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Showing papers on "Fault (geology) published in 1987"


Journal ArticleDOI
20 Nov 1987-Science
TL;DR: F Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates.
Abstract: Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at approximately 2-kilometer depth on planes parallel to the San Andreas fault.

914 citations


Journal ArticleDOI
TL;DR: In this article, the authors discuss early Mesozoic sinistral transcurrent faulting in the different regions of East Asia and discuss the characteristics of the Tancheng-Lujiang wrench fault system.

468 citations


Journal ArticleDOI
TL;DR: In this article, the authors estimate the fractal dimension of the San Andreas fault system between the northern Gabilan Range and the Salton Sea, including the postulated extent of the great 1857 Fort Tejon earthquake, from measured fault lengths.
Abstract: It has been noted that the spatial distribution of earthquakes and the mode of strain release in the San Andreas fault system is related to the complexity of fault geometry. Because of their rough appearance over many length scales, faults can be regarded as fractal surfaces. Direct estimates of fractal dimension D of portions of the San Andreas fault system between the northern Gabilan Range and the Salton Sea, including the postulated extent of the great 1857 Fort Tejon earthquake, are obtained from measured fault lengths, analogous to the lengths of coastlines as discussed by Mandelbrot. Regions characterized by complicated fault geometry are associated with larger values of D. Based on fault traces mapped at a scale of 1:750,000, D is 1.3 for this reach of the fault defined as a 30-km-wide band about a main fault trace. For that part near Parkfield which could be associated with the nucleation of the 1857 earthquake, D is 1.1; at this same scale, D is 1.4 for the San Andreas and related faults near San Bernardino where the 1857 rupture stopped, compared to 1.2 for the San Andreas-San Juan fault segments near the point of arrest of the 1966 Parkfield earthquake. At finer map scales (1:24,000 and 1:62,500) critical lengths of ∼ 500 m and 1 km are identified which might relate to the extent of off-San Andreas fault offsets. The critical lengths also suggest that fault geometry is not self-similar. If this fractal geometry persists through the seismic cycle, it may be possible to use a quantitative measure of complexity to explain the occurrence of great and characteristic earthquakes along a given reach of fault.

420 citations


Journal ArticleDOI
TL;DR: In this paper, a finite kinematic model of plate motions in the Red Sea area was constructed based on a re-evaluation of the plate boundaries in the Afro-Arabian rift system.

369 citations


Journal ArticleDOI
TL;DR: Fault displacements measured in coal mines and from seismic data are used to develop a model describing the near-field displacements associated with an ideal, single normal fault as mentioned in this paper.
Abstract: Fault displacements measured in coal mines and from seismic data are used to develop a model describing the near-field displacements associated with an ideal, single normal fault. Displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip-line. The tip-line is elliptical, with the shorter axis of the ellipse parallel to the displacement direction. Contours of equal displacement form concentric ellipses centered on the point of maximum displacement. Displacement gradients vary with fault size and with mechanical properties of host rock; fault radius to maximum displacement ratios range from 5 to 500. Plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data, both for quality control of interpretations and for quantitative extrapolation of limited data. Displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface; this decrease is seen as reverse drag in both hanging wall and footwall. Hanging-wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults; even where fault-block rotation can be demonstrated, neither listric fault geometry nor a flat detachment surface is geometrically necessary. Because faulting is accommodated by ductile deformation, rigid fault-bounded blocks cannot exist except in some special circumstances related to a free surface. The displacements within the rock volume affected by a single fault are not simply related to regional extension. Apparent horizontal extension by faulting varies from one layer to another, and a significant proportion of the extension in a basin may be due to ductile deformation.

