Deformation, Mountain Building, and the Continents - The Changing Earth: Exploring Geology and Evolution - page 258

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Figure 10.15 Faults Fractures along which movement has occurred parallel to the fracture surface
are called faults.

Striae

Fault plane

Strike
direction
of fault

Fault
plane

Dip angle
of fault

A

Footwall
block

B Fault offset

(displacement)

Hanging
wall block

a Terms used to describe the orientation of a fault plane. Striae are
scratch marks that form when one block slides past another. You can
measure offset or displacement on a fault wherever the truncated end of
one feature (point A) can be related to its equivalent across the fault (B).

b A polished, scratched fault plane and fault scarp near
Klamath Falls, Oregon.

Fault breccia

c Fault breccia, the zone of rubble along a fault in the Bighorn Mountains, Wyoming. The arrows
show the direction of movement along the fault.

sea level that were deposited in shallow and deep marine
environments.
Deformation and associated activities at convergent
plate boundaries are certainly important processes in moun-
tain building. They account for a mountain system's location
and geometry, as well as complex geologic structures, plu-
tons, and metamophism. Yet, the present-day topographic
expression of mountains is also related to several surface
processes, such as mass wasting (gravity-driven processes
including landslides), glaciers, and running water. In other
words, erosion also plays an important role in the evolution
of mountains.

Most of Earth's geologically recent and present-
day orogenies are found in two major zones or belts: the
Alpine-Himalayan orogenic belt and the circum-Pacifi c orogenic
belt (see Chapter 23). In fact, we can explain most of Earth's
past and present orogenies in terms of the geologic activity
at convergent plate boundaries.

Orogenies at Oceanic-Oceanic Plate Boundaries
Deformation, igneous activity, and the origin of a volcanic is-
land arc characterize orogenies that take place where oceanic
lithosphere is subducted beneath ocean lithosphere. Sedi-
ments derived from the island arc are deposited in an adjacent