Structural analysis of the Subbriançonnais and Valais units in the area of Moûtiers (Savoy, Western Alps): paleogeographic and tectonic consequences

Valais and Subbriançonnais units of the Western Alps of Savoie underwent a common structural evolution, postdating peak pressure conditions associated with high-pressure metamorphism of internal parts of the Valais units. The first two phases, due to roughly north/south-directed shortening, are interpreted to be related to a NNE/SSW-striking corridor of sinistral transpression between the internal Western Alps and the European foreland. Both phases led to nappe formation, isoclinal folding and north–south elongation. Only the third phase of deformation is related to WNW-directed orogen-perpendicular shortening, thus far regarded as the predominant thrusting direction in the Western Alps. Late (post 5?Ma) normal faulting, evidenced by fission-track dating, reactivated the Houiller Front in the north and the Penninic Front in the south. Kinematics of movement, observed along the present-day Houiller Front and Penninic Front, change from north to south. In the north the Houiller Front indicates post-D3 normal faulting while the Penninic Front preserved WNW-directed thrusting (D3). In the south the Houiller Front preserves syn-D2 north-directed thrusting, whereas the Penninic Front is partly reactivated by post-D3 normal faulting. Our observations clearly favor tectonic reasons for the disappearance of the Valais units south of Moûtiers in present-day map view.     

Evolution of Alpine eclogites in the Eastern Alps: Implications for Alpine geodynamics

In the Eastern Alps Alpine eclogites are generally associated with rocks of continental lithosphere, while eclogites that are associated with oceanic assemblages are restricted to minor exposures. Such eclogites are exposed both in the Penninic unit of the Tauern Window and in the Austroalpine nappe complex. (1) In the central southern part of the Tauern Window (Eclogite Zone) eclogites and associated high pressure metasediments of a distal continental margin are intercalated between Penninic basement units. A mylonitic eclogitic foliation and stretching lineation are contemporaneous to the high pressure metamorphism and are related to the subduction of distal Penninic continental margin sequences. Continuous subduction of cool lithosphere resulted in blueschist facies overprint of the whole Penninic nappe pile. (2) Within the Middle-AustroAlpine Koralm/Saualm region most eclogites are eclogitic mylonites documenting plastic deformation of omphacite and garnet. The meso- and macroscale structures indicate an overall extensional regime possibly related to a large-scale SE-directed ductile low-angle normal shear zone. The eclogites are associated with migmatite-like structures and are intruded by pegmatites. This indicates decreasing pressure, but isothermal or even increasing temperature conditions during exhumation.These relationships argue for the subduction of Penninic continental lithosphere in the foot-wall of the Austroalpine unit at the time of exhumation of the Koralm/Saualm eclogites. Formation of the Austroalpine eclogites is explained by subduction of continental lithosphere, and subsequent, rapid exhumation in an upper plate tectonic position within an extensional regime.     

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.     

The erosion history of the Central Alps: evidence from zircon fission track data of the foreland basin sediments

Fission track dating on detrital zircons of Alpine debris in the Swiss molasse basin provides information about the erosion history of the Central Alps and the thermal evolution of source terrains. During Oligocene times, only sedimentary cover nappes, and Austroalpine basement units were eroded. Incision into Austroalpine basement units is indicated by increasing importance of Cretaceous cooling ages in granite pebbles upsection. Erosion of Penninic basement units started between 25 and 20 Ma. Early Oligocene zircon FT ages show that Penninic basement units were exposed at ∼20 Ma. Deeper Penninic units of the Lepontine Dome became exposed first at ∼14 Ma, contemporaneously with the opening of the Tauern window in the Eastern Alps. A middle Miocene cooling rate of 40 °C Myr⁻¹ is deduced for the Lower Penninic units of the Lepontine Dome.     

