Page 121 - Fundamentals of Geomorphology

104 STRUCTURE

sedimentary prisms to continental margins through 245 million years ago
metamorphism, by the stacking of thrust sheets, by the
sweeping up of microcontinents and island arcs at their
leading edges, and by the addition of magma through
intrusions and extrusions (Condie 1989). Geologists
haveestablishedtherelativemovementofcontinentsover
the Phanerozoic aeon with a high degree of confidence,
although pre-Pangaean reconstructions are less reliable
than post-Pangaean reconstructions. Figure 4.4 charts
the probable break-up of Pangaea.
160 million years ago
Diastrophic processes

Traditionally, tectonic (or geotectonic) forces are divided
into two groups: (1) diastrophic forces and (2) vol-
canic and plutonic forces. Diastrophic forces lead to
the folding, faulting, uplift, and subsidence of the litho-
sphere. Volcanic forces lead to the extrusion of magma
on to the Earth’s surface as lava and to minor intru-
sions (e.g. dykes and sills) into other rocks. Plutonic 95 million years ago
forces, which originate deep in the Earth, produce major
intrusions (plutons) and associated veins.
Diastrophic forces may deform the lithosphere
through folding, faulting, uplift, and subsidence. They
are responsible for some of the major features of the
physical toposphere. Two categories of diastrophism are
recognized: orogeny and epeirogeny, but these terms
are a source of much confusion (Ollier and Pain 2000,
4–8). Orogeny literally means the genesis of moun-
tains, and when first used it meant just that. Later, it 30 million years ago
became associated with the idea of folding, and eventu-
ally it came to mean the folding of rocks in fold belts.
As mountain building is not associated with the fold-
ing of rocks, it cannot be synonymous with orogeny
(Ollier 2003). Epeirogeny is the upheaval or depression
of large areas of cratons without significant folding or
fracture. The only folding associated with epeirogeny is
the broadest of undulations. Epeirogeny includes iso-
static movements, such as the rebound of land after
an ice sheet has melted, and cymatogeny, which is the Figure 4.4 Changing arrangement of continents over the
last 245 million years, showing the break-up of Pangaea,
arching, and sometimes doming, of rocks with little during the Early Triassic period; during the Callovian age
deformation over 10–1,000 km. Some geomorphologists (Middle Jurassic); during the Cenomanian age (Late
believe that mountains result from the erosion of areas Cretaceous); and during the Oligocene epoch. All maps
uplifted epeirogenically (e.g. Ollier and Pain 2000, 8; use Mollweide’s equal-area projection.
Ollier 2003; see Huggett 2006, 29–30). Source: Adapted from maps in Smith et al. (1994)