Penninic Frontal Thrust Research Articles

Extensional reactivation of the Penninic frontal thrust 3 Myr ago as evidenced by U–Pb dating on calcite in fault zone cataclasite

Abstract. In the Western Alps, the Penninic frontal thrust (PFT) is the main crustal-scale tectonic structure of the belt. This thrust transported the high-pressure metamorphosed internal units over the non-metamorphosed European margin during the Oligocene (34–29 Ma). Following the propagation of the compression toward the European foreland, the PFT was later reactivated as an extensional detachment associated with the development of the High Durance extensional fault system (HDFS). This inversion of tectonic displacement along a major tectonic structure has been widely emphasized as an example of extensional collapse of a thickened collisional orogen. However, the inception age of the extensional inversion remains unconstrained. Here, for the first time, we provide chronological constraints on the extensional motion of an exhumed zone of the PFT by applying U–Pb dating on secondary calcites from a fault zone cataclasite. The calcite cement and veins of the cataclasite formed after the main fault slip event, at 3.6 ± 0.4–3.4 ± 0.6 Ma. Cross-cutting calcite veins featuring the last fault activity are dated at 2.6 ± 0.3–2.3 ± 0.3 Ma. δ13C and δ18O fluid signatures derived from these secondary calcites suggest fluid percolation from deep-seated reservoir at the scale of the Western Alps. Our data provide evidence that the PFT extensional reactivation initiated at least ∼ 3.5 Myr ago with a reactivation phase at ∼ 2.5 Ma. This reactivation may result from the westward propagation of the compressional deformation toward the external Alps, combined with the exhumation of external crystalline massifs. In this context, the exhumation of the dated normal faults is linked to the eastward translation of the HDFS seismogenic zone, in agreement with the present-day seismic activity.

Rock slope failure in the Western Alps: A first comprehensive inventory and spatial analysis

Rock slope failure (RSF) occurs in different contexts but is typically reported either as (i) single-category inventories or (ii) single-site geotechnical monographs. Few studies have sought to evaluate the spatial incidence of all modes of RSF conjointly, and to infer scenarios of regional landscape evolution from observed patterns of cumulative rock slope overstressing. Here we present the results of a systematic inventory of rock avalanches, rockfalls, rockslides, and gravitational rock slope deformations in the Western Alps (France, Italy, Switzerland) conducted using satellite imagery made available in Google Earth as a detection tool, and aided by preliminary ground-truth checks. The inventory totals 1416 montane RSFs, impacting 9.1% of the study area. Underpinned by GIS tools, the study further examines the spatial distribution of RSF with consideration for (i) predisposing factors (typically: lithology, geological structure), and (ii) preparatory factors (geomorphological process regimes that drives a given slope segment to the point of failure). The latter encompass slower variables (e.g., long-term crustal stress regime, cumulative residence time above equilibrium line altitudes) and faster variables (e.g., short-span glacier-related stresses, permafrost thaw, seismicity). RSF density patterns helped to define seven RSF super-hotspots (large diversity of RSF modes, up to 50% of displaced rock masses/unit area), which define the most intensely overstressed areas of the Western Alps. These super-hotspots occur at sites where highly dynamic, thick, warm-based glaciers above the equilibrium line either intersected (middle Maurienne) or followed the strike of (middle Isère) N-S bands of highly susceptible lithologies and structures during the Quaternary. The widespread incidence of rock slope deformation (cumulative area: 1760 km2, i.e., nearly 3 times the total of the other three RSF categories combined) appears further correlated with the low tectonic activity of the orogen and with its areas dominated by an extensional tectonic regime west of the Penninic Frontal Thrust. This contrasts with seismically active orogens, e.g., New Zealand's Southern Alps, where rock slope deformation is scarce compared to rock avalanches and shallow landslides.

