首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Between Bariloche (41°S) and El Bolsón (42°S), Neogene sediments of the Ñirihuau foreland basin and Paleogene volcanoclastic rocks have been thrust westward beneath basement rocks of the Andean cordillera. North of Bariloche (40°–41°S), Paleogene volcanoclastic rocks within the main cordillera show Neogene deformation. The large-scale Neogene tectonics of the area are revealed by superimposing geological maps with digital topographic data. Fault-slip data provide information on the relative amount of crustal thickening and strike-slip faulting. Throughout the area, major reverse faults and thrusts trend northwest, forming the edges to Cenozoic basins of foreland or ramp styles. Some of these are inverted grabens of Mesozoic age. The dominant strike-slip faults are right-lateral and trend nearly north, parallel to the cordillera. Conjugate left-lateral faults trend nearly east. At a regional scale, based on the fault-slip data, the principal direction of shortening is northeast, in areas where thrusts predominate, but swings around to the north in areas where strike-slip faults predominate. Thus the results indicate a degree of strain partitioning, but they are broadly compatible with the oblique direction of convergence between the Nazca and South American plates. This tectonic style seems to have lasted throughout the Neogene.

Abstract

Entre las localidades de Bariloche (41°S) y El Bolsón (42°S), sedimentos Neógenos de la cuenca de antepaís Ñirihuau y volcanoclastitas Paleógenas han sido cabalgados desde el oeste por el basamento de la Cordillera de los Andes. Al norte de Bariloche (40°–41°S), volcanoclastitas Paleógenas de la Cordillera también muestran deformación. La tectónica neógena de gran escala se destaca por la superposición de mapas geológicos y topográficos digitalizados. A la escala de los afloramientos, los datos de deslizamientos de falla proveen información relativa a las relaciones entre el espesamiento cortical y el fallamiento de rumbo. En este sentido, a través de toda el área, las fallas inversas y los cabalgamientos mayores se disponen con rumbos noroeste, controlando las cuencas Cenozoicas de antepaís o de tipo rampa. Algunas de ellas invierten grábenes Mesozoicos. Por su parte, las fallas transcurrentes son dominantemente dextrales y se disponen submeridianalmente de modo paralelo a la Cordillera. Juegos conjugados senestrales se orientan sublatitudinalmente. A escala regional, la dirección principal de acortamiento, a partir de datos de desplazamiento de fallas, es noreste donde dominan los cabalgamientos, aunque se desvía hacia el norte donde predominan las fallas transcurrentes. Estos resultados indican un grado de particionamiento de la deformación, que resulta compatible con la dirección oblícua de convergencia entre las placas de Nazca y Sudamérica; estilo tectónico que parece haberse instalado a partir del Neógeno.  相似文献   

2.
The Helena salient is a prominent craton–convex curve in the Cordillera thrust belt of Montana, USA. The Lombard thrust sheet is the primary sheet in the salient. Structural analysis of fold trends, cleavage attitudes, and movement on minor faults is used to better understand both the geometry of the Lombard thrust and the kinematic development of the salient.Early W–E to WNW–ENE shortening directions in the Lombard sheet are indicated by fold trends in the center of the thrust sheet. The same narrow range of shortening directions is inferred from kinematic analysis of movement on minor faults and the orientations of unrotated cleavage planes along the southern lateral ramp boundary of the salient. As the salient developed, the amount and direction of shortening were locally modified as listric detachment faults rotated some tight folds to the NW, and as right-lateral simple shear, caused by lock-up and folding of the Jefferson Canyon fault above the lateral ramp, rotated other folds northeastward. Where the lateral ramp and frontal-oblique ramp intersect, folds were rotated back to the NW. Our interpretation of dominant W–E to WNW–ESE shortening in the Lombard sheet, later altered by local rotations, supports a model of salient formation by primary parallel transport modified by interactions with a lateral ramp.  相似文献   

