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Effects of tectonism and magmatism on crack structure in oceanic crust: A seismic anisotropy study

¹ Department of Earth Science and Engineering, Imperial College of London, London SW7 2AZ, UK
² Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Cambridge CB3 0EZ, UK
³ Department of Earth Science and Engineering, Imperial College of London, London SW7 2AZ, UK
⁴ The ARAD Working Group comprises P.J. Barton, S. Bazin, A.J. Harding, R.W. Hobbs, G.M. Kent, J.A. Orcutt, J.W. Pye, S.C. Singh, M.C. Sinha, C.H. Tong, and R.S. White.

We analyzed 25,675 traveltime residuals from a three-dimensional seismic tomographic inversion to investigate crack-induced seismic anisotropy in the upper oceanic crust. The study covered two regions with contrasting levels of magmatic activity on the western limb of the 9°N overlapping spreading center on the East Pacific Rise. The level of anisotropy gradually decreases with depth in the magmatically and hydrothermally active ridge region. In contrast, we observed a highly variable anisotropic structure in the magmatically and hydrothermally less active tip region at the end of the dying ridge segment: a weakly anisotropic layer beneath strongly anisotropic extrusive volcanic rocks is likely to be the result of relatively shallow cracks closed by hydrothermal precipitation. Strongly anisotropic dikes with inferred narrow and water-saturated cracks provide important along-axis pathways for the circulation of hydrothermal fluids beneath the shallow cracks in the less magmatically active regions. Furthermore, a significant clockwise rotation (20°–30°) of fast directions occurs in both regions with increasing depth. Such a rotation provides evidence that the geometry of the underlying crack structure of the western limb is significantly different from that defined by the bathymetric ridge crest.

Key Words: dikes • East Pacific Rise • hydrothermal processes • mid-ocean ridges • ophiolite • paleotectonics

This article has been cited by other articles:

				C.H. Tong, C. Lana, R.S. White, M.R. Warner, and A. W. Group Subsurface tectonic structure between overlapping mid-ocean ridge segments Geology, May 1, 2005; 33(5): 409 - 412. [Abstract] [Full Text] [PDF]