Liquefaction phenomena associated with historical earthquakes in San Juan and Mendoza Provinces,Argentina

Mendoza and San Juan provinces which represent the most seismically active regions of Argentina have been affected by at least nine destructive earthquakes with magnitudes ⩾6.3 in the period 1861–1997. During these events, earthquake-induced liquefaction processes have caused the most severe damages in properties and fields impacting adversely on regional development and economy. Analysing historical liquefaction data we corroborated the relation between liquefaction phenomenon with sediment grain size and depth of phreatic level. We also noted that even the distance from liquefaction features to epicentres increases with earthquake magnitude, previous empirical relations for distance/magnitude are not enough accurate to predict liquefaction feature distance. Moreover, we suggest that when physical conditions of terrain are suitable, liquefaction phenomenon can occur even at greater distances than those established by empirical manner. In addition, regarding potential risk for this seismically region, the most liquefaction vulnerability areas were established taken into account the historical data and the presence of Holocene–Pleistocene deposits with present higher phreatic level. Our findings are essential for future proper territorial planning and they should be useful for minimize economical losses caused by secondary effects in these seismic regions of Argentina.     

Paleoliquefaction study of the Clarendon–Linden fault system, western New York State

The lack of earthquake-induced liquefaction features in Late Wisconsin and Holocene sediments in Genesee, Wyoming, and Allegany Counties suggests that the Clarendon–Linden fault system (CLF) did not generate large, moment magnitude, M≥6 earthquakes during the past 12,000 years. Given that it was the likely source of the 1929 M 4.9 Attica earthquake, however, the Clarenden–Linden fault system probably is capable of producing future M5 events. During this study, we reviewed newspaper accounts of the 1929 Attica earthquake, searched for earthquake-induced liquefaction features in sand and gravel pits and along tens of kilometers of river cutbanks, evaluated numerous soft-sediment deformation structures, compiled geotechnical data and performed liquefaction potential analysis of saturated sandy sediments. We found that the 1929 M 4.9 Attica earthquake probably did not induce liquefaction in its epicentral area and may have been generated by the western branch of the Clarendon–Linden fault system. Most soft-sediment deformation structures found during reconnaissance did not resemble earthquake-induced liquefaction features, and even the few that did could be attributed to non-seismic processes. Our analysis suggests that the magnitude threshold for liquefaction is between M 5.2 and 6, that a large (M≥6) earthquake would liquefy sediments at many sites in the area, and that a moderate earthquake (M 5–5.9) would liquefy sediments at some sites but perhaps not at enough sites to have been found during reconnaissance. We conclude that the Clarendon–Linden fault system could have produced small and moderate earthquakes, but probably not large events, during the Late Wisconsin and Holocene.     

内蒙古包头市区大青山山前断裂地震活动断层初步研究

通过多年来野外考察及断层活动事件年代测定,现已初步查明,包头市区座落在大青山山前断裂西段全新世地震活动断层上。该地震活动断层展布于山前Ⅱ级或Ⅰ级台地前缘,由一系列地震断层、地震陡坎、地震崩积楔、地震充填楔、地裂缝及沙土液化等地震形变遗迹组成,构成规模较大、连续性好的地震地表破裂带。地震断层呈直线型或宽缓弧形斜列延伸,错断全新统及最新地貌面,为一系列断面向南倾斜的正断层及阶状正断层,断层面平整,具垂直擦痕,断层带往往为砂砾、角砾或沙土充填,显示为主要在张应力作用下断层经历5次活动、正倾滑运动而形成。最新一次活动事件为公元849年地震。根据对全新世地震活动断层的活动特征、活动期次的划分及强震复发间隔的初步研究,以及历史和现今地震活动规律的研究,包头市区地震危险性应该引起相当的关注。     