364 citations


Journal ArticleDOI
TL;DR: The Parkfield segment of the San Andreas fault is transitional in character between the creeping segment and the locked Carrizo Plain segment to the southeast as mentioned in this paper, and the average rates of line length change and shallow fault slip were inverted to determine the slip rate at depth on the Parkfield fault segment.
Abstract: The Parkfield, California, segment of the San Andreas fault is transitional in character between the creeping segment of the fault to the northwest and the locked Carrizo Plain segment to the southeast. The rate of shallow fault slip decreases from 25–30 mm/yr northwest of the epicenter of the 1966 Parkfield earthquake to zero at the southeastern end of the 1966 rupture zone. Data from a network of trilateration lines spanning the San Andreas fault near Parkfield and extending to the Pacific coast near San Luis Obispo shed light on the rate of fault slip at depth since the 1966 earthquake. In this study, average rates of line length change and shallow fault slip were inverted to determine the slip rate at depth on the Parkfield fault segment. The fault is taken to be a vertical surface with unknown distribution of strike-slip displacement in an elastic half-space. A striking result of the inversions is that all solutions providing acceptable fits to the data exhibit a locked zone essentially coincident with the rupture surface of the 1966 Parkfield earthquake. The data require that the locked zone extend nearly as far north as the 1966 epicenter; however, the vertical extent of the locked zone is not well resolved. Over much of the Parkfield segment the fault is slipping faster at the earth's surface than it is at seismogenic depths. In order to fit the trilateration measurements it is necessary to include a component of contraction normal to the trend of the San Andreas. The inversion results suggest a spatially uniform normal strain of −0.06 μstrain/yr. The orientation of the contraction is compatible with geologic and seismic evidence of active folding and reverse faulting in the region. The magnitude of the contraction is consistent with convergence rates inferred from global plate motion models.

320 citations


Journal ArticleDOI
TL;DR: In this paper, a succession of main paleostress fields has been defined: (1) N-;S compression of late Eocene age, which induced the formation of the German-Czech triangle, bounded on the east by the NW-SE dextral strike-slip faults of Pfahl and Franconia and on the west by the NNE-SSW sinistral strike slip faults along the axis of the future west European Rift.
Abstract: An analysis of Cenozoic brittle deformation in the European platform has been carried out between the Bohemian Massif to the east and the Western Mediterranean Sea to the west. A succession of four main paleostress fields has been defined: (1) N-;S compression of late Eocene age. This event induced the formation of the “German-Czech triangle,” bounded on the east by the NW-SE dextral strike-slip faults of Pfahl and Franconia and on the west by the NNE-SSW sinistral strike-slip faults along the axis of the future west European Rift, (2) E-W Oligocene extension that opened the west European Rift. (3) NE-SW compression, early Miocene in age, that reactivated the Pfahl fault line (as a reverse fault) and the main faults of the Rhinegraben (as dextral strike-slip faults). (4) Finally, since the end of the Miocene, a fan-shaped distribution of directions of compression has developed at the periphery of the Alpine arc. However, farther from the Alpine chain, a more consistent direction of compression has dominated (first NW-SE, then NNW-SSE). A comparison with plate tectonic data demonstrates that this succession of tectonic events is compatible with the reconstruction of relative movements between Africa and Eurasia during the Cenozoic collision. However, some local stress patterns, close to the Alps, are clearly related to the local evolution of the Alpine arc.

298 citations


Journal ArticleDOI
TL;DR: In this paper, the authors applied fractal theory to the analysis of the San Andreas fault geometry, which directly measured the increase in total fault length with a decrease in ruler size.
Abstract: Fractal theory is applied in a quantitative analysis of the San Andreas fault (SAF) geometry. The method, which directly measures the increase in total fault length with a decrease in ruler size, gives the fractal dimension D and scaling properties for the chosen length band 0.5–1000 km. A physical interpretation of D is that it measures the irregularity of the fault trace in the selected band. The fault is subdivided into six segments of distinctive seismic behavior. A “main” fault trace which shows either maximum coseismic slip or creeping was selected for analysis, with three alternative branches examined for the SAF system south of San Bernardino. Branches of the fault trace are not considered in this work. Fractal dimensions calculated for the different segments range from 1.0008 to 1.0191, and are different from 1.0 to the 95% confidence interval. These small changes reflect the overall smoothness of the main fault trace. Significant variations in D among segments indicate heterogeneities in the fault smoothness along strike. D also changes significantly from the short-length band to the long-length band where the demarcation point ranges from 1 to 2 km. The short-length band has larger D values. A slight correlation is obtained between the fractal dimension of the main trace and the extent of subparallel faulting. This indicates some correspondence between the main fault trace irregularity and the complexity of subsidiary fault traces in plan view.

