Rift basins are not single, elongate grabens, but a series of linear or relay-pattern (Harding and Lowell, 1979), usually asymmetric, grabens. These grabens form as a result of pockets of extension that propagate along the rift and, in some areas such as the Red Sea, ultimately create oceanic crust and an ocean basin (Figure 1). Rift basins represent extension in the brittle crust in response to ductile deformation in the upper mantle. The processes that cause ductile deformation in the upper mantle, and the resulting crustal extension, have been debated since rift basins were first described in the last century. The process or combination of processes may be interpreted from the physical characteristics of an active or inactive rift. Each process results in different basinal elements and generates various basinal elements. Examination of these elements, the pattern of volcanism, the presence or absence of uplift, etc., provide a clue to basin origin.
Rosendahl (1987) pointed out that brittle fracturing and faulting are symptoms of processes that involve ductile deformation of the lithosphere. Two general categories of rift processes have been proposed: active and passive (Sengor and Burke, 1978; Morgan and Baker, 1983) (Table 1). Active rifting is a direct response to tension generated by uplift or doming. Passive rifting results from tensional stresses in the crust causing thinning and passive upwelling of hot asthenosphere. Allen and Allen (1990) summarized the various models of rift processes, which are outlined in Table 1. Stresses resulting in crustal extension may occur as a result of uplift (active) or lithospheric thickness changes and resulting isostatic compensation. Therefore, thermal history, lithospheric thinning, and topographic uplift are closely related. Each scenario assumes, in general, that the crust will behave brittlely and the lithospheric mantle ductilely. Figure 2 dia-grammatically illustrates the major processes that create rift basins. Actual rift systems may exhibit elements of both active and passive mechanisms, which are end members of idealized models (Allen and Allen, 1990).
During formation, rift basins are characterized by negative Bouguer gravity anomalies, high heat flow, and volcanic activity. These characteristics indicate a deep thermal anomaly. Refraction seismic has shown that rifts overlie thinned crust (perhaps 25 km instead of an average of about 40 km, but actual values vary depending on the particular rift) (Figure 1). Active crustal extension is reflected by fault-bounded halfgrabens and full grabens. Often these rift grabens are flanked by topographically high shoulders indicating uplift. Presence and timing of uplift (before or contemporaneous with graben formation) is a major factor in determining the process responsible for extension.
Processes that create extension are important to the petroleum geologist because of their impact on sediment distribution and heat flow history. The presence and timing of uplift control the presence or absence of prerift sediments in a graben, facies distribution of synrift sediments, and the distribution of unconformities in the section. The processes responsible for continental rifting will dictate distribution of faults that, in turn, define basin geometry and hydrocarbon traps. Source and distribution of heat flow control volcanism and source rock maturation and also are determined by the rift process; therefore, processes responsible for crustal extension are important in defining the exploration strategy that may be most effective in a rift basin.
Rift basins develop over a relatively very short period of time, geologically, with very rapid subsidence and rapid rates of sedimentation. Active extension and associated sedimentation in basins discussed in this volume range in duration from about 10 to 30 m.y. (Table 2). In some areas, such as in the North Sea, active extension does occur over a longer period of time. Volcanism can occur throughout the rift process as a result of thermal activity and thinning crust. Thick piles of basalt and other extrusive and intrusive igneous rocks are common. Postrift sags result from cooling of the asthenosphere and accompanying thermal contraction after extension has ceased.
Figures & Tables
Interior Rift Basins
Not only are rift basins the foundation for much of the geologic history of the earth, but they also are very attractive areas for hydrocarbon accumulations. Klemme stated that this geographic area has provided significant hydrocarbon reserves: "By area, these basins represent slightly over 5% of the world's basins (50% productive). However, high recovery has resulted, as they contain 10% of the world's present reserves (12% of the oil reserves and 4% of the gas reserves)." The rift basins discussed in this volume are only a few of the productive and, more importantly, potentially productive rift basins in the world. The term "rift" was coined by Gregory (1896) for the graben that now bears his name in the Kenyan portion of the East African rift system. The study of geology of rift basins began in the Rhine graben. The discovery of hydrocarbons in rift basins about the turn of the century provided new motivation for understanding these basins. This publication was initiated by the AAPG Publications Committee in 1985 and contributors were invited to write. AAPG designed their "World Petroleum Basins" series and sought to publish the definitive volume on each of several basin types. In this volume, "Interior Rift Basins," a detailed, 3-paper overview was written about the Suez Rift basin as representative of interior rift basins. The key papers were followed by less detailed reviews of three other selected interior basins: Pripyat and Dnieper-Donets Basins; Reconcavo Basin, Brazil; Albuquerque Basin Segment of the Rio Grande Rift.