Lizarralde Daniel

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Lizarralde

First Name

Daniel

ORCID

0000-0001-6152-6039

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Azimuthal seismic anisotropy of 70-ma Pacific-plate upper mantle.

(American Geophysical Union, 2019-01-28) Mark, Hannah ; Lizarralde, Daniel ; Collins, John ; Miller, Nathaniel C. ; Hirth, Greg ; Gaherty, James B. ; Evans, Rob L.

Plate formation and evolution processes are predicted to generate upper mantle seismic anisotropy and negative vertical velocity gradients in oceanic lithosphere. However, predictions for upper mantle seismic velocity structure do not fully agree with the results of seismic experiments. The strength of anisotropy observed in the upper mantle varies widely. Further, many refraction studies observe a fast direction of anisotropy rotated several degrees with respect to the paleospreading direction, suggesting that upper mantle anisotropy records processes other than 2‐D corner flow and plate‐driven shear near mid‐ocean ridges. We measure 6.0 ± 0.3% anisotropy at the Moho in 70‐Ma lithosphere in the central Pacific with a fast direction parallel to paleospreading, consistent with mineral alignment by 2‐D mantle flow near a mid‐ocean ridge. We also find an increase in the strength of anisotropy with depth, with vertical velocity gradients estimated at 0.02 km/s/km in the fast direction and 0 km/s/km in the slow direction. The increase in anisotropy with depth can be explained by mechanisms for producing anisotropy other than intrinsic effects from mineral fabric, such as aligned cracks or other structures. This measurement of seismic anisotropy and gradients reflects the effects of both plate formation and evolution processes on seismic velocity structure in mature oceanic lithosphere, and can serve as a reference for future studies to investigate the processes involved in lithospheric formation and evolution.
Seismicity of the incoming plate and forearc near the Mariana Trench recorded by ocean bottom seismographs

(American Geophysical Union, 2020-04-06) Eimer, Melody ; Wiens, Douglas A. ; Cai, Chen ; Lizarralde, Daniel ; Jasperson, Hope

Earthquakes near oceanic trenches are important for studying incoming plate bending and updip thrust zone seismogenesis, yet are poorly constrained using seismographs on land. We use an ocean bottom seismograph (OBS) deployment spanning both the incoming Pacific Plate and the forearc to study seismicity near the Mariana Trench. The yearlong deployment in 2012–2013 consisted of 20 broadband OBSs and 5 suspended hydrophones, with an additional 59 short period OBSs and hydrophones recording for 1 month. We locate 1,692 earthquakes using a nonlinear method with a 3D velocity model constructed from active source profiles and surface wave tomography results. Events occurring seaward of the trench occur to depths of ~35 km below the seafloor, and focal mechanisms of the larger events indicate normal faulting corresponding to plate bending. Significant seismicity emerges about 70 km seaward from the trench, and the seismicity rate increases continuously towards the trench, indicating that the largest bending deformation occurs near the trench axis. These plate‐bending earthquakes occur along faults that facilitate the hydration of the subducting plate, and the lateral and depth distribution of earthquakes is consistent with low‐velocity regions imaged in previous studies. The forearc is marked by a heterogeneous distribution of low magnitude (<5 Mw) thrust zone seismicity, possibly due to the rough incoming plate topography and/or serpentinization of the forearc. A sequence of thrust earthquakes occurs at depths ~10 km below seafloor and within 20 km of the trench axis, demonstrating that the megathrust is seismically active nearly to the trench.
Structure and serpentinization of the subducting Cocos plate offshore Nicaragua and Costa Rica

(American Geophysical Union, 2011-06-22) Van Avendonk, Harm J. A. ; Holbrook, W. Steven ; Lizarralde, Daniel ; Denyer, P.

