Microstructural analysis and P–T conditions of the Azul megashear zone, Tandilia, Buenos Aires province, Argentina

A microstructural analysis was carried out on mylonitic rocks of the Azul megashear zone (AMSZ), Tandilia, which were formed in a range of metamorphic conditions from lower greenschist to amphibolite facies. Tailed porphyroclasts are common and mostly symmetric. Scarce asymmetric rotated porphyroclasts show both sinistral and dextral senses of shear. In sections parallel to the mylonitic foliation, porphyroclasts are round. The AMSZ is probably related to the late Transamazonian orogenic cycle and may be due to NNE–SSW-directed convergence. In weakly deformed protolith and protomylonites, quartz deforms by dynamic recrystallization, mainly subgrain rotation in dislocation creep Regime 2. K-feldspar porphyroclasts and plagioclase show scarce fracturation and deform by dynamic recrystallization along grain boundaries. Quartz microstructures in mylonites indicate predominantly Regime 3 grain boundary migration recrystallization. Feldspar structures indicate recrystallization through the nucleation and growth of new grains at grain boundaries. The temperatures of deformation from mineral assemblages in the CNKFMASH system in four bulk compositions are in the range of 400–450 °C, and the pressures are more than 6 kb.     

The composition of back-arc basin lower crust and upper mantle in the Mariana Trough: A first report

Abstract The Mariana Trough is an active back-arc basin, with the rift propagating northward ahead of spreading. The northern part of the Trough is now rifting, with extension accommodated by combined stretching and igneous intrusion. Deep structural graben are found in a region of low heat flow, and we interpret these to manifest a low-angle normal fault system that defines the extension axis between 19°45' and 21°10'N. A single dredge haul from the deepest (∼5.5 km deep) of these graben recovered a heterogeneous suite of volcanic and plutonic crustal rocks and upper mantle peridotites, providing the first report of the deeper levels of back-arc basin lithosphere. Several lines of evidence indicate that these rocks are similar to typical back-arc basin lithosphere and are not fragments of rifted older arc lithosphere. Hornblende yielded an ⁴⁰Ar/³⁹Ar age of 1.8 ± 0.6 Ma, which is interpreted to approximate the time of crust formation. Harzburgite spinels have moderate Cr# (<40) and coexisting compositions of clinopyroxene (CPX) and plagioclase (PLAB) fall in the field of mid-ocean ridge basalt (MORB) gabbros. Crustal rocks include felsic rocks (70-80% SiO₂) and plutonic rocks that are rich in amphibole. Chemical compositions of crustal rocks show little evidence for a 'subduction component', and radiogenic isotopic compositions correspond to that expected for back-arc basin crust of the Mariana Trough. These data indicate that mechanical extension in this part of the Mariana Trough involves lithosphere that originally formed magmatically. These unique exposures of back-arc basin lithosphere call for careful study using ROVs and manned submersibles, and consideration as an ocean drilling program (ODP) drilling site.     

Evolution of bulk composition, mineralogy, strain style and fluid flow during an HT–LP metamorphic event: sillimanite zone of the Catalan Coastal Ranges Variscan basement, NE Iberia

Metamorphic rocks in the Osor complex (Guilleries massif, NE Iberian Peninsula) show the following structural and compositional features: strong differentiation into quartz-rich gneissic semipelitic and quartz-absent, mica-rich schistose bands, higher density of igneous (both basic and leucogranitic) and quartz veins in the schistose domains and strong strain partitioning in the pelitic bands. Garnet is present in both kinds of lithologies, showing also differential textural and chemical features interpreted to be dependent on bulk composition, deformation and fluid interaction histories. Textures, mineral composition and thermobarometry suggest the operation of concurrent mechanical, mass transfer and thermal phenomena such as: (1) variations in strain style, (2) fluid infiltration, (3) magmatic injection and (4) HT–LP metamorphic and metasomatic episodes. The following sequence of events is suggested: initially the cooling of syntectonic high-T basic quartz diorite sheets promoted high strain rates, low dP/dT thermobaric evolution, incipient anatexis in the pelitic bands and devolatilization through a pervasive to vein-channelized prograde fluid flow. The prograde flow enhanced an ongoing compositional tectono-metamorphic differentiation and produced metasomatism through depletion of the Osor rocks in alkalis and calcium. Later injection and cooling of peraluminous leucogranitoid sheets, preferentially along pelitic bands, increased the ratio of magmatic/metamorphic components in the fluids and strongly enriched them in alkalis producing a second metasomatic episode. During crystallization of quartz and leucogranitoid veins, the pelitic bands were strongly enriched back again in alkalis, promoting the blastesis of big crystals of post-peak muscovite and albite as well as the retrogression of garnet. The metasomatic mica-rich levels must have been the preferred locus for development of a new deformation style dominated by shear band fabrics in metapelites and related to a release of the gravitational instability originated previously due to crustal thickening. The increasing decompressional component of the retrograde P–T path also suggests that this style of deformation was prevalent during, if not responsible for, a phase of exhumation of the metamorphic complex. It is suggested that similar patterns of thermomechanical and mass transfer phenomena could well be a fundamental characteristic common to all HT–LP metamorphic belts.     

