软沉积物变形中负载、球—枕构造的古地震研究综述

在构造活动地区,软沉积物变形是研究古地震的一种关键证据。近年来,湖相沉积中的软沉积物变形受到越来越多的关注,但多数研究局限于软沉积物变形的形态分类。相比之下,对软沉积物变形的成因分析、触发机制和变形过程缺少系统分析,以至于软沉积物变形能否反映地震事件,以及软沉积物变形类型、强弱与地震震级和震中位置是否存在明确关联还存在较大争议。鉴于此,本文选择软沉积物变形中典型的变形构造—负载、球—枕构造,从其具体特征、成因、触发机制、变形过程、变形强弱与震级及震中距关系等方面展开讨论。统计结果显示,当沉积记录中的负载、球—枕构造为地震成因时,其代表的震级可能为6.0~7.0级,震中距约为20~70 km。就相同变形强度的负载、球—枕构造来说,湖相沉积记录的震级最强,其次为河湖相沉积和海相沉积。负载、球—枕构造变形层的宽度和厚度以及球状半径大小与地震震级具有正相关关系,而岩性与地震震级大小没有直接的对应关系。利用软沉积物变形所对应的地震震级估算距震中距离,或者采用软沉积物变形距断层距离估算地震震级的方法都是可行的。这样看来,软沉积物变形不仅能够记录地震事件,而且能够根据其变形类型、尺度大小和强度变化等,较好地确定地震震级及震中位置,为古地震研究提供了一个相对独立的研究方向。     

安徽寿县新元古界球-枕状构造的双重成因与古地震脉动旋回研究

在安徽寿县新元古界四十里长山组下部粉砂-细砂岩沉积地层中,发育一组具有双重成因的球-枕状软沉积物变形构造.对露头剖面进行实地观测研究显示,变形构造形成于浅海陆棚边缘斜坡带的地震灾变事件层中.由于滑塌砾石落入表层粉砂质软沉积层,在地震震颤应力作用下形成具砾石核心的球-枕状体,又在地震脉动旋回性连续震颤作用下,继续沉陷至下部液化砂层中,形成了具有双重成因的球-枕状软沉积变形构造.它经受了地震、海啸、滑塌、滑褶、震颤晃动沉陷、液化泄水、软塑性紧缩变形等多项复杂的同沉积变形作用过程.显示出该地震事件具有前震阶段、主震阶段、余震阶段等多次震颤脉动旋回性地质作用的地质事件,地震能量强度最大应超过里氏7级,为研究海洋震积岩的软沉积变形及脉动旋回性过程提供了重要的实物资料.     

山东省灵山岛负载构造和球-枕构造研究

本文运用野外露头考察、沉积岩石学、地质统计等方法,对灵山岛的负载构造和球-枕构造进行研究。这些负载构造和球-枕构造侧向延伸好,在整个剖面上连续分布,具有多尺度、多形态、多组合等特点。形成负载构造和球-枕构造的前提条件是地层密度的倒置(上大下小),变形的根本驱动力是沉积物自身的重力。球-枕构造和负载构造的形成机制相同,只是变形程度和阶段不同。根据不对称负载构造受横向剪切力分析,证明早白垩世时该盆地存在古斜坡,并且SE高、NW低。综合考虑地震触发形成震积岩的标志特征和灵山岛的构造地质背景,认为本研究区的负载构造和球-枕构造系古地震触发形成。     

皖北新元古界软沉积物液化变形-塌落叠合构造的古地震成因研究

在皖北新元古界四十里长山组下部粉砂岩层中, 发育有一个软沉积物液化变形-塌落叠合构造。观测剖面共分为三部分: 下部为啸积砾岩未变形层, 中部为液化均一层、球枕状层和塌落叠合层, 上部为震积不整合面, 不整合面之上为啸积砾岩未变形层。共同构成一个完整的地震-海啸震积岩序列。中部的液化均一层、球枕状层和塌落叠合层是震积事件的主旋回层。对剖面特征及成因机理研究分析后发现, 该剖面位于浅海陆棚边缘斜坡相带。在古地震多旋回脉动震颤作用下, 经液化均一变形, 负载体下沉滑覆及盖层塌落多重叠合, 最终形成具有软沉积物变形特征的叠合构造。四十里长山组沉积期, 位于浅海陆棚边缘斜坡地带的粉砂质软沉积物, 为震颤变形的能量转化提供了物质基础。而强地震的多旋回脉动作用, 给软沉积物液化均一变形、负载体下沉滑覆、盖层塌落叠合等提供了原动力。     