Quaternary tectonic faulting in the Eastern United States

Paleoseismological study of geologic features thought to result from Quaternary tectonic faulting can characterize the frequencies and sizes of large prehistoric and historical earthquakes, thereby improving the accuracy and precision of seismic-hazard assessments. Greater accuracy and precision can reduce the likelihood of both underprotection and unnecessary design and construction costs. Published studies proposed Quaternary tectonic faulting at 31 faults, folds, seismic zones, and fields of earthquake-induced liquefaction phenomena in the Appalachian Mountains and Coastal Plain. Of the 31 features, seven are of known origin. Four of the seven have nontectonic origins and the other three features are liquefaction fields caused by moderate to large historical and Holocene earthquakes in coastal South Carolina, including Charleston; the Central Virginia Seismic Zone; and the Newbury, Massachusetts, area. However, the causal faults of the three liquefaction fields remain unclear. Charleston has the highest hazard because of large Holocene earthquakes in that area, but the hazard is highly uncertain because the earthquakes are uncertainly located.Of the 31 features, the remaining 24 are of uncertain origin. They require additional work before they can be clearly attributed either to Quaternary tectonic faulting or to nontectonic causes. Of these 24, 14 features, most of them faults, have little or no published geologic evidence of Quaternary tectonic faulting that could indicate the likely occurrence of earthquakes larger than those observed historically. Three more features of the 24 were suggested to have had Quaternary tectonic faulting, but paleoseismological and other studies of them found no evidence of large prehistoric earthquakes. The final seven features of uncertain origin require further examination because all seven are in or near urban areas. They are the Moodus Seismic Zone (Hartford, Connecticut), Dobbs Ferry fault zone and Mosholu fault (New York City), Lancaster Seismic Zone and the epicenter of the shallow Cacoosing Valley earthquake (Lancaster and Reading, Pennsylvania), Kingston fault (central New Jersey between New York and Philadelphia), and Everona fault-Mountain Run fault zone (Washington, D.C., and Arlington and Alexandria, Virginia).     

Holocene displacement along branch and secondary faults in the San Andreas fault zone, southern California

Detailed geologic mapping of the San Andreas fault zone in Los Angeles County since 1972 has revealed evidence for diverse histories of displacement on branch and secondary faults near Palmdale. The main trace of the San Andreas fault is well defined by a variety of physiographic features. The geologic record supports the concept of many kilometers of lateral displacement on the main trace and on some secondary faults, especially when dealing with pre-Quaternary rocks. However, the distribution of upper Pleistocene rocks along branch and secondary faults suggests a strong vertical component of displacement and, in many locations, Holocene displacement appears to be primarily vertical. The most recent movement on many secondary and some branch faults has been either high-angle (reverse and normal) or thrust. This is in contrast to the abundant evidence for lateral movement seen along the main San Andreas fault. We suggest that this change in the sense of displacement is more common than has been previously recognized.The branch and secondary faults described here have geomorphic features along them that are as fresh as similar features visible along the most recent trace of the San Andreas fault. From this we infer that surface rupture occurred on these faults in 1857, as it did on the main San Andreas fault. Branch faults commonly form “Riedel” and “thrust” shear configurations adjacent to the main San Andreas fault and affect a zone less than a few hundred meters wide. Holocene and upper Pleistocene deposits have been repeatedly offset along faults that also separate contrasting older rocks. Secondary faults are located up to 1500 m on either side of the San Andreas fault and trend subparallel to it. Moreover, our mapping indicates that some portions of these secondary faults appear to have been “inactive” throughout much of Quaternary time, even though Holocene and upper Pleistocene deposits have been repeatedly offset along other parts of these same faults. For example, near 37th Street E. and Barrel Springs Road, a limited stretch of the Nadeau fault has a very fresh normal scarp, in one place as much as 3 m high, which breaks upper Pleistocene or Holocene deposits. This scarp has two bevelled surfaces, the upper surface sloping significantly less than the lower, suggesting at least two periods of recent movement. Other exposures along this fault show undisturbed Quaternary deposits overlying the fault. The Cemetery and Little Rock faults also exhibit selected reactivation of isolated segments separated by “inactive” stretches.Activity on branch and secondary faults, as outlined above, is presumed to be the result of sympathetic movement on limited segments of older faults in response to major movement on the San Andreas fault. The recognition that Holocene activity is possible on faults where much of the evidence suggests prolonged inactivity emphasizes the need for regional, as well as detailed site studies to evaluate adequately the hazard of any fault trace in a major fault zone. Similar problems may be encountered when geodetic or other studies, Which depend on stable sites, are conducted in the vicinity of major faults.     

