Continental rifts of the world

By:

FARHAN HAMEED

Continental rifts are regions of extensionaldeformation where the entire thickness of thelithosphere has deformed under the influenceof deviatoric tension.

Rifts represent the initial stage of continentalbreak-up where extension may lead tolithospheric rupture and the formation of anew ocean basin.

Continental rifts initiate with doming.

The doming induces tensional stress in theupper crust which results in normal faulting.

There is increased heat flow. The heat flow increases asthe crust thins by ductile shear of the lower crust andlithosphere and normal faulting in the brittle uppercrust. Bimodal (basaltic and rhyolitic) volcanism areresults from the increased heat flow.

If rifting continues the crust/lithosphere willthin to zero and seafloor spreading is initiatedforming a new ocean basin between twodrifting continents.

The East African Rift (EAR) is an activecontinental rift zone in East Africa. The EARbegan developing around the onset of theMiocene, 22-25 million years ago. In the past, itwas considered to be part of a larger Great RiftValley that extended north to Asia Minor.

Over time, many theories have tried to clarify theevolution of the East African Rift.

In 1972 it was proposed that the EAR was notcaused by tectonic activity, but rather bydifferences in crustal density.

Others proposed an African superplume causingmantle deformation.

The most recent and accepted view is the theoryput forth in 2009, that magmatism and platetectonics have a feedback with one another,controlled by oblique rifting conditions.

At that time it was suggested that lithosphericthinning generated volcanic activity, furtherincreasing the magmatic processes at play suchas intrusions and numerous small plumes.These processes further thin the lithosphere inconcentrated areas, forcing the thinninglithosphere to behave like a mid-ocean ridge.

Prior to rifting, enormous continental floodbasalts erupted on the surface and uplift of theEthiopian, Somalian, and East African plateausoccurred.

The first stage of rifting of the EAR ischaracterized by rift localization andmagmatism along the entire rift zone.

Periods of extension alternated with times ofrelative inactivity.

There was also the reactivation of a pre-Cambrian weakness in the crust, a suture zoneof multiple cratons, displacement along largeboundary faults, and the development of deepasymmetric basins.

The second stage of rifting is characterized bythe deactivation of large boundary faults, thedevelopment of internal fault segments, andthe concentration of magmatic activity towardsthe rifts.

Today, the narrow rift segments of the EastAfrican Rift system form zones of localizedstrain. These rifts are the result of the actions ofnumerous normal faults which are typical of alltectonic rift zones. As voluminous magmatismand continental flood basalts characterize someof the rift segments, while other segments, suchas the Western branch, have only very smallvolumes of volcanic rock.

The East African Rift Zone includes a number ofactive as well as dormant volcanoes, among them.

Active volcanos include Erta Ale, DallaFilla, andOl Doinyo Lengai, the former of which is acontinuously active basaltic shield volcano in theAfar Region of northeastern Ethiopia.

When DallaFilla erupted in 2008 it was the largestvolcanic eruption in Ethiopia in recorded history.

The Ol Doinyo Lengai volcano is currently theonly active natrocarbonatite volcano in the world.

The magma contains almost no silica, makingthe flow viscosity extremely low.

“Its lava fountains crystallize in midair then shatter like glass.”

(National Geographic)

Approximately 50 volcanic structures inEthiopia alone have documented activity sincethe onset of the Holocene.

The EAR is the largest seismically active riftsystem on Earth today.

The majority of earthquakes occur near theAfar Depression, with the largest earthquakestypically occurring along or near major borderfaults.

Seismic events in the past century areestimated to have reached a maximum momentmagnitude of 7.0

The seismicity trends parallel to the rift system,with a shallow focal depth of 12–15 km beneaththe rift axis.

Further away from the rift axis, focal depthscan reach depths of over 30 km.

The Rio Grande Rift is a north-trendingcontinental rift zone.

It separates the Colorado Plateau in the westfrom the interior of the North American cratonon the east.

The rift extends from central Colorado in thenorth to the state of Chihuahua, Mexico in thesouth.

The Rio Grande Rift represents at the eastern mostmanifestation of widespread extension in thewestern U.S. during the past 35 million years.

The rift consists of three major basins and manysmaller basins, less than 100 km. The three majorbasins (from northernmost to southernmost) arethe San Luis, Espanola, and Albuquerque basins.Further south, the rift is defined by a network ofsmaller, less topographically distinct alternatingbasins and ranges. The distinction between thesesmaller basins and those of the Basin and RangeProvince becomes blurred in northern Mexico.

Basin size generally decreases to the north inthe rift, where the Espanola coversapproximately 120 km north-south and 40 kmeast-west, and the San Luis is roughly 120 kmby 80 km.

The Albuquerque basin is the largest of thethree basins, spanning 160 km north-south and86 km east-west at its widest points. It is theoldest of the three major basins, and contains7,350 m of Paleocene clastic sedimentsdeposited on Precambrian basement.

The Baikal Rift Zone is a divergent plateboundary centered beneath Lake Baikal insoutheastern Russia.

Along it form a series of basins more than 2,000kilometers long. To its west are the EurasianPlate and the Siberian platform and to its east isthe Amur Plate which is moving away from therift toward Japan at about 4 mm per year.

The Gulf of Suez Rift is a continental rift zonethat was active between the Late Oligocene(28 Ma) and the end of the Miocene (5 Ma).

It represented a continuation of the Red SeaRift until break-up occurred in the middleMiocene, with most of the displacement on thenewly developed Red Sea spreading centrebeing accommodated by the Dead SeaTransform.

The Blue Nile rift is a major geologicalformation in the Sudan, a rift with a NW trendthat terminates on the Central African ShearZone. It was formed through crustal extensionduring the break-up of Gondwana.

The White Nile rift is one of several rifts incentral Sudan running in a NW direction andterminating in the Central African Shear Zone.

The rift is a Cretaceous/Tertiary structure thathas similar tectonic characteristics to theSouthern Sudan Rift.

The West Antarctic Rift is a major, active riftvalley lying between East and West Antarctica.It encompasses the Ross Sea, the area under theRoss Ice Shelf and a part of West Antarctica. Itsevolution is due to lithospheric thinning of thenon-cratonic area of West Antarctica.

Bahr el Arab rift

Red Sea Rift

Anza trough

Melut Basin

Muglad Basin