伸展作用序列及其深部背景: 以晚中生代以来鲁西隆起和济阳坳陷为例

露头区野外地质调查、隐伏区地质与地球物理资料研究表明,晚中生代-新生代鲁西隆起区和济阳坳陷区正断层发育,包括陡倾斜的控凹边界断层和缓倾斜的滑脱断层两类,两者构成伸展滑脱半地堑,且滑脱构造在隆起北部和坳陷南部最发育。K Ar和FT测年结果指示伸展断层的发育时间为176~103 Ma、67～49 Ma和42～25 Ma 3个时期。隆起区、坳陷区陡断层分别在中地壳22 km 和15 km左右变平,成为拆离滑脱断层。构造物理模拟表明,在伸展＋塑性物质上涌机制下隆起区和坳陷区正断层均具有由南向北的发育极性,大致对应中侏罗世-早白垩世、古新世-早始新世、中始新世-渐新世3个发育阶段,且伴随控凹断层发育的同时,断块掀斜引起滑脱断层同步发育。层析成像表明中生代早期扬子板块沿作为转换断层的郯庐断裂以近EW向与华北板块俯冲的残留体可能导致晚中生代地幔物质上涌,新生代地幔上涌则可能与太平洋板块与欧亚板块俯冲及印欧板块碰撞的远程效应有关。研究区正断层受控于地幔物质上涌+伸展作用,以齐河-广饶断层为界呈前展式分别由南向北发育,并控制着坳陷区油气的形成、运聚和分布向北迁移。     

鲁西地块NW走向断层的活动特征及裂变径迹证据

不同于华北克拉通东部普遍存在的NE走向断层,鲁西地块广泛发育一组特征明显的NW走向断层,包括非控盆断层和控盆断层两类。前者位于鲁西地块最南部,倾角相对较陡,错开了古生界及以下地层,下盘太古宇中发育韧性剪切带,断层碎裂岩指示断层存在多期活动;后者位于非控盆断层以北,除蒙山断层外韧性剪切带不发育,倾角相对较缓,控制了中生代以来的沉积。磷灰石/锆石裂变径迹证据分析得出NW走向断层的活动存在差异。断层上、下盘样品磷灰石裂变径迹表观年龄在在67±5~35±2Ma之间,径迹直方图表明样品在冷却过程中没有受到热扰动。通过平均径迹长度-年龄(或香蕉图)图、单颗粒峰值年龄、径迹年龄谱模式以及热史反演模拟综合分析来约束断层的活动时间,结果表明非控盆断层可能在早侏罗世约184Ma开始活动,之后在晚白垩世80~75Ma以及新生代~61Ma和51~43Ma活动,43Ma之后不再活动。控盆断层活动时间稍晚,于早白垩世约141Ma、晚白垩世80~75Ma活动,新生代活动时间为约61Ma、49~42Ma以及36~32Ma。总体上,NW走向断层由早到晚由南向北发育,非控盆断层活动时间早、结束早;控盆断层活动晚、结束晚,并控制了凹陷的向北发育。中生代以来区域构造应力场的变化和郯庐断裂带的走滑作用是导致两类NW走向断层差异演化的根本原因,在深部则受控于晚三叠世以来华北、扬子板块陆陆碰撞和古太平洋板块俯冲方向和速度的改变。印支期后挤压到伸展的转变,加上郯庐断裂带的左行走滑,使靠近华北克拉通南缘的前端NW走向断层首先发育,因倾角较大故不控制盆地发育;向北的后端相对伸展,成为控盆断层,后经早白垩世约141Ma期间的伸展、晚白垩世末80~75Ma和新生代的发育断层最终成型。NW走向断层的这种大致向北迁移的规律,隐示华北克拉通破坏可能始于早侏罗世或晚侏罗世,且由南向北逐渐拆沉。     

The Plankogel detachment of the Eastern Alps: petrological evidence for an orogen‐scale extraction fault

