MODIFICATION OF A REGIONAL STRESS-FIELD B Y A LOCAL MAGMATIC STRESS-FIELD, SKYE (SCOTLAND) AS A CASE-STUDY

Skye Tertiary magmatism in Scotland (fig. 1 and 2) is contemporaneous of constant SW-NE trend of regional minimal principal stress sigma3 th roughout magmatic activity. We showed, in a recent paper [Geoffroy et Bergerat, 1992], that two tensorial states of stress, computed from fa ult slip data [Angelier, 1983 ; 1989; 1990; 1991], were compatible wit h such sigma3 trend (fig. 3 and 4); an extensional stress regime expre ssed by normal faulting and a strike-slip regime with sigma1 trending NW-SE. This last stress regime could become truly compressive (sigma3 sub-vertical) with reverse faulting and fracture cleavage. Basing ours elves upon some few concording field observations of relative chronolo gy, we deduced that normal faulting stress regime probably predated st rike-slip faulting. We also proposed the hypothesis according to which strike-slip faulting was syn-plutonic. We therefore admitted the idea that there existed three regional palaeo-states of stresses during th e Palaeocene igneous events in Skye. We discuss here a new possible in terpretation which could explain the striking parallelism between the syn-magmatic tensors axes. 1) Modification of the strike-slip stress r egime around the plutonic centres (fig. 5) We have noticed a sub-radia l disposition of sigma1 computed from strike-slip faults around the ac tual plutonic centres (fig. 5, table). This disposition of sigma1 foll ows that of the tholeiitic dykes and, therefore, can not be an artefac t. It is also noticeable that the strike-slip faults show strong varia tions of the pitch of slickenside lineations around the plutonic compl exes. For example, at the NW of the intrusions, one meets both dominan t NW plunging striae and also SE plunging (fig. 5) ; it is possible in most sites of measurement to compute 2 separate stress tensors with t he same trend of sigma1 but with opposite trends of plunge of sigma1. We interpret these data as the result of the interference between a re gional strike-slip stress regime and a local low-magnitude magma stres s field caused by a high-level tholeiitic magma chamber of ellipsoidal shape (fig. 7B). 2) Geometry of the faults across Skye (fig. 3 and 6) The density of both normal and reverse faults increases towards the p lutonic centres (fig. 4). There are clear geometric relationships betw een normal and strike-slip faults around plutonic centres expressed by a continuous variation between both the inclination of the fault plan es and the pitch of slickenside lineations (fig. 6B). These relationsh ips disappear away from the igneous centres where the number of both d ip-slip and oblique-slip faults decreases (fig. 6C). We propose the fo llowing interpretation for these last observations : normal faulting i n Skye (and probably reverse faulting also) are born under a regional strike-slip stress regime locally strongly but perturbated around the plutonic centres during the emplacement of these intrusions (fig. 7C). During the intrusion of the Cuillins cone-sheet, the magmatic over-pr essure was high in magnitude over and around the magma chamber. The re gional vertical pressure sigma2 increased to a local value sigma1' exc eeding the magnitude of the regional horizontal stress sigma1 which th erefore became a local NW trending sigma2' (fig. 7C). During the late setting of the Eastern Red Hills granites, there was probably a decrea se of the vertical magma pressure as indicated by reverse faulting; th e regional vertical intermediate stress sigma2 decreased to a magnitud e smaller than that of the regional SW trending sigma3 and therefore b ecame a local sigma3'' whereas sigma3 changed, relatively to the verti cal stress, into a local sigma2'' (fig. 7D). Variations of stress regi mes are illustrated in figure 7, with Mohr diagrams showing the relati onships between the regional tectonic stresses and the superimposed lo cal stresses related to magmatic phenomena, which explain stress permu tations sigma1/sigma2 (fig. 7C) and sigma2/sigma3 (fig. 7D). 3) Discus sion and conclusion The new interpretation is very different from the preceding one [Geoffroy et Bergerat, 1992]. We believe that there has been an unique regional stress regime expressed in Skye (and perhaps t hroughout the BTVP) which has been strike-slip with sigma1 sub-horizon tal-trending NW-SE, sigma3 sub-horizontal trending SW-NE, and sigma2 s ub-vertical and lithostatic. After the alkaline activity, the tectono- magmatic events were Controlled by a high level local tholeiitic magma chamber situated below or at the actual emplacement of the central co mplexes. The variations of pressure in this magma chamber can explain the local change of the regional stress field around the intrusions.