Discussion of tectonic models for Cenozoic strike-slip fault-affected continental margins of mainland SE Asia

Understanding the roles of Cenozoic strike-slip faults in SE Asia observed in outcrop onshore, with their offshore continuation has produced a variety of structural models (particularly pull-apart vs. oblique extension, escape tectonics vs. slab-pull-driven extension) to explain their relationships to sedimentary basins. Key problems with interpreting the offshore significance of major strike-slip faults are: (1) reconciling conflicting palaeomagnetic data, (2) discriminating extensional, and oblique-extensional fault geometries from strike-slip geometries on 2D seismic reflection data, and (3) estimating strike-slip displacements from seismic reflection data.Focus on basic strike-slip fault geometries such as restraining vs. releasing bends, and strongly splaying geometries approach the gulfs of Thailand and Tonkin, suggest major strike-slip faults probably do not extend far offshore Splays covering areas 10,000’s km² in extent are characteristic of the southern portions of the Sagaing, Mae Ping, Three Pagodas and Ailao Shan-Red River faults, and are indicative of major faults dying out. The areas of the fault tips associated with faults of potentially 100 km+ displacement, scale appropriately with global examples of strike-slip faults on log–log displacement vs. tip area plots. The fault geometries in the Song Hong-Yinggehai Basin are inappropriate for a sinistral pull-apart geometry, and instead the southern fault strands of the Ailao Shan-Red River fault are interpreted to die out within the NW part of the Song Hong-Yinggehai Basin. Hence the fault zone does not transfer displacement onto the South China Seas spreading centre. The strike-slip faults are replaced by more extensional, oblique-extensional fault systems offshore to the south. The Sagaing Fault is also superimposed on an older Paleogene–Early Miocene oblique-extensional rift system. The Sagaing Fault geometry is complex, and one branch of the offshore fault zone transfers displacement onto the Pliocene-Recent Andaman spreading centre, and links with the West Andaman and related faults to form a very large pull-apart basin.     

Influence of host lithofacies on fault rock variation in carbonate fault zones: A case study from the Island of Malta

Relatively few studies have examined fault rock microstructures in carbonates. Understanding fault core production helps predict the hydraulic behaviour of faults and the potential for reservoir compartmentalisation. Normal faults on Malta, ranging from <1 m to 90 m displacement, cut two carbonate lithofacies, micrite-dominated and grain-dominated carbonates, allowing the investigation of fault rock evolution with increasing displacement in differing lithofacies. Lithological heterogeneity leads to a variety of deformation mechanisms. Nine different fault rock types have been identified, with a range of deformation microstructures along an individual slip surface. The deformation style, and hence type of fault rock produced, is a function of host rock texture, specifically grain size and sorting, porosity and uniaxial compressive strength. Homogeneously fine-grained micrtie-dominated carbonates are characterised by dispersed deformation with large fracture networks that develop into breccias. Alternatively, this lithofacies is commonly recrystallised. In contrast, in the coarse-grained, heterogeneous grain-dominated carbonates the development of faulting is characterised by localised deformation, creating protocataclasite and cataclasite fault rocks. Cementation also occurs within some grain-dominated carbonates close to and on slip surfaces. Fault rock variation is a function of displacement as well as juxtaposed lithofacies. An increase in fault rock variability is observed at higher displacements, potentially creating a more transmissible fault, which opposes what may be expected in siliciclastic and crystalline faults. Significant heterogeneity in the fault rock types formed is likely to create variable permeability along fault-strike, potentially allowing across-fault fluid flow. However, areas with homogeneous fault rocks may generate barriers to fluid flow.     

