Using laterally compatible cross sections to infer fault growth and linkage models in foreland thrust belts

Accepted Manuscript Using laterally compatible cross sections to infer fault growth and linkage models in foreland thrust belts Hannah Watkins, Robert W.H. Butler, Clare E. Bond PII:

S0191-8141(17)30019-6

DOI:

10.1016/j.jsg.2017.01.010

Reference:

SG 3440

To appear in:

Journal of Structural Geology

Received Date: 31 August 2016 Revised Date:

20 January 2017

Accepted Date: 26 January 2017

Please cite this article as: Watkins, H., Butler, R.W.H., Bond, C.E., Using laterally compatible cross sections to infer fault growth and linkage models in foreland thrust belts, Journal of Structural Geology (2017), doi: 10.1016/j.jsg.2017.01.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT 1

Using laterally compatible cross sections to infer fault growth and linkage models in

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foreland thrust belts

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[email protected]

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Robert W. H. BUTLER¹

[email protected]

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Clare E. BOND¹

[email protected]

+44 (0) 1224 273879

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Hannah WATKINS¹*

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¹ School of Geosciences, Meston Building, University of Aberdeen, Aberdeen. AB24 3UE,

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SCOTLAND.

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Keywords: fold-thrust; laterally compatible cross-section; thrust growth; thrust linkage;

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displacement transfer; French Alps

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ACCEPTED MANUSCRIPT Abstract

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We investigate changes in shortening, displacement and fold geometry to understand the

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detailed along-strike structural variation within fold-thrust belts, and infer thrust growth and

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linkage mechanisms. Field observations from the Vercors in SE France are used to

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characterise deformation style in the region. Parallel cross sections are constructed, analysed

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and used to create shortening and thrust displacement profiles from the northern to southern

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Vercors. Sections show changes in structural style and shortening accommodation from

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thrust-dominated in the north to fold-dominated in the south. The total shortening distance in

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the Vercors does not change significantly along strike (3400-4650 m), however

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displacements along individual thrust zones do vary significantly and displacement profiles

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show a range in displacement gradients (16-107 m/km). Despite relatively simple shortening

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patterns in the Vercors, sections show a more complex 3D internal structure of the fold-thrust

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belt. Thrust displacements and geometries suggest both large-scale thrust zones and small-

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scale thrusts are soft linked, transferring displacement along strike through transfer zones.

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Short, soft-linked thrust segments indicate an intermediate stage of thrust growth and linkage,

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well documented for normal fault systems, which form prior to the formation of thrust

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branches and hard-linked displacement transfer.

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INSERT GRAPHICAL ABSTRACT HERE

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1. Introduction

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Dahlstrom (1969) presented a theoretical construction of gradual structural change in the

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along strike-geometry and displacement of thrust systems. In this he envisaged thrusts

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relaying displacement through an array (Figure 1a). Individual thrusts lost displacement and

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died out laterally in a predicable fashion – giving rise to the so-called “bow and arrow rule”

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(e.g. Elliott 1976) where the ratio of thrust map-half-length and maximum displacement is

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ACCEPTED MANUSCRIPT about 10:1. Since then there have been various attempts to quantify the amount of shortening

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in thrust belts and its along strike variations (e.g. Alvarez-Marron, 1995; Fermor, 1999;

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Apotria & Wilkerson, 2002; Torres Carbonell et al., 2013). These variations in shortening

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effect structural style, fold geometry, internal strain, thrust displacement, and other geometric

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and kinematic aspects of fold-thrust belts (e.g. Pérez-Estaún et al., 1997; Cooley et al., 2011;

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Torres Carbonell et al., 2013; Qayyum et al., 2015). The underlying causes of along strike

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variation in fold-thrust belt structure have been attributed to pre-existing basement and basin

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structure, changes in stratigraphy causing variation in rheological properties and variation in

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shortening closer to the hinterland in the internal zones (Woodward, 1988; Pérez-Estaún et

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al., 1997; Cooley et al., 2011; Qayyum et al., 2015; Rocha & Cristallini, 2015; Bhattacharyya

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& Ahmed, 2016; Cain et al., 2016). Here we develop studies of lateral variation in thrust belt

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structure using the notion of lateral compatibility to develop an array of mutually-compatible

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cross-sections and use these to examine lateral changes in displacement.

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INSERT FIGURE 1 HERE

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Despite decades of research on thrust systems, there are few systematic descriptions of

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variations in shortening and displacement through thrust arrays. This contrasts with research

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on normal fault systems where there is a well-established fault growth and linkage model

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(Childs et al., 1995). This model suggests that small faults initiate and, as they grow, the

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stress perturbations produced by the propagation of their process zones and their strain fields

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interact so they become soft linked. As displacements increase, fault splays may form,

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transferring displacement between established fault segments that become hard linked (Figure

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1b; Childs et al., 1995; Davis et al., 2005; Fossen, 2010). Based on this model developed for

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extensional faults, it would also be expected that displacement on a large fault is made up of

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slip on multiple hard-linked smaller fault segments that have branched to form a major fault

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zone (Figure 1c). However, in contrast with this understanding from normal faults, much of

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ACCEPTED MANUSCRIPT the early work on the three dimensional structure of thrust systems treated them as fully

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connected (hard-linked). In this fashion displacement has been interpreted to have transferred

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simply across branch lines (e.g. Diegel, 1986; Hossack, 1983). While these studies of finite

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fault patterns show predicable map patterns in restored state (e.g. Butler, 1985; Fermor,

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1999), they give little information on how the thrust array developed.

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The aim of this paper is to examine a thrust system that shows rather low displacements –

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focussing on the Vercors segment of the Sub-Alpine thrust belt of SE France. The intention is

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to establish whether systematic variations of displacement exist through the thrust array that

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might inform the growth of the system. Field data are used to highlight large-scale structural

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variation along strike in the fold-thrust belt. Cross sections are used to investigate how

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shortening varies along strike, and how fold geometry changes within a fold-thrust belt.

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Thrust zone displacements are also analysed to determine the detailed nature of how

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shortening may be distributed, how this may vary along strike, and how displacement is

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transferred within and between fault zones.

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2. Vercors field area

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The Vercors is a fold-thrust belt forming part of the Sub-Alpine Chains of France, which,

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along with the Jura, represent the most westward regional-scale deformation structure

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deformation associated with the Alps. The French Sub-Alpine Chains run for over 200 km

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from Chamonix in the NE to south of Grenoble in the SW (Figure 2a) and include the Haute-

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Giffre, Bornes, Bauges, Chartreuse and Vercors subdivisions (Moss, 1992a; Bellahsen et al.,

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2014). Thrusting and folding in the Vercors is thought to have occurred in the middle-late

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Miocene (Roberts, 1994; Bellahsen et al., 2014), between 10-6 Ma (Butler, 1987), and

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represents one of the youngest phases of Alpine deformation. Prior to Alpine mountain

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ACCEPTED MANUSCRIPT building, the external Alps were part of a rift margin where the main extensional phase

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occurred during the mid-late Jurassic and regional subsidence occurred during the Cretaceous

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(Lemoine et al., 1986; Rudkiewicz, 1988; Butler, 1989). Although many associated normal

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faults are present in the Jurassic-Cretaceous succession, they are not thought to have been

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inverted during Alpine compression (e.g. Butler, 1989).

