12.1 Rift Basins
Simple shear. Many recent investigations of rift
zones have revealed that crnstal stretching is accomplished by slip along an intra-crustal detachment
surface (simple shear, Fig. l2.1b). In the detachment
model, extension is nonuniform and decreases in the
direction of shear. Simple shear allows the lateral
separation of the zone of initial rifting and the zone
of magma upwelling which may initiate ocean
spreading (Fig. l2.lb and c). Low-angle detachments
can also explain the coeval initiation of different basin types (Fig. l2.1d).
The principal differences between the basins resulting from pure and simple shear can be demonstrated
best by defining their end members (Friedmann and
Burbank 1995):
- End members of pure shear (intracontinental rifts)
evolve in old, thermally equilibrated crnst of normal
thickness and are mostly accompanied by alkali to
tholeitic magmatism. Crnstal extension is limited (1025%), and the normal geometry of the basin and
subbasins is that of asymmetric grabens or halfgrabens bounded by steeply dipping (45-60°) normal
faults. The basin centers subside deeply (6-10 km),
and their depocenters tend to develop along the active main boundary fault. Examples: East African rift
and Baikai rift (see below).
- End members of simple shear on top of detachment
zones (supra-detachment basins) form in widely extended terrains at high rates of extension. The initial
crnst seems to have been young, relatively hot and
thick promoting rapid extension and low-angle faulting. The volcanism of these basins is typically calcalkalic. The bounding faults of these basins dip
gently (15-35°) forcing the depocenters to develop
distal to the main bounding fault. The basin fills are
relatively thin (typically 1-3 km) and frequently interrnpted by unconformities. Cratonward individual rift
basins of this type tend to lack post-rift subsidence.
Their underlying crnst is still thick and not additionally heated by upwelling mantle material (e.g.
Tucano basin in Brazil, Fig. 12.1 c; Karner et a1.
1992). Many basins display characteristics of both
end members.
Detaehment zones were first postulated for the Basin-andRange Provinee of the western United States (Wernieke
and Burehfiel 1982; Wernieke 1985) and eonfirmed by
deep seismie refleetion profiling in many other regions, for
example on the Atlantie eontinental margins (e.g.,
Enaeheseu 1987; Boillot and Winterer 1988; Benson and
Doyle 1988; Manspeizer 1988b; Tankard and Welsink
1988).
To reeoneile the hypotheses of simple shear and pure
shear, Barbier et al. (1986) proposed eombined shear, i.e.,
a meehanism eomprising thinning of the erust by both pure
shear and simple shear (see also Ziegler 1988). Modified
eoneepts taking into aeeount the strain- and time-dependent
meehanieal behavior of the upper and lower erust have
545
been put forward, e.g., by Bassi et al. (1993) and Gartrell
(1997). Neugebauer (1987) emphasizes that lithospherie
thinning is eontrolled not only by one prineipa1 parameter,
but by severa1 independent processes. Dome, rift, and basin
struetures represent stages of the thinning proeess and thus
form a genetie sequenee. Long-term, slow thinning may be
superimposed by short-term episodes of inereased tectonic
and magmatie aetivity.
Today, an enormous body of new information on modem and aneient rift basins has beeome availab1e (e.g.
Landon 1994; Friedmann and Burbank 1995). These results show that many rift basins are more eomp1ex and that
their evolution is less predietab1e than was earlier assumed.
Other structural features deviating from the c1assical
rift model include:
- Rifts are not necessarily continuous features as illustrated in small-scale maps; they may be broken
into special segmentsby transverse struetures (transfer or accommodation zones, Fig. 12.2a and d). The
evolution of drainage systems and the direction of
their sediment transport tend to change from one rift
segment to the other. However, at the transition from
rifting to drifting, as e.g. in the Red Sea, older irregular rift structures are abandoned and replaced by laterally continuous depocenters and high-angle normal
faults (Fig. 12.2f).
Transfer zones in extensiona1 regions displaya wide range
of geometries from diserete fault zones to zones of broad
warping (Morley et al. 1990). The direetion of asymmetry
may alternate between neighboring segments, as observed,
for example, in the region of Lake Tanganyika, East Afriea
(Burgess et al. 1988). Transfer zones are often preferred
sites of volcanism (Maek and Seager (1995). The smaller
the seale of observation, the more eomplex rifts beeome.
Individual segments vary in their struetural style and thus
also in their basin fill. However, such rift struetures may
evolve into a full symmetrie graben (Woldegabriel et al.
1990).
- Rift zones frequently evolve along pre-existing lineaments in the crnst and may display a substantial
strike-slip component (Fig. 12.2c,e; cf. Sect. 12.8).
As long as extension exceeds strike-slip, the term
"rift basin" is still commonly used.
- Many rifts are not symmetrical, with the deepest
basin floor 10cated in the center, but display the pronounced asymmetry of a half-graben (Figs. 12.1e and
12.4)
- Rifting along a rift zone does not necessarily occur
simultaneously, but may proceed from one end to the
other, as found in the East African rift zone (see below).
- One may find both thermally induced rift zones
with initial updorning (Fig. 12.3b), as well as other
rifts caused, for example, by continental collision and
escape, which lack an early phase of uplift.
