Thick- and thin-skinned tectonics in the Pyrenees

A comparison is made between the Gavarnie thrust and the Mérens Fault in the Axial zone of the Pyrenees. The former has a gentle dip and quite a large displacement (at least 12 km) but does not cut through either Hercynian or Alpine isograds. The latter has a smaller displacement (~ 5 km) but dips steeply and cuts through both Hercynian and Alpine isograds at a high angle. On this basis and on the basis of shear zone geometries immediately north of it, it is proposed that the Mérens Fault nucleated as a steeply (65°–80°) dipping structure, while the Gavarnie thrust nucleated with a shallow attitude. The Mérens Fault is not a backward-rotated thrust fault, nor is it the root zone for any major nappe structure. Similar steep ductile structures occur within the Gavarnie nappe and may reflect considerable internal strain in basement lithologies.The relationship between steep and shallow structures is not yet clear; the shear zones may pre-date the thrusting in which case they may be thick-skinned structures affecting the whole lithosphere, or they may be contemporary with thrusting reflecting only local thickening above a décollement.Rheological models can be used to test proposed geometrical and kinematic models for the lithosphere-scale evolution of the Pyrenees. Suggested models are dominated by a cool, rigid, high-level mantle wedge beneath the North Pyrenean zone which probably controlled the location of north-dipping thrust faults. Thick-skinned shortening is possible in thick crust in the Axial zone but is very unlikely in the North Pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere.     

Structures along the Orobic thrust, Central Orobic Alps, Italy

A series of regional deformation phases is described for the metamorphic basement and the Permian cover in an area in the central Orobic Alps, northern Italy. In the basement deformation under low-grade amphibolite metamorphic conditions is followed by a second phase during retrograde greenschist conditions. These two phases predate the deposition of the Permian cover and are of probable Variscan age. An extensional basin formed on the eroded basement during the Late Carboniferous, filled with fan conglomerates and sandstones, and rhyolitic volcanic rocks. Well-preserved brittle extensional faults bound these basins. Further extension deformed basement and cover before the onset of Alpine compressional tectonics. Cover and basement were deformed together during two phases of compressional deformation of post-Triassic age, the first giving rise to tectonic inversion of the older extensional faults, the second to new thrust faults, both associated with south-directed nappe emplacement and regional folding. Foliations develop in the cover only during the first phase of deformation as part of the activity on “shortening faults”. Main activity on the Orobic thrust actually postdates the first phase of thrusting and foliation development in the cover.     

Syn-thrust deformation across a transverse zone: the Grem–Vedra fault system (central Southern Alps, N Italy)

The lateral continuity of the E?CW trending thrust sheets developed within the Lower to Middle Triassic cover of the central Southern Alps (Orobic belt) is disturbed by the occurrence of several N?CS trending transverse zones, such as the poorly known Grem?CVedra Transverse Zone (GVTZ). The GVTZ developed during the emplacement of the up to six S-verging thrust sheets consisting of Lower to Middle Triassic units, occurring immediately south of the Orobic Anticlines. The transverse zone, active during thrust emplacement related to the early Alpine compressions which pre-date the Adamello intrusion, includes three major vertical shear zones, the Grem, Pezzel and Zuccone faults. The major structure of the transverse zone is the dextral Grem fault, forming a deep lateral ramp between thrust sheets 3 and 5. A similar evolution also occurred along the Zuccone and Pezzel faults, which show a left-lateral displacement of syn-thrust folds. The Grem fault was later reactivated as an oblique tear fault during the emplacement of the Orobic Anticlines, due to back-thrusting along out-of-sequence thrust surfaces (Clusone fault). Transpressional deformations along the fault zone are recorded by the rotation of major syn-thrust folds, which also suggest a horizontal offset close to 0.5?km. Records of the first stage of evolution of the Grem fault are better preserved along its northern segment, and structural relationships suggest that it propagated southward and downward in the growing thrust stack. The study of the meso and megascopic structures developed along the GVTZ constrains the evolution of the transverse zone, illustrating the complex deformational phenomena occurring in a transpressional regime. The GVTZ probably reflects the existence of pre-existing tectonic lineaments with a similar orientation. Evidence of pre-existing structures are not preserved in the exposed units, nevertheless the N?CS extensional fault systems that characterize the Norian to Jurassic rifting history of the Lombardian basin are valid candidates.     

