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1.
Three felt earthquakes with local magnitudes 4.0 (June 29th, 2000), 4.2 (July 07th, 2005) and 3.7 (October 30th, 2007) occurred to the southeast of Cairo along the Suez-Cairo shear zone. Being the most well recorded events by the Egyptian National Seismic Network (ENSN) in this area, they provide us an excellent opportunity to study the tectonics, the stress field, the source parameters, and the expected ground motion at Cairo. The focal mechanisms of the three events based on the first motion P-wave polarities indicate mainly normal faulting with a slight strike-slip component along nodal planes trending nearly EW and NW–SE. The three focal solutions imply a rejuvenation of the pre-existing EW and NW–SE faults due to a partly transfer of rifting deformation from the Red Sea-Gulf of Suez along these trends. Dynamic source parameters of these events are estimated from the P-wave spectra of the closest ENSN stations. The average values of the seismic moment, stress drop, rupture radius, and fault dislocation are estimated from six stations and interpreted in the context of the tectonic setting. These parameters in addition to the effects of site and path are used to synthesize the ground motion seismograms using stochastic modeling technique at the recorded stations and at some strategic sites.  相似文献   

2.
The Precambrian of Madagascar is divided into two sectors by the north-west trending sinistral Ranotsara shear zone, which continues in the Mozambique belt, probably as the Surma shear zone, and in Southern India as the Achankovil shear zone. South of Ranotsara six north-south trending tectonic belts are recognized that consist largely of granulite and high amphibolite facies paragneisses, phlogopite diopsidites, concordant granites and granulites. North of Ranotsara the central-northern segment is traversed by a north-trending axial 100–150 km wide dextral shear zone of probable Pan-African age, which was metamorphosed under granulite and high amphibolite facies conditions and which has reworked older basement. This shear zone continues across southern India as the Palghat-Cauvery shear zone. Major stratiform basic -ultrabasic complexes occur in the axial zone and in the basement to the west. Well preserved low grade continental margin-type sediments (quartzites, mica schists and stromatolitic marbles) of Kibaran age are present in western Madagascar. Two partly greenschist grade sedimentary groups lie unconformably on high grade basement in north-east Madagascar. Isotopic age data suggest the presence in Madagascar of Archaean, Early and Mid-Proterozoic crustal material that was extensively reworked in Pan-African times.  相似文献   

3.
韩雨  牛漫兰 《岩石学报》2016,32(4):1049-1066
郯庐断裂带肥东段位于大别造山带与苏鲁造山带之间。在肥东段西韦采石场内发育了大规模的北北东向左行走滑韧性剪切带和一条低角度的韧性滑脱正断层。走滑韧性剪切带为郯庐断裂带走滑活动的产物,低角度滑脱正断层则代表了伸展背景下的构造活动。低角度滑脱正断层上、下盘发育未变形和变形的岩脉,走滑韧性剪切带内外也发育有受剪切带活动而变形的岩脉。对采石场内岩脉的构造和同位素年代学研究表明,低角度韧性滑脱正断层在129~126Ma之间发生过剪切活动,走滑韧性剪切带的活动时间在125Ma之后。综合研究认为,郯庐断裂带的伸展活动可能开始于早白垩世早期(130Ma),但在早白垩世并非一直处于伸展活动之中,125Ma之后的左行走滑活动很可能发生在早白垩世的晚期。  相似文献   

