Showing papers on "Fault (geology) published in 1970"

Structural geology in the eastern margin of the canadian rocky mountains

Abstract: Hydrocarbon accumulations in the "foothills" at the eastern margin of the Canadian Rocky Mountains are structurally trapped. Exploration for them entails predicting the deep geometric configuration of potential reservoir beds in imperfectly understood areas. This prediction is commonly derived from analogies with the most appropriate of the region's typical structures, a pragmatic approach which is effective because the "foothills" contain a limited suite of relatively simple structural types: Concentric folds (with their attendant decollement) Low-angle thrust faults (commonly folded) Tear faults (usually transverse) Late normal faults (commonly listric) The assemblage in a particular area is also a function of the degree of deformation and of the lithology of the deformed rocks. Intensity of deformation increases from east to west. Regional stratigraphic changes alter the major lithologic units, while local isopach or facies changes alter the distribution of incompetent rocks within units. The structural styles are all "thin-skinned," as the underlying Hudsonian basement is not involved. This review article summarizes current knowledge of the geometry of "foothills" structures, their mode of occurrence, and some of the empirical rules for their interpretation. Application of this knowledge is illustrated by attempts to provide geometric solutions for the structural conundrums posed by the Brazeau Range, the Flathead Fault and the Rocky Mountain Trench.

Mechanism of the Chilean Earthquakes of May 21 and 22, 1960

Abstract: The Chilean earthquake sequence of May 21–22, 1960, was accompanied by linear zones of tectonic warping, including both uplift and subsidence relative to sea level. The region involved is more than 200 km wide and about 1000 km long, and lies along the continental margin between latitude 37° and 48° S. Significant horizontal strains accompanied the vertical movements in parts of the subsided zone for which triangulation data are available. Displacements were initiated near the northern end of the deformed region during the opening earthquake of the sequence (M s ≅ 7.5) on May 21 at 10h 02m 50s GMT and were extended over the remainder of the region during the culminating shock (M s ≅ 8.5) on May 22 at 19h llm 17s GMT. During the latter event, sudden uplift of adjacent portions of the continental shelf and much or all of the continental slope apparently generated the destructive tsunami that immediately followed the main shock. Available data suggest that the primary fault or zone of faulting along which displacement occurred probably is a complex thrust fault roughly 1000 km long and at least 60 km wide; it dips eastward at a moderate angle beneath the continental margin and intersects the surface on the continental slope. Dip slip required to satisfy the surface displacements is at least 20 m and perhaps as large as 40 m. There is some evidence that there was a minor component of right-lateral slip on the fault plane.

Structural Analysis of the Dasht-e Bayaz (Iran) Earthquake Fractures

Abstract: A structural analysis is presented of the fractures formed in the fault zone associated with the Dasht-e Bayaz earthquake of August 31, 1968. The segment of the fault zone studied in detail here is 25 km long, 2 to 3 km wide, and located in the Quaternary sediments of the Nimbluk Valley. The maximum relative displacements ohserved in the fault zone (up to 450 cm left-lateral and 250 cm vertical) are concentrated in an east-west principal displacement zone 2 to 100 m wide. This zone is in turn, on a larger scale, composed of en echelon shear zones. The fault zone contains also many small fracturesdispersed throughout the area and reflecting, to a certain extent, the dominant trends of the principal displacement zone. Both on the scale of the whole fault zone and on the larger scale of the principal displacement zone, the structures are characteristic of a simple shear type of deformation. Their sense of movement and initial directions can be interpreted in terms of the Coulomb failure criterion applied to a material with an angle of shearing resistance of 35 to 40. The analysis also shows that the Nimbluk Valley contains many fault lineaments that extend into the mountains around the valley- In many places the cast-west principal displacement zone follows very precisely one of these lineaments. Evidence of fault reactivation is also found on another important lineament that crosses the principal displacement zone in a WNW-ESE direction. The earthquake fracture pattern in the Nimbluk Valley is compatible with a predominant movement along the east-west lineament, followed by stress readjustments along the WNW-ESE lineament.

Earthquake Recurrence Intervals on the San Andreas Fault

Abstract: Possible recurrence intervals between earthquakes of different magnitude that may be generated along the San Andreas fault are derived by relating long-term offset rates since mid-Tertiary time, displacements, and lengths of breaks recorded for historic earthquakes, and tectonic creep rates. The recurrence interval for earthquakes of different magnitude at a given point on the fault is believed to follow the relation: ![Formula][1] where: Rx : recurrence interval at a point on the fault, D : displacement accompanying an earthquake of given magnitude (related empirically to Richter magnitude), S : long-term strain rate (from offset of geologic units), C : tectonic creep rate. The recurrence interval for earthquakes of different magnitudes for the total length of the fault is then derived by weighting [equation (1)][2] according to the number of break lengths in the total length as follows: ![Formula][3] where: Rt : recurrence interval for entire fault, L : length of break (related empirically to Richter magnitude), Lt : total length of fault. Tectonic creep is believed to be related to Richter magnitude, for example, small for segments of the fault characterized by earthquakes of large magnitude, and large for segments characterized by small earthquakes; and [equations (1)][2] and [(2)][4] can be weighted according to this relationship. [1]: /embed/graphic-1.gif [2]: #disp-formula-1 [3]: /embed/graphic-2.gif [4]: #disp-formula-2

