Rift is here treated as synonymous with megashear (Carey, 1958) or great fundamental fault (de Sitter, 1964). Rifting does not necessarily include fissuring, crustal extension, or the formation of graben (rift valleys). The terms graben and rift valley are not necessarily synonymous, although recently the distinction is not preserved.
The order of magnitude is in each class related to the thickness of the crust. For continental rifting the endogenic processes include faulting—normal, oblique, or strike-slip—on one or, in some cases, both flanks of the rift valley. The order of magnitude of any rift valley requires that the fault have commensurate displacement on the fault plane having depth to the limit of brittle crust. Rifting comprises movement on the plane (or planes) that is therefore designated the rift.
In recent times the rift valley, or more generally, the simpler version of this landform, the graben, has tended to be regarded as the only synonym with rift. If this were accepted there would be no place for phenomena other than those resulting from extensional stress fields where the principal axis is horizontal and across the plane of failure. However, the principal axis can be reoriented in relation to the originally preferred plane of weakness so that a normal fault with dip slip can pass through oblique slip to strike slip.
There was early acceptance, which has been retained, of the term rift to include the surface trace of strike-slip faults: e.g., the San Andreas (Lawson, 1980; Noble, 1926; Billings, 1954) and the Dead Sea Rift (Quennell, 1958). Abundant other references confirm that this usage is acceptable. Rift has sometimes been defined as a large strike-slip fault, which approximately parallels regional structure.
In addition to the nomenclatural and analytical justification for this extension of the application of the term rift, there is also that of the equality in status of strike slip and graben, the end members of the taphrogeny family.
Initiation of rifting is either passive or active (Sengor and Burke, 1978). In the former there is a passive response to a regional, usually extensional, stress field in which the continental lithosphere is subject to geometric or mechanical stretching and thinning. These stresses can be generated by plate collision and intraplate stress pattern changes with rifting. In the latter there is interaction between asthenosphere and lithosphere, with thermal thinning of the latter, heating, and uplift, with generation of deviatoric stress, which is extensional and horizontal, causing rifting (Morgan and Baker, 1983). The rift or rift valley faults extend as slip planes only to the base of the upper level of the crust, the maximum depth of most continental earthquakes (Illies and St. Mueller, 1981; Irvine, 1966; Knopoff et al., 1969; Ramberg and Neumann, 1978; Neumann and Ramberg, 1978; Morgan and Baker, 1983; Palmason, 1982).
There follows consideration of relevant terms particularly with regard to their history.
Although by derivation taphrogeny means ‘born as a trench,’ Krenkel, who introduced the term (1925, 240), clearly had in mind extensional forces acting across high-angle faults separating blocks, thus allowing relative vertical movement, resulting in grabens and horsts that make up rift zones. Krenkel states: “Rift zones are ... rents of the crusts. Only tearing or pulling forces making gaping fissures which one meets with so strikingly in the water-filled depths of the rifts.... Rift formation (as block movement, taphrogenesis?) is a contemporaneous counterpart of orogenesis.”
There has been a tendency to equate taphrogenesis with relative vertical movement of adjacent fault blocks without recognizing this movement as the result of extension stress fields. With the developments of geodynamics and the greater importance of extensional crustal movements, however, Krenkel's intended meaning of taphrogenesis can be resumed.
It would appear that a graben is a geomorphological feature and that the name clearly refers to the depression and not to the down-thrown block.
“If the outer borders of two fields of subsidence approach each other so that a ridge is left between them, on both sides of which the two areas of depression descend more or less in the form of steps, then we have what we shall distinguish, making use again of a common mining word, as a horst, in this case a horst of the first order” (Suess, 1885, trans. Sollas, 1904, 126). It should be noted that Suess has specifically stated that horst blocks are those portions of the crust that are left upstanding at the same level when adjacent areas have been depressed. This view was echoed by Geikie in 1903. Suess used the Black Forest and Vosges blocks as examples, but these are atypical and asymmetric. Davis (1905) first introduced the term into English “a mass of earthcrust which is limited by faults and which stands in relief with respect to its surrounding.” He regarded it as a topographic rather than a structural term.
From these and other statements it appears that horst is a structural form as well as a landform and that, although upstanding relative to the surrounding land, it may or may not be so in an absolute sense. It is the counterpart of graben and has even been described as a “rift valley upside down.”
Gregory (1894, 295) was the author of the term rift valley, which he used to describe the Kenya Rift Valley. As a prototype of a rift valley, perhaps even the Kenya valley should not be used. This landscape form to which Gregory gave the name probably conforms more closely with volcanic-tectonic depression. Its origin is shared by tectonic and volcanic processes. Milanovsky (1972) includes it in his classification as “arch-volcanic.” However, since its inception the landscape form of rift valley has been conceived in somewhat academic or textbook terms.
Rift valley replaces graben where the order of magnitude is without doubt that of the brittle crustal thickness. It has also been suggested that rift valley is preferable where the downfaulted block is not coherent, as in a graben, but has suffered minor longitudinal faulting, as in the Kenya Rift Valley south of the Kedong segment and in part of the upper Rhinegraben.
The varieties of grabens or rift valleys include graben with equal displacement on the marginal faults; graben with asymmetrical displacement; fault-angle depressions or half-graben with one fault absent; taphrogeosynclines with one fault replaced by a monoclinal warp; volcanic rifts (enlarged fissures or rents) of the oceanic and inter-continental rift systems; major strike-slip and transform faults where rift valleys are formed as rhombograben or where the vertical component of movement has produced fault-angle features. These are all the product of an extensional stress field acting across a zone which has a thickness equal to that of the brittle crust.
Minor graben, not to be classed as rift valleys because of a lower order of magnitude, will form when extensional stress operates for only a limited depth.
