Crustal accretion and hydrothermal circulation at the East Pacific Rise: insights from waveform inversion techniques

The East Pacific Rise (EPR) is one of the magmatically most dynamic ocean spreading center along which ~6 km thick oceanic crust has been accreting. Along the EPR spreading center 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 magma emplacement on the seafloor (that together build the upper crust) is commonly accepted. While the large portion of mobilized melt from the AML is intruded into preexisting crust forming ~1.3 km thick dike section, a part of the melt erupts on the seafloor forming a porous layer of pillow basalts. Due to limited information on the upper crustal velocities, it is rather difficult to clearly delimit the contact between pillow basalts and dikes due to the presence of high-velocity gradient, the origin of which is not clear. In addition, the EPR 9º50’N is a portion of the mid-ocean ridge system characterized by prolific hydrothermal activity with a number of venting sites where the hydrothermal fluid exits the crust. However, the sites where hydrothermal fluid enters the crust and its pathways within the crust are unknown. In 2008, a 3-D multichannel seismic survey was conducted at the EPR, to study physical properties of its tectono-magmatic system and its relationship with other processes. Here, I will first present results of 2D full-waveform inversion on an axis centered seismic line to investigate hydrothermal circulation and then I will present results of the first 3D full-waveform inversion to address physical properties of the upper crustal structure and a possible mechanism for the upper crust accretion.