Major Tectonic Contacts Research Articles

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Detrital zircon geochronology is frequently used for sediment provenance analysis in orogenic belts, but it can also be applied to locate the boundary between basement and sedimentary cover in successions and tectonometamorphic units where the paleontological record is missing, and primary stratigraphic relationships have been erased by intense deformation and/or metamorphism. This is critical to differentiate metamorphic mineral associations, P-T conditions and deformational events associated with superposed orogenic cycles. In this work, detrital zircon geochronology has been applied to the Ronda peridotites’ crustal hanging wall (hinterland of the Alpine Betic Cordillera, southern Spain), apparently made up of two major tectonic units – the Jubrique unit and the Maláguide complex – separated by a major Alpine tectonic contact camouflaged in between low-grade metamorphic rocks.Our results show that pre-Paleozoic peaks (c. 650, 1000, 2000, and 2600 Ma) are common along the entire studied crustal sequence, and Paleozoic ages are only recorded in the upper part of the Jubrique unit (c. 290, 350, and 475 Ma) and in the Maláguide complex (c. 350 and 475 Ma). Thus, the Paleozoic rocks of the Maláguide complex, constituting the highest crustal unit overlying the Ronda peridotites, represent syn-to-late orogenic Variscan sediments deformed and slightly metamorphosed prior to the Alpine cycle. In contrast, the underlying Jubrique unit is made of a post-Variscan sedimentary cover deposited on a Variscan basement, with the contact between both (hidden unconformity?) located within low- to medium-grade schists. These data support the presence of a major tectonic contact between the Maláguide complex (Late Carboniferous maximum depositional ages) and the Jubrique unit (Early Permian maximum depositional ages), i.e. the Maláguide complex would have overthrust the Jubrique unit during the Alpine orogeny. These results indicate a Permian or younger exhumation/emplacement up to crustal levels of the Ronda peridotites subcontinental mantle slab.Altogether, these detrital zircon age spectra support a relatively, but not entirely local derivation of the studied crustal sequences (fragments of the Iberian Massif) and suggest some Mesozoic drifting with respect to its present position. Nonetheless, the different age distribution of the Paleozoic populations between the Jubrique (Alpujárride complex) and Maláguide samples suggests that the former deposited relatively close to other samples from the eastern-central Alpujárride complex and Nevado-Filábride complex, which, in turn, show strong similarities with the Cantabrian zone and NE Iberia (Iberian Massif). In contrast, the Maláguide samples were probably deposited close to the Ossa-Morena Zone (Iberian Massif) . Thus, Alpujárride/Nevado-Filábride and Maláguide complexes would have been juxtaposed during the Triassic-Cretaceous Neotethys rifting/drifting and/or the subsequent Alpine collisional events.

Cenozoic Pb–Zn–Ag mineralization in the Western Alps

Abstract Metallogenic models of polyphase mountain belts critically rely on robust geochronology. We combine petrology with Rb–Sr and U–Th–Pb in situ geochronology, paired at thin-section scale, to date mineralization in deformed hydrothermal Pb–Zn–Ag deposits along an east-west transect in the Western Alps, France. The Pb–Zn–Ag veins occur in shear zones with kinematic structures consistent with the mylonitized host rocks. The ore consists mainly of galena in a quartz-phengite gangue. The paragenesis can be related to hydrothermal crystallization during periods of variable strain. Both isotope systems yield only Cenozoic ages (ca. 35 Ma and 15–20 Ma) without any pre-Alpine inheritance, clearly indicating orogenic mineralization. The metallogenic model proposed here includes significant fluid circulation along major tectonic contacts between basement and sedimentary cover during Alpine convergence.

Schistes Lustrés in a hyper-extended continental margin setting and reinterpretation of the limit between the Mont Fort and Tsaté nappes (Middle and Upper Penninics, Western Swiss Alps)

