what controlled mid late miocene long term aridification in central asia global cooling or tibetan plateau uplift a review abstract debate continues over whether global cooling or uplift of the tibetan plateau provided the first order driver for the aridification moisture levels of central asia throughout the mid late miocene between about 17 and 5 ma this review attempts to throw new light on the relations between the aridification and these two key factors this paper examines the evolution of miocene climate both moisture and temperature within five separate regions of eurasia to help understand the large scale controls of long term moisture in central asia the five regions include 1 europe 2 high latitude asia 3 the east asian monsoon region 4 the south asian monsoon region and 5 central asia itself because moisture reaching central asia has to firstly cross at least one of the other four regions temperature proxy data compiled from these five regions correlate with ocean temperatures from the global deep sea oxygen isotope records furthermore compiled moisture proxy data from the four regions surrounding central asia co vary and correlate with each other this can be explained by positive feedbacks between drying and cooling and supports the assumption that global cooling provides a dominant driving factor for the drying of eurasia global cooling reduces the amount of water vapor held in the atmosphere and thereby can cause terrestrial drying however in central asia the moisture evolution shows less similarity with its surroundings the uplift of the tibetan plateau including the tianshan mountains could provide a possible explanation for this difference the changing topography resulting from uplift events over time and space strongly influenced the moisture patterns in central asia during miocene times future research on the spatial timing and amplitude of tibetan plateau uplift should be useful to understand the moisture processes in central asia during the miocene

tectonic implications of paleomagnetic and geochronologic data from the yukon koyukuk province alaska the paleomagnetic and geochronologic record of alaska is complicated by overprints that make it difficult to relate the terranes of northern and southern alaska to each other and to north america to better understand these relationships and the overprinting paleomagnetic and k ar dating samples were analyzed from the three major geologic units of the yukon koyukuk province ykp of central alaska these units consist of a basal jurassic early cretaceous volcanic island arc assemblage albian cenomanian clastic sedimentary rocks and bimodal eocene volcanic rocks k ar age determinations of the early tertiary volcanic rocks form two distinct groups the older group 65 49 ma is mostly felsic and probably represents the maximum inboard penetration of low angle subduction related volcanism these older rocks were deformed during the regional deformation of the ykp the younger group 44 43 ma is post deformational and composed dominantly of basalt these results bracket a time 49 44 ma when deformation ceased in the province this time interval is coincident with a major transition from convergence to strike slip and extensional tectonic style throughout alaska the geologic units of the ykp display two characteristic paleomagnetic signatures primary directions based on positive reversal conglomerate and or bedding tilt stability tests were obtained from the albian cenomanian sedimentary rocks and eocene volcanic rocks the mid cretaceous sedimentary rocks show about 15 of poleward motion between 90 56 ma a time of high rates of convergence between alaska and the pacific and eurasian plates age equivalent rocks from neighboring regions st matthew island and the alaskan north slope also indicate similar amounts of poleward northward motion which suggests that the whole of western and northern alaska may have moved northward about 10 relative to north america as a more or less coherent block paleomagnetic results from the eocene volcanic rocks indicate that the ykp was in place by 56 ma and formed part of the accretionary nucleus that served as a backstop for the accreted terranes of southern alaska secondary remagnetized directions are recorded in the basal island arc assemblage and in albian cenomanian sediments near younger igneous occurrences the clustering of secondary directions in the geographic reference frame indicates that they were acquired after deformation ceased 49 44 ma which is consistent with the coincidence of the mean of these directions and the 54 44 ma north american reference direction

mountains and arid climates of middle latitudes simulations from a global climate model with and without orography have been used to investigate the role of mountains in maintaining extensive arid climates in middle latitudes of the northern hemisphere dry climates similar to those observed were simulated over central asia and western interior north america in the experiment with mountains whereas relatively moist climates were simulated in these areas in the absence of orography the experiments suggest that these interior regions are dry because general subsidence and relatively infrequent storm development occur upstream of orographically induced stationary wave troughs downstream of these troughs precipitation bearing storms develop frequently in association with strong jet streams in contrast both atmospheric circulation and precipitation were more zonally symmetric in the experiment without mountains in addition orography reduces the moisture transport into the continental interiors from nearby oceanic sources the relative soil wetness of these regions in the experiment without mountains is consistent with paleoclimatic evidence of less aridity during the late tertiary before substantial uplift of the rocky mountains and tibetan plateau is believed to have occurred

