Metamorphic or igneous rocks below a sedimentary platform or cover
In
geology
,
basement
and
crystalline basement
are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment.
[1]
The basement rocks lie below a
sedimentary
platform
or cover, or more generally any rock below
sedimentary rocks
or
sedimentary basins
that are
metamorphic
or
igneous
in origin. In the same way, the
sediments
or sedimentary rocks on top of the basement can be called a "cover" or "sedimentary cover".
Crustal rocks are modified several times before they become basement,
[
clarification needed
]
and these transitions alter their composition.
[1]
Continental crust
[
edit
]
Basement rock is the thick foundation of ancient, and oldest,
metamorphic
and
igneous rock
that forms the
crust
of
continents
, often in the form of
granite
.
[2]
Basement rock is contrasted to overlying
sedimentary rocks
which are laid down on top of the basement rocks after the continent was formed, such as
sandstone
and
limestone
. The sedimentary rocks which may be deposited on top of the basement usually form a relatively thin veneer, but can be more than 5 kilometres (3 mi) thick. The basement rock of the crust can be 32?48 kilometres (20?30 mi) thick or more. The basement rock can be located under layers of sedimentary rock, or be visible at the surface.
Basement rock is visible, for example, at the bottom of the
Grand Canyon
, consisting of 1.7- to 2-billion-year-old
granite
(
Zoroaster Granite
) and
schist
(
Vishnu Schist
). The Vishnu Schist is believed to be highly metamorphosed igneous rocks and
shale
, from
basalt
,
mud
and
clay
laid from volcanic eruptions, and the granite is the result of magma intrusions into the Vishnu Schist. An extensive cross section of sedimentary rocks laid down on top of it through the ages is visible as well.
The basement rocks of the continental crust tend to be much older than the oceanic crust.
[3]
The oceanic crust can be from 0?340 million years in age, with an average age of 64 million years.
[4]
Continental crust is older because continental crust is light and thick enough so it is not subducted, while oceanic crust is periodically subducted and replaced at
subduction
and
oceanic rifting
areas.
Complexity
[
edit
]
The basement rocks are often highly metamorphosed and complex, and are usually
crystalline
.
[5]
They may consist of many different types of rock ? volcanic, intrusive igneous and metamorphic. They may also contain
ophiolites
, which are fragments of
oceanic crust
that became wedged between plates when a
terrane
was accreted to the edge of the continent. Any of this material may be folded, refolded and metamorphosed. New igneous rock may freshly intrude into the crust from underneath, or may form
underplating
, where the new igneous rock forms a layer on the underside of the crust. The majority of continental crust on the planet is around 1 to 3 billion years old, and it is theorised that there was at least one period of rapid expansion and accretion to the continents during the Precambrian.
Much of the basement rock may have originally been oceanic crust, but it was highly metamorphosed and converted into
continental crust
. It is possible for oceanic crust to be subducted down into the
Earth's mantle
, at subduction fronts, where oceanic crust is being pushed down into the mantle by an overriding plate of oceanic or continental crust.
Volcanism
[
edit
]
When a plate of oceanic crust is subducted beneath an overriding plate of oceanic crust, as the underthrusting crust melts, it causes an upwelling of magma that can cause
volcanism
along the subduction front on the overriding plate. This produces an oceanic
volcanic arc
, like
Japan
. This volcanism causes
metamorphism
, introduces
igneous intrusions
, and thickens the crust by depositing additional layers of
extrusive igneous rock
from volcanoes. This tends to make the crust thicker and less dense, making it immune to subduction.
[6]
Oceanic crust can be subducted, while continental crust cannot. Eventually, the subduction of the underthrusting oceanic crust can bring the volcanic arc close to a continent, with which it may collide. When this happens, instead of being subducted, it is
accreted
to the edge of the continent and becomes part of it. Thin strips or fragments of the underthrusting oceanic plate may also remain attached to the edge of the continent so that they are wedged and tilted between the converging plates, creating
ophiolites
. In this manner, continents can grow over time as new
terranes
are accreted to their edges, and so continents can be composed of a complex quilt of terranes of varying ages.
As such, the basement rock can become younger going closer to the edge of the continent. There are exceptions, however, such as
exotic terranes
. Exotic terranes are pieces of other continents that have broken off from their original parent continent and have become accreted to a different continent.
Cratons
[
edit
]
Continents can consist of several continental
cratons
? blocks of crust built around an initial original core of continents ? that gradually grew and expanded as additional newly created terranes were added to their edges. For instance,
Pangea
consisted of most of the Earth's continents being accreted into one giant
supercontinent
. Most continents, such as Asia, Africa and Europe, include several continental cratons, as they were formed by the accretion of many smaller continents.
