Showing papers on "Fault (geology) published in 2000"

Tectono-sedimentary evolution of active extensional basins

Abstract: We present conceptual models for the tectono-sedimentary evolution of rift basins. Basin architecture depends upon a complex interaction between the three-dimensional evolution of basin linkage through fault propagation, the evolution of drainage and drainage catchments and the effects of changes in climate and sea/lake level. In particular, the processes of fault propagation, growth, linkage and death are major tectonic controls on basin architecture. Current theoretical and experimental models of fault linkage and the direction of fault growth can be tested using observational evidence from the earliest stages of rift development. Basin linkage by burial or breaching of crossover basement ridges is the dominant process whereby hydrologically closed rifts evolve into open ones. Nontectonic effects arising from climate, sea or lake level change are responsible for major changes in basin-scale sedimentation patterns. Major gaps in our understanding of rift basins remain because of current inadequacies in sediment, fault and landscape dating.

Neotectonics of the Sumatran fault, Indonesia

Abstract: The 1900-km-long, trench-parallel Sumatran fault accommodates a significant amount of the right-lateral component of oblique convergence between the Eurasian and Indian/Australian plates from 10°N to 7°S. Our detailed map of the fault, compiled from topographic maps and stereographic aerial photographs, shows that unlike many other great strike-slip faults, the Sumatran fault is highly segmented. Cross-strike width of step overs between the 19 major subaerial segments is commonly many kilometers. The influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry. Geomorphic offsets along the fault range as high as ~20 km and may represent the total offset across the fault. If this is so, other structures must have accommodated much of the dextral component of oblique convergence during the past few million years. Our analysis of stretching of the forearc region, near the southern tip of Sumatra, constrains the combined dextral slip on the Sumatran and Mentawai faults to be no more than 100 km in the past few million years. The shape and location of the Sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere. Nonetheless, active volcanic centers of the Sumatran volcanic arc have not influenced noticeably the geometry of the active Sumatran fault. On the basis of its geologic history and pattern of deformation, we divide the Sumatran plate margin into northern, central and southern domains. We support previous proposals that the geometry and character of the subducting Investigator fracture zone are affecting the shape and evolution of the Sumatran fault system within the central domain. The southern domain is the most regular. The Sumatran fault there comprises six right-stepping segments. This pattern indicates that the overall trend of the fault deviates 4° clockwise from the slip vector between the two blocks it separates. The regularity of this section and its association with the portion of the subduction zone that generated the giant (M_w 9) earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike-slip faulting and unusually large subduction earthquakes.

Geodynamics of the northern Andes: Subductions and intracontinental deformation (Colombia)

Abstract: New regional seismological data acquired in Colombia during 1993 to 1996 and tectonic field data from the Eastern Cordillera (EC) permit a reexamination of the complex geodynamics of northwestern South America. The effect of the accretion of the Baudo-Panama oceanic arc, which began 12 Myr ago, is highlighted in connection with mountain building in the EC. The Istmina and Ibague faults in the south and the Santa Marta-Bucaramanga fault to the northeast limit an E-SE moving continental wedge. Progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the Cordilleras (northward of 5°N) and transpressive deformation in the Santander Massif. Crustal seismicity in Colombia is accurately correlated with active faults showing neotectonic morphological evidences. Intermediate seismicity allows to identify a N-NE trending subduction segment beneath the EC, which plunges toward the E-SE. This subduction is interpreted as a remnant of the paleo-Caribbean plateau (PCP) as suggested by geological and tomographic profiles. The PCP shows a low-angle subduction northward of 5.2°N and is limited southward by a major E-W transpressive shear zone. Normal oceanic subduction of the Nazca plate (NP) ends abruptly at the southern limit of the Baudo Range. Northward, the NP subducts beneath the Choco block, overlapping the southern part of the PCP. Cenozoic shortening in the EC estimated from a balanced section is ∼120 km. Stress analysis of fault slip data in the EC (northward of 4°N), indicates an ∼E-SE orientation of σ1 in agreement with the PCP subduction direction. Northward, near Bucaramanga, two stress solutions were observed: (1) a late Andean N80°E compression and (2) an early Andean NW-SE compression.