The so‐called Plankogel detachment is an east‐west trending, south‐dipping low‐angle structure that juxtaposes the high‐P rocks of the eclogite type locality of the eastern European Alps against amphibolite facies rocks to the south. It occurs in both the Saualpe and Koralpe Complex in eastern Austria. During Cretaceous intracontinental subduction, the footwall and the hangingwall units of the Plankogel detachment were buried to different crustal levels as inferred by pseudosection modelling and conventional thermobarometry: ~23–24 kbar and 640–690 °C for the eclogite facies units in the footwall of the detachment and ~12–14 kbar and 550–580 °C for the amphibolite facies metapelites in the hangingwall. Despite the different peak metamorphic conditions, both sides of the detachment display a common overprint at conditions of ~10 kbar and 580–650 °C. From this, we infer a two‐stage exhumation process and suggest that this two‐stage process is best interpreted tectonically in terms of slab extraction during Eoalpine subduction. The first stage of exhumation occurred due to the downward (southward) extraction of a lithospheric slab that was localized in the trace of the Plankogel detachment. The later stage, however, is attributed to more regional erosion‐ or extension‐driven processes. Since the Plankogel detachment is geometrically related to a crustal‐scale shear zone further north (the Plattengneiss shear zone), we suggest that both structures are part of the same extraction fault system along which the syn‐collisional exhumation of the Eoalpine high‐P units of the Eastern Alps occurred. The suggested model is consistent with both the mylonitic texture of the Plattengneiss shear zone and the overall ambiguous shear sense indicators present in the entire region.     

银额盆地及周缘的两期晚新生代伸展构造:喜马拉雅碰撞造山远程效应的两个构造间歇期

银额盆地是中亚造山带南缘的一个中-新生代沉积盆地。在2017～2018年的野外地质调查期间,我们在银额盆地及周缘发现了多处晚新生代的伸展构造。这些伸展构造主要是一系列的正断层,还有同期的“X”共轭节理。相邻的两条倾向相对的正断层形成地堑。杭乌拉南正断层和鼎新镇北地堑形成于第四纪晚期,北山煤矿的正断层活动时间可能也是第四纪晚期。第四纪晚期正断层走向近E-W;伴生的鼎新镇北“X”共轭节理由走向NE-SW和NW-SE两组近直立的节理组成。苏宏图背斜上发育的正断层形成于新近纪晚期—第四纪早期,走向近E-W。古应力场分析,两期伸展构造的最大主拉张应力方向都是近N-S。这两期晚新生代伸展构造属于喜马拉雅碰撞造山的远程效应的组成部分,代表了远程效应脉动式演化过程的两个构造间歇期。     

The geometry of planar domino-style normal faults above a dipping basal detachment

Geometrical analysis of planar domino-style normal faults rooted into a dipping basal detachment fault allows derivation of equations which relate: (1) horizontal extension within the upper plate; (2) dip of the detachment; (3) final fault dips; (4) rotation that faults and beds undergo; and (5) net slip on domino-style faults. Past geometrical models have focused on extremely idealized and non-unique geometries, in which domino-style faults are parallel and similar cut-off points are all at the same elevation after faulting. This corresponds to evenly spaced domino-style faults above a horizontal detachment. Considering non-parallel faults and dipping detachments introduces unique geometries, which allows calculation, for example, of permissible depth ranges to the detachment. Horizontal extension varies significantly: (1) with dip of the detachment; and (2) for synthetic and antithetic cases, in which the domino-style faults dip in the same or opposite direction, respectively, as the detachment. When other factors (e.g. rotation and fault dips) are held constant, horizontal extension greatly increases for moderate detachment dips and moderately decreases for antithetic detachment dips, as compared to the horizontal detachment case. This is important because the synthetic case has been widely reported.     

Geodynamic evolution of the northern Molucca Sea area (Eastern Indonesia) constrained by 3-D gravity field inversion

In order to better understand the tectonic framework of the Northern Molucca Sea area, we inverted satellite and sea-surface gravity data into an iterative scheme including a priori seismological and geological data. The resulting 3-D density model images the various tectonic units from the surface down to 40 km. We proceed to various tests to assess the stability and robustness of our inversion. In particular, we performed an offset and average smoothing method to properly refine our results. The resulting model shows a striking vertical regularity of the structures through the different layers, whereas the density contrasts appear strongly uneven in the horizontal direction.The density model emphasizes the complexity of the upper lithospheric structure in the northern Molucca Sea, which is clearly dominated by the interaction between ophiolitic ridges, sedimentary wedges and rigid blocks of the Philippine Sea Plate. It also provides new, hard information that can be used in discussion of the evolution of the region.Large density variations are concentrated in the central part of northern Molucca Sea and dominate the upper lithospheric. North–south trending density structures along the Central Ridge and west dipping thrust faults on the western side of the region are clearly imaged. In the eastern part of the region, we distinguish several blocks, especially the Snellius Plateau which seems to be split into two parts. We interpret this as an oceanic plateau associated with thicker crust that previously belonged to the Philippine Sea Plate. This crust is now trapped between the Molucca Sea complex collision zone and the Philippine Trench, due to the development of a new subduction zone in its eastern side.     

