Insubric Line Research Articles

Large sackung along major tectonic features in the Central Italian Alps

Deep-seated gravitational slope deformation (sackung) is common in alpine mountain belts. It is controlled by lithological, geomorphological, and structural features. We describe and discuss five examples of DSGSD along the Insubric Line (Periadriatic Fault) in the Central Italian Alps. The five examples are different in morphology, size, state of activity, and distribution, and are located along both primary and secondary valleys. We demonstrate relations between structural and gravitational features and discuss the possibility that structural has both passive and active roles in sackung development. Analysis of PS–SAR data shows that the studied examples are not active today. Numerical modelling demonstrates the importance of lithological and structural constraints, postglacial debuttressing, groundwater fluctuations, and weathering on sackung triggering and development.

Orogenic structure of the Eastern Alps, Europe, from TRANSALP deep seismic reflection profiling

The TRANSALP Group, comprising of partner institutions from Italy, Austria and Germany, acquired data on a 340 km long deep seismic reflection line crossing the Eastern Alps between Munich and Venice. Although the field work was split into four campaigns, between fall 1998 and summer 2001, the project gathered for the first time a continuous profile across the Alps using consistent field acquisition and data processing parameters. These sections span the orogen itself, at its broadest width, as well as the editor Fred Davey and the two adjacent basins. Vibroseis and explosion data, complementary in their depth penetration and resolution characteristics, were obtained along with wide-angle and teleseismic data. The profile shows a bi-vergent asymmetric structure of the crust beneath the Alpine axis which reaches a maximum thickness of 55 km, and 80–100 km long transcrustal ramps, the southward dipping ‘Sub-Tauern-Ramp’ and the northward-dipping ‘Sub-Dolomites-Ramp’. Strongly reflective patterns of these ramps can be traced towards the north to the Inn Valley and towards the south to the Valsugana thrust belt, both of which show enhanced seismicity in the brittle upper crust. The seismic sections do not reveal any direct evidence for the presence of the Periadriatic Fault system, the presumed equivalent to the Insubric Line in the Western Alps. According to our new evolutionary model, the Sub-Tauern-Ramp is linked at depth with remnants of the subducted Penninic Ocean. The ‘crocodile’-type model describes an upper/lower crustal decoupling and wedging of both the European and the Adriatic–African continents.

The development of the North Giudicarie fault zone (Insubric line, Northern Italy)

The kinematics of transpressive fault zones is generally characterized by a variable degree of strain partitioning. This process is enhanced by the presence of older faults, which partition the simple shear strain component more easily. The North Giudicarie line can be taken as a good example of strain partitioning because it is a left-lateral transpressive fault developed during the Oligocene–Miocene along an inherited normal fault system of late-Triassic to Cretaceous age. This paper analyzes two main points related to the kinematics of the North Giudicarie line: (1) the role of the inherited faults in the development of the North Giudicarie fault zone, and (2) the mechanisms of strain partitioning. These processes have been analyzed by structural analysis and detailed mapping of the North Giudicarie fault zone. The main tectonic contact of the North Giudicarie line is a northwest dipping reverse fault, which juxtaposes the Austroalpine basement rocks against the Southalpine cover. Oligocene foliated tonalites, together with basement- and limestone-derived mylonites, are found along the main fault plane. The brittle fault zone, up to 1 km thick, is mostly developed within the Southalpine cover rocks. This area is made up of several tectonic lenses, up to 1 km thick and several km long, bounded by anastomosing faults characterized by a normal offset. North-striking faults (e.g., the Trento-Cles line) branch from the North Giudicarie fault zone. Thrusting along the North Giudicarie line and left-lateral strike-slip along the anastomosing faults and the Trento-Cles line has been documented from shear-sense indicators observed in mylonites and small-scale fault surfaces. Stratigraphic data reveal that the Trento-Cles line reactivated pre-existing late Triassic or jounger normal faults. Therefore, it is suggested that the left-lateral slip component took place more easily along inherited faults. Deformation and metamorphic features of mylonites, and the estimated emplacement depth of the Oligocene foliated tonalites, can give a bulk estimate of the vertical displacement along the North Giudicarie line. The calculated thrust slip (∼21 km) is lower than the cumulative left-lateral slip along the whole Giudicarie fault system, as inferred from published regional data.

