Evidence and implications of crustal magmatism on the flanks of the East Pacific Rise

a r t i c l e i n f o Keywords: East Pacific Rise mid-ocean ridge oceanic crust off-axis magmatism waveform modeling Formation of oceanic crust along the East Pacific Rise is thought to be complete within a few kilometers of the rise axis. Here, however, we present evidence for magmatism 20 km from the spreading center in 300-ka-old crust. Seismic data reveal an intrusive complex ~ 2 km beneath the seafloor, which is limited in lateral extent (b 5 km) and comprises a melt lens underlain by low-velocity, high-attenuation crust. The crustal magma body overlies a region of off-axis delivery of mantle melt that results from a long-lived skew between the axes of mantle upwelling and plate spreading. Magmatism on the flanks of the EPR may be commonplace, thus affecting the architecture of oceanic crust and providing the necessary conditions to drive off-axis hydrothermal activity. Crustal accretion along the fast-spreading East Pacific Rise (EPR) occurs primarily within a narrow cross-axis window centered on the axis of plate spreading. Seafloor mapping shows that the eruptive fissures defining the neovolcanic zone occur within a kilometer-wide region centered on the axial high (Haymon et al. Geophysical studies further reveal that directly beneath the axial high lies a narrow, steep-sided, crustal magma chamber and a seismically detectable crust-mantle transition zone or Moho (Detrick et al., 1987; Kent et al., 1990; Dunn et al., 2000). These observations have been used to infer that mantle melt is efficiently focused beneath the rise axis and that the emplacement of crust is effectively complete at zero age (e.g., Korenaga and Kelemen, 1997). Yet there is evidence that magmatic and hydrothermal activity do occur off-axis. Seafloor mapping reveals venting of hydrothermal fluids at off-axis locales (Haymon et al., 2005) and the ages of some off-axis lavas are anomalously young (Zou et al., 2002; Sims et al., 2003). Here we present evidence of off-axis crustal magmatism from seismic refraction data (Fig. 1) that constrain the intrusive complex's location, size and physical properties. The seismic experiment used 28 four-component ocean-bottom seismometers (OBSs) and 29 ocean-bottom hydrophones (OBHs) to record rise parallel and perpendicular shooting lines; the source was the 20-gun, 8420 in. 3 R/V Ewing airgun array. Previous analyses of the data constrain regional scale variations in crustal thickness (Canales et al., 2003) and the structure of a mantle-low velocity zone (MLVZ) that is skewed beneath the spreading axis …