Adria Plate Research Articles

The 1627 Gargano earthquake (Southern Italy): Identification and characterization of the causative fault

We present the results of a study of the subsurface tectonic features of the Basso Molise, Western Gargano and Northern Capitanata regions (Southern Italy) aimed at the identification of the source of the disastrous 1627 Gargano earthquake. In the maximum-damage area of this earthquake we have recognised a normal fault, here called the Apricena Fault, which has been identified as the fault that caused the seismic event. The Apricena Fault, striking WNW-ESE and dipping towards SSW, extends in the subsurface for about 30 kilometres from Serracapriola to Santa Maria di Stignano cutting through the whole Quaternary sequence. Other important tectonic structures trending WNW-ESE recognized in the area belong to an inactive Pleistocene strike-slip-fault system that is linked to the Mattinata Fault and to its offshore continuation in the Gondola-Grifone structural high. The Mattinata Fault and the Gondola-Grifone High form a quite complex structural feature whose kinematic behaviour is still matter of debate in the regional geological literature. NW-SE structural features recognized in the area are extensional faults whose activity was probably related to the late flexure-hinge retreat of the Adria plate margin during the Late Pliocene-Early Pleistocene eastward migration of the thrust belt-foredeep-foreland system.

Late Cretaceous Carbonate Turbidites of the Northern Apennines: Shaking Adria at the Onset of Alpine Collision

Abstract Thick and regionally widespread deepwater carbonate turbidite successions, the Upper Cretaceous Helminthoid Flysch, were deposited during the initial stages of Alpine collision. Despite the dominant intrabasinal origin, bulk composition and heavy minerals of minor terrigenous mode indicate provenance from different crustal levels of the European and Adria plates. These data allow us to place the Helminthoid Flysch units within a Late Cretaceous paleogeographic reconstruction of the Northern Apennine Tethys. The depositional characters of these turbidites indicate the coexistence of an active tectonic setting, a paleogeographic location prone to carbonate ooze production and storage, and limited terrigenous sources outside the basin.

AN UPDATE OF THE JURASSIC OPHIOLITES AND ASSOCIATED CALC-ALKALINE ROCKS IN THE SOUTH APUSENI MOUNTAINS (WESTERN ROMANIA)

This paper presents a synthesis of the researches so far carried out by the authors on the Jurassic magmatic sequences of South Apuseni Mountains. The Apuseni Mountains represent an alpine orogenic belt located in the hinterland of the Southern Carpathians. The Apuseni Mountains include a pile of basement nappes affected by Hercynian metamorphism (Bihor, Biharia, Baia de Aries, Codru nappe complexes). These nappes are overlain by an imbricated stack of tectonic units mainly consisting of Late Cretaceous clastic deposits that are, in turn, topped by the Mures nappe. This nappe includes a Middle Jurassic ophiolite sequence covered by Upper Jurassic calc-alkaline volcanics. The ophiolite sequence consists of a gabbroic complex overlain by a sheeted dike complex and a volcanic sequence including massive and pillow-lavas. Cherts associated to pillow-lava basalts have provided Callovian to Oxfordian radiolarian associations. According to the geological and geochemical evidences, we propose that the ophiolite sequence preserved in the Southern Apuseni Mountains is representative of an oceanic lithosphere formed in a mid-ocean ridge setting. The calc-alkaline series is mostly characterized by volcanic rocks including: basalts, basaltic andesites, andesites, dacites and rhyolites showing geochemical features typical of an intra-oceanic arc setting, and is thought to be related to Late Jurassic convergence between Eurasia and Adria plate. The calc-alkaline series is in turn overlain by Late Jurassic shallow-water limestones showing a gradual transition to Cretaceous carbonate deposits. The geological and geochichemical features are consistent with a possible linkage of the Apuseni Mountains ophiolites with northern continuation of the Vardar oceanic domain of the Hellenic-Dinaric belt. The present-day location in the hinterland of the Carpathian area achieved in the Late Paleogene – Early Neogene time span, when the escape tectonics produced a large-scale displacement of blocks originated from the northernmost edge of the Adria plate.