The Schistes Lustrés form a large and complex unit at the top of the Penninic nappe stack of the Alpine belt. Calcschists, partly of Late Cretaceous age, constitute the dominant lithology. They are closely associated both with blueschist facies Piemont-Ligurian ophiolites and continent-derived Mesozoic metasediments. The question of whether the Schistes Lustrés originated on continental or oceanic crust has been extensively debated among Alpine geologists and is locally still controversial. We present here new structural and stratigraphic observations, as well as Raman graphite thermometry (RSCM) data, for the Schistes Lustrés complex of the Combin zone in the Hérens, Dix and Bagnes valleys. Our observations indicate that the basal part of this Schistes Lustrés complex (defined as the Série Rousse) is systematically devoid of ophiolitic material, and rests in stratigraphic contact on the underlying Triassic - Lower Cretaceous metasediments and Paleozoic basement of the Mont Fort nappe (Prepiemont paleogeographic domain). The unconformity at the base of the Schistes Lustrés complex is interpreted as resulting from the sedimentation of the Série Rousse on a paleorelief formed by remnants of Jurassic normal fault scarps, and not as an Alpine tectonic contact, as previously proposed. The lithostratigraphic comparison with the Breccia nappe in the Prealps, as well as a foraminifer discovery, allows us to better constrain the age of the Série Rousse. It extends from the middle of the Early Cretaceous (Aptian?) to the Late Cretaceous (Campanian to earliest Maastrichtian?). In contrast, the upper contact of the Série Rousse with the ophiolite-bearing Schistes Lustrés clearly corresponds to an Alpine thrust. The thrust zone is underlined by thin and discontinuous slices of highly strained continental-margin derived Mesozoic metasediments (Frilihorn slices). RSCM data show that the recrystallization of the organic matter progressively increases on both sides towards this contact. This contact, internal to the Schistes Lustrés complex, is reinterpreted as the major tectonic contact separating the Middle Penninic Mont Fort nappe from the Upper Penninic Tsaté nappe (defined here as including only the ophiolite-bearing Schistes Lustrés and associated meta(ultra-)basites). This study clearly documents that the Schistes Lustrés consist of sediments either deposited on oceanic crust, showing locally preserved stratigraphic contacts with ophiolitic or serpentinized sub-continental mantle slivers, or sediments still resting stratigraphically on a former hyper-extended continental margin.

Reuniting the \xd6tztal Nappe: the tectonic evolution of the Schneeberg Complex

The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction.

Low-topography deep-seated gravitational slope deformation: Slope instability of flysch thrust fronts (Outer Western Carpathians)

Unlike high-relief mountain areas, low-relief hilly landscapes are usually rarely affected by deep-seated gravitational slope deformations (DSGSDs). However, low-topography flysch thrust fronts can create suitable structural conditions for DSGSDs. The study area of the Kavalčanky ridge represents a relatively low-lying (<120 m of local relief) DSGSD-affected ridge situated at the thrust front in the flysch Outer Western Carpathians (Czech Republic). With the aim of revealing the main controlling factors and temporal constraints of mass-movement activity, a multidisciplinary investigation of DSGSDs was performed. Typical DSGSD landforms were mapped using high-resolution LiDAR-based mapping. Structural analysis revealed the presence of specific flysch thrust structural conditions with competent sandstone units overthrusted on a weak tectonically disrupted claystone basement. Geophysical measurement with the use of electrical resistivity tomography (ERT) and ground penetrating radar (GPR) profiling confirmed the deep reach (>50 m) of the studied DSGSD. Together with slope stability finite element modelling, geophysics suggests that the shear zone of DSGSDs is represented by one of the thrust faults, whereas lateral limits are formed by a set of conjugate strike-slip faults. Radiocarbon dating of bogs within the DSGSD body showed two phases of mass-movement activity corresponding to the Late Glacial-Holocene transition and Middle Holocene. The recent activity was excluded by dendrogeomorphic analysis. We concluded that the structural conditions involving high lithological complexity, tectonic weakening and the presence of major tectonic contacts might create conditions prone to DSGSDs even in relatively low-topography settings with mass movement activity, especially during humid and warmer Late Quaternary periods.

Trace metals dispersion from 1000 years of mining activity in the northern French Alps

This paper investigates the long-term dispersal of trace metals and their impact on the environment long after mine closure in the French Alps. Three statistical methods derived from modern mining (Exploratory Data Analysis, fractals and Sediment Quality Index) were applied to geochemical analyses of streambed sediments to produce mineral prospectivity maps. About 5% of the samples have a poor or marginal quality and may have an effect on biota. Positive anomalies are either related to the geological background (lithology, major tectonic contacts) or to ancient mining sites. The local impact of former mining was investigated on a medieval Pb–Ag district (Brandes; Pbmax ~8 000 mg kg−1), a small district mined during the 17th–19th c. (Peisey-Nancroix; Pbmax = 583 mg kg−1), and a major modern Pb–Ag mine (Macôt-La Plagne; Pbmax ~ 3 200 mg kg−1).The mining heritage is still traceable in the northern French Alps. The anomalies observed at local scale are related to the volume of extracted ore rather than to the timing and duration of mining, suggesting that old mining sites still release trace metals over centuries and up to a thousand years. The statistical methods used provide a strong tool for archaeological prospection and environmental studies.

