ALBERT

Description: A uniform 3-D crustal model is developed by use of published two-dimensional crustal models from previous active seismic surveys. The parameterisation of this 3-D crustal model is designed to adequately represent those crustal structures that mostly influence the propagation of teleseismic wavefronts. The 3-D model includes lateral variation in velocity structure, Moho topography, and large and deep sedimentary basins. The teleseismic forward problem for this local 3-D model is solved by calculation of travel times to the base of the model using a standard whole Earth model and by subsequent propagation of spherical wavefronts using finite difference methods. Travel time calculations for an event near Japan reveal significant lateral variations in the range between -0.3 s and +0.5 s due to crustal structures. Being able to obtain the full travel time field at the surface of the model has the additional advantage of improving the identification and timing of seismic phases observed at the TOR seismic array.

Description: Recent excavations at the Eocene maar lake of Mahenge (Tanzania) by a German/Tanzanian team produced more than 1,900 fossil vertebrate, plant and trace fossils, among which fishes (51 %) make up the largest part, followed by plant remains (36 %). Surface outcrops of lacustrine sediments suggest that the Eocene crater lake was about 400 m in diameter. A 3.8 meter long section of lacustrine deposits has been documented, among which are 1.1 meter of basal lacustrine sediments that never before have been exposed. Despite intense and most probably early diagenetic dolomitization sedimentary fabrics are well preserved. Sediments from the central part of the basin are well-bedded and in places show a fine and distinctive lamination. The overall sedimentation rate in the lake centre is estimated at 0.93 mm/year. Plant fossils are characterized by an abundance of (mostly) unattached Leguminosae leaflets representing at least eight taxa out of the total of 22-23 leaf taxa distinguished so far. The Eocene vegetation at Mahenge obviously was structurally similar to the modern "miombo" woodlands of Tanzania, which are also dominated by caesalpinioid legumes. Combined with the sedimentological data, the fossil flora indicates an overall dry climate with pronounced seasonality. The Mahenge maar lake has provided five different groups of fishes plus others still undescribed. These comprise the oldest known cichlids which may represent an early species flock. Ecological requirements of the extant relatives indicate redundant evidence of the existence of a shallow water area at the lakes margins. Mammals are so far only represented by the type specimen of Tanzanycteris mannardi Gunnell et al. (2003). T. mannardi differs from all known comparable Eocene microbats in having a much larger cochlear diameter relative to basicranial width. The extremely enlarged cochlea indicates that this bat had already developed sophisticated echolocation abilities. It is possible that Tanzanycteris represents a relatively primitive ancestral rhinolophoid but the balance of its other character states aligns it with the middle Eocene Messel Hassianycteridae. Mahenge and related maar lake deposits represent a rarely sampled temporal and geographical window, which is expected to produce more significant new information about the evolution of Paleogene ecosystems in Africa, the origin of the Malagasy biota and the origin of several modern groups of plants and vertebrates, especially mammals. ---------------- German ---------------- Neue Ausgrabungen im eozänen Maarsee von Mahenge (Tansania) durch ein deutsch/tansanisches Team haben mehr als 1900 fossile Vertebraten, Pflanzen und Spurenfossilien erbracht, unter denen Fische (51 %) den größten Teil ausmachen, gefolgt von Pflanzenresten (36 %). Aufschlüsse lakustriner Sedimente belegen, dass der eozäne Kratersee ungefähr einen Durchmesser von 400 m hatte. Ein 3,8 m langes Profil in lakustrinen Sedimenten wurde dokumentiert, darunter sind 1,1 m basale lakustrine Sedimente, die noch niemals zuvor aufgeschlossen waren. Obwohl eine intensive, sehr wahrscheinlich postdiagenetische Dolomitisierung stattgefunden hat, ist das sedimentäre Gefüge gut erhalten. Sedimente aus dem zentralen Abschnitt des Beckens sind gut geschichtet und zeigen stellenweise eine feine Laminierung. Die Sedimentationsrate im Seezentrum wird auf 0,93 mm/Jahr geschätzt. Unter den pflanzlichen Fossilien sind die meist isoliert gefundenen Blattfiedern von Leguminosen am häufigsten. Sie repräsentieren mindestens acht Taxa aus der Gesamtheit von 22-23 Blatttaxa, die bislang unterschieden werden konnten. Die eozäne Vegetation von Mahenge war offensichtlich ähnlich strukturiert wie die modernen Miombowälder von Tansania, die auch von caesalpinoiden Leguminosen dominiert werden. Kombiniert mit den sedimentologischen Daten, zeigt die fossile Flora ein insgesamt trockenes Klima mit ausgeprägter Saisonalität an. Der Kratersee von Mahenge hat fünf verschiedene Gruppen von Fischen geliefert, sowie andere, die noch unbeschrieben sind. Die Fischfossilien umfassen die ältesten bekannten Cichliden, bei denen es sich um einen frühen Artenschwarm handeln könnte. Die ökologischen Anforderungen der rezenten Verwandten liefern redundante Hinweise auf die Existenz einer Flachwasserzone an den Seerändern. Säugetiere sind bislang nur durch das Typusexemplar von Tanzanycteris mannardi Gunnell et al. 2003 belegt. T. mannardi unterscheidet sich von allen bekannten vergleichbaren eozänen Kleinfledermäusen durch einen im Verhältnis zur Basicranialbreite viel größeren Durchmesser der Cochlea. Die extrem vergrößerte Cochlea belegt, dass die Fledermaus schon die komplexe Fähigkeit der Echoorientierung entwickelt hatte. Es ist möglich, dass T. mannardi eine relativ ursprüngliche Rhinolophide ist. Die Ausgewogenheit ihrer sonstigen Merkmale stellen sie jedoch in eine Reihe mit den Hassianycteridae aus dem Mitteleozän von Messel. Mahenge und assoziierte Maarseesedimente repräsentieren ein bisher selten belegtes zeitliches und geographisches Fenster, von dem weitere bedeutsame neue Informationen zur Evolution paläogener Ökosysteme in Afrika, dem Ursprung der Malagasi-Biota und dem Ursprung verschiedener moderner Gruppen von Landpflanzen und Vertebraten, insbesondere der Säugetiere, erwartet werden können.