Fault location and fault activity assessment by analysis of historic level line data, oil-well data and groundwater data, Hollywood area, California

Transecting the Los Angeles metropolitan area in a general E-W direction are major north-dipping reverse faults comprising the Santa Monica—Raymond Hill fault zone, a segment of the frontal fault system separating the Transverse Ranges from the Peninsular Ranges geomorphic provinces of southern California. Pleistocene or Holocene movement is evident along some segments of these faults, but urban development precludes accurate location and assessment of Quaternary movement by conventional mapping techniques. At present no conclusive evidence of Holocene surface rupture has been found onshore west of the Raymond Hill segment of the fault zone, but the geologic conditions and urban development in the area are such that the possibility of Holocene movement cannot be excluded at this time. Groundwater barriers in Pleistocene sediments are indicative of Quaternary faulting on the Santa Monica fault segment west of the Newport—Inglewood fault zone. Most literature indicates that movement along the Beverly Hills—Hollywood segment east of the Newport—Inglewood fault zone terminated in Late Miocene or Pliocene time, and there is no general agreement on the location of faults in this segment. However, recent work by the Division of Mines and Geology, by Geotechnical Consultants, Inc., and others suggests that the Santa Monica fault transecting the Hollywood area is associated with a zone of differential subsidence that varies from 100 to 400 m wide, depending on the resolution of repeated leveling survey data and with a groundwater barrier determined from analysis of oil-well and water-well data. Additional exploration is essential to test our present geologic model and to evaluate the earthquake hazard and seismic risk of faults in the area.     

Hologene activity and tectonic setting of the Maacama fault zone, Mendocino County, California

The Maacama fault zone is a north-northwest trending, right-lateral zone of structural weakness in eastern Sonoma and Mendocino Counties, California, and fault-related landforms along its trace suggest that it may well have been active during Holocene time. Urbanization and dam construction in the vicinity of the zone have made it especially desirable to determine the extent and history of such geologically recent movements in order to better understand the seismic capability of faults within the zone.

The Maacama fault zone is a segment of one of two north-northwest trending zones of right-lateral faulting that diverge as major branches from the easterly side of the San Andreas fault zone north of Hollister. These branches extend at least as far north as Eureka and may mark the eastern boundaries of slivers of a separate tectonic plate.

This investigation was conducted to evaluate Holocene activity along the part of the Maacama fault zone in Mendocino County. The investigation shows that this zone comprises discontinuous, subparallel fault strands which extend from points south of the Sonoma—Mendocino county line to points north of Laytonville. Some fault segments are expressed by alignments of topographic features clearly caused by Holocene strike-slip movements. Other topographic features are less distinct and may well represent fault segments with pre-Holocene activity.

The Maacama fault zone creeps at a current rate of possibly as high as 2 mm/yr. The total amounts of Holocene offset are unknown.

Evidence obtained from exploratory trenches indicates that at least two surface rupture events have occurred within the past 16,200 years, that at least one has occurred within the past 8,310 years, and that there probably has been no surface rupturing within the past 1,140 years.

I conclude that surface offsets have occurred along major portions of the Maacama fault zone during Holocene time, and that this fault zone should be regarded as capable of producing a moderate to strong future earthquake with accompanying surface rupture in Mendocino County.     

新疆伊犁盆地西北缘红山嘴断裂特征与最新活动

红山嘴断裂位于新疆霍城县,是伊犁盆地北缘前陆褶冲带的前锋断裂。断裂走向NWW,倾向NNW。野外地质调查表明,断裂在地表主要表现为高角度逆冲断层,倾角超过50°。断裂上盘主要由新近系红层组成,下盘为上更新统黄土和全新统。综合野外地层接触关系和区域构造研究表明,红山嘴断裂可能形成于早更新世末,错断地层表明红山嘴断裂在晚第四纪以来发生过强烈的活动,最新活动表现在断错全新统松散砂砾石层。     

Repeated palaeoseismic activity of the Ventas de Zafarraya fault (S Spain) and its relation with the 1884 Andalusian earthquake

