Rift Faults Research Articles

Statistical analysis of tectonic patterns in areal volcanism: The bunyaruguru volcanic field in West Uganda

A quantitative analysis was made of the spatial arrangement of 149explosion craters in the western rift of Uganda. A variety of methods demonstrate that the spatial pattern of the craters reveals significant structural patterns that have guided volcanism to the surface. It is shown that the east-west elements in the field affected location, and the main rift fault is resolved into two main components. Tentatively, a possible dextral transform fault is identified that affected the relative location of the two main zones of activity. Grouping techniques demonstrate that crater groups obey an exponential rank-size rule and allow a mapping of the craters into energy classes that reveals a concentric pattern of energy in the field. The effect of the topography on energy levels and crater size show that only topography greater than 11,000ft could have prevented all eruptive activity, but the smaller energies and craters are sensitive to height differences on the order of the height of the rift wall, about 1000ft. Total energy in each crater class size is roughly constant, and the field energy could create one or two single craters comparable in size to small central volcanoes.

The structure of the Gulf of Suez (Clysmic) rift, with special reference to the eastern side

The stratigraphical sequence includes Palaeozoic (resting unconformably upon a Pre-Cambrian peneplane), Mesozoic and Tertiary rocks. These were deposited uniformly over the entire region although in Egypt the succession thins towards the south. Miocene rocks rest unconformably on earlier strata with which they contrast sharply in showing remarkable facies changes closely related to structural movements. Igneous activity was on a subdued scale and confined to Oligocene times. Dykes generally run parallel to the main Clysmic rift trend. Rift faulting was initiated in Oligocene and continued into post-Miocene times. In addition to major marginal faults there are many tilted blocks which typify the pre-Miocene faulting. On the west some of these were eroded down to Basement before the deposition of Miocene sediments. In the east many such blocks exhibit only slight erosion and were probably not uplifted until late Oligocene times. The eastern boundary is marked by synthetic faults with downthrows of 1000 metres or more, but in one restricted area there is a downwarp broken by antithetic faults. The Miocene is characterized by anticlinal flexures generally associated with rejuvenation of Oligocene faults. There is no evidence of lateral movement but many of the faults follow ancient Pre-Cambrian trends. On the Sinai peninsula trend-lines show a twofold directional pattern, parallel to the Gulf of Akada and Gulf of Suez respectively. The angle between them suggests a series of conjugate pairs, originating as transcurrent faults in response to north-south compression. If this was the case, such strike-slip movements ceased before the deposition of the earliest Palaeozoic strata. These are at least as old as Carboniferous. The area seems to have remained insulated from the great movements of lateral shiftwhich are claimed for the Akaba-Jordan rift and which are considered to be linked with the divergence of the Red Sea and the counter- clockwise rotation of Arabia. However, there is evidence to suggest that, since Eocene times, there has been a growing divergence of the margins of the Clysmic rift towards the south, producing a small counter-clockwise rotation of Sinai.

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Eastern margin of the Red Sea and the coastal structures in Saudi Arabia

