Ligurian Ocean Research Articles

Gibraltar subduction zone is invading the Atlantic

Abstract Subduction initiation is a cornerstone of the Wilson cycle. It marks the turning point in an ocean's lifetime, allowing its lithosphere to be recycled into the mantle. However, formation of new subduction zones in Atlantic-type oceans is challenging, given that it commonly involves the action of an external force, such as the slab pull from a nearby subduction zone, a far-field compression, or the impact of a plume. Notwithstanding, the Atlantic already has two subduction zones, the Lesser Antilles and the Scotia arcs. These subduction zones have been forced from the nearby Pacific subduction zones. The Gibraltar arc is another place where a subduction zone is invading the Atlantic. This corresponds to a direct migration of a subduction zone that developed in the closing Mediterranean Basin. Nevertheless, few authors consider the Gibraltar subduction to be still active because it has significantly slowed down in the past millions of years. Here, we use new gravity-driven geodynamic models that reproduce the evolution of the Western Mediterranean, show how the Gibraltar arc formed, and test if it is still active. The results suggest that the arc will propagate further into the Atlantic after a period of quiescence. The models also show how a subduction zone starting in a closing ocean (Ligurian Ocean) can migrate into a new opening ocean (Atlantic) through a narrow oceanic corridor. Subduction invasion is likely a common mechanism of subduction initiation in Atlantic-type oceans and a fundamental process in the recent geological evolution of Earth.

Geology of Montecastelli Pisano (inner Northern Apennines, Italy): normal and transfer fault zones affecting a dismantled ophiolite bearing orogenic wedge

ABSTRACT We present the geological map (1:10.000) of the Montecastelli Pisano area, where (i) the relationships between Neogene-Quaternary transfer and normal faults have been carefully mapped (Sheet 1), and (ii) the mantle sequence of the Ligurian Ocean has been analysed (Sheet 2). Fault analyses allowed to define: (i) NE-trending fault system (left-lateral strike- to oblique-slip faults) belonging to a transfer shear zone; and (ii) NW-trending normal to oblique-slip faults. Their evolution is framed in the extensional tectonics affecting the Northern Apennines since Early-Middle Miocene. The ophiolite complex is constituted by serpentinized spinelharzburgite with minor serpentinized dunite channels cut by gabbro dykes. A km-scale cataclastic zone cut the mantle section, locally hosting Cu-Fe mineralization. The goal of this work is to present the distribution of the main fractures affecting an area where the occurrence of ophiolite can favor the success of the CO2 reinjection project.

Deformation and Material Transfer in a Fossil Subduction Channel: Evidence From the Island of Elba (Italy)

AbstractWe document an exhumed plate boundary shear zone—a subduction channel—developed at the contact between a fossil Cretaceous‐Eocene accretionary prism (Ligurian Units) and the underlying continent‐derived nappes of the Northern Apennines. The subduction channel, referred to as the Norsi‐Cavo Complex (NCC), is continuously exposed for ∼10 km along strike on the island of Elba. The NCC consists of oceanic sediments and serpentinites, coupled at the base of the prism. In its northernmost exposures the NCC crops out as a serpentinite mélange with blocks of sedimentary rocks. There, evidence of pervasive fluid‐rock interaction is present. We interpret this mélange as the result of material transfer from the upper plate to the subduction channel through tectonic erosion at the base of the prism. The southern part of the NCC preserves a series of tectonic slices of shales and limestones (Palombini shales) coupled with ultramafic rocks through serpentinite shear zones containing lenses of massive serpentinites, mafic rocks, and sediments. Since sedimentary slices preserve the same structures of the overlying Ligurian Units, produced by accretionary prism deformation, we interpret this complex as the result of material transfer from the prism to the subduction channel and its subsequent underplating at the base of the prism. Based on our interpretation, the NCC fossil subduction channel formed during E‐verging deformation over the W‐directed subduction of the Ligurian Ocean during the early development stages of the Apennines. This deformation terminated when the continental margin entered the subduction.

