ALBERT

Description: The idea for a special publication on non-volcanic margins arose during Ocean Drilling Program (ODP) Leg 173 off West Iberia, prompted by ODP's decision to cease publishing the scientific results volumes as hard copy. The Shipboard Scientific Party favoured an open scientific meeting and associated publication. But they did not want to produce a book that was a scientific results volume by another name, but rather contribute to a publication that had a much broader scope than just reporting results obtained off West Iberia. These thoughts, and many scientific discussions during the Leg, were influenced by the presence on board of scientists who also work on Alpine geology: hence the evidence from land and sea' approach that underlies the content of this publication. However, when planning the meeting, we were very conscious of the fact that the West Iberia and Alpine examples might not be typical of other non-volcanic margins. We were keen, therefore, to ensure that margins in other parts of the world were discussed, including a margin that is active today, and that was visited by the JOIDES Resolution not long after Leg 173 took place (Leg 180: Woodlark Basin). We caution, therefore, that it may be premature to use models based on the Iberia and Tethyan margins as the paradigm for all non-volcanic margins. The first paper in this book, by Boillot & Froitzheim, reviews the synergies that have occurred between investigations of the eastern Atlantic non-volcanic margins and remnants of similar Mesozoic margins preserved in the ... This 250-word extract was created in the absence of an abstract.

Description: A TOBI sidescan sonar survey in the Irish sector of the Rockall Trough reveals the presence of a range of slope failure features of various sizes and extent along both the eastern and western margins. A number of different types are identified. These include incipient cuspate slides, slab failures and evolved slides, and debris flows. It is suggested that the incipient cuspate slides, slab failures and evolved slides represent slope failure of muddy sediments whereas the failures that gave rise to debris flows lie on steeper slopes and may be of less muddy composition. Many of the slope failure features are relatively recent (probably 〈15 ka), although some evidence points towards either a prolonged period of movement or a number of phases of slope movement locally along the margins. A comprehensive understanding of the nature, distribution, age and controls on the formation of the slope failure features will be necessary in planning the likely location of sea-bed structures in the event of petroleum development in the region.

Description: The timing of Cenozoic surface uplift in NW Europe relies on the assumption that the sedimentary response in basins is synchronous with tectonic processes in the source areas. However, many of the phenomena commonly used to infer recent uplift may as well be a consequence of climate change and sea-level fall. The timing of surface uplift therefore remains unconstrained from the sedimentary record alone, and it becomes necessary to consider the constraints imposed by physically and geologically plausible tectonic mechanisms, which have a causal relation to an initiating agent. The gradual reversal of the regional stress field following the break-up produced minor perturbations to the thermal subsidence on the Norwegian Shelf and in the North Sea. Pulses of increased compression cannot be the cause of Cenozoic land surface uplift and accelerated Neogene basin subsidence. Virtually deformation-free regional vertical movements could have been caused by changes in the density column of the lithosphere and asthenosphere following the emplacement of the Iceland plume. A transient uplift component was produced as the plume displaced denser asthenosphere at the base of the lithosphere. This component decayed as the plume material cooled. Permanent uplift as a result of igneous underplating occurred in areas of a thin lithosphere (some Palaeozoic and Mesozoic basins) or for lithosphere under extension at the time of plume emplacement (the ocean-continent boundary). In areas of a thicker lithosphere (East Greenland, Scotland and Norway) plume emplacement may have triggered a Rayleigh-Taylor instability, causing partial lithospheric delamination and associated transient surface uplift at a decreasing rate throughout Cenozoic time. A possible uplift history for the adjacent land areas hence reads: initial transient surface uplift around the break-up time at 53 Ma caused by plume emplacement, and permanent tectonic uplift caused by lithospheric delamination and associated lithospheric heating. The permanent tectonic uplift increased through Cenozoic time at a decreasing rate. Denudation acted on this evolving topography and reduced the average surface elevation, but significantly increased the elevation of the summit envelope. The marked variations in the sedimentary response in the basins were caused by climatic variations and the generally falling eustatic level. This scenario bridges the gap between the ideas of Paleocene-Eocene uplift versus repeated Cenozoic tectonic activity: the tectonic uplift history was initiated by the emplacement of the Iceland plume, but continued throughout Cenozoic time as a consequence of early plume emplacement, with climatic and eustatic control on denudation. The mechanism is consistent with topography, heat flow, crustal structure, and the Bouguer gravity of Norway, and may be applicable also to East Greenland.

Description: Smoothing algorithms provide a means of identifying significant patterns in sets of orientated data, eliminating local perturvations within the observations and predicting patterns of orientated data in places which lack observations. Here we present the smoothing of orientation data with a distance-related method of data weighting as an alternative to previous weighting algorithms. The data weighting and smoothing method presented here in theory and practice is developed on the basis of a statistical smoothing algorithm. The method can be applied to orientation data of 180{degrees} periodicity such as maximum horizontal tectonic stresses (SH) as compiled in the World Stress Map database. Our smoothing algorithm enables discrimination between local (〈250 km of lateral extent) and regional (c. 250-5000 km of lateral extent) stress fields, and allows comparison of SH with other directional data such as fault trends or strain data. We present smoothed stress maps for northeastern America, the Himalayas and western Europe. By varying the scale and smoothing parameters we illustrate their influence on the accuracy and smoothness. We give recommendations for the appropriate choice of these parameters.

