SEAFLOOR SPREADING AND TECTONICS AT THE CHARLIE GIBBS TRANSFORM SYSTEM (52-53ºN, MID ATLANTIC RIDGE): PRELIMINARY RESULTS FROM R/V A. N. STRAKHOV EXPEDITION S50

Mid-ocean ridges generally are thought of as simple and well-defined geological structures. Straight ridge segments of greater or lesser extent whose faults trend perpendicular to the direction of spreading are offset by strike slip transform faults trending parallel to the direction of spreading. In a few places along ultraslow spreading mid-ocean ridges such as the Southwest Indian Ridge (15-18 mm/yr full rate), these rules seem to be violated, as the distinction between ridge segment and transform becomes blurred (as at the oblique segment of the SW Indian Ridge). This is characteristic of ultraslow spreading ridges. The Arctic Ridges violate the “rules” of mid-ocean ridge behavior almost from the moment the N. Atlantic ridge crosses the Arctic Circle in Iceland. There, north of the Jan Mayen fracture zone, the ridges deepen, become oblique and lose their stairstep geometry. The Mohns ridge, beginning about 73 degrees N, is about 15-20 degrees oblique to the spreading direction, then swings around through 0 degrees obliquity to become the Knipovich ridge, which is almost 30 degrees oblique in the opposite direction. Both Knipovich and Mohn’s ridges are devoid of fracture zones. Little or no peridotite outcrop has been reported from Mohn’s or Knipovich ridge. Basalts from these ridges tend to be enriched relative to basalts from the North Atlantic. The northern end of Knipovitch ridge is delineated by one of the few arctic fracture zones, the Molloy Fracture Zone. Immediately to the north, Molloy Deep forms an oblique, deep rift structure, from which numerous recorded dredge hauls have reported only peridotite. Molloy deep is the deepest point on the global mid-ocean ridge system (6000m), and is the first known example of amagmatic oceanic rifting. The northern part of Molloy deep contains a basement ridge structure which also consists of mantle peridotite, mostly plagioclase-bearing. These peridotites have as yet been little studied. The Spitzbergen Fracture Zone connects Molloy Deep with the Southern end of Lena Trough. Though many authors have speculated as to the nature and configuration of Lena Trough, recent work has shown that Lena Trough is a single amagmatic oblique spreading segment, devoid of fracture zones. Instead of a classical segmentation, the rift is divided into a relay zone of steep-sided basement blocks consisting entirely of mantle peridotite. The only basalts are from a single location and show a highly enriched, K-rich composition which bears little chemical similarity to other MORB, and shows a strong garnet trace element signature in their source. The peridotites of Lena Trough are highly variable, and in the basalt location also with trace element signatures suggesting garnet residue during partial melting. The minimum chrome numbers of residual peridotites are low, 13-14 in nearly every dredge haul, and are strongly variable, up to Cr# 40-50 in some stations. There appears to be a magmatic segmentation represented by the occurrence of magmatic veins and plagioclase peridotites, which are more common in the center of Lena Trough than at the ends. Lena Trough veers abruptly East at its northern end, becoming the orthogonal-spreading Gakkel Ridge. The chemistry of basalts from western Gakkel Ridge more closely approximates normal MORB, though some sites near Lena Trough show evidence of the high-K component. Between the termination of Lena Trough and a marked discontinuity at 3E, there are no deviations of axial linearity, no fracture zones, but there is evidence of primary magmatic segmentation because of vertical undulations along the ridge. At 3E another amagmatic zone begins, somewhat shorter than Lena Trough. Peridotites from the western end of this region are uniformly fertile (Cr# 12-14), but become more variable to the east. Two dredge hauls from this region contained ultra fresh peridotites, with no visible serpentinization in many samples. A reference “fresh abyssal peridotite” sample set is being prepared so that bulk and microanalysis of these critical samples can proceed in an organized fashion. The eastern edge of the 3E zone marks a dramatic change both in tectonics and in the geochemistry of peridotites and basalts. A narrow zone of basalt enrichement is flanked by a broader zone of peridotite major element depletion. Minimum Cr#’s here reach 45-50 in several spatially associated dredge hauls. One of these, HLY D40 also contains some fresh material, as small rounded clasts in a carbonate-hasted breccia, but the amount is quite small, only a few kg total. East of this zone, as the spreading rate decreases to around 10mm/yr full rate, fertile peridotites return, and basalt chemistry becomes similar to that observed along other ultraslow spreading ridges. Overall the arctic ridges are a surprise because of what they don’t show. Low degrees of partial melting would imply that basalt have a chemistry dominated by early-melting components (often referred to as “mantle veins”), the basalt geochemical database to date for the arctic rules this out except for one location in Lena Trough. Second, thin to missing crust would imply a uniformly fertile mantle, perhaps corresponding to completely unmelted mantle. Except for one small region at the western end of the 3E amagmatic zone this is not the case. Instead, nearly all locations show a dramatic variablility in chemical compositions of mantle rocks. Some of these variations are too extreme to have plausibly originated by melting beneath Gakkel Ridge, and must instead be the product of older events. Finally, the basaltic crust turned out to be surprisingly robust. Though there are several large occurrences of bare mantle outcrop along the high arctic ridges, the basaltic crust in between is surprisingly thick. Possible reasons for this include a long-lived zone of enhanced melting beneath the western Gakkel Ridge, an increase in melt extraction efficiency when highly focused, or the fact that heavy sedimentation in the eastern Gakkel Ridge covers up peridotite exposures that we would otherwise be able to sample between the basaltic edifices. The future of exploration of the high Arctic ridges is likely to include dredging of the eastern end of Gakkel Ridge, as drilling in the sediment covered basins in the East, and drilling investigation of the causes of the abrupt change in crustal thickness at 3E