LACUNA STRATIGRAFICA TRA LE FORMAZIONI DI RANZANO E DI ANTOGNOLA NELLE ZONE DI ROTEGLIA E MONTEBARANZONE (APPENNINO REGGIANO E MODENESE)

The Monte Piano — Blsmantova sequence (Middle Eocene — Late Miocene) in the Northern Apenmnes represents a new sedimentary cycle after the tectonic phase of the Early—Middle Eocene. This phase principally affected the structural Units of Ligurian domain. This sedimentary sequence (the postorogenic Ligurian Sequence), shows evidence of the progression of the Apennine orogenesis mainly through the development of unconformities of regional extension. In fact, this new sedimentary cycle began after the definitive closure of the Ligurian ocean (Oceanic stage) and sedimentation continued durIng the subsequent development of a collisional margin (Ensialic stage) between the European and African Adriatic plates.
 The stratigraphic and structural features of the Apenmne margin between Tresinaro Valley and Lavino Valley are described brteflv in this paper. Moreover, the contact between the Ranzano Formation and the Antognola Formation, parts of the postorogenic Ligurian Sequence, is discussed in terms of the micropaleontologica] data obtained from three stratigraphical sections situated In the lower valley of the Secchia River. The contact coincides with an unconformity that is marked by a biostratigraphical hiatus.
 In fact, foraminifera] associations indicate that the top of the Ranzano Formation is attributable to Zone PI 8 (Lower Oligocene) and that the base of the Antognola Formation is already attributable to Zone P 21 (Upper Oligocene). There was no evidence of the presence of Zone PI 9/20. The chronological range of the hiatus is estimated as 2.4 to 1 0 Ma, depending upon the chronological scale adopted and the exact position of the "top" of the Ranzano Formation and the base of the Antognola Formation.
 On the basis of the stratigraphic relations existing among the various units of the postorogenic Ligurian Sequence outcropping in the studied area and on the basis of the data collected on the contact between the Ranzano Formation and Antognola Formation, it is clear that the hiatus is attributable to submarine erosion of a part of the turbiditic deposits of the Ranzano Formation, which are affected by pronounced tectono—sedimentary phenomena. The tectonic movements developed in the Ligurian domain in a per10d preceding the deposition of the Antognola Formation. They are thus contemporary with the beginning of the deposition of the Macigno in the Tuscan paleogeographical domain.

Towards a Southern European Tethyan Palaeomargin provenance signature: sandstone detrital modes and detrital zircon U–Pb age distribution of the Upper Cretaceous–Paleocene Monte Bignone Sandstones (Ligurian Alps, NW Italy)

Constraining the source terranes of Alpine siliciclastic flysch sequences is crucial for building a clearer picture of the palaeogeography and geodynamic evolution of the Western Tethys in the framework of impending continental collision. This paper presents an integrated study that involves palaeocurrent dispersal analysis, sandstone petrography and detrital zircon geochronology of the Upper Cretaceous–Paleocene Monte Bignone Sandstones, a siliciclastic turbidite system deposited during the pre-collisional evolution of the Ligurian Alps. Palaeocurrent analysis illustrates an overall eastward transport of the proximal sediments in the present configuration. Considering the ca. 45°–50° counter-clockwise rotation of the Tertiary Piedmont Basin and of the Corsica–Sardinia block in the late Paleogene, this indicates the derivation of the sediments from the northern margin of the Piedmont–Ligurian Ocean. Sandstone petrography records a stratigraphic evolution from quartzose sandstones towards lithic and then to lithic sub-arkosic composition. This trend is interpreted to reflect the gradual unroofing of the provenance terrane. The lithotypes of the recycled sedimentary rock fragments and the up-section increase in dolostone and carbonate clast proportions suggest the erosion of the sedimentary cover of the southern European palaeo-margin. New geochronological data (U–Pb detrital zircon ages) correspond to the pre-Alpine stages of crustal growth recorded in the Variscan Maures-Tanneron Massif, and therewith confirm the derivation of the sediments from the passive palaeo-European margin. This conclusion highlights the importance of the lower plate in providing the source of coarse-clastic deep-water successions during pre-collisional convergent steps. Results from this multi-proxy provenance analysis contribute to better defining the detrital signatures associated to the continental micro-fragments that constituted the palaeo-European plate as it supplied deep-sea siliciclastic sediments into the Piedmont–Ligurian Ocean prior to continental collision.

