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RILEVAMENTO GEOLOGICO DEL PROMONTORIO DEL CIRCEO: ANALISI DI FACIES ED EVOLUZIONE TETTONO-STRATIGRAFICA
No full textSul Promontorio del Circeo è stata condotta una campagna di rilevamento geologico tra l’ottobre del 2005 e luglio del 2006 su supporto topografico alla scala 1:10.000. L’adozione di metodi di analisi di facies per lo studio delle successioni sedimentarie ha permesso di offrire un contributo alla comprensione dei temi paleogeografici e strutturali, posti dall’area in studio. Il prodotto cartografico del rilevamento è una Carta delle litofacies in scala 1:10.000 che rivolge particolare attenzione ai terreni mesozoici. Il Promontorio del Circeo è un klippe, strutturato in pieghe e sovrascorrimenti, successivamente dislocato da faglie dirette ad alto e basso angolo. Esso rappresenta l’unico elemento in affioramento del bacino umbro-marchigiano, sito a Sud Ovest della piattaforma Laziale Abruzzese ed è stato suddiviso in tre unità tettono-stratigrafiche mesozoiche (unità delle Crocette, unità del Circeo, unità Monticchio-Peretto) ed una terrigena, posta al footwall del piano di thrust principale. Le formazioni affioranti sono in gran parte assimilabili da un punto di vista litostratigrafico a quelle tipiche del dominio pelagico Umbro-Marchigiano. Le formazioni mesozoiche sono state suddivise in litofacies in modo da ottenere maggiori informazioni, sia sugli ambienti di sedimentazione, sia sulla successione degli eventi deformativi che hanno portato all’attuale assetto strutturale. Sul Promontorio del Circeo affiorano dolomie, calcari e calcari marnosi di età giurassica in facies sia di piattaforma carbonatica (Calcare Massiccio) che in facies bacinali (Corniola, Rosso Ammonitico, Calcari e marne a Posidonia). Il Calcare Massiccio include dal basso verso l’alto le litofacies dei calcari bianchi cristallini e dei calcari ciclotemici. La formazione della Corniola contiene le dolomie farinose con selce, i calcari torbiditici e le dolomie scure con selce, che sono caratterizzate da una forte componente risedimentativa. I calcari nodulari marnosi ad ammoniti e i calcari micritici grigi a lamellibranchi pelagici sono le litofacies mesozoiche più giovani. Evidenze di terreno hanno dato luogo al riconoscimento di una inedita paleoscarpata liassica completamente sigillata dalla Corniola. Le litofacies oligo–mioceniche dell’unità terrigena sono terreni sinorogenici a composizione mista silicoclastico-calcarenitica mentre, quelle pleistoceniche sono di natura post orogenica e poggiano in discordanza sui terreni più antichi
Struttura del fianco occidentale del Massiccio del Gran Sasso d’Italia
No full textSul fianco occidentale del Gran Sasso è stata condotta una campagna di rilevamento geologico tra il Luglio 2007 e l’Agosto 2008 su supporto topografico alla scala 1:10.000. L’adozione di metodi di analisi di facies per lo studio delle successioni sedimentarie e la caratterizzazione delle strutture, associate ai lineamenti tettonici principali, hanno permesso di offrire un contributo alla comprensione dei temi paleogeografici e strutturali, posti dall’area in studio. Il prodotto cartografico del rilevamento è stata una Carta Geologica in scala 1:10.000, completa di sei transetti geologici. Il fianco occidentale del Gran Sasso d’Italia è il massiccio montuoso più elevato dell’Appennino ed è definito da pieghe e sovrascorrimenti, successivamente dislocati dalla tettonica estensionale quaternaria. Esso è costituito da una successione sedimentaria meso-cenozoica di piattaforma, base-of-slope, avanfossa silicoclastica, spessa complessivamente 5000 metri circa. La struttura è interessata da particolari rapporti tettono-stratigrafici, ereditati dalla tettonica pre-orogenica, che influenzano la propagazione dei sovrascorrimenti. Particolare attenzione è stata rivolta alla revisione della stratigrafia, confrontando i dati e le interpretazioni degli Autori con quelli raccolti in campagna. Nuovi dati sono emersi dallo studio delle formazioni giurassiche. Nell’area di Acqua San Franco, Versante S di Pizzo Cefalone, è presente un inedito alto strutturale liassico, delimitato da almeno tre settori ribassati e sigillato dal Verde Ammonitico. La Corniola ospita una evidente variazione laterale di facies. Il Corno Grande persiste come alto strutturale per tutto il mesozoico, vincolando la distribuzione dei risedimenti di piattaforma, che scorrevano sui blocchi ribassati. Un’area relativamente rialzata corrispondente alla zona di Campo Pericoli ospita una successione condensata e discontinua durante l’intervallo Cretaceo basale-Paleogene Medio, evidenziando il persistere di una “zona di alto” sud-orientale. La zona