A reflection observed on multi-channel seismic profiles along and across the East Pacific Rise between 8°50′ N and 13°30′ N is interpreted to arise from the top of a crustal magma chamber located 1.2–2.4 km below the sea floor. The magma chamber is quite narrow (<4 – 6 km wide), but can be traced as a nearly continuous feature for tens of kilometres along the rise axis.

Multi-channel seismic imaging of a crustal magma chamber along the East Pacific Rise

Geophysical evidence precludes the existence of a large, mainly molten magma chamber beneath portions of the East Pacific Rise (EPR). A reasonable model, consistent with these data, involves a thin (tens to hundreds of meters high), narrow (<1-2 km wide) melt lens overlying a zone of ctystal mush that is in turn surrounded by a transition zone of mostly solidified crust with isolated pockets of magma. Evidence from the superfast spreading portion of the EPR suggests that the composition of the melt lens is mainly moderately fractionated ferrobasalt

Mid-ocean ridge magna chambers

Three-dimensional images of crustal seismic structure beneath the East Pacific Rise show pronounced axial heterogeneity over distances of a few kilometres. A linear high-velocity anomaly, approximately 1–2 km in width and restricted to the uppermost 1 km of the crust, is centred on the rise axis. An axial low-velocity anomaly at depths of 1–3 km varies in amplitude along the axis, consistent with a zone of higher crustal temperatures midway between two discontinuities in the morphology of the rise axis. This apparent thermal segmentation along axis is consistent with injection of mantle-derived melt midway along a locally linear, 12-km-long segment of the rise.

The three-dimensional seismic velocity structure of the East Pacific Rise near latitude 9° 30′ N

We present the results of the analysis of expanding spread profiles (ESPs) collected on and near the axis of the East Pacific Rise at 13°N. These profiles were collected at 0, 1.1, 2.1, 3.6, and 9.5 km from the rise axis, and all but the most distant profile show a distinct low-velocity zone (LVZ) located within layer 3 of the oceanic crust. At the ridge crest, the top of the magma chamber is at the base of layer 2, while 3.6 km off axis, the roof of the LVZ is 1.1 km below the top of layer 3. The profile farthest from the ridge could possibly have a residual LVZ confined to the lower 1.5 km of the crust. The total width of the LVZ, as determined from the ESP data, is at least 6 km, and possibly much greater. This wide LVZ apparently contradicts multichannel seismic data which show cross-axis reflections from the magma chamber with a width of <5 km. We suggest that a resolution of this apparent contradiction lies in a model of the rise axis with a small and transient central magma chamber of high partial melt fraction surrounded by a much larger and permanent region of hot rock with only isolated pockets of partial melt. The ESP data are sensitive to this larger region, while the reflection data accurately map the presence or absence of the central magma chamber with its high impedance contrast. We identify the presence of a layer at the top of the oceanic crust with initial P wave velocities between 2.35 and 2.6 km/s, while the S wave velocity is estimated as being ≤0.8 km/s. The layer thickness lies between 100 and 200 m. These velocities are consistent with previous estimates for the Pacific and recent results for the Atlantic. The thickness of this layer is consistent with that of layer 2A determined from geophysical measurements at Deep Sea Drilling Project hole 504B.

Structure of young oceanic crust at 13°N on the East Pacific Rise from expanding spread profiles

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 < 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 between the Clipperton Transform and the 9°17′N Deval from forward modeling of common depth point data

Computer-processed multichannel seismic reflection profiles from the crestal zone of the East Pacific Rise near the Siqueiros Fracture Zone have revealed distinct layering in the structure not previously observed in single-channel reflection records. After processing, which included common depth point gathering, normal move-out and stack, time-varying predictive deconvolution, and wave equation migration, three distinct crustal layers emerge as coherent events across the rise crest. The base of the third layer appears to be at a depth of about 2 km below the sea floor and may represent the top of a low-velocity magma chamber, as postulated by Orcutt et al. (1975, 1976) from the results of ocean bottom seismometer refraction data. This layer appears to thicken away from the rise crest toward the edge of the raised axial block that characterizes the East Pacific Rise (Anderson and Noltimier, 1973). Illustrated in this article is the ability of modern processing techniques as applied to multichannel seismic data to enhance weak arrivals, to minimize sound source bubble-pulse reverberations, and to remove the diffraction patterns caused by rough topography.

Structure of the East Pacific Rise crest from multichannel seismic reflection data

A multichannel seismic reflection profile of stacked and migrated common depth point data across the East Pacific Rise near the Siqueiros Fracture Zone supports and extends previous observations (at two crossings 27 to 50 km to the south) of reflections assumed to be from the top of a magma chamber and of reflections from the M-discontinuity. The reflection assumed to be associated with the top of the magma chamber is 1-1/2 to 2 km below the sea floor. The combined results of the three crossings suggest that the chamber is continuous along the Rise crest and that its width, which varies from 2 to 8 km, correlates with the width of the Rise crest as indicated by the bathymetric contours. The reflections from the crust-mantle boundary can be detected beneath the raised axial block of the East Pacific Rise and although weak, can be detected beneath the magma chamber.

Magma chamber and mantle reflections - East Pacific Rise

A multichannel seismic reflection profile across the East Pacific Rise near 9°N reveals an approximately 400 m thick layer at the top of the oceanic crust with seismic velocity of about 2.8 km/s at the Rise crest. This velocity increases to about 3.8 km/s, 210 km east of the Rise crest. These low velocities suggest that this layer is probably a porous lava flow layer (layer 2A of seismic refraction studies) generated by volcanic activity at the spreading center. Reflections from the base of the layer imply a relatively sharp transition between the lava flows and the underlying layer (presumably a sheeted dike complex). The underlying layer is a zone about 1.4 km thick (with velocity about 5.7 km/s) whose base may coincide with the top of a low velocity magma chamber from which the crust develops.

Lava flow layer‐East Pacific Rise

Mantle reflections beneath the crestal zone of the East Pacific Rise from multi-channel seismic data

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Structure of the East Pacific Rise crest from multichannel seismic reflection data

Magma chamber and mantle reflections - East Pacific Rise

Lava flow layer‐East Pacific Rise

Mantle reflections beneath the crestal zone of the East Pacific Rise from multi-channel seismic data