Tectonic/volcanic segmentation and controls on hydrothermal venting along Earth's fastest seafloor spreading system, EPR 27°–32°S

Abstract

[1] We have collected 12 kHz SeaBeam bathymetry and 120 kHz DSL-120 side-scan sonar and bathymetry data to determine the tectonic and volcanic segmentation along the fastest spreading ($150 km/Myr) part of the global mid-ocean ridge system, the southern East Pacific Rise between the Easter and Juan Fernandez microplates. This area is presently reorganizing by large-scale dueling rift propagation and possible protomicroplate tectonics. Fracture patterns observed in the side-scan data define structural segmentation scales along these ridge segments. These sometimes, but not always, correlate with linear volcanic systems defining segmentation in the SeaBeam data. Some of the subsegments behave cohesively, with in-phase tectonic activity, while fundamental discontinuities occur between other subsegments. We also collected hydrothermal plume data using sensors mounted on the DSL-120 instrument package, as well as CTDO tow-yos, to determine detailed structural and volcanic controls on the hydrothermal vent pattern observed along 600 km of the Pacific-Nazca axis. Here we report the first rigorous correlation between coregistered hydrothermal plume and high-resolution marine geophysical data on similar scales and over multisegment distances. Major plume concentrations were usually found where axial inflation was relatively high and fracture density was relatively low. These correlations suggest that hydrothermal venting is most active where the apparent magmatic budget is greatest, resulting in recent eruptions that have paved over the neovolcanic zone. Areas of voluminous acoustically dark young lava flows produced from recent fissure eruptions correlate with many of the major hydrothermal vent areas. Increased crustal permeability, as gauged by increased fracture density, does not enhance hydrothermal venting in this area. Axial summit troughs and graben are rare, probably because of frequent volcanic resurfacing in this superfast spreading environment, and are not good predictors of hydrothermal activity here. Many of the hydrothermal areas are found in inflated areas near the ends of segments, suggesting that abundant magma is being supplied to these areas.

Origin of the Easter Submarine Alignment: morphology and structural lineaments

“…The average spreading rate between the Pacific and Nazca plates is close to 150 km Myr -1 (Naar & Hey, 1986;Hey et al, 1995); and at latitude 28.5°S it is of 180 km Myr -1 according to Schilling et al (1985). Therefore this region has the highest rate of crust formation in the Pacific Basin (Rappaport et al, 1997;Hey et al, 2004).…”

“…Macdonald (1989) explains that the formation of the observed segment for the EPR follows the emplacement of magmatic chambers spaced and shallow, with a discrete distribution along the axis. In the case of the region close to the study area, this is located to the south (between 28.5° and 30°S) and the EPR is separated into large segments (Hey et al, 2004). These segments propagate in the same way as the segments of the Easter Microplate and form an OSC separated by 120 km.…”

“…5) or SOEST (Hey et al, 1995), including other parallel topographic alignments. From Figure 5 it is also possible to identify V-shaped pseudofaults (e.g., Naar & Hey, 1986;Hey et al, 1995), which have their bases at 27°S, and ridge segments, including the spreading overlapping centers of the seafloor (OSC) (e.g., Naar & Hey, 1986;Martínez et al, 1997;Baker et al, 2002;Hey et al, 2004). Other minor linear trends can be identified through the use of multibeam bathymetry.…”

Metalliferous sediments from the H.M.S. Challenger voyage (1872–1876)

“…1) at the present triple junction of East Pacific Rise (EPR), Pacific-Antarctic Ridge (PAR) and Chile Ridge (CR) (Bird et al, 1998). The seafloor spreading rates at the active plate boundaries in this region are currently the fastest on the planet ($15 cm/yr; Hey et al, 2004). This unusual tectonic setting creates a high rate of tectonic and volcanic activity giving rise to unusual magmatic-hydrothermal systems (Baker et al, 2002;Hey et al, 2004).…”

“…The seafloor spreading rates at the active plate boundaries in this region are currently the fastest on the planet ($15 cm/yr; Hey et al, 2004). This unusual tectonic setting creates a high rate of tectonic and volcanic activity giving rise to unusual magmatic-hydrothermal systems (Baker et al, 2002;Hey et al, 2004). The extremely high hydrothermal activity in this region has resulted in the development of the largest field of metalliferous sediments in the Ocean (Gurvich, 2006).…”