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1.
The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation.  相似文献   

2.
《Gondwana Research》2006,9(4):457-471
The Arabian-Nubian-Shield (ANS) is composed of a number of island arcs together with fragments of oceanic lithospere and minor continental terranes. The terranes collided with each other until c. 600 Ma ago. Subsequently, they were accreted onto West Gondwana, west of the present River Nile. Apart from widespread ophiolite nappe emplacement, collisional deformation and related lithospheric thickening appear to be relatively weak. Early post-collisional structures comprise not only extensional features such as fault-bounded (molasse) basins and metamorphic core complexes, but also major wrench fault systems, and thrusts and folds. The Hammamat Group was deposited in fault-bounded basins, which formed due to N-S to NW-SE directed extension. Hammamat Group sediments were intruded by late orogenic granites, dated as c. 595 Ma old. A NNW-SSE-oriented compression prevailed after the deposition of the Hammamat Sediments. This is documented by the presence of NW-verging folds and SE-dipping thrusts that were refolded and thrusted in the same direction. Restoration of a NNW-SSE- oriented balanced section across Wadi Queih indicates more than 25% of shortening. Transpressional wrenching related to the Najd Fault System followed this stage. The wrenching produced NW-SE sinistral faults associated with positive flower structures that comprise NE-verging folds and SW-dipping thrusts. Section restoration across these late structures indicates 15 17% shortening in the NE-SW direction. At a regional scale, the two post-Hammamat compressional phases produced an interference pattern with domes and basins. It can be shown that the Najd Fault System splays into a horsetail structure in the Wadi Queih area and loses displacement towards N and NW. The present study shows a distinct space and time relationship between deposition of Hammamat Group/late-Pan-African clastic sediments and late stages of Najd Fault wrench faulting: Hammamat deposition is followed by two episodes of compression, with the second episode being related to Najd Fault transpression. Therefore, the Hammamat sediments do not represent the latest tectonic feature of the Pan-African orogeny in the ANS. The latest orogenic episodes were the two successive phases of compression and transpression, respectively. It is speculated that extension during (Hammamat) basin formation was sufficiently effective to reduce the thickness of the orogenic lithosphere until it became gravitationally stable, and incapable of further gravitational deformation.  相似文献   

3.
The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.  相似文献   

4.
Summary  The Permo-Triassic Cape Fold Belt around the southern tip of Africa consists of a thick sequence of Palaezoic siliciclastic sedimentary and pre-Cape basement rocks believed to be of Pan-African age. Both the basement rocks and the supracrustal rocks of the Cape Supergroup display only low metamorphic grades. Application of chlorite, chlorite-chloritoid Fe-Mg exchange, and calcite-graphite carbon isotope geothermometry to rocks from the unconformable contact between pre-Cape basement and the Cape Supergroup made it possible to distinguish pre-Cape and syn-Cape metamorphic overprints. During Pan-African metamorphism temperatures of up to middle greenschist facies conditions (around 400 °C) were reached, whereas lowermost greenschist facies conditions (around 300 °C) were not exceeded during the 220–290 Ma Cape orogeny. In the past, most if not all of the pre-Cape basement rocks, which form the Pan-African Saldania Belt, were considered to be of Neoproterozoic age. A hiatus of about 100 °C observed between two adjacent limestone horizons that previously had been grouped together into a single formation at the bottom of the allegedly Neoproterozoic Kango Group indicates that almost all of this group is syn- to post-orogenic with respect to the Pan-African orogeny. A revision of the stratigraphy of the Kango Group is therefore suggested. Only its lowermost member is truly Pan-African and probably related to about 620–740 Ma post-Sturtian cap carbonates in other Pan-African belts of southern Africa. The remainder of the Kango Group reflects the successive development of two stages of orogen-related intra-continental basins: The older stage led to a typical syn-orogenic foreland basin related to tectonic loading in the Gariep and Damara orogenic belts further north(west) between 570 and 540 Ma; the younger is believed to have formed either a further foreland basin or an intra-orogen pull-apart basin caused by later tectonic loading in the Ross orogenic belt and its continuation into the southern Saldania Belt between 510 and 480 Ma. Received May 7, 2000;/revised version accepted January 15, 2001  相似文献   