Active Tectonics in the Central Apennines (Italy) – Input Data for Seismic Hazard Assessment

Abstract: Quaternary tectonics and paleoseismologicalinvestigations have defined a reliable framework ofactive faults in the southern Umbria and AbruzziApennines. Two sets of NW–SE to NNW–SSE trending, 16to 33 km-long, normal and normal-oblique faults orfault systems have caused the displacement of LatePleistocene–Holocene deposits and landforms within theinvestigated sector. Available data on verticaloffsets indicate that both Late Pleistocene–Holoceneand Quaternary (since the later part of the EarlyPleistocene; 0.9–1 Ma) slip rates range between 0.4and 1.2 mm/yr (range 0.6–0.8 mm/yr preferred).Paleoseismological investigations show that recurrenceintervals for surface faulting events are alwaysgreater than 1,000 years and are usually greater than2,000 years. Both paleoseismological data andlong-term seismicity show that activation of theinvestigated faults may result in earthquakes ofM = 6.5–7.0. The extension rate across the two sets ofprimary faults ranges between 0.7 and 1.6 mm/yr.Horizontal seismic strain has been calculated to be0.5–0.6 mm/yr, based on the summation of the seismicmoment of M > 5.3 earthquakes which have affected theinvestigated area since 1200 AD. This value may belower than that inferred through geological data,probably because the seismological record reliable forthe addition of the seismic moments covers a too shorttime window (about 800 years) to be consideredrepresentative of the tectonic activity in theinvestigated area. This conclusion iscorroborated by the large recurrence intervalper fault (>1,000–2,000 years) inferred frompaleoseismological analysis. A comparison of theactive-fault framework and historical-seismicitydistribution indicates that the entire eastern set ofactive faults has likely not been activated since 1000AD, thus indicating that the elapsed time since thelast activation for several faults of the investigatedarea may be greater than 1,000 years. In terms ofhazard, the highest probability of activation isrelated to the eastern set faults, due to theobservation that the elapsed time for some of thesefaults may be similar to the recurrence interval. Asan example, paleoseismological andarchaeoseismological data indicate that the elapsedtime for the Mt. Vettore and Mt. Morrone Faults may begreater than 1,650 and 1,850 years, respectively.These data may have significant implications for riskrelated to a number of towns in central Italy and tothe city of Rome. As for the latter, in fact,monumental heritage has suffered significant damagedue to earthquakes of M > 6.5 which originated in theinvestigated Apennine sector.

Exhumation of the ultrahigh‐pressure continental crust in east central China: Cretaceous and Cenozoic unroofing and the Tan‐Lu fault

Abstract: The orogenic architecture of the world's largest ultrahigh-pressure exposure, the Hong'an-Dabie Mountains of the Triassic Qinling-Dabie orogenic belt, is dominated by Cretaceous and Cenozoic structures that contributed to its exhumation from ≤30 km depth. Cretaceous magmatic crustal recycling (≥50% for the entire Dabie) and heating (>250° to >700°C) were most prominent in Dabie, and exhumation, magmatism, and cooling were all controlled by Cretaceous transtension. Exhumation was accomplished principally by an asymmetric Cordilleran-type extensional complex in the northern Dabie (Northern Orthogneiss unit) between 140 and 120 Ma, at rates as fast as 2 mm/yr and average horizontal stretching rates of up to 6 mm/yr. Cretaceous reactivation occurred within a regional transtensional strain field as a result of far-field collisions and Pacific subduction. The onset of crustal extension was preceded and possibly facilitated by a reheating of the Hong'an-Dabie crust (∼140 Ma) coeval with the onset of voluminous magmatism in eastern China (∼145 Ma), which resulted from a change in Pacific subduction from highly oblique to orthogonal. The Tan-Lu continental-scale fault was a normal fault zone in the mid-Cretaceous (∼110-90 Ma) and underwent ≥5.4 km dip slip and ≥4 km throw in the Cenozoic. During the India-Asia collision the Qinling-Dabie belt acted as the structural discontinuity between the strike-slip-dominated escape tectonics south of the Qilian-Qinling-Dabie belt and the rifting-dominated tectonism north of it. The most prominent Cretaceous and Cenozoic structures of the Hong'an-Dabie, the Xiaotian-Mozitang and the Jinzhai fault zones, respectively, reactivated major lithospheric structures of the Triassic orogen, i.e., the Huwan detachment zone and the suture.