Large transcurrent faults in Southeast Asia with special reference to Indonesia

Abstract: Recent geological studies in areas west of the Pacific Ocean byAllen, Krause, Visser, andHermes have revealed the existence of large transcurrent faults in the Taiwan-Philippine region and in the area between the Celebes and East New Guinea. Physiographic evidence from the Philippines, bathymetric and geological data from New Guinea demonstrate that the sense of movement along these faults is sinistral. Another sinistral transcurrent fault has recently been described byBurton from Malaya. The Sumatran fault-system which shows clearly a right-handed lateral sense of displacement could be observed along the 500 kilometer long Ulu-Aer fault in North Sumatra and along another fault-zone situated between Lake Dibaruh in Central Sumatra and Lake Ranau in South Sumatra covering a length of more than 500 kilometers. Although the Island of Java is mostly covered by young volcanic material, it is still possible to detect smaller transcurrent faults possessing a strike more or less parallel to the island. The Fossa Sarasina in Central Celebes displaying smoothness of curvature over literally hundreds of kilometers could also be interpreted as being a sinistral transcurrent fault. Another dextral transcurrent fault of about 100 km length has been postulated in the Gorontalo area, northern Celebes. Two groups of transcurrent faults could be discerned in Southeast Asia viz a group trending more or less northwest-southeast and another one striking approximately east-west. The data presented in this paper tend to supportVening Meinesz theory that the Indonesian Archipelago is being protruded southeastward. The major block movements are taking place along the Philippine and Sumatran fault-zone, while differential movements within this block seem to occur along the Fossa Sarasina and the Gorontalo fault-zone. The approximately east-west trending sinistral transcurrent faults might be associated with the eastward drift of the Australian continent and the westward convection flow originated from the East Pacific Rise as envisaged byWilson.

Microearthquake seismicity and tectonics of Coastal Northern California

Abstract: This paper demonstrates that high-gain, high-frequency portable seismographs operated for short intervals can provide unique data on the details of the current tectonic activity in a very small area. Five high-frequency, high-gain seismographs were operated at 25 sites along the coast of northern California during the summer of 1968. Eighty per cent of 160 microearthquakes located in the Cape Mendocino area occurred at depths between 15 and 35 km in a well-defined, horizontal seismic layer. These depths are significantly greater than those reported for other areas along the San Andreas fault system in California. Many of the earthquakes of the Cape Mendocino area occurred in sequences that have approximately the same magnitude versus length of faulting characteristics as other California earthquakes. Consistent first-motion directions are recorded from microearthquakes located within suitably chosen subdivisions of the active area. Composite fault plane solutions indicate that right-lateral movement prevails on strike-slip faults that radiate from Cape Mendocino northwest toward the Gorda basin. This is evidence that the Gorda basin is undergoing internal deformation. Inland, east of Cape Mendocino, a significant component of thrust faulting prevails for all the composite fault plane solutions. Thrusting is predominant in the fault plane solution of the June 26 1968 earthquake located along the Gorda escarpement. In general, the pattern of slip is consistent with a north-south crustal shortening. The Gorda escarpment, the Mattole River Valley, and the 1906 fault break northwest of Shelter Cove define a sharp bend that forms a possible connection between the Mendocino escarpment and the San Andreas fault. The distribution of hypocenters, relative travel times of P waves, and focal mechanisms strongly indicate that the above three features are surface expressions of an important structural boundary. The sharp bend in this boundary, which is concave toward the southwest, would tend to lock the dextral slip along the San Andreas fault and thus cause the regional north-south compression observed at Cape Mendocino. The above conclusions support the hypothesis that dextral strike-slip motion along the San Andreas fault is currently being taken up by slip along the Mendocino escarpment as well as by slip along northwest trending faults in the Gorda basin.