ALPINE-APENNINE OPHIOLITIC PERIDOTITES: NEW CONCEPTS ON THEIR COMPOSITION AND EVOLUTION

Ophiolites exposed in the Alpine-Apennine mountain range represent the oceanic lithosphere of the Ligurian Tethys, a small oceanic basin separating the Europe and Adria plates in Mesozoic times. Most of the peridotites represent former subcontinental mantle which was: (i) isolated from the convective mantle at different times (from Proterozoic to Permian); (ii) accreted to the thermal lithosphere, where it cooled along a conductive geothermal gradient under spinel peridotite facies conditions. Our investigations reveal important records of melt/peridotite interaction and melt impregnation (i.e. formation of plagioclase peridotites), which were related to asthenosphere/lithosphere interaction occurred during lithospheric extension leading to rifting and drifting of the Jurassic Ligurian Tethys. The early asthenosphere/ lithosphere interaction was caused by the reactive percolation of asthenospheric melts, which induced significant depletion, refertilization and heating of the lithospheric mantle peridotites. The plagioclase peridotites of the Alpine-Apennine ophiolites mostly derive from melt impregnation, whereas part of the depleted spinel peridotites result from reactive percolation of depleted MORB-type melts, rather than being solely refractory residua after nearfractional melting. The presence of large areas of impregnated peridotites indicates that significant volumes of melts were trapped in the lithospheric mantle. Subsequently, the asthenospheric melts reached the surface, both intruding as MORB gabbroic bodies or extruding as MORB lava flows. These peridotites record two magmatic cycles: (1) An early magmatic, non-volcanic stage: the early diffuse porous flow percolation and impregnation by single melt increments, focused percolation in dunite channels and intrusion of MORB-type melts; (2) A late magmatic, volcanic stage: the late intrusion and extrusion of magmas deriving from aggregated MORB liquids. The sequence of periods characterized by absence (non-volcanic or a-magmatic or magma-starved stages) and presence (volcanic or magmatic stages) of volcanism represents one of the most peculiar feature of slow and very-slow spreading ridges. Melt stagnation in the oceanic lithospheric mantle has been proposed as the dominant mechanism of peridotite impregnation and plagioclase peridotite formation along slow spreading systems. It could be that amagmatic periods of (ultra-) slow spreading ridges are characterized by melt stagnation in the thermal lithosphere, leading to plagioclase peridotite formation. Our results evidence the great variability in terms of melt composition and regime of melt percolation during the rift evolution. They provide, moreover, a mechanism to explain non-volcanic and volcanic stages during the rift evolution of the Ligurian Tethys and might be equally applicable to modern slow spreading ridges, which are characterized by variable magmatic (volcanic) and amagmatic (non-volcanic) stages.

COMPARISON AMONG THE ALBANIAN AND GREEK OPHIOLITES: IN SEARCH OF CONSTRAINTS FOR THE EVOLUTION OF THE MESOZOIC TETHYS OCEAN

In this paper the stratigraphical, structural, geochemical and petrological features of the Mirdita (Albania) and Pindos, Vourinos, Koziakas, Othrys and Argolis (Greece) ophiolitic nappes are summarised and then compared. These ophiolitic nappes occur as a 700 km long belt running from Albania to Greece. These ophiolitic nappes are located between the west-verging imbricate stack of thrust sheets derived from the Adria plate continental margin to the west and the Pelagonian zone to the east. Each ophiolitic nappe is represented by several end-members represented by the sub-ophiolite melange, the ophiolite sequence( s) with their sedimentary oceanic cover and the supra-ophiolite deposits. The latter can be divided in syn- and post-emplacement deposits, the first ones are recognised only in Albania. All the described ophiolite sequences are characterised at their base by a well-developed metamorphic sole that represents a further end-member of the ophiolitic nappe. The comparison among the features of all the end-members recognised in the studied ophiolitic nappes allows providing further constraints for the geodynamic reconstructions of the Mesozoic Tethyan oceanic basin located eastwards of the Adria plate.

Cr-spinel supply in the Brkini, Istrian and Krk Island flysch basins (Slovenia, Italy and Croatia)

In Late Cretaceous times, subduction of oceanic crust occurred to the north of the Adria plate and was followed by the formation of ophiolitic complexes. Continental collision in Alpine orogenic belts lasted from Late Cretaceous to Early Tertiary times. The progressive contraction of oceanic crust caused the uplift of previously rifted continental margin and platforms and the formation of foredeep flysch basins. Detrital Cr-spinels are widespread in Eocene sandstones of the Brkini, Istrian and Krk Island foredeep flysch basins. On the basis of their TiO2 content and FeO/Fe2O3 ratio, spinels derived from peridotites and mantle-derived magmatic rocks were distinguished. The first are statistically more abundant and are considered to have been derived from type I and II peridotites. The second appear to be mainly related to backarc basin products. These results suggest that Cr-spinels were derived from the erosion of the Internal Dinarides, where type II and III peridotites are present, and also from the Outer Dinarides, where type I peridotites crop out.

Fracture networks on the Belluno syncline, a fault-propagation fold in the footwall of the Belluno thrust, Venetian Alps, NE Italy

Abstract The Belluno syncline is a structure located in the Venetian fold-and-thrust belt in the Italian Southern Alps. This SSE-vergent chain mainly developed during the Neogene as a back-thrust belt of the Alps deforming the northern margin of the Adria plate. The syncline is asymmetric in shape, with a steep backlimb, as the forelimb of the paired Monte Coppolo Anticline; it is the result of fault-propagation folding during the development of the underlying Belluno thrust. In plan view the Belluno thrust shows a prominent curvature interpreted as reactivation of inherited Mesozoic extensional structures. The hinge of the Belluno syncline presents a similar curvature. Rocks cropping out in the syncline core are Upper Chattian to Langhian fine-grained molassic sediments, mainly siltstones and marls. Most fractures on the Belluno syncline have formed at high angles to bedding. The dominant fracture trends are subparallel and orthogonal to the fold hinge. Sometimes these fractures show plumose structures revealing extension (Mode I) fracture mechanism and therefore they are referred to as longitudinal ( bc ) joints and cross ( ac ) joints respectively. Other fracture trends oblique to the hinge line are found in measurement stations located on limbs distant from the fold hinge. They are referred to as oblique (shear) fractures. Throughout the western part of the Belluno syncline the fracture network maintains a roughly symmetrical distribution with respect to the fold hinge, suggesting a development in association with folding.