One of the most destructive historical earthquakes (M 6.7) in Spain occurred in 1884 along the normal Ventas de Zafarraya Fault located in the Central Betic Cordilleras. Palaeoseismic and radiocarbon data presented in this study are the first to constrain the timing of the pre-1884 fault history in the last 10 ka. These data yield a recurrence interval of between 2 and 3 ka for major earthquakes, under the assumption of uniform return periods along the normal fault. The Holocene slip rate is estimated to be in the order of 0.35±0.05 mm/year, which is significantly higher than the mean slip rate of 0.17±0.03 mm/year since the Tortonian. Several of the most important deformations and secondary features, such as landslides and liquefaction, are related to strong ground motion and document the Holocene activity of the Ventas de Zafarraya Fault.     

The sinkhole enigma in the Alpine Foreland, Southeast Germany: Evidence of impact-induced rock liquefaction processes

Sudden collapse of the Quaternary soil to form sinkholes on the order of meters and tens of meters has been a geologic phenomenon within living memory in a localized area north of Lake Chiemsee in Southeast Germany. Failing a satisfying explanation, a relation with an undefined glaciation process has always been proposed. Excavations and geophysical measurements at three newly affected sites show underground features such as prominent sandy-gravelly intrusions and extrusions typical of rock liquefaction processes well known to occur during strong earthquakes. Since strong earthquakes can reasonably be excluded to have affected the area under discussion, it has been suggested that the observed widespread liquefaction is related with the recently proposed Holocene Chiemgau meteorite impact event. Except for one earlier proposed but unassertive relation between impact and liquefaction, the obviously direct association of both processes in the Chiemgau area emphasizes that observed paleoliquefaction features need not necessarily have originated solely from paleoseismicity but can provide a recognizable regional impact signature.     

Neotectonics of Boconó fault, Western Venezuela

Neotectonic morphologic evidence along the Boconó fault (with a mapped length of 500 km) consists of the typical features found along strike-slip faults; offset alluvium and drainage, shutterridges, closed depressions, sag ponds and marshes, fault scarps and trenches, triangular facets, and zones of mylonite and fault gouge. Evidence on fault planes, such as slickensides, suggests a predominant strike-slip displacement, and morphologic evidence suggests that this offset is right-lateral, with a magnitude of 60–100 m during the Holocene, and of several kilometers during the Quaternary. Calculations based on different empirical relationships suggest maximum expected Richter magnitudes of 7.2–7.9 for earthquakes along the fault (using rupture length estimates) and probable intervals of less than 200 years for events of magnitude 8 (using observed total displacement during the Holocene).     

Liquefaction evidence for strong earthquakes of Holocene and latest Pleistocene ages in the states of Indiana and Illinois, USA

Sand- and gravel-filled clastic dikes of seismic liquefaction origin occur throughout much of southern Indiana and Illinois. Nearly all of these dikes originated from prehistoric earthquakes centered in the study area. In this area at least seven and probably eight strong prehistoric earthquakes have been documented as occurring during the Holocene, and at least one during the latest Pleistocene. The recognition of different earthquakes has been based mainly on timing of liquefaction in combination with the regional pattern of liquefaction effects, but some have been recognized only by geotechnical testing at sites of liquefaction.

Most paleo-earthquakes presently recognized lie in Indiana, but equally as many may have occurred in Illinois. Studies in Illinois have not yet narrowly bracketed the age of clastic dikes at many sites, which sometimes causes uncertainty in defining the causative earthquake, but even in Illinois the largest paleo-earthquakes probably have been identified.

Prehistoric magnitudes were probably as high as about moment magnitude M 7.5. This greatly exceeds the largest historic earthquake of M 5.5 centered in Indiana or Illinois. The strongest paleo-earthquakes struck in the vicinity of the concentration of strongest historic seismicity. Elsewhere, paleo-earthquakes on the order of M 6–7 have occurred even where there has been little or no historic seismicity.

Both geologic and geotechnical methods of analysis have been essential for verification of seismic origin for the dikes and for back-calculating prehistoric magnitudes. Methods developed largely as part of this study should be of great value in unraveling the paleoseismic record elsewhere.     