3.
In the Appalachian thrust belt in Alabama, thrust sheets of Paleozoic strata generally strike northeastward and are imbricated northwestward; four transverse zones cross the regional strike of the thrust belt. The large-scale Pell City thrust sheet ends southwestward at an oblique lateral ramp within the Harpersville transverse zone, where the leading edge of the thrust sheet (the Pell City fault) curves abruptly 55° counterclockwise. The northwest-striking segment of the Pell City fault conforms to the geometry of an oblique lateral ramp in the footwall. Furthermore, the Pell City fault cuts up section in the hanging wall southwestward toward the transverse zone, indicating a hanging-wall lateral ramp emplaced over the footwall oblique lateral ramp.In the hanging wall adjacent to the northwest-trending segment of the Pell City fault, a pervasive train of upright, isoclinal folds (with 50% apparent shortening) trends N15°W, oblique to the regional translation direction. The fold train is limited to the southwestern part of the Pell City thrust sheet; farther northeast, the regional northeasterly strike prevails. The isoclinal folds in the hanging wall indicate contractional crowding perpendicular to the footwall oblique lateral ramp.  相似文献   

4.
The Montsec unit is one of the most important detached South-verging nappes within the South Pyrenean Central Unit (SPCU, Southern Pyrenees). A N–S cross-section of its Western sector, based on seismic reflection profiles, shows a hangingwall ramp geometry in Mesozoic strata, overlain by a syntectonic series of Lower Eocene sediments with growth geometry. The geometry of growth strata constrains the age of its movement between the Paleocene and the Middle Eocene. The geometry of the Western, oblique ramp of the South Pyrenean Central Unit is defined by a series of N–S folds, in some cases associated with underlying West-verging thrusts, as indicated by seismic reflection profiles and field data. In this paper, we propose that the geometry of the thrust wedge of Mesozoic units, progressively thinning from East to West, strongly contributed to constrain the location and geometry of the Western termination of the Montsec thrust. The hypothesis proposed is checked by a series of experimental wedges developed in a sandpack with lateral and three-dimensional thickness variations. Oblique structures form as thrusting progresses at the tip of the sand wedge.  相似文献   

5.
The coalescence and spatial variability of different thrust‐related folding mechanisms involving the same mechanical multilayer along a curved thrust system are documented in this study. The field‐based analysis of thrust‐related folds spectacularly exposed in the Gran Sasso thrust system, Central Apennines of Italy, allowed us to reconstruct the interference fold pattern between fault‐bend and fault‐propagation folding. These two thrust‐related folding mechanisms exhibit spatial variability along the differently oriented ramps of the curved Gran Sasso thrust system, passing from one style to the other. Their selective development is controlled by contrasting styles of compressional normal‐fault reactivation related to positive tectonic inversion. Fault‐bend and fault‐propagation folding interact with a characteristic interference fold pattern in the salient apex zone of the curved thrust system due to their synchronous/in‐sequence growth. This interference fold pattern might be helpful and predictive when reconstructing lateral variations in different thrust‐related folds in similar subaerial or submarine thrust belts.  相似文献   