Shortening of the European Dauphinois margin (Oisans Massif, Western Alps): New insights from RSCM maximum temperature estimates and 40Ar/39Ar in situ dating

The Oisans Massif, located in the external zones of the western Alps, experienced significant shortening during the Alpine collision. While a series of major top-to-the west shear zones was recently described, the general low grade of the metamorphism has not attracted much petrological and geochronological studies. This paper provides combined temperature and age constraints on the evolution of the Oisans Massif. Temperature was estimated with the Raman Spectrometry of Carbonaceous Material (RSCM) method and chlorite geothermometry. Maximum temperature reached by the Mesozoic cover (Tmax) from Grenoble to the Galibier pass (E-W) and from Saint Jean de Maurienne to Embrun (N-S) yielded almost constant ca. 330°C temperatures all through the Massif. Temperatures however strongly decrease either westward towards the top of the Vercors sedimentary sequence or eastward towards the Penninic Frontal Thrust. Age constraints were retrieved using 40Ar/39Ar in situ analyses performed on variously strained samples from Alpine shear zones. Over strain gradients, incipient Alpine recrystallizations progressively develop at the expense of former Variscan parageneses. A combined textural and EPMA approach permitted to identify newly formed chlorite and phengite that unequivocally grew in response to deformation or to low grade metamorphism. Chlorites recorded temperatures from ca. 350–150°C during the activity of shear zones. In parallel, 40Ar/39Ar in situ experiments enabled dating deformation using both synkinematic phengites crystallized below the closure temperature of white-micas and former Variscan muscovite whose isotopic system have been, at least partially re-opened. Activity of top-to-the-west shear zones responsible for the shortening and thickening of the Oisans massif thus occurred between 34 and 33 and 25Ma. Integrating these new age-constraints, Tmax estimates, and published geological data on the Oisans Massif and neighbouring areas allow proposing a new shortening scenario for the external zones. From ca. 34–33Ma, the Oisans Massif was buried as a rigid block below more internal units and reached a temperature of ca. 330°C. Shearing progressively localized along a series of top-to-the-W shear zones in the basement until 25Ma. Deformation then localized along a major thrust at the base of the ECM massif and propagated along the basement-cover interface below the Vercors massif after 16Ma.

Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt (SW Alps, France): Insights from balanced cross-sections

This study proposes a reappraisal of the role of the basement tectonics in the structuration of the Alpine foreland, across the Castellane fold-and-thrust belt located in southwestern Alps. We construct three 30km length N–S balanced cross-sections across the entire fold-and-thrust belt, in order to quantify the amount of horizontal shortening due to the Pyrenean and Alpine deformations. We then assess the role of the basement inherited structures during the compressional phases which resulted in the exhumation of the Argentera–Mercantour External Crystalline Massif and the Barrot Dome. The construction of these balanced cross-sections suggests a dominant thick-skinned deformation style, which includes the reactivation of inherited Permian and Early Cretaceous basement normal faults. We reconstructed three restoration stages: the oldest one highlights the geometry of Permian and Early Cretaceous extensional structures; the intermediate one after the Late Cretaceous evidences a first compressional episode linked to the Pyrenean compression; and the latest one in the Oligocene shows the first Alpine folds resulting from the southward thrusting of the internal units over the external ones along of the Penninic Frontal Thrust. Balanced cross-sections suggest very moderate crystalline basement–sedimentary cover decoupling. On the western and central cross-sections, the estimated amount of shortening ranges from 9.5 to 10km (21%) whereas on the easternmost one shows ~5km of shortening (9%). These shortening values are consistent with previously published estimates in the surrounding foreland subalpine chains. They highlight a decreasing value of Pyrenean shortening toward the east, while the Alpine shortening dominates and amplifies this first phase in a similar direction. We interpret this dominantly thick-skinned structural style as a possible consequence of the Neogene thermal weakening in the European passive margin above the Ligurian slab rollback.

