Uplift Processes Research Articles

Differential uplift of the Chinese Tianshan since the Cretaceous: constraints from sedimentary petrography and apatite fission-track dating

The Chinese Tianshan is one of the important international natural laboratories for studying continental geodynamics, but its uplift time, kinematics and mechanism of formation are key unresolved questions. In order to constrain the timing, kinematics and mechanism of uplift of the Chinese Tianshan, we applied sedimentary petrography and apatite fission-track dating to Cretaceous-Tertiary series from the northern Tarim Basin and the Tianshan Mountains. We collected and analyzed 79 sandstone samples and 75 detrital heavy mineral samples from the Kezilenuer-Kuqa profile (northern Tarim Basin) with a well-defined magnetostratigraphy. Our data indicate that detrital mineral maturity abruptly decreased at the disconformity between the Upper and Lower Cretaceous, and decreased again after 15(−12) Ma. The sediments in the northern Tarim Basin changed their provenance at 124 Ma, 26(−24) Ma and 15(−12) Ma, respectively. In addition, we collected and analyzed 36 primary apatite samples from the South, Central and North Tianshan. Our results show that the Chinese Tianshan underwent three phases of differential uplift. The first phase of uplift started at the southern Central Tianshan during the Early Cretaceous and propagated southward. The second phase of uplift started at the northern Central Tianshan during the Late Cretaceous and propagated also southward. The third phase of uplift started at the northern Central Tianshan during the Eocene and propagated both northward and southward. These differential uplifts have caused development of disconformity and drops in mineral maturity of detrital sediments in the northern Tarim Basin. Such a differential and heterogeneous uplift process might have been trigged by collisions of different microcontinents (i.e., Lhasa, Kohistan-Dras and India) at the southern margin of Asia. These collisions reactivated the South Tianshan Fault and then the North Tianshan Suture, and uplifted the Chinese Tianshan step by step.

Neogene-Quaternary tectonics and sedimentation in the Central Apennines

The Central Apennines consist of a fold and thrust belt developed during the Neogene, involving sedimentary successions of different paleogeographic domains. The geodynamic evolution of this sector of the chain was controlled by several important tectonic events, highlighted by numerous depositional sequences recognized in the Neogene marine successions. Time-scanning of the stratigraphic events has allowed us to reconstruct: the different stages of the eastward shift of the chain-foredeep-foreland system; the deformation chronology; the recent effects of the interactions between the regional uplift and extensional tectonics in the chain and the progressive eastward tilting of the Periadriatic domain. During the Miocene-Early Pliocene interval the Tyrrhenian area constituted a remarkable topographic swell, linked to an upwelling mantle dome. As a consequence, intense erosional processes developed on this area, together with crustal delamination and lateral gravitational spreading. The Tyrrhenian-derived siliciclastic detritus fed the Apennine foredeep and thrust-top basins; the continuous asthenospheric uplift in the Tyrrhenian margin favoured the formation of axial crest grabens, crust delamination and the emplacement of a stack of gravitational nappes in the Apennine sector. The maximum uplift of the mantle doming was recorded at the beginning of the Upper Messinian. The remarkable uplift of several asthenospheric domes, located in the Mediterranean area, favoured the closing of the sea-ways, connecting the Atlantic with Peri-mediterranean basins. Consequently the salinity crisis developed, with a relative diachrony: the evaporites sedimented at first in the crestal areas and later in the deep ones. The marine episodes recognized in the lago-mare sediments represent the end of uplift processes and a progressive dome deflation with sporadic connections with the Atlantic. During the Middle Pliocene-Quaternary, the intense thinning of the Tyrrhenian crust induced the collapse of the axial dome and subsidence of the rift belt.

Upwarped high velocity mafic crust, subsurface tectonics and causes of intraplate Latur-Killari (M 6.2) and Koyna (M 6.3) earthquakes, India – A comparative study

A unique attempt is made to understand the genesis of intraplate seismicity in the Latur-Killari and Koyna seismogenic regions of India, through derived crustal structure by synthesizing active and passive seismic, magnetotelluric, gravity and heat flow data. It has indicated presence of relatively high velocity/density intermediate granulite (and amphibolite) facies rocks underneath the Deccan volcanic cover caused mainly due to a continuous geodynamic process of uplift and erosion since Precambrian times. These findings have been independently confirmed by detailed borehole geological, geochemical and mineralogical investigations. The crystalline basement rock is found to contain 2 wt% of carbon-di-oxide fluid components. The presence of geodynamic process, associated with thermal anomalies at subcrustal depths, is supported by a high mantle heat flow (29–36 mW/m 2) beneath both regions, although some structural and compositional variations may exist as evidenced by P- and S-wave seismic velocities. We suggest that the stress, caused by ongoing uplift and a high mantle heat flow is continuously accumulating in this denser and rheologically stronger mafic crust within which earthquakes tend to nucleate. These stresses appear to dominate over and above those generated by the India–Eurasia collision. The role of fluids in stress generation, as advocated through earlier studies, appears limited.

