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1.
The Sylhet Basin of Bangladesh is a sub-basin of the Bengal Basin. It contains a very thick (up to 22 km) Tertiary stratigraphic succession consisting mainly of sandstones and mudstones. The Sylhet succession is divided into the Jaintia (Paleocene–late Eocene), Barail (late Eocene–early Miocene), Surma (middle–late Miocene), Tipam (late Miocene–Pliocene) and Dupitila Groups (Pliocene–Pleistocene), in ascending order. The origin of the organic matter (OM) and paleoenvironment of deposition have been evaluated on the basis of C, N, S elemental analysis, Rock-Eval pyrolysis and gas chromatography–mass spectrometry (GC–MS) analysis of 60 mudstone samples collected from drill core and surface outcrops. Total organic carbon (TOC) content ranges from 0.11% to 1.56%. Sulfur content is low in most samples. TOC content in the Sylhet succession varies systematically with sedimentation rate, with low TOC caused by clastic dilution produced by high sedimentation rates arising from rapid uplift and erosion of the Himalaya.The OM in the succession is characterized by systematic variations in pristane/phytane (Pr/Ph), oleanane/C30 hopane, n-C29/n-C19 alkane, Tm/Ts [17α(H)-22,29,30-trisnorhopane/18α(H)-22,29,30-trisnorhopane] and sterane C29/(C27 + C28 + C29) ratios during the middle Eocene to Pleistocene. Based on biomarker proxies, the depositional environment of the Sylhet succession can be divided into three phases. In the first (middle Eocene to early Miocene), deposition occurred completely in seawater-dominated oxic conditions, with abundant input of terrestrial higher plants, including angiosperms. The second phase (middle to late Miocene) consisted of mainly freshwater anoxic conditions along with a small seawater influence according to eustasic sea level change, with diluted OM derived from phytoplankton and a lesser influence from terrestrial higher plants. Oxygen-poor freshwater conditions prevailed in the third phase (post-late Miocene). Planktonic OM was relatively abundant in this stage, while a high angiosperm influx prevailed at times. Tmax values of ca. 450 °C, vitrinite reflectance (Ro) of ca. 0.66% and methylphenanthrene index (MPI 3) of ca. 1 indicate the OM to be mature. The lower part (middle Eocene to early Miocene) of the succession with moderate TOC content and predominantly terrestrial OM could have generated some condensates and oils in and around the study area.  相似文献   

2.
Mineral exploration of prospective areas concealed by extensive post-mineralization cover is growing, being very complex and expensive. The projection of rich and giant Paleocene to early Oligocene porphyry-Cu-Mo belts in northernmost Chilean Andes (17.5–19.5°S) has major exploration potential, but only a few minor deposits have been reported to date, due to the fact that the area is largely covered by post-mineral strata. We integrate the Cenozoic stratigraphic, structural and metallogenic evolution of this sector, in order to identify the most promising regions related to lesser post-mineral cover and the projection of different metallogenic belts. The Paleocene to early Eocene metallogenic belt extends along the Precordillera, with ca. 30 km wide, and includes porphyry-Cu prospects and small Cu (±Mo-Au-Ag) vein and breccia-pipe deposits. Geochronological data indicate an age of 55.5 Ma for an intrusion related to one deposit and ages from 69.5 to 54.5 Ma for hydrothermal alteration in one porphyry-Cu prospect and largest known Cu deposits. The middle Eocene to early Oligocene porphyry belt, in the Western Cordillera farther east, is associated with 46–44 Ma intrusions. It is estimated to be 40-km wide, but is largely concealed by thick post-mineral cover. The youngest Miocene to early Pliocene metallogenic belt, also in the Western Cordillera, is well-exposed and includes Au-Ag epithermal and polymetallic veins and manto-type deposits.The Oligocene-Holocene cover consists of a succession of continental sedimentary and volcanic rocks that overall increase in thickness from 0 to 5000 m, from west to east. These strata are subhorizontal in the west and folded-faulted towards the east. Miocene gentle anticlines and monocline flexures extend along strike for 30–60 km in the Precordillera and were generated by propagation of high-angle east-dipping blind reverse faults with at least 300–900 m of Oligocene bedrock offset. The thickness of cover exceeds 2000 m in the eastern Central Depression, whereas it is generally less than 1000 m in the Precordillera along the Paleocene to early Eocene porphyry-Cu belt and it can reach locally up to 5000 m in the Western Cordillera, above the middle Eocene to early Oligocene belt.In the studied Andean segment, the Miocene to early Pliocene metallogenic belt is superimposed on the Paleocene to Oligocene belts in a 40–50 km wide zone. This overlap may be explained by an accentuated migration of the magmatic front, from east to west, since ca. 25 Ma, as a consequence of subduction slab steepening after a period of magmatic lull and flat subduction from ca. 30–35 to 25 Ma. The identified areas of lesser cover thickness are prone to exploration for concealed deposits, especially along the projection of major porphyry-Cu-Mo belts.  相似文献   

