External French Alps Research Articles

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.

DIRECT AND INDIRECT EFFECTS OF SHADE ON FOUR FOREST TREE SEEDLINGS IN THE FRENCH ALPS

A number of authors have documented a higher occurrence of tree seedlings below the canopy of adult trees than in openings, particularly in mesic conditions, where increases in resources in openings stimulate the growth of competing shade-intolerant forbs. These patterns may be explained by indirect facilitation. Indirect facilitation has been mainly explored using models, and too few experimental studies have been conducted to understand the conditions under which it is likely to occur. We test here the indirect facilitation model in natural openings of subalpine forests and explore the relationship that may exist between species response to indirect interactions and life-history traits of target seedlings. Two evergreen conifers (Picea abies and Abies alba) and two deciduous broad-leaved angiosperms (Fagus sylvatica and Acer pseudoplatanus) that dominate the mixed mesic forests of the northern external French Alps were planted in a tall forb community invading natural forest openings. Seedlings were gro...

Reworked pollen assemblages and the Eocene-Oligocene boundary in the Paleogene of the western external French Alps

A study of the palynological associations of various outcrops from the Eocene-Oligocene of the western area of the “Delphino-helvetique zone” has been carried out in the French Alps. The Le Vît section, near Castellane and the Le Clouet section near Barrême, both in the Southern Subalpine Ranges, and several outcrops in the Le Déserts syncline (Savoy) in the Northern Subalpine Ranges have been examined. All these sections clearly show taxa reworked from earlier Paleogene deposits. The first occurrence of the reworked material was found at different levels in the various sections. However, a tentative correlation highlights a coincidence between reworking, shown in all sections, and the Eo-Oligocene boundary.

Belemnites, strain analysis and regional tectonics: a critical appraisal

Deformed belemnites offer great potential for estimating strain in deformed rocks. There are two fundamental approaches: (1) fragmentation and (2) reorientation of the belemnite guard. The fragmentation approach is based on measurements of the original (fragment) and final (fragment plus intervening gap) belemnite lengths. Three variants exist: standard (Ramsay), strain-reversal (Ferguson) and modified-standard (Hossain) methods, of which the second is the most accurate but also the slowest; the third method is both simple and relatively accurate. Strain ellipse estimates are conventionally determined from the best-fit envelope to plots of belemnite elongation (1 + e) against orientation (Badoux/Beach method), but this demands a large number of belemnites per locality. We describe an improved strain ellipse construction which requires only few belemnites. The reorientation approach is based on strain-induced shape modification of belemnite orientation distributions. Two variants exist: the standard (Sanderson) and modified-standard (Lloyd) methods, but they are essentially similar. We describe an adaption of the Lloyd method, which involves constructing distribution moment-ratio plots contoured in terms of strain-ratio and initial standard deviation. The relative merits of the two approaches are considered using data from the Rivet thrust sheet exposed in the La Grave region of the external French Alps. We find that whereas the fragmentation approach yields relatively simple strain estimates (true extensions and a strain ellipse), the reorientation approach only provides an estimate of the in-plane strain-ratio if the exact shape of the belemnite orientation distribution before deformation is known. The implications of these results are discussed in terms of local (belemnite) deformation histories and the evolution of the La Grave thrust belt and the Alpine orogeny.

Structural inversion in the external French Alps

Structural inversion in the external French Alps

The influence of intrabasinal faults on the development of a linked thrust system

The Mesozoic and Cenozoic rocks exposed in the Arve valley region of the External French Alps are used to assess the role of early intrabasinal faults on later thrust fault evolution. The early intrabasinal faults produced at some time from latest Upper Cretaceous to Tertiary strike parallel or subparallel to later Neo-Alpine thrusts. Where early faults dip away from the thrusts they are generally cut through and occasionally are overturned during this process. In one example extreme overturning has allowed partial reactivation. Early faults dipping in the same direction as thrusts may: a) be reactivated b) initiate ramping of the thrust ahead of the preexisting fault c) be cut through by the thrust d) cause pinning of the thrust at the footwall of the fault and folding against the fault as displacement continues (buttressing). From this work it is evident that intrabasinal faults exerted a major influence on the distribution of mechanical heterogeneities. These heterogeneities include variations of stratigraphic thickness and type across faults, fault-related unconformities and the presence of the fault itself. During the period of contraction such features strongly controlled the development of the stress field produced ahead of an advancing thrust and hence influenced the position of thrust fault propagation within the stratigraphy.