Sur-Nacimiento Fault Zone of California: Continental Margin Tectonics

Abstract: The Sur-Nacimiento fault zone extends northwest through the southern and central Coast Ranges of California, and presumably continues offshore on the Continental Shelf. In part, or perhaps in its entirety, it forms the approximate boundary between the Franciscan trench(?) assemblage on the southwest and the granitic and regionally metamorphosed basement rocks of the Salinian block on the northeast. The Sur-Nacimiento fault zone includes the Sur fault zone, the Nacimiento fault, and a number of other faults of various kinds and various ages. There has been a long history of recurrent activity characterized by sequential changes in the types of movement. The Sur fault zone is here considered to have originated at the former margin of the continent during latest Cretaceous or Early Tertiary time as the culmination of processes that operated during the entire Late Mesozoic. Throughout the latest Jurassic, Early Cretaceous, and mid-Cretaceous, Franciscan deposits were probably carried into a trench and pushed against or partly beneath the crust to the east, by ocean-floor spreading. The Franciscan assemblage has a melange-like aspect which is appropriate to this kind of disturbance. Some rocks within the assemblage are about the same age (Early or mid-Cretaceous) as the adjacent Salinian granite, but they are unaffected by the granite. The postulated trench containing the Franciscan assemblage was probably far offshore during the time of granitic intrusion, but westward drift of North America narrowed the intervening distance. Conveyor-belt action of eastward-moving oceanic crust impinging against and beneath the edge of the continent may have ripped off and carried downward blocks of sialic crust. Perhaps this was the mechanism that eventually brought the Franciscan assemblage against the Salinian granite, creating the ancestral Sur fault zone. The Franciscan terrain is locally tectonically covered by allochthonous masses of Great Valley-type Late Mesozoic clastic sedimentary rocks predominantly of two ages: (1) Tithonian-Valanginian, and (2) Campanian-Maestrichtian(?). It is tentatively suggested that these allochthonous rocks were deposited between the continent and the trench in which the Franciscan accumulated, and that they were underthrust by the Franciscan during the same action that produced the Sur fault, in latest Cretaceous or Early Tertiary time. In the Tertiary, normal faulting occurred along the Sur-Nacimiento zone, following the annihilation of the active trench system. There are indications that strike-slip faulting and reverse faulting ensued at various times.

Kermadec arc—New Zealand tectonic confluence

Abstract: The transition in tectonic style from that of the Kertnadec island arc to that of continental crust in New Zealand has been ascribed, in global tectonic schemes, to the reduction of “shortening” rate as the pole of rotation between the Australian and Pacific crustal blocks is approached. However, the recognition of a west-facing island arc south of New Zealand and of widespread Quaternary crustal shortening within the New Zealand continental crust leads to the conclusion that the differing tectonic styles reflect instead the variation in mechanical response of oceanic and continental crust. The Alpine Fault is not considered to be a trench-trench transform because it parallels and co-exists with island arc features. Displacement of the early Tertiary Vening Meinesz fracture zone by the Alpine Fault indicates 70 km of late Tertiary-Quaternary horizontal motion along this fault. It is possible that the rest of the 500 km displacement along the South Island trace occurred during the late Mesozoic wh...

Cover and Basement Tectonics in the Cordillera Oriental of Colombia, South America, and a Comparison with Some Other Folded Chains

Abstract: The Cordillera Oriental of Colombia is built of Mesozoic and Tertiary strata, deformed chiefly during Tertiary orogenesis. These materials form a sedimentary cover, overlying a basement of lower Paleozoic (and Precambrian?) metamorphic and igneous rocks, and of little deformed and nonmetamorphic Upper Paleozoic rocks, which often behaved mechanically as part of the cover. No metamorphism took place during the Tertiary orogenesis, and the main structural features of the chain are determined by the relationship between basement and cover. The basement crops out within the chain in two massifs, Santander and Quetame-Garzon, arranged en echelon. The general structure of the chain is that of a raised block between the Magdalena Valley and Los Llanos of eastern Colombia. Tilting, differential lift, and faulting produced differences in height of the order of 10,000 m in the upper surface of the basement. The faults in the basement are reverse faults, often high-angle reverse faults (some of them reactivated normal faults of pre-Cretaceous age), some of which have been active until Quaternary times (Suarez fault). It is possible that strike-slip occurred along some faults, as has been suggested for the Santa Marta (= Bucaramanga) fault and for many faults in the Caribbean area. A rather narrow strip in the central part of the chain, between the two massifs shows a more tightly folded structure. The sedimentary cover shows different reactions to basement tectonics: (1) passive behavior (undeformed or tilted cover on basement blocks); (2) simple draping structures (monoclinal flexures overlying basement faults; asymmetrical gentle folds); (3) more complex, tight structures, where the basement has a less rigid behavior; (4) independent behavior of the cover, due to gravity (collapse structures), to injection (salt of the “Sabana de Bogota”), and to disharmonic relationships. Thus, the Cordillera Oriental of Colombia shows a series of different examples of the relationship between basement and cover structures. The Cordillera Oriental constitutes the external zone of the Andean System in Colombia, in sharp contrast to the internal zones (eugeo-syncline), with their volcanism of Cretaceous age, their plutonism, and their metamorphism. Such mountain chains as the Appalachians, the Cantabrian Chain, the Alps and the Betic System are characterized by tangential tectonics (nappes), structures facing regularly toward the outside of the chain, and a general decollement of the cover. However, the Cordillera Oriental shows a fan structure, more related to the structure of such mountain chains as the Pyrenees.

