Adriatic Margin Research Articles

Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps

The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65–80 Ma) subduction to different depths at high‐pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0–1.2, 1.0–1.5, and 1.5–2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal‐scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E‐W trending mylonitic shear zone under retrograde blueschist‐ to greenschist‐facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15–20 km occurred in the footwall of a greenschist‐facies, top‐SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb‐Sr mineral ages constrain D2 to have occurred at about 60–65 Ma and D3 at about 45–55 Ma. Thus top‐SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro‐Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra‐HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo‐Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature‐dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high‐stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low‐stress subduction of the Liguro‐Piemont oceanic lithosphere.

Short-distance variability in slope bed-forms along the Southwestern Adriatic Margin (Central Mediterranean)

The slope of SW Adriatic Margin (SAM) is impinged today by two bottom-water masses, of thermohaline origin, that interact constructively along a preferred path: the Levantine Intermediate Water (LIW), generated in the eastern Mediterranean through intense evaporation and flowing along the contour, and the North Adriatic Dense Water (NAdDW), formed on the shallow north Adriatic and cascading episodically off the shelf. These two southward-flowing bottom-water masses impact the upper slope, which is characterized by an irregular physiographic setting, and together create the bottom-current deposits (e.g. fine-grained contourite drifts and sediment waves) and related erosional features (e.g. furrows, scours, comet marks). This study is focussed on a small open-slope area of the SAM (ca. 35 × 25 km), where high-resolution seismic data allow observe elongated contour-parallel scours, widespread erosion surfaces (accompanied by wide crescent-shape scours and furrow fields) and giant comet-marks spatially associated and genetically linked along a common current path. Further downslope, and away from this highly energetic current path, a field of sediment waves migrates upcurrent and shows bifurcated crests. In some cases the downcurrent limbs of the sediment waves display a well-defined erosional character on seismic profiles and furrows on side scan sonar mosaics. All bottom-current features observed are in equilibrium with the present day oceanographic regime, as documented by the available current-metre data. Moreover, sediment cores suggest that the sediment waves are actively migrating during the present interglacial, possibly implying that bottom waters formation was enhanced after the shallow Adriatic shelf was drowned during the late-Quaternary eustatic rise. The SAM slope represents, therefore, an ideal site to improve our understanding of the depositional impact of bottom currents along the Mediterranean margins and may provide useful hints on the activity of bottom currents elsewhere on the Mediterranean margins.

Active foreland deformation evidenced by shallow folds and faults affecting late Quaternary shelf–slope deposits (Adriatic Sea, Italy)

ABSTRACTA succession of depositional sequences, recording middle‐late Pleistocene and Holocene glacial–interglacial cycles, documents the impact of short‐term tectonic deformation on the western Adriatic margin. The western Adriatic margin is part of the Apennine foreland which was intensely, though variably, deformed during the Meso‐Cenozoic evolution of the Adriatic region from a passive margin to a foreland basin. The study area extends offshore Gargano Promontory, an uplifted sector of the Adriatic foreland, and includes three major deformation belts located along or cross‐strike to the margin: (1) the NW‐SE Gallignani‐Pelagosa ridge, (2) the WSW‐ENE Tremiti‐Pianosa high (both located north of Gargano) and (3) the W‐E to NW‐SE Gondola fault deformation belt (in the south Adriatic). Long‐term deformation along these tectonic lineaments is documented on conventional low‐frequency seismic profiles by regional folds and faults affecting Eocene–Miocene units overlain by dominantly draping Plio‐Quaternary deposits. At this scale of observation, only north of Gargano Promontory there is some evidence of Plio‐Quaternary units thinning against structural highs, thus suggesting that tectonic deformation was protracted through this interval. Based on new high‐resolution seismic data, we show that deformation along these pre‐existing tectonic structures continued during the Quaternary, affecting middle‐late Pleistocene and even Holocene units on the shelf and upper slope north and south of Gargano Promontory. These recent deformations consist of gentle folds and high‐angle faults, locally producing topographic relief that affects the stratigraphy and thickness of syn‐tectonic deposits. We interpret the small‐scale, shallow faults and gentle folds affecting middle‐late Pleistocene and Holocene deposits, north and south Gargano Promontory, as the evidence of ongoing foreland deformation along inherited regional fold and fault systems.