Results of many investigations since 1950 show that the eastern margin of the Red Sea and associated coastal structures in Saudi Arabia have a long geologic history, starting with the deposition of Precambrian eugeosynclinal sedimentary and volcanic rocks before 1000 Ma ago and extending to recent geologic time. The northeastern flank of the Red Sea rift valley is in a shield area affected by possibly four plutonic events at 1000, 720 to 735 (?), 660 to 670, and about 570 Ma. Cratonization of the shield occurred during all or part of the span 520 to 590 Ma. Nubian-type sandstone of Cambrian and Ordovician age laps up on the shield from Jordan southeastward around the rim of the Great Nafud basin, and along the eastern edge of the shield southeastward to 45° E longitude where it is overlapped by Permian limestone. The sandstone reappears to the south and extends southward and westward to the Asir Mountains at the Yemen border. Isolated sandstone outliers are present in the central shield, proving that lower Paleozoic sandstone covered most, if not all, of the basement as now exposed. The Mesozoic era was almost entirely a time of uplift and non-deposition except a middle to late Jurassic fringe marine invasion in the south and a possible narrow invasion from the Gulf of Suez at the end of the era. Marine and non-marine sedimentary deposits of middle and late Tertiary age are found along the Red Sea coast, and Oligocene basaltic flows are present at both low and high altitudes in the coastal ranges. Evidence for important volcanism during Oligocene and earliest Miocene time is widespread, and within the eastern rift fault zone early Miocene hypabyssal intrusives cut the shear zones. Major rifting occurred just before or during early Miocene when the flanks of the rift valley were ramped upward. Shortly after this volcanism a thickness on the order of 3500 m of middle Miocene marl and evaporite beds filled the Red Sea trough. Evidence also exists for widespread subaerial erosion in the Pliocene. Younger lava flows are Pliocene in age but the youngest, near Al Medinah, came as late as A. D. 1250. Lake-bed deposits are very probably in large part Pliocene throughout the shield. The Red Sea coastal plain in Saudi Arabia rises gently eastward from a 3 m littoral surface, generally underlain by dead reef from the Yemen border northward to Al Wajd, a distance of 1400 km. At Jizan, in the south, a salt dome has pushed the 3 m surface up to an elevation of about 50 m. From Al Wajd northward, Pleistocene terraces have been faulted, culminating in several surfaces as high as 520 m above the Red Sea at Tiran Island. Ramping of major fault-bounded blocks along the eastern side of the Red Sea trough—the Midian block in the north, a poorly defined central block, and the Asir block in the south—is connected with renewed movement on regional Precambrian faults. Drainage patterns of wadis in these blocks are characteristically affected by the ramping, and stream capture is common in the Midian and Asir blocks.

Evolution of Rifting in Africa

WHILE not disagreeing with Girdler et al.'s basic premise of thinning and breaking up of the African plate, I cannot see how the increase in age of rift faulting and vulcanicity of the East African rift valleys away from the rift can be compared with the increase in age of islands on the oceanic floor away from mid-oceanic ridges. The latter phenomena can be explained by crustal movement away from the mid-oceanic ridge, which remains the centre of seismic activity, whereas in East Africa there is certainly no corresponding increase in age of rocks away from the rift valleys. Could not the apparent increase in age of seismic activity away from the centres of the rift valleys be explained more simply ?

Structural Reorientation as Evidence of Basement Warping Associated with Rift Faulting in Uganda

A tectonic sequence has been determined for Precambrian basement structures of Ki-baran age (1100 ± 200 m.y.) in an area adjacent to the margin of the Western Rift Valley in Ankole District, southwestern Uganda. Plunges of one set of the structures in the sequence, tight folds (F 4 ), and associated lineations, are dispersed about a 35 ° -trending subhorizontal axis which is subparallel to the rift trend of N. 30 ° E. in this area. This direction is not parallel to a local fold axial trend, however, and the dispersion of F 4 is attributed, therefore, to basement warping related to Tertiary rifting.

Some Recent Geophysical Studies of the Rift System in East Africa

Seismicity maps are used to demarcate the African plate and regions where the plate maybe breaking into smaller plates. One of these is the Gregory rift through East Africa. In this region, complete crustal separation has not taken place as in the Red Sea and Gulf of Aden. Instead, extension seems to have caused attenuation of the lithosphere with accompanying rifting and volcanism. The ages of rift faults and volcanoes tend to increase with distance from the axis of the rift. Three sets of geophysical data can be used to support plate attenuation. These are (i) the long wavelength negative Bouguer anomaly which is found over a large region of East Africa; (ii) the attenuation of Sn for paths crossing the rift north of 10°S. and (iii) microseismicity studies of the rift floor. The long wavelength negative Bouguer anomaly has been interpreted as being due to the low-Q asthenosphere engulfing the lower part of the lithospheric plate. It is suggested that the areal extent of the regional negative Bouguer anomaly might be used to map the region of plate attenuation.

