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

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

Modeling of in-situ crystallization processes in the Permian mafic layered intrusion of Mont Collon (Dent Blanche nappe, western Alps)

The Mont Collon mafic complex is one of the best preserved examples of the Early Permian magmatism in the Central Alps, related to the intra-continental collapse of the Variscan belt. It mostly consists (> 95 vol.%) of ol + hy-normative plagioclase-wehrlites, olivine- and cpx-gabbros with cumulitic structures, crosscut by acid dikes. Pegmatitic gabbros, troctolites and anorthosites outcrop locally. A well-preserved cumulative sequence is exposed in the Dents de Bertol area (center of intrusion). P–T calculations indicate that this layered magma chamber emplaced at mid-crustal levels at about 0.5 GPa and 1100 °C. The Mont Collon cumulitic rocks record little magmatic differentiation, as illustrated by the restricted range of clinopyroxene mg-number (Mg#_cpx = 83–89). Whole-rock incompatible trace-element contents (e.g. Nb, Zr, Ba) vary largely and without correlation with major-element composition. These features are characteristic of an in-situ crystallization process with variable amounts of interstitial liquid L trapped between the cumulus mineral phases. LA-ICPMS measurements show that trace-element distribution in the latter is homogeneous, pointing to subsolidus re-equilibration between crystals and interstitial melts. A quantitative modeling based on Langmuir's in-situ crystallization equation successfully duplicated the REE concentrations in cumulitic minerals of all rock facies of the intrusion. The calculated amounts of interstitial liquid L vary between 0 and 35% for degrees of differentiation F of 0 to 20%, relative to the least evolved facies of the intrusion. L values are well correlated with the modal proportions of interstitial amphibole and whole-rock incompatible trace-element concentrations (e.g. Zr, Nb) of the tested samples. However, the in-situ crystallization model reaches its limitations with rock containing high modal content of REE-bearing minerals (i.e. zircon), such as pegmatitic gabbros. Dikes of anorthositic composition, locally crosscutting the layered lithologies, evidence that the Mont Collon rocks evolved in open system with mixing of intercumulus liquids of different origins and possibly contrasting compositions. The proposed model is not able to resolve these complex open systems, but migrating liquids could be partly responsible for the observed dispersion of points in some correlation diagrams. Absence of significant differentiation with recurrent lithologies in the cumulitic pile of Dents de Bertol points to an efficiently convective magma chamber, with possible periodic replenishment.     

Geometry and kinematics of early Alpine nappes in a Briançonnais basement (Ambin Massif,Western Alps)

Petrological and structural observations from the Ambin pre-alpine basement dome and from its Briançonnais and Piedmont covers show an early D1 nappe-forming event overprinted by a major D2 (+?D3) ductile shearing deformation. The D1 event is characterised by garnet-blueschist facies metamorphic assemblages retrogressed to greenschist facies conditions during D2 then D3 stages near the top of the dome. North-verging D1 structures preserved in the core of the dome are consistent with alpine evolutionary models, in which exhumation of HP–LT metamorphic alpine rocks occurs initially in a north–south direction. To cite this article: J. Ganne et al., C. R. Geoscience 336 (2004).     

The Sierra Ballena Shear Zone in the southernmost Dom Feliciano Belt (Uruguay): evolution, kinematics, and deformation conditions

The Sierra Ballena Shear Zone (SBSZ) is part of a high-strain transcurrent system that divides the Neoproterozoic Dom Feliciano Belt of South America into two different domains. The basement on both sides of the SBSZ shows a deformation stage preceding that of the transcurrent deformation recognized as a high temperature mylonitic foliation associated with migmatization. Grain boundary migration and fluid-assisted grain boundary diffusion enhanced by partial melting were the main deformation mechanisms associated with this foliation. Age estimate of this episode is >658 Ma. The second stage corresponds to the start of transpressional deformation and the nucleation and development of the SBSZ. During this stage, pure shear dominates the deformation, and is characterized by the development of conjugate dextral and sinistral shear zones and the emplacement of syntectonic granites. This event dates to 658–600 Ma based on the age of these intrusions. The third stage was a second transpressional event at about 586 to <560 Ma that was associated with the emplacement of porphyry dikes and granites that show evidence of flattening. Deformation in the SBSZ took place, during the late stages, under regional low-grade conditions, as indicated by the metamorphic paragenesis in the supracrustals of the country rocks. Granitic mylonites show plastic deformation of quartz and brittle behavior of feldspar. A transition from magmatic to solid-state microstructures is also frequently observed in syntectonic granites. Mylonitic porphyries and quartz mylonites resulted from the deformation of alkaline porphyries and quartz veins emplaced in the shear zone. Quartz veins reflect the release of silica associated with the breakdown of feldspar to white mica during the evolution of the granitic mylonites to phyllonites, which resulted in shear zone weakening. Quartz microstructures characteristic of the transition between regime 2 and regime 3, grain boundary migration and incipient recrystallization in feldspar indicate deformation under lower amphibolite to upper greenschist conditions (550–400°C). On the other hand, the mylonitic porphyries display evidence of feldspar recrystallization suggesting magmatic or high-T solid-state deformation during cooling of the dikes.     

