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

285

Tectonophysics, 206 (1992) 285-305

Elsevier Science Publishers B.V., Amsterdam

A kinematic analysis incorporating incremental strain data for the Frontal Pennine Zones of the western French Alps Sara Spencer Department of Geology, University of Newcastle upon Tyne, Newcastle upon Tyne, UK

(Received October 12, 1990; revised version accepted November 12, 1991)

ABSTRACT

Spencer,S., 1992. A kinematic analysis incorporating incremental strain data for the Frontal Pennine Zones of the western French Alps. Tectonophysics, 206: 285-305. 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 Creve T&e (the southern termination of the Valais basin). The zones to the south of Creve T&te (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 Creve T&e 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 Creve T&e, 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.

Introduction which require Finite-strain investigations knowledge of the original shape of an object compared to its shape after deformation, have frequently been used to develop an understanding of how rocks behave during deformation (e.g., Lisle, 1977, 1979; Graham, 1978). Although such investigations have allowed reasonable strain histories to be determined, this is an exception since in most cases the available knowledge of the

Correspondence to: S. Spencer, Department of Geology, University of Newcastle upon Tyne, Newcastle upon Tyne, UK. 0040-1951/92/$05.00

initial and final product of the deformation is insufficient. Geologists also require the incremental and finite-strain magnitudes and the orientations of these strains and the rotational components of the deformation. Deformational paths and histories are crucial objectives if the tectonic development of an area is to be fully understood. Interest in deformation paths has in recent years led to a growing awareness that syntectonic fibres, developed in either veins or pressure shadows, may indicate the incremental strain history. Microstructural studies (e.g., Spencer, 1991 and references therein) have shown that in many cases such fibres develop parallel to the displacement path of their boundaries (i.e. vein walls or ob-

Q 1992 - Elsevier Science Publishers B.V. All rights reserved

286

S. SPENC‘ER

ject-matrix interfaces) as these move apart. Furthermore, detailed analysis allows the identification of increments of fibre growth. These increments are identified either by inclusion trails, discrete breaks in the fibre or where deformation is non-coaxial, by changes in fibre orientation (Ramsay, 1980; Ramsay and Huber, 1983). Strain analysis techniques using syntectonic fibres have been developed (e.g., Elliot, 1972; Wickham, 1973; Durney and Ramsay, 1973; Ramsay and Huber, 1983; Casey et al., 1983; Ellis, 1984, 1986; Beutner and Diegel, 1985; Etchecopar and Malavielle, 1987; Law and Potts, 1987), which allow the magnitude and orientation of incremental and finite strains to be determined. The present study uses the rigid fibre technique of Ramsay and Huber (19831, which assumes that the fibres parallel the maximum extension direction, to determine in-

cremental and finite-strain patterns in the Frontal Pennine Zones of the Western French Alps. Regional settings and locations The rocks described here belong to the External, Frontal Pennine and Subbriangonnais Zones of the Western French Alps (Fig. 1). The relevant External Zones are the Ultradauphinois and the Ultrahelvetic sheets which lie in the footwall to the Frontal Pennine Thrust (Fig. 1). They comprise an ancient Mesozoic passive margin environment (Graciansky et al., 1979; Lemoine et al., 1981; Tricart, 1984; Tricart and Lemoine, 1986) in which the basement was extended to produce a series of half-grabens. Inversion of these halfgrabens during the Late Cretaceous and Tertiary compressional ‘events’ (Davies, 1982; Barfety and

External crystalline basement Valais Zone ]

External Zones A@uille dANeS

Fiysch

Exiernal Slices NielarfYMoni

Fui

Valbuche Grand Mondaz Permn des Encombres Sub-Alpine k-

Chains

SOkm

FPT‘

\

I

_._

Saint Maurice

SJ P

SI Jean de Mawlenne Peivoox

Fig. 1. (a) Sketch map of the Western Alps. Boxes refer to later figures. (b) Geological sketch map showing the zones adjacent to the Pennine Front, French Alps.

A KINEMATIC

ANALYSIS

INCOR~RATIN~

INCREME~AL

STRAIN

Gidon, 1983; Tricart and Lemoine, 1986; Gillchrist et- al., 1987) resulted in a ~mbination of foreland directed piggy-back thrusting (Butler et al., 19861, strike-slip faulting (Ricou and Siddans, 1986, Gourlay, 1986) and reactivation of the Mesozoic extensional fault system. The Frontal Pennine Zones (FPZ) are defined as a thin linear zone bordered by the Subbrian~onnais Front to the east and the Pennine or Valais Front to the west (Fig. 1). These zones are subdivided into the Northern Frontal Pennine Zones (NFPZ: the Valais or Breche de Tarentaise Zone; Barbier, 19481, and the Southern Frontal Pennine Zones (SFPZ: Nielard, Valbuche, Mont Fut and Aiguille d’A.rves Zones; Parish, 1985). ~llectiveiy they comprise an intermediate palaeogeographic realm, representing the most complete transition from the Dauphinois/ Helvetic domains in the west, to the Subbrianqonnais and Brian~onnais domains in the east. In effect, they represent a transition from the passive margin/shelf sequences to the deep ocean basin sequences of the Alpine chain (e.g., Debelmas and Lemoine, 1970). The FPZ are, for convenience, generally grouped together when discussing the regional geology and kinematics of the Western Alps. However, detailed study shows that although the zones show a similar early history, they display marked differences in their post-Cretaceous evolution (Antoine, 1971; Parish, 1985; Spencer, 1989). This is reflected both in the geological successions of the zones and in their deformational style. In the NFPZ the mid-cretaceous was a period of extension, with the continued and rapid development of the Valais basin (e.g., Antoine, 1971). Basin inversion in the Late Cretaceous/Te~ia~ led to the development of a complex fold, thrust, strike-slip zone with variable amounts of internal strain between adjacent thrust sheets. Structural vergence is recognised as being northeast, northwest, west-no~hwest and southeast (e.g., Antoine, 1971). The SFPZ remained unaffected by the Cretaceous development of the Valais basin. Their response to the subsequent compressional deformation followed an essentially foreland-directed (WNW) piggy-back thrust sequence (e.g., Butler et al., 1986). Local variations in transport directions do occur but these