Aulacogens “are long-lived, deeply subsiding troughs, at times fault-bounded, that extend at high angles from geosynclines far into adjacent foreland platforms” (Hoffman et al., 1974). The term was first applied by Shatsky to the Pachelma Trough, and later he recognized the Donetz Basin, the Timan Trench, and the Greater Donbas. Milanovsky (1976), comparing them with modern, actively developing rifts, regards aulacogens as similar in many ways but indicates that they are ancient, inactive tectonic elements of platforms. They are recorded in North America, Asia, Europe, and Africa. It is uncertain if they ever had the geomorphic expression required to qualify them as rift valley landforms.
The term has acquired an extended meaning in recent years to apply to troughs that are formed in the initial stages of continental fragmentation. The uprise of basic igneous material to form a median dike is regarded as a characteristic. These are the successful arms of triple junction systems, while Burke (1977) regards the failed arm as type aulacogen. Milanovsky (1981) had already defined the latter as the penetrating type, one of three, the others being the trough, bisecting the platform, and the blind, lying within the platform.
In oceanic rift valley systems, the ridge-ridge class links the segments of the median valleys of the mid-oceanic ridges. When relative movement occurs it is strike slip. It should be noted that the sense and the rate of the relative movement depend on the difference in rates, or balance, of emission of volcanic material on two sides of the central clef; that the transform extensions beyond the segments of the median valley are scars where there is no strike-slip movement; and that upwelling and spreading can occur without relative movement.
Transform faults of the continents are usually of the ridge-trench class and link the ends of these features, known as the transforms (Wilson, 1965). Transform faults may pass from continental crust into oceanic crust.
Leaky Transform, or Wrench, Faults.
These faults are openings or gaps formed by separation of the walls of major strike-slip faults (rifts). They are of nonlinear or sinuous plan, and certain other tectonic landforms associated with major continental wrench faults, specifically those with vertical components of movement, give rise to the description “leaky.”
Rhombochasm, Rhombograben, Pull-apart, Sagpond.
These terms are given to a parallelsided gap in the sialic crust. The gap is formed by rifting when an extensional fracture occurs across the block between two en echelon wrench (strike-slip) faults that then form a rift system (Quennell, 1958). The fracture would be vertical and probably oblique to the wrench faults. The fissure would widen with movement on the faults, thus forming a rhombochasm. On a smaller scale the term used is pull-apart, and the smallest are called sagponds (Crowell, 1974).
Fault-line valleys result from differential erosion of a crush zone along a major fault and can be included here because such features are found along rift zones.
“Taphrogeosynclines are sediment-filled, deeply depressed rift blocks, bounded by one or more high-angle faults; late Triassic geosynclines along the Atlantic seaboard are of this kind” (Kay, 1951, 107). The age of their infilling in relation to that of their formation is important in determining their classification.
Paleorift is applied in a general sense to rift valleys and dissected rift valley structures that show no tectonic geomorphic evidence of movement on the bordering faults; i.e., in effect, they are tectonically dead. Their presence can be revealed by differential erosion.
Continental Rift Valleys.
The stage reached in the geomorphic cycle when rift valley faulting takes place is the same for nearly all rift zones in continental (platform) environments. Continental rift valley zones typically lie within peneplaned or pediplaned regions that have reached the stage of old age or near senility. For example, in the case of the Cenozoic East African Rift Valley, faulting appears to have been initiated during the late Paleogene, dislocating the surface of the senile stage of the cycle that began in the late Cretaceous. The youngest rift faulting appears to belong to the diastrophic episode that initiated the cycle that is still running its course and has not reached maturity. Where the rift faulting lies within an uplifted region that is not greatly dissected, rift valleys and block faulting are easily recognized. Where, however, the faulting is on the flanks, the tectonic landforms are not always easily distinguished from retreating erosion scarps of the new cycle.
Oceanic Rift Systems.
Here, geomorphic processes are significantly different from continental. Exogenic processes that modify the tectonic landforms do not operate on the abyssal plains or on the mid-oceanic rises. Tectonic sub-marine forms, resulting from crustal movements of the ocean floor at depths beyond the influence of wave action, are not subject to exogenic processes other than those that operate on oversteepened slopes, with migration under gravity and bottom currents of submarine volcanics (pillow lava), graywacke, and chemical deposits. Exogenic processes of the land—i.e, the reduction of tectonic landforms—have no submarine counterpart.
The continual buildup by eruption of lava that tends to fill the rift valley and the uplift of the flanks is accompanied by concomitant spreading and the removal of the excess by the conveyor belt mechanism. Steady-state, (a balance of opposing endogenic processes), or dynamic equilibrium maintains a more or less constant submarine topography. This is virtually an open system, a situation entirely different from the closed system that applies in the case of continental rift valleys.
Recognition that a fault-intersected surface belongs to a particular cycle allows a lower limit to be put to the age of the faulting. If erosion surfaces on the two flanking blocks can be correlated—i.e., if the surface can be shown to have been continuous prior to the most recent faulting—then the rift valley is younger than that erosion surface. Although self-evident, this situation is sometimes overlooked.
Differences in elevation of the same erosion surface on the blocks flanking a rift valley or of the surface of the downfaulted block (if exposed or if its level beneath sediments or a lake surface is known) can give precise information regarding the vertical displacement on the faults. Relative movement of the flanking blocks and of the depressed block can be determined.
Planation can have been carried across a sediment-filled rift valley. By differential erosion the valley can be exhumed, the scarps being either resurrected fault scarps or recurrent fault line.