Description: The existing published data, combined with our own new field, petrographic, and geochemical observations and data show that ophiolites of the West Koryak fold system originated in a variety of tectonic environments. This fold system stretches along the boundary shared by two of NE Asia's largest tectonic units, the Verkhoyansk-Chukotka and Koryak-Kamchatka foldbelts. The fold system abounds in Palaeozoic and Mesozoic ophiolites and sedimentary and volcanic island-arc assemblages. The ophiolites are Palaeozoic and Mesozoic in age. The variety of geological and geochemical signatures implies ophiolite origin in diverse tectonic settings. The Early Palaeozoic ophiolites of the Ganychalan accreted terrane and Devonian(?) ophiolites of the Ust-Belaya accreted terrane are fragments of the Panthalassan oceanic lithosphere. Serpentinite melange in the Ust-Belaya terrane contains some blocks of island-arc provenance. They are probably Late Palaeozoic-Early Mesozoic in age as determined by K-Ar measurements, which require validation by other techniques. Mesozoic, chiefly Late Jurassic-Early Cretaceous ophiolites of the Beregovoi and Kuyul accreted terranes, originated in a suprasubduction-zone (SSZ) setting (ensimatic island arc and back-arc basin). Among the Mesozoic ophiolites, one finds blocks of oceanic assemblages in serpentinite melanges as well. Basalt and chert blocks of clearly oceanic derivation are viewed as detached fragments of the upper part of the oceanic lithosphere. The ophiolites have experienced a variety of accretionary scenarios. Palaeozoic ophiolites docked onto the Koni-Taigonos island arc (of Late Palaeozoic-Early Mesozoic age), probably in the Late Palaeozoic or Early Mesozoic, whereas Mesozoic ophiolites accreted onto the Uda-Murgal island arc (of Late Jurassic-Early Cretaceous age) in the terminal Early Cretaceous. Sedimentary deposits, whose base is late Albian in age, make a post-accretionary sequence. These island arcs portray the overall history of the convergent boundary between the North Asian continent and NW Pacific. Ophiolites of the Ganychalan and Ust-Belaya terranes consist of thrust sheets and, jointly with Yelistratov Peninsula ophiolites, make up the basement to the forearc of the Uda-Murgal island arc, ophiolites of Cape Povorotny and Kuyul terrane being incorporated in accretionary prisms of the same arc. Ophiolites and associated metamorphic, volcanic, and sedimentary rocks of Palaeozoic-Early Cretaceous age underwent three deformation phases, each reflecting a different stage in the evolution of the NE Asian continental margin and readily correlative with principal tectonic events in the northern Circum-Pacific region.

Description: This paper provides a summary of selected diamond exploration techniques used in the glaciated terrain of Canada, focusing on indicator mineral methods and till geochemistry but also including geochemistry of lake sediments and vegetation. Diamond exploration in Canada focuses on kimberlite, the primary host rock for diamonds in this country. Kimberlite is a mineralogically and chemically distinct point source which may yield discrete dispersal trains in glacial sediments. Understanding the ice flow history and depositional history of glacial sediments and identifying multiple till sheets in areas covered by thick glacial sediments are essential for successful sampling, interpretation and follow-up of indicator mineral and geochemical anomalies related to these rocks. Orientation studies over known kimberlites provide important information on the mineralogical and geochemical signatures of kimberlite, and the size fractions of glacial sediments that are best suited to indicator mineral and geochemical analysis. Kimberlite indicator minerals survive glacial transport over long distances and the relative abundance of each mineral in till is a function of the primary mineralogy of individual kimberlites. Indicator mineral distributions observed at a regional scale define the net effect of glacial dispersal, often along different ice flow directions. Local scale distributions define individual dispersal trains. The finer (0.25 to 0.5 mm) fraction of heavy mineral concentrates prepared from till samples is best suited for indicator mineral surveys. Till geochemistry is gaining popularity in diamond exploration because it is significantly cheaper than indicator mineral analysis and it can be performed quickly. Important kimberlite pathfinder elements that provide good contrast in till geochemical surveys include Ni, Cr, Ba, Co, Sr, Rb, Nb, Mg, Ta, Ca, Fe, K, Ti and REE, the relative importance of which will depend on kimberlite composition as well as that of the surrounding bedrock. Biogeochemical studies over kimberlites in Canada reveal geochemical signatures in vegetation despite the glacially transported substrate.