DATA

FOR THE FRONTAL

PENNINE

ZONES

287

are interpreted in terms of lateral and oblique ramps to the west-northwest thrust system (Parish, 1985). The relevant Subbriansonnais Zones are the Petit Saint Bernard, the Grande Moendaz and the Perron des Encombres. These form the most internal of the zones described (Fig. 1). They comprise a series of nappes/thrust sheets with a dominantly Jurassic succession. St~cturally they combine to form a linked fold thrust system which, though originally W- to WNW-verging, has been complicated and in some places overprinted by the late E- to ESE-verging structures (Parish, 1985). A comparison of the structures adjacent to the Pennine Front is given in Table 1. Data collection and analysis Syntectonic fibres were collected from all the zones both within and external to the Pennine Front, from Courmayeur in the north to Pelvoux in the south (Fig. 1). However, despite thorough sampling it proved impossible to obtain completely representative and uniform data coverage. This results from the differing lithologies of the zones adjacent to the Pennine Front (Fig. 2). The overall paucity of fibre complexes has made it necessary to augment this approach with finitestrain data derived from deformed reduction spots in Permian shales, pebbles in Eocene, Jurassic and Cretaceous conglomerates, stretching lin-

% 80 60

Fig. 2. Bar chart displaying percentage of samples collected against stratigraphic horizon for the zones both internal and external to the Pennine Front.

of structural

of backfolds

strike-slip

folds (F2) and an S2

of minor strike-

NW- SE extension

not constrained.

massifs. Some folding

of the external

(Fl), direction

crystalline

Inversion

slip faults.

the development

and

The folds are associated

with along strike extension

cleavage.

WNW vergent

thrusts.

with the

in

of an Sl cleavage,

D4

piggy-back

development

directed

of foreland

zones, resulting

of the FPZ over

the development

the external

Thrusting

thrusts.

of the Subbriansonnais

in breaching

locally resulting

of a series of minor

of

commun., Sub-Briansonnais

1991)

along

associated

ramp?

of extension.

of the Subbrian-

of a strike-slip

NW- SE extension

extensional busin.

Development

cleavage.

with an Sl

in the thrusts

has

nature

zones.

NW- SE extension

ened (Fi?).

being locally back-steep-

resulted

(F2).

of downward

sheet on to the more external

qonnais

Thrusting

of this sequence

A lateral

strike-parallel

and SE associated

hW - SE extension

strike extension.

is no associated

structures,

with large amounts

development

facing structures

The piggy-back

folds (Fl) which

with associated

folds FL Note there

sequence

of a foreland

piggy-back

The backfolding

with SE

of backfolds

has locally led to the

vergence.

and backthrusts

verge WNW, NE, NW

cumbent

of re-

of the basin leading

to the development

Inversion

ment of S2 and 53 cleavages.

with the develop-

faults. These events

are associated

strike-slip

development

directed

development

and the

with along

ting are associated strike extension

NW-SE

phase. This involves the

trending

The development

Main WNW compressional

These show a SE facing.

phase. The folding 072) and thrus-

faults trending

thrusts.

in

thrust

asso-

backthrusts.

Main WNW compressional

N-S.

Extensional

breaching

sheet locally resulting

ciated with the arrival

thrust sheet

Wave of deformation

system.

with a linked backthrust

These are

of an 54 cleavage. associated

Development

with ESE facing.

Subbriansonnais

associated

faults.

of

(F2) and

(pers.

Development

of backfolds

S

(1985) and B.D. Trudgill

Development

N

SF PZ

data from Parish

Development

(F3)

additional

N - S extension

of backfolds

Front;

N - S extension

with the arrival of the

and

Dl

(F3) with

of an S3

to the Pennine

N - S extension

NFPZ

in the zones adjacent

Local development

Wave of deformation

en-echelon

cleavage.

ESE facing. Development

This is associated

D2

histories

extension

Development

N- S

External

1

D3

D5

D6

Summary

TABLE

g T x

” % T

A KINEMATIC

ANALYSIS

INCORPORATING

INCREMENTAL

STRAIN

eations in mylonites and belemnites from Jurassic malms. Within the Internal Zones, where syntectonic fibres are less common, most of the fibres measured were from the flysch-type facies of the Eocene, the Jurassic and the Cretaceous. Veins, rather than pressure shadows, dominate although some excellent pressure shadows are preserved (Fig. 3). The Cretaceous flysch units are generally more competent than those of the Jurassic and there the veins tend to be either isolated or occur within tension gash arrays. Multiple veins are known but are infrequent, being restricted to the hinges of late (F3) southeast verging backfolds (see Table 1). Within the Jurassic and Eocene strata, fibre development is common. Isolated examples of fibre complexes are found within the Carboniferous sandstones and shales, Permian slates and Triassic quartzites and dolomites of the NFPZ and Subbrian~onnais Zones. Locally, the facies of the Lower and Middle Jurassic limestones and marls show concentrations of fibre complexes, but these occur within well delineated brittle-ductile shear zones and are not ubiquitous. The zones external to the Pennine Front produce large amounts of data. This is pa~icularly true of the Liassic shales and limestones which dominate the thrust sheets of the External Zones. The low competence of these shales, and the

DATA

FOR THE FRONTAL

PENNINE

ZONES

289

relative abundance of both framboidal and euhedral pyrites, has given rise to excellent, three-dimensional pressure shadows with well-preserved fibres. Vein complexes are common, though they are generally not as well developed. Locally, Middle and Upper Jurassic marl units provide abundant fibre complexes. There is only a limited development of fibre complexes within the preJurassic formations. Analysis The fibres are composed of quartz and/or calcite. These minerals are abundant within the host rock and since both are stable over a wide range of temperatures and pressures it was not possible to accurately constrain the P-T conditions. Analysis of the fibre complexes was thus purely geometrical. The fibre structures were all studied, unless otherwise stated, within the cleavage plane. Since rotation out of the plane of section within shale horizons (the dominant host rock) seldom exceeds lo-15”, structures observed within this plane display the most complete deformation history. The measurement technique chosen was the rigid fibre model outlined by Ramsay and Huber (1983). Not only is this model relatively simple to apply in the field, but it is also frequently used in Alpine geology (e.g., Durney and Ramsay, 1973;

Fig. 3. Pyrite-type (antitaxial) pressure shadow from the Cretaceous strata of the Valais Zone.

290

Parish, 1985; Dietrich, 1989). It is, therefore, possible to compare the results here with those obtained by other workers. The RF/+ method of strain analysis (e.g., Lisle, 1977, 1979) was applied to the reduction spots and the pebbles, whilst the strain data for the belemnites was calculated using the ‘strainreversal’ technique of Ferguson (1981) and Ferguson and Lloyd (1984). The results of these analyses are all displayed in Figure 4 (see also Appendix).

