Geology Reference
In-Depth Information
Fig. 2.12
Uniform stretching in McKenzie's (
1978
) model of rifting
Important modern examples of transcurrent
boundaries are the San Andreas (e.g., Beck
1986
)
and North Anatolian (Sengör
1979
) faults men-
tioned above, the East Anatolian Fault of south-
ern Turkey (McKenzie
1976
), and the Dead Sea
Fault Zone in Middle East (e.g., Garfunkel
1981
;
Butler et al.
1997
).
Rift zones are extensional plate boundaries,
which usually exhibit complex systems of normal
faults, linked together by transfer zones. The
basic model of evolution of the basins associated
with these fault systems is McKenzie's (
1978
)
model, which assumes that the crustal stretching
generated by an extensional force field occurs
through a uniform continuous thinning of the
ductile lower crust, a process that is known as
pure shear. Conversely, extension in the brittle
upper crust would be accommodated symmetri-
cally by listric (upward-concave) normal faults,
as shown in Fig.
2.12
. In the model of McKenzie
(
1978
), rifting events start with rapid stretching
of the continental lithosphere, which determines
passive upwelling of asthenosphere (Fig.
2.12
).
This stage is characterized by faulting of the up-
per crust and tectonic subsidence. Then, on a time
scale of 50-100 Myrs conductive cooling of the
lithosphere determines an increase of thickness
and a phase of slow thermal subsidence that is
not accompanied by faulting.
The amount of thinning is measured by the
stretching factor
, “, which is simply given by:
“
D
H
1
/
H
2
,where
H
1
and
H
2
are respectively
the average thicknesses of the lithosphere at the
beginning and at the end of the phase of tec-
tonic subsidence. Although McKenzie's model
furnishes a simple and elegant picture of the
thermal evolution of rift basins, many conjugate
pairs of continental margins show an asymmetric
pattern of faulting and the presence of exhumed
lower crust, which cannot be explained by the
symmetric model. Wernicke (
1985
) proposed an
alternative asymmetric model of rifting that de-
scribes accurately many geological features of
these zones (Fig.
2.13
).
The key difference of Wernicke's model with
respect to McKenzie's model is the recognition of
low-angle detachment faults at crustal scale and
simple shear (that is, localized, non-distributed
shear) as the main mechanisms of lithospheric
thinning during rifting. In general, several suc-
cessive studies (e.g., Buck
1991
;Brun
1999
;
Cortietal.
2003
) have shown that the models of
McKenzie and Wernicke must be considered as
descriptions of distinct modes of rifting. Depend-
ing from crustal thickness, heat flow, and rate of
extension, we can distinguish two basic modes
of rifting: (a) a wide rift mode, characterized
by high thinning of the crust and the mantle
lithosphere over an area larger than
100 km,
and (b) a narrow rift mode, in which extension
by normal faulting is concentrated in a limited
area (less than
100 km wide). Classic modern
examples of wide and narrow rifts are respec-
tively the Basin and Range region of western
US (Hamilton
1987
) and the East African Rift
(e.g., Jestin et al.
1994
). A transitional mode of
