Emilie E. E. Hooft

Crustal thickness and structure along three contrasting spreading segments of the Mid‐Atlantic Ridge, 33.5°–35°N

The crustal thickness and crustal and upper mantle structure along the rift valleys of three segments of the northern Mid-Atlantic Ridge with contrasting morphologies and gravity signatures are determined from a seismic refraction study. These segments lie between the Oceanographer and Hayes transforms and from north to south have progressively deeper axial valleys with less along-axis relief and smaller mantle Bouguer gravity lows. Major variations in seismic crustal thickness and crustal velocity and density structure are observed along these segments. The thickest crust is found near the segment centers, with maximum crustal thicknesses of 8.1, 6.9, and 6.6±0.5 km, decreasing from north to south. However, the mean crustal thickness is similar for each segment (5.6±0.4, 5.7±0.4 and 5.1±0.3 km). Near the segment ends, crustal thickness is 2.5 to 5±0.5 km with no systematic variation from north to south. At segment ends, both crustal velocities and vertical velocity gradients are anomalous and may indicate fracturing and alteration of thin igneous crust and underlying mantle. Away from segment ends, the thickness of the upper crust is relatively uniform along axis (∼3 km), although its internal structure is laterally heterogeneous (velocity anomalies of ±0.6 km s−1 over distances of 5 km), possibly related to the presence of discrete volcanic centers. The along-axis crustal thickness variations are primarily accommodated in the lower crust. The center of the northern segment (OH-1) has an unusually thick crustal root (excess thickness of 2–4 km and along-axis extent of 12 km). Our results are consistent with an enhanced supply of melt from the mantle to the segment centers and redistribution of magma along axis at shallow crustal levels by lateral dike injection. Along this portion of the Mid-Atlantic Ridge, our results suggest that differences in axial morphology, seismic crustal thickness, and gravity anomalies are correlated and the result of variations in melt flux from the mantle. A surprising result is that the melt flux per segment length is similar for all three segments despite their different morphologies and gravity signatures. This argues against excess melting of the mantle beneath segment OH-1. Instead, we suggest that the thickened crust at the segment center is a result of focusing of melt, possibly due to the influence of the thermal structure of the Oceanographer fracture zone on melt migration in the mantle.

Seismic structure and indicators of magma budget along the Southern East Pacific Rise

In this paper we re-examine the relationship between seismically constrained variations in crustal structure along the southern East Pacific Rise (SEPR) and the segment-scale variations in axial depth, morphology, basalt geochemistry, and hydrothermal activity that have often been attributed to along-axis differences in the supply of magma to the mid-ocean ridge. Along >800 km of the fast spreading SEPR, good correlations exist between axial depth, ridge cross-sectional area, mantle Bouguer anomaly, and the MgO weight percent of basalts recovered from the rise axis. These correlations indicate along-axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of ∼100,000 years. In contrast, we show that the depth and width of the midcrustal magma sill, the thickness of seismic layer 2A, and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross-sectional area. We suggest that the emplacement geometry (width of the intrusion zone and flow lengths), not magma supply, controls extrusive layer (seismic layer 2A) thickness. We hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events (dike intrusion, eruptions, faulting) which occur on much shorter timescales (∼10–100 years) than the longer-term variations in magma supply reflected in along-axis changes in the shape and depth of the ridge axis.

The role of density in the accumulation of basaltic melts at mid‐ocean ridges

It is commonly assumed that magma ponds at a level of neutral buoyancy in the shallow crust where melt densities are equal to the bulk density of the surrounding crust. At the East Pacific Rise this neutral buoyancy level lies only 100–400 m below the sea floor, significantly shallower than the depths (> 1–2 km) of the magma bodies imaged in multichannel reflection data, suggesting that other factors must control the collection of melt in these reservoirs. The apparent inverse relationship between magma chamber depth and spreading rate at intermediate and fast spreading ridges suggests that the thermal structure of the rise axis, not the buoyancy of melt, is the primary factor that controls the depth at which melt ponds in crustal magma chambers beneath mid-ocean ridges.