S SPENCER

Early strain indicators Early strain indicators are not displayed in Figure 4. They comprise those veins and antitaxial pressure shadows formed during an early stage of Alpine compression (Dl-D3, Table 1). The sensitivity of syntectonic fibres within these systems to superimposed deformation events often results in their recrystallisation and/or reorientation. Early formed veins and pressure shadows are thus radically altered (Fig. 5), and much of

SAINT MAURICE

External Crystalline

Strain data lrom fibres

Fig. 4. Map of the zones adjacent to the Pennine Front showing strain data as derived from syntectonic fibres, pebbles within conglomerates, reduction spots and extended belamnites. The incremental strains derived from the syntectonic fibres were all measured in the cleavage plane with no dip correction. The strains are depicted by arrows. The length of the arrow is propofiional to the incremental strain (logn + 2) in a given orientation (Fkamsay and Huber, 1983). Changes in the orientation of the Gbre are n at the end of the f&al incsswt. The congWates and reflected in the curvature of these arrows. The arrow head is reduction spots are represented by ellipses indicative df the finite strain ratio. The extended bekrmites are represented by lines whose length and orientation reflect the elongation of the belemnite.

A KINEMATIC ANALYSIS ~NCOKFORATING INCREMENTAL STRAiN DATA FOR THE FRONTAL PENNINE ZONES

291

tures described in the literature (Fitches et al., 1986; Roering and Smit, 19871, suggests that these early veins could have formed as the sedimentary

Pyrite type pressure sh cienul&d by F2 0-

2cm

Fig. 5. Early vein and pyrite-type pressure shadows, reoriented as a results of crenulation about an I?2 fold. From the external French Alps.

the evidence, pa~i~larly with respect to the strain orientations and magnitudes is lost. This is obviously a major problem in determining the exact nature of the deformation path. Recognition of such fibre complexes, however, remains significant, in that they serve at least to constrain the early deformation history. Early layer-parallelveins Bedding-parallel and associated layer-perpendicular quartz calcite veins are common to all of the formations adjacent to the Pennine Front. Although showing a similar distribution to the s~tectonic fibre complexes, they show their greatest abundance in the Liassic strata of the Dauphinois Zone. After their formation these veins have been locally folded, sheared, thrust, boudinaged or a mixture of all four. Their origin is therefore not easily determined, although a variety of models may be considered. Comparison with similar but essentially undefo~ed struc-

pile dewatered and underwent subsequent diagenesis. The flow of the pore fluids, liberated by dewatering through the sedimentary pile was impeded by beds of lower permeability. Following the model of Fyfe et al. (1978), the fluids pond below the impermeable barriers resulting in an increase in pore fluid pressure. Pore fluid pressure increased to a point where fracturing occurred in the rock These fractures generally occur parallel to the bedding due to the lower tensile strength in this direction. Once the fracture is formed and the layers are forced apart, the fluids precipitate minerals on the free surfaces. Another explanation is hydraulic fracturing, Gratier (1987) showed that the orientation of hydraulic fractures is a function of the ratio of elastic anisotropy to the deviatoric stress. Subsequently, many veins develop oblique, parallel or perpendicular to the bedding, depending upon the orientation of inherent fabrics within the rock with respect to the principal stress axis. Furthermore, Gratier (1987) showed that the boudinage often seen in association with layer-parallel veins is common to the final stages of vein and cleavage development (Gratier, 1987, fig. 4). This is particularly relevant when examining veins thought to have formed as a by-product of folding cleavage development in association with the development of recumbent folds (e.g., Tanner, 1989). Such structural relations are commonly seen in the large-scale isoclinal folds seen within the NFPZ (see Fig. I>. It has also been suggested that the veins could be extensional veins developed in a shear zone environment that have been rotated to become sub-parallel to the Sl cleavage plane, which is itself almost parallel to the bedding (e.g., Beach, 1974, 1982; Kerrich, 1977). The veins thus coalesce, overlap or form a discontinuous band after and during rotation giving the post-formational structures described above. Certainly some of the layer-parallel veins, particularly those proximal to large thrust planes where a strong deformational fabric exists, may have formed in this way. Whatever their origin, the nature of these veins

292

S. SPENC‘EK

is important in unravelling the overall tectonic history, since their formation is closely linked with the development of other structures. High pore fluid pressures sustain lithostatic loads during deformation, thus reducing the effects of overburden (Rubey and Hubbert, 1959). Under favourable circumstances of temperature and pressure they may be important in increasing deformation rates and the depths at which tensile stresses can exist. Field evidence shows that many of the layer-parallel veins, particularly those in the Liassic shales of the Dauphinois Zone, have acted as easy slip horizons for later thrust faults. Early pressure shadows

Early pressure shadows are those which have been demonstrably re-oriented as a result of subsequent deformation, for example the folded pressure shadows shown in Figure 5. Although it remains impossible to quantify these pressure shadows with respect to strain values or orientation, something can be said about both their origin and the implications that these have for the development of related structures. Most early pressure shadows are folded and are generally recognised in two dimensions only, usually perpendicular to the cleavage plane. This gives a distorted picture of their true nature, since pressure shadows usually show their greatest development in the plane of cleavage. Samples from the Dogger limestones to the north of Courmayeur (Fig. 11, however, show structure in three dimensions, from which it appears that their development was coaxial. Unfortunately, their original orientation relative to the dip and strike of the cleavage is not easily determined due to subsequent folding. Unfolding of these structures using conventional stereographic techniques does not help because it is known from younger strain indicators that the later folding involved variable amounts of strike-parallel extension. Quantifiable fibres The Figure plexes planes.

