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1.
We present new,geological,metamorphic,geochemical and geochronological data on the East Anatolian-Lesser Caucasus ophiolites.These data are used in combination with a synthesis of previous data and numerical modelling to unravel the tectonic emplacement of ophiolites in this region.All these data allow the reconstruction of a large obducted ophiolite nappe,thrusted for>100 km and up to 250 km on the Anatolian-Armenian block.The ophiolite petrology shows three distinct magmatic series,highlighted by new isotopic and trace element data:(1)The main Early Jurassic Tholeiites(ophiolite s.s.)bear LILEenriched,subduction-modified,MORB chemical composition.Geology and petrology of the Tholeiite series substantiates a slow-spreading oceanic environment in a time spanning from the Late Triassic to the Middle-Late Jurassic.Serpentinites,gabbros and plagiogranites were exhumed by normal faults,and covered by radiolarites,while minor volumes of pillow-lava flows infilled the rift grabens.Tendency towards a subduction-modified geochemical signature suggests emplacement in a marginal basin above a subduction zone.(2)Late Early Cretaceous alkaline lavas conformably emplaced on top of the ophiolite.They have an OIB affinity.These lavas are featured by large pillow lavas interbedded a carbonate matrix.They show evidence for a large-scale OIB plume activity,which occurred prior to ophiolite obduction.(3)Early-Late Cretaceous calc-alkaline lavas and dykes.These magmatic rocks are found on top of the obducted nappe,above the post-obduction erosion level.This series shows similar Sr-Nd isotopic features as the Alkaline series,though having a clear supra-subduction affinity.They are thus interpreted to be the remelting product of a mantle previously contaminated by the OIB plume.Correlation of data from the Lesser Caucasus to western Anatolia shows a progression from back-arc to arc and fore-arc,which highlight a dissymmetry in the obducted oceanic lithosphere from East to West.The metamorphic P-T-t paths of the obduction sole lithologies define a southward propagation of the ophiolite:(1)P-T-t data from the northern Sevan-Akera suture zone(Armenia)highlight the presence and exhumation of eclogites(1.85±0.02 GPa and 590±5℃)and blueschists below the ophiolite,which are dated at ca.94 Ma by Ar-Ar on phengite.(2)Neighbouring Amasia(Armenia)garnet amphibolites indicate metamorphic peak conditions of 0.65±0.05 GPa and 600±20 C with a U-Pb on rutile age of 90.2±5.2 Ma and Ar-Ar on amphibole and phengite ages of 90.8±3.0 Ma and 90.8±1.2 Ma,respectively.These data are consistent with palaeontological dating of sediment deposits directly under(Cenomanian,i.e.>93.9 Ma)or sealing(Coniacian-Santonian,i.e.,≤89.8 Ma),the obduction.(3)At Hinis(NE Turkey)PT-t conditions on amphibolites(0.66±0.06 GPa and 660±20℃,with a U-Pb titanite age of80.0±3.2 Ma)agree with previous P-T-t data on granulites,and highlight a rapid exhumation below a top-to-the-North detachment sealed by the Early Maastrichtian unconformity(ca.70.6 Ma).Amphibolites are cross-cut by monzonites dated by U-Pb on titanite at 78.3±3.7 Ma.We propose that the HT-MP metamorphism was coeval with the monzonites,about 10 Ma after the obduction,and was triggered by the onset of subduction South of the Anatolides and by reactivation or acceleration of the subduction below the Pontides-Eurasian margin.Numerical modelling accounts for the obduction of an"old"~80 Myr oceanic lithosphere due to a significant heating of oceanic lithosphere through mantle upwelling,which increased the oceanic lithosphere buoyancy.The long-distance transport of a currently thin section of ophiolites(<1 km)onto the Anatolian continental margin is ascribed to a combination of northward mantle extensional thinning of the obducted oceanic lithosphere by the Hinis detachment at ca.80 Ma,and southward gravitational propagation of the ophiolite nappe onto its foreland basin.  相似文献   

