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.     

Fluid migration through thrust faults in the Helvetic nappes (Western Swiss Alps)

The δ¹⁸O, δ¹³C and ⁸⁷Sr/⁸⁶Sr values of calcite and organic matter were measured for carbonate mylonites from numerous thrusts in the Helvetic Alps. Carbonate mylonites in most of the thrusts retain essentially unaltered protolith δ¹⁸O and δ¹³C values, consistent with there having been little to no advection of isotopically distinct fluid through these faults. Only carbonate mylonites from the basal thrusts of the largest nappes have δ¹⁸O and/or δ¹³C values that differ from those of their protoliths. The zone of isotopic alteration/exchange is confined to c. 10 to 20 meters of these fault contacts. We propose the fluids that migrated through these faults contained variable amounts of organically derived carbon and radiogenic strontium, and were probably derived from dewatering of the sedimentary rocks and prograde metamorphic reactions in the nappes' root zones. Apart from the basal thrusts of the largest nappes that behaved as narrow, laterally extensive conduits for fluids, there is little isotopic evidence that large quantities of fluids passed through most of the carbonate-hosted thrusts in the Helvetic Alps. Received: 25 August 1998 / Accepted: 26 February 1999     

Transversal and oblique structures at the Serres de Llevant thrust belt (Mallorca Island)

The Serres de Llevant thrust-and-fold segment outcrops along the SE coast of Mallorca (Balearic Isles, Spain) and represents the eastern continuation of the external zones of the Alpine Betic Range. The structure of the Serres de Llevant consists mainly of northwest-directed thrusts which involve a sedimentary series ranging in age from Upper Triassic to Middle Miocene. The thrusts form a thrust system developed both by piggy-back and out-of-sequence thrusting. Thrust sheets are imbricated to the SE and to the SW and, in the northern part, also to the NE, giving rise to a complex 3-D pattern. The thrusts and related folds developed during the Oligocene and the Lower and Middle Miocene. The thrust transport direction has been inferred from the stratigraphic facing directions of all mapped ramps, and has been determined with the aid of other transversal structures. The resulting transport direction is from SE to NW (N310E). One of the most significant structural features of the Serres de Llevant thrust segment is the existence of several lateral and oblique structures. These structures are ramps which display stratigraphic facing directions both to the NE and SW. Other lateral structures represented in the northern part of the thrust segment are tear and drop faults. In the southern part, kilometric-scale oblique folds are exposed.

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Zusammenfassung Der Serres de Llevant überschiebungs- und Faltenbereich ist an der südöstlichen Küste Mallorcas (Ballearen, Spanien) aufgeschlossen. Er stellt die östliche Fortsetzung der Externzone der betischen Gebirgskette dar. Die Struktur des Serres de Llevant besteht im wesentlichen aus einem NW gerichteten Deckenbau. Das Alter der betroffenen Sedimentserien umfa\t Obere Trias bis Mittleres Miozän.Das überschiebungssystem ist aus »piggy-back« und »out-of-sequence« überschiebungen entstanden. Die Dekken greifen dachziegelartig übereinander, so da\ nach SE und SW höhere strukturelle Einheiten zu finden sind. Die dreidimensionale Geometrie dieses Deckenbaus ist im nordöstlichen Teil der Ketten besonders komplex.Die überschiebungen und die zugehörigen Fakten entwickelten sich während des Oligozäns und des Unteren und Mittleren Miozäns. Die Transportrichtung ist von der Polarität aller kartierten Rampen abgeleitet worden und mit Hilfe anderer Querstrukturen genauer bestimmt worden. Die so bestimmte Transportrichtung ist N310E.Einer der bedeutendsten strukturellen Züge des überschiebungssystemes der Serres de Llevant ist das Vorhandensein von Seiten- und Schrägstrukturen, vor allem NE und SW gerichteten Rampen. Im nördlichen Teil der Ketten kann man »tear«- und »drop«-Störungen beobachten. Im südlichen Teil sind kilometergro\e Schrägfalten aufgeschlossen.