291 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of frictional heating on the thermal, hydrologic, and mechanical response of a small patch of the failure surface were investigated and a fault model was proposed to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant.
Abstract: The mechanical response of a fault zone during an earthquake may be controlled by the diffusion of excess heat and fluid pressures generated by frictional heating. In this study we formulate a fault model which incorporates the effects of frictional heating on the thermal, hydrologic, and mechanical response of a small patch of the failure surface. This model is used to examine the parameters that control the fault response and to determine their critical range of values where thermal pressurization is significant. The problem has two time scales: a characteristic slip duration and a characteristic time for thermal pressurization. The slip duration is set by the fault geometry. The characteristic time for thermal pressurization is set by the slip rate, the friction coefficient, and the thermal and hydraulic characteristics of the medium. The response of the fault depends on the relative magnitude of these two times. Results suggest that the fault width and hydraulic characteristics of the fault zone and adjacent medium are the primary parameters controlling the mechanical response. For earthquakes occurring across zones of low porous medium compressibility (< 10−9 Pa−1) and permeability (< 10−18 m2) the characteristic time for thermal pressurization is small. In this case, frictional heating can cause fluid pressures to approach lithostatic values, the shear strength to approach zero, and the temperature rise to stabilize at a maximum value dependent on the pore dilatational and transport properties of the porous medium. Whether the patch acts as a barrier to slip or exhibits substantial strain weakening is dependent on the shear strain across the fault. Moderate slip events where shear strains exceed two cause substantial strain weakening and, consequently, large stress drops, accelerations, and displacements. Thus it is possible for the patch to act as a barrier for small earthquakes but not for large ones. Both the dynamic stress drop and total displacement decrease for zones with larger compressibility, permeability, or width. If the compressibility or permeability exceeds 10−8 Pa−1 or 10−14 m2 or the shear strain is less than one, then the effects of frictional heating may be negligible and the fault will exhibit no strain-weakening characteristics. Consequently, the patch acts as barrier that halts or resists further fault motion. Extrapolation of these results suggests that spatial variations in fault width and hydraulic characteristics will cause a heterogeneous stress drop and fault slip over the failure surface, explaining many of the features of active faulting (e.g., barriers, nonuniform slip, rupture stoppage, random ground accelerations, strong motions, and frequency-magnitude relations).

251 citations


Journal ArticleDOI
TL;DR: In this paper, a new method to calculate the stress tensor associated with slip along a population of faults is derived, which incorporates two constraints: First, the stresses in the slip direction satisfy the Coulomb yield criterion, and second, the slip occurs in the direction of maximum shear stress along the fault.
Abstract: A new method to calculate the stress tensor associated with slip along a population of faults is derived here. The method incorporates two constraints: First, the stresses in the slip direction satisfy the Coulomb yield criterion, and second, the slip occurs in the direction of maximum shear stress along the fault. The computations provide the complete stress tensor (normalized by the vertical stress), and evaluation of the mean coefficient of friction and the mean cohesion of the faults during the time of faulting. The present method is applied to three field cases: Dixie Valley, Nevada, Wadi Neqarot, Israel, and Yuli microearthquakes, Taiwan. It is shown that the coefficieet of friction of the three cases varies between μ < 0.1 to μ = 0.8. Further, it is demonstrated that previous stress inversion methods implicitly assume zero friction along the faults.

235 citations


Journal ArticleDOI
01 Oct 1987-Geology
TL;DR: In this article, the orientations of intrusive rocks from a carbonatitic lamprophyre dike swarm and the history of emplacement relative to country-rock schist structures are compatible with intrusion into tension fractures and Riedel shears formed during initiation of the dextral wrench system of the Alpine fault.
Abstract: The orientations of intrusive rocks from a carbonatitic lamprophyre dike swarm and the history of emplacement relative to country-rock schist structures are compatible with intrusion into tension fractures and Riedel shears formed during initiation of the dextral wrench system of the Alpine fault. New U-Pb and Rb-Sr dates indicate a late Oligocene-early Miocene time of intrusion which, in turn, suggests a mid-Tertiary history for propagation of the Alpine fault plate boundary through South Island, New Zealand.