Reactivity Monitoring of Accelerator-Driven Nuclear Reactor Systems

Sediments of two Gondwana glaciations in Ethiopia: Provenance of the Enticho Sandstone and the Edaga Arbi Glacials

How the FEUP Removal to the New Premises Was Facilitated by the Internal Information System

High-dimensional time series analysis has become an active area of research, because of its increasing prevalence in applications as well as the unique challenges it presents. This thesis first explores a range of questions in this area, related to how high dimensionality invalidates conventional statistical methods. It seeks to find out how to extend the existing models to cope with increasing dimensionality; how to develop a reliable as well as efficient estimation approach under the complex structure of high-dimensional datasets; and how to gain some benefits from the current information explosion. We have addressed these problems through a theoretical study using M−estimation for high-dimensional time series linear models. One advantage of M−estimation is that it can act as a robust alternative for some classical statistical methods that are sensitive to outliers or distributional deviations. Furthermore, M−estimation features generality by covering a variety of estimation approaches, for example, the least squares estimation and quantile regression. Hence, the first objective of this thesis is to study the asymptotic properties of the M−estimator for analysing high-dimensional time series data. We also evaluate the finite sample performance of the proposed M−estimator by conducting a series of Monte Carlo experiments. Compared with the least squares estimator, the M−estimator has been found to perform much better, with high resistance to the presence of non-normal errors. Another central issue discussed in this thesis lies in panel data analysis. The increasing availability of panel datasets makes this an intriguing area to investigate. The advantage of panel datasets over univariate time series or cross-sectional data enables researchers to study the comprehensive interactions within socioeconomic networks. One remarkable feature of panel data is that cross-sectional units are likely to be correlated with each other, arising from spatial spillover, unobservable common factors and economic interactions among them. In the literature of late, econometricians have paid considerable attention to propose new methodologies for modelling the cross-sectional dependence, for instance, the multifactor error structure. However, the assumptions in the existing literature are overly restrictive (see, for example Bai 2009). Consequently, the second objective of this thesis is to develop the least squares estimator for a large panel data model with a multifactor error structure under a relaxed assumption framework. The estimator is shown to be consistent and asymptotic normal while its asymptotic distribution is non-centred. Hence, the closed expressions of the estimators for the asymptotic bias are given. Monte Carlo simulations not only investigate the small-sample performance of our estimator, but also show how well our bias estimators can centre the asymptotic distributions under different sample sizes and various strengths of dependence. In addition to large panels, short panels have also become pervasive in empirical studies, of which the cross-sectional dimension is relatively large compared with its temporal dimension. However, for the yearly data, fixed time periods can still span a rather long time span. The assumption of parameter stability becomes questionable due to the dramatically changing economic environment. A distance type estimator is introduced in this thesis for a short panel data model with a multifactor error structure, of which the asymptotic properties have been discussed. Monte Carlo simulations show that: (1) this structural break detection method performs well in terms of size and power; (2) it can also locate the break successfully; and (3) its setup incorporates the short panel data models with fixed effects as a special case. The proposed method, which uses a dataset consisting of more than 4,000 US financial institutions, has been further employed to provide empirical evidence of the well-known Gibrat’s ‘Law’.