Indenter growth in analogue models of Alpine‐type deformation

A series of analogue experiments were carried out to simulate continental convergence, as seen in a profile through the Central Alps. A rigid indenter, representing the Adriatic plate, was driven laterally into a sand pack representing the brittle upper crust of Europe, detached and thickening above its subducting ductile lower crust. The rigid indenter advanced at the same steady rate in each experiment, but the dip of its front face was steepened in 15° increments from 15° to 90°. Where the rigid indenter face dipped at 45° or less, a sand wedge rose and was bound by a series of forekinks that nucleated at the toe of the indenter. Where the face of the rigid indenter dipped 60° or more, the wedge was defined by a single forekink and one or more backkinks that nucleated from a point advancing in front of the indenter toe. We interpret these results as indicating that slices of the sand pack and rising wedge are transferred across kink bands to build an “effective” indenter with a frontal dip closer to that dictated by the changing shear strength of the sand pile, which thickens vertically as it shortens laterally. One of our models (with a rigid indenter dipping 75°) simulates most of the major structures shown in recent syntheses of surface geology and deep seismic data in the Central Alps, without the isostatic lithospheric depression. This model accounts for the late collisional stage (Oligocene to Present) complex strain and metamorphic histories in the core of the orogenic wedge, the rapid rise and extrusion of small pips of Alpine eclogites, and the current passivity of the Insubric Line. It also emphasizes that lateral extension along gently dipping “thrusts” (orogen‐normal horizontal escape) is confined to the extruded portion of the rising wedge.

Strike‐slip movements and thrusting along a transpressive fault zone: The North Giudicarie line (Insubric line, northern Italy)

This paper analyzes the kinematic evolution and the deformation partitioning within an important transpressive fault zone located in the central part of the Alpine chain. The North Giudicarie line is a NNE trending fault which offsets the dextral Insubric line with an apparent left‐lateral displacement of about 70 km. The main fault plane of the North Giudicarie line dips about 35°–45° to the NW. The footwall is characterized by N‐S striking strike‐slip faults, which reactivate extensional faults of Early Jurassic to Late Cretaceous age. The early deformation history of the North Giudicarie line is revealed by basement‐and limestone‐mylonites. Shear sense of mylonites indicates on average top‐to‐the‐east thrusting. These movements took place during the late Oligocene‐early Miocene, when the Insubric line was active as a right‐lateral strike‐slip fault. Therefore, in this time span the North Giudicarie line can be interpreted as a dextral transpressive bend of the Insubric line. Mylonites have later been overprinted by brittle faults related to top‐to‐the‐SE thrusting of middle‐late Miocene age. During this event the shape of the Insubric line was strongly modified by left‐lateral transpression along the Giudicarie fault zone. Deformation was partitioned between prevailing compression along the Giudicarie line and left‐lateral strike‐slip movements along the N‐S striking faults. These faults transferred the strike‐slip component of the Giudicarie line into a wider area of the central southern Alps.

Controls of erosional denudation in the orogen on foreland basin evolution: The Oligocene central Swiss Molasse Basin as an example

