Graham M. Kent

Rift topography linked to magmatism at the intermediate spreading Juan de Fuca Ridge

New seismic observations of crustal structure along the Juan de Fuca Ridge indicate that the axial rift topography reflects magma-induced deformation rather than alternating phases of magmatism and tectonic extension, as previously proposed. Contrary to predictions of the episodic models, crustal magma bodies are imaged beneath portions of all ridge segments surveyed at average depths of 2.1–2.6 km. The shallow rift valley or axial graben associated with each Juan de Fuca segment is ∼50–200 m deep and 1–8 km wide and is well correlated with a magma body in the subsurface. Analysis of graben dimensions (height and width) shows that the axial graben narrows and graben height diminishes where the magma body disappears, rather than deepening and broadening, as expected for rift topography due to tectonic extension. We propose an evolutionary model of axial topography that emphasizes the contribution of dike intrusion to subsidence and fault slip at the seafloor. In this model an evolving axial topography results from feedbacks between the rheology of the crust above a magma sill and dike intrusion, rather than episodic magma delivery from the mantle.

Seismic evidence for a hydrothermal layer above the solid roof of the axial magma chamber at the southern East Pacific Rise

A full-waveform inversion of two-ship, wide-aperture, seismic reflection data from a ridge-crest seismic line at the southern East Pacific Rise indicates that the axial magma chamber here is about 50 m thick, is embedded within a solid roof, and has a solid floor. The 50--60-m-thick roof is overlain by a 150--200-m-thick low-velocity zone that may correspond to a fracture zone that hosts the hydrothermal circulation, and the roof itself may be the transition zone separating the magma chamber from circulating fluids. Furthermore, enhanced hydrothermal activity at the sea floor seems to be associated with a fresh supply of magma in the crust from the mantle. The presence of the solid floor indicates that at least the upper gabbros of the oceanic lower crust are formed by cooling and crystallization of melt in magma chambers.

Upper crustal seismic structure of the slow spreading Mid-Atlantic Ridge, 35°N: Constraints on volcanic emplacement processes

[1]xa0The upper crustal seismic structure of the slow spreading Mid-Atlantic Ridge is studied using a genetic algorithm-based waveform inversion of multichannel streamer data. Four single-ship multichannel profiles from 35°N are analyzed: one in the rift valley and three in the rift mountains along 0.7, 1.6, and 1.9 Ma crust. A layer 2A horizon is continuously imaged along three profiles and is associated with a sharp velocity increase from extrusives to dikes. Its depth and regularity in the rift valley indicate that most of the extrusive section is built on the inner valley floor through a pattern of deposition and fault-bounded uplift into the rift mountains. Its variability along one line, however, shows that this process is disrupted during tectonically dominated periods. A thickening of layer 2A toward the Oceanographer fracture zone may be the result of along-axis magma transport. The interval of rapid velocity increase at the base of layer 2A thins with age, a possible response to enhanced hydrothermal mineralization within the zone of mixed dikes and extrusives. Transition zone (∼200 m) and off-axis layer 2A thicknesses (350–600 m) are similar to those at other spreading centers. This indicates that equivalent extrusive volumes are produced at all spreading rates along a relatively narrow zone of dike emplacement. However, differences in on-axis layer 2A thickness between this area and fast spreading ridges suggest that the exact pattern of thickening varies between spreading regimes. Relative to fast spreading ridges, the moderate velocity increase with age recorded in the upper crust (from 2.3 to >2.7 km s−1 within ∼2 Myr) may be due to a less active hydrothermal system and hence slower porosity reduction.

