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1.
The Longmen Shan Foreland Basin developed as a flexural foredeep during the Late Triassic Indosinian orogeny, spanning the time period c. 227–206 Ma. The basin fill can be divided into three tectonostratigraphic units overlying a basal megasequence boundary, and is superimposed on the Palaeozoic–Middle Triassic (Anisian) carbonate‐dominated margin of the South China Block. The remains of the load system responsible for flexure of the South China foreland can be seen in the Songpan‐Ganzi Fold Belt and Longmen Shan Thrust Belt. Early in its history the Longmen Shan Foreland Basin extended well beyond its present northwestern boundary along the trace of the Pengguan Fault, to at least the palinspastically restored position of the Beichuan Fault. The basal boundary of the foreland basin megasequence is a good candidate for a flexural forebulge unconformity, passing from conformity close to the present trace of the Beichuan Fault to a karstified surface towards the southeast. The overlying tectonostratigraphic unit shows establishment and drowning of a distal margin carbonate ramp and sponge build‐up, deepening into offshore marine muds, followed by progradation of marginal marine siliciclastics, collectively reminiscent of the Alpine underfilled trinity of Sinclair (1997) . Tectonostratigraphic unit 2 is marked by the severing of the basin's oceanic connection, a major lake flooding and the gradual establishment of major deltaic‐paralic systems that prograded from the eroding Longmen Shan orogen. The third tectonostratigraphic unit is typified by coarse, proximal conglomerates, commonly truncating underlying rocks, which fine upwards into lacustrine shales. The foreland basin stratigraphy has been further investigated using a simple analytical model based on the deflection by supracrustal loads of a continuous elastic plate overlying a fluid substratum. Load configurations have been partly informed by field geology and constrained by maximum elevations and topographic profiles of present‐day mountain belts. The closest match between model predictions and stratigraphic observations is for a relatively rigid plate with flexural rigidity on the order of 5 × 1023 to 5 × 1024 N m (equivalent elastic thickness of c. 43–54 km). The orogenic load system initially (c. 227–220 Ma) advanced rapidly (>15 mm yr?1) towards the South China Block in the Carnian, associated with the rapid closure of the Songpan‐Ganzi ocean, before slowing to < 5 mm yr?1 during the sedimentation of the upper two tectonostratigraphic units (c. 220–206 Ma).  相似文献   

2.
The Sichuan Basin and the Songpan‐Ganze terrane, separated by the Longmen Shan fold‐and‐thrust belt (the eastern margin of the Tibetan Plateau), are two main Triassic depositional centres, south of the Qinling‐Dabie orogen. During the Middle–Late Triassic closure of the Paleo‐Tethys Ocean, the Sichuan Basin region, located at the western margin of the Yangtze Block, transitioned from a passive continental margin into a foreland basin. In the meantime, the Songpan‐Granze terrane evolved from a marine turbidite basin into a fold‐and‐thrust belt. To understand if and how the regional sediment routing system adjusted to these tectonic changes, we monitored sediment provenance primarily by using detrital zircon U‐Pb analyses of representative stratigraphic samples from the south‐western edge of the Sichuan Basin. Integration of the results with paleocurrent, sandstone petrology and published detrital zircon data from other parts of the basin identified a marked change in provenance. Early–Middle Triassic samples were dominated by Neoproterozoic (~700–900 Ma) zircons sourced mainly from the northern Kangdian basement, whereas Late Triassic sandstones that contain a more diverse range of zircon ages sourced from the Qinling, Longmen Shan and Songpan‐Ganze terrane. This change reflects a major drainage adjustment in response to the Late Triassic closure of the Paleo‐Tethys Ocean and significant shortening in the Longmen Shan thrust belt and the eastern Songpan‐Ganze terrane. Furthermore, by Late Triassic time, the uplifted northern Kangdian basement had subsided. Considering the eastward paleocurrent and depocenter geometry of the Upper Triassic deposits, subsidence of the northern Kangdian basement probably resulted from eastward shortening and loading of the Songpan‐Ganze terrane over the western margin of the Yangtze Block in response to the Late Triassic collision among Yangtze Block, Yidun arc and Qiangtang terrane along the Ganze‐Litang and Jinshajiang sutures.  相似文献   

