Why do Continents Break Up?

The Earth's interior is hot. The heat comes from the heat of formation of the Earth that has not yet dissipated and heat generated by decay of unstable isotopes distributed through the mantle and crust. While the lithosphere cools primarily by conduction, the mantle cools by convection. Most of the convective heat from the mantle is dissipated at the midocean ridges and through cooling seafloor. Beneath large continents, however, heat builds up in the mantle. This excess heat should weaken the continental lithosphere making it easier to rift.

But what forces could cause a continent to come apart?

membrane stresses? The Earth's curvature is greater at the equator than at the poles and continents drifting across latitudes can experience tensional stresses.

trench suction? Trench rollback at a subduction zone can generate tensional stresses within a continent.

hotspots? A number of hotspots initiated along the line of breakup of Pangea. It has been proposed that they may have caused rifting or at least determined where the breakup occurred.

Features of Continental Rifts

Continental rifts initiate with doming. The doming induces tensional stress in the upper crust which results in normal faulting. There is increased heat flow. The heat flow increases as the crust thins by ductile shear of the lower crust and lithosphere and normal faulting in the brittle upper crust. Bimodal (basaltic and rhyolitic) volcanism also results from the increased heat flow. Subsiding basins collect sediments. If rifting continues the crust/lithosphere will thin to zero and seafloor spreading is initiated forming a new ocean basin between two drifting continents. Passive margins form on the separating continents with sedimentary strata draped over normal faulted basement. The margins continue to subside for tens of millions of years as a result of continued cooling subsidence and loading subsidence after the initial thinning subsidence.

Failed Rifts

If rifting stops before complete continental breakup, the failed rift or aulocogen will fill in with sediments and be buried in the subsurface, perhaps to be re-exposed by some later episode of erosion or discovered by seismic explorations of the crust. Aulocogens are common features associated with continental breakup. Continental rifts seem to start as a number of rift-rift-rift triple junctions. Two of the rift arms become new ocean basin and the third becomes a failed rift. The East African rift (EAR) appears to be a modern example. The EAR is the failing arm from the triple junction including the Red Sea and Gulf of Aden.

Rift Geometry

The geometry of continental rifts was assumed to be a typical graben structure with major planar normal faults on both sides of a subsiding graben, like in the Basin and Range extensional province of western North America. However, studies of the East African rift and some well-known ancient rifts show that continental rifts are commonly asymmetric; they are characterized by having a normal fault on only one side of the subsiding hanging-wall block. This border fault is curved (listric). The subsiding block is as if hinged on the far side of the basin and rotate downward on the border fault. The Newark Basin is a well-known ancient example of a rift valley. It formed in the Triassic and Jurassic, around 200 m.y.a. prior to the separation of Africa and North America.

Basin and Range Style Extension
Normal faults, which form due to crustal extension, are high-angle (~60°) faults where the block lying on top of the steeply-dipping fault surface (the hanging wall block) slides down the fault surface. The other block (the footwall block) rises up because of the reduced weight as the hanging wall block is removed from it. The uplifted block forms a fault-block mountain or mountain range. A fault block mountain is called a horst. In some areas of broad regional extension, a large number of normal faults cut the crust into a series of sub-parallel fault block mountain ranges with linear basins in between. The downdropped basins are called grabens. A good example of this kind of faulting is found in the Basin and Range of the western United States.

Half-Graben Style Extension
In other extended regions, normal faulting is limited to a narrow belt(s). For example, in the East African Rift system and in the ancient Newark structural basin and other coeval rift basins of eastern North America, half-grabens have subsided and rotated downward along major border faults, single normal faults bounding just one side of the basin. The border faults are curved (listric faults); they are steep at the surface but flatten at depth. This allows the basin to tilt downward toward the fault as it subsides. Rift lakes lie in the depression near the border fault. African rift lakes include Lake Malawi, Lake Tanganyka, Lake Turkana, and Lake Kivu. Volcanoes also develop in rift systems as the continental crust is thinned. Mt Kilimanjaro is the most famous of the African rift system volcanoes, but many others dot the African landscape, including Nyiragonga which erupted in January, 2002 sending lava flows through the city of Goma (eastern Congo) and into Lake Kivu. As the footwall block rises along the border fault it also tilts because only its free end is rising, yielding a tilted fault-block mountain. Inspection of a physical map of Africa will show that on the western side of the rift in this region of Africa stream drainage is westward, away from the tilted fault-block mountain that borders Lake Kivu and the region to the north and south of it. Streams to the east of the rift flow westward into the half-graben, toward the lakes and toward the border fault.

Passive Margins, Salt, and Petroleum Reserves

When Pangea separation began, restricted ocean basins formed. Where these ocean basins lay in low-latitude hot, arid belts (enhanced by continental/orogenic rain shadows) evaporation exceeded rainfall. The seawater got saltier and evaporite (salt) layers were deposited during the Jurassic and Cretaceous [similar evaporites are forming today in the arid eastern Mediterranean]. Very thick Jurassic salt were deposited in the present day Gulf Coast. Salt, being less dense than other sedimentary rocks, is forms diapirs (inverted teardrop shaped bodies) that ascend through the overlying strata causing the adjacent strata to bow up. The salt if it rises near to the surface may form salt domes.

Petroleum and methane form as the decay products of organic matter in marine sediments. These volatile liquids and gasses migrate upward through the sedimentary layers until they are stopped by an impermeable stratum such as a shale. But they are in a very dilute state. In areas where the sedimentary layers are folded, such as around a salt diapir, the oil and gas will migrate upslope and become trapped by the folded layers around the salt dome. It is an easy matter using seismic sounding to image likely oil traps in the subsurface and drill wells to tap off the oil and gas. Unfortunately, most of the easy onshore and shallow water oil traps have already been exploited in the Gulf Coast area.