If evidence of former cycles, erosional and diastrophic, can be found, then it may become possible to trace the history of rift faulting through these cycles. Cases are found along the Western Rift System where exhumed ancestral rift valleys of Cretaceous or even Karroo age, floored by remnants of senile or mature surfaces belonging to early cycles, are found on sunken blocks. The excavated sediments can either have been infaulted with the rift valley or deposited within it. The disposition of surfaces and bedding of sediments, whether level or tilted, appear as angular unconformities, giving the nature of the movement.
Rise to the Rift.
This concept was inherited from the model derived by (Cloos (1939). The formerly level or near-level surfaces on the flanking blocks are supposedly tilted away from the rift valley shoulder with slopes concave upward. The model is of a wedge flanked by deformed trapezoidal blocks whose inner ends float higher. This hypothetical, oversimplified model has not been adequately tested against known geology, and misconceptions have been inherited. More often than not, the slope immediately adjoining a rift valley is toward rather than away from it, as in central Kenya. The rise to the rift concept is not to be confused with the doming or arching of the surface where it precedes rifting.
Oceanic and Continental Rift Valleys Contrasted.
The rift valley of the oceanic system is not a continuous feature as at first was believed. It is divided into relatively short segments resembling continental rift valleys only in that they are elongated depressions of tectonic origin. The oceanic rift valleys are floored by young lava, room for the eruption and emplacement of which is afforded by the drawing apart of the flanks of the ridge. Linking the segments are the transform faults. They do not have pronounced geomorphic expression as do the rift valleys. Their pattern is largely known from seismic and magnetic evidence. Their strike-slip movement is known from earthquake fault plane solutions that are in contrast to the dip-slip solutions of the earthquakes on the rift valley segments.
If the rate of rise of volcanic material is symmetrical, the rift valley remains stationary. If it is asymmetrical, the valley will in effect shift laterally toward the slowest rate of rise. If, along the length of the ridge, the rate of upwelling of mantle material and its asymmetry varies, then the ridge and the rift valley will be segmented and offset along the boundaries between currents of different velocity, and the reflection in the brittle crust will be a transform fault. Displacement on the transform extensions beyond the rift valley will cease, and the extensions become scars. This pattern of segmented rift valleys, linked by active transforms, is the Oceanic Rift System. The continuity of the system is established from seismicity as well as bathymetry. It is believed to extend unbroken throughout the oceans of the world.
Continental rift valleys, like, e.g., that in east Africa, are discontinuous but in a different manner. Rift valleys within zones can be offset. The zones can be limited in length and situated en echelon. Rift valley zones in some cases end in pitching block faulting. As examples, there is no visible structural connection between the northern end of the Western Rift System and the Gregory System; there is also none between the Albert—Kivu and Tanganyika zones or between the latter and the Rukwa—Nyasa Zone. These continental rift zones are not related or linked by transform or strike-slip faults (as are the segments of the oceanic rift valley). In general, their en echelon relationship is by a bridge, a term introduced by Suess (1885).
Margins Oceanic rift valleys have crater wall margins and may have shallow gravity step faults and rising floors of volcanic material. Conversely, continental rift valleys ideally have a graben structure bordered by deep-seated normal faults, with the intervening space being occupied by a downfaulted block, the surface of which is the floor of the valley. The flanking blocks normally do not move apart more than the horizontal components of movement on the normal faults. They are only rarely symmetrical.
Volcanism is continuous along the length of the oceanic system, but in continental systems it is localized and, for the greater part of their total length, completely absent. The volcanics are of contrasting composition. Oceanic volcanics are potash-poor tholeiites, whereas the continental volcanics are strongly alkaline, and carbonatite-cored volcanoes are usual. Volcanism of oceanic and continental rift zones have little in common chemically, which is an argument against comparing the African rift systems with mid-oceanic rifts (Gilluly, 1971).
The elevation of the flanks of the oceanic rift system—i.e., the Mid-Oceanic Rise—originates from the heat flow pattern within and from the mantle. For the continental systems, volcanism and heat energy emanation accompany or are related to the rift faulting on domal uplifts. Elsewhere, isostatic recovery and epeirogenic uplift account for elevated flanks and plateaux in the rift zones (Knopoff et al., 1969; Vinogradev and Udentsiv, 1975).
The classification developed by Milanovsky (1972, 1976) is compatible with Earth models where vertical crustal movement predominates. While comprehensive as far as taphrogenic (graben) forms are concerned, Milanovsky's classification does not provide for those cases where both spreading (extensional) phenomena and/or rifting (strike slip) have operated. The reality of great strike-slip faults (megashears, transforms, plate boundaries) embodied in plate tectonics requires that any classification must include provision for them.
A rift valley is the geomorphic and structural unit; a zone is a visibly structurally related belt of rift valleys, continuous but not necessarily in alignment and may be offset; a system is a series of zones, or individual rift valleys, not necessarily visibly in structural continuity. In some cases the zones are en echelon and separated by a bridge (continental), or by a transform (oceanic or intercontinental), apparently lying within a continuous tectonic zone.
The principal rift valleys of Karroo age are the Luangwa-Luano-mid-Zambezi aligned troughs of Zambia and Zimbabwe and the Ruhuhu Mbamba Bay and the Kidodi troughs of Tanzania. They are aligned northeast-southwest. The lower Zambezi Karroo trough has a general east-west orientation but continues to the south as the Lower Shire and Urema troughs.
The Karroo (Lukuga Beds) of eastern Zaire appear to occupy troughs and also to have been downfaulted. Karroo troughs all lie transverse to the direction of present rift valleys and even across them.
Western Rift System
There are three principal zones, offset sinistrally from each other. From the south these are Malawi—Rukwa, Tanganyika, and Kivu—Edward—Semliki—Albert. The general trend is meridional, with curvature around the western margin of the East African Swell. The system is set within Archean and Proterozoic orogenic belts whose trends it follows.
