Geology Reference
In-Depth Information
Fig. 11.57.
Cross section in the transport direction of the Livingstone half graben (Fig. 11.56b), inter-
preted with the oblique simple shear model (Groshong 1995).
Darker shaded unit
is basement,
lighter
shaded unit
is sedimentary basin fill. No vertical exaggeration.
Dashed line
is predicted fault
the north end of Lake Malawi (Nyasa), one of the East African Rift valleys. The master
fault of the half graben is on the east side of the basin. The basin is broken by a number
of second-order normal faults. The north-south trend of the faults within the basin
seems to imply that the extension direction is east-west, however, the extension direc-
tion is interpreted to be northwest-southeast, oblique to the trend of the rift axis on
the map (Scott et al. 1992; Wheeler and Rosendahl 1994). This interpretation is based
on west-northwest to northwest plunging slickenlines on exposures of the master fault
and on the presence of flower structures (wrench-fault indicators) along west-north-
west trending faults in the basin. The cross section along the basin axis (Fig. 11.56b)
is thus in the transport direction.
Using the relationship between layer-parallel extension and shear angle, it is pos-
sible to find the shape of the master fault, the depth to detachment, and the displace-
ment that formed the graben. The amount of extension in the hangingwall is directly
related to the dip change of the median surface and the angle of shear. Second-order
normal faults are relatively evenly distributed throughout the hangingwall, as ex-
pected for the simple-shear model. The amount of extension along the median sur-
face is determined by measuring the length of the top basement surface with and
without the fault offsets and is
e
L
= 0.08 (Eq. 11.2). The dip of the median surface with
respect to regional is
= 25°. Given the shear angle, the shape
of the master fault can be constructed by the oblique-shear method (Sect. 11.7.2.2)
and is shown in Fig. 11.57. Measured from the figure,
H
= 18.5 km. By the oblique
simple-shear model (e.g., Fig. 11.50), the displacement that formed the graben system
is 11.8 km.
A depth of 18.5 km places the lower detachment of the Livingstone half graben in
the middle of the crust. This depth is a reasonable possibility because it is the depth
at which most seismic activity ceases below active cratonic rifts (Chen and Molnar
1983), even though the base of the crust is the expected location of the major strength
minimum (Molnar 1988; Harry and Sawyer 1992) and rare deep earthquakes occur in
the area (Shudofsky 1985; Jackson and Blenkinsop 1977).
ψ
= 2°. From Eq. 11.48,
α
