Multistage tectono-stratigraphic evolution of the Canavese Intracontinental Suture Zone: New constraints on the tectonics of the Inner Western Alps

The Canavese Intracontinental Suture Zone (CISZ) within the Inner Western Alps represents the remnant of a long-lived minor subduction zone involving a narrow, thinned continental crust/oceanic lithosphere seaway between two continental domains of the Adria microplate (i.e., the Sesia Zone and the Ivrea-Verbano Zone). As opposed to many suture zones, the CISZ mostly escaped pervasive tectonic deformation and metamorphism, thus preserving the original stratigraphy and allowing the relationships between tectonics and sedimentation to be defined. Through detailed geological mapping (1:5000 scale), structural analysis, stratigraphic and petrographic observations, we document evidences for the late Paleozoic to late Cenozoic tectonic evolution of the CISZ, showing that it played a significant role in the context of the tectonic evolution of the Inner Western Alps region from the early to late Permian Pangea segmentation, to the Jurassic Tethyan rifting, and up to the subduction and collisional stages, forming the Western Alps. The site of localization/formation of the CISZ was not accidental but associated with the re-use of structures inherited from regional-scale wrench tectonics related to the segmentation of Pangea, and from the subsequent extensional tectonics related to the Mesozoic rifting, as documented by crosscutting relationships between stratigraphic unconformities and tectonic features. Our findings document that evidences derived from stratigraphy, facies indicators, and relationships between tectonics and sedimentation in the shallow crustal portions of suture zones, such in the CISZ, are important to better constrain the tectonic history of those metamorphic orogenic belts around the world in which evolutionary details are commonly complicated by high-strain deformation and metamorphic transformations.     

Tectono-metamorphic evolution of the European continental margin involved in the Alpine subduction: New insights from Alpine Corsica,France

In Corsica, continental units (the Lower Units) affected by high-pressure metamorphism represent the remains of the European margin deformed during the Alpine orogeny. In order to document how Alpine deformation and metamorphism changed along the European margin involved in the Alpine subduction, we selected three key areas: the Corte, Cima Pedani, and Ghisoni transects. The three transects show a broadly similar lithostratigraphy. They are characterized by a Variscan basement intruded by Permo-Carboniferous metagranitoids, and by a sedimentary cover including Mesozoic carbonates and middle to late Eocene breccias and sandstones. The three transects recorded a similar deformation history with three deformation phases. Thermo-baric estimations, instead, reveal that each unit was exhumed along an independent retrograde path within the orogenic Alpine wedge. In particular, the lowest units of the Lower Units stack were exhumed along an isothermal path, whereas those located at upper structural levels experienced progressive heating.     

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).     

Tectonic significance of different block-in-matrix structures in exhumed convergent plate margins: examples from oceanic and continental HP rocks in Inner Western Alps (northwest Italy)

In the Inner Western Alps, three different types of block-in-matrix structures (BIMs) formed sequentially through time at a convergent plate margin. These show the superposition of progressive deformation from (i) subduction to eclogite-facies depths, (ii) collision, accretion, and exhumation of oceanic crust, represented by the Monviso Meta-ophiolite Complex, to (iii) collision, accretion, and exhumation of the continental Dora Maira units. The Type 1 occurs in the metasedimentary cover of the Dora Maira Unit and consists of a map-scale broken formation with boudinaged ‘native’ blocks of marble (Early Jurassic) in a calcschist matrix. It results from the tectonic overprinting of exhumation-related folding (D2-stage) on an earlier subduction-related dismembered succession (D1-stage). Type 1 also includes ‘non-mappable’ BIMs with ‘exotic’ blocks, resulting from the gravitational collapse of the Triassic carbonate platform of European Continental Margin, triggered by the Early Jurassic rifting. In the Monviso Meta-ophiolite Complex, Types 2 and 3 represent tectonically induced broken and dismembered formations, respectively. They differ from each other in the degree of stratal disruption of primary interbedded horizons of mafic metabreccia (Type 3) and mafic metasandstone (Types 2 and 3) sourced by the Late Jurassic–Early Cretaceous denudation of an oceanic core complex. Dismembered interbeds (Type 2) and isolated blocks were mixed together (Type 3) by the overlap of D2 tectonics and late- to post-exhumation extensional shearing (D3-stage). Development of these types of BIMs may be common in many exhumed convergent plate margins, where severe tectonics and metamorphic recrystallization under high-pressure conditions normally prevent the reconstruction of BIMs or mélange-forming processes. Our findings show that documenting the mode and time of the processes forming BIMs is highly relevant in order to reconstruct the oceanic seafloor morphology and composition of associated stratigraphic successions, and their control in the evolution of those convergent plate margins.     