Upper crustal evolution across the Juan de Fuca ridge flanks

[1]xa0Recent P wave velocity compilations of the oceanic crust indicate that the velocity of the uppermost layer 2A doubles or reaches ∼4.3 km/s found in mature crust in <10 Ma after crustal formation. This velocity change is commonly attributed to precipitation of low-temperature alteration minerals within the extrusive rocks associated with ridge-flank hydrothermal circulation. Sediment blanketing, acting as a thermal insulator, has been proposed to further accelerate layer 2A evolution by enhancing mineral precipitation. We carried out 1-D traveltime modeling on common midpoint supergathers from our 2002 Juan de Fuca ridge multichannel seismic data to determine upper crustal structure at ∼3 km intervals along 300 km long transects crossing the Endeavor, Northern Symmetric, and Cleft ridge segments. Our results show a regional correlation between upper crustal velocity and crustal age. The measured velocity increase with crustal age is not uniform across the investigated ridge flanks. For the ridge flanks blanketed with a sealing sedimentary cover, the velocity increase is double that observed on the sparsely and discontinuously sedimented flanks (∼60% increase versus ∼28%) over the same crustal age range of 5–9 Ma. Extrapolation of velocity-age gradients indicates that layer 2A velocity reaches 4.3 km/s by ∼8 Ma on the sediment blanketed flanks compared to ∼16 Ma on the flanks with thin and discontinuous sediment cover. The computed thickness gradients show that layer 2A does not thin and disappear in the Juan de Fuca region with increasing crustal age or sediment blanketing but persists as a relatively low seismic velocity layer capping the deeper oceanic crust. However, layer 2A on the fully sedimented ridge-flank sections is on average thinner than on the sparsely and discontinuously sedimented flanks (330 ± 80 versus 430 ± 80 m). The change in thickness occurs over a 10–20 km distance coincident with the onset of sediment burial. Our results also suggest that propagator wakes can have atypical layer 2A thickness and velocity. Impact of propagator wakes is evident in the chemical signature of the fluids sampled by ODP drill holes along the east Endeavor transect, providing further indication that these crustal discontinuities may be sites of localized fluid flow and alteration.

Fine-scale seismic structure of young upper crust at 17°20′S on the fast spreading East Pacific Rise

[1]xa0The detailed upper crustal structure of the East Pacific Rise (EPR) at 17°20′S is examined by applying a genetic algorithm-based waveform inversion to five multichannel seismic lines: one on-axis and two to either side along 42- and 85-kyr-old crust. On-axis, a double-stepped velocity pattern is recorded beneath 70–100 m of low-velocity extrusives (2.1–2.4 km s−1). We define the upper velocity contrast as the base of seismic layer 2A due to its severity and continuity along and across axis. The more subdued and intermittent lower-velocity step is not observed off-axis. Material between the two high-gradient intervals is proposed to represent the pillow/dike transition, bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and/or deformation. Extrusive velocities increase quite rapidly in this area, with velocities ∼3 km s−1 common in crust ≤85 kyr old. This, plus a rapid (300–400 m) thickening of layer 2A observed within 1–4 km of the rise axis, indicates that this segment is undergoing focused melt delivery (<500-m-wide dike intrusion zone) and elevated hydrothermal activity. These findings demonstrate the ability of single-ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information.

Preliminary results are in from mid‐ocean ridge three‐dimensional seismic reflection survey

The first three-dimensional (3-D) seismic reflection survey of a mid-ocean ridge was shot in 1997 and, while it is still too early for firm interpretations of the data, it can be confirmed that significant crustal melt bodies have been located and one widely accepted model does not seem to apply to the presence of a robust magma supply there. n nThe survey the Anatomy of a Ridge-Axis Discontinuity (ARAD) experiment, was centered over an overlapping spreading center (OSC) system that offsets the ridge axis at 9°03′ north latitude on the East Pacific Rise (EPR) (Figure 1). It was conducted aboard the R/V Maurice Ewing during September and October and included a coincident 3-D crustal seismic tomography experiment.

Processed multi-channel seismic data (stacks and migrations) offshore California acquired during R/V Melville expedition MV1316 (2013)

Part of the SONGS project, this 2013 R/V Melville expedition conducted a 2D shallow seismic survey of the Newport Inglewood/Rose Canyon and Oceanside Blind Thrust fault systems in the vicinity of the San Onofre Nuclear Generating Station (SONGS), offshore California. The objectives included identifying the level of activity of the Newport Inglewood/Rose Canyon and Oceanside Blind Thrust faults, including their specific locations, geometries, and type of fault. These data were processed by University of Nevada, Reno and Scripps Institution of Oceanography. Funding was from Southern California Edison.