A Bistacchi

Interpretation and processing of ASTER data for geological mapping and granitoids detection in the Saghro massif (eastern Anti-Atlas, Morocco)

Satellite remote sensing analysis is extensively used for geological mapping in arid regions. However, it is not considered readily applicable to the mapping of metamorphic and igneous terrains, where lithological contacts are less predictable. In this work, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data were used to clarify the geological framework of the Precambrian basement in the Saghro massif (eastern Anti-Atlas, Morocco). The Saghro basement is composed of low-grade metasedimentary sequences of the Saghro Group (Cryogenian), intruded by calc-alkaline plutons of late Cryogenian age. These rocks are unconformably covered by volcanic to volcaniclastic series of Ediacaran age that are broadly coeval with granitoid plutons. All of these units are cut by a complex network of faults associated with hydrothermal fluid flows, which developed during and shortly after the emplacement of the volcanic rocks. The geological mapping of the Precambrian units was challenging in particular for the Edicaran granitoid bodies, because they are characterized by very similar compositions and a widespread desert varnish coating. For this reason, a two-stage approach has been adopted. In the first step, false color composites, band ratios, and principal components analyses on visible and near infrared (VNIR) and shortwave infrared (SWIR) bands were chosen and interpreted on the basis of the field and petrographic knowledge of the lithologies in order to detect major lithological contacts and mineralized faults. In the second step, a major effort was dedicated to the detection of granitoid plutons using both thermal infrared (TIR) and VNIR/SWIR data. The ASTER TIR bands were used to evaluate Reststrahlen and Christiansen effects in the granitoid rocks spectra, whereas VNIR/SWIR false color composite and ratio images were chosen directly on the basis of the granitoid spectra (derived from both spectrophotometric analyses of samples and selected sites in the ASTER image). Finally, spectral angle mapper (SAM) and supervised maximum-likelihood classifications (MLL) were carried out on VNIR/SWIR data, mainly to evaluate their potential for discriminating granitoid rocks. The results have further demonstrated the value of ASTER data for geological mapping of basement units, particularly if the processing has been based on a detailed knowledge of the rock mineral assemblages. In addition, the analytical comparison of ASTER TIR and VNIR/SWIR data has demonstrated that the latter are very effective in the distinction of granitoids with very similar silica content, because they can be recognized by secondary effects related to their hydrothermal and surface alterations (K-feldspar kaolinitization, plagioclase saussiritization, substitution of mafic minerals with oxides, inhomogeneous desert varnish coating, and clay/oxide proportions).

Interpretation and processing of Aster data for geological mapping of the Precambrian Basement in the Saghro Massif (Eastern Anti-Atlas, Morocco)

Satellite remote sensing analysis is extensively used for geological mapping in arid regions. However, it is not considered readily applicable to the mapping of metamorphic and igneous terrains, where lithological contacts are less predictable. In this work, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data were used to clarify the geological framework of the Precambrian basement in the Saghro massif (eastern Anti-Atlas, Morocco). The Saghro basement is composed of low-grade metasedimentary sequences of the Saghro Group (Cryogenian), intruded by calc-alkaline plutons of late Cryogenian age. These rocks are unconformably covered by volcanic to volcaniclastic series of Ediacaran age that are broadly coeval with granitoid plutons. All of these units are cut by a complex network of faults associated with hydrothermal fluid flows, which developed during and shortly after the emplacement of the volcanic rocks. The geological mapping of the Precambrian units was challenging in particular for the Edicaran granitoid bodies, because they are characterized by very similar compositions and a widespread desert varnish coating. For this reason, a two-stage approach has been adopted. In the first step, false color composites, band ratios, and principal components analyses on visible and near infrared (VNIR) and shortwave infrared (SWIR) bands were chosen and interpreted on the basis of the field and petrographic knowledge of the lithologies in order to detect major lithological contacts and mineralized faults. In the second step, a major effort was dedicated to the detection of granitoid plutons using both thermal infrared (TIR) and VNIR/SWIR data. The ASTER TIR bands were used to evaluate Reststrahlen and Christiansen effects in the granitoid rocks spectra, whereas VNIR/SWIR false color composite and ratio images were chosen directly on the basis of the granitoid spectra (derived from both spectrophotometric analyses of samples and selected sites in the ASTER image). Finally, spectral angle mapper (SAM) and supervised maximum-likelihood classifications (MLL) were carried out on VNIR/SWIR data, mainly to evaluate their potential for discriminating granitoid rocks. The results have further demonstrated the value of ASTER data for geological mapping of basement units, particularly if the processing has been based on a detailed knowledge of the rock mineral assemblages. In addition, the analytical comparison of ASTER TIR and VNIR/SWIR data has demonstrated that the latter are very effective in the distinction of granitoids with very similar silica content, because they can be recognized by secondary effects related to their hydrothermal and surface alterations (K-feldspar kaolinitization, plagioclase saussiritization, substitution of mafic minerals with oxides, inhomogeneous desert varnish coating, and clay/oxide proportions).