Upwelling of deep mantle material through a plate window: Evidence from the geochemistry of Italian basaltic volcanics

The isotopic compositions of basaltic lavas from the Quaternary and Plio‐Pleistocene Italian volcanics (Tuscan Magmatic Province, Roman Magmatic Province, Vesuvius, Aeolian Islands, Etna, and Iblean Basin) define binary hyperbolic relationships in the 87Sr/86Sr–206Pb/204Pb–εNd–εHf space. The isotopic compositions of the two end‐members of these mixing arrays are assessed by least‐squares regression. The mantle‐derived component (206Pb/204Pb = 19.8, 87Sr/86Sr = 0.7025, εNd = +8, εHf = +9) is a rather homogeneous mixture of the standard high‐μ (HIMU) and depleted mantle (DM) components. The crust‐derived component (206Pb/204Pb = 18.5, 87Sr/86Sr > 0.715, εNd = −12, εHf = −11) accounts for the enrichment of K and other large‐ion‐lithophile elements in the Italian volcanics. As shown by the relationship in εHf–εNd space and the lower‐than‐chondritic Hf/Sm ratio, this crustal component is dominated by pelagic sediments rather than terrigenous material. The overall scarcity of calc‐alkaline compositions in the Italian volcanics and the presence of a HIMU component, which is the hallmark of hot spot basalts, raise the question of how plume mantle source contributes to volcanism in a subduction environment. At about 13 Ma, the Apennine collision terminated the westward subduction of the Adria plate under the European margin and rotated the direction of convergence to the northwest. The cumulative differential of subduction between the fossil plate under Tuscany and the active plate under Sicily since the opening of the Tyrrhenian Sea amounts to at least 300 km and is large enough to rift the dipping plate and open a plate window beneath the southern part of the peninsula. This model is consistent with recent high‐resolution seismic tomography. We propose that the counterflow of mixed upper and lower mantle passing the trailing edge of the rifted plate is the source of Italian mafic volcanism. Alternatively, material from a so‐far unidentified plume may be channeled through the plate window. The crustal signature is probably acquired by interaction of the mantle advected through the window with the upper part of the subducted plate.

Petrogenesis and geotectonic setting of ultramafic rocks in the Eastern Alps: constraints from geochemistry

In the Eastern Alps, a heterogeneous pile of Pre-Mesozoic and Mesozoic nappes was emplaced during several distinct orogenic events. Ultramafic rocks are preserved in: (1) Pre-Mesozoic basement units of the Penninic Tauern Window and in Austroalpine basement complexes, and (2) in Mesozoic units within Penninic windows and in overlying Austroalpine segments. Geochemical data (major, trace and rare earth elements, REE, of bulk rock samples) of ultramafic rocks from 18 regions covering all the major tectonic units are presented. Metamorphic olivine-rich and clinopyroxene-rich ultramafic rocks and serpentinites from the Cambro-Ordovician Lower Schist cover units of the Tauern Window (Stubach Group, “Greiner Series”) are grouped into peridotites (#Mg values=molecular 100×Mg/(Mg+Fe tot), 87–92; <2 wt.% Al 2O 3) and clinopyroxene-rich pyroxenites (#Mg=80–92; 1–3 wt.% Al 2O 3). Major and trace element characteristics indicate a residual origin of most peridotites as restites after 10–20% partial melting. Most probably, the pyroxenites are crystal segregates from a basaltic melt in a mantle transition zone. Small lenses of completely serpentinised ultramafic rocks in the Cambrian Habach Group (#Mg=90–92) also have residual character, although they are strongly metasomatised. The Stubach and Habach Groups developed in a back arc and volcanic arc setting, respectively, along the northern margin of Gondwana. Metamorphic harzburgite and dunite (#Mg=89–90; 0.5–2.4 wt.% Al 2O 3) veined by metagabbroic dikes were sampled in the Austroalpine Silvretta nappe (Hochnörderer). The harzburgites represent restites after 12% partial melting in suboceanic mantle, most probably during the Cambrian. In the Austroalpine basement east of the Tauern Window, metamorphosed ultramafic rocks occur in association with amphibolite and eclogite in the Proterozoic (about 750 Ma) Speik Complex. Harzburgite and dunite (#Mg=89–92; 0.1–0.8 wt.% Al 2O 3) are highly depleted in incompatible elements (2.6–35 ppb Yb) and are interlayered with small amounts of more fertile peridotite (amphibole-rich harzburgite and lherzolite, #Mg=89; up to 3.0 wt.% Al 2O 3), metaclinopyroxenite (#Mg∼85) and hornblendite. Coarse-grained orthopyroxenite (#Mg=89–92; 0.5–0.7 wt.% Al 2O 3; 19–36 ppb Yb) is present as veins and stocks in the harzburgite–dunite sequence. Chondrite-normalised rare earth element patterns of harzburgite–dunite and orthopyroxenite are characteristically U-shaped. The geochemical data indicate that harzburgites and some dunites are products of multiple melting events. They can be modelled as restites after 20–30% partial melting from already depleted mantle in a suprasubduction zone geotectonic setting. Orthopyroxenites crystallised from high-(Si, Mg) melts with a significant crustal component, probably during a Cambrian subduction event. In Mesozoic units of the Penninic windows (Lower Engadine, Tauern and Rechnitz windows), serpentinised ultramafic rocks of harzburgitic composition (#Mg=90–92; 1.1–2.0 wt.% Al 2O 3) are associated with metagabbro, metabasalt, radiolarite and ophicarbonate. Furthermore, serpentinised lherzolites (#Mg=88–90; 2.9–4.6 wt.% Al 2O 3) are exposed in the Matrei Zone (a tectonic mélange zone between Penninic and Austroalpine nappes), in the Lower Austroalpine Reckner Complex and in a body of unclear tectonic affiliation at the southern margin of the Lower Engadine Window at Nauders. The harzburgites are residual mantle left behind after 10–15% partial melting of a fertile source, and form part of the Mesozoic (Jurassic) incomplete ophiolite sequences of the Ligurian–Piemontais ocean. The lherzolites, having experienced less than 10% partial melting, are either attributable to an early (Permian) rifting episode during break-up of Pangaea, or represent subcontinental mantle from the Adria plate tectonically incorporated into the Neo-Tethys.