Brittle–ductile–brittle deformation during cooling of tonalite (Adamello, Southern Italian Alps)

Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature ( 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at  250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures ( 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by ³⁹Ar–⁴⁰Ar ages of  30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition.     

Structural inversion and its controls : examples from the Alpine foreland and the French Alps

Abstract

Positive structural inversion involves the uplift of rocks on the hanging-walls of faults, by dip slip or oblique slip movements. Controlling factors include the strike and dip of the earlier normal faults, the type of normal faults — whether they were listric or rotated blocks, the time lapsed since extension and the amount of contraction relative to extension. Steeply dipping faults are difficult to invert by dip slip movements; they form buttresses to displacement on both cover detachments and on deeper level but gently inclined basement faults. The decrease in displacement on the hanging-walls of such steep buttresses leads to the generation of layer parallel shortening, gentle to tight folds — depending on the amount of contractional displacement, back-folds and back-thrust systems, and short-cut thrust geometries — where the contractional fault slices across the footwall of the earlier normal fault to enclose a “floating horse”. However, early steeply dipping normal faults readily form oblique to strike slip inversion structures and often tramline the subsequent shortening into particular directions.

Examples are given from the strongly inverted structures of the western Alps and the weakly inverted structures of the Alpine foreland. Extensional faulting developed during the Triassic to Jurassic, during the initial opening of the central Atlantic, while the main phases of inversion date from the end Cretaceous when spreading began in the north Atlantic and there was a change of relative motion between Europe and Africa. During the mid-Tertiary well over 100 km of Alpine shortening took place; Alpine thrusts, often detached along, or close to, the basement-cover interface, stacking the late Jurassic to Cretaceous sediments of the post-extensional subsidence phase. These high level detachments were joined and breached by lower level faults in the basement which, in the external zones of the western Alps, generally reactivated and rotated the earlier east dipping half-graben bounding faults. The external massifs are essentially uplifted half-graben blocks. There was more reactivation and stacking of basement sheets in the eastern part of this external zone, where the faults had been rotated into more gentle dips above a shallower extensional detachment than on the steeper faults to the west.

There is no direct relationship between the weaker inversion of the Alpine foreland and the major orogenic contraction of the western Alps; the inversion structures of southern Britain and the Channel were separated from the Alps by a zone of rifting from late Eocene to Miocene which affected the Rhone, Bresse and Rhine regions. Though they relate to the same plate movements which formed the Alps, the weaker inversion structures must have been generated by within plate stresses, or from those emanating from the Atlantic rather than the Tethyan margin.     

The role of pre-existing faults in the structural evolution of thrust systems: Insights from the Ligurian Alps (Italy)

New structural and stratigraphic data for a selected area of the Ligurian Alps are combined in order to assess and discuss the role played by extensional structures in the southernmost segment of the Western Alps during thrusting. Restored cross-sections and field data suggest that the structural style in the external sector of the chain may depend upon the presence of pre-orogenic normal faults ascribed to three extensional events linked to different geodynamic contexts: (i) Permian post-Variscan plate reorganisation, (ii) Mesozoic rifting–drifting phases leading to the opening of the Alpine Tethys, and (iii) Eocenic development of the European foreland basins. During positive inversion in Eocene times, a thin-skinned thrust system developed in this area, followed by a thick-skinned phase. In both situations the inherited extensional structures played fundamental roles: during the thin-skinned phase they conditioned the thrusting sequence, also producing large-scale buckle folds and partial reactivations; during the thick-skinned phase the strain was compartmentalized and partitioned by pre-existing faults.The kinematic model of the external sectors of the Ligurian chain also allows the re-assessment of the Alpine evolution of the front-foreland transition, including: (i) indirect confirmation that in the Eocene the Ligurian Briançonnais and Dauphinois domains were not separated by the Valais-Pyrenean oceanic basin; (ii) that the thin-skinned phase progressively changed into thick-skinned; (iii) the assertion that there were no significant deformations from the Oligocene to the present-day, and the Corsica–Sardinia block rotation only produced a change in orientation of previously formed structures and normal fault system development.     

