| Summary: | A long range (1000 km) seismic refraction was carried out in the western North Atlantic Ocean over lithosphere aged 85-110 Ma. An array of 18 ocean bottom seismographs (OBS), deployed at 60 km intervals, recorded P and S-wave arrivals, at a single azimuth (000°), from a series of large explosions. The lithospheric velocity structure to depths of 60 km was derived from studies of these arrivals. Throughout the experiment, earthquakes occurring in the vicinity of the nearby Antilles island arc were monitored by a network of 53 island stations. P-wave arrivals from 7 located earthquakes were detected by one or more OBS. A separate analysis of these recordings highlighted apparent velocities consistently 0.2-0.3 kms-1 less than the corresponding explosion data. This discrepancy is best explained by the presence of a 2t velocity reduction in the upper 50 km of the downdipping lithospheric slab beneath the Puerto Rico Trench. The earthquake velocity observations were constrained to the uppermost 22 km of the lithosphere and showed no evidence of large-scale azimuthal velocity anisotropy. Deeper lithospheric anisotropy seems unlikely but cannot be conclusively excluded. The sub-crustal ocean lithospheric velocity structure can be divided into two zones separated by a sharp discontinuity at a depth of 48 km. The upper zone consists of a sequence of low (8.2 kms-1), high (8.4 kms-1) and l ov (8.2 kms-1) P-wave velocity layers. The lower zone exhibits high velocities of 8.5-8.6 kms-1. The discontinuity separating the two zones is the best defined element of the model and suggests the occurrence of a major compositional or phase change. The S-wave data are less reliable but generally support this model. The upper zone shows broad qualitative and quantitative agreement with existing petrologic models for the generation of oceanic lithosphere from an initial 'pyrolite' type composition. However, none of the 'pyrolite' type models can easily explain the P-wave velocities over 8.5 kms- observed in the lower zone. A better ...
|
|---|