Showing papers on "Fault (geology) published in 1971"

Alpine Deformation and the Oceanic Areas of the Tethys, Mediterranean, and Atlantic

Abstract: A preliminary least-squares fit of the 1,000-m contour of the continental fragments around the Mediterranean is presented It extends the Atlantic reassembly of Bullard and others to the Balearic Islands, Corsica, Sardinia, Italy, Yugoslavia, Greece, and Turkey Geological and geophysical data suggest that the reconstruction applies to Permo-Triassic time The large gap between southern Eurasia and the northern and eastern parts of the reassembled fragments shows the probable shape of the Tethyan ocean of this period During Permo-Triassic time the Mediterranean is thought not to have existed, and therefore it should not be regarded as a remnant of the Tethys As in several other syntheses, the formation of the Mediterranean, together with that of the Alpine orogenic belts around it, are attributed principally to the relative movements between Africa and Eurasia Unlike these syntheses, the nature of this motion has been estimated much more precisely by constructing a simple sea-floor spreading model of the central and northern Atlantic Ocean The model permits the relative movement hypothesis to be tested more rigorously than previously Available spreading data show that at least three phases of relative movement must have taken place Relative to Eurasia, the opening model shows the earliest phase consists of an eastward movement of Africa in Lower Jurassic to early Upper Cretaceous time, followed by a westward movement in later Upper Cretaceous to late Eocene time, finally terminating with latest Eocene to present-day northward movement Anomalous Early Permian and Cretaceous paleomagnetic data, the contrasting age differences between ophiolite-chert sequences in the eastern and western Mediterranean region, and the positions of the Alpine orogenic belts are all shown to be consistent with general features of the reassembly and the opening model The reassembly and the model also permit a self-consistent plate tectonic interpretation to be made of Lower Jurassic geology, which suggests the existence of a single, irregular plate margin between Africa and Eurasia, whose rotation pole was close to the best-fit pole of Africa and North America on the Atlantic reassembly It is believed to link such diverse features as central Atlantic opening, sea-floor spreading in Yugoslavia and Albania, transcurrent faulting in the eastern Alps, and compressional features in the Crimea and the Great Caucasus Although Lower Jurassic geology is consistent with a single irregular plate margin, it is shown that, for the reassembly and opening model adopted, all the larger fragments (Spain, Corsica-Sardinia, Italy, and Yugoslavia-Greece-Turkey) must at times have belonged to a plate or plates independent of Africa and Eurasia Because the Atlantic spreading pattern has been grossly simplified, and also because verifiable reassemblies of later times cannot yet be made, plate tectonic interpretations of such periods are premature, although partial interpretations are still possible A possible interpretation is given of a phase of movement of the south Atlas fault and of the relationship of the ophiolite-chert sequences in the Oman Mountains and the Zagros Range of Iran to Atlantic sea-floor spreading A number of fundamental problems emerge, which include the necessity to postulate different mechanisms for eliminating oceans at compressive plate margins, and the conclusion that present interpretations of structures within orogenic belts provide poor estimates of the relative displacement vectors that have produced them

Regional and Global Fault Slip Rates from Seismicity

Abstract: Rates of fault slip calculated from the seismicity of the world's chief seismic regions agree well with the predictions of “rigid plate tectonics”. Global fault activity seems to have decreased fivefold in the past seventy years, indicating that earthquakes are coupled on a global scale within decades. This article also discusses the possible recurrence of a period of high activity.

