Magnetic fabrics study and inferred flow directions of lavas of the Old Pali Road, O′ahu, Hawaii

A section 600–700 m deep in the Koolau tholeiitic lava-shield volcano consists of mostly thin pahoehoe and aa flows cut by occasional dikes and outward-dipping gravity-driven intrusive sheets. A magnetic fabric study of 250 specimens from 18 sites shows a tight clustering of K1 axes for some of the lava flows and is thought to fairly reliably define the lava flow direction. A systematic plunge of the K1 axes appears to define an imbrication and yields a flow azimuth. The azimuth differs by 60° from that anticipated but is readily explained by a change in shape of the Koolau Volcano during growth. The tightest clustering is in massive aa lava flows 5 m thick. Samples collected from near the flow base give the most consistent orientations. Compound pahoehoe lavas with many small flow units give the least tightly clustered anisotropy of magnetic susceptibility (AMS) axes and, in some flows, the orientation is random. The gravity-driven sheet has an AMS fabric indicating a down-dip magma flow direction. After the Koolau Volcano was deeply eroded, a cinder cone and lava flows of nephelinite were formed in the rejuvenation stage of volcanism. The nephelinite is normally magnetized and the AMS fabric indicates a generally downslope flow.