G.R. Keller

POLONAISE '97 — an international seismic experiment between Precambrian and Variscan Europe in Poland

Abstract The Trans-European Suture Zone (TESZ) comprises a series of sutures that developed by Palaeozoic amalgamation of the Phanerozoic Central Europe onto the Proterozoic Baltica and East European Plate during the formation of Pangea. Within TESZ, the Permian Basin in Poland forms the easternmost part of the Permian Central European Basin, which is bordered on the east by the East European Craton (EEC) and on the southwest by the Bohemian Massif. The axis of the basin, the Mid-Polish Trough, parallels the edge of the EEC. The Teisseyre–Tornquist Zone (TTZ) is a geological inversion zone in the Polish Trough. A large seismic experiment, the POLONAISE97 project, was conducted in Poland during May of 1997 and targeted the deep structure of the TESZ and the complex series of upper crustal features associated with it. It included contributions from the geophysical communities in Poland, Denmark, the USA, Lithuania, Germany, Finland, Sweden and Canada. This large lithospheric seismic experiment deployed 613 instruments to record 64 shots along five profiles with a total length of about 2000xa0km. Moreover, five multichannel seismic reflection stations (90 and 120 channels) recorded all signals from shots. Two dimensional velocity models were derived by P-wave tomographic inversion. These models provide an initial interpretation of this massive data set and allow us to present a few significant seismic and tectonic observations. One of the most important is a very distinct asymmetry between the maximum thickness of the sedimentary cover in the Polish Trough (16–20xa0km) and the crustal root (∼50xa0km) associated with TESZ/TTZ.

Crustal and uppermost mantle structure of the Bohemian Massif based on CELEBRATION 2000 data

[1]xa0The deep structure of the Bohemian Massif (BM), the largest stable outcrop of Variscan rocks in central Europe, was studied using the data of the international seismic refraction experiment Central European Lithospheric Experiment Based on Refraction (CELEBRATION) 2000. The data were interpreted by seismic tomographic inversion and by two-dimensional (2-D) trial-and-error forward modeling of P and S waves. Additional constraint on crustal structure was given by amplitude modeling using the reflectivity method and gravity modeling. Though consolidated, the BM can be subdivided into several tectonic units separated by faults, shear zones, or thrusts reflecting varying influence of the Cadomian and Variscan orogeneses: the Saxothuringian, Barrandian, Moldanubian, and Moravian. Velocity models determine three types of crust-mantle transition in the BM reflecting variable crustal thickness and delimiting contacts of tectonic units in depth. The NW area, the Saxothuringian, has a highly reflective lower crustal layer above Moho with a strong velocity contrast at the top of this layer. This reflective laminated lower crust reaches depths of 26–35 km and is characteristic for the Saxothuringian unit, which was subject to eastward subduction. The Moldanubian in the central part is characterized by the deepest (39 km) and the most pronounced Moho within the whole BM with a strong velocity contrast 6.9–8.1 km s−1. A thick crust-mantle transition zone in the SE, with velocity increase from 6.8 to 7.8 km s−1 over the depth range of 23–40 km, seems to be the characteristic feature of the Moravian overthrusted by the Moldanubian during Variscan collision.

The influence of pre-existing structures on the evolution of the southern Kenya Rift Valley — evidence from seismic and gravity studies