Basement shear zones development and shortening kinematics in the Ecrins Massif, Western Alps

AbstractIn the Western Alps, Oligocene shortening affected a highly heterogeneous European crust with Liassic half‐grabens inherited from the rifting stage and the finite deformation was strongly partitioned between the rigid basement and the weak Jurassic sediments. In the Ecrins Massif (Oisans, external crystalline massifs), where the half‐grabens are best exposed and preserved, compressional structures within the basement have to date never been described in details. This massif was shortened under moderate metamorphic conditions (250–350°C and 0.1–0.5 GPa), and the rheological contrast between the basement and the cover is strong. While the sediments are intensely folded, the cover‐basement interface presents apparent open folds underlined by the Lower Triassic layers. The basement itself shows a more localized deformation along several brittle‐ductile shear zones. We here report new evidences of such brittle‐ductile shear zones characterized by anastomosed phyllonitic shear bands rich in phengite and quartz, a low‐strength material where strain has localized. New detailed maps of reverse shear zones, faults, schistosity, and stretching lineations in both the cover and the basement are provided. We show that the Oligocene crustal shortening was mainly E‐W to ENE‐WSW. Local N‐S to NW‐SE shortening occurred and was limited to the eastern border of the Ecrins Massif, around the Penninic Frontal Thrust, which likely was a sinistral transpressive structure in this area. Finally, new balanced cross sections show that these basement shear zones have accommodated more than 50% of the Oligocene crustal shortening.

A multidisciplinary approach to the study of the fault networkin the internal Cottian Alps (Western Alps)

The architecture of the fault network and the late- to post-metamorphic structural evolution in the internal Cottian Alps have been studied by an integrated approach combining morphostructural analysis, field mapping and detailed mesostructural analysis between the Susa and Pellice Valleys. Available apatite fission track data indicate that this sector of the Western Alps reached shallow crustal levels, where brittle deformation mechanisms prevail, by Late Oligocene times. The analysed area comprises two main N-S striking regional, polyphasic semi-brittle to brittle deformation zones: the Colle delle Finestre Fault Zone (CFFZ) and the Col del Lis-Trana Deformation Zone (LTZ). The activity of those long-lived structures evolved, respectively, from dextral and dextral-reverse to extensional. In the block bounded by these two regional structures, the late- to post-metamorphic tectonic evolution is characterized by two faulting stages: (a) the development of an E-W left-lateral fault zone with a minor normal component of movement; at the regional scale, this fault zone may be considered as an antithetical shear of the major LTZ and the CFFZ; (b) the extensional reactivation and development of N-S normal faults, coeval with those recorded both by the LTZ and the CFFZ. The geometry of the fault and fracture systems has been compared with the remote sensed lineaments pattern, characterized by four main systems: Ln1 (N0°-30°E), Ln2 (N45°-70°E), Ln3 (N80°-100°E) and Ln4 (N100°-120°E). The lineament systems display similarities in terms of orientation, geometry and length with the fault systems observed at different scales. These similarities allowed us to extrapolate the structural model outlined in the mapped area to the adjoining sectors. Comparison between the different data suggests that the pattern of the lineaments detected between the LTZ and the CFFZ agrees with the architecture of a strike-slip fault network, which accommodated the internal deformation of the block bounded by these two major N-S transcurrent deformation zones. The transcurrent activity of these faults is here interpreted as coeval with the late dextral movements along the Canavese Line and with the propagation, in the Late Oligocene-Early Miocene, of the roughly E-vergent South-Alpine Thrusts beneath the Tertiary Piemonte Basin. The extensional event that followed may be related to the uplift with the subsequent isostatic readjustment of the chain which may have also induced the normal reactivation of the Penninic Frontal Thrust. This extensional regime is also in agreement with focal mechanisms of recent earthquakes.

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

AbstractDirect 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.

Tectonic control on diagenesis in a foreland basin: combined petrologic and thermochronologic approaches in the Grès d’Annot basin (Late Eocene–Early Oligocene, French–Italian external Alps)

AbstractCombined petrographic study and apatite fission track analysis (apatite FTA) across the Grès d’Annot basin document maximum temperatures reached by this turbiditic sandstone formation, from around 60 °C at the SW basin margin up to around 200 °C below the Penninic frontal thrust, in relation to burial below the front of Alpine nappes. Increasing diagenetic grade across the basin is primarily expressed by the development of silica transfer by pressure solution and correlated porosity loss. We infer the nappe thickness profile, from the nappe front in the SW up to around 8–10 km in the NE. Apatite FTA dates exhumation of the Grès d’Annot during the Late Miocene, in relation to thrusting of the underlying Argentera and Barrot Palaeozoic massifs. This study illustrates the influence of thrust front propagation on foreland basin diagenesis and exhumation, and highlights potential implications for the evolution of sediment reservoir properties in this context.