data from syntectonic fibres displayed in 4 are by necessity restricted to those comwhich lie within the dominant cleavage Over much of the region this is not a

problem since the first cleavage (SD usually dominates (with no overprinting) and therefore the fibres may record the strain increments of the entire compressional deformation history. However, elsewhere (particularly close to the hinges of the F2 and F3 folds, see Table 1) other cleavages (e.g., S2 and S3) become dominant and the syntectonic fibres preserved within these planes show only those increments of deformation that are contemporary with or post-date the later cleavage formation. In such cases, fibres usually display a simple pattern which does not reflect the true deformation. Care must be taken to assess the significance of earlier fibre complexes, even where it is impossible to quantify the amount of strain that they represent. Local and regional strain patterns The strain data displayed in Figure 4 may be divided into three trends of which the first two are dominant. These are as follows: (1) NNESSW-oriented, parallel to both the strike of the bedding, the fold axis and the major thrust sheets (note that this is also parallel to the transport direction of the early NNE-vergent folds/thrusts within the NFPZ Zone (see Table 1); (2) W- to WNW-oriented, parallel to the dominant thrust transport direction; (3) N-S-oriented, perpendicular to the strike of late normal faults. The first group has its largest data set within the NFPZ and incorporates the samples which record the highest strain values for the region. This is clearly shown by the orientation of the strain ellipses (derived from conglomeratic horizons and reduction spots) and arrows representing syntectonic fibre complexes. The paths shown by these arrows reflect the incremental rotation of the strain ellipse for syntectonic fibres during deformation. Hence, where deformation is coaxial, the strain ellipse lies parallel to the tectonic strike (NE-SW). However, where deformation is non-coaxial, increments of strain though initially parallel to the tectonic strike (NNE-SSW) show a late rotation towards the regional transport direction (WNW) (D3 and D4, Table 11. The early strike-parallel increments within the NFPZ are represented by the highest values of strain.

A KINEMATIC

ANALYSIS

INCOR~~TING

INCREMENTAL

STRAfN

Conversely, the second group (strain data with a west to west-nor~west orientation) is most frequent within the SFPZ, External and Subbrianpnnais domains, Within the SFPZ, strain data all show orientations at a high angle to the regional strike and parallel to the W to WNW

DATA

FOR THE

FRONTAL

PENNiNE

ZONES

293

transport direction (Parish, 1985). Where deformation is non-coaxial, increments initially parallel the transport direction but later rotate to become strike-parallel. Such late increments where seen represent minor strain values only. Similarly, strain data from the Jurassic marls of both the

Pennine Nappes

Fig. 6. (a) Incremental strain directions in the Western Helvetic Nappes of the Valais, Switzerland, as indicated by fibres about pyrites. The arrows represent fibre increments measured within the cleavage plane and normalised relative to a standard pyrite size. Changes in the orientation of the fibre are reflected in the curvature of these arrows. After Dietrich and Casey, 1989. Note that most of the data show an initiaf extension direction parallel to the transport direction followed by a rotation into strike-parallelism. (b) Incremental strain directions as indicated by fibres about pyrites for the Mont Blanc, Aigulles Rouges, Belledonne junction. After Gourlay, 1986. The length of the arrow is proportional to the incremental strain in a given orientation. Changes in the orientation of the fibre are reflected in the curvature of these arrows. The arrow head is shown at the end of the final increment.

NW/SE extension parallel to the movement direction. Oblique strains result from a local variation in shear strain and growth of oblique folds which display extension parallel to their fold axes. NE/SW extension within the S2 cleavage planes, parallel to the trend of the thrusts and the folds. NW/SE extension parallel to the movement direction. Oblique strains result from a local variation in shear strain and growth of oblique folds which display extension parallel to their fold axes.

NW/SE extension subparallel to the strike of the local fold axes.

S

Many of the early veins and pressure shadows developed in the Sl cIeavage planes have either been overprinted or reorientated, representing these early strains has been lost.

NW/SE extension parallel to the movement direction. Oblique strains result from a local variation in shear strain and growth of oblique folds which display extension parallel to their fold axes. (not true far the Pehroux massif to the south)

NE/SW extension within the 52 cleavage planes, parallel to the trend of the thrusts and the folds.

Late N-S extension in veins.

Late N-S extension in veins.

Late N-S extension in veins.

Local extension in veins NE/SW parallel to the axis of the backfolds.

SF PZ N

NFPZ

External

Summary of the strain analysis in the zones adjacent to the Pennine Front (additional data from Parish, 1985 and Beach, 1982)

TABLE 2

thus the shape and orientation

of the ellipse

Extension in the Subbrianpnnais reflects extension in the lower F.P.T.Z. All extension is NW/SE, though its relationship to the larger structures changes from being strike parallel in the south to strike perpendicular in the north.

NW/SE extension related to the shearing associated with the backthrusting. Locally some show an anticlockwise rotation.

Late N-S extension in veins.

SubbrianGonnais

A KINEMATIC

ANALYSIS

IN~OR~~~NG

fNCREMENTAL

STRAIN

External and SubbrianSonnais domains show their greatest values parallel to the transport direction. Late strike-parallel increments are common but generally represent low strain values. Where anomalies to this pattern occur, these can generally be related to local folding and/or faulting which effectively overprints the regional trends. Data from the NFPZ within this group tend to show a coaxial deformation path and finite-strain values are low. The final N-S trend for which the largest data set was achieved in the NFPZ, comprises those samples which record the lowest strain values. The strain markers, which are mainly syntectonic vein fibres showing coaxial development, formed late with respect to the structural evolution of the zone. A comparison of the strain data adjacent to the Pennine Front is given in Table 2.

Comparable strain data from other sources Although the techniques of incremental strain analysis have not previously been applied to the NFPZ, such techniques have been successfully adopted in other regions adjacent to the Pennine Front (Durney and Ramsay, 1973; Parish, 1985; Gourlay, 1986; Dietrich and Casey, 1989). The results of these studies have, where possible, been incorporated into the data set displayed in Figure 4. An exception are the results of Dietrich and Casey (1989), which were collected from the Helvetic Nappes in the footwall to the Pennine Front, to the north of the research area. These results are shown on a map of the Helvetic Nappes (Fig, 6a) which represent the lateral equivalents of the Dauphinois and hence display simiIar strain patterns to those seen along strike to the south. The initial extension direction in the Helvetic Nappes (the Morcles, Diablerets and Wildhorn) is NWSE. This direction is perpendicular to the orientation of the fold hinges and, therefore, to the local transport direction. In much of the nappe pile it is this initial extension direction which records the highest strain values. Further increments of extension vary between samples taken both from (a) different nappes and (b) from within

