Oceanic detachment faulting and core complex development and evolution: Insights from the mid-cayman spreading center

Detailed study of deformed fault rocks collected from surface exposures of two on-axis oceanic core complexes (Mt. Dent and Mt. Hudson) at the Mid-Cayman Spreading Center (MCSC) provide constraints on the nature and evolution of oceanic detachment faulting at slow- and ultraslow-spreading mid-ocean ridges. Deformation related to oceanic detachment faulting spans the range from upper-amphibolite facies conditions, marked by the assemblage of recrystallized plagioclase + brown hornblende and high intensities of crystal-plastic deformation, to sub-greenschist facies conditions associated with brittle fracturing and fluid infiltration. Plagioclase deformation and recrystallization mechanisms indicate that gabbroic samples deformed at peak temperatures of ~600-800°C. Strain localization is pervasive in high-temperature samples and is marked by discrete, cm-scale mylonite zones enriched in Fe-Ti oxides and brown Ti-rich hornblende. Crystallographic preferred orientation (CPO) of plagioclase from electron backscatter diffraction (EBSD) indicate that slip at Mt. Dent is dominated by (001)[100]. Limited evidence of (010)[100] slip in two samples ~3km from the detachment breakaway suggests slightly lower deformation temperatures in shallow exposures of the detachment surface. No plagioclase CPO is observed in high-strain mylonitic horizons suggesting that strain localization is accompanied by a transition to grainsize-sensitive creep mechanisms such as diffusion creep and grain-boundary sliding. Interfacial, disseminated ilmenite in high-strain shear zones preserve a weak CPO indicating basal slip during fault slip and shear zone formation. U-Pb zircon geochronology and Ti-in-zircon thermometry are used to further constrain the conditions and timing of fault slip at the MCSC. Trace element concentrations indicate that zircon collected from gabbroic samples and felsic veins are within the range of normal oceanic zircon. Crystallization temperatures estimated using the Ti-in-zircon thermometer range from 797-947°C at Mt. Dent and 749-785°C at Mt. Hudson. Crystallization temperatures of zircon combined with estimates of temperatures during peak deformation, suggest that deformation associated with fault slip initiated at near solidus conditions. 230Th-corrected 206Pb/238U ages of broadly spaced samples at Mt. Dent reveal a history of asymmetric plate spreading over a period of ~1m.y. during core complex formation. As a consequence, the footwall cutoff at Mt. Dent has migrated eastward ~3.44 -- 8.25 km with respect to the North American Plate. Together, our observations support a model for detachment faulting and core complex formation at the MCSC. We suggest that detachment faults at the MCSC steepen downward past the footwall cut off and root into an axial mush zone or melt lens. The presence of a late-stage melt at depth locally elevated the ambient temperature gradient, effectively raising the brittle-plastic transition and promoting extensive crystal-plastic deformation. This model supports core complex formation through periods of active magmatism wherein material is constantly added to the detachment footwall with continued slip. Accretion of magmatic material into the footwall of oceanic detachment faults may promote asymmetric plate spreading as commonly noted at oceanic core complexes.