Journal ArticleDOI
TL;DR: The nonuniformity of the occurrence of large slip events producing surface ruptures on seismogenic faults and variations in slip rate probably characterize seismogenic faulting in the Great Basin province, Western United States as discussed by the authors.
Abstract: Nonuniformity of the occurrence of large slip events producing surface ruptures on seismogenic faults and variations in slip rate probably characterize seismogenic faulting in the Great Basin province, Western United States. Examples include: the grouping of faulting events along the Lost River fault, Idaho; changes in tilt rates of the East Range and Cortez Mountains, Nevada; extension of slip along a fault on the northwest flank of the Humboldt Range, Nevada; and migration or shifting of slip back and forth from one fault to another along subparallel range-front faults, in Dixie Valley, Nevada.

Journal ArticleDOI
01 Apr 1987-Geology
TL;DR: The geometries of extensional fault systems have been studied by use of experimental sand analogues and are compared with examples in the literature as discussed by the authors, and experiments show that hanging-wall blocks in listric extensional faults must undergo significant internal strains in order to accommodate progressive deformation over nonplanar fault surfaces.
Abstract: The geometries of extensional fault systems have been studied by use of experimental sand analogues and are compared with examples in the literature. Extensional faulting above a uniformly extending basement produces a domino-style fault array involving both planar rotational faults and listric faults. The faults evolve with time and may change from listric geometries (concave upward) through planar fault segments to convex-upward geometries. At high extensional strains early faults are cut by later high-angle planar extensional faults. Hanging-wall deformation above a simple listric extensional detachment is characterized by faults that nucleate and propagate into the hanging wall and produce crestal collapse grabens. Listric extensional faults with a ramp/flat geometry also produce hanging-wall crestal collapse grabens and local reverse faults. The experiments show that hanging-wall blocks in listric extensional fault systems must undergo significant internal strains in order to accommodate progressive deformation over nonplanar fault surfaces.

Journal ArticleDOI
TL;DR: A 150 μm thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 rp.m. under an axial load of 330 kg for 11 s using friction welding apparatus as discussed by the authors.

Journal ArticleDOI
TL;DR: In this paper, the authors interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on January 5, 1946 (MS = 7.3), August 11, 1965 (MS= 7.5), October 27, 1971 (MS>7.1) and December 29, 1973 (MS >7.5).
Abstract: In the central Vanuatu arc, living and recently deceased reef corals act as natural tide gauges which have allowed us to map vertical tectonic deformation patterns. As corals grow, the density of the aragonite coral skeletons varies on an annual cycle, producing annual growth bands similar to tree rings. Using coral growth bands, we can determine the year coral surfaces died due to emergence. We interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on January 5, 1946 (MS = 7.3), August 11, 1965 (MS = 7.5), October 27, 1971 (MS = 7.1) and December 29, 1973 (MS = 7.5). The 1965 and 1973 events caused maximum uplifts of 120 and 60 cm, respectively, in the frontal arc. Also related to these events are uplifts of 10 cm and 6 cm in the back arc on Pentecost and Maewo islands, which lie east of the volcanic chain and the primary forearc zones of uplift and subsidence. Similar secondary zones of uplift occurred with the great 1960 Chile and 1964 Alaska earthquakes. The amplitude of these secondary uplifts is significantly larger than that predicted by models having a single fault in an elastic half-space. However, the amount of secondary uplift is comparable to that predicted if the fault occurs in a plate of constant thickness overlying a viscoelastic half-space. At various places in 1957, 1969–1970, 1977, and 1978–1981 there was about 5–10 cm of emergence not associated with major earthquakes, which may indicate nonseismic tectonic uplift. However, oceanographically lowered sea levels, as in El Ninos, may have determined the times when corals died and recorded these events. Nevertheless, the accumulation of emergence, its persistence, the limited geographic extent of each event, and occurrence in areas of rapid Holocene uplift suggest that the causes of the uplifts are tectonic. These events suggest that in some areas a third or more of the total accumulated uplift in central Vanuatu takes place as aseismic motion. However, in some areas we find only coseismic emergence. In central Vanuatu, contemporary coseismic vertical deformation, Holocene uplift, and topography have remarkably similar patterns. This suggests that the mechanisms and processes causing vertical deformation have varied little over the last 106years. Apparently, the topography, structure, and seismotectonics are controlled by the subduction of the d'Entrecasteaux ridge, a major bathymetric feature underthrusting this part of the arc. The influence of this ridge may have been especially extensive because it migrates very slowly along the arc trend, and thus it interacts for a long time with a single portion of the arc system. Our previous studies of reef terraces indicated the existence of at least four seismotectonic arc segments or blocks along the Santo-Malekula interval of the arc, and our present results further support this conclusion. Each block has uplifted at different times, by different amounts, at different rates, and tilted in a different direction. Boundaries between the north Santo and the south Santo segment and between the north Malekula and the south Malekula segment correlate with the north and south flanks of the d'Entrecasteaux ridge, as does the absence of a physiographic trench west of Santo.