3.
Western Tibet, between the Karakorum fault and the Gozha–Longmu Co fault system, is mostly internally drained and has a 1.5–2 km amplitude relief with km-large valleys. We investigate the origin of this peculiar morphology by combining a topography analysis and a study of the Cenozoic sedimentation in this area. Cenozoic continental strata correspond to a proximal, detrital fan deposition, and uncomformably rest on a palaeorelief similar to the modern one. Zircon U–Pb dating from trachytic flows interbedded within the Cenozoic continental sediments indicates that detrital sedimentation occurred at least between ca 24 and 20 Ma in the Shiquanhe basin, while K/Ar ages suggest it may have started since ~ 37 Ma in the Zapug basin. The distribution of continental deposits shows that present-day morphology features, including km-large, 1500 m-deep valleys, were already formed by Early Miocene times. We suggest that today's internally drained western Tibet was externally drained, at least during late Miocene, contemporaneously with early motion along the Karakorum Fault. Detailed study of the present day river network is compatible with a dextral offset on the Karakorum Fault of 250 km at a rate of ~ 10 ± 1 mm/yr. Displacement along the Karakorum fault possibly induced the shift from external to an internal drainage system, by damming of the Bangong Co ~ 4 Ma ago, leading to the isolation and preservation of the western Tibet relief.  相似文献   

4.
Lake Issyk-Kul occupies a large Late Mesozoic–Cenozoic intramontane basin between the mountain ranges of the Northern Kyrgyz Tien Shan. These ranges are often composed of granitoid basement that forms part of a complex mosaic assemblage of microcontinents and volcanic arcs. Several granites from the Terskey, Kungey, Trans-Ili and Zhetyzhol Ranges were dated with the zircon U/Pb method (SHRIMP, LA-ICP-MS) and yield concordant Late Ordovician–Silurian (~ 456–420 Ma) emplacement ages. These constrain the “Caledonian” accretion history of the Northern Kyrgyz Tien Shan in the amalgamated Palaeo-Kazakhstan continent. The ancestral Tien Shan orogen assembled in the Early Permian when final closure of the Turkestan Ocean ensued collision of Palaeo-Kazakhstan and Tarim. A Late Palaeozoic structural basement fabric formed and Middle–Late Permian post-collisional magmatism added to crustal growth of the Tien Shan. Permo‐Triassic cooling (~ 300–220 Ma) of the ancestral Tien Shan was unraveled using 40Ar/39Ar K-feldspar and titanite fission-track (FT) thermochronology on the Issyk-Kul granitoids. Apatite thermochronology (FT and U–Th–Sm/He) applied to the broader Issyk-Kul region elucidates the Meso-Cenozoic thermo-tectonic evolution and constrains several tectonic reactivation episodes in the Jurassic, Cretaceous and Cenozoic. Exhumation of the studied units occurred during a protracted period of intracontinental orogenesis, linked to far-field effects of Late Jurassic–Cretaceous accretion of peri-Gondwanan blocks from the Tethyan realm to Eurasian. Following a subsequent period of stability and peneplanation, incipient building of the modern Tien Shan orogen in Northern Kyrgyzstan started in the Oligocene according to our data. Intense basement cooling in distinct reactivated and fault-controlled sections of the Trans-Ili and Terskey Ranges finally pinpoint important Miocene–Pliocene (~ 22–5 Ma) exhumation of the Issyk-Kul basement. Late Cenozoic formation of the Tien Shan is associated with ongoing indentation of India into Eurasia and is a quintessential driving force for the reactivation of the entire Central Asian Orogenic Belt.  相似文献   

