ALBERT

Beschreibung: The combined passive and active seismic TRANSALP experiment produced an unprecedented high-resolution crustal image of the Eastern Alps between Munich and Venice. The European and Adriatic Mohos (EM and AM, respectively) are clearly imaged with different seismic techniques: near-vertical incidence reflections and receiver functions (RFs). The European Moho dips gently southward from 35 km beneath the northern foreland to a maximum depth of 55 km beneath the central part of the Eastern Alps, whereas the Adriatic Moho is imaged primarily by receiver functions at a constant depth of about 40 km. In both data sets, we have also detected first-order Alpine shear zones, such as the Helvetic detachment, Inntal fault and Sub-Tauern ramp in the north. apart from the Valsugana thrust, receiver functions in the southern part of the Eastern Alps have also observed a north dipping interface, which may penetrate the entire Adriatic crust [Adriatic Crust Interface (ACI)]. Deep crustal seismicity may be related to the ACI. We interpret the ACI as the currently active retroshear zone in the doubly vergent Alpine collisional belt.

Beschreibung: This 1D/2D petroleum system modelling study intended to reconstruct the temperature, maturation, migration and accumulation history within the „Tight Gas Area“ on the southern edge of the Pompeckj Block in northern Germany. To better understand hydrocarbon migration and to generate a more reliable predictive model of reservoir characteristics, we integrated detailed information from (i) sedimentological analysis of core and log data, and (ii) a structural reconstruction of the Rotliegend reservoir horizons. One cross section and 12 wells were examined using the 1D/2D simulation software PetroMod® 7.1 of IES GmbH, Jülich; subsequently the results of the 1D modelling were incorporated in the 2D model. The W-E section connects well-known gas fields of the Nordhannover concession and runs perpendicular to the prevalent N-S striking horst-and-graben structures within the Paleozoic rocks. The depth to the major source rocks within the area, the coal-bearing Westphalian strata, varies between 5800 m and around 4700 m in some areas (e.g. Top Westphalian B). Along the section, a pre-Permian erosion of between 400 to 1200 m of Upper Carboniferous sediments was assumed, the actual amount depending on the position relative to horst and grabens. During the Upper Carboniferous and the Rotliegend the temperatures within Westphalian B and Lower Westphalian C sediments remained below 100°C and 75°C, respectively. The temperatures within these sediments increased by about 50°C, before they decreased again to 125°C and 100°C, respectively, due to the Late Jurassic inversion. During on-going burial in the Upper Cretaceous, temperatures rose until they reached their present-day values of about 175°C in Lower Westfalian C, 210°C in Westfalian B, and more than 250°C in Namurian C sediments. According to the temperature history, the maturation of organic matter within these rocks occurred in two important stages. During the Triassic, vitrinite reflectance reached values of 1.5% and 1.0% in Westphalian B and Lower Westphalian C sediments, respectively. The second period of intense maturation was coeval with enhanced subsidence during the Upper Cretaceous, when vitrinite reflectance within Westfalian B and C sediments increased from to 3.0% and about 2.2%, respectively. Maturity within the horst block sediments remained lower by about 1.0%. Within Westphalian B sediments gas generation started during the Middle Triassic and lasted until the Late Tertiary. These rocks became overmature whilst Lower Westfalian C rocks generated gas until the present time and transformed 60-90% of their convertible kerogen into hydrocarbons, the actual amount depending on the structural position. During the Jurassic and the Triassic, when the regional stress field allowed N-S striking faults to be dilatant and to act as migration pathways, high gas columns along the fault zones led to a breakthrough and loss of gas, where thinned Zechstein salt exists.

Beschreibung: It is well known that the Seychelles microcontinent resulted from episodic rifting between Africa and India. However, the mechanics of such rifting are poorly understood. The aim of the SEISM project (Seismic Experiment to Investigate the Seychelles Microcontinent) is a better understanding of microcontinent formation through a seismic study of the upper-mantle and crustal structure beneath the Seychelles. From Feb. 2003 and Jan. 2004, 8 broadband and 18 intermediate-period three-component seismometers were deployed on 18 islands in the Seychelles. During this time period 240 events of Mb $〉$ 5.8 were recorded. Microseismic noise was a problem on most islands and resulted in low signal to noise ratios in seismic recordings. A polarisation filter developed by Du et al. (GJI, 2000) has been used to enhance teleseismic signals. The first stage of data analysis has been a study of upper-mantle anisotropy using shear-wave splitting in core phases such as SKS. In general the degree of splitting is similar to global averages (i.e., $sim$1 second). Variability in the orientation of the anisotropy suggests a mechanism related to deformation associated with microcontinent formation during the breakup of the Madagascar/Seychelles/India land mass, rather than a mechanism associated with current plate motions. The granitic inner-islands of the Seychelles plateau show a coherent NNE-SSW trend in anisotropy, whilst stations near the Mascarene plateau show more E-W trends. Such trends may support ideas of an anticlockwise rotation of the Seychelles during rifting events, or influence from local plume upwelling. The boundaries between oceanic and continental parts of the plateau are poorly know. Crustal thicknesses can be used to determine oceanic and contenental parts. Receiver functions will be used to estimate crustal depths and deeper mantle discontinuities.