5.
Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.  相似文献   

6.
Paleogene sediments in fault-bounded basins on Judge Daly Promontory, northeast Ellesmere Island, Canadian High Arctic, are rich in volcanogenic material. Volcanic pebbles within the Cape Back basin near Nares Strait were studied for their petrography, geochemistry, Sr and Nd isotopes, and geochronology to identify and characterize their parent rock. The pebbles are derived from lava flows and ignimbrites of a continental rift-related, strongly differentiated, highly incompatible element enriched, alkaline volcanic suite, the proposed Nares Strait volcanic suite, which is distinct from other alkaline volcanic suites on the northern coasts of Ellesmere Island and Greenland. 40Ar/39Ar amphibole and alkali feldspar ages indicate that volcanism was active around 61–58 Ma and was probably contemporaneous with sedimentation resulting in Middle to Late Paleocene age for deposition within the Cape Back basin and the other Paleogene basins on Judge Daly Promontory.  相似文献   

7.
40 Ar/39Ar–mica and 207Pb/206Pb–zircon dates are presented and combined with existing P–T data and the sedimentary record. These data indicate that the RCC was faulted into three segments which underwent different exhumation histories during the Late Carboniferous/Early Permian. The eastern segment shows 40Ar/39Ar–biotite data of336 ±4 and 323±3 Ma. Furthermore, it is intruded by the Thuringian Hauptgranite dated at 337±4 Ma by the 207Pb/206Pb single zircon method. At approximately 300 Ma rocks of the eastern segment were finally exposed and, subsequently, subsided as part of the Oberhof pull-apart basin, filled by Late Carboniferous/Early Permian molasse sediments and volcanic rocks (296–285 Ma; Goll 1996). A similar Late Carboniferous evolution is inferred for the western segment, since it is also overlain by Upper Carboniferous volcanic rocks. In contrast to the eastern and western segments, distinctly younger intrusion and cooling ages were recorded for the central segment of the RCC (40Ar/39Ar muscovite: 311±3 Ma; 40Ar/39Ar biotite: 293–288±3 Ma) that was intruded by the Trusetal Granite, the Ruhla Granite and Brotterode Diorite (207Pb/206Pb single zircon: 298±2, 295±3, 289±4 Ma, respectively). These young data are unique in the MGCR and testify that plutonic activity and cooling of basement rocks took place simultaneously with basin formation and volcanism in the eastern and western segments. Overlying Upper Permian (Zechstein) and Triassic sediments indicate final exposure of the central segment by approximately 260 Ma, as a part of the Ruhla-Schleusingen Horst. Combination of these results with P–T data from the contact aureole of the Trusetal granite indicate that the central segment was unroofed by at least 8.5 km during the Late Carboniferous. The Late Carboniferous/Early Permian horst-basin formation, documented in the RCC, is due to dextral transtensional movements along the NW-trending Franconian fault system. It may have been enhanced by mantle upwelling widespread in Central Europe during the Early Permian that also caused intensive magmatism in the Thuringian Forest region. Received: 2 February 1999 / Accepted: 15 November 1999  相似文献   

8.
The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, São Francisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of several stratigraphic units, which have been deformed and metamorphosed along the western margin of the São Francisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenile magmatic arc is exposed (the Goiás Magmatic Arc), consisting of calc-alkaline plutonic suites as well as volcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, west-dipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, and clearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of the sedimentary units comprising the Brasília Belt have been a matter of continuous debate over the last three decades. In the present paper, recent provenance data based on LA-ICPMS U–Pb ages of detrital zircon grains from several of these units, are reviewed and their significance for the age of deposition of the original sediments and tectonic evolution of the Brasília Belt are discussed.The Paranoá, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircon grains with ages older than ca. 900 Ma and are interpreted as representative of the passive margin sequence deposited on the western margin of the São Francisco Craton. On the other hand, samples from the Araxá and Ibiá groups have a much younger population of Neoproterozoic zircon grains, as young as 650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambuí Group, exposed in the easternmost part of the belt and covering large areas of the São Francisco Craton also has young zircon grains and is interpreted, at least in part, as the foreland basin of the Brasília Belt.  相似文献   

