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1.
We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (～1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e1 and e2). The better-developed e1 sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e2 lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e1 and e2 strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.  相似文献   

2.
A new U?CPb SHRIMP age of 551?±?4?Ma on a mylonitic porphyry that intruded into the Sierra Ballena Shear Zone (Southernmost Dom Feliciano Belt, Uruguay) and a review of relevant published data make possible a more refined correlation and reconstruction of Brasiliano/Pan-African transpressional events. Paleogeographic reconstruction, kinematics and timing of events indicate a connection between the shear systems of the Dom Feliciano and Kaoko Belts at 580?C550?Ma. Sinistral transpression recorded in shear zones accommodates deformation subsequent to collision between the Congo and Río de la Plata Cratons. The correlation is strengthened by the similarity of magmatic and metamorphic ages in the Coastal Terrane of the Kaoko Belt and the Punta del Este Terrane of the Dom Feliciano Belt. This post-collisional sinistral transpression brought these units near to their final position in Gondwana and explains the different evolution at 550?C530?Ma. While in the Kaoko Belt, an extensional episode resulted in exhumation as a consequence of collision in the Damara Belt, in the Dom Feliciano Belt, sinistral transpression occurred associated with the closure of the southern Adamastor Ocean due to Kalahari-Río de la Plata collision.  相似文献   

3.
The Tongbai orogenic belt has an overall antiformal geometry and the hinge of the antiform is sub-horizontal and trends NW–SE. The Tongbai complex (TBC) in the core of the antiform is bounded by the S-dipping Yindian–Malong shear zone in the south, the sub-horizontal Taibaiding shear zone at the top and the N-dipping Hongyihe–Tongbai shear zone in the north. The three shear zones have dextral, top-to-NW and sinistral movement, respectively. They are parts of a single shear zone, termed the Tongbai shear zone, that has a uniform top-to-NW sense of shear. Three samples of deformed granitoid (mylonite or protomylonite) from the shear zone have U–Pb zircon ages of 145 ± 6 Ma, 142 ± 2 Ma and 131 ± 6 Ma, respectively. An L-tectonite in the TBC yielded a metamorphic age of 137 ± 8 Ma and a migmatite an age of 137 ± 1 Ma. The Tongbai shear zone is intruded by undeformed Early Cretaceous granite and dykes and deformation in the shear zone is constrained to ca. 140–135 Ma, synchronous with metamorphism and migmatization in the TBC. Early Cretaceous magma emplacement and the associated uplift modified the TBC into a gentle antiform and the uplift may have continued to ca. 102–85 Ma. Similar geometry and kinematics have been documented in the Dabie orogenic belt to the east, which suggests that the Central Orogenic Belt in China probably experienced a uniform orogen-parallel extension and top-to-NW shearing in the ductile lithosphere in the Early Cretaceous.  相似文献   

4.
《Gondwana Research》2014,25(2):775-796
The Damara Orogeny is a late Neoproterozoic to Cambrian (ca. 570–480 Ma) intracratonic event that affected the Kaoko Belt, the inland branch of the Damara orogen and the Gariep Belt in Namibia and South Africa. This study focuses on the Pan-African evolution of part of the Kaoko Belt between the Puros shear zone and the Village mylonite zone which consists of Mesoproterozoic migmatitic para- and orthogneisses with minor granulite and amphibolite. Pseudosection modeling combined with thermobarometric calculations indicate that the para- and orthogneisses equilibrated at about 670–800 °C and ca. 0.6–0.8 GPa. Some garnets display a pronounced bell-shaped Ca, HREE, Y and Sr zoning, flat zoning profiles of Mn and Fe and concave upward concentration profiles of Sm and Nd. Pressure–temperature estimates obtained on these garnets reveal similar temperatures of 700–750 °C but slightly higher pressures of ca. 0.9 GPa. The preservation of distinct major and trace element zoning in garnet and the existence of broadly similar (near prograde) Sm–Nd and Lu–Hf garnet–whole rock ages of ca. 525 Ma obtained on the same sample indicate an extremely fast cooling path. Retrograde conditions persisted until ca. 490 Ma indicating a slow, late stage near isobaric cooling path. The resulting clockwise P–T–t path is consistent with crustal thickening through continent–continent collision followed by post-collisional extension and suggests that the upper amphibolite to granulite facies terrain of the central Kaoko Belt formed initially in a metamorphic field gradient of ca. 25–35 °C km− 1 at moderately high pressures.  相似文献   