FORMATION AND COMPOSITION OF THE OCEANIC LITHOSPHERE OF THE LIGURIAN TETHYS: INFERENCES FROM THE LIGURIAN OPHIOLITES

Ophiolites exposed along the Western Alpine - Northern Apennine (WA-NA) orogenic chain represent the oceanic lithosphere of the Ligurian Tethys which separated, during Late Jurassic - Cretaceous times, the Europe and Adria plates. WA-NA ophiolites show peculiar compositional, structural and stratigraphic characteristics: 1) mantle peridotites are both fertile, cpx-rich lherzolites, and depleted, cpx-poor peridotites; 2) gabbroic intrusives and basaltic volcanites have a MORB affinity and show a high-Ti character of primary magmas; 3) gabbroic rocks were intruded into mantle peridotites; 4) mantle rocks underwent decompressional, not adiabatic, subsolidus evolution, starting from subcontinental lithospheric mantle depths (spinel-facies conditions) towards the sea-floor. The Jurassic Ligurian Tethys was floored by an older peridotite-gabbro basement, subsequently covered by extrusion of discontinuous basaltic flows and by sedimentation of radiolarian cherts. The Ligurian ophiolites represent the spatial association of: 1) old (Proterozoic and Permian) subcontinental lithospheric mantle peridotites; 2) Lower - Middle Jurassic MORB-type gabbroic rocks, intruded in the peridotites; 3) Upper Jurassic MORB-type basaltic volcanics, interlayered with radiolarian cherts, i.e. the first oceanic sediments. Present knowledge on the Western Alps ophiolites suggest that this association is well representative of the oceanic lithosphere of the Ligurian Tethys. This peculiar ophiolitic association, which shows no cogenetic relationships between the different lithological components, cannot be reconciled with a mature oceanic lithosphere formed at present-day mid-oceanic ridges, where the mantle peridotites and the associated gabbroic-basaltic crust are linked by a direct cogenetic relationship. The large exposure of subcontinental mantle peridotites to the sea-floor, and the long history of subsolidus decompressional upwelling recorded by peridotites, are in favour of a geodynamic evolution driven by the passive extension of the Europe-Adria continental lithosphere. Passive extension caused: 1) the progressive exhumation and the tectonic unroofing at the sea-floor of the lithospheric subcontinental mantle, and 2) the passive upwelling of the asthenospheric mantle, which underwent decompressional partial melting and produced MORB-type parental melts for the gabbroic intrusions and for the basaltic extrusions. During the late stages of lithosphere extension, most probably in Jurassic times and prior to complete oceanization, the conductive lithospheric mantle was permeated and impregnated by strongly depleted single melt increments and, later on, it was intruded by aggregated MORB melts. Melt impregnation is mainly confined to the more depleted, more “internal” peridotites massifs (i.e. Internal Ligurides and Lanzo South) which were, most probably, closer to the transition to more typical oceanic lithosphere (i.e. cogenetic mantle and crustal rocks), which has not, so far, been found in the ophiolites derived from the Ligurian Tethys Ocean.

Crustal section based on CROP seismic data across the North Tyrrhenian–Northern Apennines–Adriatic Sea

Using deep seismic reflection data from the Italian lithospheric exploration project CROP in the Central Mediterranean region, a 400-km-long section, composed of three different profiles crossing the Northern Tyrrhenian Sea (CROP M-12A profile), the Northern Apennines (CROP-03) and the Adriatic Sea (CROP M-16) is reconstructed and discussed. New data allow us to outline a seismically consistent tectono-stratigraphic setting for the crust and upper mantle of the Northern Apennines thrust–belt system and its Adriatic foreland. Time–space analysis of the deformation of the investigated chain and identification of existing macrostratigraphic crustal intervals and tectonic units allow a reasonably controlled interpretation of the geodynamic evolution and of the main orogenic stages. Careful seismic reprocessing and application of advanced techniques to key zones of the explored area (such as the Tuscan Archipelago) were determinants in obtaining fundamental information for understanding of the complex lithospheric structures and their evolution. Profile interpretation supports that the Northern Apennine chain is dominated by a compressive thrust system. Crustal extension, assumed by some authors as the dominating tectonic process for the whole Tuscan Apennine area, represents a subordinate geodynamic event of the last stage (Tyrrhenian). In the Early Cretaceous–Late Jurassic, the paleogeographic framework consisted of the Europe and Adria plates separated by the Alpine Tethys Ocean. During the Late Cretaceous–Early Eocene, Adria–Europe convergence (eo-Alpine stage) and subduction beneath the Adria plate closed the Alpine Tethys Sea, with the Tethyan slab being clearly seismically imaged. The first Apenninic geodynamic stage occurred in the Late Oligocene–Early Miocene with the opening of the Balearic Basin, which generated a first “lithospheric root” of the Apenninic chain in the Tuscan Archipelago area. This root is represented by Adria-verging thrust faults that progressively flatten eastward. Upper parts of the west-verging eo-Alpine thrust blocks were truncated by the east-verging thrust faults of the Balearic stage. A deeper seismic reflector, attributed to the top of the asthenosphere, forms a mantle high below the Elba Island. From the Late Miocene to Present, the Corsica basin and western hinterland area were affected by extensional tectonics related to the Tyrrhenian opening, whereas compressional tectonics continued in the eastern hinterland and mostly on the eastward migrating foreland, with development of a second “lithospheric root” constituted by high-angle thrust faults. These faults give rise to a huge basement culmination below the main Apennines watershed. Impressive E-directed gravity-sliding of sedimentary blocks over their sloping basement occur, generating the Umbria–Marche shallow seismicity. Crustal shortening of the Apennines system amounts to 170 km, 14 km of which are due to the eo-Alpine stage, 71 km to the Balearic and 85 km to the Tyrrhenian one. In the frame of Africa–Europe convergence, the Tyrrhenian–Apennines tectonodynamics were mainly conditioned by the Mesozoic paleogeography.

