Abstract

Seafloor spreading is a fundamental component of the plate tectonic cycle, responsible for producing two-thirds of the Earth’s surface, but our understanding of this process is still far from complete. Recent exploration of slow and ultraslow spreading systems has revealed a style of spreading that is radically different from the “normal” magmatic accretion seen at fast spreading rates. In these systems, large-offset detachment faults are responsible for accommodating plate divergence. These faults exhume deep lithosphere, forming oceanic core complexes that expose lower oceanic crust and upper mantle rocks on the seafloor. Their widespread occurrence in slow-spreading systems has led to the definition of a new “detachment-mode” of seafloor spreading, representing a major advance in understanding plate tectonics. This talk will show how palaeomagnetic analysis of core samples recovered from the footwalls of oceanic detachment faults by scientific ocean drilling has allowed the tectonic evolution of oceanic core complexes to be determined. Using magnetic remanences act as a marker, it has been possible to demonstrate that oceanic detachment faults initiate as steep normal faults at depth, and shallow into low angle extensional faults through rotation of their footwalls. This characteristic rolling-hinge style of rotation provides a key to recognizing this important tectonic process in ophiolites, allowing the record of detachment-mode seafloor spreading to be extended back beyond the age of the oldest crust in the modern oceans.

Key references:

Escartín, J. & Canales, J. P., 2011. Detachments in oceanic lithosphere: Deformation, magmatism, fluid flow, and ecosystems. Eos Trans. AGU , 92(4), 31, doi:10.1029/2011EO040003.

Maffione, M., Morris, A. & Anderson, M. W., 2013. Recognizing detachment-mode seafloor spreading in the deep geological past. Scientific Reports 3, doi:10.1038/srep02336.

Morris, A., Gee, J. S., Pressling, N., John, B. E., MacLeod, C. J., Grimes, C. B. & Searle, R. C., 2009. Footwall rotation in an oceanic core complex quantified using reoriented Integrated Ocean Drilling Program core samples. Earth Planet. Sci. Lett. 287, 217–228, doi:10.1016/j.epsl.2009.08.007.

Smith, D. K, Escartín, J., Schouten, H. & Cann, J. R., 2008, Fault rotation and core complex formation: Significant processes in seafloor formation at slow spreading mid-ocean ridges (Mid-Atlantic Ridge, 13–15° N). Geochem. Geophys. Geosyst. 9, doi:10.1029/2007GC001699.