2.
Costa Rica forms part of an intra-oceanic arc between the Pacific and Caribbean oceans; the Nicoya Ophiolite Complex is located along its Pacific border. In this study, evidence is given that the Nicoya Complex is composed of ridge-formed oceanic crust that suffered a strong compressional stress during Late Santonian times. As a result of this, isoclinal folding and large-scale nappe emplacement occurred at a shallow crustal depth. The principal component of this compressional stress was E-W-directed. It is also demonstrated that, from this time, the complex was situated between a subducting plate and a volcanic arc. From that Campanian until the Middle Eocene the zone was undulated, and generally at a great depth below sea level. During the Eocene—Oligocene epoch a new tectonic stress affected the area. It produced open folding with upthrusting in the ophiolite complex and overthrust folding of the overlying rock series. As a result of crustal thickening during this tectonic phase, the area was uplifted. From Miocene times, the zone was shaped into a dome and a synform. These undulations are attributed to compression of the subducting Coco Plate, west of the area.The Upper Santonian tectonic phase demonstrates how compressional stress produced the break-up of the Caribo-Pacific plate west of the study area, as a result of which, a Caribbean plate without an associated oceanic ridge and a Pacific plate originated. The compressional stress in question was presumably generated by the opposed spreading directions of the new Mid-Atlantic Ridge and an older ridge to the west of the study area.Furthermore, it is argued that the Cretaceous obduction of the ophiolite belt along the Pacific coast of the American continents, was produced by the directional change of these continents during the birth of the Mid-Atlantic Ridge. This created intra-plate compressional stress and converted originally passive continental margins into active zones, where thrusting of oceanic crust on to a continental margin (obduction) could occur. When the Mid-Atlantic Ridge started spreading, the obduction phase ended due to subduction of the oceanic plate below the leading edge of the continent.  相似文献   

3.
The Late Triassic and Jurassic platform and the oceanic complexes in Evvoia, Greece, share a complementary plate-tectonic evolution. Shallow marine carbonate deposition responded to changing rates of subsidence and uplift, whilst the adjacent ocean underwent spreading, and then convergence, collision and finally obduction over the platform complex. Late Triassic ocean spreading correlated with platform subsidence and the formation of a long-persisting peritidal passive-margin platform. Incipient drowning occurred from the Sinemurian to the late Middle Jurassic. This subsidence correlated with intra-oceanic subduction and plate convergence that led to supra-subduction calc-alkaline magmatism and the formation of a primitive volcanic arc. During the Middle Jurassic, plate collision caused arc uplift above the carbonate compensation depth (CCD) in the oceanic realm, and related thrust-faulting, on the platform, led to sub-aerial exposures. Patch-reefs developed there during the Late Oxfordian to Kimmeridgian. Advanced oceanic nappe-loading caused platform drowning below the CCD during the Tithonian, which is documented by intercalations of reefal turbidites with non-carbonate radiolarites. Radiolarites and bypass-turbidites, consisting of siliciclastic greywacke, terminate the platform succession beneath the emplaced oceanic nappe during late Tithonian to Valanginian time.  相似文献   

4.
The Aladag region of eastern Taurides, Turkey, is characterized by an imbricated thrust structure developed during late stage emplacement of the Pozanti-Karsanti ophiolite onto the Menderes-Taurus block in the late Cretaceous. The mid to late Cretaceous dynamothermal metamorphic sole and the underlying unmetamorphosed mélange, here named the Aladag accretionary complex, were accreted to the base of the Pozanti-Karsanti ophiolite during intra-oceanic subduction, transport and final obduction of the ophiolite onto the Menderes-Taurus block.In the dynamothermal metamorphic sole, intensity of deformation and degree of metamorphism increase from the base to the top, and at least three episodes of foliation, lineation and fold development are recognized. The asymmetry of quartz c-axis fabrics, tightness and asymmetry of folds of the same generation, and curvature of fold hinge lines increase from base to top, indicating that non-coaxial progressive deformation prevailed during the development of the metamorphic sole. The mélange is divided into three major thrust fault-bounded tectonic slivers, each of which is characterized by distinctive types of matrix and block lithologies, structures and deformation style. Kinematic analyses of the dynamothermal metamorphic sole and the mélange reveal that the tectonic transport direction of the Pozanti-Karsanti ophiolite during its emplacement was from north-northwest to south-southeast, suggesting that the Pozanti-Karsanti ophiolite was derived from a Neo-Tethyan ocean to the north of the Menderes-Taurus block.  相似文献   