The Cocos plate experiences extensional faulting as it bends into the Middle American Trench (MAT) west of Nicaragua, which may lead to hydration of the subducting mantle. To estimate the along strike variations of volatile input from the Cocos plate into the subduction zone, we gathered marine seismic refraction data with the R/V Marcus Langseth along a 396 km long trench parallel transect offshore of Nicaragua and Costa Rica. Our inversion of crustal and mantle seismic phases shows two notable features in the deep structure of the Cocos plate: (1) Normal oceanic crust of 6 km thickness from the East Pacific Rise (EPR) lies offshore Nicaragua, but offshore central Costa Rica we find oceanic crust from the northern flank of the Cocos Nazca (CN) spreading center with more complex seismic velocity structure and a thickness of 10 km. We attribute the unusual seismic structure offshore Costa Rica to the midplate volcanism in the vicinity of the Galápagos hot spot. (2) A decrease in Cocos plate mantle seismic velocities from ∼7.9 km/s offshore Nicoya Peninsula to ∼6.9 km/s offshore central Nicaragua correlates well with the northward increase in the degree of crustal faulting outboard of the MAT. The negative seismic velocity anomaly reaches a depth of ∼12 km beneath the Moho offshore Nicaragua, which suggests that larger amounts of water are stored deep in the subducting mantle lithosphere than previously thought. If most of the mantle low velocity zone can be interpreted as serpentinization, the amount of water stored in the Cocos plate offshore central Nicaragua may be about 2.5 times larger than offshore Nicoya Peninsula. Hydration of oceanic lithosphere at deep sea trenches may be the most important mechanism for the transfer of aqueous fluids to volcanic arcs and the deeper mantle.
Influence of late Pleistocene glaciations on the hydrogeology of the continental shelf offshore Massachusetts, USA

(John Wiley & Sons, 2014-12-05) Siegel, Jacob ; Person, Mark ; Dugan, Brandon ; Cohen, Denis ; Lizarralde, Daniel ; Gable, Carl

Multiple late Pleistocene glaciations that extended onto the continental shelf offshore Massachusetts, USA, may have emplaced as much as 100 km3 of freshwater (salinity <5 ppt) in continental shelf sediments. To estimate the volume and extent of offshore freshwater, we developed a three-dimensional, variable-density model that couples fluid flow and heat and solute transport for the continental shelf offshore Massachusetts. The stratigraphy for our model is based on high-resolution, multichannel seismic data. The model incorporates the last 3 Ma of climate history by prescribing boundary conditions of sea level change and ice sheet extent and thickness. We incorporate new estimates of the maximum extent of a late Pleistocene ice sheet to near the shelf-slope break. Model results indicate that this late Pleistocene ice sheet was responsible for much of the emplaced freshwater. We predict that the current freshwater distribution may reach depths up to 500 meters below sea level and up to 30 km beyond Martha's Vineyard. The freshwater distribution is strongly dependent on the three-dimensional stratigraphy and ice sheet history. Our predictions improve our understanding of the distribution of offshore freshwater, a potential nonrenewable resource for coastal communities along recently glaciated margins.
Variation in styles of rifting in the Gulf of California

( 2007-06-18) Lizarralde, Daniel ; Axen, Gary J. ; Brown, Hillary E. ; Fletcher, John M. ; Gonzalez-Fernandez, Antonio ; Harding, Alistair J. ; Holbrook, W. Steven ; Kent, Graham M. ; Paramo, Pedro ; Sutherland, Fiona ; Umhoefer, Paul J.

The rifting of continental lithosphere is a fundamental solid-earth process that leads to the formation of rifted continental margins and ocean basins. Understanding of this process comes from observations of the geometry of rifted margins and the magmatism resulting from rifting, which inform us about the strength of the lithosphere, the state of the underlying mantle, and the transition from rifting to seafloor spreading. Here we describe results from the PESCADOR seismic experiment in the southern Gulf of California and present the first crustal-scale images across conjugate margins of multiple segments within a single rift that has reached the stage of oceanic spreading. A surprisingly large variation in rifting style and magmatism is observed between these segments, from wide rifting with minor syn-rift magmatism to narrow rifting in magmatically robust segments. These differences encompass much of the variation observed across nearly all other non-end-member continental margins. The characteristics of magmatic endmember margins are typically explained in terms of mantle temperature. Our explanations for the variation in the Gulf of California, in contrast, invoke mantle depletion to account for wide, magma-poor rifting and mantle fertility and possibly the influence of sediments to account for robust rift and post-rift magmatism in the Gulf of California. These factors may vary laterally over small distances in regions that have transitioned from convergence to extension, as is the case for the Gulf of California and many other rifts.
Ice sheet–derived submarine groundwater discharge on Greenland's continental shelf