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Résumé Les Serres de Llevant occupent la partie sud-orientale de Majorque (Îles Baléares, Espagne); elles font partie d'une ceinture de plis et chevauchements, et représentent la continuation vers le NE des zones externes de la ChaÎne Bétique. La structure des Serres de Llevant est caractérisée par des chevauchements dirigés vers le NW, affectant des matériaux sédimentaires d'âges compris entre le Trias supérieur et le Miocène moyen. Les chevauchements forment un système de failles à déplacement à la fois vers l'avant-pays et vers l'arrière-pays. Les lames sont empilées les unes sur les autres vers le SE et vers le SW; cependant, dans la partie septentrionale cet empilement s'opère aussi vers le NE, donnant lieu à une disposition tridimensionnelle assez complexe. Les chevauchements et les plis associés se sont développés pendant l'Oligocène et le Miocène inférieur et moyen. La direction de transport a été déduite de la polarité stratigraphique de l'ensemble des rampes cartographiées, et a été précisée à l'aide d'autres structures transverses. La direction de transport est du SE vers le NW (N 310 E). Une des caractéristiques la plus significative des Serres de Llevant est la présence de plusieurs structures latérales et obliques avec polarité stratigraphique vers le NE ou vers le SW D'autres structures latérales — failles décrochantes (tear faults) et failles normales (drop faults) - s'observent dans la partie septentrionale. La partie méridionale comporte des plis légèrement obliques qui atteignent des dimensions kilométriques.

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der Serres de Llevant - / , /. . -- - . . «piggy-back» «out-of-sequence». , - . - . . , . , . N 31 . -- , - . « » « » . , .

     

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.     

Inherited structures in the hangingwall of the Valsugana Overthrust (Southern Alps,Northern Italy)

In the hangingwall of the Valsugana Line (north side), the most important Neogene overthrust of the Venetian Alps, two major palaeostructural pre-thrusting elements are recognizable, the Atesina Platform to the west and the Carnico-Bellunese Basin to the east. These palaeostructures, which are oblique to the overthrust, are demonstrated by variations in thickness of the sedimentary cover. The present structural framework of the hangingwall of the Valsugana Line, with the crystalline basement more uplifted toward the west, is genetically related to these pre-thrusting Permo-Mesozoic structural elements. The apparent variation in throw along the Valsugana Line is mainly a result of these palaeostructural geometries, and was not, for example, produced by a lateral hangingwall ramp.     

Rotation velocity of a thrust: a paleomagnetic study in the External Sierras (Southern Pyrenees)

An accurate calibration of chronologic and geologic data (biostratigraphic and magnetostratigraphic) with GPTS has been performed for the middle and upper Eocene syntectonic deposits in the surrounding areas of the Pico del Aguila anticline (South Pyrenean External Sierras).This calibration shows a variable sedimentation rate that changes from 4.8 cm/ka at the bottom to 34 cm/ka at the top, with a maximum of 58.6 cm/ka in the upper part of the studied stratigraphic section, and gives a fine and continuous chronologic frame to understand the rotational kinematics of the infrajacent thrust system.At the same time, a detailed paleomagnetic study has been carried out (14 sites and a total of 157 thermally demagnetised samples) to know the amount of rotation in the area taking advantage the “tape-recording” effect of the syntectonic materials. These samples display a ChRM between 250 and 400 °C, both polarities (that agree very well with the sequence of magnetostratigraphic zones) and consistency with the magnetic reference. Due to the decreasing value of the magnetic declination (from +46° to −3°) of the primary components (checked by fold and reversal test), the calibration has been useful to assign an absolute age to every rotation value (from 40.32 to 37.05 Ma).A time versus rotation curve does not show significant differences of rotation between the flanks of the Pico del Aguila anticline (less than 8°). Thus, the clockwise rotations detected in the study area result from the kinematics of the imbricate basal thrust system, which is also responsible for the development of detachment folds (e.g. Pico del Aguila anticline). In a time against rotation graph, the simplest fit for the overall rotation values is given by a linear segment (rotation velocity of 10°/Ma); however, the best fit is given by a segment of a parabolic curve. This pattern implies that there were changes of the rotation velocities (between 21°/Ma and −5.3°/Ma) that in this segment of the External Sierras were caused by an acceleration of the rotational movement (between 9°/Ma² and 5.5°/Ma²). The thrust sheet shows maximum values of at least 40° of clockwise rotation.     

