Dynamics of a Seafloor Spreading Episode at the East Pacific Rise

Abstract:

Seafloor-spreading occurs largely unobserved, with 98% of the global mid-ocean ridge system below the ocean surface. Our understanding of the dynamic processes that control spr­­eading is thus inferred largely from geophysical observations of spreading events on land at Afar and Iceland (Wright et al., 2012). However, these are slow-spreading centres influenced by mantle plumes (Ebinger et al., 1998; Wolfe et al., 1997). The respective roles of magma pressure and tectonic stress in the development of seafloor spreading is still unresolved. Here we use seismic observations to show that the most recent eruption at the fast-spreading East Pacific Rise near 9°50’N initiated at a ~5-km long melt-rich segment (Xu et al., 2014). The static stress change then promoted almost-concurrent rupturing along at least 35-km of the ridge axis that was critically-stressed due to build-up of tectonic stress, which triggered magma movement. Identification and location of impulsive seismic events indicating lava reaching the seafloor (Wilcock et al., 2015) suggest that lava erupted from multiple isolated magma lenses with variable magma ascent rates, mostly within ~48 hours. This is contrary to the widely-accepted theory that eruptions at the East Pacific Rise happen in multiple pulses over months (Rubin et al., 1994; Rubin et al., 2012), and is different from spreading episodes at Afar and Iceland that last for months to years with dikes propagating laterally for tens of kilometers (Abdallah et al., 1979; Einarsson & Brandsdottier, 1980; Ayele et al., 2009; Sugmundsson et al., 2015). Our results suggest that even at magmatically-robust fast-spreading ridges, a significant portion of spreading may take place due to build-up of tectonic stress to a critical level rather than magma overpressure in the underlying magma lenses.