Diachronous late-stage exhumation across the western Alpine arc: constraints from apatite fission-track thermochronology between the Pelvoux and Dora-Maira Massifs

We present new apatite fission-track (AFT) data from the central western Alps that confirm the synchronicity and high cooling rates during Latest Miocene–Pliocene final exhumation of the External Crystalline Massifs but also provide evidence for diachronous Neogene evolution along and across the internal arc. To the SE of the Pelvoux Massif, across the front of the internal arc (Penninic Frontal Thrust), the jump in AFT ages ( c . 22 Ma) and in final cooling rates is significantly larger than further north. This difference results from reversal of movement along a major Oligocene thrust. In its hanging wall, the western Briançonnais Zone provides a mean AFT age of c . 27 Ma, which is older than further north. Early cooling in the southern Briançonnais Zone would result from rapid erosion of the compressional fan structure built during the Oligocene. Across the entire Briançonnais and Piémont nappe stack, with the exception of the Dora-Maira Massif, AFT ages young eastward and span the entire Miocene, a period during which this structure underwent extension. Further north a reverse gradient with ages younging northwestward has been described, prompting the question of the asymmetry of the internal western Alpine arc during its late-stage tectonic and morphological evolution.

A crustal‐scale cross‐section of the south‐western Alps combining geophysical and geological imagery

AbstractA geotransect across the south‐western Alps from the Pelvoux Massif (external French Alps) to the Dora‐Maira massif (internal Italian Alps), through the Monviso ophiolitic complex, was investigated in the framework of the ‘Géo‐France 3D Alpes’ programme. A new interpretative crustal‐scale section across the south‐western Alps is proposed, combining geological and geophysical 2D/3D data. The Apulian mantle (i.e. the Ivrea body) might be divided into two rigid pieces separated by the downward prolongation of the Penninic frontal thrust. These mantle indenters drive the decoupling of the European crust. Beneath the high to ultra‐high pressure metamorphic nappes, the deep structure results from the stacking of crustal slices extracted from the European lithosphere. The proposed structural model provides a basis for discussing the evidence of the crustal‐scale partitioning of the current strain pattern as well as the location of the seismicity.

The arc of the western Alps in the light of geophysical data on deep crustal structure

Recently, two international deep seismic campaigns in the western and central Alps (Etude Continentale et Océanique par Reflexion et Refraction Sismique ‐ Progetto Strategico Crosta Profonda, ECORS‐CROP [Roure et al. 1996b]; and National Research Program 20, NRP‐20 [Pfiffner et al. 1997]) have been completed. Here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the Alps collected during the past 40 years, including the two above mentioned geophysical projects. The tectonic reinterpretation of the ECORS‐CROP high‐resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology. Combined with previously published interpretations regarding the central Alps [Schmid et al., 1996], this study reveals substantial differences in geometry and kinematics between transects across the western and central Alps, respectively. At depth the transition between the central and western Alps is marked by the western limits of an Adriatic lower crustal wedge‐shaped structure and the northeastern limits of a similar structure made of European lower crustal material. At the surface it coincides with a corridor of dextral strike‐slip along the Tonale and Simplon lines. In addition, the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the Penninic realm of the western Alps to less than 20 km in the central Alps. The formation of the western Alpine arc was initiated during convergence and collision before 35 Myr ago, when the Adriatic micro‐plate, moving northward with respect to the European foreland, caused sinistral transpression in the western Alps while the central and eastern Alps underwent head‐on convergence and collision. During the post‐collisional stage, i.e., after 35 Myr ago, the arcuate shape of the western Alps was accentuated by WNW‐directed movement and anticlockwise rotation of the Adriatic microplate, decoupled from the central and eastern Alps along the Tonale‐Simplon dextral shear zone. This led to wedging of lower crustal slices both in the western Alps and in the central Alps. The new tectonic interpretation of the ECORS‐CROP transect allows a kinematic model to be established for crustal shortening in the western Alps during the past 35 Myr. The Ivrea mantle plays the role of a backstop in our tectonic model. We distinguish three episodes of post‐collisional crustal shortening along the ECORS‐CROP transect: From 35 to 30 Ma the Brianconnais basement was back‐thrusted over the Gran Paradiso units, leading to 30 km of crustal shortening (first episode). In the early Miocene, movements concentrated mainly along the Penninic frontal thrust and resulted in about 60 km shortening (second episode). Post‐12 Ma shortening within the external massifs is associated with folding in the Jura mountains when the crust was shortened by an additional 30 km (third episode).