The fate of subducted continental margins: Two-stage exhumation of the high-pressure to ultrahigh-pressure Western Gneiss Region, Norway

Thermobarometry suggests that ultrahigh‐pressure (UHP) to high‐pressure (HP) rocks across the Western Gneiss Region ponded at the Moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust. Eclogite across the c. 22 000 km² study area records minimum pressures of c. 8–18 kbar and temperatures of c. 650–780 °C. One orthopyroxene eclogite yields an UHP of c. 28.5 kbar, and evidence of former coesite has been found c. 50 km farther east than previously known. Despite this widespread evidence of UHP to HP, thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform ‘supra‐Barrovian’ amphibolite‐facies overprint at c. 11 kbar and c. 650–750 °C across the entire area. Chemical zoning analysis suggests that garnet in these samples grew during heating and decompression, presumably during the amphibolite‐facies event. These data indicate that the Norwegian UHP/HP province was exhumed from mantle depths of c. 150 km to lower crustal depths, where it stalled and underwent a profound high‐temperature overprint. The ubiquity of late‐stage supra‐Barrovian metamorphic overprints suggests that large‐scale, collisional UHP terranes routinely stall at the continental Moho where diminishing body forces are exceeded by boundary forces. Significant portions of the middle or lower crust worldwide may be formed from UHP terranes that were arrested at the Moho and never underwent their final stage of exhumation.     

Reconstructing P–T paths during continental collision using multi-stage garnet (Gran Paradiso nappe, Western Alps)

Garnet–chloritoid-bearing micaschists from the Gran Paradiso massif (Western Alps) contain evidence of a polymetamorphic evolution. Detailed textural observations reveal that two stages of garnet growth are present in the micaschists, interpreted as: (i) relics of an early metamorphism of pre-Alpine age and (ii) newly grown Alpine garnet, respectively. Both generations of garnet preserve growth zoning. From thermocalc -based numerical modelling of mineral assemblages in pressure–temperature ( P – T ) pseudosections, we infer that garnet 1 grew at increasing temperature and slightly increasing pressure, whereas garnet 2 grew at decreasing pressure and slightly increasing temperature. Estimated P – T conditions are ∼620 °C, 6 kbar for the peak of the pre-Alpine event, and of 490 °C, 18–20 kbar for the pressure peak of the Alpine event. Modelling of the modal proportion and chemical composition of garnet (i) shows that the subsequent decompression (to 14–15 kbar at 550 °C) must have been accompanied by moderate heating and (ii) does not support a stage of final temperature increase following decompressional cooling. This argues against a late thermal pulse associated with mantle delamination. Preservation of growth zoning in both generations of garnet and the limited amount of diffusive re-equilibration at the boundary between the two garnets suggests that the rocks were subjected to fast burial and exhumation rates, consistent with data obtained from other internal Alpine units.     

The denudation history of the Argentera Alpine External Crystalline Massif (Western Alps,France-Italy): an overview from the analysis of fission tracks in apatites and zircons

Apatite/zircon fission track (FT) records of the Argentera external crystalline massif (Western Alps) show three tectonic pulses, respectively at 22 Ma (zircons), 6 and 3.5 Ma (apatites). The first pulse is consistent with the basement exhumation and initiation of the major deformation recorded in the foreland of the belt from Middle to early Upper Miocene. The two others might be respectively local expressions of the syncollisional extension mainly controlled by a westward sedimentary cover detachment and a Plio-Quaternary uplift acceleration. Zircon ages of 50-80 Ma in a limited NW area and evidence of an uplift elsewhere show that in a large fraction of the massif, temperatures in post-Variscan times never reached 320°C. Finally, FT data show that the Argentera massif did not behave as a single block during its denudation. First, in the NW of the massif, a small fault-limited block was already separated since the Cretaceous and later on recorded the 6 Ma denudation event, the 22 Ma pulse being recorded only in the remaining part of the massif. Second, less than 3.5 Ma ago, the northeastern part of the massif overthrust the southwestern block along the Bersézio-Veillos fault zone.     

