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

Stacked magma lenses beneath mid-ocean ridges: insights from new seismic observations and synthesis with prior geophysical and geologic findings

Author Posting. © American Geophysical Union, 2021. 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 126(4), (2021): e2020JB021434, https://doi.org/10.1029/2020JB021434.Recent multi-channel seismic studies of fast spreading and hot-spot influenced mid-ocean ridges reveal magma bodies located beneath the mid-crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate-spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′-52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub-horizontal, extend continuously for along-axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub-AML bodies found at the East Pacific Rise. At both ridges, stacked sill-like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub-vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi-level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events.This research was supported by NSF OCE 0002488 and 0648303 (LDEO), 0002551 (WHOI), 1658199 and 1357076 (UTIG). S. M. Carbotte was partially supported by Columbia University and J. P. Canales by the Independent Research & Development Program at WHOI

Anatomy of an active submarine volcano

Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Geological Society of America for personal use, not for redistribution. The definitive version was published in Geology 42 (2014): 655-658, doi:10.1130/G35629.1.Most of the magma erupted at mid-ocean ridges is stored in a mid-crustal melt lens that lies at the boundary between sheeted dikes and gabbros. Nevertheless, images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive. Here, we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date, which is principally attributed to the juxtaposition of the Juan de Fuca Ridge with the Cobb hotspot. Our results reveal a complex melt body beneath the summit caldera, which is ~14 km long, 3 km wide and up to 1 km thick. The estimated volume of the reservoir is 18–30 km3, more than two orders of magnitude greater than the erupted magma volumes of the 1998 and 2011 eruptions. Our images show a network of sub-horizontal to shallow dipping (<30°) features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites.This research was funded through a National Science Foundation grant, OCE- 0002600, and additionally supported through the Cecil H. and Ida M. Green Foundation at the Scripps Institution of Oceanography.2015-06-0

Detection of Magma Beneath the Northern and Southern Rift Zones of Axial Seamount at the Juan de Fuca Ridge

Abstract Axial Seamount is an active hotspot‐related volcanic system located along the Juan de Fuca Ridge (JdFR) that includes a central volcano and bounding northern and southern rift zones (NRZ and SRZ). Three documented volcanic eruptions in 1998, 2011, and 2015 included dike propagation into and eruptions within the rift zones that are believed to have been sourced from the well‐imaged large magma reservoir found beneath the central volcano. However, areas beyond the central volcano have not been explored for potential magma sources that could have contributed to these events, and geochemical studies of older rift zone lavas indicate differences in compositions suggestive of magma reservoirs fed by more mid‐ocean ridge‐dominated mantle sources. In this study, we analyze multichannel seismic data acquired in 2002 to characterize the internal crustal structure of the rift zones. The new reflectivity images reveal small (<5 km wide) and discontinuous crustal magma bodies at depths of ∼1.5 to 4 km beneath and in the vicinity of the rift zone lava flows from the three eruptions. We also image wide magma bodies within the overlap regions between the rift zones and neighboring segments of JdFR including a 6.4 km wide body under the east flank of NRZ and a 1‐km wide, ∼400 to 500 m, thick body near the base of the crust under the SRZ‐Vance overlap basin. Collectively the new observations indicate that multiple small crustal magma bodies underlie Axial segment, in addition to the main reservoir, and likely contribute to rift zone magmatism with implications for interpretations of seismicity patterns and lava flow compositions

Structure de la couche 2A sous le volcan Lucky Strike, sur la médiane médio-Atlantique, au moyen d'une méthode de tomographie haute résolution et d'une méthode d'inversion des formes d'onde

The surficial layer of the oceanic crust is formed by a heterogeneous process of magmatic eruptions and tectonic deformation. Historically, it has been difficult to obtain seismic images of this region with resolutions comparable to the length scales of the geologic processes. Traditional travel time tomography methods over-smooth the vertical and horizontal velocity structure of the upper crust while multichannel seismic (MCS) processing concentrates on the wide-angle reflection imaging of the base of layer 2A with little emphasis on velocity. As a consequence, questions remain about the relationship between seismic descriptions of the upper crust in terms of Layer 2A & B and the geologic observations. In this thesis, the SISMOMAR 3-D multichannel seismic (MCS) dataset, acquired at the center of the Lucky Strike segment of the Mid-Atlantic ridge, is analyzed in a transformative way, using a new method called the Synthetic Ocean Bottom Experiment (SOBE), which can extrapolate seismic sources and receivers down to the seafloor, simulating an on-bottom seismic experiment. This downward extrapolation stage has several advantages: it collapses the seafloor diffractions and filters the background noise, improving the imaging condition, and it unwraps the Layer 2A/2B triplication, which places the refracted energy in front of the water-wave, providing information about upper crustal velocities. The first half of this thesis presents a first application of downward continuation of the seismic wavefield to near seafloor and 3-D high-resolution traveltime tomography to resolve upper crustal structure. New features within the upper crust (Layer 2A) are revealed and interpreted in terms of spatial variations in magmatic, hydrothermal, and tectonic processes across the site. The second half of this thesis presents an application of 2-D full waveform inversion (FWI) of part of the previously analyzed dataset to push the spatial resolution down to a few tens of meters. The velocity models obtained reveal an even finer-scale structure of the Lucky Strike volcano than had been obtained with the high-resolution tomography method. Several sub-horizontal velocity layers are resolved within the shallow crust that are attributed to different piled up lava sequences, consistent with surface geology that shows diverse lava deposits. The detection of distinct velocity gradient discontinuities within the top kilometer of crust further allows tracing of faults at depth that are well correlated with the surface-expressed faulting. Finally, different characteristics of the FWI models are used to address the geological significance of the Layer 2A/2B boundary in this region, which is a long-standing controversy in studies of oceanic crust.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Physical conditions and frictional properties in the source region of a slow-slip event

Recent geodetic studies have shown that slow-slip events can occur on subduction faults, including their shallow (<15 km depth) parts where tsunamis are also generated. Although observations of such events are now widespread, the physical conditions promoting shallow slow-slip events remain poorly understood. Here we use full waveform inversion of controlled-source seismic data from the central Hikurangi (New Zealand) subduction margin to constrain the physical conditions in a region hosting slow slip. We find that the subduction fault is characterized by compliant, overpressured and mechanically weak material. We identify sharp lateral variations in pore pressure, which reflect focused fluid flow along thrust faults and have a fundamental influence on the distribution of mechanical properties and frictional stability along the subduction fault. We then use high-resolution data-derived mechanical properties to underpin rate–state friction models of slow slip. These models show that shallow subduction fault rocks must be nearly velocity neutral to generate shallow frictional slow slip. Our results have implications for understanding fault-loading processes and slow transient fault slip along megathrust faults