Fissural volcanism, polygenetic volcanic fields, and crustal thickness in the Payen Volcanic Complex on the central Andes foreland (Mendoza, Argentina)

Shield volcanoes, caldera-bearing stratovolcanoes, and monogenetic cones compose the large fissural Payen Volcanic Complex, located in the Andes foreland between latitude 35°S and 38°S. The late Pliocene-Pleistocene and recent volcanic activity along E-W trending eruptive fissures produced basaltic lavas showing a within-plate geochemical signature. The spatial distribution of fractures and monogenetic vents is characterized by self-similar clustering with well defined power law distributions. Vents have average spacing of 1.27 km and fractal exponent D = 1.33 defined in the range 0.7–49.3 km. The fractal exponent of fractures is 1.62 in the range 1.5–48.1 km. The upper cutoffs of fractures and vent fractal distributions (about 48–49 km) scale to the crustal thickness in the area, as derived from geophysical data. This analysis determines fractured media (crust) thickness associated with basaltic retroarc eruptions. We propose that the Payen Volcanic Complex was and is still active under an E-W crustal shortening regime.

On the nucleation of non-Andersonian faults along phyllosilicate-rich mylonite belts

Abstract The weakness of fault zones is generally explained by invoking an elevated fluid pressure or the presence of extremely weak minerals in a continuous fault gouge horizon. This allows for faults to slip under an unfavourable normal to shear stress ratio, in contrast to E. M. Andersons theory of faulting. However, these mechanisms do not explain why faults should nucleate in such an orientation as to make them misoriented and non-Andersonian. Here we present a weakening mechanism, involving the mechanical anisotropy of phyllosilicate-bearing mylonite belts, which is likely to influence the nucleation of faults in addition to their subsequent activity. Considering three natural examples from the Alps (the Simplon, Brenner and Sprechenstein-Mules fault zones) and a review of laboratory tests on anisotropic rocks, we apply anisotropic slip tendency analysis and show that misoriented weak faults can nucleate along a sub-planar phyllosilicate-rich mylonitic foliation, constituting a large-scale mechanical anisotropy belt and preventing the development of Andersonian optimally oriented faults.

Photogrammetric digital outcrop reconstruction, visualization with textured surfaces, and three-dimensional structural analysis and modeling: Innovative methodologies applied to fault-related dolomitization (Vajont Limestone, Southern Alps, Italy)

Different remote sensing technologies, including photogrammetry and LIDAR (light detection and ranging), allow collecting three-dimensional (3D) data sets that can be used to create 3D digital representations of outcrop surfaces, called digital outcrop models (DOM). The main advantages of photogrammetry over LIDAR are represented by the very simple and lightweight field equipment (a digital camera), and by the arbitrary spatial resolution, that can be increased simply getting closer to the outcrop or by using a different lens. The quality of photogrammetric data sets obtained with structure from motion (SFM) techniques has shown a tremendous improvement over the past few years, and this is becoming one of the more effective ways to collect DOM data sets. The Vajont Gorge (Belluno Dolomites, Italy) provides spectacular outcrops of jurassic limestones (Vajont Limestone Formation) in which mesozoic faults and fracture corridors are continuously exposed. Some of these faults acted as conduits for fluids, resulting in structurally controlled dolomitization. A 3D DOM study, based on a photogrammetric SFM data set, was carried out, aimed at enabling interdisciplinary characterization and reconstruction of coupled brittle deformation and fluid flow processes. For this study we used a DOM (730 m × 360 m × 270 m) consisting of continuous triangulated surfaces representing the outcrop, textured with high-resolution images. Interpretation and modeling performed on this data set include (1) georeferencing of structural measurements and sampling stations; (2) tracing of stratigraphic boundaries, structural surfaces, and dolomitization fronts (ground-truthed); (3) correlation and extrapolation of realistic 3D surfaces from these traces; and (4) development of a 3D geological model at the scale of the Vajont Gorge, including stratigraphy, faults, dolomitization fronts, and volumetric meshes suitable for the statistical analysis of structural, diagenetic, and geochemical parameters. The DOM study highlighted the close relationship between faults and dolostone geobodies, demonstrating that dolomitization was guided by fluid infiltration along Mesozoic normal faults. In order to explore the uncertainty associated with the 3D model of irregularly shaped dolostone bodies, three different 3D dolostone geobody realizations have been modeled, providing a minimum, intermediate, and maximum estimate of the dolostone/limestone volumetric facies ratio, while honoring the field constraints.

