An official website of the United States government
Abstract Axial Seamount is an active submarine volcano located at the intersection of the Cobb hot spot and the Juan de Fuca Ridge (45°57′N, 130°01′W). Bottom pressure recorders captured co‐eruption subsidence of 2.4–3.2 m in 1998, 2011, and 2015, and campaign‐style pressure surveys every 1–2 years have provided a long‐term time series of inter‐eruption re‐inflation. The 2015 eruption occurred shortly after the Ocean Observatories Initiative (OOI) Cabled Array came online providing real‐time seismic and deformation observations for the first time. Nooner and Chadwick (2016,https://doi.org/10.1126/science.aah4666) used the available vertical deformation data to model the 2015 eruption deformation source as a steeply dipping prolate‐spheroid, approximating a high‐melt zone or conduit beneath the eastern caldera wall. More recently, Levy et al. (2018,https://doi.org/10.1130/G39978.1) used OOI seismic data to estimate dip‐slip motion along a pair of outward‐dipping caldera ring faults. This fault motion complicates the deformation field by contributing up to several centimeters of vertical seafloor motion. In this study, fault‐induced surface deformation was calculated from the slip estimates of Levy et al. (2018,https://doi.org/10.1130/G39978.1) then removed from vertical deformation data prior to model inversions. Removing fault motion resulted in an improved model fit with a new best‐fitting deformation source located 2.11 km S64°W of the source of Nooner and Chadwick (2016,https://doi.org/10.1126/science.aah4666) with similar geometry. This result shows that ring fault motion can have a significant impact on surface deformation, and future modeling efforts need to consider the contribution of fault motion when estimating the location and geometry of subsurface magma movement at Axial Seamount.
more »
« less
Global Trends of Axial Relief and Faulting at Plate Spreading Centers Imply Discrete Magmatic Events
Abstract Observed variations in across‐axis topographic relief and faulting style at spreading centers have been challenging to explain. Axial highs are seen at fast‐spreading centers, while valleys occur for slow‐spreading centers. Fault offsets range from tens of meters at fast‐spreading ridges to tens of kilometers at some slow‐spreading ridges. Models that fit the axial relief fail to produce observed fault patterns, while models that fit the fault patterns fail to produce observed variations in axial relief. A recent mechanical analysis (Liu & Buck, 2018,https://doi.org/10.1016/j.epsl.2018.03.045) suggests that including the effect of many discrete diking events can result in a gradual change in axial relief with crustal thicknesses. To compare this mechanical model directly with observations requires us to couple it with a two‐dimensional thermal model. This allows us to estimate the axial lithospheric thickness consistently as a function of the spreading rate and crustal thickness. For thinner axial lithosphere the model predicts an axial high with relief supported by low‐density material beneath the axial lithosphere. For axial lithospheric thickness between approximately one half and approximately three fourths of the crustal thickness, the axial depth decreases with magma supply increase. For thicker axial lithosphere the axial valley relief is controlled by axial brittle lithospheric thickness and near‐axis lithospheric geometry. We compared model predictions to data by compiling observations on axial relief and faulting mode for all spreading centers where seismic crustal thickness has been measured. Good fit to the data is obtained for model parameters giving dike widths in the axial lithosphere close to a meter.
