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Lateral displacement during Neogene convergence in the western and central Alps Mary Hubbard Department of Geological Sciences, University of Maine, Orono, Maine 04469 Neil S. Mancktelow Geologisches Institut, ETH-Zentrum, CH-8092 Zurich, Switzerland

ABSTRACT Although major deformation in the western Alps is clearly the result of north-northwest-directed thrust tectonics, there is evidence suggesting that orogen-parallel deformation may have been important in the late tectonic history of the western Alps. We propose that this northeast-trending deformation includes (1) southwest-directed normal-fault movement along the Simplon line; (2) a diffuse zone of northeast-striking dextral strike-slip deformation along the Rhône Valley in Switzerland, between the Mont Blanc and Aiguilles Rouges massifs, and through the Belledonne massif; and (3) southwestdirected thrusting in the Embrunais-Ubaye and Digne nappe systems of southeastern France. The correlation of these three broad regions of deformation is based on similar amounts of minimum displacement, consistent kinematics, and timing of deformation.

INTRODUCTION Shortening in collisional orogens is typically the dominant, and certainly the most studied, deformation. Recent research, however, shows the importance of strike-slip and/or extensional deformation in the development of many collisional orogens (Molnar and Tapponnier, 1975; Royden and Burchftel, 1989; Steck, 1987). The kinematic and dynamic relation of extensional and strike-slip movements to the overall convergence is not well understood but is important to a complete understanding of mountain-building processes in a collisional setting. Whereas thrust faults and folds have been the major focus of structural research in the Alps, strike-slip faults and intraorogenic extensional faults have been identified locally (e.g., Mancktelow, 1985,1990; Mugnier and Gidon, 1988; Schmid et al., 1987; Selverstone, 1988; Steck, 1984, 1987,1990). In this paper we summarize evidence for a large-scale zone of late deformation in the western Alps with movement direction parallel to the orogen—that is, generally southwest (Fig. 1). The trace of this zone, in plan view, makes two nearly right-angle bends, and as a result local deformation is extensional in the northeast and compressional in the southwest. Extension on the northeast segment is well documented in the Simplon Pass area (Mancktelow, 1985, 1990; Mancel and Merle, 1987), and southwest-vergent thrust deformation has been documented on the southwest segment in the Parpaillon and Digne areas (Merle and Brun, 1984; Faucher et al, 1988). In this paper we present evidence for dextral strike-slip deformation in the central segment that is kinematically consistent with deformation in the Simplon and Parpaillon-Digne areas. REGIONAL GEOLOGY The major lithotectonic units of the western Alps include the Helvetic nappes, the lower Penninic nappes, the upper Penninic nappes, and the crystalline basement massifs (Fig. 1). The Helvetic nappes in Switzerland GEOLOGY, v. 20, p. 943-946, October 1992

are comparable to the Subalpine massifs of the French Alps and consist of a series of thrust slices of Mesozoic sedimentary rocks of shelf origin (Triimpy, 1980). Thrusting of the Helvetic nappes was to the northwest and west-northwest (Dietrich and Durney, 1986). The amphibolite-grade lower Penninic nappes are in the Lepontine dome, which is the footwall to the Simplon fault (Milnes, 1974). The greenschist- to amphibolite-grade upper Penninic nappes form the hanging wall to the Simplon. The crystalline basement massifs are found within the Helvetic zone, but consist of Precambrian-Paleozoic granitic and gneissic rocks. In the western Alps, these massifs include the Argentera, Pelvoux, Belledonne, Mont Blanc, Aiguilles Rouge, and Aar. Simplon Fault Zone The Simplon fault zone, which is well exposed between the Rhône Valley near Visp and the Isorno Valley near Domodossola (e.g., Bearth, 1956), is a major low-angle, southwest-dipping, extensional fault across which more than 7 km of crustal section was eliminated (Mancktelow, 1985, 1990, and unpublished). Extension toward - 2 4 0 ° began - 1 9 Ma (Mancktelow, 1985) and continued until at least 3 Ma (Soom, 1990), during a period of overall uplift and cooling. A value of ~ 1 5 km for dip-slip displacement is considered to be a minimum (Mancktelow, 1990). The northwestern end of the Simplon fault zone can be followed toward the Rhône Valley (Burkhard, 1986; Mancktelow,1990; Steck, 1987), although the fault trace is concealed at its projected intersection with the valley. Along the Rhône Valley there is evidence for continued northeast-trending movement, but a change in fault orientation dictates a greater component of strike-slip deformation than in the Simplon area (Fig. 1). Dextral Strike-Slip Deformation In the Helvetic units adjacent to the Rhône Valley, pressure shadows around pyrite grains indicate stretching in a northeast direction (Steck, 1984; Burkhard, 1986; Dietrich and Durney, 1986). A study of the incremental strain history in the Helvetic nappes, as measured from fibrous mineral growth in veins and on pyrite, was made by Durney and Ramsay (1973) and Dietrich and Durney (1986). Their results demonstrate a consistent pattern of early north to north-northwest stretching, transverse to the general trend of the fold axes, followed by a later component of axis-parallel extension. The magnitude of this late extension is greatest in the upper Wildhorn nappe and toward the so-called Helvetic "root zone" in the Rhône Valley. The component of orogen-parallel extension thus increases toward the projected position of the Simplon fault zone. Farther west, near Martigny (Fig. 1), horizontal calcite fibers have grown along slip surfaces on the subvertical, northeast-striking Malm limestone unit. Stepping of these fibers indicates dextral movement in this area as well. Calcite textures from a calc-mylonite on the Col de Forclaz 943

