ation of E-W-trending folds and thrusts during the Alpine ... data (Ghandriche, 1991), a component of sinistral ... (EM) software (Mercier, 1990-1994), may be.
TECTONOPHYSICS ELSEVIER
Tectonophysics245 (1995) 111-113
G e o m e t r y of fault-propagation folds: m e t h o d and application - - C o m m e n t Eric Mercier a, Fatima Outtani a, Dominique Frizon de Lamotte a, Hac~ne Ghandriche b a Universitd de CERGY-PONTOISE, Ddpartement Sciences de la Terre (URA 1369, groupe de Cergy) 8, Le Campus, av. du Parc 95 033 Cergy-Pontoise Cedex, France b SONATRACH, Division exploration, rue Capitaine Azzoug, C~te rouge, Hussein Dey, Alger, Algeria
Received 24 January 1994; accepted 1 July 1994 Al Saffar (1993) proposes an interesting model focusing on a possible secondary evolution of fault-propagation folds by development of a second fold on the back of the first and major one. This model is designed as a new alternative to the modes of evolution of fault-propagation folds previously envisaged [transport on the upper flat (Jamison, 1987; Mercier, 1992) and development of a breakthrough thrust (Suppe and Medwedeff, 1990)]. The purpose of this comment is not to discuss the theorical background of this model but to point out that it is not suitable for explaining the Kasserou structure of northeast Algeria. In this region, the main problem with any model is the integration of the change in the tectonic transport direction which is well established in the field. Al Saffar's model disregards this point because, in particular, it assumes that the thrust planes cutting throught the sedimentary pile remained blind thrusts during the building of the structure, or, in other words, that the allochthonous and autochthonous plates remained frontally stuck. The Atlas fold-and-thrust belt where the
The editors have been unable to obtain a reply from the author of the original paper.
Kasserou structure is situated results from the superimposition of two main tectonic events, the Atlas and Alpine phases (Vila and Guellal, 1977, and numerous other maps by the same authors; Vila, 1980; Durand-Delga and Fontbot6, 1980; Ghandriche, 1991). The Atlas phase is recorded by syn- to post-tectonic deposits resting unconformably on NE-SW-trending folds. These sediments, which are of Oligocene to Upper Miocene age, were themselves involved in a second generation of E - W - t r e n d i n g folds and thrusts during the Alpine phase (Vila, 1980; Frizon de Lamotte et al., 1990; Ghandriche, 1991). The NE-SW-trending Kasserou is an Atlas anticline as is evident from the unconformity of the Miocene deposits that is well exposed along its forelimb (Vila and Guellal, 1977). On the other hand, however, the thrust plane which emerges in the core of the structure above overturned beds laterally affects the Miocene. This leads us to interpret this thrust as an important breakthrough thrust (Suppe and Medwedeff, 1990) developed during the Alpine phase. As this phase is characterized by bulk N - S shortening, which is demonstrated by numerous kinematic data (Ghandriche, 1991), a component of sinistral strike-slip along the frontal thrust is likely. No kinematic data are directly available from the
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E. Mercier et al. / Tectonophysics 245 (1995) 111-113
NNW
........
.... "............... : , ' S S E
% - -............./ /
Alpine transport
Sectiont'plane of cross-sectionC ..... J i l l . _. - - -.
..118°
'--7 .),-'-..
-;,
•,: :49' .-/: /-' J__
Barremian Neocomian Up. Jurassic--
Section C
1 km
Lias
Triassic
--
km Fig. 1. Kinematic model of the Kasserou anticline. (A) Possible situation after the Atlas phase. The Kasserou structure is a simple fault-propagation fold. Post-Atlas erosion, leading to the unconformity of the Oligo-Miocene deposits, is suggested. (B) Present-day structure after the Alpine phase and post-tectonic erosion (Oligo-Miocene deposits are not shown). (C) Cross-section through (B). IThis may be usefully compared to the field cross-section (Vila and Guellal, 1977) reported by Al Saffar (1993, fig. 11 A).
breakthrough fault, although the steep dip (up to 50°) (Vila and Guellal, 1977) is mechanically difficult to explain in the context of pure thrusting. This point has already been mentioned by Creuzot et al. (1993) for similar structures from the Tunisian Atlas, 200 km east of the Kasserou. Figure 1 proposes a possible evolution which accounts for the main structural features of the Kasserou anticline. This working hypothesis, balanced and drawn with the help of the Rampe (EM) software (Mercier, 1990-1994), may be compared to the kinematic evolution proposed by A1 Saffar (1993) and to the field data reported by this author from Vila and Guellal (1977). We
emphasise the succession of two non-coaxial tectonic phases and the existence of overturned beds above the emerging thrust (black arrow in Fig. 1) that were not taken into account by A1 Saffar (1993).
References A1 Saffar, M., 1993. Geometry of fault-propagation folds: method and application. Tectonophysics, 223: 363-380. Creuzot, G., Mercier, E., Ouali, J. and Tricart, P., 1993. La tectonogenbse altlasique en Tunisie centrale: Apport de la mod~lisation g6om&rique. Eclogae Geol. Helv., 86(2): 609-627.
E. Mercier et al. / Tectonophysics 245 (1995) 111-113
Durand-Delga, M. and Fontbot6, J.M., 1980. Le cadre structural de la M6diterran6e occidentale. In: J. Auboin, J. Debelmas and M. Latreille (Editors), Geology of Alpine Chains Born of the Tethys. 26th Int. Geological Congress, Paris, Colloque C5, pp. 65-85. Frizon de Lamotte, D., Ghandriche, H. and Moretti, I., 1990. La flexure saharienne: trace d'un chevauchement aveugle post-Plioc~ne de fl~che plurikilom6trique au Nord du Sahara (Aurbs, Alg6rie). C.R. Acad. Sci. Paris, 310, II: 15271532. Ghandriche, H., 1991. ModalitEs de la superposition de structures de plissement-chevauchement d'5.ge alpin dans les Auras (Alg6rie). Thesis, Univ. Paris-Sud, Orsay, 189 pp. Jamison, W.R., 1987. Geometric analysis of fold development in overthrust terranes. J. Struct. Geol., 9(2): 207-219.
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Mercier, E., 1992. Les plis de propagation transport6s: mod61isation et exemple. Bull. Soc. G6ol. Ft., 163 (6): 713-720. Mercier, E., 1990-1994. Rampe(EM): software for modelling fault-related folds. Public Domain for Macintosh, version 1.5.2 available from the author. Suppe, J. and Medwedeff, D.A., 1990. Geometry and kinematics of fault-propagation folding. Eclogae Geol. Helv., 83(3): 409-454. Vila, J.M., 1980. La cha]ne alpine d'Alg6rie orientale et des confins alg6ro-tunisiens. Thesis, Univ. Paris VI, Paris, 665 PP. Vila, J.M. and Guellal, S. 1977. Geological map of Merouana, 1/50 000. Service de la carte g6ologique d'Alg~rie et SONATRACH, first edition.