5.
A similar succession of Foliation Inflection/Intersection Axis (FIAs) trends preserved within porphyroblasts is present in two areas separated by 200 km along the Rocky Mountains. The Precambrian rocks in Central Colorado and Northern New Mexico were affected by deformation and metamorphism from ～1506 to 1366 Ma. A succession of five FIAs trending W–E, SSW–NNE, NNW–SSE, NW–SE and WSW–ENE is distinguished in Central Colorado and dated at 1506 ± 15 Ma, 1467 ± 23 Ma, 1425 ± 18 Ma, not dated and 1366 ± 20 Ma respectively. To the south in Northern New Mexico, a succession of five FIAs trending SSW–NNE, WNW–ESE, NNW–SSE, NW–SE and WSW–ENE is distinguished and dated at 1482 ± 48 Ma, 1448 ± 12 Ma, 1422 ± 35 Ma, not dated and 1394 ± 22 Ma. The excellent correlation of the sequence of FIA trends and their ages between regions reveals a sixfold‐FIA succession across the region with the first developed FIA set in Central Colorado not present in Northern New Mexico and the third FIA set in the region not present in Central Colorado. Preferential partitioning of W–E trending deformation into the Central Colorado region ～1506 ± 15 Ma was followed by SSW–NNE trending deformation that affected both regions at 1470 ± 20 Ma. However, preferential partitioning of WNW–ESE trending deformation into Northern New Mexico at 1448 ± 12 Ma left Central Colorado unaffected. Both regions were then affected by the three remaining periods of orogenesis, the first trending NNW–SSE at 1424 ± 15 Ma followed by one trending NW–SE that has not yet been dated, and then one trending WSW–ENE at 1390 ± 19 Ma. This suggests that the Yavapai terrane was tectonized at ～1506 Ma, prior to amalgamation with the Mazatzal terrane ～1470 Ma. Subsequent orogenesis was initially partitioned preferentially into the Mazatzal terrane, but the following three periods of tectonism affected both terranes in a similar manner.  相似文献   

6.
40Ar/39Ar age data from the boundary between the Delamerian and Lachlan Fold Belts identify the Moornambool Metamorphic Complex as a Cambrian metamorphic belt in the western Stawell Zone of the Palaeozoic Tasmanide System of southeastern Australia. A reworked orogenic zone exists between the Lachlan and Delamerian Fold Belts that contains the eastern section of the Cambrian Delamerian Fold Belt and the western limit of orogenesis associated with the formation of an Ordovician to Silurian accretionary wedge (Lachlan Fold Belt). Delamerian thrusting is craton-verging and occurred at the same time as the final consolidation of Gondwana. 40Ar/39Ar age data indicate rapid cooling of the Moornambool Metamorphic Complex at about 500 Ma at a rate of 20 – 30°C per million years, temporally associated with calc-alkaline volcanism followed by clastic sedimentation. Extension in the overriding plate of a subduction zone is interpreted to have exhumed the metamorphic rocks within the Moornambool Metamorphic Complex. The Delamerian system varies from a high geothermal gradient with syntectonic plutonism in the west to lower geothermal gradients in the east (no syntectonic plutonism). This metamorphic zonation is consistent with a west-dipping subduction zone. Contrary to some previous models involving a reversal in subduction polarity, the Ross and Delamerian systems of Antarctica and Australia are inferred to reflect deformation processes associated with a Cambrian subduction zone that dipped towards the Gondwana supercontinent. Western Lachlan Fold Belt orogenesis occurred about 40 million years after the Delamerian Orogeny and deformed older, colder, and denser oceanic crust, with metamorphism indicative of a low geothermal gradient. This orogenesis closed a marginal ocean basin by west-directed underthrusting of oceanic crust that produced an accretionary wedge with west-dipping faults that verge away from the major craton. The western Lachlan Fold Belt was not associated with arc-related volcanism and plutonism occurred 40 – 60 million years after initial deformation. The revised orogenic boundaries have implications for the location of world-class 440 Ma orogenic gold deposits. The structural complexity of the 440 Ma Stawell gold deposit reflects its location in a reworked part of the Cambrian Delamerian Fold Belt, while the structurally simpler 440 Ma Bendigo deposit is hosted by younger Ordovician turbidites solely deformed by Lachlan orogenesis.  相似文献   