Geological aspects of the May 31, 1970, Perú earthquake

Abstract: Geological effects of the destructive May 31, 1970, Peru earthquake (M s = 7.7) extended over roughly 65,000 km 2 of west-central Peru. Earthquake-triggered slope failures of all types that occurred throughout the mountainous parts of the region extensively damaged transportation routes and irrigation canals and temporarily dammed some rivers and lakes. The geologically most important and spectacular of these, a cataclysmic debris avalanche from between 5,500 and 6,400 m altitude on the north peak of Huascaran Mountain, buried the city of Yungay and part of Ranrahirca (elevation about 2,500 m) with a loss of more than about 18,000 lives. The debris avalanche involved 50-100 million m 3 of rock, ice, snow, and soil that traveled 14.5 km from the source to Yungay at an average velocity of between 280 and 335 km/hr. Exceptionally rapid movement of the avalanche is indicated by eyewitness accounts, by topographic irregularities as high as 140 m that were overridden, and locally by boulders weighing several tons that were hurled as much as 1,000 m beyond its margins. A pulse of muddy water from the debris avalanche that swept down the Rio Santa 160 km to the sea inundated farms and small settlements, buried highway and railroad routes, and destroyed the diversion dam and access bridge to a major hydroelectric plant. The extensive destruction to communities and an additional estimated 20,000 casualties resulted primarily from failure of adobe and masonry structures which had little shear resistance to lateral forces imposed by seismic shaking. The degree of damage to buildings and to transportation routes was aggravated in some areas of saturated unconsolidated deposits by differential compaction, downslope slumping or sliding, lateral spreading of liquefied sediments toward free faces, and possibly amplification of seismic vibrations. The absence of surface tectonic displacements or of a significant tsunami and the spatial distribution of the main shock and aftershocks suggest that the earthquake originated by movement on a fault, or faults, beneath the continental shelf at depths between 45 and 65 km.

2. Faulting of the Great Kanto Earthquake of 1923 as Revealed by Seismological Data

Abstract: The fault parameters of the Great Kanto earthquake of September 1, 1923, are determined on the basis of the first-motion data, aftershock area, and the amplitude of surface waves at teleseismic stations. It is found that the faulting of this earthquake is a reverse right-lateral fault on a plane which dips 34° towards N20°E. The auxiliary plane has a dip of 80° towards S55°E. This means that the foot-wall side moves approximately north-west with respect to the hanging wall side. The strike of the fault plane is almost parallel to that of the Sagami trough, and the slip direction is more or less perpendicular to the trend of the Japan trench. This earthquake is therefore considered to represent a slippage between two crustal blocks bounded by the Sagami trough. A seismic moment of 7.6×1027 dyne-cm is obtained. If the fault dimension is taken to be 130×70 km2, the average slip on the fault plane and the stress drop are estimated to be 2.1m and 18 bars respectively. This slip is about 1/3 of that estimated from geodetic data. This discrepancy may indicate an existence of a pre-seismic deformation which did not contribute to the seismic wave radiation, but the evidence from other observations is not very firm.

The Excitation of the Chandler Wobble by Earthquakes

Abstract: Summary Elastic dislocation theory has been extended to deal with point tangential displacement dislocations in a spherically symmetric, self-gravitating Earth model with a fluid core and with arbitrary radial variations in density and in the elastic velocities. This theory may be used to compute the changes in the products of inertia of a realistic Earth model caused by the displacement on a fault of arbitrary location and orientation. It is found that, in general, the changes are several times greater than similar computations for a spherical, homogeneous Earth model would seem to suggest. The results are used to examine the hypothesis that earthquakes are responsible for the excitation of the Chandler wobble. An empirical earthquake moment-magnitude relationship is used together with the theory to estimate the total excitation of the Chandler wobble by all observed large earthquakes since 1904. Unfortunately, the uncertainties in the estimation of the moments of past large earthquakes preclude a really definite conclusion, but it appears likely from the results that the cumulative effect of past earthquakes is sufficient to maintain the observed level of excitation of the Chandler wobble. The theoretical effects on the Earth'spolar path of two large recent earthquakes, the 1960 Chilean earthquake and the 1964 Alaskan earthquake, may be computed using fault parameters determined from reliable field observations, and these are compared with the observed polar motion data.

Quaternary Faulting in the Eastern Alaska Range

Abstract: Quaternary faulting is well displayed along the Denali fault system and the recently recognized and related Totschunda fault system in the eastern Alaska Range. The principal movement on both fault systems is right-lateral strike-slip. Offset glacial features of Wisconsin age indicate minimum Holocene slip rates of 1.1 to 3.5 cm per year along parts of the Denali fault system, and 0.9 to 3.3 cm per year along the Totschunda fault system. Strike-slip movement along the Denali fault system may be no older than early Pliocene and, southeast of the Totschunda fault system junction, may have terminated by the middle Pleistocene. The strike-slip Totschunda fault system, a much younger feature probably no older than middle Pleistocene, exhibits 9 to 10 km of right-lateral offset and 1,500 m of relative vertical movement. The Totschunda fault system is aligned with, and has the same sense of slip as, the Fairweather fault in the Gulf of Alaska. The Denali fault system and the Queen Charlotte Islands fault are part of a major transform fault system separating the North American and Pacific plates. Continental southern Alaska between the Aleutian arc and the Denali fault system is now largely coupled to the Pacific plate. The Totschunda-Fairweather alignment probably represents the beginning of a new transform fault by-passing the southeast part of the Denali fault system.