Geomorphic process signatures reshaping sub-humid Mediterranean badlands: 2. Application to 5-year dataset

Badland landscapes exhibit high erosion rates and represent the main source of fine sediments in some catchments. Advances in high-resolution topographic methods allow analysis of topographic changes at high temporal and spatial scales. We apply the Mapping Geomorphic Processes in the Environment (MaGPiE) algorithm to infer the main geomorphic process signatures operating in two sub-humid badlands with contrasting morphometric attributes located in the Southern Pyrenees. By interrogating a 5-year dataset of seasonal and annual topographic changes, we examine the variability of geomorphic processes at multiple temporal scales. The magnitude of geomorphic processes is linked to landform attributes and meteorological variables. Morphometric differences between both adjacent badlands allow us to analyse the role of landform attributes in the main geomorphic process reshaping landscapes subjected to the same external forcing (i.e. rainfall and temperature). The dominant geomorphic process signatures observed in both badlands are different, despite their close proximity and the same rainfall and temperature regimes. Process signatures determining surface lowering in the gently sloping south-facing badland, characterized by lower connectivity and more vegetation cover, are driven by surface runoff-based processes, both diffuse (causing sheet washing) and concentrated (determining cutting and filling, rilling and gullying). The steeper, more connected north-facing slopes of the other badland are reshaped by means of gravitational processes, with mass wasting dominating topographic changes. In terms of processes determining surface raising, both mass wasting and cutting and filling are most frequently observed in both badlands. There is a clear near-balanced feedback between both surface-raising and -lowering processes that becomes unbalanced at larger temporal scales due to the thresholds overcome, as the volume associated with surface lowering becomes higher than that associated with raising-based processes. Rainfall variables control surface flow processes, while those variables associated with low temperature have a significant relation with mass movement-based processes and other localized processes such as regolith cohesion loss. Finally, our results point out that morphometry (slope and connectivity) together with vegetation cover are key factors determining geomorphic processes and associated topographic changes. © 2020 John Wiley & Sons, Ltd.     

Across‐axis variations in petrophysical properties of the North Porcupine Basin,offshore Ireland: New insights from long‐streamer traveltime tomography

The Porcupine Basin is a Mesozoic failed rift located in the North Atlantic margin, SW of Ireland, in which a postrift phase of extensional faulting and reactivation of synrift faults occurred during the Mid–Late Eocene. Fault zones are known to act as either conduits or barriers for fluid flow and to contribute to overpressure. Yet, little is known about the distribution of fluids and their relation to the tectono‐stratigraphic architecture of the Porcupine Basin. One way to tackle this aspect is by assessing seismic (V_p) and petrophysical (e.g., porosity) properties of the basin stratigraphy. Here, we use for the first time in the Porcupine Basin 10‐km‐long‐streamer data to perform traveltime tomography of first arrivals and retrieve the 2D V_p structure of the postrift sequence along a ~130‐km‐long EW profile across the northern Porcupine Basin. A new V_p–density relationship is derived from the exploration wells tied to the seismic line to estimate density and bulk porosity of the Cenozoic postrift sequence from the tomographic result. The V_p model covers the shallowest 4 km of the basin and reveals a steeper vertical velocity gradient in the centre of the basin than in the flanks. This variation together with a relatively thick Neogene and Quaternary sediment accumulation in the centre of the basin suggests higher overburden pressure and compaction compared to the margins, implying fluid flow towards the edges of the basin driven by differential compaction. The V_p model also reveals two prominent subvertical low‐velocity bodies on the western margin of the basin. The tomographic model in combination with the time‐migrated seismic section shows that whereas the first anomaly spatially coincides with the western basin‐bounding fault, the second body occurs within the hangingwall of the fault, where no major faulting is observed. Porosity estimates suggest that this latter anomaly indicates pore overpressure of sandier Early–Mid Eocene units. Lithological well control together with fault displacement analysis suggests that the western basin‐bounding fault can act as a hydraulic barrier for fluids migrating from the centre of the basin towards its flanks, favouring fluid compartmentalization and overpressure of sandier units of its hangingwall.     

3D hydrodynamic characterisation of a meso-tidal harbour: The case of Bilbao (northern Spain)

A background knowledge of marine dynamics helps harbour managers to control pollution and to manage dredging and traffic operations. This contribution studies the hydrodynamic conditions within Bilbao Harbour, which is enclosed by the Nervión Estuary (in the Basque Country, Spain). The results obtained from hydrographical surveys are compared with numerical simulations obtained using the Regional Ocean Modelling System (ROMS). Hydrodynamic modelling was carried out to determine the inner harbour currents for a specific period in which data were available. Then, numerical experiments were designed in order to quantify the importance of different driving mechanisms in the harbour hydrodynamics. The results show that, in addition to the strong tidal influence on water circulation, the wind forcing and freshwater discharge also have a non-negligible influence on the currents. The computational domain is complex due to the presence of harbour infrastructures (i.e. breakwaters and piers). As a result, topographic eddies are therefore observed in the results. The freshwater influence of the Nervión river can also be observed in residual currents. This paper presents an improvement to the application of numerical modelling to a complex geometry domain, contributing to our understanding of the behaviour of the marine systems in meso-tidal harbours. This can be used to deal with harbour engineering and management problems.     

