Tectonic significance of different block-in-matrix structures in exhumed convergent plate margins: examples from oceanic and continental HP rocks in Inner Western Alps (northwest Italy)

In the Inner Western Alps, three different types of block-in-matrix structures (BIMs) formed sequentially through time at a convergent plate margin. These show the superposition of progressive deformation from (i) subduction to eclogite-facies depths, (ii) collision, accretion, and exhumation of oceanic crust, represented by the Monviso Meta-ophiolite Complex, to (iii) collision, accretion, and exhumation of the continental Dora Maira units. The Type 1 occurs in the metasedimentary cover of the Dora Maira Unit and consists of a map-scale broken formation with boudinaged ‘native’ blocks of marble (Early Jurassic) in a calcschist matrix. It results from the tectonic overprinting of exhumation-related folding (D2-stage) on an earlier subduction-related dismembered succession (D1-stage). Type 1 also includes ‘non-mappable’ BIMs with ‘exotic’ blocks, resulting from the gravitational collapse of the Triassic carbonate platform of European Continental Margin, triggered by the Early Jurassic rifting. In the Monviso Meta-ophiolite Complex, Types 2 and 3 represent tectonically induced broken and dismembered formations, respectively. They differ from each other in the degree of stratal disruption of primary interbedded horizons of mafic metabreccia (Type 3) and mafic metasandstone (Types 2 and 3) sourced by the Late Jurassic–Early Cretaceous denudation of an oceanic core complex. Dismembered interbeds (Type 2) and isolated blocks were mixed together (Type 3) by the overlap of D2 tectonics and late- to post-exhumation extensional shearing (D3-stage). Development of these types of BIMs may be common in many exhumed convergent plate margins, where severe tectonics and metamorphic recrystallization under high-pressure conditions normally prevent the reconstruction of BIMs or mélange-forming processes. Our findings show that documenting the mode and time of the processes forming BIMs is highly relevant in order to reconstruct the oceanic seafloor morphology and composition of associated stratigraphic successions, and their control in the evolution of those convergent plate margins.     

Segmented, curved faults: the example of the Balduini Thrust Zone, Northern Apennines, Italy

Meso- and micro-structural studies of the well-exposed Balduini Thrust (Northern Apennines, Italy) indicate that the structure formed during a single folding event, contemporaneous with diagenesis, and is a zone comprising curved, en-échelon fault segments. The geometry of each segment is arcuate with pure compression at one end and right-lateral displacement along the other. The thrust developed during the Tortonian within a single mud-rich formation, the Upper Eocene–Upper Oligocene Scaglia Cinerea, but rheological variations within the unit led to differences in deformation style; zones of scaly fabric are discontinuous and calcite veins vary in abundance. The mesoscopic morphology of the veins and the distribution of calcium carbonate along the formation indicate variations in the distribution of fluids at the time of deformation, which affected both diagenesis and the structural response of the material. Systematic variations of mechanical properties within the thickness of the Scaglia Cinerea Formation account for the curvature of the propagating thrust. Together with the heterogeneity of the stress field, the confinement of the arcing thrust to this single, weak unit lead to repeated initiation of new fractures and hence segmentation of the propagating thrust. Geometric analysis of the calcite veins and their microscopic characters suggests that hydrofracturing was involved, with the Scaglia Cinerea Formation experiencing high fluid pressure followed by rapid water expulsion. The Balduini Thrust is therefore an example of a fluid-driven, refracted compound shear zone. The analysis presented here provides insights into the three-dimensional arrangement of fault zones and fluid-migration patterns during regional faulting.     

Lagrangian sources of frontogenesis in the equatorial Atlantic front

Estimating the processes that control the north equatorial sea surface temperature (SST)-front on the northern edge of the cold tongue in the tropical Atlantic is a key issue for understanding the dynamics of the oceanic equatorial Atlantic and the West African Monsoon. Diagnosis of the frontogenetic forcings on a realistic high-resolution simulation was used to identify the processes involved in the formation and evolution of the equatorial SST-front. The turbulent forcing associated with the mixed-layer turbulent heat flux was found to be systematically frontolytic while the dynamic forcing associated with currents was found to be frontogenetic for the equatorial SST-front. Nevertheless, the low-frequency component of the turbulent forcing was frontogenetic and initiated the SST-front which was then amplified and maintained by the leading dynamic forcing. This forcing was mainly driven by the meridional convergence of the northern South Equatorial Current (nSEC) and the Guinea Current, which points out the essential role played by the circulation in the equatorial SST-front evolution. The quasi-biweekly variability of the equatorial SST-front and its forcings were found to be more strongly coupled to the wind energy flux (WEF) than to the surface wind stress. In fact the WEF controlled the convergence/divergence of the nSEC and Guinea Current and thus the meridional component of the leading dynamic forcing. The WEF explains the equatorial SST-front development better than the wind does because it is a coupled ocean-atmosphere process.     

