Base Of Layer 2A Research Articles

Nature of upper crust beneath the Lucky Strike volcano using elastic full waveform inversion of streamer data

Seismic full waveform is an emerging technique for determining the fine-scale velocity structure of the subsurface. Here, we present results of elastic full waveform inversion (FWI) along three multichannel seismic lines at the Lucky Strike volcano on the Mid-Atlantic ridge that provides a velocity image of the upper oceanic crust with unprecedented resolution (50–100 m). We have used a two-step process combining downward continuation with a time-domain, elastic FWI. The downward continuation procedure enhances the refracted arrivals and wide-angle reflections, and reduces the scattering noise due to rough seafloor. Since both sources and receivers are downward continued to the seafloor, the computational cost of FWI is reduced, as one does not need to model the thick water layer. Our results clearly demarcate two layers within seismic Layer 2A; a low-velocity, highly heterogeneous layer likely reflecting the complexity of accretion that is underlain by a more homogeneous high-velocity gradient layer. The base of Layer 2A is defined as a lithological boundary that can be offset by faulting. Thick (>400 m) units of anomalously low-velocity material (<2.5 km s−1) beneath different summital edifices on the central volcano indicate that a thick pile of high-porosity extrusive rocks can be supported without collapsing, suggesting that while in general there is pore closure with depth this is not the cause of high velocities we observe. Hydrothermal deposition sealing of small-scale porosity is shown to be a secondary process, which likely explains the upper crustal velocity increase with age, but is not responsible for the high-velocity gradient Layer 2A. Finally, the rapid thinning of the entire Layer 2A in the vicinity of the main normal faults suggests the tectonic thinning of a geologically defined layer, further confirming the lithological origin of the high-velocity gradient zone at the base of seismic Layer 2A.

Shallow oceanic crust: Full waveform tomographic images of the seismic layer 2A/2B boundary

We present results of full‐waveform tomographic inversions of four profiles acquired over young intermediate‐ and fast spreading rate oceanic crust. The mean velocity‐depth functions from our study include a 0.25–0.30 km‐thick low‐velocity, low‐gradient region beneath the seafloor overlying a 0.24–0.28‐km‐thick high‐gradient region; together these regions compose seismic layer 2A. Mean layer 2A interval velocities are 3.0–3.2 km/s. The mean depth to the layer 2A/2B boundary is 0.49–0.54 km, and mean velocities within the upper 0.25 km of layer 2B are 4.7–4.9 km/s. Previous velocity analyses of the study areas using 1‐D ray tracing underestimate the thickness of the high‐gradient region at the base of layer 2A. We observe differences in the waveform inversion velocity models that correspond to imaging of the layer 2A event; regions with a layer 2A event have higher velocity gradients at the base of layer 2A. Intermittent high velocities, which we interpret as massive flows, are observed in the waveform inversion velocity models at 0.05–0.10 km below the seafloor (bsf) over 10–25% of the intermediate‐spreading profiles and 20–45% of the fast spreading profiles. The high‐gradient region located 0.25–0.54 km bsf at the base of layer 2A may be associated with an increased prevalence of massive flows, the first appearance of dikes (lava‐dike transition zone), or with increased crack sealing by hydrothermal products. The upper portion of layer 2B, which begins at 0.49–0.54 km bsf, may correspond to sheeted dikes or the top of the transition zone of lavas and dikes.

Central anomaly magnetization high: constraints on the volcanic construction and architecture of seismic layer 2A at a fast-spreading mid-ocean ridge, the EPR at 9°30′–50′N

The central anomaly magnetization high (CAMH) is a zone of high crustal magnetization centered on the axis of the East Pacific Rise (EPR) and many other segments of the global mid-ocean ridge (MOR). The CAMH is thought to reflect the presence of recently emplaced and highly magnetic lavas. Forward models show that the complicated character of the near-bottom CAMH can be successfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava magnetization intensity with age. This lava magnetization function is the product of geomagnetic paleofield intensity, which has increased by a factor of 2 over the last 40 kyr, and low-temperature alteration which decreases the remanence of lava with exposure to seawater. The success of the forward modeling justifies the inverse approach: deconvolution of the magnetic data for lava distribution and integration of that distribution for magnetic layer thickness. This approach is tested on two near-bottom magnetic profiles AL2767 and AL2771, collected using Alvin across the EPR axis at 9°31′N and 9°50′N. Our analysis of these data produces an estimate of the relative thickness of the magnetic lava layer which is remarkably consistent with existing multichannel estimates of layer 2A thickness from lines CDP31 and CDP27. The similarity between magnetic layer and seismic layer 2A at the 9°–10°N segment of the EPR crest provides independent support to the notion that seismic layer 2A in young oceanic crust represents the highly magnetic lava layer, and that the velocity gradient at the base of layer 2A is related to the increasing number of higher-velocity dikes with depth in the lava–dike transition zone. The near-bottom magnetic anomaly character of the CAMH is a powerful indicator of the emplacement history of upper crust at MORs which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraints.