The Palestina Fault, Colombia

Abstract: The Palestina fault is an inactive right-lateral wrench fault more than 350 km long in the largely metamorphic and igneous terrain of the northern Central Cordillera of the Colombian Andes It strikes north to north-northeast, nearly parallel to the regional Andean structures Much of the fault is followed by straight canyons 50 to 600 m deep A zone of breccia and mylonite less than 50 m thick marks the fault Rock is intensely fractured within 50 m of this breccia zone Deformational effects of the Palestina include large fault-block slivers of megabreccia and drag “tails” that contain some allochthonous material A gravity profile across the Palestina shows no associated anomaly Horizontal displacement on the Palestina of 277 km is well documented because the fault has offset ten unique lithologic, metamorphic, and structural features that have been mapped on both blocks These offset features are: marble, quartzite, feldspathic and aluminous gneiss, diorite, zones of diorite mixed with Precambrian gneiss, hornblende gabbro, Cretaceous shale, metamorphic isograds, a major wrench fault, and several minor faults The Palestina fault is only one of several recently recognized wrench faults in the northern Central Cordillera, and its documentation should call attention to the possible importance of these features in the tectonics of the area Published analyses of wrench-fault tectonics in northern Colombia and Venezuela assume a genetic relationship between all the wrench faults: they originated in response to a single unchanging regional stress system A more meaningful analysis might result from the individual study of smaller geographic areas and consideration of the relative ages of the wrench faults

Microscopic Feather Fractures

Abstract: Certain microfractures adjacent to faults in experimentally deformed cylinders of Westerly Granite form as a result of shear displacement along the primary fault, and they are oriented consistently relative to the fault surface. These fractures are described in detail because they are: (a) symptomatic of a homogeneous state of stress in the region of the fault at the time of faulting; (b) criteria to demonstrate that shear displacement has occurred along a fault; and (c) criteria to establish unequivocally the sense of shear.

Boconó Fault, Venezuelan Andes: Evidence of Postglacial Movement

Abstract: Postglacial, right-lateral, strike-slip movement along the Bocono fault, measured on detailed topographic maps, averages 66 meters. The rate of movement was approximately 0.66 centimeter per year, as indicated by carbon-14 dating of associated soil. This evidence suggests that postglacial movement between the Caribbean and Americas plates occurred mainly along the Bocono fault and the north coast of Venezuela.

Focal mechanisms and techtonics in the vicinity of Taiwan

Abstract: The island of Taiwan is an anomalous member of the Ryukyu-Philippine arc. Focal mechanisms of large earthquakes under the ocean to the east of the island are found to be strike-slip type. Together with the thrust type events to the northeast and left-lateral fault in the Longitudinal Valley, a local northerly downthrust is inferred.

Late Paleozoic Structural Directions in Southern Permian Basin, West Texas and Southeastern New Mexico

Abstract: This study shows the possibility of extensive lateral movement along pre-Permian faults in the Permian basin and determines, largely from subsurface evidence, the direction of the forces responsible for Pennsylvanian and Permian tectonic deformation. A review of the relation of faulting to orthogonal stress axes shows that changes in the geometry and genesis of faults are the consequences of changes in the orientation of the stress axes. Two tectonic systems can be discerned. One consists of folds and faults. The fold axes strike N35°W, whereas the fault systems include one that strikes N55-80°E with indications of right lateral movement and another that strikes N50-65°W with left lateral movement. Thus the fault pattern indicates stress in a nearly east-west direction and the folds indicate direction of stress to be about N65°E. The age of this deformation is thought to be early Late Mississippian to late Middle Pennsylvanian. Another fault system which seems to be younger is composed of faults striking slightly west of north. The most prominent example of this system is the West Platform fault, the movement on which is right lateral with a displacement of tens of miles. The stress giving rise to this system seems to have been nearly north and south and may be related to the force that powered the early and middle Wolfcampian (Permian) thrusting on the south in the Marathon salient of the Ouachita belt. The jagged pattern of the eastern edge of the Central Basin platform may be caused by conjugate faults of this system. The later tectonic history of the region is marked by relaxation of stress and subsequent normal fault movement on the previously formed fault planes. This movement deepened preexisting basins; reefs and other sedimentary features formed around the peripheries of these depressions. Formation of salt pans in later Permian time was related to minor movement along the faults. Remnants of the basins persisted throughout Triassic time, but were obliterated by the great change in geography during Cretaceous time. Renewed movement along the old lines of weakness during Tertiary time caused the eastward tilting of the Delaware basin and the formation of grabens along its west edge. Deep channels in Pleistocene and Holocene sand deposits in the eastern part of the Delaware basin reflect soluti n of the underlying Permian evaporites over these lines.