Overthrust shear deformation of a foreland basin; structural studies south-east of the Pelvoux massif, SE France

The Late Tertiary Champsaur-Prapic fold zone lies to the southeast of the Pelvoux basement massif in the footwall of the Frontal Pennine Thrust of the western Alps. The 0.8–1.3 km thick Late Eocene-Oligocene foreland basin succession lies directly on crystalline basement and was deformed by WSW-directed shear below the overthrusting internal alpine units on the Frontal Pennine Thrust. This deformation decouples from the basement in a top to WSW-SW basal shear zone. Above, the turbidite sequence (Grès du Champsaur) is folded in chevron-like folds. A 12 km long continuous profile across the Champsaur fold zone documents variations in chevron fold shape, axial plane dips and zonal distribution of folding which indicate progressively higher shear strains towards the Frontal Pennine Thrust in the east. Chevron fold axes trend generally NNWSSE except close to the Pelvoux massif where fold axes curve to N-S possibly due to lateral drag. In the western part of the investigated area SW-directed thrusting on the Champsaur Thrust almost doubles the Tertiary stratigraphy. The Champsaur Thrust and chevron folding in its footwall deform the overlying internal thrust sheets. The Champsaur-Prapic fold zone is a particular example of overthrust shear deformation in the footwall of internal alpine nappes in which deformation is concentrated in diffuse ramp zones. To the north, late alpine deformation changes to WNW-directed partly post-dating the WSW-SW-directed deformation further south. Therefore, in the core of the external western Alpine are, two sub-perpendicular transport directions which barely overlap can be distinguished.

Constraints on the timing of crustal imbrication in the central Trans-Hudson Orogen from single zircon 207Pb/206Pb ages of granitoid rocks from the Pelican thrust zone, Saskatchewan

The Pelican thrust is a major ductile high-strain zone in the Reindeer Zone, Trans-Hudson Orogen, northern Saskatchewan. It is interpreted as the main sole thrust separating stacked juvenile Paleoproterozoic allochthons and underlying Archean microcontinental crust in this central part of the orogen. Exposed nonmylonitic rocks in the footwall of the thrust consist of the Sahli monzocharnockite and the smaller, more highly retrograded MacMillan Point granite. Protomylonitic to ultramylonitic gneisses in the thrust zone derive from a variety of prethrust protoliths. A footwall "internal suite" mainly comprises quartzofeldspathic orthogneisses ("Q" gneisses) and high-grade migmatitic paragneisses. Hanging-wall "external suite" mylonitic gneisses include feldspar-porphyroclastic hornblendic grey gneisses probably derived from arc plutons, and laminated amphibolites derived from volcanic rocks. The overlying allochthon mainly comprises protoliths equivalent to those of the porphyroclastic orthogneisses and laminated amphibolites, together with interfolded and overlying Paleoproterozoic paragneisses of the Kisseynew domain. The Sahli monzoeharnockite yields 207Pb/206Pb zircon and whole-rock Rb–Sr ages of ca. 2500 Ma, and the "Q" gneisses give 207Pb/206Pb zircon ages of up to ca 2900 Ma, implying that most of the internal suite (footwall) mylonite protoliths are Archean. In contrast, external suite (hanging wall) porphyroclastic orthogneisses yield ca. 1880–1840 Ma 207Pb/206Pb zircon ages. Main, peak-metamorphic displacement on the Pelican thrust is interpreted to have occurred mainly between 1840 and 1820 Ma, as indicated by 207Pb/206Pb zircon ages from small, highly deformed synthrusting granite–pegmatite neosomal bodies in the thrust zone. Undeformed postcollisional granites and pegmatites were emplaced~1789 Ma. Total duration from arc development to completion of arc–continent collision was ~100 Ma. The Pelican thrust zone may be similar in significance and style to younger, major, ocean closure related thrusts such as the Frontal Pennine thrust of the western Alps and the Main Mantle, Main Boundary, and Main Central thrusts of the Himalayas. As for the Pelican thrust, these displace oceanic rocks over older basement.

