Development of crystal morphology during unitaxial growth in a progressively widening vein: I. The numerical model P.D. Bons * Earth Processes Simulation Laboratory, Department of Earth Sciences, Monash University, Clayton Melbourne), VIC 3168, Australia Received 26 January 1999; accepted 23 November 1999 Abstract A two-dimensional numerical model for the simulation of the formation of microstructures in crack-seal veins is presented here. The grain aggregate in the vein is described by grain boundary nodes that link short, straight boundary segments. Growth of crystals into a crack takes place by many small incremental movements of nodes. The rate of growth depends on the chosen growth morphology of the vein-forming material. Different growth morphologies can be de®ned. For every user-de®ned number of steps, the wall rock is moved a user-de®ned distance and direction, which simulates the opening of a crack, after which sealing can proceed again. The shape of the crack-surface e.g. smooth or with ridges) can be set by the user and can be made to match fracture surfaces found in real rocks. The model is capable of reproducing realistic crack-seal textures and can be used to systematically investigate the role of different parameters on the microstructures of veins. First applications of the model are presented in Part II of this paper this volume). q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction Veins are of interest to geologists as each vein represents a stage in the overall geological history of its host rock. The mineral content, shape of the vein and internal micro- structure can reveal the geological conditions and operating processes during this stage. Those veins that have very elongate grains form one class of veins that is of particular interest to structural geologists. Such veins may not only preserve information about the average conditions during the vein growth stage, but also the progressive development and changes in conditions during that stage, in particular the maximum incremental stretching or minimum stress direction Durney and Ramsay, 1973; Ramsay, 1980; Ramsay and Huber, 1983; Passchier and Urai, 1988; Spencer, 1991; Passchier and Trouw, 1996). This class of veins is generally termed `®brous' as indi- vidual vein crystals can have rod-shapes with high to very high length/width ratios Mu Ègge, 1928; Durney and Ramsay, 1973). Fisher and Brantley 1992), and more recently Bons and Jessell 1997), proposed that this class should be subdivided into two: ®brous veins sensu-stricto and elongate-blocky veins. Fibrous veins s.s. have rod- shaped grains with length/width ratios up to the order of 100:1 and very little variation in individual grain shape Fig. 1a). Elongate-blocky veins have a larger variation in grain shape and the average grain width increases in the growth direction Fig. 1b). Many veins that are described in the literature as `®brous' would fall into the elongate- blocky category, as are the veins described by Ramsay 1980), which he suggested were formed by the crack-seal mechanism. Crack-seal veins grow by many increments of fracture opening crack), followed by ®lling of the fracture by overgrowth of existing vein crystals, exposed to the open ¯uid-®lled fracture seal). The repeated crack-sealing is usually revealed by vein surface parallel arrays of wall rock inclusions or detached over-growths and ¯uid inclusions Ramsay, 1980; Cox and Etheridge, 1983; Ramsay and Huber, 1983; Jessell et al., 1994). Stretched crystals Durney and Ramsay, 1973) are also sometimes referred to as `®bres' Cox and Etheridge, 1983). Stretched crystals are formed by repeated crack-sealing, with subse- quent cracks cutting grains at different sites. As a result, very elongate crystals may form Fig. 1c), which typically have serrated grain boundaries. Cracks cutting stretched crystals probably ®ll by growth from both crack surfaces, for which Hilgers et al. 2000) propose the term `bitaxial'. The term `unitaxial' then refers to cases where growth is only from one side. It is proposed here not to use the term `®brous', but only `stretched crystals' for crystals that grow from repeated crack-sealing at different sites. Grains in ®brous and elongate-blocky veins often show a Journal of Structural Geology 23 2001) 865±872 0191-8141/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0191-814100)00159-0 www.elsevier.nl/locate/jstrugeo * Now at: Tektonophysik ± Institut fu Èr Geowissenschaften, Universita Èt Mainz, 55099 Mainz, Germany. Fax: 149-6131-3923863. E-mail address: bons@mail.uni-mainz.de P.D. Bons).