MAPPING THE ELLICE BASIN: PRELIMINARY RESULTS FROM CRUISE KM1609

The Ellice Basin is thought to have formed via seafloor spreading during the breakup of previously proposed oceanic superplateau Ontong Java-Manihiki-Hikurangi (Taylor, 2006), also known as Ontong Java Nui (OJN). Unraveling the tectonic history of OJN is hindered by the lack of magnetic anomaly identifications during the Cretaceous Normal Superchron and therefore requires details of the deep abyssal hill fabric, which remain well below the resolution of satellite altimetry. Furthermore, the seafloor is overprinted by seamount chains and partly buried by sediments, so high resolution bathymetric data are crucial to decipher the abyssal hill fabric. The breakup of OJN has previously been modeled as a two-stage reconstruction from fracture zone trends apparent in satellite gravity (Chandler et al., 2012), but high-resolution bathymetric data reveals a possible third stage. In an attempt to resolve the breakup of OJN and formation of the Ellice Basin, we present this new data acquired during a 37-day survey aboard the R/V Kilo Moana. Over 7500 nautical miles of bathymetric data was collected, concurrent with the collection of seafloor sediment thickness and gravity and magnetic data. Due to the lack of magnetic anomaly identifications and limited age constraints for this region we identified and dredged 16 targets. Recovered rocks include both basalts and gabbros and will be analyzed using ⁴⁰Ar/³⁹Ar and zircon age dating to determine a more definitive age and rate of seafloor spreading. Bathymetric soundings were processed using MB-System to reduce the influence of outliers caused by relatively high noise levels. The survey revealed that most of the early opening occurred on N-trending abyssal hills offset by major W-trending transform faults, supporting the Taylor hypothesis. Our mapping documented a late-opening rotation of the abyssal hills to NNE-trending and of the transform faults to WNW, supporting the two-stage model. Changes in the spreading direction appear to have involved a complex interplay of propagating/overlapping ridges, together with the rearrangement and sometimes termination of closely-spaced sets of right-stepping transform faults. Finally, limited crosscutting ENE faults connecting centrally located NNW abyssal hills indicate a brief, late stage of opening before spreading ceased.