Pliocene lahar deposits in the Coastal Cordillera of central Chile: Implications for uplift, avalanche deposits, and porphyry copper systems in the Main Andean Cordillera

Lahar deposits occur within a shallow marine sedimentary succession of the Pliocene La Cueva Formation in the Coastal Cordillera of central Chile (33°40′–34°15′S). Provenance studies of the abundant volcanic material in the lahar deposits suggest that they derive from denudation by mass wasting of Oligocene–Miocene volcanic rocks on the western slopes of the Main Andean Cordillera at the same latitude. Pliocene rock debris deposits preserved in the region of El Teniente (34°S) and scattered along the westernmost part of the Andes of central Chile indicate catastrophic erosive events related to the rapid uplift of the cordilleran block. This rock debris was deposited by avalanches and transformed further downslope into lahars by dilution with stream water. Lahars were channeled along the ancient drainage system that reached a shallow Pliocene sea at the site of the present Coastal Cordillera. The exceedingly rapid exhumation of active porphyry systems during the Early Pliocene in this part of the Andes may have played a role in affecting hydrothermal processes, brecciation, and diatreme formation at the porphyry systems of El Teniente and Río Blanco–Los Bronces.     

Composition,Abundance, and Life History of Mysids (Crustacea: Mysida) in the Coastal Lagoons of MD,USA

The composition, abundance, biomass, and life history of mysid species were investigated and described for the first time in the Maryland Coastal Bays (38° N, 75° W), Mid-Western Atlantic, using data collected from 2010 to 2013. Three species of mysids were collected, with Neomysis americana being the most abundant species (maximum mean abundance 6.7 ± 6.4 numbers (nos.) m^?2 in July 2013 and biomass 2.78 ± 2.76-mg dry weight (DW) m^?2 in July 2012). Americamysis bahia was the second most abundant species (maximum mean abundance: 0.7 ± 0.4 nos. m^?2 and biomass: 0.23 ± 0.14 mg DW m^?2 in March 2012). Metamysidopsis swifti made up 0.02 to 2 % of mysids and were found in samples collected mainly from southern Chincoteague Bay close to that Bay’s inlet in the fall of 2012. The two most abundant mysid species reproduced continuously from March to July (Neomysis) and May to October (Americamysis). N. americana had larger body and brood sizes than A. bahia. Mysids were relatively low in abundance in late summer, a period of relatively high biomass of fish predators, than during other seasons, suggesting that intense predation might be controlling their abundance. The increase in mysid abundance in the fall following their disappearance in late summer without evidence of reproductive activities suggests species migration from coastal waters into the Maryland Coastal Bays. This annual mysid subsidy perhaps helps to sustain their populations within the bays.     

Evaluation of an anisotropic elastoplastic–viscoplastic bounding surface model for clays

An anisotropic time-dependent bounding surface model for clays is developed by generalizing a previous time-independent model that adopts a flexible bounding surface. It is based on the framework for coupled elastoplasticity–viscoplasticity for clays and Perzyna’s overstress theory. Three viscoplastic parameters were introduced and explained in detail. The model was validated against undrained creep tests for both isotropically and anisotropically consolidated clays, undrained and drained stress relaxation tests on some undisturbed clays, and undrained triaxial tests with varying strain rates on natural Hong Kong marine deposit clay. The general agreement between the model simulations and test results was satisfactory. The varying effects of lower-level parameters were discussed on the undrained multistage stress relaxation response for normally consolidated soils which had been ignored in literature. The flexibility of the model in capturing the shear strengths, which is the unique feature of the current model, was shown in the simulations of time-dependent triaxial tests on Taipei silty clay. All the simulations show that the proposed model is a relatively practical model considering both anisotropy and time dependency of clays.     

