Stefan Hussenoeder

Seismic analysis of the axial magma chamber reflector along the southern East Pacific Rise from conventional reflection profiling

The thickness and internal properties of the magma sill located at the top of the axial magma chamber (AMC) along the southern East Pacific Rise (EPR) have been investigated through a combination of waveform modeling the near-vertical incidence reflections from this body and analysis of reflection amplitude variation as a function of source-receiver offset (or slowness). Our results show that the AMC reflector observed along the southern EPR is best modeled by a thin (<100 m thick) sill of partial melt (Vs ≠ 0 km/s) sandwiched between higher-velocity material, and that the thickest sills are generally associated with the lowest P and S wave velocities. The comparatively high P wave velocities and nonzero shear wave velocities inferred for this sill indicate that it is filled with partially molten magma which in some locations has a high crystal content. This may have important implications for eruption mechanisms and along-axis mixing of magma at the EPR. There is no simple relationship between morphologic indicators of magma supply (e.g., axial depth or volume) and sill thickness, depth, or velocity. Magma sill properties may be closely tied to the eruption and replenishment cycle of the AMC and thus may vary on a much shorter spatial and temporal scale than axial morphology, which reflects longer-term variations in magma supply to the ridge.

Near‐bottom magnetic survey of the Mid‐Atlantic Ridge axis, 24°–24°40′N: Implications for crustal accretion at slow spreading ridges

Near-bottom magnetic field measurements provide increased spatial resolution over sea surface magnetic data and allow a detailed analysis of the accretionary and tectonic processes at work in a slow spreading environment. Through the use of magnetic inversion methods and forward modeling, we investigate the fine-scale magnetic structure of young oceanic crust along four near-bottom profiles that cross the two bathymetric segments immediately north of the Kane transform on the Mid-Atlantic Ridge. Our results show the presence of a narrow, peaked central anomaly magnetic high (CAMH) located over the zone of most recent volcanism. We hypothesize that the pattern of high magnetization at segment ends and relatively low values at their centers is largely the result of increased iron and titanium content away from segment centers. Magnetization lows, which are not observed in the sea surface data, flank the CAMH and are associated with the axial valley walls. These lows may locate areas of intense magnetic source layer disruption. Blocks of relatively high magnetization found outside the valley walls suggest a cyclic faulting process, whereby discrete blocks of relatively unaltered crust are transported out of the rift valley by adjacent zones of faulting. These observations of fine-scale magnetic structure show that while the emplacement of crust at slow spreading ridges is highly focused, subsequent tectonic disruption and alteration at the rift valley walls degrades the crustal magnetization signal and may be the primary reason for Atlantic magnetic anomaly variability.