6.
Backstripping analysis and forward modeling of 162 stratigraphic columns and wells of the Eastern Cordillera (EC), Llanos, and Magdalena Valley shows the Mesozoic Colombian Basin is marked by five lithosphere stretching pulses. Three stretching events are suggested during the Triassic–Jurassic, but additional biostratigraphical data are needed to identify them precisely. The spatial distribution of lithosphere stretching values suggests that small, narrow (<150 km), asymmetric graben basins were located on opposite sides of the paleo-Magdalena–La Salina fault system, which probably was active as a master transtensional or strike-slip fault system. Paleomagnetic data suggesting a significant (at least 10°) northward translation of terranes west of the Bucaramanga fault during the Early Jurassic, and the similarity between the early Mesozoic stratigraphy and tectonic setting of the Payandé terrane with the Late Permian transtensional rift of the Eastern Cordillera of Peru and Bolivia indicate that the areas were adjacent in early Mesozoic times. New geochronological, petrological, stratigraphic, and structural research is necessary to test this hypothesis, including additional paleomagnetic investigations to determine the paleolatitudinal position of the Central Cordillera and adjacent tectonic terranes during the Triassic–Jurassic. Two stretching events are suggested for the Cretaceous: Berriasian–Hauterivian (144–127 Ma) and Aptian–Albian (121–102 Ma). During the Early Cretaceous, marine facies accumulated on an extensional basin system. Shallow-marine sedimentation ended at the end of the Cretaceous due to the accretion of oceanic terranes of the Western Cordillera. In Berriasian–Hauterivian subsidence curves, isopach maps and paleomagnetic data imply a (>180 km) wide, asymmetrical, transtensional half-rift basin existed, divided by the Santander Floresta horst or high. The location of small mafic intrusions coincides with areas of thin crust (crustal stretching factors >1.4) and maximum stretching of the subcrustal lithosphere. During the Aptian–early Albian, the basin extended toward the south in the Upper Magdalena Valley. Differences between crustal and subcrustal stretching values suggest some lowermost crustal decoupling between the crust and subcrustal lithosphere or that increased thermal thinning affected the mantle lithosphere. Late Cretaceous subsidence was mainly driven by lithospheric cooling, water loading, and horizontal compressional stresses generated by collision of oceanic terranes in western Colombia. Triassic transtensional basins were narrow and increased in width during the Triassic and Jurassic. Cretaceous transtensional basins were wider than Triassic–Jurassic basins. During the Mesozoic, the strike-slip component gradually decreased at the expense of the increase of the extensional component, as suggested by paleomagnetic data and lithosphere stretching values. During the Berriasian–Hauterivian, the eastern side of the extensional basin may have developed by reactivation of an older Paleozoic rift system associated with the Guaicáramo fault system. The western side probably developed through reactivation of an earlier normal fault system developed during Triassic–Jurassic transtension. Alternatively, the eastern and western margins of the graben may have developed along older strike-slip faults, which were the boundaries of the accretion of terranes west of the Guaicáramo fault during the Late Triassic and Jurassic. The increasing width of the graben system likely was the result of progressive tensional reactivation of preexisting upper crustal weakness zones. Lateral changes in Mesozoic sediment thickness suggest the reverse or thrust faults that now define the eastern and western borders of the EC were originally normal faults with a strike-slip component that inverted during the Cenozoic Andean orogeny. Thus, the Guaicáramo, La Salina, Bitúima, Magdalena, and Boyacá originally were transtensional faults. Their oblique orientation relative to the Mesozoic magmatic arc of the Central Cordillera may be the result of oblique slip extension during the Cretaceous or inherited from the pre-Mesozoic structural grains. However, not all Mesozoic transtensional faults were inverted.  相似文献   

7.
The inversion of the Middle Proterozoic Belt sedimentary basin during Late Cretaceous thrusting in Montana produced a large eastwardly-convex salient, the southern boundary of which is a 200 km-long oblique to lateral ramp subtended by a detachment between the Belt rocks and Archean basement. A 10 km-long lateral ramp segment exposes the upper levels of the detachment where hanging wall Belt rocks have moved out over the Paleozoic and Mesozoic section. The hanging wall structure consists of a train of high amplitude, faulted, asymmetrical detachment folds. Initial west-east shortening produced layer parallel shortening fabrics and dominantly strike slip faulting followed by symmetrical detachment folding. “Lock-up” of movement on the detachment surface produced regional simple shear and caused the detachment folds to become asymmetrical and faulted. Folding of the detachment surface after lock-up modified the easternmost detachment folds further into a southeast-verging, overturned fold pair with a ramp-related fault along the base of the stretched mutual limb.  相似文献   

8.
The Eastern Cordillera (Central Andes,  24°S) consists of a basement-involved thrust system, resulting from Miocene–Quaternary eastward migrating compression, separating the Puna plateau from the Santa Barbara System foreland. The inferred Tertiary strains arising from shortening in the Eastern Cordillera and Santa Barbara System are similar, higher than in the Puna. Slip data collected on the major  N–S trending faults of Eastern Cordillera show a westward progression from dip-slip (contraction) to dextral and sinistral motions. This, consistently with established tectonic models, may result from partitioning due to the oblique Mio-Quaternary underthrusting of the Brazilian Shield north of 24°S. This strain partitioning has three main implications. (1) As the dextral and sinistral shear in the Eastern Cordillera are  62% and 29% of the compressive strain respectively, the Eastern Cordillera results more strained than Santa Barbara System foreland, contrary to previous estimates. (2) The partitioning in the Eastern Cordillera may find its counterpart in that to the west of the Central Andes, giving a possible structural symmetry to the Central Andes. (3) The easternmost N–S strike-slip structures in the Eastern Cordillera coincide with the easternmost Mio-Pliocene magmatic centres in the Central Andes, at  24°S. Provided that, further to the east, the crust is partially molten, the absence of magmatic centres may be explained by the presence of pure compressive structures in this portion of the Eastern Cordillera.  相似文献   