more »
« less
- PAR ID:
- 10363905
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 125
- Issue:
- 8
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Fissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid‐ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast‐spreading 9°50′N segment of the East Pacific Rise were mapped using bathymetric data collected at 1‐m horizontal resolution by autonomous underwater vehicleSentry. Fault dip estimates from the bathymetric data were calibrated using co‐registered near‐bottom imagery and depth transects acquired by remotely operated vehicleJason. Fissures are classified as either eruptive or non‐eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (<4 km from the ridge axis). Pre‐ and post‐eruption surveys show that most fissures were reactivated during the eruptions in 2005–2006. Variable eruptive fissure geometry could be explained by the frequency with which each fissure is reactivated and partially infilled. Fissure swarms and lava plateaus in low‐relief areas >2 km from the ridge are spatially associated with off‐axis lower‐crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on‐axis fissures and inferred subsurface dike geometry imply a ∼9‐year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate.more » « less
-
Abstract Magnetic anomaly variations near mid‐ocean ridge spreading centers are sensitive to a variety of crustal accretionary processes as well as geomagnetic field variations when the crust forms. We collected near‐bottom vector magnetic anomaly data during a series of 21 autonomous underwater vehicleSentrydives near 9°50′N on the East Pacific Rise (EPR) covering ∼26 km along‐axis. These data document the 2–3 km wide axial anomaly high that is commonly observed at fast‐spreading ridges but also reveal the presence of a superimposed ∼800 m full wavelength anomaly low. The anomaly low is continuous for ≥13 km along axis and may extend over the entire survey region. A more detailed survey of hydrothermal vents near 9°50.3′N reveals ∼100 m diameter magnetic lows, which are misaligned relative to active vents and therefore cannot explain the continuous axial low. The axial magnetization low persists in magnetic inversions with variable extrusive source thickness, indicating that to the extent to which layer 2A constitutes the sole magnetic source, variations in its thickness alone cannot account for the axial low. Lava accumulation models illustrate that high geomagnetic intensity over the past ∼2.5 kyr, and decreasing intensity over the past ∼900 years, are both consistent with the broad axial anomaly high and the superimposed shorter wavelength low. The continuity of this axial low, and similar features elsewhere on the EPR suggests, that either crustal accretionary processes responsible for this anomaly are common among fast‐spread ridges, or that the observed magnetization low may partially reflect global geomagnetic intensity fluctuations.more » « less
-
Abstract We show that atmospheric gravity waves can generate plasma ducts and irregularities in the plasmasphere using the coupled SAMI3/WACCM‐X model. We find the equatorial electron density is irregular as a function of longitude which is consistent with CRRES measurements (Clilverd et al., 2007,https://doi.org/10.1029/2007ja012416). We also find that plasma ducts can be generated forL‐shells in the range 1.5–3.0 with lifetimes of ∼ 0.5 hr; this is in line with observations of ducted VLF wave propagation with lifetimes of 0.5–2.0 hr (Clilverd et al., 2008,https://doi.org/10.1029/2007ja012602; Singh et al., 1998,https://doi.org/10.1016/s1364-6826(98)00001-7).more » « less
-
Abstract Mantle convection models based on geophysical constraints have provided us with a basic understanding of the forces driving and resisting plate motions on Earth. However, existing studies computing the balance of underlying forces are contradicting, and the impact of plate boundary geometry on surface deformation remains unknown. We address these issues by developing global instantaneous 3‐D mantle convection models with a heterogeneous density and viscosity distribution and weak plate boundaries prescribed using different geometries. We find that the plate boundary geometry of the Global Earthquake Model (GEM, Pagani et al., 2018,https://doi.org/10.1177/8755293020931866), featuring open plate boundaries with discrete lithospheric‐depth weak zones in the oceans and distributed crustal faults within continents, achieves the best fit to the observed GPS data with a directional correlation of 95.1% and a global point‐wise velocity residual of 1.87 cm/year. A good fit also requires plate boundaries being 3 to 4 orders of magnitude weaker than the surrounding lithosphere and low asthenospheric viscosities between 5 × 1017and 5 × 1018 Pa s. Models without asthenospheric and lower mantle heterogeneities retain on average 30% and 70% of the plate speeds, respectively. Our results show that Earth's plate boundaries are not uniform and better described by more discrete plate boundaries within the oceans and distributed faults within continents. Furthermore, they emphasize the impact of plate boundary geometry on the direction and speed of plate motions and reaffirm the importance of slab pull in the uppermost mantle as a major plate driving force.more » « less
