Upper crustal seismic structure along the Southeast Indian Ridge: Evolution from 0 to 550 ka and variation with axial morphology

The seismic structure of uppermost crust evolves after crustal formation with precipitation of alteration minerals during ridge-flank hydrothermal circulation. However, key parameters of crustal evolution including depth extent and rates of change in crustal properties, and factors contributing to this evolution remain poorly understood. Here, long-offset multichannel seismic data are used to study the evolution of seismic layer 2A and uppermost 2B from 0 to 550 ka at three segments of the intermediate spreading rate Southeast Indian Ridge. The segments differ in on-axis morphology and structure with crustal magma bodies imaged at axial high and rifted high segments P1 and P2, but not at axial valley segment S1 and marked differences in thermal conditions within the upper crust are inferred. One-dimensional travel time modeling of common midpoint supergathers is used to determine the thickness and velocity of layer 2A and velocity of uppermost 2B. At all three segments, layer 2A velocities are higher in 550 ka crust than on-axis (by 7–14%) with the largest increases at segments P1 and P2. Velocities increase more rapidly (by 125 ka) at P1 with spatial variations in velocity gradients linked to location of the underlying crustal magma body. We attribute these differences in crustal evolution to higher rates of fluid flow and temperatures of reaction at these ridge segments where crustal magma bodies are present. Layer 2A thickens off-axis at segments P1 and P2 but not at S1; both off-axis volcanic thickening and downward propagation of a cracking front linked to the vigor of axial hydrothermal activity could contribute to these differences. In zero-age crust, layer 2B velocities are significantly lower at segments P1 and P2 than S1 (5.0, 5.4, and 5.8 km/s respectively), whereas similar velocities are measured off-axis at all segments (5.7–5.9 km/s). Lower on-axis 2B velocities at segments P1 and P2 can be partly attributed to thinner layer 2A, with lower overburden pressures leading to higher porosities in shallowest 2B. However, other factors must also contribute. Likely candidates include subaxial deformation due to magmatic processes and enhanced cracking with axial hydrothermal activity at these segments with crustal magma bodies

Variations in upper crustal structure due to variable mantle temperature along the Southeast Indian Ridge

There is a systematic variation in axial morphology and axial depth along the Southeast Indian Ridge (SEIR) with distance away from the Australian Antarctic Discordance, an area of cold uppermost mantle. Since spreading rate (72–76 mm/yr) and mantle geochemistry appear constant along this portion of the SEIR, the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature. In this study, we report results from a multichannel seismic investigation of on-axis crustal structure along this portion of the SEIR. Three distinct forms of ridge crest morphology are found within our study area: axial highs, rifted axial highs, and shallow axial valleys. Axial highs have a shallow (~ 1500 m below seafloor (bsf)) magma lens and a thin (~ 300 m) layer 2A along the ridge crest. Rifted axial highs have a deeper (~ 2100 m bsf) magma lens and thicker (~ 450 m) layer 2A on-axis. Beneath shallow axial valleys, no magma lens is imaged, and layer 2A is thick (~ 450 + m). There are step-like transitions in magma lens depth and layer 2A thickness with changes in morphology along the SEIR. The transitions between the different modes of axial morphology and shallow structure are abrupt, suggesting a threshold-type mechanism. Variations in crustal structure along the SEIR appear to be steady state, persisting for at least 1 m.y. Portions of segments in which a magma lens is found are characterized by lower relief abyssal hills on the ridge flank, shallower ridge flank depths, and at the location of along-axis Mantle Bouguer Anomaly (MBA) lows. The long-wavelength variation in ridge morphology along the SEIR from axial high segments to the west to axial valley segments to the east is linked to the regional gradient in mantle temperature. Superimposed on the long-wavelength trend are segment to segment variations that are related to the absolute motion of the SEIR to the northeast which influence mantle melt production and delivery to the ridge

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

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 12 (2011): Q05009, doi:10.1029/2010GC003462.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 covered, direct pathways from the igneous basement through the seafloor are cut off, forcing convective cells to find alternate paths.This research was supported by National Science Foundation grants OCE0002488 and OCE0648303 to S.M.C. and M.R.N., NSERC Discovery grant to M.R.N., and a Bruce C. Heezen Graduate Research Fellowship (Office of Naval Research grant N00014‐02‐1‐0691) to K.R.N

Dry Juan de Fuca slab revealed by quantification of water entering Cascadia subduction zone