The Desert Locust (Schistocerca gregaria) is an insect whose distribution area extends from North Africa through the Near East to Southwest Asia. During invasion periods, adults form swarms that can fly or be carried by wind over great distances. These swarms can threaten crops located thousands of kilometres from their places of origin. Supported by its member countries, FAO established the "Emergency Prevention System for Transboundary Animal and Plant Pests and Diseases" (EMPRES programme ““ Desert Locust Component) to minimize the risk of emergencies developing as a result of Desert Locust plagues. EMPRES has created an operational early warning system involving surveying for Locusts in order to find Desert Locust outbreaks as early as possible and prevent them from developing into serious upsurges or plagues. The system includes the use of SPOT-VEGETATION images received on a ten-day basis at FAO through ARTEMIS. The S10 SPOT-VEGETATION NDVI product and single channels (RED-NIR-SWIR) are analysed at the Desert Locust Information Service, FAO HQ for conditions favourable for Desert Locust reproduction and development. The images and analyses are then provided to each of the twenty countries via a FTP site where the Locust Information Officers can have direct access. The analyses aim to identify two sorts of error: i) Commission error (when NDVI indicates that there is vegetation but there is no real vegetation on the ground) ii) Omission error (when NDVI indicates no presence of vegetation but there is vegetation on the ground). To reduce the first type of error, S10 single channels (Red, NIR, and SWIR) are needed and provided to the users. To reduce the second type of error, TERRA-MODIS images are used to detect sparse vegetation not identified with SPOT-VEGETATION. The satellite images (SPOT-VEGETATION and TERRA-MODIS) are now used operationally by some Locust survey teams to direct their surveys toward high-risk regions and allow them to optimize use of available resources. Once in the field, survey teams introduce field observations into a palm-top computer and send the information to the National Locust Unit (LCU) in real-time via radio signal. The information arriving at the centre is then automatically integrated into a GIS system that was developed specifically to manage Locust and environmental data. This system, called the Reconnaissance And Management System of the Environment of Schistocerca (RAMSES), allows Locust Information Officers to orient field surveys geographically, to predict Desert Locust breeding and migration and to develop control strategies in case of emergency. The integration of satellite images within RAMSES also allows the officers to assess the areas covered by vegetation where control operations must be carried out. At the Desert Locust Information Service (DLIS), FAO HQ (Rome, Italy), all of the data collected at national levels are automatically imported into a much larger UNIX-based GIS called SWARMS (Schistocerca WARning Management System) where short and medium-term forecasts are prepared indicating potential locust migrations and areas of breeding.