Fault plane processes and mesoscopic structure of a strong-type seismogenic fault in tonalites (Adamello batholith, Southern Alps)

The Gole Larghe Fault is an exhumed paleoseismic fault crosscutting the Adamello tonalites (Italian Southern Alps). Ambient conditions of faulting were 9–11 km in depth and 250–300 °C. In the study area the fault accommodates 1100 m of dextral strike-slip over a fault thickness of 550 m. Displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons (faults_1–2–3). Fault displacement is in the range of few centimeters (faults₃) to a maximum of a few tens of meters in major faults₁. Faults_1–2 nucleated on pre-existing joints, whereas faults₃ are newly generated fractures produced during slip along faults_1–2. Each fault within the Gole Larghe Fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production. Pseudotachylytes are usually generated at the host rock/cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins. Pseudotachylytes and cataclasites have a similar chemical composition which is enriched in Loss On Ignition, K, Rb, Ba, U and Fe³⁺ compared to host rock.We envision two models for the evolution of the Gole Larghe Fault. In both models synkinematic fluid–rock interaction along a fault causes fault hardening by precipitation of abundant K-feldspar+epidote (and minor chlorite) in the cataclasite matrix conducive to final production of pseudotachylyte. In the first model, induration occurs progressively by differential precipitation related to fabric evolution in cataclasites. In the second model, induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir. Whatever the model, the Gole Larghe Fault represents a strong fault, where hardening processes resulted in a low displacement/fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement/fault thickness ratios.     

Shallow SH-wave seismic investigation of the Mt. Angel Fault, Northwest Oregon, USA

The Mt. Angel Fault is likely one of the most active faults near the Portland metropolitan area, and was probably associated with the 1993 Scotts Mills earthquake. SH-wave seismic techniques used to image the Mt. Angel Fault suggest that the fault offsets late Pleistocene gravel (22 to 34 ka) at several locations. Within the study area, displacement of the late Pleistocene gravel along the strike of the Mt. Angel Fault increases from no obvious displacement on the northwest to approximately 18 m on the southeast. This trend of increasing offset along the strike of the fault is paralleled by topographic and geomorphic trends. A reconnaissance geologic investigation at an anomalous bend in the Pudding River near the projected trace of the Mt. Angel Fault revealed potential tectonic deformation in sediments younger than the late Pleistocene gravel imaged by SH-wave data. The results of this study have contributed to the paleoseismic record of the Mt. Angel Fault, laid the groundwork for future geologic investigations along the Pudding River, and determined potential sites for future paleoseismic trenching investigations.     

Late Quaternary activity of the Feldbiss Fault Zone, Roer Valley Rift System, the Netherlands, based on displaced fluvial terrace fragments

The Meuse River crosses the Feldbiss Fault Zone, one of the main border fault zones of the Roer Valley Graben in the southern part of the Netherlands. Uplift of the area south of the Feldbiss Fault Zone forced the Meuse River to incise and, as a result, a flight of terraces was formed. Faults of the Feldbiss Fault Zone have displaced the Middle and Late Pleistocene terrace deposits. In this study, an extensive geomorphological survey was carried out to locate the faults of the Feldbiss Fault Zone and to determine the displacement history of terrace deposits.The Feldbiss Fault Zone is characterized by an average displacement rate of 0.041–0.047 mm a⁻¹ during the Late Pleistocene. Individual faults show an average displacement rate ranging between 0.010 and 0.034 mm a⁻¹. The spatial variation in displacement rates along the individual faults reveals a system of overstepping faults. These normal faults developed by reactivation of Paleozoic strike-slip faults.As fault displacements at the bases of the younger terrace deposits are apparently similar to the tops of the adjacent older terrace, the age of these horizons is the same within thousands of years. This implies that the model of terrace development by rapid fluvial incision followed by slow aggradation does apply for this area.     

Neotectonic deformation in the transition zone between the Dead Sea Transform and the East Anatolian Fault Zone, Southern Turkey: a palaeomagnetic study of the Karasu Rift Volcanism

