Microseismicity and tectonics of Sinai

Five microearthquake seismographs were used at 11 sites in northern Sinai in the period February 1987 to February 1988 to study the microseismicity of the area around the Maghara coal mine for mine-development studies. A total of 270 events were recorded on 850 records. The magnitude, epicenter and depth of each event were calculated using suitable software for an Egyptian Geological Survey and Mining Authority's (EGSMA) XT computer. The interpretation of the seismic activity in the area is given in view of plate tectonics, the Sinai subplate boundary follows the Gulf of Aqaba and the Dead Sea. The plate and subplate boundaries are presently active, and there seems to be a diffuse zone of deformation between “NW Africa” and “Nubia” affecting the Cairo-Suez district.     

Sedimentological/Palaeogeographic Data as Fundamental Building Blocks of the DDE Project:Critical Underpinning of Reconstructing Deep-time Earth Systems

正1 Introduction Sedimentary rocks archive important information for understanding how the earth system operates and how life and environments have evolved through earth history.Properly identifying characteristics of sedimentary rocks,along with the subsequent interpretation of depositional processes and sedimentary environments in a basin or locality,are fundamental to inter-basinal correlation of sedimentary successions as well as to understanding and deconvolving regional and global processes     

Ibiza Absolute Calibration Experiment: Survey and Preliminary Results

Within the framework of a project comprising part of the Spanish Space Program related to the JASON-1 CNES (Centre National d'Etudes Spatiales)/NASA (National Aeronautics and Space Administration) mission, a campaign was conducted from June 9–17, 2003, on the Absolute Calibration Site of the island of Ibiza. The objective was to determine the local marine geoid slope under the ascending (187) and descending (248) Jason-1 ground tracks, in order to allow a better extrapolation of the open-ocean altimetric data with on-shore tide gauge locations, and thereby improve the overall precision of the calibration process. For this we have used a catamaran with two GPS antennas onboard, following the Corsica/Senetosa design (Bonnefond et al. 2003a Bonnefond, P., Exertier, P., Laurain, O., Menard, Y., Orsoni, A., Jeansou, E., Haines, B., Kubitschek, D. and Born, G. 2003a. Leveling Sea Surface using a GPS catamaran. Marine Geodesy, 26(3–4): 319–334. [Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]). Five GPS reference stations were deployed in order to reduce the distance between the areas covered by the catamaran and the fixed GPS receiver used in the kinematic process. The geodetic activities (e.g., GPS, leveling) have enabled the building of a very accurate (few mm) network in a reference frame compatible with the satellite altimetry missions (ITRF 2000). The GPS kinematic data were processed using two different software programmes, allowing checking of the consistency of the solutions. If the standard deviation of the differences (3.3 cm) is close to the kinematic process precision, they exhibit some large values (up to 14 cm). These large discrepancies have been reduced using a weighting based on the crossover differences. Inasmuch as the distances between the tide gauges and the areas covered by the GPS catamaran were becoming large, we have used the MOG2D ocean model (Carrère and Lyard 2003 Carrère, L. and Lyard, F. 2003. Modelling the barotropic response of the global ocean to atmospheric wind and pressure forcing—comparisons with observations. Geophys. Res. Letters, 30(6) [Google Scholar]) to correct the sea surface from tides. In the farthest areas, the crossover differences show an improvement by a factor of two. Finally, we also present preliminary results on Jason-1 altimeter calibration using the derived marine geoid. From this analysis, the altimeter bias is estimated to be 120 ± 5 mm. The quality of this first result validates the whole GPS-based marine geoid processing, for which the accuracy is estimated to be better than 3 cm rms at crossovers.     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

Neoproterozoic contaminated MORB of Wadi Ghadir ophiolite,NE Africa: Geochemical and Nd and Sr isotopic constraints

The ophiolitic metabasalts (pillowed and sheeted dikes) of Wadi Ghadir area, Eastern Desert, Egypt, were analyzed for their major, trace and rare earth elements, Nd and Sr isotopes and the chemistry of their plagioclase, amphibole and chlorite was also reported. Geochemically these rocks range from tholeiitic basalt to basaltic andesite. The generally low MgO, Cr and Ni and high Zr contents are consistent with derivation of these rocks from an evolved magma. The high TiO₂ contents (mostly between 1.76% and 2.23%) classify Wadi Ghadir ophiolitic metabasalts as MORB ophiolite. The chondrite-normalized REE patterns of most samples display small LREE-enrichment with (La/Yb)_n ranging from 1.44 to 2.56. The MORB-normalized spider diagram shows variable LILE abundances, which are either similar to or enriched relative to MORB, and most samples display small Nb depletion. The abundances of some LILE (Ba, Rb and K) as well as Na and Si were modified by post-magmatic seafloor hydrothermal alteration. Enrichment of the least mobile LILE (Th & U) indicates that Wadi Ghadir ophiolitic metabasalts are akin to C (contaminated)-MORB. These geochemical characteristics are similar to BABB modified by contamination.Wadi Ghadir metabasalts have low initial Sr ratios (0.7010–0.7034) which are similar to those of MORB, while their ε_Nd(t) values (+7.7–+4.5) are either more or less positive than the value of depleted mantle (DM). The more positive ε_Nd(t) values indicate DM source for these basalts, while the less positive ε_Nd(t) values reflect the involvement of slightly older component in Wadi Ghadir ophiolite. We suggest that the parent magma of metabasalts was contaminated by slightly older material, most probably oceanic-arc crustal rocks, which caused enrichment in LREE, and by analogy LILE, but did not significantly affect Nd isotopic systematics or modify Sr isotopes.Such contaminated MORB character also revealed by other ophiolitic metavolcanics in the Central Eastern Desert, contrasting the N-MORB character of the Gerf ophiolite in the South Eastern Desert. Moreover, the present work suggests the increase of the degree of contamination of the ophiolitic metabasalts from south to north in the Central Eastern Desert.     

