Alvin, Jason II, DSL-120 Investigation of Super-Fast EPR Crust Exposed at the Pito Deep Rift, Easter Microplate, SE Pacific

Rift propagation along the NE corner of the Easter Microplate over the past 1 Ma has created a tectonic window into 3-Ma-old oceanic crust generated at the EPR at a full-rate of 144 mm/yr. Rift-related faults cut EPR abyssal hill lineaments and magnetic anomaly boundaries at a high angle revealing the internal structure and composition of the upper oceanic crust. Nested-scale investigations with the submersible Alvin, ROV Jason II, DSL-120 side-scan sonar and near-bottom magnetic surveys were carried out in 2 study areas each several kilometers long and spanning a depth range of about 1500 m. Direct observations, imaging, and sampling with these systems augment results from earlier Nautile dives in both areas. As anticipated, the major rock units mapped in the study areas include (from top down): 1) poorly consolidated pelagic ooze and chalk (<2 m); 2) pillow, lobate, and sheeted basaltic lavas (300-500 m); 3) sheeted diabase dikes (700-1000 m); and 4) a diverse suite of gabbroic rocks (>900 m exposed above talus at the base of the scarp). Transitional units both above and below the sheeted dikes are highly variable but generally in the range of 100-250 m vertically. The internal structure of the lavas and sheeted dike units is complex with crustal sections tilted slightly toward or away from the EPR spreading axis. Faulting is highly localized and marked by intense brecciation and hydrothermal alteration, locally with abundant syn- to post-tectonic quartz vein networks. Variations in local crustal structure are interpreted in terms of temporal fluctuations in magma supply, faulting, and hydrothermal processes. Compared to crustal sections in other tectonic windows and deep drill holes in the Pacific region, super-fast EPR crust at Pito Deep has a relatively thin basaltic lava unit, a relatively thick sheeted dike unit, and less intense fracturing. These results provide a different perspective on the internal structure, composition, and variability of crust formed at very high spreading rates and processes at very fast-spreading ridges like the southern EPR.