Giancarlo Molli

The association of continental crust rocks with ophiolites in the Northern Apennines (Italy): implications for the continent-ocean transition in the Western Tethys

Abstract The Late Cretaceous sedimentary melanges from the External Liguride Units of the Northern Apennines include large slide-blocks of ophiolites and lower and upper continental crust rocks representative of a continent-ocean transition between the Internal Liguride oceanic domain and the thinned continental margin of the Adria plate. The slide-blocks preserve a record of the long-lived history of rifting which led to opening of the Jurassic Western Tethys Basin. The External Liguride ophiolites consist of: (1) undepleted spinel-peridoties, partly re-equilibrated under plagioclase-facies conditions, which were interpreted as unroofed subcontinental mantle; (2) rare gabbroic rocks (mainly troctolite to olivine-bearing gabbro) probably crystallised from N-MORB magmas; and (3) basalts with N- to T-MORB affinity covered by late Callovian-early Oxfordian radiolarian cherts. Both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re-equilibration to plagioclase-facies conditions. The slide-blocks of lower continental crust are composed of gabbro-derived mafic granulites and felsic granulites. The latter include quartzo-feldspathic granulites and rare quartz-poor to quartz-free charnockitic rocks. In both mafic and felsic granulites, granulite-facies re-equilibration was followed by a retrograde metamorphic evolution to amphibolite-, greenschist- and subgreenschist-facies conditions. Retrogression is commonly accompanied by deformations progressively changing from plastic to brittle. The upper crustal rocks occurring as slide-blocks consist of Hercynian granitoids with orogenic affinity, mainly biotite-bearing granodiorites and peraluminous two-mica leucogranites. Locally, the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts. The granitoids underwent polyphase brittle deformations under subgreenschist-facies conditions which predated the basalt emplacement. The tectono-metamorphic evolution recorded by the slide-blocks of the External Liguride Units started in the Late Carboniferous-Early Permian (about 290 Ma), with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites. The associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro-derived granulites. Mafic and felsic granulites subsequently underwent tectonic exhumation in Permo-Triassic times, as testified by the development of granulite- to amphibolite-facies ductile shear zones. The granulites were finally exhumed to shallow levels, probably in association with the subcontinental mantle, in Late Triassic-Middle Jurassic times. The latter period was most likely characterized by extensive brittle faulting at shallow crustal levels, thus giving rise to extensional allochthons formed by stretched slices of granitoids. The Western Tethys opening is finally testified by the basalt intrusion and effusion in the Late Jurassic, followed by deep-sea pelagic sedimentation. The External Liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent-ocean boundary. This reconstruction fits well with the available data on the present-day continental margins derived from passive lithosphere stretching.

Tectono-sedimentary evolution of the External Liguride units (Northern Apennines, Italy): insights in the pre-collisional history of a fossil ocean-continent transition zone

Abstract In the Northern Apennines, the External Liguride (EL) units are interpreted as derived from the domain that joined the Ligure–Piemontese oceanic basin to the Adriatic plate continental margin. The EL units can be divided into two different groups according to the lithostratigraphic features of the basal complexes underlying the Upper Cretaceous–Lower Tertiary carbonate flysch (e.g. Helminthoid flysch). The first group includes the western successions characterized by Santonian–Campanian sedimentary melanges where slide blocks of lherzolitic mantle, gabbros, basalts, granulites, continental granitoids are represented. The second group is represented by the eastern successions where the Cenomanian–Campanian basal complexes mainly consist of sandstones and conglomerates where the mafic and ultramafic rocks are scarce or completely lacking. Their original substrate is represented by the Middle Triassic to Lower Cretaceous, mainly platform carbonate deposits, found as slices at the base of the eastern successions. The stratigraphic features shown by the basal complexes allow the reconstruction of their source area that is assumed to be also representative for the pre-Upper Cretaceous setting. The proposed reconstruction suggests the occurrence in the EL domain of two distinct domains. The eastern domain was characterized by a thinned and faulted continental crust belonging to the Adriatic continental margin. The western domain was instead floored by subcontinental mantle associated with lower and upper continental crust, representing the ocean–continent transition. This setting is interpreted as the result of the opening of the Ligure–Piemontese oceanic basin by passive rifting, mainly developed by simple shear, asymmetric extension of the continental crust.