The Kenya Rift is an active continental rift that has developed since the Late Oligocene. Although a thermal origin for the rifting episode is indicated by the scale of volcanism and its relative timing with uplift and faulting, the influence of pre-existing lithospheric structural controls is poorly understood. The interpretation of a 430-km-long seismic refraction and gravity line across the southern part of the Kenya Rift shows that the rift is developed across a transition zone, thought to represent the sheared Proterozoic boundary between the Archaean Nyanza Craton and the mobile Mozambique Belt. This zone of weakness has been exploited by the recent thermal rifting event. The Moho is at a depth of 33 km beneath the Archaean craton in the western part of the profile, and 40 km beneath the Mozambique Belt in the east. A few kilometres of localised crustal thinning has developed across the transition from thin to thick crust. At the surface, brittle faulting has formed an asymmetric rift basin 3.6 km deep, filled with low-velocity volcanic rocks. Basement velocities show a transition across the same area from low velocities (6.0 km s−1) in the Archaean, to high velocities (6.35 km s−1) in the Proterozoic. Mid-crustal layers show no deformation that can be attributed to the rifting event. Poorly constrained upper mantle velocities of 7.8 km s−1 beneath the southern rift confirm the continuation of the axial low-velocity zone imaged in previous seismic experiments. This is interpreted as the effect of small degrees of partial melt caused by elevated mantle temperatures. Gravity modelling suggests a contribution to the Bouguer anomaly from below the Moho, invoking the need for deep density contrasts. The regional gravity gradient necessary to model the Bouguer anomaly is used as supporting evidence for mantle-plume type circulation beneath the uplifted East African Plateau to the west of the Kenya Rift.

Crustal structure of the northern Main Ethiopian Rift from the EAGLE controlled-source survey; a snapshot of incipient lithospheric break-up

Abstract The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) was undertaken to provide a snapshot of lithospheric break-up above a mantle upwelling at the transition between continental and oceanic rifting. The focus of the project was the northern Main Ethiopian Rift (NMER) cutting across the uplifted Ethiopian plateau comprising the Eocene-Oligocene Afar flood basalt province. A major component of EAGLE was a controlled-source seismic survey involving one rift-axial and one cross-rift c. 400 km profile, and a c. 100 km diameter 2D array to provide a 3D subsurface image beneath the profiles’ intersection. The resulting seismic data are interpreted in terms of a crustal and sub-Moho P-wave seismic velocity model. We identify four main results: (1) the velocity within the mid- and upper crust varies from 6.1 km s−1 beneath the rift flanks to 6.6 km s−1 beneath overlying Quaternary axial magmatic segments, interpreted in terms of the presence of cooled gabbroic bodies arranged en echelon along the axis of the rift; (2) the existence of a high-velocity body (Vp 7.4 km s−1) in the lower crust beneath the northwestern rift flank, interpreted in terms of about 15 km-thick, mafic under-plated/intruded layer at the base of the crust (we suggest this was emplaced during the eruption of Oligocene flood basalts and modified by more recent mafic melt during rifting); (3) the variation in crustal thickness along the NMER axis from c. 40 km in the SW to c. 26 km in the NE beneath Afar. This variation is interpreted in terms of the transition from near-continental rifting in the south to a crust in the north that could be almost entirely composed of mantle-derived mafic melt; and (4) the presence of a possibly continuous mantle reflector at a depth of about 15–25 km below the base of the crust beneath both linear profiles. We suggest this results from a compositional or structural boundary, its depth apparently correlated with the amount of extension.

Crustal structure beneath the Kenya Rift from axial profile data

Abstract Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5–7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5–6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1–6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4–6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s.

Lower lithospheric structure beneath the Trans-European Suture Zone from POLONAISE'97 seismic profiles

Abstract The large-scale POLONAISE97 seismic experiment investigated the velocity structure of the lithosphere in the Trans-European Suture Zone (TESZ) region between the Precambrian East European Craton (EEC) and Palaeozoic Platform (PP). In the area of the Polish Basin, the P-wave velocity is very low (Vp 8.25 km/s in the Palaeozoic Platform and ∼8.1 km/s in the Precambrian Platform. Good quality record sections were obtained to the longest offsets of about 600 km from the shot points, with clear first arrivals and later phases of waves reflected/refracted in the lower lithosphere. Two-dimensional interpretation of the reversed system of travel times constrains a series of reflectors in the depth range of 50–90 km. A seismic reflector appears as a general feature at around 10 km depth below Moho in the area, independent of the actual depth to the Moho and sub-Moho seismic velocity. “Ringing reflections” are explained by relatively small-scale heterogeneities beneath the depth interval from ∼90 to 110 km. Qualitative interpretation of the observed wave field shows a differentiation of the reflectivity in the lower lithosphere. The seismic reflectivity of the uppermost mantle is stronger beneath the Palaeozoic Platform and TESZ than the East European Platform. The deepest interpreted seismic reflector with zone of high reflectivity may mark a change in upper mantle structure from an upper zone characterised by seismic scatterers of small vertical dimension to a lower zone with vertically larger seismic scatterers, possible caused by inclusions of partial melt.