DATA

FOR THE FRONTAL

PENNINE

ZONES

295

different positions in the same nappe. As a general rule, however, there is a rotation of the strain elhpse into an orientation sub-parallel to the fold axes. The strain patterns determined by Gourlay (1986) for the junction between the Mont Blanc, Aiguilles Rouges and Belledonne massifs, differ from those seen elsewhere. Gourlay recognised two phases of ‘superimposed’ or incremental deformation: an initia1 N-S extension was followed by a rotation of the extension direction to the west-northwest (Fig. 6b). The incremental and finite-strain data of Parish (1985) have also been included in the data set shown in Figure 4. Parish determined these values from the analysis of pyrite-type pressure shadows, reduction spots in Permian shales and from conglomerate horizons of the Eocene flysch. He also recognised the existence of a sub-horizontal stretching tineation defined by pebble and mineral elongation. The strain patterns that Parish (op. cit.) recognised in the External Zone in the St. Jean de Maurienne area show similar relationships to the major structures observed in the Helvetics (Dietrich and Casey, 1989). The highest values are recorded by the WNW-oriented initial strain increments, parallel to the local transport direction. Later increments show a rotation into NE-SW strike-parallelism. Within the SFPZ to the east of Co1 de Madelaine (Fig. l), the strain pattern varies depending on the location relative to the Bellachat-Tete de Fer inflection (Parish, 1985). To the north of the Bellachat inflection, the stretching and mineral lineations have the standard WNW orientation, but there is no record of late strike-parallel growth increments within the pyrite-type pressure shadows. South of the Bellachat inflection there is a widespread extension sub-parallel to the fold axes with axial strain ratios of 3 : 1 recorded from congiomeratic sequences, whilst strain ratios from Permian Conglomerates in the VaIbuche unit reach 4: 1 (Parish, 1985). Here the axes of maximum elongation lie parallel to the axes of oblique NE-verging backfolds. Late incremental fibre growths also record this trend (Parish, 1985). Finally, in the Grande Moendaz unit of the Subbrian~onnais domain, Parish (1985) recognised a

2%

coaxial extension direction sub-parallel to the plunge of northeast verging isoclinal backfolds. Other finite-strain data for this part of the Alpine chain come from the Liassic strata to the north and northwest of La Grave in the footwall to the Pennine Front (in this case the Aiguille d’Arve thrust). Studies of extended belemnites, pyrite-type pressure shadows and syntectonic veins by Beach (1982), Beach and Jack (1982) and more recently by Lloyd and Ferguson (19891, show two groups of data. The first group indicates extension parallel or sub-parallel to the local NW-SE strike of the thrust belt. The second group consists of stretching lineations parallel to the local southwest thrust transport direction swinging around towards strike-parallelism (NW-SE). Maximum extensions calculated by Lloyd and Ferguson (1989) range from 140 to 450%, with the strike-parallel component varying from 80 to 270%. Interpretation Strain analyses applied to areas with a complex deformation history have to be interpreted with respect to both local and regional structural evolution. Strain data are thus related to the orientation of the larger structure. Using these techniques, previous workers (e.g., Durney and Ramsay, 1973; Beach, 1982; Beach and Jack, 1982; Butler, 1982; Parish, 1985; Dietrich and Casey, 1989; Lloyd and Ferguson, 1989) have been able to interpret the results of their own strain analyses in terms of the following: (1) Elongation in the thrust transport direction, with oblique strains reflecting local variation in shear strain and the growth of oblique folds in which extension is parallel to the fold axes. (2) Elongation in the thrust transport direction where non-coaxial deformation reflects rotation of a thrust sheet about a sticking point. (3) Strike-parallel extension necessitated by radial transport directions. (4) Bedding-parallel shear associated with strike-slip faulting and/or oblique convergence. (5) Irrotational strike-parallel extension, reflecting the effects of extension on lateral culmination walls. The strain patterns developed during thrust sheet propagation are reasonably well understood

S CPt:N(‘FK

(e.g., Elliot, 1976; Coward and Kim, 1981; Sanderson, 1982) and can be readily adapted to explain the development of transport-parallel extension. Such a feature is common to most orogenie belts (e.g., Shackleton and Ries, 1984; Ellis and Watkinson, 19871, and has been used as a criterion in the Alps to determine regional movement directions (e.g., Choukroune et al., 1986; Platt et al., 1989; Gratier et al., 1989). Strikeparallel extension is also a common feature of many erogenic terrains, although the amount of calculated extension is variable. Within the context of the Alpine chain, a variety of models have been proposed to explain strike-parallel extension. Model 1

The first of these models is a reflection of the arcuate nature of the Alpine chain. It has long been recognised that there is no unique transport direction in the cover formations along the Alpine arc (e.g., Merle and Brun, 1984; Coward and Dietrich, 1989). Transport directions vary from north in the Austro/Helvetic Alps, to northwest to west, in the region between Mont Blanc and Pelvoux, and to the southwest between Pelvoux and the Argenterra (e.g., Vialon et al., 1989; Coward and Dietrich, 1989). Where such radial transport directions are a result of a single pulse of compressive deformation, strike-parallel extension is a necessary structural accommodation mechanism. Detailed examination of structures along the length of the chain, however, shows that many of these transport directions are superimposed on one another and are not the result of a single radial compressive event. Structures adjacent to the Mont Blanc massif show the superimposition of an early northward movement by a transport direction which is dominantly westnorthwest (Gourlay, 1986, see also Fig. 6b). Similarly, Merle and Brun (1984) recognised the superposition of a southwest transport direction on to a northwest movement in the region within and to the south of Pelvoux. Furthermore, the radial model requires the amount of strike-parallel extension to increase towards the more external parts of the Alpine arc. The strain patterns

A KINEMATIC ANALYSIS INCORPO~TING

INCREME~AL

STRAIN DATA FOR THE FRONTAL PENNINE ZONES

shown in Figure 4 do not support this. The radial trans~~/e~~nsion model is thus considered to have little application on the erogenic scale, although calculations of arcuate strain show that it may be invoked to explain strike-parallel extension within singIe thrust sheets. Model 2 The present geometry of the Pennine Front is a result of a complex interplay between extensional and compressional forces associated with oblique slip. Recent work (Graciansky et al., 1979; Lemoine et al., 1981; Tricart, 1984; Tricart and Lemoine, 1986) has led to the recognition of an Early Jurassic extensional template which developed in conjunction with the opening of the Tethys Ocean. The subsequent closure of the Tethys Ocean was necessarily associated with the development of positive inversion statures and

297

oblique slip (Davies, 1982; Barfety and Gidon, 1983; Tricart and Lemoine, 1986; Gillchrist et al., 1987). Figure 7 shows the position and the relative effects of such early extensional faults upon bedding under compression. Figure 7a depicts a situation in which the original faults dip in the same direction as the structural vergence and subsequently act as pressure shadows. In this situation, rocks in the hanging wall will remain 1argeIy unaffected by the compressional deformation. This is seen in strain data from the Plateau de Paris (north of Pelvoux, Fig. lb), where rocks and belemnites in the hanging wall of NW-dipping faults are relatively unstrained whilst the surrounding strata record high strain values (Trudgill and Spencer, in prep.). Where the dip of the original fault plane opposes and where the fault plane is orthogonal or oblique to the strnctural vergence, the result is often an amplification of the strain caused by a ~mbination of buttressb.