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

geochronological framework and tectonic setting of the granitic magmatism in the chaihe moguqi region central great xing an range china abstract we compiled and analyzed 56 u pb zircon ages including 38 ages obtained during the present study and 18 from other sources for granitic rocks of the chaihe moguqi region central great xing an range china magmatism in this area can be divided into eight stages early devonian 399 3 ma late devonian 365 358 ma late carboniferous 322 299 ma early permian 295 282 ma late triassic 231 227 ma early middle jurassic 179 172 ma late jurassic 152 149 ma and early cretaceous 137 120 ma granites were mainly emplaced during the latter four mesozoic stages of magmatism paleozoic granites formed during several stages and were associated with oceanic subduction and the amalgamation of crustal blocks in the eastern fragment of the central asian orogenic belt the late triassic is an important period with respect to the change from the paleo asian ocean tectonic regime to the circum pacific tectonic regime the formation of late triassic granites may have been related to post orogenic lithospheric extension after closure of the paleo asian ocean early middle jurassic granites resulted from either subduction of the paleo pacific plate or subduction during the amalgamation of the jiamusi massif and the songliao terrane whereas the intrusion of late jurassic early cretaceous granites was associated with both the subduction of the paleo pacific plate and the closure of the mongol okhotsk ocean

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

carboniferous and cretaceous mafic ultramafic massifs in inner mongolia china a shrimp zircon and geochemical study of the previously presumed integral hegenshan ophiolite abstract we present shrimp zircon ages and whole rock geochemical data to identify mafic ultramafic massifs of early carboniferous and early cretaceous ages from northern inner mongolia china the early carboniferous massifs chongenshan xiaobaliang wusnihei comprise predominantly lherzolite and minor gabbroic rocks zircons from a microgabbro e nd t 9 8 and a plagiogranite e nd t 9 yielded weighted mean 206 pb 238 u ages of 354 7 ma and 333 4 ma that we interpret as reflecting the time of zircon and magma crystallization a volcanic breccia resting unconformably on the ultramafic rocks serpentinites has an eruption age of 300 2 ma and provides a younger minimum time limit for the emplacement of the early carboniferous mafic ultramafic massifs in contrast the early cretaceous mafic ultramafic massifs hegenshan chaogenshan consist predominantly of harzburgite a pegmatoid gabbro and a rodingite e nd t 10 6 from the hegenshan massif have magmatic zircon ages of 139 2 ma and 132 7 ma respectively the chaogenshan massif is concentrically zoned with an ultramafic core dominantly harzburgite dunite subordinate lherzolite surrounded by gabbro with minor hornblendite and an outermost volcano sedimentary sequence metabasaltic flows metapelites we dated zircons from a leuocogabbro 125 2 ma e nd t 10 6 a coarse grained gabbro 142 4 ma and a metabasalt 134 2 ma zircon xenocrysts ca 152 492 ma are ubiquitous in these and other associated rocks a coarse grained gabbro and a diabase as in the hegenshan rocks ca 152 468 ma the youngest detrital zircon in outer zone garnet schist i e metapelite has an age of 132 2 ma accordingly we conclude that the northern inner mongolia lherzolite dominant and harzburgite dominant massifs are temporally and genetically unrelated their emplacement is attributed to asthenospheric upwelling and consequent lithospheric extension during periods of orogenic quiescence in the southern central asian orogenic belt

