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)

Frictional Melting in Hydrothermal Fluid-Rich Faults: Field and Experimental Evidence From the Bolfín Fault Zone (Chile)

Tectonic pseudotachylytes are thought to be unique to certain water-deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid-rich faults hosted in the continental crust. Pseudotachylytes were found in the >40 km-long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m-thick (ultra)cataclastic strands hosted in a damage-zone made of chlorite-epidote-rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite-, epidote- and calcite-bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s−1) resulted in the production of vesiculated pseudotachylytes both at dry and water-pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid-deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co-seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust

Association of cone sheets and radial dykes on Ascraeus Mons (Mars): structural analysis and modelling

Ascraeus Mons was one of the first of the Martian volcanoes to be imaged by the High Resolution Stereo Camera (HRSC) experiment onboard the ESA Mars Express spacecraft. These images show much of the volcano at a higher resolution than previously (12 m/px), and details of its lava flows, sinuous rilles, flank vents and tectonic features. Concentric fractures systems (pit chains, grabens) around the volcano, changing transitionally into radial structures systems have been recognized and cartographed using a HRSC mosaic. The structural interpretation showed strong analogies with concentric dykes (cone sheets) on many terrestrial volcanoes such as Isla Fernandina (Galapagos), Tejeda Complex (Canary Islands) and Cuillins Complex (Isle of Skye, Scotland). In particular this last terrestrial analogue has been studied in detail and with the use of a Finite Element Method (FEM) modeling the shape of the magma chamber that originated the cone sheets on the Cuillins Complex was discovered (Bistacchi et al. 2010). An oblate inflating magma chamber is responsible for the formation of those structures. By analogy, we tested the presence of an oblate magma chamber below Ascraeus Mons. We measured the diameter of the transition zone between concentric and radial structures on Ascraeus Mons flanks, that is strongly related to the diameter of the deep magma chamber (1-1.2 magma chamber diameters). With a FEM model built for Ascraeus we have been able to discover the average depth of the oblate magma chamber, which could have originated the concentric structures. The presence of a plume with an oblate summit instead of the magma chamber has also been tested. Moreover an additional oblate shallow magma chamber that likely originated the summit caldera has been verified. In addition, the deformation event that originated the structures on Ascraeus flanks has been dated by crater counting, resulting very recent

Introduzione alla tettonica fragile neoalpina e sua influenza sull'instabilità dei versanti

Questo lavoro mira a sottolineare l'importanza che l'evoluzione tettonica a carattere regionale e locale assume sul controllo dei ben più tangibili processi di deformazione dei versanti. Dopo una breve sintesi sulla composizione e sulla struttura delle Alpi, se ne ripercorre la storia genetica, ponendo maggior enfasi sulle ultime fasi evolutive (Neoalpine), responsabili dell'attuale assetto della catena in particolare per quanto riguarda i rilievo topografico e le deformazione di tipo fragile. Partendo quindi dall'assioma che i grandi fenomeni di dissesto sono legati alla concomitanza di elevata energia di rilievo e cattive condizioni geomeccaniche dell'ammasso roccioso, gli autori evidenziano come queste caratteristiche si trovino, non a caso, lungo il settore assiale della catena ed, in particolare, in corrispondenza delle aree in cui il sollevamento differenziale è stato più rapido in tempi recenti ed attuali. L'ubicazione dei più estesi processi di instabilità di versante non è quindi da considerarsi casuale, ma è invece da ricondurre ad una complessa evoluzione tettonica, sviluppatasi nel corso di milioni di anni. Infine, vengono brevemente descritti tre casi, rispettivamente in media Val d'Aosta, in Trentino nord-occidentale ed in Alto Adige, in cui si è riconosciuto che le relazioni tra assetto tettonico fragile e Deformazioni Gravitative Profonde di Versante (DGPV) sono particolarmente significative

Post-nappe brittle tectonics and kinematic evolution of the north-western Alps: an integrated approach