Prograde high- to ultrahigh-pressure metamorphism and exhumation of oceanic sediments at Lago di Cignana, Zermatt-Saas Zone, western Alps

Pelagic metasediments and MORB-type metabasalts of the former Tethyan oceanic crust at Cignana, Valtournanche, Italy, experienced UHP metamorphism and subsequent exhumation during the Early to Late Tertiary. Maximum PT conditions attained during UHP metamorphism were 600–630 °C, 2.7–2.9 GPa, which resulted in the formation of coesite-glaucophane-eclogites in the basaltic layer and of garnet-dolomite-aragonite-lawsonite-coesite-phengite-bearing calc-schists and garnet-phengite-coesite-schists with variable amounts of epidote, talc, dolomite, Na-pyroxene and Na-amphibole in the overlying metasediments. During subduction the rocks followed a prograde HP/UHP path which in correspondance with the Jurassic age of the Tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating. Inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle-plastic transition in the quartz-aragonite-dominated rocks, induced convection in the asthenospheric mantle wedge and/or heat consumption by endothermic reactions over a restricted PT segment during subduction. After detachment from the downgoing slab some 50–70 Ma before present, the Cignana crustal slice was first exhumed to ca. 60 km and concomitantly cooled to ca. 550 °C, tracing back the UHP/HP prograde path displaced by 50–80 °C to higher temperatures. Exhumation at this stage is likely to have occurred in the Benioff zone, while the subduction of cool lithosphere was going on. Subsequently, the rocks were near-isothermally exhumed to ca. 30 km, followed by concomitant decompression and cooling to surface conditions (at < 500 °C, < 1 GPa). During this last stage the UHPM slice arrived at its present tectonic position with respect to the overlying greenschist-facies Combin zone. In contrast to the well-preserved HP/UHPM record of the coesite-glaucophane eclogites, the HP/UHP assemblages of the metasediments have been largely obliterated during exhumation. Relics from which the metamorphic evolution of the rocks during prograde HP metamorphism and the UHP stage can be retrieved are restricted to rigid low-diffusion minerals like garnet, dolomite, tourmaline and apatite.     

Western Alpine back-thrusting as subsidence mechanism in the Tertiary Piedmont Basin (Western Po Plain, NW Italy)

Basin formation dynamics of the Tertiary Piedmont Basin (TPB) are here investigated by means of cross-section numerical modelling. Previous works hypothesised that basin subsidence occurred due first to extension (Oligocene) and then to subsequent loading due to back-thrusting (Miocene). However, structural evidence shows that the TPB was mainly under contraction from Oligocene until post Pliocene time while extension played a minor role. Furthermore, thermal indicators strongly call for a cold (flexure-induced) mechanism but are strictly inconsistent with a hot (thermally induced) mechanism. Our new modelling shows that the TPB stratigraphic features can be reproduced by flexure of a visco-elastic plate loaded by back-thrusts active in the Western Alps in Oligo-Miocene times. Far-field compression contributed to the TPB subsidence and controlled the basin infill geometry by enhancing basin tilting, forebulge uplift and erosion of the southern margin of the basin. These results suggest that the TPB subsidence is the result of a combination of mechanisms including thrust loading and far-field compressional stresses.     

The P-T-d development at the basement-cover boundary in the north-eastern Tauern Window (Eastern Alps): Alpine continental collision

Abstract At the basement-cover boundary of the north-eastern Tauern Window (Eastern Alps), the following Alpine P-T-d development has been reconstructed on the basis of macro- and micro-structures as well as preferred crystallographic orientations, mineral parageneses and compositions.
During increasing P-T conditions in the greenschist facies a first period of deformation produced imbrication of the basement gneisses and cover sediments, and then monoclinal folds up to the kilometre scale. Tectonic transport was continuously top-to-the-ENE. A second period of deformation began at about peak P-T conditions of 9 kbar and c. 540–560°C in the south, and about 7–9 kbar and 490–500° C in the north; this continued locally to lower temperature. During the second period, transport was continuously top-to-the-SE. Crystallographic orientations of white mica and plagioclase give particularly useful information on the kinematic framework. In addition, data on the ductile behaviour of dolomite and plagioclase can be inferred. At c. 7–9 kbar, dolomite recrystallization starts at 450–480° C, and the beginning of plagioclase recrystallization coincides with the oligoclase boundary.
In general, the Alpine geodynamic history of the basement-cover boundary may be related to continental collision processes between a northerly plate (European or Briançonnais) and a southerly (Adriatic) one. The first deformation period possibly reflects subduction of the gneiss-sediment boundary toward the WSW, to a depth of 31–32 km. The second period may be a result of obduction toward the NW, followed by late-stage uplift. Most of the basement domes of the eastern Tauern Window appear as a result of the final stage of the first deformation, formed prior to the peak of metamorphism, possibly partly influenced by the final collision between the northern and the southern continents.     

