ALBERT

Beschreibung: Rare earth element distribution of mineral, geothermal and surface waters are discussed. The causes of the evidently natural Ce, Eu, and Y anomalies as well as the anthropogenic Gd anomaly are described. For crustal waters it is suggested to apply the REE/Ca abundance ratios rather than the REE abundances because of the less scatter of patterns achieved. The rock-normalised patterns show rock-specific signatures of the waters. The Gd anomaly can be applied as a conservative tracer in hydrological studies.

Beschreibung: The motion of hotspots and the deformation of their underlying plume conduits as calculated within models of global mantle flow are presented. A new list of 44 possible hotspots with associated tracks has been compiled. For all of them, calculations have been performed under consideration of individual age and anomalous mass flux for three different models of plume buoyancy and mantle flow. Plume source depth has usually been assumed to be the top of D”, but an alternative source depth at the 670-km discontinuity has also been considered. Using models of relative plate motions and boundaries, hotspot tracks on plates have been calculated and compared with age data, ocean floor topography, and distribution of volcanics on continents. Absolute plate motions have been redetermined under consideration of hotspot motion, using a new least squares method. For the Hawaiian and Yellowstone hotspots, source locations and hotspot motion have been computed for a total of up to 23 different models. The results show plume conduits being tilted, with source regions at the D” moving in the lowermost mantle flow, generally toward large-scale upwellings under southern Africa and the south central Pacific. Hotspot surface motion often represents the horizontal component of midmantle flow, which is frequently opposite to plate motion, toward ridges and away from subduction zones. In particular, almost all models tested predict southward motion of the Hawaii and Kerguelen hotspots and westward motion of the Iceland hotspot. For models including hotspot motion the agreement between calculated and observed hotspot tracks is frequently about as good as, or better than, for the fixed hotspot model, but sometimes fixed hotspots give the best fit. In some cases where the track ends at a subduction zone, e.g., for the Bowie hotspot, results can give indications about the otherwise unknown age of the hotspot. In other cases, especially for the Tahiti hotspot, results suggest an origin shallower than D”, and in yet other cases, particularly East Africa, the failure of the hotspot models used supports other evidence indicating the presence of comparatively broad upwellings rather than localized plumes.

Beschreibung: The Lindås Nappe, Caledonides W-Norway was affected by two major tectonometamorphic events. A Precambrian granulite facies event at T=800–900°C, P〈10 kbar was followed by localized Caledonian eclogite facies (T=650–700°C and P〉15 kbar) and localized amphibolite facies reworking. During the granulite–eclogite facies transition, anorthositic rocks were converted from garnet granulites to kyanite eclogites, while phlogopite-bearing spinel lherzolite reacted to garnet lherzolite. The eclogite and amphibolite facies reequilibration took place along shear zones and fluid pathways. In the unhydrated and undeformed parts, the minerals preserved their granulite facies composition with constant Fe/Mg ratios from core to rim, suggesting diffusional reequilibration. Rb/Sr age dating was carried out on relict granulite facies minerals from three lenses of ultramafites (Alvfjellet, Hundskjeften and Kvamsfjellet). Phlogopite from phlogopite lherzolite at Alvfjellet give 857±9 Ma, while clinopyroxene, amphibole, phlogopite and whole rock from a lherzolite at Hundskjeften yield an age of 842±12 Ma (MSWD=1.9). Clinopyroxene, feldspar, orthopyroxene phlogopite and whole rock from websterite, Kvamsfjellet, yield an age of 835±7 Ma (MSWD〈1), while clinopyroxene, phlogopite and whole rock from a lherzolite from the same lens gives a result of 882±9 Ma. These results are interpreted as minimum ages for the granulite facies event and only slightly younger than, or overlap with previous U–Pb zircon ages (929±1 Ma) and Sm–Nd garnet–pyroxene ages (890–923 Ma) interpreted to date the end of the granulite facies event. By contrast, ages obtained for the eclogite and amphibolite facies range from 460 (U–Pb, sphene), 440 (Ar–Ar), 419 (U–Pb, zircon) to 410 Ma (Rb/Sr mineral ages). These results demonstrate that the reopening temperature for the Rb/Sr system in phlogopite–biotite under dry and static high-pressure conditions is, in the given mineral assemblages, at least 650°C, considerably higher than the 300–400°C assumed as the closure temperature of this system. We ascribe this elevated reopening temperature to fluid absent conditions that prevented element transport and rehomogenization.