9.
Two models with different boundary conditions were carried out to simulate the structural evolution of the Kekeya-Hetian fold-and-thrust belt and Kashi-Yecheng strike-slip belt in the eastern margin of Pamir salient, respectively. The analogue modeling results show that: (1) Both of the Kekeya-Hetian fold-and-thrust belt and Kashi-Yecheng strike-slip belt in the eastern margin of Pamir salient were formed under compressive shearing. Strike-slip faults occurred within both of the belts, but the displacement of these strike-slip faults in the Kekeya-Hetian fold-and-thrust belt is less than that in the Kashi-Yecheng strike-slip belt; (2) The Kekeya-Hetian fold-and-thrust belt is mainly under the influence of compression stress with weaker shearing stress while the Kashi-Yecheng strike-slip belt is mainly under the influence of shearing stress with oblique compressive stress. The strike-slip faults are mainly located in the piedmont within these two belts. The effect of the strike-slip fault diminishes towards the front of the thrust belt (to the interior basin); (3) In the front of the boundary strike-slip faults (to the interior basin), the intersecting arc thrust faults occurred successively along the shortening direction. These structural features demonstrated that the structures evolved northwards in the eastern margin of Pamir salient; (4) The oblique compression does not necessarily result in high angle faults or vertical faults, whereas low-middle angle thrust faults with strike-slip displacement are also possible. Hence, more attention should be paid to such thrust faults during the structural analysis of seismic profiles in the eastern margin of Pamir salient (e.g. the structural belts in piedmont of western Tarim Basin). © 2017, Science Press. All right reserved.  相似文献   

10.
利用物理模拟实验,建立了两个不同边界条件的模型分别模拟帕米尔突刺东缘柯克亚-和田褶皱冲断带和喀什-叶城转换断层带的逆冲走滑构造演化过程,进而分析和讨论了研究区构造变形特征和变形机制。物理模拟实验结果表明:(1)帕米尔突刺东缘的柯克亚-和田褶皱冲断带和喀什-叶城转换断层带均形成于压扭应力场作用下,发育明显断层走滑现象,前者逆冲前缘断层兼具左行走滑特征,后者逆冲前缘断层则具右行走滑特征,但前者总体走滑量明显小于后者;(2)帕米尔突刺东缘的柯克亚-和田褶皱冲断带和喀什-叶城转换断层带走滑作用均主要位于山前边界断层带,越靠近逆冲前缘(盆地内部),走滑效应越微弱,挤压效应越明显;(3)在边界走滑断层前缘(往盆地方向),弧形断裂由挤压方向向前依次产生,并且斜向相交,验证了帕米尔东缘冲断带构造演化符合自南向北依次变新的规律;(4)在斜向压扭作用过程中,走滑断层构造带不一定发育明显的高角度甚至直立的断层,也可能表现为逆冲叠瓦构造楔样式,形成走滑逆断层,故在进行帕米尔突刺东缘(如塔西南山前)地震剖面构造解析时应充分关注这种构造类型。  相似文献   

11.
In the Pyrenees, the development of mylonites zones is one of the most striking structural features. Two sets of mylonites of regional extent have been recognized: large longitudinal E-W to N110°E trending zones (e.g. Mérens fault and North Pyrenean fault) and oblique NW-SE trending zones cross-cutting both the Hercynian and the post-Hercynian terrains. The longitudinal zones limit the major structural zones of the Pyrenees and are associated with NW-SE “en échelons” folds in the Mesozoic terrains and rotations of rootless plutonic or gneissic massifs, acting as competent inclusions in a more ductile matrix, in the Hercynian basement. The oblique mylonite zones limit map-scale fold-bands and appear as the sheared limbs of these folds.The age of the oblique zones and of the major movements along the longitudinal zones is clearly Alpine and the “en échelons” folds seem to have controlled the sedimentation during the Upper Albian and possibly during the Upper Cretaceous. Early movements along the longitudinal zones may have been Hercynian.The analysis of the structures at all scales leads us to interpret these mylonite zones and associated structures as the ultimate result of a transcurrent simple shear acting during the whole Mesozoic period. This strike-slip shearing was probably associated with an extension perpendicular to it from the Permian to the Upper Cretaceous and then to a shortening component also perpendicular to it from the Late Cretaceous to the Eocene.The development of the mylonite zones appears to have predated the major Alpine thrusting but to have been reactivated during this thrusting, acting as initiation sites for the thrusts or as oblique ramps in the case of the oblique mylonite zones.  相似文献   

