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Table T1. Predicted and actual depths to gabbros at Site 1256.

  Depth to axial
low-velocity zone
100 m of
off-axis lavas
300 m of
off-axis lavas
450 m of
off-axis lavas
575 m of
off-axis lavas
depth to gabbro
msb mbsf msb mbsf msb mbsf msb mbsf msb mbsf msb mbsf
Minimum 725 975 825 1075 1025 1275 1175 1425 1300 1550 1157 1407
Maximum 1000 1250 1100 1350 1300 1550 1450 1700 1575 1825

Depths to the axial low-velocity zone estimated from the relationship with spreading rate shown in Figure F5 (following Purdy et al., 1992, and Carbotte et al., 1997) for ocean crust spreading at 200 mm/y. The thickness of off-axis lavas estimated on textural grounds from cores recovered during Leg 206 was 284 m. Greater thicknesses of off-axis lavas have been estimated from wireline imaging logs from the occurrence of a pillow lava–rich zone between 700 and 825 mbsf in Hole 1256D (Tominaga and Umino, 2010). The actual depth where the first gabbro (Gabbro 1) was recovered was ~1407 mbsf (1157 msb). This is in excellent agreement with the observed relationship between spreading rate and depth to axial low-velocity zones and the thickness of off-axis lavas estimated from Hole 1256D. Depths in mbsf include the 250 m thick sediment cover at Site 1256. — = not available.

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