The building blocks of continental crust: Evidence for a major change in the tectonic setting of continental growth at the end of the Archean

Oceanic arcs are commonly cited as primary building blocks of continents, yet modern oceanic arcs are mostly subducted. Also, lithosphere buoyancy considerations show that oceanic arcs (even those with a felsic component) should readily subduct. With the exception of the Arabian–Nubian orogen, terranes in post-Archean accretionary orogens comprise < 10% of accreted oceanic arcs, whereas continental arcs compose 40–80% of these orogens. Nd and Hf isotopic data suggest that accretionary orogens include 40–65% juvenile crustal components, with most of these (> 50%) produced in continental arcs.Felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs. They have lower Th/Yb, Nb/Yb, Sr/Y and La/Yb ratios, reflecting shallow mantle sources in which garnet did not exist in the restite during melting. The bottom line of these geochemical differences is that post-Archean continental crust does not begin life in oceanic arcs. On the other hand, the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs.During the Archean, however, oceanic arcs may have been thicker due to higher degrees of melting in the mantle, and oceanic lithosphere would be more buoyant. These arcs may have accreted to each other and to oceanic plateaus, a process that eventually led to the production of Archean continental crust. After the Archean, oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges, hence, oceanic lithosphere is more subductable. Widespread propagation of plate tectonics in the late Archean may have led not only to rapid production of continental crust, but to a change in the primary site of production of continental crust, from accreted oceanic arcs and oceanic plateaus in the Archean to primarily continental arcs thereafter.     

Flood hazards in the upper and middle Odra River basin – A short review over the last century

During the 20th century many floods of different intensity and extent have occurred on the Odra River and its tributaries. On the basis of long-term water level observations five major floods, that affected the entire upper and middle Odra River basin, were chosen for further analysis: June 1902, July 1903, August 1977, August 1985 and July 1997. However, hazardous floods were not only those that covered the whole upper and middle Odra River basin, so several local floods were also studied. Detailed historical analysis was made of meteorological conditions, with special emphasis on precipitation patterns and amounts. Then, on the basis of flood peak time occurrence, the stages of flood wave formation were formulated. The natural flood wave of the Odra River is often modified by hydro-technical infrastructure, the development and improvement of which is briefly described in this paper. In conclusion, a comparison of flood wave characteristics such as rising time, falling time, duration, peak flow and volume is presented.     

Present limitations of accurate satellite Doppler positioning for tectonics. An example: Djibouti

One of the possibilities of the Doppler positioning from satellite is to provide geodetic measurements continuous in time without the need for reference stations. If measurements of sufficient accuracy can be achieved they may be used to study local surface displacements in relation to tectonic activity. A Doppler receiver of the MEDOC network is located near Djibouti in the Ghoubhat-Asal rift region which corresponds to the accreting plate boundary between the Arabian and African plates. In November 1978, a seismic and volcanic crisis occured in this area. Surface geodetic measurements and levellings performed in 1973 and 1978–79, just after the crisis, reveal a 60–80 cm sinking of the graben floor and a lateral extension of about 2 meters. Here we analyse Doppler measurements for the period January 1977 to November 1980. Point positions are computed for 7 to 10 day intervals using a precise ephemeris, and a moving window analysis is applied to the data. An apparent 2 meter uplift preceding the November 1978 seismic crisis is detected at Djibouti, whereas no similar phenomenon is observed at the two closest stations, Pretoria and Uccle-Brussels. However, field observations rule out a tectonic origin for this uplift. In Djibouti, the correlation between the apparent vertical station position and the electron density in the ionospheric F-layer reveals that a bias may be induced by the third order term of the ionospheric refractive index not previously taken into account, or more probably by the ray curvature through the ionosphere. This bias is particularly strong for our data set, from a station located close to the magnetic equator, and related to a period of rapid increase in the solar activity. Although our analysis fails to detect any tectonic displacement related to the rift seismic activity, it shows that similar tectonic studies by the Doppler method will be possible once the residual ionospheric errors are removed, for example by use of higher radio frequencies.     

Pliocene and late Pleistocene (MIS 5e) decapod crustaceans from Santa Maria Island (Azores Archipelago: Central Atlantic): systematics,palaeoecology and palaeobiogeography

The presence of decapod crustaceans in the Pliocene and Pleistocene (MIS 5e) fossil record of Santa Maria Island (Azores Archipelago) is herein reviewed. Our study raises the number of fossil decapods from this island from one species to 10 taxa (three for the Pliocene and seven for the Last Interglacial). Four of these 10 taxa are reported for the first time in the fossil record, worldwide. A new species of a mud shrimp is also described (Upogebia azorensis n. sp.). Our study suggests that the Plio–Pleistocene decapod assemblages of the Azores did not differ significantly from modern ones, being dominated by species that are today widespread across the Webbnesia ecoregion, the Mediterranean Sea, and the eastern Atlantic shores, including the Azores. As far as can be judged from the limited fossil record, apparently no tropical crab species with a Cabo Verdean/Senegalese provenance reached the Azores during windows of opportunity associated with Glacial Termination 2 or with the initial setting of the Last Interglacial period. This contribution increases the total number of marine taxa reported for the Pliocene and Pleistocene outcrops of Santa Maria Island to 218 and 155, respectively, highlighting the scientific relevance of its palaeontological heritage.     

