Page 183 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 169

kilometers into the middle and, possibly, the large-offset normal faults may evolve from
lower crust. high-angle faults by fl exural rotation (Section
Extensional detachment faults are low-angle 7.6.4). As the hanging wall is removed by slip
(<30°), commonly domed fault surfaces on the fault, the footwall is mechanically
of large areal extent that accommodate unloaded and results in isostatic uplift and
displacements of 10–50 km (Axen, 2004). The doming (Buck, 1988; Wernicke & Axen,
footwalls of these faults may expose a thick 1988). The doming can rotate the normal
(0.1–3 km) ductile shear zone that initially fault to gentler dips and lead to the
formed in the middle or lower crust and formation of new high-angle faults.
later evolved into a frictional (brittle) slip The variety of Cenozoic fault patterns
surface as it was unroofed during the that typify the Basin and Range is illustrated
extension (Wernicke, 1981). In the Basin and in Fig. 7.14, which shows a segment of the
Range, these features characterize regions eastern Great Basin in Utah and eastern
that have been thinned to such an extent Nevada (Niemi et al., 2004). The 350 km long
(100–400% extension) that the upper crust Wasatch Fault Zone is composed of multiple
has been completely pulled apart and segments with the largest displaying dips
metamorphic rocks that once resided in the ranging from 35° to 70° to the west. Its
middle and lower crust have been exhumed. subsurface geometry is not well constrained
These domed regions of deeply denuded but it probably penetrates at least through
crust and detachment faulting are the the upper crust. The Sevier Desert
hallmarks of the Cordilleran extensional Detachment Fault dips 12° to the west and
metamorphic core complexes (Crittenden et al., can be traced continuously on seismic

1980; Coney & Harms, 1984). Core refl ection profiles to a depth of at least 12–
complexes are relatively common in the 15 km (Fig. 7.14b). The range-bounding
Basin and Range (Figs 7.13, 7.14), although Spring Valley and Egan Range faults
they are not unique to this province. Their penetrate to at least 20 km depth and possibly
ages are diverse with most forming during through the entire 30 km thickness of the
Late Oligocene–Middle Miocene time crust at angles of ∼30°. The Snake Range
(Dickinson, 2002). Similar features occur in Detachment also dips ∼30° through most of
many other settings, including the southern the upper crust. Large-magnitude extension
Aegean Sea, in rifts that form above along the Snake Range (Miller et al., 1999)
subduction zones, such as the and Sevier Desert (Stockli et al., 2001)
D’Entrecasteaux Islands (Section 7.8.2), near detachment faults began in Early Miocene
oceanic spreading centers (Section 6.7), and time and Late Oligocene or Early Miocene
in zones of extension within collisional time, respectively. In most areas, high-angle
orogens (Section 10.4.4). normal faults are superimposed on these
Most authors view core complexes as older structures.
characteristic of regions where weak crustal
rheologies facilitate lateral fl ow in the deep
crust and, in some cases, the mantle, causing
upper crustal extension to localize into 7.4 VOLCANIC
narrow zones (Sections 7.6.2, 7.6.5).
Nevertheless, the mechanics of slip on low-
angle normal faults is not well understood. ACTIVITY
Much of the uncertainty is centered on

whether specific examples initially formed at
low angles or were rotated from a steep 7.4.1 Large igneous provinces
orientation during deformation (Axen, 2004).
The consensus is that both types probably Many rifts and rifted margins (Section 7.7.1) are associ-
occur (Section 7.8.2). Some low-angle, ated with the subaerial eruption of continental fl ood