GIS-based assessment of landslide susceptibility on the base of the Weights-of-Evidence model

The major scope of the study is the assessment of landslide susceptibility of Flysch areas including the Penninic Klippen in the Vienna Forest (Lower Austria) by means of Geographical Information System (GIS)-based modelling. A statistical/probabilistic method, referred to as Weights-of-Evidence (WofE), is applied in a GIS environment in order to derive quantitative spatial information on the predisposition to landslides. While previous research in this area concentrated on local geomorphological, pedological and slope stability analyses, the present study is carried out at a regional level. The results of the modelling emphasise the relevance of clay shale zones within the Flysch formations for the occurrence of landslides. Moreover, the distribution of mass movements is closely connected to the fault system and nappe boundaries. An increased frequency of landslides is observed in the proximity to drainage lines, which can change to torrential conditions after heavy rainfall. Furthermore, landslide susceptibility is enhanced on N-W facing slopes, which are exposed to the prevailing direction of wind and rainfall. Both of the latter geofactors indirectly show the major importance of the hydrological conditions, in particular, of precipitation and surface runoff, for the occurrence of mass movements in the study area. Model performance was checked with an independent validation set of landslides, which are not used in the model. An area of 15% of the susceptibility map, classified as highly susceptible, “predicted” 40% of the landslides.     

Southern Westerlies during the last glacial maximum

Vegetation and climate over approximately the past 13,000 yr are reconstructed from fossil pollen in a 9.4-m mire section at Caleta Róbalo on Beagle Channel, Isla Navarino (54°56′S, 67°38′W), southern Tierra del Fuego. Fifty surface samples reflecting modern pollen dispersal serve to interpret the record. Chronologically controlled by nine radiocarbon dates, fossil pollen assemblages are: Empetrum-Gramineae-Gunnera-Tubuliflorae (zone 3b, 13,000–11,850 yr B.P.), Gramineae-Empetrum-assorted minor taxa (zone 3a, 11,850-10,000 yr B.P.), Nothofagus-Gramineae-Tubuliflorae-Polypodiaceae (zone 2, 10,000–5000 yr B.P.), Nothofagus-Empetrum (zone 1b, 5000-3000 yr B.P.), and Empetrum-Nothofagus (zone 1a, 3000-0 yr B.P.). Assemblages show tundra under a cold, dry climate (zone 3), followed by open woodland (zone 2), as conditions became warmer and less dry, and later, with greater humidity and lower temperatures, by closed forest and the spread of mires (zone 1). Comparisons drawn with records from Antarctica, New Zealand, Tasmania, and the subantarctic islands demonstrate broadly uniform conditions in the circumpolar Southern Hemisphere. The influences of continental and maritime antarctic air masses were apparently considerable in Tierra del Fuego during cold late-glacial time, whereas Holocene climate was largely regulated by interplay between maritime polar and maritime tropical air.     

Late Pleistocene Vertebrates and Other Fossils from Epiguruk, Northwestern Alaska

Sediments exposed at Epiguruk, a large cutbank on the Kobuk River about 170 km inland from Kotzebue Sound, record multiple episodes of glacial-age alluviation followed by interstadial downcutting and formation of paleosols. Vertebrate remains from Epiguruk include mammoth, bison, caribou, an equid, a canid, arctic ground squirrel, lemmings, and voles. Radiocarbon ages of bone validated by concordant ages of peat and wood span the interval between about 37,000 and 14,000 yr B.P. The late Pleistocene pollen record is dominated by Cyperaceae, with Artemisia, Salix, Betula, and Gramineae also generally abundant. The fossil record from Epiguruk indicates that the Kobuk River valley supported tundra vegetation with abundant riparian willows during middle and late Wisconsin time. Large herbivores were present during the height of late Wisconsin glaciation as well as during its waning stage and the preceding interstadial interval. The Kobuk River valley would have been a favorable refugium for plants, animals, and possibly humans throughout the last glaciation.     

