En echelon veins

In structural geology, en échelon veins or "en échelon gash fractures" are structures within rock caused by noncoaxial shear.
They appear as sets of short, parallel, planar, mineral-filled lenses within a body of a rock. They originate as tension fractures that are parallel to the major stress orientation, σ1, in a shear zone. They are subsequently filled by precipitation of a mineral, typically quartz or calcite. As soon as they form, they begin to rotate in the shear zone. Subsequent growth of the fracture therefore causes the vein to take on a sigmoidal shape. They can be used to determine the incremental kinematics of the deformation history of the rock.