(American Geophysical Union, 2011-07-28) DeFoor, Whitney ; Person, Mark ; Larsen, Hans Christian ; Lizarralde, Daniel ; Cohen, Denis ; Dugan, Brandon

Isotopically light (−1‰ to −8‰ δ18O) and fresh pore fluids (300–520 mM Cl−) were found in continental shelf sediments up to 100 km offshore of southeastern (SE) Greenland, suggesting infiltration and mixing of ice sheet meltwater with seawater to depths of 260 m. These geochemical anomalies may be associated with ice sheet–derived submarine groundwater discharge (SMGD). We present a continental-scale reconstruction of the late Pleistocene hydrogeology of SE Greenland using finite element analysis that incorporates ice sheet loading and solute and isotope transport. Results indicate that subglacial infiltration and SMGD are of the same order of magnitude and are highly dependent on the permeability of the subaerial basalt. Simulated infiltration and SMGD almost doubled during the Last Glacial Maximum, compared to ice-free conditions. Much of the present-day glacially induced groundwater discharge occurs on land. Subice infiltration on the continental shelf likely represents a mixture of seawater and meltwater during past glacial maximums. Simulated SMGD during the most recent interval of glacial retreat is about 4% of the total ice sheet melting. At present, the simulated rate of SMGD is about 11% of the estimated current melting rate.
Microbial communities under distinct thermal and geochemical regimes in axial and off-axis sediments of Guaymas Basin

(Frontiers Media, 2021-02-12) Teske, Andreas P. ; Wegener, Gunter ; Chanton, Jeffrey P. ; White, Dylan ; MacGregor, Barbara J. ; Hoer, Daniel ; de Beer, Dirk ; Zhuang, Guangchao ; Saxton, Matthew A. ; Joye, Samantha B. ; Lizarralde, Daniel ; Soule, S. Adam ; Ruff, S. Emil

Cold seeps and hydrothermal vents are seafloor habitats fueled by subsurface energy sources. Both habitat types coexist in Guaymas Basin in the Gulf of California, providing an opportunity to compare microbial communities with distinct physiologies adapted to different thermal regimes. Hydrothermally active sites in the southern Guaymas Basin axial valley, and cold seep sites at Octopus Mound, a carbonate mound with abundant methanotrophic cold seep fauna at the Central Seep location on the northern off-axis flanking regions, show consistent geochemical and microbial differences between hot, temperate, cold seep, and background sites. The changing microbial actors include autotrophic and heterotrophic bacterial and archaeal lineages that catalyze sulfur, nitrogen, and methane cycling, organic matter degradation, and hydrocarbon oxidation. Thermal, biogeochemical, and microbiological characteristics of the sampling locations indicate that sediment thermal regime and seep-derived or hydrothermal energy sources structure the microbial communities at the sediment surface.
Fracture propagation to the base of the Greenland Ice Sheet during supraglacial lake drainage

( 2008-02-20) Das, Sarah B. ; Joughin, Ian ; Behn, Mark D. ; Howat, Ian M. ; King, Matt A. ; Lizarralde, Daniel ; Bhatia, Maya P.

Surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change. The process whereby such a pathway is created through thick, cold ice has not, however, been previously observed. We describe the rapid (<2 hours) drainage of a large supraglacial lake down 980 m through to the bed of the Greenland Ice Sheet initiated by water-driven fracture propagation evolving into moulin flow. Drainage coincided with increased seismicity, transient acceleration, ice sheet uplift and horizontal displacement. Subsidence and deceleration occurred over the following 24 hours. The short-lived dynamic response suggests an efficient drainage system dispersed the meltwater subglacially. The integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup.
Seismic structure of the southern Gulf of California from Los Cabos block to the East Pacific Rise

(American Geophysical Union, 2008-03-15) Paramo, Pedro ; Holbrook, W. Steven ; Brown, Hillary E. ; Lizarralde, Daniel ; Fletcher, John M. ; Umhoefer, Paul J. ; Kent, Graham M. ; Harding, Alistair J. ; Gonzalez, A. ; Axen, Gary J.

