PULL-APARTS, SHEAR FRACTURES AND PRESSURE SOLUTION

Vein arrays are often composed of pull-aparts which are linked by shea r fractures, good examples of which occur in the Lower Jurassic limest ones of Somerset, southwestern England. Such pull-apart arrays have di splacement-distance characteristics which are similar to fault zones, with maximum displacement (indicated by the largest pull-apart widths) near the centre of the array, and with displacement decreasing toward s the tips. Pull-apart arrays usually die out into en echelon or pinna te veins. Evidence for pressure solution along the shear fractures whi ch connect pull-aparts include their dark and braided nature, their ob liquity to the displacement direction, the high dihedral angles(often > 90 degrees) between conjugate shear fractures, and the dissolution o f earlier structures. A range of geometries occurs, with varying relat ive amounts of veins and pressure solution being related to varying am ounts of transtension or transpression. There is a general trend for a n increase in the angle between vein segments and the shear fractures as contraction increases. There is therefore a trend for increased pre ssure solution on the shear fractures in more contractional arrays. Th e concentration of insoluble material along shear fractures has import ant implications for the mechanics and sealing of faults.