Geological and Geophysical Studies of the Atacama Fault Zone in Northern Chile

Abstract: The Atacama fault, a suggested regional strike-slip fracture parallel to the coastline of northern Chile, is a major structural break that has undergone a complicated history of movement since Mesozoic time. Abundant Quaternary scarps attest to continuing activity; however, the Atacama fault does not appear to be an active strike-slip fault -- at least not along its central segment between 24°5. and 26°S. In the map area, the Coast Ranges are underlain predominantly by crystalline rocks that have been intensely broken by high-angle faults of the Atacama fault system. A eugeosynclinal section, chiefly comprising Jurassic-Cretaceous andesites, overlies Paleozoic metamorphic and plutonic rocks and has been intimately intruded by batholithic rocks of Late Jurassic to Tertiary (?) age. An older more voluminous suite of rocks ranging from gabbro to adamellite, with abundant diorite and pyroxene granodiorite, is discordantly cut by stocks of more homogeneous hornblende-and-biotite granodiorite, adamellite, and tonalite. In the Taltal area, the Atacama fault zone was obliquely cut and left-laterally off set 10 km by the northwest-trending Taltal fault during mid- to late Tertiary time. Lateral motion on the Taltal fault appears to have ceased before the Pliocene epoch. A structural "knot" presently constrains lateral motion along truncated branches of the Atacama fault. A northwest-curving branch that escapes truncation by the Taltal fault has been more recently active both onshore and offshore, with recent motion predominantly vertical. Strike-slip displacement occurred during the early history of the Atacama fault. Subsequent to its segmentation, individual sectors have been reactivated independently and have accommodated vertical block motions in their recent history. No convincing evidence was found for any recent lateral displacement. Aspects of the origin of alluvial scarps displaying an anomalous ridge-trench-ridge morphology remain enigmatic, but the features are not necessarily indicative of strike-slip movement as had earlier been considered. Offshore studies near the map area indicate major faulting on the upper continental slope, beginning within 10 to 20 km of the coastline, that has resulted in the tilting and down-dropping of blocks toward the trench axis. Late Cenozoic tectonics in the Coast Ranges appear to be directly related to the development of the Peru-Chile trench. A reconnaissance micro-earthquake survey along the Atacama fault shows virtually no activity, or at most very little, in the immediate vicinity of the fault. Abundant micro-after shock activity from an offshore after shock zone related to the magnitude 7.5 Taltal earthquake (Dec. 28, 1966) was detected. Gravity and magnetic profiles across the Atacama fault zone reveal no distinctive anomalies that would allow correlation of major branches over large distances. No large-scale gravity anomalies occur across the fault zone. Schemes of consistent lateral offset along the Atacama fault and its branches have not emerged from systematic regional mapping. Large vertical separation along the faults is a chief control of the regional distribution of rocks. No need is seen to hypothesize strike­-slip displacements of hundreds of kilometers along the Atacama fault zone, but suggestive lines of evidence call for a component of lateral displacement measured at least in tens of kilometers. The vertical component of total displacement probably amounts to several kilometers of uplift of the east side. Taking into account map relations which appear to constrain the amount of allowable lateral displacement, it is judged that total strike-slip displacement may be moderate, amounting only to a few tens of kilometers of right-lateral slip. Photographic materials on pp. 11, 13, 15, 72, 73, 78, 79, 129, 144, 162, 177 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

A Theory of Creep Waves propagating along a transform fault

Abstract: The movements of crustal blocks in the new global tectonics can be described in terms of edge dislocations flowing along a transform fault. The dislocations are created at the oceanic ridges and absorbed at the oceanic trenches, but along the transform fault they are conserved. The total creep strain rate across the fault zone at any point on the transform fault will be proportional to q, the flow of dislocations past that point. A simple physical model of fault creep suggests that q = q(k, x, k′), where k is the concentration of dislocations per unit length of fault, x is the position coordinate measured along the fault length, and k′ is the gradient of k. Thus, the flow q is governed by a kinematic wave equation. The propagation of kinematic waves is characterized by amplitude dispersion that leads to the formation of propagating shock waves, i.e. abrupt changes in the creep rate. The model suggests that the arrival of these shock waves, which presumably is connected with high earthquake hazard, might be forecast in much the same way as flood crests on river systems are now predicted. The very large earthquakes along the transform-fault system that bounds the west coast of North America exhibit a migration pattern suggesting kinematic shock propagation. A similar pattern has been observed along the Anatolian fault.