5.
In the Central Rhodopes of southern Bulgaria, an eclogite-bearing rock sheet belonging to the Middle Allochthon (Starcevo Unit) is over- and underlain by eclogite-free, amphibolite-facies rock units along low-angle shear zones, the Borovica Shear Zone at the top and the Starcevo-Ardino Shear Zone at the base. The age of these shear zones is determined by U–Pb zircon dating of pre-, syn- and posttectonic magmatic rocks, mostly pegmatite veins, using LA–SF–ICP–MS. Zircons from pre- to syntectonic pegmatites within the Borovica Shear Zone yielded ages of ca. 45–43?Ma, indicating that the shear zone was active at that time, and zircons from a pretectonic pegmatite and a posttectonic granitoid body within the Starcevo-Ardino Shear Zone yielded ages of ca. 45 and ca. 36?Ma, respectively, giving a time frame for the activity of that shear zone which probably rather postdated the activity of the Borovica Shear Zone. By combining the ages with the kinematics of the shear zones and the metamorphic history of the rock units, the following scenario is sketched: Soon after the Starcevo Unit reached peak pressure (eclogite facies), it was exhumed to a mid-crustal level by top-to-the-north-west, extensional unroofing along the Borovica Shear Zone, in a kinematic framework of orogen-parallel extension. Beginning at ca. 40?Ma, the partly exhumed Starcevo Unit was underthrust from the south-west by continental crust of the foreland (Apulia), forming the Lower Allochthon of the Rhodopes, along the Starcevo-Ardino Shear Zone. These results underline the significance of orogen-parallel extension for the exhumation of high-pressure rocks. With respect to regional geology of the Hellenides and the Aegean, it is found that the tectonic architecture of the Rhodopes is essentially of Tertiary age. Cretaceous syn-metamorphic shear zones do exist but are largely restricted to higher levels of the nappe stack (Upper Allochthon). The Rhodopes do not represent an older essentially Mesozoic core of the Hellenides but are formed by the internal, higher-metamorphic portions of the same major nappe systems as occur in the Hellenides.  相似文献   

6.
The closure of the western part of the Neotethys Ocean started in late Early Jurassic. The Middle to early Late Jurassic contraction is documented in the Berchtesgaden Alps by the migration of trench-like basins formed in front of a propagating thrust belt. Due to ophiolite obduction these basins propagated from the outer shelf area (=Hallstatt realm) to the interior continent (=Hauptdolomit/Dachstein platform realm). The basins were separated by nappe fronts forming structural highs. This scenario mirrors syn-orogenic erosion and deposition in an evolving thrust belt. Active basin formation and nappe thrusting ended around the Oxfordian/Kimmeridgian boundary, followed by the onset of carbonate platforms on structural highs. Starved basins remained between the platforms. Rapid deepening around the Early/Late Tithonian boundary was induced by extension due to mountain uplift and resulted in the reconfiguration of the platforms and basins. Erosion of the uplifted nappe stack including obducted ophiolites resulted in increased sediment supply into the basins and final drowning and demise of the platforms in the Berriasian. The remaining Early Cretaceous foreland basins were filled up by sediments including siliciclastics. The described Jurassic to Early Cretaceous history of the Northern Calcareous Alps accords with the history of the Western Carpathians, the Dinarides, and the Albanides, where (1) age dating of the metamorphic soles prove late Early to Middle Jurassic inneroceanic thrusting followed by late Middle to early Late Jurassic ophiolite obduction, (2) Kimmeridgian to Tithonian shallow-water platforms formed on top of the obducted ophiolites, and (3) latest Jurassic to Early Cretaceous sediments show postorogenic character.  相似文献   