Heat flow in Eastern Egypt: The thermal signature of a continental breakup

The Red Sea is a modern example of continental fragmentation and incipient ocean formation. Heat flow data have been collected from eastern Egypt to provide information relating to the mode and mechanism of Red Sea opening. Preliminary heat flow data, including new data reported here, are now available from twenty-five sites in eastern Egypt and one site in western Sinai. A pattern of low to normal heat flow (35–55 mW m⁻²) inland with high heat flow (75–100 mW m⁻²) in a zone within 30 to 40 km of the coast is indicated.Moderately high heat flow (around 70 mW m⁻²) is indicated for the Gulf of Suez. The coastal zone thermal anomaly appears continuous with high heat flow previously reported for the Red Sea shelf. Heat production data indicate that the coastal thermal anomaly is not primarily related to crustal radiogenic heat production. The effects of rapid erosion may contribute to the anomaly, but are not thought to be the primary cause of the anomaly. If the anomaly is caused by lateral conduction from hot, extended, offshore lithosphere, the extension must have been active for the last 30 Ma or so, and a minimum of 100% extension is indicated. Alternatively, the anomaly is primarily caused by high mantle heat flow causing lithospheric thinning, centred beneath the Red Sea. The Red Sea is probably underlain by dominantly basic crust, formed either by intrusion into attenuated continental crust or sea-floor spreading, and for most purposes the crust formed in these two modes of extension may be essentially indistinguishable. Fission-track ages from eastern Egypt indicate that uplift started prior to, or at latest at the time of initial Red Sea opening, and this result, together with thermo-mechanical considerations, suggests an active asthenospheric upwelling beneath the Red Sea and high temperature in the lithosphere prior to extension.     

Variable style of transition between Palaeogene fluvial fan and lacustrine systems, southern Pyrenean foreland, NE Spain

Two Palaeogene fluvial fan systems linked to the south‐Pyrenean margin are recognized in the eastern Ebro Basin: the Cardona–Súria and Solsona–Sanaüja fans. These had radii of 40 and 35 km and were 800 and 600 km² in area respectively. During the Priabonian to the Middle Rupelian, the fluvial fans built into a hydrologically closed foreland basin, and shallow lacustrine systems persisted in the basin centre. In the studied area, both fans are part of the same upward‐coarsening megasequence (up to 800 m thick), driven by hinterland drainage expansion and foreland propagation of Pyrenean thrusts. Fourteen sedimentary facies have been grouped into seven facies associations corresponding to medial fluvial fan, channelized terminal lobe, non‐channelized terminal lobe, mudflat, deltaic, evaporitic playa‐lake and carbonate‐rich, shallow lacustrine environments. Lateral correlations define two styles of alluvial‐lacustrine transition. During low lake‐level stages, terminal lobes developed, whereas during lake highstands, fluvial‐dominated deltas and interdistributary bays were formed. Terminal lobe deposits are characterized by extensive (100–600 m wide) sheet‐like fine sandstone beds formed by sub‐aqueous, quasi‐steady, hyperpycnal turbidity currents. Sedimentary structures and trace fossils indicate rapid desiccation and sub‐aerial exposure of the lobe deposits. These deposits are arranged in coarsening–fining sequences (metres to tens of metres in thickness) controlled by a combination of tectonics, climatic oscillations and autocyclic sedimentary processes. The presence of anomalously deeply incised distributary channels associated with distal terminal lobe or mudflat deposits indicates rapid lake‐level falls. Deltaic deposits form progradational coarsening‐upward sequences (several metres thick) characterized by channel and friction‐dominated mouth‐bar facies overlying white‐grey offshore lacustrine facies. Deltaic bar deposits are less extensive (50–300 m wide) than the terminal lobes and were also deposited by hyperpycnal currents, although they lack evidence of emergence. Sandy deltaic deposits accumulated locally at the mouths of main feeder distal fan streams and were separated by muddy interdistributary bays; whereas the terminal lobe sheets expand from a series of mid‐fan intersection points and coalesced to form a more continuous sandy fan fringe.