9.
Summary The Cretaceous Eclogite-Gneiss unit and its tectonic overburden (Micaschist, Phyllite and Lower Magdalensberg units) and the underlying Preims subunit of the Saualpe, Eastern Alps, have been investigated in order to constrain the mode of exhumation of the type locality of eclogites. 40Ar/39Ar ages of white mica from the eclogite-bearing unit suggest rapid, uniform cooling and exhumation between 86 and 78 Ma (Santonian-Campanian). Overlying units show upwards increasingly older ages with an age of 261.7 ± 1.4 Ma in the uppermost, low-grade metamorphic unit (Lower Magdalensberg unit). We consider this Permian age as geologically significant and to record a Permian tectonic event. Rocks of phyllite and micaschist units along western margins of the Saualpe block yield amphibole and white mica ages ranging from 123 to 130 Ma. These are considered to closely date the age of nappe stacking, whereas a single biotite age of 66–68 Ma from a shear zone is interpreted to date retrogression during normal faulting. Biotite and amphibole of Micaschist and Eclogite-Gneiss units show variable contents of extraneous argon. Consequently, their ages are in part geologically meaningless whereas other samples yield meaningful ages. The white mica ages from the Eclogite-Gneiss unit range from 78 to 85 Ma and argue for cooling through ca. 400 °C during the time as the westerly adjacent Upper Cretaceous Krappfeld collapse basin formed. The Preims subunit with paragneiss and marbles is considered to represent a large synmetamorphic shear zone at the base of the overthrusting Eclogite-Gneiss unit. The unit comprises a flat-lying foliation and a SE-trending lineation. This zone is interpreted to represent a zone of top-NW thrusting. A major ductile low-angle normal fault with top to ESE shear has been detected between the Eclogite-Gneiss and overlying units, and between the Micaschist and Phyllite units. The ductile thrust at the base and the low-angle normal fault at the top are considered to confine a NW-ward extruding high-pressure wedge. The new observations argue for rapid exhumation of a subducted high-pressure wedge within a subduction channel. Rapid surface erosion of the exhuming wedge might have facilitated exhumation. Eroded sedimentary rocks are preserved within adjacent Gosau basins, although only pebbles of low-grade metamorphic rocks of the uppermost tectonic unit can be found in these basins.  相似文献   

10.
 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   

11.
 In the Neoarchaean intracratonic basin of the Kaapvaal craton, between approximately 2640 Ma and 2516 Ma, two successive stromatolitic carbonate platforms developed. Deposition started with the Schmidtsdrif Subgroup, which is probably oldest in the southwestern part of the basin, and which contains stromatolitic carbonates, siliciclastic sediments and minor lava flows. Subsequently, the Nauga formation carbonates were deposited on peritidal flats located to the southwest and were drowned during a transgression of the Transvaal Supergroup epeiric sea, around 2550 Ma ago. This transgression led to the development of a carbonate platform in the areas of the preserved Transvaal and Griqualand West basins, which persisted for 30–50 Ma. During this time, shales were deposited over the Nauga Formation carbonates in the southwestern portion of the epeiric sea. A subsequent period of basin subsidence led to drowning of the stromatolitic platform and to sedimentation of chemical, iron-rich silica precipitates of the banded iron formations (BIF) over the entire basin. Carbonate precipitation in the Archaean was largely due to chemical and lesser biogenic processes, with stromatolites and ocean water composition playing an important role. The stromatolitic carbonates in the preserved Griqualand West and Transvaal basins are subdivided into several formations, based on the depositional facies, reflected by stromatolite morphology, and on intraformational unconformities; interbedded tuffs and available radiometric age data do not yet permit detailed correlation of units from the two basins. Thorough dolomitisation of most formations took place at different post-depositional stages, but mainly during early diagenesis. Partial silicification was the result of diagenetic and weathering processes. Karstification of the carbonate rocks was related to periods of exposure to subaerial conditions and to percolation of groundwater. Such periods occurred locally at the time of carbonate and BIF deposition. Main karstification, however, probably took place during an erosional period between approximately 2430 Ma and 2320 Ma. Received: 15 September 1996 · Accepted: 12 May 1998  相似文献   