4.
The Teisseyre-Tornquist Zone that separates the East European Craton from the Palaeozoic Platform forms one of the most fundamental lithospheric boundaries in Europe. Devonian to Cretaceous-Paleogene evolution of the SE segment of this zone was analyzed using high-quality seismic reflection data that provided detailed information regarding entire Palaeozoic and Mesozoic sedimentary cover, with particular focus on problems of Late Carboniferous and Late Cretaceous-Paleogene basin inversion and uplift. Two previously proposed models of development and inversion of the Devonian-Carboniferous Lublin Basin seem to only partly explain configuration of this sedimentary basin. A new model includes Late Devonian-Early Carboniferous reverse faulting within the cratonic area NE from the Kock fault zone, possibly first far-field effect of the Variscan orogeny. This was followed by Late Carboniferous inversion of the Lublin Basin. Inversion tectonics was associated with strike-slip movements along the Ursynów-Kazimierz fault zone, and thrusting along the Kock fault zone possibly triggered by deeper strike-slip movements. Late Carboniferous inversion-related deformations along the NE boundary of the Lublin Basin were associated with some degree of ductile (quasi-diapiric) deformation facilitated by thick series of Silurian shales. During Mesozoic extension and development of the Mid-Polish Trough major fault zones within the Lublin Basin remained mostly inactive, and subsidence centre moved to the SW, towards the Nowe Miasto-Zawichost fault zone and further to the SW into the present-day Holy Cross Mts. area. Late Cretaceous-Paleogene inversion of the Mid-Polish Trough and formation of the Mid-Polish Swell was associated with reactivation of inherited deeper fault zones, and included also some strike-slip faulting. The study area provides well-documented example of the foreland plate within which repeated basin inversion related to compressive/transpressive deformations was triggered by active orogenic processes at the plate margin (i.e. Variscan or Carpathian orogeny) and involved important strike-slip reactivation of crustal scale inherited fault zones belonging to the Teisseyre-Tornquist Zone.  相似文献   

5.
The direction of convergence between the Rivera and North American plates becomes progressively more oblique (in a counter-clockwise sense as measured relative to the trench-normal direction) northwestward along the Jalisco subduction zone. By analogy to other subduction zones, the forces resulting from this distribution of convergence directions are expected to produce a NW moving, fore-arc sliver and a NW–SE stretching of the fore-arc area. Also, a series of roughly arc parallel strike-slip faults may form in the fore-arc area, both onshore and offshore, as is observed in the Aleutian arc.In the Jalisco subduction zone, the Jalisco block has been proposed to represent such a fore-arc sliver. However, this proposal has encountered one major problem. Namely, right-lateral strike-slip faulting within the fore-arc sliver, and between the fore-arc sliver and the North American plate, should be observed. However, evidence for the expected right-lateral strike-slip faulting is sparse. Some evidence for right-lateral strike-slip faulting along the Jalisco block–North American plate boundary (the Tepic–Zacoalco rift system) has been reported, although some disagreement exists. Right-lateral strike-slip faulting has also been reported within the interior of the Jalisco block and in the southern Colima rift, which forms the SE boundary of the Jalisco block.Threefold, multi-channel seismic reflection data were collected in the offshore area of the Jalisco subduction zone off Manzanillo in April 2002 during the FAMEX campaign of the N/O L'Atalante. These data provide additional evidence for recent strike-slip motion within the fore-arc region of the Jalisco subduction zone. This faulting offsets right-laterally a prominent horst block within the southern Colima rift, from which we conclude that the sense of motion along the faulting is dextral. These data also provide additional evidence for recent subsidence within the area offshore of Manzanillo, as has been proposed.  相似文献   

6.
 Basins within the African sector of Gondwana contain a Late Palaeozoic to Early Mesozoic Gondwana sequence unconformably overlying Precambrian basement in the interior and mid-Palaeozoic strata along the palaeo-Pacific margin. Small sea-board Pacific basins form an exception in having a Carboniferous to Early Permian fill overlying Devonian metasediments and intrusives. The Late Palaeozoic geographic and tectonic changes in the region followed four well-defined consecutive events which can also be traced outside the study area. During the Late Devonian to Early Carboniferous period (up to 330 Ma) accretion of microplates along the Patagonian margin of Gondwana resulted in the evolution of the Pacific basins. Thermal uplift of the Gondwana crust and extensive erosion causing a break in the stratigraphic record characterised the period between 300 and 330 Ma. At the end of this period the Gondwana Ice Sheet was well established over the uplands. The period 260–300 Ma evidenced the release of the Gondwana heat and thermal subsidence caused widespread basin formation. Late Carboniferous transpressive strike-slip basins (e.g. Sierra Australes/Colorado, Karoo-Falklands, Ellsworth-Central Transantarctic Mountains) in which thick glacial deposits accumulated, formed inboard of the palaeo-Pacific margin. In the continental interior the formation of Zambesi-type rift and extensional strike-slip basins were controlled by large mega-shear systems, whereas rare intracratonic thermal subsidence basins formed locally. In the Late Permian the tectonic regime changed to compressional largely due to northwest-directed subduction along the palaeo-Pacific margin. The orogenic cycle between 240 and 260 Ma resulted in the formation of the Gondwana fold belt and overall north–south crustal shortening with strike-slip motions and regional uplift within the interior. The Gondwana fold belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dextral transpressive motions. Associated with the orogenic cycle was the formation of mega-shear systems one of which (Falklands-East Africa-Tethys shear) split the supercontinent in the Permo-Triassic into a West and an East Gondwana. By a slight clockwise rotation of East Gondwana a supradetachment basin formed along the Tethyan margin and northward displacement of Madagascar, West Falkland and the Gondwana fold belt occurred relative to a southward motion of Africa. Received: 2 October 1995 / Accepted: 28 May 1996  相似文献   