The Ramapo Fault System in New York and Adjacent Northern New Jersey: A Case of Tectonic Heredity

Abstract: The Ramapo fault system forms the northwestern boundary of the Newark Triassic basin in New York and adjacent northern New Jersey, and is commonly attributed to Mesozoic crustal fracturing. However, the detailed geology in the vicinity of this fault indicates an earlier, complex tectonic ancestry, perhaps dating from late Precambrian time. Intense cataclastic effects found in Precambrian to Middle Ordovician rocks on both sides of the Ramapo fault are absent from the bordering Triassic rocks of the footwall block. Significant post-Middle Ordovician right-lateral-transcurrent faulting is recorded by actual offset of distinctive units and by the megascopic fabric in mylonite zones. The total cumulative offset since the late Precambrian on these faults is unknown but could be large. Right-lateral displacement of 4 km is indicated for the Canopus Valley area. Intrusive relationships of probable late Precambrian to Late Ordovician diorite plutons and dikes, as well as pegmatites of probable Devonian age, help date the strike-slip movements as Paleozoic and older. Block faulting along the Ramapo fault in the Lower Ordovician affected Middle Ordovician sedimentation and resulted in unconformable relationships between a Middle Ordovician gneiss-boulder conglomerate (Bucher, 1957) and Precambrian gneisses on the upthrown block west of the fault north of Peekskill, New York. Repeated movements of the Highlands block during Late Triassic time produced the coarse clastic fanglomerates of the Hammer Creek Formation adjacent to the Ramapo fault. This syndepositional faulting was followed by post-Brunswick downdropping along the Ramapo fault south of the Hudson River. Recent seismic activity along this fault system reported by Page and others (1968) suggests that the fault may still be active. These observations suggest a possible tectonic longevity of some 700 m.y. for the “Triassic” border fault system here. This fracture system had deep crustal connections along which basic magma rose toward the surface in Late Precambrian, Late Ordovician (Cortlandt), and Late Triassic time (Palisades sill and Triassic flows). The Ramapo fault also marks the boundary between tectonic blocks having different Paleozoic deformational histories. These observations, along with the remarkable longevity, suggest that this fault zone may be part of a fundamental crustal fracture system that was operative during formation of the Appalachian orogen. The ideas presented here require a degree of tectonic permanence not commonly reported in orogenic belts. Activity along this fracture system in pre-Middle Ordovician time as documented here suggests strongly that the northern end of the Reading Prong is not allochthonous in the sense proposed by Isachsen (1964).

Some Geophysical Profiles in the Eastern Mediterranean

Abstract: Continuous seismic profiles in the Mediterranean Sea east of 22° E. show that deformation of the sedimentary strata increases toward the north and the west. The crest and northern flank of the East Mediterranean Ridge is characterized by fine-textured topographic relief associated with intensely fractured sub-bottom layers, whereas the southern flank is marked by coarser relief and strongly folded sub-bottom layers. These observations, the lack of extensive abyssal plains, and the presence of tilted sedimentary basins in the eastern Mediterranean, suggest that sedimentation has not kept pace with tectonic activity except perhaps in the Nile Cone. The magnetic field over the area is remarkably smooth except over Eratosthenes Sea-mount and near the eastern and western ends of Cyprus. This smoothness could be the result of deep burial of the magnetic basement or the absence of susceptibility contrasts in the basement rock. Evidence in the form of crustal deformation has been found for a postulated transcurrent fault which passes between Cyprus and Eratosthenes Seamount into Antalya Bay (southern coast of Turkey). This fault is thought to have formed when the African plate underthrust the European shield and hinterland and when the East Mediterranean Ridge was created.