The Malawi—Rukwa Zone has a meridional to north-west trend and is 700 mi (1100 km) long. Lake Rukwa is bordered by eroded fault scarps, discontinuous and varying in amount of throw.
Between Rukwa and Malawi is a fault-angle depression that contains both Karroo and Cretaceous terrestrial sediments separated by unconformities. This rift valley therefore belongs to two or even three generations. The mid-Tertiary erosion surface provides a reference plane on both flanks for most of the length of the zone.
Lake Malawi drains through the Shire Trough to the Zambezi. Lake Rukwa is dischargeless.
Between Malawi and Rukwa is the Rungwe volcanic field. The composition ranges through basalt, trachyte, and phonolite.
The Tanganyika Zone to the west is almost wholly occupied by the lake. There is fresh faulting at both the north and south ends. High fault scarps, in some cases multicyclic, are found on the west, but in general, the coasts are drowned. The western flank has since mid-Tertiary time risen more than the eastern, with partial downcutting of the outlet to the Congo River Basin. There are deep troughs in Lake Tanganyika, 1,440 m and 1,290 m respectively below lake level. Sediments of probable Cretaceous age are found at depth at the northern end. This zone is therefore at least in part a second or even third generation rift valley. There is no volcanic activity in the Tanganyika Rift zone.
The Kivu—Albert Zone is offset to the west from Tanganyika with visible structural connection, although Lake Kivu now drains south into Lake Tanganyika, the level of which is 700 m below.
Lake Kivu has a drowned coast except for a low scarp on the east. Lake Edward, the Semliki River Valley, and Lake Albert are rift valleys, successively offset to the west. High scarps typically face low scarps. The rise of the western flank of Tanganyika continues for this zone, and this has diverted the formerly westward drainage into the Congo northward to the Nile Basin.
The Virunga volcanics dammed the Rutschuru Valley to form Lake Kivu. Volcanism extends to the Fort Portal field north of Ruwenzori. Composition progresses from basaltic to trachytic and highly alkaline.
In general there is gravitational isostatic compensation along the Western Rift System, even for the elevated horst blocks. There are, however, strong negative isostatic anomalies with positive flanking zones over sediment-filled depressions of Lakes Albert, Edward, Tanganyika, Rukwa, and Malawi. There is no structural or tectonic continuity between the Western Rift System at its northern end and the Western Rift System.
The Gregory System
The Gregory System is continuous from Lake Rudolf southward through Kenya into northern Tanzania where it branches into block faulting. The Kavirondo Rift Valley extends westward into Lake Victoria as the Kavirondo Gulf. This relationship has been cited as a triple junction, with the main valley as an aulacogen.
The main rift valley margins are interrupted, en echelon, stepped scarps in volcanic flows and interbedded pyroclastics, with a very great thickness overlying crystalline basement.
From Lake Rudolf (Turkana) to Lake Balangida in Tanzania, there are nine lakes, of which two are on perched water tables. All have inland drainage.
The Eastern Zone, chiefly block faulting, branches from the Gregory Zone near Kilimanjaro and extends in a southeasterly direction to the coast and southward to the Uluguru Mountains. This zone is continued by parallel and offset faults, southeastward to the Mozambique border.
In the Gregory Rift System Tertiary volcanism of olivine basalt composition accompanied prerift warping. Phonolites, trachytes, and kenyites dominate in the later phases.
Gravity anomalies across the Gregory rift valleys are much less pronounced than in the west. In one section in Kenya is a central zone of positive anomaly, supporting the suggestion that this arm is an aulacogen. The hypothesis that this is the initiation of the breakup of Africa has serious objections.
Uplift of the Horn of Africa during the Eocene was followed by the rapid rise of the Arabo-Ethiopian Dome or Swell that had a maximum rise of more than 2,000 m. Rift faulting followed a triple-junction pattern of which the three arms are the Red Sea, the Gulf of Aden, and the main Ethiopian Rift Valley. The Danakil horst block is now separated from Ethiopia by the Afar Depression, which appears to be underlain by oceanic crust. The Ethiopian Rift Valley has been described as an aulacogen, the failed arm of the triple junction.
The Ethiopian System begins in the south with three parallel rift valleys, en echelon: the northern half of Lake Rudolf and the lower Omo Valley, Lake Stefanie, and a belt of faulting lying farther east. Northward, there is minor faulting at Lake Chomo and a single, 900 m high, eroded eastern scarp at Lake Awash. The dissected western margin rises to the north. The rising floor contains a central zone of faulting and volcanics called the Wonji Fault Belt.
In the Awash the margins diverge. To the northeast are erosion scarps in the Trap Series basic volcanics, and this becomes the southern margin of the Afar and the Gulf of Aden. On the west the rift wall is warped, step faulted, and dissected. It descends to the floor of the Afar Depression, which consists of mixed sediments, evaporites, and young lava flows, faulted as elongated blocks.
The Wonji Fault Belt is a zone of volcanic activity and widening fissures and resembles the oceanic rift pattern. From Lake Abaya northward it may be a young active spreading ridge.
Post-rift-faulting volcanism is active only in the rift valley and the Afar. The youngest volcanics of the Aden Series petrologically resemble the Oligocene Trap Series but are more silicic or trachytic.
The Benue Valley, Nigeria, is an infilled rift valley, with Cretaceous sediments, chiefly marine, flanked by Precambrian basement. There may be 5,000 m thickness of sediments. The Chad Basin extends the structure to the northeast. The Benue Valley has been described as an aulacogen, the one failed branch of a triple junction that existed when the separation of Africa and South America began in Cretaceous times.