Evidence of slope instability in the Southwestern Adriatic Margin

Abstract. The Southwestern Adriatic Margin (SAM) shows evidence of widespread failure events that generated slide scars up to 10 km wide and extensive slide deposits with run out distances greater than 50 km. Chirp-sonar profiles, side-scan sonar mosaics, multibeam bathymetry and sediment cores document that the entire slope area underwent repeated failures along a stretch of 150 km and that mass-transport deposits, covering an area of 3320 km2, are highly variable ranging from blocky slides to turbidites, and lay on the lower slope and in the basin. The SAM slope between 300–700 m is impacted by southward bottom currents shaping sediment drifts (partly affected by failure) and areas of dominant erosion of the seafloor. When slide deposits occur in areas swept by bottom currents their fresh appearence and their location at seafloor may give the misleading impression of a very young age. Seismic-stratigraphic correlation of these deposits to the basin floor, however, allow a more reliable age estimate through sediment coring of the post-slide unit. Multiple buried failed masses overlap each other in the lower slope and below the basin floor; the most widespread of these mass-transport deposits occurred during the MIS 2-glacial interval on a combined area of 2670 km2. Displacements affecting Holocene deposits suggest recent failure events during or after the last phases of the last post-glacial eustatic rise. Differences in sediment accumulation rates at the base or within the sediment drifts and presence of downlap surfaces along the slope and further in the basin may provide one or multiple potential weak layers above which widespread collapses take place. Neotectonic activity and seismicity, together with the presence of a steep slope, represent additional elements conducive to sediment instability and failure along the SAM. Evidence of large areas still prone to failure provides elements of tsunamogenic hazard.

Pleistocene “muddy” forced-regression deposits on the Adriatic shelf: A comparison with prodelta deposits of the late Holocene highstand mud wedge

The upper Pleistocene–Holocene stratigraphy of the Adriatic margin consists of a succession of depositional sequences separated by shelf-wide unconformities. The most recent of these depositional sequences encompasses the last ca. 25 kyr, and consists of lowstand, transgressive and highstand units formed, respectively, during the Last Glacial Maximum lowstand, the subsequent sea level rise and the modern highstand. Older deposits are grouped into four depositional sequences, each recording ca. 100 kyr glacio-eustatic cycles during an interval between ca. 450 and 25 kyr BP. Each of these Pleistocene sequences is composed dominantly by muddy progradational units deposited during prolonged phases of sea level fall; therefore, these sequences represent a unique example of well-preserved muddy forced-regression deposits. Detailed seismic–stratigraphic analyses show that forced-regression deposits composing the four sequences are not directly connected to nearshore and/or fluvio-deltaic environments. In contrast, typical forced-regression deposits defined in outcrop and reported in the literature consist of sand-rich, nearshore/deltaic facies preserved as patchy remnants of ancient shorelines. We suggest that muddy forced-regression deposits on the Adriatic shelf represent the offshore counterpart of shoreface deposits recognised in outcrop as the product of typical forced regressions. Interestingly, muddy forced-regression deposits on the Adriatic shelf bear clear similarities with the late Holocene highstand mud wedge, suggesting a common depositional mechanism through lateral advection of fine-grained sediment. In this view, subaqueous progradation of muddy deposits occurs below storm wave base, with a subaqueous roll-over point that, during proceeding sea level fall, likely positioned at decreasing distance from the time-equivalent shoreline.

Early Tertiary evolution of the North Alpine Foreland Basin of the Swiss Alps and adjoining areas

AbstractWe present a new palaeogeographic reconstruction of the Helvetic zone based on the palinspastic restoration of 18 recently published and new retrodeformed structural cross‐sections through the Swiss Alps, Haute Savoie (France) and Vorarlberg (Austria). The reconstruction resulted in two palaeogeographic maps, one of the pre‐Mesozoic basement, the other for the sedimentary cover of the Helvetic shelf including the Nummulitic deposits of the Palaeocene–Eocene, which mark the onset of the North Alpine Foreland Basin of the Alps.Based on the palaeogeographic maps and a precise dating of the Nummulitic deposits, we established maps of the facies distribution including the estimated positions of the ancient coastlines and their evolution through time. The North Alpine Foreland Basin started as a narrow flysch basin in Palaeocene–Eocene times. Emplacement of the Penninic nappes led to the formation of a mélange on the active margin of this basin. This early foreland basin and its active margin migrated to the NW in Early Eocene times at a rate of about 10 mm yr−1. The maps also reveal a general progressive north‐ and westward propagation of the Eocene coastline between 50–34 Ma and during the Oligocene until approximately 32 Ma. Coastline propagation reveals strongly varying rates both spatially and temporally, and is ca. 1–2 mm yr−1 between 50 and 37 Ma and approximately 20 mm yr−1 between 37 and 32 Ma. Evolution and orientation of the Tertiary coastlines infers that the early development of the North Alpine Foreland Basin was mainly controlled initially by eustatic sea‐level fluctuations superimposed on flexural subsidence. After 37 Ma, we suggest a tectonically controlled coastline evolution in response to the collision of the European and Adriatic margins.