Slip rate of trench‐parallel normal faulting along the Mejillones Fault (Atacama Fault System): Relationships with the northern Chile subduction and implications for seismic hazards

The recent tectonics of the arid northern Chile Andean western forearc is characterized by trench‐parallel normal faults within the Atacama Fault System (AFS). Since the 1995‐Mw 8.1 Antofagasta earthquake, the mechanism driving this recent and localized extension is considered to be associated with the seismic cycle within the subduction zone. Analyzing morphotectonic patterns along these faults allows examining the seismic potential associated with the subduction zone. Using field Digital Elevation Models and in situ‐produced cosmogenic ¹⁰Be, we determined a 0.2 mm/a long‐term slip rate along the Mejillones Fault, one of the most prominent structures within the AFS. This result suggests that the AFS corresponds to slow slip rate faults despite the rapid subduction context. However, the size of coseismic slips observed along the AFS faults suggests that larger subduction earthquakes (Mw > 8.1) may occur episodically in the area.     

Subduction of the Woodlark Basin at New Britain Trench, Solomon Islands region

The Woodlark Basin, located south of the Solomon Islands arc region, is a young (5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone.     

Three-dimensional model and late stage warping of the Plattengneis Shear Zone in the Eastern Alps

The Plattengneis shear zone is a 250–600 m thick, flat lying, Cretaceous, eclogite facies, mylonitic shear zone, with north-over-south transport direction, that is exposed over almost 1000 km² in the Koralpe region along the eastern margin of the Alps. Although the shear zone is one of the largest in the Alps, its role in the Eoalpine metamorphic evolution and the subsequent exhumation of the region, remain enigmatic and its large-scale geometry is not well understood. The outcrop pattern suggests that the shear zone is made up of a single sheet that is folded into a series of open syn- and antiforms with wavelengths of about 10 km. Eclogite bodies occur above, within and below the shear zone and there is no metamorphic grade change across the shear zone. In the south, the fold axes strike east–west and plunge shallowly to the east. In the north, the fold axes are oriented in north–south direction and form a dome shaped structure of the shear zone. Total shortening during this late stage warping event was of the order of 5%. Indirect evidence constrains this folding event to have occurred between 80 and 50 Ma and the fold geometry implies that the final exhumation in the Koralpe occurred somewhat later than further north. Interestingly, the shear zone appears to strike out of the topography in the south and dip into the topography in the north, so that north of the shear zone only hanging-wall rocks are exposed and south of it only foot-wall rocks. Possibilities for the geometric relationship of the Plattengneis shear zone with the surrounding south dipping detachments are discussed.     