Crustal accretion at the Reykjanes Ridge, 61°–62°N

We report results of a seismic, gravity, and magnetic survey of the Reykjanes Ridge spreading center at 61°–62°N, about 600 km from the center of the Iceland mantle plume. Anomalously shallow water on the ridge crest enabled us to record seismic refractions on a 2.4 km hydrophone streamer. The velocity within layer 2A is 2.4±0.3 km s−1, and its mean thickness is 400±100 m. The velocity at the base of layer 2A is 3.3±0.3 km s−1 on the ridge axis, increasing with crustal age to ∼4.0 km s−1 at 1.5 Ma and ∼4.5 km s−1 at 5 Ma. Assuming that seismic layer 2A on the ridge axis is also the extrusive layer, i.e., the magnetic source layer, we have successfully modeled the variations in amplitude of the magnetic field. The best magnetic model includes enhanced magnetization within layer 2A at the sites of recent volcanic activity as independently recognized in side‐scan sonar data. We also present a full crustal seismic model, based on wide‐angle seismic recordings on digital ocean bottom hydrophones and disposable sonobuoys. The seismic model is complemented by gravity modeling, which further suggests that the ridge crest is in isostatic equilibrium. The zero age crust is 10.0 km thick, while crust of age 5 Ma is 7.8 km thick. These crustal thicknesses are greater than those of normal oceanic crust, which we attribute to the presence of anomalously hot asthenospheric mantle beneath the spreading center. We suggest that the variation in thickness between 0 Ma and 5 Ma crust is caused by temporal variation in the plume‐fed asthenospheric temperature beneath the Reykjanes Ridge.

Shear and compressional wave structure of the East Pacific Rise, 9°–10°N

We use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9°–10°N region of the East Pacific Rise. Modeling indicates that shear conversion occurs at the base of layer 2A, producing Ps (energy that travels through oceanic crust as P and converts to S at the base of layer 2A on upgoing path) and pS (energy that travels as P through layer 2A and converts to S at the base of layer 2A on downgoing path) arrivals. The travel times of these arrivals require a shear wave velocity within layer 2A of 0.4–0.8 km/s (Poisson's ratio of 0.46–0.49). Waveform inversion was used to model the Poisson's ratio structure of layers 2B and 3 for nine instruments with good shear arrivals. Poisson's ratio within layer 2B was highly variable, with values as low as 0.24 and a mean value of 0.263. Some of this variability might be due to lateral variability in layer 2A structure. The mean Poisson's ratio of layer 3 was 0.271. According to the cracking model of Shaw [1994], the low Poisson's ratios within the upper portion of layer 2B indicate that thick cracks (aspect ratio α = 0.1) extend to depths of ∼ 1.5–1.7 km in this region. Two‐dimensional travel time modeling indicates that the southwestern portion of our study area is associated with anomalously low seismic velocities, with layer 2B and layer 3 velocities reduced by up to 11% from the regional values. Cracking theory suggests that these low velocities could be caused by porosities of 0.3–5.5%, depending on the crack aspect ratio; a maximum porosity of 1.5% is predicted from our one Poisson's ratio measurement in this area. The anomalous velocities are located near the western discordant zone left by the 9°03′N overlapping spreading center. We suggest that shearing associated with on‐axis rotation of the overlap basins is responsible for the low seismic velocities. The pattern of anomalies indicates that faulting extends to distances of 10–15 km from the basins.

Extrusive thickness variability at the East Pacific Rise, 9°–10°N: Constraints from seismic techniques