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The Vercors forms a plateau elevated 1000-2300 metres above its foreland (regional) level,

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some 15-20 km across. At outcrop it is dominated by the “Urgonian” limestone (Hauterivian-

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Barremian) that is the youngest carbonate platform that built out from this part of the

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European continent towards the Tethyan ocean. The Urgonian is c 250-300m thick. It is

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underlain by older Cretaceous and Jurassic strata. These strata comprise of marls, shales,

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mudstones and competent limestones (Ramsay, 1963; Lemoine et al., 1986; Arnaud-Vanneau

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& Arnaud, 1990). The Mesozoic strata within the thrust belt form a marked mechanical

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multilayer, which is likely to have a significant control on present-day fold and thrust

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geometries. Collectively the stratigraphy achieves thicknesses in excess of 6 km in the east of

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the Vercors. However, well-data (see Butler 1987) for the foreland show the equivalent

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stratigraphy to be less than 3 km thick and dominated by carbonates. Thus the Mesozoic

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strata formed a broad westward-tapering wedge. During the Miocene this wedge was pushed

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westwards, along a basal detachment established in Triassic evaporites (Bayer et al., 1987;

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Mugnier et al., 1987; Roberts, 1991) and lower Jurassic shales, forming a high plateau offset

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by internal thrusts. The front of the Vercors appears to be pinned by pre-existing normal

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faults (so-called “faille de l’Isere” Butler, 1987, 1989).

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This study builds upon previous analyses of the Vercors. Butler (1987) presents a single

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balanced cross-section through the Vercors that established that plateau uplift and its internal

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ACCEPTED MANUSCRIPT thrusting resulted from about 10 km of shortening, with an additional 15 km of displacement

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being accommodated on a large backthrust that emerges to the east of the Drac Valley.

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Deville & Sassi (2005) presented a cross section through the Vercors, as part of a larger study

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on structural variation and thermal evolution of the Sub-Alpine Chains. As part of their

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regional study of the Subalps, Gratier et al. (1989) presented four cross-sections through the

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Vercors and demonstrated the southward decline in net displacement across the system. This

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is in contrast to the much higher displacements interpreted further north in the external

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Alpine thrust belt, where hard-linked interpretations have been applied (Butler, 1985; Deville

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et al., 1994; Affolter et al., 2008; Bellahsen et al., 2012; Bellahsen et al., 2014). The position

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of the Vercors, at the southern end of this declining chain, is ideal for testing the thrust

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linkage model proposed by Dahlstrom (1969), and the fault growth and linkage model

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originally established for normal fault systems (Figure 1b, Childs et al., 1995).

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3. Field observations

Field observations, photographs, sketches and bedding orientation data were collected in the

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field. We now describe some field relationships and fold geometries from different structural

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zones throughout the fold-thrust belt, which have been influential in regional cross section

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construction. The Vercors was divided into four main structural zones (see Figure 2b), which

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were chosen based on the positions of major thrusts, well-developed fold forelimbs marking

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the edge of large fold structures, or topographical features.

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3.1.Pont-en-Royans: thrust zone splays, backthrusts and disharmonic folding (structural zone 2)

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ACCEPTED MANUSCRIPT The frontal thrust of structural zone 2 (see Figure 2b) can be observed in the village of Pont-

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en-Royans where, instead of a single thrust surface, several thrust splays can be seen cutting

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up through folded Upper Cretaceous units (Figure 3a), defining a distributed thrust zone.

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From the outcrop, it is unclear how these thrusts link at depth. The fold geometry at Pont-en-

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Royans is also of particular interest due to its very short wavelength. Figure 3b shows a very

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tight fold to the east of the village; we have interpreted this as being caused by backthrusts

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branching off the main foreland-directed thrust at depth. Backthrusts can be seen elsewhere

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throughout the Vercors at a range of scales. The presence of backthrusts, particularly in fold

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forelimbs, have a significant control on resulting fold geometry and potentially damage

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distribution.

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The tight fold geometry at Pont-en-Royans is associated with high density fracturing in the

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Urgonian Limestone, and small-scale, disharmonic folding in the Hauterivian shales below

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(Figure 3c). Disharmony in weak underlying layers (Hauterivian shales) appears to have

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allowed the Urgonian limestone to fold tightly, resulting in the geometry seen on Figure 3c.

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This difference in deformation style between more competent units such as the Urgonian and

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Upper Valanginian, and more incompetent units such as the Lower Valanginian and

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Hauterivian can be seen throughout the Vercors at a range of structural positions. The Pont-

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en-Royans structure is a good example of the relationship between deformation distribution

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in strong versus weak units in the Vercors, caused by rheological differences. The competent

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Urgonian Limestone defines the large-scale fold geometry, and much smaller scale folds

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form in the incompetent Hauterivian below in response to compression in the fold core.

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3.2.Petits Goulets: pre-existing structure in fold forelimbs (structural zone 2)

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ACCEPTED MANUSCRIPT The forelimb structure at Petits Goulets (see Figure 2b for location) is another example of the

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importance of backthrusts in fold-thrust belts. This forelimb exposure has a very different

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geometry to at Pont-en-Royans, despite being located just 1.5 km along strike to the south. In

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this example a prominent fault can be seen in cross section in the forelimb of the fold (Figure

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4a). We have interpreted this as a westward dipping backthrust that pre-dates the main

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folding and thrusting phase, based on the fold geometry and steep fault angle observed in the

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field. However Butler (1987) suggests the backthrust formed late and offsets the main

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eastward-dipping thrust. Initially the dip of this feature would have been much shallower, but

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it has since been rotated within the forelimb of a larger fold-thrust structure (see inset Figure

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4a). The presence of this backthrust has created small-scale folding within the fold forelimb,

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which causes structural complexity and could potentially affect the strain distribution within

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this region.

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3.3.Rencurel Thrust (structural zone 3)

The Rencurel Thrust zone marks the western edge of structural zone 3 (Figure 2b). Above the

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thrust is a well-developed forelimb of the Rencurel structure. It is best exposed in the Gorges

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de la Bourne (Figure 2b) where a gently westward-dipping forelimb can be seen in the

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Urgonian (Figure 4b). Minor backthrusting, associated with the Rencurel Thrust appears to

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have developed at this locality, but in this case the backthrust displacement is minimal

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meaning it does not have a significant impact on the fold geometry, unlike at Pont-en-Royans

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(Figure 3b).

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3.4.Plateau de Sornin (structural zone 3) 8

ACCEPTED MANUSCRIPT At the northern tip of the Vercors the Urgonian rises sharply from NW of Sassenage, where it

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is exposed in the Isere Valley, to the Plateau de Sornin, 1300 m above (see Figure 2b for

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location). The Urgonian flattens on the plateau before being cut by the Rencurel Thrust Zone

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5.5 km further west (Figure 5a). This geometry is quite different from anything seen further

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south in the Vercors and suggests a more complex thrust geometry at depth. Unlike further

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south where exposure of Urgonian Limestone suggest smooth, curved fold profiles, the

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Urgonian above Sassenage appears to have a kinked geometry with distinct hinge zones.

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3.5.Urgonian plateau and the Drac Valley (structural zones 3 and 4) The eastern edge of the Vercors is marked by the Urgonian plateau and ridge that runs

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continuously for over 50 km from St-Nizier-du-Moucherotte in the north to Chatillon-en-

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Diois at the southern edge of the Vercors (Figure 2). Urgonian bedding on the ridge dips

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moderately to the west, probably due to the presence of an underlying backthrust. The

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presence of such a continuous, high-elevation ridge suggests this backthrust must be a

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continuous feature, with relatively high displacement. The Drac Valley to the east of the ridge

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exposes multiple ridges of Jurassic sediments (Figure 5b, all dipping to the west. These ridges

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are also likely to have formed due to eastward vergent backthrusts. The backthrusts forming

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the Drac Valley structures and the Urgonian ridge probably splay off the large-displacement

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backthrust bounding the eastern edge of the Drac Valley, suggested by Butler (1989).

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4. Cross sections through the Vercors

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ACCEPTED MANUSCRIPT To analyse structural variation along strike, a series of 6 parallel, regional-scale cross sections

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were constructed through the Vercors, from the Bas Dauphine in the west to the Drac Valley

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in the east (see Figure 2b). The orientations of the cross sections are ESE-WNW (110-290°).