Journal ArticleDOI
TL;DR: In this article, the authors proposed a numerical method to study the kinematics of seismic faults provided by focal mechanisms, which is based on the simple mechanical model used for fault population analysis.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the three-dimensional geometry of fault movement in the destructive earthquake (Ms= 6.9) of 1980 November 23 in Campania-Basilicata (southern Italy).
Abstract: Summary Teleseismic waveforms, local ground acceleration, elevation changes, surface faulting and aftershocks are used to investigate the three-dimensional geometry of fault movement in the destructive earthquake (Ms= 6.9) of 1980 November 23 in Campania–Basilicata (southern Italy). Twelve kilometres of surface faulting has been identified following this earthquake. The re-determined epicentre and focal mechanism, and the focal depth of 10 km, determined by modelling long-period teleseismic body waves, show that the hypocentre was on a downward projection of the surface faulting, and that the seismogenic normal fault was approximately planar, with a dip of 60°, from the hypocentre to the surface. Further analysis of the long-period body-waves indicates that, within 10s of the origin time of the earthquake, motion occurred on three discrete fault-segments extending for 30 km along strike. Fault rupture in the earthquake propagated predominantly towards the NW. An overall moment tensor for the earthquake is obtained from the inversion of long-period GDSN and WWSSN data, and shows that the total scalar moment of 26×1018 Nm is approximately double that accounted for by the fault motion in these first three subevents. We use teleseismic body-waves, locally recorded ground acceleration and aftershocks to investigate the position, timing and orientation of the additional seismic sources responsible for the remaining seismic moment. These data suggest that a fourth subevent occurred about 13 s after the first motion, approximately 20 km SE of the hypocentre of the first subevent. Two later fault ruptures also occurred, beneath the hanging wall of the earlier ruptures, about 20 and 40 s after the first motion. Long-period body waves and elevation changes are consistent with these occurring on normal faults, dipping at about 20°NE, at the base of the upper-crustal seismogenic zone. The total of six subevents that we identify for this earthquake account for almost all of the scalar moment in the overall moment tensor.

Journal ArticleDOI
TL;DR: In the Malaŵi and Tanganyika rifts as mentioned in this paper, uplifted segments of the border fault system flank basins with different acoustic stratigraphies, sediment thicknesses and styles of faulting.