鄂尔多斯盆地富县探区三叠系延长组震积岩及其地质意义

通过对26口井岩心观察,结合区域构造背景,在鄂尔多斯盆地富县探区三叠系延长组中识别出典型的震积岩。该区震积岩代表性沉积构造有液化砂脉、液化卷曲变形、环状层理、负载构造、球枕构造、微断层、微裂缝、震裂岩与震碎角砾岩,其中液化砂脉和液化卷曲变形是地震引起的主要软沉积变形构造。震积岩垂向序列划分为两类:一种以脆性变形为主,自下向上依次为下伏未震层,扰动层,微断层、微裂缝及震裂岩层段,液化卷曲变形层段,震碎角砾岩层段,液化砂脉层段及上覆未震层;另一种以软沉积变形为主,自下至上依次为下伏未震层,环状层段,扰动层,球枕构造及负载构造层段,液化砂脉及液化卷曲变形层,枕状层及上覆未震层。富县探区延长组中的震积岩说明在该盆地晚三叠世存在一个强地震事件活跃期;震积岩中的微裂缝、微断层、震裂岩及液化砂脉可改善储集层物性,同时震碎角砾岩和网状砂脉也是良好的储集层。     

龙门山中、新生界软沉积物变形及构造演化

龙门山是由三条主要断裂组成的山体。汶川—茂县断裂,也称后山断裂,构成龙门山的西部边界;映秀—北川断裂为龙门山的中央断裂;灌县—安县断裂为龙门山的东部边界,也称前山断裂。龙门山断裂带以东为始自晚三叠世末的不同时期的前陆盆地。前陆盆地中从晚三叠世至2008年5月12日汶川地震(MS8.0),在不同年代地层中均有丰富的软沉积物变形构造(SSDS)记录,包括液化变形、重力作用变形、水塑性变形及其他相关的变形。这些变形层的地点紧邻龙门山的三条断裂,这些断裂在不同时期的活动诱发不同时期的强地震,导致当时尚未固结的沉积物变形(震积岩)。上三叠统小塘子组的软沉积的变形构造有液化角砾岩、液化滴状体、液化底辟、触变底辟、卷曲变形、拉伸布丁、负载、球-枕构造、枕状层及粒序断层等。侏罗系、白垩系主要为粗粒沉积物,除少数层位发现有液化变形外,主要的软沉积变形类型为各种形态、大尺度的砾岩负载构造。古近系为湖相沉积,沉积物粒度较细,软沉积物变形又出现大量液化变形构造,如液化混插、液化角砾岩等。2008年5月12日汶川地震(MS8.0)诱发大规模地表以下沙层液化,形成一系列液化变形构造与微地貌:液化沙堆、液化席状沙、沙火山、液化丘、坑状地形与混杂堆积。应用龙门山反射地震成果、古地震记录,结合区域构造可以给出龙门山断裂带发生的时间顺序与地震造山时期:(1)松潘—甘孜造山带与扬子板块的碰撞发生于晚三叠世早期,二者的边界即现在的汶川—茂县断裂;汶川—茂县断裂于晚三叠世末逆冲推覆造山,三叠纪末龙门山地区的山地可称松潘-甘孜山,在其东侧形成前陆盆地;晚三叠世印支造山旋回的大陆动力作用是龙门山诞生与孕育的阶段。(2)映秀—北川断裂与灌县—安县断裂的逆冲活动时间为侏罗纪—早白垩世,形成高山与前陆盆地。(3)早白垩世的龙门山已是一个由三条逆冲断裂组成的断裂带山体,可称古龙门山,山高约3 500m。(4)三条断裂在古近纪的活动诱发古近系软沉积物变形,但断裂未发生逆冲推覆造山,沉积物为湖相细粒沉积,古近纪是一个地震活动期,但不是造山的阶段。(5)中生代龙门山经历了多次瞬时地震造山与平静期山脉剥蚀降低的过程,现在的龙门山是晚新生代期间多次地震瞬时造山的产物。与众多的龙门山地学研究者不同,本文系采用另一种思维——软沉积物变形构造,即通过古地震途径讨论龙门山地区的构造演化。     