5.
《Gondwana Research》2014,25(3):1186-1201
The modern Koala Phascolarctos cinereus is the last surviving member of a once diverse family Phascolarctidae (Marsupialia, Phascolarctomorphia). Nine genera and at least 16 species of koala are known. Late Oligocene sediments of central Australia record the oldest fossils and highest species diversity. Five species are known from the early to middle Miocene rainforest assemblages of the Riversleigh World Heritage Area, Queensland. With the onset of dryer conditions after the middle Miocene climatic optimum (~ 16 Ma), rainforest habitats contracted resulting in the apparent extinction of three koala lineages (Litokoala, Nimiokoala, Priscakoala). Phascolarctos first appears in the fossil record during the Pliocene and the modern species around 350 ka. Despite a dramatic decline in taxonomic diversity to a single extant species, the fossil record indicates that at most only three koala species coexisted in any given faunal assemblage throughout their 24 million year history. Within these assemblages, the vast majority of extinct koalas are extremely rare (some known from only a single specimen) which may reflect a general rarity within their palaeohabitats compared with the modern species which is represented by an estimated 400,000 individuals spread over most of eastern mainland Australia. Be that as it may, P. cinereus, although once geographically more widespread, occurring for example in Western Australia in the Pleistocene, underwent significant range contractions and localized population extinctions during the stressful climatic conditions of the late Pleistocene and more recently through human-induced habitat destruction. Combined with threats of disease, reduced genetic diversity and climate change, the survival of this iconic Australian marsupial is arguably a cause for concern.  相似文献   

6.
Late glacial and early Holocene summer temperatures were reconstructed based on fossil chironomid assemblages at Lake Brazi (Retezat Mountains) with a joint Norwegian–Swiss transfer function, providing an important addition to the late glacial quantitative climate reconstructions from Europe. The pattern of the late glacial temperature changes in Lake Brazi show both similarities and some differences from the NGRIP δ18O record and other European chironomid-based reconstructions. Our reconstruction indicates that at Lake Brazi (1740 m a.s.l.) summer air temperature increased by ~ 2.8°C at the Oldest Dryas/Bølling transition (GS-2/GI-1) and reached 8.1–8.7°C during the late glacial interstade. The onset of the Younger Dryas (GS-1) was characterized by a weak (< 1°C) decrease in chironomid-inferred temperatures. Similarly, at the GS-1/Holocene transition no major changes in summer temperature were recorded. In the early Holocene, summer temperature increased in two steps and reached ~ 12.0–13.3°C during the Preboreal. Two short-term cold events were detected during the early Holocene between 11,480–11,390 and 10,350–10,190 cal yr BP. The first cooling coincides with the Preboreal oscillation and shows a weak (0.7°C) temperature decrease, while the second is characterized by 1°C cooling. Both cold events coincide with cooling events in the Greenland ice core records and other European temperature reconstructions.  相似文献   

7.
The Paleocene (66–56 Ma) was a critical time interval for understanding recovery from mass extinction in high palaeolatitudes when global climate was warmer than today. A unique sedimentary succession from Seymour Island (Antarctic Peninsula) provides key reference material from this important phase of the early Cenozoic. Dinoflagellate cyst data from a 376 m thick stratigraphical section, including the Cretaceous–Paleogene boundary, is correlated with biozones from New Zealand, the East Tasman Plateau and southeastern Australia. A detailed age model is suggested for the López de Bertodano (LDBF) and Sobral (SF) formations based on dinoflagellate cyst biostratigraphy and U–Pb dating of zircons, supported by correlated magnetostratigraphy and strontium isotope values from macrofossils. The top of the LDBF is confirmed as latest Maastrichtian to earliest Danian (~ 66.2–65.65 Ma) in age. The overlying SF is mostly Danian in age, with an inferred hiatus near the top overlain by sediments dated as ?late Thanetian. Rare Apectodinium homomorphum first appear in the uppermost SF; the earliest in situ record from Antarctica. The distribution of marine and terrestrial fossils from uppermost Cretaceous to Eocene sediments in Patagonia, Antarctica, New Zealand and Australia required both sea and land connections between these fragments of Gondwana. Fossil evidence and reconstructions of Antarctic palaeogeography and palaeotopography reveal evidence for persistent embayments in the proto-Weddell and Ross Sea regions at this time. We conclude that a coastal dispersal route along the palaeo-Pacific margin of Gondwana could explain the fossil distribution without requiring a transAntarctic strait or closely spaced archipelago. A region in the West to East Antarctic boundary zone, elevated until the early Paleogene, perhaps acted as a site for high elevation ice caps. This supports fossil, geochemical and sedimentological evidence for cold climate intervals and significant sea level falls during the Maastrichtian and Paleocene.  相似文献   