7.
Near-critical angle and refraction studies were performed at IFP as piggyback studies during a wider programme of crustal imagery operated by WesternGeco on behalf of the Ministry of Energy of the United Arab Emirates. The main objective is to illuminate the base of the Semail Ophiolite along part of a regional transect (D1) crossing the Northern Emirates from the Gulf of Oman in the east up to the Arabian Gulf in the west. Results confirm that the sole thrust of the ophiolite has been folded during the Miocene stacking of the underlying Arabian Platform. The thickness of the ophiolite grades from zero in the core of the Masafi tectonic window, up to a maximum of 1.7 km below the axial part of a successor basin which has been preserved on top of the serpentinite west of the current exposure of the main ultramafic bodies. Apatite grains extracted from plagiogranites of the Semail ophiolite also provide evidences for an early unroofing of the gabbros and plagiogranites during the Late Cretaceous, with cooling ages of 72–76 Ma at the top of the ophiolite in the east (not far from the Fujairah coast line), which are coeval and also consistent with the occurrence of Late Cretaceous paleo-soils, rudists and paleo-reef deposits on top of serpentinized ultramafics in the west. Younger cooling ages of 20 Ma have been also found at the base of the ophiolite near Masafi, in the core of the nappe anticline, thus providing a Neogene age for the refolding of the allochthon and stacking of underlying parautochthonous platform carbonate units. These results, together with the occurrence of a thick sedimentary pile illuminated below the metamorphic sole along the north-trending, strike-profile D2 running parallel to the axis of the Masafi window, should stimulate a renewal of the exploration in the central part of the Emirate foothills, where the ophiolite thickness is currently limited, and was already drastically reduced by the end of its Late Cretaceous obduction.  相似文献   

8.
东秦岭松树沟蛇绿岩构造变形及其演化   总被引：1，自引：0，他引：1  
东秦岭松树沟蛇绿岩是沿西沟早期韧性剪切带和界岭韧性剪切带构造侵位于早元古代秦岭群变质杂岩南缘之上的蛇绿岩推覆体，主要遭受了４期构造变形，反映蛇绿岩演化过程中四期主要的构造运动，分别为：１０亿年左右上地幔部分熔融残余体──蛇绿橄榄岩块底辟侵位于上覆玄武岩、晋宁期蛇绿岩构造侵位、加里东─早华力西期由南向北的过冲及晚华力西─印支期的左行走滑运动。  相似文献   

9.
Tectonic observations in the northern Saih Hatat,Sultanate of Oman   总被引：1，自引：0，他引：1  
The Saih Hatat region, in northeastern Oman, is characterized by a large tectonic window, tectonically overlain during the upper Cretaceous by nappes composed of sedimentary rocks from the Mesozoic Hawasina ocean and the Samail ophiolite. In this window, the autochthonous sedimentary cover of the eastern Arabian Platform from the Late Neoproterozoic to the Cenomanian is well exposed. The oldest of these strata, the Hatat schists, were deformed into a NE-facing fold nappe during the upper Cretaceous. Within the overturned and thrusted lower limb of that fold nappe, we identified three small windows exposing stratigraphically younger Hiyam carbonates and Ordovician sandstone. The structural inventory of the windows and the surrounding area indicates three major tectonic phases. The first deformation led to NNE-SSW trending fold structures which probably formed simultaneously with the major fold nappe of the Hatat schists, followed by the extreme attenuation and thrusting of the lower limb of the fold nappe. The second phase was a gentle folding of the thrust with N-S oriented fold axes and third deformation phase that formed WNW-ESE oriented open folds. The windows are situated in the intersection of anticline axes of these two superposed fold generations and represent a mini basin-and-dome structure with an extension of just 1 km?×?1 km.  相似文献   

10.
The Southeast Anatolian orogen is a part of the eastern Mediterranean-Himalayan orogenic belt. Development of the Southeast Anatolian orogen began with the first ophiolite obduction onto the Arabian platform during the Late Cretaceous, and it continued until the Miocene. Its lingering effects continue to be discernible at present. During the Late Cretaceous-Miocene interval, three major deformational phases occurred, related to Late Cretaceous, Eocene, and Miocene nappe emplacements. The Miocene nappes are composed of ophiolites and metamorphic massifs.

For a decade, field studies in the region have shown that strike-slip tectonics played a role complementary to the major horizontal effects of the nappe movement, as indicated by: (1) fault systems active during the Eocene; (2) different Eocene rock units composed of coeval continental and deep-sea deposits and presently tectonically juxtaposed; and (3) other stratigraphic and structural data obtained across the present strike-slip fault zones.