Mattlgny

Aiguillas Rouges

CALCITË axis

Bel led on ne

Figure 1. Simplified geologic map of western Alps. Large arrows indicate Neogene kinematics. Inset is pole figure of calcite texture (sample 88AI9i). The c-axis is perpendicular to contoured < a > axis. In twinning field of calcite, which appears to be most important deformation mechanism in this sample, sense of obliquity relative to foliation suggests dextral movement (Schmid et al., 1987). Plot is upper hemisphere, equal area. Short arrow points to sample top. Section cut parallel to lineation (long arrow, 44°N, 55°E), perpendicular to foliation (N35°E, 70°SE). Sample from Col de la Forclaz in Chamonix syncline.

GrenoW»

CRYSTALLINE MASSIFS

HELVETIC and SUBALPINE NAPPES Pelvoux UPPER PENNINE NAPPES

LOWER PENNINE NAPPES

Argent era

Fig. 1) are also consistent with dextral strike-slip deformation (Hubbard and Mancktelow, 1989). Gourlay and Ricou (1983) interpreted asymmetric folds in the Chamonix syncline (Mesozoic sedimentary units between the Mont Blanc and Aiguilles Rouges massifs) to be the result of dextral movement parallel to the northeast-trending Chamonix valley. The Belledonne and Subalpine massifs are cut by a series of en echelon dextral faults (Fig. 1). In the Subalpine massifs, the dextral faults cut west- to northwest-directed thrust faults of Miocene age, thus constraining the age of these dextral faults as middle to late Neogene (Mugnier and Gidon, 1988; J. L. Mugnier, 1988, personal commun.). The age of the dextral strike-slip faults in the Belledonne massif is not determined. The Belledonne faults, however, have the same orientation and movement sense as the dextral faults in the Subalpine massifs, which suggests that they may be coeval. The faults in the Belledonne massif are post-Mesozoic because they involve Mesozoic sediments both in the medial syncline and at the internal margin of the Belledonne crystalline rocks. The en echelon fault geometry in the Belledonne and Subalpine massifs implies a dextral wrench-fault system oriented ~N40°E. Further evidence for dextral strike-slip fault movement comes from drag folding adjacent to mylonites, shear bands, and asymmetric clasts in the crystalline rocks of the Belledonne massif. Permian-Carboniferous 944

30 km

rocks and Mesozoic sedimentary rocks adjacent to the Belledonne massif and within the medial syncline show evidence for strike-slip deformation associated with horizontal (northeast) stretching of pyrite framboids and fossils. At the Col de la Madeleine, within the Belledonne crystalline rocks, a north-striking mylonite is folded at the contact with a younger northeaststriking mylonite in a sense consistent with drag along a dextral shear zone (Fig. 2). Strike-slip faulting in the Belledonne massif occurred under both ductile and brittle conditions. This variation in deformation mechanism implies either that conditions changed through time, the deformation occurring at different times, or that the variation in fault mechanism is a function of different rock types or different local conditions. In crystalline rocks near Col de la Madeleine, the ductile deformation occurred under greenschist facies conditions, suggesting that the rocks were buried, though not deeply. In contrast, deformation in adjacent sedimentary rocks involved stretching of limestone and shale and brittle fracturing of pyrite and fossils. Throughout the area there was slip along discrete surfaces accompanied by fiber growth on those surfaces. South of Beaufort, deformation of Triassic sandstone produced a fault gouge, suggesting deformation at near-surface conditions. Although it remains unclear whether these different styles of deformation occurred at different times or coevally under GEOLOGY, October 1992

