Page 130 - Fundamentals of Geomorphology

LARGE-SCALE TECTONIC AND STRUCTURAL LANDFORMS 113

a continent. The result is a modified continental- () Transtension
a
margin orogen.
3 Continent–island arc collisions occur when conti- Pull-apart
basin
nents drift towards subduction zones connected with
intra-oceanic island arcs.The continent resists signif-
icant subduction and a modified passive continen-
tal margin results. Northern New Guinea may be an
example.
4 Island arc–island arc collisions are poorly understood
because there are no present examples from which () Transpression
b
to work out the processes involved. However, the Transverse
outcome would probably be a compound intra- orogen
oceanic island arc.

Transform margins
Rather than colliding, some plates slip by one another
along transform or oblique-slip faults. Convergent and
divergent forces occur at transform margins. Divergent Figure 4.12 Landforms associated with oblique-slip faults.
or transtensional forces may lead to pull-apart basins, (a) Pull-apart basin formed by transtension. (b) Transverse
orogen formed by transpression.
of which the Salton Sea trough in the southern San
Andreas Fault system, California, USA, is a good exam-
ple (Figure 4.12a). Convergent or transpressional forces attachtoanotherbodyofcontinentalcrust.Suchwander-
may produce transverse orogens, of which the 3,000-m ing slivers go by several names: allochthonous terranes,
San Gabriel and San Bernardino Mountains (collectively displaced terranes, exotic terranes, native terranes, and
called the Transverse Ranges) in California are examples suspect terranes. Exotic or allochthonous terranes orig-
(Figure 4.12b). As transform faults are often sinuous, inate from a continent different from that against which
pull-apart basins and transverse orogens may occur near they now rest. Suspect terranes may be exotic, but
to each other. The bending of originally straight faults their exoticism is not confirmable. Native terranes man-
alsoleadstospaysandwedgesofcrust.Alonganastomos- ifestly relate to the continental margin against which
ing faults, movement may produce upthrust blocks and they presently sit. Over 70 per cent of the North
down-sagging ponds (Figure 4.13). A change in the dom- American Cordillera is composed of displaced terranes,
inant direction of stress may render all these transform most of which travelled thousands of kilometres and
margin features more complex. A classic area of trans- joined the margin of the North American craton dur-
form margin complexity is the southern section of the ing the Mesozoic and Cenozoic eras (Coney et al. 1980).
San Andreas fault system. Some 1,000 km of movement Many displaced terranes also occur in the Alps and the
has occurred along the fault over the last 25 million years. Himalayas, including Adria and Sicily in Italy (Nur and
The individual faults branch, join, and sidestep each Ben-Avraham 1982).
other, producing many areas of uplift and subsidence.

Terranes TECTONIC GEOMORPHOLOGY AND
CONTINENTAL LANDFORMS
Slivers of continental crust that somehow become
detached and then travel independently of their parent Important interactions between endogenic factors and
body, sometimes over great distances, may eventually exogenic processes produce macroscale and megascale