8.
Porphyry copper deposits (PCDs) in Iran are dominantly distributed in Arasbaran (NW Iran), the middle segment of the Urumieh–Dokhtar Magmatic Arc (UDMA), and Kerman (central SE Iran), with minor occurrences in eastern Iran and the Makran arc. This paper provides a temporal–spatial and geodynamic framework of the Iranian porphyry Cu (Mo–Au) systems, based on geochronologic data obtained from zircon U–Pb and molybdenite Re–Os dating of host porphyritic rocks and molybdenites in 15 major PCDs. The dating results define a long metallogenic duration (39–6 Ma), and suggest a long history of tectonic evolution from the accretionary orogeny related to early Cenozoic closure of the Neo-Tethys Ocean to subsequent collisional orogeny for the Iranian porphyry copper systems.The oldest porphyry mineralization occurred in the eastern part of Iran after the closure of a branch of the Neo-Tethyan (Sistan) Ocean between the Lut and Afghan blocks in the late Eocene (39–37 Ma). This was followed by mineralization in the Kerman porphyry copper belt over a time interval of about 20 m.y., where two metallogenic epochs have been recognized, including late Oligocene (29–27 Ma) and Miocene (18–6 Ma). The Bondar-e-Hanza deposit formed in the late Oligocene, while and the remaining dated deposits belong to Miocene epoch. According to the deposits' characteristics and their ages, the Miocene epoch can be divided into early, middle, and late stages. The Darreh Zar, Bakh Khoshk, Chah Firouzeh and Sar Kuh deposits formed during the early–middle Miocene. The largest porphyry deposits occur in the middle stage during the middle Miocene (14–11 Ma) and include the Sar Cheshmeh, Meiduk, Dar Alu and Now Chun deposits. These deposits were formed during crustal thickening, uplift, and rapid exhumation of the belt. The final stage of porphyry mineralization occurred during the late Miocene (9–6 Ma), and formed the Iju, Kerver, Kuh Panj and Abdar deposits.There were two porphyry mineralization stages in the Arasbaran porphyry copper belt in NW Iran, including an older late Oligocene (29–27 Ma) and a younger early Miocene (22–20 Ma) events. The Haft Cheshmeh deposit belongs to the older stage, and the world-class Sungun and Masjed Daghi deposits formed during the early Miocene.In the middle segment of the UDMA (Saveh–Yazd porphyry copper belt), PCDs formed during middle Miocene time (17–15 Ma). The geochronological results reveal that the porphyry mineralization moved from the northwest to southeast of UDMA over the time.Our dating results, combined with the possible late Eocene–Oligocene timing for collision between the Arabian and Iranian plates, support a model for Iranian PCD formation by partial melting of previously subduction-modified lithosphere in a post-subduction and post-collisional tectonic setting.  相似文献   

9.
Anza-Borrego Desert State Park® (ABDSP) has an exceptional proboscidean fossil record. The remains of Gomphotheriidae and Elephantidae span over 10 Ma and encompass over eighty productive sites. The most numerous of the proboscideans are the mammoths. Significant specimens include one of the youngest Gomphotherium and the most complete skeleton of Mammuthus meridionalis in North America.Over 6 km of fossiliferous sediments spans the Miocene–Pliocene and Pliocene–Pleistocene boundaries and provides a continuous record of changing environments. Over 550 taxa of aquatic and terrestrial plants, marine and lacustrine invertebrates, and marine, fresh water and terrestrial vertebrates are represented. Present are both Asian and South American immigrant mammals. M. meridionalis and M. columbi (=M. imperator) regionally co-existed here. A specimen from the Diablo Formation extends the temporal range of Gomphotherium in North America nearly one million years into the middle Blancan.  相似文献   