These strike-slip faults possibly resulted from oblique subduction of the mid-oceanic ridge underneath the northerly situated Yuksekova ensimatic island-arc complex, causing a gradual cessation of the island-arc system. The subduction also led to the development of a back-arc pull-apart basin, i.e., the Maden basin, which opened on the upper plate. The geologic history in Southeast Anatolia resembles the development of the San Andreas fault system and subsequent tectonic evolution.  相似文献   

11.
The Tertiary Mineoka ophiolite occurs in a fault zone at the intersection of the Honshu and Izu forearcs in central Japan and displays structural evidence for three major phases of deformation: normal and oblique-slip faults and hydrothermal veins formed during the seafloor spreading evolution of the ophiolite at a ridge-transform fault intersection. These structures may represent repeated changes in differential stress and pore-fluid pressures during their formation. The second series of deformation is characterized by oblique thrust faults with Riedel shears and no significant mineral veining, and is interpreted to have resulted from transpressional dextral faulting during the obduction of the ophiolite through oblique convergence and tectonic accretion. This deformation occurred at the NW corner of a TTT-type (trench–trench–trench) triple junction in the NW Pacific rim before the middle Miocene. The third series of deformation of the ophiolite is marked by contractional and oblique shear zones, Riedel shears, and thrust faults that crosscut and offset earlier structures, and that give the Mineoka fault zone its lenticular (phacoidal) fabric at all scales. This deformation phase was associated with the establishment and the southward migration of the TTT Boso triple junction and with the kinematics of oblique subduction and forearc sliver fault development. The composite Mineoka ophiolite hence displays rocks and structures that evolved during its complex geodynamic history involving seafloor spreading, tectonic accretion, and triple junction evolution in the NW Pacific Rim.  相似文献   

12.
Whole rock, electron microprobe analyses and 40Ar/39Ar geochronology of certain ophioliterelated metamorphic rocks from beneath the Pindos, Vourinos, Othris and Euboea ophiolites of Greece show that they were formed mainly from ocean-type basalts, in part under P-T conditions of the upper mantle and that they have ages between 170–180 m.y. The evidence presented is inconsistent with the view that these sub-ophiolite metamorphic rocks were produced by the obduction of ocean-type crust onto a continental margin, or that they are remnant slices of Palaeozoic ‘basement’, but is consistent with their formation by thrusting and related metamorphism occurring within ocean lithosphere during the Lower to Middle Jurassic. It is proposed that this intraoceanic metamorphism was caused by the inception of a fault zone which subsequently became the transport surface for the main phase of ophiolite emplacement that occurred in the Hellenides from the Late Jurassic to Early Cretaceous.  相似文献   

13.
This progress report is based on investigations of the tectonometamorphic development of crystalline complexes and is restricted to a few key problems of the Hellenides:
  1. ‘Hinterland’. Rhodopia is strongly affected by Alpidic metamorphism, granitoid intrusions, orogenic deformation and intracrustal delamination. Therefore, close relations between the Balkanides and Hellenides have to be considered.
  2. External zones. The Phyllite-Quartzite Series probably originated in a Late Palaeozoic rift within Apulia. In Middle Triassic times rifting stopped and the area became the basement on an extended carbonate platform (Late Triassic-Liassic). From the Dogger to Palaeocene, parts of that platform subsided, forming the Ionian pelagic basin. The Eocene orogenesis within the central Hellenides then caused an inversion of the buried Phyllite-Quartzite rift zone, whereas from the Late Oligocene onwards the previous rift zone underwent continental (A-) subduction. Finally, uplift of the Phyllite-Quartzite Series and nappe emplacement started in Miocene times.
  3. Late orogenic intracrustal shearing. Structural analyses of crystalline complexes of Attica have shown that neotectonic extension and the large vertical displacement of the Aegean region were caused by low angle faulting and large-scale shearing within the deeper crust, probably along former overthrust planes.
These results reveal the mechanisms of intracratonic tectonics, remobilization of continental crust and intracrustal detachment throughout the evolution of the Hellenides.  相似文献   

14.
The Jurassic and Cretaceous sedimentary history of northern Somalia and the Morondava Basin of south-western Madagascar have been studied. Both regions display an independent facial development; however, a comparison of the sequential evolution of the Mesozoic sedimentary successions in these two presently widely separated areas reveals a surprisingly high level of similarity, which probably reflects major events during the disintegration of Eastern Gondwana during the Jurassic and Cretaceous. Although in Jurassic times the onset of transgressions and regressions in both areas compares well with eustatic development, major deviations in combination with the tectonic activities of different degrees are observed in the Early and Late Cretaceous synchronously in both regions. Transgressions are observed in Toarcian, Bajocian (not dated in northern Somalia), Callovian, Valanginian (Madagascar only), Aptian and Campanian times. Tectonism is noted before the Aptian and Campanian transgressions in northern Somalia and the Morondava Basin of south-western Madagascar.  相似文献   