7.
Foliation inflexion/intersection axes(FIAs)preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE-SW,N-S,E-W to NW-SE.This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny.Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined.Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE-SW shortening,1645±7 Ma for N-S shortening,and 1591±10 Ma for that directed E-W.Inclusion trail asymmetries indicate shear senses of top to the SW for NW-SE FIAs and dominantly top to the N for E-W FIAs,reflecting thrusting towards the SW and N.No evidence for tectonism related to early NE-SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier.Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE-SW and N-S-directed thrusting as the ages of tectonism are similar.Overlapping dates,tectonic,metamorphic,and metallogenic similarities between eastern Australia(Mount Isa and Broken Hill terranes)and the southwest part of Laurentia imply a most probable connection between both continental masses.Putting Australia in such position with respect to North America during the Late-Paleo-to-Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent.  相似文献   

8.
《International Geology Review》2012,54(13):1602-1629
Widespread Cretaceous volcanic basins are common in eastern South China and are crucial to understanding how the Circum-Pacific and Tethyan plate boundaries evolved and interacted with one another in controlling the tectonic evolution of South China. Lithostratigraphic units in these basins are grouped, in ascending order, into the Early Cretaceous volcanic suite (K1V), the Yongkang Group (K1-2), and the Jinqu Group (K2). SHRIMP U-Pb zircon geochronological results indicate that (1) the Early Cretaceous volcanic suite (K1V) erupted at 136–129 Ma, (2) the Yongkang Group (K1-2) was deposited from 129 Ma to 91 Ma, and (3) the deposition of the Jinqu Group (K2) post-dated 91 Ma. Structural analyses of fault-slip data from these rock units delineate a four-stage tectonic evolution of the basins during Cretaceous to Palaeogene time. The first stage (Early to middle Cretaceous time, 136–91 Ma) was dominated by NW–SE extension, as manifested by voluminous volcanism, initial opening of NE-trending basins, and deposition of the Yongkang Group. This extension was followed during Late Cretaceous time by NW–SE compression that inverted previous rift basins. During the third stage in Late Cretaceous time, possibly since 78.5 Ma, the tectonic stress changed to N–S extension, which led to basin opening and deposition of the Jinqu Group along E-trending faults. This extension probably lasted until early Palaeogene time and was terminated by the latest NE–SW compressional deformation that caused basin inversion again. Geodynamically, the NW–SE-oriented stress fields were associated with plate kinematics along the Circum-Pacific plate boundary, and the extension–compression alternation is interpreted as resulting from variations of the subducted slab dynamics. A drastic change in the tectonic stress field from NW–SE to N–S implies that the Pacific subduction-dominated back-arc extension and shortening were completed in the Late Cretaceous, and simultaneously, that Neo-Tethyan subduction became dominant and exerted a new force on South China. The ongoing Neo-Tethyan subduction might provide plausible geodynamic interpretations for the Late Cretaceous N–S extension-dominated basin rifting, and the subsequent Cenozoic India–Asia collision might explain the early Palaeogene NE–SW compression-dominated basin inversion.  相似文献   