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This orientation was chosen by calculating the average dip direction of surface bedding data

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(see Figure 2d), which is parallel to the thrust transport direction (Philippe et al., 1998). The

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six cross section lines were chosen in locations where exposure is good enough to analyse

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fold geometry in cross section, for example along WNW-ESE oriented gorges. The bedding

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data used to construct the cross sections was made up of our own field data, along with data

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from published geological maps of the Vercors (B.R.G.M. map sheets 1967, 1968, 1975,

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1978, 1983). Lithological unit boundaries and fault positions on the published maps

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(B.R.G.M. map sheets 1967, 1968, 1975, 1978, 1983) were also used for cross section

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construction. Bedding data was projected from between 1500-2000 m either side of each

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section line; the projection distance was chosen to ensure high enough data density on each

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cross section.

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Field observations have shown the variation in deformation style in different units must be

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taken into account when constructing a regional-scale cross section. In weaker units, such as

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the Hauterivian, the bedding orientation may relate either to large scale fold geometry, and

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therefore parallel stronger unit horizons, or to much smaller scale folds which are not

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representative of regional scale structure (e.g. Figure 3c). For this reason, where

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discrepancies between bedding data in competent and incompetent units are found, the data

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from the more competent unit is assumed to be more representative of larger scale structure,

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and

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Aptian/Maastrichtian) were restored using line-length restoration with a fixed pin at the

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WNW end of each cross section (e.g. Dahlstrom, 1969) to ensure they balanced and had

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realistic fault geometries.

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is

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4.1.Barbieres cross section (Figure 6) The Barbieres cross section is the most southerly section presented here. It runs through the

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south-central Vercors, across the wide Urgonian plateaus. The cross section features two

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high-displacement thrusts, both of which are towards the western end of the section (thrust

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zones 1 & 2, Figure 2b). Backthrusts splaying off eastward-dipping thrusts are also a key

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feature of this section; some are emergent and some have been interpreted at depth due to

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asymmetric folding at the surface. Structural zone 3 is bounded by the Rencurel Thrust Zone

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(thrust zone 3) which, in this section, is non emergent and instead is inferred at depth due to

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the presence of a well-developed, steeply dipping forelimb at the surface. The section also

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includes a large number of out-of-zone thrusts (i.e. thrusts located outside of the main thrust

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zones 1-3).

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4.2.Beauregard-Baret cross section (Figure 7)

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The Beauregard-Baret section is 5.8 km north, along strike of the Barbieres section. It also

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features the two high-displacement thrusts at the western end, bounding structural zones 1

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and 2, as well as the well-developed forelimb above the non-emergent Rencurel thrust at the

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western edge of structural zone 3. Thrusting in this section is mainly foreland-directed, but

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there are some minor backthrust splays inferred in structural zone 1. The section contains a

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few out-of-zone thrusts, as well as minor folding in larger-scale fold backlimbs.

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4.3.Ste-Eulalie-en-Royans cross section (Figure 8) The Ste-Eulalie-en-Royans section is located 10 km north, along strike of the Beauregard-

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Baret section. There is no surface evidence for the main thrust controlling the development of

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the frontal structure (zone 1) seen on the Barbieres and Beauregard-Baret sections therefore it

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is assumed that displacement has decreased and this structure no longer exists as far north as

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the Ste-Eulalie-en-Royans line. The most striking feature of this section is the complication in

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the forelimb of the frontal structure (zone 2). This is the oversteepened backthrust described

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at Petits Goulets in section 3.2 (Figure 4a); based on the steep thrust dip this is interpreted as

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an older thrust with an eastward transport direction that has later been oversteepened by the

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formation of a larger forelimb. The presence of this backthrust structure has been attributed to

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a buttressing effect (as suggested by Butler, 1987) due to the presence of a basement-

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involved normal fault at the level of the regional detachment. The buttressing effect has been

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described by Welbon (1988) and Butler (1989), where a rigid obstacle at depth (i.e. the

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basement fault scarp) acts as a buffer, inhibiting propagation of a regional detachment layer

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towards the foreland. In response to this a hinterland-directed thrust is inferred to have

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propagated up from this buffering obstacle at depth. Alternatively this feature could be a

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‘rabbit-ear’ structure, in which the hinterland-dipping thrust and the backthrust are coeval;

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the backthrust ramps out of a bedding slip surface, forming asymmetrical fault-propagations

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folds within the foreland-dipping fold forelimb (Neely & Erslev, 2008). Since the major

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eastward-dipping thrust is not exposed it is not possible to distinguish between the two

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interpretations. The Ste-Eulalie section also reflects the tight fold geometry above thrust zone

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2 seen at Pont-en-Royans (Figure 3b). The other key feature of the St-Eulalie section is the

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Rencurel Thrust zone which marks the western end of structural zone 3. On this section it is

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an emergent feature with several major thrust splays and no significant fold forelimb

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development.

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INSERT FIGURE 8 HERE

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4.4. St-Romans cross section (Figure 9) 6.7 km north along strike from the Ste-Eulalie-en-Royans section line is the St-Romans cross

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section. There is little field evidence for backthrusting in the forelimb of the frontal structure,

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as seen on the Ste-Eulalie-en-Royans section; instead a simpler, steeply dipping forelimb is

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seen on the St-Romans section. The Rencurel Thrust zone (thrust zone 3) on the western edge

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of structural zone 3 is again emergent on this cross section, with both foreland and hinterland

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directed thrust splays. A major feature of this cross section is a high-displacement, westward-

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dipping backthrust at the eastern end of the section. This thrust is probably associated with

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the westward dip of bedding on the Urgonian Plateau.

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4.5.Cognin-les-Gorges cross section (Figure 10) The Cognin-les-Gorges cross section lies 7.8 km north, along strike from the St-Romans

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section. This cross section features rotated and steepened backthrusts in the forelimb of the

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frontal structure, similar to that seen at Petits Goulets (Figure 4a) and on the St-Eulalie cross

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section (Figure 8). The deep structure of this feature has been interpreted in the same way as

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further south, initiating at a basement fault scarp at depth. The Rencurel thrust zone (thrust

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zone 3) in this cross section is represented as a single structure rather than fault splays, unlike

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further south. The eastern edge of the Vercors is again marked by a high-displacement,

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westward-dipping backthrust below the Urgonian Plateau, presumably associated with

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backthrusting in the Drac Valley (structural zone 4).

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INSERT FIGURE 10 HERE

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4.6.Sassenage cross section (Figure 11) The Sassenage cross section is the most northerly of the six regional sections presented, and

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is located 11 km along strike, to the north of the Cognin-les-Gorges section. It runs through

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the narrow northern tip of the Vercors, and is therefore a relatively short section. The most

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prominent feature of the Sassenage section is the large displacement Rencurel thrust zone

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(thrust zone 3) that emplaces the Hauterivian and Valanginian on top of Miocene rocks.

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Another distinctive feature of this cross section has already been described in section 3.4; the

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Urgonian limestone of Plateau de Sornin, which rises sharply from the Valley bottom (Figure

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5a). This geometry has been interpreted in the cross section as being associated with a flat-

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ramp-flat thrust geometry at depth. Further east towards the hinterland a series of closely

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spaced thrusts that splay off a major thrust at depth can be seen. These low displacement

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thrusts are associated with well-developed forelimbs, unlike the large-displacement thrust

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further west, which has no significant fold forelimb developed.

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4.7.Deep structure, the Drac Valley and pre-existing, Mesozoic normal faults

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On each of the cross sections presented (Figures 6-11), a large scale (>1 km displacement)

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normal fault is shown to offset the top basement horizon; this is the Faille de L’Isere fault.