Description: A series of measurements of the structure of a variety of porous materials has been made using synchrotron computed microtomography (SCMT). The work was carried out at the Brookhaven National Synchrotron Light Source (NSLS), the Argonne Advanced Photon Source (APS) and the European Synchrotron Radiation Facility (ESRF). The experiments at Brookhaven and Argonne were carried out on bending magnet beam lines using area detectors to obtain CT images based on determination of X-ray absorption coefficients. The work at the ESRF used an undulator beam line, a 13 KeV pencil X-ray beam of 2 {micro}m and an energy dispersive X-ray detector to make tomographic sections of trace element distributions by X-ray fluorescence tomography. Most of the work was done with a pixel/voxel size ranging from 0.002 to 0.010 mm. We examined the structure of unconsolidated estuarine sediments, whose structure is relevant to transport of contaminants in rivers and estuaries. Fluorescent tomography with 2-3 {micro}m resolution was used to ascertain whether or not metals were concentrated on the surface or throughout the volume of a single sediment particle. Sandstone samples were investigated to obtain a set of values describing their microstructures that could be useful in fluid flow calculations relevant to petroleum recovery or transport of environmental contaminants. Measurements were also made on sandstone samples that had been subjected to high-pressure compression to investigate the relation between the microgeometry and the magnitude of the applied pressure. Finally, a Wood's metal-filled sample was scanned for demonstration of resolution enhancement and fluid flow studies.

Description: The use of 4500 km2 of amalgamated 3D seismic surveys allowed for an improved intra-Carboniferous seismic interpretation of the Dutch Cleaver Bank High, which is part of the Southern North Sea Carboniferous Basin. The observations of faults that were active during the Late Carboniferous are reviewed in the context of what is described in literature about the regional structural framework of the basin. The high quality seismic data show at least three distinct fault trends, namely east-west, NE-SW and NW-SE, active before Rotliegend times. All of these trends are inherited from older existing zones of weakness, and furthermore, all three trends have been reactivated again during the Mesozoic or Cenozoic to some extent. The interpretation of major controlling east-west shear zones that have been reactivated in different senses throughout geological history is the result of careful examination of the data. The dominance of such systems is not obvious on simple fault maps. Their role in the regional plate tectonics fits the model of escape tectonics of the North Sea-Baltic plate. The distinction of different styles of Late Carboniferous normal fault systems results in a better understanding of the different tectonic phases during the period from Westphalian D to Autunian.

Description: Tectonic stress is one of the fundamental data sets in Earth sciences comparable with topography, gravity, heat flow and others. The importance of stress observations for both academic research (e.g. geodynamics, plate tectonics) and applied sciences (e.g. hydrocarbon production, civil engineering) proves the necessity of a project like the World Stress Map for compiling and making available stress data on a global scale. The World Stress Map project offers not only free access to this global database via the Internet, but also continues in its effort to expand and improve the database, to develop new quality criteria, and to initiate topical research projects. In this paper we present (a) the new release of the World Stress Map, (b) expanded quality ranking schemes for borehole breakouts and geological indicators, (c) new stress indicators (drilling-induced fractures, borehole slotter data) and their quality ranking schemes, and (d) examples for the application of tectonic stress data.

Description: During ODP Leg 173 (April-May 1997) five new sites (1065, 1067-1070) were drilled at the ocean-continent transition (OCT) zone off the West Iberia margin in the Iberia Abyssal Plain. At Site 1067 the 92 m of cored amphibolites were subdivided into three units based on textural criteria. Unit 1 consists of highly foliated and folded amphibolites and acidic gneiss concordant to and folded along with the foliation of the amphibolite. Unit 2, in the middle part of the section, consists of brecciated amphibolites. Unit 3, at the bottom of the hole, is a weakly deformed zone where magmatic textures are observed in the amphibolites and in associated anorthosites. The amphibolites contain tschermakitic to magnesio-hornblende amphibole, plagioclase, zircon, apatite {+/-} titanite {+/-} Fe-oxide {+/-} quartz. The acidic gneiss consists of garnet, plagioclase, alkali-feldspar, quartz, biotite and zircon. Chlorite, sericite and ilmenite occur as secondary phases. The metamorphic evolution of the amphibolite and acidic gneiss started under amphibolitefacies conditions (Stage I: 670 {+/-} 40{degrees}C and 7 {+/-} 1 kbar). Further exhumation took place through low-grade amphibolite-facies (Stage II: 550 {+/-} 50{degrees}C and 5.5 {+/-} 1 kbar) to green-schist-facies (Stage III: 〈 500{degrees}C and 〈 3 kbar) conditions contemporaneous with the development of ductile structures. The late metamorphic evolution of the amphibolite ended under ocean-floor conditions. Oxygen isotope ratios and studies of fluid inclusion indicate that a magmatic water-rich fluid in equilibrium with the igneous protolith predominated during Stage I. During Stages II and IIII low-temperature water-rich fluids of metamorphic origin predominated, with a probable contribution of sea water. Apatite fission-track dating indicates that the amphibolites record two thermal excursions below 120{degrees}C. The first took place at c. 113-100 Ma and could be related to Cretaceous rifting. The second was brief, and so did not anneal older tracks; it occurred between 75 and 55 Ma and could be related to the Pyrenean orogeny.