Quaternary fault kinematics and chronology in intraplate northeastern Brazil

Northeastern Brazil has experienced earthquake swarms, which include events up to m_b = 5.2 in the last 30 years. Ground-rupturing events, however, have not been reported in the last 200 years. We have used a multidisciplinary approach to describe the geometry, kinematics, age, paleostress field, and paleoseismological significance of ground ruptures filled by clastic sediments in the region. Methods included GIS- and remote sensing-assisted structural mapping, borehole and resistivity sounding, grain size and X-ray analysis, and Optically Stimulated Luminescence (OSL) and Single-Aliquot Regenerative-Dose (SAR) chronology. Sediment-filled faults concentrate on the hanging wall of the Jundiaí fault, a 35 km long, NE-trending, late Cretaceous to Quaternary fault. These sediment-filled faults cut across Precambrian granites and overlying alluvial and colluvial deposits and exhibit steeply planar geometry. The largest faults exhibit sediment fills that extend downward to at least 20 m. The faults alternate between normal and right-oblique normal slip, which allowed the gravitational influx of unlithified gravels to gravelly sediments into the granites. Field evidence indicates seismogenic origin associated with multistage opening and infilling. On the basis of OSL and SAR chronology, we distinguish six generations of infills, which represent the main periods of fault activity: 8.0–9.0, 11.0–15.0, 16.0–24.0, 37.0–45.5, 65.0–67.9, and 84.5–93.5 ka. These dates suggest a 15.8 ka recurrence period. A few OSL dates may represent maximum ages due to poor bleaching of sediments, which implies that the recurrence period may be underestimated. We conclude that the Jundiaí fault has been continuously active in respect of surface-rupturing during at least the last ～100 ka. The size of the surface ruptures is consistent with earthquake magnitude M ～ 5.3, close to magnitudes observed in northeastern Brazil in the short instrumental and historical record. We suggest that seismogenic-fault recurrence in intraplate settings is longer than human settlement and should be assessed by multidisciplinary methods, mainly those that provide subsurface data and chronology of clastic sediments. Faults that trap sediments during movements are ideal places for these studies. The structures we describe here may have analogs in both modern and ancient intraplate settings.     

Probable tsunami origin for a Shell and Sand Sheet from marine ponds on Anegada, British Virgin Islands

A distinctive Shell and Sand Sheet found beneath the marine ponds of Anegada, British Virgin Islands, was formed by a post-1650 AD overwash event, but its origin (tsunami or hurricane) was unclear. This study assesses the taphonomic characters of the shell and large clast material (>2?mm) to determine its provenance and origin. Pond-wide stratigraphic units (Shelly Mud, Shell and Sand Sheet, Mud Cap) were analyzed (12 samples) at four sites in Bumber Well and Red Pond along with eight samples from the Shell and Sand Sheet in a 2-km transect of Bumber Well. Mollusks in the pond muds include Anomalocardia spp. and cerithids with no allochthonous shells from the offshore reef-flat. Results show that the shells and clasts (>2?mm) are derived from the erosion and winnowing of the underlying Shelly Mud of the former marine pond, forming a distinctive sheet-like deposit with Homotrema sand. The Shell and Sand Sheet contains articulated Anomalocardia bivalves and moderate numbers of angular fragments (approximately 35%) that are likely from crab predation. Radiocarbon dates of articulated Anomalocardia specimens from the Shell and Sand Sheet range widely (approximately 4000?years), with shell condition (pristine to variably preserved) showing no correlation with age. The articulated condition of the bivalves with the wide-ranging dates suggests erosion and winnowing of the underlying Shelly Mud but minimal transport of the bivalves. The Shell and Sand Sheet has taphonomic characteristics indicative of a widespread tsunami overwash (sheet-like extent and articulated specimens) but lacks allochthonous reef-flat shells. Reef-flat shell material may not have penetrated the pond, as a tsunami would have to cross the reef-flat and overtop high dunes (2.2?m) hindering transport of larger shell material but allowing the Homotrema sand to penetrate. Processes including hurricane overwash, pond wave action, or tidal channel opening and closure are not favoured interpretations as they would not produce extensive sheet-like deposits. Taphonomic analysis is hampered by the limited (400?C500?years BP) depositional history from Anegada??s ponds and the lack of comparative data from other Caribbean locations.     