Scales of fluid-rock interaction and carbon mobility in the deeply underplated and HP-Metamorphosed Schistes Lustrés, Western Alps

The Schistes Lustrés metasedimentary complex, exposed in the Italian and French Alps, is a regionally extensive unit of blueschist to eclogite facies marble, calc-schist and metapelite providing a record of sediment underplating and fluid-rock interaction at 40–65 km of a paleo-subduction zone. Limestones, carbonate-rich mudstones, and black shales in SE France and central Switzerland, herein used as a proxy for the unmetamorphosed Schistes Lustrés, bear a striking resemblance to sediments currently subducting into the E. Sunda margin.The Schistes Lustrés, with estimated P-T conditions ranging from ∼1 GPa and 350 °C to ∼2.3 GPa and 550 °C, have on a regional scale had their average carbonate δ18OVSMOW lowered to +20.4‰, relative to the average of +28.5‰ of their likely protoliths. This decrease in δ18O can be most easily explained by varying combinations of: (1) closed-system equilibration of carbonate O with O in silicate minerals in the same rocks (i.e., at the μm to cm scale); and (2) interaction between carbonates and H2O-rich fluids released during prograde metamorphic devolatilization of the metapelitic rocks in the section (at up to km scales). Regional-scale decrease in carbonate δ18O in the Schistes Lustrés could have occurred without the need for infiltration by fluids generated outside of the Schistes Lustrés complex. More dramatic reductions in δ18O (as low as +8‰; average = +15.5‰) at/near major tectonic contacts, inferred to be fossil transient subduction interfaces, likely reflect deformation-enhanced infiltration by fluids derived in mafic or ultramafic rocks at greater depths.Pressure solution and concomitant cleavage formation in the Schistes Lustrés presumably were associated with some carbonate removal, and higher-grade rocks show some evidence for decarbonation. However, within the Schistes Lustrés, C is deposited as carbonate in locally abundant veins and it has proven difficult to identify clear evidence of wholesale carbonate removal from the complex (i.e., at scales of kms to 10s of kms). This study demonstrates that large fractions of subducted sedimentary carbonate sections could persist to depths approaching those beneath volcanic fronts, if they are not accreted or underplated and depending on the extents to which they are infiltrated by H2O-rich fluids capable of driving both decarbonation reactions and carbonate dissolution. Sediments such as these could supply C for additions to arc source regions or, with further subduction, convey C into the deeper mantle. The C removal and deposition reported by others for sites of particularly high fluid flux, along large fault systems, in zones of high fracture/vein density, and in metasomatized contacts with mafic/ultramafic rocks, contrast with the relative retention and lower degrees of mobilization of C observed within km-scale packets of Schistes Lustrés away from zones of enhanced deformation and behaving as relatively closed systems. The fragmented and potentially lithologically biased nature of HP/UHP metamorphic suites greatly complicates attempts to derive fluid and C fluxes scalable to modern subduction margins.

Geochemistry and architecture of the host sequence of the massive sulfides in the northern Iberian Pyrite Belt

The northernmost Iberian Pyrite Belt hosts several small felsic volcanogenic massive sulfide orebodies (<15 Mt), except the Aguas Teñidas and Magdalena each with ca. 45 Mt, which are economically important because of their high base and precious metal grades if compared with the bulk of the IPB. They occur aligned in a E-W belt located just south of the major suture that limits the South Portuguese Zone with the Pulo do Lobo Unit. The mineralization is related to a specific felsic volcanic unit that occurs in different tectonically dismembered units, but has a specific geochemical signature and a characteristic hydrothermal alteration. Six principal tectonostratigraphic units can be differentiated: (1) The Lower Felsic Unit is composed of feldspar-quartz-phyric rhyodacite (crypto)-dome complexes, intruded by sills and dykes of similar composition. Pumice and glass-rich breccia beds occur at the top and on the lateral margins of the dome complexes. A major shear zone separates the Lower Felsic Unit from the overlying Volcano-Sedimentary Unit; (2) The Volcano-Sedimentary Unit consists of vesicular basaltic lava and associated mafic epiclastic sandstone and siltstone, intercalated with thin beds of shale. A major tectonic contact marks the upper boundary; (3) The Middle Felsic Unit represents a series of intercalated coherent rhyolite domes with associated volcaniclastic rocks, and polymictic epiclastic sedimentary rocks. They are cut by numerous volcanic dykes and geochemically equivalent sub-volcanic intrusions; (4) The Sedimentary Unit is a highly deformed package of grey siltstone with interlayered shale and fine-grained epiclastic sedimentary rocks; (5) The Upper Felsic Unit is a package of dacitic to rhyolitic dome complexes with similar characteristics to the Lower Felsic Unit. Both the basal and upper contacts of this unit are narrow and high-strain tectonized zones. (6) The uppermost unit, the Andesite Unit, is comprised of andesitic dome complexes rich in hyaloclastite breccias and andesitic volcaniclastic rocks. Massive sulfides are hosted dominantly by the Lower Felsic Unit with only some orebodies related spatially to the Upper Felsic Unit (Lomero Poyatos, San Telmo and El Carpio).Geochemically, the Volcanic Sedimentary Complex is dominated by andesitic to rhyolitic calc-alkaline magmatic rocks with a significant proportion of dacite and andesite. The Lower Felsic and Upper Felsic Units are geochemically identical and probably represent dismembered parts of a single volcanic felsic complex. If that holds true, massive sulfide deposits of the northern IPB are hosted by volcaniclastic rocks located in the hanging wall of a single unit dominated by dacite-rhyolite dome complexes. This volcanic unit is characterized by Zr contents <200 µg/g (ppm) which are significantly lower than those of the more evolved Middle Felsic Unit (200–750 µg/g).Structural analysis and the geochemistry suggest that the volcanic sequence is tectonically imbricated, with the youngest units located at the lowest structural position. The proposed reconstructed stratigraphic sequence comprises an andesitic footwall, overlain by the felsic complexes of the Middle Felsic and Lower-Upper Felsic Units. Thus, the massive sulfide deposits formed toward the top of the whole sequence, and associated with the youngest felsic rocks having a specific geochemistry.