Petrology of Peter I Øy (Peter I Island), West Antarctica

Peter I Øy is located in the Bellinghausen Sea, 400 km NE of Thurston Island, West Antarctica. It is a Pleistocene volcanic island situated adjacent to a former tranform fault on the continental rise of the presently passive margin between the Pacific and Antarctica. New K-Ar age determinations ranging from 0.1 to 0.35 Ma show that the volcanism responsible for this island took place at the same time as post-subduction, rift-related volcanism occurred in the nearby Marie Byrd Land and the Antarctic Peninsula. The rocks of the island are alkalic basalt and hawaiite, benmoreite and trachyte. The basic tocks typically contain phenocrysts of olivine (Fo_61–84), diopsidic augite, and plagioclase (ca. An₆₀). Small xenoliths are present and consist of mantle-type spinel lherzolite, cumulate clinopyroxenite and gabbro and felsic inclusions that consist of medium-grained strained quartz, plagioclase, and abundant colorless glass. Chemically, the basic rocks are characterized by rather high MgO (7.8–10.2 wt.%) and TiO₂ (3.1–3.7 wt.%) and relatively low CaO (8.4–9.5 wt.%) contents. They have steep REE patterns, [(La/Yb)_N = 20] with HREE only 5 x chrondrite. Y and Sc are almost constant at relatively low levels. Compatible trace elements such as Ni and Cr show considerable variation (190–300 and 150–470 ppm, respectively.), whereas V shows only little variation. Sr and Nd isotope ratios vary slightly with ⁸⁷Sr/⁸⁶Sr averaging 0.70388 and ¹⁴³Nd/¹⁴⁴Nd 0.512782, both typical for ocean island volcanism. Lead isotope ratios are consistently high in basalts; ²⁰⁶Pb/²⁰⁴Pb = 19.194, ²⁰⁷Pb/²⁰⁴Pb = 15.728 and ²⁰⁸Pb/²⁰⁴Pb = 39.290, whereas benmoreïte is somewhat less radiogenic. Oxygen isotope analyses average δ¹⁸O = +6.0‰. Incompatible trace elements vary by a factor of 1.5–2.0 within the range of the basic rocks. It is proposed that the incompatible trace-element variations represent different degrees (<10%) of partial melting, and that these melts were later modified by minor (<15‰) olivine and spinel fractionation. The very small variation in Y (and Sc) and the very fractionated REE pattern indicate that the source had an Y- and HREE-rich residual phase, most probably garnet. Furthermore, it is suggested that the source was slightly hydrous and that melting took place at 18–20 kbar. Trachyte was derived by multiphase fractionation of ne-normative basalts, and benmoreite from hy-normative parental liquids. The rocks of Peter I Øy are generally of the same type and age as those outcropping in extensional regimes on the nearby continent, and therefore, these occurrences may be related to each other in some way. However, the Peter I Øy rocks are considerably more radiogenic in strontium and less radiogenic in neodymium than the rocks of the Antarctic Peninsula and Marie Byrd Land. Possible explanations are that Peter I Øy represent asthenospheric hot spot activity, or transtensional rifting as subduction ceased.     

Depositional mechanics of turbulent nuées ardentes (surges) from their grain sizes

The ability of turbulent nuées ardentes (surges) to transport coarse pyroclasts has been questioned on the basis that settling velocities of coarse fragments in the deposits are much too high for them to have been supported by turbulence in a dilute gas suspension. A computer model is used to evaluate the settling velocity of pyroclasts in suspensions of varying concentration and temperature. Since suspension of grains in low-concentration surges occurs if the shear velocity exceeds the settling velocity, the shear velocities related to the 16th and 84th percentiles, and the mean of the grain-size distribution are compared in surge deposits of the Vulsini, with the shear velocity necessary to move the coarsest grain on the bed surface (the Shields criterion). The results show that the settling velocities do not vary significantly in gaseous suspensions having volume concentrations lower than 15%, and that an increase in concentration to 25% is not sufficient to decrease the settling velocity of the coarser fraction, if it represents flow shear velocity. It is shown that the settling velocity of the mean grain size (M _z ) best depicts the shear velocity of a dilute turbulent suspension. Applying the results to the May 1902 paroxysmal nuées ardentes of Mount Pelée shows that the estimated mean velocities are well within the observed velocities, and sufficient to support all the clasts in dilute, turbulent suspensions.     

