Page 201 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 187

Crust: Weak seed: 12 x 10 km Coupled Decoupled
(a) Wet quartz von Mises plastic 0
Frictional plastic p m = 3300 kg m -3
p c = 2800 kg m -3 σ = 10 7 Pa 500 C
o
0 km V ext Wet quartz
V ext
35 km
85 km Depth (km)
120 km o Dry olivine
1330 C
V b Mantle Lithosphere and Sublithospheric Mantle V b
Dry Olivine 120
Frictional plastic 0 150 0 150
p m = 3300 kg m -3
Stress (MPa)
o
o
Isothermal mantle, Temperature = 1330 C 1330 C
600 km 1200 km (d)
(b) t = 28 Myr, Δx = 84 km 0 t = 22 Myr, Δx = 66 km S1A c S2A S1B
0 S1A S2A S2B S1B 20
C
o
20 40 550 C LC
o
40 550 C 60 800 C T1B
o
60 800 C T1B T1A 80
o
o
80 S1A S1B 100 1000 C P S1A S2A
o
1000 C C S1B
100 T1B T1A 120 1200 C T1B
o
120 o
1200 C 400 500 600 700 800
400 500 600 700 800
(e) t = 40 Myr, Δx = 120 km P
(c) t = 50 Myr, Δx = 150 km 0 S1A S2A S3B S2B S1B
0 S1A S2A S3A S3B S2B S1B LC
20
20 40 550 C
o
o
40 550 C T1B 60 T1B T3B
o
60 T1A 800 C
o
800 C 80
o
80 S1A S2A S2B S1B 1000 C
o S3A S3B 100 S2A S3B S1B
100 1000 C T1B T1A 120 S1A S2B
o
120 1200 C T1B T3B
o
1200 C 400 500 600 700 800
400 500 600 700 800
(f) t = 25 Myr, Δx = 75 km
S1A S1B
0 S2A S2B
20
o
40 550 C
60 800 C T1B T2B
o
80 S1B
o
1000 C P S1A
100
o
120 1200 C T1B T2B
400 500 600 700 800
(g) t = 50 Myr, Δx = 150 km
0 S1A S2A S2B S1B
20
o
40 550 C
60 T1B T2B
o
800 C
80 S2A S2B
S1B
o
100 1000 C S1A T2B
120 T1B
o
1200 C
400 500 600 700 800
Figure 7.26 (a) Model geometry showing temperature structure of the crust, mantle lithosphere and sublithospheric

mantle (images provided by R. Huismans and modified from Huismans & Beaumont, 2003, by permission of the
American Geophysical Union. Copyright © 2003 American Geophysical Union). Initial (solid lines) and strain softened
−1
(dashed lines) strength envelopes are shown for an imposed horizontal extensional velocity of V ext = 3 mm a , with V b
chosen to achieve mass balance. Decoupling between crust and mantle is modeled using a wet quartzite rheology for
the lower crust. (b,c) Reference model of extension when strain softening is absent. Models of extension involving (d,e)
frictional-plastic (brittle) strain softening and (f,g) both frictional-plastic and viscous weakening mechanisms. Models
in (b–g) show a subdivision of the crust and mantle into an upper and lower crust, strong frictional upper mantle
lithosphere, ductile lower lithosphere, and ductile sublithospheric mantle. Scaling of quartz viscosity makes the three
upper layers frictional-plastic in all models shown. t, time elapsed in millions of years; Δx, amount of horizontal
−1
extension. Vertical and horizontal scales are in kilometers. V ext = 3 mm a for every model.