Back to Normal: Direct Evidence of the Cretan Detachment as a North‐Directed Normal Fault During the Miocene

AbstractThe Cretan Detachment represents a major tectonic contact between an Alpine high‐pressure Lower Nappe System in the footwall and an Alpine unmetamorphosed Upper Nappe System in the hanging wall. Interestingly, the kinematics and the tectonic nature of this contact is still highly controversial and has been interpreted either as a top‐to‐the S thrust or as a normal fault with top‐to‐the N and/or top‐to‐the S displacement sense. Whereas some models suggest exhumation of the high‐pressure rocks synchronous with out‐of‐sequence thrusting or Himalayan‐type extrusion tectonics, other models propose that Crete represents a metamorphic core complex that exhumed below the extensional Cretan Detachment. In this work, we show that the Cretan Detachment in eastern Crete is an up to 100‐m‐thick cataclastic low‐angle fault zone, which localized below the Tripolitza Unit (Upper Nappe System) within the upper part of the Upper Violet Slates (top of Lower Nappe System) cutting downsection to lower structural levels. The unequivocal kinematic indicators in this fault zone clearly suggest a top‐to‐the N displacement of the hanging wall. New low‐temperature geochronological data and numerical modeling suggest rapid cooling of the rocks in the footwall of the Cretan Detachment at circa 15 Ma. Top‐to‐the S ductile shear sense toward lower structural level in the Lower Nappe System support the idea that the Cretan Detachment facilitated the exhumation of the high‐pressure rocks in an extrusion tectonic setting. Therefore, in the early to middle Miocene, subduction‐related extrusion in Crete was simultaneously active with back‐arc extension‐related metamorphic core formation in the Cyclades.

Polyphase greenschist-facies reactivation of the Dent Blanche Basal Thrust (Western Alps) during progressive Alpine orogeny

This study assesses the significance, geometry, and kinematics of greenschist-facies deformation along the Dent Blanche Basal Thrust (DBBT), a major tectonic contact in the Internal Western Alps of Switzerland and Italy. The DBBT separates continental units of the Dent Blanche nappe, the structurally highest unit in the Western Alps, from underlying Piemont-Ligurian ophiolites. Mylonites and deformation structures along the contact provide a record of its retrograde greenschist-facies evolution after earlier high-pressure metamorphism. A first phase of foreland-directed, reverse-sense, top-(N)W shearing (D1) occurred between ca. 43 and 39 Ma, related to exhumation of the Dent Blanche nappe from high-pressure conditions. It led to the formation of mylonitic fabrics under high- to medium-grade greenschist-facies conditions along the entire DBBT. A phase of ductile normal-sense top-SE shearing (D2) at ca. 38–37 Ma was mainly localized within underlying ophiolitic units and only partly affected the DBBT. Another phase of ductile deformation (D3) under medium- to low-grade greenschist-facies conditions at ca. 36–35 Ma occurred in response to underthrusting of European continental margin units and resulted in the updoming of the nappe stack. Especially the southeastern DBBT was characterized by bulk top-NW shearing, partly conjugate top-NW/top-SE shearing, and resulting orogen-perpendicular crustal extension. Subsequently, the DBBT was affected by a phase of orogen-perpendicular shortening (D4) and formation of folds and crenulations at ca. 34–33 Ma due to increasing compressional tectonics. Finally, a phase of semi-ductile to brittle normal-sense top-NW and conjugate shearing (D5) from ca. 32 Ma onwards particularly affected the southeastern segment and indicates exhumation of the DBBT through the ductile–brittle transition. This was followed by brittle NW–SE extensional deformation. This study suggests that the DBBT experienced a polyphase deformation and reactivation history under decreasing greenschist-facies metamorphic conditions during which different segments of this major shear zone were variably affected.