15.
If ophiolite complexes originate as oceanic crust and mantle generated by sea-floor spreading at oceanic ridges or in marginal basins, the tectonic emplacement (obduction) of ophiolite sheets and slices must involve some form of decoupling of oceanic lithosphere prior to emplacement and the expulsion of relatively dense oceanic rocks onto lighter continental rocks. The major problems are the mechanism by which this decoupling takes place, the extent to which the decoupling fractures penetrate the entire lithosphere, and the mechanism and geometry of the tectonic emplacement process, that is — the extent to which compressional versus gravity-sliding mechanisms predominate. Several writers (Coleman, 1971; Stevens, 1970; Church and Stevens, 1971; Temple and Zimmerman, 1969; Dewey and Bird, 1970, 1971; Williams, 1971) have discussed these obduction problems and offered various kinds of solution. These solutions, among others, are discussed in this paper. It is concluded that several convergent plate-margin mechanisms may be responsible for ophiolite obduction, none of which involve gravity sliding.  相似文献   

16.
South of the Matterhorn the Valtournanche cuts through Alpine serpentinites, metagabbros, meta-pillowbasalts and metasediments—dismembered remnants of the Jurassic Tethyan oceanic crust, reassembled in the Piemonte ophiolite nappe. This study deals with a serpentinized ultramafic to mafic layered complex stemming from a spreading ridge environment. Cumulus fabrics of various kinds can be read through antigorite pseudomorphs, still allowing the detailed reconstruction of deep oceanic crust. Relics of igneous and metamorphic olivine prove crustal conditions during deformation. Fracturing of cumulus olivine was succeeded by plastic flow that activated low-temperature slip systems. Concomitant recrystallization produced metaperidotite only along shear zones, which are ascribed to subduction of the oceanic crust. At the turning point from subduction to obduction a static metamorphic event resulted in recovery and grain growth of recrystallized olivine. Afterwards serpentinization of the complex took place still under static conditions. Deformation of the serpentinite led to a sequence of four phases, involving non-penetrative cleavage formation, stretching and folding. This deformation is structurally related to obduction of the complex although partly accompanied by subduction zone metamorphism. Final movements of the ophiolites were due to cataclastic thrusting forming subnappe boundaries.  相似文献   

17.
《Tectonophysics》1999,301(1-2):145-158
The Mersin ophiolite is located on the southern flank of the E–W-trending central Tauride belt in Turkey. It is one of the Late Cretaceous Neotethyan oceanic lithospheric remnants. The Mersin ophiolite formed in a suprasubduction zone tectonic setting in southern Turkey at the beginning of the Late Cretaceous. The Mersin ophiolite is one of the best examples in Turkey in order to study reconstruction of ophiolite emplacement along the Alpine–Himalayan orogenic belt. 40Ar/39Ar incremental-heating measurements were performed on seven obduction-related subophiolitic metamorphic rocks. Hornblende separates yielded isochron ages ranging from 96.0±0.7 Ma to 91.6±0.3 Ma (all errors ±1σ). Five of the seven hornblende age determinations are indistinguishable at the 95% confidence level and have a weighted mean age of 92.6±0.2 (2σ) Ma. We interpret these ages as the date of cooling below 500°C. Intraoceanic thrusting occurred (∼4 Ma) soon after formation of oceanic crust. The sole was crosscut by microgabbro–diabase dikes less than 3 m.y. later. The final obduction onto the Tauride platform occurred during the Late Cretaceous–Early Paleocene. Our new high-precision ages constrain intraoceanic thrusting for a single ophiolite (Mersin) in the Tauride belt.  相似文献   