We calculate synthetic shot gathers and their corresponding common depth point (CDP) profiles over plausible East Pacific Rise (EPR) shallow velocity structures, based on the structures obtained from high‐resolution on‐bottom seismic refraction experiments. We then use these results to analyze the variability in layer 2A thickness at the EPR 9°–10°N region, as measured by CDP, wide‐aperture profile (WAP), on‐bottom seismic refraction experiments, and conventional air gun refraction data. The synthetics indicate that the accuracy of correlating the prominent shallow reflector observed in CDP and wide‐angle data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low‐velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two‐way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may originate from a gradient within layer 2A if the upper crust contains more than one high‐gradient region (our step model). This implies that independent estimates of layer 2A velocity structure are needed to properly interpret CDP and wide‐angle data. We also determine that the travel time to the layer 2A reflector, for identical velocity structure, can vary by as much as 50 ms (about 125 m) for differing experimental geometries. This can explain the discrepancy in two‐way travel time to the layer 2A reflector imaged on zero‐age CDP and WAP lines. The depths to a shallow reflector calculated from CDP and wide‐angle data in the 9°–10°N region of the EPR generally correlate with estimated layer 2A thicknesses from on‐bottom refraction profiles and conventional air gun refraction lines, which suggests that the upper crustal structure in this area is similar to the gradient model. WAP and conventional air gun refraction data indicate that there is a 100–200 m decrease in off‐axis layer 2A thickness at 9°35′N on the EPR, the present‐day location of a deviation in axial linearity (deval). There is no bathymetric expression of the 50% decrease in layer 2A thickness. Layer 2A can be interpreted to consist of the extrusive section and transition zone, with the layer 2A/2B boundary corresponding to the top of the sheeted dikes. We suggest that buoyancy forces associated with the axial‐magma chamber (AMC) are supporting the extrusive layer and sheeted dikes at the neovolcanic zone. With distance from the rise axis, the AMC solidifies, the crust cools, the buoyancy forces are reduced, and the sheeted dike complex subsides. Concurrently, the extrusive layer thickens, resulting in significantly less subsidence of the seafloor. We speculate that the 50% decrease in dike subsidence and extrusive thickness at the 9°35′N deval is due to a local reduction in magma supply within the axial magma chamber. The off‐axis pattern of layer 2A thickness suggests that the 9°35′N deval has persisted for 175,000–275,000 years.

Distribution of magma beneath the East Pacific Rise between the Clipperton Transform and the 9°17′N Deval from forward modeling of common depth point data

We have reprocessed seven cross‐axis common depth point (CDP) profiles between the Clipperton transform and the 9°17′N Deval (deviation in axial linearity) on the East Pacific Rise (EPR) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Forward modeling of cross‐axis CDP profiles suggests a segmented AMC in which significant variations in width occur across minor rise axis discontinuities (e.g., Devals). The modeled rise segment widths bounded by the 9°53′N‐9°35′N Devals, the 9°35′N‐9°17′N Devals, and south of the 9°17′N Deval were &lt; 0.7 km, 1.0–1.2 km, and 4.15 km, respectively. Transition in AMC width across these discontinuities is unclear due to the sparseness of cross‐axis line spacing; however, a simple association of Devals with decreased magma supply is doubtful: the minimum (250 m) and maximum (4150 m) AMC widths are found near the 9°35′N and 9°17′N Devals, respectively. The reprocessing of CDP profiles included repicking stacking velocities to ensure a proper stack of the AMC reflector and its associated diffractions, imaging postcritical reflections from the base of layer 2A, finite difference time migration, ray theoretical depth migration, and travel time modeling of AMC diffraction patterns. Constraints on AMC width were derived from forward modeling techniques based on analytic raytracing. Velocity models were constructed from SeaBeam bathymetry and modified expanding spread profile (ESP) velocity‐depth functions. ESP velocity models were altered to compensate for off‐axis thickening of layer 2A as imaged in the CDP reflection data. Continuous two‐dimensional velocity models constructed from modified ESP velocity‐depth functions and SeaBeam bathymetry should account for ray bending at the seafloor/basalt interface and any lateral velocity gradients induced by a thickening layer 2A. Stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration. Reflector positions were input into our forward modeling scheme to produce a zero‐offset travel time response of our migrated solution. The travel time response was then overlain on the stacked section to ensure an adequate match, especially to diffractions generated at the AMC edge. Forward modeling of AMC diffraction patterns reveals that original AMC width estimates were too large. The under‐migration of the AMC reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual CMP gathers, thus resulting in improperly collapsed diffraction hyperbolae. Ship wandering relative to the AMC edge can account for variations in AMC reflector amplitude and dropout on the along‐axis CDP line. The continuity of the AMC appears unbroken across several ridge axis discontinuities between the Clipperton transform and the 9°17′N Deval which suggests an AMC whose along‐axis dimension exceeds 75 km. Reflectivity modeling of CMP gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush. Such a thin layer of melt might inhibit along‐axis mixing of magmas and thereby account for variations in magma composition along the rise crest. This melt lens model for the AMC would also produce strong diffraction patterns as imaged in the CDP data.