7.
We summarize here the occurrence of a number of semiprecious stones within a major gemstone belt in the Palghat-Cauvery shear zone system close to the northern margin of the Madurai Granulite Block, southern India. The gem mineralization in this belt includes different varieties of corundum (star ruby, sapphire), cordierite (iolite), feldspar (moonstones and sunstone of various hues), beryl (emerald, aquamarine), chrysoberyl (alexandrite), kornerupine, topaz, spinel, crystal quartz and amethyst, among others. Gem mineralisation has not been directly dated, but is associated with pegmatites that crosscut rocks metamorphosed in late Neoproterozoic/Early Cambrian times. Similar aged gem mineralization occurs south of the Achankovil Shear Zone in southern India, as well as in Sri Lanka and Madagascar and strengthen the view of a broad late Neoproterozoic-early Cambrian gemstone province in central Gondwana. The ruby mineralization within this belt can be correlated with similar occurrences in eastern Madagascar, supporting a correlation between the Malagasy Betsimisaraka suture zone and the Palghat-Cauvery shear zone system in southern India.  相似文献   

8.
The unlined Bedretto tunnel in the Central Swiss Alps has been used to investigate in detail the fault architecture and late Alpine brittle faulting processes in the Rotondo granite on macroscopic and microscopic scales. Brittle faults in the late Variscan Rotondo granite preferentially are situated within the extent of preexisting ductile shear zones. Only in relatively few cases the damage zone extends into or develops in the previously undeformed granite. Slickensides suggest a predominant (dextral) strike-slip movement along these steeply dipping and NE–SW-striking faults. Microstructures of these fault rocks illustrate a multi-stage retrograde deformation history from ductile to brittle conditions up to the cessation of fault activity. In addition these fabrics allow identifying cataclastic flow, fluid-assisted brecciation and chemical corrosive wear as important deformation mechanisms during this retrogressive deformation path. Based on the analysis of zeolite microfabrics (laumontite and stilbite; hydrated Ca–Al- and Na–Ca–Al–silicate, respectively) in fault breccias, cataclasites and open fractures we conclude, that the main phase of active brittle faulting started below 280°C and ceased ca. 14 Ma ago at temperatures slightly above 200°C. This corresponds to a depth of approx. 7 km.  相似文献   

9.
The high-temperature metamorphism recorded in the Valuengo and Monesterio areas constitutes a rare occurrence in the Ossa-Morena Zone of Southwest Iberia, where low-grade metamorphism dominates. The metamorphism of the Valuengo area has been previously considered either Cadomian or Variscan in age, whereas that of Monesterio has been interpreted as a Cadomian imprint. However, these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution. The metamorphism of the Valuengo and Monesterio areas affects Late Proterozoic and Early Cambrian rocks, and is syn-kinematic with a top-to-the-north mylonitic foliation, which was subsequently deformed by early Variscan folds and thrusts. The U–Pb zircon age (480±7 Ma) we have obtained for an undeformed granite of the Valuengo area is consistent with our geological observations constraining the age of the metamorphism. We propose that this high-temperature metamorphic imprint along a NW–SE ductile extensional shear zone is related to the crustal extension that occurred in the Ossa-Morena Zone during the Cambro-Ordovician rifting. In the same way, the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one.  相似文献   