10.
Reconstruction of Mesozoic and Cenozoic sedimentary ‘cover’ on the Precambrian shield in the Lac de Gras diamond field, Northwest Territories, Canada, has been achieved using Cretaceous and early Tertiary sedimentary xenoliths and contemporaneous organic matter preserved in volcaniclastic sediments associated with late Cretaceous to early Tertiary kimberlite pipe intrusions, and in situ, Eocene crater lake, lacustrine and peat bog strata. Percent reflectance in oil (%Ro) of vitrinite within shale xenoliths for: (i) Albian to mid-Cenomanian to Turonian ranges from > 0.27 to 0.42 %Ro (mean = 0.38 %Ro), (ii) Maastrichtian to early Paleocene from 0.24 to < 0.30%; (iii) latest Paleocene to early middle Eocene 0.15 to < 0.23 %Ro (mean = 0.18 %Ro). These levels of thermal maturity are corroborated by Rock Eval pyrolysis Tmax (°C) and VIS region fluorescence of liptinites, with wavelengths of maximum emission for sporinite, prasinophyte alginite and dinoflagellates consistent with vitrinite reflectance of 0.20 to < 0.50 %Ro. Burial–thermal history modeling, constrained by measured vitrinite reflectance and porosity of shale xenoliths, predicts a maximum burial temperature for Mid to Late Albian strata (∼115 Ma) of 60 °C with ∼1.2 to 1.4 km of Cretaceous strata in the Lac de Gras kimberlite field region prior to major uplift and erosion, which began at 90 Ma. Late Paleocene to middle Eocene volcanic crater lake lacustrine to peat bog strata were only buried to a few hundreds of meters and are in a peat-brown coal stage of thermal maturation.  相似文献   

11.
The Yellow River system, the largest river system in northern China, generally flows northeasterly through a series of linear mountain belts in the northeastern margin of the Tibetan plateau, the youngest of which are the Laji–Jishi Shan and Riyue Shan ranges, formed during late Cenozoic time due to NE–SW oblique shortening. As the product of the interaction between the tectonic process and the climate, the incision of the Yellow River system is a crucial parameter in models of the scale and timing of the crustal uplift and erosion in northeastern Tibetan plateau. Thus, whether the along-strike topographic feature of the Laji-Jishi Shan that is cut through by the Yellow River system and related streams is controlled by structural deformation or by erosion needs to be constrained. Our mapping shows that the variation in deformation along this mountain belt formed two structural saddles with relative low elevation in late Cenozoic time, through which the Yellow and Yaoshui Rivers cut into the plateau and drained a series of the Tertiary basins. The Yaoshui River is the tributary of the Huangshui River which itself flows into the Yellow River in the Lanzhou area. One saddle is present along the Yaoshui River valley, formed by NW–SE extension along the Riyue Shan Pass (RSP) normal fault, along which the Miocene and Mesozoic rocks were subsided against Proterozoic metamorphic rocks. These deformed rocks in the hanging wall are truncated by a sub-horizontal erosion surface at an elevation of 3200 m, on which terrace deposits are locally present, presumably middle Pleistocene in age. This terrace is incised by the Yaoshui River to an elevation of 3000 m, which yields 300 m of incision. Another saddle is along the Yellow River valley (the Xunhua-Linxia gouge) between the southern tip of the Laji Shan and the northern tip of the Jishi Shan, generated by en echelon folding. This structural saddle is underlain by the lower Cretaceous and Pliocene clastic rocks, which are truncated on the top by a rugged erosion surface at an average elevation of 3000 m. The Yellow River incised into this surface to an elevation of 1900 m, which yields 1100 m of incision. These two saddles, featured by topographic and structural low, were formed in the middle or late Miocene, and facilitated the headward propagation of the Yellow and Yaoshui Rivers, which initiated in early and middle Pleistocene time, respectively.  相似文献   

12.
The Tibetan Plateau (TP) is the highest plateau in the world, which has been the focus of Cenozoic geological studies. The Northeast Tibetan Plateau (NETP) is a key location to decipher the Cenozoic evolution history of the TP. Understanding the building of the Qimen Tagh Mountains located in NETP will help to constrain the development of the northern boundary of the main TP, test the existence of a Paleo-Qaidam Basin and test the eastward growth model of the TP. In this study, granite samples from the Qimen Tagh Mountains were dated by LA-ICPMS and apatite fission track (AFT). The LA-ICPMS zircon U–Pb ages give two magmatic events around ~ 405 and ~ 255 Ma from two different sites. AFT modeling shows that the initial uplift took place at ~ 40–30 Ma in these mountains, which should be controlled by the Altyn Tagh Fault. Compiling previously low-temperature thermochronometry results, it reveals that the initial Cenozoic uplift of the northern boundary of the TP (Qimen Tagh and East Kunlun mountains), soon after the India–Eurasia collision in the southern TP, has divided the Paleo-Qaidam Basin into several sub-basins. The approximate NE–E growth process occurred along the lithospheric Altyn Tagh and Kunlun faults. The current basin and range morphology of the NETP took place around ~ 8 Ma.  相似文献   

