Deformation, Mountain Building, and the Continents - The Changing Earth: Exploring Geology and Evolution - page 263

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Figure 10.18 Origins of Horsts and Grabens

Horst

Graben

Normal fault

Magma

Block-faulting and the origin of horsts and grabens. Many of the mountain ranges in the Basin
and Range Province of the western United States and northern Mexico formed in this manner.

a

The Stillwater Range in Nevada is a horst bounded by normal faults.

b

Terranes and the Origin of Mountains
In the preceding section, we discussed orogenies along conver-
gent plate boundaries that result in adding material to a con-
tinent, a process termed continental accretion . Much of the
material added to continental margins is eroded older con-
tinental crust, but some plutonic and volcanic rocks are new
additions. During the 1970s and 1980s, however, geologists

discovered that parts of many mountain systems are also made
up of small, accreted lithospheric blocks that clearly originated
elsewhere. These terranes ,* as they are called, are fragments

*Some geologists prefer the terms suspect terrane, exotic terrane, or displaced
terrane . Notice also the spelling of terrane as opposed to the more familiar
terrain , the latter a geographic term indicating a particular area of land.