The exhumed Eocene Sultepec-Goleta Volcanic Center of southern Mexico: record of partial collapse and ignimbritic volcanism fed by wide pyroclastic dike complexes     

B. A. Díaz-Bravo  D. J. Morán-Zenteno 《Bulletin of Volcanology》2011,73(7):917-932

The Sultepec-Goleta area in southern Mexico hosts one of the major Eocene silicic volcanic centers that make up the Cenozoic volcanic record of the north-central part of the Sierra Madre del Sur. This center is represented by a partially exhumed NNE-SSW trending ignimbritic field that covers an area of ∼400 km² with a preserved volume of ∼200 km³. A remarkable feature of volcanic center is the exposure of pyroclastic dike complexes up to 1 km wide that extend almost continuously along the western and southern flanks of the volcanic field. The ignimbritic record comprises four units with different textural features as well as variable proportions of the main components (i.e. phenocrysts, pumice and lithic clasts). The most extensive unit is the basal Goleta Ignimbrite, of which the middle and upper members are richer in phenocrysts than the lower member. It has a thickness that ranges from ∼200 m in the north to at least 600 m in the south of the volcanic center. The overlying ignimbrite units (Cienega, Lobera and Potrero ignimbrites) are phenocryst-poor and are distributed in the northern sector of the volcanic field. The semi-curvilinear trend of the pyroclastic dikes along the southern flank of the Goleta Range, coupled with the greater thickness of the Goleta Ignimbrite encircled by the dikes, is indicative of the development of a partial-collapse caldera. Based on spatial relationships as well as analogies in the nature and abundances of the components, it has been recognized that the central and southern dike complexes fed the relatively phenocryst-rich Goleta Ignimbrite, whereas the phenocryst-poor units of the northern sector were extruded through the pyroclastic conduits distributed around Sultepec. We suggest that the stirring produced by the partial collapse of the magma chamber roof beneath the southern sector of the study area triggered the tapping of phenocryst-rich portions of the zoned magma chamber, resulting in an increase of phenocryst content in the upper members of the Goleta Ignimbrite. The presence of breccias and lithic-rich facies dominated by wallrock fragments at the borders of the pyroclastic dikes, including those of the northern extra-caldera sector, indicate that erosion of the conduit walls above the fragmentation level contributed to widening of the conduits and the construction of wide dike complexes.  相似文献   

Evaluating restoration of man‐made slopes: a threshold approach balancing vegetation and rill erosion     

Mariano Moreno‐de las Heras  Rubén Díaz‐Sierra  José M. Nicolau  Miguel A. Zavala 《地球表面变化过程与地形》2011,36(10):1367-1377

The management of reclaimed slopes derived from industrial and civil activities (e.g. surface mining and road construction) requires the development of practical stability analysis approaches that integrate the processes and mechanisms that rule the dynamics of these ubiquitous emerging ecosystems. This work describes a new modelling approach focused on stability analysis of water‐limited reclaimed slopes, where interactive relationships between rill erosion and vegetation regulate ecosystem stability. Our framework reproduces two main groups of possible trends along the temporal evolution of reclaimed slopes: successful trends, characterized by widespread vegetation development and the effective control of rill erosion processes; and gullying trends, characterized by the progressive loss of vegetation and a sharp logistic increase in erosion rates. Furthermore, this analytical approach allows the determination of threshold values for the state variables (i.e. vegetation cover and rill erosion) that drive the system's stability, facilitating the identification of critical situations that require specific human intervention (e.g. revegetation or, in very problematic cases, revegetation combined with rill network destruction) to ensure the long‐term sustainability of the restored ecosystem. The application of our threshold analysis framework in Mediterranean‐dry reclaimed slopes derived from surface coal mining (the Teruel coalfield in central‐eastern Spain) showed a good field‐based performance. Therefore, we believe that this model is a valuable contribution for the management of water‐limited reclaimed systems, including those associated with rill erosion, as it provides a tool for the evaluation of restoration success and can play an important role in decision‐making during ecosystem restoration in severely disturbed landscapes. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

LES of turbulent surface shear stress and pressure-gradient-driven flow on shallow continental shelves     

Guillaume Martinat  Ying Xu  Chester E. Grosch  Andrés E. Tejada-Martínez 《Ocean Dynamics》2011,61(9):1369-1390