Tectonics of the Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central Alps

The Lepontine dome represents a unique region in the arc of the Central and Western Alps, where complex fold structures of upper amphibolite facies grade of the deepest stage of the orogenic belt are exposed in a tectonic half-window. The NW-verging Mont Blanc, Aar und Gotthard basement folds and the Lower Penninic gneiss nappes of the Central Alps were formed by ductile detachment of the upper European crust during its Late Eocene–Early Oligocene SE-directed underthrust below the upper Penninic and Austroalpine thrusts and the Adriatic plate. Four underthrust zones are distinguished in the NW-verging stack of Alpine fold nappes and thrusts: the Canavese, Piemont, Valais and Adula zones. Up to three schistosities S1–S3, folds F1–F3 and a stretching lineation XI with top-to-NW shear indicators were developed in the F1–F3 fold nappes. Spectacular F4 transverse folds, the SW-verging Verzasca, Maggia, Ziccher, Alpe Bosa and Wandfluhhorn anticlines and synclines overprint the Alpine nappe stack. Their formation under amphibolite facies grade was related to late ductile folding of the southern nappe roots during dextral displacement of the Adriatic indenter. The transverse folding F4 was followed since 30 Ma by the pull-apart exhumation and erosion of the Lepontine dome. This occurred coevally with the formation of the dextral ductile Simplon shear zone, the S-verging backfolding F5 and the formation of the southern steep belt. Exhumation continued after 18 Ma with movement on the brittle Rhone-Simplon detachment, accompanied by the N-, NW- and W-directed Helvetic and Dauphiné thrusts. The dextral shear is dated by the 29–25 Ma crustal-derived aplite and pegmatite intrusions in the southern steep belt. The cooling by uplift and erosion of the Tertiary migmatites of the Bellinzona region occurred between 22 and 18 Ma followed by the exhumation of the Toce dome on the brittle Rhone–Simplon fault since 18 Ma.     

Lu-Hf garnet geochronology of eclogites from the Balma Unit (Pennine Alps): implications for Alpine paleotectonic reconstructions

Three samples of eclogite from the Balma Unit, an ophiolite sheet on top of the Monte Rosa Nappe in the Pennine Alps, were investigated in terms of their P-T evolution, geochemistry, and Lu-Hf geochronology. The paleogeographic origin of this unit is controversial (North Penninic vs. South Penninic). It has been interpreted as a piece of Late Cretaceous oceanic crust, on the basis of ca. 93 Ma U-Pb SHRIMP ages of synmagmatic zircon cores in an eclogite. Trace element and isotope data suggest a mid ocean ridge (MOR) rather than an intraplate or OIB setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 550–600 Cº most likely did not exceed the closure temperature of the Lu-Hf system. Hence, Lu-Hf ages most likely reflect garnet growth in the studied samples. To minimize inclusion effects on age determinations, a selective digestion procedure for garnet was applied, in which zircon and rutile inclusions are not dissolved. The ages obtained for three samples, 42.3 ± 0.6 Ma (MSWD: 0.47), 42 ± 1 Ma (MSWD: 3.0) and 45.5 ± 0.3 Ma (MSWD: 0.33), are younger than all Lu-Hf ages reported so far for South Penninic Units. Metamorphic zircon domains of the 42.3 Ma sample (PIS1) were previously dated by U-Pb SHRIMP at 40.4 ± 0.7 Ma, indicating that the growth of metamorphic zircon post-dated the onset of garnet growth.These new data put important constraints on the paleogeographic reconstruction of the Alps. The MORB character of the rocks, together with their previously published protolith age, imply that oceanic spreading was still taking place in the Late Cretaceous. This supports a North Penninic origin for our samples because plate tectonic models predict Cretaceous spreading in the North Penninic but not in the South Penninic Ocean. If the Balma Unit is indeed North Penninic, the new Lu-Hf data, in combination with published geochronological data, require that two independent subduction zones consumed the South and North Penninic oceans.     