Neogene kinematics of the central and western Alps: Evidence from fission-track dating

Zircon and apatite fission-track analysis has established the continuation of a zone of rapid change in mineral ages extending from the Simplon fault zone in the Simplon Alps, through the Rhone Valley, and along the southeastern side of the Mont Blanc and Belledonne massifs. Apatite fission-track ages to the northwest of this zone generally range from 1.4 to 6 Ma, whereas those to the southeast range from 6 to 13 Ma: corresponding values for zircon are 11-13 and 16-26 Ma, respectively. The zone corresponds to the position of the Frontal Pennine thrust in the Mont Blanc-Belledonne region. This structure, a prominent feature on recent deep seismic profiles, may have been reactivated in the Neogene with an important normal-fault component. This is suggested by apatite and zircon fission-track ages in the footwall that are younger than those in the hanging wall and by the observed elimination of the lower Pennine (or Simplon) nappes along this fault zone in the Rhone Valley-Mont Blanc-Belledonne region, resulting in the juxtaposition of North Penninic and Helvetic-Ultra-helvetic nappes at the current erosion level. The distribution of fission-track ages reflects this movement and the related Neogene exhumation of the Mont Blanc and Belledonne external massifs, which has continued until less than 1.4 m.y. ago (i.e., it is almost certainly still active)and has tilted the overlying nappe sequence.

A kinematic analysis incorporating incremental strain data for the Frontal Pennine Zones of the western French Alps

An incremental and finite-strain study of the zones adjacent to the Frontal Pennine Thrust (FPT) has been undertaken, using syntectonic fibres taken from veins, pyrite-type pressure shadows and similar fringes from around pebbles in conglomeratic rocks. The resultant strain data, together with detailed mapping of local deformational geometries and regional trends has shown that the Pennine Zones show a more complex structural history than previously envisaged. The structural analysis divides the Pennine Zones into two regions, north and south of Crevé Tête (the southern termination of the Valais basin). The zones to the south of Creve Tête (Mont Fut, Valbuche, Nielard and Aiguille d'Arves) show an essentially piggy-back thrust sequence directed towards the foreland in which the elongation of the strain ellipse initially parallels the transport direction (WNW), but finally rotates to become sub-parallel to the tectonic strike (NNE-SSW). The structures of the Valais Zone to the north of Crevé Tête are more complex, with both N- and WNW-verging folds, thrusts and strike-slip faults. Here the long axis of the strain ellipse appears initially to parallel the tectonic strike (NE-SW). This is followed by a rotation into the late transport direction (WNW). This localised alternation between strike and transport parallelism is problematic. Specific examples of strike-parallel extension may be explained in terms of radial transport, buttressing against earlier extensional structures, strike-slip motion and/or stretching over lateral culmination walls. The major division in fibre pattern north and south of Crevé Tête, however, is best explained as being a function of compartmentalization of deformation styles between adjacent areas. Models which invoke a dominant deformation mechanism e.g., thrust tectonics along the entirety of the Alpine chain, thus need to be rethought. Similarly, the recognition of widespread strike-parallel extension suggests that two-dimensional balanced-section techniques are simplistic giving only minimum shortening estimates in a single direction and thus failing to reflect the true situation.