Structural

Structural

/-z!!!Y

d. Structural

Strike slip

Fig. 7. Tbe effect of early normal faults on subsequent compressional strain patterns. (a) Where the earty fault dips in the same direction as the structural vergence, the fault may act as a wall to the compression creating a pressure shadow. (b) Where the early fault dip opposes the structural vergence, then buttressing against the fault may cause strain amplification. (c) Where the early fault is oblique to the structural vergence, strain patterns are varied due to localised buttressing and strike slip. (d) Where the early fault is parallel to the structural vergertee, trance-pa~llel strain patterns dominate.

298

ing and strike-slip motion (Trudgill and Spencer, in prep.; see also Fig. 7). The strain patterns displayed in Figure 4 may be similarly interpreted as reflecting the positions of early extensional faults. Strike-parallel extension would thus result from buttressing and strike-slip motion.

x ~---________________-_-__ A0

x-

~:.:.~.:.:~:.:B~:.:.~.:.:.~.:.~.:.~~:.:.:~:.:.~.. &:&.:.;.:.:~ :.:.;.:.:.:.:.:.~.~.;.;.;.~~ WYy ____--____------------t :~:;.‘,‘.‘,‘.‘.‘,‘.~.‘.‘.‘.‘.~.‘.’.’ ~:,~:.‘_~_~.~.~.‘_‘.‘.~.‘.‘.‘.‘.’

~~.~...::::,...,.,..~.~. i~...~;.~....:...~....,~...~

1

Model 3

Strike-parallel extension may result from strike-slip motion on large faults, similar to those envisaged by Ricou and Siddans (1986). These may be either recent faults propagating through previously unfaulted terrain or more commonly are early extensional faults which control the position of the later strike-slip faults developed in response to oblique compression. Such faults have been identified by Gratier et al. (1989) and Vialon et al. (1989) at the eastern limit of the Sub-Alpine chains adjacent to the Massif Central where NESW- to N-S-trending normal faults of Jurassic age have been reactivated as dextral strike-slip faults thus deflecting the deformation in a counter-clockwise manner. Indeed Vialon et al. (1989) argue on the basis of palaeomagnetic data taken from either side of the Frontal Pennine Thrust that this must also be a major dextral strike-slip zone, in which strike-slip motion was initiated after the northwestward thrusting. Gourlay (1986) proposed that the development of strike-parallel extension in the Chamonix syncline was related to strike-slip motion. Similar features have also been recognised in the NFPZ in association with the relatively small strike-slip zones at Cormet de Roseland (northwest of Bourg Saint Maurice1 and to the west of Courmayeur (Fig. lb). Model 4 An alternative method of developing strikeparallel extension was proposed by Butler (1982). He suggested that constant area stretching over culmination walls could be responsible for the strike-parallel extension seen within thrust sheets (Fig, 8). In this way, higher thrust sheets are progressively stretched as a thrust system propa-

y trx.x~~(;,___-___________~ AlaA

y t---__________-_--_-----~y(x-+-(Y-Y-)

A2=AO

D

r---7

r-------\

Fig. 8. The culmination model. (A-C) Balancing the hangingwall above a horse AO, Al and A2 are areas. The shaded regions in (0 are the extended culmination walls. After Butler, 1982. (D) Along strike extension as a result of stretching over culmination walls. This takes the stages (A-C) a few steps further and shows how the earliest thrust sheets in the piggy-back system accumulate the greatest amount of strain. This is a result of stretching over culmination wails in the lower thrust sheets. Thus strike-parallel extension will be greater in layer (A) than in layer (0.

gates in a piggy-back fashion. Figure 8D shows such a development in sectional view. If this model is applied to an erogenic belt in which the thrust sequence is known to be piggy-back, the strain pattern should reveal a decrease in the amount of strike-parallel extension towards the foreland. The strain pattern displayed in Figure 4

A KINEMATIC ANALYSIS INCOR~~TING

I~~REME~AL

STRAIN

supports this interpretation for the NFPZ and External Zones. However, the amount of strikeparallel extension within the Subbrian$onnais adjacent to the NFPZ (Fig. 4) is limited. In the context of the ‘culmination’ model there are two possible explanations. The first is that deformation within the Subbrianc;onnais did not involve the development of obvious tectonic fabrics. Similar deductions were made by Freeman (1985) in the Dalradian sandstones of Scotland. The second is that the SubbrianSpnnais thrust is either out of sequence or a late breaching structure. Since both tectonic fabrics and fibres have been recognised in the SubbrianFpnnais Zone, the lack of significant strike-parallel extension is most likely to be due to anomalous thrust sequence development. This is supported by field data from the NFPZ (Spencer et al., in prep.) which suggests that the Subbrianqonnais Front is a late thrust. Viable models for strike-parallel extension are therefore varied, and the extent to which any one may be applied depends on the inte~retation of the local structural features. Radial transport directions, though not applicable to the Alpine chain on a regional scale, may be used to explain strike-parallel extension within indi~dual thrust sheets. The effects of early extensional faults, positive basin inversion and oblique slip cannot be disregarded when interpreting strain patterns, where such an early extensional template is known to exist. Indeed the lack of strike-parallel extension within the Subbrianqonnais domain adjacent to the Valais could alternatively be explained by the absence of such structures. With respect to the culmination model, Parish (1985) has demonstrated that the strain patterns of the Grande Chateiard and Grande Rousses massifs are in fact compatible with stretching over lateral culmination walls. These models may be used to explain the existence of strike-parallel extension; however, they do not explain the pattern of strike and transport-parallel deformation reversal seen within the NFPZ and the SPPZ, External and Subbrian9onnais domains. In the former, the first increments are strike-parallel rotating to transport-parallel, whereas in the latter domains, the