12.
Based on sedimentological, geochronological and geochemical investigations, a Paleozoic orogenic belt, called the Heihe-Dashizhai orogenic belt (HDOB), has been recognized, which consists of three tectonic units: Duobaoshan-Dashizhai arc belt, Wolihe back-arc basin and Sankuanggou-Jinshuishan molasse basin, representing a northwesternward subduction system of the Heihe-Nenjiang Ocean (HNO) between the Xing'an-Airgin Sum Block (XAB) and the Songliao-Hunshandake Block (SHB) in Great Xing'an area of the northeast China. The Duobaoshan-Dashizhai arc belt includes arc volcanic-sedimentary sequence and pluton belt composed by granodiorites, diorites and quartz diorites, which can be divided into the early (506–469 Ma) and late periods (463–426 Ma). Geochemical research indicates that the primary magma of the early and late period arc rocks was derived from the partial melting of depleted mantle to a relatively enriched lithospheric mantle related with thickened continental crust, and a depleted mantle wedge, respectively. The Wolihe back-arc basin is composed of basalt with pillow structure, gabbro, serpentinized ultramafic rocks and thin-bedded chert in lower part and turbidity with double direction provenance from both arc belt and older continent in upper part. The Sankuanggou-Jinshuishan molasse basin contains several cycles, revealing a transformation from flysch in lower part to marine molasse with rapid proximal accumulation in upper part, indicating a change from neritic to littoral sedimentary environments. The Early-Middle Paleozoic tectonic evolution of the HDOB can be divided into three stages: the early arc stage (506–469 Ma), the late arc stage (463–426 Ma) and molasse basin development (426 Ma to Early Devonian), representing the early and late subduction of the HNO and formation of the HDOB, respectively.  相似文献   

13.
Strongly deformed volcaniclastic metasediments and ophiolitic slices hosting the Sukari gold mineralization display evidence of a complex structural evolution involving three main ductile deformational events (D1–D3). D1 produced ENE-trending folds associated with NNW-propagating thrust slices and intrusion of the Sukari granite (689 ± 3 Ma). D2 formed a moderately to steeply dipping, NNW-trending S2 foliation curved to NE and developed arcuate structure constituting the Kurdeman shear zone (≤ 595 Ma) and East Sukari imbricate thrust belt. Major NE-trending F2 folds, NW-dipping high-angle thrusts, shallow and steeply plunging mineral lineation and shear indicators recorded both subhorizontal and subvertical transport direction during D2. D3 (560–540 Ma) formed NNE-trending S3 crenulation cleavage, tight F3 folds, Sukari Thrust and West Sukari imbricate thrust. The system of NW-trending sinistral Kurdeman shear zone (lateral ramps and tear faults) and imbricate thrusts (frontal ramps) forming the actuate structure developed during SE-directed thrusting, whereas the prevailing pattern of NNE-trending dextral Sukari shear zone and imbricate thrusts forming Sukari thrust duplex developed during NE-directed tectonic shearing. Sukari granite intruded in different pluses between 689 and 540 Ma and associated with at least four phases of quartz veins with different geometry and orientation. Structural analysis of the shear fabrics indicates that the geometry of the mineralized quartz veins and alteration patterns are controlled by the regional NNW- and NE-trending conjugate zones of transpression. Gold-bearing quartz veins are located within NNW-oriented sinistral shear zones in Kurdeman gold mine area, within steeply dipping NW- and SE dipping thrusts and NE- and NS-oriented dextral and sinistral shear zones around Sukari mine area, and along E-dipping backthrusts and NW-SE and N-S fractures in Sukari granite. The high grade of gold mineralization in Sukari is mainly controlled by SE-dipping back-thrusts branched from the major NW-dipping Sukari Thrust. The gold mineralization in Sukari gold mine and neighboring areas in the Central Eastern Desert of Egypt is mainly controlled by the conjugate shear zones of the Najd Fault System and related to E-W directed shortening associated with oblique convergence between East and West Gondwana.  相似文献   

14.
Propagation of faults and folds in the foreland basins of Tian Shan is an important process accommodating Cenozoic crustal shortening and mountain building, but little is known about the accurate time of the Cenozoic tectonic deformation. Based on growth strata and age determination, we show that syntectonic growth strata began to develop in the middle part of Tian Shan since 6 Ma ago. Geometry analysis indicates that formation of the growth strata is associated with progressive fold-limb rotation. Formation of the growth strata is contemporaneous with the tectonic deformation in the thrusting and folding zones. Together with the remarkable increase of sedimentation rate as well as the accumulation of coarse molasse deposits, we conclude that the late Cenozoic crustal shortening and mountain building in the region initiated since about 6 Ma and lasted to the early Pleistocene, as a consequence of intracontinental deformation within the India–Eurasia convergent system.  相似文献   