Postglacial faulting and seismicity in New York and Quebec

Abstract: Within the last few years studies of relationships among faulting, fault creep, and seismicity in active areas such as California and Nevada have progressed rapidly, and have contributed greatly to our understanding of tectonics and the earthquake mechanism. This success prompted a field and literature search for similar phenomena in New York State and adjoining areas, a region of only moderate seismicity but of sufficient population density so that the seismic hazard should be thoroughly studied using all relevant information.Evidence for postglacial bedrock faulting, dated by offset striations, is common on outcrops of a belt of Paleozoic shales and slates that extends from near Hyde Park, New York, northerly along the east side of the Hudson and Champlain valleys, and continues northeasterly into Quebec at least as far as St. Georges. Faults tend to be in the plane of slaty cleavage with the south or east side upthrown in postglacial time. The observed faults are thus high-angle reverse faults with lit...

Surface deformation and fault displacement associated with an earthquake swarm in Iceland

Abstract: Repeated precision leveling across a fracture zone in southwest Iceland showed significant surface deformation over a period of three years, 1966 to 1969. Most of the observed deformation is assumed to have taken place during an earthquake swarm in September 1967, originating about 25 km to the southwest of the leveling profile. The observed deformation includes a dip-slip displacement of about 6 mm on the fault Hrafnagja, and subsidence of up to 8 mm of a narrow zone centered near the same fault. The rate of change of deformation away from the fault shows that the dip-slip displacement is confined to a thin surface layer, probably less than 500 meters thick, and the subsidence can be explained as resulting from tensional displacement of about 7 mm in the fault zone between 400 and 3000 meters depth. During the year 1968 to 1969 the Hrafnagja fault zone rose slightly, probably 1 to 2 mm, indicating a reversal of the subsidence observed during the previous two years. It is suggested that this uplift is due either to local heating caused by change of flow of ground water and associated thermal expansion or to lateral compression triggered by an earthquake of magnitude 6 that occurred on December 5, 1968, about 30 km east of the leveling profile.

Dating Episodes of Faulting from Tree Rings: Effects of the 1958 Rupture of the Fairweather Fault on Tree Growth

Abstract: Surface faulting associated with major earthquakes commonly kills, damages, or otherwise disturbs the growth of trees along the fault trace. Because severe disturbances are reflected in the annual growth, tree-ring sequences provide a basis for dating recent episodes of faulting. The validity of this method is confirmed by data collected from the Fairweather fault zone in southeastern Alaska. Marked changes in growth that correlate with large-scale faulting in 1958 resulted from tilting, felling, and topping of trees and changes in ground water level and exposure to sunlight.

Delineation of shallow salt domes and surface faults by temperature measurements at a depth of approximately 2 metres

Abstract: The feasibility of using temperature measurements at a depth of about 2 m for locating and delineating salt domes and faults has been investigated both theoretically and in experimental field surveys. It is shown that measurable temperature anomalies in the soil are to be expected over shallow salt domes. In a field survey over a salt-dome area bordering the Groningen gas field, a large number of temperature measurements were made in small holes (2 m deep, 3 cm in diameter) within a relatively short time (some weeks). The results clearly indicate several temperature anomalies with differential temperatures of about 1°C. Comparison of our thermal contour map with interpretations of available seismic or gravity data, or with direct evidence from wells, showed an excellent correlation. Seismic data even support the shape of the thermal contours. Results in similar agreement with gravity or well data were obtained over salt ridges in a tropical area. Experiments showed that the technique worked as well in lakes and marshes as on dry land. In addition, some experimental evidence collected so far over shallow and surface faults is presented. In several cases, strong thermal anomalies coincided with known surface faults. A thermal model for a surface-fault zone is suggested which accounts satisfactorily for the observed thermal data. It suggests some diagnostic value for the fault's geometry. For shallow faults, however, lack of knowledge of subsurface detail prevented any unambiguous correlation with observed thermal anomalies. Accordingly any geological use of thermal analysis over shallow faults remains debatable. The field technique is simple, needs little correction and can, where useful, easily be included in routine gravity work to provide additional local information.