Turbulent shear flows on shallow continental shelves (here shallow means that the interaction with the solid, no-slip bottom is important) are of great importance because tide- and wind-driven flows on the shelf are drivers of the transfer of momentum, heat, and mass (gas) across the air–sea interface. These turbulent flows play an important role because vertical mixing and current are vectors for the transport of sediment and bioactive material on continental shelves. Understanding the dynamics of this class of flows presents complications because of the presence of a free surface and also because the flow can be driven by a pressure gradient (a tidal current), a stress at the free surface (a wind-driven current), or a combination of both. In addition, the flow can be modified by the presence of a wave field that can induce Langmuir circulation (Langmuir, Science 87:119–123, 1938). Large eddy simulation is used to quantify the effects of pressure gradient and wind shear on the distinctive structures of the turbulent flow. From these computations, an understanding of the physics governing the turbulence of pressure-driven and wind-driven flows, how they can interact in a normal or a tangential direction, and the effect of wave forcing on these flows is obtained.  相似文献   

Shear wave crustal velocity model of the Western Bohemian Massif from Love wave phase velocity dispersion     

Petr?KolínskyEmail author  Ji?í?Málek  Johana?Broke?ová 《Journal of Seismology》2011,15(1):81-104

We propose a new quantitative determination of shear wave velocities for distinct geological units in the Bohemian Massif, Czech Republic (Central Europe). The phase velocities of fundamental Love wave modes are measured along two long profiles (~200 km) crossing three major geological units and one rift-like structure of the studied region. We have developed a modified version of the classical multiple filtering technique for the frequency-time analysis and we apply it to two-station phase velocity estimation. Tests of both the analysis and inversion are provided. Seismograms of three Aegean Sea earthquakes are analyzed. One of the two profiles is further divided into four shorter sub-profiles. The long profiles yield smooth dispersion curves; while the curves of the sub-profiles have complicated shapes. Dispersion curve undulations are interpreted as period-dependent apparent velocity anomalies caused both by different backazimuths of surface wave propagation and by surface wave mode coupling. An appropriate backazimuth of propagation is found for each period, and the dispersion curves are corrected for this true propagation direction. Both the curves for the long and short profiles are inverted for a 1D shear wave velocity model of the crust. Subsurface shear wave velocities are found to be around 2.9 km/s for all four studied sub-profiles. Two of the profiles crossing the older Moldanubian and Teplá-Barrandian units are characterized by higher velocities of 3.8 km/s in the upper crust while for the Saxothuringian unit we find the velocity slightly lower, around 3.6 km/s at the same depths. We obtain an indication of a shear wave low velocity zone above Moho in the Moldanubian and Teplá-Barrandian units. The area of the Eger Rift (Teplá-Barrandian–Saxothuringian unit contact) is significantly different from all other three units. Low upper crust velocities suggest sedimentary and volcanic filling of the rift as well as fluid activity causing the earthquake swarms. Higher velocities in the lower crust together with weak or even missing Moho implies the upper mantle updoming.  相似文献   

Acigöl rhyolite field,central Anatolia (part II): geochemical and isotopic (Sr–Nd–Pb, δ<Superscript>18</Superscript>O) constraints on volcanism involving two high-silica rhyolite suites     

W.?SiebelEmail author  A.?K.?Schmitt  E.?Kiemele  M.?Dani?ík  F.?Aydin 《Contributions to Mineralogy and Petrology》2011,162(6):1233-1247

The Acigöl rhyolite field erupted the most recent high-silica rhyolites within the Cappadocian Volcanic Province of central Anatolia, Turkey. It comprises two sequences of domes and pyroclastic rocks with eruption ages of ~150–200 ka (eastern group) and ~20–25 ka (western group). Compositionally, the eastern rhyolite group lavas are less evolved (SiO₂ = 74–76 wt%), whereas the western group has higher silica abundance (SiO₂ = ~77 wt%) with extremely depleted feldspar-compatible trace elements. Within each group, compositional variability is small and ¹⁴³Nd/¹⁴⁴Nd (0.51257–0.51265) and Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.87–18.88, ²⁰⁷Pb/²⁰⁴Pb = 15.65–15.67 and ²⁰⁸Pb/²⁰⁴Pb = 38.94–38.98) are homogeneous. The western group rhyolites have δ¹⁸O(zircon) overlapping mantle values (5.7 ± 0.2‰), whereas eastern group rhyolites are enriched in δ¹⁸O by ~0.5‰, consistent with a tendency to lower εNd values. By contrast, western group rhyolites have markedly more radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7065–0.7091) compared to those of the eastern group (0.7059–0.7065). The presence of angular granitic xenoliths and a correlation between hydration (based on loss on ignition data) and ⁸⁷Sr/⁸⁶Sr in the western lavas, however, indicates that Sr was added during the eruption or post-eruption alteration. Isotope constraints preclude the possibility that the rhyolite magmas formed by partial melting of any known regional crystalline basement rocks. Basalts and andesites erupted in the periphery of the Acigöl field are characterised by ⁸⁷Sr/⁸⁶Sr ratios between 0.7040 and 0.7053, ¹⁴³Nd/¹⁴⁴Nd = 0.51259–0.51300, ²⁰⁶Pb/²⁰⁴Pb = 18.85–18.87, ²⁰⁷Pb/²⁰⁴Pb = 15.646–15.655, ²⁰⁸Pb/²⁰⁴Pb = 38.90–38.97. The isotopic and trace element data favour an origin of the rhyolites by mixing of basaltic/andesitic magmas with minor amounts of crustal melts and followed by extensive fractional crystallization.  相似文献   