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Nature Geoscience 10 (2017): 864-870, doi:10.1038/ngeo3050.Water is carried by subducting slabs as a pore fluid and in structurally bound minerals, yet no comprehensive quantification of water content and how it is stored and distributed at depth within incoming plates exists for any segment of the global subduction system. Here we use seismic data to quantify the amount of pore and structurally bound water in the Juan de Fuca plate entering the Cascadia subduction zone. Specifically, we analyse these water reservoirs in the sediments, crust and lithospheric mantle, and their variations along the central Cascadia margin. We find that the Juan de Fuca lower crust and mantle are drier than at any other subducting plate, with most of the water stored in the sediments and upper crust. Variable but limited bend faulting along the margin limits slab access to water, and a warm thermal structure resulting from a thick sediment cover and young plate age prevents significant serpentinization of the mantle. The dryness of the lower crust and mantle indicates that fluids that facilitate episodic tremor and slip must be sourced from the subducted upper crust, and that decompression rather than hydrous melting must dominate arc magmatism in central Cascadia. Additionally, dry subducted lower crust and mantle can explain the low levels of intermediate-depth seismicity in the Juan de Fuca slab.This research was funded by the US NSF

Recent seismic studies at the East Pacific Rise 8°20'–10°10'N and Endeavour Segment : insights into mid-ocean ridge hydrothermal and magmatic processes

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 100–112, doi:10.5670/oceanog.2012.08.As part of the suite of multidisciplinary investigations undertaken by the Ridge 2000 Program, new multichannel seismic studies of crustal structure were conducted at the East Pacific Rise (EPR) 8°20'–10°10'N and Endeavour Segment of the Juan de Fuca Ridge. These studies provide important insights into magmatic systems and hydrothermal flow in these regions, with broader implications for fast- and intermediate-spreading mid-ocean ridges. A mid-crust magma body is imaged beneath Endeavour Segment underlying all known vent fields, suggesting that prior notions of a tectonically driven hydrothermal system at this site can be ruled out. There is evidence at both sites that the axial magma body is segmented on a similar 5–20 km length scale, with implications for the geometry of high-temperature axial hydrothermal flow and for lava geochemistry. The new data provide the first seismic reflection images of magma sills in the crust away from the axial melt lens. These off-axis magma reservoirs are the likely source of more-evolved lavas typically sampled on the ridge flanks and may be associated with off-axis hydrothermal venting, which has recently been discovered within the EPR site. Clusters of seismic reflection events at the base of the crust are observed, and localized regions of thick Moho Transition Zone, with frozen or partially molten gabbro lenses embedded within mantle rocks, are inferred. Studies of the upper crust on the flanks of Endeavour Segment provide new insights into the low-temperature hydrothermal flow that continues long after crustal formation. Precipitation of alteration minerals due to fluid flow leads to changes in P-wave velocities within seismic Layer 2A (the uppermost layer of the oceanic crust) that vary markedly with extent of sediment blanketing the crust. In addition, intermediate-scale variations in the structure of Layers 2A and 2B with local topography are observed that may result from topographically driven fluid upflow and downflow on the ridge flanks.This research was supported by NSF OCE grants 0002488, 0002551, 0648303, 0648923, 0327872 and 0327885

Vp/Vs ratio of incoming sediments off Cascadia subduction zone from analysis of controlled-source multicomponent OBS records

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 125(6), (2020): e2019JB019239, doi:10.1029/2019JB019239.P‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vs structure of Cascadia Basin sediments. We used P‐to‐S waves converted at the basement to derive an empirical function describing the average Vp/Vs of Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vs functions from semblance velocity analysis of S‐converted intrasediment and basement reflections, which we used to define an empirical Vp/Vs versus burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vs structure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs. These zones with elevated Vp/Vs are likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vs functions of Cascadia Basin sediments.This research was funded by National Science Foundation (NSF) Grant OCE‐1657237 to J. P. C, OCE‐1657839 to A. F. A. and S. H., and OCE‐1657737 to S. M. C. Data used in this study were acquired with funding from NSF Grants OCE‐1029305 and OCE‐1249353. Data used in this research were provided by instruments from the Ocean Bottom Seismic Instrument Center (http://obsic.whoi.edu, formerly OBSIP), which is funded by the NSF. OBSIC/OBSIP data are archived at the IRIS Data Management Center (http://www.iris.edu) under network code X6 (https://doi.org/10.7914/SN/X6_2012). Data processing was conducted with Emerson‐Paradigm Software package Echos licensed to Woods Hole Oceanographic Institution under Paradigm Academic Software Program and MATLAB package SeismicLab of the University of Alberta, Canada (http://seismic-lab.physics.ualberta.ca), under GNU General Public License (MATLAB® is a registered trademark of MathWorks).2020-11-2

A multi-sill magma plumbing system beneath the axis of the East Pacific Rise

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 7 (2014): 825-829, doi:10.1038/ngeo2272.The mid-crust axial magma lens detected at fast and intermediate spreading mid-ocean ridges is believed to be the primary magma reservoir for formation of upper oceanic crust. However, the mechanism behind formation of the lower crust is a subject of ongoing debate. The sheeted sill model proposed from observations of ophiloites requires the presence of multiple lenses/sills throughout lower crust but only a single lens is imaged directly beneath the innermost axial zone in prior seismic studies . Here, high-fidelity seismic data from the East Pacific Rise reveal series of reflections below the axial magma lens that we interpret as mid-lower crustal lenses. These deeper lenses are present between 9°20-57′N at variable two-way-travel-times, up to 4.6 s (~1.5 km beneath the axial magma lens), providing direct support for the sheeted sill model. From local changes in the amplitude and geometry of the events beneath a zone of recent volcanic eruption, we infer that melt drained from a lower lens contributed to the replenishment of the axial magma lens above and, perhaps, the eruption. The new data indicate that a multi-level sill complex is present beneath the East Pacific Rise that likely contributes to the formation of both the upper and lower crust.This research was supported by NSF awards OCE0327872 to J. C. M., S. M. C., OCE- 0327885 to J. P. C., and OCE0624401 to M. R. N.2015-04-1