Red Sea, Gulf of Aden, and Gulf of Suez.
In the south, coastal plains border the Red Sea. On the east is the Yemen plateau at 4,000 m, while on the west is the back slope of the Danakil tilted horst block. The shelves are very shallow, with the median zone about 1,500 m deep. The central part has a similar coast. The highlands rise to 1,700 m beyond erosion scarps. There is a median trough, 1,500–2,900 m deep, widening to the south.
A straight western margin with a narrow coastal plain is backed by a dissected scarp. The seafloor deepens rapidly. The eastern margin is less regular. The deep median zone continues from the south with a maximum depth of 2,500 m but shallows in the north.
Marine transgression in the Miocene was from the north and was followed by dessication and deposition of evaporites. Pliocene marine incursion was from the Indian Ocean.
The Gulf of Aden has a deep median zone which is the continuation of the Indian Ocean rifts. It is crossed and offset by northeast-trending transforms.
Regarding the history of formation, one view is that the shelves of both the Red Sea and the Gulf of Aden may be underlain by thinned continental crust, with incursion of the sea and deposition of evaporites developed in the early phase before the final rifting took place. The scarps along the coast would thus be erosional and not fault scarps. Another view is that the rifting took place in two stages, with a downfaulted block that later split to give the central rift or graben. The third view is that the coasts were initiated by true Wegenerian rifting with exposure of subcrustal material, which later separated to give the central cleft.
The Red Sea, the Gulf of Aden, and the Main Ethiopian Rift made up the three arms of the triple junction that trisected the Arabo-Ethiopian Dome or Swell. There is a suggestion that in addition to the Ethiopian rift being an aulacogen, the other arms, the Red Sea and the Gulf of Aden, were at one time also classed as aulacogens before the final separation into an intercontinental sea took place.
The Gulf of Suez has a depth of less than 60 m, increasing southward. This depth is in great contrast to that of the Red Sea, 1,000 m at the entrance, and continental crust appears to be continuous across the gulf, with Nubia and Sinai-Palestine as one plate. The basement is overlain by marine and terrestrial sediments as old as Carboniferous. As an embayment or furrow, the gulf may have existed before the rift faulting that began not earlier than Oligocene. Longitudinal faults separate blocks.
The main faulting phase took place from Miocene time onward. The mid-Tertiary planation surface originally extended across the site of the gulf and was broken by the faulting. The gulf has a moderate negative anomaly, in contrast to the Gulf of Aqaba that has a strong negative Bouguer anomaly. References: Irvine, 1966; Pilger and Rosler, 1976.
Western Arabia Rift System (Dead Sea Rift).
This system extends from the Red Sea to the Toros Belt in southern Turkey. There are seven changes in structural style along its length. One view, now obsolescent, held that the structures are taphrogenic graben. The modern view is that the system is a rift (in the sense in which San Andreas is a rift)—that is, strike slip—with associated rift valley structures but not a graben.
The Gulf of Aqaba has steep margins, the eastern margin typically being a coastal plain. The gulf is 900–1,800 m deep, in contrast to the Gulf of Suez with which it is sometimes erroneously paired.
On the eastern wall of Wadi Araba, scarps expose Precambrian. On the west, complex faulting near the head of the gulf gives place to monoclinal scarps of Cretaceous sediments and a cover of Neogene sediments. A well-defined fault trace with a left-lateral displacement runs on a broken course obliquely from the west of the gulf to the southeast corner of the Dead Sea. Northward the margin is a major fault scarp merging into a steep monoclinal scarp. The faulted west wall is lower.
The Jordan Valley is not a true rift valley or fault trough. The flanks are typically monoclinal flexures, descending beneath infilling sediments. Fault trace features reflect a left-lateral strike-slip fault similar to that in the Wadi Araba. It runs from the west coast of the Dead Sea and continues to the east coast of Lake Tiberias and, continuing, becomes the eastern flank of the Upper Jordan (Hule) Valley. The Yammoune Fault runs obliquely along the Lebanon Range. It has an eastfacing scarp and evidence of some strike-slip movements.
The Bekaa Valley is a synform or syncline between the Lebanon and the Anti-Lebanon ranges. The Palmyra Fold System or Belt trends northeastward beyond the Anti-Lebanon ranges. The Palmyra Fold System or Belt trends northeast beyond the Anti-Lebanon. The folds are asymmetric.
The northward-striking Syrian section commences at the northern end of the Lebanon range. It forms the eastern margin of the Gharb Depression, 45 km long and 12 km wide, in which flows the Orontes River. The western marginal fault of the depression continues northward and is lost in the wide alluvial plain but probably continues due north and ends against the Amanos Fault Belt, which joins the East Anatolian Fault. All these faults have left-lateral strike-slip displacement.
The Arabian Rift System from the Red Sea to its junction with the Amanos Fault is a left-lateral strike-slip fault zone. It is regarded as a transform joining the Red Sea spreading zone with the buried Bitlis-Zagros subduction zone, but this is an oversimplification. It separates the Arabia Plate from the Sinai-Palestine-Levant subplate.
The Gulf of Aqaba, an oblique crustal separation that extends northward obliquely across the Wadi Araba;
The Rhombochasm of the Dead Sea;
The opposed monoclinal downwarps of the Jordan Valley;
The rhombochasm of Lake Tiberias and the upper Jordan Valley;
A rhombochasm at Yammoune;
The Gharb rhombochasm.
Regarding volcanism, east of the Wadi Araba are minor flows of olivine basalt from vents on post-Miocene east-west faults. This volcanism continued into the Pleistocene. An olivine-basalt laccolith in the Jordan Valley is the only known center of eruption within the rift system. The extensive Hauran lava flows in Syria extending across the present rift valleys are olivine and tholeiitic basalts. The composition of the volcanics is entirely different from the extreme alkalinity of the African rifts.