DATA

FOR THE FRONTAL

PENNINE

ZONES

299

first increments are transport-parallel rotating to strike-parallel. Indeed such a variation in strain pattern between adjacent areas negates the application of an all encompassing model for strikeparallel extension. Furthermore, it negates models which attempt to describe the Frontal Pennine Thrust as it appears at the surface as a single entity with respect to its response to larger-scale movements. For example, the strain patterns observed do not support the proposal of Vialon et al. (1989) that the Frontal Pennine Thrust is a strike-slip fault which postdates the northwest thrusting. The variations in strain pattern, therefore, must be the result of more fundamental controls. One explanation proposed by Spencer (1989; see also Table 1) suggests that the variation in strain patterns directly reflected the different structural styles identified within the two zones, and that the patterns, though different, are in fact s~chronous. Table 1 shows that the initial transport direction within the NFPZ was actually to the north, a direction which was overprinted by W- to ~-vergent structures and is now strike-parallel. In the SFPZ transport was originally west to west-northwest and though strike-parallel extension is present this can be explained in terms of the models previously outlined. It is suggested that the strain patterns though varied share a similar origin, resulting from extension within the transport direction. General implications of the results for local and regional modelling The strain patterns displayed in Figure 4 have a number of implications which are significant to the larger-scale interpretation of the Alps. Previous Alpine studies have placed much emphasis on shortening estimates derived by techniques using two-dimensional balanced sections particularly within the cover sequences (e.g., Butler et al., 1986). The large components of strike-parallel extension identified in the Frontal Pennine Zones and throughout the Alpine chain (e.g., Merle et al., 1989, Lacassin, 1989) suggests that this approach is simplistic, even where it is possible to calculate both the thrust sequence and the amount of displacement. Any calculations based

300

on such restorations give only minimum shortening estimates and fail to reflect the true situation. Balanced section techniques should therefore be applied with care and three-dimensional restoration should be attempted where possible. A variety of geological and geophysical markers has been used to determine motion histories and subsequent thrust/plate tectonic models within the Alpine chain on both local and regional scales (e.g., Dewey et al., 1973, 1989; BijuDuval et al., 1977; Tapponier, 1977; Choukroune et al., 1986; Platt et al., 19891. The aim of this study was not to propose a similar Alpine model but to highlight the fact that finite-strain markers alone do not give sufficient data to constrain all motions. Only studies using incremental strain markers or methods by which strain can be compartmentalised into separate successive events (e.g., Gratier et al., 1989) will give the required data for this constraint. The parity between stretching directions, fibre orientations and transport directions has been confirmed for this region. Fibres are thus a useful tool in predicting relative movement directions within erogenic belts providing that the larger structure is well understood. Acknowledgements I would like to thank Professor M.P. Coward, Dr. G. Lloyd and Dr. M. Ford for their useful discussion and appraisal of the manuscript. I should also like to thank my colleagues both at Imperial College, the University of Newcastle upon Tyne and ETH, Zurich for their comments and support. My thanks go to the referees for their comments. This research was supported by an NERC grant (GT4 85 GS 58).

S. SPkN(:EK

TABLE Al Incremental strain data from the analysis of fibre complexes from the zones adjacent to the Pennine Front, French Alps; the strain was determined using the Ramsay and Huber (1983) rigid fibre model Coaxial fibres: Sample

Grid reference

Fibre orientation

Strain

4700 5051

30” 30 50 35”

1.09 4.5 3.06 2.17

D4 36A D4 36B

4692 505 1

70 22

0.17 1.2

D4 B39B

4768 5968

120

2.08

D4 41B

4697 5956

25”

0.63

D4 16

4698 505 1

350”

1.9

D4 45

4750 5061

202

2.14

D5 L6

4475 5647

120

2.5

D5 CC

4600 5058

45”

2.15

D5 SQBA D5 SQBB

4575 5055

165” 330

2.4 3

D5 1lAA D5 1lAB

4473 5047

90” 110

0.38 0.26

D5 30AA D5 30AB D5 30AC

46015047

5 20 65” 355”

13.25 7.5 3.42 0.94

D5 31A D5 31B D5 31C

4600 5046

45” 340 10

7.67 1.55 3.86

D5 M

4596 5056

85”

CDM 1

4489 5048

115”

2.92

CDM 2A

4430 5042

120” 170” 160”

1.08 0.35 0.51

CMlA CMlB CMlC

4490 5047

15 15” 15

3.6 7 6

Dl Cl

5120 5099

125”

1.92

Dl C2

5118 5099

63

1.6

Dl C3

5100 5089

10

1.47

D4 D4 D4 D4

18B 18AA 18AB 18AC

CDM 2B

2

A KINEMATIC ANALYSIS INCORPORATING

Appendix

TABLE Al (continued)

TABLE Al (continued)

Coaxial fibres: Sample

Coaxial fibres: Sample

Grid reference

Fibre orientation

301

INCREMENTAL STRAIN DATA FOR THE FRONTAL PENNINE ZONES

Grid reference

Fibre orien-’ tation

Strain

D5 L2A D5 L2CA D5 L2CB

4590 5057

30” 20” 130”

1.57 3.5 1.5

Strain

Dl C5

5190 5094

15

2.38

D2 07

4976 5077

50” 145”

2.59 1.5

D5 L3A

4590 5055

5 40

1.25 0.76

D2 08A D2 088

4977 5077

95” 82” 340 65” 30 80”

1.28 2.5 0.7 3.07 0.55 4.58

D5 OA

4595 5056

295” 25

0.55 0.64

D5 DB

4492 5048

125”

17

D5 THETA

4490 5048

75

1

D5 TRIA D5 TRIB

4600 5057

1.9

60” 45

3.36 2.77

D2 08C D2 08D SP 41

4697 5056

100

Z

4878 5077

10

9

D5 GA

45915057

10”

2.29

49B

4839 5069

80

1.13

D6 LGSA

4480 5004

BML

4806 5077

80”

1.41

75 325”

1 0.6

4750 5069

55 55

0.91 1.78

D6 CDL1

4513 5006

57

0.5

D6 CDWB

4500 5004

300”

0.79

ZL

4882 5077

360

1

D6 CDL4

4435 5005

124

1.63

R3

4250 5006

280”

1.97

D6 CDL5

4436 5005

120

3.29

R4

4252 5006

290”

0.37

D6 CDL8

4420 5006

140”

2.48

R6

4250 5004

260

2.72

4228 5003

240”

0.33

B 17A B 17B

4200 5023

R7

20 45”

1 1.85

D5 L13A D5 L13B

4602 5046

43” 45

1.97 1.5

BDO 1A BDO 1B

4150 5003

65 90”

0.8 2.6

D5 28B

46015046

95”

2

BDO 2

4230 5005

30”

1.55

D5 CB

4600 5058

110

1.12

BDO 3

42015001

75

3.33

D5 GSY

4570 5009

5

1.68

BDO 4A BDO 4B

42015004

95 75”

1.33 1.36

D5 27A

46015046

40”

0.625

4270 5022

80” 80” 80 80

6 7.25 7.5 6.33

B 14A B 14B B 14C

4310 5205

D5S4A D5 S4B D5 S4C D5 S4D

100” 110” 345”