13.
The Cenozoic terrestrial, intermontane Qaidam Basin on the northeastern edge of the Tibetan Plateau contains > 12 km of sedimentary rocks that potentially document the accommodation of India-Asia convergence and the growth of the plateau. The chronology remains incomplete, hindering cross-basin correlation between lithostratigraphic units and their further interpretation. Here we present a high-resolution magnetostratigraphy spanning > 5 km of Paleogene-Neogene sequence at Dahonggou in the Northern Qaidam Basin. Based on correlation with the geomagnetic polarity time scale (GPTS), we have dated the section to being between ~ 52 and ~ 7 Ma. The bottom conglomeratic unit, ranging from > 52 Ma to ~ 44 Ma, was deposited in high-energy environments (e.g., alluvial fan or braided river), reflecting the earliest deformation and uplift of the basin-bounding Qilian Shan fold-thrust belt in response to India-Asia collision. In addition, we identified two major increases in sedimentation rate at 25–16 Ma and after ~ 9.5 Ma and three phases of lesser increases at 52–44 Ma, 38–33 Ma, and 14.6–12.0 Ma. These increases in sedimentation rate are consistent with regional thermochronology and basin analysis studies, which revealed enhanced motion on basin-bounding thrust faults. We argue that these accelerated sedimentation rates indicate pulsed tectonism in the northeastern Tibetan Plateau. The pulse at 25–16 Ma may further relate to phases of strong rainfall linked to an intense monsoon at that time.  相似文献   

14.
The formation of the eastern Pontides orogenic belt has been widely assigned to a northward subduction of the Neotethyan oceanic slab during the late Mesozoic–Cenozoic. Here we provide an alternate model based on new geological, geochemical and isotopic data. The magmatic activity in the far south of the belt started in the early Campanian with shoshonitic trachyandesites and associated pyroclastics. This sequence is covered by the late Campanian–early Maastrichtian reefal limestones and another stage of high-K volcanism represented by analcimized leucite-rich ultrapotassic rocks of the Maastrichtian–early Paleocene (?) ages. The shoshonitic and ultrapotassic rocks, with K2O contents ranging from 0.26 to 6.95 wt.%, display broadly similar rare earth and multi-element distribution patterns. Both rock types are enriched in LILE and LREE and depleted in HFSE (Nb, Ta and Ti), suggesting a subduction-enriched mantle source for the magma generation. Subsequently, during the late Paleocene, a stage of acidic magmatism (SiO2 of 53.25–73.61 wt.%) that shows adakitic geochemical characteristics including high Sr/Y (46–416) and La/Yb (11–51) and low Y (2.6–12.2 ppm), is documented characterized by melting of a mafic source such as the MORB crust with garnet in the residue. The adakitic magmatism began at ~ 56 Ma and migrated toward the north through time, culminating with porphyritic andesites (~ 47 Ma) that were emplaced in the Gumushane–Bayburt line and its vicinity. North of this line, coeval magmas show typical calc-alkaline nature and continued to develop toward further north until the middle to late Eocene. Based on the spatial and temporal variations in the magmas generated in the eastern Pontides orogenic belt, we propose a new geodynamic model to explain the tectonomagmatic evolution of these rocks and correlate the adakitic magmatism to ridge subduction and slab window process within a south-dipping subduction zone. Our model is in contrast to the previous proposals which envisage partial melting or delamination of thickened lower continental crust due to the collision in the south during the Paleocene–Eocene.  相似文献   

15.
The Eucla Basin including the vast Nullarbor Plain lies on the margins of the Yilgarn, Musgrave and Gawler cratons in southern Australia and owes its distinctive landscape to a unique set of interactions between eustatic, climatic and tectonic processes over the last ~ 50 Ma. Understanding of the history of the basin and the palaeovalleys that drained from the surrounding cratons are important because they contain major mineral deposits, and the sediments derived from them contain remobilised gold, uranium, and heavy minerals. In particular, a remarkably preserved palaeoshoreline sequence along the north-eastern margin of the Eucla Basin is highly prospective for heavy mineral placer deposits. The record of marine, marginal marine, estuarine, fluvial and lacustrine environments, as constrained mainly by an extensive borehole dataset, reflects major depositional events during the Palaeocene–Early Eocene, Middle–Late Eocene, Oligocene–Early Miocene, Middle Miocene–Early Pliocene and Pliocene–Quaternary. These events reflect the key role of eustatic sea-level variation which, during highstands, inundated the craton margins, flooding palaeovalleys to up to 400 km inboard of the present coastline. However, a systematic eastward migration of the depocentre across the Eucla Basin during the Neogene, together with apparent flow reversals in a number of palaeovalley systems draining the Gawler Craton, suggest that the Eucla Basin has also been subject to differential vertical movements, expressed as a west-side up, east-side down tilting of ~ 100–200 m. This differential movement forms part of a broader north-down–southwest-up dynamic topographic tilting of the Australian continent associated with relatively fast (6–7 cm/yr) northward plate motion since fast spreading commenced in the Southern Ocean at ~ 43 Ma. We suggest that the evolving dynamic topography field has played a key role in facilitating development of placer deposits, largely through multistage, eastward reworking of near-shore sequences during highstand transgressive cycles on a progressively tilting platform under the influence of persistent westerly weather systems.  相似文献   