Plate tectonic processes that governed the mineralization of the Eastern Alps

The notably limited post-Hercynian mineralization of the Eastern Alps is shown to be a direct function of the particular plate tectonic history of the region. The Alpine orogeny can be viewed as consisting of two separate, although overlapping events. Formation and then destruction of a "Penninic Ocean" of Jurassic age left a root of subduced oceanic crust in the Asthenosphere. Cyprus style copper deposits and submarine exhalative tungsten and base metal ore-bodies are the main mineralization episodes that can be related to this Penninic event. In the second stage of the Alpine orogeny the northward subducing Tethyan ocean floor collided with the Penninic remnant causing steepening and deflection of the Benioff-zone. The Alps were thus insulated by the Penninic root from many of the thermal events typical of normal subduction induced orogenies. Minor transport of earlier disseminated mineralization into faults formed in the Alpine tectonism is the dominant manifestation of the limited Alpine thermal event. However, although the geometry of the Alpine orogeny favoured only minor metallization it is also noted that earlier orogenies of the Alpine region are metal poor compared to many orogenic regions. It is suggested therefore that inheritance of the metallogenetic character of the preceeding basement may also play a role in the metalliferous nature of later orogenic episodes.     

Geochemical evidence for the nature of the crust beneath the eastern North Penninic basin of the Mesozoic Tethys ocean

The North Penninic basin was a subbasin in the northern part of the Mesozoic Tethys ocean. Its significance within the framework of this ocean is controversial because it is not clear whether it was underlain by thinned continental or oceanic crust. Remnants of the eastern North Penninic basin are preserved in the Alps of eastern Switzerland (Grisons) as low metamorphic "Bündnerschiefer" sediments and associated basaltic rocks which formed approximately 140–170 Ma ago (Misox Bündnerschiefer zone, Middle Jurassic to Early Cretaceous). Nb/U, Zr/Nb, and Y/Nb ratios, as well as Nd–Sr isotopic and REE data of most of the metabasalts point to a depleted MORB-type mantle origin. They have been contaminated by magmatic assimilation of Bündnerschiefer sediments and by exchange with seawater, but do not prove the existence of a subcontinental lithospheric mantle or continental crust beneath the North Penninic basin. This suggests that the studied part of the North Penninic realm was underlain by oceanic crust. Only the metabasalts from two melange zones (Vals and Grava melanges) show a more important contamination by crustal material. Since this type of contamination cannot be observed in the other tectonic units, we suggest that its occurrence is related to melange formation during the subduction of the North Penninic basin in the Tertiary. The North Penninic basin was probably, despite the occurrence of oceanic crust, smaller than the South Penninic ocean where the presence of oceanic crust is well established. Modern analogues for the North Penninic basin could be the transitional zone of the Red Sea or the pull-apart basins of the southernmost Gulf of California where local patches of oceanic crust with effusive volcanism have been described.     

40Ar/ 39Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31–34 Ma)

Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the ⁴⁰Ar/³⁹Ar technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite–phengite thermobarometry yields 10–15 km and T ∼280 °C, while ⁴⁰Ar/³⁹Ar phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde ⁴⁰Ar/³⁹Ar ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites.     

The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps

The Valais units in Savoy (Zone des Brèches de Tarentaise) have been re-mapped in great detail and are subject of combined stratigraphic, structural and petrological investigations summarized in this contribution. The sediments and rare relics of basement, together with Cretaceous age mafic and ultramafic rocks of the Valais palaeogeographical domain, represent the heavily deformed relics of the former distal European margin (External Valais units) and an ocean–continent transition (Internal Valais unit or Versoyen unit) that formed during rifting. This rifting led to the opening of the Valais ocean, a northern branch of the Alpine Tethys. Post-rift sediments referred to as “Valais trilogy” stratigraphically overlie both External and Internal Valais successions above an angular unconformity formed in Barremian to Aptian times, providing robust evidence for the timing of the opening of the Valais ocean. The Valais units in Savoy are part of a second and more external mid-Eocene high-pressure belt in the Alps that sutured the Briançonnais microcontinent to Europe. Top-N D1-deformation led to the formation of a nappe stack that emplaced the largely eclogite-facies Internal Valais unit (Versoyen) onto blueschist-facies External Valais units. The latter originally consisted of, from internal to external, the Petit St. Bernard unit, the Roc de l’Enfer unit, the Moûtiers unit and the Quermoz unit. Ongoing top-N D2-thrusting and folding substantially modified this nappe stack. Post 35 Ma D3 folding led to relatively minor modifications of the nappe stack within the Valais units but was associated with substantial top-WNW thrusting of the Valais units over the Dauphinois units along the Roselend thrust during W-directed indentation of the Adria block contributing to the formation of the arc of the Western Alps.     