15.
The results of geological, structural, tectonic, and geoelectric studies of the dry basins in the Baikal Rift Zone and western Transbaikalia, combined under the term Baikal region, are integrated. Deformations of the Cenozoic sediments related to pulsing and creeping tectonic processes are classified. The efficiency of mapping of the fault-block structure of the territories overlapped by loose and poorly cemented sediments is shown. The faults mapped at the ground surface within the basins are correlated with the deep structure of the sedimentary fill and the surface of the crystalline basement, where they are expressed in warping and zones of low electric resistance. It is established that the kinematics of the faults actively developing in the Late Cenozoic testifies to the relatively stable regional stress field during the Late Pliocene and Quaternary over the entire Baikal region, where the NW-SE-trending extension was predominant. At the local level, the stress field of the uppermost Earth’s crust is mosaic and controlled by variable orientation of the principal stress axes with the prevalence of extension. The integrated tectonophysical model of the Mesozoic and Cenozoic rift basin is primarily characterized by the occurrence of mountain thresholds, asymmetric morphostructure, and block-fault structure of the sedimentary beds and upper part of the crystalline basement. The geological evolution of the Baikal region from the Jurassic to Recent is determined by alternation of long (20–115 Ma) epochs of extension and relatively short (5.3–3.0 Ma) stages of compression. The basins of the Baikal Rift System and western Transbaikalia are derivatives of the same geodynamic processes.  相似文献   

16.
Conventional and SHRIMP U-Pb analyses of zircon, monazite, titanite and apatite from the high grade rocks of the Northampton Complex in Western Australia provide constraints on the timing of metamorphic processes and deformation events in the northern Darling Mobile Belt (western margin of the Archean Yilgarn Craton). Paragneisses and mafic volcanics and/or intrusions have undergone granulite facies metamorphism in a probable extensional tectonic setting prior to formation of W- to NW-verging folds and thrusts cut by normal shears (interpreted as late collapse structures) during the main deformation event (D1). These structures are folded by open to tight folds with NW-striking axial surfaces developed in a second, NE-SW contractional event (D2). Zircons from a mafic granulite provide an age of 1079 ± 3 Ma attributed to new zircon growth prior to, or at the peak of regional granulite facies metamorphism. Metamorphic monazites extracted from a paragneiss yield an identical age of 1083 ± 3 Ma. The similarity of ages between zircons from the mafic granulite (1079 ± 3 Ma) and monazites from the paragneiss (1083 ± 3 Ma) is interpreted to reflect fast cooling and/or rapid uplift, which is consistent with thrusting of the gneissic units during the first deformation event (D1) associated with the onset of retrograde metamorphism. Granitic activity at 1068 ± 13 Ma was followed by intrusion of post-D2 pegmatite (989 ± 2 Ma), which constrains the end of metamorphism and associated deformation. Cooling of the complex to about 500 °C is timed by the apatite age of 921 ± 23 Ma. SHRIMP U-Pb ages of detrital zircons from a paragneiss sample yield a maximum age of 2043 Ma, with no evidence of an Archean Yilgarn signature. A majority of ages between 1.6 and 1.9 Ga are consistent with derivation from the Capricorn Orogen on the northern margin of the Yilgarn Craton. Younger detrital zircons with 1150–1450 Ma ages, however, indicate an additional source that had undergone early Grenvillian igneous or metamorphic event(s) and also places a maximum age constraint upon deposition. The source of this clastic material may have been from within the southern Darling Mobile Belt or from Greater India (adjacent to the Northampton Complex in Rodinia reconstructions). This study documents an extended Grenvillian history, with basin formation, sedimentation, granulite facies metamorphism, contractional tectonics (two periods with orthogonal directions of shortening) and late pegmatite emplacement taking place between 1150–989 Ma on the western margin of the Yilgarn Craton. Ages recorded in this study indicate that the proposed global distribution of Grenvillian belts during assembly of the Rodinia supercontinent should be reassessed to include the Darling Mobile Belt. Received: 7 January 1998 / Accepted: 10 March 1999  相似文献   

