Page 238 - Physical Principles of Sedimentary Basin Analysis

220 Subsidence

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Figure 7.17. (a) Wernicke (1985) simple shear model for extension and crustal thinning by a low-
angle detachment fault. (b) The crustal configuration in the case of isostatic equilibrium. The
temperature contours are shown for the case of instantaneous extension.

towards zero with increasing extension.) Show that the β-factors for the two rift phases
become
1 sin φ 1
β 1 = and β 2 = . (7.93)
sin φ 1 sin φ 2

Exercise 7.14 Extension takes place along a low-angle detachment fault in Wernicke’s
simple shear model, as shown in Figure 7.17, where the fault penetrates into the litho-
spheric mantle (Wernicke, 1985). The detachment fault is an asymmetric tectonic structure.
Both the pure and simple shear models predict crustal thinning of the rifted continental
margins.
(a) Show that the β-factor for crustal thinning becomes
c 0 c 0
β = = (7.94)
c c 0 − d tan θ
where d is the horizontal offset of the plates, θ is the angle of the planar detachment, c is
the crustal thickness and c 0 is the initial thickness. What is the maximum horizontal offset
d before the mantle becomes exhumed?
(b) Show that the isostatic subsidence from crustal thinning is

( m − c )
s = d tan θ. (7.95)
( m − s )