Page 213 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 199

Present-day seismically observed section 0 Ma a specific density and elastic thickness (T e) (Section
Syn-rift 7.6.4) and then summing the effects of each layer for
erosion
surfaces successive time intervals. Corrections due to sediment

compaction, fluctuations in sea level, and estimates of
50 Ma
water depth using fossils or other sedimentary indica-
tors are then applied. This approach generally involves
using information derived from post-rift sediments
100 Ma rather than syn-rift units because the latter violate
assumptions of a closed system during extension
(Kusznir et al., 2004). The results usually show that the
Water 150 Ma depth of rifted margins at successive time intervals
depends upon both the magnitude of stretching factor
Post-rift (β) and the flexural strength of the lithosphere. Most

applications indicate that the elastic thickness of the
Syn-rift Reverse modeled to the
base of post-rift sequences 200 Ma lithosphere increases as the thermal anomaly associated
with rifting decays.
0 25
Investigations of lithospheric-scale stretching factors
km
at both volcanic and nonvolcanic margins have revealed
Figure 7.35 Schematic diagram showing application several characteristic relationships. Many margins
of flexural backstripping and the modeling of post-rift show more subsidence after an initial tectonic phase

subsidence to predict sequential restorations of due to stretching than is predicted by thermal subsid-
stratigraphy and paleobathymetry. Restored sections are ence curves for uniform stretching. Rifted margins off
dependent on the β stretching factor used to define the Norway (Roberts et al., 1997), near northwest Austra-

magnitude of lithospheric extension and lithospheric lia (Driscoll & Karner, 1998), and in the Goban Spur

flexural strength (after Kusznir et al., 2004, with and Galicia Bank (Davis & Kusznir, 2004) show sig-
permission from Blackwell Publishing). nificantly more subsidence than is predicted by the

magnitude of extension indicated by upper crustal
faulting. In addition, many margins show that the
loading (Section 7.6.4) and thermal subsidence. One of magnitude of lithospheric stretching increases with
the most commonly used approaches to obtaining litho- depth within ∼150 km of the ocean–continent bound-
spheric-scale stretching factors employs a technique ary (Kusznir et al., 2004). Farther toward the conti-
known as fl exural backstripping. nent, stretching and thinning estimates for the upper
Flexural backstripping involves reconstructing crust, whole crust, and lithosphere converge as the
changes in the depth to basement in an extensional stretching factor (β) decreases. These observations
sedimentary basin by taking into account the isostatic provide important boundary conditions on the pro-
effects of loading. The concept behind the method is cesses that control the transition from rifting to sea

to exploit the stratigraphic profi le of the basin to deter- floor spreading. However, the causes of the extra sub-
mine the depth at which basement rock would be in sidence and depth-dependent stretching are uncertain.
the absence of loads produced by both water and all One possibility is that the extra subsidence results
the overlying layers. This is accomplished by progres- from extra uplift during the initial stage of sea fl oor
sively removing, or backstripping, the loads produced by spreading, perhaps as a result of upwelling anoma-
each layer and restoring the basement to its depth at lously hot asthenosphere (Hopper et al., 2003; Buck,
the time each layer was deposited (Fig. 7.35). These 2004). Alternatively, greater stretching in the mantle
results combined with knowledge of water depth theo- lithosphere than in the crust, or within a zone of
retically allow determination of the stretching factor mantle lithosphere that is narrow than in the crust,
(β). Nevertheless, as discussed further below, relation- also may result in extra uplift. Once these initial effects
ships between stretching factor and subsidence curves decay the ensuing thermal subsidence during cooling
may be complicated by interactions between the litho- would be greater than models of uniform stretching
sphere and the sublithospheric mantle. In practice, fl ex- would predict. These hypotheses, although seemingly
ural backstripping is carried out by assigning each layer plausible, require further testing.