Page 234 - Global Tectonics

220 CHAPTER 8

The Transverse Ranges in southern California over 10 km farther south (Fig. 8.9a). The
(Figs 8.1, 8.8a) illustrate the characteristics of current smooth shape of the releasing bends
a large restraining bend in the San Andreas formed later, after unconformity DB3, as
Fault. These ranges have been uplifted in subsequent strike-slip motion formed faults
response to a combination of dextral motion that joined across the gap between the step-
and compression across a portion of the fault overs (Fig. 8.10a,b).
that strikes more westerly than the general In contrast to the extension that characterizes
strike of the fault system. Seismic refl ection the northern Dagg Basin, the southern end of
profiles and information from wells indicate the basin shows evidence of reverse faulting

that thrust faults dip northward at 25–35° and uplift. A combination of shortening and
beneath the San Gabriel Mountains and strike-slip faulting associated with a
intersect the near vertical (83°) San Andreas restraining bend in this region formed the
Fault at mid-crustal depths of ∼21 km (Fuis et Dagg Ridge, which has been squeezed
al., 2001, 2003). Earthquake focal upward between the main trace of the Alpine
mechanisms show thrust solutions on fault Fault on the west and a curved oblique-slip
splays that branch upward off the dipping fault beneath its eastern margin (Fig. 8.9c).
décollement surface (Fig. 8.8b). This South of the ridge, the Breaksea Basin
combination of motion has resulted in a preserves features that indicates it was once
zone of transpression and topographic uplift continuous with the Dagg Basin, suggesting
commonly referred to as the Big Bend. that the reverse faulting occurred after the
Examples of active releasing bends and strike- pull apart had formed (Barnes et al., 2005). As
slip basins occur along the southernmost part the total plate motion and amount of slip
of the Alpine Fault in southwest New increased, some faults were abandoned and
Zealand. Near Fiordland, three others formed linkages that cut through the
semicontinuous fault segments accommodate extensional basins, resulting in localized push-
dextral strike-slip motion between the ups and ridges where they formed restraining

Australian and Pacific plates. Along the bends (Fig. 8.10c). These relationships
Resolution segment of the plate boundary illustrate how large strike-slip faults typically
(Fig. 8.9a), geophysical surveys have revealed evolve very rapidly and that localized strike-
the presence of a Pleistocene pull-apart slip basins and uplifts develop along different
called the Dagg Basin (Barnes et al., 2001, parts of the fault zone (Fig. 8.10c) on

2005). A seismic refl ection profile across the timescales of tens to hundreds of thousands
northern part of the basin (Fig. 8.9b) shows of years.
that it is bounded on the northwest by a

ridge above an active reverse fault. Inactive 4 Strike-slip duplexes, fans, and flower structures. A
faults are buried beneath the ridge. At the strike-slip duplex is an imbricate array of two
center of the basin, upward splaying faults or more fault-bounded blocks and basins that
accommodate oblique extension, forming a occur between two or more large bounding
graben. Some west-dipping splays (labeled IA faults (Woodcock & Fischer, 1986). These
in Fig. 8.9b) presently are inactive, although structures are analogous to the duplexes that
the deposition of wedge-shaped strata form on the ramps of dip-slip faults but differ in
between the development of two that vertical movements are not constrained at
unconformities (surfaces DB3 and DB4) the upper (ground) surface. The fault-bounded
indicates that they once were active basins that characterize the duplex typically are
simultaneously with the east-dipping splays. lens-shaped. The individual blocks defi ned by
This geometry suggests that the pull-apart the strike-slip faults are shortened and uplifted
basin initially formed in an extensional step- when the faults converge and stretched and
over prior to unconformity DB3 (Barnes et downthrown where the faults diverge (Fig.
al., 2001, 2005). Another pull-apart, called the 8.5c). This tendency for strike-slip faults to
Five Fingers Basin, formed in a similar step- diverge and converge creates a characteristic