Paleoliquefaction features on Tenerife (Canary Islands) in Holocene sand deposits

On Tenerife, one of the Canary Islands, a series of clastic dikes and tubular vents is attributed to liquefaction of sediments during a high-intensity paleoearthquake. Geotechnical, geological, tectonic, and mineralogical investigations have been carried out to identify the soil composition and structure, as well as the geological processes operating in the area. Geochronological analysis has indicated an age ranging from 10,081±933 to 3490±473 years BP for the liquefaction features. The area in which these liquefaction features are found has undergone tectonic uplift and is affected by two faults. One of these faults was responsible for displacing the Holocene materials. The paleoearthquake responsible for this liquefaction has been analysed in terms of its peak ground acceleration (pga) and magnitude by back calculation analysis based on the cyclic stress and Ishihara methods. A range of 0.22–0.35g was obtained for the pga, with the value of 0.30g being selected as most representative. From this, an earthquake-modified Mercalli intensity of I_MM=IX was estimated for the liquefaction site. The magnitude-bound method and energy-based approaches were used to determine the magnitude of the paleoearthquake, providing a moment magnitude M in the range of 6.4–7.2; M=6.8 is taken as the representative figure.     

Late Quaternary Activity of the Huashan Piedmont Fault and Associated Hazards in the Southeastern Weihe Graben, Central China

The Weihe Graben is not only an important Cenozoic fault basin in China but also a significant active seismic zone. The Huashan piedmont fault is an important active fault on the southeast side of the Weihe Graben and has been highly active since the Cenozoic. The well–known Great Huaxian County Earthquake of 1556 occurred on the Huashan piedmont fault. This earthquake, which claimed the lives of approximately 830000 people, is one of the few large earthquakes known to have occurred on a high–angle normal fault. The Huashan piedmont fault is a typical active normal fault that can be used to study tectonic activity and the associated hazards. In this study, the types and characteristics of late Quaternary deformation along this fault are discussed from geological investigations, historical research and comprehensive analysis. On the basis of its characteristics and activity, the fault can be divided into three sections, namely eastern, central and western. The eastern and western sections display normal slip. Intense deformation has occurred along the two sections during the Quaternary; however, no deformation has occurred during the Holocene. The central section has experienced significant high–angle normal fault activity during the Quaternary, including the Holocene. Holocene alluvial fans and loess cut by the fault have been identified at the mouths of many stream valleys of the Huashan Mountains along the central section of the Huashan piedmont fault zone. Of the three sections of the Huashan piedmont fault, the central section is the most active and was very active during the late Quaternary. The rate of normal dip–slip was 1.67–2.71±0.11 mm/a in the Holocene and 0.61±0.15 mm/a during the Mid–Late Pleistocene. As is typical of normal faults, the late Quaternary activity of the Huashan piedmont fault has produced a set of disasters, which include frequent earthquakes, collapses, landslides, mudslides and ground fissures. Ground fissures mainly occur on the hanging–wall of the Huashan piedmont fault, with landslides, collapses and mudslides occurring on the footwall.     