Multichannel reflection and coincident wide-angle seismic data collected during the 2002 Premier Experiment, Sea of Cortez, Addressing the Development of Oblique Rifting (PESCADOR) experiment provide the most detailed seismic structure to date of the southern Gulf of California. Multichannel seismic (MCS) data were recorded with a 6-km-long streamer, 480-channel, aboard the R/V Maurice Ewing, and wide-angle data was recorded by 19 instruments spaced every ∼12 km along the transect. The MCS and wide-angle data reveal the seismic structure across the continent-ocean transition of the rifted margin. Typical continental and oceanic crust are separated by a ∼75-km-wide zone of extended continental crust dominated by block-faulted basement. Little lateral variation in crustal thicknesses and seismic velocities is observed in the oceanic crust, suggesting a constant rate of magmatic productivity since seafloor spreading began. Oceanic crustal thickness and mean crustal velocities suggest normal mantle temperature (1300°C) and passive mantle upwelling at the early stages of seafloor spreading. The crustal thickness, width of extended continental crust, and predicted temperature conditions all indicate a narrow rift mode of extension. On the basis of upper and lower crust stretching factors, an excess of lower crust was found in the extended continental crust. Total extension along transect 5W is estimated to be ∼35 km. Following crustal extension, new oceanic crust ∼6.4-km-thick was formed at a rate of ∼48 mm a−1 to accommodate plate separation.
Seismological evidence for girdled olivine lattice‐preferred orientation in oceanic lithosphere and implications for mantle deformation processes during seafloor spreading

(American Geophysical Union, 2022-10-03) Russell, Joshua B. ; Gaherty, James B. ; Mark, Hannah F. ; Hirth, Greg ; Hansen, Lars N. ; Lizarralde, Daniel ; Collins, John A. ; Evans, Rob L.

Seismic anisotropy produced by aligned olivine in oceanic lithosphere offers a window into mid‐ocean ridge (MOR) dynamics. Yet, interpreting anisotropy in the context of grain‐scale deformation processes and strain observed in laboratory experiments and natural olivine samples has proven challenging due to incomplete seismological constraints and length scale differences spanning orders of magnitude. To bridge this observational gap, we estimate an in situ elastic tensor for oceanic lithosphere using co‐located compressional‐ and shear‐wavespeed anisotropy observations at the NoMelt experiment located on ∼70 Ma seafloor. The elastic model for the upper 7 km of the mantle, NoMelt_SPani7, is characterized by a fast azimuth parallel to the fossil‐spreading direction, consistent with corner‐flow deformation fabric. We compare this model with a database of 123 petrofabrics from the literature to infer olivine crystallographic orientations and shear strain accumulated within the lithosphere. Direct comparison to olivine deformation experiments indicates strain accumulation of 250%–400% in the shallow mantle. We find evidence for D‐type olivine lattice‐preferred orientation (LPO) with fast [100] parallel to the shear direction and girdled [010] and [001] crystallographic axes perpendicular to shear. D‐type LPO implies similar amounts of slip on the (010)[100] and (001)[100] easy slip systems during MOR spreading; we hypothesize that grain‐boundary sliding during dislocation creep relaxes strain compatibility, allowing D‐type LPO to develop in the shallow lithosphere. Deformation dominated by dislocation‐accommodated grain‐boundary sliding (disGBS) has implications for in situ stress and grain size during MOR spreading and implies grain‐size dependent deformation, in contrast to pure dislocation creep.
High-resolution constraints on pacific upper mantle petrofabric inferred from surface-wave anisotropy.

(American Geophysical Union, 2018-12-26) Russell, Joshua B. ; Gaherty, James B. ; Lin, Pei-Ying Patty ; Lizarralde, Daniel ; Collins, John A. ; Hirth, Greg ; Evans, Rob L.