In southern Turkey ongoing differential impingement of Arabia into the weak Anatolian collisional collage resulting from subduction of the Neotethyan Ocean has produced one of the most complex crustal interactions along the Alpine–Himalayan Orogen. Several major transforms with disputed motions, including the northward extension of the Dead Sea Fault Zone (DSFZ), meet in this region. To evaluate neotectonic motion on the Amanos and East Hatay fault zones considered to be northward extensions of the DSFZ, the palaeomagnetism of volcanic fields in the Karasu Rift between these faults has been studied. Remanence carriers are low-Ti magnetites and all except 5 of 51 basalt lavas have normal polarity. Morphological, polarity and K–Ar evidence show that rift formation occurred largely during the Brunhes chron with volcanism concentrated at 0.66–0.35 Ma and a subsidiary episode at 0.25–0.05. Forty-four units of normal polarity yield a mean of D/I=8.8°/54.7° with inclination identical to the present-day field and declination rotated clockwise by 8.8±4.0°. Within the 15-km-wide Hassa sector of the Karasu Rift, the volcanic activity is concentrated between the Amanos and East Hatay faults, both with left lateral motions, which have rotated blocks bounded by NW–SE cross faults in a clockwise sense as the Arabian Block has moved northwestwards. An average lava age of 0.5 Ma yields a minimum cumulative slip rate on the system bounding faults of 0.46 cm/year according with the rate deduced from the Africa–Arabia Euler vector and reduced rates of slip on the southern extension of the DSFZ during Plio-Quaternary times. Estimates deduced from offsets of dated lavas flows and morphological features on the Amanos Fault Zone [Tectonophysics 344 (2002) 207] are lower (0.09–0.18 cm/year) probably because they are limited to surface fault breaks and do not embrace the seismogenic crust.Results of this study suggest that most strike slip on the DSFZ is taken up by the Amanos–East Hatay–Afrin fault array in southern Turkey. Comparable estimates of Quaternary slip rate are identified on other faults meeting at an unstable FFF junction (DSFZ, East Anatolian Fault Zone, Karatas Fault Zone). A deceleration in slip rate across the DSFZ and its northward continuation during Plio-Quaternary times correlates with reorganization of the tectonic regime during the last 1–3 Ma including tectonic escape within Anatolia, establishment of the North and East Anatolian Fault Zones bounding the Anatolian collage in mid–late Pliocene times, a contemporaneous transition from transpression to transtension and concentration of all basaltic magmatism in this region within the last 1 Ma.     

Morpho-Sedimentary evidence of the Huoshan Fault’s late Cenozoic right-lateral movement in the Linfen Graben, Shanxi Graben System, North China

The Huoshan Fault, being of NEN strike, is one of the most important faults in the Shanxi Graben System of North China; it is the location of the 1303 A.D. Hongtong earthquake (M_S = 8.0). The late Pleistocene and Holocene offset of some gullies that cross the fault and some fault scratches have proved its right lateral movement during the late Quaternary; however, until now, geological evidence to support the movement in the Neogene and early Quaternary was scarce. Our work provides further crucial evidence that supports both its movement in the late Cenozoic and the total right-lateral displacement since the Pliocene. The difference in the outcrop heights of the Pliocene sediment along the fault, the difference in the geomorphological development along the fault, the inconsistency in the lithological composition of the Pliocene proluvial gravels with the rock types within the catchments of the current upper stream, and the offset of the Pliocene alluvial gravels all completely indicate that the fault has always moved right-laterally since the Pliocene. Additionally, this evidence indicates that the accumulative displacement is up to 12.5 km. Based on the horizontal and vertical displacement of the fault since the Pliocene, the time-averaged horizontal slip rate of the fault is estimated to be about 3.5 mm/a, while the ratio of the horizontal to vertical offsets is about 3.8; these data are roughly close to the results that were acquired from the Holocene and the present movement of the fault. This similarity in the tectonic movement parameters may imply that the intensity as well as the configuration of the regional stress field has remained constant, and that no significant changes have taken place since the Pliocene.     