Reply to discussion on ‘Middle Jurassic shear zones at Cap de Creus (eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte–Ligurian Ocean?’ Journal of the Geological Society, London , 174, 289–300

We welcome the discussion by Elena Druguet, Jordi Carreras and Jochen Mezger (Druguet et al. 2017) of our paper (Vissers et al . 2017 b ) in which we report Jurassic 40Ar/39Ar ages from shear zones at Cap de Creus, NW Spain. The authors argue that our interpretation of these results and the inferred development of the Cap de Creus shear zones during Jurassic stretching and opening of the Piemonte Ligurian ocean contradicts structural and tectonic evidence, and that it is not sufficiently supported by geochronological data. Below we first consider their structural and tectonic arguments from the tectonic–palaeogeographical scale down to the smaller field scale, and then proceed to discuss our Ar/Ar results. Druguet et al. (2017) note that prior to the Oligo-Miocene rifting event the Corso-Sardinian block was fixed east of NE Iberia, hence that the rocks at Cap de Creus were not located at the Jurassic pre-drift continental margin and that the northern Cap de Creus shear zones could not have formed as normal faults related to continental rifting. This palaeogeographical argument builds on a previous hypothesis by Stampfli & Borel (2002), Stampfli et al. (2002) and Stampfli & Hochard (2009) in which Iberia, the Corso-Sardinian block and the Brianconnais domain formed one single microcontinent. This hypothesis, however, must be discarded on the basis of recent palaeomagnetic work on Sardinia by Advokaat et al. (2014) showing that between Late Jurassic and Eocene time Sardinia underwent no vertical-axis rotations relative to Eurasia and was fixed to Europe in a rotated position that results from post-Eocene deformation, whereas all available evidence indicates that Iberia was located much further west and underwent some 35° counterclockwise rotation during the Cretaceous (van Hinsbergen et al. 2017; Vissers et al . 2017 a ). Sardinia, …

Discussion on ‘Middle Jurassic shear zones at Cap de Creus (eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte–Ligurian Ocean?’ Journal of the Geological Society, London , 174, 289–300

Vissers et al . (2017) performed muscovite 40Ar/39Ar geochronology on shear zones from the Northern Cap de Creus shear belt (Variscan basement of the eastern Pyrenees). They propose a model according to which these shear zones formed during Middle Jurassic extension associated with the opening of the Piemonte–Ligurian Ocean to the east. We argue that this model contradicts structural and tectonic evidence and is not sufficiently supported by geochronological data.

Structural, stratigraphic, and petrological clues for a Cretaceous–Paleogene abortive rift in the southern Adria domain (southern Apennines, Italy)

This study provides new structural, stratigraphic, and geochemical data and a literature review of the Cretaceous–Paleogene stratigraphy, biostratigraphy, tectonics, and magmatism in the southern Apennines belt, Italy, with the aim to demonstrate the occurrence of an Albian to Eocene abortive rifting stage in the southern Adria domain. During this time, the tectono‐stratigraphic evolution of the Adria domain is characterized by episodes of coeval uplift and drowning. Different sectors of the Apennine and Apulian platforms were so characterized by changes in the paleoenvironments, leading to different stratigraphic records (from shallow‐water to slope and basin), as well as the development of thick bauxitic levels. Contemporaneously, a large amount of calciclastic sediments supply from the emerging sectors was deposited in the basins surrounding the carbonate platforms (i.e., Ligurian and Lagonegro–Molise basins). The Albian–Eocene interval was also characterized by the occurrence of anorogenic magmatism and synsedimentary extensional faulting that, along with the changed sedimentary facies distribution, points out for a crustal‐scale extensional tectonics. We suggest that such tectonics is the result of a rifting episode, characterized by limited anorogenic magmatism, starting in the Albian and reaching its climax in the uppermost Cretaceous–Eocene times. In this scenario, the extensional tectonics recorded in the Adria domain was the product during an event of a single abortive rift system, which extended toward the south, from the southern margin of the Ligurian Ocean to the Hyblean (Sicily), Pelagian (Tunisia), and Sirte Basin Province Rift (Libya).

