SEGMENTATION OF THE SOUTHERN MARIANA BACK-ARC SPREADING CENTER

SeaBeam multibeam bathymetry obtained during cruise SO-69 of research vessel (R/V) Sonne defines the segmentation and structure of similar t o 300 km of the Mariana back-are spreading center south of the Pagan f racture zone at 17 degrees 33'N. Eight ridge segments, ranging from 14 to 64 km in length, are displaced as much as 2.7-14.5 km by both righ t- (predominantly) and left-lateral offsets and transform faults. An a xial ridge commonly occupies the middle portion of the rift valley and rises from 200 to 700 m above the adjacent sea floor, in places shoal ing to a water depth of 3200 m. An exception is the 60-km-long segment between 16 degrees 58' and 17 degrees 33'N where single peaks only a few tens of meters high punctuate the rift axis. Photographic evidence and pock samples reveal the presence of mostly pillow lavas outcroppi ng on the axial ridges or peaks whereas the deeper parts of the rift v alley floor (max. depth 4900 m) ape heavily to totally sedimented. Abu ndant talus ramps along fault scarps testify to ongoing disruption of the crust. Lozenge-shaped collapse structures are covered by layers of sediment up to tens of centimeters thick on the rift valley floor. Th e presence of discrete volcanic ridges in the southern Mariana back-ar c spreading region suggests that emplacement of oceanic crust at this slow spreading center occurs by 'multi-site' injection of magma. Along -axis variations in length, crestal depth, and size of the axial ridge s call be best explained by different stages in the cyclicity of mag-m a supply along-axis.