بررسی منشأ و شرایط شکل گیری آگات های خور و بیابانک (استان اصفهان)

Sustainable use of water resources has become increasingly important in the past decades. The water resources that we use influence almost the entire hydrological cycle, in particular shallow and deeper aquifers, as well as rivers and lakes. The interactions between aquifers in a sedimentary basin and the connectivity of these aquifers with rivers are complex. To understand these interactions in relation to climate, geology and anthropogenic use, a sound hydrogeological framework is required. The Gippsland Basin in southeast Australia is a good example of the complexity of hydrological and hydrogeological inter-connectivity. The basin contains valuable energy resources including brown coal, gas and oil. Furthermore, it is an important area for agriculture in Australia. Extractions of groundwater to drain the open pit mines and for irrigation are suspected to have led to a decline in water levels in many aquifers across the basin. While the energy reserves have been well studied, information on the hydrogeology of the basin is scarce and understanding the complexity of the aquifer system is essential for a sustainable water management in the area. This study focused on three particular aspects of inter-aquifer and aquifer-river connectivity; (i), deeper aquifer systems, (ii) the connectivity of major rivers with the shallow aquifers in the area and (iii) the role of short to medium-term reservoirs. A methodological study on the use of radionuclides from the U/Th-series in surface water groundwater interactions studies is also included as this method was developed in the process of investigating the three main aims. Hydrogeochemical data were collected from the main aquifers in the Latrobe Valley in the western part of the Gippsland Basin to understand inter-aquifer mixing in deeper aquifer units in the basin. Major ion chemistry and environmental isotopes were used to discuss a geochemical and chronological framework of the most abundant aquifers. Major ion chemistry is heterogeneously distributed within and across aquifers, indicating that the major aquifer units are hydraulically connected and that mixing between the units occurs. 14C and 3H data was used to discuss a chronological structure and carbon ages range from ∼36000 years to modern. Ages are also heterogeneously distributed and there is no significant trend along the major flow path in the basin, supporting the hypothesis of intensive inter-aquifer mixing. Surface water groundwater interactions were studied along the Avon and the Mitchell Rivers in the centre of the Gippsland Basin, using hydrogeochemistry and in particular radon (222Rn) as a tracer. Radon as a short-lived radioactive gas can be used to determine short- to medium- term reservoirs contributions, such as bank storage in the direct proximity of rivers, to the total discharge of a river. The study has shown that groundwater discharge and discharge from the riverbanks to the streams is temporally and spatially variable. It was concluded that most rivers have gaining and losing sections and these may invert depending on the flow conditions of the river. The knowledge gained from this study lead to the investigation of other tracers for surface water groundwater interactions (e.g. U/Th-series) and the development of a continuous radon-monitoring device.

<p>The Charlie Gibbs offsetting by ~340 km the Mid Atlantic Ridge (MAR) axis between 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced to the south by the Azores and to the north by the Iceland mantle plume, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ‘70 and ‘80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54552 km<sup>2</sup> covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. In this contribution we present preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system.</p>

An effort to evaluate the oil potential of the Brazilian Continental Margins is currently under way through a plate tectonic model. The aims of the work are not only to describe the continental margin fundamental rifting tectonic framework and its evolution, but also its thermal history. The basic work correlates the South Atlantic fracture zones with marginal ridges, marginal offsets, marginal transverse structures, and high land regional structures and/or lineaments. These last elements, pre-existing lines of weakness or newly formed structures, are not always at the same transform directions. In general, the thermal subsidence of both the quasioceanic crust and the quasicontinental crust is much greater than that of the continental and the oceanic crusts. In this region the behavior of marginal fracture zones that may reach the adjacent ocean basins induces the formation of coastal basins on the adjacent shelf and continent. These basins or' marginal segments are the result of local differential subsidence, possibly controlled by those fractures. On the whole, the Brazilian Continental Margins subsided gradually from the south to the northeast and from the northwest to the northeast. The subject of this paper concerns the oil potential evaluation of the Atlantic-type margins. As a result of the work done, the following conclusions can be stated:in general, the transform directions intersect the eastern continental margin, where transverse structural features and/or lineaments were identified; but the same does not hold for the northern continental margin, where the pre-existing or newly formed structures are systematically deviated from the transform directions;the geothermal gradients were possibly very high for the rifting phases and systematically lower and lower for the subsequent phases (mio-eugeocline phases). Finally, this work is an attempt to complement with new data the work developed by Francheteau and Le Pichon.15 INTRODUCTION Since the transform-fault concept was brought to light by Wilson,1 a great deal of geological work has been done not only to understand the offsets of the accreting plate margins, but also the evolutionary plate history as explained by the theory of sea-floor spreading of Hess2 and Dietz,3 which was later carried out to a further extent by Le Pichon.4 In order to bring these two concepts to a successful conclusion within the plate-tectonic theory, many works have contributed their fruitful efforts. Among these is the pioneer bathymetric work of Heezen et al.5 on the Mid-Atlantic Ridge, later supported by the magnetometric works of Le Pichon4 and Heirtzler et al.6 These works showed the offsets of the South Mid-Atlantic Ridge and their transform-fault directions or fracture zones. The first attempt to explain the evolution of marginal offsets and ridges of the South Atlantic Continental Margins and correlate them to the South Mid-Atlantic Ridge transform-fault system or fracture zones was carried out by Le Pichon and Hayes.7 In recent years, many workers studied the tracing of fracture zones and their effects at the South American and African Continental Margins.