Distribution of magma beneath the East Pacific Rise near the 9°03′N overlapping spreading center from forward modeling of common depth point data

We have reprocessed six cross‐axis and three along‐axis common depth point (CDP) profiles near the 9°03′N overlapping spreading center (OSC) to understand the relationship between axial magma chamber (AMC) width and seafloor morphology. Travel time modeling of the AMC reflector reveals an asymmetric distribution of melt across the 9°03′N OSC. The variation of modeled AMC width beneath either OSC limb is minimal, but the width increases nearly fourfold across the offset attaining an estimated maximum width of 4.15 km near the 9°17′N ridge axis discontinuity. Additionally, melt distribution underlying the eastern rise limb is not symmetric with respect to the rise axis/neovolcanic zone but is displaced toward the western rise flank. Depth migration, based on a continuum velocity model consistent with postcritical reflections from the base of layer 2A, places the skewed AMC reflector beneath a nearly constant thickness sheeted dike section which dips ∼ 10° away from the rise axis. To confirm AMC continuity beneath the western rise flank, we use the Maslov synthetic seismogram method to show that amplitude enhancement of the AMC reflector is consistent with a continuous melt body underlying a thickening extrusive layer. Analysis of along‐strike CDP profiles indicates an AMC which is neither overlapping nor discontinuous when projected onto the along‐strike plane. Identifying intracrustal events on along‐axis CDP lines, however, requires extreme caution; we have modeled out‐of‐plane scattering using a Kirchhoff formulation, and we show that a coherent event identified beneath the overlap basin results from diffraction off the AMC which lies nearly 3 km to the west of the profile. We attribute the asymmetric pattern of melt to a decoupling of melt supply from preexisting weaknesses in the brittle upper crust. In this model, melt ascends upward (buoyancy forces) until deflected by the impermeable sheeted dike complex; melt then migrates updip, beneath the base of the sheeted dikes, toward the neovplcanic zone where fissuring produces a temporary conduit for emplacement. Discrete jumps in modeled AMC width toward the overlap basin represent a further displacement/defocusing of melt supply (western AMC edge) relative to the neovolcanic zone (eastern AMC edge). The asymmetric pattern of melt therefore represents a gradual, en‐echelon accommodation of melt supply across the 9 km of ridge axis offset at 9°03′N. Thus for asymmetric configurations, AMC width may not correlate solely with magmatic robustness but may signify the amount of decoupling which exists between melt supply and extrusive emplacement within the neovolcanic zone. Here we present a new model for OSC development which invokes a significant component of cross‐axis melt migration. Moreover, abrupt changes in AMC width near ridge axis discontinuities (e.g., 9°17′N deviation in axial linearity) suggest that any along‐axis melt migration is confined to subsegments of the ridge and seem to preclude the segment length migration of melt proposed in some current models. The transition of melt supply beneath the overlap basin might favor a continuous low‐velocity zone underlying this feature; if true, basin development may be related to the subsidence of a mechanically weak crustal lid. The proposed model for OSC development therefore views ridge axis discontinuities as the surficial response of misalignment and/or defocusing of melt supply in the uppermost mantle.

Upper crustal structure as a function of plate age

Each Lamont-Doherty sonobuoy located on well-dated crust has been carefully analyzed to determine crustal structure down to oceanic layer 3. Results from the Atlantic and the Pacific are compiled separately in order to study crustal structure as a function of plate age in both oceans, since they have very different spreading rates. Layer 2A (refraction velocity about 3.6 km/s) in the North Atlantic is 1.5 km thick at the ridge crest and thins consistently to about 100 m as the crust ages to about 60 m.y. Layer 2A in the east Pacific is 0.7 km thick at the ridge and thins to about 100 m at about 30 m.y. This difference in thickness is probably attributable to the much faster spreading rate in the Pacific. A poorly refractive acoustic basement layer about 200 m thick with similarities to layer 2A but not necessarily composed of the same materials is measured sporadically in the Pacific M-Series plates and even less consistently in the Atlantic. This layer is not recorded in the Cretaceous or the Jurassic quiet zones. Regressions of refraction velocities in layer 2A as a function of age show that its velocity increases from about 3.3 km/s at the ridge crests to that of layer 2B on crust about 40 m.y. old. There is no corresponding increase of velocity with age in any of the deeper layers. The high resolution of the air gun/sonobuoy records shows that the layer 2B refraction line breaks directly to layer 3 velocities (46 times in the present work) or to a line with a velocity of 6.1 km/s (114 times in the present work), which we call layer 2C. The variance of the velocities in 2C is one fourth that of 2A and 2B, which indicates relative lithological uniformity in 2C. It does not seem likely that layer 2A really thins; what appears to be a thinning of the layer may actually be the result of an increase of its refraction velocity with age. However, the 2A/2B interface is well-defined by large amplitude refractions from 2B on crust that is younger than about 30 m.y., which seems to rule out a transitional zone at the base of layer 2A where it ‘converts’ to 2B. The seismic observations seem to require a diagenetic process or repeated basaltic intrusions; both processes raise serious objections.