High-grade metamorphism of ironstones in the Mesoarchaean of southwest Swaziland

A sequence of high-grade rocks collectively referred to as Mkhondo Valley Metamorphic Suite is exposed around the Nhlangano Gneiss Dome in southwest Swaziland and also less extensively further north. Meta-ironstones are part of this sequence and have been studied specifically with two main objectives in mind, firstly, to test the hypothesis of the high-grade metamorphic sequence correlating with lower-grade Pongola-age rocks nearby, and secondly, to assess the potential of these meta-ironstones for retrieving P-T conditions of metamorphism. The studied ironstones contain substantial Al due to terrigenous clastic input and also include Mn-rich varieties. Three compositional groups of meta-ironstones can be distinguished: garnet-orthopyroxene-bearing ones, garnet-bearing, orthopyroxene-free ones, and a garnet-poor to garnet-free type. A comparison of bulk rock compositions between these high-grade meta-ironstones and low-grade Mn-poor to Mn-rich ironstones from the Mozaan Group supports a stratigraphic correlation. Pseudosection modelling demonstrates that aluminous meta-ironstones provide useful P-T constraints at high peak temperatures as they do not tend to produce significant amounts of melt, if any. The presence of Al is critical for introducing garnet to amphibolite and granulite-facies assemblages, which also allows to apply conventional thermobarometers. The combination of pseudosection modelling and conventional thermobarometry yielded peak metamorphic conditions of 850–890 °C at 7 to 8 kbar. These data are compatible with the ones obtained from associated metapelitic gneisses, except that the latter indicate decompression to lower pressure granulite-facies conditions. The petrological, structural and geochronological data from southwest Swaziland confirm major orogenic activity in the eastern Kaapvaal Craton during the Neo-Archaean.     

Stabilization of systems of galaxies by subclustering

We investigate the contribution of gravitationally bound subsystems to the potential energy of systems of galaxies. We derive a statistical correction factor to the virial mass using Holmberg's observed frequency function of multiple galaxies. It is of the order of ten for groups of galaxies and negligible for rich clusters. However, we emphasize that an individual treatment is necessary. As an illustration we calculate the contribution to the binding energy by observed binary galaxies in the de Vaucouleurs groups 48 and 49, and the Coma Cluster. In all three cases a single double galaxy removes the virial mass paradox. We conclude that corrections for subclustering are substantial in stability analyses of systems of galaxies, that they can remove easily mass discrepancies of the order of ten in groups and rich clusters, and that they should be made individually and not statistically.     

Post-Brasiliano (Pan-African) high-K granitic magmatism in Central Brazil: the role of late Precambrian-early Palaeozoic extension

In western Goiás, Brazil, the emplacement of large, high-K postorogenic granites and associated small gabbro-dioritic intrusions, followed immediately after the last deformational events of the Brasiliano-Pan-African orogeny at 600 Ma. Well-fitted whole-rock Rb---Sr isochrons indicate ages which suggest two discrete intrusive events: the older between 588 and 560 Ma and the younger between 508 and 485 Ma. The older granites display general petrographic and geochemical characteristics of highly differentiated calc-alkaline I-type granitoids, whereas the younger intrusions are more alkaline, similar to A-type granites.Initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.703 to 0.710 and initial Nd isotope ratios yield ε_Nd(T) values in the range between −4.0 and +3.0. There are no major differences in initial isotopic compositions between the two granite groups, suggesting that the parental magmas for both groups of rocks mostly originated by refusion of crustal sources isotopically similar to the 940-640 Ma basement arc-type metatonalites-metagranodiorites and associated arc metavolcanics.The major and trace element compositional differences between the two granite groups is explained in terms of modifications in the melting conditions within the crust, with younger melts being produced by the refusion of anhydrous, depleted crustal sources left after the extraction of previous batches of more hydrated calc-alkaline magmas. The heat input required to promote extensive remelting of the continental crust was, most likely, provided by mantle-derived mafic magmas that invaded and probably underplated the crust, during uplift and extension.The two intrusive events are bimodal in nature and are interpreted as shallow-level extension-related events associated with regional uplift and denudation occurring just after two orogenic pulses at the end of the Proterozoic and early Palaeozoic, the older at 600 Ma and the younger between 550 and 510 Ma. The onset of the granite magmatism at 590 Ma, shortly post-dating the Brasiliano orogeny (600 Ma), is broadly coeval with the initial stages of sedimentation of the terrigenous and carbonatic rocks of the ensialic Paraguay Belt in Brazil and its correlative in Bolivia, the Tucavaca Belt, which probably correspond to rift deposits related to the break up of Laurentia from Gondwana at the end of the Proterozoic and beginning of the Palaeozoic. Granites of the younger group cut the deformational structures of the Paraguay Belt metasediments and pre-date, by between 40 and 20 Ma, the initial stages of subsidence and sedimentation of the Paraná Basin.     