A high‐resolution three‐dimensional reconstruction of the 25‐m.y.‐old central Swiss Molasse Basin reveals two sedimentary domains separated by a ∼5‐km‐wide flood‐plain. The proximal domain of the basin attained a width of 20 km, and its basement is steeply flexed (6°‐7° dip). Petrographic data indicate that it was filled by sediment from the Rigi dispersal system derived from the central Alps of eastern Switzerland and by locally sourced bajadas. In contrast, the distal sedimentary domain, located farther north, was gently dipping (<2°) and was filled by the meandering Lac Léman and Honegg dispersal systems. Chronological data reveal that sedimentation in the northern proximal part of the basin started at ∼27 Ma, when sediment supply to the basin started to increase. Deflection of the foreland plate at ∼25 Ma is successfully simulated by flexural modeling of the thrust load and the sediment load. The model reveals that the Lac Léman and Honegg dispersal systems are located on a buried flexural bulge. Furthermore, it shows that burial and suppression of the flexural bulge at ∼27 Ma as well as an increase of the basin wavelength were controlled by the contemporaneous increase in the sediment supply rate of the Rigi system. The model presented suggests that the tectonic subsidence of the Molasse Basin was mainly controlled by tectonic events in the northern part of the orogen, within ∼70 km distance from the tip of the orogenic wedge. Crustal thickening in this part of the orogen is reflected in the proximal Molasse by sedimentary cycles characterized by an increase in the sediment accumulation rates up section and by the presence of locally sourced bajada fans at the top of each cycle. Although south vergent back thrusting along the Insubric Line ∼150 km south of the foreland basin contributed little to flexure, it resulted in an increase of the sediment supply to the foreland basin. This is reflected in the Molasse by coarsening and thickening upward trends, an increase of the basin wavelength, basinward shifts of the depocenters of the dispersal systems, and uplift and erosion of the proximal basin border.

Metamorphic field gradients in the Central Alps

Metamorphic field gradients were determined across the entire amphibolite grade Central Alps (c. 50×100 km). P–T were calculated from 116 samples acquired from our own field work, from samples provided to us by others, and from rocks with mineral compositions described in the literature. Only fluid‐conserved equilibria were used to determine P–T . The use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors. The results are presented in contour maps. Temperature increases from 500 to 550 °C along the limit of amphibolite grade metamorphism in the north and west, to c. 675 °C toward the south at the Insubric line near the town of Bellinzona. Maximum recorded pressures of c. 7 kbar are in a central region c. 20 km north of the Insubric line, and decrease both to the north (5.5 kbar) and south (4.5 kbar). The P–T results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite; this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep (<40 °C) of the kyanite‐sillimanite phase boundary. Comparison of P–T conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source. Examination of the two‐dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses affected the Central Alps during the Tertiary. In the second, heat affected only the southern parts of the area and overprinted the previously established P–T gradients.

Magnetostratigraphic constraints on relationships between evolution of the central Swiss Molasse basin and Alpine orogenic events

Magnetostratigraphic chronologies, together with lithostratigraphic, sedimentological, and petrological data enable detailed reconstruction of the Oligocene to Miocene history of the North Alpine foreland basin in relation to specific orogenic events and exhumation of the Alps. The Molasse of the study area was deposited by three major dispersal systems (Rigi, Hohronen, Napf). Distinguished by characteristic heavy mineral suites, conglomerate clast populations, and the presence of key clasts, these systems record three major phases of denudation of the Alpine edifice. The Rigi system eroded the Austroalpine and Penninic nappes of eastern Switzerland from 30 to 25.5 Ma as a result of backthrusting and uplift of these units along the Insubric Line. Subsequent uplift of the Aar massif some 40 km to the north appears to have controlled the duration of the Hohronen and Napf dispersal systems, spanning 24–22 Ma and 21.5–15 Ma, respectively. They record downcutting into the crystalline cores of the Penninic and Austroalpine nappes of eastern (Hohronen) and western (Napf) Switzerland. High-resolution reconstruction of the structural and geometrical evolution of the proximal Molasse reveals in-sequence and out-of-sequence thrusting events at the Alpine front and incorporation of the Molasse into the orogenic wedge by in-sequence thrusting and underplating. Furthermore, it reveals close relationships between periods of rapid denudation in the central Alps and phases of increased sediment accumulation rates at the proximal basin border. An initial increase in Molasse accumulation rates to >1 km/m.y. occurred between 30 and 25.5 Ma and coincides with the Insubric phase of backthrusting along the eastern Insubric Line, where >10 km of vertical displacement is interpreted. During the same time span, the Alpine wedge propagated forward along the basal Alpine thrust, as indicated by the coarsening- and thickening-upward megasequence and by occurrence of bajada fans derived from the Alpine border. The end of this tectonic event is marked by a basinwide unconformity, interpreted to have resulted from crustal rebound after initial loading. A subsequent increase in accumulation rates to >1 km/m.y. between 23 and 21.5 Ma coincides with initial uplift of the eastern Aar massif by at least 4 km. This phase of high accumulation rates is associated with incorporation of early Chattian conglomerates into the orogenic wedge. The third advance of the Alpine wedge between 21 and 15.5 Ma caused underplating of Molasse deposits, resulting in synsedimentary backthrusting of previously deposited Molasse sequences and in the development of a progressive unconformity. A rapid increase in accumulation rates from 0.35 to >1 km/m.y. between 15.5 and 15 Ma marks the final loading event in the wedge, which may be caused by further major displacement and loading of the Aar massif. This deformation is coeval with out-of-sequence thrusting of the Helvetic border chain and of the piggyback stack of North Penninic and Ultrahelvetic Flysch nappes along the basal Alpine thrust.