12.
敦密断裂带白垩纪两期重要的变形事件   总被引：1，自引：1，他引：0  
本文报道了敦密断裂带糜棱岩中黑云母~(40)Ar/~(39)Ar定年结果和大规模走滑-逆冲断裂的几何学、运动学特征及其形成时代,以便揭示断裂带两期变形事件的构造属性。黑龙江省密山市花岗质糜棱岩中黑云母~(40)Ar/~(39)Ar加权平均年龄为132.2±1.2Ma,它是敦密断裂带经历伸展事件的冷却年龄,也是东北亚大陆边缘在早白垩世Hauterivian期-Albian期发生强烈区域伸展作用的产物。密山市至辽宁省清原县系列大型走滑-逆冲断层和断层相关褶皱揭示出在晚白垩世晚期-末期发生右旋走滑-逆冲事件,该事件规模大,影响范围广,导致整个断裂带遭受到强烈改造,形成对冲式断裂系统。将研究区走滑-逆冲断裂与山东省郯庐断裂带中段挤压构造对比,认为郯庐断裂带北段和中段在晚白垩世末期都发生了强烈的走滑-逆冲事件,它们具有相同的构造特征和构造属性。  相似文献   

13.
The Umbria-Marche-Sabina foreland fold and thrust belt (Northern Apennines, Italy) provides excellent test-cases for the hypothesis of ancient syndepositional structural features controlling thrust ramp development. The sedimentary cover, Late Triassic to Miocene in age, is made of platform and pelagic carbonates, whose deposition was controlled by significant synsedimentary extension. Normal faulting, mainly during the Jurassic and the Late Cretaceous-Palaeogene, determined sensible lateral thickness variations within the relative sequences. By late Miocene the sedimentary cover was detached from its basement along a mainly evaporitic horizon, and was deformed by means of eastward-verging folds and thrusts.
In order to locate the points where thrust ramps branch-off the basal detachment, both line-length and equal-area techniques were used in the construction of a balanced cross-section through three major fault-related folds in southeastern Umbria. The nucleation of thrust ramps was controlled by the occurrence of Jurassic and Cretaceous-Palaeogene synsedimentary normal faults. These interrupted the lateral continuity of the evaporitic unit (the Late Triassic Anidriti di Burano Fm.) at the base of the sedimentary cover, and acted as obstacles to the eastward propagation of the thrust system, giving rise to major folds which originated from tip-line folding processes.
Therefore, the inferred relationships between ancient normal faults and late thrusts indicate that synsedimentary tectonic structures and the related lateral stratigraphic variations can be envisaged as mechanically important perturbations, which effectively control the nucleation and development of thrust ramps.  相似文献   

14.
《International Geology Review》2012,54(12):1075-1085
The modern Andean Cordillera has proven to be a good modern analog for the Mesozoic and early Tertiary tectonic evolution of the US Cordillera, particularly for the transition between the Sevier and Laramide orogenies. A detailed version of this analogy, based on the tectonic evolution of the northern Chilean Andes, may explain the tectonic style of intra-arc exhumation and the southward migration of tectonism associated with arc extinction in southern California. Two regionally extensive episodes of deformation and exhumation are identified in southern California; the first occurred in an intra-arc setting in mid-Late Cretaceous time, and the second followed extinction of the magmatic arc and tectonic underplating by a blueschist/greenschist-grade metagraywacke terrane. We develop a model of Laramide oblique subduction of an aseismic oceanic ridge to explain these observations, based on modern subduction of the Juan Fernandez Ridge beneath the northern Chilean Andes. Laramide oblique ridge collision and consequent shallow subduction beneath southern California extinguished the magmatic arc and its intra-arc thrust belt and caused tectonic burial of the forearc beneath the extinct magmatic arc.  相似文献   