Middle Jurassic shear zones at Cap de Creus (eastern Pyrenees, Spain): a record of pre-drift extension of the Piemonte–Ligurian Ocean?

The Cap de Creus peninsula in NE Spain consists of greenschist- to amphibolite-facies metasediments and granitoid bodies of the Variscan Axial Zone of the Pyrenees, overprinted in the north by anastomosed greenschist-facies shear zones. Current tectonic interpretations ascribe these shear zones to the waning stages of the Variscan orogeny. We present muscovite 40 Ar/ 39 Ar data from the shear zones, yielding Middle Jurassic ages between 159.33 ± 0.43 and 175.18 ± 1.10 Ma and one Tertiary age of 58.57 ± 0.55 Ma. We suggest that the present-day structure at Cap de Creus resulted from Variscan deformation and HT–LP metamorphism, followed during the Jurassic by crustal stretching and development of ductile normal faults reflecting pre-drift continental extension related to opening of the Piemonte–Ligurian basin east of Iberia. Tilting during Alpine convergence caused steepening in the northern part of the penisula, with the ductile normal faults rotated to their present orientations appearing as dextral reverse shear zones. The shear zone yielding a Tertiary age could reflect either an Alpine structure or reactivation of an earlier, presumably Jurassic shear zone. The Cap de Creus structure may thus represent a continental margin that has undergone ductile stretching equivalent to the now-buried west Iberian or Newfoundland margin.

Fluid inclusion study of the Boccassuolo VMS-related stockwork deposit (Northern-Apennine ophiolites, Italy)

Several Cyprus-type volcanogenic massive sulphide (VMS) deposits are found in the Jurassic ophiolitic series of the Northern Apennines. Stratabound, stratiform and stockwork deposits were formed in the western limb of the Neotethys (Ligurian Ocean) and are found today in basalt, gabbro and serpentinised peridotite host rocks. The studied stockwork deposit at Boccassuolo is found in basalt and basalt breccia. Detailed petrography, fluid inclusion study, Raman spectroscopy analyses and chlorite thermometry calculations were used to determine the P, T, X conditions of the fluid circulation system. The veins contain three quartz generations, calcite, chlorite, epidote and sericite as gangue minerals and pyrite, chalcopyrite, sphalerite, pyrrhotite and galena as ore minerals. Based on the fluid inclusion study, the earlier defined three vein types (1., 2. and 3.) precipitated from the same type of evolving fluid, though at slightly different stratigraphical positions. The determined ranges of temperature (370 60°C), salinity (6.2 11.4 NaCl equiv. wt%), pressure (30-44 MPa) ranges and methane content (average 0.28 mol/kg) suggest an evolved seawater origin for the hydrothermal fluid, modified by fluid-rock interaction and possibly by mixing of magmatic volatile. The fluid characteristics and the mineralogical observations have proven a slightly distal position in relation to the centre of the fluid flow for all studied locations, but less and more distal blocks were also recognised. The temporal evolution of the system developed into a low temperature event, occurring after the main mineral stage, but still participating to the same process as a whole.