9.
The Lower Ugab and Goantagab structural domains are located at the junction between the N–S trending Kaoko and the E–W trending Damara belts (NW Namibia), where Neoproterozoic metavolcano-sedimentary sequences were intruded by several syenitic/granitic plutons. We present here new U–Pb ages on zircon grains from the Voetspoor and Doros plutons. Together with petrological, geochemical and structural data we evaluate the timing of the deformation and relation to the geodynamics during the final stage of Gondwana amalgamation.The plutons are composed of three main rock types: hornblende quartz-syenite, syenodiorite and biotite granite. The two former are predominant and show genetic correlation such as magma mingling structures and similar geochemical signatures. The biotite granite occurs in the SW parts of the intrusions and clearly cuts the syenitic rocks. Although the plutons are mainly isotropic, the structures around them demonstrate that their intrusion occurred during a second deformation phase (D2) with a component of sinistral solid state rotation with respect to the wall rocks in response to D2 transpression. Four samples were dated using U–Pb SHRIMP methodology in single zircon grains. A hornblende monzodiorite from the Voetspoor pluton yielded an age of 534 ± 4.5 Ma. A hornblende monzonite from the Doros pluton produced an age of 528 ± 5 Ma. The biotite granite facies was sampled in the Doros intrusion and yielded an age of 530 ± 4.5 Ma. In addition, a granitic vein folded by D2 close to the northeastern contact of the Doros pluton with the encasing phyllites (Amis River Formation) was also dated, yielding an age of 533 ± 6 Ma. The data show that all granite–syenite from Doros and Voetspoor intrusions are contemporaneous and crystallized in the period between 539 and 522 Ma within the errors. D1–D2 deformational phases took place under greenschist facies (biotite zone) conditions and during D3 the metamorphic grade was slightly lower. We interpret that the plutons are coeval to peak metamorphism of the region (530–520 Ma) and that D2 and D3 sinistral transpressional phases are due to collision in the Damara Belt. The E–W compressional event and second metamorphic episode in the Kaoko Belt occurred between 580 and 560 Ma and are apparently unrelated to the thermo-tectonic evolution described here, although D1 might be partially related to this event. The sinistral transpressional D2 phase resulted probably from the position of the area considered at the junction between the belts, and not in the frontal Damara collision further to the east. This new interpretation is consistent with the Ar–Ar ages for the region (about 500 Ma), interpreted to reflect cooling of the orogen. The enrichment in LREE, K, Rb, Ba and Sr, and depletion in Nb of these basic to intermediate alkalic rocks could indicate that they partially derived from melting of a subcontinental lithospheric mantle that was affected by subduction and the granitic rock types represent lower crust contamination. We interpret that they could be related to heating in the mantle caused by asthenosphere influx in a zone of slab-breakoff during collision between Kalahari and Congo cratons.  相似文献   

10.
The Maria da Fé Shear Zone (MFSZ) is a sinistral strike-slip kilometric-scale structure developed in the late Neoproterozoic during the assembly of Gondwana. The MFSZ development is related to the NW–SE collision between the São Francisco Paleocontinent and the Rio Negro Magmatic Arc, which formed the Ribeira Belt. This paper describes the shear zone in detail, concluding that the orientation and age are consistent with NW–SE shortening during the afore mentioned collision. A U–Pb SHRIMP Concordia age of 586.9 ± 8.7 Ma is reported from zircon grains of a granitic dyke that crystallised synkinematically to the main tectonic activity of the shear zone. Another group of zircon grains from the same sample generated an upper intercept age of 2083 ± 43 Ma anchored in the younger Concordia age. These zircon grains are interpreted as relict grains of the basement from which the granite dyke was generated by partial melting. The temperature during mylonitization in the MFSZ was estimated in the range from 450 to 600 °C, based on microstructures in quartz and feldspar. An earlier collision in the same region, between 640 and 610 Ma, led to an extensive nappe-stack with tectonic transport to ENE, integrating the southern Brasilia Belt. One of the thrust zones between these nappes in the studied area is the Cristina Shear Zone with mylonites that were generated under upper amphibolite to granulite facies conditions. Brittle-ductile E–W metric-scale shear zones are superimposed on the MFSZ, which were active in similar, but probably slightly cooler, metamorphic conditions (≈500 °C).  相似文献   