0.85 2.06 0.38

4228 5006

DS S2A D5 S2B

4475 5038

125” 125”

2.88 0.67

RlA RlB RlD

330” 330 270

3.54 2.47 1.5

D5 S3

4770 5081

219”

2

D5 SS

4270 5018

70”

D5 31A D5 31B

4600 5046

120” 120”

LAAB

4.5 2 6

302

S. SPENCER

TABLE Al kontinued)

TABLE Al (continued)

Non-coaxial fibres: Sample

Non-coaxial fibres:

Grid

Fibre

reference

orien-

Efl

Ef2

Sample

Grid

Fibre

reference

orien-

tation D2 07A

D2 09

D3 BM2

D4 CDLC

D4 17B

D4 41Bl

D4B

B39B

4976 5077

4995 5076

4806 5069

4288 5260

4700 505 1

4697 5056

4768 5051

4768 5068

D4 B13B

D4 49

L63

4622 5050

4280 5260

4652 5053

4475 5647

Ef2

tation

75

0.29

0

295”

0.57

0

65”

0.56

0

200

0.58

0.3

55”

0.82

0.01

190

0.74

0.48

65”

1.91

0.02

180”

1.08

0.5

110”

1.95

0.04

130

2.01

0.11

0

65”

1.07

70” 80

CD

G

4600 5055

60”

0.33

45”

0.66

0

0

40”

1.06

0.01

1.14

0

30

1.25

0.02

1.21

0

15”

1.52

0.06

100

1.26

0.01

120

1.28

0.02

100”

0.5

0

115

0.9

0.01

75”

0.44

0

125”

1.2

0.02

82”

0.7

0

135

2

0.07

CDL9

4591 5057

4410 5001

94”

0.86

0

145”

2.4

0.09

111”

1.01

0

165”

2.3

0.11

124”

1.18

0

130”

1.48

0.15

290”

1

0

140

2.48

0.31

280”

1.65

0

270”

2.35

0.02

40”

1

0

50”

1.22

0

60”

1.28

0

K

45% 5056

300

1.33

0

310

1.6

0

330

1.99

0.3

35”

0.62

0

95”

1.35

0.05

50

0.96

0

120”

1.39

0.26

30

0.78

0

170

1.09

0.07

25”

0.55

0

290

0.43

0

50

0.9

0.02

300”

0.63

0

120

1.56

315”

0.77

0.01

110

2.35

0 0

340”

0.97

0.08

3.42

0.08

65” D4 77D

Efl

40

0.33

0

355

0.53

0

80

0.91

0

50

1.1

0.02

80

2.86

0.03

30

0.53

0

45

0.79

0

70

0.92

0

50

1.29

0

70

0.33

0

80

0.63

0

100

1.01

0.03

110

1.41

0.06

G78

LG3

CDL3

RIG

RlE

R2

R5

4425 5005

4510 5005

4405 5005

4228 5006

4228 5006

4228 5005

4254 5005

285”

0.09

0

305”

5.17

0.01

300

1.3

0

280

4.9

0.01

330

7.23

0.2

50”

0.98

0

60”

2.05

0

310”

2.5

0

320”

5.9

Ob

250”

1.6

0

260”

3.4

0

220”

5.28

0.01

A KINEMATIC ANALYSIS INCORPORATING INC~ME~~

303

STRAIN DATA FOR THE FRONTAL PENNINE ZONES

TABLE A2 Results of RF# analysis on reduction spots and pebbles from locations proximal to the Pennine Front (see Spencer, 1989 for original plots) Sample

Grid ref.

RF max.

RF min.

Orientation

Rs

Ri

15a 15b La12 Lm182a Lm182b B44a B44b B44c B44d B48 B49a B49b B52 B60 Xi Aime Moutiers G GLMl GLM4 GLM4G GLM5 GLMIO

4757 5070 4757 5070 4762 5069 4770 5069 4775 5069 4746 5068 4746 5068 4746 5068 4746 5068 4744 5069 4839 5069 4839 5069 4849 5069 4849 5069 4849 5069 4763 5061 4762 5061 4580 5009 4576 5009 4555 5012 4555 5012 4556 501.5 4560 5009

12.50 5.00 4.00 14.00 2.50 3.32 5.00 1.70 17.00 6.33 5.33 8.00 21.25 3.20 10.50 7.78 10.00 13.00 10.75 10.00 4.86 9.00 16.00

1.16 1.00 1.27 1.50 1.17 1.08 1.16 1.10 4.00 1.25 1.25 2.05 1.33 2.00 1.86 2.25 1.33 3.08 3.00 1.33 1.18 1.33 2.00

052” 012”

3.81 2.24 2.25 4.58 1.71 1.89 2.41 1.37 8.25 2.81 2.58 8.00 5.32 2.53 4.42 4.18 3.65 6.33 5.86 3.65 2.39 3.46 5.66

3.28 2.24 1.77 3.06 1.46 1.75 2.08 1.24 2.06 2.25 2.06 2.05 4.00 1.26 2.38 1.86 2.74 2.05 1.89 2.74 2.03 2.06 2.83

018” 098” 146” 062” 085” 076” 050” 076” 044” 160” 125” 170” 168” 165” 171”

Note the change in the orientation of the long axis of the specimens collected in the Valais Zone (samples 15a-Moutiers) compared to those collected farther south in the region of Galibier (G-GLMlO).

TABLE A3 Results from the analysis of belemnite samples from areas adjacent to the Pennine Front Sample

Grid ref.

Ramsay

Hossain

Ferguson

Orient. long axis

48.000 77a 77b CDL7 G38a G38b L5c La Lb Lc N LM

4652 5053 4622 5050 4622 5050 4512 5005 4509 5008 4509 5008 4600 5056 4610 5056 4610 5056 4610 5056 4742 5069 4739 5068

1.248 1.088 1.124 1.435 1.667 2.050 1.298 1.275 1.206 1.330 1.116 1.164

1.281 1.118 1.181 1.620 2.115 2.375 1.380 1.550 1.226 1.530 1.124 1.166

1.320 1.151 1.255 1.880 3.649 4.087 1.530 2.100 1.310 1.547 1.145 1.194

025” 025” 085” 010” 159O 120” 054” 030° 040” 040” 008” 014”

Note that those samples taken from south of the Valais Zone (G38a and G38b) have a southwestern extension direction, whilst those within or adjacent to the Valais Zone have a no~hwestem extension direction.

304

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