundamental questions are identified.

Viscous and adiabatic heating effects in three-dimensional compressible convection at infinite Prandtl number

A three-dimensional direct numerical simulation (using a fully spectral method) of compressible convection of an infinite Prandtl number fluid in a wide box with dimensions 5×5×1 was conducted. Depth-dependent viscosity, thermal expansivity, and thermal conductivity have been included in order to model deep-seated processes in the Earth’s mantle. Solutions have been obtained up to a surface Rayleigh number of 4×107. There is a remarkable contrast between the dynamics of the upper and lower boundary layers. Very few cylindrical plumes are developed at the bottom but they merge collectively to form a strong upwelling, which pulses chaotically. Viscous and adiabatic heating are found to become important at high Rayleigh numbers, larger than 107. These results have important implications on the thermal structure of early Earth, where there might have been dramatic effects from intense mechanical heating near the top boundary layer.

Laser microprobe study of sulfur isotope variation in a sea-floor hydrothermal spire, Axial Seamount, Juan de Fuca Ridge, eastern Pacific

The Axial Seamount site is located on the Juan de Fuca Ridge near the intersection of the Cobb-Eikenberg-Brown BEar seamount chain and the axial rift of the ridge. Several large (to 12 m) silica+sulfide+sulfate hydrothermal spires reflect high-temperature (to 300°C) hydrothermal activity. Laser microprobe analyses along traverses of five temporally distinct fluid conduits within an inactive high-temperature spire reveal significant temporal variation inδ34S, both within individual conduits and between conduits. The maximum intra-conduitδ34S variation (on wurtzite) is 5‰ and the maximum inter-conduit variation (on sphalerite) is 7.4‰ Inter-conduitδ34S variation occurs primarily due to variable amounts of seawater mixing with hydrothermal fluids within the spire; generallyδ34Smineral decreases through time due to lesser amounts of seawater that invade more mature, heavily mineralized spires. Intra-conduitδ34S variation has not been documented at this scale (5‰ within 1 mm), and the potential mechanisms responsible for this variation include: (1) within-spire seawater-hydrothermal fluid mixing, which also produces the inter-conduit variations, or (2) more deep-seated convection and redox processes in the sea-floor subsurface that alter theδ34Sfluid.Mixing of hydrothermal fluid with seawater cannot produce a range ofδ34Smineral-values of this magnitude (5‰), and deeper subsurface processes are required. Such deep-seated processes may involve the opening of new fluid conduits in the subsurface, exposing fresh basalt which will increase the reduction potential of the rock system. This in turn will promote increased reduction of seawater sulfate in the hydrothermal fluid and attendant increases ofδ34Sfluid. The fluids will ulti precipitate34S-enriched sulfide phases, although this excursion inδ34S-values is ephemeral and will last only until the fluid has exhausted the reducing potential of the new conduit. At this point, sulfate reduction will be sharply reduced andδ34S-values will correspondingly decrease. This process may explain the major (to +5‰) isotope excursions that occur within individual conduits on a very small (<1m) scale.