11.
The Eastern Tianshan Orogenic Belt of the Central Asian Orogenic Belt and the Beishan terrane of the Tarim Block, NW China, host numerous Fe deposits. The Cihai Fe deposit (>90 Mt at 45.6 % Fe) in the Beishan terrane is diabase-hosted and consists of the Cihai, Cinan, and Cixi ore clusters. Ore minerals are dominantly magnetite, pyrite, and pyrrhotite, with minor chalcopyrite, galena, and sphalerite. Gangue minerals include pyroxene, garnet, hornblende and minor plagioclase, biotite, chlorite, epidotite, quartz, and calcite. Pyrite from the Cihai and Cixi ore clusters has similar Re–Os isotope compositions, with ～14 to 62 ppb Re and ≤10?ppt common Os. Pyrrhotite has ～5 to 39 ppb Re and ～0.6 ppb common Os. Pyrite has a mean Re–Os model age of 262.3?±?5.6 Ma (n?=?13), in agreement with the isochron regression of 187Os vs. 187Re. The Re–Os age (～262 Ma) for the Cihai Fe deposit is within uncertainty in agreement with a previously reported Rb–Sr age (268?±?25 Ma) of the hosting diabase, indicating a genetic relationship between magmatism and mineralization. Magnetite from the Cihai deposit has Mg, Al, Ti, V, Cr, Co, Ni, Mn, Zn, Ga, and Sn more elevated than that of typical skarn deposits, but both V and Ti contents lower than that of magmatic Fe–Ti–V deposits. Magnetite from these two ore clusters at Cihai has slightly different trace element concentrations. Magnetite from the Cihai ore cluster has relatively constant trace element compositions. Some magnetite grains from the Cixi ore cluster have higher V, Ti, and Cr than those from the Cihai ore cluster. The compositional variations of magnetite between the ore clusters are possibly due to different formation temperatures. Combined with regional tectonic evolution of the Beishan terrane, the Re–Os age of pyrite and the composition of magnetite indicate that the Cihai Fe deposit may have derived from magmatic–hydrothermal fluids related to mafic magmatism, probably in an extensional rift environment.  相似文献   

12.
The type locality for high-temperature,low-pressure regional metamorphism,the Buchan Block in NE Scotland,exhibits profound differences to the rest of the Grampian Terrane.These differences have led some to regard the Buchan Block as an exotic crustal fragment comprising Precambrian basement gneisses and cover rocks thrust into their current position during Grampian orogenesis.Although rocks of the Buchan Block are now generally correlated with Dalradian strata elsewhere,the origin of the gneisses and the cause of the high heat flow and associated magmatism is debated.We report SIMS U-Pb and LA-ICPMS Hf isotopic data in zircon from high-grade rocks from the northeast(Inzie Head Gneiss)and northwest(Portsoy)corners of the Buchan Block.Around Inzie Head,upper amphibolite to granulite facies metasedimentary gneisses coexist with diorite sheets that were emplaced contemporaneously with partial melting of their host rocks,at least locally.U-Pb geochronology indicates a crystallisation age for the diorite of 486±9 Ma.Highly-deformed diorites within the Portsoy Gabbro have a crystallisation age of 493±8 Ma.Ages of ca.490 Ma for magmatism and high-grade metamorphism,which are broadly contemporaneous with ophiolite obduction and the onset of orogenesis,are significantly older than the established peak of Grampian metamorphism(ca.470 Ma).We propose a new model for the Grampian Orogeny involving punctuated tectonothermal activity due to tectonic switching during accretionary orogenesis.Rollback of a NW-dipping subduction zone at ca.490 Ma produced a back-arc environment(the Buchan Block)with associated arc magmatism and high dT/dP metamorphism.Arrival of an outboard arc resulted in shortening(the initial phase of the Grampian Orogeny)at ca.488 Ma.Rollback of a NW-dipping subduction zone to the SE of the ca.488 Ma suture began at 473 Ma and led to lithospheric-scale extension,decompression melting and advective heating of the middle crust,producing the widespread ca.470 Ma Grampian(classic Barrovian and Buchan)regional metamorphism.Resumed hinge advance and the final phase of shortening cut off the heat supply at ca.465 Ma,marking the end of the Grampian Orogeny.  相似文献   