Fracture and separation histories of stretched belemnites and other rigid-brittle inclusions in tectonites

Apparent extension of a stretched rigid inclusion ( e R = L f, L 0−1 where L 0 and L f are original and final lengths respectively) is always less than strain-reversal extension ( e f ) calculated by sequentially closing inter-fragment gaps until the inclusion is restored to its original length. Differences between the two measures are strongly influenced by the number of fractures, n, and their timing within the total stretch history, and weakly influenced by the fracture location. Several fracture-and-separation models are established to explore these differences using Monte Carlo simulations, all normalized to e R = 0.5. When fragment and gap lengths are random and uncorrelated, e F values fall in the range 0.5 < e F < 1.0. More highly structured models yield a smaller range of possible e f values; the expected (mean) value is still much greater than 0.5 when n is small but approaches e R with increasing n and with decreasing time interval between successive fractures. Model predictions are compared with stretched belemnites in the external French Alps. Results indicate that belemnite fracturing continued throughout a substantial proportion of total stretch history, the proportion varying from locality to locality and correlating strongly with maximum stretch recorded. This probably reflects variations in stress and suggests that, if constant rheology is assumed, stretched belemnites may be used to estimate relative strain rates as well as absolute strains.

Extension analysis of stretched belemnites: a comparison of methods

Stretched belemnites, and other fragmented rigid-brittle inclusions, offer great potential for estimating true extensions in deformed rocks. In the standard (Ramsay) method of analysis the elongation is calculated as 1 + e = L/L o where L is the final end-to-end length of the fragmented belemnite and L o is the original length (sum of the individual fragment lengths). This method always underestimates the true extension, usually by a substantial amount. An apparently trivial modification of the standard method, as used by Hossain, gives much improved extension estimates without any additional cost in time or effort. The strain-reversal method proposed by Ferguson is the most accurate method currently available but involves more laborious data collection, and subsequent computer processing. The three methods were applied to 120 stretched belemnites in the external French Alps. The different extension values calculated by the three methods arise because Ferguson's method is sensitive to the entire fracture-and-separation history, Hossain's method is sensitive to only part of that history, and Ramsay's method is history insensitive. If strain history is of prime interest Ferguson's method should be used, but the ease, speed, and relatively good performance of Hossain's method will make it attractive to many field geologists.

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.

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

The Mont Blanc massif is one of a chain of basement culminations which crop out along the external French Alps. Its southwestern margin is interpreted as being a major thrust belt which propagated in a piggy-back sequence towards the foreland. These imbricates have developed in the footwall of the high-level Valais thrust. The depth to the floor thrust and shortening within imbricates above this thrust are estimated by a series of partially balanced cross-sections drawn between the ‘synclinal median’ and the Valais thrust. These sections restore to a pre-thrust length of at least 50 km, probably exceeding 100 km, above a floor thrust never deeper than 1 km below the sub-Triassic unconformity. All this thrust displacement is transferred via a series of lateral branch lines onto the Mont Blanc thrust in the Chamonix area. A corollary of this is that the Aiguilles Rouges and the main part of the Mont Blanc massif were separated by probably as much as 100 km prior to Alpine thrusting. Such large shortening estimates imply a hitherto unsuspected Dauphinois stratigraphic consistency in both thickness and lithology. To achieve a balance a restored crustal cross-section must show an equal length of both lower and upper crust. Thus a high-level basal detachment which floors large thrust displacements must overlie a long, undeformed lower crustal wedge. A restored section 100 km long requires such a lower crustal wedge to exist beneath the entire Alpine internal zones. Perrier & Vialon's crustal velocity profile through the western Alps is reinterpreted in these terms. The Ivrea body is considered to be a portion of an external lower crustal wedge which has been uplifted by thrusts after most of the displacement on the external thrust belt.