Stress field changes in the Rhone Valley from the Miocene to the present

Study of the deformation of Neogene and Quaternary deposits in the Rhône Valley shows that the end-Miocene E-W greatest principal compression ( σ 1) that deformed the Western Alps did not continue into the Quaternary, at least between Vienne and Montélimar. The Middle to Upper Miocene molasse on the east side of the Rhône Valley was affected by E-W compression at the end of the Miocene, causing reverse faulting and N-S folding (St. Lattier and Varacieux anticlines). In contrast, the Pliocene sands and clays of the Rhône Valley are unaffected by folding and show only small normal faults with a maximum throw of l m trending between N-S and NW-SE. These were caused by E-W to NE-SW extension, which is incompatible with the E-W compression that characterises the latest-Miocene Alpine phase. These structures are the surface expression of extension that was probably induced locally by the overall compressive regime that exists to the present day in this part of Western Europe [the “transcurrent extensional regime” of Philip (1983), USTL thesis, Montpellier]. This stress field results from the collision of the African and European plates that began in the Tertiary. West of the frontal Pennine thrust, the present-day direction of compression, which is deduced from calculated focal mechanisms and from in-situ stress measurements, varies between N-S and NW-SE. In the Grenoble region and the northern Massif Central, the focal mechanisms indicate transcurrent movement, with σ 1 horizontal and oriented between N-S and NW-SE, and with σ 2 vertical. In the southwestern Massif Central and the Vendée, however, they indicate an extensional tectonic regime, with σ 1 vertical and σ 2 horizontal and oriented NW-SE, σ 3 trends NE-SW. This orientation is similar to that which would have caused the small Pliocene normal faulting observed in the Rhône Valley between Vienne and Montélimar. In this part of the Rhône Valley, west of the frontal Pennine thrust, the E-W stress resulting from the collision of Apulia (Italy) with the West European plate has not been in evidence since the Pliocene. The E-W compression has been replaced by N-S to NW-SE compression resulting directly from the collision between the African and European plates transmitted through the Straits of Gibraltar and the Iberian microplate in the west and the Corsica-Sardina block in the east. The piercing effect of the Apulian block is no longer dominant in the Rhône Valley.

The influence of pre-existing basin structure on thrust system evolution in the Western Alps

Summary Collision mountain belts have recently been interpreted as being large scale stacked sedimentary basins, where the wholesale reactivation of pre-existing normal faults exerts the primary control on orogenic style. This model is explored here using examples from the Western Alps. The use of stratigraphic separation across faults, in the absence of corroborative sedimentological data, provides an ambiguous criterion to determinate fault reactivation. Variations between ‘compressional’ and ‘extensional’ stratigraphic separations across the Frontal Pennine thrust are more likely to be due to regional unconformity patterns developed in lower Tertiary times rather than normal fault reactivation along the eastern margin of the external zones. Mesozoic-Tertiary basin structure has been important in controlling the sites of major thrust ramps where stratigraphic horizons suitable for activation as thrust detachment zones have been offset. Examples include the thrust front in the Jura (Triassic detachment dropped to W by faults bounding the Bresse basin) and Vercors (basement uplifted to W at Faille de l’Isere, bounding the Mesozoic sub-Alpine basin to W). Buttressing structures, where thrust displacement is converted into vertical pure shear, may indicate buried normal faults but can occur at either hinterland or foreland-directed basin faults. The overall structural style in the sub-Alpine thrust belt indicates detachment between basement and cover with basement involvement along the Sole thrust which carries the external basement massifs. There is no indication from surface geology that this thrust originates from reactivation of a Mesozoic basin fault although this process may have occurred at depth. Similar styles of detachment-dominated thrusting with large horizontal relative to vertical displacements also characterize internal Alpine structure. Preliminary results from the ECORS-CROP deep seismic reflection profile across the Western Alps support the detachment thrusting model although they do not rule out minor normal fault reactivation.

