Tectonics, structural analysis and geodynamic evolution of the Maghrebian Flysch Basin and Ligurian Accretionary Complex Units: Examples in the Western Mediterranean Area

This work provides a structural study on some successions of the Ligurian Accretionary Complex (LAC; southern Italy), Maghrebian Flysch Basin (MFB; Morocco) and External Dorsale Calcaire (Morocco). The LAC Units, cropping out in the southern Apennines include the sedimenary deep basin successions of Nord Calabrese, Parasicilide and Sicilide. Presently they are the highest tectonic units of the South Apennine fold and thrust belt. They are all characterized by a polyphasic and progressive deformation related to the Early Miocene inclusion in the tectonic accretionary wedge, by means of a frontal accretion mechanism, with a mean E/SE tectonic vergence. A subsequent deformation stage, associated to the eastward migration of the thrust front, affecting also the Middle-Upper Miocene unconformable wedge-top basin deposits, was characterized by a mean E/NE tectonic transport. In this orogenic phase the Apennine thrust sheet pile, formed by LAC and Apennine Platform Units, tectonically covered the successions located in the westernmost sector of the Lagonegro-Molise Basin. Finally a Pliocene-Middle Pleistocene regional fold set deformed the whole orogenic prism, comprised the LAC Units as consequence of a thick-skinned tectonics expressed by means of deeply rooted thrusts in the buried Apulian Platform carbonates. The metamorphic units of LAC, analyzed in this study, are the Frido and Diamante-Terranova Units, cropping out at Calabria-Basilicata boundary and northern Calabria, respectively. Both units are characterized by a HP/LT metamorphism reaching pressures of ca. 1.4/1.2 and 1.0 GPa and temperatures of 350 360 and 380 °C, respectively. The HP/LT parageneses include the Fe-carpholite, chlorite and phengite for the Frido and glaucophane, lawsonite, epidote and chlorite for the Diamante-Terranova Unit. The tectonic exhumation was recorded by Ca amphiboles. The P-T-paths, presented below, of both units indicate a cool and rapid exhumation. This is testified also by the preservation of HP/LT mineral parageneses and by non-isothermal exhumation such as marked in the P-T-paths of the Frido(this work) and Diamante-Terranova (Liberi and Piluso, 2009) Units. These units were subducted in the latest Oligocene and Early Eocene, respectively, with their complete exhumation in the middle Tortonian. The comparable geodynamic evolution of the LAC Units suggests an origin of all successions in a common oceanic domain (Ligurian Ocean) characterized by a western sector floored by oceanic crust (Diamante-Terranova domain), a central sector represented by an Ocean Continent Transition (Frido and Nord-Calabrese domain) and an eastern area formed by thinned continental crust (Parasicilide and Sicilide domain). A further aim of this study is the reconstruction of the tectonic evolution of some successions of the Maghrebian Flysch Basin (MFB) domain (Predorsalian and Massylian Units) and the External Dorsale Calcaire in a key area (Chefchaouen) of the Rif chain in the northern Morocco. Maghrebian Flysch Basin successions show a comparable stratigraphy with the sedimentary LAC successions, suggesting paleogeographic continuity between LAC, located to E/NE, and the MFB to the W. The Triassic-Lower Miocene External Dorsale Calcaire succession overthrust the Predorsalian Unit through a regional thrust fault well-exposed in Chefchaouen area. The kinematic analysis of this structure and all minor structures in the footwall, indicate a SW-tectonic vergence. The Predorsalian unit in turn overthrust the Massylian succession characterized by a similar progressive deformation. The whole tectonic pile was subsequentely deformed by thrust and folds verging to NW. Like the sedimentary LAC units, the MFB Units were deformed by frontal accretion in the Burdigalian-Langhian time. The External Dorsale Calcaire provides a good example of Inversion Tectonics. The Liassic succession (cherty limestones and conglomerates) recorded the extension related to the Jurassic rifting of the Neotethys Domain as normal faulting and veining.The subsequent inclusion of these rocks in the orogenic wedge, which mainly occurred in the Miocene time, deformed the most of pre-orogenic structures in a passive manner, with only few cases of reverse reactivation; whereas, frequently, pre-orogenic normal fault planes show only an indentation of hanging-wall and footwall (buttressing effect). The orogenic deformation includes two main stages; the first tectonic pulse, which occurred during the Burdigalian-Langhian interval, was characterized by a NE-SW shortening and recorded by folds, thrust and back-thrust faults. During this stage the carbonates of the External Dorsale Calcaire tectonically covered the Predorsalian succession, producing, in the thrust front, a SW verging regional fold. The second orogenic deformation, consisting of a NW-SE shortening, was expressed by thrust faults and related folds both verging to NW and SE, which probably occurred in the Late Miocene-Pliocene time