Variations in axial magma lens properties along the East Pacific Rise (9°30′N–10°00′N) from swath 3-D seismic imaging and 1-D waveform inversion

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 119 (2014): 2721–2744, doi:10.1002/2013JB010730.We use three-dimensional multistreamer seismic reflection data to investigate variations in axial magma lens (AML) physical properties along the East Pacific Rise between 9°30′N and 10°00′N. Using partial-offset stacks of P- and S-converted waves reflecting off the top of the AML, we image four 2–4 km long melt-rich sections spaced 5–10 km from each other. One-dimensional waveform inversion indicates that the AML in a melt-rich section is best modeled with a low Vp (2.95–3.23 km/s) and Vs (0.3–1.5 km/s), indicating >70% melt fraction. In contrast, the AML in a melt-poor section requires higher Vp (4.52–4.82 km/s) and Vs (2.0–3.0 km/s), which indicates <40% melt fraction. The thicknesses of the AML are constrained to be 8–32 m and 8–120 m at the melt-rich and -poor sites, respectively. Based on the AML melt-mush segmentation imaged in the area around the 2005–2006 eruption, we infer that the main source of this eruption was a 5 km long section of the AML between 9°48′N and 51′N. The eruption drained most of the melt in this section of the AML, leaving behind a large fraction of connected crystals. We estimate that during the 2005–2006 eruption, a total magma volume of 9–83 × 106 m3 was extracted from the AML, with a maximum of 71 × 106 m3 left unerupted in the crust as dikes. From this, we conclude that an eruption of similar dimensions to the 2005–2006, one would be needed with a frequency of years to decades in order to sustain the long-term average seafloor spreading rate at this location.This research was supported by NSF grants OCE-0327885 and OCE-0327872 through the RIDGE-2000 program.2014-10-2

Crustal thickness and Moho character of the fast-spreading East Pacific Rise from 9°42′N to 9°57′N from poststack-migrated 3-D MCS data

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 634–657, doi:10.1002/2013GC005069.We computed crustal thickness (5740 ± 270 m) and mapped Moho reflection character using 3-D seismic data covering 658 km2 of the fast-spreading East Pacific Rise (EPR) from 9°42′N to 9°57′N. Moho reflections are imaged within ∼87% of the study area. Average crustal thickness varies little between large sections of the study area suggesting regionally uniform crustal production in the last ∼180 Ka. However, individual crustal thickness measurements differ by as much as 1.75 km indicating that the mantle melt delivery has not been uniform. Third-order, but not fourth-order ridge discontinuities are associated with changes in the Moho reflection character and/or near-axis crustal thickness. This suggests that the third-order segmentation is governed by melt distribution processes within the uppermost mantle while the fourth-order ridge segmentation arises from midcrustal to upper-crustal processes. In this light, we assign fourth-order ridge discontinuity status to the debated ridge segment boundary at ∼9°45′N and third-order status at ∼9°51.5′N to the ridge segment boundary previously interpreted as a fourth-order discontinuity. Our seismic results also suggest that the mechanism of lower-crustal accretion varies along the investigated section of the EPR but that the volume of melt delivered to the crust is mostly uniform. More efficient mantle melt extraction is inferred within the southern half of our survey area with greater proportion of the lower crust accreted from the axial magma lens than that for the northern half. This south-to-north variation in the crustal accretion style may be caused by interaction between the melt sources for the ridge and the Lamont seamounts.This research was supported by the National Science Foundation grants OCE0327872 to J. C. M., S. M. C., OCE327885 to J. P. C., OCE0624401 to M. R. N., and NSERC Discovery, CRC and CFI grants to M. R. N.2014-09-1

Recent advances in multichannel seismic imaging for academic research in deep oceanic environments

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 1 (2012): 113–115, doi:10.5670/oceanog.2012.09.Academic research using marine multichannel seismic (MCS) methods to investigate processes related to Earth's oceanic crust has made substantial advances in the last decade. These advances were made possible by access to state-of-the-art MCS acquisition systems, and by development of data processing and modeling techniques that specifically deal with the particularities of oceanic crustal structure and the challenges of subseafloor imaging in the deep ocean. Among these methods, we highlight multistreamer three-dimensional (3D) imaging, streamer refraction tomography, synthetic ocean bottom experiments (SOBE), and time-lapse (4D) studies.The studies highlighted here were supported by NSF OCE grants 0327885, 0327872, 0621660, and 0826481