Palaeostress and neotectonic analysis of sheared conglomerates: Southwest Alps and Southern Apennines Jean-Claude Hippolyte * Laboratoire de ge Âodynamique des Chaõ Ãnes Alpines (CNRS, UMR 5025), Universite  de Savoie, Campus Scienti®que, 73376 Le Bourget du Lac Cedex, France Received 10 January 2000; accepted 17 July 2000 Abstract Geometrical and mechanical characteristics of the deformation of poorly cemented conglomerates are described. Using striated pebbles for analysis of palaeostresses, it is crucial to distinguish radial striation patterns, which result from deformation of the matrix around a rigid pebble, from unidirectional striation patterns that represent shear zones crossing the conglomeratic material. Examples of palaeostress determinations from striations of the latter type are given for extensional settings (Provence) and compressional settings (Southern Apennines, Southwest Alps). Their comparison with fault analyses in brittle rocks that underlie the conglomerates validates their usefulness for palaeostress analyses and suggests that some conglomerates behave as materials containing pre-existing surfaces of mechanical anisotropy that fail by sliding on some suitable oriented surfaces. These examples show that sheared conglomerates can be used for stratigraphic dating of the deformation, for studies of syndepositional deformation and for neotectonic analysis. q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction Brittle tectonic analysis of mesofractures and palaeostress determination permits the deciphering of the evolution of structures from outcrop to tectonic plate scale (cf. Hancock, 1985; Angelier, 1994; Dunne and Hancock, 1994). In particular, if one wants to precisely date the deformation by ®nding the most recent faulted rocks, or collect evidence of recent deformation, syntectonic deposits which are often clays, sands and conglomerates have to be studied. However, some dif®culties may arise in carrying out fault analysis in these materials. The sandy material usually does not exhibit slip indicators along fault planes. The sense of slip on faults cutting clays and marls is often ambiguous if not indicated by stratigraphic offsets. In contrast, conglomerates often show well-preserved striations and sense indicators on pebble surfaces, and sometimes on fractures cutting the pebbles. In deformed conglomerates, different structures will result from variations in the contrast between the mechanical properties of the pebbles with those of the matrix. Faults cutting pebbles and matrix indicate a brittle behaviour of the whole material and can clearly be used in palaeostress determination (e.g. Petit et al., 1985). In poorly cemented conglomerates, fractures cutting only the pebbles may result from intra-pebble ampli®cation of the tectonic stresses (e.g. Eidelman and Reches, 1992). These tensile fractures are most common whenever the strain rate is high, the matrix is weak and the pebbles rigid (e.g. quartzitic) (McEwen, 1981). In compressional settings, these tensile fractures can form in pebbles at a depth of a few hundred metres or deeper and they indicate the trend of s 1 (Eidelman and Reches, 1992). More frequently, the poorly cemented conglomerates are deformed by shear movements. In this case, the different value of the elastic moduli of the pebbles and of the matrix, together with the low strength of the pebble±matrix bond (McEwen, 1981), result in deformation which is mainly localized at the pebble±matrix boundary. The resulting striated outer surfaces of pebbles have often been used to determine palaeostress axes, considering them as similar to fault planes (e.g. Campredon et al., 1977; Petit et al., 1985; Combes, 1984; Fesce, 1986; Ritz, 1992). Furthermore, Schrader (1988) has shown that striation ®elds made by the sandy matrix around a rigid pebble can be used in deformation analysis. This paper presents examples show- ing that some striation patterns can be good tectonic and neotectonic indicators, and can be used in palaeostress determination. Journal of Structural Geology 23 (2001) 421±429 0191-8141/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0191-8141(00)00120-6 www.elsevier.nl/locate/jstrugeo * Fax: 133-4-79-75-87-77. E-mail address: jean-claude.hippolyte@univ-savoie.fr (J.-C. Hippolyte).