Source Parameters of Earthquakes from Spectra of Rayleigh Waves

Abstract: Summary A generalized form of the directivity function has been used to determine the length and the rupture velocity of four earthquakes with known vertical strike slip faulting. For body wave magnitudes between 5.7 and 7.0, the lengths vary from 18 to 35 km with a rupture velocity of 1.5 kms−1. Seismic moments have been determined from the spectral amplitude densities and the theoretical response to a point source model. From these values, stress drops of 2 to 30 bars and average dislocations of 4 to 160cm have been derived. The products of the average stresses acting at the fault by the seismic efficiency factor are of the order of 107 dyne cm−2 for all four earthquakes; this seems to indicate a minimum strength of the crust under horizontal shear stress of the same order.

Potassium-argon age of some New Zealand lamprophyre dikes near the Alpine Fault

Abstract: Lamprophyre dikes in north-west Otago and north Westland yield whole rock potassium-argon ages mainly lying between 136 and 107 m.y. These ages correspond roughly to the ages inferred by others for the wall rock, suggesting that the ages give the latest major cooling event and that the dikes were possibly intruded earlier. In the dikes, the mica and amphibole ages are younger than their host whole rock age, while the plagioclase ages are older. The mineral ages are best explained as due to loss of argon from micas and amphiboles, and absorption by plagioclase and possibly other tectosilicates, the whole rock remaining approximately a closed system. The redistribution of argon is thought to have occurred at slightly lower temperatures than that causing outgassing of the whole rock. The whole rock dates support an earlier suggestion that the dikes were intruded as a single belt that has since been separated into two parts by a 120 km displacement along the Alpine Fault. The Alpine Fault displacemen...

The Ceres, South Africa, earthquake of September 29, 1969: I. Report on some aftershocks

Abstract: Aftershocks following the Ceres earthquake of September 29, 1969, (Magnitude 6.3) were monitored using a number of portable seismic recording stations. Earthquakes of this magnitude are rare in South Africa. The event occurred in a relatively densely-populated part of the Republic, and resulted in nine deaths and considerable damage. Accurate locations of some 125 aftershocks delineate a linear, almost vertical fault plane. The volume of the aftershock region is 3 × 9 × 20 km 3 with the depth of the aftershocks varying from surface to 9 km. Aftershocks following the September event had almost ceased when another large earthquake (Magnitude 5.7) occurred on April 14, 1970. Following this event, the frequency and magnitude of aftershocks increased, and they were located on a limited portion of the same fault system delineated by the September 29th aftershocks. Previously-mapped faults do not correlate simply with the fault zone indicated by the aftershock sequence.

Sea‐floor spreading in the southwest Indian Ocean

Abstract: Results are presented from a detailed bathymetric and magnetic survey of a selected area of the southwest Indian Ocean ridge near 44°S, 38°E. The survey area is traversed by two fault zones, believed to be active transform faults. Outside these fault zones a 1000–2000 gamma peak to peak central magnetic anomaly trends roughly east-west. Some findings (deep troughs, fault scarps, and apparent offset of magnetic anomalies) are typical of the intersection of a fracture zone with a section of oceanic ridge. Two unusual features of this particular intersection are the width of the zone of fault affected magnetics and a topographic high that is subparallel to the transform faults. The surveyed branch of ridge is currently spreading in a N 20°E direction at about 1.0 cm per year. This agrees closely with an earlier estimate by Morgan and Johnson.

Earthquake distribution and mechanism of faulting in the Rainbow Mountain‐Dixie Valley‐Fairview Peak Area, central Nevada

Abstract: The distribution of microearthquakes in west-central Nevada correlates well with fault-plane solutions for this area and defines a zigzag series of crustal fractures that vary in length from a few to several tens of kilometers. The main Fairview fault strikes northwest, and motion on this fault is right lateral oblique slip. In other parts of the active zone northeast-striking faults have mainly dip-slip motion. Focal mechanisms are generally consistent with an interpretation of simple block faulting, faults of different orientation having the same slip direction. For the Rainbow Mountain and Fairview Peak areas, crustal blocks to the east of the fracture zone move down and southeast with respect to blocks on the west side. Faulting in this region appears to be related to regional extension, acting in the direction N60°W–S60°E.