North American Feldspathoidal Rocks in Space and Time

Feldspathoidal rocks are generally considered to be postorogenic and concentrated in the stable portions of continents. These erroneous ideas have biased petrogenetic hypotheses. Of the more than 120 feldspathoidal rock occurrences in North America, only one-third are in the shield and stable craton; the remainder are within Phanerozoic orogenic belts. Approximately 20 percent of all occurrences are pretectonic or syntectonic. In several regions (usually considered petrographic provinces) superficially identical feldspathoidal rocks were emplaced over spans of several hundred million years. Time correlation based upon petrologic similarity and geographic proximity is as risky for these igneous rocks as it is for sedimentary rocks. Hypotheses stating that feldspathoidal rocks are generated during “crustal tension” and rift faulting seem inapplicable to North America.

Peripheral Gulf Rifting in Northeast Texas

The Gulf Coast exhibits a unique association of rift faulting, a tectonic belt, and geosynclinal development. The peripheral graben system of the Gulf Coast is compared with the classic rift systems of Africa and Europe, and is shown to have many characteristic features in common. This rift system is composed of the Balcones-Luling, Mexia-Talco, South Arkansas, and Pickens-Gilbertown rifts. The Mexia-Talco rift is composed of en echelon grabens and half-grabens. These faults are the resultant of post-orogenic taphrogenic movement of the Ouachita mobile belt.

The Potassium-Argon Dating of Late Cenozoic Rocks in East Africa and Italy [and Comments and Reply

A technique for the potassium-argon dating of high potassium feldspars of less than 50,000 years age is described. The technique is applied to the obtaining of high precision ages in the time-range 60,000-2,000,000 years. Sufficient data are presented to show that the time-scale of Plio-Pleistocene glaciations is greater than 10 years and that the time-scale of hominoids capable of fashioning tools by the working of stone is at least 1.75 10 years. Several other points on the time-scale of human evolution are presented. The time-scale of rift faulting in Kenya is established and the ages of several Italian volcanoes are presented.

Major Zones of Transcurrent Dislocation and Superposition of Orogenic Belts in Part of Central Africa

Compression and uplift of the Katanga geosyncline produced the Lufilian fold belt, which sweeps in a broad curve across central Africa. At the close of the main orogenic phase the geosyncline had reached its limit of compressibility, and continued compressive stress ruptured the basement along major tear zones. The crustal blocks were displaced laterally for distances of up to 120 miles. The locations of the zones and their sense of movement were determined by an old orogenic belt at right angles to the Lufilian. In turn, the dislocation zones partly controlled the location of rift faulting of some of the Karroo basins. Consideration of the tectonic history suggests that the Lomagundi and Umkondo systems of Rhodesia are equivalent to the Katanga system, and that the Great Dyke was disrupted at its northern end by movements along the dislocation zones.

The geological structure of the Wadi el Dirba area of Sinai

Summary The Egyptian province of Sinai can be divided into three geological regions: (1) the northern plain, the isthmus, whose surface outcrops are confined chiefly to strata of Mesozoic age, (2) the mountainous peninsula, formed of a complex of Pre-Cambrian rocks, and (3) the western coastal belt, flanking the Gulf of Suez and largely made up of a series of rocks belonging to the Mesozoic and Tertiary. The rocks of the coastal belt have been involved in the rift-faulting movement which produced the Gulf of Suez trough. The effect of these movements was to break up the whole area into a series of tilted blocks. The rocks of the isthmus dip gently towards the Mediterranean, but this regional dip is broken by a number of fold-axes and associated faults which trend WSW.-EXE. They form the western extension of a similar group in Palestine. The particular interest in the Wadi el Dirba area lies in the fact that it is there possible to establish the relationship between the isthmus structures and the Rift movements. The field evidence within the area mapped may be summarized as follows:– 1. There are four normal faults belonging to the Rift system. One of them has a dyke associated with it (both dyke and faults can be established in age as Oligocene from evidence outside the area); one fault also appears to show some horizontal movement. 2. Another normal fault, the Dirba fault, running through the area in a direction parallel to the isthmus structures and assumed to be associated with them is displaced by the Rift faults. Movement along the Dirba fault must therefore have ceased before the main Rift faulting began. Reference is also made to other evidence beyond the area mapped which indicates (1) that the main Rift faulting movement was active during and after Miocene as well as during Oligocene times, and (2) that the isthmus folds may owe their origin predominantly to vertical rather than to horizontal movement.