Venacquaro-Monte Corvo registra sempre caratteri depocentrali. L’attività della Faglia delle Tre Selle inizia probabilmente già nell’Eocene inf., caratterizzando facies e spessori dell’hangingwall. I thrusts frontali sono associati a pieghe per propagazione di faglia e a roto-traslazioni antiorarie. Il plunge della piega immerge verso la zona depocentrale, dimostrando l’importanza dell’eredità giurassica nella propagazione dei thrusts. Il basculamento complessivo della struttura Laga-Gran Sasso è legato a importanti sovrascorrimenti retrovergenti profondi, che definiscono una zona a triangolo. I sistemi di faglie dirette sono tuttora attivi ed interagiscono tra loro attraverso il trasferimento del rigetto, operato su faglie transtensionali ad alto angolo
Transtensive faulting in carbonates at different crustal levels: examples from SW Helvetics and Central Apennines
No full textFault rocks at different crustal depths generate different fault structures and tectonites mostly depending on lithology, fluid flow and temperature. In this note we address the fault zone development and deformation mechanisms that occur in carbonate lithologies at different crustal levels. We compare faults exhumed near the brittle-ductile transition in the Helvetic Nappes in theAlps and similar faults from the upper crust in the Gran Sasso area in the Central Appenines. We observe the different role of deformation processes at different depth and lithology and assess the tendency for localization at the Brittle Ductile Transition
From Mesozoic rifting to Apennine orogeny: The Gran Sasso range (Italy)
No full textThe Apennines are a low-temperature accretionary prism generated by the west-directed subduction of the Adriatic–Ionian plate, whose structural origin is still to be fully understood. The highest and best-exposed segment of the Apennines, the Gran Sasso range is here documented to unravel the tectonic history of the northern tip of Gondwana. It is located along a NE-verging salient of thrust sheets decoupling the sedimentary cover of the subducting Adriatic lithosphere. Field mapping and structural analysis along the E–W trending left-lateral transpressive segment of the salient highlight the interplay of the inheritedMesozoic passive margin stratigraphic and tectonic framework with the Neogene contraction. The rheological differences between the massive carbonate platform and the well-bedded turbiditic and pelagic limestones determined along-strike undulations of the thrusts geometries and fold styles during shortening. Heterogeneities are due to inherited syn- and post rift Mesozoic tectonics. The Gran Sasso overturned anticline shows a backlimb anomalously tilted toward the foreland and we infer this dip as being related to a deeper back-thrust of a triangle zone. The pinching out of the foredeep sequence on the growth anticline forelimb dates the contractional phases of the region to the late Messinian. From the late Pliocene to Present, the area has been uplifted and extended about 2 km by oblique normal faults cross-cutting the accretionary prism. Some of them are seismically active, as shown by the 2009 Mw 6.3 L'Aquila earthquake
A trip through the Wildhorn Nappe from Cretaceous to Neogene time (Helvetic Nappes, Switzerland)
No full textIn questa guida presentiamo un percorso attraverso la Falda del Wildhorn (Svizzera sudoccidentale), una delle falde
strutturalmente più alte delle coperture elvetiche. L’escursione ha preso luogo tra l’8 e il 9 agosto 2015 in quanto
scelta come escursione annuale del Gruppo Studi Tettonici della Società Geologica Svizzera. Il percorso è stato
organizzato come un itinerario di 2 giorni lungo i fianchi meridionali delle Alpi Elvetiche tra il Canton Berna e il Vallese
(Fig. 1). Questa guida contiene anche un breve inquadramento strutturale e stratigrafico, seguito dalla descrizione
degli Stop lungo una sezione geologica di riferimento che da sud a nord mostra sia le strutture del Neogene che
quelle Cretacee, concentrandosi sulle loro diverse caratteristiche strutturali. Le faglie neogeniche sono associate a
vene e strutture da duttili a fragili. Le faglie cretacee invece sono associate a fratture non riempite, sono discrete e
relazionate a paleo-scarpate associate a contatti stratigrafici inconformi, geometrie di faglie di crescita, slumps e
dicchi sedimentari. L’evoluzione temporale del sistema di faglie sin-sedimentarie cretacee è documentato anche da
foto panoramiche spettacolari ed è brevemente discusso in rapporto anche allo sviluppo del sistema a pieghe e
sovrascorrimenti e alla successiva estensione parallela all’asse principale dell’orogene alpino.A field trip across the Wildhorn Nappe (SW Switzerland), forming part of the Helvetic nappes, is presented here. The
