The seismic structure of 140 Myr old crust in the western central Atlantic Ocean

A detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone (OBH) receivers was carried out over Mesozoic magnetic anomaly M17 about 300 km south-west of Bermuda. Amplitude and travel-time interpretations show this 140 Myr old crust to be uniform, on the seismic scale, over a lateral distance of almost 100 km. The best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7.2 km. The principle components, beginning with a 5 km s−1 velocity at the top of layer 2, are a 0.4 km thickness with gradient 1.1 s−1, a 1.9 km thickness with gradient 0.64 s−1, 1.7 km thickness with gradient 0.1 s−1, a 3.2 km thickness with zero gradient and constant velocity of 7 km s−1 below which lies a 0.5 km thick moho transition zone. The uppermost few kilometres of the upper mantle apparently has little or no velocity gradient. The normal incidence two-way reflection time through this structure agrees to better than 0.1 s with the location of the Moho reflection seen on the IPOD/USGS multichannel reflection profile that passes within 1 km of this experiment. The travel times of the converted shear wave arrivals constrain Poisson's ratio in the igneous crust to ∼ 0.28 ± 0.01. A delay time study of a 20 km radius circle of shots fired around a small (∼ 6 km) five-element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres. The observed differences in shallow crustal delay time between the five OBH instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile. These experiments were located clear of fracture zones or other structural anomalies in a region of well-defined linear constant amplitude magnetic anomalies. We propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres. We suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies (unknown at the time) and the data density and sediment thickness information available for each experiment was insufficient to overcome the errors introduced by the basement topography.