Eclogitic metamorphism in carbonate rocks: the example of impure marbles from the Sesia-Lanzo Zone,Italian Western Alps1

The impure marbles of the internal Sesia-Lanzo Zone underwent a multi-stage metamorphic evolution of Alpine age and retain early-Alpine eclogitic assemblages, partially recrystallized under blueschist to greenschist facies conditions. These high-P assemblages consist of carbonates, phengite, quartz, omphacite, grossular-rich (locally spessartinic) garnet, zoisite and Al-rich titanite. Retrogressive stages are characterized by the growth of glaucophane, paragonite, phlogopite, tremolite and albite. Halogen-rich biotite and amphibole are also present. P-T estimates of the early-Alpine metamophism have been calculated from these unique high-P assemblages, in order to test the applicability of some calibrations to impure carbonate systems. In particular, some Gt-Cpx calibrations and the phengite geobarometer give results (T= 575 ± 45° C at 15 kbar for the eclogitic climax and T≤ 500° C at PH₂O ≤ 9 kbar for early-Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies. Phase relationships in P-T-XCO₂ space indicate that mineral assemblages in the impure marbles coexisted with H₂O-rich fluids (XCO₂ <0.03) during their entire Alpine evolution.     

Faulted backfold versus reactivated backthrust: the role of inherited structures during late extension in the frontal Piémont nappes east of Pelvoux (Western Alps)

In the central part of the internal Western Alps, widespread multidirectional normal faulting resulted in an orogen-scale radial extension during the Neogene. We revisit the frontal Piémont units, between Doire and Ubaye, where contrasting lithologies allow analysing the interference with the N–S trending Oligocene compressive structures. A major extensional structure is the orogen-perpendicular Chenaillet graben, whose development was guided by an E–W trending transfer fault zone between the Chaberton backfold to the north and the Rochebrune backthrust to the south. The Chaberton hinge zone was passively crosscut by planar normal faults, resulting in a E–W trending step-type structure. Within the Rochebrune nappe, E–W trending listric normal faults bound tilted blocks that slipped northward along the basal backthrust surface reactivated as an extensional detachment. Gravity-driven gliding is suggested by the general northward tilting of the structure in relation with the collapse of the Chenaillet graben. The stress tensors computed from brittle deformation analysis confirm the predominance of orogen-parallel extension in the entire frontal Piémont zone. This can be compared with the nearby Briançonnnais nappe stack where the extensional reactivation of thrust surfaces locally resulted in prominent orogen-perpendicular extension. Such a contrasting situation illustrates how the main direction of the late-Alpine extension may be regionally governed by the nature and orientation of the pre-existing structures inherited from the main collision stage.     

Application of K-feldspar–jadeite–quartz barometry to eclogite facies metagranites and metapelites in the Sesia Lanzo Zone (Western Alps, Italy)*

The eclogite facies assemblage K-feldspar–jadeite–quartz in metagranites and metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the equilibration pressure by dilution of the reaction jadeite+quartz=albite. The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm₆₃Prp₂₆Grs₁₀)–K-feldspar–plagioclase–biotite±sillimanite to the Eo-Alpine high-pressure assemblage garnet (Alm₅₀Prp₁₄Grs₃₅)–jadeite (Jd_80–97Di_0–4Hd_0–8Acm_0–7)–zoisite–phengite. Plagioclase is replaced by jadeite–zoisite–kyanite–K-feldspar–quartz, and biotite is replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K-feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calculations on the stability of the assemblage K-feldspar–jadeite–quartz using available mixing data for K-feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high-pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O/P_T. The inferred limiting a(H₂O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.     

西秦岭-松潘构造结古洋地幔地球化学特征及构造归属探讨

西秦岭-松潘构造结是中国大陆中重要的构造转换域,为探讨不同陆块及古洋幔的构造归属提供了非常有利的条件。该构造结早古生代蛇绿岩单元内变质玄武岩的地球化学特征表明:其源区不具有高场强元素(HFSE)相对于大离子亲石元素(LILE)亏损的特征;稀土元素(REE)及其它不活泼元素特征表明蛇绿岩形成于与岛弧无关的环境。变质玄武岩的 Sr 同位素组成受到海水热液蚀变或后期变质作用的影响而有所改变。Nd-Pb 同位素组成特征表明其地幔源区具有典型的印度洋MORB 型同位素组成特征(传统意义上的"Dupal"异常),从而说明其地幔源区属于原特提斯构造域。西秦岭-松潘构造结古洋幔构造归属的研究不仅为研究该构造结内各微陆块构造归属提供重要的约束,也为研究有关陆块间造山带的性质、发展与演化提供了动力学背景。