Termination of major ductile strike-slip shear and differential cooling along the Insubric line (Central Alps): UPb, RbSr and [formula omitted] ages of cross-cutting pegmatites

To constrain the age of strike-slip shear, related granitic magmatism, and cooling along the Insubric line, 29 size fractions of monazite and xenotime were dated by the UPb method, and a series of 25 RbSr and 40Ar 39Ar ages were measured on different size fractions of muscovite and biotite. The three pegmatitic intrusions analyzed truncate high-grade metamorphic mylonite gneisses of the Simplon shear zone, a major Alpine structure produced in association with dextral strike-slip movements along the southern edge of the European plate, after collision with its Adriatic indenter. Pegmatites and aplites were produced between 29 and 25 Ma in direct relation to right-lateral shear along the Insubric line, by melting of continental crust having 87Sr 86Sr between 0.7199 and 0.7244 at the time of melting. High-temperature dextral strike-slip shear was active at 29.2 ± 0.2 (2σ) Ma, and it terminated before 26.4 ± 0.1 Ma. During dike injection, temperatures in the country rocks of the Isorno-Orselina and Monte Rosa structural units did not exceed ≈ 500°C, leading to fast initial cooling, followed by slower cooling to ≈ 350°C within several million years. In one case, initial cooling to ≈ 500°C was significantly delayed by about 4 m.y., with final cooling to ≈ 300°C at 20-19 Ma in all units. For the period between 29 and 19 Ma, cooling of the three sample localities was non-uniform in space and time, with significant variations on the kilometre scale. These differences are most likely due to strongly varying heat flow and/or heterogeneous distribution of unroofing rates within the continuously deforming Insubric line. If entirely ascribed to differences in unroofing, corresponding rates would vary between 0.5 and 2.5 mm/y, for a thermal gradient of 30°/km.

