Seismic properties of layer 2 basalts

Johnston, Joel E.; Christensen, Nikolas I.

doi:10.1111/j.1365-246x.1997.tb01555.x

Cited by 27 publications

(27 citation statements)

References 51 publications

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“…Thus, the variations in 2B velocity could be due to either differences in the amount of fracturing acquired at the time of formation, or the subsequent extent of alteration that was able to take place before the crustal column was carried away from the region of “active” hydrothermal circulation. Conversely, the composition of alteration minerals has an effect on rock velocities [ Johnston and Christensen , 1997] and thus variations in 2B velocity could perhaps be related to the types of precipitated hydrothermal minerals.…”

Section: Discussionmentioning

confidence: 99%

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

Newman

Nedimović

Canales

et al. 2011

Geochem Geophys Geosyst

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[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

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Section: Discussionmentioning

confidence: 99%

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

Newman

Nedimović

Canales

et al. 2011

Geochem Geophys Geosyst

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“…However, V p variations may also be due to variation in porosity as well as degree of alteration [Han et al, 1986;Johnston and Christensen, 1997]. Increases in both alteration and porosity cause significant decreases in V p velocity of the order observed in Montserrat.…”

Section: Discussionmentioning

confidence: 99%

Montserrat geothermal system: A 3D conceptual model

Ryan

Peacock

Shalev

et al. 2013

Geophysical Research Letters

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[1] We constructed a three-dimensional conceptual model of a geothermal system on the Caribbean island of Montserrat. The model was generated using magnetotelluric resistivity data, earthquake hypocenter data, and a three-dimensional P wave velocity model, all plotted using a shared geographical reference. The results of the study suggest a high-temperature fracture-controlled geothermal system at the intersection of two faults in the SW of the island. We also present a "prospectivity index" map that represents a proxy of the spatial variation in harvestable heat flux at 1500 m depth. The index is the product of relative permeability around modeled faults and a proxy for the subsurface temperature calculated using P wave velocity anomalies. Citation: Ryan, G. A., J. R. Peacock, E. Shalev, and J. Rugis (2013), Montserrat geothermal system: A 3D conceptual model, Geophys.

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“…In our DEM model, the effective medium consists of two components of basalt and seawater‐filled crack as a host and inclusion, respectively. The physical properties of components are Vp = 6.4 km/s, Vs = 3.5 km/s, and density ρ = 2,971 kg/m 3 for basalt (Johnston & Christensen, ), and Vp = 1.5 km/s, Vs = 0 km/s, and ρ = 1,030 kg/m 3 for seawater (Telford & Sheriff, ). The inclusions are introduced in an aligned manner since the seismic anisotropy in the upper oceanic crust is attributed to vertical cracks aligned parallel to the ridge axis (Stephen, ).…”

Section: Methodsmentioning

confidence: 99%

Upper Crustal Vp/Vs Ratios at the Endeavour Segment, Juan de Fuca Ridge, From Joint Inversion of P and S Traveltimes: Implications for Hydrothermal Circulation

Kim

Toomey

Hooft

et al. 2019

Geochem Geophys Geosyst

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The Endeavour segment of the Juan de Fuca Ridge is one of the most active and long‐lived hydrothermal areas of the mid‐ocean ridge system. However, the permeability structure that gives rise to long‐term venting at well‐established fields, such as the High Rise, Main Endeavour, and Mothra fields, is not fully understood. Here we jointly invert Pg and Sg traveltimes from a seismic refraction experiment conducted at the Endeavour segment using P‐to‐S coupling constraints. We then calculate porosity and crack density as a function of crack aspect ratio by applying the differential effective medium theory to the seismic velocities. At 1.4‐km depth, average Vp ~5 km off axis increases by ~0.4 km/s compared to the ridge axis. The average Vp/Vs has a minimum of ~1.75 on the ridge axis and increases to a maximum of ~1.84 off axis. The inferred porosity and crack density distributions show that the proportion of thick versus thin cracks decreases from the ridge axis to the flanks, since theoretical models indicate that Vp/Vs increases going from thick to thin cracks (aspect ratio decreasing from 0.1 to 0.01). The dominant presence of thick cracks on the axis may provide long‐term conduits for upflow in high‐temperature hydrothermal circulation potentially forming the vent fields. The increased proportion of thin cracks on the flanks, coupled with the increased seismic velocity, indicates a decrease in permeability caused by progressive clogging of thick cracks due to mineral precipitation likely in the downflow zone of hydrothermal circulation.

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Seismic properties of layer 2 basalts

Cited by 27 publications

References 51 publications

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

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

Montserrat geothermal system: A 3D conceptual model

Upper Crustal Vp/Vs Ratios at the Endeavour Segment, Juan de Fuca Ridge, From Joint Inversion of P and S Traveltimes: Implications for Hydrothermal Circulation

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