The role of rift‐inherited hyper‐extension in Alpine‐type orogens

Alpine‐type orogens are interpreted to result from the collision of former rifted margins. As many present‐day rifted margins consist of hyper‐extended domains floored by thinned continental crust (<10 km) and/or exhumed mantle, this study explores the influence of rift inheritance on the architecture and final evolution of Alpine‐type orogens. We propose that rift‐related necking zones, separating weakly thinned 25‐ to 30‐km‐thick crust from hyper‐extended domains, may act as buttresses during the transition from subduction to collision. As a result, former necking zones may now be found at the boundary between a highly deformed and overthickened nappe stack, made of relics of hyper‐extended domains, and an external, weakly deformed fold‐and‐thrust belt, which largely escaped significant rift‐related crustal thinning and orogeny‐related thickening. Therefore, the role of rift inheritance is of critical importance and is largely underestimated in controlling the architecture and evolution of Alpine‐type orogens.     

The lithospheric geodynamics of plate boundary transpression in New Zealand: Initiating and emplacing subduction along the Hikurangi margin, and the tectonic evolution of the Alpine Fault system

In contrast to the normal ‘Wilson cycle’ sequence of subduction leading to continental collision and associated mountain building, the evolution of the New Zealand plate boundary in the Neogene reflects the converse—initially a period of continental convergence that is followed by the emplacement of subduction. Plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes. Relative plate motions and reconstructions since the Early to Mid-Miocene require significant continental convergence in advance of the emplacement of the southward migrating Hikurangi subduction—a sequence of tectonism seen in the present plate boundary geography of Hikurangi subduction beneath North Island and convergence in the Southern Alps along the Alpine Fault. In contrast to a transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable, the transition from a continental convergent regime, with its associated crustal and lithospheric thickening, to subduction of oceanic lithosphere requires substantial thinning (removal) of upper plate continental lithosphere to make room for the slab. The simple structure of the Wadati–Benioff zone seen in the present-day geometry of the subducting Pacific plate beneath North Island indicates that this lithospheric adjustment occurs quickly. Associated with this rapid lithospheric thinning is the development of a series of ephemeral basins, younging to the south, that straddle the migrating slab edge. Based on this association between localized vertical tectonics and slab emplacement, the tectonic history of these basins records the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting Pacific slab. Although the New Zealand plate boundary is often described as simply two subduction zones linked by the transpressive Alpine Fault, in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction.     

Gondwana-derived microcontinents — the constituents of the Variscan and Alpine collisional orogens

The European Variscan and Alpine mountain chains are collisional orogens, and are built up of pre-Variscan “building blocks” which, in most cases, originated at the Gondwana margin. Such pre-Variscan elements were part of a pre-Ordovician archipelago-like continental ribbon in the former eastern prolongation of Avalonia, and their present-day distribution resulted from juxtaposition through Variscan and/or Alpine tectonic evolution. The well-known nomenclatures applied to these mountain chains are the mirror of Variscan resp. Alpine organization. It is the aim of this paper to present a terminology taking into account their pre-Variscan evolution at the Gondwana margin. They may contain relics of volcanic islands with pieces of Cadomian crust, relics of volcanic arc settings, and accretionary wedges, which were separated from Gondwana by initial stages of Rheic ocean opening. After a short-lived Ordovician orogenic event and amalgamation of these elements at the Gondwanan margin, the still continuing Gondwana-directed subduction triggered the formation of Ordovician Al-rich granitoids and the latest Ordovician opening of Palaeo-Tethys. An example from the Alps (External Massifs) illustrates the gradual reworking of Gondwana-derived, pre-Variscan elements during the Variscan and Alpine/Tertiary orogenic cycles.     

