[1] We use 2-D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off-axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off-axis lithospheric thickening produces long-lived, large-offset faults, similar to oceanic core complexes. Such long-lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across-axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.

[1] As the Galápagos hot spot is approached from the west along the Galápagos Spreading Center there are systematic increases in crustal thickness and in the K/Ti, Nb/Zr, 3 He/ 4 He, H 2 O, and Na 2 O content of lavas recovered from the spreading axis. These increases correlate with progressive transitions from rift valley to axial high morphology along with decreases in average swell depth, residual mantle Bouguer gravity anomaly, magma chamber depth, average lava Mg #, Ca/Al ratio, and the frequency of point-fed versus fissure-fed volcanism. Magma chamber depth and axial morphology display a ''threshold'' effect in which small changes in magma supply result in large changes in these variables. These correlated variations in geophysical, geochemical, and volcanological manifestations of plume-ridge interaction along the western Galápagos Spreading Center reflect the combined effects of changes in mantle temperature and source composition on melt generation processes, and the consequences of these variations on magma supply, axial thermal structure, basalt chemistry, and styles of volcanism.Components: 6355 words, 4 figures, 1 table.

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Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Gala ¤pagos Spreading Center (GSC) between 97 ‡W and 91 ‡25PW thickens by 2.3 km as the Gala ¤pagos plume is approached from the west. This crustal thickening can account for V52% of the 700 m amplitude of the Gala ¤pagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Gala ¤pagos swell shows a minimum of 325 mGal near 92 ‡15PW, the area where the GSC is intersected by the WolfD arwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50^100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of V30 ‡C. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Gala ¤pagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Gala ¤pagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Gala ¤pagos plume (700 kg s 31 ) is lower than previous estimates, while the total crustal production rate of the Gala ¤pagos plume (5.5 m 3 s 31 ) is comparable to that of the Icelandic and Hawaiian plumes. ß