Geodetic measurements at the northern border of the Adria plate

This work describes measurements and results relevant to crustal movements at the northern border of the Adria plate. The measurements taken into consideration are subsurface observations made with extensometers and tiltmeters and episodic and continuous GPS (CGPS) observations. The subsurface measurements cover a remarkably long period of 34 years, while the episodic GPS campaigns have been performed over a time period of 7 years. The CGPS observations are available since July 1996, and 3.5 years of data have been considered in this work. The main characteristics of each different type of measurement are illustrated, and a first approach is made to compare the results. It is shown that environmental effects are present both in subsurface and surface measurements. Methods to interpret and remove these signals are also proposed. The correlation between crustal deformation and seismic events is investigated, and some examples are presented. The long-term observations are discussed with respect to a well-known model of plate motion. The subsurface crustal long-term observations reveal inversions in the direction of deformation with time scales of 10 or more years, providing important boundary conditions for future geodynamic models of the northern Adria plate. The results show that the comparison between subsurface deformation and CGPS measurements, if made over a sufficiently long period of time, in the order of 10 years, can give important contributions to the understanding of the geodynamical and physical mechanisms related to the earthquake occurrence in this area.

Subduction-related episodes of K-alkaline magmatism (15–0.1 Ma) and geodynamic implications in the north Tyrrhenian — central Italy region: a review

In the north Tyrrhenian region and adjoining shores of Alpine Corsica and central Italy, four magmatic phases (15–14; 8–6; 5–2; and 1.3–0.1 Ma) are present which consist prevailingly of either old acidic and lamproitic rocks or young (1.3–0.1 Ma) basic volcanics with potassic–ultrapotassic affinity (the rocks of the K and HK series of the Roman province, and the kamafugitic rocks). Composition, age and location of the igneous phases reveal a geochemical polarity oriented from west to east. The episodes of magmatic activity reflect the evolution of the geodynamic environment (ensialic plate convergence): backarc stretching and attenuation of an Alpine orogenic wedge were accompanied by the eastward roll-back and progressive steepening of the descending Adria plate. The latter process led to enhanced metasomatic modifications of the mantle wedge composition and to eruption of the large volumes of KS and HKS volcanic rocks in central Italy (0.6–0.1 Ma).

The Aptian–Albian of the Apulia Carbonate Platform (Gargano Promontory, southern Italy): evidence of palaeoceanographic and tectonic controls on the stratigraphic architecture of the platform margin

Impressive megabreccia levels that were deposited mainly on by-pass, steep margins of carbonate platforms have usually been linked to syn-sedimentary tectonics and, more recently, to other mechanisms including sea-level low-stands. The Apulia Carbonate Platform, a main palaeogeographic domain of the Mediterranean Tethys, is characterized by huge megabreccia bodies which were deposited along its flank at various intervals during the Cretaceous. This paper deals with the stratigraphy and the genetic meaning of a peculiar early Cretaceous megabreccia level occurring on the Apulia platform margin exposed in the southern Gargano Promontory (southern Italy). The stratigraphic implications of the megabreccias within the tectonic evolution of the Apulia platform are also discussed along with a comparison with previous models. A sub-vertical blind contact separates a tectonized Berriasian shallow-water succession (Lower S. Giovanni Rotondo Limestones) from a thick rock-fall megabreccia succession pertaining to the Late Aptian–Albian Posta Manganaro Megabreccias. The contact, which is interpreted as a syn-sedimentary fault plane, is sealed by undeformed late Albian slope bioclastic sediments (lower Monte S. Angelo Rudist Limestones). Tensile tectonics downfaulted an external sector of the platform generating an articulated, steep, retreating margin. Field analysis and stratigraphic reconstructions show that the syn-tectonic wedge-shaped megabreccias partly filled an asymmetric depocenter (half-graben) resulting from the activity of a normal fault system located behind the previous prograding, low-energy, leeward margin (Montagna degli Angeli Limestones). Nevertheless, syn-sedimentary tectonics severely conditioned the composition, depositional processes and dispersal pattern of sediments supplied towards the original platform-to-basin transition (Upper Mattinata Bioclastic Limestones). This stratigraphic framework is believed to reflect a local response of the sedimentation to a late Early Cretaceous syn-sedimentary tectonic phase representing an important turning point in the evolution of the Apulia Carbonate Platform. It is supposed here that the observed tectonics, which are assumed to be related to the onset of the Austrian orogenic phase at the distant Adria plate margin, could only control the extension of the down-faulted platform. More probably, its drowning was decisively caused by the coeval global-scale Selli palaeoceanographic event (OAE 1a), which reduced the healing power of the platform by poisoning the shallow-water, sediment-donor communities. As a major causal mechanism, the OAE Selli event controlled the development of a rapid facies evolution during the late Early Aptian within the shallow-water successions of the Apulia and many other Tethyan carbonate platforms through the onset of nutrient-enriched water conditions. In this light, a stratigraphic correlation between the bases of the late Early Aptian shallow-water Orbitolina Level and the basinal Fucoid Marls is proposed. Their deposition within distinctive tracts of many platform-to-basin transitions on the southern Tethyan margin is viewed as the response of a previously pure carbonate sedimentation to the onset of environmental control on a global scale during the late Early Aptian.