Lithospheric seismic anisotropy illuminates mid‐ocean ridge dynamics and the thermal evolution of oceanic plates. We utilize short‐period (5–7.5 s) ambient‐noise surface waves and 15‐ to 150‐s Rayleigh waves measured across the NoMelt ocean‐bottom array to invert for the complete radial and azimuthal anisotropy in the upper ∼35 km of ∼70‐Ma Pacific lithospheric mantle, and azimuthal anisotropy through the underlying asthenosphere. Strong azimuthal variations in Rayleigh‐ and Love‐wave velocity are observed, including the first clearly measured Love‐wave 2θ and 4θ variations. Inversion of averaged dispersion requires radial anisotropy in the shallow mantle (2‐3%) and the lower crust (4‐5%), with horizontal velocities (VSH) faster than vertical velocities (VSV). Azimuthal anisotropy is strong in the mantle, with 4.5–6% 2θ variation in VSV with fast propagation parallel to the fossil‐spreading direction (FSD), and 2–2.5% 4θ variation in VSH with a fast direction 45° from FSD. The relative behavior of 2θ, 4θ, and radial anisotropy in the mantle are consistent with ophiolite petrofabrics, linking outcrop and surface‐wave length scales. VSV remains fast parallel to FSD to ∼80 km depth where the direction changes, suggesting spreading‐dominated deformation at the ridge. The transition at ∼80 km perhaps marks the dehydration boundary and base of the lithosphere. Azimuthal anisotropy strength increases from the Moho to ∼30 km depth, consistent with flow models of passive upwelling at the ridge. Strong azimuthal anisotropy suggests extremely coherent olivine fabric. Weaker radial anisotropy implies slightly nonhorizontal fabric or the presence of alternative (so‐called E‐type) peridotite fabric. Presence of radial anisotropy in the crust suggests subhorizontal layering and/or shearing during crustal accretion.
Seismicity on the western Greenland Ice Sheet : surface fracture in the vicinity of active moulins

(John Wiley & Sons, 2015-06-25) Carmichael, Joshua D. ; Joughin, Ian ; Behn, Mark D. ; Das, Sarah B. ; King, Matt A. ; Stevens, Laura A. ; Lizarralde, Daniel

We analyzed geophone and GPS measurements collected within the ablation zone of the western Greenland Ice Sheet during a ~35 day period of the 2011 melt season to study changes in ice deformation before, during, and after a supraglacial lake drainage event. During rapid lake drainage, ice flow speeds increased to ~400% of winter values, and icequake activity peaked. At times >7 days after drainage, this seismicity developed variability over both diurnal and longer periods (~10 days), while coincident ice speeds fell to ~150% of winter values and showed nightly peaks in spatial variability. Approximately 95% of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near-surface ice (<330 m deep) that generated Rayleigh waves. Icequakes occurring before and during drainage frequently were collocated with the down flow (west) end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage. We interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage, followed by enhanced seismicity and ice flow on the downstream side of the lake basin. This region collocates with interferometric synthetic aperture radar-measured speedup in previous years and could reflect the migration path of the meltwater supplied to the bed by the lake. The diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction, thereby promoting elevated strain in the surficial ice.
Finite-frequency wave propagation through outer rise fault zones and seismic measurements of upper mantle hydration

(John Wiley & Sons, 2016-08-14) Miller, Nathaniel C. ; Lizarralde, Daniel

Effects of serpentine-filled fault zones on seismic wave propagation in the upper mantle at the outer rise of subduction zones are evaluated using acoustic wave propagation models. Modeled wave speeds depend on azimuth, with slowest speeds in the fault-normal direction. Propagation is fastest along faults, but, for fault widths on the order of the seismic wavelength, apparent wave speeds in this direction depend on frequency. For the 5–12 Hz Pn arrivals used in tomographic studies, joint-parallel wavefronts are slowed by joints. This delay can account for the slowing seen in tomographic images of the outer rise upper mantle. At the Middle America Trench, confining serpentine to fault zones, as opposed to a uniform distribution, reduces estimates of bulk upper mantle hydration from ~3.5 wt % to as low as 0.33 wt % H2O.
Constraints on the depth, thickness, and strength of the G Discontinuity in the Central Pacific from S Receiver Functions

(American Geophysical Union, 2021-03-09) Mark, Hannah F. ; Collins, John A. ; Lizarralde, Daniel ; Hirth, Greg ; Gaherty, James B. ; Evans, Rob L. ; Behn, Mark D.