Metamorphic evolution of the Sesia-Lanzo Zone,Western Alps: time constraints from multi-system geochronology

 The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowing multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb–Sr white mica and U–Pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60–70 Ma. This substantially alters the common view of early- to mid-Cretaceous eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny, since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate post-Cretaceous high-pressure metamorphism. Step-heated ⁴⁰Ar–³⁹Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb–Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under the fluid/pressure conditions acting during this high-pressure metamorphic event. The U–Pb sphene ages are variable in polymetamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-pressure event. Two monometamorphic assemblages yield concordant ages at 66±1 Ma, reflecting crystallisation of the eclogite facies assemblage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclogitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb–Sr white mica ages at 38–39 Ma. This greenschist event did not affect the majority of the EMC. The ⁴⁰Ar–³⁹Ar ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their ⁴⁰Ar–³⁹Ar system reset. The greenschist event did not strongly affect U–Pb systematics in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but was juxtaposed against the EMC later in the orogeny. This model still requires that the locus of deformation and metamorphism (and possibly fluid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge. Received: 12 October 1995/Accepted:25 June 1996     

Tectonic progradation and plate tectonic evolution of the Alps

Rifting and spreading, trench formation, flysch deposition, subduction and nappe formation prograde from internal to external parts of the Alpine orogen. The progradation is a characteristic feature of the evolution of the Alps. A plate tectonics model based on this cognition is presented and an attempt is made to integrate the plate movements of the Alpine region during the Mesozoic and Cenozoic into the plate pattern of the Western Mediterranean.

Important events in the evolution of the Alps are the successive opening and closing of the Piedmont (South Penninic) and Valais (North Penninic) oceans, and the two continental collisions related to this. The southward drift of the Briançonian plate in the Cretaceous closes the Piedmont and opens the Valais ocean. The evolution of these oceans is related to the plate movements in the North Atlantic. The second continental collision is followed by the formation of an exogeosyncline, the molasse foredeep.

Prograding orogens like the Alps are most likely to evolve in an originally continental environment by rifting. Retrograding orogens, however, indicate an originally oceanic environment with well-developed magmatic arcs and back-arc basins.     

Palaeomagnetic data on the andesitic cover of the Sesia-Lanzo Zone (Western Alps)

The remanent magnetization of the andesitic cover of the Sesia-Lanzo Zone has been studied from 16 sites along three transversal sections. The remanent magnetization of the rock is stable and it appears to be primary after a conglomerate test. The mean direction of magnetization is consistent for all sites, but for one exception, with satisfactory values of statistical parameters. Therefore no deformation structures postdating the acquisition of remanent magnetization can be inferred from palaeomagnetic data. The mean direction of magnetization (11 sites, 152 specimens) is: D=135.9, I=–2.9, with ₉₅=8.8. No tectonic correction can yet be made.

---

Zusammenfassung Die natürliche remanente Magnetisierung der Andesiten, die die Bedeckung der Sesia-Lanzo Zone bilden, ist an 16 Orten gemessen worden, die entlang drei Querdurchschnitten liegen. Das Gestein besitzt eine stabile NRM, die auf Grund eines Konglomeratstest die ursprüngliche sein müßte. Alle Orte, die annehmbare statistische Werte aufweisen, außer einem, haben die gleiche Magnetisierungsrichtung. Die paläomagnetischen Messungen zeigen keine Strukturen aufeinanderfolgender Deformation bei dem Magnetisierungsvorgang. Die durchschnittliche Magnetisierungsrichtung (11 Orte, 152 Gesteinsproben) ist: D = 135,9, I = –2,9, mit ₉₅ = 8,8. Keine tektonische Korrektur wird bisher eingerechnet.