Source Process of Deep and Intermediate Earthquakes as Inferred from Long-Period P and S Waveforms

Abstract: The dynamical processes at the source of eleven deep-focus and intermediate-depth earthquakes that occurred around Japan have been investigated from the analysis of long-period P and S waveforms. The recorded P waveforms have been equalized at some distance around the focal region to get the source function, eliminating the combined effects of wave propagation in the earth and of the seismograph characteristics. It is found that the source process times derived from the source function, as well as from the recorded first half-periods, indicate some azimuthal dependence with respect to the orientation of one nodal plane and of the null vector, in most of the earthquakes. This dependence is interpreted as a result of shear faulting over a finite fault area, and used to determine the slip plane, slip direction, fault length and width. The seismic moment and the average dislocation over the plane are evaluated from P wave amplitudes together with the estimated fault dimension. A close agreement in general features between the recorded and synthesized waveforms including the absolute amplitudes of both P and S waves supports the above shear dislocation model. The overall distribution of the orientations of the slip planes and slip vectors of these earthquakes does not seem to be definitely related to the local dip or strike of seismic zones in this region. The calculated stress drops during these earthquakes, together with some available data, appear to show a gradual increase with focal depths down to 400 km. A tentative interpretation of this increase is that the stress drop might be related, at least qualitatively, to partial loss of the intrinsic (cohesive) shear strength of the material under increasing hydrostatic pressures, if these earthquakes are caused by brittle fractures in the lithosphere.

The earthquake sequence near Danville, California, 1970

Abstract: Several thousand small earthquakes, with magnitudes ranging up to 4, occurred near Danville, California, during May, June, and July 1970. Seven temporary seismographs were installed near the epicentral region to augment an existing telemetered network within 1 day after the first felt earthquake. The dense concentration of 10 seismographs within 12 km of the seismic activity and the existence of a reversed seismic-refraction profile through the region permitted a very detailed study of this earthquake sequence. Over 400 events with magnitude greater than 0.5 were located with an average accuracy of ±1 km. The main earthquake region was approximately spherical: 1 km in radius, 6 km deep, and centered about 5 km southeast of Danville, 40 km east of San Francisco. Fault-plane solutions indicate that faulting associated with the earthquakes was of the right-lateral strike-slip type, with a strike of N 35°W. The earthquakes cannot readily be related to any previously identified geological structure or fault. They suggest that active right-lateral faulting is not restricted to the mapped fault trace of the Calaveras fault. This result implies that not all of the sources of future earthquakes in the San Francisco Bay region can be indentified by surface mapping alone. The use of small earthquakes to map active faulting at depth may help to identify areas of potential earthquake hazards.

Finite‐element analysis of shallow temperature anomalies*

Abstract: Finite-element analysis has been used in calculating shallow temperature anomalies due to both the presence of salt domes and near-surface faults. The results of the numerical analysis are in good agreement with field observations reported earlier by Poley and van Steveninck. The calculations show how surface emissivity must be responsible for the pronounced temperature anomalies above salt domes. Faulting is a mechanism that disorders the layering sequence of sedimentary deposits. If the disordered sedimentary layers show notable differences in heat conductivity, the layers are thick enough and the displacements along the fault plane large enough, temperature anomalies close to the free surface may be apparent because of the surface-emissivity mechanism. In order to explain certain characteristic temperature anomalies where a fault reaches or approaches the free surface, an additional mechanism has to be assumed; namely a distortion of texture, and consequently a change in the heat conductivity of the rock material surrounding the fault plane. The consequences of both a reduced and an increased heat conductivity of the surroundings of a near-surface fault have been calculated, and they show remarkable agreement with temperature anomalies observed in various parts of the world.

Superimposed Deformations in the Missi Meta-sedimentary Rocks near Flin Flon, Manitoba

Abstract: Within the Flin Flon Basin, Precambrian meta-sandstones and conglomerates belonging to the Missi Group have been complexly deformed as a result of three periods of deformation. The first two periods (P1 and P2) involved folding but no apparent faulting, the last (P3) involved both folding and faulting. Progressive metamorphism (M2), within the greenschist facies, occurred during the second phase (P2) and aided in the formation of a pronounced axial-plane foliation (S2). Small-scale folding and retrograde metamorphism (M3) occurred along faults formed during the last phase (P3).The faults in this area all appear to be high angle, oblique-slip reverse faults and can be interpreted as having formed during a single orogenic event (P3) although there is sequence in their initiation. The net-slip directions of different faults are approximately the same, and plunge moderately to the southeast, with either the east or south side upthrown, depending on the attitude of the fault.

