A crustal suture and lineament in North Africa

A possible suture between an old craton in West Africa and the Arabian—Nubian shield is marked by a persistent zone of Phanerozoic weakness. This zone extends between the Nile and Niger deltas and passes through the Qattara embayment, along the Bahariya fault, through the Kufra basin, between the Tibesti and Jabal al Uwaynat massifs, through the Bahr el Ghazal and Lake Chad areas, and along the Benoue trough.     

Seismic studies of the crustal structure in West Antarctica 1979–1980—Preliminary results

The Polish Geophysical Expedition to West Antarctica in the summer of 1979–1980 was organized by the Institute of Geophysics of the Polish Academy of Sciences. The purpose of the expedition was to carry out studies of deep structures of the Earth's crust by reflection, refraction and deep seismic sounding methods. Special attention was paid to tectonically active zones and to the contact zones between the blocks of the Earth's crust and the lithospheric plates. Geophysical measurements were carried out in the area extending between 61° and 65°S and between 56° and 66°W. The measurements covered the southern Shetlands, the Antarctic Peninsula, the Bransfield Strait, the Drake Passage, the Palmer Archipelago, the Gerlache Strait and the Bismarck Strait towards the southern Pacific.Deep seismic soundings were made along profiles with a total length of about 2000 km. Seismic reflection measurements were made along profiles about 1100 km long. A detailed analysis of the seismic wave field shows that the structure of the Earth's crust in this part of West Antarctica is very complex. Numerous deep fractures divide the Earth's crust into blocks of different physical properties. The thickness of the Earth's crust changes from 32 km in the region of the South Shetland Islands to 40–45 km in the region of the Antarctic Peninsula. A preliminary geodynamical model of this part of West Antarctica is presented.     

The Elbe Fault System in North Central Europe—a basement controlled zone of crustal weakness

The Elbe Fault System (EFS) is a WNW-striking zone extending from the southeastern North Sea to southwestern Poland along the present southern margin of the North German Basin and the northern margin of the Sudetes Mountains. Although details are still under debate, geological and geophysical data reveal that upper crustal deformation along the Elbe Fault System has taken place repeatedly since Late Carboniferous times with changing kinematic activity in response to variation in the stress regime. In Late Carboniferous to early Permian times, the Elbe Fault System was part of a post-Variscan wrench fault system and acted as the southern boundary fault during the formation of the Permian Basins along the Trans-European Suture Zone (sensu [Geol. Mag. 134 (5) (1997) 585]). The Teisseyre–Tornquist Zone (TTZ) most probably provided the northern counterpart in a pull-apart scenario at that time. Further strain localisation took place during late Mesozoic transtension, when local shear within the Elbe Fault System caused subsidence and basin formation along and parallel to the fault system. The most intense deformation took place along the system during late Cretaceous–early Cenozoic time, when the Elbe Fault System responded to regional compression with up to 4 km of uplift and formation of internal flexural highs. Compressional deformation continued during early Cenozoic time and actually may be ongoing. The upper crust of the Elbe Fault System, which itself reacted in a more or less ductile fashion, is underlain by a lower crust characterised by low P-wave velocities, low densities and a weak rheology. Structural, seismic and gravimetric data as well as rheology models support the assumption that a weak, stress-sensitive zone in the lower crust is the reason for the high mobility of the area and repeated strain localisation along the Elbe Fault System.     