Anomalously thin transition zone beneath the Galápagos hotspot

Differential arrival times of P-to-S conversions at 410 and 660 km depth, measured from radial receiver functions, are used to determine the thickness of the mantle transition zone beneath the Galapagos hotspot. For an area of approximately 700 km 2 surrounding the Galapagos archipelago, P660s3P410s times are not significantly different from those for the Pacific Basin. In contrast, a subset of the Galapagos data yields differential times indicating thinning of the mantle transition zone by 18 3 8 km within an area approximately 100 km in radius centered about 40 km southwest of the center of the island of Fernandina. This anomaly is consistent with an excess temperature of 130 3 60 K within this volume of the transition zone, comparable to that inferred beneath the Iceland and Society hotspots. The most straightforward interpretation of this anomaly is that a mantle plume upwelling from depths greater than 410 km underlies the Galapagos hotspot.

Upper mantle structure beneath the Galápagos Archipelago from surface wave tomography

(1) We present a Rayleigh wave tomographic study of the upper mantle beneath the Galapagos Archipelago. We analyze waves in 12 separate frequency bands (8-50 mHz) sensitive to shear wave velocity (VS) structure in the upper 150 km. Average phase velocities are up to 2 and 8% lower than for 0- to 4-My-old and 4- to 20-My-old Pacific seafloor, respectively. Laterally averaged VS is 0.05-0.2 km/s lower between 75- and 150-km depth than for normal Pacific mantle of comparable age, corresponding to an excess temperature of 30 to 150C and � 0.5% melt. A continuous low-velocity volume that tilts in a northerly direction as it shoals extends from the bottom of our model to the base of a high-velocity lid, which is located at depths varying from 40 to 70 km. We interpret this low-velocity volume as an upwelling thermal plume that flattens against the base of the high-velocity lid. The high-velocity lid is � 30 km thicker than estimated lithospheric thickness beneath the southwestern archipelago, above the main region of plume upwelling. We attribute the thicker-than-normal high-velocity lid to residuum from hot spot melting. The thickness of the lid appears to control the final depth of melting and the variability of basalt composition in the archipelago. At depths less than 100-120 km, plume material spreads in directions both toward and against eastward plate motion, indicating that plume buoyancy forces dominate over plate drag forces and suggesting a relatively high plume buoyancy flux (B � 2000 kg/s).

Relationship between axial morphology, crustal thickness, and mantle temperature along the Juan de Fuca and Gorda Ridges

We analyze gravity data over the Juan de Fuca and northern Gorda Ridges to understand the lithospheric structure of two ridges with contrasting axial morphologies spreading at the same intermediate rate (28 mm/yr half rate). The Cleft Segment, at the south end of the Juan de Fuca Ridge, has an axial high morphology while the northern segment of the Gorda Ridge has a rift valley. Residual mantle Bouguer anomalies (RMBA) on the northern Gorda Ridge are elevated relative to the Cleft Segment by 10–20 mGal, indicating thinner crust and/or a colder mantle. The minimum value (−50 mGal) of the RMBA along the Juan de Fuca Ridge is over Axial Seamount and gradually increases south toward the Blanco Transform. The observed RMBA are interpreted to result from along axis variations in crustal thickness and mantle density, both of which are controlled by temperatures in the upper mantle where decompression melting occurs. We estimate that mantle temperatures are elevated by 30°–40°C beneath Axial Seamount, resulting in an excess crustal thickness of ∼1.7 km. The Cleft Segment is associated with crust that is estimated to be only 300–700 m thicker, and mantle temperatures are only 10°–15°C higher than beneath the northern Gorda Ridge. However, even these small differences in crustal thickness and mantle temperature appear to be sufficient to produce a major change in lithospheric strength and axial morphology. These results are consistent with the predicted sensitivity of recent thermo-mechanical models for rift valley formation to small changes in crustal thickness and mantle temperature at intermediate spreading rates. We attribute the systematic differences in axial morphology, crustal thickness, mantle temperature, and lithospheric strength along the Juan de Fuca/Gorda ridge system to the presence of the Cobb thermal anomaly at Axial Seamount. The hotter mantle beneath the Juan de Fuca Ridge results in greater amounts of decompression melting and the formation of a thicker crust and a thinner, weaker lithosphere than along the Gorda Ridge.