field trip, which took place on the 8-9th August 2015, was the annual excursion of the Tectonic Studies Group of the
Swiss Geological Society. It has been organized as a 2-day itinerary along the southern slopes of the Helvetic Alps
between Kanton Bern and Valais (Fig. 1). This guide contains a brief structural and stratigraphic overview followed
by Stop descriptions along a main geological cross-section that, from south to north, shows both the Neogene and
the Cretaceous faults and their different structural characteristics. Neogene faults are associated with veins and
ductile to brittle structures. Cretaceous faults are comparatively dry, discrete and related to palaeo-escarpments
associated with stratigraphic unconformities, fault-growth geometries, slumps and sedimentary dykes. The temporal
evolution of the Cretaceous syn-sedimentary fault system is also documented by spectacular panoramas and briefly
discussed in relation to nappe-stack development and subsequent Neogene orogen-parallel extension
Transtensive faulting in carbonates at different crustal levels: Examples from SW Helvetics and Central Apennines
No full textTranstensive faulting in carbonates at different crustal levels: Examples from SW Helvetics and Central Apennines
No full textFault rocks at different crustal depths generate different fault structures and tectonites mostly depending on lithology, fluid flow and temperature. In this note we address the fault zone development and deformation mechanisms that occur in carbonate lithologies at different crustal levels. We compare faults exhumed near the brittleductile transition in the Helvetic Nappes in theAlps and similar faults from the upper crust in the Gran Sasso area in the Central Appenines. We observe the different role of deformation processes at different depth and lithology and assess the tendency for localization at the Brittle Ductile Transition
The Rawil depression: Its structural history from cretaceous to neogene
No full textIn the Rawil Depression between the Aar and Mont Blanc massifs, tectonic events ranging in age from at least the Cretaceous until recent times have produced a complex puzzle of folds and faults on the published maps. This thesis is a field based study that attempts to decipher this puzzle using a multidisciplinary approach, considering thermochronometry, anisotropy of magnetic susceptibility (AMS), paleomagnetism, and clumped- and stable-isotope data in addition to structural geological techniques including field mapping, stress inversion and microstructural analysis. The overall aim of this work is to establish the succession of events that produced and exhumed the Rawil Depression, which on its southern side is still one of the most seismically active sectors of the Alps. The geological history considered ranges from Cretaceous syn-sedimentary faulting, to nappe-stacking during the late Oligocene and early Miocene, to later up-doming and related exhumation in the late Miocene to recent. New geological maps and profiles of the Rawil Depression were constructed based on existing maps and extensive fieldwork. The characterization of different vein and fault sets allowed the progressive stress and stretching history of the area to be established in detail. The NE-striking fault set dips mainly to the SE and is the oldest. These faults developed as syn-sedimentary structures active at different stages during the Cretaceous and are marked in many places by karstification and silicification of the surface, by sedimentary dykes and by the onlap of younger basinal formations. These Cretaceous faults are no longer discernible toward the basal thrust of the Wildhorn Nappe, where their possible reactivation could be responsible for the lack of any real inverted limb. However, on the upper limb they are well preserved and not markedly reactivated. Instead, they acted as buttress, promoting shorter wavelength folding in the adjacent basinal sediments. They were also not significantly reactivated during later Neogene transtensional faulting. In a profile perpendicular to the chain, the Alpine orogeny was first characterized by in-sequence thrusting and nappe-stacking of the Ultrahelvetics and Wildhorn Nappe, followed by out-of-sequence overthrusting of the Penninic Nappes, and finally by broad antiformal doming of the whole nappe sequence. However, in reality the deformation history is much more three-dimensional. After initial NW-directed stretching related to thrusting and nappe formation, the stretching direction rotates towards the WSW to SW, generally in a counter-clockwise sense. This orogen parallel stretching is developed throughout the Rawil Depression, overprints the