Triggering of earthquake aftershocks by dynamic stresses.

Abstract: It is thought that small 'static' stress changes due to permanent fault displacement can alter the likelihood of, or trigger, earthquakes on nearby faults. Many studies of triggering in the near-field, particularly of aftershocks, rely on these static changes as the triggering agent and consider them only in terms of equivalent changes in the applied load on the fault. Here we report a comparison of the aftershock pattern of the moment magnitude Mw = 7.3 Landers earthquake, not only with static stress changes but also with transient, oscillatory stress changes transmitted as seismic waves (that is, 'dynamic' stresses). Dynamic stresses do not permanently change the applied load and thus can trigger earthquakes only by altering the mechanical state or properties of the fault zone. These dynamically weakened faults may fail after the seismic waves have passed by, and might even cause earthquakes that would not otherwise have occurred. We find similar asymmetries in the aftershock and dynamic stress patterns, the latter being due to rupture propagation, whereas the static stress changes lack this asymmetry. Previous studies have shown that dynamic stresses can promote failure at remote distances, but here we show that they can also do so nearby.

Tectonic reconstructions of New Zealand: 40 Ma to the Present

Abstract: Reconstructions of the New Zealand subcontinent from 40 Ma to the Present are presented. Assumptions that have constrained the model include: semi‐straight initial alignment of basement terranes and markers; Australian plate fixed; onset of Emerald Basin spreading at c. 45 Ma; and Pacific plate subduction north of New Zealand from c. 30 Ma. Five independent, rigid, crustal blocks are employed, including: Northland‐Taranaki‐western South Island (combined), East Coast (North Island), east Nelson (Marlborough Sounds), eastern South Island, and Fiordland. At 40 Ma the Pacific/Australian rotation pole was located close to or within the Wanganui region. A proto‐plate boundary zone was propagating through western New Zealand, as an incipient link between Emerald Basin spreading in the south and subduction in the northwest. Lateral offset on the Alpine Fault was initiated by c. 23–22 Ma, mainly as an effect of changing subduction kinematics and increasing relative motion of the Australian and Pacific pla...

Source Scaling Properties from Finite-Fault-Rupture Models

Abstract: Finite-source images of earthquake rupture show that fault slip is spatially variable at all resolvable scales. In this study we develop scaling laws that account for this variability by measuring effective fault dimensions derived from the autocorrelation of the slip function for 31 published slip models of 18 earthquakes, 8 strike-slip events, and 10 dip-slip (reverse, normal, or oblique) events. We find that dip-slip events show self-similar scaling, but that scale invariance appears to break down for large strike-slip events for which slip increases with increasing fault length despite the saturation of rupture width. Combining our data with measurements from other studies, we find evidence for a nonlinear relationship between average displacement and fault length, in which displacement increases with fault length at a decreasing rate for large strike-slip events. This observation is inconsistent with pure width or length scaling for simple constant stress-drop models, but suggests that the finite seismogenic width of the fault zone exerts a strong influence on the displacement for very large strike-slip earthquakes.

Experimental study of caldera formation

Abstract: Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (≤1), subsidence starts by flexure of the roof and the formation of outward dipping, reverse ring faults, which in turn trigger formation of peripheral inward dipping, normal ring faults. The subsidence always occurs asymmetrically. In cross section the reverse faults delimit a coherent piston, bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets. The surface depression consists of a nondeformed area (piston) surrounded by an annular extensional zone (tilted strata). For high aspect ratios (>1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2-D) or cones (3-D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.

Active Normal Faulting Beneath a Salt Layer: An Experimental Study of Deformation Patterns in the Cover Sequence

Abstract: Scaled experimental models show that the presence of a viscous layer, such as salt, facilitates the development of extensional forced folds above active normal faults. The geometries of the extensional forced folds and their associated secondary fault patterns depend on the thickness and viscosity of the viscous layer, the thickness of the cover sequence, the strength and ductility of the cover sequence, and the magnitude and rate of displacement on the underlying master normal fault. Increasing the thickness of the viscous layer and the cohesive strength and ductility of the overburden enhances the decoupling between the deep and shallow deformation. Alternatively, increasing the viscosity of the viscous layer, the thickness of the overburden, and the magnitude and rate of displacement on the master normal fault reduces the decoupling between the deep and shallow deformation. Enhanced decoupling facilitates the formation of broad extensional forced folds and the development of detached secondary faults both near and far from the master normal faults. The model-predicted deformation patterns closely resemble those observed in the Gulf of Suez, the Haltenbanken area of offshore Norway, and the Jeanne d'Arc Basin of the Grand Banks, offshore southeastern Canada.

