Signatures of melt distribution along the East Pacific Rise

Along fast and intermediate spreading centers, thin and narrow axial magma lenses (AMLs) are detected beneath much of the ridge axis, and the notion that the AML is the primary melt reservoir for dike intrusions and volcanic eruptions that build the upper crust is commonly accepted. However, some of physical properties of the AMLs and the role of AMLs in construction of the lower crust are still elusive. In 2008, a 3D multichannel seismic (MCS) survey was conducted at the East Pacific Rise (EPR), one of the most intensively studied portions of the mid-ocean ridge system. In addition to the main ridge-perpendicular survey acquired for high-resolution 3D imaging of the seafloor subsurface, a 3D along-axis MCS data were collected to facilitate examination of spatial variations in AML properties along the EPR extending between 8º20’ to 10º10’N. In the along axis data the AML is imaged along ~ 85% of the ridge axis length as a relatively bright event, sitting at ~ 1.6 km below the seafloor. Visual inspection of the AML reflector and its calculated instantaneous phase seismic attribute show that this event is disrupted. The most of thus mapped interruptions in AML are collocated with previously identified higher (third and fourth) order discontinuities in bathymetry, corroborating genetic relationship between magmatic and tectonic segmentation, and supporting the notion of crustal accretion through small magmatic units. To examine presence of liquid fraction within segments north of 9º20’N, for the first time in mid-ocean ridge environment we apply A vs. B crossplotting method based on amplitude variation with angle of incidence (AVA) behavior of a reflector and developed by oil-industry. The results suggests presence of melt within four lens segments spanning regions between ~ 9º42’ and 9º56’ N and one centered at 9º38.6’N. Detailed AVA analysis suggests that drainage related to the 2005-06 eruption appears to be limited to two narrow areas (~ 800 m) centered at 9º50.6’ and 9º45.6’N. Furthermore, the results strongly argue that magma mixing processes within a single AML are very limited and that point located 1D waveform inversions studies are not of extensive use. By careful examination of the seismic sections for the first time we show evidence for presence of sub-axial magma lens (SAML) with the most prominent found between latitudes 9º20’ and 9º56’N, where they appear as moderately bright, discontinuous reflectors, at ~ 50 to 300 ms (~ 200-600 m) below the AML. We interpret these deep melt lenses to have a low crystalline component (i.e. they are mostly molten). Disruptions in the SAML reflector, represented by relatively abrupt steps in two-way travel time are collocated with small-scale discontinuities in the AML. Moreover, within the area of documented volcanic eruptions in 1991-1992 and 2005-2006, two prominent gaps centered at 9º46’ and 9º50.5’N in the SAML reflectors are identified. We hypothesize that magma from these deeper lenses have also contributed to the eruption, implying hydraulic connectivity between the AML and SAMLs during eruption events. We suggest that the SAMLs play an important role in eruption triggering and processes of magma lens replenishment and magma fractionation beneath this fast spreading ridge.