Microfabric study on the deformational and thermal history of the Alpi Apuane marbles (Carrara marbles), Italy

Marbles from different geometrical and structural positions within the Alpi Apuane metamorphic complex show a large variability in microfabric types as indicated by microstructure, c-axis orientation and temperature analysis. Statically recrystallized samples showing a granoblastic microstructure and polygonal grain boundaries are characterized by a grain size variation from east to west from 80–100 μm to 250–300 μm. This is correlated with an equilibration calcite-dolomite temperature from 360–380°C to 420°–430°C. Two kinds of dynamically recrystallized microstructures have been investigated: a first one exhibiting coarse grains (150–200 μm) with lobate grain boundaries and a strong shape preferred orientation and a second one characterized by a smaller grain size (40–50 μm) and predominantly straight grain boundaries. These microstructural types, associated with localized post-thermal peak shear zones and meter- to kilometer-scale folds, are interpreted as related to high strain and high temperature crystal plastic deformation mechanisms (dislocation creep) associated with predominant grain boundary migration (type-B1) or subgrain-rotation recrystallization (type-B2). These differences in dynamically recrystallized microstructures are related to equilibration temperatures higher in type-B1 (390°C) than in type-B2 (370–340°C). We have been able to relate the development of the different microfabric types to the successive stages of deformation of the Alpi Apuane metamorphic complex.

‘High-temperature’ texture in naturally deformed Carrara marble from the Alpi Apuane, Italy

Abstract From deformation experiments and numerical modelling, a large type variety of crystallographic preferred orientations (textures) are known for calcite. In contrast, naturally deformed samples usually show the ‘low-temperature (LT)’-texture type with only minor texture variations. The ‘high-temperature (HT)’-texture type is rarely described and mostly not very well defined. Based on neutron diffraction measurements and a quantitative texture analysis by means of the iterative series expansion method and the texture component model, this study gives evidence for the HT-texture type in a deformed marble from the Alpi Apuane in Italy. The microstructure of the sample shows elongated grains with long/short axis ratios of up to 10:1. The long axes of the grains are oriented parallel to the general direction of transport indicating prolate strain, but no shear sense. From the texture, a shear sense can be deduced that cannot be fully brought in line with the regional deformation and thermal history. The results indicate a larger texture variety of naturally deformed calcite rocks than generally assumed. This should stimulate further systematic texture studies for a better understanding of the texture forming mechanisms and the closely connected understanding of the kinematic significance of textures for the analysis of regional deformation histories.

Microstructural features of high temperature shear zones in gabbros of the Northern Apennine Ophiolites

Abstract This paper presents preliminary data on the microstructural evolution of upper amphibolite facies mylonitic metagabbros from the Bracco Ophiolite Complex, Northern Apennines (Italy). High temperature deformation, possibly related to the initial stages of breakup and oceanization in the Ligurian Tethys, led to the development of three different types of fabrics in the coarse grained Mg-gabbros, the most widespread rock-type in this area. With progressive deformations, coarse-grained porphyroclastic mylonites (fabric type 1), banded (fabric type 2) and ‘flinty’ ultramylonites (fabric type 3) were formed. In plagioclase and clinopyroxene, the two main minerals in the studied rocks, optical microstructures and scanning electron microscopy analyses indicate crystal plasticity as the main deformation mechanism. A possible change from ‘rotation’-recrystallization regime, through grain boundary migration to diffusion-assisted high temperature grain boundary sliding, is suggested for the plagioclase grains in high temperature shear zones.

Microstructures associated with static and dynamic recrystallization of Carrara marble (Alpi Apuane, NW Tuscany, Italy)

The present contribution summarizes the first results of a study focusing on microstructures from Alpi Apuane marbles. Its aim is both an analysis of the evolution of the metamorphic complex recorded in marbles and the supply of basic material for process-oriented studies on calcite microstructures due to natural deformation. Quantitative analysis of the variations of statically recrystallized microstructures suggest a relationship with the peak metamorphic temperatures. Previously unrecognized post-thermal peak shear zones, showing overprint microstructures typical of grain-boundary migration and dynamic recrystallization, are described; they document the natural deformation of Carrara marble.

