Page 198 - Global Tectonics

184 CHAPTER 7

different types of load it has been possible to estimate (Steckler, 1985). Ebinger et al. (1999) showed that
the effective long-term elastic thickness (T e ) of continen- increases in the both T e and T s in several rift basins in
tal lithosphere (Section 2.12) using forward models of East Africa and elsewhere systematically correspond to

topography and gravity anomaly profiles (Weissel & increases in the length of border faults and rift basin
Karner, 1989; Petit & Ebinger, 2000). The value of T e in width. As the border faults grow in size, small faults
many rifts, such as the Basin and Range, is low (4 km) form to accommodate the monoclinal bending of the
due to the weakening effects of high geothermal gradi- plate into the depression created by slip on the border
ents. However, in other rifts, including those in East fault (Fig. 7.25). The radius of curvature of this bend is

Africa and in the Baikal Rift, the value of T e exceeds a measure of flexural rigidity. Strong plates result in a
30 km in lithosphere that is relatively strong (Ebinger et narrow deformation zone with long, wide basins and
al., 1999). The physical meaning of T e , and its relation- long border faults that penetrate deeper into the crust.
ship to the thickness (T s ) of the seismogenic layer, is the Weak plates result in a very broad zone of deformation
subject of much discussion. Rheological considerations with many short, narrow basins and border faults that
based on data from experimental rock mechanics do not penetrate very deeply. These studies suggest that

suggest that T e refl ects the integrated brittle, elastic, and the rheology and flexural rigidity of the upper part of
ductile strength of the lithosphere. It, therefore, is the lithosphere control several primary features of rift
expected to differ from the seismogenic layer thickness, structure and morphology, especially during the fi rst
which is indicative of the depth to which short term few million years of rifting. They also suggest that the
(periods of years) anelastic deformation occurs as crust and upper mantle may retain considerable strength
unstable frictional sliding (Watts & Burov, 2003). For in extension (Petit & Ebinger, 2000).
these reasons, T e typically is larger than T s in stable Lithospheric flexure also plays an important role

continental cratons and in many continental rifts. during the formation of large-magnitude normal faults

The deflection of the crust by slip on normal faults (Section 7.3). Large displacements on both high- and
generates several types of vertical loads. A mechanical low-angle fault surfaces cause isostatic uplift of the foot-
unloading of the footwall occurs as crustal material in wall as extension proceeds, resulting in dome-shaped
the overlying hanging wall is displaced downward and fault surfaces (Buck et al., 1988; Axen & Bartley, 1997;
the crust is thinned. This process creates a buoyancy Lavier et al., 1999; Lavier & Manatschal, 2006). Lavier
force that promotes surface uplift. Loading of the & Manatschal (2006) showed that listric fault surfaces
hanging wall may occur as sediment and volcanic mate- whose dip angle decreases with depth (i.e. concave
rial are deposited into the rift basin. These loads combine upward faults) are unable to accommodate displace-
with those that are generated during lithospheric ments large enough (>10 km) to unroof the deep crust.
stretching (Section 7.6.2). Loads promoting surface By contrast, low-angle normal faults whose dips increase
uplift are generated by increases in the geothermal gra- with depth (i.e. concave downward faults) may unroof
dient beneath a rift, which leads to density contrasts. the deep crust efficiently and over short periods of time

Loads promoting subsidence may be generated by the if faulting is accompanied by a thinning of the middle
replacement of thinned crust by dense upper mantle crust and by the formation of serpentinite in the lower
and by conductive cooling of the lithosphere if thermal crust and upper mantle. The thinning and serpentini-
diffusion outpaces heating. zation weaken the crust and minimize the force re-
Weissel & Karner (1989) showed that fl exural iso- quired to bend the lithosphere upward during faulting,
static compensation (Section 2.11.4) following the allowing large magnitudes of slip.
mechanical unloading of the lithosphere by normal
faulting and crustal thinning leads to uplift of the rift
flanks. The width and height of the uplift depend upon 7.6.5 Strain-induced

the strength of the elastic lithosphere and, to a lesser
extent, on the stretching factor (β) and the density of weakening

the basin infill. Other factors may moderate the degree
and pattern of the uplift, including the effects of erosion, Although differences in the effective elastic thickness
variations in depth of lithospheric necking (van der and flexural strength of the lithosphere (Section 7.6.4)

Beek & Cloetingh, 1992; van der Beek, 1997) and, pos- may explain variations in the length of border faults and
sibly, small-scale convection in the underlying mantle the width of rift basins, they have been much less