10.
Five stages of faulting were observed in and around the Stephanian Decazeville basin, in the SW French Massif Central, at the southern edge of the Sillon houiller fault. The older stage ends during middle Stephanian time, and corresponds to a strike-slip regime with N–S shortening and E–W extension. Before the end of the middle Stephanian, three other stages were recorded: two strike-slip regimes with NW–SE, then E–W compression and NE–SW, then N–S extension; and finally a NNE–SSW extensional regime during the main subsidence of the basin from the end of the middle Stephanian to late Stephanian. Based on mining documents, a new interpretation of the N–S striking folds of the Decazeville basin is proposed. Folding may not be associated with E–W compression but with diapirism of coal seams along syn-sedimentary normal faults during the extensional phase. A last strike-slip regime with N–S compression and E–W extension may be related to Cainozoic Pyrenean orogeny. At a regional scale, it is suggested that from the end of the middle Stephanian to the late Stephanian, the main faults in the Decazeville basin may represent a horsetail splay structure at the southern termination of the Sillon houiller fault.  相似文献   

11.
《地学前缘(英文版)》2019,10(6):2007-2019
Madagascar,a major fragment of Gondwana,is mainly composed of Precambrian basenent rocks formed by Mesoarchean to Neoproterozoic tectono-thernial events and recording a Pan-African metamorphic overprint.The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent.Here we present detailed petrology,mineral chemistry,metamorphic P-T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone.Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism.The peak mineral assemblage is interpreted as garnet+sillimanite+cordierite+quartz+plagioclase+Kfeldspar+magnetite+spinel+ilmenite,which is overprinted by a retrograde mineral assemblage of biotite+garnet+cordierite+quartz+plagioclase+K-feldspar+magnetite+spinel+ilmenite.Phase equilibria nodelling in the system Na_2 O-CaO-K_2 O-FeO-MgO-Al_2 O_3-SiO_2-H_2 O-TiO_2-Fe_2 O_3(NCKFMASHTO) indicates peak metamorphic conditions of 850-960 C and 6.9-77 kbar,and retrograde P-Tconditions of 740 C and 4.8 kbar,that define a clockwise P-T path.Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma,indicating dominantly Paleoproterozoic sources.The lower intercept age of 514±33 Ma probably indicates the timing of high-grade metamorphism,which coincides with the assembly of the Gondwana supercontinent.The comparable rock types,zircon ages and metamorphic P-T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.  相似文献   

12.
郯庐断裂带肥东段早白垩世中期走滑运动的年代学证据   总被引：3，自引：0，他引：3  
郯庐断裂带肥东段西韦地区和桃花源地区出露了大规模的北北东向韧性剪切带。桃花源地区韧性剪切带显示出2期构造变形的叠加。野外构造和显微构造分析皆指示为左行走滑韧性剪切带。糜棱岩中石英与长石的变形行为指示其变形温度分别为400~450℃和500℃。通过对这两处走滑剪切带内糜棱岩化花岗岩脉的锆石LA-ICP-MS定年,获得了3个样品的侵位年龄分别为(133.2±1.9)Ma,(131.3±2.0)Ma,(130.3±2.0)Ma。再结合已有的研究成果,认为在128~124Ma(早白垩世中期)郯庐断裂带发生过左行走滑活动。综合分析表明,郯庐断裂带在晚侏罗世和早白垩世中期分别经历了2期左行走滑活动,而期间和之后的早白垩世则处于伸展活动之中。伸展活动持续较长,控制发育了西侧的合肥盆地及断裂带内一系列岩浆活动;而区域挤压背景下出现的走滑活动则相对短暂。这些演化规律显示该断裂带在晚侏罗世—早白垩世呈现为交替式的走滑和伸展活动。新发现的早白垩世中期走滑活动,与太平洋区伊泽纳崎板块板块运动方向的调整相对应,是大洋板块运动方向短暂调整的构造响应。  相似文献   