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Butler (1987, 1989) interpreted this fault as the main control on the position of the frontal

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thrust, suggesting that the lower detachment of the fold-thrust belt was unable to propagate

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westwards from the Triassic evaporites in the Faille de L’Isere hangingwall through to the

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ACCEPTED MANUSCRIPT much stronger basement in the footwall. Instead the detachment deflected upwards over the

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basement fault scarp of the Faille de L’Isere and emerged as the frontal thrust at the surface,

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on the western margin of the Vercors. This interpretation has been used in this paper to infer

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a simple model for the deep structure. Pre-existing literature has also been used to infer the

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structure to the east of the cross sections; beyond the main field area. Bedding data from the

340

Urgonian limestone to the west of the Drac Valley shows a moderate dip to the west,

341

suggesting a backthrust may be present at depth.

342

In the Drac Valley a series of westward-dipping dip panels in Jurassic units can be seen

343

(Figure 5b), also indicating westward dipping backthrusts at depth. These are probably linked

344

to the large displacement backthrust suggested by Butler (1987). Backthrusting in the Drac

345

Valley has been represented as a single, large fault on the Sassenage cross section (Figure 11)

346

although in reality the structure is probably more complex, with multiple thrust splays.

347

The other main structural feature of the cross sections is normal faults. Each of the cross

348

sections contains several eastward or westward-dipping normal faults, most of which are

349

thought to pre-date thrusting and probably relate to Mesozoic rifting during opening of the

350

Tethys Ocean. The normal faults on the cross section are shown to be offset by younger

351

thrusts, and none are shown to have been reactivated or inverted. This is because there is little

352

field evidence along the lines of section to suggest the pre-existing normal faults significantly

353

influence fold-thrust structure; instead they appear to have behaved passively during

354

thrusting. Other authors report localised interaction between pre-existing normal faults and

355

younger thrusts in the Vercors, for example Butler (1987, 1989) shows an example where a

356

normal fault has juxtaposed the strong, competent Urgonian limestone alongside weaker,

357

incompetent Hauterivian shales. This horizontal change in lithology meant that a thrust could

358

not easily propagate from the weak shales into the strong limestones, so instead, intense,

359

localised folding occurred, with potential reactivation of the normal fault in the hangingwall

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of the newly formed thrust, a phenomenon termed buttressing (Welbon, 1988). Although

361

evidence for buttressing is seen in Mesozoic rocks in the Vercors, it appears to be quite rare

362

and is therefore not represented on the cross sections.

364

5. Along strike variation

365

5.1.Shortening and total thrust displacement

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Regional cross sections were analysed to determine changes in fold geometry, thrust

368

displacement and shortening. Displacements on each thrust were measured at the top

369

Urgonian horizon for the six regional cross sections (Figures 6-11), along with total

370

shortening distance and shortening percentage. Figure 12 shows how these estimates for

371

shortening, along with the total thrust displacement for each cross section, varies along strike.

372

The graph patterns for total thrust displacement and shortening distance are quite similar

373

(Figure 12a), indicating that, overall, the majority of shortening is accommodated by

374

thrusting and brittle deformation rather than folding. Both the total thrust displacement and

375

shortening distance increase from Sassenage to St-Romans, before decreasing in the Ste-

376

Eulalie-en-Royans section and increasing again to the Barbieres section. On the southern two

377

cross sections (Beauregard-Baret and Barbieres) the shortening distance becomes larger than

378

the total thrust displacement, probably due to the presence of well-developed folds on lower

379

displacement thrusts (Figures 10 &11).

380

INSERT FIGURE 12 HERE

381

The shortening percentage has also been calculated for each cross section and plotted on

382

Figure 12b. This value is commonly used to compare the amount of compression and strain in

383

thrust belts. Figure 12b shows the shortening percentage trend along strike is different to that

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ACCEPTED MANUSCRIPT of shortening distance (Figure 12a). The three northern sections (Sassenage, Cognin-les-

385

Gorges and St-Romans, Figures 6-8) have the highest shortening values (11-14%) and the

386

three southern sections (Ste-Eulalie-en-Royans, Beauregard-Baret and Barbieres, Figures 9-

387

11) have the lowest shortening values (5-10%). Shortening percentages suggest the two most

388

southerly sections underwent less shortening that the two most northerly sections, however

389

the shortening distance values suggest the opposite (Figure 12a). The reason for this

390

discrepancy is that the two southerly cross sections are much longer than the northern

391

sections. If you compare two cross sections with the same shortening distance but different

392

lengths, the shorter section will appear to have higher shortening percentage than the longer

393

section. Clearly when analysing thrust belt shortening using cross sections of different

394

lengths it is important to use the actual shortening distance value rather than shortening

395

percentage, for the reason shown above.

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5.2.Thrust zone displacement

Three main thrust zones were identified in the study area; zone 1 is the frontal thrust zone in

399

front of the Beauregard-Baret structure (structural zone 1). It can be traced along the

400

mountain front in the southern Vercors. Zone 2 is the thrust zone beneath the Pont-en-Royans

401

structure (structural zone 2), and zone 3 is the Rencurel Thrust zone (bounding structural

402

zone 3) that merges with the Voreppe Thrust in the southern Chartreuse (Figure 2). Total

403

displacements for each thrust zone were calculated at the top Urgonian horizon for each cross

404

section to determine whether shortening variations along strike occur along a single thrust

405

zone or are due to consistent changes in displacement within all three main thrust zones.

406

Displacement variations along strike on individual thrusts were not analysed because at the

407

surface thrusts are relatively short and usually do not intersect more than one cross section.

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ACCEPTED MANUSCRIPT INSERT FIGURE 13 HERE

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Figure 13 shows how thrust zone displacements vary along strike, as well as on out-of-zone

410

thrust in between major thrust zones. Thrust zone 1 only appears on the two most southerly

411

sections, where its displacement is 900-1100 m. This sudden increase in displacement

412

between the Ste-Eulalie-en-Royans and Beauregard-Baret sections is reflected in the fold

413

geometry; on the northern banks of the Isère river no fold is present and topography is

414

relatively flat; 3km to the south of this a large fold with a well-developed, steeply dipping

415

forelimb rising 500 m above the river in the Urgonian. Thrust zone 2 is not present on the

416

Sassenage section; its displacement gradually increases from the Cognin-les-Gorges section

417

to the south at a much steadier rate than thrust zone 1. Thrust zone 2 remains a relatively low

418

displacement zone throughout the region. Displacement on thrust zone 3 is high in the north,

419

and decreases gradually to the south where it is interpreted to have zero displacement in the

420

Urgonian; the thrust is still present at depth in the Jurassic succession. On the southern two

421

cross sections thrust zone 3 is represented at the surface as a well-developed fold with a

422

steeply dipping forelimb. This fold geometry contrasts with sections further north along

423

thrust zone 3, where thrust displacement in the Urgonian is much higher, but folds are less

424

well developed and have only shallowly dipping forelimbs. Out-of-zone displacement in

425

between thrust zones 1, 2 and 3 is relatively low in the north, increasing significantly to the

426

south.

427

The patterns of thrust displacement variation in the three thrust zones and out-of-zone are a

428

good example of how displacement may be transferred along strike in a fold-thrust belt. In

429

the north thrust zone 3 is clearly the dominant one, accommodating the majority of

430

shortening in the Vercors. As displacement within thrust zone 3 decreases to the south, both

431

the out-of-zone and thrust zone 1 displacement gradually increase to accommodate

432

displacement loss from zone 3. Displacement in thrust zone 2 remains low throughout the

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ACCEPTED MANUSCRIPT Vercors, indicating that it has little influence on along strike shortening variation (Figure 13).