Journal ArticleDOI
TL;DR: In this paper, a Y-shaped zone of surface faults that is divided into a southern, a western, and a northern section is described, with the largest amount of net slip, most complex rupture patterns, and best evidence of sinistral slip.
Abstract: On the morning of 28 October 1983, the Ms 7.3 Borah Peak earthquake struck central Idaho and formed a Y-shaped zone of surface faults that is divided into a southern, a western, and a northern section. The total length of the surface faults is 36.4 ± 3.1 km, and the maximum net throw is 2.5 to 2.7 m. The near-surface net slip direction, determined from the rakes of striations in colluvium, averaged 0.17 m of sinistral slip for 1.00 m of dip slip . The 20.8-km-long southern section is the main zone of surface faulting and coincides with the Thousand Springs segment of the Lost River fault. It has the largest amount of net throw, most complex rupture patterns, and best evidence of sinistral slip. The surface faults include zones of ground breakage as much as 140 m wide, en echelon scarps with synthetic and antithetic displacements, and individual scarps that are nearly 5 m high . The 14.2-km-long western section diverges away from the Lost River fault near the northern end of the southern section. The net throw on this section is generally less than 0.5 m but locally is as much as 1.6 m. The new ruptures are poorly developed across the crest and north flank of the Willow Creek hills; they are mostly downhill-facing, arcuate scars, perhaps incipient landslides, that may overlie a deeper zone of tectonic movement . The northern section, at least 7.9 km long, is on the Warm Spring segment of the Lost River fault and has a maximum net throw of about 1 m. The pattern of surface faulting on this section is simple compared to the other sections. A 4.7-km-long gap in 1983 surface faults separates the northern and southern sections but contains an older scarp of late Pleistocene age . Geologic, seismologic, and geodetic data from the earthquake suggest that barriers confined the primary coseismic rupture to the Thousand Springs segment of the fault. The rupture propagated unilaterally to the northwest from a hypocenter near the southeastern end of the segment. The southeastern boundary of the segment is marked by an abrupt bend in the range front, a 4-km-long gap in late Quaternary scarps, and transverse faults of Eocene age that intersect the Lost River fault . The northwestern boundary of the Thousand Springs segment is at the junction of the Willow Creek hills and the Lost River fault. Here, the southern and western sections of surface faults diverge and there is a gap in the 1983 scarps. During the first few weeks after the main shock, the large-magnitude and large stress-drop aftershocks clustered near this barrier. Later, aftershocks were mainly northwest of the barrier on the Warm Spring and Challis segments, and showed that strain adjustments eventually affected the entire northern part of the Lost River fault. Fault-scarp morphology and the bedrock geology suggest that the boundary between the Thousand Springs and Warm Spring segments has probably ruptured less frequently and had less net slip during much of the late Cenozoic than the interior of the adjacent segments. The 1983 faulting shows that although segment boundaries can stop or deflect primary ruptures, secondary surface faulting can occur on adjacent segments of the main fault. A late Pleistocene scarp in the 1983 gap suggests that infrequent earthquakes, perhaps larger than the 1983 event, might break through a segment boundary and thus release strain on two adjacent segments .

Journal ArticleDOI
TL;DR: In this paper, the effects of shear heating and displacement of cool hanging-wall rocks against hotter footwall rocks are calculated, and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed.

Journal ArticleDOI
TL;DR: In this article, it was shown that northern Panamint valley and Saline Valley, southeast California, form paired pull-apart basins on opposite sides of the right-slip Hunter Mountain Fault Zone.
Abstract: Geological studies support the interpretation that northern Panamint Valley and Saline Valley, southeast California, form paired pull-apart basins on opposite sides of the right-slip Hunter Mountain Fault Zone. Eight to ten km of late Cenozoic net slip can be established on the Hunter Mountain Fault Zone. Palinspastic reconstruction of northern Panamint Valley indicates that the valley was formed by movement on a shallow crustal, low-angle normal fault of 0–15 degree west dip during the last 3.0 Ma. This interpretation appears to contradict the notions that little extension is accomodated in the uppermost crust by low-angle faulting and that the most recent extension in the Basin and Range Province is accomodated exclusively by high-angle faulting. Saline Valley, however, is interpreted to have formed by extension on closely spaced, rotated planar normal faults. Thus, within one geometric system of paired pull-apart basins, extension appears to have been accommodated in the shallow crust in two different ways.

Journal ArticleDOI
01 Nov 1987-Geology
TL;DR: In this article, the authors show that sediment accreted at subduction zones undergo stratal disruption and form a type of melange, and the thickness of the disrupted zones grows with progressive deformation.
Abstract: Sediments accreted at subduction zones undergo stratal disruption and form a type of melange. The thickness of the disrupted zones grows with progressive deformation. This suggests that initial fault surfaces are abandoned and deformation propagates into adjacent undeformed sediment. Factors causing the abandonment of fault surfaces during continuing deformation include (1) strengthening owing to porosity loss during consolidation, (2) localized drops in fluid pressure on fault surfaces that act as dewaterinig conduits, and (3) reorientation of fault surfaces. The disruptive processes occurring in accretionary prisms result principally from the deformation of a consolidating sediment mass.