The Levantine Basin—crustal structure and origin

The origin of the Levantine Basin in the Southeastern Mediterranean Sea is related to the opening of the Neo-Tethys. The nature of its crust has been debated for decades. Therefore, we conducted a geophysical experiment in the Levantine Basin. We recorded two refraction seismic lines with 19 and 20 ocean bottom hydrophones, respectively, and developed velocity models. Additional seismic reflection data yield structural information about the upper layers in the first few kilometers. The crystalline basement in the Levantine Basin consists of two layers with a P-wave velocity of 6.0–6.4 km/s in the upper and 6.5–6.9 km/s in the lower crust. Towards the center of the basin, the Moho depth decreases from 27 to 22 km. Local variations of the velocity gradient can be attributed to previously postulated shear zones like the Pelusium Line, the Damietta–Latakia Line and the Baltim–Hecateus Line. Both layers of the crystalline crust are continuous and no indication for a transition from continental to oceanic crust is observed. These results are confirmed by gravity data. Comparison with other seismic refraction studies in prolongation of our profiles under Israel and Jordan and in the Mediterranean Sea near Greece and Sardinia reveal similarities between the crust in the Levantine Basin and thinned continental crust, which is found in that region. The presence of thinned continental crust under the Levantine Basin is therefore suggested. A β-factor of 2.3–3 is estimated. Based on these findings, we conclude that sea-floor spreading in the Eastern Mediterranean Sea only occurred north of the Eratosthenes Seamount, and the oceanic crust was later subducted at the Cyprus Arc.     

Nares Strait—A suture zone: Geophysical and geological implications

The motion of Greenland relative to Ellesmere Island along Nares Strait is determined from poles of rotation which provide control for the motion independent of local geology and geophysics. The plate kinematics around the North Atlantic Ocean, the Norwegian and Greenland Seas and the Eurasian Basin of the Arctic Ocean constrain motion along Nares Strait. These motions are checked by examining the stability characteristics of the triple junctions. These junctions are found to be stable. The motion along Nares Strait between anomalies 34 and 13 is a combination of strike-slip and compression. The regional geology is found to support the plate reconstructions. The local geology of the Nares Strait area is reviewed and found not to refute the predicted motions. The geophysical and geological data are interpreted in terms of the Wilson cycle, the opening and closing of an ocean. The Nares Strait area has the characteristics of a cryptic suture, a join between regions of collided continental crust.     

Earthquake swarms in continental rifts — A comparison of selected cases in America, Africa and Europe

The occurrence of earthquake swarms is typically related to magmatic activity in volcanoes, yet swarms are also common in other intracontinental regions such as continental rifts. We present here a summary of geophysical observations that have been made in earthquake swarm areas of the Rio Grande, Kenya, and Eger rifts, focusing on characteristic parameters for the origin and generation of the swarm earthquakes.Our compilation of seismological parameters such as spatial distribution and focal parameters of hypocenters, magnitude statistics, and the location of the swarm centres in the rift environments reveals major similarities. The earthquake swarms take place at shallow depth between 0 and 10 km. The maximum magnitudes are mostly less than 4.5. The b-values, indicating the magnitude frequency relation of the seismicity, are about 0.8. They are hence not deviating from a normal non-volcanic intraplate environment, but are considerably lower than those of volcanic earthquake swarms. Focal mechanism studies give uniform pictures of stress field orientation and faulting style for the swarm areas. In all three rifts, the centres of swarm activity seem to be restricted to rift valley sections that may be influenced by large-scale fracture or shear zones that intersect the rifts. We conclude that these deep-reaching zones of weakness allow intrusions of upper mantle material into crustal layers, where magma-related fluids or fluctuations of the magma bodies themselves cause the generation of earthquake swarms.     

Active crustal deformation in two seismogenic zones of the Pannonian region — GPS versus seismological observations

The seismicity and the associated seismic hazard in the central part of the Pannonian region is moderate, however the vulnerability is high, as three capital cities are located near the most active seismic zones. In our analysis two seismically active areas, the Central Pannonian and Mur-Mürz zones, have been considered in order to assess the style and rate of crustal deformation using Global Positioning System (GPS) and earthquake data.We processed data of continuous and campaign GPS measurements obtained during the years 1991–2007. Velocities relative to the stable Eurasia have been computed at HGRN, CEGRN and EPN GPS sites in and around the Pannonian basin. Uniform strain rates and relative displacements were calculated for the investigated regions. GPS data confirm the mostly left lateral strike slip character of the Mur-Mürz–Vienna basin fault system and suggest a contraction between the eastward moving Alpine-North Pannonian unit and the Carpathians.The computation of the seismic strain rate was based on the Kostrov summation. The averaged unit norm seismic moment tensor, which describes the characteristic style of deformation, has been obtained from the available focal mechanism solutions, whereas the annual seismic moment release showing the rate of the deformation was estimated using the catalogues of historical and recent earthquakes.Our analysis reveals that in the Central Pannonian zone the geodetic strain rate is significantly larger than the seismic strain rate. Based on the weakness of the lithosphere, the stress magnitudes and the regional features of seismicity, we suggest that the low value of the seismic/geodetic strain rate ratio can be attributed to the aseismic release of the prevailing compressive stress and not to an overdue major earthquake. In the Mur-Mürz zone, although the uncertainty of the seismic/geodetic strain rate ratio is high, the seismic part of the deformation seems to be notably larger than in the case of the Central Pannonian zone. These results reflect the different deformation mechanism, rheology and tectonic style of the investigated zones.     