On Afro-Arabian graben tectonics

Abstract: This study of the Afro-Arabian graben, which concentrates on the region between Kenya and the Levant reviews their evolution that begun not earlier than in the latest Oligocene or in the lower Miocene. The graben continue to sink in several major downfaulting phases, especially during the transitional times of Miocene to Pliocene, of Pliocene to Pleistocene and in the Mid-Pleistocene. We have no proof of erstwhile major graben structures in the crystalline Precambrian basement or in the pre-Oligocene sedimentary cover revived during the young Cenozoics that could have redeemed the formation of the Afro-Arabian megagraben. Significant faults in the pre-Oligocene sedimentary cover are seldom observed. Where they occur, their tectonic direction scarcely runs parallel to the principal trend of the Cenozoic graben. At best these ancient faults may have had an impact on the development of cross — and diagonal faulting which played also a role in the disposition of the subgraben, grabensplays and other offshoots of the main graben. Equally rarely has parallelism of trend between the basement metamorphics as well as their intrusive dykes and that of the main graben lineations been established. The Afro-Arabian graben originated in a lower Tertiary peneplain of regional dimension which had expanded over large part of Gondwana. The graben dissection thus took place regardless of former tectonic highs and lows such as swells and basins — a phenomenon that negates the theory of key-stone dropping as a result of preceding upwarping. The occasional impression of vaulting with the graben as its axis is purely of morphological significance; it is due to “isostatic” uplifting and tilting of the main horsts attached to the main graben. Besides, most megahorsts are not “halfhorsts” but huge blocks mountains (Levant, Etbai, Ethiopia, Danakil, Yemen) bordered on both flanks by major tensional faults with throws of thousands of meters. These major uplifts, which are especially noticed in the Plio-Pleistocene, caused, moreover, the removal of the sedimentary cover by erosion and denudation in the elevated parts of the graben shoulders and its hinterland. The few known outcrops of Cretaceous-Eocene in the coastal area of the Red Sea, believed to be indications of ancient marine gulfs and thus forerunners of the Rift valley are in all likelihood the relics of such coverbeds that were saved from erosion in the down dropped blocks of the graben. The surveyed marginal faults are normal dip-slips of average 70° dip. These remained in the ensuing taphrogenic episodes the “localized” fixed graben frames. In the interior of the graben they are followed by a Vorberge zone consisting of tilted blocks that implicate also subsidiary horsts and graben. In the superdimensional rifts of Afar and Red Sea, the subsidiary graben and horsts of the Vorberge zone may approach a width comparable to that of the East Levant rift valleys. In the axial part of the Afar, Red Sea and Gregory Rift valley appears as youngest, that is, Pleistocene structural element a distinct graben-in-graben or rift-in-rift. This young structure is much less developed in the graben of the Gulf of Aden. The Aden graben differs also in other aspects from the pattern of the Red Sea, particularly in its submarine axial topography and in the peculiar transversal fault offsets, both morphotectonic features reminiscent of the Indian mid-oceanic ridges. A comparison of geophysical, especially of gravity measurements, carried out in the various main graben reveals very conflicting results. While higher positive Bouguer values led to the conjecture of heavy magmatic material in the bottom of the rift-in-rift of the Red Sea, in most of the Afar fault-funnel and in the entire Gregory Rift valley there prevails an unusual large gravity deficiency that would point to an extremely thick crust. This is all the more remarkable as both rift valleys and horsts of Ethiopia and of Kenya are dominated by huge basaltic trap lavas and by other basic volcanic material. The hypothesis of anticlockwise rotation and drifting of the Arabian peninsula (including its supposed impact on the orogenic origin of the Zagros fold belt) has many pitfalls.

Refraction seismic investigations of the northern midcontinent gravity high

Abstract: Eighty-seven seismic refraction profiles have been obtained to define the geologic structure in the upper crust associated with the midcontinent gravity high in Minnesota and Wisconsin. The seismic measurements were taken across a fixed spread of seven geophones from distances up to 13 km. A structural section was prepared for each profile by interpretation of the travel-time graph, and the individual sections were compiled onto regional cross sections. Measured seismic velocities in bedrock fall in the 2.5–7.1 km/sec range. Observed velocities can be assigned to seven categories corresponding to Paleozoic, upper, middle, and lower Upper Keweenawan strata, Middle Keweenawan volcanics, pre-Keweenawan felsic-to-intermediate intrusive rocks, and pre-Keweenawan mafic intrusive rocks. These groups display good continuity through the area and suggest the lithologic equivalence of rock bodies between geologic provinces. The St. Croix horst and its flanking basins underlie the midcontinent gravity high and its parallel gravity lows north of Minneapolis. Minimum throw along the western and eastern boundary fault zones reaches about 3.0 and 2.0 km. Sedimentary rocks in the eastern basin reach a thickness of at least 2.6 km. A complex horst-like structure also underlies the midcontinent gravity high in southern Minnesota; an uplifted basaltic body is bordered by sedimentary basins having strata about 3.0 km thick. Middle Keweenawan basalts are present locally in the eastern and western basins underlying the Upper Keweenawan strata. Rocks probably lithologically equivalent to the Oronto group are rare in the western basin, common in small basins on the St. Croix horst, and abundant in the eastern basin. Rocks probably lithologically equivalent to the Bayfield group are extensive in the western and eastern basins, but they have not been found on the St. Croix horst. The Bayfield group seems to be several kilometers thick across Douglas County north of the Douglas fault, and it does not appear to increase in thickness under the Bayfield peninsula.

