Seismic signatures of up- and down-going hydrothermal pathways along the East Pacific Rise between 9º15’ and 10ºN

Hydrothermal circulation along divergent plate boundaries plays an important role in heat and mass exchange between the Earth’s lithosphere and deep ocean, evidenced by the presence of high-temperature, particle-laden plumes emanated from hydrothermal vent edifices formed on the seafloor. Whilst the spatial distribution of different types of the fluid discharge through these vent orifices is well documented, the distribution of fluid recharge zones and fluid flow pattern within the oceanic crust are still elusive. Here, we apply elastic full waveform inversion techniques to extrapolated high-fidelity 2D seismic data to characterize the nature of ~zero-age upper crust formed at the East Pacific Rise (EPR) between 9º16 and 9º56’N. The resulting velocity model shows prominent perturbation in background velocity in the northern part of the profile, where prolific hydrothermal and volcanic activities have been reported. This, ~22 km long region is represented by five low velocity anomalies (for >300 m/s lower) that are ~3 km wide and can be tracked to up to 1 km below the seafloor. Two of the low velocity zones seem to underlay vent clusters centered at 9º47’ and 9º50’ that we relate to the presence of up-going pathways of the fluid. The three remaining low velocity zones (centered at 9º44’, 9º48.5’, 9º51’) are more prominent and their extent roughly coincides with the previously identified fine-scale tectonic discontinuities. The results suggest these deviations of axial orientation observed in the seafloor, coupled with upper crustal fracturing that can be sustained for 1000s of years as ideal locations for seawater to penetrate more permeable crust on the ridge-axis and establish down-going pathway of hydrothermal flow. Similar scenario was suggested by micro-earthquakes within one small portion of the region during the last eruption. The presence of a strong axial melt lens and associated anomalous velocity zone indicate enhanced thermal regime within the area responsible for establishing and sustaining hydrothermal flow in the upper crust. Although pipe-like upper-crustal low velocity regions are imaged in the vicinity of prominent third-order discontinuities at 9º17’ and 9º37’N the underlying AML is shown to be mostly crystallized hindering the hydrothermal circulation process in the area.