Evidence of paleoearthquakes from trench investigations across Pinjore Garden fault in Pinjore Dun, NW Himalaya

The Pinjore Garden Fault (PGF) striking NNW-SSE is now considered one of the active faults displacing the younger Quaternary surfaces in the piggyback basin of Pinjore Dun. This has displaced the older Kalka and Pinjore surfaces, along with the other younger surfaces giving rise to WSW and SW-facing fault scarps with heights ranging from 2 to 16 m. The PGF represents a younger branch of the Main Boundary Thrust (MBT) system. An ~ 4m wide trench excavated across the PGF has revealed displacement of younger Quaternary deposits along a low angle thrust fault. Either side of the trench-walls reveals contrasting slip-related deformation of lithounits. The northern wall shows displacement of lithounits along a low-angle thrust fault, while the southern wall shows well-developed fault-related folding of thick sand unit. The sudden change in the deformational features on the southern wall is an evidence of the changing fault geometry within a short distance. Out of five prominent lithounits identified in the trench, the lower four units show displacement along a single fault. The basal unit ‘A’ shows maximum displacement of aboutT _o = 2.85 m, unit B = 1.8 m and unit C = 1.45 m. The displacement measured between the sedimentary units and retro-deformation of trench log suggests that at least two earthquake events have occurred along the PGF. The units A and D mark the event horizons. Considering the average amount of displacement during one single event (2 m) and the minimum length of the fault trace (~ 45 km), the behaviour of PGF seems similar to that of the Himalayan Frontal Fault (HFF) and appears capable of producing large magnitude earthquakes.     

断层内部结构及其对封闭性的影响

先前的研究多考虑断层封堵和开启的2种极端状态,近来的研究认为,在多数情况下断层处于2种之间的状态,只有在静止期具有封闭能力的断层,才有可能对油气起封堵作用。分析断层对流体运移的影响,需要分析断层在演化过程中的内部结构特征。断层可以划分出破碎带、诱导裂缝带和围岩3部分,断层岩和伴生裂缝构成破碎带的主体部分。常见的断层岩包括断层角砾岩、断层泥和部分碎裂岩,它们充填在断层裂缝空间中,断层内部结构受断层形成时的构造应力性质、断层活动强度和围岩岩性因素的控制。从动态角度看,随着断距增加,断层活动伴随着裂缝的发育和岩石的破碎混杂,可用泥质源岩层厚度和断距的比值来划分不同的发育阶段。断层活动期为油气运移通道,在静止时表现出差异性的封闭,通常用断层渗透率和排替压力2个参数来定量评价断层的封闭程度。断层岩渗透率主要受断距、泥质含量、埋深等因素的控制;断层排替压力的预测方法有2种:一种是从断层岩成岩角度分析的"等效埋深法",另一种是分析实测排替压力与主控地质因素的"拟合法"。通过简化的断层模型,建立了渗透率、排替压力与主控因素的预测关系。和储层类似,流体在断层中的运移遵循多孔介质的渗流特征。利用断层两侧的流体压力和油气柱高度并不能直接评价封闭性能,还必须考虑油气充注史和流体压力变化历史。     

Architecture and growth of normal fault zones in multilayer systems: A 3D field analysis in the South-Eastern Basin,France

3D field data on mesoscale normal faults were collected to examine the geometries and growth of faults in multilayer systems. Observation and analysis of the fractures include the collection of geometric attributes such as fault dips and fault zone thicknesses, detailed mapping in cross-sections and plan views, and the construction of individual and cumulative displacement profiles. Fault zone growth is consistent with a “coherent model” and is strongly influenced by the multilayer system. In the limestone layers, faults grew in several steps, including opening and frictional sliding on 80° dipping segments. Faulting in clay layers was in the form of 40° dipping faults and sub-horizontal faults, the latter being mostly early features developed under the same extensional regime as normal faults and disturbing the fault architecture. The fault zone thickness increases with the limestone thickness and the presence of sub-horizontal faults in clay beds. Numerous connections occur in clay units. The moderate (≈0.08) and low (<0.03) mean displacement gradients in clays and in limestones respectively indicate that the vertical propagation of faults is inhibited in clay layers. Analysis of displacement along fault strike indicates that a 0.08 displacement gradient is associated with the horizontal propagation of fault segments in limestones. According to this value, the fault zones are much longer than expected. It is associated with ‘flat topped’ displacement profiles along some fault segments and connection between segments to form complex fault zones.     