Deformation mechanisms in some cover thrust sheets from the external French Alps

Rocks from two parts of the Ultradauphinois Zone of the external French Alps have been examined, and the mechanisms by which they were deformed have been assessed from petrographic data. Jurassic and Triassic limestones deformed by pressure solution, dissolving non-ferroan-calcite and precipitating ferroan-calcite. Calcite pressure shadows are usually less elliptical in shape than pyrite pressure shadows, and grain boundary sliding is therefore thought to have played a significant role. Eocene rocks deformed by a variety of mechanisms. Limestones show mylonitic textures, whereas limestone conglomerates with a quartz-sandstone matrix deformed by pressure solution of calcite and grain boundary sliding of quartz. A model for enhanced diffusion of silica along mica seams is proposed to account for planar quartz-mica boundaries. Greywackes deformed by incongruent pressure solution, involving the metamorphic reaction of feldspar to mica and quartz, coupled with the replacement of feldspar by calcite.     

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.     

Evolution of palaeofluids at the Variscan thrust front in eastern Belgium

The geochemical evolution of the fluids migra- ting at the Variscan thrust front in eastern Belgium has been investigated by a petrographic, mineralogical and geoche-mical study of ankerite, quartz and ferroan calcite veins hosted by lower Devonian rocks. Three vein generations have been recognized. The first generation consists of quartz, chlorite and ankerite filling pre- to early Variscan extensional fractures. The second generation is present as shear veins of Variscan age, and contains quartz, chlorite and ferroan calcite. The third generation consists of ankerite filling post-Variscan fractures. The oxygen and carbon isotopic composition of the two ankerite phases and of the ferroan calcites are respectively between –16.4 and –11.4‰ PDB between –17.8 and –1.7‰ PDB. This range is greater than that of calcite nodules in the lower Devonian siliciclastic sediments (δ¹⁸O= –15.6 to –11.1‰ PDB and δ¹³C= –13.4 to –10.2‰ PDB). This suggests precipitation of the carbonate veins from a fluid which was at most only partly isotopically buffered by the calcite nodules in the host rock. The calculated oxygen isotopic composition of the ambient fluid from which the calcite veins formed is between +7.8 and +10.0‰ SMOW. Two main fluid types have been recognized in fluid inclusions in the quartz and carbonates. The first fluid type is present as secondary fluid inclusions in the first and second vein generations. The fluid has a salinity of 0.5–7.2 eq. wt.% NaCl and a high, but variable, homogenization temperature (Th=124–188°C). Two origins can be proposed for this fluid. It could have been expelled from the lower Devonian or could have been derived from the metamorphic zone to the south of the area studied. Taking into account the microthermometric and stable-isotope data, and the regional geological setting, the fluid most likely originated from metamorphic rocks and interacted with the lower Devonian along its migration path. This is in agreement with numerical simulations of the palaeofluid and especially the palaeotempera-ture field, which is based on chlorite geothermometry and vitrinite reflectance data. The second fluid type occurs as secondary inclusions in the shear veins and as fluid inclusions of unknown origin in post-Variscan ankerite veins. Therefore, it has a post-Variscan age. The inclusions are characterized by a high salinity (18.6–22.9 eq. wt.% CaCl₂). The composition of the fluid is similar to that which caused the development of Mississippi Valley-type Pb–Zn deposits in Belgium.     

Exhumed fault-bounded Alpine blocks along the Periadriatic lineament: the Eder unit (Carnic Alps, Austria)

The Eder unit in the Carnic Alps, which is situated immediately south of the Periadriatic lineament (PL), represents a fault-bounded block consisting of a low-grade (up to 400?°C, indicated by epizonal illite “crystallinity” values, recrystallized quartz, and non-recrystallized white mica) metamorphic Paleozoic metasedimentary sequence. Until now, it has been assumed to represent a separate Variscan nappe. The rocks of the Eder unit show a strong E- to W-oriented stretching lineation on steep foliation planes (D₁) subparallel to the PL. D₁ structures originated near the temperature peak of metamorphism, and shear sense indicators show dextral ductile shear parallel to the PL. Tight mesoscale D₂ folds formed on the cooling path. K–Ar and Ar–Ar ages from newly formed white mica cluster around 32–28 and 18–13 Ma and suggest a two-stage Tertiary history of the Eder unit. We interpret the Eder unit as a fault-bounded block formed during Oligocene large-scale dextral shearing along the PL (near T_max) and exhumed in mid-Miocene times during another phase of activity along the PL. Its nature as a separate Variscan nappe is questioned.     