Journal ArticleDOI
TL;DR: In this paper, a generalized Elsasser model was proposed to describe the time dependence of deep slip and crustal stress build up throughout the earthquake cycle, where loading is represented as imposed uniform dislocation slip on the fault below the locked zone.
Abstract: Periodic crustal deformation associated with repeated strike slip earthquakes is computed for the following model: A depth L (less than or similiar to H) extending downward from the Earth's surface at a transform boundary between uniform elastic lithospheric plates of thickness H is locked between earthquakes. It slips an amount consistent with remote plate velocity V sub pl after each lapse of earthquake cycle time T sub cy. Lower portions of the fault zone at the boundary slip continuously so as to maintain constant resistive shear stress. The plates are coupled at their base to a Maxwellian viscoelastic asthenosphere through which steady deep seated mantle motions, compatible with plate velocity, are transmitted to the surface plates. The coupling is described approximately through a generalized Elsasser model. It is argued that the model gives a more realistic physical description of tectonic loading, including the time dependence of deep slip and crustal stress build up throughout the earthquake cycle, than do simpler kinematic models in which loading is represented as imposed uniform dislocation slip on the fault below the locked zone.

Journal ArticleDOI
TL;DR: In this article, a structural modification of the Hibernia structure occurred in late Barremian time when a major detachment developed within the basin fill, leading to a conspicuous offset pattern and funnel-shaped geometry of the Jeanne d'Arc basin.
Abstract: The tectonic evolution of the Grand Banks involved several episodes of rifting and sequential development of the continental margin. The dominant period of rifting and basin formation in the central Grand Banks was late Callovian to Aptian. Extensional failure of the central Grand Banks crust probably occurred along a shear zone that dipped gently to the west. Listric normal basin-forming faults merge at depth with this detachment system. The listric Murre fault soles at 26 km (16 mi), creating the very deep Jeanne d'Arc basin. Cross-basin transfer faults accommodated different amounts and rates of extension and resulted in the conspicuous offset pattern and funnel-shaped geometry of the Jeanne d'Arc basin. The Hibernia oil field is associated with one of these structural salients. The stratigraphy of the Jeanne d'Arc basin and the Hibernia oil field is a direct response to this tectonic and structural framework. Large-scale, unconformity-bounded sequences correspond to major plate reorganizations and zipper-type opening of the Atlantic Ocean. Smaller scale adjustments to episodic extension are expressed in multiple stacking of depositional sequences. The broad-scale evolution of the synrift succession involved the early deposition of limestones and oil-prone shales. About 10-12 m.y. later, rifting climaxed, resulting in floods of coarse terrestrial material from the rift shoulders. The characteristics of these depositional systems reflect a pervasive structural control affecting the distribution of facies tracts. Dip-slip movement on basin-bounding faults and c oss-basin transfer fault trends were equally important. A profound structural modification of the Hibernia structure occurred in late Barremian time when a major detachment developed within the basin fill. This structural modification and associated antithetic and synthetic faulting in the hanging wall largely controlled reservoir distribution and the maturation, migration, and trapping of hydrocarbons.

Journal ArticleDOI
TL;DR: In this paper, a statistical method for identifying fault or joint planes within what may otherwise appear to be an amorphous earthquake location set is presented, where all the hypocenters in an event set are taken at a time in order to determine the strikes and dips of all possible planes within the event set.
Abstract: Although it is generally accepted that earthquakes occur along preexisting faults, the distribution of earthquake locations is often so smeared that the underlying fault or joint structures along which the earthquakes occur cannot be inferred from visual inspection of location plots. We present a statistical method for identifying fault or joint planes within what may otherwise appear to be an amorphous earthquake location set. The method takes all the hypocenters in an event set three at a time in order to determine the strikes and dips of all possible planes within the event set. A procedure for correcting for the shape of the region in which the earthquakes occur is applied. After correction, the orientation (one or a few) that is seen most often is taken as that of the zone of preexisting fault(s) or joint(s). We applied the method to a set of hypocenters determined for microearthquakes that accompanied a hydraulic injection into crystalline rock. The method was able to resolve successively five statistically significant orientations (planes) along which most of the microearthquakes occurred. The first two planes determined by the method are parallel to one nodal plane from each of the two most commonly found fault plane solutions. One of the two planes intersects the injection well bore at a location where water is known to have entered the rock during the injection. The planes identified thus coincide with the major fluid paths during the hydraulic injection.

