Open AccessCrustal Structure of Atlantic Fracture Zones ‐ Iii. the Tydeman Fracture Zone
Author(s) -
Potts C. G.,
Calvert A. J.,
White R. S.
Publication year - 1986
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1986.tb00668.x
Subject(s) - geology , seismometer , seismology , fracture zone , explosive material , fracture (geology) , crust , seismic survey , shot (pellet) , seismic tomography , seismic refraction , mantle (geology) , geophysics , geotechnical engineering , materials science , chemistry , organic chemistry , metallurgy
Summary. An extensive seismic survey was performed in March 1982 on two different parts of the Tydeman fracture zone. This survey consisted of two separate experiments: a reversed seismic refraction experiment, centred on 36°N, 26°W, using ocean‐bottom seismometers and free‐floating sonobuoys, which was shot with explosives; and a two‐ship multichannel experiment of the expanding spread type, also shot with explosives at approximately 36°N, 23°30°W. These experiments are the first, and to date only, wide‐angle seismic surveys of fracture zone crust older than 25 Myr. Slope‐intercept and tau‐ p inversion of the OBS/sonobuoy travel time data indicates low crustal and mantle seismic velocities within the fracture zone. These results are confirmed and refined by a more detailed analysis using raytracing and synthetic seismogram modelling for laterally varying structures, which implies velocities for the upper mantle beneath the fracture zone of 7.3–7.5 km s −1 . Similar analysis of a line perpendicular to the fracture zone reveals the transition from normal mantle velocities to the low values found beneath the fracture zone, and suggests that the width of the anomalous region is approximately 12 km. the geometry of the OBS experiment permitted a delay time analysis and a further simple tomographic inversion of the traveltime data, which produced an approximate image of the low velocity mantle region associated with the fracture zone. the width of the region of anomalous crust is the same as the width of the sediment filled trough, which marks the fracture zone location at the line intersection. Analysis of the two‐ship seismic data from a site some 240 km to the east, also by synthetic seismogram modelling, gave similar results to the OBS/sonobuoy experiment with a mantle velocity of 7.2 km s −1 detected beneath the crust, which possessed lower than normal seismic velocities. the close seismogram spacing of less than 0.1 km allowed us to transform the dataset into the tau‐ p domain and enabled us to detect arrivals obscured in the X‐T domain by the reflected water wave. Both experiments indicate low crustal seismic velocities within the fracture zone together with unusually low velocities of 7.2–7.5 km s −1 in the upper mantle. Comparison with other fracture zone surveys reveals that the seismic velocity in the upper mantle beneath fracture zones decreases with increasing age. We attribute this to serpentinization of mantle peridotite brought about by hydrothermal circulation to the base of the fractured and faulted crust beneath the fracture zone.