Beschreibung: We present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western South America: the Broadband Andean Joint experiment (BANJO), a 1000-km-long east-west line at 20 degrees S, and the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) and Seismic Exploration of the Deep Altiplano (SEDA), deployed several hunderd kilometers north and south of this line. We determined the splitting parameters phi (fast polarization direction) and delta t (splitting delay time) for waves that sample the above- and below-slab regions: teleseismic *KS and S, ScS waves from local deep-focus events, as well as S waves from intermediate-focus events that sample only the above-slab region. All but one of the *KS stacks for the BANJO stations show E-W fast directions with delta t varying between 0.4 and 1.5 s. However, for *KS recorded at most of the SEDA and PISCO stations, and for local deep-focus S events north and south of BANJO, there is a rotation of phi to a more nearly trench parallel direction. The splitting parameters for above-slab paths, determined from events around 200 km deep to western stations, yield small delay times (less than or equal to 0.3 a) and N-S fast polarization directions. Assuming the anisotropy is limited to the top 400 km of the mantle (olivine stability field), these data suggest the following spatial distribution of anisotropy. For the above-slab component, as one goes from east (where *KS reflects the above-slab component) to west, phi changes from E-W to N-S, and delay times are substantially reduced. This change may mark the. transition from the Brazilian craton to actively deforming (E-W shortening) Andean mantle. We see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion. The small delay times for above-slab paths in the west require the existence of significant, spatially varying below-slab anisotropy to explain the *KS results. The implied anisotropic pattern below the slab is not easily explained by a simple model of slab-entrained shear flow beneath the plate. Instead, flow induced by the retrograde motion of the slab, in combination with local structural variations, may provide a better explanation.

Beschreibung: Abstract: Seismically active fault systems may be in a state of self-organized criticality (SOC). Investigations of simple SOC models have suggested that earthquakes might be inherently unpredictable. In this paper, we analyse the question of predictability in a more complex and realistic SOC model, which consists of a spring-block system with transient creep characteristics. Additionally to the power law distribution of earthquake sizes, this model reproduces also foreshock and aftershock sequences. Aside from a short-term increase of seismicity immediately prior to large model earthquakes, these events are preceded on average by an intermediate-term period of reduced seismicity. The stronger and the longer the duration of this period, the larger on average is the subsequent main shock. We find that the detection of seismic quiescence can improve the time-independent hazard assessment. The improvement is most significant for the largest target events.

Beschreibung: Seismic interpretation and structural modelling of the Rotliegend along the northern limit of the Northeast German Basin Dirk Kossow, Holger Rieke, Tommy McCann, Robert Ondrak, *Manfred Strecker, Jörg F.W. Negendank GeoForschungsZentrum Potsdam, PB 3.3. "Sedimente und Beckenbildung", Telegrafenberg, 14473 Potsdam *Universität Potsdam, Mathematisch-Naturwissenschaftliche Fakultät, Institut für Geowissenschaften, 14469 Potsdam Following the cessation of compressive Variscan movement, Central Europe was characterized by a period of widespread basin formation accompanied by extensive magmatic activity (Ziegler 1990). In the study area, basin development is linked with activities along the TESZ, leading to a complex relationship between sedimentation and tectonics. To study the Lower Permian basin morphologies and depositional architectures along-strike of the initial northern limit of the Northeast German Basin, an extensive database consisting of core material and a network of commercial seismic profiles have been used. In the area several NW-SE-trending subbasins are developed. However, the internal structure of these basins and, therefore, the depositional pattern is controlled by the development of approximately N- to NE-trending half grabens reflecting E-W extension during this time. The observed structural style is interpreted as the brittle response to deep-seated ductile deformation along the NW-trending Trans-European Fault (Berthelsen 1992). Dextral crustal shearing had its near-surface expression in the development of N-trending normal faults. Forward modelling using STRETCH (Kuznir et al. 1991) has been used to attempt to quantify the tectono-sedimentary evolution during and after the phase of extension. The program is based on the flexural cantilever model of continental lithosphere extension and is able to compute the complex interference pattern which is induced by the presence of many faults of different size, position and polarity along an investigated profile. Sediment loading, erosional unloading as well as compaction of the syn-rift sediments may be included. Furthermore, the post-rift phase which is mainly characterised by thermal subsidence can be calculated. The analysis showed that extension by multiple faults has resulted in a complex interference pattern between the flexural footwall uplift and hangingwall collapse of each fault. In this phase of mechanical extension the basin geometry and deposition were largely controlled by the fault pattern resulting from the overall extension. Faulting led to rapid thickness variations across the tilted blocks producing a complex sedimentation pattern. The response to the subsequent post-rift stage was the gradual peneplanation of the fault-generated topography. In the study area post-rift thermal subsidence led to deepening of the basin and produced a simpler depositional pattern with a thickening towards the basin centre. The post-rift sediment package overlies the syn-faulted sediments and extends across the former basin margins. By integrating geological and geophysical data, analysis of the depositional and structural development resulted in an improved understanding of the geodynamic evolution of the Lower Permian succession in the region. REFERENCES: ZIEGLER, P. (1990): Geological atlas of western and central Europe. Elsevier, Amsterdam, 239 pp BERTHELSEN, A. (1992): From Precambrian To Variscan Europe. In: BLUNDELL, D., FREEMAN, R., MUELLER ST., A Continent Revealed - The European Geotraverse. Cambridge University Press pp153-164 KUZNIR, N.J., MARSDEN, G. & EGAN, S.S. (1991): A flexural-cantilever simple-shear/pure-shear model of continental lithosphere extension: applications to the Jeanne d' Arc Basin, Grand Banks and Viking Graben, North Sea. In: ROBERTS, A.M., YIELDING, G. & FREEMAN, B. The Geometry of Normal Faults London Geological Society Special Publication 56 41-60.