Barium-rich fluids and melts in a subduction environment (La Corea and Sierra del Convento mélanges,eastern Cuba)     

Idael Francisco Blanco-Quintero  C. Lázaro  A. García-Casco  J. A. Proenza  Y. Rojas-Agramonte 《Contributions to Mineralogy and Petrology》2011,162(2):395-413

Whole-rock compositions of muscovite-bearing amphibolite, trondhjemite, pegmatite and quartz-muscovite rocks from Sierra del Convento and La Corea mélanges (eastern Cuba), as well as mineralogy, record complex circulation of Ba-rich fluids and melts in the subduction environment. Partial melting of fluid-fluxed, MORB-derived amphibolite produced trondhjemite magmas that crystallized at depth, in some cases evolving into pegmatites. Qtz-Ms rocks probably crystallized from primary fluids derived from subducted sediments. All these rocks have elevated concentrations of large-ion lithophile elements, especially Ba (up to 11,810 ppm), presumably released from slab sediments by fluids and/or melts. Fluid–rock interaction produced crystallization of phengite in parental amphibolites. The phengite crystallized in all types of rocks is rich in Ba, with concentric zoning, characterized by Ba-rich cores and Ba-poor rims, indicating a compatible behaviour of Ba in the studied systems. Zoning in phengite is governed primarily by the celadonite (tschermak) exchange vector ((Mg,Fe)Si-(Al)_-2), with more moderate contributions of the celsian (BaAl-(KSi)_-1) and paragonite (NaK_-1) exchange vectors. Late remobilization of Ba at relatively low temperature formed retrograde celsian. The compatible behaviour of Ba in the studied rocks strengthens the importance of the stability of phengite for the transfer of LIL elements from the subduction to the volcanic arc environments.  相似文献   

Leaching of metals from copper smelter flue dust (Mufulira,Zambian Copperbelt)     

Martina Vítková  Vojtěch Ettler  Jiri Hyks  Thomas Astrup  Bohdan Kříbek 《Applied Geochemistry》2011

The leaching behaviour of electrostatic precipitator dust from the Mufulira Cu smelter (Copperbelt, Zambia) was studied using a 48-h pH-static leaching experiment (CEN/TS 14997). The release of metals (Cd, Co, Cu, Ni, Pb and Zn) and changes in mineralogical composition using X-ray diffraction and PHREEQC-2 modelling were investigated in the pH range of 3–7. The highest concentrations of metals were released at pH 3–4.5, which encompasses the natural pH of the dust suspension (～4.3). About 40% of the total Cu was leached at pH 3, yielding 107 g/kg. Chalcanthite (CuSO₄·5H₂O), magnetite (Fe₃O₄) and delafossite (CuFeO₂) represented the principal phases of the studied dust. In contact with water, chalcanthite was dissolved and hydrated Cu sulphates precipitated at pH 4–7. Gypsum (CaSO₄·2H₂O) and secondary Fe or Al phases were observed in the leached residues. Serious environmental impact due to leaching may occur in dust-contaminated soil systems in the vicinity of the smelting plants.  相似文献   

A geologic study of the Juan del Grijalva landslide,the most important mass movement during the last century in Mexico     

Moisés Dávila Serrano  Oscar Jiménez 《Natural Hazards》2011,58(1):183-197

On November 4th 2007, along the Grijalva River in the state of Chiapas, Mexico, has occurred one of the largest landslides ever known. This landslide, known as Juan del Grijalva, destroyed the town of the same name, killing 20 people, and moved 55 million cubic meters of rock and debris down slope to completely block the Grijalva River. In order to understand the characteristics and factors that triggered the Juan del Grijalva landslide, geologic studies were conducted at the site. The results indicate that the landslide was composed of a lithologic sequence of thin-bedded shales and thin to medium-thick-bedded sandstones. This was faulted into several blocks dipping in the same sense as the mass movement. The main triggering factor was the increment of the pore pressure into the lithologic unit due to water saturation after 5 days of heavy rain before the incident. According to records from the last century, the Juan del Grijalva mass movement represents one of the largest mass movements recorded all over the world. The risk conditions of the area after the landslide lead to the rapid construction of an artificial channel to drain the accumulating mass of water upstream and therefore prevent a future catastrophic inundation down stream.  相似文献   

