Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

Abstract

The upper ocean crust contains a comprehensive record of the shallow geological processes active along the world's mid-ocean ridge system. This thesis examines the magnetic and seismic structure of the upper crust at two contrasting ridges-the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR)-to build a more complete understanding about the roles of volcanic emplacement, tectonic disruption and hydrothermal alteration in the near-ridge environment.A technique that inverts potential field measurements. directly from an uneven observation track is developed and applied to near-bottom magnetic data from the spreading segments north of the Kane transform on the MAR. It is concluded that the central anomaly magnetization high marks the locus of focused volcanic emplacement. A cyclic faulting model is proposed to explain the oscillatory magnetization pattern associated with discrete blocks of crust being transported out of the rift valley between intensely altered fault zones. Seismic waveform and amplitude analyses of the magma sill along the EPR reveal it to be a thin «100 m) body of partial melt. These characteristics have important implications for melt availability and transport within the cycle of eruption and replenishment. A genetic algorithm-based seismic waveform inversion technique is developed and applied to on-and near-axis multichannel data from 17°20 's on the EPR and the spreading segment south of the Oceanographer transform (MAR) to map and compare for the first time the detailed velocity structure of the upper crust at two different spreading rates. Combined with conventionally processed seismic profiles, our results show that, while final extrusive thickness is comparable at all spreading ridges (300-500 m), the style of thickening may vary. While a thin (:S100 m) extrusive carapace quadruples in thickness within 1-4 km of the EPR crest, the extrusive section at the MAR achieves its final thickness within the inner valley. Both show evidence for a narrow zone of volcanic emplacement. Vigorous hydrothermalism at the EPR may produce a more rapid increase in basement velocities relative to the MAR. Rapid modification of the extrusive/dike transition at both ridges indicates that hydrothermalism is enhanced in this interval. Along-axis transport of lavas may thicken the extrusive pile at slow spreading segment ends, strengthening the magnetic highs generated by lava chemistry. Biographical SketchIn order to wax only partially rhapsodic, let me start at my formative age as a marine geophysicist ... the age of six. The first hint that I wasn't going to grow up to be a successful businessman was dropped in Germany, where my best friend and I rejected the idea of opening a lemonade stand in favor of selling fossils and geodes on the street corner. We soon went bankrupt. Our village in southern Germany was not far from Solnhofen, home of the famed Archaeopteryx. The countryside was full of geological treasures exposed in streambeds or along roadsides. Our final m...

Crustal evolution over the last 2 m.y. at the Mid‐Atlantic Ridge OH‐1 segment, 35°N

“…Although the uppermost 1-1.5 km of crust is poorly sampled by seismic energy relative to other regions of the model (Figure 4a), the velocities we obtain for the upper crust agree well with the velocity-depth profile obtained from analysis of multichannel seismic (MCS) data in the same area [Hussenoeder, 1998]. At 1 km depth, for example, the velocity at x=-18 km and x=5 km is 5 km/s in both the MCS velocity analysis and in our tomographic results.…”

“…Some variability does exist in the thickness and mean velocity of layer 2A at OH-1. In the western rift mountains, for example, MCS images show that layer 2A thickens by 15 0 m from the center of the segment to its northern end [Hussenoeder, 1998]. Similarly, at two locations on the eastern flank, the velocity of the upper 50 m of crust varies by 20% over an along-strike distance of only 5 km.…”

Crustal accretion and evolution at slow and ultra-slow spreading mid-ocean ridges

“…Although the uppermost 1-1.5 km of crust is poorly sampled by seismic energy relative to other regions of the model (Figure 4a), the velocities we obtain for the upper crust agree well with the velocity-depth profile obtained from analysis of multichannel seismic (MCS) data in the same area [Hussenoeder, 1998]. At 1 Ian depth, for example, the velocity at x=-18 Ian and x=5 Ian is 5 km/s in both the MCS velocity analysis and in our tomographic results.…”

“…In the western rift mountains, for example, MCS images show that layer 2A thickens by 150 m from the center of the segment to its northern end [Hussenoeder, 1998]. Similarly, at two locations on the eastern flank, the velocity of the upper 50 m of crust varies by 20%…”

“…It is interesting to note that -3 km east of Line E, MCS analysis at two locations shows that layer 2A is 275 m thinner at a site closest to a particularly large seamount [Hussenoeder, 1998]. A reduction in the volume of extrusive material beneath a volcano is counterintuitive.…”