Résumé
Crystal-plastic fabrics within peridotites of Hole BA1B, BA3A and BA4A have been described during the Oman Drilling Project Phase 2 from 5 (super th) August to 3 (super rd) September on board D/V Chikyu. The relatively high degree of alteration present in the dunite makes the recognition of crystal plastic deformation difficult at the macroscopic scale. The deformation recorded in the harzburgite units is easier to characterize than in the dunite, using the elongation of orthopyroxene and the preferred orientation of spinel grains. Harzburgites in BA1B between 170.7 and 260.7 m showed mainly porphyroclastic textures with slightly elongated pyroxene grains. The degree of plastic deformation became intense between 260.7 and 353.8 m, where harzburgites gradually showed strongly foliated to protomylonitic texture. Harzburgites in BA3A showed strongly foliated to protomylonitic texture by intensely elongated, aligned pyroxenes in relatively shallow sections between 115.5 and 120.4 m as well as in deeper sections between 263.7 and 302.78 m, whereas the other sections showed mainly protogranular to porphyroclastic texture by slightly elongated, rounded pyroxene grains. Harzburgites in BA4A showed protogranular to porphyroclastic texture in the sections between 46.9 and 52.8 m and strongly foliated to protomylonitic texture in the deeper sections between 263.8 and 266.9 m. The other sections showed manily porphyroclastic texture. Dip angles of measured foliations range from nearly horizontal to 30-40 degrees with a maximum for 1 degrees to 10 degrees in BA1B. Most dip angles of pyroxenes are around 30 degrees in BA3A. Dip angles range from 10 to 50 degrees with an average at around 30 degrees in BA4A, slightly less steep than the dip of the dunite-harzburgite contacts. This is similar to the dip of the peridotite foliation in core BA3A, but significantly steeper than for the harzburgite dominated core BA1B. The dip does not show any significant variation with depth.