汶川地震断裂带结构特征与龙门山隆升的关系

2008年汶川地震(MW7.9)发生在青藏高原东缘龙门山断裂带上,并沿映秀-北川断裂和灌县-安县断裂分别产生约270km和80km的不同性质的地表破裂带。断裂岩是断裂活动的产物,是断裂带的物质组成,其结构特征记录了断裂活动演化的历史。本文以汶川地震发震断裂映秀-北川断裂带中虹口八角庙地区地表露头和汶川地震科学钻探一号孔(WFSD-1)岩心为主要研究对象,通过详细的野外调研、显微结构及XRD分析等,识别出映秀-北川断裂带由五个次级单元组成,分别为:碎裂岩带、黑色断层泥和角砾岩带、灰色断层角砾岩带、深灰色断层角砾岩带以及断层泥和角砾岩带。断裂岩组合显示映秀-北川断裂带具有多核断裂结构特征。映秀-北川断裂带在地表出露的宽度约为240m,岩心中厚度约为105m,碎裂岩、断层角砾岩、断层泥在地表及岩心中均发育,而假玄武玻璃仅在地表碎裂岩部分出现。汶川地震主滑移带斜切了映秀-北川断裂带,不完全沿袭古地震滑移带,暗示汶川地震断裂带与映秀-北川断裂带可能不是同一个断裂体系。通过断裂岩的研究确定了映秀-北川断裂带存在着摩擦熔融、热增压、动态润滑和机械润滑等多种断裂滑移机制。低温热年代学的研究推断映秀-北川断裂带的形成时代为15～10Ma,自形成以来,映秀-北川断裂带的长期活动控制着龙门山的快速隆升。断裂带五个不同断裂岩组合的内部结构带,可能与龙门山不同的隆升速率期有着一定的联系。     

Analysis of the relationship between displacements and dimensions of faults

Displacement gradients on single fault surfaces are a function of the maximum displacement on a fault and the dimensions of the fault surface. Data on the maximum lateral dimensions (widths) and maximum displacements on normal faults and thrusts, with maximum displacements from 4 mm to 40 km, are used to derive an expression relating width, displacement and material properties. The basis of this expression is a fault growth model in which width is proportional to the square root of displacement. Width/displacement ratios vary systematically with the size of a fault from values of ca 30,000, which are characteristic of a single slip event, to about 10 in the case of thrusts with displacements of 40 km. Rocks from which the fault data are derived have a likely range of shear moduli from ca 0.1 to ca 30 GPa, which is sufficient to account for the range of data.Data on widths and maximum displacements of 308 fault traces recorded on British coalmine plans are shown to be consistent with variation of shear modulus of about half an order of magnitude. Data on 58 further fault traces are shown to be consistent with the fault growth model. Synsedimentary faults may have growth curves characteristically different from those of other faults.It is suggested that the increase in dimensions of a fault is a postseismic process of subcritical crack propagation for which the significant material property is fracture toughness.     

On the anomalous land uplift and aseismic creep prior to the 1976 Tangshan earthquake

A spatio-temporal analysis based on the data of eleven repeated levellings around the Tangshan region prior to the 1976 earthquake indicates that an uplift lasting for 2 years, from 1968 through 1969. with a magnitude of 50 mm, occurred in the epicentral area.Aseismic creep superimposed on the accumulated strain has been found in the vicinity of Tangshan and Baodi along both the Tangshan and the Jiyunhe faults.Assuming uniform strain accumulation and elastic dislocation, theoretical values of displacement at the various dislocation sites have been calculated and, using the least squares method, the optimal values of strain accumulation and the parameters of the creep faults in different years have been determined.The creep fault under Tangshan, a right-lateral normal fault, strikes N47°E and dips S87°E. and is 8 km long and 6 km wide. The upper boundary of the fault lies 2 km deep. The strike-slip and dip-slip offsets are, respectively, 104 cm and 8cm. The average rate of strain accumulation amounts to 0.9 × 10⁻⁷/yr. Creep at the fault amounted to 18.6 cm/yr and 1.4 cm/yr, respectively, in the strike and dip directions over the period 1969–1975. The Jiyunhe fault, although of smaller dimensions, has experienced a greater rate of creep than the Tangshan fault.A correlation of the above-mentioned uplift and creep with that of the Tangshan earthquake suggests that the uplift might have been a manifestation of the early development of the earthquake and that aseismic creep may be one of the precursory phenomena of shallow earthquakes. The sequence of processes preceding the Tangshan earthquake may be described as: strain accumulation-land upliftaseismic creep-inverse land deformation (or decrease in creep rate)-earthquake.     