INFLUENCE OF HETEROGENEOUS LITHOSPHERIC STRUCTURE ON CONTINENTAL RIFTING: POSSIBLE IMPLICATIONS FOR THE LIGURE-PIEMONTESE OCEAN

Continental rifting represents the thermo-mechanical process by which continents break. This process is characterised by an high variability in terms of duration, volcanicity, width and symmetry. Models formulated to explain such a structural variability consider differences in (e.g., Davison, 1997; Ziegler and Cloetingh, 2004): initial rheological or thermal structure of the lithosphere, rift kinematics (e.g., oblique vs. orthogonal) and mechanics (e.g., pure vs. simple shear), strain rate, melt volumes, presence of pre-existing weakness zones. Among these processes, the lithospheric structure inherited from previous deformation phases and the presence of weakness zones have been shown to play a major role in controlling continental extension (e.g., Dunbar and Sawyer, 1989; Ziegler and Cloetingh, 2004). As a consequence, rifting processes often affect continental lithospheres characterized by important lateral variations in rheology and these variations are able to influence parameters as the width and symmetry of the rift, the patterns of uplift and subsidence, the location and amount of volcanic products. Extension in the Ligure-Piemontese ocean has been shown to be superimposed onto a previous ercinic chain, thus configuring a lithospheric weakness zone. In this frame, lateral rheological heterogeneities is expected to have played a major role in controlling the structure of the passive margins of the Adria/Europe system. In this work, this influence is investigated through scaled analogue modelling. Small-scale experiments were performed in an artificial gravity field of 31g, by using the Large Capacity Centrifuge of the Tectonic Modelling Lab of the CNR-IGG, settled at the Earth-Science Department of Florence University. The models, with dimensions of 10cm x 16cm x 4cm, were built with suitable analogue materials (sand, silicone, oleic acid) able to reproduce the brittle/ductile stratification of the continental lithosphere and were properly scaled to be directly comparable to natural prototypes. The experimental series investigated the occurrence of three different continental lithospheres: 1) a weak orogenic lithosphere (WL; Thickness: 80-100km; Thermo-mechanic Age: 50 Ma; Effective Elastic Thickness: 20km); 2) a normal 4-layer lithosphere (NL; Th: 120km; TA: 250 Ma; EET: 40km); 3) a strong cratonic lithosphere (CL; Th: 140km; TA: 400 Ma; EET: 70km). Results show that in the simple case of WL, continental extension is taken up by regularly-spaced faulting in the upper brittle layer coupled to flow of the ductile lower crust and uppermost lithospheric mantle. No strain localisation configures with this initial rheological stratification; instead, the deformation style can be ascribed to a wide-rifting mode (e.g., Buck, 1991; Brun, 1999). The base of the lithosphere in these models was rather flat, suggesting that no major asthenospheric upraising took place during deformation. In case of lateral variation between WL and CL, deformation was mostly localised at the craton border and within the weak domain; no deformation developed within the strong lithosphere. The largest normal fault developed at the contact between WL and CL, whereas regularly-spaced faults and lower crustal flow (with development of core complex structures) characterised the weakest portion of the models. Extension was accommodated by downward flexure of the WL and elastic rebound with strong shoulder uplift of CL; limited asthenospheric upraising was mainly localised within the WL. In models with NL and CL separated by a central WL, extensional deformation was strongly localised by the weakness zone. This rheological configuration leaded indeed to a rapid necking of the lithosphere and asthenospheric upraising leading to continental break-up in the central part of model. The modelling results support an important role played by the initial lithospheric structure on rift structure and duration and suggest that lateral rheological variations may have an important influence in controlling the local pattern of uplift/subsidence and faulting in continental rift settings. These results may have also relevance for the opening of the Ligure-Pemontese ocean. Preliminarily, the structural conditions of continental extension between the European and Adriatic margin may be correlated to the experiment reproducing the strength contrast between weak and cratonic lithospheres. In particular, the downward flexure associated with low subsidence rates of the WL may well account for the development of the Calcare Massiccio fm. on the Adriatic margin, whereas the areas characterised by high subsidence at the WL-CL contact may be correlated with the coeval development of the Brianconnaise domain in the European margin.

The Swiss Alps and their peripheral foreland basin: Stratigraphic response to deep crustal processes

This paper gives a synoptic view of the Cenozoic evolution of the Swiss Alps and their northern foreland basin. In this orogen, deep crustal processes (subduction, nappe stacking, underplating, and exhumation) are intimately linked with surface processes (surface uplift, erosion, basin formation, and basin‐axis migration). Within the foreland basin the spatial pattern of subsidence and alluvial fan construction suggests that an increase in flexure of the foreland plate and the creation of relief in the orogen migrated from east to west in the course of collision. In the orogen itself, crustal thickening involved lower crust of the Adriatic margin in the east and the European margin in the west. Exhumation of upper crustal units occurred earlier in the east as compared with the west. An Adriatic mantle wedge (the Ivrea body) and its associated wedge of lower crustal material are identified as an extra lithospheric load which contributed to downward flexure of the European plate. As a result of enhanced subsidence of the foreland plate, relief was generated presumably in order to adjust to critical taper geometry. It appears, therefore, that the westward motion of the Adriatic wedge ultimately caused the contemporaneous westward propagation of the location of enhanced rates of alluvial fan construction. Coeval strike‐slip and N‐S convergence juxtaposed the Adriatic wedge sequentially to different European upper crustal units which resulted in different styles of crustal structure and evolution along strike within the orogen.