13.
《International Geology Review》2012,54(16):2036-2056
ABSTRACT

The Chinese Southwest Tianshan Orogenic Belt is located along the boundary between the Central Asian Orogenic Belt (CAOB) and the Tarim Block (TB), NW China. It records the convergence of the Tarim Block and the Middle Tianshan, and is, therefore, a crucial region for understanding the Eurasia continental growth and evolution. The Wulagen (geographical name) metasedimentary rocks of the Wuqia area (mainly metamorphic sandstones and mica schists) form one of the metamorphic terranes in the Southwestern Tianshan Orogenic Belt. The geochronology of these rocks is poorly known, which hampers our understanding of the tectonic evolution of the belt. We analyzed 517 zircon grains for detrital zircon U–Pb dating and 93 zircon grains for in situ Lu–Hf isotopic compositions from the Wulagen metasedimentary rocks. The analyzed zircon grains yield Neoarchean to late Paleozoic U–Pb ages with major age peaks at ~2543 Ma, 1814 Ma, 830 Ma, 460 Ma, and the youngest cluster of zircon (magmatogene) ages is 395 Ma. The zircon U–Pb data show that the late Paleozoic (Early Devonian) is the maximum depositional age of the Wulagen metasedimentary rocks, rather than the previously considered Precambrian period. The zircons with Paleozoic ages yield εHf(t) values of ?22.0 to +11.3 and two-stage model ages (TDM2) of 3.95 to 1.30 Ga, suggesting that the parental magmas were formed from partial melting of pre-existing crustal rocks. Our zircon U–Pb geochronology and Hf isotopic data indicate the major source regions for the Wulagen metasedimentary rocks was the Kyrgyzstan North Tianshan. The zircon age population of 600–400 Ma (peak at ~460 Ma) has negative εHf(t) values (?15.0 to ?0.6) and Mesoproterozoic two-stage model ages, suggesting that the early Paleozoic magmatism resulted mainly from the melting of ancient crust, which played an important role in crustal evolution in the southern CAOB.  相似文献   

14.
ABSTRACT

The basal conglomerates (‘Linsishan Conglomerate’, LC, herein) are exposed discontinuously along the northern part of the Sulu Orogenic Belt (SOB) and the southern part of the Jiaobei Terrane. Studying these conglomerates can offer key constrains for the formation age of the Jiaolai Basin and improve our understanding of the uplift and erosional histories of the SOB and Jiaobei Terrane, which are still in great controversy. In Huangyadi section, the LC is characterized as debris-flow deposits, channel deposits, and sheet-flow deposits. However, in Shanjiao section, the LC is changed to sheet-flow and sieve deposits, as well as debris-flow and channel deposits. These deposit characteristics indicate an unstable tectonic setting during initial opening stage of the basin. Based on the data of conglomerate component, palaeocurrent, and debris zircons ages, it can be inferred that the sediments in the Laiyang region were sourced from the Jiaobei Terrane and Northern Sulu Orogenic Belt (NSOB), and the sediments in the Zhucheng and Wulian regions were derived from the Jiaobei Terrane and the Southern Sulu Orogenic Belt (SSOB). Besides, the sediments in the Haiyang and Jimo regions were provided by the NSOB and SSOB, respectively. The significant SHRIMP U–Pb ages of a tuff developing in the LC has been obtained, indicating that 149 ± 2.5 Ma is the oldest age constraint for the Jiaolai Basin. In addition, our result shows that the Latest Jurassic (ca. 149 Ma) may be a critical time; before this time, the Jiaobei Terrane and the SOB experienced a rapid uplift with minimal uplift velocity (~0.9 km/Ma); since then, the Orogen began to collapse and a series of basins formed rapidly in its core, which indicate the tectonic stress regime of the Dabie-Sulu Orogen varied from compressional stress to tensile stress.  相似文献   

15.
The sequential growth of biotite, garnet, staurolite, kyanite, andalusite, cordierite and fibrolitic sillimanite, their microstructural relationships, foliation intersection axes preserved in porphyroblasts (FIAs), geochronology, P–T pseudosection (MnNCKFMASH system) modelling and geothermobarometry provide evidence for a P–T–t–D path that changes from clockwise to anticlockwise with time for the Balcooma Metamorphic Group. Growth of garnet at ～530 °C and 4.6 kbar during the N–S‐shortening event that formed FIA 1 was followed by staurolite, plagioclase and kyanite growth. The inclusions of garnet in staurolite porphyroblasts that formed during the development of FIAs 2 and 3 plus kyanite growth during FIA 3 reflect continuous crustal thickening from c. 443 to 425 Ma during an Early Silurian Benambran Orogenic event. The temperature and pressure increased during this time from ～530 °C and 4.6 kbar to ～630 °C and 6.2 kbar. The overprinting of garnet‐, staurolite‐ and kyanite‐bearing mineral assemblages by low‐pressure andalusite and cordierite assemblages implies ～4‐kbar decompression during Early Devonian exhumation of the Greenvale Province.  相似文献   

