Page 50 - Global Tectonics

THE INTERIOR OF THE EARTH 37

(a) 1 2 3 4 5 6 7 8 (b) 1 2 3 4 5 6 7 8

A Glide A
B plane B

(c) Slipped Unslipped (d) Slipped Unslipped

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

A Glide A
B plane B

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

(e) Slipped Unslipped (f) Slipped
1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

A Glide A
B plane B

1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
Figure 2.23 Plastic flow by the migration of a linear edge dislocation through a crystal (from Structural Geology by

Robert J. Twiss and Eldridge M. Moores. © 1992 by W.H. Freeman and Company. Used with permission).

to deformation in the lower mantle, although this lithosphere and its constituent layers can be estimated
interpretation is controversial. by integrating yield stress with respect to depth. This
integrated strength is highly sensitive to the geother-
mal gradient as well as to the composition and thick-
2.10.4 Lithospheric ness of each layer, and to the presence or absence of
fl uids.
strength profiles The results of deformation experiments and evi-

dence of compositional variations with depth (Section
In most quantitative treatments of deformation at 2.4) have led investigators to propose that the litho-
large scales, the lithosphere is assumed to consist of sphere is characterized by a “jelly sandwich” type rheo-
multiple layers characterized by different rheologies logical layering (Ranalli & Murphy, 1987), where strong
(e.g. Section 7.6.6). The rheologic behavior of each layers separate one or more weak layers. For example,
layer depends on the level of the differential stress (Δσ) Brace & Kohlstedt (1980) investigated the limits of
and the lesser of the calculated brittle and ductile yield lithospheric strength based on measurements on quartz
stresses (Section 2.10.1). The overall strength of the and olivine, which are primary constituents of the