The large-scale dynamics of grain-size variation in alluvial basins, 2: Application to syntectonic conglomerate

The concept of‘syntectonic’ conglomerate is based on the idea that gravel progradation is mainly generated by an increase in tectonic uplift and erosion of a source area with attendant increase in sediment flux supplied to a basin. However, other mechanisms, such as changes in basin subsidence rates, sorting of supplied sediment, and capability of transporting streams, can also lead to progradation and be difficult to distinguish from a syntectonic origin. Here we use our previously developed model to help understand the origin of gravel progradation in three Neogene alluvial basins - the Bermejo Basin of Argentina, the Himalayan Foreland Basin, and the San Pedro Basin of southern Arizona - all of which have available high-resolution magnetostratigraphy. Interpretation of the origin of gravel progradation in these basins begins with calculation of basin equilibrium time, which is the time-scale required for the streams to reach a steady-state profile, assuming constant conditions. We then compare the time-scale of the observed changes in the basin with the equilibrium time to determine if and how the model can be applied to the stratigraphic record. Most of the changes we have studied occur on time scales longer than the equilibrium time (‘slow variations’), in which case the key to interpretation is the relationship between overall grain-size change and sedimentation rate in vertical sections. Of the three examples studied only one, the Bermejo Basin, is consistent with the traditional model of syntectonic progradation. Overall progradation in the two other basins is most consistent with a long-term reduction in basin subsidence rates. In addition, short-term variation in diffusivity or sediment flux, probably climatically driven, is the most likely control of small-scale progradation of gravel tongues in the San Pedro Basin. These results, along with observations from other basins, suggest that subsidence is clearly an important control on clastic progradation on ‘slow’ time scales (i.e. generally a million years or more). If subsidence rates are directly linked to tectonic events, then subsidence-driven progradation marks times of tectonic quiescence and is clearly not syntectonic in the traditional sense. These examples show that the model can be useful in interpreting the rock record, particularly when combined with other traditional basin-analysis techniques. In particular, our results can be used to help discriminate between clastic progradation due to tectonic origin and progradation resulting from other mechanisms in alluvial basins.     

High-pressure structural study of muscovite

The compressibility and structural variations of two 2M₁ muscovites having compositions (Na_0.07K_0.90 Ba_0.01□_0.02)(Al_1.84Ti_0.04Fe_0.07Mg_0.04)(Si_3.02Al_0.98) O₁₀ (OH)₂ (7 mole % paragonite) and (Na_0.37K_0.60□_0.03)(Al_1.84Ti_0.02 Fe_0.10Mg_0.06)(Si_3.03Al_0.97) O₁₀(OH)₂ (37 mole % paragonite) were determined at pressures between 1 bar and 35 kbar, by single-crystal X-ray diffraction using a Merrill-Bassett diamond anvil cell. Isothermal bulk moduli, setting K′ = 4, were 490 and 540 (± 30) kbar for the Na-poor and Na-rich samples respectively. Both samples show highly anisotropic compressibility patterns, with β _a ∶β _b ∶β _c = 1∶1.15∶3.95 for the Na-poor sample and β _a ∶β _b ∶β _c = 1∶1.19∶3.46 for the Na-rich one. HP structural refinements showed that the different compressibility was largely due to the partial substitution of Na for K in the interlayer region. Moreover, the different compressibility of the tetrahedral and octahedral layers, observed in both micas, increased the a rotation of the tetrahedral layer by about 2° in 28 kbar, as also indicated by the evolution of interlayer cation bond lengths. This increases the repulsion of oxygens of the basal layers and between the high-charged cations of the tetrahedral layer. As a consequence, phengitic substitution, reducing α rotation, would increase the baric stability of mica. Comparison between the HP structures of muscovite and phlogopite indicated the lower compressibility of the latter, mainly due to the greater compressibility of the dioctahedral layer with respect to that of the trioctahedral layer. The HT and HP behaviour of di- and trioctahedral micas showed an anisotropy in the compressional pattern which was markedly greater than that observed in the dilatation pattern. This unexpected result was explained by the different evolution with P and T of alkaliO bond lengths. By combining HP and HT data, a tentative equation of state of muscovite is proposed.     