HP metamorphic belt of the western Alps

The understanding of the subduction-related processes benefited by the studies of the high-pressure (HP) meta-morphic rocks from the western Alps. The most stimu-lating information was obtained from the inner part of the western Alpine belt, where most tectonic units show an early Alpine eclogite-facies recrystallisation. This is especially true for the Austroalpine Sesia Zone and the Penninic Dora-Maira massif. From the Sesia zone,which consists of a wide spectrum of continental crust lithologies recrystallised to quartz-eclogite-facies min-eral assemblages, the first finding of a jadeite-bearingmeta-granitoid has been described, supporting evidencethat even continental crust may subduct into the mantle.From the Dora-Maira massif the first occurrence of regional metamorphic coesite has been reported, open-ing the new fertile field of the ultrahigh-pressure meta-morphism (UHPM), which is now becoming the rule in the collisional orogenic belts.     

大陆碰撞造山样式与过程：来自特提斯碰撞造山带的实例

本文选取特提斯域内比利牛斯、阿尔卑斯、扎格罗斯、喜马拉雅-青藏高原四个地球上最年轻的陆-陆碰撞造山带,对其造山带结构、类型、物质组成、构造岩浆过程等方面进行详细介绍,进而讨论各个造山带的差异性及其缘由,分析碰撞造山普遍性规律。资料分析表明,四个碰撞造山带具有不同的结构和组成。根据板块汇聚方向与造山带边界间的夹角可将造山带分为正向和斜向两种;根据造山带结构可将碰撞带分为对称式和不对称式两种。由此本文将碰撞造山带划分为四种基本式样:正向对称式、正向不对称式、斜向对称式、斜向不对称式,分别以比利牛斯、青藏高原、阿尔卑斯和扎格罗斯碰撞带为代表。综合分析四个造山带碰撞以来的岩浆构造活动,本文发现完整的碰撞过程可以划分为三个阶段,第一阶段主要发生挤压缩短、地壳加厚,高压变质和钙碱性火山岩浆活动;第二阶段以大规模走滑系统发育和高钾钙碱性或钾质火山岩浆作用为特征;第三个阶段挤压应力向碰撞带两侧扩展,同时伴有大型伸展构造系统的发育。在这三阶段演化历程中,比利牛斯只进行到第一阶段,成为幼年夭折的碰撞带;扎格罗斯进行到第二阶段,出现调节挤压应变的走滑系统和钾质超钾质岩浆活动;青藏高原和阿尔卑斯进行到第三个阶段,以发育大型伸展构造和钾质、超钾质岩浆活动为特征,但后者在造山带物质组成和汇聚速率方面显示出比前者更成熟的造山演化程度。因此认为岩石圈组成是碰撞造山带结构的主要控制因素,如果上覆板块具有相对不稳定的岩石圈,会使得碰撞带后陆发育宽广的构造岩浆带,造成造山带呈不对称式结构。     