The relative motion of the lithosphere with respect to the asthenosphere implies the existence of a boundary zone that accommodates shear between the rigid plates and flowing mantle. This shear zone is typically referred to as the lithosphere-asthenosphere boundary (LAB). The width of this zone and the mechanisms accommodating shear across it have important implications for coupling between mantle convection and surface plate motion. Seismic observations have provided evidence for several physical mechanisms that might help enable relative plate motion, but how these mechanisms each contribute to the overall accommodation of shear remains unclear. Here we present receiver function constraints on the discontinuity structure of the oceanic upper mantle at the NoMelt site in the central Pacific, where local constraints on shear velocity, anisotropy, conductivity, and attenuation down to ∼300 km depth provide a comprehensive picture of upper mantle structure. We image a seismic discontinuity with a Vsv decrease of 4.5% or more over a 0–20 km thick gradient layer centered at a depth of ∼65 km. We associate this feature with the Gutenberg discontinuity (G), and interpret our observation of G as resulting from strain localization across a dehydration boundary based on the good agreement between the discontinuity depth and that of the dry solidus. Transitions in Vsv, azimuthal anisotropy, conductivity, and attenuation observed at roughly similar depths suggest that the G discontinuity represents a region of localized strain within a broader zone accommodating shear between the lithosphere and asthenosphere.
Estimating oceanic turbulence dissipation from seismic images

(American Meteorological Society, 2013-08) Holbrook, W. Steven ; Fer, Ilker ; Schmitt, Raymond W. ; Lizarralde, Daniel ; Klymak, Jody M. ; Helfrich, L. Cody ; Kubichek, Robert

Seismic images of oceanic thermohaline finestructure record vertical displacements from internal waves and turbulence over large sections at unprecedented horizontal resolution. Where reflections follow isopycnals, their displacements can be used to estimate levels of turbulence dissipation, by applying the Klymak–Moum slope spectrum method. However, many issues must be considered when using seismic images for estimating turbulence dissipation, especially sources of random and harmonic noise. This study examines the utility of seismic images for estimating turbulence dissipation in the ocean, using synthetic modeling and data from two field surveys, from the South China Sea and the eastern Pacific Ocean, including the first comparison of turbulence estimates from seismic images and from vertical shear. Realistic synthetic models that mimic the spectral characteristics of internal waves and turbulence show that reflector slope spectra accurately reproduce isopycnal slope spectra out to horizontal wavenumbers of 0.04 cpm, corresponding to horizontal wavelengths of 25 m. Using seismic reflector slope spectra requires recognition and suppression of shot-generated harmonic noise and restriction of data to frequency bands with signal-to-noise ratios greater than about 4. Calculation of slope spectra directly from Fourier transforms of the seismic data is necessary to determine the suitability of a particular dataset to turbulence estimation from reflector slope spectra. Turbulence dissipation estimated from seismic reflector displacements compares well to those from 10-m shear determined by coincident expendable current profiler (XCP) data, demonstrating that seismic images can produce reliable estimates of turbulence dissipation in the ocean, provided that random noise is minimal and harmonic noise is removed.
Glacially generated overpressure on the New England continental shelf : integration of full-waveform inversion and overpressure modeling

(John Wiley & Sons, 2014-04-29) Siegel, Jacob ; Lizarralde, Daniel ; Dugan, Brandon ; Person, Mark

