Significance of Fractures and Veins in the Basalt at Site U1431 and U1433, International Ocean Discovery Program Expedition 349, South China Sea
Wednesday, 17 December 2014
During International Ocean Discovery Program (IODP) Expedition 349, we successfully cored basalts along the central relict spreading ridge, in both the East Subbasin at Site U1431 and the Southwest Subbasin at Sites U1433 and U1434. Due to low recovery (<20%), Site U1434 provides little opportunity to analyze fractures and veins; however, we carefully described and analyzed fractures and veins at Sites U1431 and U1433. Several hundred fractures and veins were observed and we measured ~75% of those, whereas the other ~25% are either irregular or in small branches and thus could not be measured easily. Most of the fractures and veins have no obvious lateral offset or striations on the fractured surface. The veins are generally infilled fractures, whereas many fractures are reactivated along preexisting veins. Thus, the total number of fractures and veins can be used as an indicator of the degree of rock fragmentation. Shapes and contact relationships suggest that most of the fractures and veins resulted from cooling effects. Comparison between fracture and vein frequency with lithology indicates that high fragmentation occurs in the pillow basalt, whereas low fragmentation corresponds to massive flow basalt at Site U1433. At Site U1431, the relationship is similar, but more complicated. The frequency of fractures and veins at Site U1431 is almost double that of Site U1433. The basalt is also more altered at Site U1431 than U1433. These results provide constraints on investigation of the rock fragmentation and alteration processes of basaltic basement of the South China Sea.