Fluid regimes in the deformation of the Helvetic nappes, Switzerland, as inferred from stable isotope data

The stable isotope composition of veins, pressure shadows, mylonites and fault breccias in allochthonous Mesozoic carbonate cover units of the Helvetic zone show evidence for concurrent closed and open system of fluid advection at different scales in the tectonic development of the Swiss Alps. Marine carbonates are isotopically uniform, independent of metamorphic grade, where δ 13C=1.5±1.5 (1 σ) and δ 18O=25.4±2.2 (1 σ). Total variations of up to 2‰ in δ 13C and 1.5‰ in δ 18O occur over a cm scale. Calcite in pre- (Type I) and syntectonic (Type II) vein arrays and pressure shadows are mostly in close isotopic compliance with the matrix calcite, to within ±0.5‰, signifying isotopic buffering of pore fluids by host rocks during deformation, and closed system redistribution of carbonate over a cm to m scale. This is consistent with microstructural evidence for pressure solution — precipitation deformation. Type III post-tectonic veins occur throughout 5 km of structural section, extend several km to the basement, and accommodate up to 15% extension. Whereas the main population of Type III veins is isotopically undistinguishable from matrix carbonates, calcite in the largest of these veins is depleted in 18O by up to 23‰ but acquired comparable δ 13C values. This generation of veins involved geopressurized hydrothermal fluids at 200 to 350° C where δ 18O H2O=−8 to +20‰, representing variable mixtures of 18O enriched pore and metamorphic fluids, with 18O depleted meteoric water. Calc-mylonites (δ 18O=25 to 11‰) at the base of the Helvetic units, and syntectonic veins from the frontal Pennine thrust are characterized by a trend of 18O depletion relative to carbonate protoliths, due to exchange with an isotopically variable reservoir (δ 18O H2O=20 to 4‰). The upper limiting value corresponds to carbonate-buffered pore fluid, whereas the lower value is interpreted as 18O-depleted formation brines tectonically expelled at lithostatic pressure from the crystalline basement. Carbonate breccias in one of the large scale late normal faults exchanged with infiltrating 18O-depleted meteoric surface waters (δ 18O=−8 to −10‰). During the main ductile Alpine deformation, individual lithological units and associated tectonic vein arrays behaved as closed systems, whereas mylonites along thrust faults acted as conduits for tectonically expelled lithostatically pressured reservoirs driven over tens of km. At the latest stages, marked by 5 to 15 km uplift and brittle deformation, low 18O meteoric surface waters penetrated to depths of several km under hydrostatic gradients.

The restoration of thrust systems and displacement continuity around the Mont Blanc massif, NW external Alpine thrust belt

Foreland-propagating external thrust belts may be considered as essentially plane strain phenomena so that displacements can be correlated throughout their linked, three-dimensional fault geometry. This approach has been applied to part of the northwest external French-Swiss Alps, around the Mont Blanc basement massif. Imbricates of basement and cover sequences on the SW margin of this massif restore to a width in excess of 77 km with an implicit shortening of at least 67 km. These displacements can be correlated with those in the neighbouring Helvetic nappes by transferring movements, via lateral branch lines, onto the Mont Blanc thrust. By reappraising thrust geometries, the Helvetic/Ultrahelvetic nappe complex has been restored to a width of 114 km to the ESE of the Aiguilles Rouges basement massif. Displacements on the internal (SE) margin of the Mont Blanc massif, estimated by balanced sections and a restoration of the Ultrahelvetic klippen in the sub-alps, exceed 59 km. Thrust continuity, incorporating the restorations of nappes and imbricate geometries around the Mont Blanc massif, is illustrated on a crude, restored branch-line map which also serves as a preliminary palaeogeographic reconstruction. External thrust systems, to the east of the external Belledonne/Aiguilles Rouges massif, restore to a width of at least 140 km in the footwall to the Frontal Pennine thrust.

Balanced cross-sections and their implications for the deep structure of the northwest Alps: reply

The structure of the northwest external French Alps, around the internal Belledonne and Mont Blanc massifs in the Beaufortain area, can be explained by foreland (WNW) propagating thrust tectonics. The internal Belledonne massif is interpreted as having developed by imbrication of an originally thin ‘skin’ of basement with a Mesozoic cover. Above this massif all thrusts tend to climb lateral sections to the south to converge with the frontal Pennine thrust: the southern termination of the Mont Blanc massif is merely a spectacular example of such a lateral ramp.