THE STRUCTURAL AND GEOMORPHIC EVOLUTION OF THE DEAD SEA RIFT

The two methods of projected profiles and reconstructed stream profiles are applied in a geomorphic study of the region on both sides of the Dead Sea Rift. This supplements an earlier study of the tectonics. There are three recognizable surfaces at high levels. The earliest is cut partly in Eocene marine sediments. On both the Arabia and the Sinai–Palestine blocks the third surface has been well preserved and extends far into central Arabia. Warping, tilting and local uplift which affected these early erosion surfaces are related to the first phase of horizontal movement on the wrench-faults of the Rift. Along some sectors tendency to oblique overthrusting and underthrusting caused the downwarping or upwarping of the margins of the blocks ; along others the margins separated and there was an absence of distortion. There followed a period of quiescence, and base-level changed negatively and then remained stationary on more than seven occasions. The last major still-stand was interrupted by a second phase of horizontal movement on the Rift faults, which is continuing. Except for the outpouring of the plateau basalts of the Hauran of southern Syria, volcanism in the region was confined almost entirely to the Arabia block south of Amman. It was characterized by short-lived and occasionally violent eruptions. It began shortly after the initial warping of the early surfaces.

Structural Geology of Arbuckle Mountain Region: ABSTRACT

Structural Geology of Arbuckle Mountain Region: ABSTRACT

Notes on the Geology of the Northern Part of the Ruwenzori Mountains

AbstractA description is given of the geology of the northern portion of the Ruwenzori Mountains. The rocks consist chiefly of a migmatitic series of biotite-gneiss passing to plagioclase-amphibolite uncomformably overlying these are rare occurrences of a slightly metamorphosed series of conglomerates, grits, quartzites, and tale-schists. The area is bounded by rift faults which strike N.N.E.– S.S.W. and the strike of the gneiss series is in general parallel to that direction with a steep westerly dip. Major rift faults arc described and evidence given to show that they are rejuvenations of Pre-Cambrian fault lines. Dextral tear-fault movement is implied with superimposed vertical component. Evidence indicates that faulting took place under compressional stress in Pre-Cambrian times, but that tension supervened at the later stages under light load. A steeply dipping faulting system striking W.N.W.–E.S.E. is also present, and has resulted in the southern blocks rising relative to the northern: in this system also, early compressional stress has been superseded by tension.

