The continental lithosphere - The Solid Earth - page 574

Figure 10.45. (a) A plot of relative thinning (1-1/ β ) against subsidence for the
simple stretching model of the lithosphere. The upper line represents the initial
(immediately post-stretching) subsidence S i as given by Eq. (10.9). The lower line
represents the total subsidence, S = S i + S t at infinite time as given by Eq. (10.11),
where S t is the thermal subsidence. Values of parameters are h 1 , 125 km; h c ,30km;
ρ m 0 , 3350 kg m −3 ; ρ c 0 , 2780 kg m −3 ; ρ w , 1030 kg m −3 ; α , 3.28 × 10 −5 ◦ C −1 ; T a , 1333 ◦ C.
(After Le Pichon and Sibuet (1981).) (b) Observed and computed subsidence curves
for a well on the Nova Scotia continental margin off eastern Canada. The rifting
period indicates the time during which extension was taking place and thus shows
the initial subsidence S i . Subsidence occurring since that time is due to thermal
re-equilibration. (After Keen and Cordsen (1981).)

faulting of the basement surface are, therefore, likely to be lower than the actual
value. Measurement of the crustal thickness by seismic-refraction experiments
provides another method of estimating

β

, provided that an adjacent unextended
crustal thickness is known. However, in locations with a significant thickness
of material underplated to the lower crust, estimates of the stretching factor β

obtained from changes in crustal thickness will be too low.
Figure 10.45(b) shows observed and computed subsidence curves for a well
on the continental margin off eastern Canada. Figure 10.46 shows a cross section
of the passive continental margin of the eastern U.S.A. The transition between
stretched and unstretched crust, termed the hinge zone ,isnarrow, being only
50 km across. Figure 10.47 shows observed and computed subsidence curves for
five wells situated across the central graben of the North Sea (on a line between
Scotland and Norway) and the variation in crustal thickness across the graben.