碰撞带倾角和摩擦系数对陆-陆碰撞变形的影响

借助一个简单的大陆与大陆碰撞模型,即把两个大陆的碰撞简化为两个粘弹性块体的碰撞,两个块体之间的不连续变形面对应两个大陆之间的碰撞断裂带,运用三维粘弹性拉格朗日非连续变形有限元(LDDA)方法,通过分析模型中不连续变形面的存在对两个粘弹性块体碰撞变形的影响,探讨了大陆碰撞断裂带的倾角和摩擦系数对两个大陆碰撞变形影响的一般规律,给出了大陆碰撞变形的一些显著的特点。尽管运算模型的几何尺寸、边界条件等参考了印度和欧亚大陆碰撞的实际特征,但所得结果适用于更一般的情况。研究结果表明,碰撞断裂带倾角和摩擦系数对陆-陆碰撞变形有着重要的影响。当断裂带倾角在15~30°时,两个大陆板块碰撞导致的仰冲板块一侧隆升高度相对更大,利于形成高大的山脉,其中以15°倾角对应的仰冲板块一侧隆升最高;当断裂带倾角在30~45°时,两个大陆板块碰撞导致的俯冲板块的俯冲深度相对更深,利于形成深陷的盆地,其中以45°倾角对应的俯冲陆块一侧俯冲最深;当断裂带倾角≥75°时,两个大陆板块之间的相对俯冲和仰冲作用变得不明显。碰撞断裂带摩擦系数越小,碰撞过程中两个大陆板块之间相对俯冲和仰冲作用越强,形成高大的山脉和深陷的盆地要求碰撞断裂带摩擦系数≤0.2。从大陆与大陆碰撞变形构造特征看,除发育前陆盆地-山脉系统外,在仰冲陆块一侧靠近造山带后缘还发育呈不对称结构的挤压性凹陷,当两个大陆板块碰撞断裂带倾角在15~30°时,该类型凹陷更容易形成,其中以30°倾角对应的凹陷最深,反映其形成可能是大陆碰撞过程中陆块之间相对俯冲和仰冲运动的综合结果。     

NORMAL-SLIP ALONG THE NORTHERN ALTYN TAGH FAULT, NORTH TIBET

NORMAL-SLIP ALONG THE NORTHERN ALTYN TAGH FAULT, NORTH TIBET     

Structural inversion and its controls : examples from the Alpine foreland and the French Alps

Abstract

Positive structural inversion involves the uplift of rocks on the hanging-walls of faults, by dip slip or oblique slip movements. Controlling factors include the strike and dip of the earlier normal faults, the type of normal faults — whether they were listric or rotated blocks, the time lapsed since extension and the amount of contraction relative to extension. Steeply dipping faults are difficult to invert by dip slip movements; they form buttresses to displacement on both cover detachments and on deeper level but gently inclined basement faults. The decrease in displacement on the hanging-walls of such steep buttresses leads to the generation of layer parallel shortening, gentle to tight folds — depending on the amount of contractional displacement, back-folds and back-thrust systems, and short-cut thrust geometries — where the contractional fault slices across the footwall of the earlier normal fault to enclose a “floating horse”. However, early steeply dipping normal faults readily form oblique to strike slip inversion structures and often tramline the subsequent shortening into particular directions.

Examples are given from the strongly inverted structures of the western Alps and the weakly inverted structures of the Alpine foreland. Extensional faulting developed during the Triassic to Jurassic, during the initial opening of the central Atlantic, while the main phases of inversion date from the end Cretaceous when spreading began in the north Atlantic and there was a change of relative motion between Europe and Africa. During the mid-Tertiary well over 100 km of Alpine shortening took place; Alpine thrusts, often detached along, or close to, the basement-cover interface, stacking the late Jurassic to Cretaceous sediments of the post-extensional subsidence phase. These high level detachments were joined and breached by lower level faults in the basement which, in the external zones of the western Alps, generally reactivated and rotated the earlier east dipping half-graben bounding faults. The external massifs are essentially uplifted half-graben blocks. There was more reactivation and stacking of basement sheets in the eastern part of this external zone, where the faults had been rotated into more gentle dips above a shallower extensional detachment than on the steeper faults to the west.

There is no direct relationship between the weaker inversion of the Alpine foreland and the major orogenic contraction of the western Alps; the inversion structures of southern Britain and the Channel were separated from the Alps by a zone of rifting from late Eocene to Miocene which affected the Rhone, Bresse and Rhine regions. Though they relate to the same plate movements which formed the Alps, the weaker inversion structures must have been generated by within plate stresses, or from those emanating from the Atlantic rather than the Tethyan margin.     