Simple shear. Many recent investigations of rift
zones have revealed that crnstal stretching is accomplished by slip along an intra-crustal detachment
surface (simple shear, Fig. l2.1b). In the detachment
model, extension is nonuniform and decreases in the
direction of shear. Simple shear allows the lateral
separation of the zone of initial rifting and the zone
of magma upwelling which may initiate ocean
spreading (Fig. l2.lb and c). Low-angle detachments
can also explain the coeval initiation of different basin types (Fig. l2.1d).
The principal differences between the basins resulting from pure and simple shear can be demonstrated
best by defining their end members (Friedmann and
Burbank 1995):
- End members of pure shear (intracontinental rifts)
evolve in old, thermally equilibrated crnst of normal
thickness and are mostly accompanied by alkali to
tholeitic magmatism. Crnstal extension is limited (1025%), and the normal geometry of the basin and
subbasins is that of asymmetric grabens or halfgrabens bounded by steeply dipping (45-60°) normal
faults. The basin centers subside deeply (6-10 km),
and their depocenters tend to develop along the active main boundary fault. Examples: East African rift
and Baikai rift (see below).
- End members of simple shear on top of detachment
zones (supra-detachment basins) form in widely extended terrains at high rates of extension. The initial
crnst seems to have been young, relatively hot and
thick promoting rapid extension and low-angle faulting. The volcanism of these basins is typically calcalkalic. The bounding faults of these basins dip
gently (15-35°) forcing the depocenters to develop
distal to the main bounding fault. The basin fills are
relatively thin (typically 1-3 km) and frequently interrnpted by unconformities. Cratonward individual rift
basins of this type tend to lack post-rift subsidence.
Their underlying crnst is still thick and not additionally heated by upwelling mantle material (e.g.
Tucano basin in Brazil, Fig. 12.1 c; Karner et a1.
1992). Many basins display characteristics of both
end members.
Detaehment zones were first postulated for the Basin-andRange Provinee of the western United States (Wernieke
and Burehfiel 1982; Wernieke 1985) and eonfirmed by
deep seismie refleetion profiling in many other regions, for
example on the Atlantie eontinental margins (e.g.,
Enaeheseu 1987; Boillot and Winterer 1988; Benson and
Doyle 1988; Manspeizer 1988b; Tankard and Welsink
1988).
To reeoneile the hypotheses of simple shear and pure
shear, Barbier et al. (1986) proposed eombined shear, i.e.,
a meehanism eomprising thinning of the erust by both pure
shear and simple shear (see also Ziegler 1988). Modified
eoneepts taking into aeeount the strain- and time-dependent
meehanieal behavior of the upper and lower erust have
545
been put forward, e.g., by Bassi et al. (1993) and Gartrell
(1997). Neugebauer (1987) emphasizes that lithospherie
thinning is eontrolled not only by one prineipa1 parameter,
but by severa1 independent processes. Dome, rift, and basin
struetures represent stages of the thinning proeess and thus
form a genetie sequenee. Long-term, slow thinning may be
superimposed by short-term episodes of inereased tectonic
and magmatie aetivity.
Today, an enormous body of new information on modem and aneient rift basins has beeome availab1e (e.g.
Landon 1994; Friedmann and Burbank 1995). These results show that many rift basins are more eomp1ex and that
their evolution is less predietab1e than was earlier assumed.
Other structural features deviating from the c1assical
rift model include:
- Rifts are not necessarily continuous features as illustrated in small-scale maps; they may be broken
into special segmentsby transverse struetures (transfer or accommodation zones, Fig. 12.2a and d). The
evolution of drainage systems and the direction of
their sediment transport tend to change from one rift
segment to the other. However, at the transition from
rifting to drifting, as e.g. in the Red Sea, older irregular rift structures are abandoned and replaced by laterally continuous depocenters and high-angle normal
faults (Fig. 12.2f).
Transfer zones in extensiona1 regions displaya wide range
of geometries from diserete fault zones to zones of broad
warping (Morley et al. 1990). The direetion of asymmetry
may alternate between neighboring segments, as observed,
for example, in the region of Lake Tanganyika, East Afriea
(Burgess et al. 1988). Transfer zones are often preferred
sites of volcanism (Maek and Seager (1995). The smaller
the seale of observation, the more eomplex rifts beeome.
Individual segments vary in their struetural style and thus
also in their basin fill. However, such rift struetures may
evolve into a full symmetrie graben (Woldegabriel et al.
1990).
- Rift zones frequently evolve along pre-existing lineaments in the crnst and may display a substantial
strike-slip component (Fig. 12.2c,e; cf. Sect. 12.8).
As long as extension exceeds strike-slip, the term
"rift basin" is still commonly used.
- Many rifts are not symmetrical, with the deepest
basin floor 10cated in the center, but display the pronounced asymmetry of a half-graben (Figs. 12.1e and
12.4)
- Rifting along a rift zone does not necessarily occur
simultaneously, but may proceed from one end to the
other, as found in the East African rift zone (see below).
- One may find both thermally induced rift zones
with initial updorning (Fig. 12.3b), as well as other
rifts caused, for example, by continental collision and
escape, which lack an early phase of uplift.