The Late Precambrian Timna igneous complex, Southern Israel: Evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma

New data from a geochemical, geochronological and isotopic study of the Late Precambrian Timna igneous complex suggest the formation of alkali granites from a LIL-enriched, mantle derived, sanukitoid-type monzodiorite (a silica oversaturated rock with Mg# >60). These data also provide new insights into the petrology, timing and regional tectonic control of the transition from the calc-alkaline to the alkaline magmatic activity in the northern Arabian-Nubian Shield (ANS) during the Late Precambrian.

The Timna alkali granite was formed by fractional crystallization from the monzodioritic magma in a quasi-stratified magmatic cell which formed 610 Ma ago in the 625 Ma old calc-alkaline, porphyritic granite crust. These monzodiorites are mantle-derived, as demonstrated by their high Mg# (63), Cr (230 ppm), and Ni (120 ppm). They are characterized by initial ⁸⁷Sr/⁸⁶Sr of 0.7034, ε-Nd (610 Ma) = +3.4, and are enriched in K₂O (2.9%), Sr (840 ppm), Ba (1290 ppm) and LREE [(La/Lu)_n= 10–25]. The chemical characteristics and REE patterns of the monzodiorites and andesitic dykes of Timna are very similar to Dokhan andesites from northeastern Egypt and the Archean sanukitoids from Canada. The isotopic, geochemical and geochronologic data all indicate that Timna monzodiorites are comagmatic with the alkali granite. The alkali granite is a typical post-orogenic, borderline A-type granite. It is enriched in potassium (K₂O=4.68–6.64%), has a negative europium anomaly (Eu/Eu^*=0.058–0.38) and ε-Nd (610 Ma) of +3.9. The calc-alkaline granite is a typical I-type granite with a small positive europium anomaly (Eu/Eu^*=1.02–1.16). Its age and the Sr, Nd and Pb isotopic characteristics with ε-Nd (625 Ma) of +5.6 to +5.9 are significantly different from these of the alkali granite and monzodiorites, and indicate little interaction with the monzodiorite during the formation of the alkali granite.

The alkali granites are correlative with the post-collisional extensional granites in Jordan and Egypt while the porphyritic granites can be correlated with the late orogenic types. Crustal thickening associated with orogenic compression resulted in crustal anatexis to form the I-type granitic rocks, whereas crustal thinning associated with extension allowed LIL-enriched mantle melts to rise very near to the surface, where space was available for these to pond and fractionate to alkali granite.     

Precambrian crystalline basement in southern Mongolia as revealed by SHRIMP zircon dating

Single zircon ages determined by ion microprobe (SHRIMP II) for granitoid gneisses from the southern slope of the Baga Bogd massif (Gobi-Altai, southern Mongolia) reveal several episodes of zircon growth, ranging from late Palaeoproterozoic to late Cambrian. The oldest events are documented by a zircon crystallization age for a gneiss protolith at 1519 ± 11 Ma and by a xenocrystic zircon from a dark grey augen-gneiss yielding an age of c. 1701 Ma. Discrete igneous events are recorded in granite-gneisses with protolith emplacement ages of 983 ± 6, 956 ± 3 and 954 ± 8 Ma. These ages provide the first record of early Neoproterozoic magmatic activity in this region. A much younger and discrete magmatic event is recorded by several dioritic to granitic orthogneisses which are tectonically interlayered with the older gneisses and have protolith emplacement ages between 502 and 498 Ma. These late Cambrian granitoids of calc-alkaline affinity are likely to have been emplaced along an active continental margin and suggest that the Baga Bogd Precambrian crustal fragment was either docked against the southward (present-day coordinates) growing margin of the CAOB or was a large enough crustal entity to develop an arc along its margin. We speculate that the Precambrian gneisses of this massif may be part of a crustal fragment rifted off the Tarim Craton.     

Constraints on sedimentary ages of the Chuanlinggou Formation in the Ming Tombs,Beijing, North China Craton: LA-ICP-MS and SHRIMP U–Pb dating of detrital zircons

Detrital zircons in five sedimentary samples, MC1 to MC5, from the bottom of the Chuanlinggou Formation in the Ming Tombs District, Beijing, were dated with the LA-ICP-MS and SHRIMP U–Pb methods. Age spectra of the five samples show a major peak at 2500 Ma and a secondary peak at 2000 Ma, suggesting their provenances were mainly from the crystalline basement of the North China Craton and the Trans-North China Orogen. The youngest zircon has an age of 1673 ± 44 Ma, indicating that the Chuanlinggou Formation was deposited after this age. From sample MC4 to MC5, lithology changed from a clastic rock (fine-grained sandstone) to a carbonate rock (fine-grained dolomite), suggesting that the depositional basin became progressively deeper. The age spectrum of sample MC5 shows a major peak at 2500 Ma and a secondary peak at 2000 Ma. Sample MC4, which is stratigraphically lower than sample MC5, only had one peak at 2500 Ma. We conclude that there was a transgressive event when sediments represented by MC5 was deposited, and seawater carried ca. 2000 Ma clastic materials to the basin where the Chuanlinggou Formation was deposited, leading to the addition of ca. 2000 Ma detritus. Our research indicates that the source area for the sediments became more extensive with time. We conclude that the Chuanlinggou Formation in the Ming Tombs District was deposited in a low-energy mud flat sedimentary environment in the inter-supra tidal zone because it is mainly composed of silty mudstone and fine-grained sandstone with relatively simple sedimentary structures.     

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