Strain analysis in a cover thrust zone, external French Alps

The La Grave thrust zone is situated in the external French Alps north of the Pelvoux basement massif. Jurassic cover rocks are deformed and imbricated in a region some 15 km square. Strains have been measured using the statistical reorientation of belemnites, the extension of belemnites and the shapes of pyrite pressure shadows. Orientation data are treated statistically, following Sanderson (1977). Groups of 100 measurements do not provide very accurate results and generally underestimate the strain when compared with more accurate determinations from extension measurements. In addition to providing reliable finite strain markers, pyrite pressure shadows allow interpretation of the incremental strain history. In the area studied, strains accumulated irrotationally in the plane of the bedding. The deformed autochthon to the basement massif and the lower thrust sheets were deformed by layer parallel shear and extensional flow, which produced LS fabrics with a down dip lineation. The higher thrust sheets were deformed by layer parallel shear and compressive flow, which produced LS to L fabrics with a strike-parallel lineation.

Syntectonic vein development in a thrust sheet from the external French Alps

Abstract The geometry and age relations of syntectonic veins within calcareous rocks of one imbricate sheet within a thrust belt in the external French Alps, are described. The earliest veins developed during the main ductile deformation by cleavage-parallel extension. The majority of the syntectonic veins developed towards the end of the deformation, and after the formation of second folds. They include a conjugate set of normal shears, an abundant set of upright extension veins, and en echelon sets. The dominantly simple shear strain making up the main ductile phase of deformation occurred by a mechanism of grain to grain pressure solution. The stretching lineation records the overall direction of thrust sheet movement. A change in the microchemical mechanism of pressure solution is thought to have caused the change from first to second phase deformation as recorded by slaty cleavage and crenulation folds in the field. From the shear and vein geometries, directions of principal stress have been inferred. The directions rotated throughout the deformation, the maximum principal stress being inclined to bedding during simple shear strain, becoming normal to bedding during the phase of abundant vein growth, and becoming vertical at the very end of the deformation.

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.

Thrust tectonics and crustal shortening in the external French Alps based on a seismic cross-section

Duplex geometry has an important bearing on understanding the development of orogenic zones. Based on a published seismic section through the French Alps, the external massifs are considered to form a duplex structure as a result of basement thrusting. The duplex is restored to a uniform basement thickness, giving a model estimate of shortening of 45 km. The interpretation of this figure in terms of early Alpine crustal thinning is discussed.

Thrust structures in the eastern Dauphinois Zone (French Alps), north of the Pelvoux Massif

An examination of thrust structures in the eastern part of the Dauphinois Zone of the external French Alps (referred to in the literature as the Ultradauphinois Zone) shows that major basement thrusts climb up section to produce cover-basement synclines. These thrusts also climb laterally and are continuous with thrust in the cover rocks. The external basement massifs are recognized as thrust sheets with variably deformed and thrust cover sequences. The distinction made in the previous literature between the Dauphinois and Ultradauphinois Zones is no longer tenable. Cover thrusting proceeded by both smooth slip and rough slip, the latter producing a duplex of cover thrust slices. Restoration of this duplex indicates that a shortening of 70 km in the cover occured during its formation. Possible errors in this estimate include uncertainties in the original stratigraphic thickness and in the overall shape of the duplex. Another duplex is thought to have formed at a basement ramp created by the presence of an early basement normal fault. Partial footwall collapse of this basement ramp gave rise to a basement horse at the bottom of the duplex. The overall relation between cover and basement thrusting is indicated using a hanging wall sequence diagram. Recent geophysical studies suggest that the basement thrusts developed from a mid-crustal décollement which passes down dip to offset the Moho. Model studies of thin-skinned tectonics may not be appropriate to such thrust geometries.