0191-8141/87 $03.00 + 0.00 Pergamon Journals Ltd.
Criteria for the sense o f m o v e m e n t on fault surfaces in brittle rocks
J. P . PETIT Laboratoire de Tectonique, Unit6 Associ6e C.N.R.S. 266, U. S.T.L., Place E. B ataillon, 34060 Montpellier c6dex, France
(Received 13 June 1986; acceptedin revised form 3 March 1987)
Abstract--This paper gives a detailed description of the use of minor structures on fault planes in various brittle rocks as indicators of the sense of relative movement. The main kinds of structures described involve sets of repeated secondary fractures (striated or not) which intersect the slip plane in a direction roughly perpendicular to the slip direction. Someof the most frequent criteria are new; their reliability is established in the field and by comparison with both previous and new experiments. It is shown that some of the rarer structures probably correspond to neo-rupture in intact rock, most probably at the tip of pre-existing joints, while the most frequent structures are generated by friction on joints, with very little sliding. Thepossible seismic origin of some structures is discussed.
THE DETERMINATIONof direction and sense of movement
on faults is a basic requirement during brittle tectonics analysis. It is indispensable in establishing the kinematics of faults of various scales or the geometry of faulted ore bodies, especially in cases where the slip cannot be determined from the offsets of geologicalstructures. With recent computer methods which give some characteristics of paleostress tensors from measurements of fault striation (Carey & Brunier 1974, Angelier 1975, Etchecopar et al. 1981, Armijo et al. 1982), slip-sense determination is also necessary. A most useful way of determining direction and sense is the direct observation of fault surfaces which may show not only striation but alsominor structures indicating the sense of relative movement (sense criteria). Original morphological descriptions of corresponding structures are rare in the geological literature (Dzulinski & Kotlarczyk 1965, Tjia 1971, Hobbs et al. 1976, Elliott 1976). This paper gives descriptions of such structures based on field observations of faults in the Alpine High Atlas Mountains of Morocco (Petit 1976),where there is a very dense fault network linked to the Tizi n'Test Fault zone (Mattauer et al. 1972, Proust et al. 1977). In the excellent outcrops of this semi-arid region, more than 4000 fault planes were observed in different rocks: granite, gneiss, volcanics, sandstone and limestone (Petit et al. 1983). In addition, slickensides formed in incompletely lithified sandstones, and morphologicaland microstructural comparisons between 'hydroplastic' and 'brittle' fault structures formed in the corresponding indurated sandstone, have been described (Petit & Laville 1987). Some of the features given in this paper have already been described (in French) by Petit et al. (1983); they have subsequently proved useful in other regions. The present paper re-describes these features, together withnew observations and interpretative elements. The observations were strictly limited to fault sur-
faces, excluding those completely covered with gouge or with pronounced planar anisotropies in the rock (especially cleavage). Detailed field descriptions are mainly confined to features visible to the naked eye or with a hand lens. The microstructures will be described in a separate publication.The reliability of the shear-sense criteria can be checked when different criteria are used for the same fault plane (related to the same movement). Within a strike-slip regime, the slickensides of conjugate faults show structures indicating opposite senses. If several tectonic phases are responsible for different structures linked to several striation orientations on the same surfaces, the...