Observation and interpretation of tectonic strain release mechanisms

Abstract: In four chapters various aspects of earthquake source are studied. Chapter I Surface displacements that followed the Parkfield, 1966, earthquakes were measured for two years with six small-scale geodetic networks straddling the fault trace. The logarithmic rate and the periodic nature of the creep displacement recorded on a strain meter made it possible to predict creep episodes on the San Andreas fault. Some individual earthquakes were related directly to surface displacement, while in general, slow creep and aftershock activity were found to occur independently. The Parkfield earthquake is interpreted as a buried dislocation. Chapter II The source parameters of earthquakes between magnitude 1 and 6 were studied using field observations, fault plane solutions, and surface wave and S-wave spectral analysis. The seismic moment, MO, was found to be related to local magnitude, ML, by log MO = 1.7 ML + 15.1. The source length vs magnitude relation for the San Andreas system found to be: ML = 1.9 log L - 6.7. The surface wave envelope parameter AR gives the moment according to log MO = log AR300 + 30.1, and the stress drop, τ, was found to be related to the magnitude by τ = 0.54 M - 2.58. The relation between surface wave magnitude MS and ML is proposed to be MS = 1.7 ML - 4.1. It is proposed to estimate the relative stress level (and possibly the strength) of a source-region by the amplitude ratio of high-frequency to low-frequency waves. An apparent stress map for Southern California is presented. Chapter III Seismic triggering and seismic shaking are proposed as two closely related mechanisms of strain release which explain observations of the character of the P wave generated by the Alaskan earthquake of 1964, and distant fault slippage observed after the Borrego Mountain, California earthquake of 1968. The Alaska, 1964, earthquake is shown to be adequately described as a series of individual rupture events. The first of these events had a body wave magnitude of 6.6 and is considered to have initiated or triggered the whole sequence. The propagation velocity of the disturbance is estimated to be 3.5 km/sec. On the basis of circumstantial evidence it is proposed that the Borrego Mountain, 1968, earthquake caused release of tectonic strain along three active faults at distances of 45 to 75 km from the epicenter. It is suggested that this mechanism of strain release is best described as "seismic shaking." Chapter IV The changes of apparent stress with depth are studied in the South American deep seismic zone. For shallow earthquakes the apparent stress is 20 bars on the average, the same as for earthquakes in the Aleutians and on Oceanic Ridges. At depths between 50 and 150 km the apparent stresses are relatively high, approximately 380 bars, and around 600 km depth they are again near 20 bars. The seismic efficiency is estimated to be 0.1. This suggests that the true stress is obtained by multiplying the apparent stress by ten. The variation of apparent stress with depth is explained in terms of the hypothesis of ocean floor consumption.

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 - Interpretation of satellite photographs of the Red Sea and Gulf of Aden

Abstract: Geological structures observed in Gemini and Apollo colour photographs suggest that large-scale translational movements could have taken place in the Red Sea and Gulf of Aden. In the northern Red Sea the apparent displacement of two pairs of shear zones and three pairs of serpentinite belts is consistent with a movement of Arabia towards the NNE of some 150 km. In the southern part of the Red Sea evidence of displacement is derived from correlation of Precambrian trend-lines, particularly at points where there is an abrupt change in the regional grain; at Ras Kasr-Al Lith (latitudes 18° N and 20° N) the total movement could be 225 km. Across the Gulf of Aden observations are in general agreement with the pre-Miocene fit proposed by Laughton (1965). Study of satellite photographs provide the following additional evidence: (1) The continuity of the Hadramawt folds (southern Arabia) in the Somali Plateau; the southern Hadramawt arch appears to be extended in the northern Somali arch. (2) Pre-drift correlation of several fault zones of WNW-ESE trend across the Gulf. The NE-SW faults, on the other hand, show poor correlation across the Gulf and appear to be related to fault lineaments within the Gulf of Aden. Across the Strait of Bab El Mandeb geological and morphological similarities in the distribution of Quaternary sediments, volcanic fields, intervening alluvial deposits, fault and drainage lineaments suggest a left-lateral displacement of Arabia some 40 km to the NNE since the Plio-Pleistocene. Such a movement could have resulted in the final opening of the Bab El Mandeb Strait.

Volcanism and the Ardross Fault, Fife

Abstract: Synopsis The relationship of volcanic necks and related rocks to folded and faulted Visean strata between Elie and St. Monance suggests that the Ardross Fault was the final expression of a major north-easterly tectono-volcanic structure which was active intermittently throughout Upper Carboniferous, and possibly early Permian, times. A sequence of volcanic and tectonic episodes began with folding and faulting on NNE. and SE. lines, probably early in the Namurian. The main volcanism is probably late Namurian, but may have continued into the Westphalian and Stephanian. The Ardross Fault, a dextral strike-slip structure, though with a strong vertical component, is later than the volcanism and is off-set by later SE. faulting.