Archaeology of the continental shelf: Marine resources, submerged landscapes and underwater archaeology

We provide a critical review of the evidence for the long-term use of marine resources and coastal environments in human evolution and later development. We emphasise the importance of the coastal archaeological record in understanding patterns of human settlement and dispersal and draw attention to the large potential biases introduced by the destructive or obscuring effects of Pleistocene sea-level change. We note that lowered sea levels have been the norm for most of the Pleistocene and that periods of high sea level have been too short-lived to provide other than a fragmentary coastal record and one that is beset with ambiguities and uncertainties. We examine the geological processes of coastal uplift and underwater preservation that may help to mitigate these biases. Coastlines elevated by isostatic and tectonic processes, or with very steep offshore drop-offs at plate boundaries, are important in providing a potential window into coastal landscapes and archaeology formed during periods of lowered sea level. However, we conclude that the opportunities afforded by these geological factors are too limited to obviate the need for underwater exploration. We review the evidence now available from submerged landscapes across the Africa–Eurasia interface from the Atlantic coastlines of Northwest Europe to the southern Red Sea. We show that geomorphological conditions for the preservation of archaeological and palaeoenvironmental data are commonly present, that much material has already been discovered, and that new techniques, technologies and projects are providing the momentum for a rapidly expanding field of investigation. The results do not simply add to what we already know from sites on land, but are likely to produce qualitatively different evidence for coastal adaptations and landscapes that have no analogue on present-day coastlines. We note the strong probability that many coastal landscapes exposed at lowered sea level provided relatively fertile and productive refugia for plants, land mammals and humans at a time when increased aridity would have reduced or deterred hinterland occupation. We conclude that underwater investigation is essential if hypotheses of early human adaptation and dispersal are to be fully tested.

Cenozoic sedimentary records and geochronological constraints of differential uplift of the Qinghai-Tibet Plateau

Geological mapping data (1:250000) in the Qinghai-Tibet Plateau and its adjacent regions reveal the sediment sequences, distribution and tectonic evolution of the 92 Tertiary remnant basins. Southern Tibet and the Yecheng area in Xinjiang, located at southern and northwestern margins of the Qinghai-Tibet Plateau, respectively, were parts of the Neo-Tethys remnant sea in the Paleogene. In southern Tibet, both the subabyssal and abyssal sequences occur at the Gyangze, Saga, Guoyala, and Sangmai areas. The deep-water facies successions outcrop in the west, whereas the shallow-water facies sequences in the east, indicating the east to the west retreat of the Neo-Tethys Ocean. The retreat of the Neo-Tethys Ocean in the east was contributed to the earlier tectonic uplift of the eastern Qinghai-Tibet Plateau. The uplift process of the Plateau from the Late Cretaceous to Pliocene is described as follows: During the Late Cretaceous, tectonic uplift of the Qinghai-Tibet Plateau occurred in the northeastern part and the configuration of the Qinghai-Tibet Plateau was characterized by rise in the northeast and depression in the west. In the Paleocene-Eocene interval, the Tengchong-Baingoin and Kuyake-Golmud areas experienced local tectonic uplifting, the West Kunlun uplift zone broadened easterly, the Qilian uplift zone broadened southerly, and the Songpan-Garze uplift zone shrank easterly. The Oligocene configuration of the Qinghai-Tibet Plateau was characterized by mountain chains rising along its margins and sedimentary basins in the central part because of tectonic uplifts of the Gangdise and the Himalaya blocks. Meanwhile, the Kunlun-Altyn-Qilian uplift zones have also broadened southerly and northerly. In contrast, the great uplift zones of the Gangdise, the Himalaya, the Karakorum, and the Kunlun blocks characterize the paleogeographic contours of the Qinghai-Tibet Plateau during the Miocene-Pliocene. Additionally, the thermochronological data on tectonic uplift events in southern Tibet, West Kunlun Mountains, Altyn Tagh, eastern Tibet, and western Sichuan all suggest that the most intense deformation occurred at 13-8 Ma and since 5 Ma, respectively, corresponding to two great uplift periods in Neogene. As a result, turnover of paleogeographic configuration of the Qinghai-Tibet Plateau occurred during the Neogene, experiencing a change from high contours in the east in the pre-Oligocene to high contours in the west at the end-Pliocene. The uplift of the Qinghai-Tibet Plateau during the Cenozoic was episodic, and the uplifts of various blocks within the Plateau were spatially and chronologically different.