Flexural response of the Venetian foreland to the Southalpine tectonics along the TRANSALP profile

The Venetian Basin was affected by flexure related to the Southalpine shortening phase during the Middle Miocene – Early Pliocene. This downbending is quantified here using a two‐dimensional flexural model. A recently improved data set on basin geometry based on the bottom of the Serravallian–Tortonian clastic wedge, on palaeobathymetry and gravity anomalies is used to constrain the components of flexure and to test the importance of the initial bathymetry in evaluating the contribution of surface loads to deflection. A good fit is obtained assuming a northward broken plate configuration of the downbent Adriatic plate with an effective elastic thickness of 20 km. Results highlight that, in the studied region, flexure related to the Eastern Southern Alps is totally due to surface loads (topographic load partly replacing initial bathymetry) and that no hidden loads are required. Furthermore, the palaeobathymetry contributes up to 50% to the total flexure in the studied region.     

Post-collisional tectonics in the Northern Cottian Alps (Italian Western Alps)

Field mapping and structural analysis have allowed us to characterise the fault geometry and the post-metamorphic tectonics of an area located in the Northern Cottian Alps (inner Western Alps). Two main faulting stages were distinguished here. The first (Oligocene?-Early Miocene) is related to the development of an E–W-striking left-normal shear zone. This shear zone is interpreted as an antithetical of two regional, N–S right-lateral structures: the Col del Lis-Trana Deformation Zone (LTZ) and the Colle delle Finestre Deformation Zone (CFZ). The second faulting stage (post-Early Miocene) is related mainly to the development of N–S normal faults, coeval with the extensional reactivation of the LTZ and the CFZ. We discuss this kinematic evolution in the framework of the geodynamic evolution of the Western Alps.     

The Caledonian thrust front and palinspastic restorations in the southern Norwegian Caledonides

The Osen-Røa thrust sheet of the southern Norwegian Caledonides comprises the coarse clastic late Precambrian Sparagmite region, and the folded and imbricated Cambro-Silurian rocks of the Oslo region. Ramp-flat geometries occur in the hangingwall of the Osen-Røa thrust in the Mjøsa district. Two major ramps are recognized. One coincides with the strike of the Ringsaker inversion, while the other coincides with the traditional thrust front in the Gjøvik area. The Osen-Røa thrust cuts up section in the transport direction (south), eventually cutting out all late Precambrian rocks, to lie as a 150 km long flat in the Cambrian Alum shales of the Oslo region. The now eroded detachment termination probably died out horizontally in the Alum shales to end as a buried thrust front in the southern Oslo region. Restoration of hanging- and footwall cutoffs allows the amount of overthrusting to be calculated; the Sparagmite region/Oslo region boundary restores to a minimum of 130 km to the NNW. This displacement estimate agrees with estimates of 135 km NNW transport calculated from balanced cross-section restorations through the Oslo region.     