Late Cretaceous, synorogenic, low-angle normal faulting along the Schlinig fault (Switzerland, Italy, Austria) and its significance for the tectonics of the Eastern Alps

The Schlinig fault at the western border of theÖtztal nappe (Eastern Alps), previously interpreted as a west-directed thrust, actually represents a Late Cretaceous, top-SE to -ESE normal fault, as indicated by sense-of-shear criteria found within cataclasites and greenschist-facies mylonites. Normal faulting postdated and offset an earlier, Cretaceous-age, west-directed thrust at the base of theÖtztal nappe. Shape fabric and crystallographic preferred orientation in completely recrystallized quartz layers in a mylonite from the Schlinig fault record a combination of (1) top-east-southeast simple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directed shortening during the Early Tertiary, also recorded by open folds on the outcrop and map scale. Offset of the basal thrust of theÖtztal nappe across the Schlinig fault indicates a normal displacement of 17 km. The fault was initiated with a dip angle of 10° to 15° (low-angle normal fault). Domino-style extension of the competent Late Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting.

The Schlinig fault belongs to a system of east- to southeast-dipping normal faults which accommodated severe stretching of the Alpine orogen during the Late Cretaceous. The slip direction of extensional faults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip sense is reversed and the normal faults are slightly steeper than the thrusts. In the western Austroalpine nappes, extension started at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of Austroalpine Gosau basins with fluviatile to deep-marine sediments. West-directed rollback of an east-dipping Piemont-Ligurian subduction zone is proposed to have caused this stretching in the upper plate.     

Balancing lateral orogenic float of the Eastern Alps

Oligocene to Miocene post-collisional shortening between the Adriatic and European plates was compensated by frontal thrusting onto the Molasse foreland basin and by contemporaneous lateral wedging of the Austroalpine upper plate. Balancing of the upper plate shortening by horizontal retrodeformation of lateral escaping and extruding wedges of the Austroalpine lid enables an evaluation of the total post-collisional deformation of the hangingwall plate. Quantification of the north–south shortening and east–west extension of the upper plate is derived from displacement data of major faults that dissect the Austroalpine wedges. Indentation of the South Alpine unit corresponds to 64 km north–south shortening and a minimum of 120 km of east–west extension. Lateral wedging affected the Eastern Alps east of the Giudicarie fault. West of the Giudicarie fault, north–south shortening was compensated by 50 to 80 km of backthrusting in the Lombardian thrust system of the Southern Alps. The main structures that bound the escaping wedges to the north are the Inntal fault system (ca. 50 km sinistral offset), the Königsee–Lammertal–Traunsee (KLT) fault (10 km) and the Salzach–Ennstal–Mariazell–Puchberg (SEMP) fault system (60 km). These faults, as well as a number of minor faults with displacements less than 10 km, root in the basal detachment of the Alps. The thin-skinned nature of lateral escape-related structures north of the SEMP line is documented by industry reflection seismic lines crossing the Northern Calcareous Alps (NCA) and the frontal thrust of the Eastern Alps. Complex triangle zones with passive roof backthrusts of Middle Miocene Molasse sediments formed in front of the laterally escaping wedges of the northern Eastern Alps. The aim of this paper is a semiquantitative reconstruction of the upper plate of the Eastern Alps. Most of the data is published elsewhere.     

Kinematic evolution of thrusts wedge and erratic line length balancing: insights from deformed sandbox models

Kinematic evolution of fold-thrust structures has been investigated by analogue models that include syntectonic sedimentation. Different decollement dips and basement thicknesses produced different wedge geometries and propagating characteristics. A model with one decollement level was characterized by a closely spaced thrust system during early stages of shortening as compared to the late stages. The frequency of fault nucleation was rapid during the early stages of deformation. Conversely, the frequency of fault nucleation was low and thrust spacing was significantly wider in a model with two decollement levels. Individual faults became locked at steep dips and deformation stepped forward as a new fault nucleated in-sequence in front of the older locked structure. Once the thrust system was established up to 27 % overall shortening, an overlying bed was introduced to simulate syntectonic deformation. Model sand wedge did not grow self similarly but rather its length and height increased episodically with deformation. Restoration of deformed models show that layer parallel shortening accommodated for approximately half of the total model shortening across the multilayers. Calculated error in apparent layer shortening from the restored layers revealed a direct relation with depth of the layers in the models. The experimental results are comparable to a natural example from the Northern Apennines fold-and-thrust belts.     