Syn- versus post-orogenic extension: the case study of Giglio Island (Northern Tyrrhenian Sea, Italy)

Combined structural, petrographic and fluid inclusion data from Giglio Island (Northern Tyrrhenian Sea, Italy) are used here to constrain kinematics, timing and thermal regimes associated with the post-thickening tectonic evolution of the innermost sector of the Northern Apennine belt. Giglio Island is composed of Pliocene intrusives in tectonic contact with a poorly retrograded HP/LT, Fe-carpholite bearing nappe pile. Peak pressure estimates for the carpholite bearing Verrucano metasediments range between 10 and 14 kbar for temperatures lower than 350°C. Data from this study indicate a nearly isothermal exhumation of the Giglio HP metamorphic complex, which occurred during the Early–Middle Miocene as a consequence of a top-to-the-east syn-orogenic extensional shear. This resulted in the transposition of the HP fabric while the lithostatic pressure was decreasing, with a final re-equilibration in the greenschist facies along a cool geothermal-gradient regime. This relatively cold regime is referred to the continuous underthrusting of the cool continental Adria plate during exhumation. Afterwards, during the Late Miocene–Early Pliocene, the onset of the post-orogenic crustal thinning also showed an eastward asymmetry in an HT/LP back-arc environment, leading to partial melting of previously exhumed deep crustal levels and genesis and emplacement of the Giglio intrusives. A comparison with other peri-Tyrrhenian regions allows us to propose that the post-thickening Miocene extension in the Northern Apennines was driven by the collapse of the previously thickened Northern Apennine orogenic wedge, combined with the progressive eastward retreat of the Adria subduction boundary.

ORIGIN AND SIGNIFICANCE OF LATE NORITIC DYKES IN THE BALDISSERO PERIDOTITE MASSIF (IVREA-VERBANO ZONE)

The Baldissero Peridotite Massif (Southern Ivrea-Verbano Zone) is a depleted lherzolite (ol 65÷70, opx 16÷20, cpx 10÷11, sp 2÷2.5, LaN/YbN 0.003÷0.26, eNd270 = 6.4÷14, 87Sr/86Sr = 0.7021÷0.7035). It is cross-cutted by a swarm of 5÷15 cm thick, fine grained (<1mm), NNE trending noritic dykes. Contacts peridotite-dyke are sharp. Dyke texture is hypidiomorphic and their mode consists of dominant plagioclase (31÷45 %, An 40÷48), orthopyroxene (25÷37%, En = 78.9÷87.8, Wo = 0.6÷1.1), subordinate clinopyroxene (14÷18 %, En = 42.7÷44.0, Wo = 47.9÷52.4), interstitial pargasitic amphibole (10÷15 %) and opaques (1÷2 %, Fe-Ni sulfides). Re-equilibration conditions are 0.8÷1 GPa and 800÷900°C, like those of the ambient peridotite. Major element (wt%) dyke composition is characterized by high MgO (10÷16) and high Mg# values (0.81÷0.89). In the observed MgO range, SiO2, CaO and K2O vary negligibly (52÷53, 9÷10 and <0.2, respectively). With decreasing MgO, TiO2 varies randomly from 0.8 to 0.4, Al2O3 increases from 15 to 21, Na2O from 3 to 4.5 and FeO decreases from 5 to 2. Bulk rock incompatible trace element patterns (Fig. 1) vary from weakly LREE depleted to weakly LREE enriched (LaN/YbN = 0.5÷2.4) and have marked positive Sr anomaly (Sr/Sr* = 5.6÷10.4), smaller Eu anomaly (Eu/Eu* 1.3÷1.5) and negative Nb anomaly (LaN/NbN = 1.2÷2.2). Incompatible element concentration increases, and the magnitude of the negative Nb and of the positive Sr anomalies decrease, with increasing MgO or Mg#. Main incompatible trace element carriers are clinopyroxene and hornblende, which display patterns varying from slightly LREE depleted to enriched. Orthopyroxene is LREE depleted and plagioclase is LREE and Sr enriched. Clinopyroxene is Nb-depleted (LaN/NbN = 1.0÷5.9) and amphibole is Nb enriched (LaN/NbN = 0.09÷0.35). The latter phases have negative Zr spikes. Incompatible element concentration in the mafic phases show correlations with Mg# similar to those of bulk rock (Fig. 2). The only available isotope determination refers to a Sm-Nd internal isochron which results in an age of 180Ma and eNd = 7.8. In spite of the contacts being sharp, peridotite mineral assemblage and composition varies over a distance of 7 cm. In the peridotite, toward the dyke contact, modal orthopyroxene increases and clinopyroxene decreases, whereas trace element concentration and LaN/YbN increase (from 0.003÷0.017 to 0.1÷0.26) and Mg# decreases from 0.9 to 0.8. The dyke at the peridotite contact does not show sizeable chemical or modal variations. There is no clear relationship between the Mg# variation of the dyke and the chemical trends observed in the contact peridotite. The high Mg# numbers of the dykes and the Mg# value of orthopyroxene, lower than that of bulk-rock, indicate that these norites are not melts, but rather represent mineral segregates possibly retaining some interstitial liquid. Estimates of the Mg# values of the parental melts, made assuming opx/liqDMg# = 1.2, indicates a Mg# range from 0.66 to 0.74, which is consistent with that of primary mantle-derived magmas. Since olivine does not occur even in the more MgO-rich dyke composition, the parent melt had to be, furthermore, SiO2-rich and hydrous, as indicated by the amphibole abundance. Whatever the set of solid/melt partition coefficient used for estimating trace elements in the parent melts, the calculated REE profiles would vary from slightly LREE depleted to variably LREE enriched. The HFSE characteristics of the primary melts cannot be precisely constrained because of the strong dependence of their partition coefficient on the melt composition. For example, using available partition coefficients for basaltic compositions, the estimated parental melts would have a weak Nb and Zr anomaly, which disappears using D sets for intermediate compositions and become positive if the D’s are referred to high-silica compositions. In any case, the geochemical trends shown by bulk rock and mineral phases (i.e. the increase of incompatible element concentration with increasing Mg#) are qualitatively maintained. We propose that this peculiar feature can be explained either by fluid-assisted melting of a depleted peridotite source, where melting degree increases with the increasing supply of hydrous fluid (which is the incompatible element carrier), or by fluid-peridotite reaction during porous flow percolation in a mantle column at a deeper and higher temperature level than that actually observed. If confirmed by new isotopic data, the occurrence of post-Hercynian gabbroic rocks in the Australpine and South Alpine domains will support the involvement of the marginal units of the Adria plate in the preoceanic extensional regime (Piccardo et al., 1994).

