Stress, faults, and folds - Tectonic Geomorphology - page 99

Extensional
Structures

half graben

master normal
fault

antithetic
fault

synthetic
fault

basin
fill

horst

back-
tilting

graben

hanging wall footwall

Fig. 4.22 Schematic of cross-section of normal faults in an extensional regime.
The master fault delineates footwall and hanging-wall blocks. A broad half-graben is cut by synthetic and antithetic
faults. Note both the geometry of the syntectonic growth strata that fill the faulted basins and the curvature of the
hanging wall toward the master fault (hanging-wall roll-over).

Interbasin Transfer Zones

Antithetic interbasin Antithetic interference

zone

up up

ridge

down down

extension
direction

complex
polygonal
transfer
zone

down

down

relay

ramp

up

1 km

up

0

A B

Structures in a
Continental Rift

Synthetic relay
ramp

up

up

Transfer
fault

Lake
Bogoria

down down

up

down

Fig. 4.24 Schematic fault geometry in the East
African Rift system.
Relay ramps, transfer faults, and smaller-scale synthetic
faults facilitate transfer of displacement between the
major faults. Note that, for any cross-section
perpendicular to the traces of the major faults, the net
displacement across the region is nearly equal, despite
large along-strike differences in displacement on
individual faults. Modified after Morley (1989).

up

down

C D

Fig. 4.23 Interbasin transfer zones.
Simplified geometries of structures that assist the
transfer of deformation between two regional-scale,
overlapping or non-overlapping normal faults. Modified
after Gawthorpe and Hurst (1993).

(iv) antithetic interbasinal ridges , which develop
where normal faults change polarity and step
into the footwall. Each of these structures pro-
vides a mechanism whereby the total regional
displacement can vary smoothly along strike,
despite the fact that individual faults die out, that
displacement is variable along each fault, and
that both fault lengths and amount of overlap
change through time. Any relatively broad region
that is undergoing extension will display some of
these displacement-accommodation features. For
example, during much of Cenozoic time, active
extension has been occurring across eastern

Africa. As a consequence, an elaborate network
of mature normal faults has developed. Within
many segments of the East African Rift, en
echelon normal faults joined by relay ramps and
complex transfer zones (Fig. 4.24) exemplify the
spatially varied distribution of faults and strain
that accommodates crustal extension. Where
Quaternary strain within complexly faulted rifts
has been particularly well quantified, such as in
the Taupo Rift on the North Island of New
Zealand (Nicol et al. , 2006), both the compensa-
tion that occurs between different subregions
that slip at changing local rates through time and