Pre-orogenic High Temperature Shear Zones in an Ophiolite Complex (Bracco Massif, Northern Apennines, Italy)

This paper presents the first results of a structurally-oriented study of pre-orogenic high temperature shear zone structures which are preserved in the Northern Apennines ophiolites. A detailed analysis is made of the geometry, areal distribution and kinematics of upper amphibolite facies shear zones present in the gabbroic Bracco Massif. The high temperature shear zones show distinct deformation gradients, and three characteristic fabric types are recognized: porphyroclastic coarse grained metagabbro mylonites (fabric type 1), banded ultramylonites (fabric type 2), and “flinty” ultramylonites (fabric type 3). Microstructural observations suggest that the deformation in the gneissic mylonite types (fabric types 1 and 2) was dominated by dislocation creep, whilst a possible grainsize-sensitive flow mechanism may have been important in the very fine grained “flinty” ultramylonites (fabric type 3). Reconstruction of the shear zones into their presumed original orientations suggests that they developed as low- to medium-angle extensional fault zones. This has important implications for the inferred processes of ophiolite generation.

Surface-subsurface structural architecture and groundwater flow of the Equi Terme hydrothermal area, northern Tuscany Italy

A multidisciplinary integrated approach was used to study the structural architecture influencing the circulation pattern of geother -mal fluids in the Equi Terme area (NW Alpi Apuane, Tuscany). Geological-structural surveys were carried out to define the structural setting of the area and to characterize geometries and kinematics of fault systems. Chemical (major components) and isotopic analyses (δ18O‰, δ2H‰, 3H, δ13C‰[DIC], δ34S‰[SO4]) were performed on thermal water and cold springs. A geophysical survey was also conducted by means of both Magnetotelluric and Electrical Resistivity Tomography methodologies, in order to gain insight into the resistivity distribution at depth and to indirectly image the subsurface structure. This multidisciplinary approach proved to be a powerful tool, since it unravels the complexity of this natural geothermal system and provides useful suggestion for reconstructing the fluid circulation outflowing at the Equi Terme thermal spring. Results pointed out how the E-W oriented fault system (the Equi Terme Fault) play a key role in controlling the thermal groundwater outflow, and the chemical-physical features of this resource. This structural lineament separates high permeability carbonate complexes (footwall), in which both shallow and deep flow paths develop, from a medium-low permeability succession (hangingwall) that contains evaporitic formations from which thermal water acquires a high salinity and a composition of the Na-Cl (Ca-SO4) type. During the uprising along the fault system, the thermal water is also affected by a mixing with shallow fresh-cold waters that lead to a strong seasonal variation in the chemical-physical properties of the thermal springs.

Impact of erosion and décollements on large-scale faulting and folding in orogenic wedges: analogue models and case studies

Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small- and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. Recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold–nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy). Supplementary material: Raw data of the experiments are available at www.geolsoc.org.uk/SUP18658.

Fluid pressure cycles, variations in permeability, and weakening mechanisms along low-angle normal faults: the Tellaro detachment, Italy

Classical frictional fault reactivation models indicate that slip along misoriented fault planes is not possible under most conditions. Nevertheless, active or exhumed low-angle normal faults have been described in many settings worldwide. This discrepancy is addressed by contrasting models: (1) those proposing that low-angle normal faults result from postkinematic passive rotation of former high-angle extensional faults; and (2) those proposing that specific conditions can promote slip along misoriented fault planes. This paper describes the Tellaro detachment, a mid–late Miocene low-angle normal fault that was responsible for ∼500 m of tectonic vertical thinning in the carbonate-dominated Triassic to Lower Miocene succession of the Northern Apennines, Italy. By integrating structural, petrographic, isotopic, and fluid inclusion data, we show that: (1) the main kinematic activity of the Tellaro detachment occurred between ∼8 and 4 km depths and peak temperature ∼190 °C; (2) dilational breccias, tens of cubic meters in volume, are frequently associated with major low-angle fault segments; (3) slip along misoriented planes was favored by elevated fluid pressures and low differential stress; and (4) the fault system was characterized by transient permeability pulses and overpressure buildups, associated with multiple fracturing and cementation events that caused the downward migration of master slip surfaces. Results presented in this study show that: (1) in a fluid-active regime, continental crustal thinning can occur for shallow values of fault dip; (2) low-angle normal faults have a great influence on fluid circulation within the upper crust; and (3) episodic permeability enhancement and destruction in detachment faults can promote overpressure buildups, triggering deformation episodes.