Aulacogen
Encyclopedia
In geology
, an aulacogen is a failed arm of a triple junction
of a plate tectonics
rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops spreading. The resulting failed rift is called an aulacogen and becomes a filled graben
system within the continent.
The term "aulacogen" is derived from the Greek aulax (furrow) and was suggested by the Soviet geologist Nicholas Shatski in 1946.
The crust in an aulacogen region remains weakened by previous rifting activity and thus seismic activity and, occasionally, volcanic activity may re-occur subsequently from time to time. As aulacogens remain places of weakness, given the appropriate conditions, they can reactivate into active rift valleys again, as had happened to the Ottawa-Bonnechere Graben
in Ontario
and Quebec
, Canada
, an ancient aulacogen that reactivated during the breakup of Pangaea
.
The Mississippi embayment
with the associated New Madrid Seismic Zone
is an example of an ancient aulacogen that dates back to the breakup of the ancient continent, Rodinia
. This ancient rift was the site of extreme earthquakes in the early 19th century in the region. The Southern Oklahoma Aulogen, exposed in the Wichita Mountains
, is an Eocambrian
rift system formed as a product of intracontinetal rifting during the breakup of Pannotia
. The Rio Grande Rift
is another example. On the Southwestern European margin (offshore Portugal
) is located another abandoned rift basin (Lusitanian Basin) that evolved at the same time as the Canadian
Grand Banks
region, where the Hibernia Oil Field is located.
Abandoned rift basins that have been uplifted and exposed onshore, like the Lusitanian Basin, are important analogues of deep-sea basins located on conjugated margins of ancient rift axes.
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
, an aulacogen is a failed arm of a triple junction
Triple junction
A triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction a boundary will be one of 3 types - a ridge, trench or transform fault - and triple junctions can be described according to the types of plate margin that meet at them...
of a plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops spreading. The resulting failed rift is called an aulacogen and becomes a filled graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
system within the continent.
The term "aulacogen" is derived from the Greek aulax (furrow) and was suggested by the Soviet geologist Nicholas Shatski in 1946.
The crust in an aulacogen region remains weakened by previous rifting activity and thus seismic activity and, occasionally, volcanic activity may re-occur subsequently from time to time. As aulacogens remain places of weakness, given the appropriate conditions, they can reactivate into active rift valleys again, as had happened to the Ottawa-Bonnechere Graben
Ottawa-Bonnechere Graben
The Ottawa-Bonnechere Graben , with its branch the Timiskaming Graben, is an ancient rift valley in the Canadian Shield of Northeastern Ontario and Quebec, Canada. This rift valley was formed when the Earth's crust moved downward about a kilometre between two major fault zones known as the Mattawa...
in Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
and Quebec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, an ancient aulacogen that reactivated during the breakup of Pangaea
Pangaea
Pangaea, Pangæa, or Pangea is hypothesized as a supercontinent that existed during the Paleozoic and Mesozoic eras about 250 million years ago, before the component continents were separated into their current configuration....
.
The Mississippi embayment
Mississippi embayment
The Mississippi Embayment is a physiographic feature in the south-central United States, part of the Mississippi Alluvial Plain. It is essentially a northward continuation of the fluvial sediments of the Mississippi River Delta to its confluence with the Ohio River at Cairo, Illinois. The embayment...
with the associated New Madrid Seismic Zone
New Madrid Seismic Zone
The New Madrid Seismic Zone , sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes in the southern and midwestern United States, stretching to the southwest from New Madrid, Missouri.The New Madrid fault system was responsible for the...
is an example of an ancient aulacogen that dates back to the breakup of the ancient continent, Rodinia
Rodinia
In geology, Rodinia is the name of a supercontinent, a continent which contained most or all of Earth's landmass. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago, in the Neoproterozoic era...
. This ancient rift was the site of extreme earthquakes in the early 19th century in the region. The Southern Oklahoma Aulogen, exposed in the Wichita Mountains
Wichita Mountains
The Wichita Mountains are located in the southwestern portion of the U.S. state of Oklahoma. The mountains are a northwest-southeast trending series of rocky promontories, many capped by 540 million-year old granite. These were exposed and rounded by weathering during the Permian Period...
, is an Eocambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
rift system formed as a product of intracontinetal rifting during the breakup of Pannotia
Pannotia
Pannotia, first described by Ian W. D. Dalziel in 1997, is a hypothetical supercontinent that existed from the Pan-African orogeny about six hundred million years ago to the end of the Precambrian about five hundred and fifty million years ago. It is also known as the Vendian supercontinent...
. The Rio Grande Rift
Rio Grande Rift
The Rio Grande Rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuahua, Mexico in the south. The rift zone consists of four...
is another example. On the Southwestern European margin (offshore Portugal
Portugal
Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...
) is located another abandoned rift basin (Lusitanian Basin) that evolved at the same time as the Canadian
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
Grand Banks
Grand Banks
The Grand Banks of Newfoundland are a group of underwater plateaus southeast of Newfoundland on the North American continental shelf. These areas are relatively shallow, ranging from in depth. The cold Labrador Current mixes with the warm waters of the Gulf Stream here.The mixing of these waters...
region, where the Hibernia Oil Field is located.
Abandoned rift basins that have been uplifted and exposed onshore, like the Lusitanian Basin, are important analogues of deep-sea basins located on conjugated margins of ancient rift axes.