Neural network based daily precipitation generator (NNGEN-P)

Daily weather generators are used in many applications and risk analyses. The present paper explores the potential of neural network architectures to design daily weather generator models. Focusing this first paper on precipitation, we design a collection of neural networks (multi-layer perceptrons in the present case), which are trained so as to approximate the empirical cumulative distribution (CDF) function for the occurrence of wet and dry spells and for the precipitation amounts. This approach contributes to correct some of the biases of the usual two-step weather generator models. As compared to a rainfall occurrence Markov model, NNGEN-P represents fairly well the mean and standard deviation of the number of wet days per month, and it significantly improves the simulation of the longest dry and wet periods. Then, we compared NNGEN-P to three parametric distribution functions usually applied to fit rainfall cumulative distribution functions (Gamma, Weibull and double-exponential). A data set of 19 Argentine stations was used. Also, data corresponding to stations in the United States, in Europe and in the Tropics were included to confirm the results. One of the advantages of NNGEN-P is that it is non-parametric. Unlike other parametric function, which adapt to certain types of climate regimes, NNGEN-P is fully adaptive to the observed cumulative distribution functions, which, on some occasions, may present complex shapes. On-going works will soon produce an extended version of NNGEN to temperature and radiation.     

Application of LPW and SAI SAFNWC/MSG satellite products in pre-convective environments

The Spinning Enhanced Visible and Infrared Imagery (SEVIRI) instrument, on board the Meteosat Second Generation (MSG), is a radiometer with eight infrared (IR) spectral bands. Seven of these channels are used to retrieve Layer Precipitable Water (LPW) and Stability Analysis Imagery (SAI). Both products are the PGE07 and the PGE08 of SAFNWC (Satellite Application Facility on support to Nowcasting and Very Short-Range Forecasting). The authors at Instituto Nacional de Meteorología (INM) have developed the LPW and SAI algorithms, in the SAFNWC framework. Both products are retrieved using statistical retrieval based on neural networks. The main advantage of these algorithms versus physical retrieval algorithms is the independence from the Numerical Weather Prediction (NWP) models. The LPW provides information on the water vapor contained in a vertical column of unit cross-section area in three layers in the troposphere (low, middle and high) and in the total layer in cloud free areas. The SAI provides estimations of the atmospheric instability in cloud free areas, in particular the Lifted Index (LI).The stability and precipitable water obtained with both products are routinely generated every 15 min at a satellite horizontal resolution of 3 km in NADIR. A significant advantage of these MSG products, compared to traditional measurements such as radiosondes, is their ability to measure high resolution temporal and spatial variations of atmospheric stability and moisture in pre-convective environments. The main disadvantage is that they do not have the vertical resolution of radiosonde. The MSG moisture and stability time trend fields are especially useful during the period preceding the outbreak of convection due to the high resolution. Once the outbreak of convection occurs, the products calculated in the clear air pixels surrounding the convective system will allow to foresee the evolution of the convection.     

The effect of Southwestern Atlantic burrowing crabs on habitat use and foraging activity of migratory shorebirds

Southwestern Atlantic estuaries (Southern Brazil to Northern Patagonia, Argentina) are characterized by the presence of an intertidal burrowing crab Chasmagnathus granulata. This crab species is an important bioturbator which lives in large assemblages and excavates semi-permanent burrows that affect sediment characteristics. Our observations showed that distribution of the crabs in the Mar Chiquita coastal lagoon, Argentina (37°45′S, 57°26′W) affected habitat use and feeding success of migratory shorebirds. During the migratory season the two-banded plover (Charadrius falklandicus) foraged more frequently inside crab beds, and yellowlegs (Tringa flavipes and T. melanoleuca) fed more freqeuntly outside crab beds. Focal observations on the feeding behavior of the white-rumped sandpiper (Calidris fuscicollis) and the two-banded plover inside and outside crab beds showed that the plover was a visual searcher and captured more prey inside crab beds, and the white-rumped sandpiper was a tactile feeder. Although consumption rates (prey min^?1) did not differ between sites, their efficiency (prey probe^?1) inside crab beds was less. These differences were probably related to changes in sediment characteristics and prey behavior, which vary with crab activity. Burrowing crabs alter the suitability of intertidal habitats used by shorebirds in southwestern Atlantic estuaries. We believe that the same process could be occurring with other burrowing curstaceans such as thalassinidean shrimps in other estuaries of the world and could have important implications for management of flats for shorebirds. *** DIRECT SUPPORT *** A01BY090 00007