The system is made up of fault troughs and some fault-angle depressions, of which there are eight or nine, and has a length of about 2,500 km. It extends northeastward and then eastward along the southern margin of the Aldan Shield, possibly as far as the Sea of Okhotsk.
Late Mesozoic and Tertiary rift faulting occurred within the folds of the Baykalian Geosyncline and accompanied and followed its uplift, producing a system of asymmetric rifts valleys or fault-angle depressions, the fault scarps of which faced southeast. Tertiary sediments and volcanics have been deposited in the depressions, and in Lake Baikal they reach a great thickness. Volcanism is active, not within the troughs but in the uplift zones, and is chiefly basaltic in nature.
All first-motion fault plane solutions point to extension transverse to the rift trends. There are major negative gravity anomalies over the main rift zone, indicating a great depth of low-density sediments, possibly as much as 6,000 m.
The Lake Baikal Rift Valley resembles the Tanganyika Rift Valley. For example, they both lie within orogenic belts bordering continental cratons, and their dimensions and lake depths are approximately the same. Neither has associated volcanic activity.
The Baikal Rift System is the most northeasterly of four regions affected by extensional tectonics related to the convergence of the India and Eurasia plates. The impact and penetration of India into and beneath Asia has given rise to an east-west belt of extension in Tibet resulting in north-south normal faults and graben, to normal faulting and graben oriented north-south in the Yunnan province of China and to the Shansi Graben and normal faults to the northeast near the margin of the region of continental tectonic disturbance.
The aulacogens of the Asian platforms are important taphrogenic elements in their early history. The Ura Aulacogen is aligned with the Lake Baikal Rift Valley, trending northeastward along the margin of the Siberian Platform.
In the Russian Platform, which structurally belongs with Asia, are the Timan Trough and the Pachelma Trough in the northeast and, in the southwest, the Donetz Basi and the Greater Donbas Trough, all regarded as aulacogens. They are ancient features. (Milanovsky, 1981).
The Upper Rhinegraben and the Hessische Graben are the central sections of the European System of rifts and rift valleys that extends from the Oslo and Lake Mjosen graben in southern Norway to the Rhone Trough of southern France and to the Mediterranean (Fig. 6) (Ziegler, 1981; Illies and Mueller, 1970; Knopoff et al., 1969). The Upper Rhine Rift Valley lies within the Rhine Shield. It is 300 km long and averages 40 km wide. The total subsidence is about 900 m. It is flanked by the Vosges and the Black Forest marginal blocks of crystalline rocks that prior to Paleogene times formed an unbroken shield. The southern end of this shield is masked by the outer Jura folds, and in the north is the Hunsruck-Taunus Horst Block through which the antecedent Rhine Gorge was cut. Triassic and Jurassic sediments were infaulted during subsidence that began in the middle Eocene, and marine sedimentation, with evaporites, accompanied Oligocene and early miocene rift faulting. The strike of the graben crosses the Hercynian trend. The marginal faults are high angle, but within the rift there is antithetic block faulting. Volcanism related to the rift faulting produced the Kaiserstuhl, an alkaline (carbonatite) volcano.
The Hessische Graben continues the main rift valley and is 160 km by 16 km. It contains young lava flows, and the Vogelsberg volcano. The Lower Rhine embayment is a complex system of width 32 km that enters the shield for about 60 km. The faults disappear beneath sediments in the Munster Basin, but this fault zone appears to link with the North Sea Graben.
The Rhinegraben has moderate negative gravity anomaly, 30–50 mgals. The Hessische Graben shows a short extension of this, but a negative anomaly trough branches to the northeast from Manneheim. The lower Rhine embayment has a negative anomaly zone with a Hercynian trend, extending to south Holland. The Belfort Channel and Rhone Valley have no gravity anomaly.
The Limagne Rift Valley, 130 km by 42 km, intersects the margin of the central plateau of France. The infilling marine sediments are Oligocene in age. The Loire flows in a similar fault trough.
In the continental shelf of the North Sea, the presence of the important Viking Graben and Central Graben has been established in the course of petroleum exploration. The meridionally oriented northern member lies midway between Scotland and Norway, but the southern part is directed southeastward in the mid-North Sea region. Its history began in Triassic times, but greatest subsidence took place during the Cretaceous when 2,000 m of sediments were deposited. It is a rift structure rather than a rift valley because it has probably never been an emergent landform or submarine rift valley form.
The evolution of the Viking Graben and Central Graben and of other rift structures of the North Sea Shelf Zone, within which sedimentation was probably contemporaneous with their formation, was related to the initiation of the North Atlantic rift ocean during the Jurassic and Cretaceous.
The earlier (paleorift) pre-Permian rift valleys include the Oslo Graben, which follows the order of the Fennoscandian Shield with the Caledonides and lies within crystalline rocks; the Horn Graben, lying within the Caledonides belt; and the Midland Graben of Scotland.
In the British Isles the Midland Valley of Scotland owes its present form to differential erosion of infaulted sedimentary formations. It may not have been a true rift valley. Conversely, the Great Glen of Scotland is the result of major transcurrent or transform faulting. It is, therefore, a rift system of the same classification as the Dead Sea, Alpine, and San Andreas rift systems. Its physiographic form is partly the result of erosion of a crush zone and partly the dip component of predominantly strike-slip movement. This movement was more than 100 km. There is a broad belt of crushed, sheared, and mylonitized rock up to about a mile in width. Differential erosion, in part glacial, has produced a simulated rift valley. Opinion differs as to the sense of strike-slip displacement, but it is usually accepted as sinistral.