3D crustal architecture of the Alps-Apennines join — a new view on seismic data

Seismic data from the Alps-Apennines join have usually been interpreted in the form of 2D cross-sections, passing either through the Western Alps or the Ligurian Alps-Monferrato Apennines. However, the oblique SE-NW convergence of Adria and Europa and superimposed rotations imply a distinct 3D kinematic development around the Adriatic Indenter (AI), the westernmost spur of Adria. In order to develop kinematic models, data on motion at the different margins of AI must be coordinated. Along the northern margin, the dextrally transpressive Insubric line (IL) was active between 25 and 16 Ma (Insubric-Helvetic phase of Alpine orogeny). Contemporaneously, along the southern margin (Paleo-Apenninic phase), a complementary sinistral motion took place along the Villalvernia-Varzi line (VVL). It emplaced the Monferrato Apennines westward to the north of the Ligurian Alps by carrying them westward on top of AI. Between 14 and 6 Ma (Jura-Lombardic phase of Alpine orogeny) the Lombardic thrust belt developed on the northern margin of AI, now largely hidden under the Po plain. Its continuation to the southwest is impeded by older thrust masses along the Western Alps that consist largely of basement, their sediments having been eroded, as noted on the deep reflection line CROP ALPI-1 by earlier investigators. This line, moreover, contains a deep reflection band originating in the autochthonous Mesozoic of the Apenninic foredeep. In order to better visualize this origin and the relation of further elements identified on reflection lines around the northwestern end of the Monferrato Apennines, a 3D fence diagram was constructed. It helps in establishing a 3D structural-kinematic model of the Alps-Apennines join based on the kinematics of AI. This model features an underthrust of AI under the western Alps in the Paleo-Apenninic phase. In the course of this underthrust, the Paleo-Apenninic elements of the Monferrato moved under the marginal thrusts of the western Alps. Subsequent Neo-Apenninic thrusting brought both elements together to the surface where they now form the Monferrato and Turin hills. A derivation of the Alpine Collina di Torino from the south instead of from the west, as recently proposed, meets with serious kinematic difficulties.

Alpine and pre-Alpine magmatism in the root-zone of the western Central Alps

The highest grade of metamorphism and associated structural elements in orogenic belts may be inherited from earlier orogenic events. We illustrate this point using magmatic and metamorphic rocks from the southern steep belt of the Lepontine Gneiss Dome (Central Alps). The U-Pb zircon ages from an anatectic granite at Verampio and migmatites at Corcapolo and Lavertezzo yield 280–290 Ma, i.e., Hercynian ages. These ages indicate that the highest grade of metamorphism in several crystalline nappes of the Lepontine Gneiss Dome is pre-Alpine. Alpine metamorphism reached sufficiently high grade to reset the Rb-Sr and K-Ar systematics of mica and amphibole, but generally did not result in crustal melting, except in the steep belt to the north of the Insubric Line, where numerous 29 to 26 Ma old pegmatites and aplites had intruded syn- and post-kinematically into gneisses of the ductile Simplon Shear Zone. The emplacement age of these pegmatites gives a minimum estimate for the age of the Alpine metamorphic peak in the Monte Rosa nappe. The U-Pb titanite ages of 33 to 31 Ma from felsic porphyritic veins represent a minimum-age estimate for Alpine metamorphism in the Sesia Zone. A porphyric vein emplaced at 448±5 Ma (U-Pb monazite) demonstrates that there existed a consolidated Caledonian basement in the Sesia Zone.

Mechanical model for subduction-collision tectonics of Alpine-type compressional orogens

Alpine-type orogens are characterized by three distinct convergent tectonic phases: subduction with deformation that has primarily single vergence, a transition from subduction to collision, and continental collision with double vergence. Although the Cenozoic history of the European Alps has additional complexities, a mechanical explanation for these three phases would provide the necessary crustal-scale framework in which to develop an understanding of the smaller-scale processes. We present results from a simple numerical model, which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen. The development and exhumation of a subduction complex, suture zone, and basement nappe stack (Piemont suture, Penninic Nappes); the presence of a crustal-scale back fold and thrust (Insubric Line); and uplift of basement on the pro- (European) side of the orogen (external basement massifs) may be explained as a simple consequence of changing dynamics during the transition from subduction to collision.

