Shear wave velocities and anisotropy in the Barbados accretionary complex

Abstract

S wave velocities at the base of the Barbados accretionary wedge were determined from mode‐converted S waves in two vertical seismic profiles in Ocean Drilling Program boreholes. At Hole 949C the profile extends from 92 to 397 m below seafloor (mbsf), ending ∼17 m into the décollement zone separating the wedge from the underthrust sediment on the subducting plate beneath. At Hole 948D the profile extends from 105 to 471 mbsf, ending ∼19 m above the décollement zone. S wave velocities are 303–549 m/s at Hole 949C and 453–679 m/s at Hole 948D. The profiles show that the lowest 70 m of the accreted sediment, just above the décollement, has lower S wave velocity (379±26 m/s at Hole 949C and 453±11 m/s at Hole 948D) and higher ratio of P wave to S wave velocity (Vp/Vs) (4.59±0.31 at Hole 949C and 3.99±0.10 at Hole 948D) than in the overlying layer. This coincides with high clay content that has a high proportion of smectite. Shear wave splitting observed in records from the lowest 100 m of the accretionary wedge at both sites indicates little or no anisotropy in the low‐velocity zone. A single measurement at Hole 949C of the horizontal component of polarization of the leading split S wave from an ocean bottom shot is N68°E±20°. At Hole 949C, split shear wave delays in the downward propagating S wave originating at a fault at 275 mbsf indicate strong anisotropy in the range 275–306 mbsf and little or no anisotropy in the range 306–380 mbsf. The rate of increase of delay between 275 and 306 mbsf, 0.52±0.16 ms/m, can be modeled by either a Hudson cracked medium with crack density 0.186±0.054 or (preferably) a Hornby model of shale with 44% of clay particles aligned within 5° of the symmetry plane. This strong anisotropy is attributed to “scaly fabric,” alignment of clay particles by pervasive shear in the fault zone, which is observed in core. The low anisotropy in the remainder of the section shows that there is very little overall alignment of pore space in the lower part of the accreted sediments, supporting the hypothesis that the sediments are undercompacted rather than hydrofractured. The low S wave velocities and high Vp/Vs ratios indicate undercompaction and low effective stress despite the tectonic burial of the lowermost accreted sediments.