434

Data presented on Figure 13 shows that variation in shortening along strike is accommodated

435

along certain zones rather than being due to consistent changes in displacement on all thrusts.

436

Figure 13 also shows that displacement in all three thrust zones, as well as out-of-zone

437

displacement is low on the Ste Eulalie section, causing a low shortening distance on Figure

438

12a.

439

6. Discussion

441

6.1 Shortening and structural style of the Vercors

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The six cross sections presented in section 4 suggest shortening in the Vercors is up to 5 km;

443

this is in contrast to other authors who estimate more shortening such as Butler (1989) and

444

Philippe et al. (1998) who both calculate 10 km of shortening and Bellahsen et al. (2014) who

445

suggests that the value is around 28 km. The reason for the differences in shortening

446

estimates is because the cross sections of Butler (1989) and Bellahsen et al. (2014) extend

447

much further east than our own. In this paper we focus mainly on the deformation in the

448

central Vercors (i.e. structural zones 1-3, see Figure 2b), however Butler’s (1989) estimate

449

includes large-scale backthrusting present in the Drac Valley to the east of our sections.

450

Figure 5b showed west-dipping ridges of Jurassic units which are probably the backlimbs of

451

folds formed above backthrusts. The topographic relief of these features is quite high, which

452

might indicate high thrust displacement and significant shortening on these structures, which

453

could accommodate any ‘missing’ shortening distance.

454

Total thrust displacement curves (Figure 12a) suggest a gradual increase in displacement to

455

the south from the Sassenage section to the St-Romans section, before decreasing on the Ste-

456

Eulalie-en-Royans section and increasing again in the Beauregard-Baret and Barbieres

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19

ACCEPTED MANUSCRIPT sections. This rather complicated pattern can be explained simply by the presence of

458

backthrusts at the eastern end the Cognin-les-Gorges and St-Romans sections (Figures 9 &

459

10); if the displacements of these two thrusts were removed from the thrust displacement

460

graph (Figure 12a), there would be little change in total thrust displacement along strike, with

461

the exception of the Ste-Eulalie-en-Royans section, indicating relatively consistent

462

deformation along the length of the Vercors.

463

The Ste-Eulalie-en-Royans section (Figure 8) is anomalous to the overall shortening patterns;

464

the total thrust displacement and total shortening distance (Figure 12a) are much lower than

465

on adjacent sections. If shortening did decrease suddenly around the Ste-Eulalie-en-Royans

466

section line we might expect to see localised faults and folds forming oblique to the main

467

thrusts zones, deviations in the orientation of these thrust zones, and more intense

468

deformation in this region. These features would be required to accommodate the sudden

469

change

470

The map patterns of thrust zones 1-3 show continuous, parallel features that do not show any

471

significant changes around the Ste-Eulalie-en-Royans section line. This would suggest that

472

the total shortening distance along the thrust belt is relatively consistent with no sudden

473

changes. This suggests that a significant amount of shortening (c. 2500-3000 m) is absent

474

from the Ste-Eulalie-en-Royans cross section (Figure 8). Displacements on individual thrusts

475

on this section line are well constrained by map patterns and field observations so the

476

‘missing’ shortening suggests at least one major compressional feature is absent from this

477

section. The absence of this feature was inferred by analysing and comparing cross sections

478

along strike; when observed in isolation this cross section balances when restored (Figure 8)

479

and appears as a structurally valid interpretation. Along strike cross section analysis is not

480

only important for determining large scale structural trends, but can also be used as a tool to

481

validate structural interpretation.

shortening

distance.

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ACCEPTED MANUSCRIPT The patterns for total thrust displacement and shortening distance on Figure 12a also indicate

483

a change in the nature of deformation along strike; in the north thrust displacement and

484

shortening are similar, whereas in the south shortening distance is considerably higher than

485

thrust displacement. This relationship reflects a change in structural style, as exemplified by

486

the Rencurel thrust zone (thrust zone 3). In the north brittle deformation dominates; thrust

487

displacement is high but folds are poorly developed. In the south, around the Beauregard-

488

Baret and Barbieres sections, the thrust has zero displacement at the surface but folds are well

489

developed. A similar amount of shortening is accommodated along strike but structural

490

geometries are very different.

491

There are several reasons behind this change in fold geometry, including variation in burial

492

depth and deformation conditions; rate of deformation; variation in lithology; change in thrust

493

kinematic mechanism, variation in detachment characteristics and proximity to the thrust tip.

494

The change from thrust-dominated shortening in the north to fold-dominated shortening in

495

the south could be explained by deeper burial depths, and therefore warmer deformation

496

conditions in the south. Investigations into the organic maturation and tectonic loading by

497

Moss (1992a, 1992b) suggest the southern Sub-Alpine chains (Vercors and Chartreuse) have

498

undergone shallower burial than the northern chains (Haute-Giffre and Aravis massifs),

499

however data was not collected south of the Gorges de la Bourne in the Vercors (Figure 2b),

500

meaning little is known about deformation conditions in the southern Vercors, around the

501

Beauregard-Baret and Barbieres sections.

502

Variation in the rate of deformation may also cause changes in structural style; slower rates

503

might allow well-developed folding without brittle failure to occur in the south, whereas

504

faster rates might be the cause of brittle thrusting and poorly developed folds in the north.

505

Although variation in deformation rate could be behind the observed structural variation,

506

there is no other evidence for it so it is an unlikely cause. A major change in lithology would

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21

ACCEPTED MANUSCRIPT lead to changes in the rheological behaviour of the deforming rocks, and therefore alter the

508

structural style. However, once again there is little evidence for significant changes in

509

lithology in the Vercors that might cause these changes in structural style.

510

It is probable that the variation in fold geometry could be caused by changes in kinematic

511

thrusting mechanism along strike and proximity to the thrust tip. The change from a high

512

displacement thrust in the north to a non-emergent thrust in the south suggests the Rencurel

513

Thrust (thrust zone 3) probably dies out south of the cross sections so the folds on the

514

Beauregard-Baret and Barbieres sections (Figures 10 & 11) are likely to be much closer to

515

the thrust tip than the northern sections. The sections also show a difference in thrust

516

propagation from north to south. In the northern sections (Figures 6-9) the Rencurel Thrust is

517

shown to have propagated all the way through the Upper Cretaceous whereas on the southern

518

two sections (Figures 10 & 11) the thrust tip is in the Jurassic or Lower Cretaceous. The

519

difference in structural geometry might relate to the thrust propagation rate: for the same

520

amount of shortening the thrust in the south may have had a slower thrust propagation rate,

521

giving time for the fold to develop in the hangingwall in front of the thrust tip, as in the

522

trishear mechanism (Erslev, 1991). The thrust in the north may have propagated up section

523

much faster, giving little time for folding in the hangingwall, as in the fault-parallel flow or

524

fault-bend-fold mechanisms (Suppe, 1983; Egan et al., 1997; Kane et al., 1997). It has also

525

been suggested (e.g. by Philippe et al., 1998) that the nature of the lower detachment to the

526

fold-thrust belt changes along strike; in the northern Vercors the detachment is thought to be

527

in marls and shales, whereas in the south thrusts are thought to detach in a salt horizon

528

(Philippe et al., 1998). The lithology of a detachment has been found to control the structural

529

style of a fold-thrust belt; frictional detachments create thrusts with asymmetric folds whereas

530

viscous detachments (such as salt) create more symmetric structures with less faulting (e.g.

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ACCEPTED MANUSCRIPT 531

Ruh et al., 2012). The variation in detachment lithology could therefore influence structural

532

style variations in the Vercors.

533

6.2 Thrust linkage and growth Analysing displacements in individual thrust zones has shown variations from north to south.

