Volcanism at the Galapagos Spreading Centre consists of sheet flows
and other distributed extrusives. The surficial volcanics in the axial
region should therefore be generally young over a finite area and those
on the ridge flanks should be younger, on average, than would be
predicted from simple seafloor spreading. A method for determining an
upper bound on the mean age anomaly of the flanks, which is
interpreted as equivalent to the mean extent of volcanism, is
demonstrated using sediment thicknesses digitised from Deep Tow
profiler records of the Galapagos Spreading Centre. As found
previously, the sediments thicken linearly with distance from the
ridge, reflecting simple seafloor spreading up to ~1 Ma. Deviations
from the linear trend are due to sediment redistribution (short
wavelength fluctuations) and the age distribution of extrusives, in
particular due to large volcanic flows revealed by regions of uniform
thickness. The full width of the extrusion zone is obtained by
regressing sediment thickness on across-axis distance for both flanks
and observing the separation of the two regression lines where they
cross zero thickness. Since the method is quantitative and provides an
average value for the length of the profile, it is more representative than
estimating the width from morphological observations of the spreading
axis. Due to scattering from the basement, which obscures sediment in
depressions, sediment thickness is generally under-estimated and the
measured axial width is an upper bound (10-12 km for the Galapagos
Spreading Centre). The mean sediment thickness bias is calculated
from high-resolution topography data of lava flows and is ~1.5 m,
suggesting that the true mean width of the extrusion zone is 8-10 km.
Using this corrected value, the surface of the crust is younger than
predicted by seafloor spreading by ~120-150 kyr, on average. This
large value implies that the flanks of the Galapagos Spreading Centre
are covered with a series of large overlapping volcanic flows.
Mitchell, N. C., "Characterising the extent of volcanism at the Galapagos spreading
centre using Deep Tow sediment profiler records", Earth Planet. Sci. Lett., 134, 459-472,
1995.