Morphotectonic study of the lower Sangro River valley (Abruzzi, Central Italy)

This work analyses the morphotectonic setting of the Adriatic piedmont of Central Italy, in order to outline the role of tectonics in the development of the fluvial landscape in this area. Particularly, it focuses on the Sangro River, which flows in the southern Abruzzi, from the Apennines, through the piedmont, to the Adriatic coast. The piedmont area developed in a marine environment from the Pliocene to the Early Pleistocene resulting in the deposition of a clayey-sandy sequence. Afterwards, regional uplift processes with NE tilting led to the deposition of a regressive conglomeratic sequence, as the area evolved into a continental environment. This work is based on the qualitative and quantitative study of drainage patterns and on the analysis and correlation of selected morphological evidence of tectonics on fluvial terraces, in order to define the role of tectonics in the geomorphological evolution of the lower Sangro River valley. The drainage network and its statistical azimuthal distributions are analysed; the distribution and the transverse and longitudinal profiles of fluvial terraces are investigated and the morphological evidence for tectonics is then assessed: river bends, linear valleys, beheaded and hanging valleys, counterflow confluences of streams. Space and time correlation of these features allows us to highlight the effects of regional uplift, tilting, and local tectonics on drainage development. Following the emergence of the piedmont area (Middle Pleistocene), uplift and tilting determined a SW–NE consequent and parallel drainage. Progressive incision shaped the Sangro valley as a consequent valley. Local tectonics along NNW–SSE and WNW–ESE faults and fractures resulted in the incision of linear valleys, beheading the previous SW–NE drainage, and controlled the development of a rectangular drainage network (late Middle Pleistocene). The drainage network was then rearranged by selective erosion processes, maintaining a strong inheritance from of its Middle Pleistocene configuration.

Fisiografia da região do planalto de Poços de Caldas, MG/SP

The physiographic analysis statement is that the understanding of origin and evolution of the landscape can provide the soil's occurrence and characteristics, which are very important in geoenvironmental zoning programs. This paper presents a physiographic analysis based on remote sensing, morphogenetical aspects, altitude and dissecation to decide upon the phisiography of the land. The study was developed in Poços de Caldas's volcanic alkaline plateau between Minas Gerais and São Paulo states. In this area there are some plateaus associated with regolithic soils generated from allochthonous materials. These landscapes were formed by recurrent processes of erosion and neotectonic uplift. The alluvium plains present gleisoils and buried organic matter.

Fisiografia da região do planalto de Poços de Caldas, MG/SP

The physiographic analysis statement is that the understanding of origin and evolution of the landscape can provide the soil’s occurrence and characteristics, which are very important in geoenvironmental zoning programs. This paper presents a physiographic analysis based on remote sensing, morphogenetical aspects, altitude and dissecation to decide upon the phisiography of the land. The study was developed in Pocos de Caldas’s volcanic alkaline plateau between Minas Gerais and Sao Paulo states. In this area there are some plateaus associated with regolithic soils generated from allochthonous materials. These landscapes were formed by recurrent processes of erosion and neotectonic uplift. The alluvium plains present gleisoils and buried organic matter.

Oligo-Miocene evolution of the Tuotuohe Basin (headwaters of the Yangtze River) and its significance for the uplift history of the central Tibetan Plateau