Crustal structure of the Eastern Alps along the TRANSALP profile from wide-angle seismic tomography

The objective of the TRANSALP project is an investigation of the Eastern Alps with regard to their deep structure and dynamic evolution. The core of the project is a 340-km-long seismic profile at 12°E between Munich and Venice. This paper deals with the P-wave velocity distribution as derived from active source travel time tomography. Our database consists of Vibroseis and explosion seismic travel times recorded at up to 100 seismological stations distributed in a 30-km-wide corridor along the profile. In order to derive a velocity and reflector model, we simultaneously inverted refractions and reflections using a derivative of a damped least squares approach for local earthquake tomography. 8000 travel time picks from dense Vibroseis recordings provide the basis for high resolution in the upper crust. Explosion seismic wide-angle reflection travel times constrain both deeper crustal velocities and structure of the crust–mantle boundary with low resolution. In the resulting model, the Adriatic crust shows significantly higher P-wave velocities than the European crust. The European Moho is dipping south at an angle of 7°. The Adriatic Moho dips north with a gentle inclination at shallower depths. This geometry suggests S-directed subduction. Azimuthal variations of the first-break velocities as well as observations of shear wave splitting reveal strong anisotropy in the Tauern Window. We explain this finding by foliations and laminations generated by lateral extrusion. Based on the P-wave model we also localized almost 100 local earthquakes recorded during the 2-month acquisition campaign in 1999. Seismicity patterns in the North seem related to the Inn valley shear zone, and to thrusting of Austroalpine units over European basement. The alignment of deep seismicity in the Trento-Vicenza region with the top of the Adriatic lower crust corroborates the suggestion of a deep thrust fault in the Southern Alps.     

Pseudotachylytes in the Balmuccia peridotite (Ivrea Zone) as markers of the exhumation of the southern Alpine continental crust

Two types of pseudotachylytes are observed in the Balmuccia peridotite of the Ivrea zone (Southern Alps, Italy). A-type pseudotachylytes correspond to previously studied occurrences and were formed under temperatures comprising between 550 and 900 °C and pressures comprising between 0.6 and 1.2 GPa. These conditions were met in the Ivrea crust between 350 and 270 Ma, suggesting that A-type pseudotachylytes were formed during Variscan tectonics or Permian transtensional tectonics. B-type pseudotachylytes post-date A-type pseudotachylytes. Textural characteristics of B-type veins suggest a formation in the upper continental crust, at depths of about 5–10 km or less. Petrological constraints indicate that the exhumation of the Ivrea crust at such shallow depths was achieved later than c.  70 Ma, thus providing a maximum age of 70 Ma for B-type veins. Pseudotachylytes appear as markers of the poly-orogenic evolution of the Alpine belt.     

Overprinted strike-slip deformation in the southern Valley and Ridge in Pennsylvania

Two major faults, over 32 km long and 6.4 km apart, truncate or overprint most previous folds and faults as they trend more northerly than the previous N25°E to N40°E fold trends. The faults were imposed as the last event in a region undergoing sequential counter-clockwise generation of tectonic structures. The western Big Cove anticline has an early NW verging thrust fault that emplaces resistant rocks on its NW limb. A 16 km overprint by the Cove Fault is manifested as 30 small northeast striking right-lateral strike-slip faults. This suggests major left-lateral strike-slip separation on the Cove Fault, but steep, dip-slip separation also occurs. From south to north the Cove Fault passes from SE dipping beds within the Big Cove anticline, to the vertical beds of the NW limb. Then it crosses four extended, separated, Tuscarora blocks along the ridge, brings Cambro-Ordovician carbonates against Devonian beds, and initiates the zone of overprinted right-lateral faults. Finally, it deflects the Lat 40°N fault zone as it crosses to the next major anticline to the northwest. To the east, the major Path Valley Fault rotates and overprints the earlier Carrick Valley thrust. The Path Valley Fault and Cove Fault may be Mesozoic in age, based upon fault fabrics and overprinting on the east–west Lat 40°N faults.     

Pseudotachylyte-Induced Weakness of Plate-Boundary Fault:Insight from the Indus-Tsangpo Suture between India and Asia

Although the Indus-Tsangpo Suture(ITS) is the most spectacular thrust system of continent-continent collision in the world, fundamental questions about its strength evolution and deformation behavior transition remain unanswered. Here we reported, for the first time, frictional melting-induced pseudotachylytes in the intensively deformed felsic rocks along the ITS zone in southern Tibet. This study reveals that pseudotachylytes induced profound weakness of the boundary fault between Indian and Asian plates. The intrinsically low strength of the foliated microlites crystallized from frictional melt or glass(i.e., pseudotachylyte) at seismogenic depths compared with the surrounding coarse-grained quartzofeldspathic rocks in the brittle and semi-brittle regime is sufficient to explain the localization of shear strain, the development of ductile shear zones embedded in strong wall rocks, and the transition from the strong to weak fault behaviors without invoking the presence of high fluid pressure or low friction coefficient metasomatic materials(e.g., smectite or lizardite) within the faults.     