whole nappe-stack at conditions probably close to the metamorphic peak, and is the signal generally recorded in the magnetic fabric. Based on the new thermochronometric results up-doming occurred after ca 17-15 Ma and contributed to the cooling and large scale refolding of the Helvetic units. NW- to ca. W-striking oblique normal faults crosscut nappe boundaries and constitute a dextral wrench zone that displaces the already formed axial depression. Oblique normal faults record a progressive development from (1) fault initiation on en-échelon vein sets in brittle-ductile shear zones, to (2) mylonitization of both veins and country rock, localized on these pre-existing initial fault planes, to (3) mature faults accumulating significant displacement on cataclastic fault cores and more discrete surfaces. In places, fault rocks are transitional between mylonites and cataclasites are characterized by the coexistence and/or alternation of ductile and brittle processes. The range of clumped-isotope temperatures indicates that transtensive shearing and faulting began at temperatures in the range of ~180-150°C, whereas embrittlement took place at progressively colder temperatures and shallower depths. A calculated exhumation curve based on new zircon (U-Th)/He ages and published apatite and zircon fission track ages that were used to convert clumped-isotope temperatures to corresponding estimates of time and depth, thereby proving constraints on the development of different veins and fault sets during progressive exhumation. Deformational events can be tracked in both the magnetic fabric and in the paleomagnetic directions, providing useful information about the kinematics of deformation and the thermal conditions reached during metamorphism. In a later stage, stretching direction rotates, also generally in a counter-clockwise direction, from orogen-parallel to nearly orogen-perpendicular. This SE-directed extension perpendicular to the fold axes may be associated with the Rhône Fault after ca. 5.5 Ma, when the exhumation rate increased from about 0.2 to ca. 0.8 km/Ma. During the final stage of exhumation, reactivation of the Rezli Fault Zone occurred in the ‘Alpine region’ and also at the border with the southern region in the back-limb at the Col de Puchet Fault. This event is accompanied by more open circulation of meteoric fluids together with cataclasis localized on the main slip surfaces of a few faults
Anatomy of the Cycladic Blueschist Unit on Sifnos Island (Cyclades, Greece)
No full textSince 35 Ma, the kinematics of the Aegean domain has been mainly controlled by the southward retreat of the African slab, inducing back-arc extension. The main structures and associated kinematics are well constrained, but the kinematics of deformation before 35 Ma, coeval with the exhumation of blueschists and eclogites of the Cycladic Blueschist Unit, is still poorly understood. The earlier Eocene syn-orogenic evolution is strongly debated and very different geometrical interpretations and kinematic histories have been proposed in the literature. This study focuses on the high-pressure and low-temperature (HP-LT) parageneses spectacularly exposed and well preserved on Sifnos Island. The new field work provides new structural constraints on the tectonic history of HP-LT units generated in the subduction zone during the Eocene. It further shows how lithological heterogeneities localize strain within an accretionary wedge and how the localisation of strain evolves through time during exhumation. We show, through new geological and metamorphic maps, cross-sections and analyses of kinematic indicators, that Sifnos is characterized by shallow-dipping shear zones reactivating weak zones due to competence contrasts or earlier tectonic contacts. Structures and kinematics associated with these shear zones, show a top-to-the-N to −NE ductile deformation. The lower part of the tectonic pile shows a downward gradient of shearing deformation and is actually a thick top-to-the-NE shear zone, which we name the Apollonia Shear Zone. Through time shearing deformation tends to localize downward, leaving the upper part of the subduction complex preserved from late deformation. The present-day shape and topography of the island is largely controlled by late brittle faults reworking the earlier ductile shear zones. Comparing with the nearby island of Syros, we propose a new tectono-metamorphic evolution of the Cycladic Blueschist Unit, which partly explains the different degrees of retrogression observed on the Cycladic Islands
Fault-controlled volcanic vents between the Volsci Range and the magma-rich Tyrrhenian passive margin (Italy)
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