The Charlie - Gibbs Fracture Zone: Tectonic, Magmatic and Hydrothermal Processes in a Chain of Giant Oceanic Core Complexes

Rock samples were retrieved and the seabed imaged around the axial spreading zone of the double-transform fault of the Charlie - Gibbs Fracture Zone (CGFZ) by the TRASNA and TOSCA cruises in 2016 and 2018, respectively. High resolution bathymetric data reveal complex topography that includes a median rift interpreted as a spreading centre and, perpendicular to this rift, corrugations exceeding 10 km length on elevated sections of the sea floor reaching up to only 570 m below sea level. Rock samples, retrieved by Remotely Operated underwater Vehicle (ROV) and by dredging, are interpreted as being derived from local bedrock. They consist primarily of serpentinised peridotite and variably altered gabbros, with occasional intermediate to granitic plutonic rocks, dolerites and hydrothermal talc rocks. The gabbros and other plutonic rocks are variably mylonitised and partially metamorphosed through interaction with seawater. No basalts or other extrusive rocks were retrieved. The imaging and seabed geology indicate that the median transverse ridge within the CGFZ comprises a series of overlapping oceanic core complexes. Relatively sodic plagioclase (andesine) and high modal Fe-Ti oxide content in gabbros suggest unusually advanced fractional crystallization, and perhaps unusually low degrees of partial melting, compared to typical mid-ocean ridge basalts. Mylonitic fabrics are interpreted as resulting from movement on low-angle detachment surfaces such as the imaged corrugated surfaces. A 'plum pudding' type of oceanic crust is proposed, in which relatively small volumes of mantle-derived magma underwent variable degrees of fractional crystallization as they intruded peridotite undergoing extension and serpentinisation. Copper-rich, often brecciated, colloform banded talc rocks are interpreted to have precipitated on, or perhaps just beneath, the seabed through interaction between seawater and serpentinising fluids.