华北板块南缘的变形分解:洛南-栾川断裂带与秦岭北缘强变形带研究

秦岭洛南-栾川断裂带具有左旋斜向俯冲的运动学特征,其产状一般为107°/N∠65°。华南板块的俯冲方向为80°,俯冲角度为42°;华南板块运动方向为42°,运动方向与华北板块南部边界的夹角为65°,汇聚角25°。秦岭北缘强变形带内褶皱枢纽延伸方向为290°,与洛南-栾川断裂带存在15°的夹角。逆冲断层走向与褶皱的枢纽方向基本一致,大多数断层与洛南-栾川断裂带有相同的运动学极性,性质为左行平移逆断层。平移正断层走向主要为NE SW,断层性质、展布方向、运动学特征与板块汇聚的应力作用方式吻合;片理、片麻理走向117°,与洛南-栾川断裂带走向夹角为10°。在垂直剪切带的剖面上,系统观察岩石变形特征,测量面理产状,进行岩石有限应变测量,岩石非共轴递进变形分析结果表明:秦岭北缘强变形带内由南向北面理走向与剪切带走向的夹角逐渐增大,岩石剪应变量依次递减,造山带变形具有“三斜对称”特点。     

Palaeogene structural evolution of Dongying Depression,Bohai Bay Basin,NE China

Based on the new all-covering 3D seismic data and the drilling-logging data, we established the sequence stratigraphic framework for Dongying Depression and identified two kinds of structural systems in Palaeogene, i.e. the extensional structural system and the transtensional structural system. The extensional structural system consists of different normal faults that predominantly trend NE, EW, and NW. The attitudes of the normal faults vary in different tectonic settings. However, the transtensional structural system consists of some strike–slip faults and some normal faults. According to the analysis of the relationships between the faults and the sedimentary sequences of Dongying Depression, we considered that the extensional structural system was developed mainly from the Palaeocene to the middle Eocene, whereas the transtensional structural system was mostly developed from the middle Eocene to the Oligocene. In addition, we found that the structural systems had transformed since 43.5 Ma, when the subduction direction and activity rates of the Pacific Plate changed and the dextral strike–slip movement of the large-scale Tanlu fault zone started from eastern China. The extensional structural deformation was probably derived from the back-arc extension triggered by subduction rollback of the Pacific Plate under the Eurasian Plate, whereas the transtensional structural deformation was probably related to the regional dextral strike–slip movement induced by the subduction of the Pacific Plate and the continents’ collision between the Indian Plate and the Eurasian Plate.     

大兴安岭地区上古生界变形特征及构造层划分

大兴安岭地区古生界构造变形表明,上古生界自二叠纪末以来遭受了3期构造变形改造:第一期变形为二叠纪末华北板块与佳蒙地块碰撞造成的近EW向展布的断裂和褶皱构造,强度由南向北有减弱的趋势;第二期变形为侏罗纪西太平洋板块俯冲导致的NE—NNE向左行走滑断裂和褶皱构造;第三期为NW向具有右行走滑特征的断裂构造,时间大致在晚侏罗世—早白垩世。综合区域构造、沉积岩古地理分析对比,初步将大兴安岭地区上古生界划分为早古生代、D—C1、C2—P2、P3—T14个构造层:早古生代末加里东运动之后,在D—C1期间早期以伸展为主,总体表现为北海南陆的古地理特征;早石炭世末期松嫩地块与额尔古纳—兴安地块沿嫩江—扎兰屯一线碰撞拼接;C2—P2期间总体表现为造山后伸展特征,表现为北陆南海;P3—T1时期古亚洲洋的闭合,海水退出,转为陆相。     

Late Cretaceous, synorogenic, low-angle normal faulting along the Schlinig fault (Switzerland, Italy, Austria) and its significance for the tectonics of the Eastern Alps

The Schlinig fault at the western border of theÖtztal nappe (Eastern Alps), previously interpreted as a west-directed thrust, actually represents a Late Cretaceous, top-SE to -ESE normal fault, as indicated by sense-of-shear criteria found within cataclasites and greenschist-facies mylonites. Normal faulting postdated and offset an earlier, Cretaceous-age, west-directed thrust at the base of theÖtztal nappe. Shape fabric and crystallographic preferred orientation in completely recrystallized quartz layers in a mylonite from the Schlinig fault record a combination of (1) top-east-southeast simple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directed shortening during the Early Tertiary, also recorded by open folds on the outcrop and map scale. Offset of the basal thrust of theÖtztal nappe across the Schlinig fault indicates a normal displacement of 17 km. The fault was initiated with a dip angle of 10° to 15° (low-angle normal fault). Domino-style extension of the competent Late Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting.

