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Horst and Graben
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The fabric
of the Basin and Range throughout most of the region is such that the
mountain ranges are oriented lengthwise north and south. With this in mind,
consider that if one crosses Nevada in a straight line from the east to the west, one will
"cross the fabric of the land" and repeatedly go from valley to mountain
range to valley and so on, as is diagrammed in the adjacent image. Make note
that geologists use the term "horst" for the mountain range and "graben"
for the valley. This is only true when there are faults separating the two.
Shown in the diagram are the bounding faults. These faults have been named
"listeric normal faults". These faults have a concave-upward surface because
the landscape is being pulled apart by deep-seated tensional forces. These
tensional forces are derived from the recent and on-going tectonic
characteristics of the region and give it its special character. Many of
the fault traces throughout the region show very recent signs of movement,
and most of the Great Basin has a very high potential for earthquakes because
of these active faults.
Volcanic action and the Comstock
The Great Basin has had a very complex geologic history that started in
Precambrian times. The region has been close to the continent's edge on and
off over great periods of time. It has seen numerous, sometimes major tectonic
events that at times have added crust and at other times have removed crust.
For long periods, great seas covered the land and added sediment. At other
times, such as in the Tertiary, great volumes of volcanic rock were added
while volcanoes wreaked havoc throughout the region. Within these volcanic
rocks along the focus of a normal fault the famously rich Comstock Lode formed.
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Horst and Graben
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