Localized zones of high-amplitude, discontinuous seismic reflections 100 km off the coast of Massachusetts, USA, have P wave velocities up to 190 m/s lower than those of adjacent sediments of equal depth (250 m below the sea floor). To investigate the origin of these low-velocity zones, we compare the detailed velocity structure across high-amplitude regions to adjacent, undisturbed regions through full-waveform inversion. We relate the full-waveform inversion velocities to effective stress and overpressure with a power law model. This model predicts localized overpressures up to 2.2 MPa associated with the high-amplitude reflections. To help understand the overpressure source, we model overpressure due to erosion, glacial loading, and sedimentation in one dimension. The modeling results show that ice loading from a late Pleistocene glaciation, ice loading from the Last Glacial Maximum, and rapid sedimentation contributed to the overpressure. Localized overpressure, however, is likely the result of focused fluid flow through a high-permeability layer below the region characterized by the high-amplitude reflections. These high overpressures may have also caused localized sediment deformation. Our forward models predict maximum overpressure during the Last Glacial Maximum due to loading by glaciers and rapid sedimentation, but these overpressures are dissipating in the modern, low sedimentation rate environment. This has important implications for our understanding continental shelf morphology, fluid flow, and submarine groundwater discharge off Massachusetts, as we show a mechanism related to Pleistocene ice sheets that may have created regions of anomalously high overpressure.
Constraints on Appalachian orogenesis and continental rifting in the Southeastern United States from wide-angle seismic data

(American Geophysical Union, 2019-06-24) Marzen, Rachel E. ; Shillington, Donna J. ; Lizarralde, Daniel ; Harder, Steven H.

The Southeastern United States is an ideal location to understand the interactions between mountain building, rifting, and magmatism. Line 2 of the Suwannee suture and Georgia Rift basin refraction seismic experiment in eastern Georgia extends 420 km from the Inner Piedmont to the Georgia coast. We model crustal and upper mantle VP and upper crustal VS. The most dramatic model transition occurs at the Higgins‐Zietz magnetic boundary, north of which we observe higher upper crustal VP and VS and lower VP/VS. These observations support the interpretation of the Higgins‐Zietz boundary as the Alleghanian suture. North of this boundary, we observe a low‐velocity zone less than 2 km thick at ~5‐km depth, consistent with a layer of sheared metasedimentary rocks that forms the Appalachian detachment. To the southeast, we interpret synrift sediments and decreasing crustal thickness to represent crustal thinning associated with the South Georgia Rift Basin and subsequent continental breakup. The correspondence of the northern limit of thinning with the interpreted suture location suggests that the orogenic suture zone and/or the Gondwanan crust to the south of the suture helped localize subsequent extension. Lower crustal VP and VP/VS preclude volumetrically significant mafic magmatic addition during rifting or associated with the Central Atlantic Magmatic Province. Structures formed during orogenesis and/or extension appear to influence seismicity in Georgia today; earthquakes localize along a steeply dipping zone that coincides with the northern edge of the South Georgia Basin and the change in upper crustal velocities at the Higgins‐Zietz boundary.
Along-strike structure of the Costa Rican convergent margin from seismic a refraction/reflection survey : evidence for underplating beneath the inner forearc

(John Wiley & Sons, 2016-02-24) St. Clair, James ; Holbrook, W. Steven ; Van Avendonk, Harm J. A. ; Lizarralde, Daniel

The convergent margin offshore Costa Rica shows evidence of subsidence due to subduction erosion along the outer forearc and relatively high rates of uplift (∼3–6 mm/yr) along the coast. Recently erupted arc lavas exhibit a low 10Be signal, suggesting that although nearly the entire package of incoming sediments enters the subduction zone, very little of that material is carried directly with the downgoing Cocos plate to the magma generating depths of the mantle wedge. One mechanism that would explain both the low 10Be and the coastal uplift is the underplating of sediments, tectonically eroded material, and seamounts beneath the inner forearc. We present results of a 320 km long, trench-parallel seismic reflection and refraction study of the Costa Rican forearc. The primary observations are (1) margin perpendicular faulting of the basement, (2) thickening of the Cocos plate to the northwest, and (3) two weak bands of reflections in the multichannel seismic (MCS) reflection image with travel times similar to the top of the subducting Cocos plate. The modeled depths to these reflections are consistent with an ∼40 km long, 1–3 km thick region of underplated material ∼15 km beneath some of the highest observed coastal uplift rates in Costa Rica.
Limited mantle hydration by bending faults at the Middle America Trench

(American Geophysical Union, 2020-12-15) Miller, Nathaniel C. ; Lizarralde, Daniel ; Collins, John A. ; Holbrook, W. Steven ; Van Avendonk, Harm J. A.

