Seismic evidence for hydrothermally altered mantle beneath old crust in the Tydeman fracture zone

Details Seismic evidence for hydrothermally altered mantle beneath old crust in the Tydeman fracture zone Seismic evidence for hydrothermally altered mantle beneath old crust in the Tydeman fracture zone

Nov 1985

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1985e%26psl..75..439c&link_type=abstract

Earth and Planetary Science Letters, Volume 75, Issue 4, p. 439-449.

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Scientific paper

To study the structure of old Atlantic fracture zones, seismic refraction profiles were obtained using ocean-bottom seismometers and two-ship multichannel expanding spread experiments on two inactive sections of the Tydeman fracture zone. The results indicate that: (a) crustal seismic velocities are low compared with ``normal'' oceanic crust; (b) there is a marked absence of an identifiable layer 3; (c) the depth to the Moho is shallow, though not exceptionally so; and (d) anomalously low seismic velocities of 7.2-7.5 km s-1 are observed over ranges at which upper mantle velocities are normally detected. S-wave arrivals, corresponding to both crustal and mantle refractions, are also observed, which enable the first estimates of Poisson's ratio within a fracture zone to be made. Using these results, and those from other surveys, we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age. These findings suggest that hydrothermal circulation, which can lower the upper mantle velocity by serpentinization, persists under fracture zones for at least 70 Ma, and occurs at greater depths than for normal oceanic crust. The serpentinization process is not sufficiently rapid to obscure the location of the original crust-mantle boundary.

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