Neil C. Mitchell Department of Geological Sciences, Universty of Durham, South Road, Durham DH1 3LE, UK
Roger C. Searle Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK
Constraining the timescale over which mid-ocean ridge faults evolve is important for understanding the overall mechanics of ridges. For example, at fast-spreading ridges, hydrothermal cooling may strengthen the axial lithosphere sufficiently such that gravitational spreading of the ridge may produce differential stress over a broader zone than merely the spreading axis itself, possibly reactivating near-axis faults (Lee, 1995). We present here some observations of stable sediment scarps which may constrain the maximum age of seismogenic faulting at the Galapagos spreading centre.
High rates of equatorial pelagic sedimentation result in steep accumulations over fault scarps around the Galapagos spreading centre. Figure 1 shows interpretations of profiler records from the Scripps Deep Tow system collected near the spreading centre (Klitgord and Mudie, 1974) which show that these deposits generally lack evidence for major slides, such as concave slide scars or irregular deposits at scarp bases. We propose that significant seismogenic movement on these faults would cause sediment sliding or other bulk mass-wasting, and therefore the locations of stable slopes can be used to constrain the limit of seismogenic faulting around the spreading centre. Stable sediment deposits occur on crust between ~500,000 years and 2 Ma, which is the oldest seafloor of the survey, so normal faults are probably not seismogenic on crust within this age range here.
In order to quantify this observation, we use a simple slope stability model and shear vane measurements of sediment shear strength (Wilkens, 1983) to infer the minimum horizontal ground acceleration required to produce failure. The dashed lines in Figure 1 show the slide surfaces predicted by the model and Figure 2 shows the minimum accelerations required to produce failure, which are generally 0.1-0.2g. Strong motion data for continental earthquakes of magnitude 4-5 suggest that earthquakes within distances of 5-10 km may produce accelerations of 0.1-0.2g (Westaway and Smith, 1989). Given this seismic sensitivity and the locations of stable slopes, we predict that faults are probably aseismic on seafloor older than ~300,000 years. Sediment scarps become increasingly sensitive to ground shaking with seafloor age as accumulations steepen and thicken, so long-term seismicity on ridge flanks could potentially be assessed from the extent and geometry of slides, if these were mapped with suitable deeply towed instruments. This may have applications, for example, for studying the spatial distribution of seismic release due to thermoelastic stresses in young oceanic lithosphere.
Fig. 1: A selection of interpretations of Deep Tow profiler records across fault scarps around the Galapagos spreading centre adapted from Mitchell (1996) showing the basement and sediment topography. The basement reflectors are highlighted by grey shading. The dashed line in each profile shows the potential failure surface obtained using a simple slope stability model, shear strength and density measurements from DSDP hydraulic piston cores (Karato and Becker, 1983; Wilkens and Langseth, 1983) and for the horizontal ground accelerations shown in Figure 2.
Fig. 2: Estimates of the minimum horizontal ground acceleration required to cause slope failure. Circles and crosses show estimates calculated assuming sediment angles of internal friction of 30° and 20° respectively. Scarps from both sides of the spreading axis were used.
Karato, S. & Becker, K., In: Honnorez, J., Von Herzen, R.P. et al., Init. Repts. DSDP 70, U.S. Govt. Printing Office, Washington, 355-368 (1983).
Klitgord, K.D. & Mudie, J.D., Geophys. J. R. Astr. Soc. 38, 563-586 (1974).
Lee, S.-M., Ph.D. Thesis, Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program in Oceanography, pp. 511 (1995).
Mitchell, N.C., Geophys. Res. Lett. 23, 483-486 (1996).
Westaway, R. & Smith, R.B., Geophys. J. 96, 529-559 (1989).
Wilkens, R.H. & Langseth, M.G., Deep Sea Drilling Project site 504 and 505, In: Cann, J.R., Langseth, M.G., Honnorez, J., Von Herzen, R.P., White, S.M. et al., Init. Repts. DSDP 69, U. S. Govt. Printing Office, Washington, 659-673 (1983).
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