The eastern North America paleorift system was developed in Mesozoic times. The extensional stress conditions were related to the early opening of the North Atlantic. There was major igneous activity of the type associated with the breakup of continents. This system comprises, first, the Triassic taphrogeosynclines (Kay, 1951), lying along the Atlantic coast, which from north to south are Fundy, Connecticut, Newark, Gettysburg, Culpeper, and Durham-Deep River. All are infilled with arkose and shale, flood basalts, and silts. Next are the St. Lawrence paleorifts that lie in the western St. Lawrence Basin and are, from west to east, Timiskaming, Nipissing, Ottawa, Champlain, and Saguenay. Farther east are the less well defined or authenticated structures of the lower St. Lawrence Basin. There appears to have been Precambrian structural influence.
In western North America are zones and regions of late Cenozoic extensional tectonics that lie within and adjacent to the cordillera (Smith and Eaton, 1978). The comparatively young and active Rio Grande Rift System is oriented meridionally along the eastern Rocky Mountains (Riecker, 1979). There is a discontinuous chain of graben that widens southward. It extends from the Mexican border through New Mexico and Colorado. Volcanic activity accompanied faulting.
This rift valley zone has been projected to the northwest through the chain of graben—Saratoga, Continental, Grand, Swan, Birch Creek-Lemhi, and Bitterroot grabens—to connect with the Rocky Mountain Trench. This is not a trench in the oceanic sense but a chain of block-faulted graben and erosional (glaciated) valleys. It appears to reflect a deep-seated fracture zone. It or related fractures continue into Alaska. West of the southern end of the trench is the Republican Graben, which is of the dimensions of a true rift valley.
The Basin and Range Province in Utah, Nevada, and California is described as block faulted with elongated horsts and grabens, symmetrical or tilted and oriented meridionally. One explanation is that the fault planes flatten in depth (listric) in which case the valleys, although they may be geomorphic graben, may not be true rift valleys. The other explanation is that bounding faults extend to the lower levels of the brittle crust, in which case the graben are true rift valleys. An example is the Wasatch. The vertical dimension can be as great as 3,000 m.
Following the margin of Cordilleran belt is the major right-lateral strike-slip fault system, or transform, the San Andreas Rift. It is not a rift valley in the restricted sense, even though there are sections where the physiography simulates a rift valley. Some of these result from the alternation in sense, along the rift, of small vertical displacements. Erosional retreat of the scarps took place before further horizontal shift. The San Andreas Rift has two sections, with the east-west Garlock Fault System meeting at their junction. The horizontal displacement on the two sections differs (Crowell, 1979). This variation takes place at the big bend and the branching of the Garlock and other faults. Such bends are found in other major wrench fault systems and may be due to changes in the nature of the crust.
The aulacogens of North America are almost all intracontinental. They date from late Precambrian to late Paleozoic. The centrally placed, east-west oriented Keewanawan dominates. The oldest described are in the Canadian Shield, the Athapuscow being best known. There are at least six, also oriented east-west to the east of the Cordillera belt. Others lie along a belt close to the east coast.
Rift valleys are found in the shields of Guiana, of central and coastal Brazil, and of the Pampean Massif of Argentina (Katz, 1971; King, 1956; Zeil, 1979). In Brazil are the San Francisco Rift Valley and the Paraiba Graben. The former is oriented submeridionally and is bordered in part by impressive fault scarps. The Paraiba Graben is of smaller dimension and is oriented northeast, parallel to the coast. In the Guiana Shield is an infilled graben structure, oriented east-northeast, 160 km in length and 50 km wide.
Other structures include the Ypacari Graben in Paraguay, oriented north-northeast, 65 km long, and the Chiquitos Fracture Zone, oriented north-west (probably a graben), in Bolivia. The Pampean Range Massif is characterized by basin and range structure, with tilted blocks, oriented north-south.
In the Atlantic shelves of the Argentine and the Malvinas submarine plateaus are submerged graben of which the Salado, Colorado, and San Jorge basins are most noteworthy. The Brazilian marginal basins, the Reconcaro and the Espirito Sento, are asymmetric graben.
For the greater part of the length of the Andes, from the east-west Oca Fault in Venezuela and Columbia to the Golfo de Peras in Chile, there are intra-Andean discontinuous grabenlike features lying along a central zone and between the Cordillera. They are associated with stretching or tearing processes (taphrogenic) acting generally in an east-west direction. The view that the bordering faults, where they can be discerned, are deep-seated, justifying the description rift valleys, is supported by the usual association of volcanic activity.
The northern section of the Andes, Venezuela, Colombia, Ecuador, and northern Peru has an orientation about 10° east of north. In Colombia are the Magdalena Valley Graben with ill-defined margins and the Cauca Graben, separated by the Cordillera Central. Both these depressions contain enormous thicknesses of Cenozoic sediments.
The Quito-Cuenco Graben Zone of Ecuador is the southern extension of the Cauca Graben. It lies between the Sierra and the Cordillera Real and is floored by the Altiplano, the term used for high-level plains generally. In the western coastal low-land is the Guayas Basin, a graben structure infilled with Cenozoic sediments that is apparently not related to the intra-Andean trench.
The continuity of the chain of the Andes is abruptly interrupted at about 4°S latitude. Southward in northern Peru, the trend is S20°E, and here, although there is longitudinal block faulting in the Precambrian basement reflected in the overlying volcanics, there are only ill-defined graben features containing Tertiary marine sediments but no Vallé Centrale.