Zellphysiologische Untersuchungen mittels Elodea canadensis Michx. zur Biotropie diphosphonsäurchaltiger Stabilisatoren der Wasserhärte

Elodea canadensis Michx , reagiert auf Wasser, das das Präparat FERROFOS DFL (1-Hydroxyalkan-1,1-diphosphonsäure als aktive Substanz) enthält,¹ je nach ihrer Herkunft, der Konzentration des Härtestabilisators, dem Pufferimgsvermögen des Substrates, der Einwirkungstemperatur und der Jahreszeit zellphysiologisch unterschiedlich. 0,1%ige Lösungen mindern die Wasserpermeabilität der Zellen signifikant und verlängern folglich die Deplasmolysezeit der Protoplasten, Geringere Konzentrationen stören den Deplasmolysevorgang bei den Standortvarianten unterschiedlich. Pufferung auf den Neutralbereich erhöht die Empfindlichkeit der Zellen, wenn selbst nur >0,01 ppm eine halbe Stunde auf die Elodea-Zellen eingewirkt haben. Bei 10 °C ist die Reaktivität des FERROFOS DFL gegenüber 20 °C deutlich vermindert, so daß die Protoplasten auch temperaturdifferent auf die Prüfsubstanz reagieren. Im Winter scheinen die Testpflanzen gegenüber dem Diphosphonsäurepräparat sensibler als während des Sommers zu sein. Veränderungen der Phytotoxizität von Zink-, Kadmium- oder Kupferionen können nicht nachgewiesen werden.     

Zur Erkundung der phytotoxischen Wirkung von Zink,Kadmium und Kupfer auf Elodea canadensis Michx. durch Erfassen der Deplasmolysezeit

Die zur Deplasmolyse von Elodea-Protoplasten erforderliche Zeit kann als Maßstab für die Giftigkeit von Seinvermetallionen dienen, denen die Pflanzen zuvor exponiert Waren, In Modellversuchen sind die Blätter der Testpflanzen vor der experimentellen Plasmolyse mindestens 24 h den zu beurteilenden Tonenkonzentrationen auszusetzen. Zur zufallskritischen Absicherung sollten wenigstens 200 Zellen aus dem subapikalen Blatthereich beobachtet werden, wobei die Blätter etwa 10 cm unterhalb des Vegetationskegels gleichlanger Pflanzen gleichen Entwicklungsstadiums inserieren. Als zytotoxisch erweisen sich >0,005 mg Zn²⁺ 1^?1, ≥0,05 mg Cd²⁺ 1^?1 bzw. ≥0,0005 mg Cu²⁺ 1^?1, bei Kombination schon geringere Mengen. Störende Anioneneffekte treten bei Anwendung von Sulfaten oder Nitraten nicht auf. Die geringsten zytotoxischen Metallgehalte liegen unterhalb bekannter phytotoxischer Konzentrationen. Die zytologische Störung durch solche Grenzkonzentrationen kann von den Pflanzen noch kompensiert werden. Daher ist der Deplasmolysetest nach Schwermetallexposition bei Elodea canadensis eine sehr empfindliche Methode. Ein negativer Ausfall schließt die Anwesenheit phytotoxischer Schwermetallionenkonzentrationen aus. Mit steigender Deplasmolysezeit wird die Anwesenheit phytotoxischer Substanzen immer wahrscheinlicher. Verallgemeinernde Schlußfolgerungen aus den Ergebnissen des Deplasmolysetest s unterliegen einer Reihe von Vorbehalten. So reagieren die verschiedenen Pflanzenarten unterschiedlich auf gleiche Schwermetallbelastungen, differieren die pflanzlichen Resistenzverhältnisse gegenüber Schwermetallionen mit der Jahreszeit, verstärken simultan anwesende verschiedene Schwermetalle den Hemmeffekt und mindert härteres Wasser die Schwermetallgiftigkeit. Da die Kombination dieser Wirkbedingungen stetigem Wechsel unterliegt, haben mittels Deplasmolysetest nach Schwermetallexposition an Elodea canadensis erkundete Toxizitätsgrenzkonzentrationen nur spezifischen Aktualitätswert.     

Landslide-generated Tsunamis: Geotechnical Considerations

— Landslide-induced tsunamis are particularly dangerous because the warning time is very short in the coastal area. Therefore it is critical to identify the potential for coastal landslides involving large masses of material. Almost invariably such large landslides will occur in material susceptible to liquefaction, particularly during earthquakes. This paper discusses characteristics of such materials, how they may be identified, and offers suggestions for estimating the potential for large deformations.