3.
The main Karoo Basin of South Africa is a Late Carboniferous–Middle Jurassic retroarc foreland fill, developed in front of the Cape Fold Belt (CFB) in relation to subduction of the palaeo-Pacific plate underneath the Gondwana plate. The Karoo sedimentary fill corresponds to a first-order sequence, with the basal and top contacts marking profound changes in the tectonic setting, i.e. from extensional to foreland and from foreland to extensional, respectively. Sedimentation within the Karoo Foreland Basin was closely controlled by orogenic cycles of loading and unloading in the CFB. During orogenic loading, episodes of subsidence and increase in accommodation adjacent to the orogen correlate to episodes of uplift and decrease in accommodation away from the thrust-fold belt. During orogenic unloading the reverse occurred. As a consequence, the depocentre of the Karoo Basin alternated between the proximal region, during orogenic loading, and the distal region, during orogenic unloading. Orogenic loading dominated during the Late Carboniferous–Middle Triassic interval, leading to the accumulation of thick foredeep sequences with much thinner forebulge correlatives. The Late Triassic–Middle Jurassic interval was dominated by orogenic unloading, with deposition taking place in the distal region of the foreland system and coeval bypass and reworking of the older foredeep sequences. The out of phase history of base-level changes generated contrasting stratigraphies between the proximal and distal regions of the foreland system separated by a stratigraphic hinge line. The patterns of hinge line migration show the flexural peripheral bulge advancing towards the craton during the Late Carboniferous–Permian interval in response to the progradation of the orogenic front. The orogenward migration of the foreland system recorded during the Triassic–Middle Jurassic may be attributed to piggyback thrusting accompanied by a retrogradation of the centre of weight within the orogenic belt during orogenic loading (Early Middle Triassic) or to the retrogradation of the orogenic load through the erosion of the orogenic front during times of orogenic unloading (Late Triassic–Middle Jurassic).  相似文献   

4.
四川盆地雷口坡组大量钻井资料揭示中三叠统雷口坡组主要成盐期分别为雷三2期和雷四2期。岩性组合特征、沉积环境研究表明,在川中的平昌—南充—遂宁一带发育雷三2期浓缩盐膏质蒸发泻湖微相,在川中-川西地区的南部—盐亭—成都—邛崃地区发育雷四2期盐膏湖盆微相。两期盐岩差异主要表现是,雷三2期时,盆地处于频繁的海进、海退过程中,沉积环境变化较快,成盐环境相对开放,盐盆往往多次蒸发浓缩、补给、淡化,海水表现出接近或者达到盐岩沉积浓度,其间往往夹杂石膏和云质团块沉积。雷四2期时,四川盆地处于持续的海退蒸发期,蒸发量远远大于补给量,海水以蒸发浓缩为主,海水表现为超过石盐沉积浓度,沉积石盐中夹杂卤石,局部地区钾含量极高。  相似文献   

5.
《Basin Research》2018,30(1):75-96
The Xichang Basin in southeastern Tibet provides crucial information about formation and tectonic processes affecting the eastern Tibetan Plateau. To determine when and how the uplift developed, we conducted detailed studies of structures and obtained thermochronology data from the Xichang Basin and its periphery. The Xichang Basin is characterized by gentle deformation of the strata, segmented by an E‐vergent boundary thrust fault. Two stages of deformation, strike‐slip followed by an E‐W oriented shortening resulted in oblique shortening between the southeastern Tibetan Plateau and the Sichuan Basin. New apatite fission‐track data interpreted together with (U‐Th)/He data confirm a simple burial/heating and exhumation/cooling history across the Xichang Basin and its periphery. Subsidence and burial of the Xichang Basin peaked between 80–30 Ma, followed by mountain building with a protracted cooling starting at around 40–20 Ma, with rates of ca. 2.0–8.0 °C Myr−1 (i.e. 0.1–0.3 mm year−1). Our data indicate that the Xichang Basin has experienced ca. 2.5–5 km of exhumation, much more intensive than the ca. 1–2 km of exhumation inferred for the southwestern Sichuan Basin. Restored balanced cross‐sections of post‐Late‐Triassic strata along a ca. 250 km traverse indicate ca. 10–20% east‐west shortening strain (i.e. ca. 20–30 km) at the southeastern Tibetan Plateau during Cenozoic time. Study of crustal thickening and erosion supports a tectonic shortening mechanism to account for the uplift of the Xichang Basin on the southeastern Tibetan Plateau.  相似文献   