Description: Assessment of contributions from shallow lithosphere to teleseismic wave front distortion is a prerequisite for high-resolution regional teleseismic tomography. Several methods have been proposed in the past for the correction of these effects, e.g. application of station correction terms. We propose an approach that is independent of the subsequent inversion and uses the available a priori knowledge of the crustal structure to calculate crustal traveltime effects of teleseismic wave fronts. Our approach involves the construction of a 3-D crustal model based on controlled source seismology data and calculation of the associated traveltime anomalies for incoming teleseismic wave fronts. The model for central Fennoscandia shows a maximum crustal thickness of 64 km and includes a high-velocity lower crust as derived for parts of the study area by previous authors. Traveltimes calculated using finite differences for teleseismic waves travelling through this crustal model are compared with those from the standard reference model IASP91 and the residuals are used to correct observed teleseismic arrival times at the SVEKALAPKO array. To test the performance of this approach, in a second part of the study a synthetic traveltime data set is obtained by tracing wave fronts through a mantle structure with known velocity anomalies and the 3-D crustal model. This data set is inverted with and without correction for crustal effects. The 3-D crustal effects alone with a homogeneous mantle are also inverted and the results showthat the crustal effects propagate down to 450 km. The comparison of the inversion results demonstrates the need to apply appropriate 3-D crustal corrections in high-resolution regional tomography for upper-mantle structure beneath the Baltic Shield.