Journal ArticleDOI
09 Jan 1987-Science
TL;DR: A set of events with well-constrained depth determinations shows a ring-fault structure that extends from the surface to a depth of about 4 kilometers and slopes steeply outward from the center of the caldera.
Abstract: The locations of a large number of earthquakes recorded at Rabaul caldera in Papua New Guinea from late 1983 to mid-1985 have produced a picture of this active caldera's structural boundary. The earthquake epicenters form an elliptical annulus about 10 kilometers long by 4 kilometers wide, centered in the southern part of the Rabaul volcanic complex. A set of events with well-constrained depth determinations shows a ring-fault structure that extends from the surface to a depth of about 4 kilometers and slopes steeply outward from the center of the caldera. This is the first geophysical data set that clearly outlines the orientation of an active caldera's bounding faults. This orientation, however, conflicts with the configuration of many other calderas and is not in keeping with currently preferred models of caldera formation.

Journal ArticleDOI
TL;DR: In this article, the authors examined P, S, and surface waves derived from seismograms that were collected for the 1929 Grand Banks, Canada, earthquake and found that the total volume of sedimentary slumping was about 5.5 × 10^(11) m^3, which is approximately 5 times larger than a recent estimate of volume from in situ measurements.
Abstract: We have examined P, S, and surface waves derived from seismograms that we collected for the 1929 Grand Banks, Canada, earthquake. This event is noteworthy for the sediment slide and turbidity current that broke the trans-Atlantic cables and for its destructive tsunami. Both the surface-wave magnitude, M_S, and the body-wave magnitude, m_B, calculated from these seismograms are 7.2. Fault mechanisms previously suggested for this event include a NW-SE-striking strike-slip mechanism and an approximately E-W-striking thrust mechanism. In addition, because of the presence of an extensive area of slump and turbidity current, there exists the possibility that sediment slumping could also be a primary causative factor of this event. We tested these fault models and a horizontal single-force (oriented N5°W) model representing a sediment slide against our data. Among these models, only the single-force model is consistent with the P-, S-, and surface-wave data. Our data, however, do not preclude fault models which were not tested. From the spectral data of Love waves at a 50-sec period, we estimated the magnitude of the single force to be about 1.4 × 10^(20) dynes. From this value, we estimated the total volume of sedimentary slumping to be about 5.5 × 10^(11) m^3, which is approximately 5 times larger than a recent estimate of volume from in situ measurements. The difference in estimates of overall volume is likely due to a combination of the inherent difficulty in estimating accurately the displaced sediments from in situ measurements, and of inadequacy of the seismic model; or perhaps because not only the slump but also a tectonic earthquake could have been the cause of this event and contributed significantly to the waveforms studied.

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TL;DR: The Tyrrhenian basin has developed as a marginal basin through drifting of the Calabrian-Sicilian arc system towards the east-southeast, and the main structural features of the basin result from this movement as mentioned in this paper.

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TL;DR: In this article, the authors calculated apparent reflection coefficients for the brightest of these events and obtained values around 0.1, indicating that these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite.
Abstract: Summary. Several bright reflections from structures within the mantle can be seen on BIRPS' deep seismic reflection profiles. We have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0.1. It is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle. These large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite. The brightest reflections seem to be best explained as major faults or shear zones within the mantle.

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TL;DR: The late Mesozoic Coast Range ophiolite and Great Valley sequence in California were juxtaposed against the Franciscan Complex during a long tectonic history that included imbricate thrust faulting, low-angle detachment, and high-angle reverse faulting as mentioned in this paper.
Abstract: The late Mesozoic Coast Range ophiolite and Great Valley sequence in California were juxtaposed against the Franciscan Complex during a long tectonic history that included imbricate thrust faulting, low-angle detachment, and high-angle reverse faulting. Many low-angle faults previously mapped as thrusts invariably juxtapose younger over older rocks, suggesting a normal sense of offset. We infer that serpentinite melange that is present structurally beneath the Coast Range ophiolite formed above the subduction zone during convergence and was subsequently faulted and further attenuated with upper plate rocks concurrent with extension. Franciscan blueschist-facies rock is inferred to have been transported from depth to higher structural levels concurrent with underplating and extensional unroofing in the upper plate. The present juxta-position of the Coast Range ophiolite and Great Valley sequence with Franciscan rocks is commonly controlled by Neogene high-angle faults. We propose that the term Coast Range thrust is no longer appropriate and that the name should be changed to Coast Range fault.