16.
The Damara Orogen is composed of the Damara, Kaoko and Gariep belts developed during the Neoproterozoic Pan-African Orogeny. The Damara Belt contains Neoproterozoic siliciclastic and carbonate successions of the Damara Supergroup that record rift to proto-ocean depositional phases during the Rodinia supercontinent break up. There are two conflicting interpretations of the geotectonic framework of the Damara Supergroup basin: i) as one major basin, composed of the Outjo and Khomas basins, related to rifting in the Angola-Congo-Kalahari paleocontinent or, ii) as two independent passive margin basins, one related to the Angola-Congo and the other to the Kalahari proto-cratons. Detrital zircon provenance studies linked to field geology were used to solve this controversy. U-Pb zircon age data were analyzed in order to characterize depositional ages and provenance of the sediments and evolution of the succession in the northern part of the Outjo Basin. The basal Nabis Formation (Nosib Group) and the base of the Chuos Formation were deposited between ca. 870 Ma and 760 Ma. The upper Chuos, Berg Aukas, Gauss, Auros and lower Brak River formations formed between ca. 760 Ma and 635 Ma. It also includes the time span recorded by the unconformity between the Auros and lower Brak River formations. The Ghaub, upper Brak River, Karibib and Kuiseb formations were deposited between 663 Ma and 590 Ma. The geochronological data indicate that the main source areas are related to: i) the Angola-Congo Craton, ii) rift-related intrabasinal igneous rocks of the Naauwpoort Formation, iii) an intrabasinal basement structural high (Abbabis High), and iv) the Coastal Terrane of the Kaoko Belt. The Kalahari Craton units apparently did not constitute a main source area for the studied succession. This is possibly due to the position of the succession in the northern part of the Outjo Basin, at the southern margin of the Congo Craton. Comparison of the obtained geochronological data with those from the literature shows that the Abbabis High forms part of the Kalahari proto-craton and that Angola-Congo and Kalahari cratons were part of the same paleocontinent in Rodinia times.  相似文献   

17.
Within the Albany–Fraser Orogen of southwestern Australia, the Coramup Gneiss is a NE–SW trending zone of high‐strain rocks that preserves a detailed record of orogenesis related to Mesoproterozoic convergence of the West Australian and Mawson cratons. New structural, metamorphic and U–Pb SHRIMP zircon age data establish that the Coramup Gneiss underwent high‐grade tectonism during both Stage I (c. 1290 Ma) and Stage II (c. 1170 Ma) of the Albany–Fraser Orogeny. Stage I commenced with c. 1300 Ma high‐T, low‐P M1a metamorphism during extension, and the formation of small‐scale ptygmatic folds within a subhorizontal S1a gneissosity. High‐P M1b metamorphism at c. 1290 Ma was accompanied by the transposition and shearing of S1a into a composite, shallow SE‐dipping S1b foliation, and the development of tight recumbent F1b folds with S1‐parallel axial surfaces and asymmetries indicating NW‐directed thrusting. The preservation of a similar P–T–time record in the Fraser Complex (NE of the Coramup Gneiss) is consistent with large‐scale, NW‐directed Stage I thrusting of the Mawson Craton margin over the south‐eastern edge of the West Australian Craton. Stage II tectonism in the western Coramup Gneiss involved high‐T, low‐P M2a metamorphism and the formation of subvertical SE‐dipping D2 shear zones, shallow SW‐plunging L2 mineral stretching lineations, and NW‐verging F2 folds with S2‐parallel axial surfaces. A synkinematic pegmatite dyke emplaced into a D2 shear zone yielded a U–Pb SHRIMP zircon age of 1168 ± 12 Ma. Kinematic indicators suggest a combination of pure shear flattening perpendicular to S2, and dextral simple shear. However, contemporaneous structures elsewhere in the Albany–Fraser Orogen are consistent with continued NW–SE convergence at craton‐scale during Stage II, and oblique compression in the Coramup Gneiss is attributed to the arcuate geometry of the orogen‐scale deformation front.  相似文献   

