The pattern of curved mafic dike swarm in a Hercynian(?) granitic body, eastern Tianshan orogen (Northern mountain, Gangsu province, northwestern China): genesis and implications

In the Northern mountain, northwest of Gangsu province, northwestern China there occurs a set of curved dike swarm in an elliptical Hercynian(?) granitic body (about 35km×15km). This phenomenon is only visible on satellite image. Based upon the abutting and crosscutting relationships observed in the field, four dike sets are recognized in the intrusive body. The first is felsic, and the others mafic. The second set predominates, and makes up the phenomenon of curved dike swarm in the granitic body: a set of densely spaced dikes appears to change their azimuths systematically from the north-northeast in the north to the north-to-south in the middle to the northwest in the south. A simple explanation about the phenomenon is the existence of a single buckle that consists of these curved dikes. However, our field observations do not support this model. They in turn make us believe that the curved dike swarm should have formed to propagate along the curved trajectory of the maximum horizontal principal stress that resulted from local stress perturbation by preexisting fault(s). From the divergent distribution of the first dike set, we argue the presence of a hidden strike-slip fault beneath the granitic body, trending the west to east. Poly3D numerical software was used to address this stress perturbation. The results showed that a relatively large amount of dexterous strike slip along the fault was required to produce the curved stress trajectory in the area. The consolidation of this post-orogenic intrusive body seemed not to have completely healed the hidden fault(s), probably deep-rooted, along which it might have migrated upwards and/or emplaced. A satellite image of curved dike swarm in a granitic body from the Northern mountain, Gangsu, northwestern China. (downloaded from Google Earth)