Volcanic activity and crust-mantle exposure in the ultrafast Garrett transform fault near 13°28′S in the Pacific

Abstract

The 130 km long Garrett transform fault, which offsets two ultrafast (14.5 cm/yr) EPR spreading segments, is the site of volcanic and tectonic activity. Evidence for recent volcanic events was observed during several dives on oblique ridges (3100–3500 m high) trending 040° (Alpha, Beta and Gamma), and on small constructional mounds located in the central transform basin and eastern valley at a depth of 3500–5000 m. The constructional features show a fissured terrain, suggesting a component of lithospheric opening. The strike-slip motion of the transform is inferred at several locations between the two EPR-transform intersection zones, from the presence of abundant sediment-free talus with crushed rock debris, fresh scarps, and well-developed striations on outcrops of serpentinized peridotites and metamorphosed volcanic breccia. The width of the active tectonic zone varies between 200 and 3000 m in the explored area near the eastern and western EPR-transform intersection zones, along the E-W trending Median Ridge walls in the eastern and western transform troughs of the central basin and at several sites on the south-facing wall of the transform domain first escarpment. The presence of recent volcanic activity implies that the Garrett transform behaves as a leaky transform fault. The most common volcanics, consisting of plagioclase and olivine phyric lava erupted in the transform, are compositionally more depleted [ K/Ti< 0.08 ] and less evolved (Mg#60–70) than the basalts encountered on the southern EPR segments [Mg#40–66, and K/Ti= 0.05–0.25 ]. This suggests that different magmatic conditions are operating in the transform domain. The transform strike-slip motion is responsible for the emplacement of a sliver of serpentinized peridotite located in an area about 2 km wide and 10 km long along the Median Ridge at the contact with outcrops of metamorphosed basaltic breccia. The ultramafics consist of residual harzburgites, often impregnated with irregular lenses and dykes of basaltic liquids (plagioclase-wehrlite and gabbro), and are found associated with troctolites, dunite and gabbros. The emplacement of the ultramafics and their gabbroic inclusions has resulted in the presence of metamorphosed tectonic breccia forming part of the Median Ridge in the central transform basin.