the effects of orography on midlatitude northern hemisphere dry climates abstract the role of mountains in maintaining extensive midlatitude arid regions in the northern hemisphere was investigated using simulations from the gfdl global climate model with and without orography in the integration with mountains dry climates were simulated over central asia and the interior of north america in good agreement with the observed climate in contrast moist climates were simulated in the same regions in the integration without mountains during all season but summer large amplitude stationary waves occur in response to the tibetan plateau and rocky mountains the midlatitude dry regions are located upstream of the troughs of these waves where general subsidence and relatively infrequent storm development occur and precipitation is thus inhibited in summer this mechanism contributes to the dryness of interior north america as a stationary wave trough remains east of the rockies but is not effective in eurasia due to seasonal changes in the atmospheric circulation the dryness

a late carboniferous to early early permian subduction accretion complex in daqing pasture southeastern inner mongolia evidence of northward subduction beneath the siberian paleoplate southern margin abstract a subduction accretion complex is identified from previously defined late carboniferous and early permian strata in daqing pasture southern xiwuqi inner mongolia the subduction accretion complex is composed of a matrix of siltstone and exotic blocks of bioclastic limestone pillow basalt foliated basalt chert and asbestos the pillow basalt possesses the geochemical characteristics of mid ocean ridge basalt n morb whereas the foliated basalt displays the geochemical characteristics of island arc basalt iab indicating that these basalts are of different origins u pb zircon dating indicates that the foliated basalt formed in the late carboniferous 314 5 to 318 4 ma and the bioclastic limestone formed in the early early permian combined with regional geological data the subduction accretion complex and coeval calc alkaline granitic belt to the north constitute the essential elements of the late carboniferous to early early permian subduction zone on the southern margin of the siberian paleoplate the zircon ehf t values of the foliated basalt are positive 14 4 to 23 9 suggesting that this basalt originated directly from depleted mantle the temporal spatial distribution of the subduction accretion complex and ophiolite belts in southeastern inner mongolia indicates that there was significant lateral crustal growth on the southern margin of the siberian paleoplate in the late paleozoic

hf isotopes in zircon reveal contrasting sources and crystallization histories for alkaline to peralkaline granites of temora southeastern australia peralkaline granites exhibit the hallmark features of a type igneous rocks but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins we present the first in situ hf isotope data from zircons of peralkaline granites aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous weakly peraluminous granites this study used rocks of the devonian narraburra complex in southeastern australia and found that correlations between hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites the latter have a depleted mantle like ancestry whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile probably alkaline magmas juxtaposition of crust and mantle derived magmas reflects the high heat flow and lithosphere scale faults associated with continental extension and explains the diversity of a type granites

granites of the southern mongolia carboniferous arc new geochronological and geochemical constraints the crust in southern mongolia is part of the central asian orogenic belt a vast accretionary orogen that records the opening and closure of the palaeo asian ocean in the late proterozoic to palaeozoic the crustal evolution of the region is revealed in basement inliers that also contain intrusion related porphyry ore bodies that are important mineral exploration targets the saykhandulaan inlier in southeast mongolia is a devonian carboniferous segment of island arc crust which is dominantly composed of extrusive and sedimentary lithologies but which also contains the oyut ulaan i type quartz monzonite intrusion a u pb zircon age for the oyut ulaan monzonite indicates emplacement at 330 0 0 5 ma to the east of the saykhandulaan inlier intrusive complexes dominate the neighbouring mandakh inlier new ages are presented for four of these plutons the bronze fox granodiorite 333 6 0 6 ma the narin hudag monzonite 333 2 0 6 ma the shuteen quartz monzonite 325 5 1 0 ma and the north mandakh granite 292 3 0 5 the intrusive bodies of the saykhandulaan and mandakh inliers have two distinct geochronological and geochemical associations 1 mid carboniferous i type monzonites that constitute the most easterly intrusive expression of the southern mongolia carboniferous arc and 2 early permian a type and peralkaline granites that represent a post orogenic phase of voluminous granite emplacement both groups are significantly younger than the nearby oyu tolgoi and tsagaan suvarga cu porphyry ore bodies which have previously been dated as early and late devonian respectively the new data presented here provide constraints on the timing of the transition from island arc magmatism to post collisional extension related magmatism in the region and possible controls on fertile and infertile granitoid intrusions with respect to cu au mineralisation