18.
《Cretaceous Research》1995,16(5):539-558
The Cretaceous sedimentary successions of the Ionian Zone, Hellenides, western Greece, are composed of pelagic limestones intercalated with cherty layers. The micritic and biomicritic beds with abundant chert nodules and cherty horizons, which were deposited during late Tithonian to early Santonian times, belong to the Vigla Limestone Formation, while the sediments deposited during the late Santonian to Maastrichtian, formed clastic limestone beds in which chert nodules also occur sparsely.In the Cretaceous beds calpionellids, planktonic and benthonic foraminifera characteristics of the Tethyan realm, and radiolaria have been recorded. The calpionellids, together with radiolaria, colonized the entire basin during the Berriasian to early Valanginian, the latter becoming dominant during the Hauterivian to early Albian as a result of anoxia. Planktonic foraminifera first appeared in the basin during the late Albian and persisted until the Maastrichtian. The numbers decreased, however, during the Cenomanian-early Turonian interval, when radiolaria increased owing to anoxic conditions, and during the Campanian-Maastrichtian interval because the basin became shallow. During this interval larger benthonic foraminifera colonized the basin. Zonal markers have been recognized in calpionellid and planktonic foraminiferal assemblages on the basis of which two calpionellid zones are distinguished, viz. the Calpionella alpina and Calpionellopsis Zones (Berriasian-early Valanginian) along with seven planktonic foraminiferal zones, viz. the Rotalipora ticinensis, Rotalipora appenninica (late Albian), Rotalipora brotzeni (early Cenomanian), Helvetoglobotruncana helvetica (early to middle Turonian), Marginotruncana sigali(late Turonian to early Coniacian), Dicarinella concavata (late Coniacian to early Santonian) and Dicarinella asymetrica (late early-late Santonian) Zones.The anoxic conditions that prevailed in the Ionian basin during the Barremian-early Albian, Cenomanian-early Turonian and Coniacian-Santonian intervals probably arose as a result of (a) the accumulation of large amounts of organic matter because the palaeotopography of the basin periodically hindered the circulation of water from the ocean and (b) the oxygen content of the intruding oceanic waters was low.  相似文献   

19.
蛇绿岩就位机制研究   总被引：5，自引：0，他引：5  
在对东昆仑蛇绿岩研究的基础上 ,结合其它地区资料 ,对蛇绿岩就位机制进行了研究 ,把蛇绿岩就位机制分为俯冲刮削拼贴式、俯冲折返拼贴式和仰冲推覆式三种类型 ,并对不同就位机制下形成的蛇绿岩的特征进行了讨论。  相似文献   

20.
The Bashkirian anticlinorium of the southwestern Urals shows a much more complex structural architecture and tectonic evolution than previously known. Pre-Uralian Proterozoic extensional and compressional structures controlled significantly the Uralian tectonic convergence. A long-lasting Proterozoic rift process created extensional basement structures and a Riphean basin topography which influenced the formation of the western fold-and-thrust-belt with inversion structures during the Uralian deformation. A complete orogenic cycle during Cadomian times, including terrane accretion at the eastern margin of the East European platform, resulted in a high-level Cadomian basement complex, which controlled the onset of Uralian deformation, and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the Main Uralian fault. The Uralian orogenic evolution can be subdivided into three deformation stages with differently oriented stress regimes. Tectonic convergence started in the Late Devonian with ophiolite obduction, tectonic accretion of basin and slope units and early flysch deposits (Zilair flysch). The accretionary complex prograded from the SE to the NW. Continuous NW/SE-directed convergence resulted finally in the formation of an early orogenic wedge thrusting the Cadomian basement complex onto the East European platform. The main tectonic shortening was connected with these two stages and, although not well constrained, appears to be of Late Devonian to Carboniferous age. In the Permian a final stage of E–W compression is observed throughout the SW Urals. In the west the fold-and-thrust-belt prograded to the west with reactivation of former extensional structures and minor shortening. In the east this phase was related to intense back thrusting. The East European platform was subducted beneath the Magnitogorsk magmatic arc during the Late Paleozoic collision. The thick and cold East European platform reacted as a stable rigid block which resulted in a narrow zone of intense crustal shortening, tectonic stacking and high strain at its eastern margin. Whereas the first orogenic wedge is of thick-skinned type with the involvement of crystalline basement, even the later west-directed wedge is not typically thin-skinned as the depth of the basal detachment appears below 15 km and the involvement of Archean basement can be assumed.  相似文献   

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