An eclogite-bearing continental tectonic slice in the Zermatt–Saas high-pressure ophiolites at Trockener Steg (Zermatt, Swiss Western Alps)

The Theodul Glacier Unit (TGU) at “Trockener Steg” represents a continental slice, embedded within the ophiolitic Zermatt–Saas Zone. The Zermatt–Saas Zone is the remnant of the Piemonte–Liguria oceanic lithosphere, formed in the middle Jurassic and subducted up to eclogite facies conditions in the Early Tertiary. The close spatial association of the TGU to the Zermatt–Saas Zone permits a comparison of the metamorphic evolution of the units by detailed field mapping and a petrological investigation of eclogites. The eclogites from both tectono-metamorphic units can be clearly distinguished by their textures, mineral assemblages and by mineral and bulk-rock composition. Geothermobarometry and computed assemblage stability diagrams for the TGU eclogites indicate P–T conditions of 2.2±0.1GPa and 580±50°C. These derived P–T conditions must be considered as minimum peak metamorphic conditions the rocks achieved during subduction. The P–T data are different from those derived for eclogites of Zermatt–Saas Zone adjacent to the Theodul Glacier Unit, that reached maximal burial depths at 2.3–2.4GPa and 500±50°C. While the estimates of the eclogites of Zermatt–Saas Zone are in good agreement with some of the previous studies, the contrasting P–T estimates for the TGU eclogites suggest that the Zermatt–Saas complex must be subdivided into several tectonic subunits. The non-uniform peak conditions over the “Trockener Steg” area and the maximum pressures conditions reported from ultra-high pressure localities within Zermatt–Saas Zone suggest, that individual tectonic slices have been assembled after detachment from the slab at the return-point, i.e. along the exhumation path. Detached packages of rocks may range from small tectonic slices up to several kilometer-sized fragments.The TGU is separated from the surrounding rocks of the ophiolite unit by two major tectonic contacts. In addition, the formation of biotite-rich crusts along the basal contact of the TGU is evidence of prolonged fluid channeling along the basal thrust. The presence of hydrous decompression assemblages replacing earlier formed high-pressure mineral assemblages within the studied eclogite suggests that fluids were present throughout most of the TGU exhumation history.

U–Pb dating of detrital zircons from Andros, Greece: constraints for the time of sediment accumulation in the northern part of the Cycladic blueschist belt

The Attic–Cycladic Crystalline Belt in the central Aegean region represents a major tectono‐stratigraphic unit of the Hellenides. The essential geological, magmatic and tectono‐metamorphic features are well documented. Unresolved questions concern the time of sediment accumulation and litho‐ and/or tectono‐stratigraphic relationships across the study area. In order to address this issue we have studied siliciclastic metasedimentary rocks from Andros Island, northern Cyclades. The sampling strategy aimed at covering the complete age range recorded by the Andros metamorphic succession. Detrital zircon U–Pb dating of nine samples indicates maximum depositional ages of c. 260 Ma for the topmost part of the metamorphic succession and of c. 160–140 Ma for rock sequences below a prominent serpentinite belt that is interpreted to outline a major tectonic contact. These age constraints are in accordance with interpretations suggesting that the metamorphic rocks of Andros represent different tectonic subunits (Makrotantalon Unit and Lower Unit) that are separated by a thrust fault. Modification of the internal structure of the Lower Unit by tectonic stacking can currently not be ascertained. The new data for the Lower Unit corroborate the importance of Late Jurassic–Early Cretaceous sediment accumulation for the larger study area. In contrast to some of the neighbouring islands, no evidence for transfer of Late Cretaceous (c. 80 Ma) material into the Andros sedimentary environment was found. The most striking feature of the zircon populations of the Lower Unit is a remarkable age cluster at 250–200 Ma that documents the importance of Triassic igneous sediment sources. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Measuring radon flux across active faults: Relevance of excavating and possibility of satellite discharges

Searching for gas exhalation around major tectonic contacts raises important methodological issues such as the role of the superficial soil and the possible long distance transport. These effects have been studied on the Xidatan segment of the Kunlun Fault, Qinghai Province, China, using measurement of the radon-222 and carbon dioxide exhalation flux. A significant radon flux, reaching up to 538 ± 33 mBq m −2 s −1 was observed in a 2–3 m deep trench excavated across the fault. On the soil surface, the radon flux varied from 7 to 38 mBq m −2 s −1, including on the fault trace, with an average value of 14.1 ± 1.0 mBq m −2 s −1, similar to the world average. The carbon dioxide flux on the soil surface, with an average value of 12.9 ± 3.3 g m −2 day −1, also remained similar to regular background values. It showed no systematic spatial variation up to a distance of 1 km from the fault, and no clear enhancement in the trench. However, a high carbon dioxide flux of 421 ± 130 g m −2 day −1 was observed near subvertical fractured phyllite outcrops on a hill located about 3 km north of the fault, at the boundary of the large-scale pull-apart basin associated with the fault. This high carbon dioxide flux was associated with a high radon flux of 607 ± 35 mBq m −2 s −1. These preliminary results indicate that, at the fault trace, it can be important to measure gas flux at the bottom of a trench to remove superficial soil layers. In addition, gas discharges need to be investigated also at some distance from the main fault, in zones where morphotectonics features support associated secondary fractures.