The central part of the Tibetan Plateau between the Kunlun and Tanggula ranges provides unique Eocene-Oligocene sediments in the Fenghuoshan Basin of northern part of the Hoh Xil Basin to recognize the early uplift history of Tibet (Liu et al. 2001, 2003; Wang et al. 2002). The Fenghuoshan Mountain region to the south of Jinsha River Suture Zone was recommended as Eocene Tibet and northern part to the Jinsha Suture Zone as Oligo-Miocene Tibet (Tapponnier et al. 2001). However, previous studies have failed to discover any continuous Oligocene to Miocene sequence that precludes our understanding of early evolution history of the central Tibetan Plateau, because a regional discordance developed between the overlying Early Miocene grey limestone of the the Wudaoliang Group and the underlying EoceneOligocene grey-violet sandstone, mudstone, and conglomerate complex of the Fenghuoshan and Yaxicuo groups (Liu and Wang 2001; Wang et al. 2002). Our recent fieldwork during summers of 2002 and 2003 in the Tuotuohe Basin as the southern part of the Hoh Xil Basin, about 120 km northward of Tanggula Pass, however, discovered a well-outcropped Oligo-Miocene section. The Tongtianhe section (N33o55’45, E92o37’13) in the southeastern part of the Tuotuohe Basin, situated in the modern headwaters region of the Yangtze River, consists of the underlying fan-delta-fluvial Fenghuoshan Group sandstone and conglomerate, the middle part of fluvial-lacustrine Yaxicuo Group sandstone and mudstone (~2192 m thick), and the overlying lacustrine Wudaoliang Group limestone (~200 m thick). The Fenghuoshan and Yaxicuo groups were paleomagnetically dated in the Fenghuoshan Basin as Eocene-Early Oligocene (51-30 Ma) with an age of 31.3 Ma for the boundary of the two groups (Liu et al. 2003). In this Tuotuohe Basin, because of poor-outcropped Fenghuoshan Group, our measurements started from the boundary of the Fenghuoshan and Yaxicuo groups upwards to finish at the top of the Wudaoliang Group for atotal thickness of 2392 m. 380 individually-oriented paleomagnetic samples, spaced at 125m stratigraphic intervals, were collected from the Tongtianhe section. Progressive thermal and alternating field demagnetization experiments were conducted by a 2G cryogenic magnetometer at the Paleomagnetic Laboratory of the University of California, Santa Cruz (UCSC). On the basis of distinct magnetic reversal zones with a reference of Cande and Kent (1995) and biostratigraphic data, this section is paleomagnetically dated as 31.3-21.8 Ma (Early Oligocene-Early Miocene), i.e. the Yaxicuo Group as 31.3-23.8 Ma and the Wudaoliang Group as 23.8-21.8 Ma. This result fits very well the boundary age of 31.3 Ma for the Fenghuoshan and Yaxicuo groupswhich was earlier recognized in the Fenghuoshan Basin (Liu et al. 2003). Our comprehensive basin analysis along with the new paleomagnetic data from the Tongtianhe section suggests that the basin could have been formed as a foreland basin and undergoing an (Eocene?)-Oligocene accelerated subsidence mainly with fan-delta-fluvial to fluvial-lacustrine environments. During the Oligocene, provenance analysis and predominant paleocurrents with northeasterly directions from the Yaxicuo Group suggest the uplift process of the Tanggula Mountain orogenas a major source area of the Yaxicuo Group clastics. The average sediment accumulation rate is 29.2 cm/ka for the Yaxicuo Group. However, during Early Miocene, a relative stable lacustrine environment with a low accumulation rate of about 10 cm/ka developed in the Tuotuohe Basin indicating a peneplanation process accompanying an interior water system in central Tibet. Thus, geomorphic conditions strongly suggest the Yangtze River was not established in central Tibet until after the Wudaoliang Group formation, i.e. 21.8 Ma. This work is advancing our understanding of the Oligo-Miocene tectonic-sedimentary events recorded in the central Tibetan Plateau, which have a direct relationship to the early uplift of Tibet and to the evolution of the Yangtze River water system.

Timing and processes of Himalayan and Tibetan uplift

Timing and processes of Himalayan and Tibetan uplift

Plio-Pleistocene rapid uplift process of the Nepal Himalaya revealed from fission-track ages

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 98-99

Evolution and Age Relations of Karst Landscapes

Any karst landscape is a work in progress. The observed evolution of the landscape is dictated by competing rate processes of surface denudation, stream downcutting, cave development, and tectonic uplift. Quantitative data on these processes, applied to two physiographic provinces of the Appalachian Mountains of eastern United States gives ages and time scales that are in agreement with previous geomorphic interpretation. The results are anchored, very loosely, by the few dates that have been established for cave sediments. Unfortunately, the measured rates vary over an order of magnitude as a result of local circumstances making regional interpretation a rough approximation at best.