15.
The junction angle between the western Charlie-Gibbs transform fault and the spreading axis of the Mid-Atlantic Ridge diverges by 40° from the orthogonal intersection assumed in many studies of plate boundaries. This has been established by a surface-ship reconnaissance and by mapping fault trends in a transponder-navigated deep-tow survey of the fracture valley 25 km from the intersection. One set of normal faults trends 325–330°, parallel to the obliquely spreading ridge axis, and another set trends 275°, parellel to the direction of relative plate motion. Although the near-bottom survey was in the theoretically inactive part of the fracture zone, beyond the transform fault section, there is evidence for recent motion on faults that cut the thick sediment fill of the fracture valley.Oblique spreading of a ridge axis near a transform fault may result from distortion of the regional stress field by a strike-slip couple. Tension parallel to the long axis of the strike-slip strain ellipse, which is responsible for oblique normal faulting in transform valleys, causes oblique dike injection and oblique faulting in the axial rift valley. These effects extend further from transfrom fault intersections on slow-spreading ridges than on fast-spreading rises.  相似文献   

16.
The terminology of structures in thrust belts   总被引：1，自引：0，他引：1  
A review of structures and geometric relationships recognized in thrust belts is presented. A thrust is defined as any contractional fault, a corollary being that thrusts must cut up-section in their transport direction. ‘Flats’ are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ‘ramps’ are those portions of thrusts which cut across datum surfaces. Ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust. Lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike-slip movement.An array of thrusts which diverge in their transport direction may form by either of two propagation models. These are termed ‘piggy-back’ propagation, which is foreland-directed, and ‘overstep’ propagation which is opposed to the thrust transport direction. An array of thrust surfaces is termed an ‘imbricate stack’ and should these surfaces anastamose upwards a ‘duplex’ will result; the fault-bounded blocks are termed ‘horses’. A duplex is bounded by a higher, ‘roof’ thrust and a lower, ‘floor’ thrust. The intersection of any two thrust planes is termed a ‘branch line’.Thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut. A further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one.The movement of thrust sheets over corrugated surfaces, or the local development of thrust structures beneath, will fold higher thrust sheets. These folds are termed ‘culminations’ and their limbs are termed ‘culmination walls’. Accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust. These accommodation surfaces are termed ‘hangingwall detachments’ and they need not root down into the active thrust. This category of detachment includes dip-slip ‘hangingwall drop faults’ which are developed by differential uplift of duplex roofs, and ‘out-of-the-syncline’ thrusts which develop from overtightened fold hinges. Back thrusts, as well as forming as hangingwall detachments, may also form due to layer-parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp.  相似文献   

17.
《Journal of Structural Geology》2001,23(6-7):1123-1140
The western portion of the Skeena Fold Belt, northern Canadian Cordillera, contains northeast-trending folds that are highly oblique to northwest-trending folds in the eastern portion of the fold belt, and to most Mesozoic contractional structures in the northern Cordillera. The northeast-trending folds locally interfere with the northwest-trending folds, and one region includes transected folds. Geometric relationships within and between the two fold sets are not easily reconciled by notions of the northeast-trending folds resulting from vertical axis rotation of blocks, influence of basement features, or lateral variations in magnitude of shortening. The northeast-trending folds are inferred to result from sinistral plate convergence early in the history of the fold belt (Early Cretaceous).Northeast-trending folds in the Skeena Fold Belt are the most conspicuous elements of a seldom-studied group of similarly oriented contractional structures, which collectively define a belt at least 1700 km long, within and bordering the Coast Belt. The extent of Early Cretaceous structures potentially related to sinistral convergence supports them having originated in response to the relative plate motion rather than local controls (e.g. indentors). This agrees with relative plate motion studies based on ocean floor reconstructions, which suggest a mid-Cretaceous change from sinistral to dextral convergence.  相似文献   

18.
The Umbria-Marche foreland fold-and-thrust belt in the northern Apennines of Italy provides excellent evidence to test the hypothesis of synsedimentary-structural control on thrust ramp development. This orogenic belt consists of platform and pelagic carbonates, Late Triassic to Miocene in age, whose deposition was controlled by significant synsedimentary extension. Normal faulting, mainly active from Jurassic through Late Cretaceous-Paleogene time, resulted in significant lateral thickness variability within the related stratigraphic sequences. By Late Miocene time the sedimentary cover was detached from the underlying basement and was deformed by east-verging folds and west-dipping thrusts. Two restored balanced cross sections through the southernmost part of the belt show a coincidence between the early synsedimentary normal faults and the late thrust fault ramps. These evidences suggest that synsedimentary tectonic structures, such as faults and the related lithological lateral changes, can be regarded as mechanically important controlling factors in the process of thrust ramp development during positive tectonic inversion processes.  相似文献   