Seismic hazard in the Marmara Sea region following the 17 August 1999 Izmit earthquake

Abstract: On 17 August 1999, a destructive magnitude 7.4 earthquake occurred 100 km east of Istanbul, near the city of Izmit, on the North Anatolian fault. This 1,600-km-long plate boundary(1,2) slips at an average rate of 2-3 cm yr(-1) (refs 3-5), and historically has been the site of many devastating earthquakes(6,7). This century alone it has ruptured over 900 km of its length(6). Models of earthquake-induced stress change(8) combined with active fault maps(9) had been used to forecast that the epicentral area of the 1999 Izmit event was indeed a likely location for the occurrence of a large earthquake(9,10). Here we show that the 1999 event itself significantly modifies the stress distribution resulting from previous fault interactions(9,10). Our new stress models take into account all events in the region with magnitudes greater than 6 having occurred since 1700 (ref. 7) as well as secular interseismic stress change, constrained by GPS data(11). These models provide a consistent picture of the long term spatio-temporal behaviour of the North Anatolian fault and indicate that two events of magnitude equal to, or greater than, the Izmit earthquake are likely to occur within the next decades beneath the Marmara Sea, south of Istanbul.

Implications of fault array evolution for synrift depocentre development: insights from a numerical fault growth model

Abstract: We investigate the effects of interaction between growing normal faults on the creation of accommodation in extensional half grabens. Fault evolution is simulated using a numerical model in which we calculate both the stress field around each fault and the changes in stress level on neighbouring faults caused by individual slip events (earthquakes). These stress changes govern the interaction and determine both the direction and rate of lateral fault propagation and the accumulation of displacement. The spatial distribution of subsidence resulting from fault growth is examined parallel and transverse to strike to document the three-dimensional evolution of accommodation creation. The numerical experiment permits analysis of the geometry, inception and growth history of individual fault-controlled depocentres during development of a linked normal fault array. Model results indicate that the pattern of fault-controlled subsidence, and hence hangingwall accommodation creation, is spatially and temporally variable as the timing and rates of fault movement vary considerably. In general, there is a progression from many relatively small and isolated subbasins initially, to the development of a larger laterally continuous half graben in the hangingwall to a major through-going linked fault system. Many smaller faults initially active in footwall and hangingwall areas become inactive as the extension localizes onto the major structure. This switching-off of some faults, combined with focusing of the extensional deformation, leads to an increase in the rate of displacement accumulation on the remaining active fault. As dip-slip displacement is a proxy for hangingwall subsidence, we interpret this prominent rate change in terms of the ‘rift-initiation’ to ‘rift-climax’ transition, previously recognized in synrift stratigraphy. A general picture of depocentre development relative to timing of linkage emerges from the simulated fault evolution that provides some simple conceptual models to be tested. However, the important result of this paper is that it shows the degree to which fault activity can vary in space and time both along the same fault zone and also across strike. We briefly discuss the implications of model results for stratigraphic architecture in rift basins. Our conclusion is that some of the stratigraphic complexity of rifts previously ascribed to other controls (e.g. sediment supply, eustasy, etc.) may be tectonically controlled and that with improved three-dimensional imaging of rift basins these effects may be recognized.

