Rifting and Rift Valleys

views updated

Rifting and Rift Valleys

Rifting is the process in which continental crust is extended and thinned, forming extensional sedimentary basins and/or mafic dyke-swarms. Rifts commence as intracratonic, down-thrown blocks dominated by normal or oblique-extensional (transtensional) faults (e.g., the Rhine Graben in Germany and the East African Rift). Rift flanks may be uplifted. Continued rifting results in the break-up of continental plates and creation of oceanic crust (typically 20 to 60 million years after the onset of rifting, but ranging from 7 to 280 million years). Outpouring of flood basalts (also called traps, e.g., the Deccan Traps in western India) can occur over large areas prior to break-up. Marine sedimentary rocks are deposited over the rift sequence during the ensuing phase of post-rift, thermal subsidence.

Rifts commonly develop above upwelling convection cells in the asthenosphere , such as over a mantle plume. Extensional stresses are induced or enhanced by shear-traction on the base of the lithosphere by outwards asthenospheric flow from zones of upwelling. Continents may be split along rifts linking two or more mantle plumes (e.g., South America and Africa were separated along rifts linking the St. Helena, Tristan and Bouvet plumes). Rifting may also represent the far-field reactivation of pre-existing crustal weaknesses, such as old orogenic (mobile) belts, during collision at a distant convergent plate margin. For example, Permo-Triassic 'Gondwanan' rifts in southern Africa, India, and Australia represent the orthogonal or oblique extensional reactivation of Proterozoic orogenic belts on the margins of Archaean cratons during collision on the Paleopacific margin of Gondwanaland. Rifts develop in back-arc settings where extensional stresses can be induced by decreasing rates of plate convergence and/or rollback of a subduction zone . Small oceanic basins, such as the Sea of Japan, may form by back-arc rifting. Small rifts may also form by the stepping of transcurrent faults, such as the Salton Sea area of the San Andreas fault system in California.

An understanding of rift architecture and structural styles is important as rifts contain major hydrocarbon provinces and mineral deposits. The two main styles of rifting are pure-shear and simple-shear. Many rifts, however, exhibit different elements of these two end-member styles. In pure-shear rifting, steep to moderately dipping normal faults form symmetrically either side of the rift axis. Rift valleys (such as along the East African rift) are developed in the early stages of rifting. Rift valleys are elongate, wide, and typically flat-bottomed topographic depressions along down-thrown blocks. As rift valleys are bounded by normal faults, their sides tend to be steep. Continued crustal extension results in further subsidence and formation of a sedimentary basin along the rift axis. The asthenosphere is bowed upward as an isostatic response to lithospheric thinning. Mafic dykes may be intruded along fractures in the overlying crust. The area of thinnest crust corresponds to the shallowest asthensophere.

Other, highly asymmetrical rifts display a different pattern of structures. In early simple-shear rift models, a through-going shear zone was proposed to extend from the upper crust to the upper mantle. Brittle deformation along the upper part of the extensional detachment was thought to progressively change to ductile shearing over a broader zone at greater depth. It is now thought more likely that a zone of ductile flow in the lower to middle crust separates and decouples displacement along a shallowly dipping extensional detachment in the upper to middle crust from a normal shear zone cutting lithospheric mantle (possibly reactivating a former suture) or bowed up lithospheric mantle. In simple shear rifts, areas of greatest crustal thinning may be offset from areas of greatest asthenospheric uplift. Greatest heat flow and hence volcanic activity and dyke intrusion may be offset from a zone of highly extended crust. This zone comprises highly rotated blocks between imbricate, curved (listric) normal faults. Extensional detachments may be folded by regional antiforms during asthenospheric uplift. The opposite margins of two continents rifted apart may therefore be quite dissimilar. The lower plate margin may show a broad zone of shallowly dipping listric normal faults draped by shallowly dipping sedimentary strata deposited during the post-rift, thermal subsidence phase. Dykes and volcanic rocks may be absent to sparse. In contrast, the opposite, upper plate margin may comprise a narrow zone of steeply dipping faults and contain volcanic rocks and dykes.

See also Continental drift theory; Earth, interior structure; Faults and fractures; Orogeny; Plate tectonics