Crustal structure of Atlantic Fracture Zones - III. The Tydeman fracture zone

Abstract

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 t o 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 7.3-7.5kms-'. Similar analysis of a line perpendicular t o 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 t o the east, also by synthetic seismogram modelling, gave similar results to the OBS/sonobuoy experiment with a mantle velocity of 7.2 km s-l detected beneath the crust, which possessed lower than normal seismic velocities. The close seismogram spacing of less than 0.1 km allowed us t o 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-' 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.The Tydeinan fracture z m e 91 1 anomaly 30/31 by Collette et 01. (1984), using the time-scales of La Brecque et al. (1977)). At the OBS and sonobuoy experimental site the identification of anomaly 24, on the north side of the fracture zone, gives an age between 53 and 56 Ma.There is no evidence of present day large-scale natural seismic activity along the Tydeman fracture zone itself (Anderson 1985). The location of the fracture zone from our bathymetric contour map (Fig. 1) is in close ...

The ocean-continent boundary off the western continental margin of Iberia-II. Crustal structure in the Tagus Abyssal Plain

“…The thin low-velocity crust is probably intensely fractured and hydrothermally altered ). The low mantle velocities have been attributed to serpentinization of peridotite by circulating sea-water (Potts, Calvert & White 1986) and in situ serpentinized peridotite has been found by submersible studies in several FZ's like the Cayman Trough (Stroup & Fox 1981) and the Kane FZ (Karson & Dick 1983). Traveltime modelling of Line 5 S-wave arrivals from the 7.6-7.9 km s-' layer allowed us to calculate a Poisson's ratio of 0.285.…”

Crustal structure of Atlantic fracture zones--II. The Vema fracture zone and transverse ridge

“…This phenomenon is often observed in OBS data, and there is some evidence that it may be attributable to resonance of the instruments themselves (Loncarevic 1981) or is a result of reverberations in shallow layers. It has, however, been observed particularly in experiments in fracture zones using OBSs of different designs, and also surface receivers such as sonobuoys (e.g Sinha & Louden 1983) and multichannel hydrophone streamers (Potts, Calvert & White 1986). The ringy character may thus be a feature of fracture zone structure, caused by dispersion, attenuation and reverberation of the seismic signals in the highly fractured crust which is inevitably present, especially in the active transform region.…”

Seismic Crustal Structure and Morphotectonic Features Associated With the Chain Fracture Zone and Their Role in the Evolution of the Equatorial Atlantic Region

“…Regardless of the presence/absence of tectonic activity, an FZ domain, together with its parent TF domain, can be referred to as a transform discontinuity system. Much of what we know about the nature of oceanic crust within transform discontinuity systems came from a large number of active-source seismic refraction (and some reflection) experiments conducted in the 1980s in slow-spreading (20-50 mm/yr) environments (e.g., Ambos & Hussong, 1986;Calvert & Potts, 1985;Cormier et al, 1984;Detrick & Purdy, 1980;Detrick et al, 1982Detrick et al, , 1993Fox et al, 1976;Minshull et al, 1991;Mutter et al, 1984;Potts, Calvert, et al, 1986;Sinha & Louden, 1983;White et al, 1990;Whitmarsh & Calvert, 1986). A global notion resulting from these studies is that the subsurface within slow-slipping transform discontinuity systems (including FZ domains) is represented by thin, highly fractured and altered crust, often with partially or entirely absent gabbroic layer (e.g., Cormier et al, 1984;Detrick et al, 1982Detrick et al, , 1993Detrick & Purdy, 1980;Whitmarsh & Calvert, 1986).…”