Gigantic individual lava flows in the Andean foothills near Malargüe (Mendoza, Argentina)

Payún Volcano is a Quaternary fissural edifice located in the southern part of the Mendoza Province (Argentina) and belonging to an Andean back-arc extensional basin. The volcano eastern portion is covered by a basaltic lava field composed of several very long lava flows, two of which advanced over more than 170 km from their fissural feeding vents. These two gigantic flows covered most of the distance over the nearly flat surface of the Pampean foreland, moving on a 0.3° slope. Since these lava flows are the longest ever documented on our Planet, a full understanding of the processes leading to their emplacement might result in a revision of the current theories on exceptionally long channelized lava flows, both on Earth and on terrestrial planets.RiassuntoIl Vulcano Payún è un edificio fissurale quaternario appartenente ad un bacino estensionale di retroarco della catena andina situato nella parte meridionale della provincia di Mendoza in Argentina. La porzione orientale dell’edificio è mantellata da un campo di lave basaltiche tra cui si riconoscono alcune singole colate canalizzate che si estendono per oltre 170 km di distanza dalla frattura alimentatrice, scorrendo per gran parte del loro percorso sulla superficie piana dell’avampaese pampeano con una pendenza di 0,3°. Esse costituiscono le più lunghe colate laviche individuali finora documentate sulla Terra e la comprensione dei loro processi di messa in posto potrebbe provocare la revisione delle teorie riguardanti le colate laviche di lunghezza eccezionale sia sulla Terra sia sui pianeti terrestri.

G: Fracture energy, friction and dissipation in earthquakes

Recent estimates of fracture energy G′ in earthquakes show a power-law dependence with slip u which can be summarized as G′ ∝ ua where a is a positive real slightly larger than one. For cracks with sliding friction, fracture energy can be equated to Gf: the post-failure integral of the dynamic weakening curve. If the dominant dissipative process in earthquakes is friction, G′ and Gf should be comparable and show a similar scaling with slip. We test this hypothesis by analyzing experiments performed on various cohesive and non-cohesive rock types, under wet and dry conditions, with imposed deformation typical of seismic slip (normal stress of tens of MPa, target slip velocity > 1 m/s and fast accelerations ≈ 6.5 m/s2). The resulting fracture energy Gf is similar to the seismological estimates, with Gf and G′ being comparable over most of the slip range. However, Gf appears to saturate after several meters of slip, while in most of the reported earthquake sequences, G′ appears to increase further and surpasses Gf at large magnitudes. We analyze several possible causes of such discrepancy, in particular, additional off-fault damage in large natural earthquakes.

Geology of the Brenner Pass-Fortezza transect, Italian Eastern Alps

We present a 1:30,000 geological map resulting from detailed geostructural surveys carried out along the Italian segment of the design corridor for the Brenner Pass railway base tunnel (BBT), extending from Fortezza (Italy) to Innsbruck (Austria). The map covers the southern part of the Austroalpine-Penninic collisional wedge, the Periadriatic Fault System, associated Oligocene igneous bodies (Periadriatic magmatism) and part of the Southalpine basement. The Penninic Zone in the western Tauern Window is represented by the double domal structure of the Europe-derived Tux and Venediger-Zillertal basement and cover nappe system, capped by the ophiolitic Glockner nappe. The overlying Austroalpine nappe system is here represented by the polymetamorphic Merano-Mules basement and minor cover sheets. The Southern Alps domain includes the Bressanone Granite and pre-granite quartz-phyllites. Four Alpine ductile deformation phases have been recognized, followed by ductile-brittle shear zones, and finally brittle deformations along faults with extensional and strike-slip kinematics. The Quaternary is characterized by glacial deposits, large gravitational mass movements and landslides.

Gigantic individual lava flows in the Andean foothills near Malargüe (Mendoza, Argentina)@@@Colate laviche gigantesche alle pendici delle Ande presso Malargüe (Mendoza, Argentina)

Payún Volcano is a Quaternary fissural edifice located in the southern part of the Mendoza Province (Argentina) and belonging to an Andean back-arc extensional basin. The volcano eastern portion is covered by a basaltic lava field composed of several very long lava flows, two of which advanced over more than 170 km from their fissural feeding vents. These two gigantic flows covered most of the distance over the nearly flat surface of the Pampean foreland, moving on a 0.3° slope. Since these lava flows are the longest ever documented on our Planet, a full understanding of the processes leading to their emplacement might result in a revision of the current theories on exceptionally long channelized lava flows, both on Earth and on terrestrial planets.RiassuntoIl Vulcano Payún è un edificio fissurale quaternario appartenente ad un bacino estensionale di retroarco della catena andina situato nella parte meridionale della provincia di Mendoza in Argentina. La porzione orientale dell’edificio è mantellata da un campo di lave basaltiche tra cui si riconoscono alcune singole colate canalizzate che si estendono per oltre 170 km di distanza dalla frattura alimentatrice, scorrendo per gran parte del loro percorso sulla superficie piana dell’avampaese pampeano con una pendenza di 0,3°. Esse costituiscono le più lunghe colate laviche individuali finora documentate sulla Terra e la comprensione dei loro processi di messa in posto potrebbe provocare la revisione delle teorie riguardanti le colate laviche di lunghezza eccezionale sia sulla Terra sia sui pianeti terrestri.