3D seismic analysis of the structure and evolution of a salt-influenced normal fault zone: A test of competing fault growth models

In this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3D seismic reflection dataset from the Suez Rift, Egypt. Our analysis indicates that the October Fault Zone is composed of two fault systems that are locally decoupled across a salt-bearing interval of Late Miocene (Messinian) age. The sub-salt system offsets pre-rift crystalline basement, and was active during the Late Oligocene-early Middle Miocene. It is composed of four, planar, NW–SE-striking segments that are hard- linked by N–S-striking segments, and up to 2 km of displacement occurs at top basement, suggesting that this fault system nucleated at or, more likely, below this structural level. The supra-salt system was active during the Pliocene-Holocene, and is composed of four, NW–SE-striking, listric fault segments, which are soft-linked by unbreached relay zones. Segments in the supra-salt fault system nucleated within Pliocene strata and have maximum throws of up to 482 m. Locally, the segments of the supra-salt fault system breach the Messinian salt to hard-link downwards with the underlying, sub-salt fault system, thus forming the upper part of a fault zone composed of: (i) a single, amalgamated fault system below the salt and (ii) a fault system composed of multiple soft-linked segments above the salt. Analysis of throw-distance (T-x) and throw-depth (T-z) plots for the supra-salt fault system, isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history. The fault tips were then effectively ‘pinned’ and the faults accumulated displacement via predominantly downward propagation. We interpret that the October Fault Zone had the following evolutionary trend; (i) growth of the sub-salt fault system during the Oligocene-to-early Middle Miocene; (ii) cessation of activity on the sub-salt fault system during the Middle Miocene-to-?Early Pliocene; (iii) stretching of the sub- and supra-salt intervals during Pliocene regional extension, which resulted in mild reactivation of the sub-salt fault system and nucleation of the segmented supra-salt fault system, which at this time was geometrically decoupled from the sub-salt fault system; and (iv) Pliocene-to-Holocene growth of the supra-salt fault system by downwards vertical tip line propagation, which resulted in downward breaching of the salt and dip-linkage with the sub-salt fault system. The structure of the October Fault Zone and the rapid establishment of supra-salt fault lengths are compatible with the predictions of the coherent fault model, although we note that individual segments in the supra-salt array grew in accordance with the isolated fault model. Our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre-scale, basin-bounding faults. Furthermore, we highlight the role that fault reactivation and dip-linkage in mechanically layered sequences can play in controlling the three-dimensional geometry of normal faults.     