The Titicaca Trough of southern Peru and northwestern Bolivia is a long, narrow basin containing Cretaceous and Tertiary sediments. Very late thrust and normal faulting disrupted the Pliocene Para surface and are responsible for the trough that contains Lake Titicaca that straddles the border. The trough broadens and merges into the grabenlike Altiplano partly infilled with continental sediments from the enclosing Cordilleras Oriental with Cordillera Real and the Cordillera Occidental. The Altiplano floor was block faulted.
In northern Chile on the west, commencing at the northern frontier and extending obliquely to 27°S latitude, is the Pampa del Tamarugal, an elongated tectonic depression partly occupied by salars (saline deposits).
The Vallé Longitudinal in Chile extends 1,100 km from north of Santiago (latitude 33°S) to the Golfo de Peras (latitude 47°S). It is a grabenlike depression, and while the margins are not clearly defined by single normal faults, it represents a subsided block between Coastal and High cordilleras. It is most clearly developed between 36°S and 39°S and again south of 40°S latitude and is clearly the product of extensional tectonics.
In northern Chile parallel to the coastline is the dextral strike-slip Atacama Fault Zone, which is more than 1,000 km long. The youngest displacements have been largely vertical.
Aulacogens cannot be expected in the west of South America but along the east, accompanying the opening of the south Atlantic, aulacogens have been described in east Argentina and east Brazil— in particular, the Sierras of Buenos Aires.
Australia has, on the whole, remained remarkably stable during Phanerozoic time, and intracratonic and even marginal rift valley faulting is not conspicuously present (Milanovsky, 1981; Plumb, 1979; Schneibner, 1978). In Tasmania are four graben-type rift valleys of Cenozoic age. The general trend is north by west. In central Tasmania are the Midland Graben, which bifurcates northward into the Tamar, and the Cressy Grabens, which reach the north coast. There are also the Macquarie Harbour and Derwent grabens.
In southeast Queensland is the Upper Brisbane Valley Graben, or Esk Rift, a paleorift of pre-Triassic age infilled with terrestrial sediments and volcanics. It is oriented west of north and is about 60 mi (100 km) long and 10 mi (16 km) wide.
The remaining rift structures have been described as aulacogens, but this term may not be strictly applicable to all. The younger internal basins lie in the northeast, and although elongated, as in the case of the Drummond Basin, their origin has been folding rather than faulting. The older internal elongated basins of central Australia include Officer, Amadeus, and Ngalia, all oriented east-west. In some writings they are described as inner, or blind, aulacogens, but their dimensions and dependence on folding rather than faulting are more characteristic of elongated intracratonic basins. In the west are the elongated basins or penetrating aulacogens, the Petrel Graben and the Fitzroy Graben, both of which are extensions of basins formed during crustal fragmentation of Gondwanaland. The Western Geosyncline marginal to the craton, the Yilgarn Block, is recognized as a rifted aulacogen formed during the separation of India and Tibet in late Cretaceous times.
In south Australia are the faulted blocks and graben of the Spencer Gulf, Yorke Peninsula, St. Vincent Gulf, and Mt. Lofty ranges, extending northward as the Lake Torrens and Flinders ranges. These structures occupy the Adelaide Geosyncline. The pattern of faulting was established in Precambrian or Paleozoic times, but the faults have been active during the late Cenozoic— and still are.
The Alpine fault is of the same nature as the San Andreas and the Great Glen faults (Lillie, 1980). The magnitude of their horizontal displacement is of the same order. They are all transcurrent, in part continental, in part oceanic. The Alpine Fault is now well known in the South Island of New Zealand, but there is no agreement on the course of its extension through the North Island. This is probably through the Central Volcanic region and beyond.
The Hauraki Graben in the North Island is about 220 km long and 20–30 km wide. Its subsidence is related to the late Cenozoic andesitic volcanism. In the South Island in Central Otago is a block-faulted region of classic basin and range geomorphology.
The mid-oceanic rise, along which lies the oceanic rift system, is subdivided by fracture zones into a succession of ridges, each of which bears a name (Garrett, 1981; Knopoff et al., 1969; Vinogradev and Udentsiv, 1975). The fracture zones are the major transforms on which the greatest offsets of the rift system occur. They extend across the full width of the ocean floor that came into being during the current phase of spreading. The mid-oceanic rise is virtually continuous but there are bifurcations and branches. From north of Iceland, which lies on the rise, it continues southward virtually following the median line along the north and south Atlantic Ocean. It continues around southern Africa and in mid-Indian Ocean bifurcates, one branch being northward to the Gulf of Suez. The main course continues south of Australia and the southern Pacific and then northward in the eastern Pacific to the Gulf of California. Along the length of the rise there are differences in the amount of spreading. This spreading has proceeded by the taphrogenic process of pulling apart on the rift valleys with uprise of floor material, and movement on the transforms, especially on the major transforms that connect individual rise units.
In the eastern north Pacific there is no defined rise. The transforms are all fracture zones and widely spaced. The rift valley segments are here much longer than on the rise.
The maximum width of sections of oceanic rift valleys is about 25–35 km at the highest level and 2–3 km on the deepest floor. The depth from crest to floor varies between 500 and 1,500 m. In the Mid-Atlantic, the length of a segment between transforms is about 30 km. The walls usually have the appearance of being gravity step faulted. Depth of water can vary considerably. An abundance of pillow basalt makes up the floor. There are no marked localized gravity or magnetic anomalies over the rift valleys.
There is a balance between delivery of volcanics and outward spreading velocity. The section of the central graben depends on the balance between delivery and spreading. If delivery exceeds spreading the topographic valley will disappear, and a horst will replace it.
The two places where the crustal spreading ridge, with its median rift valleys, enters the land-mass are of special interest. These are the Gulf of California with the Salton Trough and the Gulf of Aden with the Ardoukoba Rift at its head.
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