Structural analysis of fractures, faults and dykes in the Somma-Vesuvio volcano

A geological and structural survey on the Somma-Vesuvio volcanic edifice has been carried out with the main aims to define the contribution of local versus regional stress field in the evolution of the volcano and the relationships between caldera evolution, fractures and faults. Measurements of fractures, faults and dykes were collected, mainly inside the caldera sector, and analyzed to characterize the fracture network affecting volcanic rocks. Structures were recorded both in lavas and pyroclastic rocks. Most of fractures in lavas are cooling joints. Several cooling joint are reactivated as normal faults, following the caldera collapse. Generally fractures both parallel and orthogonal to caldera rim occur. Finally recent fractures are associated to gravity instability of volcano edifice are well evident in the eastern sector of the Mt. Somma. Other normal faults characterized by low angle dips occur along the cliff of Mt. Somma. Dykes appear as planar at places forming an en-echelon pattern or with a ramp-flat geometry. Fractures, dykes and fault show some preferred orientations. A field stress map was carried out from inversion of meso-scale faults and fractures

Brittle vs. ductile strain during the synorogenic exhumation of HP-LT rocks: An example from the Lungro-Verbicaro Unit mylonites (northern Calabria, Italy)

We present a study on the mylonites exposed in the Cirella area along the Tyrrhenian coast of northern Calabria (southern Italy), in order to understand the role of ductile and brittle deformation on the exhumation process of these rocks. The analyzed shear zone is located in the footwall of a regional tectonic contact between metaophiolites (Ligurian Complex) on the top, and the continental Lungro-Verbicaro Unit (Adria plate) at the bottom. The latter consists of a metasedimentary succession with Triassic shallow-water carbonates at the base of it, evolving upward to Jurassic-lower Miocene slope to deep basin carbonates and foredeep siliciclastic rocks. Both oceanic and continental successions are characterized by high-pressure and low-temperature (HP-LT) metamorphism. The analyzed mylonites show different degrees of strain, ranging from protomylonites to ultramylonites, the latter characterized by well-marked foliation and stretching lineations. Several micro- and mesoscale progressive deformation structures, including two fold sets, S-C’ structures are present in these rocks, and a late oblique foliation, indicating a NW sense of shear. A successive shortening stage produced thrust faults and related folds, indicating a tectonic transport toward SE. We associate the mylonitic deformation with the activity of the overlying detachment fault, which is responsible for the synorogenic exhumation of the Lungro-Verbicaro Unit during the Miocene time. However, the ductile strain, recorded by the studied mylonites, accounted for a small displacement during the exhumation of this tectonic slice. In fact, the calculated displacement achieved by the Lungro-Verbicaro Unit, during exhumation, is much larger than the one estimated considering only the ductile zones activity. This evidence suggests that, synchronously with ductile strain, brittle deformation occurred along the detachment fault located at the top of the thrust sheet. In addition, a severe erosion due to the tectonic exhumation of the HP-LT rocks (in the late Miocene) produced a large amount of clastic sediment that filled the extensional basins widespread in northern Calabria

Fault rocks within the blueschist metabasalts of the Diamante-Terranova unit (southern Italy): potential fossil record of intermediate-depth subduction earthquakes

We report the first evidence of fault rocks developed during high-pressure/low temperature subduction-related metamorphism, within quartz+epidote pods embedded in the glaucophane-lawsonite-bearing ophiolitic metabasalts of the Diamante-Terranova unit (Calabria, Italy). Fault rocks occur as relic injections appearing as thin dark seams, locally showing an internal foliation characterized by tabular, curvilinear and meander-like shapes, and consist of very fine grains of glaucophane and titanite, locally including survivor clasts of epidote and lawsonite. Some boudinaged veins show glaucophane fibres in the boudin necks, marking a clear HP/LT syn-metamorphic origin at ca. 30 km depth. The injected fault rocks can be alternatively interpreted either as pseudotachylytes or as fluidized ultracataclasites. Although subsequent recrystallization largely obliterated primary diagnostic features, the occurrence of (i) different coloured flow streaks, characterized by alternating layers of glaucophane and titanite, (ii) well-developed flow-folds and (iii) corroded epidote survivor crystals, could indicate a viscous flow of molten material characterized by a non-uniform chemical composition. With this in mind, we support the hypothesis that these fine-grained veins were originally pseudotachylytes generated by the frictional melting of the glaucophane-rich layers of the Diamante-Terranova metabasalts, likely related with seismic events occurring during the Eocene along thrust faults within the subducting oceanic Ligurian lithosphere. The lack of evidence for pseudotachylyte relics in the metabasalt source rock argues for a selective preservation, largely dependent on the efficient mechanical shielding action of the stiffer quartz+epidote pods

Structural analysis of Chefchaouen area in the Western Rif (Morocco)