Deep structure and kinematics of the Northern Calcareous Alps (TRANSALP Profile)

The Northern Calcareous Alps (NCA) of southern Bavaria and northern Tyrol constitute a carbonate-dominated polyphase fold-thrust wedge; together with its Grauwacken Zone Basement, it is the northernmost part of the far-travelled Upper Austroalpine thrust complex of the Eastern Alps. The present geometry developed in several kinematic stages. Jurassic extensional faults that affected large parts of the NCA and their basement originated when the main part of the NCA was still located southeast of the Central Alpine Ötztal-Silvretta complex. These faults and related facies transitions influenced the later style of detachment of the NCA thrust sheets. Mid to Late Cretaceous detachment, thrust-sheet stacking and motion over the Central Alpine complex are registered in clastic deposits of syntectonic basins. The latest Cretaceous to Cenozoic NNE- to N-directed motion of the NCA towards Europe in front of the Central Alpine complex created another set of significant contraction structures, which at depth overprinted all previous structures. During Cretaceous to Cenozoic deformation, the NCA experienced about 80 km of shortening, i.e., about 73% along the TRANSALP Profile. The European basement and autochthonous Mesozoic cover beneath the allochthonous NCA thrust sheets and flysch complexes seem to have remained relatively undeformed.     

Active deformation in the eastern Swiss Alps: post-glacial faults, seismicity and surface uplift

Post-glacial tectonic faults in the eastern Swiss Alps occur as single lineaments, clusters of faults or extensive fault zones consisting of several individual faults aligned along the same trend. The orientation of the faults reflects the underlying lithology and the pre-existing structures (joints, pervasive foliations) within these lithologies. Most post-glacially formed faults in the area around Chur, which undergoes active surface uplift of 1.6 mm/year, trend E–W and cut across Alpine and glacial features such as active screes and moraines. Additionally, there are NNW and ENE striking faults reactivating pervasive Alpine foliations and shear zones. Based on a comparison with the nodal planes of recent earthquakes, E–W striking faults are interpreted as active faults. Because of very short rupture lengths and mismatches of fault location with earthquake distribution, magnitude and abundance, the faults are considered to be secondary faults due to earthquake shaking, cumulative deformation in post- or interseismic periods or creep, and not primary earthquake-related faults. The maximum of recent surface uplift rates coincides with the youngest cooling of the rocks according to apatite fission-track data and is therefore a long-lived feature that extends well into pre-glacial times. Isostatic rebound owing to overthickened crust or to melting of glacial overburden cannot explain the observed surface uplift pattern. Rather, the faults, earthquakes and surface uplift patterns suggest that the Alps are deforming under active compression and that the Aar massif basement uplift is still active in response to ongoing collision.     

Fluid flow in cataclastic thrust fault zones in sandstones, Sub-Andean Zone, southern Bolivia

The Bolivian Sub-Andean Zone (SAZ) corresponds to a Neogene thrust system that affects an about 10-km thick Palaeozoic to Neogene siliciclastic succession. The analysis of macro and microstructures and cement distribution in thrust fault zones shows that they are sealed by quartz at depths >3 km, due to local silica transfer by pressure-solution/precipitation activated at temperatures >70–90 °C. At shallower depths, faults have remained open and could be preferential drains for lateral flow of carbonate-bearing fluids, as shown by the occurrence of carbonate cements in fractures and their host-sandstone. Due to decreasing burial, resulting from foothill erosion during fault activity, critically buried fault segments can be affected by non-quartz-sealed structures that post-date initial quartz-sealed structures. The integration of textural, fluid inclusion and isotopic data shows that carbonates precipitated at shallow depth (<3 km), low temperature (<80 °C) and relatively late during the thrusting history. Isotopic data also show that precipitation occurred from the mixing of gravity-driven meteoric water with deeper formation water bearing carbonate carbon derived from the maturation of hydrocarbon source rocks (Silurian and Devonian shales). The combined microstructural and isotopic analyses indicate that: (i) fluid flow in fault zones often occurred with successive pulses derived from different or evolving sources and probably related to episodic fault activity, and (ii) at a large-scale, the faults have a low transverse permeability and they separate thrust sheets with different fluid histories.     