Influence of Potsdam sandstone on the trace element signatures of some 19th‐century American and Canadian glass: Redwood,Redford, Mallorytown,and Como–Hudson

Potsdam sandstone from quarries and outcrops near 19th‐century glassworks sites in Redwood, NY, and Saranac, NY, Mallorytown, ON, and Como and Hudson, QC, commonly contains _97% silica, so in terms of its purity can compete with other historical producers of silica sand (e.g., Cheshire quartzite, MA; southern New Jersey sand). Exploratory analysis of trace element data using multidimensional scaling (MDS) shows that geographically distinct sources of Potsdam sandstone can be distinguished from one another and from competing sources of silica sand, particularly in terms of high field strength elements (e.g., Nb, Y, Ti, Zr), the rare earth elements, and radioactive elements (U, Th), and this geochemical signature is carried through to the glass it was used to manufacture. Other trace elements (e.g., Ba, Sr, Rb) are concentrated in various batch ingredients (e.g., limestone, alkali fluxes). The Hf/Nb, La/Ce, Nb/Th, and La/Zr ratios for each type of glass and nearby Potsdam sandstone sources cluster together in distinct fields on MDS plots. These data confirm the use of Potsdam sandstone in these important historical glassworks, and show that except for material sampled from neighboring communities (Mallorytown and Redwood), trace elements can be used to identify specific sources of silica historically used by the glassmaking industry. © 2008 Wiley Periodicals, Inc.     

Intraplate subsidence and basin filling adjacent to an oceanic arc–continent collision: a case from the southern Caribbean‐South America plate margin

The upper Campanian–Lower Eocene synorogenic sedimentary wedge of the Ranchería Basin was deposited in an intraplate basin resting on a tilted continental crustal block that was deformed by collision and subsequent subduction of the Caribbean Plate. Upper Cretaceous–Lower Eocene strata rest unconformably upon Jurassic igneous rocks of the Santa Marta Massif, with no major thrust faults separating the Santa Marta Massif from the Ranchería Basin. The upper Campanian–Lower Eocene succession includes, from base to top: foraminifera‐rich calcareous mudstone, mixed carbonate–siliciclastic strata and mudstone, coal and immature fluvial sandstone beds. Diachronous collision and eastward tilting of the plate margin (Santa Marta Massif and Central Cordillera) favoured the generation of accommodation space in a continuous intraplate basin (Ranchería, Cesar and western Maracaibo) during the Maastrichtian to Late Palaeocene. Terrigenous detritus from the distal colliding margin filled the western segments of the continuous intraplate basin (Ranchería and Cesar Basins); in the Late Paleocene, continental depositional systems migrated eastwards as far as the western Maracaibo Basin. In Early Eocene time, reactivation of former extensional structures fragmented the intraplate basin into the Ranchería‐Cesar Basins to the west, and the western Maracaibo Basin and Palmar High to the East. This scenario of continent–oceanic arc collision, crustal‐scale tilting, intraplate basin generation and fault reactivation may apply for Upper Cretaceous–Palaeogene syntectonic basins in western Colombia and Ecuador, and should be considered in other settings where arc–continent collision is followed by subduction.     

Stress transfer and nonlinear stress accumulation at subduction-type plate boundaries — Application to the Aleutians

A study of stress accumulation in seismic gaps and of stress transfer along linear plate boundaries is presented. Time-dependent reloading of plate boundaries following seismic ruptures is modeled by a modified Elsasser model of a coupled lithosphere/asthenosphere plate system. This model is applied to study a series of large earthquakes in the Aleutian Islands and the Alaska peninsula in 1938–1965. It is found that the Rat Island earthquake and the 1948 earthquake in the central Aleutians are likely to have been triggered by adjacent ruptures, in the sense that their occurrence would have come at a later time had their neighboring segments not been ruptured. Stresses in the Unalaska Gap and the Shumagin gap are at a relatively high level and these segments of the plate boundary may be expected to rupture in the near future. In general, in the ten years (about 16% of the earthquake cycle for the Aleutians) following an earthquake, the stress recovery in the rupture zone is highly nonlinear, resulting in a much more rapid stress accumulation than the linear case. Even at a later stage of an earthquake cycle, adjacent ruptures can cause an acceleration of loading rate in addition to the coseismic stress jump. A good example is the influence of the 1964 Alaska earthquake on the 1938 rupture zone. A general conclusion of this work is that long term earthquake prediction models must take into account the nonlinear stress accumulation behavior in seismic gaps. Also, we have shown the interaction of adjacent plate boundary segments, which suggests that some large earthquakes may have been triggered by nearby ruptures.     