龙门山彭县－灌县断裂的活动构造于地表破裂

2008年5月12日在龙门山发生了8.0级特大地震,彭县－灌县断裂亦发生了同震地表破裂。在前期对龙门山活动构造研究的基础上,汶川特大地震发生后,在灾区进行了多次的野外调查和国际合作考察,重点对汶川地震的地表破裂和地质灾害开展了详细的详细野外地质填图,利用全站仪和GPS对地表破裂进行了精确的测量,研究了的地表破裂地貌错位、构造组合和运动学,已实地测得地表破裂数据70余组(其中彭县－灌县断裂地表破裂数据20余组)。文章以彭县－灌县断裂地表破裂为切入点,在彭县－灌县断裂的关键部位开展了详细的野外地貌测量,主要测量了彭州磁峰、白鹿、绵竹金花和汉旺等地的地表破裂,标定了彭县－灌县断裂破裂带的垂向断距和水平断距,结果表明该地表破裂南西起于彭州磁峰,向北东延伸经白鹿、绵竹金花至绵竹汉旺,全长约 40～50km。地表破裂带沿彭县－灌县断裂带的走向断续分布,单个破裂长度在几米到500余米不等,破裂带切割了多种类型的地貌单元,包括山脉基岩、河流阶地、冲洪积扇、公路、桥梁等,同时也使道路发生拱曲、破坏和桥梁垮塌或移位。其以脆性破裂为特征,以逆冲－右旋走滑为特点,断面倾角较陡,北西盘为上升盘,南东盘为下降盘,垂直位错介于 0.39～2.70m之间,水平位错介于 0.20～0.70m,平均垂直位错为1.6m,平均水平位错为0.6m; 地表最大错动量的地点位于彭州白鹿镇,其中最大垂直断错为 2.7±0.2m,最大水平断错为 0.7±0.2m。垂直位错与水平位错量之间的比值为2 ∶1,表明该地震地表破裂带不仅存在逆冲运动分量和右旋走滑运动分量,而且逆冲运动分量大于右旋走滑运动分量,显示了彭县－灌县断裂破裂带具有以逆冲和缩短作用为主、右旋走滑作用为辅的破裂性质。其与映秀－北川断裂带的地表破裂相比较,该断裂的地表破裂程度远小于映秀－北川断裂带的地表破裂程度,主要表现在地表破裂的长度较短,垂直位错和水平位错也相对较小,而且为以逆冲作用为主。初步研究结果表明,彭县－灌县断裂与映秀－北川断裂地表破裂的平面组合样式显示为两条在平面上近于平行的北东向地表破裂带,其间由一条南北向的次级地表破裂带(小鱼洞断裂)将它们相连结,地下破裂面的剖面组合样式显示为叠瓦状,并在汶川地震震源附近或震源的上方相连的,是同“根”的。     

Deformation features within an active normal fault zone in carbonate rocks: The Gubbio fault (Central Apennines, Italy)

The Gubbio fault is an active normal fault defined by an important morphological scarp and normal fault focal mechanism solutions. This fault truncates the inherited Miocene Gubbio anticline and juxtaposes Mesozoic limestones in the footwall against Quaternary lacustrine deposits in the hanging wall. The offset is more than 2000 m of geological throw accumulated during a poly-phased history, as suggested by previous works, and has generated a complex zone of carbonate-rich fault-related structures. We report the results of a multidisciplinary study that integrates detailed outcrop and petrographic analysis of two well-exposed areas along the Gubbio fault zone, geochemical analysis (fluid inclusions, stable isotopes, and trace elements) of calcite-sealed fault-related structures and fault rocks, and biostratigraphic controls. Our aims are: (i) the characterization of the deformation features and their spatial–temporal relationships, and (ii) the determination of the P/T conditions and the fluid behaviour during deformation to achieve a better understanding of fluid–rock interaction in fault zones.We show that few of the observed structures can be attributed to an inherited shortening phase while the most abundant structures and fault rocks are related to extensional tectonics. The outcropping extensional patterns formed at depths less than 2.5–3 km, in a confined fluid system isolated from meteoric water, and the fault structures are the response to a small amount of cumulated displacement, 12–19% of the total geological throw.     

Can pseudotachylytes be used to infer earthquake source parameters? An example of limitations in the study of exhumed faults

