Page 297 - Physical Principles of Sedimentary Basin Analysis

8.7 Ductile flow and yield strength envelopes 279

0 0

−20 −20
−40
depth [mm] −40 depth [mm] −60
−60

−80 −80

−100 −100

−120 −120
0 300 600 900 1200 1500 0 300 600 900 1200 1500
YSE [MPa] YSE [MPa]
(a) time=0, beta=1 (b) time=5, beta=1.41

0 0

−20 −20 t=10 Ma
depth [mm] −40 depth [km] −40 t=5 Ma

−60
−60
−80 −80 t=0 Ma

−100 −100

−120 −120
0 300 600 900 1200 1500 0 500 1000 1500
YSE [MPa] temperature [C]
(c) time=10, beta=2 (d)
Figure 8.18. The YSE during rifting with a constant strain rate. The β-factor is 2 after 10 Ma of
rifting. Table 8.1 gives the rheological data used. (a) The YSE at time 0 Ma. (b) The YSE at time
5 Ma. (c) The YSE at time 10 Ma. (d) The temperature of the lithosphere at the times 0 Ma, 5 Ma and
10 Ma.

the lithosphere becomes gradually weaker during the stretching because of the hot mantle
that moves upwards. The lithospheric strength as a function of the β-factor is shown in
Figure 8.19 for two different durations of the rift phase. One lasts for 10 Ma and the other
for 100 Ma. Rifting over a “short ” time interval (10 Ma) leads to continuously decreasing
lithospheric strength, while the “slow” stretching (over 100 Ma) has a lithospheric strength
that reaches a minimum before halfway through the rift phase, and then stays at the mini-
mum for the rest of the rifting. Notice that the strain rate becomes zero when stretching has
ceased, which implies that the stress state in the ductile parts of the lithosphere becomes
isotropic.