---

Résumé L'aimantation rémanente des andésites qui constituent la couverture de la Zone Sesia-Lanzo a été mesurée en 16 endroits disposés le long de trois sections transversales. La roche a une aimantation rémanente stable, qui, d'après le test du conglomérat, devrait être originelle. Tous les endroits qui présentent des valeurs statistiques acceptables ont, sauf un, la même direction d'aimantation. Les mesures paléomagnétiques ne révèlent donc pas de structures de déformation consécutive à l'acquisition de l'aimantation rémanente. La direction moyenne de l'aimantation (11 sites, 152 échantillons) est: D=135.9, I=–2.9 avec ₉₅=8.8. On n'a pas apporté, pour l'instant, de correction tectonique.

---

, Sesia-Lanzo, 16- , . , , , . , , , . , . (11 , 152 ) : D=135,9; =–2,9 ₉₅=8,8. .

     

东阿尔卑斯原-古特提斯构造演化

新生代阿尔卑斯是非洲和欧洲之间的陆陆碰撞造山带。强烈的造山作用使大量前中生代基底出露地表,尽管这些基底被强烈逆冲推覆和走滑叠置,但是仍保留较丰富的前中生代基底演化信息。结合近几年对东阿尔卑斯原-古特提斯的研究,本文梳理和重建了阿尔卑斯前中生代基底的构造格局,认为前阿尔卑斯基底受原特提斯、南华力西洋、古特提斯洋构造体系影响而经历了多期造山过程。新元古代-早古生代的原阿尔卑斯作为环冈瓦纳地块群的组成部分,受原特提斯洋俯冲的制约,是新元古-早古生代环冈瓦纳活动陆缘的组成部分,其中,海尔微-彭尼内基底组成外缘增生系统,包括卡多米期地壳碎片在内的陆缘弧/岛弧以及大量增生楔组成内缘增生系统。早奥陶世瑞亚克洋打开,随后原阿尔卑斯从冈瓦纳陆缘裂离,在泥盆纪-石炭纪受南华力西洋控制,海尔微-彭尼内-中、下奥地利阿尔卑斯从冈瓦纳分离。在早石炭世(维宪期)南阿尔卑斯(或与之相当的冈瓦纳源地块)与北部阿莫里卡地块群拼贴增生于古欧洲大陆南缘,共同组成华力西造山带(广义),华力西期缝合带保留在绍山-科尔山南侧。晚石炭世-早二叠世,阿尔卑斯受古特提斯洋的俯冲影响,在华力西造山带南侧形成安第斯山型活动大陆边缘,古特提斯洋在阿尔卑斯的演化至少持续到早三叠世,消亡遗迹保留在中奥地利阿尔卑斯基底的Plankogel杂岩中。     

Structure and kinematics of the Louth-Eumarra Shear Zone (north-central New South Wales,Australia) and implications for the Paleozoic plate tectonic evolution of eastern Australia

The ～E–W-trending Olepoloko Fault and ～ENE-trending Louth-Eumarra Shear Zone in north-central New South Wales are approximately orthogonal to the dominant ～N–S-trending structural grain of Paleozoic eastern Australia. These structures have been interpreted to represent the boundary between the Thomson and Lachlan orogens, but their exact geometry and kinematics remain unclear owing to the scarcity of surface exposure. Using gridded aeromagnetic data and limited field mapping, we obtained new data on the tectonic history of the Louth-Eumarra Shear Zone, which seems to represent a broad zone of dextral shearing with a component of crustal thickening indicated by the recognition of kyanite growth in a mica-schist. The timing of deformation is relatively poorly constrained, but at least a component of the dextral shearing appears to be coeval or younger than the age of displaced late Silurian and Early Devonian granitoids. Additional indicators for dextral kinematics farther north, along the ～ENE-trending Culgoa Fault, suggest that the width of the zone that was subjected to dextral deformation is possibly >100 km. This raises the possibility that a large component of dextral displacement was accommodated in this region. In a broader geodynamic context, we discuss the possibility that the precursor of the Louth-Eumarra Shear Zone and Olepoloko Fault originated from segmentation between the northern and southern Tasmanides, perhaps during the Cambrian. The existence of such a discontinuity may have buttressed the process of oroclinal bending in the Silurian. The observed dextral kinematics has possibly resulted from reactivated deformation during the Tabberabberan and Alice Springs orogenies.     