The Pb-rich sulfide veins in the Boccassuolo ophiolite: Implications for the geochemical evolution of hydrothermal activity across the ocean-continent transition in the Ligurian Tethys (Northern-Apennine, Italy)

Galena bearing sulfide veins have been discovered coexisting with Fe–Cu–Zn dominated veins in the hydrothermal stockwork of the Boccassuolo ophiolite (External Ligurides, Northern Apennine, Italy). The galena-rich veins cut across a volcanic pile composed of pillow lava flows, pillow breccia, and ophiolitic sandstone. Bulk-ore analyses indicate significant enrichment in Pb giving raise to mantle normalized Pb–Ag–Au–Zn–Cu patterns with unusual negative slope, in contrast with the average flat pattern of most sulfide deposits in the Internal Liguride ophiolites which reflect the Fe–Cu–Zn assemblage of ophiolite-hosted Volcanic-associated Massive Sulfide (VMS) deposits all over the world. A wide literature shows that, in contrast with the Internal Ligurides, plutonic and volcanic rocks of the External Ligurides display less depleted and even enriched geochemical characters, not consistent with common oceanic crust at mid oceanic ridges (MOR), but probably originated in the ocean–continent transition of the Adria continental margin. In this geodynamic context, pillow basalts become locally enriched in Pb with high Pb/Cu ratios, and other crustal-compatible elements such as Mo and U. The Pb enrichment observed in the veins Boccassuolo is interpreted to be a result of leaching of such anomalous volcanics forming the ophiolitic substrate. The case of Boccassuolo supports the conclusion that the geochemical character of hydrothermal activity evolved from Cu–Zn rich in MOR-type assemblages of the Internal Ligurides, towards composition enriched in Pb in the External Liguride domain, representing the transition from the Ligurian ocean to the Adria continental margin.

Ophiolite‐bearing mélanges in southern Italy

AbstractIn southern Italy two ophiolite‐bearing belts, respectively involved in the Adria‐verging southern Apennines and in the Europe‐verging thrust belt of the northern Calabrian Arc, represent the southward extension of the northern Apennines and of ‘Alpine Corsica’ ophiolitic units, respectively. They form two distinct suture zones, which are characterized by different age of emplacement and opposite sense of tectonic transport. The ophiolite‐bearing units of the southern Apennines are represented by broken formation and tectonic mélange associated with remnants of a well‐developed accretionary wedge emplaced on top of the Adria continental margin, with an overall NE direction of tectonic transport. These units consist of a Cretaceous‐Oligocene matrix, which includes blocks of continental‐type rocks and ophiolites with remnants of their original Upper Jurassic to Lower Cretaceous pelagic cover. The innermost portion of the accretionary wedge is represented by a polymetamorphosed and polydeformed tectonic units that underwent a Late Oligocene high pressure/low temperature (HP/LT) metamorphism. The northern Calabria ophiolitic‐belt is indeed composed of west‐verging tectonic slices of oceanic rocks which, embedded between platform carbonate units of a western continental margin at the bottom and the basement crystalline nappes of the Calabrian Arc at the top, are affected by a Late Eocene‐Early Oligocene HP/LT metamorphism. The main tectonic features of these two suture zones suggest that they can be interpreted as the result of the closure of two branches of the western Neotethys separated by a continental block that includes the crystalline basement rocks of the Calabrian Arc. We thus suggest that the north‐east verging southern Apennine suture constituted by a well‐developed accretionary wedge is the result of the closure of a large Late Jurassic‐Early Cretaceous oceanic domain (the Ligurian Ocean) located between the African (the Adria Block) and European continental margins. The northern Calabria suture derives indeed from the deformation of a very narrow oceanic‐floored basin developed during the Mesozoic rifting stages within the European margin separating a small continental ribbon (Calabrian Block) from the main continent. Copyright © 2008 John Wiley & Sons, Ltd.

Revised radiolarian ages for the sedimentary cover of the Balagne ophiolite (Corsica, France). Implications for the palaeoenvironmental evolution of the Balano-Ligurian margin

AbstractThe age of radiolarites covering the Balagne ophiolite is reassessed based on new and revised radiolarian fossil evidence. The oldest radiolarian cherts are dated as upper Bathonian-lower Callovian in two tectonic units: San Colombano and Novella. These are amongst the oldest ages available so far from ophiolites of the Ligurian ocean. An important stratigraphic gap, spanning the Callovian-early Kimmeridgian interval, is specified between radiolarites and the overlying (and locally gullying) San Colombano shallow-water limestones (sub-unit I). We can now specify that radiolarian ooze accumulated until the late Kimmeridgian in the distal parts of the Balagne margin (sub-unit SC III and Novella unit), while fragments of Hercynian basement fell into the Balagne basin during the late Kimmeridgian-Tithonian.