The Schlinig fault belongs to a system of east- to southeast-dipping normal faults which accommodated severe stretching of the Alpine orogen during the Late Cretaceous. The slip direction of extensional faults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip sense is reversed and the normal faults are slightly steeper than the thrusts. In the western Austroalpine nappes, extension started at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of Austroalpine Gosau basins with fluviatile to deep-marine sediments. West-directed rollback of an east-dipping Piemont-Ligurian subduction zone is proposed to have caused this stretching in the upper plate.     

新疆北阿尔泰造山带早古生代花岗岩类侵入序列及其构造意义

新疆阿尔泰早古生代造山带侵入岩占构造带面积50％以上,近年大量高精度SHRIMP和LA-ICP-MS锆石U-Pb年代学资料反映其构造属性为奥陶纪碰撞前序列和中志留-早泥盆世后碰撞序列.碰撞前序列岩石组合为(石英)闪长岩-英云闪长岩/奥长花岗岩/花岗闪长岩-二长花岗岩序列,类似TTG组合,锆石U-Pb同位素年龄峰值为450～ 465Ma.后碰撞由二长花岗岩-正长花岗岩及少量碱长花岗岩组成,属于广义的GG组合,同位素年龄峰值390～ 415Ma.前者主要分布在中南部,后者主要分布中北部,分布的极性显示俯冲带在南侧.而区域南侧的阿尔曼太蛇绿岩带同位素年龄与北阿尔泰奥陶纪碰撞前序列时代相同,本文推测该蛇绿岩带与北阿尔泰岩浆链带构成洋脊俯冲带模式;其间的南阿尔泰晚古生代增生带、额尔齐斯强变形带、北准噶尔晚古生代洋内弧带都是后来的上叠产物.     

Tectonic and magmatic responses to the subduction of high bathymetric relief

Subduction of high bathymetric relief, such as aseismic ridges and magmatic plateaus, is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone. For example, the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab, modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism. In addition, the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback, a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing. Here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum-Pacific subduction systems. We demonstrate that flattening of the subduction dip angle is only significant in the eastern Pacific, where the average slab dip angle is relatively shallow. In the western Pacific, in contrast, the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction. Subduction of high bathymetric relief in the circum-Pacific is commonly associated with reduced arc volcanism, and in many cases, the area of ridge subduction coincides with a volcanic gap. In the overriding plate, ridge subduction is associated with pronounced changes in the style of deformation, involving uplift, reactivation of basement thrusts, development of orogen-perpendicular tear faults and block rotations leading to oroclinal bending. The discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes, and could help constraining the geodynamics of ancient subduction systems.     

银根-额济纳旗盆地西缘的中-新生代伸展构造

银根-额济纳旗盆地简称银额盆地,是中亚造山带南缘的一个中-新生代沉积盆地。最近的野外地质调查,在其西缘发现早侏罗世和第四纪晚期的伸展构造。早侏罗世的伸展构造为一系列走向NNW-SSE 的正断层,是下侏罗统的同沉积断层。这组正断层与银额盆地内NNE-SSW 走向的正断层组合成共轭断裂系统,指示古构造应力场的最大主拉张应力方向为近E-W。它们是中亚造山带（南缘）造山后应力伸展阶段的构造变形。第四纪晚期的伸展构造是由两条倾向相向的正断层组合成的地堑构造,走向进E-W,可能代表了喜马拉雅碰撞造山远程效应脉动式演化过程的一个构造间歇期。     

An insight into the Bucaramanga nest

We used the local seismicity for the period of 1993 to 2001, in the northeast of Colombia to show the existence of two slabs in the north and south of the Bucaramanga nest. The northern slab has a dip angle of about 25° and the southern slab has a 50° dip angle, while the dip in the Bucaramanga nest is about 29°. In order to explain the nature of the Bucaramanga nest, we proposed the scenario of collision between these two slabs. Using a 3D Finite Element Model (FEM) we show that collision can concentrate, modify and perturb the stress field. The active process of dehydration embrittlement at intermediate depths and the concentrated stress field in the collision zone may explain the high rate of seismic activity inside the Bucaramanga nest. The perturbed and modified stress field resulting from the simultaneous effect of collision between two subducted slabs and subduction of the lithosphere under its own weight can explain the variation in the focal mechanism of micro-earthquakes and the complexity in the source of the moderate size earthquakes in the Bucaramanga nest.