P10 A data management tool for tracking clinical endpoint reviews

A south-vergent fold-thrust belt of Miocene-Recent age accompanies the south slope of the Lombardian Alps and is partly buried beneath Plio-Pleistocene Po Valley basin fill. The belt is probably detached along a trans-crustal thrust, named Main South Alpine Thrust (MSAT), with an estimated dip slip of 70–100 km. Transport on this thrust piggybacks the Adamello pluton of Late Eocene age, pre-Adamello folds, and Oligocene-Miocene Insubric strike-slip structures, by ramping up through 12–15 km of Austro-Alpine (Adria) crust and through 8–10 km of Triassic to Eocene sediments. Folds in the Front Ranges are ascribed to MSAT ramping, not to pre-Adamello compression. The MSAT soles upward in a blind thrust beneath 3–4 km of Oligocene-Pliocene foredeep fill. Initial regional failure along the MSAT implies substantial and pre-existing topographic relief near the Insubric line. An average of 25% wedge thickening during MSAT transport is consistent with the requirement of Coulomb critical taper. Progression of the south-Alpine detachment from the MSAT to the base of the foreland sediments has added a thickness of 6–12 km in footwall imbrications to the base and the toe of the thrust wedge. This addition in wedge volume is consistent with wedge dynamics only if a mid-Miocene or younger spike of excess Alpine topography is admitted.

Crustal structure beneath the eastern Swiss Alps derived from seismic refraction data

The eastern part of the Swiss Central Alps is densely covered by a network of seismic refraction and wide-angle reflection profiles recorded mostly parallel to the tectonic strike of the Alps and along the newly acquired N-S-oriented European Geotraverse (EGT). To obtain a well-constrained crustal transect along the EGT an initial model was constructed from information based mainly on the coincident reflection seismic profile of the Swiss National Research Program NFP20 and short-range observations of the EGT data for the shallow structure and on the along-strike wide-angle profiles for the deep structure. The refinement of the initial model by 2-D raytracing during the subsequent interpretation of the EGT data leads to a detailed P-wave velocity distribution of the crustal cross-section beneath the Central Swiss Alps and its adjacent areas. In general, the distinctly layered crustal structure below the Alpine foreland thickens considerably as the Alps are approached, reaching a maximum thickness of nearly 60 km below the Insubric Line. The upper and middle crust has velocities between 6.0 and 6.2 km/s. Except for the area below the southern part of the Molasse Basin and the Helvetic nappes a distinct lower crust with a relatively low velocity of 6.5–6.6 km/s is found. Below the Penninic nappes the lower crust thickens remarkably, merging probably with the high-velocity zone of 6.6 km/s at a depth of about 21 km, which has been interpreted as the top of the indenting lower crust of the Adriatic promontory of the African plate. A clear vertical offset between the smoothly south-dipping European and the more rapidly rising Adriatic crust-mantle boundaries is found. The complex structures of the upper crust beneath the Alps caused by the nappe tectonics can only be partly resolved by the refraction seismic data. A south-dipping high-velocity zone within the Penninic nappe pile and a reflector beneath the northern front of the Aar Massif can possibly be interpreted as incomplete images of the shallow heterogeneous 3-D structure.

Basement-cover relationships in the Tambo nappe (Central Alps, Switzerland): geometry, structure and kinematics

In the Pennine zone of the Central Alps, the Tambo nappe forms a thin crystalline sliver embedded in the Mesozoic cover. Four Tertiary Alpine deformations are observed. The D 1 ductile deformation is linked to the progressive Eocene stacking of the nappes towards the northwest. During D 1, basement deformation is governed by imbricate tectonics whereas cover is thin-skinned and intensively folded. These different structures reflect the original strong rheological contrast between basement and cover. During the heterogeneous and ductile D 2 deformation, the behaviour of the basement and cover became similar. The strong vertical D 2 shortening, associated with a ‘top-to-the-east’ shear, led to the folding of the prior SE-dipping structures, developing SE-vergent folds with axes close to the E-W L 2 stretching lineation. D 2 corresponds to post-collisional crustal thinning following D 2 crustal thickening. The D 3 and D 4 deformations occurred under retrograde conditions and can be correlated with uplift and late dextral movement on the Insubric line, respectively.