Ridge Basin, California

Ridge Basin is a graben dropped between the San Gabriel and the San Andreas faults as a result of local tension or differential pressure where the Garlock and the San Andreas rift faults meet and westward movement causes the latter to be bent into a great arc. Following the inception of the graben in or about Middle Miocene time its point sank with unprecedented rapidity, forming an inland lake in which 21,000 feet of Pliocene and Lower Pleistocene(?) sediments were deposited to comprise the thickest post-Miocene section in California. Resting with marked unconformity on Eocene strata older than the basin is the basin sequence; reading upward, fragments of non-marine Mint Canyon (early Upper Miocene) yielding Hipparion, Merychippus, Protohippus, and Parahippus(?) near-by, then 2,000 feet of upper Monterey (late Upper Miocene) yielding a Neroly marine macrofauna at the base and grading upward to brackish strata, then 18,000 feet of Pliocene(?) lacustrine deposits more or less brackish at the bottom, and finally 3,000± feet of Lower Pleistocene(?) lacustrine beds. The 23,000-foot post-Mint Canyon succession is continuously exposed as a northwest-plunging syncline between Castaic and Gorman, the axis of which is essentially the course of the new Ridge Route highway. The southwest edge of the succession is everywhere angular conglomerate deposited along the San Gabriel fault scarp. The conglomerate grades northeasterly to shale, which, in turn, grades back to pebbly sandstone farther northeast. The vertical throw of the San Gabriel fault which bounds the basin on the southwest scissors near Castaic, and then increases from zero at the pivot to 23,000± feet 20 miles northwest. The particular throw occurred between Middle Miocene and Middle Pleistocene times concomitant with deposition. Following this extreme activity the deep-seated fault plane froze, and has now been essentially dead for a million or so years between Castaic and Beartrap Canyon. After deposition ceased the basin was deformed and degraded approximately 15,000 feet in the Coast Range revolution of Middle and Upper Pleistocene time. During this interval crystalline Frazier Mountain was dragged or thrust across the extreme northwest point of the basin from an original site computed to have been east of the junction of the Garlock and San Andreas rift faults, thus causing piracy of drainage systems to occur on a grand scale, and giving to Piru Creek its present anomalous upper course.

The Corallian Rocks of the Howardian Hills (Yorkshire)

I. Introduction In north-eastern Yorkshire, the gritstones and limestones comprising the Corallian sequence occur flanking the greater part of the Vale of Pickering. From the coastal districts about Filey and Scarborough they extend westward, forming a broad belt of flat-topped hills, the Tabular Hills, and terminating in the magnificent escarpment of the Hambleton Hills overlooking the Vale of Mowbray. To the south-west, the continuity of outcrop is broken by the Coxwold-Gilling rift which intervenes between the Hambleton Hills and the Howardian Hills, the latter lying to the south-east. It is the area of the Howardian Hills which forms the subject of the present communication. The lower ground between the rift faults is occupied by low hills of Kimeridge Clay, covered to varying degrees by glacial drift, but an area of Corallian rocks forms a prominent ridge between Coxwold and Kilburn. About 3½ miles east of Coxwold, the Corallian appears in Gilling Park, and from thence extends as a low ridge in an east-south-easterly direction through Malton to the vicinity of North Grimston, where it becomes lost under the Cretaceous overstep. The strata in this area have commonly a low dip to the north, but the presence of an intricate system of faults causes local modification in the dip and distribution of the rocks and complicates the stratigraphy. The northern boundary of the Corallian is determined by the main Gilling fault, though for the most part its course is obscured by drift. Besides the main line of outcrop, areas of Corallian

Societies and Academies

LONDON. Geological Society, May 14.—Charles Barrington Brown: The geology of north-eastern British Somali-land. Geological results of four months' field work with the Anglo-Italian Boundary Commission in Somaliland. The area discussed is a belt of country along the 49th meridian east, from the coast to about 80 miles inland. There are three structural units: (1) The sunken block or belt of the Aden Gulf, bordered on the south by an important fault of 6000 feet throw, from the next unit, (2), the faulted upraised mass of the Al Hills, which continue as a bordering scarp along the south of the Gulf as the Aroru and Afaf Hills; on the south succeeds (3), the inland plateau-region at 2000 to 3500 feet elevation. This plateau is entirely un-faulted, but has been subjected to folding of a peculiar kind, resulting in gently undulating areas, unaffected areas, and numbers of small synclinal basins the origin of which is obscure. The Jurassic and Cretaceous rocks crop out on the face of the Al scarp fronting seawards. The Lower Eocene forms all the surface of the Al Hills, a splintered block tilted to the south. The Middle Eocene covers all the inland plateau. There is a group of faults in echelon in the mountainous country, each with a throw of nil to 2000 feet in a short distance. The Aden Rift Fault was traced for a distance of 48 miles.