17.
The age and Precambrian history of the Moine Supergroup within the Caledonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granite formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its enclosed segregation pegmatites is 873 ± 7 Ma. This demonstrates the reality of a Neoproterozoic episode of high-grade metamorphism in the Glenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints on Neoproterozoic palaeogeographic reconstructions of the North Atlantic region, indicating that the Moine rocks cannot be used as a link between the Grenvillian belt of North America and the Sveconorwegian orogen in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obtained from detrital, inherited zircons and reflect the provenance of the Glenfinnan Group Moine sediments which must, therefore, have been deposited between c. 1100 and 870 Ma. Potential sources are found as relatively minor, tectonically bounded basement inliers within the British Caledonides, although more widespread source areas occur outside Britain in both Laurentia and Baltica. The most important feature of the provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement component of the western foreland to the Caledonides in Britain, was not a major contributor to the Glenfinnan Group basin. Received: 16 June 1996 / Accepted: 29 January 1997  相似文献   

18.
祁连山西段及酒西盆地区第四纪构造运动的阶段划分   总被引：25，自引：6，他引：25  
通过沉积地层、地貌、构造形变等的综合研究，对祁连山西段及酒西盆地区第四纪构造运动的期次和阶段进行了划分。上新世晚期以来，这一地区至少经历过６次显著的构造变动或构造事件，其中以玉门、酒泉和白杨河运动最为强烈。针对上述构造事件进行了古地磁、孢粉、红外释光和热释光等方法的综合研究和年龄测定，论述了各阶段构造运动的方式、性质和其它有关特征。  相似文献   

19.
The Aswa Shear Zone (ASZ) is a fundamental Precambrian lithospheric structure that has been shaped by many tectonic events in eastern Africa. It separates the Saharan Metacraton in the northeast from the Northern Uganda terrane (which represents part of the Northeastern Congo block of the Congo craton) to the southwest. Nonetheless, its tectonic evolution is not fully understood. We used high-resolution airborne magnetic and radiometric data over Uganda integrated with Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) in South Sudan to assess the extent, kinematics and contribute to the understanding of the tectonic origin of the ASZ. (1) Our results showed that the ASZ extends in a NW–SE direction for ~ 550 km in Uganda and South Sudan. (2) The airborne magnetic and radiometric data revealed a much wider (~ 50 km) deformation belt than the 5-10 km of the exposed surface expression of the ASZ. The deformation belt is defined by three NW-trending sinistral strike-slip shear zones bounding structural domains with magnetic fabric showing splays of secondary shear zones and shear-related folds. These folds are tighter close to the discrete shear zones with their axial traces becoming sub-parallel to the shear zones. A similar fold pattern is observed in South Sudan from the SRTM DEM. We interpreted these folds as due to ENE–WSW contraction associated with the sinistral strike-slip movement. (3) To the northeast, the magnetic patterns and radiometric signatures suggest the presence of a series of W-verging nappes indicative of strong E–W to NE–SW contraction deformation. (4) We relate the evolution of the ASZ to E–W to NE–SW Neoproterozoic oblique collision between East and West Gondwana. The deformation related to this collision was partitioned into E–W to NE–SW contraction resulting in W-verging thrusts in the east and a sinistral strike-slip movement along the NW-trending ASZ with the strain localized at the boundary between the Saharan Metacraton and the Northern Uganda terrane.  相似文献   

20.
吐哈盆地及邻区早二叠世沉积特征与构造发育的耦合关系   总被引：7，自引：0，他引：7  
通过对不同露头剖面和探井资料的分析,在吐哈盆地及邻区区分出三种不同类型沉积相：（1）裂谷型海相火山喷发沉积相;（2）裂谷型陆相火山喷发沉积相;（3）造山带磨拉石相;不同沉积相特征并结合构造分析认为,早二叠世在吐哈及邻区存在二种不同类型的盆地,即前陆盆地和裂谷盆地,同时,还探讨了大陆一大陆碰撞带上两种不同类型盆地的形成机理。  相似文献   

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