Evidence for a strong San Andreas fault

Abstract: Stress measurements in deep boreholes have universally shown that stresses in the Earth’s crust are in equilibrium with favorably oriented faults with friction coefficients in the range 0.6‐0.7 and with nearly hydrostatic pore-pressure gradients. Because of the lack of any fault-adjacent heat-flow anomaly as predicted by a conductive model of frictional heating, the San Andreas fault has long been thought to be an exception, i.e., far weaker than this standard case. Borehole stress measurements near the San Andreas fault have failed to confirm this weak-fault hypothesis, being either inconclusive or in conflict with it. Directions of maximum horizontal stresses reported to be nearly fault normal in central California are now known not to be regional stresses but a result of active folding within folds that have been rotated 20°‐30° clockwise from their original orientations. Everywhere in southern California it is observed that the maximum stress directions rotate to smaller angles (30°‐60°) with the San Andreas, within 20 km of it. The sense of this rotation is opposite to that expected from the weak-fault hypothesis and indicates that the shear stress on the San Andreas is comparable in magnitude to all other horizontal stresses in the system. In the “big bend” section of the fault, this rotation is predicted from a transpressional plate-boundary model in which the San Andreas is loaded by a deep shear zone with a locking depth of 10 km. If the adjacent minor thrust faults are assumed to obey Byerlee friction, the crustal-average shear stress on the San Andreas in that region must be in the range 100‐160 MPa, regardless of the pore pressure in the fault. These stresses are many times greater than permitted by the weak-fault hypothesis. In the more transcurrent regions farther south, the San Andreas shear stress will be smaller than this estimate, but similar stress rotations observed there indicate that the San Andreas cannot be weak relative to minor faults in that region. These stress rotations can only be consistent with the weakfault hypothesis if it were assumed that all faults in California were equally weak, which is known to be untrue. The conclusion is that the heat-flow model is flawed, probably in its assumption that all heat transfer is governed by conduction.

Factors controlling normal fault offset in an ideal brittle layer

Abstract: We study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two-dimensional elastic-plastic (brittle) layer floating on an inviscid fluid. The yield stress of the layer is the sum of the layer cohesion and its frictional stress. Faults are initiated by a small plastic flaw in the layer. We get finite fault offset when we make fault cohesion decrease with strain. Even in this highly idealized system we vary six physical parameters: the initial cohesion of the layer, the thickness of the layer, the rate of cohesion reduction with plastic strain, the friction coefficient, the flaw size and the fault width. We obtain two main types of faulting behavior: (1) multiple major faults with small offset and (2) single major fault that can develop very large offset. We show that only two parameters control these different types of faulting patterns: (1) the brittle layer thickness for a given cohesion and (2) the rate of cohesion reduction with strain. For a large brittle layer thickness (> 22 km with 44 MPa of cohesion), extension always leads to multiple faults distributed over the width of the layer. For a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening: a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault. When the offset is very large, the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes.

Fault reactivation and fluid flow along a previously dormant normal fault in the northern North Sea

Abstract: Detailed seismic imaging and in situ stress and pore-pressure measurements are used to analyze reverse-fault reactivation of a long-dormant normal fault in the northern North Sea. Fault reactivation is caused by three factors: (1) a recent increase in the compressional stress in the area associated with postglacial rebound, (2) locally elevated pore pressure due to the presence of natural gas in a hydrocarbon reservoir on the footwall side of the fault, and (3) a fault orientation that is nearly optimally oriented for frictional slip in the present-day stress field. We demonstrate that the combination of these three factors induces fault slippage and gas leakage along sections of the previously sealing reservoir-bounding fault. We argue that similar pore-pressure triggering of fault slip in the crust may occur because of the accumulation of gas columns of, e.g., CO2 and He in the vicinity of tectonic faults.

Late Cenozoic to Holocene deformation in southwestern Sichuan and adjacent Yunnan, China, and its role in formation of the southeastern part of the Tibetan Plateau