6.
Data from apatite fission track analysis are presented for 20 outcrop samples collected in the southern Adelaide Fold Belt, South Australia. Interpretation of these data, with the aid of numerical models which allow inference of multiphase cooling histories, indicate three discrete cooling events that are likely to correlate with sedimentation events in surrounding depositional settings. An event beginning some time after 85 Ma (Late Cretaceous) was characterized by cooling throughout the study area from temperatures of roughly 50 to 70 °C. An event beginning at 300–270 Ma (Late Palaeozoic) was characterized by cooling from temperatures >120 °C in all areas except for the Mount Lofty Ranges and Murray Bridge region, where peak temperatures were only 95–115 °C prior to Palaeozoic cooling. Some samples from these subregions of relatively cool Late Palaeozoic temperatures also retain evidence for even earlier cooling from temperatures >120 °C, beginning prior to 350 Ma. We interpret the post 85-Ma event as the consequence of regional exhumation from a depth of 1.0–1.6 km. The Late Palaeozoic event (300–270 Ma) is interpreted as cooling associated with the termination of the Alice Springs Orogeny, while cooling prior to 350 Ma probably represents the final stages of Early Middle Palaeozoic unroofing of the southern Adelaide Fold Belt.
The results highlight the importance of regional, episodic postorogenic exhumation of Palaeozoic fold belts, where – in some cases – conventional methods have erroneously suggested relatively long-term stability.  相似文献   

7.
The Limón back‐arc basin belongs to the southern Central American arc‐trench system and is situated at the east coast of Costa Rica. The basin‐fill consists of Late Cretaceous to Pleistocene sedimentary rocks. A northern and a southern sub‐basin can be defined, separated by the E–W‐trending Trans Isthmic Fault System. The North Limón Basin is nearly undeformed, whereas the South Limón Basin is characterized by a fold‐and‐thrust belt. Both sub‐basins have a very similar sedimentary fill and can act as a natural laboratory for reconstructing controlling factors of arc‐related sedimentary basins as well as the influence of deformation on a basin system. Modelling focused on burial history and temperature evolution. Two‐dimensional simulations were carried out with the software PetroMod®. The geohistory curve of the North Limón Basin is overall linear, indicating continuous subsidence. The South Limón Basin is also characterized by continuous subsidence, but rates strongly increased at the beginning of the Neogene. Despite a rapid Plio‐Pleistocene deformation of the fold‐and‐thrust belt, the present‐day temperature field is not disturbed in that area. The modelling results indicate a mean heat flow of 60 mW m?2 for the North Limón Basin and 41 mW m?2 for the South Limón Basin. These values are low compared with other back‐arc basins. The lower values are attributed to the following effects: (1) underlying basaltic crust, (2) the lack of an initial rift phase, (3) the low extension rates, (4) absence of volcanic activity and (5) insulation effects of a thick sediment pile. The reasons for the locally lower heat flow in the southern sub‐basin can be found in the low‐angle subduction of the Cocos Ridge. Owing to the low subduction angle, the cool fore‐arc mantle‐wedge below the island‐arc is pushed backwards increasing the cooled area.  相似文献   