Unravelling the widening of the earliest Andean northern orogen: Maastrichtian to early Eocene intra‐basinal deformation in the northern Eastern Cordillera of Colombia

The onset of deformation in the northern Andes is overprinted by subsequent stages of basin deformation, complicating the examination of competing models illustrating potential location of earliest synorogenic basins and uplifts. To establish the width of the earliest northern Andean orogen, we carried out field mapping, palynological dating, sedimentary, stratigraphic and provenance analyses in Campanian to lower Eocene units exposed in the northern Eastern Cordillera of Colombia (Cocuy region) and compare the results with coeval succession in adjacent basins. The onset of deformation is recorded in earliest Maastrichtian time, as terrigenous detritus arrived into the basin marking the end of chemical precipitation and the onset of clastic deposition produced by the uplift of a western source area dominated by shaly Cretaceous rocks. Disconformable contacts within the upper Maastrichtian to middle Palaeocene succession document increasing supply of quartzose sandy detritus from Cretaceous quartzose rocks exposed in eastern source areas. The continued unroofing of both source areas produced a rapid shift in depositional environments from shallow marine in Maastrichtian to fluvial‐lacustrine systems during the Palaeocene‐early Eocene. Supply of immature Jurassic sandstones from nearby western uplifts, together with localized plutonic and volcanic Cretaceous rocks, caused a shift in Palaeocene sandstones composition from quartzarenites to litharenites. Supply of detrital sandy fragments, unstable heavy minerals and Cretaceous to Ordovician detrital zircons, were derived from nearby uplifted blocks and from SW fluvial systems within the synorogenic basin, instead of distal basement rocks. The presence of volcanic rock fragments and 51–59 Ma volcanic zircons constrain magmatism within the basin. The Maastrichtian–Palaeocene sequence studied here documents crustal deformation that correlates with coeval deformation farther south in Ecuador and Peru. Slab flattening of the subducting Caribbean plate produced a wider orogen (>400 km) with a continental magmatic arc and intra‐basinal deformation and magmatism.     

Shrinking Sphere Kinetics for Batch Dissolution of Mixed Particles of a Single Substance at High Under-Saturation: Validation with Sodium Chloride, but with Biogenic Silica in Mind

The cubic equation recently derived for the increase in concentration of a solute with time, as the solid dissolves in batch according to the shrinking sphere model at high under-saturation, is extended to dissolutions of mixtures of differently sized particles. This problem needs to be solved if batch dissolutions are to play their part in the proposed amelioration of global warming and associated climate change by accelerated ‘re-burial’ of excess CO₂ in ocean sediment. The upgraded model was tested using sodium chloride dissolved in 50% aqueous propanone, whence the model fitted two separate runs with 500 and 212 μm, and 212 and 38 μm, diameter crystals, respectively. The key to simulating dissolution in this way lies in the dissolutions being independent of each other. It is further shown that although this condition was implicit in the recent derivation of the cubic equation, it was not recognised at the time. The work should be applicable to any batch dissolution of mixed particles at high under-saturation, and hence, may find use in many industrial and laboratory dissolutions. Simulations show how agglomerated mixtures can yield a straight line on the plot of ln(1 − C/C _T) versus time, as was reported to occur recently with sodium chloride taken ‘straight from the bottle’. It is shown that this probably explains why exponential dissolutions may have seemed appropriate to the dissolution of biogenic silica in earlier literature. This study suggests that a new round of biogenic silica dissolutions, but with sieved samples, would be worthwhile, with the likelihood that shrinking sphere behaviour might well be found to characterise the kinetics. The opportunity is taken to investigate a number of aspects of the shrinking sphere model not generally discussed before, e.g. the graph for the change in surface area with time. The limitations of using cubic salt crystals with the shrinking sphere model are discussed.     