18.
The western Daqingshan area, located in the eastern Yinshan belt, is dominated by the southern Daqingshan fold-and-thrust system and the northern Shiguai basin. Based on detailed structural investigations, stratigraphic controls, and geochronology, a three-stage tectonic evolution is proposed for the western Daqingshan area during the Jurassic. The discovery of syndepositional normal faults in the Early–Middle Jurassic sequences suggests that an N–S extensional regime (ca. 200–170 Ma) characterized the first deformational stage, which controlled the initial formation of the Shiguai basin. Subsequently, the relatively expansive rift basin was dissected by the initial development of the Daqingshan fold-and-thrust system that was associated with a N–S compressional regime (ca. 170–160 Ma). This phase of deformation involved the Lower–Middle Jurassic synrift sediments into a series of E–W-trending compressional structures, and controlled the deposition of Late–Middle Jurassic Changhangou growth strata ahead of the deformation front. Finally, the progression of Daqingshan fold-and-thrust system was dominated by NW–SE compression (ca. 160–145 Ma), which converted the previous E–W-trending compressional structures into a stepped geometry marked by several NE-trending oblique footwall ramps, and resulted in the depocentre of the Late Jurassic Daqingshan synorogenic conglomerate migrating markedly northeastwards. The driving mechanisms for these three palaeostress fields are considered as asthenosphere upwelling following Permian–Triassic collisional orogenesis, closure of the Mongol–Okhotsk Ocean, and NW-directed subduction of the Palaeo-Pacific plate, respectively.  相似文献   

19.
Neoproterozoic metaturbidites in the Lower Ugab Domain, Namibia, contain a complex network of four sets of quartz-calcite veins, overprinted by km-scale folds associated with four regional foliations. The veins formed by fluid overpressure predating the main deformation. Deformation structures developed at the junction of two mobile belts during the assembly of Gondwana, the NS Kaoko Belt, and the EW trending Damara Belt. Km-scale NS trending folds were initiated during EW constriction in the Kaoko Belt, while their further development and all subsequent events are related to constriction in the EW-Damara Belt, with coeval sinistral strike slip in the Kaoko Belt. Deformation of the veins, and development of four orthogonal foliations are due to gradual changes in the bulk tectonic framework rather than separate orogenic events. The veins are deformed in a complex manner allowing a full 3D reconstruction of regional sequence of events. The local complex tectonics could be reconstructed because of the perfect local exposure and the multitude of veins: it illustrates the potential complexity of tectonic events and structural evolution in apparently simple slate belts.  相似文献   

20.
In orogenic belts, a basal décollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover. In the Early Mesozoic intracontinental Xuefengshan Belt of South China, such a décollement zone is exposed in the core of anticlines formed by the emplacement of the late-orogenic granitic plutons. Our detailed, multi-scale structural analysis documents a synmetamorphic ductile deformation. In the basal décollement, the Neoproterozoic pelite and sandstone, and the intruding Early Paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss, respectively. The metamorphic foliation contains a NW–SE stretching lineation associated with top-to-the-NW kinematic indicators. The ductile shearing of these high-strained rocks can be correlated with NW-verging folds and thrusts recognized in the Neoproterozoic to Early Triassic sedimentary cover. Monazite U–Th–Pbtot chemical dating, and zircon SIMS U–Pb dating provide age constraints of the ductile shearing between 243 and 226?Ma, and late-orogenic granite emplacement around 235–215?Ma. In agreement with recent geochronological data, these new results show that the Xuefengshan Belt is an Early Mesozoic orogen dominated by the NW-directed shearing and thrusting. At the southeastern boundary of the Xuefengshan Belt, the Chenzhou-Linwu fault separates the Early Mesozoic domain to the NW from the Early Paleozoic domain to the SE. The tectonic architecture of this belt was possibly originated from the continental underthrusting to the SE of the South China block in response to northwest-directed subduction of the Paleo-Pacific plate.  相似文献   

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