Fault topography and rate of faulting along the median tectonic line in the drainage basin of the river yoshino, northeastern shikoku, japan

Abstract: The Median Tectonic Line is one of the most spectacular features in physiography of Southwest Japan, ; and lies as a boundary between the Upper Cretaceous Izumi group of the Inner Zone and the Palaeozoic crystalline schist of the Outer Zone. It trends continuously and almost straight in ENE direction, especially in the middle and western part. The writer investigated mainly fault topography and geology along the Median Tectonic Line in the lower drainage basin of the Yoshino. The results are briefly summarized as follows. (1) There are several faults which run parallel or en echelon along the so-called Median Line. All of these faults are not necessarily active, but have been dislocated in the rightlateral sense. The witer, therefore, named these faults the Median Tectonic Line fault system. In areas intervening between neighbouring two strands of these faults, several NE-trending faults of dip-slip type develop as shown in Fig. 1&2. (2) At Awa-Ikeda, horizontal and vertical displacements along the Ikeda fault amount to about 200 meters or more and 40-50 meters, respectively, during the past 30, 000 years (Okada, 1968). At Ikenoura, Mima-Town (Fig. 4), horizontal displacement along the Mino fault is 200-230 meters during about 25, 000 years. At Kami-kirai, Ichiba-Town (Fig. 8), hori-zontal and vertical displacements along the Chichio fault are 50 meters and 5-6 meters, re-spectively, in the recent 10, 000 years. From these data, the mean rate of right-lateral displace-ment along the Median Tectonic Line fault system is estimated at 6-9 meters per 1, 000 years. (3) Fault planes of the Median Tectonic Line, as far as observed at many outcrops in this area, generally dip 60°-80° N or almost vertically. Some features preserved in the shat-tered zones indicate the right-lateral faulting, accompanying with slight uplifting of the northern blocks. (4) In this area, generally speaking, northern blocks of these active faults have been up-lifted. However, rates of vertical displacement along one strand of these active faults are locally different, and recent faulting accompanied with wavy deformation or transcurrent buckling. Vertical displacement along the whole length of the Median Tectonic Line from central Japan to Kyushu shows the scissoring on both sides, that is, the northern blocks of the fault have been uplifted in its eastern part and vice versa in its western part. (5) As to stream offsets along the faults, it indicate that the longer are upper streams from faults, the larger are stream offsets, which amount to 1.5 or 2 kilometers in maximum (Fig. 10). It is concluded to have been dislocated at almost uniform rates at least during the latter half of the Quaternary. (6) No great earthquake has occurred along the Median Tectonic Line in the historical time, though there are many geological evidences of quite recent fault displacement probably accompanying with great earthquakes. Tectonic creep along the fault lines is not yet found in the investigated area. It is, therefore, highly possible that a great earthquake may occur along the Median Tectonic Line in near future.

Locations of small earthquakes near the trifurcation of the San Jacinto fault southeast of Anza, California

Abstract: About 100 small earthquakes (M ≈ 1/2 to 2) which occurred near the trifurcation of the San Jacinto fault southeast of Anza, California, have been accurately located using five- and six-station arrays with dimensions of about 10 km. The pattern of epicenters is complex and extends several km outside of the area outlined by the traces of faulting. Patterns of seismicity observed on opposite sides of the San Jacinto fault are significantly different. On the southwest side, a concentration of foci lies at a depth of about 4-7 km along the projected extension of the Coyote Creek fault a few km northwest of the last surface evidence of faulting. On the northeast side, earthquakes are concentrated at depths between 10 and 15 km. A group of the latter events recorded about 1 week after the magnitude 4.7 earthquake of May 21, 1967 forms a linear pattern parallel to the San Jacinto fault with depths from 3 to 15 km. This pattern may represent the zone of energy release or slip for that earthquake and possibly the plane of the San Jacinto fault at depth, although the epicenters are located about 2 to 3 km to the northeast of the trace of the San Jacinto fault. Most of the earthquakes located in this study are not aftershocks in the usual sense, i.e., easily correlated with a preceding large earthquake. They represent a complex pattern of seismicity which has continued at least for the last 3 years on the micro-earthquake level and for the last 30 years on the macroseismic level.

Earthquake swarm in the Santa Barbara Channel, California, 1968

Abstract: Sixty-three minor earthquakes (maximum magnitude = 5.2 ) occurred in the Santa Barbara Channel during the period June 26 to August 3 1968. The epicenters form a shot-scatter pattern upon a broad, high-standing fault block in the channel midway between Santa Cruz Island and the City of Santa Barbara. Focal mechanism studies indicate that oblique-slip movement occurred along a northwest-striking fault even though the major folds and faults strike nearly east-west. Preliminary studies of the areal hydrocarbon production data show no compelling evidence for a causal relationship with the swarm.