The Madinah eruption,Saudi Arabia: Magma mixing and simultaneous extrusion of three basaltic chemical types

During a 52-day eruption in 1256 A.D., 0.5 km³ of alkali-olivine basalt was extruded from a 2.25-km-long fissure at the north end of the Harrat Rahat lava field, Saudi Arabia. The eruption produced 6 scoria cones and a lava flow 23 km long that approached the ancient and holy city of Madinah to within 8 km. Three chemical types of basalt are defined by data point clusters on variation diagrams, i.e. the low-K, high-K, and hybrid types. All three erupted simultaneously. Their distribution is delineated in both scoria cones and lava flow units from detailed mapping and a petrochemical study of 135 samples. Six flow units, defined by distinct flow fronts, represent extrusive pulses. The high-K type erupted during all six pulses, the low-K type during the first three, and the hybrid type during the first two.Three mineral assemblages occur out of equilibrium in all three chemical types.Assemblage 1 contains resorbed olivine and clinopyroxene megacrysts and ultramafic microxenoliths (Fo₉₀ + Cr spinel + Cr endiopside) which fractionated within the spinel zone of the mantle.Assemblage 2 contains resorbed plagioclase megacrysts (An₆₀) with olivine inclusions (Fo₇₈) which fractionated in the crust.Assemblage 3 contains microphenocrysts of plagioclase and olivine in a groundmass of the same minerals with late-crystallizing titansalite and titanomagnetite; assemblage 3 crystallized at the surface and/or in the upper crust. Each assemblage represents a distinct range in PTX environment, suggesting that their coexistence in each chemical type may be a function of magma mixing. Such a process is confirmed by variable ratios of incompatible element pairs in a range of analyses.All three chemical types are products of mixing. Some of the hybrid types may have developed from surface mixing of the low-K and high-K lavas; however, the occurrence of all three types at the vent system suggests that subsurface mixing was the dominant process. We suggest that the Madinah flow was extruded from a heterogeneous magma chamber containing vertically stacked sections equivalent to the six eruptive pulses. This chamber may have developed contemporaneously with magma mixing when a crustal reservoir containing a magma in equilibrium with assemblage 2 was invaded by a more primitive magma containing cognate microxenoliths and megacrysts of assemblage 1.     

Polymerization of silicate and aluminate tetrahedra in glasses,melts, and aqueous solutions—IV. Aluminum coordination in glasses and aqueous solutions and comments on the aluminum avoidance principle

The coordination of aluminum with oxygen in crystalline and amorphous alumina, aluminates, and aluminosilicates has been determined with magic angle spinning ²⁷Al nuclear magnetic resonance. The ²⁷Al NMR spectra of crystalline materials show that ^VIAl and ^IVAl can readily be distinguished. The same is not the case for amorphous aluminosilicates due to the superposition of a narrow peak, characteristic of ^IVAl, on a broad band. Our spectroscopic results indicate that Al coordination is not the determining factor in explaining differences in devitrification behavior of albite and anorthite glasses. The coordination of Al in aqueous solutions seems to prevent precipitation of the three common Al(OH)₃ polymorphs (^VIAl) at pH above 10. There is clear evidence to suggest that, in solutions containing Si as well as Al, aluminum coordination is related to the type of precipitate formed in acid (clays, ^VIAl, ^IVAl), and basic (zeolites, ^IVAl) environments. Zeolites can be precipitated in near neutral pH environments at higher temperatures, reflecting an increase in ^IVAl under these conditions. The Al avoidance principle for aluminosilicates does not seem to be a hard principle. It is likely that the validity of this principle depends on the type of modifying cation present in the aluminosilicate framework.