Deformation and ultrafine dynamic recrystallization of quartz in pseudotachylyte-bearing brittle faults: A matter of a few seconds

Tectonic pseudotachylytes, i.e. quenched friction-induced silicate melts, record coseismic slip along faults and are mainly reported from the brittle crust in association with cataclasites. In this study, we document the occurrence of recrystallization of quartz to ultrafine-grained (grain size 1–2 μm) aggregates along microshear zones (50–150 μm thick) in the host rock adjacent to pseudotachylytes from two different faults within quartzite (Schneeberg Normal Fault Zone, Eastern Alps), and tonalite (Adamello fault, Southern Alps) in the brittle crust. The transition from the host quartz to microshear zone interior includes: (i) formation of high dislocation densities; (ii) fine (0.3–0.5 μm) polygonization to subgrains defined by disordered to well-ordered dislocation walls; (iii) development of a mosaic aggregate of dislocation-free new grains. The crystallographic preferred orientation (CPO) of quartz towards the microshear zone shows a progressive misorientation from the host grain, by subgrain rotation recrystallization, to a nearly random CPO possibly related to grain boundary sliding. These ultrafine aggregates appear to be typically associated with pseudotachylytes in nature. We refer the crystal plastic deformation of quartz accompanied by dramatic grain size refinement to the coseismic stages of fault slip due to high differential stress and temperature transients induced by frictional heating. Microshear zones localized on precursory fractures developed during the stages of earthquake rupture propagation and the very initial stages of fault slip. Thermal models indicate that the process of recrystallization, including recovery processes, occurred in a time lapse of a few tens of seconds.     

Along strike changes in the structural evolution over a brittle detachment fault: Example of the Pleistocene Corinth–Patras rift (Greece)

The Patras, Corinth, and northern Saronic gulfs occupy a 200-km-long, N120° trending Pleistocene rift zone, where Peloponnese drifts away from mainland Greece. The axes of Patras and Corinth basins are 25 km apart and linked by two transfer-fault zones trending N040°. The older one defines the western slope of Panachaïkon mountain, and the younger one limits the narrow Rion–Patras littoral plain. Between these two faults, the ca. 4-km-thick Rion–Patras series dips 20–30° SSW. It is part of the Patras gulf synrift deposits, which pile in an asymmetric basin governed by a fault dipping ca. 25–35° NNE, located in the southern Gulf of Patras. Mapping of this fault to the east in northern Peloponnese shows that it is an inactive north-dipping low-angle normal fault (0° to 30°N), called the northern Peloponnese major fault (NPMF). The structural evolution of the NPMF was different in the gulfs of Patras and Corinth. In the Gulf of Patras, it is still active. In northern Peloponnese, footwall uplift and coeval southward tilting flattened the fault and locked its southern part. Steeper normal faults formed north of the locked area, connecting the still active northern part of the NPMF to the surface. After several locks, the presently active normal faults (Psathopyrgos, Aigion, Helike) trend along the southern shore of the Gulf of Corinth. This migration of faults caused the relative 25 km northward shift of the Corinth basin, and the formation of NE–SW trending transfer-faults between the Corinth and Patras gulfs.     

Geochronological constraints on the evolution of the Periadriatic Fault System (Alps)

Fault rocks from various segments of the Periadriatic fault system (PAF; Alps) have been directly dated using texturally controlled Rb-Sr microsampling dating applied to mylonites, and both stepwise-heating and laser-ablation ⁴⁰Ar/³⁹Ar dating applied to pseudotachylytes. The new fault ages place better constraints on tectonic models proposed for the PAF, particularly in its central sector. Along the North Giudicarie fault, Oligocene (E)SE-directed thrusting (29-32 Ma) is currently best explained as accommodation across a cogenetic restraining bend within the Oligocene dextral Tonale-Pustertal fault system. In this case, the limited jump in metamorphic grade observed across the North Giudicarie fault restricts the dextral displacement along the kinematically linked Tonale fault to ~30 km. Dextral displacement between the Tonale and Pustertal faults cannot be transferred via the Peio fault because of both Late Cretaceous fault ages (74-67 Ma) and sinistral transtensive fault kinematics. In combination with other pseudotachylyte ages (62-58 Ma), widespread Late Cretaceous-Paleocene extension is established within the Austroalpine unit, coeval with sedimentation of Gosau Group sediments. Early Miocene pseudotachylyte ages (22-16 Ma) from the Tonale, Pustertal, Jaufen and Passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the Eastern Alps. This event coincides with exhumation of the Penninic units and contemporaneous sedimentation within fault-bound basins.