8.
The Yanshan fold‐thrust belt is an exposed portion of a major Mesozoic orogenic system that lies north of Beijing in northeast China. Structures and strata within the Yanshan record a complex history of thrust faulting characterized by multiple deformational events. Initially, Triassic thrusting led to the erosion of a thick sequence of Proterozoic and Palaeozoic sedimentary strata from northern reaches of the thrust belt; Triassic–Lower Jurassic strata that record this episode are deposited in a thin belt south of this zone of erosion. This was followed by postulated Late Jurassic emplacement of a major allochthon (the Chengde thrust plate), which is thought to have overridden structures and strata associated with the Triassic event and is cut by two younger thrusts (the Gubeikou and Chengde County thrusts). The Chengde allochthon is now expressed as a major east–west trending, thrust‐bounded synform (the Chengde synform), which has been interpreted as a folded klippe 20 km wide underlain by a single, north‐vergent thrust fault. Two sedimentary basins, defined on the basis of provenance, geochronology and palaeodispersal trends, developed within the Yanshan belt during Late Jurassic–Early Cretaceous time and are closely associated with the Chengde thrust and allied structures. Shouwangfen basin developed in the footwall of the Gubeikou thrust and records syntectonic unroofing of the hanging wall of that fault. Chengde basin developed in part atop Proterozoic strata interpreted as the upper plate of the Chengde allochthon and records unroofing of the adjacent Chengde County thrust. Both the Chengde County thrust and the Gubeikou thrust are younger than emplacement of the postulated Chengde allochthon, and structurally underlie it, yet neither Shouwangfen basin nor Chengde basin contain a detrital record of the erosion of this overlying structure. In addition, facies, palaeodispersal patterns and geochronology of Upper Jurassic strata that are cut by the Chengde thrust suggest only limited (ca. 5 km) displacement along this fault. We suggest that the units forming the Chengde synform are autochthonous, and that the synform is bounded by two limited‐displacement faults of opposing north and south vergence, rather than a single large north‐directed thrust. This conclusion implies that the Yanshan belt experienced far less Late Jurassic shortening than was previously thought, and has major implications for the Mesozoic evolution of the region. Specifically, we argue that the bulk of shortening and uplift in the Yanshan belt was accomplished during Triassic–Early Jurassic time, and that Late Jurassic structures modified and locally ponded sediments from a well‐developed southward drainage system developed atop this older orogen. Although Upper Jurassic strata are widespread throughout the Yanshan belt, it is clear that these strata developed within several discrete intermontane basins that are not correlable across the belt as a single entity. Thus, the Yanshan has no obvious associated foreland basin, and determining where the Mesozoic erosional products of this orogen ultimately lie is one of the more intriguing unresolved questions surrounding the palaeogeography of North China.  相似文献   

9.
The Sassa‐Guardistallo Basin (SGB) is located close to the Tyrrhenian Sea and represents one of the most internal Neogene–Quaternary hinterland basins of the Northern Apennines fold‐and‐thrust belt. Its sedimentary succession consists of ca. 400‐m‐thick Late Tortonian–Messinian continental – largely conglomeratic – units overstepping a mainly shaly substratum (Palombini Shales) and overlain by Late Messinian evaporites and marine to continental Pliocene–Pleistocene sediments. This stratigraphic succession can be approximated to a composite rheological multilayer that dictated the style of basin deformation. Detailed geological mapping and structural analysis revealed that basin deposits were affected by compressional deformations that can be found both at map and outcrop scales. Decametric splay thrusts emanating from the substratum–conglomerate interface locally double the continental succession and are bounded by a roof thrust along the Late Messinian evaporite décollement, defining a deformation pattern consistent with a duplex‐like structure. The time–space structural evolution of the basin inferred from the fieldwork was addressed and tested by analogue modelling that approximated the rheological stratification of the study area to a layered brittle–ductile system. The model results support the hypothesis that the evolution of the thrust system affecting the SGB started as an early floor imbricate fan thrust system that successively evolved to a duplex structure as the link thrusts propagated into the upper décollement layer that resulted from the deposition of the Late Messinian evaporites. Models display many structural features that may be compared with the natural prototype, and highlight the importance of syntectonic sedimentation in the development and evolution of tectonic structures. The results of this study retain relevant implications for the Neogene evolution of the Tyrrhenian Basin–Northern Apennines system. This study also supports that combining between field structural analyses and analogue modelling can give useful hints into the evolutionary history of tectonically complex areas.  相似文献   

