Vertical Stylolites Found in Paleozoic Carbonates in the Western Shandong and Their Constraints on Tectonic Evolution of the Eastern North China Plate

The author of this paper found two sets of verticlal stylolites in Paleozoic carbonates,the Middle Ordovician dolostones and the Upper Carboniferous limestones,in the Boshan area,Shandong Province,east of China. Both sets of stylolites developed along the preexisting vertical shear joints——the X conjugate joints during the process of diagenesis. The teeth of both stylolites are not perpendicular,but oblique to the stylolite surfaces. However,it is recognized that all the teeth of the stylolites should track the largest regional compressive stress (σ1). According to the direction of the teeth,crust compression events in the NNE—SSW direction happened twice separately in the Middle Ordovician and the Late Paleozoic. The first event might be related to the uniformly uplift of eastern North China Plate,which is shown by the parallel unconformity between the Middle Ordovician and the Upper Carboniferous (C2/O2). The second event coincided with the folding and uplifting of North China Plate by the end of Paleozoic,which is indicated by the angular unconformity between the Upper Permian and Jurassic. Furthermore,the two sets of vertical stylolites also show that the X-type conjugate shear jointing can develop in the early diagenesis and thus provide the sites for the pressure solution,and the shear joint planes can be rotated by the successive crust compression. Therefore,in reality,the direction of the greateast principal stress (σ1) often bisect an abtuse angle (90°) rather than an acute angle at which the two conjugate joint planes intersect.