19.
In northwest Spain thrust sheets occur in an arcuate fold belt. The fault style consists of an array of thrusts, merging downdip into a single décollement surface. Most of the thrust sheets were initiated as thrusts cutting across flat lying beds. Folds above the hanging-wall ramps and some minor structures indicate that the body of the nappes has been subjected to an inhomogeneous simple shear parallel to bedding (y = 1.15), with slip concentrated along bedding planes. This allows the rocks forming the nappe to remain unstrained. At the base of the nappes a thin zone of deformed rock exists. The thrust sheets die out laterally against an anticline-syncline couple, oblique to the thrust direction. A geometrical analysis shows that if anticline and syncline axes are oblique, the thrust sheet was emplaced with a rotational movement, which can be evaluated. As deformation progressed two sets of folds were formed: a circumferential set, following the arc, and a radial set. An arcuate trace of the thrust structures remains after unfolding the radial folds. With a rotational emplacement, the displacement vector for successive points has a progressively greater length, and forms a progressively lower angle with the thrust. The main thrust units are broken into several slices with rotational movements, so that each unit was curved as it was being emplaced, producing a first tightening of the arc. Later folding increased the arc curvature to its present shape. The palaeomagnetic data available support the above conclusions.  相似文献   

20.
Antony Morris   《Tectonophysics》2003,377(1-2):157
A compilation of available palaeomagnetic data from the Troodos (Cyprus) and Baër–Bassit (Syria) ophiolitic terranes of the eastern Mediterranean Tethyan orogenic belt is presented. The ophiolites represent fragments of oceanic lithosphere generated at a Neotethyan spreading axis in the Late Cretaceous, although debate continues over the tectonic setting of this spreading axis and its position within the eastern Mediterranean palaeogeography. Two types of model reconstructions have been proposed: Type 1—the ophiolites formed in a southerly Neotethyan basin by spreading above an oceanic subduction zone. The Baër–Bassit ophiolite was then emplaced a relatively short distance (tens of kilometers) southwards on to the Arabian continental margin, leaving the Troodos ophiolite isolated in an intra-oceanic setting to the west; and Type 2—the ophiolites formed in a northerly Neotethyan basin by spreading at a ‘normal’ oceanic ridge, with subsequent large-scale thrusting (hundreds of kilometers) to the south of emplaced ophiolites over microcontinental fragments to reach their present positions. Palaeomagnetic determination of the palaeolatitude of the Neotethyan spreading axis is, therefore, of considerable interest.Previous palaeomagnetic analyses have demonstrated the presence of significant, and in some cases extreme, relative tectonic rotations of a variety of origins in both ophiolites. To allow palaeomagnetic data from these rotated units to be combined, an inclination-only formulation of the palaeomagnetic tilt test is employed. This provides unequivocal evidence that both ophiolites retain pre-deformational remanent magnetizations, which are interpreted as original ocean-floor magnetizations acquired close to the time of crustal formation in the Late Cretaceous. The mean inclinations of 37.0±2.6° for the Troodos terrane and 41.1±3.4° for the Baër–Bassit terrane indicate respective palaeolatitudes for the spreading axes of 20.6°N±1.8° and 23.6°N±2.5°, consistent with a Late Cretaceous position between the Arabian and Eurasian margins. These data, together with a well-defined palaeolatitude of 25.5°N±4.5° for the eastern Pontides previously reported in the literature, provide constraints which must be incorporated in any successful tectonic reconstruction of the eastern Mediterranean Tethys. The implications of these constraints for Type 1 and 2 models are discussed using a series of plate tectonic cross-sections constructed along a line extending northwards from the Arabian continental margin. In the absence of palaeomagnetic data from Late Cretaceous rocks of the eastern Taurides, however, it is presently impossible to use these palaeolatitudinal constraints to resolve the root zone debate on a purely palaeomagnetic basis. Solutions which satisfy the constraints may be found for both types of model reconstruction. Additional, published field-based geological considerations, however, strongly support models in which the Troodos and Baër–Bassit (and other southerly) ophiolites were generated in a southern Neotethyan basin, rather than those involving generation in a northerly basin and subsequent large-scale thrust displacement to the south.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号