10.
Zircon U–Pb geochronometry, heavy mineral analyses and conventional seismic reflection data were used to interpret the provenance of the Lower Triassic Bunter Sandstone Formation. The succession was sampled in five Danish wells in the northern part of the North German Basin. The results show that sediment supply was mainly derived from the Ringkøbing‐Fyn High situated north of the basin and from the Variscan belt located south of the basin. Seismic reflection data document that the Ringkøbing‐Fyn High was a local barrier for sediment transport during the Early Triassic. Hence, the Fennoscandian Shield did not supply much sediment to the basin as opposed to what was previously believed. Sediment from the Variscan belt was transported by wind activity across the North German Basin when it was dried out during deposition of the aeolian part of the Volpriehausen Member (lower Bunter Sandstone). Fluvial sand was supplied from the Ringkøbing‐Fyn High to the basin during precipitation events which occurred most frequently when the Solling Member was deposited (upper Bunter Sandstone). Late Neoproterozoic to Carboniferous zircon ages predominate in the Volpriehausen Member where the dominant age population with a peak age of 337 Ma corresponds to the culmination of Variscan high‐grade metamorphism, whereas a secondary age population with a peak at 300 Ma matches the timing of volcanism and magmatism at the Carboniferous/Permian boundary in the northern Variscan belt. Parts of the basement in the Ringkøbing‐Fyn High were outcropping during the Early Triassic and zircon ages similar to this Mesoproterozoic basement are present in the Bunter Sandstone. The heavy mineral assemblage of the Solling Member is uniform and has a high garnet content compared to the contemporaneous sediments in the Norwegian‐Danish Basin and in the southern part of the North German Basin. This finding confirms that a local source in the Ringkøbing‐Fyn High supplied most of the fluvial sediment in the northern part of the North German Basin. The northernmost part of the Bunter Sandstone is situated on a platform area that is separated from the basin area by a broad WNW–ESE‐oriented fault zone. The most promising reservoir in the basin area is the aeolian Volpriehausen Member since the sandstone has a wide lateral distribution and a constant thickness. The alluvial to ephemeral fluvial Solling Member may be a good reservoir in the platform area and marginal basin area, but the complex sand‐body architecture makes it difficult to predict the reservoir quality.  相似文献   

11.
The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T2). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T1) and the present-day flood plain (terrace-T0). Terrace-T1 shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T0 and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T2 show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka.  相似文献   

12.
Sedimentological and geochronological studies along a north–south traverse across the Bangong‐Nujiang suture zone (BNSZ) in Gaize, Tibet provide evidence for a Late Triassic–Jurassic accretionary wedge accreted to the south margin of Qiangtang. This wedge, preserved as the Mugagangri Group (MG), records evidence for the northward subduction of the Bangong‐Nujiang Ocean (BNO) beneath Qiangtang. The MG strata comprise two coarser intervals (lower olistostromes and upper conglomerates) intercalated within sandy turbidites, which are consistent with timing and forearc stratigraphy during subduction initiation predicted by geodynamic modelling. Following the model, the northward subduction of the BNO beneath Qiangtang and subsequent arc‐magmatism are inferred to have begun, respectively, at ca. 220 Ma and ca. 210 Ma, with respect to depositional ages constrained by youngest detrital‐zircon ages. The initiation of arc‐magmatism is also supported by provenance transition reflected by sandstone detrital modes and age patterns of detrital zircons. Previously, evidence for an incipient arc was lacking, but the timing of Late Triassic BNO subduction and related arc‐magmatism is coincident with an important Late Triassic magmatic event in central Qiangtang that probably represents the ‘missing’ arc. Other Qiangtang events, such as exhumation of the Qiangtang metamorphic belt as a source area, and development of the Late Triassic Nadigangri deposits and bimodal volcanism, are more easily explained in the tectonic context of early northward subduction of the BNO beneath Qiangtang, beginning at about 220 Ma.  相似文献   

