5.8-Three Examples of Rifting Systems

5.8-Three Examples of Rifting Systems
1) young East African Rift, including the Red Sea; 2) mature mid-Atlantic Ridge; 3) ancient Keweenawan failed rift in the US mid-continent

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The development of a new ocean basin begins with the formation of a continental rift, an elongated depression in which the entire thickness of the lithosphere has been deformed. Examples of continental rifts include the East African Rift, the Baikal Rift in south central Siberia, the Rhine Valley in northwestern Europe, the Rio Grande rift, and the Basin and Range province in the Western United States. It appears that continental rifts form in a variety of tectonic settings and may result in the breakup of a land mass.

In those settings where rifting continues, the rift system will evolve into a young, narrow ocean basin, exemplified by the present day Red Sea. If spreading continues, the Red Sea will grow wider and develop an elevated oceanic ridge similar to the Mid-Atlantic Ridge. Eventually, seafloor spreading results in the formation of a mature ocean basin bordered by rifted continental margins. The Atlantic Ocean is such a feature.

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The East African Rift

An example of an active continental rift is the East African rift, which extends through eastern Africa for approximately 2,000 miles. Rather than being a single rift, the East African Rift consists of several somewhat interconnected rift valleys that split into an eastern and western section around Lake Victoria. Whether this rift will develop into a spreading center where the Somali subplate separates from the continent of Africa is still being debated. Nevertheless, the East African Rift is thought to characterize the initial stage in the breakup of a continent.

The most recent period of rifting began about 20 million years ago as upwelling in the mantle forcefully intruded the base of the lithosphere. Buoyant uplifting of the heated lithosphere led to doming of the crust. As a consequence, the upper crust was broken along steep angle normal faults, producing down-faulted blocks, or grabens, while the lower crust deformed by ductile stretching. Thus, this continental rift system closely resembles rifts found along slow spreading centers.

In its early stage of formation, magma generated by decompression melting of the rising mantle plume intrudes the crust. Some of the magma migrates along fractures and erupts at the surface. This activity produces extensive basaltic flows within the rift, as well as volcanic cones, some forming more than 100 km from the rift axis. Examples include Mount Kenya and Mount Kilimanjaro, which is the highest point in Africa, rising almost 20,000 ft above the Serengeti plane.

The Red Sea

Research suggests that if tensional forces are maintained, a rift valley will lengthen and deepen, eventually extending out to the margin of the continent, thereby splitting it in two. At this point, the continental rift becomes a narrow linear sea with an outlet to the ocean, similar to the Red Sea.

The Red Sea formed when the Arabian peninsula rifted from Africa beginning about 30 million years ago. Steep fault scarps that rise as much as 3 km above sea level flank the margins of this water body. Thus, the escarpments surrounding the Red Sea are similar to the cliffs that border the East African rift. Although the Red Sea only reaches oceanic depths up to 5 km in a few locations, symmetrical magnetic stripes indicate that typical seafloor spreading here has been taking place for the past 5 million years.

The Atlantic Ocean

If spreading continues, the Red Sea will grow wider and develop an elevated oceanic ridge similar to the Mid-Atlantic Ridge. As new oceanic crust is added to the diverging plates, the rifted continental margins move ever so slowly away from one another. As a result, the rifted continental margins that were once situated above the region of upwelling are displaced toward the interior of the growing plates. Consequently, as the continental lithosphere moves away from the source of heat, it cools, contracts, and subsides.

In time, these continental margins will subside below sea level. Simultaneously, material eroded from the adjacent land mass will be deposited atop the faulted topography of the submerged continental margin. Eventually, this material will accumulate to form a thick, broad wedge of relatively undisturbed sediment and sedimentary rock. Continental margins of this type are called passive continental margins. Because passive margins are not associated with plate boundaries, they experience little volcanism and few earthquakes. However, this was not the case when the lithospheric blocks made up the flanks of the continental rift.

The Ancient Keweenawan Failed Rift in the US Mid-continent

Not all continental rift valleys develop into full-fledged spreading centers. Running through the central United States is a failed rift that extends from Lake Superior into central Kansas. This once active rift valley is filled with volcanic rock that was extruded onto the crust more than a billion years ago. Why some rift valleys are abandoned while others develop into active spreading centers is not yet known.

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