冲断构造与正反转构造物理模拟实验的研究进展

冲断构造和正反转构造是我国西部叠合盆地中典型的构造样式,其构造物理模拟实验是研究和模拟自然界冲断构造（正反转构造）变形特征和动力学过程的一种有效的实验方法。在查阅大量相关资料的基础上,本文概述了物理模拟实验的发展历史,以及国内外在实验理论、实验技术和从二维到三维转变等方面的研究现状。目前,构造物理模拟实验已被广泛应用于构造地质学、石油构造地质学等众多研究领域,是油气勘探研究由定性描述跨入半定量-定量分析的有效途径之一。本文分别阐述了冲断构造和正反转构造在近年来取得的进展：1）在冲断构造物理模拟实验方面,介绍了双指向冲断构造和推覆体、地表作用对冲断构造的影响以及冲断构造中地层缩短量和应变等3方面的进展;2）在正反转构造物理模拟实验方面主要讲述了基底对反转构造演化型式的制约以及反转临界条件的三维构造模拟取得的进展。同时,认为物理模拟在未来的发展过程中应紧密结合数值模拟、高精度成像技术和数据采集技术等。作为研究构造变形机制的重要媒介,冲断构造物理模拟在塑性变形对冲断构造的影响、如何反映现代构造地质学成果以及模拟过程中如何加入化学物质迁移等问题方面仍存在不足。此外,高端实验室的建设和模拟技术也是构造物理模拟实验面临的一个问题。     

北大巴山逆冲推覆构造带前缘构造特征及变形年代学研究

在北大巴山推覆构造带前缘,北—北东倾斜的城口断裂和南—南西倾斜的高桥断裂形成一背冲式逆冲推覆体系,以两断层为界,其间为一巨型冲起构造。该冲起构造因边界断裂的产状特征、活动强度沿走向的差异,而表现出明显的分段性:其北西段为双向逆冲的"正花状"结构;而南东段为反向逆冲为主的"半花状"结构。同构造低温变质绢云母40Ar-39Ar年龄显示,城口断裂在燕山期(约143.3Ma)存在一次强烈的构造活动,导致北大巴构造带进一步逆冲扩展和强烈隆升。在逆冲-推覆向前扩展过程中,受锋缘外侧阻抗的影响,沿前缘部位形成一系列反向逆冲断层,从而形成巨型冲起构造。     

Fluid evolution at the Variscan front in the vicinity of the Aachen thrust

Quartz–carbonate–chlorite veins were studied in borehole samples of the RWTH-1 well in Aachen. Veins formed in Devonian rocks in the footwall of the Aachen thrust during Variscan deformation and associated fluid flow. Primary fluid inclusions indicate subsolvus unmixing of a homogenous H₂O–CO₂–CH₄–(N₂)–Na–(K)–Cl fluid into a H₂O–Na–(K)–Cl solution and a vapour-rich CO₂–(H₂O, CH₄, N₂) fluid. The aqueous end-member composition resembles that of metamorphic fluids of the Variscan front zone with salinities ranging from 4 to 7% NaCl equiv. and maximum homogenisation temperatures of close to 400°C. Pressure estimates indicate a burial depth between 4,500 and 8,000 m at geothermal gradients between 50 and 75°C/26 MPa, but pressure decrease to sublithostatic conditions is also indicated, probably as a consequence of fracture opening during episodic seismic activity. A second fluid system, mainly preserved in pseudo-secondary and secondary fluid inclusions, is characterised by fluid temperatures between 200 and 250°C and salinities of <5% NaCl equiv. Bulk stable isotope analyses of fluids released from vein quartz, calcite, and dolomite by decrepitation yielded δD_H2O values from −89 to −113 ‰, δ¹³C_CH4 from −26.9 to −28.9‰ (VPDB) and δ¹³C_CO2 from −12.8 to −23.3‰ (VPDB). The low δD and δ¹³C range of the fluids is considered to be due to interaction with cracked hydrocarbons. The second fluid influx caused partial isotope exchange and disequilibrium. It is envisaged that an initial short lived flux of hot metamorphic fluids expelled from the epizonal metamorphic domains of the Stavelot–Venn massif. The metamorphic fluid was focused along major thrust faults of the Variscan front zone such as the Aachen thrust. A second fluid influx was introduced from formation waters in the footwall of the Aachen thrust as a consequence of progressive deformation. Mixing of the cooler and lower salinity formation water with the hot metamorphic fluid during episodic fluid trapping resulted in an evolving range of physicochemical fluid inclusion characteristics.     

Three steps of serpentinization in an eclogitized oceanic serpentinization front (Lanzo Massif – Western Alps)

The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean–continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta‐ophicarbonates bordering the foliated serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P–T (estimated in eclogitized metagabbros at 2–2.5 GPa and 550–620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into serpentinite during alpine subduction and collision. Thus, the serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo‐Moho.