Reactivated strike-slip faults: examples from north Cornwall, UK

Several strike-slip faults at Crackington Haven, UK show evidence of right- lateral movement with tip cracks and dilatational jogs, which have been rea ctivated by left-lateral strike-slip movement. Evidence for reactivation in cludes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or n egative (left-lateral) displacements at fault tips. Evidence for the relati ve age of the two strike-slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks for med during left-lateral slip show no deformation; (2) some of the tip crack s associated with right-lateral movement show left-lateral reactivation; an d (3) left-lateral displacement is commonly recorded at the tips of dominan tly right-lateral faults. The orientation of the tip cracks to the main fau lt is 30-70 degrees clockwise for right-lateral slip, and 20-40 degrees cou nter-clockwise for left-lateral slip. The structure formed by this process of shrike-slip reactivation is termed a "tree structure" because it is simi lar to a tree with branches. The angular difference between these two group s of tip cracks could be interpreted as due to different stress distributio n (e.g., transtensional/transpressional, near-field or far-field stress), d ifferent fracture modes or fractures utilizing pre-existing planes of weakn ess. Most of the d-x profiles have similar patterns, which show low or nega tive displacement at the segment fault tips. Although the d-x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements sh ow various shapes depending on the amount of displacement and the slip sequ ence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fa ult tips due to late-stage tip propagation. Whereas for a smaller slip foll owed by larger slip with opposite sense, the d-x profile would be flatter w ith no reverse displacement at the tips. Reactivation also decreases the ra tio of d-x since for an original right-lateral fault, left lateral reactiva tion will reduce the net displacement (d(max)) along a fault and increase t he fault length (L). Finally, we compare Crackington Haven faults with thos e in the Atacama system of northern Chile. The Salar Grande Fault (SGF) for med as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven. (C) 2001 Published by Elsevier Science B.V.