Transtensional deformation in the Lake Tahoe region, California and Nevada, USA

Dextral transtensional deformation is occurring along the Sierra Nevada–Great Basin boundary zone (SNGBBZ) at the eastern edge of the Sierra Nevada microplate. In the Lake Tahoe region of the SNGBBZ, transtension is partitioned spatially and temporally into domains of north–south striking normal faults and transitional domains with conjugate strike-slip faults. The normal fault domains, which have had large Holocene earthquakes but account only for background seismicity in the historic period, primarily accommodate east–west extension, while the transitional domains, which have had moderate Holocene and historic earthquakes and are currently seismically active, primarily record north–south shortening. Through partitioned slip, the upper crust in this region undergoes overall constrictional strain.Major fault zones within the Lake Tahoe basin include two normal fault zones: the northwest-trending Tahoe–Sierra frontal fault zone (TSFFZ) and the north-trending West Tahoe–Dollar Point fault zone. Most faults in these zones show eastside down displacements. Both of these fault zones show evidence of Holocene earthquakes but are relatively quiet seismically through the historic record. The northeast-trending North Tahoe–Incline Village fault zone is a major normal to sinistral-oblique fault zone. This fault zone shows evidence for large Holocene earthquakes and based on the historic record is seismically active at the microearthquake level. The zone forms the boundary between the Lake Tahoe normal fault domain to the south and the Truckee transition zone to the north.Several lines of evidence, including both geology and historic seismicity, indicate that the seismically active Truckee and Gardnerville transition zones, north and southeast of Lake Tahoe basin, respectively, are undergoing north–south shortening. In addition, the central Carson Range, a major north-trending range block between two large normal fault zones, shows internal fault patterns that suggest the range is undergoing north–south shortening in addition to east–west extension.A model capable of explaining the spatial and temporal partitioning of slip suggests that seismic behavior in the region alternates between two modes, one mode characterized by an east–west minimum principal stress and a north–south maximum principal stress as at present. In this mode, seismicity and small-scale faulting reflecting north–south shortening concentrate in mechanically weak transition zones with primarily strike-slip faulting in relatively small-magnitude events, and domains with major normal faults are relatively quiet. A second mode occurs after sufficient north–south shortening reduces the north–south S_hmax in magnitude until it is less than S_v, at which point S_v becomes the maximum principal stress. This second mode is then characterized by large earthquakes on major normal faults in the large normal fault domains, which dominate the overall moment release in the region, producing significant east–west extension.     

Tectonic and liquefaction structures in the Loreto basin,Baja California (Mexico) : synsedimentary deformation along a fossil fault plane

Abstract

Complex fault assemblages associated to liquefaction structures have been analyzed in a Pliocene basin located along the Gulf of California. The studied outcrop shows a fossil fault plane formed ill soft flat lying sediments. The liquefaction and fluidification structures have been recognized in voleaniclastic layers deposited ill a lagoonal environment and are potentially related to seismic wave shaking and to successive dewatering along fractures. This strati-graphic record is explained by the progressive development of a seismic fault zone, related to the transtensional regime still active in the Gulf. The present analysis can he considered as an useful case study for the reconnaissance of the different types of structures formed during synsedimentary deformation in hydroplastic conditions.     

Liquefaction hazard mapping at the town of Edessa,Northern Greece

The town of Edessa is located on Northern Greece at a region that is characterized as low seismicity zone due to the fact that few moderate events of M < 6 occurred during the last century. According to the Greek Seismic Code, the expected acceleration having a 10% probability of being exceeded in 50 years is equal to 0.16g. However, an amplification of ground motion is likely to occur due the local geology that is consisted of Holocene fluvio-torrential deposits. The basic aim of this paper is to evaluate the site amplification due to geological conditions and to assess the liquefaction hazard. In order to achieve this, 1-D site response analyses were performed. The data that were employed for the construction of the numerical models have been collected from borings with standard penetrations tests (SPT) that were drilled for construction purposes. Afterward, the liquefaction potential of the subsoil layers was evaluated taking into consideration two seismic scenarios. The first scenario was based on the seismic parameters, earthquake magnitude and PGA, assigned by the Greek Seismic Code. On the second seismic model, we employed the values of acceleration, resulted from the 1-D analyses and the earthquake magnitude as it was defined by the Greek Seismic Code. In order to compile the liquefaction hazard maps, we initially estimated the liquefaction potential index (LPI) of the soil columns using the parameters provided by SPT, for both seismic loadings, and afterward we correlated these values with the proposed classification of the severity of liquefaction-induced deformations. In addition, having computed the value of probability based on the LPI, liquefaction manifestations probability maps were compiled for both scenarios. The result of this study was that liquefaction-induced ground disruptions are likely to occur at the center of the city, among the branches of Voda River, only when the amplified values of acceleration are taken into account to the computation of liquefaction potential.     