535

Thrust zone displacement curves (Figure 13) show evidence for displacement transfer along

536

strike; zone 3 dominates in the north and decreases to the south, whereas displacement in

537

zone 1 and on out-of-zone thrusts increases to the north. Displacement in thrust zone 2

538

appears not to change significantly along strike. These patterns can be compared with the

539

model of Dahlstrom (1969) showing thrusts relaying displacements through an array (Figure

540

1a). In the model the total shortening is equal on all sections, suggesting that thrust belt

541

structure is relatively simple on a large scale. However, displacement on individual thrusts

542

varies significantly, which creates structural complexity within the thrust belt. Dahlstrom’s

543

model has been reconstructed for the Vercors (Figure 14), showing that displacement loss in

544

thrust zone 3 to the south is accommodated by displacement gain in thrust zone 1. Thrust

545

zones are not isolated systems, but instead these large features are soft linked. Displacement

546

is transferred between thrusts zones to maintain a relatively constant shortening distance

547

along strike. Although thrusting in the Vercors occurs on a much smaller scale to the

548

Canadian Rockies, on which Dahlstrom’s (1969) thrust transfer zone model was based, we

549

can see the same mechanisms for large-scale thrust linkage.

550

INSERT FIGURE 14 HERE

551

The relatively low thrust displacements in the Vercors has allowed us to analyse the

552

mechanisms of thrust growth and linkage. Map patterns within thrust zones show them to be

553

made up of multiple short thrust segments rather than single, long thrust structures. These

554

short thrust segments within individual thrusts zones do not branch off one-another and

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23

ACCEPTED MANUSCRIPT appear isolated on the geological map (Figure 2b). These patterns suggests that thrust zones

556

must be made of short, soft linked thrusts, which transfer displacement along strike and

557

contribute towards the total thrust zone displacement. These observations suggest the

558

established model for normal fault growth and linkage (Childs et al., 1995, Figure 1b-c) could

559

also be applied to thrust systems. The soft linked thrusts observed in the Vercors are an

560

under-reported thrusting style, which may represent the early stages of thrust development. In

561

other thrust belt examples (e.g. Diegel, 1986; Hossack, 1983) high displacement thrusts are

562

shown to transfer displacement across branch lines (i.e. they are hard-linked). If the fault

563

growth model (Childs et al., 1995) for extensional systems can be applied to thrust belts, then

564

we have might expected thrust branches to have formed within the Vercors thrust zones, had

565

displacements increased, leading to hard linked, through-going thrust zones.

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7. Conclusions

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Analysis of parallel cross sections constructed through the Vercors fold-thrust belt has shown

569

the total shortening distance is relatively consistent, however the internal structure is more

570

complex. Cross sections show changes in structural style along strike and significant

571

variation in individual thrust zone displacement with displacement gradients varying

572

significantly from 16-107 m/km. Large-scale thrust zone displacement patterns suggest they

573

are soft linked; as displacement decreases in one zone, it increases in another to maintain a

574

constant total shortening within the fold-thrust belt. Individual thrust zones are comprised of

575

short, soft-linked thrust segments. They probably represent an intermediate stage in thrust

576

development at low displacements (up to 2000 m in the Vercors), where thrust splays have

577

not yet formed to create larger, through-going thrust structures, as seen in other global thrust

578

belt examples, such as the Rockies, where thrust displacements are much higher (over 10 km,

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ACCEPTED MANUSCRIPT 579

Price, 1981). The question is are the thrust displacement gradients and linkage patterns in the

580

Vercors applicable to carbonate thrust belts in general or are they dependent on the specific

581

nature of the deforming multiplayer?

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Acknowledgements

584

This research is partially funded by a CGG-financed pathfinder project, and formed part of a

585

study by the Fold-Thrust Research Group at the University of Aberdeen, co-funded by

586

InterOil, Oil Search and Santos. We also thank Nicolas Bellahsen and Stefano Tavani for

587

constructive reviews.

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28

ACCEPTED MANUSCRIPT Fermor, P., 1999. Aspects of the three-dimensional structure of the Alberta Foothills and

673

Front Ranges. GSA Bulletin, 111, 3, 317-346.

674

Fossen, H., 2010. Structural Geology. Cambridge University Press.

675

Gratier, J-P., Ménard, G. & Arpin, R., 1989. Strain-displacement compatibility and

676

restoration of the Chaines Subalpines of the western Alps. In: Alpine Tectonics edited by

677

Coward, M. P., Dietrich, D. & Park, R. G. Geological Society, London, Special Publications,

678

45, 65-81.

679

Hossack, J. R., 1983. A cross-section through the Scandinavian Caledonides constructed with

680

the aid of branch-line maps. Journal of Structural Geology, 5, 2, 103-111.

681

Kane, S. J., Williams, G. D., Buddin, T. S., Egan, S. S., and Hodgetts, D., 1997, Flexural-

682

slip based restoration in 3D, a new approach. AAPG Annual Convention Official

683

Program, A58.

684

Lemoine, M., Bas, T., Arnaud-Vanneau, A., Arnaud, H., Dumont, T., Gidon, M., De

685

Graciansky, P. C., Rudkiewicz, J. L., Megard-Galli, J. & Tricart, P. 1986. The continental

686

margin of the Mesozoic Tethys in the western Alps. Marine Petroleum Geology, 3, 179-199.

687

Moss, S., 1992a. Organic maturation in the French Subalpine Chains: regional differences in

688

burial

689

history and the size of tectonic loads. Journal of the Geological Society, London, 149, 503-

690

515.

691

Moss, S. J., 1992b. Burial diagenesis, organic maturation and tectonic loading in the French

692

subalpine chains, S.E. France, Durham theses, Durham University.

693

Mugnier, J. L., Arpin, R. & Thouvenot, F., 1987. Coupes Equilibrees a travers le massif sub

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alpine de la Chartreuse. Geodinamica Acta, Paris, 1, 125-137.

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29

ACCEPTED MANUSCRIPT Neely, T. G. & Erslev, E. A., 2008. The interplay of fold mechanisms and basement

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weaknesses at the transition between Laramide basement-involved arches, north-central

697

Wyoming, USA. Journal of Structural Geology, 31, 1012-1027.

698

Qayyum, M., Spratt, D. A., Dixon, J. M. & Lawrence, R. D., 2015. Displacement transfer

699

from fault-bend to fault-propagation fold geometry: An example from the Himalayan thrust

700

front. Journal of Structural Geology, 77, 260-276.

701

Pérez-Estaún, A., Alvarez-Marrón, J., Brown, D., Puchkov, V., Gorozhanina, Y. & Baryshev,

702

V., 1997. Along-strike structural variations in the foreland thrust and fold belt of the southern

703

Urals. Tectonophysics, 276, 265-280.

704

Philippe, Y., Deville, E. & Mascle, A., 1998. Thin-skinned inversion tectonics at oblique

705

basin margins: example of the western Vercors and Chartreuse Subalpine massifs (SE

706

France). In: Cenozoic Foreland Basins of Western Europe edited by Mascle, A.,

707

Puigdefàbregas, C., Luterbacher, H. P. & Fernàndez, M. Geological Society of London,

708

Special Publications, 134, 239-262.

709

Price, R. A., 1981. The Cordilleran foreland thrust and fold belt in the southern Canadian

710

Rocky Mountains. In: Thrust and Nappe Tectonics edited by McClay, K.R. & Price, N.J.

711

Geological Society, London, Special Publications, 9, 427-448.

712

Ramsay, J. G., 1963. Stratigraphy, Structure and Metamorphism in the Western Alps.

713

Proceedings of the Geological Association, 74, 3, 357-391.

714

Roberts, G., 1991. Structural controls on fluid migration through the Rencurel thrust zone,

715

Vercors, French Sub-Alpine Chains. In: Petroleum Migration edited by England, W. A. &

716

Fleet, A. J. Geological Society, London, Special Publications, 59, 245-262.