Data from remote sensing, structural geology and thermochronology provide the basis for this integrated reconstruction of the Oligocene to Present kinematic evolution of the north-western Alpine nappe stack. Two brittle tectonic phases post-date the Cretaceous\u2013Eocene ductile deformation. A NW\u2013SE extension developed in the Oligocene (D1) along three main conjugate fault systems arranged in orthorhombic symmetry (N-, NW- and SE-dipping). Cooling rate contour maps, from published apatite and zircon fission-track ages and Rb/Sr biotite ages, highlight the differential exhumation of large fault-bounded blocks during this phase, whilst synkinematic hydrothermal veins and calc-alkaline dykes (29\u201332 Ma) help to constrain its age. From the Miocene onwards, a general rearrangement of the strain pattern led to SW-directed lateral extrusion (D2) of the Pennine-Graian Alps block, bounded by a network of seismogenic shear zones, the most important being the Ospizio Sottile, Simplon, Rhone and Chamonix faults. The internal deformation of the Pennine-Graian Alps block is characterised by an overall more or less homogeneous NE\u2013SW extension. The approach undertaken, integrating remote sensing, structural analysis on different scales, and thermochronology (with the cooling rate map representation), is therefore effective in reconstructing the late-orogenic extensional tectonic evolution of metamorphic nappe stacks

Geological outline of the Alps

The Alps were developed from the Cretaceous onwards by subduction of a Mesozoic ocean and collision between the Adriatic (Austroalpine-Southalpine) and European (Penninic-Helvetic) continental margins. The Austroalpine-Penninic wedge is the core of the collisional belt, a fossil subduction complex which floats on the European lower plate. It consists of continental and minor oceanic nappes and is marked by a blueschist-to-eclogite-facies imprint of Cretaceous- Eocene age, followed by a Barrovian overprint. The collisional wedge was later accreted by the Helvetic basement and cover units and indented by the Southalpine lithosphere, which in turn was deformed as an antithetic fold-and-thrust belt

FEM modelling and fractal analysis of concentric and radial structures on Ascraeus Mons (Mars)

Ascraeus Mons \ue8 l\u2019edificio vulcanico situato pi\uf9 a nord nella provincia dei Tharsis su Marte. In questo lavoro vogliamo verificare l\u2019ipotesi che afferma che l\u2019ultima attivit\ue0 vulcano-tettonica su Ascraeus possa essere stata indotta dalla sovrapressione di una camera magmatica di forma oblata. Il prodotto \ue8 un campo di stress che ha causato l\u2019apertura di fratture utili all\u2019iniezione di dicchi radiali e cone sheets, aventi una distribuzione simile a quelli rinvenuti sul Cuillins Complex (isola di Skye, Scozia). In quest\u2019ultimo caso tramite un modello a elementi finiti (FEM) si \ue8 dimostrato che questa particolare distribuzione di dicchi e fratture \ue8 compatibile solamente con la presenza di una camera magmatica oblata a una data profondit\ue0. Basandoci su un\u2019accurata mappatura di lineamenti e strutture concentriche e radiali su immagini HRSC (High Resolution Stereo Camera con risoluzione 12 m/pixel) combinata con parametri fisici e reologici marziani, e soprattutto con la posizione della zona di transizione concentrico-radiale nei sistemi di fratture intorno al vulcano, abbiamo potuto costruire un modello FEM di Ascraeus Mons. Si \ue8 cos\uec potuta testare la possibile presenza e profondit\ue0 di una camera magmatica oblata al momento dell\u2019ultimo evento vulcano-tettonico (datato con la tecnica del conteggio di crateri). Inoltre le strutture concentriche cartografate essendo direttamente correlate ai sistemi di fratture in profondit\ue0 collegati alla sorgente magmatica e al campo di stress, sono state analizzate secondo la loro distribuzione spaziale. Questa distribuzione \ue8 stata descritta da una legge di potenza con esponente frattale, che permette di ricavare la profondit\ue0 della camera magmatica e confrontarla con quella ottenuta dal modello FEM. L\u2019incrocio e la reciproca verifica di risultati simili ottenuti con metodi indipendenti forniscono un forte constrain sulla presenza e posizione della camera magmatica, oltre che sulla bont\ue0 dei metodi stessi

Geological and structural map of the southeastern Pag Island, Croatia: field constraints on the Cretaceous - Eocene evolution of the Dinarides foreland

The sedimentary succession exposed in the Northern Dalmatia Islands mainly consists of Cretaceous to Neogene shallow water carbonates, folded and imbricated within the External Dinarides thrust belt. During Cretaceous times, carbonate sediments were deposed on a heterogeneous, tectonically-influenced carbonate platform, which was then uplifted and eroded, as evidenced by a regional unconformity embracing the Late Cretaceous and Paleocene. Sedimentation resumed during the Eocene, when the area was part of the foreland basin of the Dinaric belt. With our geological and structural map of the southeastern Pag Island at the 1:25,000 scale, we refined the stratigraphic and structural setting and the tectono- sedimentary evolution of the area.Applied Geolog