Comments on “The crustal structure of the Balearic Sea” — in light of deep-sea drilling in the Mediterranean

Supplementary to the paper by K. Hinz on The crustal structure of the Balearic Sea, some results are reported from the deep-sea drilling cruise of the “Glomar Challenger”.     

TRANSALP—deep crustal Vibroseis and explosive seismic profiling in the Eastern Alps

The TRANSALP consortium, comprising institutions from Italy, Austria and Germany, carried out deep seismic reflection measurements in the Eastern Alps between Munich and Venice in 1998, 1999 and 2001. In order to complement each other in resolution and depth range, the Vibroseis technique was combined with simultaneous explosive source measurements. Additionally, passive cross-line recording provided three-dimensional control and alternative north–south sections. Profits were obtained by the combination of the three methods in sectors or depths where one method alone was less successful.The TRANSALP sections clearly image a thin-skinned wedge of tectonic nappes at the northern Alpine front zone, unexpected graben or half-graben structures within the European basement, and, thick-skinned back-thrusting in the southern frontal zone beneath the Dolomite Mountains. A bi-vergent structure at crustal scale is directed from the Alpine axis to the external parts. The Tauern Window obviously forms the hanging wall ramp anticline above a southward dipping, deep reaching reflection pattern interpreted as a tectonic ramp along which the Penninic units of the Tauern Window have been up-thrusted.The upper crystalline crust appears generally transparent. The lower crust in the European domain is characterized by a 6–7 km thick laminated structure. On the Adriatic side the lower crust displays a much thicker or twofold reflective pattern. The crustal root at about 55 km depth is shifted around 50 km to the south with respect to the main Alpine crest.     

Towards an understanding of the southern Appalachian Piedmont crustal transition—A multidisciplinary approach

The Piedmont of the southern Appalachians is characterized by significant geophysical and geological anomalies which indicate there is a major crustal transition. Multiple hypotheses, including a suture zone and a subducted continental margin, have been presented to explain the variations. Although crustal seismic reflection data have provided significant constraints, there are ambiguities inherent in the interpretation of such data. The ambiguities can be reduced by careful consideration of related geophysical and geological observations. Although the importance of correlating crustal reflection data with known geologic features by tracing reflections to drill holes or to the surface cannot be overestimated, only rarely are such correlations possible. In almost all interpretations of crustal reflection structure it is necessary to constrain the model with methods such as seismic refraction, gravity, magnetics, conductivity, and surface geology (including palinspastic reconstructions). When information from these techniques is incorporated into interpretations of the Piedmont crustal structure, the model which appears to be most consistent with the observations is one in which the upper crust of the Piedmont is decoupled from the lower crust, and in which the lower crust thins eastward. The lower crust may be a subducted Precambrian continental margin and its associated transition toward thinner, basinal crust.     

The indigenous growth towards a cash economy in Africa — A case study of a lacustrine economy in Malawi

Chilwa, an endohreic lake in East Central Africa, lies in a basin peripheral to the developed areas of the Shire Highlands in Malawi. In 1965 the catch of fish accounted for half the estimated catch of Malawi, while the seasonally inundated grasslands sustain 35,000 head of cattle. The receding lake has left a series of terraces favourable to cotton and tobacco production. This study examines the responses of the people in the southern section of the plain to circumstances which could offer an integrated rural economy with the additional advantage of an entry into the money nexus through commercial fisheries.