从变质作用观看板块构造何时在华北克拉通开始

了解板块构造在地球上何时和怎样开始的是地球科学领域还没有解决的重要问题之一。作为板块运动的最终结果,大陆碰撞造山带是识别地球历史演化中板块构造机制起主导作用的重要标志。大陆碰撞带变质作用一般以顺时针p-T轨迹演化为特征,尤其伴有峰期变质之后的等温减压过程。这样,具有峰后等温减压过程的顺时针p-T轨迹是识别地球早期的板块构造作用的重要标志之一。作为世界上最古老的克拉通陆块之一,华北克拉通基底岩石变质作用p-T演化在过去几年已得到广泛深入的研究,使得该克拉通可能成为应用大规模变质作用p-T轨迹途径来探讨构造环境和构造演化过程的最佳场所。构造上,华北克拉通可划分为三个小的陆块(东部陆块、阴山陆块和鄂尔多斯陆块)和三个古元古代活动带(华北中部碰撞带、孔兹岩带和胶—辽—吉带)。东部陆块和阴山陆块新太古代基底岩石变质作用具有等压冷却型逆时针p-T演化轨迹特征,反映变质作用热源与大量地幔岩浆底板垫托或侵位有关。尽管理论上这样大规模的地幔岩浆可形成在大陆岩浆弧、地幔柱或大陆裂谷环境,只有地幔柱模式才能合理地解释东部陆块和阴山陆块新太古代基底岩石时空分布、岩石组合和构造特征。这样,地幔柱可能是主导东部陆块和阴山陆块新太古代地壳形成和演化的主要构造机制,而板块构造在晚太古宙并不是其主要的构造机制。古元古代孔兹岩带和华北中部碰撞带基底岩石变质作用均具有等温减压型顺时针p-T演化特征,反映两造山带都经历地壳加厚和随后的隆升剥蚀构造过程。这样的构造过程是板块构造体制下的碰撞造山带的典型标志。古元古代胶—辽—吉带可进一步划分为南部带和北部带,其中南部带基底岩石具有逆时针p-T演化特征,而北部带基地岩石具有顺时针p-T演化特征,也反映板块构造机制下的产物。现代规模的板块构造在华北克拉通上的启动时间可以由三个活动带中最老的与板块俯冲有关的新生地壳形成时间来大致标定。目前,华北克拉通内部三个活动带中可识别出来的最老的与板块俯冲有关的新生地壳是华北中部碰撞带2·56Ga五台花岗岩,它们的形成可能大致标志着现代样式的板块构造在华北克拉通大规模作用的开始。     

Formation, subduction, and exhumation of Penninic oceanic crust in the Eastern Alps: time constraints from Ar/Ar geochronology

New ⁴⁰Ar/³⁹Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186±2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine–Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57±3 Ma amphibole from the Penninic subduction–accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through ≤15 kbar and >500 °C at 42 Ma to 10 kbar and 400 °C at 39 Ma. The Tauern Eclogite Zone pressure–temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure–ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at ≤45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at 35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscan-style subduction–accretion processes that are recorded in the Rechnitz Complex.     

Earthquake focal mechanisms and stress orientations in the eastern Swiss Alps

This study presents an updated set of earthquake focal mechanisms in the Helvetic and Penninic/Austroalpine domains of the eastern Swiss Alps. In eight cases, based on high-precision relative hypocentre locations of events within individual earthquake sequences, it was possible to identify the active fault plane. Whereas the focal mechanisms in the Helvetic domain are mostly strike-slip, the Penninic/Austroalpine domain is dominated by normal-faulting mechanisms. Given this systematic difference in faulting style, an inversion for the stress field was performed separately for the two regions. The stress field in the Penninic/Austroalpine domain is characterized by extension oriented obliquely to the E–W strike of the orogen. Hence, the Penninic nappes, which were emplaced as large-scale compressional structures during the Alpine orogenesis, are now deforming in an extensional mode. This contrasts with the more compressional strike-slip regime in the Helvetic domain towards the northern Alpine front. Relative to the regional stress field seen in the northern Alpine foreland with a NNW–SSE compression and an ENE–WSW extension, the orientation of the least compressive stress in the Penninic/Austroalpine domain is rotated counter-clockwise by about 40°. Following earlier studies, the observed rotation of the orientation of the least compressive stress in the Penninic/Austroalpine region can be explained as the superposition of the regional stress field of the northern foreland and a uniaxial extensional stress perpendicular to the local trend of the Alpine mountain belt.