13.
Abstract

Variscan convergence produced two-sided (bivergent) crustal-scale thrusting in the Vosges Mountains. In the northern Vosges the central polymetamorphic crystallines were thrust to the NW over Cambrian to Silurian low-grade and very low-grade metamorphic clastics. Synorogenic upper Devonian - lower Carboniferous turbidites and volcanics were folded into NW-vergent structures which display SE-dipping slaty cleavage. The entire sequence shows increasing metamorphism and deformation from NW to SE. Late right-lateral strike-slip faulting along the Lalaye-Lubine fault zone outlasted thrusting. In the southern Vosges a lower Carboniferous turbiditic basin that was fringed on the south by a volcanic arc was tectonically shortened by south-directed tectonic imbrication of slivers of varied rocks including ultramafics, gneissic basement, and synorogenic elastics. The increasing degree of deformation and metamorphism towards the north suggests a thrust contact with the polymetamorphic gneisses of the central Vosges. The final stages of Variscan convergence were accompanied by voluminous granitic plutonism and by faulting along NNE-SSW and E-W-trending strike-slip faults. The tectonic evolution reflects progressive Variscan closure of a previously extended basinal crust in a high-temperature regime.  相似文献   

14.
An integrated study of fission-track (FT) dating and structural geology revealed a complex tectono-thermal history preserved in basement rocks of central Madagascar since the amalgamation of Gondwana at the end of the Cambrian. A detailed study of five domains argues for several cooling steps with associated brittle deformations during the separation of Madagascar.Titanite and apatite FT ages range between 483 Ma and 266 Ma and between 460 Ma and 79 Ma, respectively. The titanite FT data indicate that the final cooling after the latest metamorphic overprint was terminated at c. 500 Ma (FC1). A 150 Myr phase of minor cooling (SC2), possibly related to a phase of tectonic quiescence and isostatic compensation, followed episode FC1. Between the Carboniferous and Early Jurassic, when an intracontinental rift developed between East Africa and Madagascar, complex brittle deformation effected the western margin of Madagascar and led to differential cooling of small basement blocks (FC3–FC5). During this period, ductile structural trends were reactivated at the western basement margin and in the centre of the island.A Late Cretaceous thermal event (T1) affected apatite FT data of samples from western–central and the eastern margin of Madagascar. These ages are related to the Madagascar–India/Seychelles break-up, whereby the thermal penetration along the eastern coast was restricted to the west by the Angavo shear zone (AGSZ). The Cretaceous evolution of the eastern margin was associated with minor erosion and was triggered by vertical displacements along brittle structures.  相似文献   

15.
The Tauern Window exposes a Paleogene nappe stack consisting of highly metamorphosed oceanic (Alpine Tethys) and continental (distal European margin) thrust sheets. In the eastern part of this window, this nappe stack (Eastern Tauern Subdome, ETD) is bounded by a Neogene system of shear (the Katschberg Shear Zone System, KSZS) that accommodated orogen-parallel stretching, orogen-normal shortening, and exhumation with respect to the structurally overlying Austroalpine units (Adriatic margin). The KSZS comprises a ≤5-km-thick belt of retrograde mylonite, the central segment of which is a southeast-dipping, low-angle extensional shear zone with a brittle overprint (Katschberg Normal Fault, KNF). At the northern and southern ends of this central segment, the KSZS loses its brittle overprint and swings around both corners of the ETD to become subvertical, dextral, and sinistral strike-slip faults. The latter represent stretching faults whose displacements decrease westward to near zero. The kinematic continuity of top-east to top-southeast ductile shearing along the central, low-angle extensional part of the KSZS with strike-slip shearing along its steep ends, combined with maximum tectonic omission of nappes of the ETD in the footwall of the KNF, indicates that north–south shortening, orogen-parallel stretching, and normal faulting were coeval. Stratigraphic and radiometric ages constrain exhumation of the folded nappe complex in the footwall of the KSZS to have begun at 23–21 Ma, leading to rapid cooling between 21 and 16 Ma. This exhumation involved a combination of tectonic unroofing by extensional shearing, upright folding, and erosional denudation. The contribution of tectonic unroofing is greatest along the central segment of the KSZS and decreases westward to the central part of the Tauern Window. The KSZS formed in response to the indentation of wedge-shaped blocks of semi-rigid Austroalpine basement located in front of the South-Alpine indenter that was part of the Adriatic microplate. Northward motion of this indenter along the sinistral Giudicarie Belt offsets the Periadriatic Fault and triggered rapid exhumation of orogenic crust within the entire Tauern Window. Exhumation involved strike-slip and normal faulting that accommodated about 100 km of orogen-parallel extension and was contemporaneous with about 30 km of orogen-perpendicular, north–south shortening of the ETD. Extension of the Pannonian Basin related to roll-back subduction in the Carpathians began at 20 Ma, but did not affect the Eastern Alps before about 17 Ma. The effect of this extension was to reduce the lateral resistance to eastward crustal flow away from the zone of greatest thickening in the Tauern Window area. Therefore, we propose that roll-back subduction temporarily enhanced rather than triggered exhumation and orogen-parallel motion in the Eastern Alps. Lateral extrusion and orogen-parallel extension in the Eastern Alps have continued from 12 to 10 Ma to the present and are driven by northward push of Adria.  相似文献   

