Computers & Geosciences 28 (2002) 775–781 DIPSLIP: a QuickBasic stress inversion program for analysing sets of faults without slip lineations $ Tobore Orife*, Luis Arlegui, Richard J. Lisle Laboratory for Strain Analysis, Department of Earth Sciences, Cardiff University, Cardiff, Wales CF10 3YE, UK Received 10 May 2000; received in revised form 30 June 2001; accepted 5 July 2001 Abstract A simple computer program is described for estimating palaeostress tensors from orientation data from a set of fault planes. The computation is based on a novel technique that allows the tensor to be estimated in situations where directions of slip on the faults cannot be determined, but where the senses of the dip-slip component of slip on the faults are known. The new technique greatly broadens the scope of palaeostress analysis, permitting the analysis of faults lacking slickenlines but exhibiting offsets of horizontal marker beds. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Structural geology; Palaeostresses; Stress quadric; Slip sense; Stress tensor 1. Introduction Faulting is the brittle response of rocks to tectonic stresses. The geometrical properties of faults and their movements are thought to be controlled by the nature of the active stresses. Half a century ago Anderson (1951), using the Navier–Coulomb theory of brittle fracturing, suggested how the orientation of faults are controlled by the directions of the principal stress axes. Geologists realised that if faults are governed by stresses, then the orientation information collected from faults in the field could be used to characterise the palaeostress tensor. This process has become known as stress inversion. Stress inversion based on Anderson’s theory is limited to a consideration of stresses responsible for forming the original fracture of the fault surface. It is known, however, that fault slip often takes place along favourably oriented pre-existing planes of weakness rather than always on newly formed fracture surfaces. Wallace (1951) and Bott (1959) developed a theory relating the direction of slip on reactivated faults to the imposed stress state. Their theory assumes that the fault- slip vector is parallel to the direction of resolved shear stress on the plane of weakness. They demonstrate that on a given plane of weakness the direction of slip depends on four variables; three of these describe the orientations of the principal stress axes and the fourth is f; the ratio of the principal stress differences (f ¼½s 2  s 3 =½s 1  s 3 ). The Wallace–Bott hypothesis forms the theoretical basis of most methods of stress inversion in current use (see Angelier, 1994, for a comprehensive review of methods). The advantage of Bott–Wallace methods of stress inversion is that they offer the potential of computing four of the six components of the full stress tensor. On the other hand, these methods require data on the orientations of the slip directions as well as the attitude of the fault surfaces. These requirements can be often fulfilled in faults exposed at the surface, where lineations $ Code available from server at http://www.iamg.org/ CGEditor/index.htm. *Corresponding author. E-mail address: orife@cardiff.ac.uk (T. Orife). 0098-3004/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0098-3004(01)00099-1