Magma supply and its rise axis expressions: Time constraints for magma supply on the East Pacific Rise and Juan de Fuca Ridge

The morphology and thermal structure of two distinctly different mid-ocean spreading centers (the East Pacific Rise and Juan de Fuca Ridge) are studied by morphotectonic and water column data analyses to explore variabilities in magmatic cooling processes.; The East Pacific Rise at 10{dollar}spcirc{dollar}-13{dollar}spcirc{dollar}N, a fast spreading ridge (60 mm/yr half-rate), is marked by robust magma chambers between overlapping spreading centers (OSCs). Analyses of the water column above this section of the EPR reveal undulations in density of water surfaces on the order of several hundred meters. Cold-dense water (colder and denser than ambient water at this depth) is found to reside (at least during the time frame of the survey) over OSCs while warm-buoyant water resides above and adjacent to bathymetrically shallow and broad sections of the rise axis. This suggests that bottom water is being drawn into highly fractured crust at OSCs. In this thesis, a model is developed to explain how entry of cold water into crust near OSCs modifies and cools the neighboring robust magma chamber by percolating along rift-parallel faults into and above the magma chamber at mid-segment, finally exiting by hot-venting into the ocean. Results from this model predict that the timescale for cooling a magma body along this section of the EPR is approximately 7,500 years.; In contrast, magma supply along the medium spreading rate Juan de Fuca Ridge (29 mm/yr half-rate) can be shown to be intermittent on a much longer timescale of approximately 60,000 years. In this case, bathymetry, temperature and structural data from individual axial segments are used to estimate magma supply. Along the Juan de Fuca Ridge, hydrothermal areas are generally associated with closely-spaced fault zones and shallow lava fields whereas cooler, deeper areas are marked by desiccated remains of previously-active terrain. A near-surface continuously elongate magma chamber has not been detected beneath the Juan de Fuca Ridge so such a chamber cannot supply the axis with a continuous magma source. It is therefore proposed that either magma rises from depth at intermittent intervals or near-surface magma supplies are extremely localized point sources which must migrate along the rise axis.