Beschreibung: An analysis of 108,058 atmospheric refractivity profiles observed by CHAMP during 2002 and 2003 reveals a negative bias compared to ECMWF meteorological fields at altitudes below 5 km. The bias is most pronounced in the tropical Pacific, Central Africa, Indonesia, and India with values reaching -4%. In order to separate bias contributions caused by critical refraction from contributions induced by the receiver tracking process a comprehensive end-to-end simulation study was performed using radio sonde profiles obtained regularly by Alfred Wegener Institute aboard research vessel POLARSTERN since 1982. Within a subset of 2917 profiles recorded between 60N and 60S on the Atlantic ocean between 29 December 1982 and 14 November 2003, 40.2% (1172 profiles) indicate the presence of critical refraction with vertical refractivity gradients below -157 km^-1. Layers exceeding the critical refractivity value are mainly located between 1 to 2 km altitude, above 3 km the occurrence of critical refraction can be disregarded. Simulations including a receiver signal tracking model and using these 2917 sonde observations confirm that four quadrant carrier phase extraction outperforms the arctangent method currently implemented on CHAMP. Within regions of low signal-to-noise ratio an interesting alternative to 'fly-wheeling' and open-loop tracking methods is carrier loop band width reduction. Changing the band width from 30 to 10 Hz improves data yield at 0.5 km altitude by about 16%.

Beschreibung: The Louisville seamount trail has been recognized as one of the key examples of hot spot volcanism, comparable to the classic volcanic Hawaiian-Emperor lineaments. The published total fusion 40Ar/39Ar data of Watts et al. [1988] showed an astonishing linear age progression, firmly establishing Louisville as a fixed hot spot in the South Pacific mantle. We report new 40Ar/39Ar ages based on high-resolution incremental heating 40Ar/39Ar dating for the same group of samples, showing a marked increase in both precision and accuracy. One of the key findings in our reexamination is that the age progression is not linear after all. The new data show a significantly decreased “apparent” plate velocity for the Louisville seamount trail older than 62 Ma but confirm the linear trend between 47 Ma and the present day (although based on only three samples over 2150 km). The most recent volcanic activity in the Louisville seamount trail has now been dated at 1.11 ± 0.04 Ma for the most southeastern seamount located at 50°26′S and 139°09′W. These results indicate that the Louisville age progression should be interpreted on the basis of both plate and hot spot motion. In this paper we examine our new results in conjunction with the numerical mantle flow models of Steinberger et al. [2004] that also predict marked deviations from simple linear age progressions. With these models we can achieve a good fit to the geometry of both the Hawaiian and Louisville seamount trails and their age progressions as well as the ∼15° paleolatitudinal shift observed by Tarduno et al. [2003] for the Hawaiian hot spot between 80 and 47 Ma. If the model is restricted to Pacific hot spots only, we can improve the fit to the nonlinear age trend for the Louisville seamount trail by allowing an additional rotation change of the Pacific plate around 62 Ma and by decreasing the initiation age of the Louisville plume from 120 to 90 Ma. This improved model features a significant eastward hot spot motion of ∼5° between 80 and 30 Ma for the Louisville hot spot, which is quite dissimilar to the southward motion of the Hawaiian hot spot during the same time interval, followed by a minor ∼2° latitudinal shift over the last 30 Myr. If hot spot tracks are considered globally, the age trend observed for the oldest part of the Louisville seamount trail does not entirely follow the numerical model predictions. This may indicate some remaining inaccuracies in the global plate circuit, but it may also indicate that the Louisville hot spot experienced a motion somewhat different than in the numerical model: faster in the interval between 62 and 47 Ma but slower before that.