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 1. The Miagliano Pluton

The Canavese Line in the Western Alps represents the position in the Alpine chain, where alkaline and calc-alkaline magmatism occur in close spatial and temporal association. In addition to available data on the alkaline Valle del Cervo Pluton, we present petrological and geochemical data on the Miagliano tonalite. The latter is of special interest, because it is located in the south-eastern side of the Canavese Line, in contrast to most Periadriatic Plutons. The dioritic to tonalitic rocks of the Miagliano Pluton represent an intermediate stage of a calc-alkaline differentiation, demonstrated by relics of two different pyroxenes as well as the texture of allanite. Hornblende barometry indicates pressures of ~0.46?GPa consistent with the presence of magmatic epidote. Field relationships between the two Plutons, the volcanic and volcaniclastic rocks of the Biella Volcanic Suite and numerous dykes cross-cutting the different units, allow reconstruction of a more refined chronology of the calc-alkaline and alkaline magmatic series. High precision zircon geochronology yields an age of 33.00?±?0.04?Ma for the central tonalitic part of the Miagliano Pluton and 30.39?±?0.50?Ma for the granitic core of the Valle del Cervo Pluton. The difference in age combined with cooling data and intrusion depth indicates dissimilar tectonic transport east and west of the Canavese Line. The earlier emplaced Miagliano Pluton has to be exhumed from an intrusion depth of ~12?C15?km, whereas the neighbouring and younger Valle del Cervo Pluton is exhumed from a depth of 5?C7?km. This tectonic scenario is related to upper crustal rigid block rotation responsible for the burial of the lowermost Rupelian paleosurface of the Sesia?CLanzo Zone. Thus, the new ages constrain the paroxysm of the orogenic magmatism in the internal Western Alps to an extremely short lapse of time in the first half of the Rupelian.     

Tectonic setting, evolution and orogenic gold potential of the late Mesoarchaean Mosquito Creek Basin, North Pilbara Craton, Western Australia

The geology, evolution, and metallogenic potential of the Mesoarchaean Mosquito Creek Basin remains poorly understood, despite the presence of several orogenic gold deposits. The basin is dominated by medium- to coarse-grained, poorly sorted and chemically immature sandstone and conglomerates, characterised by very high Cr/Th, high Th/Sc, and low Zr/Sc relative to average continental crust. These features are consistent with the presence of significant mafic rocks in the source terrain(s), a limited role for sediment recycling, and deposition in an increasingly distal passive margin setting on the southeastern edge of the Palaeo- to Mesoarchaean East Pilbara Terrane.New U–Pb SHRIMP data on 358 detrital zircons indicate a conservative maximum depositional age of 2972 + 14/−37 Ma (robust median; 96.1% confidence). Zircon provenance spectra from conglomeratic rocks near the base of the unit are consistent with substantial derivation from the East Pilbara Terrane, but finer-grained sandstones higher in the stratigraphy appear to have been sourced elsewhere, as their zircon age spectra are not well matched by any of the exposed Pilbara terranes.The Mosquito Creek Basin was deformed before and during collision with the northern edge of the Mesoarchaean Kurrana Terrane, which resulted in the development of macroscopic north-verging folds, thrust faulting, and widespread sub-greenschist to greenschist facies metamorphism. This collisional event probably took place at ca. 2900 Ma, based on two identical Pb–Pb model ages of 2905 ± 9 Ma from epigenetic galena associated with vein-hosted gold–antimony mineralization. The metallogenic potential of the Mosquito Creek Basin remains largely unevaluated; however, the possibility of a passive margin setting and continental basement points to relatively limited potential for the formation of major orogenic gold deposits.     

Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone,Eastern Alps,Italy)

We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO₂-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO₂-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO₂-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. In memory of Lauro Morten 1941–2006.     