Seismic anisotropy measurements show that upper mantle hydration at the Middle America Trench (MAT) is limited to serpentinization and/or water in fault zones, rather than distributed uniformly. Subduction of hydrated oceanic lithosphere recycles water back into the deep mantle, drives arc volcanism, and affects seismicity at subduction zones. Constraining the extent of upper mantle hydration is an important part of understanding many fundamental processes on Earth. Substantially reduced seismic velocities in tomography suggest that outer rise plate‐bending faults provide a pathway for seawater to rehydrate the slab mantle just prior to subduction. Estimates of outer‐rise hydration based on tomograms vary significantly, with some large enough to imply that, globally, subduction has consumed more than two oceans worth of water during the Phanerozoic. We found that, while the mean upper mantle wavespeed is reduced at the MAT outer rise, the amplitude and orientation of inherited anisotropy are preserved at depths >1 km below the Moho. At shallower depths, relict anisotropy is replaced by slowing in the fault‐normal direction. These observations are incompatible with pervasive hydration but consistent with models of wave propagation through serpentinized fault zones that thin to <100‐m in width at depths >1 km below Moho. Confining hydration to fault zones reduces water storage estimates for the MAT upper mantle from ∼3.5 wt% to <0.9 wt% H20. Since the intermediate thermal structure in the ∼24 Myr‐old MAT slab favors serpentinization, limited hydration suggests that fault mechanics are the limiting factor, not temperatures. Subducting mantle may be similarly dry globally.
Seismic evidence for fluids in fault zones on top of the subducting Cocos Plate beneath Costa Rica

(John Wiley & Sons, 2010-03-09) Van Avendonk, Harm J. A. ; Holbrook, W. Steven ; Lizarralde, Daniel ; Mora, Mauricio M. ; Harder, Steven H. ; Bullock, Andrew D. ; Alvarado, Guillermo E. ; Ramirez, Carlos J.

In the 2005 TICOCAVA explosion seismology study in Costa Rica we observed crustal turning waves with a dominant frequency of ~10 Hz on a linear array of short-period seismometers from the Pacific Ocean to the Caribbean Sea. On one of the shot records, from Shot 21 in the backarc of the Cordillera Central, we also observed two seismic phases with an unusually high dominant frequency (~20 Hz). These two phases were recorded in the forearc region of central Costa Rica and arrived ~7 s apart and 30 to 40 s after the detonation of Shot 21. We considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active-source seismic experiment. Such high-frequency phases following Shot 21 were not recorded after Shots 22, 23, and 24, all in the backarc of Costa Rica, which might suggest that they were produced by some other source. However, earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude. In addition, we would have expected to see more arrivals from such an earthquake on other seismic stations in central Costa Rica. We therefore investigate whether the high-frequency arrivals may be the result of a deep seismic reflection from the subducting Cocos plate. The timing of these phases is consistent with a shear wave from Shot 21 that was reflected as a compressional (SxP) and a shear (SxS) wave at the top of the subducting Cocos slab between 35 and 55 km depth. The shift in dominant frequency from ~10 Hz in the downgoing seismic wave to ~20 Hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle in order to produce a substantial increase in reflection coefficients with frequency. The spectral amplitude characteristics of the SxP and SxS phases from Shot 21 are consistent with a very high Vp/Vs ratio of 6 in ~5 m thick, slab-parallel layers. This result suggests that a system of thin shear zones near the plate interface beneath the forearc is occupied by hydrous fluids under near-lithostatic conditions. The overpressured shear zone probably takes up fluids from the downgoing slab, and it may control the lower limit of the seismogenic zone.

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