Structural and metamorphic evolution of the Ambin massif (western Alps): toward a new alternative exhumation model for the Briançonnais domain

Abstract The basement domes of the central part of western Alps may result either from a multistage tectonic evolution with a dominant horizontal shortening component, an extensional behaviour, or both. The Ambin massif belongs to the “Briançonnais” domain and is located within the HP metamorphic zone. It was chosen for a reappraisal of the tectonic evolution of the Internal Alps in its western segment. Structural investigations have shown that Alpine HP rocks were exhumed in three successive stages. The D1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds to a non-coaxial regime with dominant horizontal shortening and north movement direction. Petrological observations and P-T estimates show that the exhumation process was initiated during D1, the corresponding mechanism being still poorly understood. The D2 stage took place under low-blueschist facies conditions and culminated under greenschist facies conditions. It developed a retrogressive foliation and pervasive shear-zones at all scales that locally define major tectonic contacts. D2 shear zones show a top-to-east movement direction and correspond actually to large-scale detachment faults responsible for the juxtaposition of less metamorphic units above the Ambin basement and thus to a large part of the exhumation of HP rocks toward the surface. D2 shear zones were subsequently deformed by D3 open folds, large antiforms (e.g. the Ambin dome) and associated brittle-ductile D3 shear-bands. The D1 to D3 P-T conditions and P-T path of the blueschists occurring in the deepest part of the Ambin dome, was estimated by using the multi-equilibrium thermobarometric method of the Tweeq and Thermocalc softwares. Peak pressure conditions, estimated at about 14–16 Kb, 500oC, are followed by a nearly-isothermal decompression that occurred concurrently with the major D1–D2 change in the ductile deformation regime. Eastwards, the Schistes Lustrés units exhibit a similar geometry on top of the Gran Paradiso dome but exhibit opposite D2 movement direction. Lower-grade units are lying above higher-grade units, the shear zones occurring in between being similar to Ambin’s D2 detachments. Thus at regional scale, the D2 detachments seem to form together with the Ambin shear-zones, a network of conjugate detachments. Such a pattern suggests that the exhumation history is mostly controlled by a D2+D3 crustal-scale vertical shortening resulting in the thinning of the previous tectonic pile formed during D1. The slab-break off hypothesis may explain such an extensional behaviour within the western Pennine domain. It is suggested that the thermo-mechanical rebound of the residual European slab initiated between 35 and 32 Ma the fast exhumation of the previously thickened orogenic wedge (stack of D1 HP slices). It was immediately followed by a collapse of the wedge that may correspond to the E-W Oligocene extensional event responsible for the opening of rifts in the West European platform.

Aquisição e interpretação de anomalias gravimétricas do Quadrilátero Ferrífero, SE do Cráton São Francisco

Um novo levantamento gravimétrico foi realizado na região delimitada pelas coordenadas 39º a 49ºW e 17º a 23ºS. Foram coletados 176 novos dados gravimétricos, localizados na parte SE do Cráton São Francisco. As estações gravimétricas foram posicionadas por GPS (Global Positioning System), através do modo relativo estático. Os novos dados foram adicionados ao banco de dados gravimétricos do IAG/USP. Assim, 12.339 dados gravimétricos foram processados e interpretados. A geologia da área é representada pelos terrenos metamórficos arqueanos e proterozóicos do sudeste do Cráton São Francisco, incluindo as faixas de dobramentos Araçuaí e Ribeira. Para separar fontes rasas de fontes profundas do campo gravimétrico, o campo regional continuado para cima à altura de 50 km foi removido das anomalias Bouguer. O campo gravimétrico residual do Quadrilátero Ferrífero foi correlacionado com as unidades geológicas aflorantes. Os complexos metamórficos existentes nessa região possuem assinaturas gravimétricas distintas. Os complexos metamórficos Bação e Caeté possuem anomalias gravimétricas negativas com amplitudes muito próximas. O complexo metamórfico Bonfim possui anomalias gravimétricas positivas e seus limites estendem-se além da área mapeada pela geologia de superfície, sob seqüências sedimentares mais jovens. Modelagem gravimétrica direta utilizando como vínculos os contatos geológicos aflorantes permitiu estimar as direções e mergulhos de dois contactos tectônicos importantes. O contato entre os complexos de Barbacena e Bonfim tem mergulho de 21º para norte, enquanto o complexo Belo Horizonte mergulha com ângulo de 31º para oeste sob o Supergrupo Rio das Velhas.

