School of Earth and Environment

Institute of Geophysics and Tectonics (IGT) PhD Projects

Thermal evolution of oceanic detachment faults

Supervisors: Dr Andrew McCaig, Prof Greg Housemann and Prof Joe Cann

Probably the most important new discovery in Plate Tectonics in the last 30 years is the fact that much of the seafloor at slow spreading ridges has formed by processes involving slip on long-lived, convex-upward detachment faults, rather than purely by emplacement of magma (see Smith et al 2008; MacLeod et al 2009).  These faults can accommodate 50-100% of the plate separation rate. At depth they are steep, evolving to shallow dips at the seafloor through flexure. They are associated with large hydrothermal systems (Fig. 1), with evidence for discharge of black smoker fluids up active fault zones (deMartin et al. 2007; McCaig et al., 2007, 2010).

Sophisticated mechanical models have been developed for the growth and flexure of detachment faults (eg. Tucholke et al., 2008), but so far there are no models that predict the thermal evolution of detachment fault footwalls in any detail. These models need to include advection of material, intrusion and solidification of magma, and hydrothermal cooling from above. The thermal structure predicted by such models is expected to be asymmetric, with significant consequences for the modeling of hydrothermal systems at slow spreading ridges, which currently invariably assume a symmetrical thermal structure.

Aims of the project are:

  1. To develop numerical models for the thermal evolution of detachment faults at mid-ocean ridges
  2. To evaluate the relative importance of fault slip, hydrothermal cooling, and igneous intrusion in controlling the thermal evolution
  3. To test the models against petrological and palaeomagnetic data on the cooling history of detachment fault footwalls
  4. To provide input data on thermal structures for hydrothermal models of fluid flow in and around detachment faults

Modeling (Fig. 2) will be undertaken using COMSOL multiphysics, and models will be tested against petrological data on thermal evolution and cooling rates from ocean drill core. Mark Behn (Woods Hole Oceanographic Institute) has agreed to assist with setting up the models and would host the student for a period. Results will be used as input to hydrothermal models under development at IPGP, Paris.

This project would suit a numerically capable geologist or geophysicist interested in a multidisciplinary project at the cutting edge of tectonics research.

References
deMartin, B. J., Canales, R. A. R., Canales, J. P. & Humphris, S. E. Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge. Geology 35, 711-714 (2007).

MacLeod, C. J. et al. Life cycle of oceanic core complexes. Earth and Planetary Science Letters 287, 333-344, doi:10.1016/j.epsl.2009.08.016 (2009).

McCaig, A. M., Cliff, R. A., Escartin, J., Fallick, A. E. & MacLeod, C. J. Oceanic detachment faults focus very large volumes of black smoker fluids. Geology 35, 935-938, doi:10.1130/g23657a.1 (2007).

McCaig, AM, Delacour A, Fallick AE, Castelain T and Fruh-Green GL (2010), Detachment Fault Control on Hydrothermal Circulation Systems: Interpreting the Subsurface Beneath the TAG Hydrothermal Field Using the Isotopic and Geological Evolution of Oceanic Core Complexes in the Atlantic. In: Diversity of Hydrothermal Systems on Slow Spreading Ocean Ridges, PA Rona, CW Devey, J Dyment and BJ Murton (eds), Geophysical Monograph 108, p. 207-240.

Smith, D. K., Cann, J. R. & Escartin, J. Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge. Nature 442, 440-443, doi:10.1038/nature04950 (2006)

Tucholke, B. E., Behn, M. D., Buck, W. R. & Lin, J. Role of melt supply in oceanic detachment faulting and formation of megamullions. Geology 36, 455-458, doi:10.1130/g24639a.1 (2008).