Tertiary age and paleostructural inferences of the eclogitic imprint in the Austroalpine outliers and Zermatt–Saas ophiolite, western Alps

The Austroalpine Sesia–Lanzo inlier and upper Austroalpine Dent Blanche, Mt. Mary and Pillonet outliers occur on top of the western-Alpine orogenic wedge and, as a whole, override the structurally composite ophiolitic Piemonte zone. Instead, the Mt. Emilius, Glacier–Rafray, Etirol–Levaz and other lower Austroalpine eclogitic outliers are inserted within the Piemonte zone, between its upper (Combin) and lower (Zermatt–Saas) tectonic elements, or within the latter. Rb–Sr dating on phengitic micas show that the eclogitic imprint in the lower Austroalpine outliers, conventionally regarded as Late Cretaceous by comparison with the Sesia–Lanzo inlier, is of Eocene age (49–40 Ma), like the underlying Zermatt–Saas ophiolite (45–42 Ma) between the Aosta valley and Gran Paradiso massif. 40Ar–39Ar plateau ages on the same mica concentrates of the ophiolitic Zermatt–Saas nappe (46–43 Ma) are consistent with Rb–Sr dating, whereas that on the Austroalpine Glacier–Rafray klippe (92 Ma) is influenced by argon excess. The lower Austroalpine outliers underwent the subduction metamorphism concurrently with the Zermatt–Saas nappe, 20–25 Ma later than the eclogitic Sesia–Lanzo inlier and blueschist Pillonet klippe. The temporal gap and present intra-ophiolitic position mean that the lower Austroalpine outliers were probably derived from an intraoceanic extensional allochthon (Mt. Emilius domain) stranded inside the Piemonte–Ligurian ocean far from the Dent Blanche–Sesia domain and Adriatic margin.

Continental collision history from arenites of episutural basins in the Northern Apennines, Italy

Integration of sandstone petrography, sedimentology, stratigraphy, and structural and paleogeographic restorations of clastic sediments deposited in episutural basins has allowed us to unravel the collisional history of the Northern Apennines from middle Eocene to Holocene time, as illustrated in seven paleogeographic maps. The episutural successions were deposited in two types of basins. 1. Late Eocene–early Oligocene basins perched on deformed Ligurian units (ophiolites and their sedimentary cover) of the southwestern side of the Alps (i.e., part of the future northern Apennine accretionary wedge). In these basins, provenance evolved from an extrawedge north-to-south supply from the Austroalpine (Adriatic) continental block to an intrawedge supply with progressive unroofing of the collisional tectonic stack composed of Ligurian units plus minor high-pressure–low-temperature metamorphic units (Pennidic units). These basins developed after the middle Eocene continental collision between the Adriatic margin and the southern European margin, a period dominated by magmatism, uplift, and block faulting of the Pennidic-Ligurian orogen. 2. Late Oligocene–Holocene basins formed on top of the migrating Apenninic orogenic wedge, which was progressively thrust onto the Adriatic margin, where thick, turbiditic successions were being deposited on the foreland. Sandstone composition is characterized by an overall increase in detritus from Pennidic units both up-section and from south to north and by detritus recycled from older sedimentary units and their Ligurian substratum. These basins developed during continental subduction of the Adriatic plate toward the west underneath the Corsica-Sardinia block coupled with extension in the future western Mediterranean area. Sedimentation patterns indicate that paleobathymetry and source rocks were markedly different north and south of the Val Secchia line, a structural lineament that does not correspond to a present-day observable structure across the Apennines.

2 - ELBA ISLAND - B - EASTERN ELBA

The geology of the Elba Island is very interesting for its structural complexity, for the relationships between Mio- Pliocene magmatism and tectonics and for its location between Corsica and Northern Apennines. In fact, it is the southwesternmost outcrop of the Northern Apennines chain. The first tectono-sedimentary events come back to the Palaeozoic. In fact, in the Palaeozoic basement are clearly recorded the Hercynian and, probably, also the Caledonian events. The Alpine sedimentary succession began during the Triassic and its tectonic evolution begins in Late Cretaceous - Early Tertiary and goes on until Late Miocene - Pliocene. The compressive movements started with the consumption of the Mesozoic Western Tethys (Liguria-Piedmont Basin), went up to the collision of European (Corsica) and Adriatic (Tuscany Domain) margins (Abbate et al., 1980, Boccaletti et al., 1980; Bortolotti et al., 2001a), and terminated with the successive polyphase deformations, which originated the Apennine orogenic chain. Trevisan (1950), proposed a geological frame of the Elba I. which, successively slightly modified by Barberi et al. (1967; 1969), which has been the geological starting-point for all successive studies. This model divides the nappe pile of the Island into five Complexes. New stratigraphical and structural data modify this frame. The new scheme we propose (Bortolotti et al., 2001a), comprises nine tectonic units, built up of Tuscan (Adria continental margin), Ligurian and Liguria-Piedmont (Jurassic-Eocene oceanic domains) successions, piled up with a very complex frame. This frame is complicated by post-orogenic extensional events, that produced the thinning of the Tuscan crust that underlies the Elba Units, the uplift of the Moho and the birth and evolution of the Tyrrhenian Basin (Boccaletti et al., 1985; Bartole et al., 1991; Bartole, 1995 cum bibl.; Carmignani et al., 1995). To this phase is linked the emplacement of the monzogranitic bodies and the formation of ore deposits and skarns. These latter constitute one of the best known geological features of the Island (Marinelli, 1975; Serri et al., 1991; Tanelli, 1977; 1983 cum bibl.).

