CRUSTAL STRUCTURE VARIATION ALONG THE CENTRAL RIFT RIDGE AXIS IN THE NORTH FIJI BASIN - IMPLICATIONS FROM SEISMIC-REFLECTION AND REFRACTIONDATA

Seismic reflection and refraction surveys carried out in the accretion ary region of the North Fiji Basin (NFB) have revealed several feature s of the crustal structure along the axial domain, some of which can b e interpreted as the results similar to the phenomena of the active ri fting and spreading in the mid-ocean ridges system. Although the multi -channel seismic (MCS) reflection profiles recorded along the central axial graben and ridge, where the most active hydrothermal activities were observed and medium to fast crustal spreading rate was implied, d o not show clear structural evidence of the axial magma chamber (AMC) reflector, very shallow Moho (approximately 3 km below the sea floor, unusually thin crustal thickness) is deduced from the refraction data obtained from the site around station 14, where the most dominant spre ading is occurring to date in the NFB. The seafloor in this region is sediment free. This is interpreted to be simply because the crust is y oung and the volcanic material is directly exposed at the seafloor as a result of the recent volcanism. This is also confirmed from the velo city profiles obtained by refraction experiments. Although the station distribution of the refraction experiments is not dense enough to cov er all along the axial region, the along-axis lateral variation of the velocity depth profile of the crust is implied. In general, the featu re of the crustal velocity structure in the axial domain deduced from seismic reflection and refraction experiments indicates that the centr al spreading/rifting system of the NFB is very similar to that of mid- ocean ridge crust rather than those of island- or back-arc type or the typical mature oceanic crust, which are observed in the NFB, outside of the central domain.