ORIGIN OF POST-VARISCAN GABBRO-DERIVED GRANULITES (NORTHERN APENNINES, ITALY): CRUSTAL CONTAMINATION PROCESSES AND IMPLICATIONS FOR MANTLE SOURCES

The Late Cretaceous sedimentary melanges from the External Liguride Units of Northern Apennines include large slide-blocks of subcontinental mantle peridotites, MORbasalts and lower and upper continental crust rocks. The slide-block association has been interpreted as representative of a continent-ocean transition between the Internal Liguride oceanic domain (Late Jurassic Western Tethys) and the thinned continental margin of the Adria plate (Marroni et al., 1998). The slide-blocks of lower continental crust consist of mafic and felsic granulites, which locally preserve primary contacts. The mafic granulites commonly display a metamorphic layering, but undeformed rocks preserving a gabbroic fabric are locally found. Undeformed mafic granulites are mostly represented by spinel-bearing gabbronorites, usually containing significant amounts of either olivine or Fe-Ti-oxides. Olivine- and Fe-Ti oxide-bearing rocks locally show spinel-pyroxene symplectites and garnet coronas, respectively. The felsic granulites are mainly quartzo- feldspathic rocks consisting of mesoperthitic to perthitic feldspar, quartz and garnet. The gabbroic protoliths of the granulites were emplaced at about 290 Ma at deep crustal levels, where they underwent slow cooling and recrystallisation under granulite-facies conditions (P = 0.7-0.8 GPa, T = 800-900°C). They were exhumed to upper levels, in association with the felsic granulites, in late Triassic-middle Jurassic times. The gabbro-derived granulites can be recognized as cumulus rocks with negligible amounts of residual trapped liquid, on the basis of low SiO2/Al2O3 ratios and overall low contents of incompatible trace elements. The Mg# value ranges from 80 to 52, and point to negative correlations with TiO2 and MnO, thus indicating a tholeiitic differentiation trend. Most gabbro-derived granulites have slightly LREEenriched patterns showing decreasing Eu positive anomaly with increasing total REE abundances. Chondrite normalization of incompatible trace elements reveals spikes at Ba and Sr, and a slight Zr depletion. The quartzo-feldspathic granulites have LREE enriched patterns, with nearly flat HREE and no or slightly positive Eu anomaly; Ba is abruptly enriched relative to REE, whereas Nb and Ti are depleted. The gabbro-derived granulites show a wide range in Sr and Nd isotopic compositions. The Sr isotopic ratio recalculated at 290 Ma varies between 0.7031 and 0.7077, and the initial eNd ranges between +6.8 and -4.5. Two samples of quartzo-feldspathic granulite yield age-corrected Sr isotopic ratios of 0.7107 and 0.7109, and eNd of -8.0 and -5.7. As a whole, the Nd and Sr isotopic data at 290 Ma form a hyperbolic array, in which the olivine-bearing gabbronorites have the highest eNd values and the lowest Sr isotopic ratios. Clinopyroxenes have been analyzed for trace elements by ion microprobe. Clinopyroxene from olivine-bearing gabbronorites shows peculiar compositions that indicate a metamorphic origin through olivine-plagioclase reaction, i.e. the igneous protoliths of the olivine-bearing gabbronorites were most likely troctolite-type cumulates. Clinopyroxenes from Fe-Ti oxide bearing gabbronorites show igneous geochemical trends, thus suggesting that these rocks contained clinopyroxene as original igneous phase. Petrography, bulk-rock and mineral composition indicate that the gabbro-derived granulites can be related to a fractional crystallization process, with early separation of olivine and plagioclase, followed by the replacement of olivine by pyroxene at the liquidus. Trace element modelization of the parental liquid compositions applied to the olivine-bearing rocks yields LREE- and LILE-enriched liquids, with absence of negative Nb anomaly, similar to plume-type MOR-basalts and continental tholeiites. However, a P-MORB origin seems to contrast with the initial Nd and Sr isotopic compositions, which are close to depleted mantle values at the time of emplacement. AFC modelization was successfully applied to obtain the isotopic compositions of the most contaminated samples, starting from the trace element and isotopic compositions of the parental liquids of the olivine gabbronorites and assuming a crustal contaminant with low Sr/Nd and isotopic composition comparable to that of the quartzo-feldspathic granulites. AFC calculations also indicate that the parental liquids of the olivine-bearing gabbronorites cannot be ascribed to N-MORB primary liquids. The primary mantle magma was necessarily characterized by moderate LILE enrichment, although an increase in LILE concentrations could have been enhanced by a small crustal contribution. The LILE enrichment in the parental liquids of the gabbro-derived granulites may be explained with a low degree partial melting of a rather fertile lithospheric mantle source. Alternatively, the primary liquids of the gabbro-derived granulites were related to a mantle source enriched in LILE as a result of the Variscan subduction event.