13.
During the Early Triassic the Jameson Land Basin (Central East Greenland) was located around 30° N, in the Northern arid belt, but by the Early Jurassic was positioned at a latitude of approximately 50° N. This study examines the record of this transition through a largely continental succession using clay mineralogy, sedimentology, petrography and heavy mineralogy. The Jameson Land Basin is aligned north–south and is 280 km long and 80 km wide. Following an Early Triassic marine phase the basin was filled by predominantly continental sediments. The Early‐to‐Late Triassic succession comprises coarse alluvial clastics (Pingo Dal Formation) overlain by a succession of fine‐grained evaporite‐rich playa/lacustrine sediments (Gipsdalen Formation), indicative of arid climatic conditions. The overlying buff, dolomitic and then red lacustrine mudstones with subordinate sandstones (Fleming Fjord Formation) record reduced aridity. The uppermost Triassic grades into dark organic‐rich, and in places coaly, mudstones and buff coarse‐grained sandstones of lacustrine origin that belong to the Kap Stewart Group, which spans the Triassic–Jurassic boundary, and appear to record more humid climatic conditions. Clay mineralogy analyses highlight significant variations in the kaolinite/illite ratio, from both mudstone and sandstone samples, through the Triassic and into the earliest Jurassic. Complementary heavy mineral analyses demonstrate that the variations recognised in clay mineralogy and sandstone maturity through the Triassic–Early Jurassic succession are not a product of major provenance change or the effect of significant diagenetic alteration. The observed variations are consistent with sedimentological evidence for a long‐term trend towards more humid conditions through the Late Triassic to Early Jurassic, and the suggestion of a significant pluvial episode in the mid‐Carnian.  相似文献   

14.
The Andean Orogen is the type‐example of an active Cordilleran style margin with a long‐lived retroarc fold‐and‐thrust belt and foreland basin. Timing of initial shortening and foreland basin development in Argentina is diachronous along‐strike, with ages varying by 20–30 Myr. The Neuquén Basin (32°S to 40°S) contains a thick sedimentary sequence ranging in age from late Triassic to Cenozoic, which preserves a record of rift, back arc and foreland basin environments. As much of the primary evidence for initial uplift has been overprinted or covered by younger shortening and volcanic activity, basin strata provide the most complete record of early mountain building. Detailed sedimentology and new maximum depositional ages obtained from detrital zircon U–Pb analyses from the Malargüe fold‐and‐thrust belt (35°S) record a facies change between the marine evaporites of the Huitrín Formation (ca. 122 Ma) and the fluvial sandstones and conglomerates of the Diamante Formation (ca. 95 Ma). A 25–30 Myr unconformity between the Huitrín and Diamante formations represents the transition from post‐rift thermal subsidence to forebulge erosion during initial flexural loading related to crustal shortening and uplift along the magmatic arc to the west by at least 97 ± 2 Ma. This change in basin style is not marked by any significant difference in provenance and detrital zircon signature. A distinct change in detrital zircons, sandstone composition and palaeocurrent direction from west‐directed to east‐directed occurs instead in the middle Diamante Formation and may reflect the Late Cretaceous transition from forebulge derived sediment in the distal foredeep to proximal foredeep material derived from the thrust belt to the west. This change in palaeoflow represents the migration of the forebulge, and therefore, of the foreland basin system between 80 and 90 Ma in the Malargüe area.  相似文献   

15.
The tectonic evolution of the Tian Shan, as for most ranges in continental Asia is dominated by north‐south compression since the Cenozoic India‐Asia collision. However, precollision governing tectonic processes remain enigmatic. An excellent record is provided by thick Palaeozoic – Cenozoic lacustrine to fluvial depositional sequences that are well preserved in the southern margin of the Junggar Basin and exposed along a foreland basin associated to the Late Cenozoic rejuvenation of the Tian Shan ranges. U/Pb (LA‐ICP‐MS) dating of detrital zircons from 14 sandstone samples from a continuous series ranging in age from latest Palaeozoic to Quaternary is used to investigate changes in sediment provenance through time and to correlate them with major tectonic phases in the range. Samples were systematically collected along two nearby sections in the foreland basin. The results show that the detrital zircons are mostly magmatic in origin, with some minor input from metamorphic zircons. The U‐Pb detrital zircon ages range widely from 127 to 2856 Ma and can be divided into four main groups: 127–197 (sub‐peak at 159 Ma), 250–379 (sub‐peak at 318 Ma), 381–538 (sub‐peak at 406 Ma) and 543–2856 Ma (sub‐peak at 912 Ma). These groups indicate that the zircons were largely derived from the Tian Shan area to the south since a Late Carboniferous basin initiation. The provenance and basin‐range pattern evolution of the southern margin of Junggar Basin can be generally divided into four stages: (1) Late Carboniferous – Early Triassic basin evolution in a half‐graben or post‐orogenic extensional context; (2) From Middle Triassic to Upper Jurassic times, the southern Junggar became a passively subsiding basin until (3) being inverted during Lower Cretaceous – Palaeogene; (4) During the Neogene, a piedmont developed along the northern margin of the North Tian Shan block and Junggar Basin became a true foreland basin.  相似文献   