Geometry, kinematics and evolution of the Tongbai orogenic belt

The Tongbai orogenic belt (TOB) is composed of six tectonic units. From south to north these units are: Tongbai gneiss rise (TGR); Hongyihe-Luozhuang eclogite belt (HLE); Maopo-Hujiazhai igneous rock belt (MHI); Zhoujiawan flysch belt (ZFB); Yangzhuang greenschist belt (YGB); and Dongjiazhuang marble belt (DMB).The geometry and kinematic images of the TOB include: the antiformal structures caused by a later uplift process, the top-to-north ductile shear structure that related to a process that the ultrahigh pressure rocks are brought to surface, the top-to-south ductile shear thrust and the sinistrial shear structures related to a south-north direction compression, and the east-west direction fold structures in the upper crust. In the view of the multistage subduction-collision orogenic belt, according to the characters of petrology and its distribution, geometry, kinematics and structural chronology in these tectonic units, tectonic evolution of the TOB can be divided into four stages: oceanic crust subduction during 400―300 Ma, continental collision during 270―250 Ma, continental deep subduction and uplift during 250―205 Ma and doming deformation during 200―185 Ma.

Fission track evidence for the Mesozoic-Cenozoic tectonic uplift of Mt. Bogda, Xinjiang, northwest China

Fission-track dating evidence from 5 apatite samples and 4 zircon samples, and modeled time-temperature thermal history indicate that since Late Jurassic-Cretaceous (150 – 106 Ma), the uplift process of Mt. Bogda can be divided into four stages of thermal evolution: 150 – 106, 75 – 65, 44 – 24 and 13 – 9 Ma. Before 44 – 24 Ma, the cooling rate and uplifting rate of the southern and northern segments of Mt. Bogda are almost the same, showing that the uplifting of Mt. Bogda is an overall process. Since 44 – 24 Ma, the uplifting of the southern and northern segments of Mt. Bogda has shown differences. During 42 – 11 Ma, the northern segment of Mt. Bogda was at a steady stage, with the cooling rate being 0.03°C/Ma and the uplifting rate being 0.001 mm/a. From 11 Ma to the present, the northern segment of Mt. Bogda was at a rapid cooling and uplifting stage, with the cooling rate being 5.72°C/Ma and the uplifting rate being 0.19 mm/a. However, the southern segment of Mt. Bogda has been at a rapid cooling and uplifting stage since 26 Ma, with the cooling rate being 1.24°C/Ma and the uplifting rate being 0.041 mm/a during 26 – 9 Ma; 4.88°C/Ma and 0.163 mm/a from 9 Ma till now.

Holocene hydrologic balance of tropical South America from oxygen isotopes of lake sediment opal, Venezuelan Andes

Precipitation in the South American Andes is derived from Atlantic Ocean evaporation which is modified by passage over lowland South America. The isotopic composition of Andean precipitation reflects evaporation conditions over the Atlantic Ocean, moisture recycling during advection across the South American lowlands and uplift to the Andes. Records of the oxygen isotope composition of precipitation in the Venezuelan Andes, derived from lake sediment diatom δ 18 O measurements, show a 2.4‰ decrease during the past 10,000 yr. A simple model of the evaporation, advection and uplift processes is used to understand the cause of the isotope shift. The data and model suggest that the decreasing δ 18 O reflects a decrease in the fraction of moisture entering South America that reaches the Andes. Ice cores from Peru and Bolivia exhibit similar isotope trends indicating that the shift occurred in both hemispheres. An isotopic record of Amazon River discharge is consistent with the Andean records, indicating increasing continental runoff was associated with the decreasing export of water vapor. Orbital changes in solar insolation cannot explain the synchronous trends in both hemispheres. Changing climate in the tropical Pacific is an attractive explanation for the trends because modern interannual variability in this region has similar effects in both hemispheres.