The Curinga-Girifalco fault zone (northern Serre, Calabria) and its significance within the Alpine tectonic evolution of the western Mediterranean

Abstract Hercynian basement rocks and Mesozoic ophiolites of the Calabria-Peloritani terrane drifted in the present position during the opening of western Mediterranean basins (namely Liguro-Provencal and Tyrrhenian basins) since the Oligocene. Basement rocks were partly involved by Alpine (late Cretaceous—Eocene) deformation and metamorphism before the onset of the drifting process. Even though the kinematics of the Alpine deformation in Calabria has been already defined, restoration of structural and kinematic data to the original position and orientation before the opening of the western Mediterranean has never been performed. In this work we present new structural and petrological data on a major tectonic contact of Alpine age exposed in central Calabria (Serre Massif). Structural and kinematic data are then restored at the original orientation in the early Oligocene time, to allow a correct tectonic interpretation. In the Serre Massif the Hercynian basement is sliced into three nappes emplaced during the Alpine orogeny. The upper nappe is formed by a nearly continuous section of the Hercynian crust, consisting of medium- to high-grade metamorphic rocks in the lower portion. The intermediate nappe mainly consists of orthogneisses, whereas the lower nappe is chiefly composed of phyllites. The contacts between the Alpine nappes are outlined by well developed mylonitic and cataclastic rocks. The Curinga-Girifalco Line is a well exposed shear zone that overprints mainly metapelitic rocks of the upper nappe and granitoid orthogneisses of the intermediate nappe. Mylonites of the intermediate nappe typically show overgrowths on garnet and hornblende with grossular-rich and tschermakitic composition, respectively. The Alpine mineral assemblage indicates that deformation took place in epidote-amphibolite facies at pressures ranging from 0.75 to 0.9 GPa. In the investigated area mylonites strike roughly WNW–ESE, with shallow dips towards SSW. Kinematic indicators in mylonites are mostly consistent with a top-to-the-SE shear sense in the present geographic coordinates. The mylonitic belt is affected by later extensional faults outlined by South-dipping cataclasite horizons. Published geochronological data indicate that mylonites and cataclasites developed in Eocene and early Miocene times, respectively. Considering rotational parameters coming from paleomagnetic studies and large-scale palinspastic reconstructions, the shear sense of the Curinga-Girifalco Line has been restored to the early Oligocene position and orientation. Through restoration a top-to-the-S shear sense is obtained. This result is in striking agreement with the convergence direction between Africa and W-Europe/Iberia during Eocene, computed from the North Atlantic magnetic anomalies. Our geodynamic reconstruction, combined with structural and petrological evidence, allows to relate the Curinga-Girifalco mylonites to a thrust related to the southeastern front of the double-verging Alpine chain. The adopted method could be used also for other exotic terranes, such as the Kabylie or the Corsica-Sardinia, to better constrain geometry and evolution of the southern Alpine belt.

THE PRESENCE OF VARI - KIROU PIRA UNIT AT PANION HILL (SE ATTICA, GREECE)

Detailed mapping in the area of Panion Hill (SE Attica, Greece), revealed the existence of three metamorphosed lithological units which are separated by major tectonic contacts. The lower unit is located at the western part of Panion Hill and comprises marble - schist alternations that pass transitionally to massive dolomitic marbles of probable Thassic age. The intermediate unit comprises exclusively massive to thick bedded marbles that occupy the central and eastern part of the hill. The upper unit can be observed mainly at the foothills of Panion and comprises HP/LT schists, exhibiting greenschsist fades retrograde metamorphism. The contact between the lower and the intermediate unit is a low-angle normal fault, dipping to the east-southeast whereas the upper unit, in most cases is juxtaposed against the intermediate and lower units by high angle normal faults. These contacts accommodated the exhumation of the lowermost metamorphic formations of Attica at least in the last stages. The overall structure resembles that of Mt. Hymittos (east of Athens), where the Vari Schists and Dolomitic Marble formations (Vari - Kirou Pira Unit, VKPU) underlie tectonically the Lower Marble, Kesariani Schists and Upper Marble formations (Hymittos Unit, HU) Based on lithological and structural similarities the lower unit of Panion Hill is considered to be part of the Vari - Kirou Pira Unit and the intermediate unit part of the Hymittos Unit, respectively. The upper unit of Panion Hill belongs to the allochthonous Lavrion Unit. The existence of a major tectonic contact separating formations of VKPU and HU in several outcrops in SE Attica signifies that it is of no local character. Furthermore, it supports the suggestion of dividing the relatively autochthonous unit of Attica into two distinct units: the upper Hymittos Unit and the lower Vari - Kirou Pira Unit, the latter standing for the actual autochthonous unit of SE Attica.