717

Roberts, G. P., 1994. Displacement localization and palaeo-seismicity of the Rencurel Thrust

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Zone, French Sub-Alpine Chains. Journal of Structural Geology, 16, 5, 633-646.

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ACCEPTED MANUSCRIPT Rocha, E. & Cristallini, E. O., 2015. Controls on structural styles along the deformation front

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of the Subandean zone of southern Bolivia. Journal of Structural Geology, 73, 83-96.

721

Suppe, J., 1983. Geometry and kinematics of fault-bend folding. American Journal of

722

Science, 283, 684-721.

723

Rudkiewicz, J. L., 1988. Quantitative subsidence and thermal structure of the European

724

continental margin of the Tethys during Early and Middle Jurassic times in the Western Alps

725

(Grenoble-Briancon transect). Bulletin de la Société Géologique de France, 4, 4, 623-632.

726

Ruh, J. B., Kaus, B. J. P. & Burg, J-P., 2012. Numerical investigation of deformation

727

mechanics in fold-thrust belts: Influence of rheology of single and multiple décollements.

728

Tectonics, 31, TC3005.

729

Torres Carbonell, P. J., Dimieri, L. V. & Olivero, E. B., 2013. Evaluation of strain and

730

structural style variations along the strike of the Fuegian thrust-fold belt front, Argentina.

731

Andean Geology, 40, 3, 438-457.

732

Welbon, A., 1988. The influence of intrabasinal faults on the development of a linked thrust

733

system. Geologische Rundschau, 77, 1-11.

734

Woodward, N. D., 1988. Primary and secondary basement controls on thrust sheet

735

geometries. Geological Society of America Memoirs, 171, 353-366.

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Figure captions

739

Figure 1: a) Dahlstroms (1969) model for soft-linked thrusts; as displacement on one thrust

740

decreases, it must increase on another to maintain a constant shortening distance along the

741

fold-thrust belt. In this case displacement on thrust A increases towards the top, displacement

742

on thrust C decreases towards the top and the maximum displacement on thrust B is on the

31

ACCEPTED MANUSCRIPT central section. b) Fault growth and linkage model. At stage 1 the stress and strain fields of

744

two small faults do not interact; the faults are unlinked. At stage 2 the faults have grown and

745

their dispalcements increased; their stress and strain field begin to interact so they are soft

746

linked. By stage 3 the thrust displacement has increased and a thrust splay has formed,

747

linking the two faults; the faults are now hard linked. b) High displacement thrust fault zones

748

may be expected to be made up of multiple, smaller fault segments. Total thrust zone

749

displacement (dashed line) is made up of slip on individual small fault segments or series of

750

linked fault segments (adapted from Fossen, 2010).

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Figure 2: a) Location map and simplified geological map of the French Sub-Alpine Chains in

753

SE France. Adapted from Moss, 1992a. b) Geological map of the Vercors showing the

754

boundaries of major structural zones (dashed lines) and cross section traces, adapted from

755

B.R.G.M. map sheets 1967, 1968, 1975, 1978, 1983. c) Key to geological map and cross

756

sections (Figures 6-11). d) Equal area stereonet showing contoured poles to bedding for the

757

Jurassic-Cretaceous succession (using a 0.75 % contour interval). The orientation of the best

758

fit section plane is derived by calculating the average bedding dip direction using Move

759

software.

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Figure 3: a) Looking north onto eastward dipping thrust splays in the forelimb of the fold at

762

Pont-en-Royans. b) Looking NNE from Ste-Eulalie-en-Royans to the tight Urgonian fold at

763

Pont-en-Royans, possibly caused by the presence of west-dipping backthrusts causing a pop-

764

up structure. c) Looking north into the fold core at Pont-en-Royans. Small-scale folding in the

765

Hauterivian is probably caused by compression within the fold core. See Figure 2 for

766

locations.

767

32

ACCEPTED MANUSCRIPT Figure 4: a) Looking north from Petits Goulets; a steep, west-dipping backthrust probably

769

formed early during Vercors deformation, and was later rotated as the larger fold forelimb

770

developed. Inset: suggested evolution of the Petits Goulets structure: stage 1: backthrusting

771

followed by rotation of the resultant fold due to folding above a larger, eastward-dipping

772

thrust at depth; stage 2: eastward-dipping thrust offsets the older backthrust . b) The Urgonian

773

fold forelimb above the Rencurel Thrust in the Gorges de la Bourne; backthrusts are seen

774

cutting through the fold but they do not alter fold geometry significantly. See Figure 2 for

775

location.

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Figure 5: a) Looking south from the southern Chartreuse onto the northern Vercors. The

778

Urgonian shows a clear kinked geometry as it rises from the Valley bottom to the Plateau de

779

Sornin, possible due to a kinked thrust geometry at depth. b) Looking SE from Pic St-Michel

780

to the Drac Valley. Forested ridges represent the dip panels of Jurassic folds, probably caused

781

by west-dipping backthrusts at depth. See Figure 2 for locations.

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Figure 6: Top: Barbieres cross section running WNW-ESE from the Bas Dauphine (BD) to

784

the Drac Valley (zone 4). F: Faille de L’Isere fault. See Figure 2b for section trace line.

785

Bottom: cross section following line-length restoration. Line length restoration has been

786

applied only to the top Jurassic-Cretaceous horizons; the internal structure of the Jurassic and

787

basement is not restored.

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Figure 7: Top: Beauregard-Baret cross section running WNW-ESE from the Bas Dauphine

790

(BD) to the Drac Valley (zone 4). F: Faille de L’Isere fault. See Figure 2b for section trace

791

line. Bottom: cross section following line-length restoration. Line length restoration has been

33

ACCEPTED MANUSCRIPT 792

applied only to the top Jurassic-Cretaceous horizons; the internal structure of the Jurassic and

793

basement is not restored.

794

Figure 8: Top: Ste-Eulalie-en-Royans cross section running WNW-ESE from the Bas

796

Dauphine (BD) to the Drac Valley (zone 4). F: Faille de L’Isere fault. See Figure 2b for

797

section trace line. Bottom: cross section following line-length restoration. Line length

798

restoration has been applied only to the top Jurassic-Cretaceous horizons; the internal

799

structure of the Jurassic and basement is not restored.

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Figure 9: Top: St-Romans cross section running WNW-ESE from the Bas Dauphine (BD) to

802

the Drac Valley (zone 4). F: Faille de L’Isere fault. See Figure 2b for section trace line.

803

Bottom: cross section following line-length restoration. Line length restoration has been

804

applied only to the top Jurassic-Cretaceous horizons; the internal structure of the Jurassic and

805

basement is not restored.

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806

Figure 10: Top: Cognin-les-Gorges cross section running WNW-ESE from the Bas Dauphine

808

(BD) to the Drac Valley (zone 4). F: Faille de L’Isere fault. See Figure 2b for section trace

809

line. Bottom: cross section following line-length restoration. Line length restoration has been

810

applied only to the top Jurassic-Cretaceous horizons; the internal structure of the Jurassic and

811

basement is not restored.

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Figure 11: Top: Sassenage cross section running WNW-ESE from the Bas Dauphine (BD) to

814

the Drac Valley (zone 4). F: Faille de L’Isere fault. See Figure 2b for section trace line.

815

Bottom: cross section following line-length restoration. Line length restoration has been

34

ACCEPTED MANUSCRIPT 816

applied only to the top Jurassic-Cretaceous horizons; the internal structure of the Jurassic and

817

basement is not restored.