鄂尔多斯盆地早中侏罗世延安组软沉积物变形及成因分析

古地震相关的软沉积物变形构造在盆地演化中具有指示盆地及其周缘构造活动的作用.在鄂尔多斯盆地延安组岩心描述和野外调查过程中,于定边西南部DT3522井、安塞延河剖面中,发现并识别出软沉积物液化变形层,包括液化作用相关的枕状层、液化砂岩脉、液化角砾岩、泥火山,以及负载构造、球枕构造等9种变形构造.通过软沉积物变形层位对比,变形特征研究,结合区域构造背景认为,鄂尔多斯盆地延安组延7油层组沉积末期,发生了3期古地震活动,且呈现地震强度先弱后强的特征.     

新疆境内塔拉斯 费尔干纳断裂早侏罗世走滑的古地震证据

在野外考察过程中,于新疆乌恰地区早侏罗世康苏组沼泽相砂岩层中,发现并识别出软沉积物液化变形层,变形包括负载构造,球枕构造及卷曲变形构造。通过模拟试验的对比研究认为,该软沉积物变形机制与液化作用有关,触发沉积物液化的动力是古地震,并且根据地震震级与液化最大震中距的关系,推测出造成早侏罗世软沉积物变形的里氏地震震级为6相似文献   

新疆境内塔拉斯-费尔干纳断裂早侏罗世走滑的古地震证据

在野外考察过程中,于新疆乌恰地区早侏罗世康苏组沼泽相砂岩层中,发现并识别出软沉积物液化变形层,变形包括负载构造,球-枕构造及卷曲变形构造。通过模拟试验的对比研究认为,该软沉积物变形机制与液化作用有关,触发沉积物液化的动力是古地震,并且根据地震震级与液化最大震中距的关系,推测出造成早侏罗世软沉积物变形的里氏地震震级为6相似文献   

柴达木盆地西南缘新近系与地震沉积有关的软沉积物变形构造及其地质意义

在区域构造背景研究和岩心观察的基础上,在柴达木盆地西南缘新近纪地层中识别出与地震沉积有关的软沉积物变形构造。软沉积物变形构造包括液化砂岩脉、泄水构造、重荷模、火焰构造、震积砂枕、砂球构造、枕状层、层内错断、地裂缝、串珠状构造、震褶层、混合层及地震角砾状构造等。液化砂岩脉有喉道状、脉络状、飘带状、尖突状及“V”字形五种,主要是由振动流体化作用、振动液化挤压作用和振动拉张裂缝充填作用形成的;重荷模、火焰构造、枕状构造、球状构造是受地震颤动在砂、泥岩界面上由于砂层下沉、泥层上穿形成的;地裂缝、层内错断、震褶层是地震颤动直接引起的断裂、错断和褶皱;枕状层是地震振动引起的砂层脱水、下沉、变形形成的;混合层构造的完整性取决于地震强度和地震持续时间;地震角砾状构造是由地震振动使原始沉积层断裂形成的自碎屑角砾、脆性角砾和塑性角砾组成。该成果从沉积学角度证明了新近纪是昆仑山造山带北侧断裂活动较强烈时期,也为柴达木盆地新生代构造演化研究提供了依据。地震作用极大地提高了储层的渗透率,改善了油气储层的储集物性。     

西藏许如错地区中全新世地震触发软沉积物变形构造及其地质意义

湖相沉积古地震研究是对地表破裂古地震研究的重要补充.通过详细的野外地质调查,在西藏许如错地区全新统湖相地层内新发现大量地震触发软沉积物变形构造(震积岩),层内发育液化脉、液化曲卷变形、液化角砾岩、液化水压构造、滴状体与锥状体、砾石丘、负载构造和火焰构造等软沉积变形标志,还发育同震断层、震裂缝和同震褶皱等同震构造标志.根据软沉积变形标志与震级之间的关系,结合历史地震统计液化颗粒范围,通过C14和光释光年龄测定,推测古地震事件发生在±7.5 ka,M_S>7.5级;填补了该区历史地震的空缺,为恢复青藏高原南北向地堑地震活动历史及迁移规律提供了素材.震积岩中见大量砾石液化现象,这对现阶段以砂土-粉砂土研究为主的砂土液化调查工作提出了新挑战.      

