Abstract. In contrast to the along-axis uniformity observed at the East Pacific Rise (EPR), crustal accretion at the Mid-Atlantic Ridge (MAR) appears to be a highly complex and heterogeneous process. Besides spreading rate, one of the first-order differences between the EPR and the MAR is the much higher degree of ridge segmentation observed in the Atlantic. Circular lows in the mantle Bouguer anomaly (MBA bull' s-eyes) are common at centers of spreading segments of the MAR, suggesting crustal thickness variations of up to 4 km along individual segments. We use a three-dimensional numerical model of mantle flow to examine the effect of ridge segmentation on mantle upwelling and the resulting overall crustal production and along-axis variations in crustal thickness. Mantle flow in our model is driven by both buoyant forces and segmented plate spreading. Various asthenospheric viscosity structures, plate spreading geometries, and mantle potential temperatures are explored. We find that a combination of buoyant mantle flow and three-dimensional melt migration can reproduce crustal thickness variations similar to those inferred from gravity. Buoyant flow gives rise to variations in upwelling velocity at along-axis wavelengths greater than 150 km but does not contribute to short-wavelength variations. However, three-dimensional melt migration may greatly enhance crustal thickness variations along all segments, independent of the wavelength of buoyant upwelling. We present an idealized model, in which melt first rises vertically and then flows along the base of the lithosphere toward the ridge axis, that easily produces crustal thickness variations greater than 4 km. The models also predict that the average crustal thickness should decrease with increasing amount of segmentation and decreasing spreading rate. Therefore the thinner, more heterogeneous crust observed at the MAR may result from the combined effects of slower spreading rate and more pervasive ridge segmentation.
We report the results of a seismic tomography experiment which images the three-dimensional nature of the crustal melt delivery system beneath a segment of the slow-spreading Mid-Atlantic Ridge. In the lower crust ( s 3.5 km depth) near the segment center, inversion of first-arriving crustal P-waves reveals a pair of vertical pipe-like ( 6 10-kmdiameter) low-velocity anomalies (30.4 km/s). In the upper crust, these two features, which are physically isolated from each other below 3 km, both connect to a 10-km-wide, 45-km-long, axis-parallel, low-velocity zone (30.2 km/s). Three higher-amplitude low-velocity anomalies (30.6 km/s) are observed in the upper crust ( 6 2 km depth), and are located directly beneath seafloor volcanic features. We interpret the overall image to represent the thermal/melt signature of a magma feeding system in which focused injections of magma from the mantle travel upward until they intersect the brittle-ductile transition, where they are then diverted along-axis to supply shallow intrusive bodies and seafloor eruptions along much of the ridge segment. ß
This paper reassesses the crustal and upper mantle contribution to the axial gravity anomaly and isostatic topography observed at two segments (14°S and 17°S) of the southern East Pacific Rise (SEPR) in order to determine what constraints these data place on the amount of melt present in the underlying mantle. Gravity effects due to seafloor topography and relief on the Moho (assuming a constant crustal thickness and density) overpredict the amplitude of the gravity high at the EPR by 8–10 mGal. About 70% of this mantle Bouguer anomaly (MBA) low (6–7 mGal) can be explained by a region of partial melt and elevated temperatures in the mid‐to‐lower crust beneath the rise axis. Compositional density reductions in the mantle due to melt extraction are shown to make a negligible contribution to the amplitude of the observed MBA. Temperature‐related mantle density variations predicted by a simple, plate‐driven, passive flow model with no melt retention can adequately account for the mantle contribution to the observed MBA within the experimental uncertainty (±1 mGal). However, the retention of a small amount of melt (≤1–2% at 14°S; ≤4% at 17°S) in a broad region (tens of kilometers wide) of upwelling mantle is also consistent with the observed gravity data given the uncertainty in crustal thermal models. The anomalous height of the narrow, topographic high at the EPR provides the strongest evidence for the existence of significant melt fractions in the underlying mantle. It is consistent with the presence of a narrow (∼10 km wide) partial melt conduit that extends to depths of 50–70 km with melt concentrations up to 2% higher than the surrounding mantle. Along‐axis variations in mantle melt fraction that might potentially indicate focused upwelling are only marginally resolvable in the gravity data due to uncertainties in crustal thermal models. The good correlation between along‐axis variations in depth, and changes in axial volume and gravity, argue against the mantle melt conduit as being the major source of this along‐axis variation. Instead, this variability can be adequately explained by a combination of along‐axis changes in crustal thermal structure and/or along‐axis crustal thickness changes of a few hundred meters.
The axial zone of the Reykjanes Ridge is covered with small (0.5-3 km in diameter) volcanoes that pile together to form larger axial volcanic ridges. This style of volcanism is similar to that at the Mid-Atlantic Ridge (MAR) and may be common to slow spreading ridges despite proximity of the Reykjanes Ridge to the Iceland hot spot. In this study we quantitatively investigate the population of seamounts in three study areas at the Reykjanes Ridge. Areas A and B are centered at 62øN and 60øN, respectively. Area C is centered at 58øN and is located south of the transition in ridge morphology from an axial high to an axial graben. Using multibeam bathymetry data, 541 seamounts (summit height H > 50 m) were identified in the three areas, and their size and shape statistics were compiled. Additionally, 105 seamounts in areas B and C were recognized in deep-towed side scan images, and their surface morphologies (hummocky or smooth) were recorded. On the basis of estimated population parameters, we find that seamounts at the Reykjanes Ridge are more abundant (310 + 20 per 103 km2), on average, than at the MAR between 24 ø and 30øN (200 + 10 per 103 km2). Significant along-axis variations exist at the Reykjanes Ridge, however, which are not simply related to distance from the hot spot: area B has nearly twice the abundance of seamounts as either area A or area C. Variation in the characteristic height of the seamount population is also observed between the Reykjanes Ridge (68 + 2 m) and the MAR (58 + 2 m), but no significant variation is found between our three study areas. A dramatic change in seamount surface morphology occurs between areas B and C (there are no side scan data from area A). Area C has 78% hummocky seamounts (similar to the proportion observed at the MAR), while area B has 83% smooth seamounts. On the basis of these results, we present a conceptual model for building the shallow crust at the slow spreading Reykjanes Ridge that takes into account the possible influence of the Iceland hot spot on the crustal melt delivery system and its influence on variables that control seamount abundances, sizes, shapes, and surface morphologies. In this model we suggest that the increased seamount production and proliferation of smooth seamounts in area B may be associated with a pulse of hot spot material, in the form of asthenosphere of higher temperature, that has recently affected area B. Paper number 95JB00048. 0148-0227/95/95JB-00048 $05.00 [e.g., Fornari et al., 1987], where the style of volcanism is characterized by low-relief flows [e.g., Macdonald et al., 1989]. The formation of small volcanoes at the axis of the Reykjanes Ridge suggests that the shallow crustal plumbing system is the same as that at other slow spreading ridges. The Reykjanes Ridge is not a typical slow spreading ridge, however. It is located next to the Iceland hot spot, and while small volcanoes are built at the axis as at other slow spreading ridges, the large-scale topography north of 59øN is an axial high similar to that observed at fast...
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