Implications of management strategies and vegetation change in the Mount St. Helens blast zone

The 1980 eruptions of Mount St. Helens provided an excellent opportunity for scientists to investigate the recovery of vegetation communities following a major geologic disturbance. An important and often overlooked aspect in these studies is the human factor in recovery processes, and specifically, the different management approaches taken towards re-establishment of vegetation on lands under the control of various owners. This study examines vegetation changes throughout the 1980 blast zone using a time series of Landsat-derived Normalized Difference Vegetation Index (NDVI) images and change detection methods to assess the changes over 25 years, from 1980 to 2005, as a function of human management combined with ecological factors. This long-term tracking of change indicates that differences in the speed of vegetation re-establishment and consequent rates of change substantially reflect human involvement and varying management strategies.     

New evidence for Jurassic continental rifting in the northern Sanandaj Sirjan Zone,western Iran: the Ghalaylan seamount,southwest Ghorveh

ABSTRACT

We address the growing controversy about the tectonic setting in which Jurassic magmatism of Iran occurred: arc or continental rift. In the Ghorveh area of the northern Sanandaj Sirjan zone (SaSZ), the Ghalayan metabasites are interlayered with marble and schist and locally cut by acidic dikes. Zircon U-Pb dating of the metabasitic rocks shows that these crystallized at ca. 145–144 Ma ago in the Late Jurassic (Tithonian). This complex was metamorphosed in the lower greenschist facies, however, some protolithic structures such as pillow lava and primary minerals are preserved. The metabasites are tholeiites with low SiO₂ (45.6–50.5 wt.%), moderate Al₂O₃ (11.3–17.0 wt.%), and high TiO₂ (0.7–2.9 wt.%) and Fe₂O₃ (9.4–14.1 wt.%). The Ghalayan metabasites are enriched in Light rare earth elements (LREEs) without significant Nb, Ta, Pb, Sr and Ba anomalies, similar to modern continental intra-plate tholeiitic basalts such as Afar and East African rifts. The Ghalaylan metabasites show wide ranges for ⁸⁷Sr/⁸⁶Sr_(i) (0.7039–0.7077) and positive ε_Nd(t) values (+0.1 to +4.6). These isotopic compositions are similar to those expected for slightly depleted subcontinental lithospheric mantle sources. Independently built discrimination diagrams indicate an intra-continental rifting regime for the source of Jurassic metabasites in the northern SaSZ. Geochemical and tectonic evidence suggests that rifting or a mantle plume was responsible for volcanic activity in the Upper Jurassic SaSZ. Considering the variation of ages of basaltic volcanism along the SaSZ, we suggest that Ghalayan basaltic magmatism reflected a submarine volcano that formed as part of the late stage continental rift, similar to Afar in the East African Rift system. Our results indicate that an extensional tectonic regime dominated SaSZ tectonics in the Middle to Late Jurassic.     

Surface runoff in a torrent catchment area in Middle Europe and its prevention

The Schesa, a sinister contributory torrent to the Ill river near Bludenz (federal province of Vorarlberg) is the largest basin-shaped gully of Middle Europe and endangers the underlying villages by torrential debris flow and gigantic mass movements. The catchment is characterized by a complex geological situation, high annual precipitation and torrential rains from spring to early autumn, which cause enormous amounts of surface runoff. Based on field investigations comprising rain simulation experiments on representative plots, investigations on land-use, vegetation cover, soil physical characteristics, geology, hydrogeology and other features of the catchment area, surface runoff coefficient maps were developed. They formed the basis for assessment of runoff potential for different scenarios in vegetation cover and land-use intensity. Calculation of runoff for the recurrent design event by use of an improved run-time method showed the urgent necessity of runoff reduction measures in large parts of the catchment area above the gully. Based on the modelling results a concept for reduction of both, surface runoff and amount of deep percolating water has been elaborated.     