滦平盆地下白垩统西瓜园组震积软沉积变形

为完善对滦平盆地西瓜园组地震引发的软沉积变形构造的认识,综合利用野外观测与室内分析相结合、宏观沉积体系与微观软沉积变形构造等分析相结合的方法,对研究区震积成因的软沉积变形构造进行了研究.结果表明,在盆内主、次控盆断层夹角位置的两处剖面中,可见枕状构造、液化砂岩脉、液化砂岩侵位、液化角砾岩、液化卷曲变形、砂岩滴落体、火焰构造、球枕构造、枕状层、负荷构造和震积不整合的组合发育,且同一剖面具有垂向多次震积作用的连续发育特点,这些震积作用均被识别在扇三角洲前缘相带内.根据这些软沉积变形发育的位置、彼此组合伴生、连续发育和区域分布的特点,可识别出连续2次大地震的发生.      

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.     

Experimental soft-sediment deformation: structures formed by the liquefaction of unconsolidated sands and some ancient examples

The effects of liquefaction in saturated sand bodies under a variety of driving forces are described from shaking table experiments, and structures from the geological record are presented which are analogous to the experimental structures. The collapse of sloping heaps of cross-bedded sand under a gravitational body force generates low-angle, essentially uncontorted stratification. A basal zone of shearing may be present, with steepened and folded foresets. Stretching of foresets may be accommodated on normal faults, and bottomsets may be contorted into inclined folds. In natural systems the substrate may also liquefy, causing deformation driven by an unevenly distributed confining load. Stratification in the surface bedform is flattened, and stratification in the substratum contorted. Experiments failed to produce relative displacement at the interface between stacked sand bodies. Liquefaction of gravitationally unstable systems in sands generates load structures comparable to those from sand-mud systems. Recumbent-folded deformed cross-bedding is formed by current shear over a liquefied bed, as has been inferred from field and theoretical analyses. Shear of nonliquefied sand forms angular folds. Other deformation mechanisms, such as fluidization or seepage, may generate structures similar to all of these. Local water-escape structures driven by fluidization occur in the upper parts of some liquefied sand bodies. They include cusps, sand volcanoes and clastic dykes. Transient cavities formed in some experiments and seemed to be preserved as breached cusps. Although the experiments tried to isolate individual driving forces, driving forces may operate together, and there may be a continuum between deformation driven by water escape and deformation driven by loading. Different structures from those described here may form where liquefaction develops in a buried layer as opposed to at the sediment surface.     

Formation of fluidization pipes during liquefaction: examples from the Uratanna Formation (Lower Cambrian), South Australia

ABSTRACT Usually well preserved fluidization pillars and sand filled fluidization pipes occur within submarine channel sands of the basal Uratanna Formation (Lower Cambrian) in the Adelaide Geosyncline of South Australia. The morphology of these structures reflects complex lateral and vertical movement of fluids during liquefaction and dewatering. Fluidization pipes acted as conduits for highly concentrated, upward directed fluid flow. The formation and maintenance of these pipes was dependent upon the development of a pipe wall composed of clay plugged fine sand. Formed during initial fluidization, this lining acted as a permeability barrier, confining and concentrating fluidized flow within the pipe. Each of the pipes is surrounded by a cylindrical fluidization halo in which leakage through the pipe lining produced partial fluidization of the surrounding sediment. Fine scale structures within these haloes indicate that fluids flowed radially and upward out of the fluidization pipes at an acute angle. These fluids merged with and influenced the orientation and size of adjacent fluidization pillars. The fluidization pipes of the Uratanna Formation may represent unusual preservation of the unstable fluid flow conditions that occur during incipient fluidization of sand beds.     