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 2. The Biella Volcanic Suite

The high-pressure metamorphic rocks of the Sesia?CLanzo zone are partly covered by a volcano-sedimentary unit, the Biella Volcanic Suite. Calc-alkaline and shoshonitic lavas extruded sub-aerially on the Oligocene surface. Uranium?CLead zircon dating yields 32.44?C32.89?Ma for the eruption of the calc-alkaline lavas and therefore fixes a minimum age for the paleosurface. The Biella Volcanic Suite consists mainly of epiclastic rocks deposited in a high-energy fluvial environment and minor lava flows. The rocks of the suite display widespread post-eruption transformations. Illite and chlorite thermometry as well as fission track dating suggest a thermal overprint related to burial of the Biella Volcanic Suite. An upper crustal rigid block tilting in the area causes this burial. Hydrothermal tourmaline and ankerite veins related to the intrusion of the nearby Valle del Cervo Pluton crosscut the already tilted Biella Volcanic Suite. The intrusion age of Valle del Cervo Pluton at 30.39?±?0.50?Ma sets therefore the lower time limit for tectonic processes responsible for the tilting and burial. After the burial, the Biella Volcanic Suite remained for around 20?million years between the zircon and the apatite partial annealing zone. The apatite fission track ages spread between 16 and 20?Ma gives the time frame for the second exhumation of these units. The Biella Volcanic Suite and the adjacent rocks of the Sesia?CLanzo zone were the second time exhumed to the surface in Messinian times, after a long residence time within the apatite partial annealing zone.     

Geometry and Kinematics of the Afogados da Ingazeira Shear Zone,Northeast Brazil

Numerous transcurrent NE-SW, mainly sinistral, and E-W dextral-trending shear zones transect Borborema Province, northeastern Brazil. The most important kinematic event involving those shear zones reflects deformation related to the Brasiliano (Pan-African) tectonic cycle. The Afogados da Ingazeira shear zone (AISZ) is probably the most important of the NE-SW-trending lineaments, having continuity for over 250 km. Field work associated with petrographic studies (including quartz c-axis patterns), conducted in an area encompassing the north-central part of this shear zone, indicate that sinistral transcurrent deformation was responsible for the main banding/foliation (C-S) observed in the rocks. Mineral assemblages and microstructural features are suggestive of a deformational history starting under low- to medium-amphibolite facies (thermal peak conditions) and decreasing until greenschist facies.     

Along-strike shear-sense reversal in the Vals-Scaradra Shear Zone at the front of the Adula Nappe (Central Alps,Switzerland)

The Adula Nappe in the Central Alps comprises pre-Mesozoic basement and minor Mesozoic sediments, overprinted by Paleogene eclogite-facies metamorphism. Peak pressures increase southward from ca. 1.2 GPa to values over 3 GPa, which is interpreted to reflect exhumation from a south-dipping subduction zone. The over- and underlying nappes experienced much lower Alpine pressures. To the north, the Adula Nappe ends in a lobe surrounded by Mesozoic metasediments. The external shape of the lobe is simple but the internal structure highly complicated. The frontal boundary of the nappe represents a discontinuity in metamorphic peak temperatures, between higher T in the Adula Nappe and lower T outside. A shear zone with steeply dipping foliation and shallowly-plunging, WSW-ENE oriented, i.e. orogen-parallel stretching lineation overprinted the northernmost part of the Adula Nappe and the adjacent Mesozoic metasediments (Vals-Scaradra Shear Zone). It formed during the local Leis deformation phase. The shear sense in the Vals-Scaradra Shear Zone changes along strike; from sinistral in the W to dextral in the E. Quartz textures also vary along strike. In the W, they indicate sinistral shearing with a component of coaxial (flattening) strain. A texture from the middle part of the shear zone is symmetric and indicates coaxial flattening. Textures from the eastern part show strong, single c-axis maxima indicating dextral shearing. These relations reflect complex flow within the Adula Nappe during a late stage of its exhumation. The structures and reconstructed flow field indicate that the Adula basement protruded upward and northward into the surrounding metasediments, spread laterally, and expelled the metasediments in front towards west and east.     

Tectonic Evolution of the Dongbo Ophiolite in Western Yarlung Zangbo Suture Zone,Xizang(Tibet)

正The Dongbo ophiolite in the western part of the Yarlung-Zangbo suture zone in southern Tibet rests tectonically on the middle-late Triassic and Cretaceous flysch units,and consist mainly of peridotites,mafic dikes,