En echelon vein arrays

1. En echelon vein arrays Faculty of geosciences 01000l-05 Du Shijun

2. INTRODUCTION There has been considerable debate over the nature of fractures which host en echelon arrays of veins. Mechanisms which have been proposed include extensional fracturing, shear fracturing (faulting) with the opposite sense of shear to the array (antithetic shear fractures), such as what we have seen in the Ya Kou of Nan Wang mountain. shear fracturing with the same sense as that of the array (synthetic shear fractures) and a hybrid of extensional and shear fracturing.

3. Morphological evidence supporting a shear fracture origin for the host fractures of en echelon veins include: (1)continuation of the host fractures outside the vein array with evidence of shear displacement on host fractures (2) shear fibres in veins. Although these features indicate shear displacement on a fracture they don’t exclude the possibility of its nucleation as an extension fracture.

4. Brittle –ductile shear zones often contain en echelon vein arrays which can be used to determine the sense of shear on the zone, since the extension direction is indicated by the dilation of the veins. Earlier-formed veins may be folded by the later movement on the shear zone to produce a sigmoidal pattern , which is also a good indicator of the sense of movement.

5. The veins have a sigmoidal shape below show evidence that the host fractures are dextral fault. Sigmoidal shape veins

6. The fractures continue beyond the tips of sigmoidal veins and displace bedding along the fracture.

7. Pinnate extension veins occur on the host fractures, particularly at terminations.

8. Quartz shear fibres fill some of these fractures.

9. conclusion Pinnate veins have extensional fabrics and sigmoidal veins have both shear and extension fabrics . The sigmoidal curvature of the veins produced contraction jogs along the dextral faults. Thus dextral slip could not occurred during or after sigmoidal vein opening. As deformation proceeded, and movement on dextral faults ceased, dextral arrays of extension fractures formed.

10. The problem on the determination of stress and strain trajectories For the conjugate array of en echelon veins, most authors agreed that conjugate zones enclose an acute bisector parallel to Z (where Z is the maximum incremental shorting strain direction), many equating this direction with that of sigma1(where sigma1 is the maximums effective compressive stress direction) in coaxial deformation. However, many en echelon arrays don’t occur in conjugate sets, and many workers have proposed that en echelon array may form during propagations of an extension fracture. These workers view the above method of inferring stress orientations with scepticism.

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