Abstract: From at least 2–4 Ma to present, crust in the southeastern part of the Tibetan Plateau west of the convex-east Xianshuihe-Xiaojiang fault system has deformed internally and rotated clockwise around the eastern Himalayan syntaxis. The northwest-striking Ganzi fault zone bounds the rotating crust on the north and has a total left slip of 78–100 km, of which ∼60 km is transferred to the Xianshuihe fault zone across a diffuse transfer zone, and ∼22–40 km is absorbed by bending of older structures and crustal shortening. Crustal shortening is expressed along and east of the eastern end of the Ganzi fault zone by mountains capped by permanent glaciers locally rising nearly 1000 m above the average elevation of the Tibetan Plateau. A similar transfer of left slip into shortening occurs farther south across the Xianshuihe fault in the high mountains around and east of Gongga Shan (7556 m). The northwest-striking, convex-east, left-lateral Litang fault zone lies southwest of the Ganzi-Xianshuihe-Xiaojiang fault zone and appears to be less well developed but otherwise similar to the Ganzi fault zone. The Batang, Chenzhi, and other northeast-striking right-lateral faults of small displacement occur within the rotating crustal fragment. Together with the left-slip faults, they accommodate east-west shortening northeast of the eastern Himalayan syntaxis. South of this region of shortening, the crust is extending to form grabens within the Dali and southern Xiaojiang fault systems and in the Tengchong volcanic province. The progressive change from shortening southward into extension is related to variations in strain that characterize the region from northeast to southeast of the eastern Himalayan syntaxis. The assemblage of structures in southwestern Sichuan geometrically resembles structures of Eocene to Miocene age in southern Yunnan that were positioned northeast of the eastern Himalayan syntaxis, similar to present-day southwestern Sichuan, at the time of their development. The similarity in the structural development in the two areas indicates that crust northeast of the syntaxis underwent a common evolution as the syntaxis migrated northward during the past ∼50 m.y. Structures in Sichuan are less fully developed than older structures in southwestern Yunnan and can serve as a guide to reconstruct the progressive tectonic development in the region of the syntaxis. Deformation in these areas indicates that plateau formation has been complex, inhomogeneous, and diachronous at scales from 1000 km to less than 100 km.

The propagation and linkage of normal faults: Insights from the Strathspey-Brent-Statfjord fault array, northern North Sea

Abstract: Through examination of the scaling relations of faults and the use of seismic stratigraphic techniques, we demonstrate how the temporal and spatial evolution of the fault population in a half-graben basin can be accurately reconstructed. The basin bounded by the ≫62-km-long Strathspey–Brent–Statfjord fault array is located on the western flank of the Late Jurassic northern North Sea rift basin. Along-strike displacement variations, transverse fault-displacement folds and palaeo-fault tips abandoned in the hangingwall all provide evidence that the fault system comprises a hierarchy of linked palaeo-segments. The displacement variations developed while the fault was in a prelinkage, multisegment stage of its growth have not been equilibrated following fault linkage. Using the stratal architecture of synrift sediments, we date the main phase of segment linkage as latest Callovian – middle Oxfordian (10–14 Myr after rift initiation). A dense subpopulation of faults is mapped in the hangingwall to the Strathspey–Brent–Statfjord fault array. The majority of these faults are short, of low displacement and became inactive within 3–4 Myr of the beginning of the extensional event. Subsequently, only the segments of the proto-Strathspey–Brent–Statfjord fault and a conjugate array of antithetic faults located 3.5 km basinward continued to grow to define a graben-like basin geometry. Faults of the antithetic array became inactive ∼11.5 Myr into the rift event, concentrating strain on the linked Strathspey–Brent–Statfjord fault; hence, the basin evolved into a half-graben. As the rift event progressed, strain was localized on a smaller number of active structures with increased rates of displacement. The results of this study suggest that a simple model for the linkage of 2–3 fault segments may not be applicable to a complex multisegment array.

Full interseismic locking of the Nankai and Japan-west Kurile subduction zones: An analysis of uniform elastic strain accumulation in Japan constrained by permanent GPS

Abstract: We analyze permanent Global Positioning System (GPS) data obtained over Japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the Pacific and Philippine Sea plates below the Japanese islands. We first derive the GPS strain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction velocity along the central Nankai trough on one side and the Japan-west Kurile trench on the other. These “reference free” velocities are close to those predicted by plate motion models with respect to Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction motion and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows local variations within the seismogenic zones. To do this, we divide the subduction interface into 35 km×30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to derive the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan-west Kurile seismogenic zones are homogeneously fully locked. Most of the coupling ratios are close to 1 and a few are close to 0; intermediate values are rare. The zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 Sanriku-Oki interplate earthquake (Japan trench) or to a small overestimation of the actual lower limit of the locked zone. We conclude that within the resolution of the GPS data and the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japan-west Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip; and (3) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading. Finally, we discuss the nonelastic residual deformation within the frame of the long-term deformation of the Japanese islands.