Anisotropy of magnetic susceptibility constraints on Variscan obduction processes in the Bragança Massif (NE Portugal)

In this paper, we present an anisotropy of magnetic susceptibility (AMS) and structural study of high-grade metamorphic and ophiolitic rocks from the Bragança Massif, trying to contribute to the debate concerning the identification of transport directions during the closure of the Variscan Ocean. The study area comprises high-grade metamorphic rocks of the Continental Allochthonous Terrane (CAT) and the Northern Ophiolite Terrane (NOT) in the Bragança Massif. Mineral parageneses and fabrics were characterized through conventional petrographic and microstructural studies, electron microprobe, and high field and thermomagnetic analysis. With very few exceptions, the magnetic lineations and foliations were coincident, within limited error, with the observed mesoscopic mylonitic and metamorphic foliations, and with the mineral and stretching lineations. In places of the studied major tectonic contacts where the mineral and stretching lineations were obliterated by post-kinematic recrystallization, AMS data revealed a magnetic lineation. AMS results confirmed the NNW–SSE to N–S amphibolite facies lineation common to CAT and NOT, and revealed an E–W greenschist facies lineation in NOT. AMS results combined with detailed structural and metamorphic data from previous work showed that there are two E–W lineations separated in time: (1) an E–W mesoscopic and magnetic lineation in competent granulites that is older than the N–S magnetic lineation in more ductile gneisses of a major shear zone within the CAT; (2) an E–W magnetic lineation in incompetent greenschists of the NOT, younger than the NNW–SSE to N–S mesoscopic and magnetic lineations found in more competent amphibolites of CAT and NOT. The NNW–SSE to N–S lineations can be interpreted as the result of the uppermost allochthonous terranes transport to the NNW over the Iberian Terrane, dated with 40Ar/ 39Ar at ca. 390 Ma.

Tectonics of SE China: New insights from the Lushan massif (Jiangxi Province)

In south China the Lushan massif forms a topographic high of the South China Block south of the Qinling‐Dabie belt. The Lushan massif consists of two main lithotectonic units separated by a major tectonic contact: a Neoproterozoic (upper Sinian)‐Paleozoic unit comprising primarily unmetamorphosed sandstones overlies a Paleoproterozoic unit mainly composed of low‐pressure, high‐temperature gneisses and micaschists. Both units are cut by Cretaceous granitic intrusions. Three primary tectono‐metamorphic and magmatic events are recognized. The eastern part of the Lushan massif is cut by a NNE‐SSW trending ductile normal fault (D3 deformation) coeval to the emplacement of a 100–110 Ma leucogranite dated by 40Ar/39Ar laserprobe on biotite and muscovite. D2 deformation is responsible for the formation of a decakilometer‐scale NE‐SW trending upright anticline characterized by NE‐SW stretching and NW‐SE shortening. The age of this folding event is defined by a 127±1 (2σ) Ma U/Pb titanite date obtained for a syntectonic granodiorite and 40Ar/39Ar ages of 133 Ma for amphibole. This Cretaceous age also corresponds to the 40Ar/39Ar ages of 126 Ma found on syntectonic muscovites at the base of the Sinian unit. An older deformation event, D1, characterized by a top‐to‐the‐NW extensional decollement of the Sinian‐Paleozoic series above Proterozoic metamorphic rocks is related to the Triassic tectonics of the Dabieshan. Lastly, in the lower part of Sinian rocks, the occurrence of kyanite cataclased during D1 documents an older, poorly preserved, late Paleozoic‐early Mesozoic tectonometamorphic event (Dx) related to a blind thrust in the continental crust of the South China Block in the southern foreland of the Dabieshan.

Exhuming the Sanbagawa metamorphic belt: the importance of tectonic discontinuities

Tectonic processes that have been proposed to explain the transport to the surface of regional metamorphic belts can be broadly divided into two types. (i) Corner‐flow within a convergent margin bounded by two essentially rigid plates associated with extension at shallow levels. This type of model assumes deformation is distributed throughout the margin and that any discontinuities are of secondary importance. (ii) Expulsion or extrusion of coherent metamorphic nappes. In this second idea, tectonic discontinuities are fundamental in the transport to the surface of metamorphic rocks. The wealth of geological data available from a variety of studies in the Sanbagawa metamorphic belt, southwest Japan makes it well‐suited for studying the relative importance of continuous vs. discontinuous deformation in the process of exhumation. In the Sanbagawa belt a sudden decrease in metamorphic pressure going down section of several kilobars suggests the presence of a major tectonic contact separating two major regional nappes: an overlying higher‐pressure Besshi nappe and an underlying lower‐pressure Oboke nappe. Major tectonic discontinuities have also been proposed within the Besshi nappe, however, indicators of metamorphic temperature, the results of radiometric age dating, and microstructural studies all suggest that post‐metamorphic discontinuities are minor and that this nappe formed and remained as an essentially coherent unit. Lithological associations and petrological studies suggest the following positions for the two nappes. The Besshi nappe formed deep within the former accretionary wedge, adjacent to the overlying mantle wedge, and with a dip of roughly 30 °C. In contrast, the Oboke nappe formed at moderate depths within the accretionary wedge, was distant from the mantle wedge, and was roughly horizontal. Penetrative deformation that post‐dates the peak of metamorphism has affected nearly all of the Sanbagawa belt and has played an important role in its exhumation. However, the presence of a broad coherent Besshi nappe overlying the lower‐pressure Oboke nappe suggests that some process such as buoyancy‐driven extrusion was also important in the exhumation process and in forming the structure of the Sanbagawa metamorphic belt.