818

Figure 12: a) Shortening distance and total thrust displacement profiles showing along strike

820

variations from north to south. A good correlation between the two lines in the north shows

821

thrusting accommodates the majority of shortening; in the south shortening distance is higher

822

than thrust displacement indicating more folding is accommodating shortening. b) Shortening

823

percentage profile suggesting a general decrease in shortening from north to south. S:

824

Sassenage section; C: Cognin-les-Gorges section; SR: St-Romans sections; SE: Ste-Eulalie-

825

en-Royans section; BB: Beauregard-Baret section; B: Barbieres section. Shortening

826

percentages have been calculated by measuring the difference in line length between the

827

present day and restored cross sections (Figures 6-11) for the top Urgonian horizon, dividing

828

by the original (restored) length and multiplying by 100.

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Figure 13: Thrust zone displacement profiles for thrust zones 1-3 and out-of-zone

831

displacement. Out-of-zone displacement refers to displacement on thrusts that are located

832

within structural zones 1-3 (see Figure 2b) but not within thrust zones 1-3. A decrease in

833

thrust zone 3 displacement coincides with increases in thrust zone 1 and out-of-zone

834

displacement, indicating displacement transfer between thrust zones. S: Sassenage section; C:

835

Cognin-les-Gorges section; SR: St-Romans sections; SE: Ste-Eulalie-en-Royans section; BB:

836

Beauregard-Baret section; B: Barbieres section.

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837 838

Figure 14: Model for thrust zone displacement transfer along strike in the Vercors.

839

Displacement in thrust zone 3 gradually decreases to the south, which is accommodated by a

35

ACCEPTED MANUSCRIPT gradual increase in thrust zone 1 displacement to maintain constant fold-thrust belt

841

shortening. Displacement in thrust zone 2 varies very little along strike.

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36

ACCEPTED MANUSCRIPT FIGURE 1 A

a A

B

B C

C

C

B

A

C

B

A

b

c

3

2

1

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Displacement

Displacement

Horizontal distance along faults

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Horizontal distance along faults

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Horizontal distance along faults

Displacement

stress/strain field

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Displacement

fault

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B

Horizontal distance along fault zone

ACCEPTED MANUSCRIPT

FIGURE 2 a Ge

B.

RT

SR

C: Chamonix

Gr: Grenoble A: Annecy au

PeR

sD

PG SE on

e2

Chartr.: Chartreuse DR: Drome River

EP AC C

thr u

st

zo

ne

1

B

Hauterivian Valanginian Berrasian Jurassic (& Triassic) Basement Structural zone boundary

Fault

Zone 2

St-R

d best fit section plane

om

ans

Ste Roy Eulalie ans -en -

Bea u Bar regard et Bar

bie

Zone 3

Cog les- ninGor ges

n = 781

UR

CR

Zone 1

Barremian (non platform carbonate)

age

G

SNi

GG CL

thrust zone 3

thr

BB

TE D

ine

SN

Ba

H.-G.: Hautes-Giffre

Urgonian

PM

GB ph

Ch: Chatillon-en-Diois

Aptian-Maastrichtian

RI PT

e nn

C

Cr: Crest

Ge: Geneva

Sas

M AN U

Cr Dr

Palaeocene-Miocene

sen

N

M. B.: Mont Blanc

S

PS

Brianconnais Internal Zone

Internal Zones

50 km

Key to geological map of the Vercors and cross sections N

External Zone Massifs

do

Ba

Ch

SQ

Dauphinois Mesozoic

ust z

b

c 10 km

Jurassian Mesozoic

lle Be

Ch art r.

C

Ve rco rs

Gr

H.-

G.

Tertiary

M.

es Bo rne s ug

A

Bas Dauphine

b Geological map of the Vercors

SC

Jura

res

Zone 4: Drac Valley

B. Barbieres BB. Beauregard-Baret C. Cognin-les-Gorges CL. Combe Laval CR. Col de Rousset G. Grenoble GB. Gorges de la Bourne GG. Grands Goulets PeR. Pont-en-Royans PG. Petits Goulets PM. Pic St-Michel PS. Plateau de Sornin

S. Sassenage SE. Ste-Eulalie-enRoyans SN. St-Nazaire-enRoyans SNi. St-Nizier-duMoucherotte SQ. St-Quentin-surIsère SR. St-Romans RT. Rencurel Thrust UR. Urgonian Ridge

ACCEPTED MANUSCRIPT FIGURE 3 a

N

b

offset thrust: flexural slip?

ESE WNW tight fold

thrust splays

c

N

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Urgonian Hauterivian

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bedding

800

SC

thrust trace

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10 m

bedding

backthrust

m

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photo W WNW

E

ESE ESE

WNW

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b

260 m

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backthrusts

FIGURE 4

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SC

a

ACCEPTED MANUSCRIPT FIGURE 5 a

WNW

ESE fold hinges

Plateau de Sornin

Rencurel Thrust

inian

er Upp

n

Urgonia

5 km Jurassic ridges

ssic

Jura

M AN U

SC

E

Jurassic bedding

17 km

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Drac Valley

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b

RI PT

g Valan Upper

Vercors

Urgonian ridge (W-dipping)

W

ACCEPTED MANUSCRIPT

Barbieres zone 3

zone 2

zone 1 thrust zone 2

thrust zone 1

thrust zone 3 topography

Internal structure not shown

SC

F

M AN U

5000 m WNW

zone 4 (Drac Valley)

RI PT

BD

Internal structure not restored

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FIGURE 6

ESE

ACCEPTED MANUSCRIPT

Beauregard-Baret zone 1 thrust zone 1

zone 2

zone 4 (Drac Valley)

zone 3

thrust zone 2

thrust zone 3

RI PT

BD

topography

SC

Internal structure not shown

5000 m F

EP AC C

FIGURE 7

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ESE

Internal structure not restored

ACCEPTED MANUSCRIPT Ste-Eulalie-en-Royans BD

zone 4 (Drac Valley)

zone 3

zone 2 thrust zone 3

Internal structure not shown

5000 m

ESE

F

SC

WNW

topography

RI PT

thrust zone 2

M AN U

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FIGURE 8

ACCEPTED MANUSCRIPT

BD

zone 2

thrust zone 3

RI PT

St-Romans zone 3

topography

SC

thrust zone 2

zone 4 (Drac Valley)

Internal structure not shown

WNW

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5000 m F

EP AC C

FIGURE 9

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Internal structure not restored

ESE

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Cognin-les-Gorges zone 3

zone 2

BD

zone 4 (Drac Valley)

thrust zone 3 thrust zone 2

Internal structure not shown

5000 m F

ESE

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topography

Internal structure not restored

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FIGURE 10

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Sassenage BD

zone 4 (Drac Valley)

zone 3

thrust zone 3

RI PT

topography

5000 m

SC

F

ESE

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WNW

Drac Valley backthrusting (simplified)

Internal structure not shown

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ACCEPTED MANUSCRIPT a.

Shortening distance/total thrust displacement

Shortening distance

4000 3000

Total thrust displacement (top Urgonian horizon)

2000 1000 0 North S

C

SR

SE

BB

B South

b.

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cross section

Shortening percentage

8 4 0 North S

C

SR

SE

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FIGURE 12

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shortening (%)

16 12

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shortening/displacement (m)

5000

B South

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Thrust zone displacement

1500 Zone 1

RI PT

displacement (m)

2000

Zone 2

1000

Zone 3

Out-of-zone

0 North

S

C

SR

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BB

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FIGURE 13

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South

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FIGURE 14

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3 2

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ACCEPTED MANUSCRIPT Using laterally compatible cross sections to infer fault growth and linkage models in foreland thrust belts Highlights

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We construct parallel cross sections through the Vercors foreland thrust belt Cross sections are analysed to ensure lateral continuity Displacement profiles show displacement transfer between thrust zones Fault properties suggest thrusts evolve through soft-linked and hard-linked stages

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• • • •