Asymmetric plume‐ridge interaction around Iceland: The Kolbeinsey Ridge Iceland Seismic Experiment

We present the results of a seismic refraction experiment that constrains crustal structure and thickness along 225 km of the Kolbeinsey Ridge and Tjornes Fracture Zone and thus quantifies the influence of the Iceland hot spot on melt flux at the spreading center north of Iceland. North of the Iceland shelf, crustal thickness is relatively constant over 75 km, 9.4 ± 0.2 km. Along the southern portion of the Kolbeinsey Ridge, on the Iceland shelf, crustal thickness increases from 9.5 ± 0.1 km to 12.1 ± 0.4 km over 90 km. Gravity inversion indicates a residual crustal gravity anomaly that decreases by about 30–40 mGal toward Iceland. We infer that the variations in crustal thickness and gravity are accompanied by mantle temperature changes of 40° to 50°C. At similar distances from the Iceland hot spot, crustal thickness along the Kolbeinsey Ridge is 2–2.5 km less than at the Reykjanes Ridge, consistent with the asymmetry in plume-ridge interaction that has been inferred from the axial depth and geochemistry of these ridges. Average lower crustal velocities are also higher along the Kolbeinsey Ridge, consistent with a lower degree of active upwelling than along the Reykjanes Ridge. Topography and crustal thickness patterns at the spreading centers around Iceland are consistent with isostatic support for normal crustal and mantle densities. However, we infer that the lower crust beneath central Iceland is considerably denser than that beneath the adjacent ridges. Crustal thickness and geochemical patterns suggest that deep melting is spatially limited and asymmetric about Iceland while shallow melting is enhanced over a broad region. This asymmetry may be due to a mantle plume that is tilted from south to north in the upper mantle and preferentially melts deeper enriched material beneath the Reykjanes Ridge.

Shear-wave splitting beneath the Galápagos archipelago

Shear-wave splitting measurements in the Galapagos archipelago show a rapid change from consistently oriented anisotropy to no measurable anisotropy. At the western edge of the archipelago delay times are 0.4– 0.9 s and fast polarization directions are 81– 109°E. These directions are consistent with anisotropy resulting from shear of the asthenosphere by the overlying plate; there is no indication of fossil lithospheric anisotropy in the plate spreading direction. In contrast, beneath the center of the archipelago the upper mantle is isotropic or weakly anisotropic. The isotropic region coincides approximately with a volume of anomalously low upper mantle velocities, suggesting that the presence of melt may weaken the effects of fabric on anisotropy or that melt preferred orientation generates a vertical fast polarization direction. Alternatively, the complex flow field associated with a near-ridge hotspot may result in apparent isotropy.

Upper crustal seismic structure of the Endeavour segment, Juan de Fuca Ridge from traveltime tomography: Implications for oceanic crustal accretion

The isotropic and anisotropic P wave velocity structure of the upper oceanic crust on the Endeavour segment of the Juan de Fuca Ridge is studied using refracted traveltime data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the segment ends. These low velocities are indicative of pervasive tectonic fracturing and persist off axis, recording the history of ridge propagation. Near the segment center, velocities within the upper 1 km show ridge-parallel bands with low velocities on the outer flanks of topographic highs. These features are consistent with localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault-tilted blocks that form the abyssal hill topography. On-axis velocities are generally relatively high beneath the hydrothermal vent fields likely due to the infilling of porosity by mineral precipitation. Lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures. Seismic anisotropy is high on-axis but decreases substantially off axis over 5–10 km (0.2–0.4 Ma). This decrease coincides with an increase in seismic velocities resolved at depths ≥1 km and is attributed to the infilling of cracks by mineral precipitation associated with near-axis hydrothermal circulation. The orientation of the fast-axis of anisotropy is ridge-parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the OSCs.

Seismic anisotropy beneath the Juan de Fuca plate system: Evidence for heterogeneous mantle flow

Here we use SKS shear wave splitting observations from ocean-bottom seismometer data to infer patterns of mantle deformation beneath the Juan de Fuca plate and its adjoining boundaries. Our results indicate that the asthenosphere beneath the Juan de Fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone. Geographically restricted deviations from this pattern indicate the presence of secondary processes. At discrete plate boundaries, such as the Blanco transform fault, seismic anisotropy is attributed to relative plate motion within a narrow zone (<50 km). Beneath the deforming southern Gorda plate region—a diffuse plate boundary—splitting observations similarly suggest deformation dominated by relative motion between the rigid Juan de Fuca and Pacific plates but distributed over a broad zone (∼200 km). Our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback, as suggested by onshore studies. Instead, reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern Cascadia.