Page 349 - Global Tectonics

332 CHAPTER 10

10.29). This geometry has allowed geoscientists to the Luzon arc and forearc sequences onto the accretion-
use spatial variations in the patterns of deformation, ary wedge and Asian continent (Fig. 10.30c). These
uplift, and sedimentation to piece together the pro- events have led to the uplift and exhumation of the
gressive evolution of an oblique collision. C.-Y. Huang underthrust Eurasian continental crust in the Coastal
et al. (2000, 2006) used this approach to propose four Range of eastern Taiwan. The last stage in the colli-
stages of arc–continent collision beginning with intra- sion/accretion process is recorded north of about 24°N
oceanic subduction and evolving through initial and where the collapse and subsidence of the accreted arc
advanced stages before the arc and forearc collapse and forearc has occurred over the last one or two million
and subside. years (Fig. 10.30d), possibly as a result of the northward
Off southern Taiwan, near latitude 21°N (Fig. subduction of the northernmost Coastal Range at the
10.29), subduction of South China Sea oceanic litho- Ryukyu Trench (Fig. 10.29). C.-Y. Huang et al. (2000)
sphere beneath the Philippine Sea plate results in volca- postulated that the Longitudinal Valley–Chingshui
nism and has formed an accretionary prism and forearc faults mark the collapsed trace of the arc where it
basin (Figs 10.29, 10.30a). The Hengchun Ridge/ approaches the subduction zone. This sequence of
Kaoping slope and North Luzon Trough represent events suggests that orogens formed by arc–continent
these two tectonic elements, respectively. Farther north, collision can progress rapidly through the initial stage
near 22°N, the North Luzon Trough narrows at the of collision to an advanced stage and even collapse of
expense of an expanding accretionary prism (Fig. 10.29). the arc and forearc in only a few million years.
In this latter region arc–continent collision began about
5 Ma and resulted in the formation of a suture between
the arc and prism. The suture records both convergent
and sinistral strike-slip motion (Malavieille et al., 2002)
and separates two zones of contrasting structural ver- 10.6 TERRANE
gence. To the east, forearc sequences have been thrust
eastward toward the arc, forming the Huatung Ridge ACCRETION AND
(Fig. 10.29). To the west, forearc material on the Asian
continental slope and South China Sea basin is carried CONTINENTAL
westward within a growing accretionary prism. On the
Hengchun Peninsula, Miocene slates and turbidites of GROWTH
the prism have been uplifted and exposed. These and
other observations suggest that the initial stage of
oblique arc–continent collision involves the following
processes (Fig. 10.30b): 10.6.1 Terrane analysis

1 uplift and erosion of the accretionary prism and Many orogens are composed of a collage of fault-
the continued deposition of forearc basin bounded blocks that preserve geologic histories unre-
sequences; lated to those of adjacent blocks. These units are known
as terranes and may range in size from a few hundreds
2 waning arc volcanism and the build-up of
to thousands of square kilometers. Terranes usually are
fringing reefs on inactive volcanic islands;

classified into groups according to whether they are
3 arc subsidence, strike-slip faulting, and the
native or exotic to their adjacent continental cratons
development of intraarc pull-apart basins;
(e.g. Section 11.5.5). Exotic (or allochthonous) terranes
4 suturing, clockwise rotation, and shortening of are those that have moved relative to adjacent bodies
forearc sequences to form a syn-collisional fold and, in some cases, have traveled very great distances.
and thrust belt. For example, paleomagnetic investigations have dem-
onstrated that some terranes have a north–south com-
North of the Huatung Ridge, near 23°N, arc–conti- ponent of motion of several thousand kilometers (Beck,
nent collision has reached an advanced stage (Huang et 1980; Ward et al., 1997) and have undergone rotations
al., 2006). Here, collision since the Plio-Pleistocene has of up to 60° (Cox, 1980; Butler et al., 1989). The bound-
resulted in the west-directed thrusting and accretion of aries of terranes may be normal, reverse, or strike-slip