Prograde and retrograde thermal histories from the central Swiss Alps

The “Lepontine” phase of metamorphism in the central Alps is a result of thermal relaxation following continental collision and overthrusting. The timing of the peak remains poorly known, largely because the existing chronological data relate to cooling or to phases for which the pressure and temperature of growth is only poorly constrained. Here we report U Pb, Sm Nd and Rb Sr analyses of garnets from the central Swiss Alps, as well as major element data on garnets and other phases. The data shows that the climax of the metamorphism was diachronous across grade, from 30 Ma in the lower grade northwest to 27 Ma in the higher grade in the southeast. Consideration of these data in conjunction with existing chronological constraints suggests a period of about 10 Ma between the attainment of the metamorphic climax in greenschist facies and the upper amphibolite facies rocks. The time constraints obtained here yield a garnet growth interval of 2.9 ± 1.5 Ma, corresponding to a growth rate of 0.2 cm Ma −1, a heating rate of 12°C Ma −1 and a burial rate of 2.9 km Ma −1. The strain rate obtained from measurement of the rotation of the garnet relative to the matrix fabric during growth is ∼ 1.9 × 10 −14s −1. This strain rate, as well as the heating and burial rates, are in the range of values derived in a similar fashion from other Barrovian metamorphic terrains. Consideration of these data in conjunction with published cooling ages leads to cooling rates of about 20°C Ma −1 for the Steinental samples but much higher rates for the Bellinzona area (> 100°C Ma −1). In both cases it is necessary to invoke tectonic exhumation rather than isostatic recovery and erosion to explain the observed temperature-time trends. In the case of the Bellinzona area, the most likely cause of the very rapid cooling rates is late backthrusting over the Insubric Line.

Deformation partitioning during polyphase oblique convergence in the Karawanken Mountains, southeastern Alps

The Karawanken Mountains of southeastern Austria straddle the Insubric Line, a major fault zone that corresponds to the Austroalpine-South Alpine border of the eastern Alps. They display polyphase structural patterns that resulted from deformation partitioning at shallow crustal levels. Following a poorly constrained late Cretaceous deformation history, Paleogene contraction produced NW—SE-trending fold—thrust structures and possibly related sinistral (?) cross faults. Miocene deformation produced both NW- and SE-directed thrust faults and NW-striking high-angle dextral cross faults which merged with or displaced the W-striking Insubric Line. Distributed dextral strike-slip displacements of the order of 30–40 km and thrust displacement of 4–5 km along the northern Karawanken front initiated subsidence of a small foreland basin north of the Karawanken Mountains. Recent regional deformation seems to be concentrated along cross-cutting NNW-striking dextral faults within a shifting system of seismogenic transfer faults that probably merge with the convergent South Alpine thrust front to the west. There are conspicuous parallels in the style of deformation along the Karawanken Mountains with that found along other seismogenic convergent strike-slip faults, such as those of southern California, New Zealand and Anatolia.

Thrusting and wedge growth, Southern Alps of Lombardia (Italy)

A south-vergent fold-thrust belt of Miocene-Recent age accompanies the south slope of the Lombardian Alps and is partly buried beneath Plio-Pleistocene Po Valley basin fill. The belt is probably detached along a trans-crustal thrust, named Main South Alpine Thrust (MSAT), with an estimated dip slip of 70–100 km. Transport on this thrust piggybacks the Adamello pluton of Late Eocene age, pre-Adamello folds, and Oligocene-Miocene Insubric strike-slip structures, by ramping up through 12–15 km of Austro-Alpine (Adria) crust and through 8–10 km of Triassic to Eocene sediments. Folds in the Front Ranges are ascribed to MSAT ramping, not to pre-Adamello compression. The MSAT soles upward in a blind thrust beneath 3–4 km of Oligocene-Pliocene foredeep fill. Initial regional failure along the MSAT implies substantial and pre-existing topographic relief near the Insubric line. An average of 25% wedge thickening during MSAT transport is consistent with the requirement of Coulomb critical taper. Progression of the south-Alpine detachment from the MSAT to the base of the foreland sediments has added a thickness of 6–12 km in footwall imbrications to the base and the toe of the thrust wedge. This addition in wedge volume is consistent with wedge dynamics only if a mid-Miocene or younger spike of excess Alpine topography is admitted.