Description: Abstract: The temporary seismic station array (TOR) was designed to study the lithosphere-asthenosphere system across the north-western part of the Trans-European Suture Zone (TESZ) by teleseismic tomography. Teleseismic wave fronts, when propagating through complex crustal structure, undergo severe distortion that may result in travel time residual anomalies of significant amplitude. The inversion of teleseismic travel time residuals for deep structures without accounting for such crustal-related anomalies may erroneously map these travel time anomalies into features at greater depth. In this study we apply a three-dimensional (3-D) technique to estimate effects of a priori known 3-D crustal structure on travel times of teleseismic waves observed at the TOR seismic array across the TESZ to correct for these effects in future tomographic studies. A uniform 3-D crustal model is developed by use of published two-dimensional crustal models from previous active seismic surveys. The parameterisation of this 3-D crustal model is designed to adequately represent those crustal structures that mostly influence the propagation of teleseismic wave fronts. The 3-D model includes lateral variation in velocity structure, Moho topography, and large and deep sedimentary basins. The teleseismic forward problem for this local 3-D model is solved by calculation of travel times to the base of the model using a standard whole Earth model and by subsequent propagation of spherical wave fronts using finite difference methods. Travel time calculations for an event near Japan reveal significant lateral variations in the range between -0.3 s and +0.5 s due to crustal structures. Being able to obtain the full travel time field at the surface of the model has the additional advantage of improving the identification and timing of seismic phases observed at the TOR seismic array.

Description: A number of different geodynamic models have been proposed to explain the early tectonic evolution of the Baltic Shield. To provide additional geophysical constraints on these models, we performed a teleseismic tomography traveltime inversion for the central part of the Baltic Shield. The SVEKALAPKO project is focused on the investigation of the lithosphere-asthenosphere structure down to 400 km depth under central Fennoscandia (Baltic Shield). A total of 143 stations were deployed including 15 permanent stations from the Finnish seismic network. The temporal network was composed of 40 broad-band and 88 short-period instruments distributed in a rectangular array of 1000 km by 900 km from 1998 August to 1999 May. The results are based on a non-linear teleseismic tomography algorithm. They reveal significant P-velocity variations (up to 4 per cent) throughout the SVEKALAPKO array. The most prominent feature is a positive anomaly that can be followed down to 250 km depth beneath the centre of the array. We interpret this anomaly as the signature of the tectosphere (Jordan 1978) beneath the Fennoscandian Shield. It correlates spatially with an anomalous high-velocity lower crust. Other shallow (crustal) anomalies can be correlated with magmatic events surrounding this nucleus of high velocity. Comparison of images before and after correction by crustal structure proves that this methodology yields solid and coherent tomographic results. Further observations of relative P traveltime residuals from six teleseismic events with different azimuths show delay variations of ±2.0 s between stations located in the North German basin and stations on the Svecofennian Shield.

Description: Regional seismic tomography provides valuable information on the structure of shields, thereby gaining insight to the formation and stabilization of old continents. Fennoscandia (known as the Baltic Shield for its exposed part) is a composite shield for which the last recorded tectonic event is the intrusion of the Rapakivi granitoids around 1.6 Ga. A seismic experiment carried out as part of the European project Svecofennian-Karelia-Lapland-Kola (SVEKALAPKO) was designed to study the upper mantle of the Finnish part of the Baltic Shield, especially the boundary between Archean and Proterozoic domains. We invert the fundamental mode Rayleigh waves to obtain a three-dimensional shear wave velocity model using a ray-based method accounting for the curvature of wave fronts. The experiment geometry allows an evaluation of lateral variations in velocities down to 150 km depth. The obtained model exhibits variations of up to ±3% in S wave velocities. As the thermal variations beneath Finland are very small, these lateral variations must be caused by different rock compositions. The lithospheres beneath the Archean and Proterozoic domains are not noticeably different in the S wave velocity maps. A classification of the velocity profiles with depth yields four main families and five intermediate regions that can be correlated with surface features. The comparison of these profiles with composition-based shear wave velocities implies both lateral and vertical variations of the mineralogy.