No abstract available.

An analysis is given of air-gun profiler and magnetic data obtained in the central North Atlantic between 12° and 18°N. Eight fracture zones were crossed, one of which (the 15°20′N fracture zone) was traced over a distance of 1500 km. The mode of adjustment of fracture zones to a change in direction of spreading is discussed. It is shown that also if this new direction would lead to an opening of the fracture zone, and adjustment fracture can originate and actually does so in several instances. The about E-W fracture zones dominate the structure of the Ridge province entirely, both with regard to the topography and to the magnetics. A magnetic model is proposed accounting for the different types of anomalies found over fracture zones. No intrusive bodies are needed to explain these anomalies. The origin of fracture zones is related to thermal contraction of a cooling lithosphere while moving from the ridge. Thermal contraction may also explain how the American and the African plates are freed from the grip they are caught in by the fanning of the fracture zones in the central North Atlantic. The fanning of fracture zones has consequences for the determination of the pole of spreading. This pole can only be found as a best fit from a synthesis of the total plate boundary, i.e. from the Azores to Bouvet Island. ‘Local’ poles have only restricted value, since deviations up to 5 deg occur from a small circle pattern based on existing data. Several huge structures, viz. Researcher Ridge and Royal Trough, are found in the area which seem to parallel the flow lines of the fracture zone system. No adequate explanation exists for these structures.

پترولوژی، شیمی کانیها و محیط تکتونوماگمایی سنگهای آتشفشانی شمال شرق فرمهین (شمال اراک)