GEOLOGICAL SETTING AND STRUCTURAL STYLES OF VOLCANIC MASSIVE SULFIDE DEPOSITS IN THE NORTHERN APENNINES (ITALY): EVIDENCE FOR SEAFLOOR AND SUB-SEAFLOOR HYDROTHERMAL ACTIVITY IN UNCONVENTIONAL OPHIOLITES OF THE MESOZOIC TETHYS

This paper is an overview of the geological setting and structural styles of Cu-sulfide deposits of the VMS-type, associated with Tethyan ophiolites (the Ligurides) in the northern Apennines (Italy). The Italian deposits represent a rare example of VMS associated with both the mantle-peridotite basement and the overlaying volcanic pile, within a single ophiolite sequence. This peculiar feature is due to the particular geodynamic evolution of the Mesozoic Ligurian ocean that allowed the upper mantle to be exposed on the seafl oor for a long period before the outfl ow of MORBtype lava and the deposition of pelagic sediments (cherts, Palombini shales and Calpionella limestones). Middle to Upper Jurassic uprising of the basaltic magma initially provided the heath source for convective circulation of hydrothermal fl uids through the overlaying mantle section, giving rise to sulfi de deposition in crosscutting stockworkveins and seafl oor-stratiform ore bodies within the serpentinized mantle peridotite and the serpentinite breccia formed by submarine erosion of the upper mantle. The setting and structure of VMS associated with the volcanic pile indicate that hydrothermal activity continued during and after the eruption of pillow basalts at the ocean fl oor, forming stockwork veins and conformable stratabound ore bodies within the basalt unit. Furthermore, hydrothermal activity formed seafl oor-stratiform deposits, at the top of the volcanic pile, covered with a thick horizon of cherts containing exhalative deposits of Mn.

MINERALS OF AU, AG AND U IN VOLCANIC-ROCK-ASSOCIATED MASSIVE SULFIDE DEPOSITS OF THE NORTHERN APENNINE OPHIOLITE, ITALY

Ore samples from five copper sulfide deposits associated with Jurassic ophiolites in the Northern Apennine belt of Italy were found to contain gold, acanthite or argentite (Ag2S), and freibergite (Ag,Cu,Fe)12(Sb,As)4S13. These precious-metal minerals are part of a complex assemblage of microscopic to submicroscopic (^lt;10 mm) accessory phases, including uraninite, barite, galena, smithsonite, siderite, a La–Ce carbonate and a Sn hydroxide, attributable to the rare mineral hydroromarchite Sn3O2(OH)2. The textures of the sulfide host are indicative of metal deposition in a range of conditions, involving initial precipitation from hot, metal-charged solutions (syngenesis), followed by postdepositional replacement of biological material and resedimentation of clasts, at low temperature (epigenesis). Syngenetic gold is Ag-rich and associated with uraninite in Fe–Cu-rich ore. The fineness of native gold increases by loss of Ag into secondary Ag2S during seafloor weathering of the sulfides. Freibergite forms during recrystallization of sphalerite in massive Zn–Fe ore. Hydroromarchite occurs in massive Zn–Fe ore, closely related with late deposition of siderite and secondary pyrite, possibly due to the action of neutral to basic and reducing waters flushing through the ore in the initial stages of seafloor diagenesis. The widespread evidence for seafloor resedimentation and for biogenic- activity-driven reworking of the sulfides suggests a transition from proximal VMS to a more distal type of deposit formed by resedimentation processes in a dynamic tectonic environment. This scenario seems to be consistent with the interpretation of the Jurassic Ligurian ocean as a narrow basin in the early stage of its opening, having its modern analogue in the Red Sea.