16.
The ENE–WSW Autun Shear Zone in the northeastern part of the French Massif Central has been interpreted previously as a dextral wrench fault. New field observations and microstructural analyses document a NE–SW stretching lineation that indicates normal dextral motions along this shear zone. Further east, similar structures are observed along the La Serre Shear Zone. In both areas, a strain gradient from leucogranites with a weak preferred orientation to highly sheared mylonites supports a continuous Autun–La Serre fault system. Microstructural observations, and shape and lattice-preferred orientation document high-temperature deformation and magmatic fabrics in the Autun and La Serre granites, whereas low- to intermediate-temperature fabrics characterize the mylonitic granite. Electron microprobe monazite geochronology of the Autun and La Serre granites yields a ca. 320 Ma age for pluton emplacement, while mica 40Ar-39Ar datings of the Autun granite yield plateau ages from 305 to 300 Ma. The ca. 300 Ma 40Ar-39Ar ages, obtained on micas from Autun and La Serre mylonites, indicate the time of the mylonitization. The ca. 15-Ma time gap between pluton emplacement and deformation along the Autun–La Serre fault system argue against a synkinematic pluton emplacement during late orogenic to postorogenic extension of the Variscan Belt. A ductile to brittle continuum of deformation is observed along the shear zone, with Lower Permian brittle faults controlling the development of sedimentary basins. These results suggest a two-stage Late Carboniferous extension in the northeastern French Massif Central, with regional crustal melting and emplacement of the Autun and La Serre leucogranites around 320 Ma, followed, at 305–295 Ma, by ductile shearing, normal brittle faulting, and subsequent exhumation along the Autun–La Serre transtensional fault system.  相似文献   

17.
The present paper correlates the southern Madgascar terrain, south of the Ranotsara shear with the granulite terrain of southern India, occurring south of the Palghat-Cauvery (P-C) shear zone. Both the terrains have witnessed high temperature to ultra high temperature granulite metamorphism at 550 Ma and are traversed by shear zones and deep crustal faults. The 550 Ma old granulite terrains of Madagascar and southern India have similar lithologies, in particular, sapphirine bearing pelitic assemblages. Graphite deposits and gem occurrences are common to both these terrains. The 550 Ma old southern granulite terrain of southern India comprises of different blocks, the Madurai and the Kerala Khondalite belt, but all the blocks have similar lithologies with pelite—calc silicate rocks inter-banded with two pyroxene granulite bodies. These lithologies occur amidst an essentially charnockitic terrain. The protolith ages of the southern granulite terrain, south of the P-C shear zone ranges between 2400–2100 Ma. The terrain as a whole has witnessed the 550 Ma old granulite event. The granulite metamorphism took place under temperatures of 800–1000°C and at pressures of 9.5 to 5 Kbar.The source of heat for the high temperature granulite event of the southern Madagascar terrain has been linked to advective heat transfer along mantle deep faults. The source for the high temperature granulite metamorphism for the southern granulite terrain may be attributed to high temperature carbonatite and alkaline intrusives in an extensional setting which followed an initial crustal thickening.Many workers have linked Madagascar to southern India by connecting the Ranotsara shear either to the P-C shear zone or to the Achankovil shear zone, further south. The important factor is the lithologies of the Madagascar terrain, south of Ranotsara shear zone and the 550 Ma. old southern Indian granulite terrain are similar in many aspects. It will be more appropriate to link the Ranotsara shear to the curvilinear lineament bounding the Anaimalai-Kodaikanal ranges and which merges with the southern margin of the P-C shear zone.However, north of the Ranotsara shear/fault, the northern Madagascar terrain comprises of a dominant Itremo sequence (< 1850 Ma) and 780 Ma old calc-alkaline intrusives. The latter have similarities with that of Aravallis and the Sirohi, Malani sequences occurring further north east. The Rajasthan terrain has witnessed igneous intrusive activity at 1000–800 Ma. If we can broaden the area of investigations and include the above areas, the Madagascar-India connection can be better understood.  相似文献   