<p> Large-offset transform faults (TFs) in the Atlantic juxtapose hot spreading segments against older, colder oceanic lithosphere, leave permanent traces as fracture zones in ageing oceanic crust and represent a significant proportion of the plate boundary along the Mid-Atlantic Ridge (MAR). The manifestation of the thermal contrast and the structure and composition of TFs however, are not well understood. The Romanche TF, situated in the Equatorial Atlantic, offsets the MAR by ~950 km, has a slip of ~1.7 cm/yr, and divides the northern MAR from its equatorial and southern spreading systems. Close to the eastern ridge-transform intersection (RTI), shallowing of the seafloor from north to south across the TF reflects the change from old, cold African lithosphere to the warmer and younger South American plate close to the MAR axis, however the bathymetry and structures across the fault itself are complex. Over 100 km distance, a large northern transverse ridge reaches depths of <1000 m and contains a fossil transform trace, before steeply descending into a 45‑km wide transform valley containing ~7000 m‑deep basins, which is bounded to the south by a further shallow structure reaching ~2500 m‑depth. Previous studies using seafloor sampling, seismic reflection and bathymetry data have suggested these features comprise a mix of uplifted magmatic crustal blocks and serpentinized mantle peridotites. However, these studies cannot effectively determine the sub‑seafloor structure.</p><p>The ILAB-SPARC experiment in 2018 obtained an active-source wide-angle refraction profile across the eastern Romanche TF, consisting of twenty-eight ocean-bottom seismometers spaced at ~14 km. We present a P-wave velocity model produced by the inversion of seismic travel time picks which reveals variations in crustal structure from ~40 My lithosphere to the north to ~7 My lithosphere to the south. Within the TF, a ~15 km-wide low-velocity anomaly extends from the top basement through to >10 km below basement. A lack of Moho reflections suggests no abrupt crust/mantle boundary exists beneath the TF, likely indicating the presence of a deep column of fractured and sheared basalts, breccias and peridotites. Low mantle velocities suggest faulting and water penetration to depths of ~16 km, causing widespread and extensive serpentinization. The crust to the south of Romanche is relatively thin (~5 km‑thick) compared to north of Romanche (~6 km‑thick), and contains areas of high velocity indicative of a predominantly gabbroic crust. This may be attributed to the irregularity of the MAR segment as it approaches the RTI, as it jumps to the west in several non-transform discontinuities and exhibits seafloor fabric indicative of magma-starved, tectonic spreading with exhumation along detachment faults.</p><p>These results suggest the shearing and transtensional/transpressional forces present at large-offset transform faults result in mantle exhumation and form deep conduits for fluid circulation. At Romanche, these tectonic forces combined with the thermal contrast and magma-starved ridge axis, stretch and deform magmatic oceanic crust within the TF such that it is thin and patchy. This may suggest that crustal structure within transforms is linked to the fault offset, valley width, and the magma supply at the closest ridge segment.</p>

New data are obtained on the structure, evolution, and origin of zones of nontransform offsets of adjacent segments in the Mid-Atlantic Ridge (MAR), which, in contrast to transform fracture zones, so far are studied insufficiently. The effects of deep mantle plumes developing off the crest of the MAR on the processes occurring in the spreading zone are revealed. These results are obtained from the geological investigation of the crest of the MAR between 19.8 ° and 21° S, where bottom sampling, bathymetric survey, and magnetic measurements have been carried out previously. Two segments of the rift valley displaced by 10 km relative to each other along a nontransform offset are revealed. A volcanic center of a spreading cell, which has been active over the last 2 Ma, is located in the northern part of the southern segment and distinguished by a decreased depth of the rift valley and increased thickness of the crust. Magnesian, slightly evolved basalts of the N-MORB type are detected in this center, whereas evolved and high-Fe basalts are found beyond it. The variation in the composition of the basalts indicates that the volcanic center is related to the upwelling of the asthenospheric mantle, which spread along and across the spreading ridge. In the lithosphere, the melt migrated off the volcanic center along the rift valley. In the northern segment, a vigorous volcanic center arose 2.5 Ma ago near its southern end; at present, the volcanic activity has ceased. As a result of the volcanic activity, an oval rise composed of enriched T-MORB-type basalts was formed at the western flank of the crest zone. The isotopic signatures show that the primary melts are derivatives of the chemically heterogeneous mantle. The mixing of material of the depleted mantle with the mantle material pertaining either to the Saint Helena or the Tristan da Cunha plumes is suggested; the mixture of all three sources cannot be ruled out. The conclusion is drawn that the mantle material of the Saint Helena plume was supplied to the melting zone beneath the axial rift near the oval rise along a linear permeable zone in the mantle extending at an azimuth of 225° SW. The blocks of mantle material that got to the convecting mantle from the Tristan da Cunha plume at the stage of supercontinent breakup were involved in melting as well. The nontransform offset between the two segments arose on the place of a previously existing transform fracture zone about 5 Ma ago. The nontransform offset developed in the regime of oblique spreading at the progressive propagation of the southern segment to the north. The zone of nontransform offset is characterized by recent volcanic activity. Over the last 2 Ma, spreading of the studied MAR segment was asymmetric, faster in the western direction. The rates of westward and eastward half-spreading in the northern segment are estimated at 1.88 and 1.60 cm/yr, respectively.