The role of in situ stress in determining hydraulic connectivity in a fractured rock aquifer (Australia)

Fracture network connectivity is a spatially variable property that is difficult to quantify from standard hydrogeological datasets. This critical property is related to the distributions of fracture density, orientation, dimensions, intersections, apertures and roughness. These features that determine the inherent connectivity of a fracture network can be modified by secondary processes including weathering, uplift and unloading and other mechanisms that lead to fracture deformation in response to in situ stress. This study focussed on a fractured rock aquifer in the Clare Valley, South Australia, and found that fracture network connectivity could be discriminated from several geological, geophysical and hydrogeological field datasets at various scales including single well and local- to regional-scale data. Representative hydromechanical models of the field site were not only consistent with field observations but also highlighted the strong influence of in situ stress in determining the distribution of fracture hydraulic apertures and the formation of hydraulic chokes that impede fluid flow. The results of this multi-disciplinary investigation support the notion that the hydraulic conductivity of a fracture network is limited to the least hydraulically conductive interconnected fractures, which imposes a physical limit on the bulk hydraulic conductivity of a fractured rock aquifer.     

Stable isotope evidence for regional-scale fluid migration in a Barrovian metamorphic terrane,Vermont, USA

We measured the C- and O-isotopic composition of carbonate minerals in the Waits River Formation, eastern Vermont, to determine the extent of fluid infiltration during regional metamorphism over an approx. 2000 km² area in a deep-seated (>25 km) Barrovian terrane. From a petrologic study of this terrane, Ferry proposed the existence of a large regional metamorphic hydrothermal system with two first-order features: (1) on the scale of the entire terrane fluid flow was focused into the axes of two major antiforms of regional extent; (2) on a smaller scale (about 100 km²) flow was further focused around synmetamorphic granitic plutons that intruded along the axes of the antiforms. We find isotopic evidence for both the regional hydrothermal activity along the antiforms and the more intense fluid flow around synmetamorphic plutons. The evidence for hydrothermal activity around the plutons is large heavy isotope depletions, up to 6–9 in ¹⁸O_carb and ¹³C_carb, in diopside zone rocks adjacent to the plutons. These isotopic shifts are greater than can be explained solely by prograde metamorphic reactions. We find two lines of evidence for the more diffuse regional flow that was focused into axes of the antiforms. First, ¹⁸O_carb and ¹³C_carb, within individual outcrops become increasingly homogeneous with increasing grade towards antiform axes, indicating that the rocks equilibrated with a permeating fluid. Second, there are depletions in ¹⁸O near the margins of the Waits River Formation which can be interpreted as a dispersed, advective infiltration front displaced toward the antiform axes. These fronts were modelled using Eq. 13 of Bickle and Baker and imply time-integrated fluid of 10⁵–10⁶ cm³/cm², which are consistent with values derived by Ferry from measured progress of prograde devolatilization reactions.     

Composition, age, and origin of the ~620?Ma Humr Akarim and Humrat Mukbid A-type granites: no evidence for pre-Neoproterozoic basement in the Eastern Desert, Egypt

The Humr Akarim and Humrat Mukbid plutons, in the central Eastern Desert of Egypt, are late Neoproterozoic post-collisional alkaline A-type granites. Humr Akarim and Humrat Mukbid plutonic rocks consist of subsolvus alkali granites and a subordinate roof facies of albite granite, which hosts greisen and Sn–Mo-mineralized quartz veins; textural and field evidence strongly suggest the presence of late magmatic F-rich fluids. The granites are Si-alkali rich, Mg–Ca–Ti poor with high Rb/Sr (20–123), and low K/Rb (27–65). They are enriched in high field strength elements (e.g., Nb, Ta, Zr, Y, U, Th) and heavy rare earth elements (La _n /Yb _n ?=?0.27–0.95) and exhibit significant tetrad effects in REE patterns. These geochemical attributes indicate that granite trace element distribution was controlled by crystal fractionation as well as interaction with fluorine-rich magmatic fluids. U–Pb SHRIMP zircon dating indicates an age of ~630–620?Ma but with abundant evidence that zircons were affected by late corrosive fluids (e.g., discordance, high common Pb). εNd at 620?Ma ranges from +3.4 to +6.8 (mean?=?+5.0) for Humr Akarim granitic rocks and from +4.8 to +7.5 (mean?=?+5.8) for Humrat Mukbid granitic rocks. Some slightly older zircons (~740?Ma, 703?Ma) may have been inherited from older granites in the region. Our U–Pb zircon data and Nd isotope results indicate a juvenile magma source of Neoproterozoic age like that responsible for forming most other ANS crust and refute previous conclusions that pre-Neoproterozoic continental crust was involved in the generation of the studied granites.     