Seismic exploration in complex terrains: A processing experience in the Southern Apennines

We discuss a data‐processing sequence adopted to reprocess a seismic line that crosses the Italian southern Apennines from the Tyrrhenian Sea to the Adriatic margin and investigate both the overthrust and foreland areas. We first determine the main causes of the very low S/N ratio in the field data and then propose a processing sequence aimed at exploiting the signal content, also making use of a priori geological knowledge of this area. Our work indicates a combination of causes for the very low quality of the seismic data. These include length of the spread (about 20 km) that is unfavorable because of the rapid variation in the near‐surface geology, tectonic complexity, crooked‐line acquisition, and the rough topography associated with outcropping rocks characterized by highly variable velocities. Based on the outcome of this data analysis, we present a processing sequence driven by knowledge of the regional tectonic setting and by knowledge of the shallow subsurface geology. The main effort is in removing the large, near‐surface related noise components. The low S/N ratio makes it impossible or nearly impossible to successfully apply highly sophisticated techniques such as depth migration or wave equation datuming. Thus, we used robust techniques specifically designed to solve each problem that degraded data quality. The most relevant of these techniques were the removal of bad traces where unacceptably low quality was detected by energy and frequency decay criteria; estimation and correction for static time shifts attributable to near‐surface conditions; optimization of common midpoint (CMP) sorting to attenuate the deleterious effects of the crooked‐line acquisition; application of a weighted stacking technique to maximize stack power and application of prestack f-x deconvolution to attenuate uncorrelated noise. The outcome of this processing sequence is compared with the result of a more standard sequence that was previously applied to the same data and is also discussed in terms of the possible geological model it might evidence. The realization of a seismic section showing rather continuous and structured events down to 8 s which, depending on the interpretation, may be related to Moho discontinuity or to very deep sedimentary layers supports the efficacy of the processing approach we propose.

Geodynamic cycles of subcontinental lithosphere in the Central Alps and the Arami enigma

Major masses of metamorphosed ultramafic rocks in the Central Alps, Western Alps and Liguria are former subcontinental lithospheric mantle. It occurs in two, tectonically distinct ophiolitic zones which paleogeographically correspond to the Jurassic, South Penninic, Piemontese ocean belt and to the more external, Cretaceous, North Penninic, Valais ocean belt, respectively. In both belts, the geodynamic cycle of pieces of subcontinental lithospheric mantle started with its exhumation during continental rifting and resulted in the exposure of denuded mantle in the Tethyan ocean basins. With the onset of oceanic closure, these mantle rocks were subducted and, after continental collision, integrated in the Alpine mountain chain and again exhumed. Combined evidence from two key areas of the Central Alps is presented. At Val Malenco, a stable continental crust-mantle transition (1.0–0.9 GPa; 800–600°C) welded by a 270 Ma gabbro intrusion existed into the Triassic. During Jurassic rifting, Malenco mantle with attached pieces of lower crust was rapidly exhumed and exposed in the Piemontese ocean basin near its Adriatic margin. There, an ‘Alpine type ophiolite suite’ developed, containing serpentinized ex-subcontinental mantle, rodingitized Permian gabbro, ophicarbonate rocks and MOR basalts. During Cretaceous convergence, the rifted margin of Val Malenco was only moderately subducted (∼450°C, 0.6 GPa) as it remained in a supra-subduction wedge. In the Western Alps and Liguria, rifted margins of the Piemontese belt were subducted to eclogite conditions (∼3 GPa, 650°C), but never beyond the stability of Antigorite. In the North-Penninic Adula-Cima Lunga nappe of the Central Alps, however, a suite analogous to Val Malenco was subducted to much higher metamorphic conditions (∼3 GPa, 830°C) during Eocene collision. Successive regional metamorphic prograde stages are proven by overgrowth textures in mafic rocks up to kyanite eclogite and in ultramafic rocks up to garnet peridotite. The inferred ultradeep garnet lherzolite of Alpe Arami forms part of the prograde Alpine suite of Adula-Cima Lunga. Ilmenite rods in olivine, as at Arami, invoked as ‘proof’ for an ultradeep origin, have been detected in over 10 localities. They have been shown to form from planar, Ti-bearing, (OH)-defects at conditions below 3 GPa. No direct evidence for an ultradeep origin for Alpe Arami has been presented so far.

Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps)

The Braccia gabbro of Val Malenco, Italian Alps, intruded 275 My ago during Early Permian lithospheric extension. The intrusion took place along the crust–mantle transition zone and caused granulite metamorphism of lower-crustal and upper-mantle rocks. The magmatic crystallization of the gabbro was outlasted by ductile deformation, which is also observed in the other rocks of the crust–mantle transition. Two stages of retrograde metamorphism followed. Mineral parageneses in garnet–kyanite gneiss, metagabbro, and metaperidotite record a first stage of near-isobaric cooling under anhydrous conditions. The stabilized crust–mantle transition then persisted over a period of about 50 My into the Late Triassic. Exhumation of the crust–mantle complex began with the onset of continental rifting during Early Jurassic. This stage of retrograde metamorphism is recorded by near-isothermal decompression and partial hydration of the granulitic mineral assemblages. The whole crust-to-mantle complex was then exposed in the Tethyan ocean near its Adriatic margin. The magmatic assemblage of the Braccia gabbro formed at 1–1·2 GPa and 1150–1250°C. Microstructures show that the gabbroic rocks evolved from olivine gabbros through spinel to garnet granulite whereas the peridotites recrystallized within the spinel peridotite field and the pelitic granulites remained in the stability field of kyanite. Such an evolution is characteristic of isobaric cooling after magmatic underplating. Granulitic mineral assemblages record cooling from 850°C to 650°C with decompression to 0·8 ± 0·1 GPa, and dP/dT < ∼0·15 GPa/100°C. During later hydration, Cl-rich amphibole and biotite + plagioclase formed in the gabbros, clinozoisite + phengite + paragonite ± staurolite ± chloritoid in the metapelites and olivine + tremolite + chlorite ± talc in the ultramafic rocks at metamorphic conditions of 0·9 ± 0·1 GPa and 600 ± 50°C. Subsequent retrograde metamorphism involved decompression of ∼0·3 GPa and cooling to ∼500°C, consistent with the preservation of the olivine + tremolite + talc assemblage in ultramafic rocks. Estimated uplift rates of 1–2 mm/year indicate a 15–30 My exhumation related to Jurassic rifting. The two-stage retrograde path of the Malenco granulites separated by >50 My suggests that Permian extension and Jurassic rifting are two independent tectonic processes. The presence of hydrous, Cl-rich minerals at 600 ± 50°C and 0·8 ± 0·1 GPa requires input of externally derived fluids at the base of 30 km thick continental crust into previously dry granulites at the onset of Jurassic rifting. These fluids were generated by dehydration of continental crust juxtaposed during rifting with the hot, exhuming granulite complex along a active shear zone.

Spatial variation in stresses in peninsular Italy and Sicily from background seismicity

We have retrieved the stress tensor orientation in the Italian region, applying the FMSI technique (Gephart, J., 1990b. FMSI: a FORTRAN program for inverting fault/slickenside and earthquake focal mechanism data to obtain the regional stress tensor. Comput. Geosci. 16, 953–989) to fault plane solutions of 298 small earthquakes (2.5< M d<4.8) recorded by the National seismic network of the Istituto Nazionale di Geofisica in the period 1988–1995. First, we verified that the stress inversion procedure, generally applied to a large number of focal mechanisms of earthquakes recorded in a local seismic network, could be successfully used in Italy, using background seismicity recorded over large (∼100–200 km) regions. Then, comparing the stress inversion results with other stress indicators, we find that they are geologically sound for many regions in Italy, even if the misfit values are relatively large. We also find that the largest misfit values are determined by a few earthquakes (only 10–20% of the total number). Excluding these earthquakes from inversion, the average misfit becomes much lower (by ∼30–40%). The results obtained in this study confirm the large variations of the stress field over a relatively short distance in Italy, but also confirm the existence of regions extending for a few hundred kilometres within which the stress is homogeneous. Among these, most of the Apennines are dominated by extension perpendicular to the trend of the mountain belt. Only in the northern Apennines is there any clear distinction between an area of extension in the inner portion of the belt and an area under horizontal compression along the Adriatic margin of the belt. More rapid variations occur around Calabria and Sicily, as expected because of the complexities linked with the Tyrrhenian subduction and with active volcanism. In the Ionian Calabrian coastal region, there are hints of active NW–SE compression, although constrained by a few data, whereas in the Messina Strait, we observe ∼E–W extension and a vertical σ 1. In northern and western Sicily, our results indicate N–S to NW–SE compression, roughly consistent with the general convergence between the European and the African plates.

