ENORMOUS RIFT ZONES, GIANT LANDSLIDES, AND UBIQUITOUS PALEO-REEF TERRACES: DETAILED SUBMARINE GEOLOGIC MAPPING REVEALS COMPLEXITY OF THE NORTHWESTERN HAWAIIAN RIDGE

An extensive set of geophysical data was acquired from the Northwestern Hawaiian Ridge over 72 days aboard R/V Falkor within the Papahānaumokuākea Marine National Monument (PMNM) in the Northwestern Hawaiian Islands (NWHI). A derivative of this project has been to reexamine the nearly 50 giant submarine landslides (GSLs) mapped in the NWHI by the U.S. Geological Survey using the GLORIA sidescan system and other underway geophysics between 1988-1991. A renewed study of these GSLs has also included analysis of the edifices and structures they originate from, in order to make best use of this high-resolution dataset of extraordinary quality. Thus, we have carried out detailed interpretation of the older GSLs to provide a larger statistical base, a greater variety of source edifice morphologies and structural settings, and a longer temporal comparison with the GSLs in the main islands and worldwide. This presentation focuses on the middle section of the PMNM, dominated by big rift zone ridges, landslides in the form of large slumps and debris avalanches, and broad carbonate platforms with associated drowned reef terraces at a range of depths. The rift zones located here are comparable in shape and size with those off Maui and the Island of Hawai‘i in the main islands. Several of these rifts are oriented north-south, nearly orthogonal to the hot spot trace superposed on the motion of the overriding Pacific plate. The new data have also revealed the detail of numerous landslides, their transport pathways, and basal deposits all along the chain. One remarkable feature is a rift zone flank failure creating a knife-edge ridge off Pioneer Bank with sediment filled basins bounded by normal faults and a steep-faced terminus, interpreted as a thrust toe delineating the foot of the slump. As for the carbonate platforms, two such units are differentiated based on the current water depths. The deepest, and most likely the paleo-shoreline during volcanic construction, lies at a water depth of about 200 m, while the second one begins at approximately 100-150 m. These two distinct carbonate units are present on all but one of the Hawaiian seamounts in this region of the NWHI. Interestingly, the magnitude of the largest carbonate platform of Gardner Pinnacles suggests its original high island may have met or exceeded the enormity of the Island of Hawai‘i.