16.
《Gondwana Research》2014,25(3):1202-1215
The South China Block, consisting of the Yangtze and the Cathaysia blocks, is one of the largest Precambrian blocks in eastern Asia. However, the early history of the Cathaysia Block is poorly understood due largely to intensive and extensive reworking by Phanerozoic polyphase orogenesis and magmatism which strongly overprinted and obscured much of the Precambrian geological record. In this paper, we use the detrital zircon U–Pb age and Hf isotope datasets as an alternative approach to delineate the early history of the Cathaysia Block. Compilation of published 4041 Precambrian detrital zircon ages from a number of (meta)sedimentary samples and river sands exhibits a broad age spectrum, with three major peaks at ~ 2485 Ma, ~ 1853 Ma and ~ 970 Ma (counting for ~ 10%, ~ 16% and ~ 24% of all analyses, respectively), and four subordinate peaks at ~ 1426 Ma, ~ 1074 Ma, ~ 780 Ma and ~ 588 Ma. Five of seven detrital zircon age peaks are broadly coincident with the crystallisation ages of ~ 1.89–1.83 Ga, ~ 1.43 Ga, ~ 1.0–0.98 Ga and ~ 0.82–0.72 Ga for known igneous rocks exposed in Cathaysia, whereas, igneous rocks with ages of ~ 2.49 Ga and ~ 0.59 Ga have not yet been found. The Hf isotopic data from 1085 detrital zircons yield Hf model ages (TDMC) between ~ 4.19 Ga and ~ 0.81 Ga, and the calculated εHf(t) values between − 40.2 and 14.4. The Archean detrital zircons are exclusively oval in shape with complicated internal textures, indicating that they were sourced by long distance transportations and strong abrasion from an exotic Archean continent. In contrast, the majority of detrital zircons in age between ~ 1.9 and ~ 0.8 Ga are euhedral to subhedral crystals, indicative of local derivation by short distance transportations from their sources. The oldest crustal basement rocks in Cathaysia were most likely formed by generation of juvenile crust and reworking of recycled Archean components in Late Paleoproterozoic at ~ 1.9–1.8 Ga, rather than in the Archean as previously speculated. Reworking and recycling of the continental crust are likely the dominant processes for the crustal evolution of Cathaysia during the Mesoproterozoic to Neoproterozoic time, with an intervenient period of significant generation of juvenile crust at ~ 1.0 Ga.Precambrian crustal evolutions of the Cathaysia Block are genetically related to the supercontinent cycles. By comparing detrital zircon data from Cathaysia with those for other continents, and integrating multiple lines of geological evidence, we interpret the Cathaysia Block as an orogenic belt located between East Antarctica, Laurentia and Australia during the assembly of supercontinent Columbia/Nuna at ~ 1.9–1.8 Ga. The Cathaysia Block amalgamated with the Yangtze Block to form the united South China Block during the Sibao Orogeny at ~ 1.0–0.89 Ga. The Laurentia–Cathaysia–Yangtze–Australia–East Antarctica connection gives the best solution to the paleo-position of Cathaysia in supercontinent Rodinia. The significant amount of ~ 0.6–0.55 Ga detrital zircons in Cathaysia and West Yangtze have exclusively high crustal incubation time of > 300 Ma, indicating crystallisation from magmas generated dominantly by crustal reworking. This detrital zircon population compares well with the similar-aged zircon populations from a number of Gondwana-derived terranes including Tethyan Himalaya, High Himalaya, Qiangtang and Indochina. The united South China–Indochina continent was likely once an integral part of Gondwanaland, connected to northern India by a “Pan-African” collisional orogen.  相似文献   