16.
民勤盆地位于石羊河下游,现今大面积地区已被风沙覆盖.前人的研究报道了在民勤盆地的东部曾经发育过一个面积巨大的"腾格里古大湖",然而尚不明确民勤盆地是否也曾被湖水覆盖.为此,本文收集整理了民勤盆地中8个水文地质钻孔的地层资料,利用GIS技术重现了研究区地表以下50 m深度内的湖相地层的空间分布特征.分析发现,在盆地的不同地区普遍发育了2~3层深湖亚相地层,部分地区甚至多达4层,各湖相层厚度介于0.5~32 m.区域地层对比结果显示,在某个时段研究区大范围地区普遍发育了深水湖泊,而其他时段则在不同地区零星发育了小湖泊.结合研究区已有的测年资料,本文认为在晚第四纪某个时段(倒数第二层深湖亚相发育阶段),整个研究区内曾经存在一个规模较大的连通湖泊;但民勤盆地晚第四纪不同阶段深湖亚相地层差异明显的空间分布特征,可能反映了湖盆沉积中心自东向西、再向北东的迁移过程.  相似文献   

17.
新构造应力场在研究区域边坡稳定性及地表水土流失等方面具有重要作用。该文用地貌学方法——Scheidegger法,研究了三峡库区的新构造应力场及其与库岸滑坡滑动的优势方向问题。库岸滑坡发育的优势方向主要与新构造应力场的Maxl剪切带密切相关。  相似文献   

18.
Swath bathymetry, single‐channel seismic profiling, gravity and box coring, 210Pb down‐core radiochemical analyses and sequence stratigraphic analysis in the Gulf of Alkyonides yielded new data on the evolution of the easternmost part of the Gulf of Corinth. Three fault segments, the South Strava, West Alkyonides and East Alkyonides faults, dipping 45, 30 and 45°, respectively, northwards, form the southern tectonic boundary of the Alkyonides Basin. Two 45° southwards dipping segments, the Domvrena and Germeno Faults, form the northern tectonic margin. The Alkyonides Basin architecture is the result of a complex interaction between fault dynamics and the effects of changes in climate and sea/lake level. Chrono‐stratigraphic interpretation of the seismic stratigraphy through correlation of the successive seismic packages with lowstands and highstands of the Late Quaternary indicates that the evolution of the basin started 0.40–0.45 Ma BP and can be divided in two stages. Subsidence of the basin floor during the early stage was uniform across the basin and the mean sedimentation rate was 1.0 m kyear?1. Vertical slip acceleration on the southern tectonic margin since 0.13 Ma BP resulted in the present asymmetric character of the basin. Subsidence concentrated close to the southern margin and sedimentation rate increased to 1.4 m kyear?1 in the newly formed depocentre of the basin. Actual (last 100 year) sedimentation rates were calculated to >2 mm year?1, but are significantly influenced by the presence of episodic gravity flow deposits. Total vertical displacement of 1.1 km is estimated between the subsiding Alkyonides Basin floor and the uplifting Megara Basin since the onset of basin subsidence at a mean rate of 2.4–2.75 m kyear?1, recorded on the East Alkyonides Fault. Gravity coring in the Strava Graben and in the lower northern margin of Alkyonides Basin proved the presence of whitish to olive grey laminated mud below thin marine sediments. Aragonite crystals and absence of the marine coccolithophora Emiliania huxleyi indicate sedimentation in lacustrine environment during the last lowstand glacial interval.  相似文献   