Multiple mantle metasomatism in central-southern Italy: Geochemical effects, timing and geodynamic implications

Research Article| April 01, 1999 Multiple mantle metasomatism in central-southern Italy: Geochemical effects, timing and geodynamic implications Angelo Peccerillo Angelo Peccerillo 1Dipartimento di Scienze della Terra, Università degli Studi di Perugia, Piazza Università, 06100 Perugia, Italy Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (4): 315–318. https://doi.org/10.1130/0091-7613(1999)027<0315:MMMICS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Angelo Peccerillo; Multiple mantle metasomatism in central-southern Italy: Geochemical effects, timing and geodynamic implications. Geology 1999;; 27 (4): 315–318. doi: https://doi.org/10.1130/0091-7613(1999)027<0315:MMMICS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The regional petrological and geochemical variations observed in the Recent magmatism along the Italian peninsula are interpreted to testify to the coexistence of distinct sectors of upper mantle. These are suggested to result from at least three compositionally and temporally distinct metasomatic events that affected petrologically different premetasomatic mantle sources. Geological and geochemical evidence suggests that metasomatism in the northern sector occurred during the Alpine subduction by introduction of a composition similar to the deeply subducted Dora Maira metagranites into a residual lithospheric mantle of upper crustal material. Metasomatic events beneath the Roman and Neapolitan areas are younger. These are related to the addition of melts and fluids into a fertile asthenospheric mantle during the latest stage of the west-directed Apennine subduction of the Adria plate and during the current northwest subduction of the Ionian sea floor. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Relationships between ultrapotassic and carbonate-rich volcanic rocks in central Italy: petrogenetic and geodynamic implications

The Pleistocene intra-Apennine volcanic (IAV) centres occurring east of the potassium-rich Roman comagmatic province show variable petrological and geochemical composition. Some rocks have a strongly undersaturated ultrapotassic kamafugitic affinity with K 2O/Na 2O=8–20, whereas the rocks from the southern center of Mt. Vulture are still strongly undersaturated in silica but are enriched in both Na 2O and K 2O with K/Na around unity. Carbonate-rich pyroclastic rocks, believed to represent carbonatitic magmas, are found in the IAV centers. Kamafugites have high abundances of LILE and high LILE/HFSE ratios, and their incompatible element patterns resemble closely those of ultrapotassic rocks from the adjoining Roman province. The Vulture volcanics also display high contents of LILE, but their LILE/HFSE ratios are intermediate between intraplate alkaline rocks and kamafugites. The carbonate-rich rocks exhibit an exotic mineralogy and high enrichments in LILE, which speaks for a carbonatitic affinity. However, they have similar incompatible element patterns but consistently lower abundances of almost all the elements than the associated silicate volcanics. These data favour the hypothesis that the IAV carbonate rocks may represent mixtures of silicate magmas and geochemically depleted carbonate material. The sedimentary carbonates that crop out extensively along the Apennine chain may be the source of barren carbonate material. Overall, geochemical data of IAV centres and of the rocks from the Roman province display strong geochemical and isotopic evidence of being generated in an upper mantle that was modified by addition of upper crustal material brought down by subduction processes. A possible exception is represented by Mt. Vulture which, however, occurs east of the main axis of the Apennines, on the western margin of the foreland Adria plate. The occurrence of strongly undersaturated alkaline rocks requires magma generation at high pressures and X CO 2 . This is in agreement with the hypothesis that subduction processes under the Apennines occurred by consumption of poorly hydrated thinned or delaminated continental crust.

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

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.