Page 169 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 155

basalts. Others, such as the Western branch of the East with flexural isostatic compensation of the
African Rift system (Fig. 7.2) and the Baikal Rift, are lithosphere (Section 7.6.4) leads to uplift of the

magma starved and characterized by very small rift flanks, creating a characteristic asymmetric

volumes of volcanic rock. topographic profile. The lower relief side
In this chapter, several well-studied examples of of the basin may be faulted and exhibits a
rifts and rifted margins are used to illustrate how monocline that dips toward the basin center.
strain and magmatism are distributed as rifting pro- Deposition during slip on the bounding

ceeds to sea floor spreading. The examples also show normal faults produces sedimentary and
how geoscientists combine different data types and volcanic units that thicken towards the fault
use spatial and temporal variations in the patterns of plane (Fig. 7.3b). The age of these syn-rift
rifting to piece together the tectonic evolution of units, as well as units that pre-date rifting,
these features. provide control on the timing of normal
faulting and volcanism. In plan view,
displacements decrease toward the tips of
border faults where they interact with other
7.2 GENERAL faults bounding adjacent basins. Within these
transfer zones faults may accommodate
CHARACTERISTICS OF differential horizontal (including strike-slip)
and vertical displacements between adjacent
NARROW RIFTS basins.

2 Shallow seismicity and regional tensional stresses.
Beneath the axis of most continental rifts
Some of the best-studied examples of tectonically

earthquakes generally are confined to the
active, narrow intracontinental rifts occur in East
uppermost 12–15 km of the crust, defi ning a
Africa (Fig. 7.2). Southwest of the Afar triple junction,
seismogenic layer that is thin relative to other
the Nubian and Somalian plates are moving apart at a
−1
rate of approximately 6–7 mm a (Fernandes et al., regions of the continents (Section 2.12). Away
from the rift axis, earthquakes may occur to
2004). This divergent plate motion results in exten-
depths of 30 km or more. These patterns
sional deformation that is localized into a series of
imply that rifting and thinning locally weaken
discrete rift segments of variable age, including the
the crust and affect its mechanical behavior
Western Rift, the Eastern Rift, the Main Ethiopian Rift,
(Section 7.6).
and the Afar Depression. These segments display char-
In Ethiopia, the record of seismicity from 1960
acteristics that are common to rifts that form in rela-
to 2005 (Fig. 7.4a) shows that the majority of
tively strong, cool continental lithosphere. Key features
large earthquakes occur between the Afar
include:
Depression and the Red Sea. Analyses of
1 Asymmetric rift basins flanked by normal faults. seismic moment release for this period

Continental rifts are associated with the shows that more than 50% of extension
formation of sedimentary basins that are across the Main Ethiopian Rift is
bounded by normal faults. Most tectonically accommodated aseismically (Hofstetter &
active rift basins show an asymmetric half Beth, 2003). The earthquakes show
graben morphology where the majority of the combinations of normal, oblique and strike-
strain is accommodated along border faults slip motions. North of the Afar Depression,
that bound the deep side of the basins (Fig. the horizontal component of most axes of
7.2b–e). The polarity of these half grabens minimum compressive stress strike to the
may change along the strike of the rift axis, north and northeast at high angles to the
resulting in a segmentation of the rift valley trend of the rift segments.
(Fig. 7.3a). In plan view, the border faults Keir et al. (2006) used nearly 2000 earthquakes
typically are the longest faults within each to determine seismicity patterns within
individual basin. Slip on these faults combined the northern Ethiopian Rift and its fl anks