Marked ecological shifts during 6.2–2.4 Ma revealed by a terrestrial molluscan record from the Chinese Red Clay Formation and implication for palaeoclimatic evolution

A new molluscan fossil record from the late Miocene–Pliocene Xifeng Red Clay sequence in the central Loess Plateau, northern China, reveals the process of ecological environmental changes during ∼ 6.2–2.4 Ma. Three marked ecological shifts in mollusk assemblages occurred at about 5.4, 4.5, and 3.4 Ma, which reflect changes in the East Asian monsoon circulation. A predominantly cold-aridiphilous group reveals an extremely cold dry climate due to strengthened East Asian winter monsoon during ∼ 6.2–5.4 Ma. A predominantly thermo-humidiphilous group from ∼ 5.4–4.5 Ma indicates a strengthening of the East Asia summer monsoon circulation. The marked occurrence of meso-xerophilous taxa during ∼ 4.5–3.4 Ma implies mild climatic conditions. From 3.4 Ma, the cold-aridiphilious species again became dominant in the fossil assemblages, indicating that the winter monsoon was gradually intensifying towards the Quaternary glaciations. Climate changes in this region were characterized by stepwise cooling from 5.4 to 2.4 Ma with a 1 Ma rhythmicity. The cooling trend is in good agreement with the general global cooling trend during this period, as documented by marine δ 18O records. The formation and development of the Northern Hemisphere ice sheets during the late Pliocene accelerated the climate deterioration towards the Quaternary glaciations. The history of environmental changes revealed by the Red Clay molluscan fossils is coupled in phase with the process of Tibetan Plateau uplift, which may be the major cause and forcing mechanism of late Miocene–Pliocene environmental changes in the Loess Plateau.

Palaeomonsoon variability in the southern fringe of the Badain Jaran Desert, China, since 130 ka BP

AbstractTaking the Chagelebulu Stratigraphic Profile as a typical example, a comprehensive study has been conducted to elucidate the palaeoclimatic and geomorphic evolution patterns in the southern fringe of the Badain Jaran Desert, which were found to be complex and polycyclic in the past 130 ka. However, the fluctuating magnitude is not as remarkable as that in the eastern China sandy region. The shift in climate from interglacial to glacial and the uplift process of the Qinghai–Xizang Plateau are the two leading forces driving the evolution of the climate and desert landforms in this area. Seventeen cycles of cold, dry and warm, humid climatic stages were recognized in the Upper Pleistocene Series of the profile. The sharp uplift of the Qinghai–Xizang Plateau superimposed a cool and arid climatic trend in this area. As a result of the climatic changes, the desert in this area has undergone multiple stages of expansion and contraction since 130 ka bp. The middle Holocene Epoch and the early stage of the Late Pleistocene Period were the main periods when the sand dunes became stabilized, and the early and late phases of the Holocene Epoch and late phase of the Pleistocene Epoch were the main periods when the previously stabilized sand dunes became mobile. The late phase of the Pleistocene Epoch was the most mobile stage, when the aeolian sand activities formed the essential geomorphic pattern of the Badain Jaran Desert. Copyright © 2005 John Wiley & Sons, Ltd.

Red clay sediment in the central Chinese Loess Plateau and its implication for the uplift of the Tibetan Plateau

The widely distributed red clay sediment underlying the Chinese Loess Plateau truly records the Neogene environmental evolution, and its genesis and development are intrinsically related to the uplift processes of the Tibetan Plateau and the evolution of East Asia monsoon system. In this paper, a detailed magnetostratigraphy of a loess-red clay section (107°13′E, 35°02′N) from the central Loess Plateau is reported. The loess-red clay sequence is composed of 175 m Quaternary loess-paleosol sequence and 128 m Neogene red clay sediments. Based on the correlation with the standard geomagnetic polarity time scale, the paleomagnetic results indicate that the age of Chaona red clay sequence extends to 8.1 Ma, which is the older red clay deposition in the central Chinese Loess Plateau. The commencement of red clay at ∼8.1 Ma may imply that the Ordos planation surface was broken by the movement of the Haiyuan-Liupanshan Faults, which was related to the uplift of the Tibetan Plateau induced by the collision of India Plate and Eurasian Plate. And the western part adjacent to the Tibetan Plateau was uplifted to form the embryo of the Liupan Shan (Mts.) and the eastern part was down-faulted to receive red clay deposition. We link this faulting to an initial uplift of the Tibetan Plateau. The undulating nature of the broken Ordos planation surface may explain the chronological differences and depth discrepancies among various cross-sections of red clay.