Recognition on space photographs of structural elements of Baja California

Abstract: Gemini and Apollo photographs provide illustrations of known structural features of the peninsula and some structures not recognized previously. An apparent transform relationship between strike-slip and normal faulting is illustrated by the overlapping vertical photographs of northern Baja California. The active Agua Blanca right-lateral strike-slip fault trends east-southeastward to end at the north end of the Valle San Felipe and Valle Chico. The uplands of the high Sierra San Pedro Martir are a low-relief surface deformed by young faults, monoclines, and warps, which mostly produce west-facing steps and slopes; the topography is basically structural. The Sierra Cucapas of northeasternmost Baja California and the Colorado River delta of northwesternmost Sonora are broken by northwest-trending strike-slip faults. A strike-slip fault is inferred to trend northward obliquely from near Cabo San Lucas to La Paz, thence offshore until it comes ashore again as the Bahia Concepcion strike-slip fault.

A Tertiary thrust fault on the southern margin of the Bovey Basin

Abstract: Geological investigations and borehole drilling on the southern margin of the Bovey basin around Mainbow ball clay mine and Ringslade open pit have shown that Devonian slate up to 30 m thick is superimposed on Tertiary sediments. A drag fold beneath the slate indicates that the slate has moved eastwards over the Tertiary material. As the slate is relatively intact and not disaggregated, hillcreep cannot be responsible for the superimposition. A large landslip involving a rotational shear cannot be responsible, as the movement plane dips into the hill over too large an area. The southern margin of the Bovey basin is therefore interpreted as a thrust fault of Tertiary age, with thrust plane dipping 10° to 30° S. It is suggested that this is a rejuvenation of the Variscan thrust recognized in the Bickington area and further west in the Holne area and that the movement on the Tertiary thrust is related to the dextral wrench movement on the Sticklepath fault.

Active displacement on the Calaveras fault zone at Hollister, California

Abstract: The Calaveras fault zone, which is a major branch of the San Andreas fault system in northern California, passes through the City of Hollister 160 km (100 miles) southeast of San Francisco. Active fault displacement (fault creep slippage) has occurred in and near Hollister along a fault trace within the Calaveras fault zone. Various man-made structures crossing the fault trace have been deformed and gradually offset in a right-lateral sense. The amount of offset varies directly with age of the structure. The maximum offset is 33 cm (13 in) of a sidewalk constructed in the period 1909 to 1914. Offsets on dated structures indicate displacement rates of approximately 2 mm/yr (0.08 in/yr) from 1909 to 1925 and 6 mm/yr (0.24 in/yr) from 1925 to 1967. Data obtained from periodic measurement of specially designed survey lines and instruments have indicated a displacement rate of 9 mm/yr (0.4 in/yr) since May 1967. Displacements of the survey lines are not associated with local earthquake events. Rates of active fault displacement vary with time and position along the Calaveras and San Andreas fault zones in the Hollister area. The pattern of this variation suggests that active displacement on the San Andreas fault zone may be transferring northeastward to the Calaveras fault zone.

Faults in South-East Asia, and the Andaman Rhombochasm

Abstract: SOUTH-EAST Asia is a region where, since Mesozoic times, three crustal plate systems have met in a complicated pattern which includes large-scale transform faults where adjacent plates have moved laterally relative to each other. One such fault belt is that which crosses the Thai-Malay Peninsula, of which the major dislocation has been named the Khlong Marui Fault by Garson and Mitchell1. On regional grounds they infer that the sinistral wrench movement took place in the late Jurassic to early Cretaceous.

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

Abstract: 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.

Detection of Active Faulting Using Oblique Infrared Aerial Photography in the Imperial Valley, California

Abstract: Low-angle infrared aerial photography, in conjunction with conventional aerial photographs and ground truth, has proved effective for locating active faults in the Imperial Valley. Slight differences in soil moisture, which may depend upon faulting, affect the vigor of plants within a field, and plant vigor can be recorded on infrared Ektachrome film. The effectiveness of the method depends on the presence of vegetation, and upon crop type and plant maturity. A rectilinear pattern of fields aids in detecting faults which do not parallel section lines. The method should be useful in other vegetation-covered areas, including alluvial plains and prairie areas blanketed with glacial till. Oblique infrared photo coverage was obtained at altitudes of 2,000 to 5,000 ft for much of the east and west sides of the Imperial Valley. Two previously unmapped zones of active faulting were located along the eastern side of the valley, and also a probable fault along the southwest side of the Superstition Hills.