The Role of Rifting in the Tectonic Development of the Midcontinent, U.S.A.

Abstract Recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of North America. Although the dating of some of these rifts (and even the rift interpretations) are subject to question, an analysis of these “paleo-rifts” reveals three major episodes of rifting: Keweenawan (~ 1.1 b.y. B.P.), Eocambrian (~ 600 m.y. B.P.), and early Mesozoic (~ 200 m.y. B.P.). The extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region. This role goes well beyond the initial rifting event because these features display a strong correlation with Paleozoic basins and a strong propensity for reactivation. For example, the Eocambrian Reelfoot rift was reactivated in the Mesozoic to form the Mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ENE compression. Even though the importance of rifting can be established, recognition of rifts and delineation of their complexities remain a major problem which requires more study.

An overview of recent seismic refraction experiments in Central Europe

WORKING GROUPS: K. ARIC, S. ACEVEDO, I. ASUDEH, M. BEHM, A. A. BELINSKY, T. BODOKY, R. BRINKMANN, M. BROx8e, E. BRÜCKL, W. CHWATAL, R. CLOWES, W. CZUBA, T. FANCSIK, B. FORKMANN, M. FORT, E. GACZYŃSKI, H. GEBRANDE, H. GEISSLER, A. GOSAR, M. GRAD, H. GRASSI, R. GRESCHKE, A. GUTERCH, Z. HAJNAL, S. HARDER, E. HEGEDÜS, A. HEMMANN, S. HOCK, V. HOECK, P. HRUBCOVÁ, T. JANIK, G. JENTZSCH, P. JOERGENSEN, G. KAIP, G.R. KELLER, K. KOMMINAHO, M. KORN, O. KAROUSOVÁ, S.L. KOSTIUCHENKO, F. KOHLBECK, D. KRACKE, M. MAJDAŃSKI, M. MALINOWSKI, K.C. MILLER, A.F. MOROZOV, E.-M. RUMPFHUBER, CH. SCHMID, C. SNELSON, A. x8aPIČÁK, P. ŚRODA, F. SUMANOVAC, E. TAKACS, H. THYBO, T. TIIRA, Č. TOMEK, J. VOZÁR, F. WEBER, M. WILDE-PIÓRKO, J. YLINIEMI, A. ŻELAŹNIEWICZ

Geophysical project in Ethiopia studies continental breakup

As continental rift zones evolve to sea floor spreading, they do so through progressive episodes of lithospheric stretching, heating, and magmatism, yet the actual process of continental breakup is poorly understood. The East African Rift system in northeastern Ethiopia is central to our understanding of this process, as it lies at the transition between continental and oceanic rifting [Ebinger and Casey, 2001]. n nWe are exploring the kinematics and dynamics of continental breakup through the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), which aims to probe the crust and upper mantle structure between the Main Ethiopian (continental) and Afar (ocean spreading) rifts, a region providing an ideal laboratory to examine the process of breakup as it is occurring. EAGLE is a multidisciplinary study centered around the most advanced seismic project yet undertaken in Africa (Figure l). Our study follows the Kenya Rift International Seismic Project [e.g., KRISP Working Group, 1995],and capitalizes on the IRIS/PASSCAL broadband seismic array [Nyblade and Langston, 2002], providing a telescoping view of the East African Rift within this suspected plume province.

Seismic structure of the uppermost mantle beneath the Kenya rift

A major goal of the Kenya Rift International Seismic Project WRISP) 1990 experiment was the determination of deep li~ospheric structure. In the re~action/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle vyere recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrikals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amoungs of extrusive volcanics associated with the rift suggest comp~itional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneaith the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lith~phere due to stretching in the north.