Tectonic pseudotachylytes might be used to constrain earthquake source parameters, such as dynamic shear stress resistance, average dynamic friction and slip-weakening distance. Estimation of dynamic shear stress resistance and dynamic friction from field studies is based on the assumption that the volume of melt produced during coseismic slip is proportional to the frictional work converted to heat on the fault surface. Conditions conducive to a realistic estimate of dynamic shear resistance are: (i) the presence of large outcrop exposures that allow for estimation of the volume of pseudotachylyte; (ii) the presence of structural markers offset by faults in order to relate the displacement accommodated by the fault with the volume of melt produced; (iii) data that provide an estimate of the initial melt temperature; and (iv) determination of host-rock temperature and pressure conditions that may have existed during seismic faulting. An independent indication that steady-state friction in the presence of melts might be achieved during coseismic slip arises from the dependence of the fractal dimension of the fault profile (intersection of the fault surface with the outcrop surface) with displacement. This relation could also indicate the slip-weakening distance (Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574).The above conditions are all satisfied in the case of the Gole Larghe Fault Zone, which consists of hundreds subparallel strike-slip faults that cut tonalites of the Adamello batholith (Italy). The thickness of pseudotachylyte-bearing faults increases with displacement. From displacement/thickness ratios and energy balance calculations, we determined the dynamic shear resistance for several pseudotachylyte-bearing faults. In the same faults, the fractal dimension of the fault profile increases from 1.0 to 1.16 with displacement. This was also observed in experiments where steady-state friction in the presence of melt was achieved (Hirose, T., Shimamoto, T., 2003. Fractal dimension of molten surfaces as a possible parameter to infer the slip-weakening distance of faults from natural pseudotachylytes. Journal of Structural Geology 25, 1569–1574). However, we will show that the estimate of the dynamic shear stress resistance, average dynamic friction and slip-weakening distance in the studied faults is limited by the uncertainties to attribute the measured displacement to a single seismic rupture. Since many pseudotachylytes in the upper seismogenic crust overprint preexisting cataclasites, it is suggested that future field and experimental work should be addressed to determine microstructural indicators (i.e. evolution of cataclastic fabric with displacement) within cataclasites, which might constrain the contribution of the cataclastic, pre-pseudotachylyte displacement to the total displacement accommodated by the fault.     

Structure of pseudotachylyte vein systems as a key to co-seismic rupture dynamics: the case of Gavilgarh–Tan Shear Zone,central India

The secondary fractures associated with a major pseudotachylyte-bearing fault vein in the sheared aplitic granitoid of the Proterozoic Gavilgarh–Tan Shear Zone in central India are mapped at the outcrop scale. The fracture maps help to identify at least three different types of co-seismic ruptures, e.g., X–X′, T₁ and T₂, which characterize sinistral-sense shearing of rocks, confined between two sinistral strike-slip faults slipping at seismic rate. From the asymmetric distribution of tensile fractures around the sinistral-sense fault vein, the direction of seismic rupture propagation is predicted to have occurred from west-southwest to east-northeast, during an ancient (Ordovician?) earthquake. Calculations of approximate co-seismic displacement on the faults and seismic moment (M ₀) of the earthquake are attempted, following the methods proposed by earlier workers. These estimates broadly agree to the findings from other studied fault zones (e.g., Gole Larghe Fault zone, Italian Alps). This study supports the proposition by some researchers that important seismological information can be extracted from tectonic pseudotachylytes of all ages, provided they are not reworked by subsequent tectonic activity.     

Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault,Wollaston Forland,NE Greenland)

Structurally controlled, syn-rift, clastic depocentres are of economic interest as hydrocarbon reservoirs; understanding the structure of their bounding faults is of great relevance, e.g. in the assessment of fault-controlled hydrocarbon retention potential. Here we investigate the structure of the Dombjerg Fault Zone (Wollaston Forland, NE Greenland), a syn-rift border fault that juxtaposes syn-rift deep-water hanging-wall clastics against a footwall of crystalline basement. A series of discrete fault strands characterize the central fault zone, where discrete slip surfaces, fault rock assemblages and extreme fracturing are common. A chemical alteration zone (CAZ) of fault-related calcite cementation envelops the fault and places strong controls on the style of deformation, particularly in the hanging-wall. The hanging-wall damage zone includes faults, joints, veins and, outside the CAZ, disaggregation deformation bands. Footwall deformation includes faults, joints and veins. Our observations suggest that the CAZ formed during early-stage fault slip and imparted a mechanical control on later fault-related deformation. This study thus gives new insights to the structure of an exposed basin-bounding fault and highlights a spatiotemporal interplay between fault damage and chemical alteration, the latter of which is often underreported in fault studies. To better elucidate the structure, evolution and flow properties of faults (outcrop or subsurface), both fault damage and fault-related chemical alteration must be considered.