Beschreibung: The two main segments of the Central Andean plateau, Altiplano and Puna, demonstrate since the Late Miocene different styles of tectonic shortening. Initially pure shear shortening in the Altiplano plateau switched at 13-9 Ma into the simple shear mode accompanied by formation of one of the world largest thin skinned foreland belt. Further to the south, in the Puna, the pure shear shortening continued until much more recently, gradually transforming into mixed pure and simple shear mode with thick skinned deformation in the foreland (the Santa Barbara System). Through numerical simulation of thermo-mechanical processes we show that different shortening modes - pure and simple shear accompanied by thin or thick skinned tectonics - might be controlled by (i) strength of the foreland uppermost crust and (ii) temperature of the foreland lithosphere. As a numerical tool we use a 2-D parallel thermo-mechanical finite element code LAPEX-2D. The code combines explicit lagrangian finite element FLAC algorithm with particle-in-cell technique. Particles track not only material properties but also full strain and stress tensors minimizing numerical diffusion. We employ Maxwell visco-elastic rheology with temperature- and stress-dependent viscosity, simulating ductile flow, as well as Mohr-Coulomb elasto-plastic rheology, simulating brittle deformation. Both rheological models may experience strain softening. Previous geodynamic models indicated the importance of the lateral temperature variations in the lithosphere on the style of tectonic shortening. However, they failed to reproduce migration of the deformation from the Altiplano plateau into its foreland before the major uplift of the plateau. We show that deformation may easily migrate from the plateau into the foreland by rapidly propagating thin skinned thrust belt as a consequence of dramatic mechanical weakening of the Palaeozoic sediments overlying the cold lithosphere of the Altiplano foreland. The processes in the foreland sediments, which triggered their mechanical failure, remain unclear. We speculate that they might be related to the onset of the hydrocarbon maturation in the foreland sediments after the initial stage of the plateau uplift. Lack of the thick sediments further south, in the Puna foreland, precluded formation of the thin skinned thrusting. If the Puna foreland would have been as cold as the foreland at the latitude of the Altiplano, the Puna plateau would be continuously deformed in a pure shear mode with almost no deformation in the foreland; which is not the case. We show that the existing thick skinned deformation pattern of the Puna foreland might be explained by warmer foreland at the latitude of Puna than that at the latitude of Altiplano, which is in accord with the existing seismic Q-factor observations.

Beschreibung: Trees, as long living plants, are governed by environmental and/or climate changes within their habitat. Their growth rings record to a large extent the temporal dynamics of these changes either directly or through tree physiological reactions. They render the highest time resolution thus far possible for environmental or climate reconstructions of the past 10,000 years (exactly dated, annually resolved, see: [828], [994]). Trees are a substantial part of the human environment with a high socio-economic value. Their large geographical extension over various regions of the world, including those with greatest population densities but also marginal areas allows to gain unique informations about local and regional consequences of global climate change.

Beschreibung: River-borne quartz carries a cosmogenic nuclide memory that is a function of the catchment-wide erosion rate. This record may be preserved in fluvial deposits such as river terraces. If the age of a terrace is independently known and transport time in the river system is relatively short, then the upstream erosion rate at the time of terrace deposition can be determined. We have used cosmogenic nuclides to date river terraces in the lower Meuse catchment, the Netherlands, and to obtain a 1.3 Ma record of paleoerosion rates in a 104-km2 drainage basin comprising the Ardennes Mountains. Paleoerosion rates were uniform within the range of 25–35 mm/ka from 1.3 to 0.7 Ma. After 0.7 Ma, erosion rates have increased progressively to Late Pleistocene values of around 80 mm/ka. Around 0.7 Ma, both climatic and tectonic boundary conditions changed. The amplitude and duration of climate cycles increased significantly, resulting in long periods of sustained low temperatures in the Meuse catchment. In addition, an episode of magmatic underplating and mafic volcanism in the nearby Eifel caused up to 250 m of surface uplift in the Meuse catchment. The main streams in the region have responded to the perturbation at 0.7 Ma within a few 105 yr. Our data indicate that the catchment-wide response time is much longer. Further investigations are required to attribute the observed increase in paleoerosion rates to one or the other mechanism discussed.

Beschreibung: We introduce a new method to measure differential travel times and attenuation of seismic body waves. The problem is formulated as a nonlinear inverse problem, which is solved by simulated annealing. Using this technique, we have analyzed triplicated PKP waves recorded by the temporary Eifel array in central Europe. These examples demonstrate the potential of the technique, which is able to determine differential travel times and waveforms of the core phases, even when they interfere on the seismograms of when additional depth phases are present. The PKP differential travel times reveal the presence of large-amplitude and small-scale heterogeneities along the PKP(AB) ray paths and favor a local radial inner core model with ~0.9% velocity perturbation in its top 150 km and small velocity perturbations below. The quality factor in the top 300 km of this inner core region is estimated from PKP differential attenuation. Its preferred value is 330 with a lower bound of 75.