Rift Valleys: Continental Breakup and Sedimentary Processes
A rift is a linear zone where the Earth's crust and lithosphere are being pulled apart, a process known as extensional tectonics. This phenomenon occurs in continental settings, marking the early stages of continental break-up. Rift systems form primarily through normal faulting, and some of the most iconic examples include the Red Sea, the African Great Rift Valley, and the Gulf of Aden, which converge at a triple junction in the Afar region.
Rift valleys are narrow, elongated depressions formed by the stretching and pulling apart of the Earth's crust, typically 30-60 km wide. They start above sea level, where continental or volcanic influenced sediments accumulate. Large dropped blocks create basins filled with thick sediment from nearby highlands, leading to flat valley floors. These valleys often contain major lakes or rivers, shaping unique ecosystems and landscapes.
Characteristics of Active Rifts
Active rift systems are characterized by long, narrow depressions with unique geological and topographical features:
- Seismic activity that extends to depths of around 15 km.
- High heat flow, indicating intense tectonic and volcanic processes.
- Topographically elevated shoulders caused by flexural isostatic compensation of the lithosphere.
Notable Features of Rifts
Narrow, elongated depressions: Rift valleys typically have a width of 30–60 km, approximately the thickness of the continental crust being stretched. These valleys are often filled with thick sediments derived from surrounding highlands, creating flat, yet irregular topographies.
Elevated flanks: The steep flanks of rift systems, often 3–5 km higher than the rift floor, result from the accumulation of normal-fault scarps. These faults generally step down toward the central lowlands, with some fault systems extending for great distances. Shoulder uplift, a key feature, results from the mechanical unloading of footwall blocks, and its magnitude is linked to the elastic thickness (Te) of the lithosphere. For instance, the Rhine Graben has a Te of 15 km, resulting in shoulder widths of about 80 km, while the Baikal rift zone, with a Te of 50 km, has shoulder widths of up to 200 km.
Segmented half-grabens: Rifts are not of uniform depth along their length, due to crustal thinning across sets of listric or planar faults. These faults often break rift zones into smaller segments, creating a series of half-grabens with varying polarities. The dimensions of these half-grabens typically range from 25 to 100 km in length and 20 to 50 km in width. Each half-graben evolves as extensional blocks rotate around horizontal axes, widening the rift.
Sedimentary Processes in Rifts
Sediment accumulation in rift valleys occurs rapidly, especially during episodic pulses of crustal extension, creating large spaces for sediment to settle. The drainage patterns feeding these basins often produce asymmetric sedimentary deposits:
- Coarse-grained sediments like gravel and sand collect in aprons and alluvial fans along the fault escarpments.
- Finer-grained materials, such as silt and mud, settle on the gentler slopes of the rift.
Over time, early rift sediments (often alluvial and fluvial) transition into lacustrine facies, including conglomerates, sandstones, and coal, as lakes intermittently form along the basin floor. As the rift widens and deepens, it may eventually sink to sea level, creating shallow marine environments where evaporites can precipitate. With continued extension, rifts can evolve into narrow seas or even oceans, where marine sediments bury earlier deposits, including evaporites.
Volcanism and Magmatism in Rift Zones
Rifting is often accompanied by significant volcanic activity. Normal faulting in these settings is frequently associated with parallel dyke swarms and volcanic flows, which include both tholeiitic and alkaline basalts. Some of the largest volcanic edifices in East Africa, such as Mount Kenya and Mount Kilimanjaro, are products of this rift-related volcanism.
The magmatic processes in rift zones also produce a bimodal volcanic association, with both acidic (rhyolitic) and basic (basaltic) compositions. In some cases, rift systems give rise to unusual rock types, such as highly sodic and potassic rocks. A famous example of this is the carbonatite complexes of East Africa, notably at Oldoinyo Lengai in Tanzania, which erupts sodium carbonate lavas and ash—a rare occurrence in global volcanic activity.
Post-Rift Evolution
Once rifting ceases, thermal subsidence begins as the asthenospheric material, which had upwelled during rifting, cools and contracts. This phase of subsidence affects regions far beyond the immediate rift zone. The sedimentary deposits in these post-rift environments are predominantly marine and pelagic, with relatively little input of terrigenous material. However, the rugged morphology of the earlier rift phase may still influence the sedimentation patterns.
In some areas, pelagic swells develop, which are sediment-starved and characterized by condensed facies, sedimentary gaps, and features like neptunian dikes. The basins, in contrast, are filled with hemipelagic and eupelagic sediments, occasionally disrupted by resedimentation events caused by material sliding in from nearby elevations.