Usage
[
edit
]
In
European geology
, the basement generally refers to rocks older than the
Variscan orogeny
. On top of this older basement
Permian
evaporites
and
Mesozoic
limestones
were deposited. The evaporites formed a
weak
zone on which the harder (stronger) limestone
cover
was able to move over the hard basement, making the distinction between basement and cover even more pronounced.
[
citation needed
]
In
Andean
geology the basement refers to the
Proterozoic
,
Paleozoic
and early
Mesozoic
(
Triassic
to
Jurassic
) rock units as the basement to the late Mesozoic and
Cenozoic
Andean sequences developed following the onset of
subduction
along the western margin of the
South American Plate
.
[7]
When discussing the
Trans-Mexican Volcanic Belt
of
Mexico
the
basement
include Proterozoic, Paleozoic and Mesozoic age rocks for the Oaxaquia, the Mixteco and the Guerrero
terranes
respectively.
[8]
The term basement is used mostly in disciplines of geology like
basin geology
,
sedimentology
and
petroleum geology
in which the (typically
Precambrian
) crystalline basement is not of interest as it rarely contains
petroleum
or
natural gas
.
[9]
The term
economic basement
is also used to describe the deeper parts of a cover sequence that are of no economic interest.
[10]
See also
[
edit
]
- Shield
? Large stable area of exposed Precambrian crystalline rock
- Bedrock
? Solid rock under loose surface material
References
[
edit
]
- ^
a
b
This article incorporates
public domain material
from
Map Shows Content and Origins of the Nation’s Geologic Basement
.
United States Geological Survey
. April 23, 2015.
- ^
"Bedrock | Encyclopedia.com"
.
www.encyclopedia.com
. Retrieved
2019-04-09
.
- ^
"Basement domain list by region"
. USGS.
- ^
Seton, M; Muller, RD; Zahirovic, S; Williams, S; Wright, NM; Cannon, J; et al. (2020).
"A global data set of present-day oceanic crustal age and seafloor spreading parameters"
.
Geochemistry, Geophysics, Geosystems
.
21
(10): e2020GC009214.
Bibcode
:
2020GGG....2109214S
.
doi
:
10.1029/2020GC009214
.
hdl
:
1885/289849
.
S2CID
224967179
.
- ^
Burwash, RA (1987). "Basement".
Structural Geology and Tectonics. Encyclopedia of Earth Science
. Berlin, Heidelberg: Springer. pp. 26?29.
doi
:
10.1007/3-540-31080-0_6
.
ISBN
0-442-28125-0
.
- ^
"Volcanism | geology"
.
Encyclopedia Britannica
. Retrieved
2019-04-09
.
- ^
Teresa Moreno, et al.,
The geology of Chile,
Geological Society of London, 2007, Ch. 2 Metamorphic and Igneous Basement Complexes, p. 5,
ISBN
978-1-86239-220-5
- ^
A. Gomez-Tuena, Ma.T. Orozco-Esquivel, and L. Ferrari
Igneous petrogenesis of the Trans-Mexican Volcanic Belt
, Ch 5, in Susana A. Alaniz-Alvarez and Angel F. Nieto-Samaniego, eds.,
Geology of Mexico,
Geological Society of America Special Paper 422, 2007, pp. 142?145
ISBN
978-0-8137-2422-5
- ^
Gay, Parker (2002)
Mapping Geologic Structure of Basement and Role of Basement in Hydrocarbon Entrapment,
Search and Discovery Article #40052 (adapted from: AAPG Explorer (November and December, 1999)
- ^
Mulhadiano J.A.S. (1984).
"The Determination of Economic Basement of Rock Formation in Exploring the Langkat?Medan Area, North Sumatra Basin"
. AAPG: 75?107
. Retrieved
2019-04-09
.
Sources
[
edit
]
- Parker, Sybil P., ed. (2003).
McGraw-Hill dictionary of geology and mineralogy
(2nd ed.). New York: McGraw-Hill.
ISBN
0-07-141044-9
.
OCLC
50731349
.
.
- Bates, Robert L.; Jackson, Julia A., eds. (1984).
Dictionary of geological terms
. Bates, Robert Latimer, 1912-, Jackson, Julia A., 1939-, American Geological Institute. (3rd ed., Anchor books ed.). Garden City, N.Y.: Anchor Press/Doubleday.
ISBN
0-385-18100-0
.
OCLC
9412868
.
- Basement and basins of eastern North America
. Van der Pluijm, Ben A., 1955-, Catacosinos, Paul A., 1933-. Boulder, Colo.: Geological Society of America. 1996.
ISBN
0-8137-2308-6
.
OCLC
35068112
.
{{
cite book
}}
: CS1 maint: others (
link
)
- "Western metamorphic and sedimentary basement / New Zealand's Rocks / NZ Geology / Science Topics / Learning / Home - GNS Science"
.
www.gns.cri.nz
. Retrieved
2019-12-26
.
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