Extension- and compression-related basins in central Italy during the Messinian Lago- Mare event

During the Messinian Lago- Mare event, in the central Mediterranean area, compression- and extension-related basins developed as a consequence of the post-Tortonian evolution of the Tyrrhenian–Apennines system. All these syn-tectonic sedimentary basins were characterized by different subsidence rates and sedimentary processes. Several sections located in different tectonic domains from the Tyrrhenian to the Adriatic margin of the Apennine chain have been investigated: S. Valentino in Abruzzo Citeriore, Roccamorice, Penscosansonesco–Roccacaramanico and Le Vicenne sections are located in compression-related basins (foreland basin system), while the Tolfa–Blera sections and the sedimentary succession of Mondragone 1 well, drilled in the Garigliano coastal plain, are located in extension-related basins (hinterland or back-arc basins). The recognition of ostracod assemblages with Paratethyan affinities allows them to be referred to the Messinian Lago- Mare event ( Loxoconcha djaffarovi Zone). This result allows us to consider the Tyrrhenian rifting, the accretion of the Apennine thrust belt and the increase in subsidence rate during the flexuration of the foreland domain as synchronous processes due to the same geodynamical process. Taking into account the thicknesses of the analysed sedimentary basins, different total subsidence rates have been computed for both the compression- and extension-related basins. The syn-rift basins that developed on the eastern margin of the northern Tyrrhenian Basin (Volterra, Val di Fine and Tolfa-Blera Basins) recorded a mean total subsidence rate of 1.25 mm/a, whereas higher subsidence rates affected the eastern margin of the central Tyrrhenian Basin (5 mm/a, Garigliano coastal plain). The differences in horizontal stretching and total subsidence rates that affected the Tyrrhenian extensional area correspond to different migration rates of the Apennine orogenic system in the northern (10 mm/a) and central Apennines (40 mm/a). This difference in the total syn-rift subsidence rate fits in well with the segmentation model of the Tyrrhenian–Apennines system along a NNE–SSW lithosphere discontinuity, which crosses the system from Ancona (northern Apennines) to the R. Selli lineament, in the Tyrrhenian Basin.

Magnetic anisotropy of Plio–Pleistocene sediments from the Adriatic margin of the northern Apennines (Italy): implications for the time–space evolution of the stress field

The anisotropy of magnetic susceptibility (AMS) of the marine fine-grained Plio–Pleistocene sediments that crop out at the eastern (Adriatic) front of the central–northern Apennines (Italy) indicates a prevalent sedimentary-compactional magnetic fabric with variable overprint of the tectonic strain. The degree of anisotropy and the geometry of the AMS ellipsoids suggest a subdivision of the studied sediments in two distinct ages: Early–middle Pliocene and Late Pliocene–Early Pleistocene. The Early–middle Pliocene sediments show a weak but well defined magnetic lineation parallel to the main fold and thrust axes throughout the region, analogously to the Messinian sediments located in a more internal (western) position of the Apenninic chain. Since the Late Pliocene, the AMS data are not regionally coherent and indicate a reduced and locally variable tectonic influence on the magnetic fabrics. The AMS data have been integrated with the available geophysical information of the present-day stress in the region, with the aim to reconstruct the time–spatial evolution of the stress field in the Adriatic margin of the central-northern Apennines, during the past 5 Ma. The present-day stress field was evaluated using borehole breakout in deep wells (about 50 wells), focal mechanisms of crustal earthquakes (2.5< M d<5) that occurred in the last 10 years and a review of the historical seismicity. The synthesis of these data suggests significant changes in the time–space evolution of the stress field through the area in the last 5 Ma. In the northern sector the tectonic regime shows a maximum compressional axis trending NE–SW, that is constant at least since the Early Pliocene. In the southern sector the stress field has drastically changed: a compressional stress with a maximum compressional axis trending E–W was active during the Pliocene and was likely preserved until Early Pleistocene; then the stress field changed, presently showing a NE–SW minimum horizontal stress that mirrors the trend of the active extension observed in the southern Apennines. Between these two sectors a transitional area is identified at about 43°N, that we associate with a main NNE–SSW lithospheric discontinuity pointed out by the available geological and geophysical data.

Dating of eclogite-facies zircons: the age of Alpine metamorphism in the Sesia–Lanzo Zone (Western Alps)

Zircons from eclogite-facies rocks of the Sesia–Lanzo Zone have been investigated by cathodoluminescence and dated by ion microprobe (SHRIMP). Most of the zircons show a oscillatory or sector zoned core, which is surrounded by a weakly zoned or unzoned rim, irregular in shape. The U–Th–Pb analyses revealed that, opposite to the cores, the zircon rims have low U- and Th-contents, very low Th/U ratios and that their U–Pb systems were reset or they lost lead at the time of metamorphism. This study provides evidence that the zircon rims recrystallized and changed their internal zoning as well as their chemical and isotopic composition during high-pressure (15–18 kbar) moderate temperature (550–600°C) metamorphism. In the Sesia–Lanzo Zone, eclogite-facies zircons from three different localities and four different rock types were analysed. The age of metamorphic zircon rims was obtained from an eclogite from Monte Mucrone (65±5 Ma), an eclogitic micaschist from the Aosta Valley (65±3 Ma) and two mafic rocks from Cima di Bonze (68±7 Ma). The ∼65 Ma age is further supported by the ages of the youngest zircon rims in three other samples. These results constrain the age of eclogite-facies metamorphism in the Sesia–Lanzo Zone. An age of 76±1 Ma determined on zircons from a hydrothermal vein is interpreted as dating the prograde formation of the vein at temperatures well below 550°C. The age results suggest a maximum rate of 0.7 cm/y for the subduction of the Sesia–Lanzo Zone to eclogite-facies conditions. The Cretaceous–Tertiary age for the eclogite-facies metamorphism in the Sesia–Lanzo Zone implies that this continental unit was not subducted together with either portions of the Adriatic margin or the neighbouring Mesozoic Tethys. We suggest that the Sesia–Lanzo Zone behaved as a separate continental block at the time of Alpine convergence.

