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

Abstract

Summary. 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 140Myr old crust to be uniform, on the seismic scale, over a lateral distance of almost 1OOkm. 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-l velocity at the top of layer 2, are a 0.4 km thickness with gradient 1.1 s-', a 1.9km thickness with gradient 0.64s-', 1.7 km thickness with gradient 0.1 s-', a 3.2 km thickness with zero gradient and constant velocity of 7 km s-' 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 k 0.01. A delay time study of a 20km 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