Water escape structures in coarse-grained sediments

Three processes of water escape characterize the consolidation of silt-, sand-and gravel-sized sediments. Seepage involves the slow upward movement of pore fluids within existing voids or rapid flow within compact and confined sediments. Liquefaction is marked by the sudden breakdown of a metastable, loosely packed grain framework, the grains becoming temporarily suspended in the pore fluid and settling rapidly through the fluid until a grain-supported structure is re-established. Fluidization occurs when the drag exerted by moving pore fluids exceeds the effective weight of the grains; the particles are lifted, the grain framework destroyed, and the sediment strength reduced to nearly zero. Diagenetic sedimentary structures formed in direct response to processes of fluid escape are here termed water escape structures. Four main types of water escape structures form during the fluidization and liquefaction of sands: (1) soft-sediment mixing bodies, (2) soft-sedimsnt intrusions, (3) consolidation laminations, and (4) soft-sediment folds. These structures represent both the direct rearrangement of sediment grains by escaping fluids and the deformation of hydroplastic, liquefied, or fluidized sediment in response to external stresses. Fundamental controls on sediment consolidation are exerted by the bulk sediment properties of grain size, packing, permeability, and strength, which together determine whether consolidation will occur and, if so the course it follows, and by external disturbances which act to trigger liquefaction and fluidization. The liquefaction and fluidization of natural sands usually accompanies the collapse of loosely packed cross-bedded deposits. This collapse is commonly initiated by water forced into the units as underlying beds, especially muds and clays, consolidate. The consolidation of subjacent units is often triggered by the rapid deposition of the sand itself, although earthquakes or other disturbances are probably influential in some instances. Water escape structures most commonly form in fine- to medium-grained sands deposited at high instantaneous and mean sedimentation rates; they are particularly abundant in cross-laminated deposits but rare in units deposited under upper flow regime plane bed conditions. Their development is favoured by upward decreasing permeability within sedimentation units such as normally graded turbidites. They are especially common in sequences made up of alternating fine-(clay and mud) and coarse-grained (sand) units such as deep-sea flysch prodelta, and, to a lesser extent, fluvial point bar, levee, and proximal overbank deposits.     

2018年5月28日中国吉林松原M5.7级地震引起的液化构造*

2018年5月28日,吉林松原市宁江区毛都站镇牙木吐村发生M5.7级地震(45°16'12″N,124°42'35″E),震源深度13 km,震中位于郯庐断裂带西北侧的扶余/松原—肇东断裂带、第二松花江断裂带和扶余北断裂带交汇处。地震诱发震中距3 km范围内普遍的液化和地表裂缝,给当地居民带来严重灾害。可见液化构造以砂火山为主,其次为液化砂堆、液化砂脉和液化砂席等。液化砂火山又可分为有火山口型砂火山、无火山口型砂火山和无砂型(水)火山。地震液化伴生软沉积物变形构造有变形层理、负载构造和火焰构造、滑塌褶皱、碟状构造和包卷层理等。地震诱发液化砂火山形成过程包括液化层内超孔隙流体压力形成、上覆低渗透层破裂和水、砂喷出地表后砂涌3个阶段。液化和流化砂体在上涌过程中会注入低渗透黏土层形成各种形态的砂脉、砂席和多种类型的变形构造。垂向上地震液化结构可划分为底部松散可液化层、下部液化变形层、上部液化变形层和地表砂火山4层结构。液化层埋深2~5 m,液化层厚度2 m。松原M5.7级地震发震机制为NE-SW(35°~215°)方向挤压应力使断层活跃,推测扶余/松原—肇东断裂是主要的发震断层。松原地震液化构造研究为现代地震活动区和灾害易发区预测提供依据,为地震引发的现代软沉积物变形构造研究提供丰富的素材,兼具将今论古意义,为揭示本世纪以来郯庐断裂带北段进入了一个强断裂和地震活跃阶段提供了最新的实际资料。     

黏性沉积物中的古地震触变流动变形*

粒径小于0.005mm的饱和淤泥和黏土等对地震和外力扰动产生的敏感变化特性被称为触变性。地震触发的软沉积物流动变形构造包括液化流动变形与触变流动变形两大类,前者多指沙层和碳酸盐沉积物的液化流动变形,后者指饱和的泥质沉积物触变流动变形。在地层剖面中,饱和淤泥、淤泥质土、黏土、硅泥(胶体)、碳酸盐灰泥等黏性沉积物的触变流动变形构造广布,它们多与沙层等的液化变形构成复合变形构造,但中外地质学家对触变流动变形构造注意较少,往往把它们笼统解释为液化流动构造。近年来地震触发饱和淤泥的触变流动变形现象逐渐引起地质学家的关注。作者对国内多个地层剖面中地震触发的饱和淤泥流动变形记录进行了描述和成因解释,并按照触变流动变形的方向性归纳出4类模式,即①向上流动、②向下流动、③同时向上及向下流动和④近水平方向流动,希望引起从事软沉积物变形和古地震研究的地质学家的关注。