Page 196 - Global Tectonics

182 CHAPTER 7

(a) Narrow rift mode Q s 60 mW m 2
C MPa log (Pa-s)
0 1200 0 600 17 23
0
Crust
20
Depth (km) 40
Mantle

60
Lithosphere
80
Asthenosphere
Temperature Yield strength Viscosity

(b) Wide rift mode Q s 80 mW m 2
C MPa log (Pa-s)
0 1200 0 600 17 23
0

Crust Depth (km) 20
Mantle
Lithosphere 40

Asthenosphere 60
Temperature Yield strength Viscosity

(c) Core complex mode Q s 100 mW m 2
C MPa log (Pa-s)
0 1200 0 600 17 23
upper crust Lithosphere 0
Asthenosphere 20
Depth (km)

lower crust 40

60
Temperature Yield strength Viscosity
Straining
region 40 km V.E. 2
Figure 7.24 Sketches of the lithosphere illustrating three modes of extension emphasizing the regions undergoing the

greatest amount of extensional strain (modified from Buck, 1991, by permission of the American Geophysical Union.
Copyright © 1991 American Geophysical Union). (a) Narrow mode, (b) wide mode, (c) core complex mode. Lithosphere
−1
21
is defined as areas with effective viscosities of >10 Pa s . The plots to the right of each sketch show initial model

−15 −1
geotherms, yield strengths (for a strain rate of 8 × 10 s ) and effective viscosities for a dry quartz crust overlying a dry
olivine mantle. From top to bottom the crustal thicknesses are 30 km, 40 km, and 50 km. Q s, initial surface heat fl ow.
(c) shows layers labeled at two scales: the upper crust and lower crust labels on the left side of diagram show a weak,
deforming lower crust (shaded); the lithosphere and asthenosphere labels on the right side of diagram show a scale
emphasizing that the zone of crustal thinning (shaded column) is localized into a relatively narrow zone of weak
lithosphere.