Fault-propagation folding in extensional settings: Examples of structural style and synrift sedimentary response from the Suez rift, Sinai, Egypt

Abstract: Field data from the Oligocene–Miocene Gulf of Suez rift demonstrate that coeval growth faults, folds, and transfer zones exerted a major control on synrift stratigraphic sequence development. Growth folds in the Suez rift are related to steeply dipping normal faults that propagated upward, resulting in broad, upward-widening monoclines in overlying strata. Folding during fault propagation was accommodated by layer-parallel slip and detachment along mudstone horizons as well as by normal and rare reverse secondary faults that propagated away from the master fault. The eventual propagation of the master fault through to the surface left the steep limb of the monocline and most of the secondary faults in the hanging wall. This evolving structural style exerted a marked control on the geometry and stacking patterns of coeval synrift sediments. Synrift sediments display onlap and intraformational unconformities toward the growth monoclines and buried faults, whereas they diverge into broadly synclinal expanded sections away from the growth monocline. Continued movement across buried faults resulted in the progressive rotation of the monoclinal limb and associated synrift sediments, each successively younger sequence dipping basinward at a shallower angle than the previous one. The resulting synrift geometries differ significantly from stratal geometries normally anticipated adjacent to normal faults. Along-strike variations in facies stacking patterns are also commonly associated with decreasing displacement across faults and associated folds toward low-relief transfer zones. Data from other rift basins indicate that fault-propagation folds are not unique to the Gulf of Suez.

Slip rate on the Dead Sea transform fault in northern Araba valley (Jordan)

Abstract: The Araba valley lies between the southern tip of the Dead Sea and the Gulf of Aqaba. This depression, blanketed with alluvial and lacustrine deposits, is cut along its entire length by the Dead Sea fault. In many places the fault is well defined by scarps, and evidence for left-lateral strike-slip faulting is abundant. The slip rate on the fault can be constrained from dated geomorphic features displaced by the fault. A large fan at the mouth of Wadi Dahal has been displaced by about 500 m since the bulk of the fanglomerates were deposited 77–140 kyr ago, as dated from cosmogenic isotope analysis (^(10)Be in chert) of pebbles collected on the fan surface and from the age of transgressive lacustrine sediments capping the fan. Holocene alluvial surfaces are also clearly offset. By correlation with similar surfaces along the Dead Sea lake margin, we propose a chronology for their emplacement. Taken together, our observations suggest an average slip rate over the Late Pleistocene of between 2 and 6 mm yr^(−1), with a preferred value of 4 mm yr^(−1). This slip rate is shown to be consistent with other constraints on the kinematics of the Arabian plate, assuming a rotation rate of about 0.396° Myr^(−1) around a pole at 31.1°N, 26.7°E relative to Africa.

Architecture and seismotectonics of a regional low‐angle normal fault zone in central Italy

Abstract: Information from surface geology, subsurface geology (boreholes, seismic reflection, and refraction profiles), and seismicity are used to depict the geometry and the possible seismogenic role of the Altotiberina Fault (AF), a low-angle normal fault in central Italy. The AF extends along the inner Umbria region, for a length of ∼70 km, with an average dip of ∼30° and an horizontal displacement up to 5 km. It emerges west of the inner border of the Tiber basin and deepens beneath the Umbria-Marche carbonate fold-and-thrust belt to a depth of 12–14 km. Close to the AF surface trace, low-angle synthetic east dipping normal faults extensively outcrop, whereas high-angle antithetic west dipping normal faults prevail farther east. Integrating geological and seismologic information, it can be stated that the AF behaves as an active extensional fault zone and represents the basal detachment of the west dipping seismogenic normal faults of the Umbria-Marche region. The AF belongs to a regional NE dipping low-angle normal fault system (Etrurian Fault System (EFS)), which extends for ∼350 km from northwestern Tuscany to southern Umbria. Early preliminary considerations suggest that the EFS may play an important role in controlling active extension and related seismicity in northern central Italy.

