DEPENDENCE OF RIDGE-AXIS MORPHOLOGY ON MAGMA SUPPLY AND SPREADING RATE

WHY do some mid-ocean-ridge axes have as their topographic expression a median valley 1-2 km deep and 15 30 km wide, whereas others have an axial high 100-200 m high and 1-2 km wide? In general, median valleys exist at the axes of ridges spreading at half-rates less than approximately 20-25 mm yr-1, whereas axial highs are found at faster-spreading ridges1,2. A recent crustal genesis model3, incorporating hydrothermal cooling and crustal accretion by means of a magma lens, agrees well with observations of the spreading-rate dependence of axial morphology1,2, the depth of the axial magma chamber3,4 and the thickness of the ridge-axis lithosphere5,6. But there are several notable exceptions to the correlation between spreading rate and axial morphology, such as the Reykjanes Ridge, which spreads at 10 mm yr-1 but has an axial high. Here we show that by extending the model of ref 3 to include variations in crustal thickness, we can explain the observed dependence of axial morphology on crustal thickness and spreading rate. Our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis, which is a function of both spreading rate and magmna supply.