Element Mobility During Incipient Granulite Formation at Kabbaldurga, Southern India

At Kabbaldurga, infiltration of carbonic fluids along a systemof ductile shears and foliation planes has led to partial transformationof Archaean grey biotite–hornblende gneiss to coarse-grainedmassive charnockite at about 2.5 b.y. ago. The dehydration ofthe gneiss assemblage was induced by a marked metasomatic changeof the reacting system from granodioritic to granitic, and obviouslytook place under conditions of an open system at 700–750?C and 5–7 kb. Extensive replacement of plagioclase (An_16–30)by K-feldspar through Na, Ca–K exchange reactions withthe ascending carbonic fluids led to strong enrichment in K,Rb, Ba, and SiO₂, and to a depletion in Ca. Progressive dissolutionof hornblende, biotite, magnetite, and the accessory mineralsapatite and zircon resulted in a marked depletion in Fe, Mg,Ti, Zn, V, P, and Zr. Most important is the recognition of REEmobility: with advancing charnockitization, the moderately fractionatedREE distribution patterns of the grey gneisses (La_N270; La_N/Yb_N= 5–20; Eu_N27; Eu/Eu^* = 0.6–0.3) give way to stronglyfractionated REE patterns with a positive Eu-anomaly (La_N200;La_N/Yb_N = 20–80; Eu_N22; Eu/Eu^* = 0.6–1.8). The systematicdepletion especially in the HREE is due to the progressive dissolutionof zircon, apatite (and monazite), which strongly concentratethe REE. Stable isotope data (¹⁸O of 6.9–8.0 per mille for gneissesand charnockites; ¹³C of –8.5 and –6.5 per millefor late carbonate) indicate a magmatogenic source for the carbonicfluids. In contrast to the currently favoured derivation ofcarbonic fluids by decarbonation of the upper mantle or degassingof underplated basaltic intrusions, it is discussed here thatabundant fluid inclusions in lower crustal charnockites providedan extensive reservoir of ‘fossil’ carbonic fluids.Shear deformation has tapped this reservoir and generated thechannel-ways for fluid ascent. Charnockitization of the Kabbaldurgatypethus appears to be a metasomatic process which is tectonicallycontrolled and restricted to the crustal level of the amphiboliteto granulite transition.     

Geochemistry of the Sweetwater Wash Pluton,California: Implications for “anomalous” trace element behavior during differentiation of felsic magmas

Field relations, mineralogy and major and trace element data for the very felsic, peraluminous Sweetwater Wash pluton establish a differentiation sequence dominantly controlled by fractional crystallization processes. Elements Ba and Sr show depletion by factors of 50–60X from the earliest granite unit analyzed to the late-stage pegmatites and aplites. The strong Ba depletion is largely due to the partitioning behavior of this element in K-feldspar, while the Sr depletion is due to the combined effects of the two feldspars. The 4-fold increase in Rb during crystallization is also predictable from mineral/ melt partition coefficients.Coefficients for the light rare-earth elements (LREE) in major mineral species predict that these elements should behave incompatibly during crystallization and increase with fractionation. In fact, the LREE abundances decrease by a factor of 10–20X during crystallization. This anomalous behavior is commonly observed in felsic plutonic and volcanic sequences. In the Sweetwater Wash pluton monazite occurs in minute quantities, but it is sufficiently abundant to govern the partitioning of LREE and Th during crystallization. Petrographic observations indicate that monazite was on the liquidus throughout most of the crystallization. Analyses of silicate mineral separates suggest that the monazite contains more than 75% of the LREE in the whole rocks.Fractionation of REE-rich accessories (in particular monazite) from felsic magmas may be the general cause of REE depletion during differentiation of these melts. Monazite can easily be mistaken for zircon and, because it typically contains 50% LREE, extremely minute and easily overlooked quantities of monazite can control LREE abundances.     

Tectono-thermal modeling of the Yinggehai Basin,South China Sea

Based on the observed data, the average value of surface heat flow in the Yinggehai Basin is calculated and it turns out to be 84.1 mW/m². The thermal evolution of the basin since the Cenozoic era has been attempted by tectono-thermal modeling. Three-phase extension made the basin become hotter and hotter, reaching its climax in paleo-temperature history since 5.2 Ma. And nowadays, the basin is in the heat flow decreasing period. During the Cenozoic era, the basement heat flow remained at 50–70 mW/m² all the time. This is related to the degree of each extension phase, stretching rate mode and also the limited basin scale. Modeling results also show that, the surface heat flow is controlled mainly by the basement heat flow, and less than 20% comes from radiogenic heat production in the sediments of the basin