The origin of orthogonal vein network in Paleocene-Eocene flysch south of Birjand, eastern Iran

This paper presents the results of a field study aiming to describe and to interpret origin of the vein network in the south of Birjand. Adjacent to Birjand ophiolite in eastern Iran, the Paleocene-Eocene flysch facies was deposited in the Birjand foreland area concomitant with the Alpine orogeny (Laramide). The sandstone unit of this facies contain two orthogonal sets of quartzite veins. The N310-340 striking veins (set 1) are arranged parallel to the Bagheran Kuh range front and perpendicular to the vein set 2 (N215-240). The paleostress reconstruction in the Paleocene-Eocene shows that the regional compression direction N240 is perpendicular to the Bagheran Kuh range front. Structurally, to create orthogonal veins, σ1 should be perpendicular to the layering and σ2 and σ3 should be horizontal. This situation was created in the middle Eocene-Oligocene. The post-collision extensions of this period caused a decrease in regional pressure in the region. As a resault, the maximum principal stress (σ1), was changed to a vertical state and intermediate stress (σ2) was parallel to the orogenic pressure. In these conditions, orthogonal tensile openings (state I) were formed as a result of the local inversion of stress and fluid pressure in the flysch facies.