ORIGIN OF THE ABSENCE OF MAGNETIC LINEATIONS IN THE YAMATO BASIN OF THE JAPAN SEA - MAGNETIC-PROPERTIES OF MAFIC ROCKS FROM OCEAN DRILLING PROGRAM HOLE 794D

We investigated magnetic properties of mafic rocks recovered from Ocea n Drilling Program Hole 794D in the Yamato Basin of the Japan Sea. In contrast to other back-are basins in the western Pacific, lineated mag netic anomaly patterns have not been recognized in the Yamato Basin. O ur objective is to clarify the reason for the absence of lineated magn etic anomalies by direct magnetic measurements of crustal rocks. The d ominant magnetic mineral of Hole 794D is titanomaghemite as commonly o bserved for typical oceanic basalts, but the grain size is much larger (greater than several tens of microns). Remanence properties, i.e., l ow natural remanent magnetization intensities (approximate to 1.5 A/m) , low Koenigsberger ratios (approximate to 1.0), high viscous remanent magnetization, and mixed magnetic polarities, indicate that the mafic rocks cannot be a source of lineated magnetic anomalies. While Curie temperature, saturation magnetization, and susceptibility values are n ot significantly different from those of typical oceanic basalts, magn etic properties related to stability are greatly reduced. The large gr ain size of the mafic rocks from Hole 794D plays a primary role in dif ferentiating the magnetic properties from those of typical oceanic bas alts, and is caused by the formation of sill-sediment complexes beneat h the Yamato Basin. Sill-sediment complexes, rather than extrusive bas alts in typical oceanic crust, are responsible for the absence of line ated magnetic anomalies in the Yamato Basin.