Geological setting and structural styles of Volcanic Massive Sulfide deposits in the northern Apennines (Italy): evidence for seafloor and sub-seafloor hydrothermal activity in unconventional ophiolites of the Mesozoic Tethys

This paper is an overview of the geological setting and structural styles of Cu-sulfi de deposits of the VMS-type, associated with Tethyan ophiolites (the Ligurides) in the northern Apennines (Italy). The Italian deposits represent a rare example of VMS associated with both the mantle-peridotite basement and the overlaying volcanic pile, within a single ophiolite sequence. This peculiar feature is due to the particular geodynamic evolution of the Mesozoic Ligurian ocean that allowed the upper mantle to be exposed on the seafl oor for a long period before the outfl ow of MORB-type lava and the deposition of pelagic sediments (cherts, Palombini shales and Calpionella limestones). Middle to Upper Jurassic uprising of the basaltic magma initially provided the heath source for convective circulation of hydrothermal fl uids through the overlaying mantle section, giving rise to sulfi de deposition in crosscutting stockworkveins and seafl oor-stratiform ore bodies within the serpentinized mantle peridotite and the serpentinite breccia formed by submarine erosion of the upper mantle. The setting and structure of VMS associated with the volcanic pile indicate that hydrothermal activity continued during and after the eruption of pillow basalts at the ocean fl oor, forming stockwork veins and conformable stratabound ore bodies within the basalt unit. Furthermore, hydrothermal activity formed seafl oor-stratiform deposits, at the top of the volcanic pile, covered with a thick horizon of cherts containing exhalative deposits of Mn.

The Mesozoic continental rifting in the Mediterranean area: insights from the Verrucano tectofacies of southern Tuscany (Northern Apennines, Italy)

In the Alpine-Mediterranean region, the continental redbeds and shallow-marine siliciclastics related to the early depositional phases of the Late Permian-Mesozoic continental rifting are referred to as the most common representative of the “Verrucano tectofacies”. The Verrucano-type successions exposed in southern Tuscany are diachronous, spanning from Triassic to earliest Jurassic in age, and accumulated within the Tuscan domain, a paleogeographic region of continental crust that due to the opening of the Piedmont–Ligurian ocean formed part of the Adria passive-margin. They belong to the metamorphic Verrucano Group and the non-metamorphic Pseudoverrucano fm. Viewed overall, these Verrucano-type successions appear to manifest five episodes or pulses of an ongoing continental rifting. With the exception of the first episode that developed entirely within a terrestrial setting, each one is represented by basal Verrucano-type continental siliciclastics overlain by compositionally mixed marine deposits, which resulted from four diachronous, post-Middle Triassic transgressions. This suite of tectonic pulses produced the progressive westward widening (backstepping) of the Tuscan domain in the rifting south-Tuscany area.

MINERALS OF Au, Ag AND U IN VOLCANIC-ROCK-ASSOCIATED MASSIVE SULFIDE DEPOSITS OF THE NORTHERN APENNINE OPHIOLITE, ITALY

Ore samples from five copper sulfide deposits associated with Jurassic ophiolites in the Northern Apennine belt of Italy were found to contain gold, acanthite or argentite (Ag2S), and freibergite (Ag,Cu,Fe)12(Sb,As)4S13. These precious-metal minerals are part of a complex assemblage of microscopic to submicroscopic (<10 μm) accessory phases, including uraninite, barite, galena, smithsonite, siderite, a La–Ce carbonate and a Sn hydroxide, attributable to the rare mineral hydroromarchite Sn3O2(OH)2. The textures of the sulfide host are indicative of metal deposition in a range of conditions, involving initial precipitation from hot, metal-charged solutions (syngenesis), followed by post-depositional replacement of biological material and resedimentation of clasts, at low temperature (epigenesis). Syngenetic gold is Ag-rich and associated with uraninite in Fe–Cu-rich ore. The fineness of native gold increases by loss of Ag into secondary Ag2S during seafloor weathering of the sulfides. Freibergite forms during recrystallization of sphalerite in massive Zn–Fe ore. Hydroromarchite occurs in massive Zn–Fe ore, closely related with late deposition of siderite and secondary pyrite, possibly due to the action of neutral to basic and reducing waters flushing through the ore in the initial stages of seafloor diagenesis. The widespread evidence for seafloor resedimentation and for biogenic-activity-driven reworking of the sulfides suggests a transition from proximal VMS to a more distal type of deposit formed by resedimentation processes in a dynamic tectonic environment. This scenario seems to be consistent with the interpretation of the Jurassic Ligurian ocean as a narrow basin in the early stage of its opening, having its modern analogue in the Red Sea.