Interhemispheric synchrony of Late-glacial climatic instability as recorded in proglacial Lake Mascardi,Argentina

Several high-resolution continental records have been reported recently in sites in South America, but the extent to which climatic variations were synchronous between the northern and southern hemispheres during the Late-glacial–Holocene transition, and the causes of the climatic changes, remain open questions. Previous investigations indicated that, east of the Andes, the middle and high latitudes of South America warmed uniformly and rapidly from 13 000 ¹⁴C yr BP, with no indication of subsequent climate fluctuations, equivalent, for example, to the Younger Dryas cooling. Here we present a multiproxy continuous record, radiocarbon dated by accelerated mass spectroscopy, from proglacial Lake Mascardi in Argentina. The results show that unstable climatic conditions, comparable to those described from records obtained in the Northern Hemisphere, dominated the Late-glacial–Holocene transition in Argentina at this latitude. Furthermore, a significant advance of the Tronador ice-cap, which feeds Lake Mascardi, occurred during the Younger Dryas Chronozone. This instability suggests a step-wise climatic history reflecting a global, rather than regional, forcing mechanism. The Lake Mascardi record, therefore, provides strong support for the hypothesis that ocean–atmosphere interaction, rather than global ocean circulation alone, governed interhemispheric climate teleconnections during the last deglaciation. © 1997 John Wiley & Sons, Ltd.     

Three-dimensional compressional deformations in the Zailiski Alatau mountains,Kazakstan

Abstract

The classical model of faulting predicts that slip planes occur in two conjugate sets. Theoretically, more sets can be contemporarily active if pre-existing structures are reactivated in a three-dimensional strain field. Four to six sets of faults have been active in the Holocene in the Zailiski Alatau mountain range, Kazakstan. Faults strike with the highest frequency ENE and ESE and show mostly left-lateral reverse and right-lateral reverse motions, respectively. These faults have a bimodal distribution of dips, forming four sets arranged in orthorhombic symmetry. Locally, NNW- to NNE- striking vertical faults have also been active in the Holocene and show right-lateral strike-slip and left-lateral strike-slip motions, respectively. All these fault sets accommodated the general three-dimensional deformation, given by N-S-directed horizontal shortening, vertical extension, and E-W-directed horizontal extension. Field evidence also shows that the reverse motions, even if with a minor strike-slip component, occurred on high-angle planes with inclination of 65°-85°. ENE- and ESE-striking faults reactivated older fracture zones, whereas the other sets are newly formed. Comparison of these field results with the structures obtained from published analogue models shows a strong similarity of fault geometry and kinematics.     

Soft-sediment deformation structures in late Albian to Cenomanian deposits, São Luís Basin, northern Brazil: evidence for palaeoseismicity

Soft-sediment deformation structures from the Alcântara Formation (late Albian to Cenomanian), São Luís Basin, northern Brazil, consist of (1) contorted structures, which include convolute folds, ball-and-pillow structures, concave-up paths with consolidation lamination, recumbently folded cross-stratification and irregular convolute stratification that grades into massive beds; (2) intruded structures, which include pillars, dykes, cusps and subsidence lobes; and (3) brittle structures, represented by fractures and faults displaying planes with a delicate, ragged morphology and sharp peaks. These structures result from a complex combination of processes, mostly including reverse density gradients, fluidization and liquefaction. Reverse density gradients, promoted by differential liquefaction associated with different degrees of sediment compaction, led to the genesis of convolute folds. More intense deformation promoted the development of ball-and-pillow structures, subsidence lobes and sand rolls, which are attributed to denser, and thus more compacted (less liquefied), portions that sank down into less dense, more liquefied sediments. Irregular convolute stratification that grades into massive beds would have formed at periods of maximum deformation. The subsidence of beds was accompanied by lateral current drag and fluid escape from water-saturated sands. In addition, the fractures and faults record brittle deformation penecontemporaneous with sediment deposition. All these mechanisms were triggered by a seismic agent, as suggested by a combination of criteria, including (1) the position of the study area at the edge of a major strike-slip fault zone that was reactivated several times from the Albian to the Holocene; (2) a relative increase in the degree of deformation in sites located closer to the fault zone; (3) continuity of the deformed beds over large distances (several kilometres); (4) restriction of soft-sediment deformation structures to single stratigraphic intervals bounded by entirely undeformed strata; (5) recurrence through time; and (6) similarities to many other earthquake-induced deformational structures.