TECTONIQUE/ TECTONICS zyx Analyse géométrique et cinématique d’un zyxwv (( duplex )) issu d’un modèle analogique visualisé en 3D par tomographie aux rayons X Bertrand Guillier, Patrice Baby, Bernard Colletta, Edgar Mendez, Rodrigo Limachi, Jean Letouzey et Martin Specht C.R. Acad. Sci. Paris, t. 321, série zy II a, zy p. 901 à 908, 1995 B. G. et zyxw P. B. : Convention YPFB- ORSTOM, casilla 1659, Santa Cruz, Bolivie ; B. C. et J. L. : IFP Géologie- Géochimie, BP no 31 1,92506 Rueil- Malmaison Cedex, France ; M . S. : Total, tour Total, 92069 Paris- LaDefense, France ; E. M. et R. L. : YPFB, casilla 1659, Santa Cruzt Résumé Nous présentons un modèle analogique ayant des propriétés mécaniques similaires à celles d’une série sédimentaire avec deux niveaux de décollement potentiel. Les expériences conduisent au déve- loppement de structures de type zyxwvutsrqpo duplex caractéristiques de nombreux avant- paysde chaînes. L’étude d e ces duplexes analogiques montre, entre autre, que la propagation des chevauchements y est tou- jours conforme et que le premier (( horse )) fonctionne en coin intracutané. Mots- clés : Duplex, Horse, Cinématique, Modèle analogique, Tomographie aux rayons X. zyxwv Abstract Geometry and kinematics of an experimental duplex analysed by X- raytomography Small- scaled multilayered models with the mechanical properties of a sedimentary pile including two levels of potential décollement were performed to study the kinematics and the geometry of duplex formation. The models reproduce the main features of the duplexes seen in the foreland of many mountain belts. Thrust propagation in the duplex is always in a forward sequence and the earliest horse develops as a triangle zone. Keywords: Duplex, Horse, Kinematics, Sandbox model, X- ray tomography. -_ =i zyxwvutsrqponm zz--T< -” Abridged INTRODUCTION English Version N a compressional regime a duplex is de- fined by two décollements or thrusts, a I basal “floor thrust” and a “roof thrust” at the top, which bound a set of “horses”.Horses are in turn bounded by subsidiary thrust faults curved asymptotically to merge the floor and roof thrusts (Dahlstrom, 1970 ; Boyer and Elliot, 1982 ; Mitra and Boyer, 1986). Boyer and Elliot (1982) classified the duplexes according to the ratio (u/ s) where (u) is the displacement and (s) the horse size (fig. 1 A). A series of four multilayered models, produ- cing duplex analogues, has been performed at the Institut Français du Pétrole. Each experi- ment was carried out in a normal uavitv field “ I with a shortening velo’city of 30 mm . h-l. The i l models were composed of sand interlayered with silicone putty which simulated a weak vis- cous layer and the roof décollement. GIass microbeads at the base of the sand cake simu- lated the floor décollement (fig. 1 zyx B). Compu- terized X-ray tomography (Colletta et a l., 1991) was used to visualise cross-sections during deformation and a complete 3D image of the final deformed model. Four or five sec- tions have been scanned in each model every 4 mm (1.7 %) of shortening, with a total shor- I UilLxi u~ifJlllcliE,ilL L a Y , I-, tening of 55 mm (22 %). cote: (s* I 9 5% Ex:d GEOMETRICAL ANALYSIS AND NATURAL EXAMPLE Note présentée par Jean Dercourt. The horizontal cross-sections obtained ~~~~~~~~ thanks to X-ray tomography, show that the le 12 mai 1995. 901 1250- 8050/ 95/ 03210901 0 $2.00 O Académie des Sciences