Evolution of seismic layer 2B across the Juan de Fuca Ridge from hydrophone streamer 2‐D traveltime tomography

Abstract

[1] How oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the Earth's interior and the oceans. Although considerable work has been done characterizing the evolution of seismic layer 2A, there has been little analysis of the processes that affect layer 2B after formation. Here we present high-resolution 2-D tomographic models of seismic layer 2B along ∼300 km long multichannel seismic transects crossing the Endeavour, Northern Symmetric, and Cleft segments of the Juan de Fuca Ridge. These models show that seismic layer 2B evolves rapidly following a different course than layer 2A. The upper layer 2B velocities increase on average by 0.8 km/s and reach a generally constant velocity of 5.2 ± 0.3 km/s within the first 0.5 Myr after crustal formation. This suggests that the strongest impact on layer 2B evolution may be that of mineral precipitation due to "active" hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber. Variations in upper layer 2B velocity with age at time scales ≥0.5 Ma show correlation about the ridge axis indicating that in the long term, crustal accretion processes affect both sides of the ridge axis in a similar way. Below the 0.5 Ma threshold, differences in 2B velocity are likely imprinted during crustal formation or early crustal evolution. Layer 2B velocities at propagator wakes (5.0 ± 0.2 km/s), where enhanced faulting and cracking are expected, and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2B. Analysis of the layer 2B velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2B velocities, respectively. Additionally, the distance between present up-flow and down-flow zones is related to the amount of sediment cover because, as sediment cover increases and basement outcrops become

The hydrothermal power of oceanic lithosphere

“…The Juan de Fuca Ridge flank is geophysically well characterized, but there is strong evidence of ventilated discharge over much of the sampled area (Davis et al, 1997(Davis et al, , 1999. The elevated heat flow in this area may reflect this discharge, persistent deep hydrothermal circulation along faults (Nedimovic et al, 2009), heat release from hydration (Lowell and Rona, 2002), or advection from younger seafloor. Heat flow on the Costa Rica Rift is probably the most important data point as it (1) samples seafloor young enough to potentially differentiate models, (2) is heavily sedimented with no evidence of thermally significant ventilated transport, and (3) is well characterized for heat flow and basement depth.…”

“…5), the young-age end of the data is in best agreement with model GH, while the bin of data around 3.5 My is in between model GHC and GH. The conventional hydrogeological interpretation of heat flow along the Endeavour flank includes recharge over the young (< 0.66 My old) unsedimented seafloor and discharge at a basement high around 1.3 My, about 20 km from the recharge site (Davis et al, 1997(Davis et al, , 1999Newman et al, 2011). Borehole measurements and correlations of surface heat flux and sediment column thickness indicate that the temperature of crustal basement is roughly constant over this region.…”

“…Possible sources of elevated heat flow are (1) continued deep hydrothermal circulation, perhaps associated with faulting as suggested by Nedimovic et al (2009); (2) heat release from hydration of the crust (Lowell and Rona, 2002); (3) heat transported from younger seafloor and discharged, consistent with models (Davis et al, 1997(Davis et al, , 1999Newman et al, 2011); or (4) microbial thermogenesis, which is difficult to quantify but could be significant if nutrient supplies are adequate (D. LaRowe, personal communication, 2012). Davis et al (1989Davis et al ( , 1997Davis et al ( , 1999 suggested that advective heat loss around 3.5 My old seafloor is large enough to depress regional heat flow below that of basement heat flux.…”

Geophysical and geochemical constraints on the evolution of oceanic lithosphere from formation to subduction

“…Audet et al, 2009;Bostock et al, 2002;Hacker et al, 2003;Kao et al, 2005;Preston et al, 2003;Rogers and Dragert, 2003;Walowski et al, 2015]. Prior studies within the interior of the JdF plate provide evidence for hydration of the upper crust associated with ridge flank hydrothermal circulation and spatially correlated with local zones of ridge propagation [McClymont and Clowes, 2005;Nedimović et al, 2009;Nedimović et al, 2008;Newman et al, 2011] but the full extent of when and how water is incorporated into the lithosphere during the evolution of the JdF plate, and how water is distributed at depth within the plate prior to subduction remains unknown. To address this important question, we conducted in 2012 an activesource seismic study to characterize crustal and shallow mantle velocities and distribution of faulting along two ridge-perpendicular transects spanning the full width of the JdF plate, and a trench-parallel line to characterize along-margin variations in the architecture and velocity structure of the down-going plate Han et al, 2016].…”

“…In the interior of the plate, hydration of the upper crust is inferred from observations of increasing P-wave velocities within seismic layer 2A and 2B with increasing crustal age and sediment cover [Nedimović et al, 2008;Newman et al, 2011].…”