Evaluation of the ERA-40 air–sea surface heat flux spin-up

Ocean models depend strongly on surface fluxes. When computed from atmospheric models, fluxes are affected by spin-up, i.e. they increase (or decrease) with forecast length. Such behavior may bias ocean models. The European Centre for Medium Range Forecasts (ECMWF) 40-year re-analysis (ERA-40) has been used to quantify short-range spin-ups of radiative and turbulent heat fluxes. Fluxes are compared as differences between two runs with the same initialization time. This method allows flux analysis over short-range forecasts as a function of distance from the initialization time. Results indicate that (i) latent heat flux spin-up increases with time but levels off after 24 h; (ii) sensible and radiative flux spin-ups remain constant after 6 h; (iii) regional spin-up of turbulent fluxes are systematic and can be larger than 30% for sensible heat but never exceeds 15% for latent heat; (iv) spin-up depends upon the season. The same analysis has been carried out with the ECMWF 15-year re-analysis (ERA-15); spin-ups in ERA-40 have been generally smaller than those in ERA-15.     

Exploration tree approach to estimate historical earthquakes Mw and depth,test cases from the French past seismicity

The estimation of the seismological parameters of historical earthquakes is a key step when performing seismic hazard assessment in moderate seismicity regions as France. We propose an original method to assess magnitude and depth of historical earthquakes using intensity data points. A flowchart based on an exploration tree (ET) approach allows to apply a consistent methodology to all the different configurations of the earthquake macroseismic field and to explore the inherent uncertainties. The method is applied to French test case historical earthquakes, using the SisFrance (BRGM, IRSN, EDF) macroseismic database and the intensity prediction equations (IPEs) calibrated in the companion paper (Baumont et al. Bull Earthq Eng, 2017). A weighted least square scheme allowing for the joint inversion of magnitude and depth is applied to earthquakes that exhibit a decay of intensity with distance. Two cases are distinguished: (1) a “Complete ET” is applied to earthquakes located within the metropolitan territory, while (2) a “Simplified ET” is applied to both, offshore and cross border events, lacking information at short distances but disposing of reliable data at large ones. Finally, a priori-depth-based magnitude computation is applied to ancient or poorly documented events, only described by single/sporadic intensity data or few macroseismic testimonies. Specific processing of “felt” testimonies allows exploiting this complementary information for poorly described earthquakes. Uncertainties associated to magnitude and depth estimates result from both, full propagation of uncertainties related to the original macroseismic information and the epistemic uncertainty related to the IPEs selection procedure.     

Geology of the Corbetti Caldera area (Main Ethiopian Rift Valley)

The Corbetti Caldera area, a recent volcanic complex in the Main Ethiopian Rift Valley, is described. In the area, most of the volcanic products are peralkaline pyroclastics (ignimbrites and pumice). The volcanological history of this complex has been reconstructed. It comprises fissure eruptions, which were followed by a volcano-tectonic collapse. Finally the activity resumed with the birth of two recent peralkaline volcanoes (Urji and Chabbi) inside the caidera. Both these volcanoes are at present in a fumarolic stage. Relations between the tectonic of the Rift Valley and the volcanological evolution of the Caldera Corbetti Area are discussed.     

Representing the visibility structure of a polyhedral terrain through a horizon map

Abstract

We present a model for describing the visibility of a polyhedral terrain from a fixed viewpoint, based on a collection of nested horizons. We briefly introduce the concepts of mathematical and digital terrain models, and some background notions for visibility problems on terrains. Then, we define horizons on a polyhedral terrain, and introduce a visibility model, that we call the horizon map. We present a construction algorithm and a data structure for encoding the horizon map, and show how it can be used for solving point visibility queries with respect to a fixed viewpoint.     

Magma contamination by direct wall rock interaction: constraints from xenoliths from the walls of a carbonate-hosted magma chamber (Vesuvius 1944 eruption)

During the 1944 eruption of Vesuvius different types of xenoliths were ejected. They represent fragments of the walls of a low volume (<0.5 km³) shallow (3–4 km depth) magma chamber. The study of these xenoliths enables us to estimate the amount of contamination occurring at the boundary of a high-T alkaline magma chamber hosted in carbonate rocks. The process of contamination of the magma by carbonates can be modelled, using isotopic and chemical data, as a mixing between magma and marbles. Mass exchanges occur at the boundary between the crystallizing magma and marble wall rocks, where endoskarn forms. The contamination of the solidification front of the chamber is very limited. The solidification front and the skarn shell effectively isolate the interior of the magma chamber from new inputs of contaminants from the carbonate wall rocks. Therefore, the main volume of magma, hosted in the magma chamber, did not undergo any significant mass exchange with the wall rocks.