Structure and stratigraphy of the Teplá–Barrandian Neoproterozoic,Bohemian Massif: A new plate-tectonic reinterpretation     

Jaroslava Hajná  Jiří Žák  Václav Kachlík 《Gondwana Research》2011,19(2):495-508

The Teplá–Barrandian unit (TBU) of the Bohemian Massif exposes a section across the once extensive Avalonian–Cadomian belt, which bordered the northern active margin of Gondwana during late Neoproterozoic. This paper synthesizes the state-of-the-art knowledge on the Cadomian basement of the TBU to redefine its principal component units, to revise an outdated stratigraphic scheme, and to interpret this scheme in terms of a recent plate-tectonic model for the Cadomian orogeny in the Bohemian Massif. The main emphasis of this paper is on an area between two newly defined fronts of the Variscan pervasive deformation to the NW and SE of the Barrandian Lower Paleozoic overlap successions. This area has escaped the pervasive Variscan (late Devonian to early Carboniferous) ductile reworking and a section through the Cadomian orogen is here superbly preserved.The NW segment of the TBU consists of three juxtaposed allochthonous belts of unknown stratigraphic relation (the Kralovice–Rakovník, Radnice–Kralupy, and Zbiroh–?árka belts), differing in lithology, complex internal strain patterns, and containing sedimentary and tectonic mélanges with blocks of diverse ocean floor (meta-)basalts. We summarize these three belts under a new term the Blovice complex, which we believe represents a part of an accretionary wedge of the Cadomian orogen.The SE segment of the TBU exposes the narrow Pi?ín belt, which is probably a continuation of the Blovice complex from beneath the Barrandian Lower Paleozoic, and a volcanic arc sequence (the Davle Group). Their stratigraphic relation is unknown. Flysch units (the ?těchovice Group and Svrchnice Formation) overlay the arc volcanics, and both units contain material derived from volcanic arc. The former was also sourced from the NW segment, whereas the latter contains an increased amount of passive margin continental material. In contrast to the Blovice complex, the flysch experienced only weak Cadomian deformation.The new lithotectonic zonation fits the following tectonic scenario for the Cadomian evolution of the TBU well. The S- to SE-directed Cadomian subduction beneath the TBU led to the involvement of turbidites, chaotic deposits, and 605 ± 39 Ma ocean floor in the accretionary wedge represented by the Blovice complex. The accretionary wedge formation mostly overlapped temporally with the growth of the volcanic arc (the Davle Group) at ~ 620–560 Ma. Upon cessation of the arc igneous activity, the rear of the wedge and some elevated portions of the arc were eroded to supply the deep-water flysch sequences of the ?těchovice Group, whereas the comparable Svrchnice Formation (~ 560 to < 544 Ma) was deposited in a southeasterly remnant basin close to the continental margin. The Cadomian orogeny in the TBU was terminated at ~ 550–540 Ma by slab breakoff, by final attachment of the most outboard ~ 540 Ma oceanic crust, and by intrusion of ~ 544–524 Ma boninite dikes marking the transition from the destructive to transform margin during the early/middle Cambrian.  相似文献   

Recent plumbing system of the Krakatau volcano revealed by teleseismic earthquake distribution     

Aleš Špičák  Jiří Vaněk  Václav Hanuš 《International Journal of Earth Sciences》2011,100(6):1375-1381

Spatial and temporal analysis of global seismological data 1964–2005 reveals a distinct teleseismic earthquake activity producing a columnar-like formation in the continental wedge between the Krakatau volcano at the surface and the subducting slab of the Indo-Australian plate. These earthquakes occur continuously in time, are in the body-wave (m _b) magnitude range 4.5–5.3 and in the depth range 1–100 km. The Krakatau earthquake cluster is vertical and elongated in the azimuth N30°E, suggesting existence of a deep-rooted fault zone cutting the Sunda Strait in the SSW-NNE direction. Possible continuation of the fault zone in the SW direction was activated by an intensive 2002/2003 aftershock sequence, elongated in the azimuth of N55°E. Beneath the Krakatau earthquake cluster, an aseismic gap exists in the Wadati-Benioff zone of the subducting plate at the depths 100–120 km. We interpret this aseismic gap as a consequence of partial melting inhibiting stress concentration necessary to generate stronger earthquakes, whereas the numerous earthquakes observed in the overlying lithospheric wedge beneath the volcano probably reflect magma ascent in the recent plumbing system of the Krakatau volcano. Focal depth of the deepest events (~100 km) of the Krakatau cluster constrains the location of the primary magma generation to greater depths. The ascending magmatic fluids stress fault segments within the Sunda Strait fault zone and change their friction parameters inducing the observed tectonic earthquakes beneath Krakatau.  相似文献