A Geophysical Comparison between Fast and Slow Spreading Centers: Constraints on Magma Chamber Formation and Hydrothermal Activity

Abstract

While there are many similarities in the geologic structure of various spreading centers, there are some important differences which appear to be related to spreading rate. Taking recent studies of the fast-spreading East Pacific Rise and slow-spreading Mid-Atlantic Ridge, I have compiled a list of properties (summarized in Table 1) which distinguish the two spreading centers. These studies include seismic reflection and refraction, microearthquake studies, gravity and magnetic measurements, electromagnetic sounding, thermal models, observations of hydrothermal activity and geomorphic/tectonic studies. For each of the contrasting properties listed in Table 1, I briefly explain in the text the origin of the observation and emphasize possible limitations and areas of disagreement in the associated interpretation. Many of the important contrasts between fast and slow spreading centers are related to the increased thermal budget at fast spreading rates which allows for the maintenance of a steady state chamber along most of the length of the rise. At slow spreading rates, the axial magma chamber may persist at distances greater than 15–20 km from transform fault intersections, but given the finite width and spacing of transform faults on slow spreading centers, the axial magma chamber may be transient along most of the ridge’s length. The mechanics and deformation of the lithosphere are also affected by the thermal budget as manifested by along strike topographic continuity, transform fault spacing and style of deformation along spreading center offsets. High temperature vents (350°C) are common at intermediate to fast spreading rates, but may be rare occurrences at slow speading rates (with the possible exception of the Reykjanes ridge and other hot-spot influenced ridges). At most slow-spreading ridges, the frequency and duration of such hydrothermal events may not be adequate to sustain the chemosynthetic benthic faunal communities which thrive on faster spreading centers.