southwestward transport of the Digne thrust system also occurred in the Neogene (Aquitanian, Tabianian) (Gidon et al., 1970; Siddans, 1979; Fry, 1989), and the time of emplacement of the Parpaillon nappe, though subject to some debate (e.g., see Fry, 1989), is also generally taken to be post-early Miocene (Kerckhove, 1969). It follows that all these movements were broadly coeval and probably occurred throughout much of the Neogene. Fault-plane solutions frr recent earthquakes suggest that this kinematic framework has been maintained to the present (Pavoni and Roth, 1990). Major movements on the Insubric-Periadriatic line began in the Oligocene, but dextral movements have continued into the Neogene (Schmid et al., 1989).

Figure 2. Drag folding associated with dextral deformation in Belledonne massif. Northeast-trending mylonite zone is seen across top of photo. Older, light-colored mylonite is folded at contact with younger mylonite in sense consistent with dextral movement on northeasttrending younger mylonite zone. Photo taken looking down at horizontal outcrop surface at Col de la Madeleine (45°40'N, 6°30'E). Northeast is to right.

different local conditions, we can be reasonably sure that all of this strikeslip deformation is Alpine in age, probably Neogene. Thrust Deformation of Embrunais-Ubaye and Digne Nappes Northeast-striking strike-slip deformation was reported from the Pelvoux massif by Gillcrest et al. (1987) and A. Pêcher (personal commun.). It has been suggested that these faults in the Pelvoux massif are connected with the southwest-directed thrust faults of the Digne nappe (G. Menard, personal commun.). The structural history of the Digne region is complex, but the late Alpine history includes southwest-directed thrust faults that have been dated by fossils in molassic rocks of the lower plate as postearly Miocene (Clauzon et al., 1987; Faucher et al., 1988). Another important region of thrusting in the southwestern Alps is the Embrunais-Ubaye nappe system, which occupies a more internal position relative to the Digne nappe. The Embrunais-Ubaye nappes have undergone several episodes of deformation. Merle and Brun (1984) interpreted curved fibers from pressure shadows in the Parpaillon nappe, one of the thrust sheets of the Embrunais-Ubaye system, to represent a change in thrust direction from northwest to southwest. Orientations of large-scale recumbent folds are consistent with these two movement directions in the Parpaillon nappe. Merle and Brun (1984) used the ages of the deformed units to determine the timing of northwest-directed thrusting to be Oligocene and the southwest-directed thrusting to be Miocene. TIMING Timing of movements on the Simplon-Rhône fault zone in the region of the Simplon Pass and Rhône Valley is well constrained by Rb-Sr, K-Ar, and fission-track mineral ages (see the summary of radiometric ages in Soom, 1990). These results allow construction of cooling curves for both the footwall and hanging wall of the Simplon fault zone in the Simplon Alps (e.g., Wagner et al., 1977), from which it can be concluded that the retrograde mylonites of the footwall developed between 19 and 12 Ma (Mancktelow, 1985,1990). Offset in apatite fission-track ages near Brig in the Rhône Valley (Soom, 1990) demonstrates that the fault accommodated significant relative vertical movements until after 3 Ma. Recent apatite fission-track ages from the Belledonne massif indicate relative vertical displacements of a similar young age (Lelarge et al., 1991). The 944 GEOLOGY, October 1992