Beschreibung: Abstract: The role of Special Study Groups (SSGs) in the IAG is briefly discussed. The assessment is largely based on the author"s experience gained while chairing SSG 4.176 during the period 1995--1999. Specific points of discussion are (1) the relation between SSGs and Special Commissions, (2) the appropriate procedure of establishing new SSGs and (3) the relations between SSGs covering similar topics.

Beschreibung: Microscopic, geochemical and pollen analysis of sediment samples of a Lateglacial/Holocene profile from Lago di Mezzano, a maar lake in central Italy, reveals evidence of significant climatic and human-induced environmental changes. Time control is provided by a combination of varve chronology and radiocarbon dating. The well-known Lateglacial climatic variations, a warmer interstadial and the Younger Dryas cold phase are clearly represented in all the parameters. During the interval between 9200 and 5600 cal. BP of the Holocene climatic optimum, annually laminated, organic-rich diatom gyttja was deposited. Two periods of diminished total organic carbon are identified within this interval. The first one (P1) ranges from 8200 to 7800 cal. BP while the second (P2) is centred around 6500 cal. BP. During P1, a predominance of diatoms (Stephanodiscus parvus and S. minutulus) over other algae (represented by the total organic carbon content) is observed. The timing of this period coincides with the early- to mid-Holocene climatic transition, reported from ice cores and lake sediments (Stager and Mayewski, 1997). P2 is characterized by a decrease in all biogenic parameters including biogenic opal, organic carbon as well as arboreal pollen. From 5000 cal. BP to date, the sediment pattern changes coincide with the mid-Holocene climatic deterioration. In addition to these natural variations, human impact has been recorded and recognized from sedimentological features such as turbidites and charcoal, as well as from reduced arboreal pollen content. Two Middle Bronze Age (3700 cal. BP and 3300 cal. BP), Etruscan/early Roman (2500 cal. BP), Longobardic (ad 900) and ‘modern settlements’ (since ad 1700) have been distinguished on the basis of these data.

Beschreibung: Additional keywords: data communication, moment tensor determination, strong-motion parameters, catalogue completeness, Poison distribution, Gutenberg-Richter relation, b-value determination, intensity estimation, seismic hazard assessment, exercises, Seismometrie, Seismometereichung, digital Datenerfassung, seismische Registriergeraete, physikalische Netz, virtuelle Netze, Datenkommunikation, Planung seismische Netze, Installation seismischer Netze, Betrieb und Abstimmung seismischer Netze, Erdbebenortung, Bestimmung des Momententensors, Parameter starker Erdbebenbewegungen, Vollstaendigkeitspruefung von Erdbebenkatalogen, Poisson-Verteilung, Gutenberg-Richter-Beziehung, b-Wert-Bestimmung, Intensitaetabschaetzung, Abschaetzung der Erdbebengefaehrdung, Uebungen,

Beschreibung: The Central Andes are the Earth's highest mountain belt formed by ocean-continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening Here we use P-to-S (compressional-to shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro-eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. WE see an intracrustal low-velocity zone, 10-20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.