17.
Cenozoic conglomerates are exposed discontinuously along the length of the Yarlung Tsangpo suture zone on the southern margin of the Gangdese arc. These conglomerates (the “Gangdese Conglomerates” herein) record a crucial stage in the uplift and erosion histories of the southern Tibet after the initial India–Asia collision. In the Mt. Kailas area, the Gangdese Conglomerates strata consist of multiple sedimentary cycles and each cycle is a fining-upward sequence that was deposited by alluvial fan, braided-river and delta systems. Whereas in the Xigaze area, the Gangdese Conglomerates strata comprise a coarsening-upward sequence that was deposited by delta, braided-river and alluvial fan systems. Based on the detrital and igneous zircon U–Pb ages, the depositional ages of the Gangdese Conglomerates are late Oligocene to early Pliocene (ca. 26–5 Ma) in the Mt. Kailas area, late Oligocene to middle Miocene (ca. 26–15 Ma) in the Xigaze area, and late Oligocene to early Miocene (ca. 26–19 Ma) in the Zedong area. Paleocurrent measurements and provenance data (i.e., conglomerate clast composition, sandstone petrology and detrital zircon age) indicate that the initial detritus of the Gangdese Conglomerates were entirely derived from the north (mainly from the Gangdese arc). Sediment resulting from denudation to the south (the Xigaze forearc basin, the Yarlung Tsangpo suture zone and the northern margin of the Indian plate) first appeared by the early Miocene (ca. 19 Ma) and subsequently increased in abundance gradually. Our new results, together with previous data from the Xigaze area, reveal 3 major stages in the evolution of the Yarlung Tsangpo River system: (1) the southward-flowing stage (ca. 26–19 Ma) featured southward-draining transverse rivers that transported materials from the Gangdese arc southward. Southward paleocurrents in the Gangdese Conglomerates indicate a northern source. (2) The westward-flowing stage (ca. 19–15 Ma) developed due to the uplift of the suture zone and Tethys Himalaya to the south. Northward-draining rivers began to develop, and lakes resembling a string of beads formed and finally connected together, initiating the westward-flowing paleo-Yarlung Tsangpo River. Westward paleoflows were recorded in the Gangdese Conglomerates. (3) The eastward-flowing stage (ca. 15 Ma–present) resulted from differential uplift and denudation of the southern Tibet, which reversed the direction of the young Yarlung Tsangpo River by ca. 15 Ma. The deposition of the Gangdese Conglomerates was controlled by eastward paleoflows. At this point, the modern eastward-flowing Yarlung Tsangpo River system was established.  相似文献   

18.
The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27 Ma.Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, 206Pb/204Pb vs. Th/Nb, Th/Nb vs. δ18O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event.  相似文献   

19.
The Western foreland basin in Taiwan originated through the oblique collision between the Luzon volcanic arc and the Asian passive margin. Crustal flexure adjacent to the growing orogenic load created a subsiding foreland basin. The sedimentary record reveals progressively changing sedimentary environments influenced by the orogen approaching from the East. Based on sedimentary facies distribution at five key stratigraphic horizons, paleogeographic maps were constructed. The maps highlight the complicated basin-wide dynamics of sediment dispersal within an evolving foreland basin.The basin physiography changed very little from the middle Miocene (∼12.5 Ma) to the late Pliocene (∼3 Ma). The transition from a passive margin to foreland basin setting in the late Pliocene (∼3 Ma), during deposition of the mud-dominated Chinshui Shale, is dominantly marked by a deepening and widening of the main depositional basin. These finer grained Taiwan derived sediments clearly indicate increased subsidence, though water depths remain relatively shallow, and sedimentation associated with the approach of the growing orogen to the East.In the late Pleistocene as the shallow marine wedge ahead of the growing orogen propagated southward, the proximal parts of the basin evolved into a wedge-top setting introducing deformation and sedimentation in the distal basin. Despite high Pleistocene to modern erosion/sedimentation rates, shallow marine facies persist, as the basin remains open to the South and longitudinal transport is sufficient to prevent it from becoming overfilled or even fully terrestrial.Our paleoenvironmental and paleogeographical reconstructions constrain southward propagation rates in the range of 5–20 km/Myr from 2 Ma to 0.5 Ma, and 106–120 km/Myr between late Pleistocene and present (0.5–0 Ma). The initial rates are not synchronous with the migration of the sediment depocenters highlighting the complexity of sediment distribution and accumulation in evolving foreland basins.  相似文献   

20.
《Gondwana Research》2016,29(4):1482-1499
The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. 40Ar/39Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.  相似文献   

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