Recent advances in Alpine studies: tracking the Caledonian–Variscan belt in the internal western Alps

Recent advances in western Alps knowledge haveconcerned several key geodynamic issues. The deepstructure of the belt, and then the implication ofthe lower crust and lithospheric mantle in the colli-sional deformation were recognized through seismic–reflection and tomographic studies. The origin ofthe Piedmont–Ligurian Ocean and adjacent palaeo-margins was thoroughly elaborated through compar-ison with central Atlantic and the Galicia margin(it is worth to point here another striking similarity,i.e. with the oceanic margin of southwestern Aus-tralia [4]). A wealth of field and laboratory stud-ies have been devoted to the Alpine synkinematicmetamorphism, and particularly to the early, high-pressure–low-temperature (HP–LT) metamorphism,which increases from the Fe–Mg carpholite-bearing,low-grade blueschist-facies units of internal Briancon-nais, to the eclogite and ultra-high pressure (UHP)units of the internal massifs (IM) and overthrust-ing ophiolites [9,22 (and references therein)]. Thepeak pressure mineral assemblages from the ophio-lites and IM are now clearly dated from the Earlyto Late Eocene. Kinematic and thermal models havebeen coined taking into account the chemical compo-sition of the crust [14]. The HP–UHP metamorphism

Detrital Modes of the Ionian Forearc Basin Fill (Oligocene-Quaternary) Reflect the Tectonic Evolution of the Calabria-Peloritani Terrane (Southern Italy)

Abstract During Oligocene–Neogene time, the Calabria–Peloritani terrane (CPT), a small block of continental lithosphere, rifted from the southern margin of the Eurasian continent and drifted southeastward to collide with the African continent. Along the leading edge of the CPT, a late Oligocene to Quaternary, mostly siliciclastic succession (> 2,000 meters thick) was deposited in the Ionian forearc basin. The detritus forming the succession was derived directly from the erosion of the nearby CPT, composed of Hercynian plutons intruding metasedimentary host rocks. Contrary to most modern and ancient forearc basins, penecontemporaneous volcanic detritus is virtually absent; this results from rift-related topographic barriers which separated the forearc basin from the associated volcanic arc. In spite of an overall quartzofeldspathic, continental-block provenance, the analysis of a large sandstone and conglomerate petrological dataset (228 samples) points to three discrete detrital inputs of aphanitic lithic fragments in (i) late Oligocene–early Miocene, (ii) late Messinian, and (iii) Quaternary times. These petrological variations can be linked to independently determined tectonic events in the western Mediterranean, which likely modified the lithologic composition of the sediment source terranes. Input (i) documents the progressive, widespread unroofing of the continental basement of the southern European margin, when northwestward subduction of the remnants of the Ligurian ocean began. Input (ii) is related to widespread intra-Messinian tectonism possibly due to out-of-sequence thrusting induced by isostatic readjustments of the perimediterranean orogenic wedges following the Messinian drawdown. Input (iii) reflects the rapid vertical uplift experienced by the CPT since mid-Pleistocene time, possibly the result of slab detachment along the Ionian subduction zone. This study indicates that detailed statistical analysis of the detrital modes of ancient terrigenous deposits can be used to identify the tectonic events affecting somewhat petrologically monotonous sediment source areas within small lithospheric terranes.