18.
The Karakoram Shear Zone is a northwest-southeast trending dextral ductile shear zone, which has affected the granitic and granodioritic bodies of the southern Asian Plate margin in three distinct episodes. The ductile shearing of the granitic bodies at Tangste and Darbuk has resulted in the development of mylonites with mylonitic foliation and stretching lineation. More intense deformation is noted in the Tangste granite grading up to orthomylonite, as compared to the Darbuk granite. Kinematic indicators include S-C foliation, synthetic C′ and C″ antithetic shear bands, Type A s-mantled porphyroclasts, oblique quartz foliation, micro-shears with bookshelf gliding, mineral fishes including Group 2 mica fishes, and Type 1 and 2a pull-apart microstructures, and exhibit strong dextral sense of ductile shearing towards southeast. The textural features of the minerals, especially that of quartz and feldspar, indicate temperature of mylonitisation ranging between 300 and 500°C in the upper greenschist facies, and appear to have been evolved during exhumation as a consequence of oblique strike-slip movements along the Karakoram shear zone.  相似文献   

19.
We describe an active right-lateral strike-slip fault zone along the southern margin of the Japan Sea, named the Southern Japan Sea Fault Zone (SJSFZ). Onshore segments of the fault zone are delineated on the basis of aerial photograph interpretations and field observations of tectonic geomorphic features, whereas the offshore parts are interpreted from single-/multichannel seismic data combined with borehole information. In an effort to evaluate late Quaternary activity along the fault zone, four active segments separated by uplifting structures are identified in this study. The east–northeast-trending SJSFZ constitutes paired arc-parallel strike-slip faults together with the Median Tectonic Line (MTL), both of which have been activated by oblique subduction of the Philippine Sea plate during the Quaternary. They act as the boundaries of three neotectonic stress domains around the eastern margin of the Eurasian plate: the near-trench Outer zone and NW–SE compressive Inner zone of southwest Japan arc, and the southern Japan Sea deformed under E–W compression from south to north.  相似文献   

20.
The Antucoya porphyry copper deposit (300 Mt at 0.45% total Cu) is one of the largest deposits of a poorly known Early Cretaceous porphyry belt in the Coastal Cordillera of northern Chile. It is related to a succession of granodioritic and tonalitic porphyritic stocks and dikes that were emplaced within Jurassic andesitic rocks of the La Negra Formation immediately west of the N–S trending sinistral strike-slip Atacama Fault Zone. New zircon SHRIMP U–Pb data indicate that the porphyries of Antucoya crystallized within the time span from 142.7 ± 1.6 to 140.6 ± 1.5 Ma (±2 σ), and late, unmineralized, NW–SE trending dacite dikes with potassic alteration and internal deformation crystallized at 141.9 ± 1.4 Ma. The Antucoya porphyry copper system appears to be formed after a change of stress conditions along the magmatic arc from extensional in the Late Jurassic to transpressive during the Early Cretaceous and provides support for an Early Cretaceous metallogenic episode of porphyry-type mineralization along the Coastal Cordillera of northern Chile.  相似文献   

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