Transformative left-wing parties’ and grassroots organizations: Unpacking the politics of “top-down” and “bottom-up” development

Academic analyses of the potential for agents to transform development processes have been dominated by social movement theorists who focus on the prospects for creating an alternative to development that challenges the status quo. This has downplayed the role of political parties in the transformation process. This article takes the South African Communist Party (SACP) as a unit of analysis by drawing from a case study in Alexandra township, Johannesburg, where the local SACP has assisted with mobilizing a community-based organization in an informal settlement called S’swetla where the ruling African National Congress (ANC) purportedly imposed development onto residents. The local SACP viewed its intervention as pro-poor and bottom-up. It appeared initially to offer a transformative alternative to the official approach taken by the Alexandra Development Forum (ADF), an invited participatory space adopted by the Alexandra Renewal Project (ARP) – a flagship project of the ANC. This paper uses this example to problematize the simplistic dichotomy between top-down and bottom-up development in the context of a political party that claims to be committed to pro-poor and people-driven development. In doing so, it argues that theorists must pay closer empirical attention to the politics of both invited and invented participatory spaces in order to understand the implications that this has for transforming development.     

Petrogenesis of a Late Precambrian (575–600 Ma) bimodal suite in Northeast Africa

Late Precambrian crustal evolution in the North Eastern Desert of Egypt occurred in a strongly extensional tectonic environment and was accompanied by abundant bimodal igneous activity. The extrusive and intrusive expressions of this magmatism, known as the Dokhan Volcanics and Pink Granites, respectively, were studied in detail from two areas. The Dokhan Volcanics and associated feeder dikes consist of a mafic suite dominated by andesites (60% SiO₂) and smaller volumes of basalt and a felsic suite composed of rhyolite tuffs, ignimbrites and hypabyssal intrusions (72–78% SiO₂). The rocks of the mafic suite display calc-alkaline trends on an AFM diagram but are enriched in incompatibles such as TiO₂, P₂O₅, K₂O, Rb, Sr, Ba, Zr, Y, Nb, and LREE. Rare earth element patterns are steep, with (Ce/Yb)_n = 7.7 to 16.8. They contain moderate Ni (60 ppm) and Cr (95 ppm), indicating limited low-P fractionation. The melts of the mafic suite are interpreted to have formed either by 25% batch melting of eclogite or by 10% batch melting of LREE-enriched garnet lherzolite. The rocks of the felsic suite include Dokhan rhyolites and the epizonal Pink Granites. These contain 72–78% SiO₂, are metaluminous and peraluminous, and have the high K₂O/Na₂O and FeO^*/(FeO^*+MgO) characteristic of post-tectonic, A-type granites. They are moderately enriched in incompatible elements, but their REE patterns overlap with those of the mafic suite, from which they can be distinguished by deep europium anomalies (Eu/Eu^*=0.08–0.64) and flat HREE patterns=((Yb/Er)_n=0.90–1.16). They share with the rocks of the mafic suite isotopic characteristics of depleted mantle, precluding anatexis of much older continental crust. The europium anomalies covary with Sr contents and indicate that plagioclase control was important, while the flat HREE patterns preclude residual garnet in the source. Hence the felsic melts could not have formed by anatexis of garnet-bearing mafic lower crust. Such melts could have formed by anatexis of amphibolite-facies crust, an interpretation which is not favored because the melts are not saturated with P₂O₅. Alternatively, the felsic melts may have formed via low-P fractional crystallization of the mafic melts, with about 2/3 removal of mostly plagioclase and amphibole along with minor apatite and zircon. This may have been accompanied in the latest stages of magmatic evolution by liquid-state fractionation such as thermo-gravitational diffusion or halide complexing.