Subduction and obduction processes in the Swiss Alps

The significance of the Briançonnais domain in the Alpine orogen is reviewed in the light of data concerning its collision with the active Adriatic margin and the passive Helvetic margin. The Briançonnais which formerly belonged to the Iberian plate, was located on the northern margin of the Alpine Tethys (Liguro-Piémont ocean) since its opening in the early-Middle Jurassic. Together with the Iberian plate the Briançonnais terrane was separated from the European plate in the Late Jurassic–Early Cretaceous, following the northern Atlantic, Bay of Biscay, Valais ocean opening. This was accompanied by the onset of subduction along the northern margin of Adria and the closure of the Alpine Tethys. Stratigraphic and metamorphic data regarding this subduction and the geohistory of the Briançonnais allows the scenario of subduction–obduction processes during the Late Cretaceous–early Tertiary in the eastern and western Alps to be specified. HP–LT metamorphism record a long-lasting history of oceanic subduction-accretion, followed in the Middle Eocene by the incorporation of the Briançonnais as an exotic terrane into the accretionary prism. Middle to Late Eocene cooling ages of the Briançonnais basement and the presence of pelagic, anorogenic sedimentation lasting until the Middle Eocene on the Briançonnais preclude any sort of collision before that time between this domain and the active Adria margin or the Helvetic margin. This is confirmed by plate reconstructions constrained by magnetic anomalies in the Atlantic domain. Only a small percentage of the former Briançonnais domain was obducted, most of the crust and lithospheric roots were subducted. This applies also to domains formerly belonging to the southern Alpine Tethys margin (Austroalpine–inner Carpathian domain). It is proposed that there was a single Palaeogene subduction zone responsible for the Alpine orogen formation (from northern Spain to the East Carpathians), with the exception of a short-lived Late Cretaceous partial closure of the Valais ocean. Subduction in the western Tethyan domain originated during the closure of the Meliata ocean during the Jurassic incorporating the Austroalpine–Carpathian domain as terranes during the Cretaceous. The subduction zone propagated into the northern margin of Adria and then to the northern margin of the Iberian plate, where it gave birth to the Pyrenean–Provençal orogenic belt. This implies the absence of a separated Cretaceous subduction zone within the Austro-Carpathian Penninic ocean. Collision of Iberia with Europe forced the subduction to jump to the SE margin of Iberia in the Eocene, creating the Apenninic orogenic wedge and inverting the vergence of subduction from south- to north-directed.

Jurassic formation and Eocene subduction of the Zermatt-Saas-Fee ophiolites: implications for the geodynamic evolution of the Central and Western Alps

The Zermatt–Saas-Fee ophiolites (ZSFO) are one of the best preserved slices of eclogitic oceanic crust in the Alpine chain. They formed during the opening of the Mesozoic Tethys and underwent subduction to HP/UHP conditions during Alpine compression. A cathodoluminescence-based ion microprobe (SHRIMP) dating of different zircon domains from metagabbros and oceanic metasediments was carried out to constrain the timing of formation and subduction of this ophiolite, two fundamental questions in Alpine geodynamics. The formation of the ophiolitic sequence is constrained by the intrusion ages of the Mellichen and the Allalin metagabbros (164.0 ± 2.7 Ma and 163.5 ± 1.8 Ma) obtained on magmatic zircon domains. These data are in line with the maximum deposition age for Mn-rich metasediments which overlie the mafic rocks at Lago di Cignana (161 ± 11 Ma) and at Sparrenflue (ca. 153–154 Ma). An Eocene age of 44.1 ± 0.7 Ma was obtained for whole zircons and zircon rims from an UHP eclogite and two metasediments at Lago di Cignana. One of the Eocene zircons contains a rutile inclusion indicating formation at HP conditions. As the temperature and pressure peak of these rocks nearly coincide, the Eocene zircons probably constrain the age for the deepest subduction of the ZSFO. This Eocene age for the UHP metamorphism implies that the ZSFO were subducted later than the Adriatic margin (Sesia-Lanzo Zone) and before the Late Eocene subduction of the European continental crust below Apulia. A scenario with three subduction episodes propagating in time from SE to NW is proposed for the geological evolution of the Central and Western Alps.