Geodynamic Studies of Faulting, Magmatism, and the Morphology of Mid-Ocean Ridges

G. Ito¹* & M.D. Behn²

Corresponding author: gito@hawaii.edu
¹Department of Geology and Geophysics, University of Hawaii-SOEST, Honolulu, HI, 96822
²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543

Abstract:
Two-dimensional numerical models are used to explore the thermal and mechanical effects of magma intrusion on the morphology of mid-ocean ridges. The models are time-dependent, simulating alternating tectonic and magmatic periods where extension is accommodated by faulting and by magmatism, respectively. During tectonic phases, faults are predicted to grow on either side of the ridge axis; during magmatic phases, magmatic extension is simulated by allowing the axial lithosphere to open freely in response to extension. Results show that fault spacing and heave increase with increasing time fraction spent in the magmatic phase FM, and with decreasing rate of lithosphere thickening away from the ridge axis. Contrary to what is often cited in the literature, we find that thicker axial lithosphere tends to reduce, not enhance fault spacing and heave. Thus the large and widely spaced faults that are typically associated with lower spreading rates and magma budgets are predicted to be caused by lower magmatic time fractions FM (FM approaching 0.5 for long-lived detachment faults) and, indirectly from cooler axial lithosphere because it thickens more gradually off-axis.

Contributions to Integration and Synthesis:
Models also successfully predict axial topographic highs, axial valleys, as well as the transition between the two. The main process is the competition between topographic growth during magmatic phases versus sinking of the axis during tectonic phases. This competition is quantified by the “rise-sink ratio”, (FM /F.T)(tT/tM) where FM /FT is the ratio of the time spent in the magmatic and tectonic periods and tT/tM is the ratio of the characteristic rates for growing topography during magmatic phases (1/tT) and for reducing topography during tectonic phases (1/tT). Models predict the tallest axial highs when (FM /F.T)(tT/tM) >>1, faulted “transitional” topography without a high or valley when (FM /F.T)(tT/tM) ~ 1, and deep median valleys when (FM /F.T)(tT/tM) << 1. New scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and micro earthquakes. Exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the Lau Spreading Center. Still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions, as evident at the Mid-Atlantic and Juan de Fuca ridges, influence the rise-sink-ratio (FM /F.T)(tT/tM) and thus axial morphology. Our results provide a quantitative context within which to interpret the wealth of detailed bathymetric and seismic data arising out of the R2K integrated studies. New 3-D models offer the opportunity to more fully use these data to understand the mechanical coupling of faulting and magmatism along individual ridges segments and near segment offsets.