19.
An unconventional, continuous petroleum system consists of an accumulation of hydrocarbons that is found in low-matrix-permeability rocks and contain large amounts of hydrocarbons. Tight-sand gas in the Jurassic and shale gas within the fifth member of Xujiahe Formation (T3x5) are currently regarded as the most prolific emerging unconventional gas plays in China. The conventional and systematical evaluation of T3x5 source rocks was carried out for the first time in the western Sichuan basin (WSD). Hydrocarbon generation and expulsion characteristics (including intensity, efficiency, and amount) of T3x5 source rocks were investigated. Results show that T3x5 source rocks are thick (generally >200 m), have high total organic content (TOC, ranging from 2.5 to 4.5 wt%), and dominated by III-type kerogen. These favorable characteristics result in a great hydrocarbon generating potential under the high thermal evolution history (R o > 1.2%) of the area. An improved hydrocarbon generation potential methodology was applied to well data from the area to unravel the hydrocarbon generation and expulsion characteristics of T3x5 source rocks in the WSD. Results indicate that the source rocks reached hydrocarbon expulsion threshold at 1.06% R o and the comprehensive hydrocarbon expulsion efficiency was about 60%. The amount of generation and expulsion from T3x5 source rocks was 3.14 × 1010 and 1.86 × 1010 t, respectively, with a residual amount of 1.28 × 1010 t within the source rocks. Continuous-type tight-sand gas was predicted to develop in the Jurassic in the Chengdu Sag of the WSD because of the good source-reservoir configuration (i.e., the hydrocarbon generation and expulsion center was located in Chengdu Sag), the Jurassic sandstone reservoirs were tight, and the gas expelled from the T3x5 source rocks migrated for very short distances vertically and horizontally. The amount of gas accumulation in the Jurassic reservoirs derived from T3x5 source rocks is up to 9.3 × 108 t. The T3x5 gas shale has good accumulation potential compared with several active US shale-gas plays. Volumetrically, the geological resource of shale gas is up to 1.05 × 1010 t. Small differences between the amounts calculated by volumetric method compared with that by hydrocarbon generation potential methodology may be due to other gas accumulations present within interbedded sands associated with the gas shales.  相似文献   

20.
The syntectonic continental conglomerates of the South‐Central Pyrenees record the late stages of thin‐skinned transport of the South‐Pyrenean Central Units and the onset of exhumation of the Pyrenean Axial Zone (AZ) in the core of the orogen. New magnetostratigraphic data of these syntectonic continental conglomerates have established their age as Late Lutetian to Late Oligocene. The data reveal that these materials were deposited during intense periods of tectonic activity of the Pyrenean chain and not during the cessation of the deformation as considered previously. The magnetostratigraphic ages have been combined with new detrital apatite fission track (AFT) thermochronology from AZ‐derived granite cobbles within the syntectonic conglomerates. Distribution of the granitic cobbles with different AFT ages and track lengths combined with their depositional ages reveal information on the timing and rate of episodes of exhumation in the orogen. Some AFT ages are considerably older than the AFT ages of the outcropping AZ granitic massifs, indicating erosion from higher crustal levels within the massifs than presently exposed or from completely eroded plutons. Inverse thermal modelling reveals two well‐defined periods of rapid cooling in the hinterland at ca. 50–40 and ca. 30–25 Ma, with another poorly defined cooling episode at ca. 70–60 Ma. The lowest stratigraphic samples experienced postburial annealing caused by the deposition of younger syntectonic sediments during progressive burial of the south Pyrenean thrust and fold belt. Moreover, samples from the deeper stratigraphic levels also reveal postorogenic cooling during the Late Miocene as a response to the excavation of the Ebro River towards the Mediterranean Sea. Our data strongly support previous ideas about the burial of the South Pyrenean fold and thrust belt by Late Palaeogene syntectonic conglomerates and their subsequent re‐excavation and are consistent with other thermochronological data and thermal modelling from the interior part of the orogen.  相似文献   

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