KINEMATIC MODEL As outlined above, the upper Pennine nappes were displaced across the low-angle extensional Simplon fault zone by at least 15 km toward the southwest, relative to the underlying units of the Lepontine dome. Dextral offsets shown on a generalized map of the Belledonne and Subalpine massifs give a minimum dextral displacement of ~ 15 km across a 25-kmwide zone that is oriented ~N45°E (Gidon, 1977). Gidon's map is simplified, so probably not all offsets are shown, but this minimum value is similar to that for the Simplon fault zone. Zones of dextral displacement appear to bend rapidly in the area south of the Pelvoux massif into southwest-directed thrusts (e.g., Fry, 1989). Total Neogene thrust displacements in this region are not well established: Fry (1989) estimated displacement of about 20 km south of Digne. As summarized above, these linked movements observed from the Simplon area to the Digne region are considered to have been broadly synchronous throughout the Neogene. Two basic models could be invoked to explain the Neogene kinematics of the western Alps: oblique convergence and dextral transpression (Steck, 1984, 1990) or continental escape (cf. Ratschbacher et al., 1989, for the eastern Alps). The models are not mutually exclusive. In a model of dextral transpression, the Simplon fault zone would represent a pull-apart structure transferring the dextral displacement on the Insubric or Periadriatic line to the more northerly Rhóne-Belledonne line (e.g., Steck, 1990, Fig. 6). The gradual decrease in dextral offset on the Insubric line would be balanced by the gradual transfer to the Simplon fault zone west of Locarno across a broad ductile zone at the midcrustal levels currently exposed. As discussed in detail by several authors (e.g., Walcott, 1978; Oldow et al., 1990), there will be a partitioning of displacements in such transpressive zones into components parallel and normal to the plate boundary. The degree to which shear stresses are partitioned on the transcurrent shear zones depends on their resistance. In the extreme of very easy boundaryparallel shear, shear stress will be almost totally partitioned on these zones and the stresses adjacent to the strike-slip fault will show normal compressive stresses almost perpendicular to the fault. This was suggested for the San Andreas fault (Zoback et al., 1987). In the western Alps evidence for compressive stresses may be the zones of crustal thickening and rapid uplift adjacent to the Periadriatic-Simplon-Rhóne-Belledonne line. The uplift is broadly synchronous with transcurrent movement and has exposed deeper structural levels immediately adjacent to the strike-slip boundary (e.g., mid-crustal levels with migmatite metamorphism to the north of the Insubric line; Schmid et al., 1989; the external massifs of Aar, Mont Blanc, Belledone, and Pelvoux). These zones of uplift can be explained by a transpressive model. The continental escape model differs from the transpressive regime in that lateral movement accommodates gravitational instability or space problems that accompany convergence. The zones of lateral movement are therefore adjacent to the zones of maximum convergence. It has been proposed that many of the strike-slip faults and extensional faults in the eastern Alps accommodate lateral movement or "escape" away from the convergent zones in the central Alps (Ratschbacher et al., 1991). Application of this model to the western Alps implies that the western and central Alps are detached from the southern Alps and should be separated from

the southern Alps by sinistral faults. Field evidence does not support such strike-slip detachment. A model of dextral transpression is thus favored for the western Alps. Gidon (1974) proposed a rotational model to explain strike-slip faults in the Belledonne massif. Although his original model implied rotation of a relatively small block in the western Alps, it may be more consistent with Neogene kinematics to consider some amount of rotation of a large southern block accompanying transpression. CONCLUSIONS Though dominant deformation in the western Alps involves northwest-directed thrusting, there is increasing evidence that orogenparallel deformation played a role in the later (Neogene) stages of the Alpine tectonic evolution. In the vicinity of Simplon Pass, that orogenparallel deformation takes the form of extensional faulting on the southwest-dipping Simplon fault. In the Rhône Valley of Switzerland, the Chamonix valley, and the Belledonne massif, southwest-striking strike-slip faults have accommodated orogen-parallel deformation. Farther southwest, in the Haute Provence region, there is a late component of thrust deformation on the Digne and Parpaillon thrusts that is southwest directed. We propose that the orogen-parallel deformation from Simplon Pass to Digne is kinematically and tectonically related. Age determinations suggest that deformation occurred s; 18 Ma. A minimum estimate for the amount of orogen-parallel displacement is ~ 15 km. This lateral displacement is due to oblique convergence, which produces regional dextral transpression within the central and western Alps.

ACKNOWLEDGMENTS Partially supported by the Petroleum Research Fund, administered by the American Chemical Society. We thank Martin Burkhard, Stefan Schmid, Albrecht Steck, Arnauld Pêcher, Jean-Pierre Gratier, Niko Froitzheim, Gilles Menard, JeanLouis Mugnier, Rudolf Triimpy, and Lydia LeLarge for inspiration, insight, and advice. Hubbard thanks field assistants L. Ko, T. Slocum, S. Neustadtl, M. Davis, and J. Yanasak. We also thank J. Selverstone and M. Handy for thorough and constructive reviews.

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