Three‐dimensional mantle upwelling, melt generation, and melt migration beneath segment slow spreading ridges

Abstract

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.

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…Large spatial gradients in the thickness of the axial lithosphere are expected both across and along axis at slow and ultraslow ridges (Figs 13b & 14b) that may act to efficiently focus mantle melts that migrate along the base of the lithosphere (e.g. Sparks & Parmentier 1991;Magde & Sparks 1997). Furthermore, strong positive feedback between pooling of mantle melts and lithospheric structure is expected, and magmatic segmentation could be primarily controlled by spatial variations in lithospheric thickness.…”

Thermal structure of oceanic transform faults

“…These effects result in an increase of as much as ~75% in lithospheric thickness beneath the center of a transform fault relative to a half-space cooling model, as well as signifi cant cooling of the mantle beneath the ends of the adjacent spreading centers. This characteristic ridge-transform thermal structure is invoked to explain the focusing of crustal production toward the centers of ridge segments (e.g., Magde and Sparks, 1997), geochemical evidence for colder upper mantle near segment ends (e.g., Niu and Batiza, 1994), and increased normal fault displacement near segment ends (Shaw and Lin, 1993). However, correlating the maximum depth of earthquakes on transform faults with this colder thermal structure implies that the transition from stable to unstable frictional sliding occurs at ~350 °C, which is inconsistent with both laboratory studies and the depth of intraplate earthquakes.…”

Interactions between mantle plumes and mid-ocean ridges : constraints from geophysics, geochemistry, and geodynamical modeling

“…This ridge geometry is significantly different from that along the western SWIR, where long transform offsets separate relatively short ridge segments. It is possible that much of the variability in MBA around Bouvet reflects segment-scale processes, such as magmatic focusing toward the center of segments [Magde and Sparks, 1997] and thermal effects of transform cooling…”