Geologic factors controlling patterns of small‐volume basaltic volcanism: Application to a volcanic hazards assessment at Yucca Mountain, Nevada

Abstract: The proposed high-level radioactive waste repository at Yucca Mountain, Nevada, is located within an active volcanic field. Probabilistic volcanic hazard models for future eruptions through the proposed repository depend heavily on our understanding of the spatial controls on volcano distribution at a variety of scales. On regional scales, Pliocene-Quaternary volcano clusters are located east of the Bare Mountain fault. Extension has resulted in large-scale crustal density contrast across the fault, and vents are restricted to low-density areas of the hanging wall. Finite element modeling indicates that this crustal density contrast can result in transient pressure changes of up to 7 MPa at 40 km depth, providing a mechanism to generate partial melts in areas where mantle rocks are already close to their solidus. On subregional scales, vent alignments, including one alignment newly recognized by ground magnetic mapping, parallel the trends of high-dilation tendency faults in the Yucca Mountain region (YMR). Forty percent of vents in the YMR are part of vent alignments that vary in length from 2 to 16 km. Locally, new geological and geophysical data show that individual vents and short vent alignments occur along and adjacent to faults, particularly at fault intersections, and left-stepping en echelon fault segments adjacent to Yucca Mountain. Conditions which formed these structures persist in the YMR today, indicating that volcanism will likely continue in the region and that the proposed repository site is within an area where future volcanism may occur. On the basis of these data the probability of volcanic disruptions of the proposed repository is estimated between 10−8/yr and 10−7/yr.

Magnitude of post–Middle Jurassic (Bajocian) displacement on the central Altyn Tagh fault system, northwest China

Abstract: The Altyn Tagh fault system, a key structural feature in the tectonic collage of central Asia, is an active left-lateral strike-slip fault system. However, its age of initiation, kinematic history, and total magnitude of displacement are not well known. Middle Jurassic nonmarine sedimentary rocks that crop out along the central segment of the Altyn Tagh fault were deposited in a contiguous Tarim-Qaidam foreland-style basin that was geodynamically linked to Mesozoic contraction in the Tian Shan, Bei Shan, Qilian Shan, and Kunlun Shan. This study documents the best evidence of a piercing point to date, consisting of an offset lacustrine shoreline demarcated by Aalenian-Bajocian open lacustrine strata to the west and fluvial and alluvial strata of comparable age to the east. Detrital compositions, palynology, paleocurrents, and lithostratigraphy support correlation of these sections. Restoration of this Jurassic facies boundary indicates 400 ± 60 km of post-Bajocian left-lateral separation on the Altyn Tagh fault. Consistent with this result, restoration of the Jurassic shoreline aligns felsic plutons across the western segment of the Altyn Tagh fault system that may have been left-laterally offset by 360 km. These two pairs of offset features suggest that the current best estimate for net magnitude of post-Bajocian left-lateral separation on the Altyn Tagh fault is about 360 km.

Localization of bedding plane slip and backthrust faults above blind thrust faults: Keys to wrinkle ridge structure

Abstract: A mechanically based model for wrinkle ridge development is developed that combines wrinkle ridge morphologies, regional topographic offsets suggestive of subsurface thrust faults, and folding of near-surface layers. This model provides explicit relationships between observed morphologic elements characteristic of wrinkle ridges and plausible mechanisms in the subsurface, a key component that is absent in previous qualitative fault-based scenarios for these structures. As developed in this paper, wrinkle ridges are the surface expression of anticlines that grow above a blind thrust fault as a result of both flexural slip folding of near-surface strata and the nucleation and growth of echelon arrays of backthrust faults. Calculations of displacements (both horizontal shortening and vertical uplift) and Coulomb stress change related to slip along blind thrust faults in the model demonstrate physically important spatial inhomogeneities in these quantities, with revealing and useful implications. (1) The ratio of shortening due to folding at the surface to the shortening due to faulting at depth is characteristically small for coupled wrinkle ridge-blind thrust fault systems and decreases with increasing fault depth; the depth of the blind thrust fault's upper tip thus profoundly influences the surface strains. (2) Folding and uplifted topography, forming the topographic ridge, are produced above the area of the slipping blind thrust fault plane. Horizontal and vertical deformation at the surface extend over several ridge widths, or a total of at least 50 km for a 10-km-wide ridge, implying that topographic profiles and geologic studies must extend sufficiently far from the wrinkle ridge to fully characterize the surface deformation. (3) Calculations of Coulomb stress changes suggest that fault slip can localize both bedding plane slip in overlying strata and new backthrust faults that propagate upward to become wrinkles on the trailing side of the ridge. Initiation of bedding plane slip in association with slip along the blind thrust fault likely determines whether the resulting surface structure becomes a wrinkle ridge or a lobate scarp. Wrinkle ridge spacing may also be related to stress changes associated with slip along the underlying blind thrust fault.