This paper provides a structural analysis of the Chefchaouen area, in the Northern Rif. Here the Dorsale Calcaire succession tectonically superpose the Predorsalian Unit. Both successions overthrust the Massylian Unit. The carbonates of the Dorsale Calcaire are characterized by pre-orogenic structures, such as normal faults, veins and fractures, resulting from extension related to the Liassic-Dogger rifting of the Neotethys domain. The successive inclusion of these rocks in the orogenic wedge, mainly occurred in the Miocene time, has deformed the most of pre-orogenic structures in a passive fashion, without an overall reverse reactivation. The Predorsalian Unit is characterized by a two hundred meters thick shear zone located close to the contact with the Dorsale Calcaire carbonates. Here the dominantly pelitic levels are highly deformed by (i) C’ type shear bands indicating a mean WSW tectonic transport and (ii) conjugate extensional shear planes marking an extension both orthogonal and parallel to the shear direction. Such as the previous succession, conjugate extensional shear bands and normal faults indicate a horizontal extension parallel to the thrust front synchronous with the mainly WSW-directed overthrusting. Also the Massylian succession shows a progressive deformation characterized by two superposed fold sets. The whole thrust sheet pile recorded a further shortening, characterized by a NW-SE direction, expressed by several reverse and thrust faults and related folds. Finally strike-slip and normal faults were the last deformation structures recorded in the analyzed rocks

Tectonic evolution of the Dorsale Calcaire, Pre - Dorsalian and Massylian units in the Chefchaouen area (Northern Rif, Morocco)

In questo lavoro è presentata l'analisi strutturale delle successioni bacinali e di piattaforma calcarea affioranti nell’area di Chefchaouen (Rif settentrionale). L'analisi ha permesso la ricostruzione della storia tettonica delle successioni studiate, caratterizzata dalla sovrapposizione di più fasi deformative. Le prime due fasi sono associate al sovrascorrimento, vergente a SW, della Dorsale Calcaire sulle unità Pre-Dorsalian e Massylian, registrata come deformazione progressiva nelle successioni poste a letto. La terza fase è contraddistinta da un raccorciamento diretto NW-SE, mentre le ultime due fasi sono caratterizzate da faglie trascorrenti e normali che deformano l'intera pila tettonica

Deformation characterization of a regional thrust zone in the northern Rif (Chefchaouen, Morocco)

This paper provides the structural analysis of the Chefchaouen area in the northern Rif. Here the Dorsale Calcaire superposes, by means of an excellently exposed thrust fault, onto the Predorsalian succession in turn tectonically covering the Massylian Unit. Hanging wall carbonates of the Dorsale Calcaire Unit form a WSW-verging regional fold with several parasitic structures, deformed by late reverse faults in places indicating an ENE vergence. A two hundred meters thick shear zone characterizes the upper part of the Predorsalian succession, located at footwall of the Dorsale Calcaire Unit. Here the dominantly pelitic levels are highly deformed by (i) C’ type shear bands indicating a mean WSW tectonic transport and (ii) conjugate extensional shear planes marking an extension both orthogonal and parallel to the shear direction. The Massylian Unit is characterized by a strain gradient increasing toward the tectonic contact with the overlying Predorsalian succession, where the dominantly pelitic levels are so highly deformed so as appearing as a broken formation. Such as the previous succession, conjugate extensional shear bands and normal faults indicate a horizontal extension parallel to the thrust front synchronous with the mainly WSW-directed overthrusting. The whole thrust sheet pile recorded a further shortening, characterized by a NW-SE direction, expressed by several reverse and thrust faults and related folds. Finally strike-slip and normal faults were the last deformation structures recorded in the analyzed rocks. A possible tectonic evolution for these successions is provided. In the late Burdigalian, the Dorsale Calcaire Unit tectonically covered the Predorsalian succession and together the Massylian Unit. The latter two successions were completely detached from their basement and accreted in the orogenic wedge within a general NE-SW shortening for the analyzed sector of the northern Rif. At lithosphere scale the thrust front migration was driven by roll back and slab tear mechanisms producing a synchronous arching and related counterclockwise rotation of the tectonic prism along the African margin. Radial displacement involved extension parallel to the thrust front well-recorded in the analyzed rocks. The NE-SW shortening, probably acting in the Tortonian-Pliocene interval, was related to the final compression of the Rif Chain resulting in out-of-sequence thrusts affecting the whole orogenic belt

From Jurassic extension to Miocene shortening: An example of polyphasic deformation in the External Dorsale Calcaire Unit (Chefchaouen, Morocco)

Liassic rocks of the External Dorsale Calcaire succession, cropping out in the northern Rif close to Chefchaouen city, host pre-orogenic structures, such as normal faults, veins and fractures, resulting from extension related to the Jurassic rifting of the Neotethys Domain. The successive inclusion of these rocks in the orogenic wedge, which mainly occurred in the Miocene time, deformed themost of pre-orogenic structures in a passivemanner, without an overall reverse reactivation. The orogenic deformation includes two main stages; the first tectonic pulse, which occurred during the Burdigalian–Langhian interval, was characterized by a NE–SW shortening (in the current coordinates) and recorded by folds, thrust and back-thrust faults. During this stage the carbonates of the External Dorsale Calcaire tectonically covered the Predorsalian succession, producing, in the thrust front, a SW-verging regional fold. The second orogenic deformation, consisting of a NW–SE shortening,was expressed by thrust faults and related folds both verging to NWand SE,which probably occurred in the Late Miocene-Pliocene time