The lithosphere in central Europe—seismological and petrological aspects

The lithosphere thickness in the Variscan belt of central Europe varies between about 60 and 150 km with typical values of 100–120 km. Our estimates, derived from directionally independent representative P-wave residuals, are in good agreement with magnetotelluric determinations of a layer with increased conductivity in the upper mantle. The large-scale anisotropies of the subcrustal lithosphere beneath four seismological stations determined from spatial variations of relative P residuals vary between 6.5 and 15.2% for P velocities; the S-wave anisotropies determined from SKS polarizations vary between 2.2 and 6.7%. These values are in reasonable agreement with the anisotropies of peridotites determined in laboratory. Systematic spatial variations of the directional terms of relative residuals in dependence on azimuths and incidence angles suggest the existence of large dipping anisotropic structures in the subcrustal lithosphere. The residual patterns at most stations in the Saxothuringicum, Rhenohercynicum and in the Massif Central imply northwesterly orientated dips of the anisotropic structures while stations in the Moldanubicum, the Alpine Foredeep and most of the Alps north of the Insubric line, suggest southeasterly orientated dips. In our interpretation the dipping anisotropic structures may represent paleosubductions which retain olivine preferred orientations originating from an ancient oceanic lithosphere. The Variscides of central Europe may thus represent a collision zone characterized by two systems of paleosubductions divergent relative to the suture between the Moldanubicum and the Saxothuringicum.

Archean geodynamics and the Abitibi-Pontiac collision: implications for advection of fluids at transpressive collisional boundaries and the origin of giant quartz vein systems

Giant quartz vein systems of all ages, from the Archean to Cenozoic, are characterised by similarities of geodynamic setting, local structure, paragenesis, metal budget, P-T-t paths, and chemical, fluid dynamic and isotopic properties that collectively imply a singular hydrothermal process. Such quartz vein provinces form in regional brittle-ductile shear zones that define terrane boundaries, including closure of back-arc basins, and arc-continent or continent-continent collisions. Examples include the SVZ of the Archean Abitibi belt; the Norseman Wiluna belt, Yilgarn block; Kolar schist belt, India; Foothills Metamorphic Belt, California, and Coast Ranges Megalineament, in the Cordillera; and the Cenozoic Monte Rosa district, Insubric line. Evidence for an Archean collisional environment comes from the Abitibi and Pontiac Subprovince tectonic boundary, which hosts giant quartz vein systems. In this region, the amphibolite facies Lacorne tectonic block in the Archean Abitibi greenstone belt is anomalous with respect to the prevalent low-grade supracrustal sequences in neighbouring blocks. The Lacorne block has mature clastic sediments with a zircon provenance age spectrum from 3040-2691 Ma, and two granitic magma series; a late syntectonic monzodiorite-monzonite-granodiorite-syenite series formed over 2685-2670 Ma, similar to Phanerozoic volcanic arc granites, and post-tectonic garnet-muscovite granites emplaced at 2650-2630 Ma, which compositionally resemble Phanerozic collisional S-type granites. The Pontiac subprovince to the south of the Abitibi greenstone belt shares all the above features with the Lacorne block, and collided with and was locally thrust under the Abitibi belt, with differential uplift of the Lacorne block following collision to generate a tectonic window. Following collision of allochthonous terranes, the conjuction of large volumes of subcreted oceanic crust and sediments between accreted terranes, displaced isotherms rise, and metamorphic dehydration are all necessary conditions for forming giant quartz vein systems. Metamorphic fluids are expelled along the terrane boundary structures at deep levels, and focussed into second and higher order splays at mid-crustal levels where quartz and Au precipitation occurs. Hydrothermal fluids that formed the giant quartz veins are remarkably uniform in H, O, C, and Sr isotopic compositions, albeit with small provincial variations. The veins and gold precipitated at 270°-360°C, from fluids with low salinity and moderate CO 2 at 2–3 kbar in the brittle-ductile transition.