Episodes of Cenozoic Gold Mineralization on the Eastern Margin of the Qinghai-Tibet Plateau: 40Ar/39Ar Dating and Implication for Geodynamic Events

Abstract A lot of new gold deposits have been found on the eastern margin of the Qinghai-Tibet Plateau during the past two decades. Among them, three main types of gold deposits have been recognized, including quartz-vein-type, shearzone-type and porphyry-type. The former two types of gold deposits are mainly hosted within metamorphic rocks, while the latter is related to Cenozoic magmatism. Although all of these gold deposits are believed to have been formed during the uplift process of the Qinghai-Tibet Plateau in the Cenozoic era (Wang et al., 2002b), precise isotopic age constraints have still been lacking until quite recently. This paper presents new 40Ar/39Ar data of some gold deposits on the eastern margin of the Qinghai-Tibet Plateau, which indicate that gold mineralization in the region occurred in response to the episodic stages of the orogenies. Recently obtained 40Ar/39Ar data on quartz and feldspars from several gold deposits, such as the Sandiao deposit, the Baijintaizi deposit, the Pusagang deposits, provide new constraints on gold mineralization on the eastern margin of the Qinghai-Tibet Plateau. Geochronological studies of gold deposits along the Daduhe River indicate that there are three stages of gold mineralization. The early two stages occurred as early as 65.1 Ma in the Shuibaiyang deposit and 58.95 Ma in the Ruoji deposit, while the latter stage occurred as late as 25.35 Ma in Baijintaizi and 24.70 Ma in Sandiao. Isotopic dating of three plagioclases from the Beiya deposit, Zhifanggou deposit and Luobodi deposit and a K-feldspar from the Jinchangqing deposit in Yunnan Province indicates that these deposits were formed at two stages. The Zhifanggou and Jinchangqing deposits have early stage records as old as 58.82 Ma in Zhifanggou and 55.49 Ma in Jinchangqing, but all of the above four deposits in Yunnan have late stage records of 23.18 Ma in Jinchangqing, 24.54 Ma in Zhifanggou, 24.60 Ma in Luobodi and 24.56 Ma in Hongnitang. The above results suggest that the gold deposits on the eastern margin of the Qinghai-Tibet Plateau were formed concentratedly at two main episodes, i.e. the end of the Paleocene (about 58 Ma) and the boundary between the Paleogene and the Neogene (about 25 Ma). The later episode appears to be looks like more important and was coupled with the Sichuan movement, which was extensively activated at that period. The beginning of the Cenozoic Era (about 65 Ma) might be another episode of gold mineralization, but only one deposit (Shuibaiyang) in this study has been proved to have been be formed at this stage and might be earlier than the initial collision between the Indian Plate and the Eurasia Plate. In view of geology, the above three episodes of gold mineralization are associated with three events of tectonic-magmatism and/or fluid events. Even though the gold deposits (for example, the Shuibaiyang deposit, Ruoji deposit and Pusagang deposit) were formed at different episodes, all of them are genetically related to tectonic movements in large-scale shear zones. It looks like theat tectonic events (including large-scale strike-slip) between Paleogene and Neogene had a wide influence upon gold mineralization, with new deposits formed and old deposits enriched or superimposed to be a higher grade by new stage of mineralization. The above data suggest that gold deposits were not only concentrated in some areas, but also formed mainly at different boundaries of geological times, indicating that there existed some peak stages of gold mineralization (metallogenic episodes), and that the gold deposits were formed mainly by episodic mineralization.

The history of crustal uplift and metamorphic evolution of Panzhihua-Xichang micro-palaeoland, SW China: Constraints on Sm-Nd, 40Ar/39Ar and FT ages of granulites

Panzhihua-Xichang (Panxi) micro-palaeoland is the oldest terrane on the western margin of the Yangtze Block. Some intermediate-basic granulites are considered to be the crystalline basement of lower crust in the terrane. Granulite-facies metamorphism of the granulites was developed in the period from 1186 Mato 1128 Ma. The origin of granulites was related to the collision orogenic process occurring when the micro-palaeolands merged to form the Rodinia Supercontinent. Amphibolite-facies retrogressive metamorphism of granulites took place in the period from 877 Ma to 825 Ma. This period was consistent with the breakup time of the Rodinia Supercontinent. 40Ar/39Ar ages and fission track (FT) ages of granulites in the Panxi micro-palaeoland show that the vertical movement history of crustal rocks was a slow uplift process of the rigid terrane in the time from Neoproterozoic to Mesozoic. The subduction of India Plate towards Euroasia Plate resulted in the rapid uplift of the Qinghai-Tibetan Block in Cenozoic. Meanwhile, the Qinghai-Tibetan Block moved towards east. Consequently the Panxi terrane was uplifted rapidly. As a result of the collision orogeny between the Qinghai-Tibetan Block and the Panxi terrane, the granulite-facies crystalline basement in this region was exhumed and exposed to the surface.