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths

elemental and sr nd isotopic geochemistry of the uradzhongqi magmatic complex in western inner mongolia china a record of early permian post collisional magmatism abstract the magmatic complex in uradzhongqi inner mongolia is located in the western segment of the northern margin of the north china craton ncc the dominant components in the complex include syenogranite monzogranite granodiorite diorite and gabbro mafic microgranular enclaves mmes are common in syenogranite and granodiorite zircon u pb dating shows that the ages of these rocks range from 283 to 270 ma suggesting an early permian emplacement the syenogranite and monzogranite are peraluminous i type granites exhibiting conspicuous negative eu anomaly enrichment in large ion lithophile elements lile and light rare earth elements lree depletion in high field strength elements hfse the granodiorites diorites and mmes are metaluminous in composition show high al 2 o 3 mgo and fe 2 o 3 t contents and weak negative eu anomaly as well as lree and lile enrichment and hfse depletion the gabbros show weak positive eu anomaly and slight ree differentiation the sr nd isotope compositions show that the source of mafic magma was depleted mantle dm with possible involvement of enriched mantle ii em ii whereas the felsic magma was derived from the archean lower crust petrographic observation and analytical results of mineralogy geochronology geochemistry and sr nd isotopes indicate that the main petrogenesis of these magmatic rocks is the mixing of underplating mafic magma and felsic magma tectonically the complex pluton was formed within a post collisional regime and the underplating in this area provides another piece of evidence for the vertical growth of the western segment of the northern margin of the ncc

impacts of uplift of northern tibetan plateau and formation of asian inland deserts on regional climate and environment abstract based on the geological evidence that the northern tibetan plateau ntp had an uplift of a finite magnitude since the miocene and the major asian inland deserts formed in the early pliocene a regional climate model regcm4 1 with a horizontal resolution of 50 km was used to explore the effects of the ntp uplift and the related aridification of inland asia on regional climate we designed three numerical experiments including the control experiment representing the present day condition the high mountain experiment representing the early pliocene condition with uplifted ntp but absence of the asian inland deserts and the low mountain experiment representing the mid miocene condition with reduced topography in the ntp by as much as 2400 m and also absence of the deserts our simulation results indicated that the ntp uplift caused significant reductions in annual precipitation in a broad region of inland asia north of the tibetan plateau tp mainly due to the enhanced rain shadow effect of the mountains and changes in the regional circulations however four mountainous regions located in the uplift showed significant increases in precipitation stretching from the pamir plateau in the west to the qilian mountains in the east these mountainous areas also experienced different changes in the rainfall seasonality with the greatest increases occurring during the respective rainy seasons predominantly resulted from the enhanced orographically forced upwind ascents the appearance of the major deserts in the inland asia further reduced precipitation in the region and led to increased dust emission and deposition fluxes while the spatial patterns of dust deposition were also changed not only in the regions of uplift impacted topography but also in the downwind regions one major contribution from this study is the comparison of the simulation results with 11 existing geological records representing the moisture conditions from miocene to pliocene the comparisons revealed good matches between the simulation results and the published geological records therefore we conclude that the ntp uplift and the related formation of the major deserts played a controlling role in the evolution of regional climatic conditions in a broad region in inland asia since the miocene

early permian a type granites from central inner mongolia north china magmatic tracer of post collisional tectonics and oceanic crustal recycling abstract a type granitoids constitute volumetrically minor but genetically important building blocks of most post archean continental landmasses on earth and their diversified genesis encapsulates key information for ascertaining critical geodynamic controls on continental crustal formation and differentiation this zircon u pb dating and geochemical study documents the early permian peralkaline granites from central inner mongolia of china eastern central asian orogenic belt caob these granites show an a type magmatic affinity with typical enrichment in alkalis ga zr nb and y depletion in sr and p and fractionated ree patterns showing strong negative eu anomalies they have meaningful initial 87 sr 86 sr ratio of 0 70594 e nd t of 4 25 to 6 05 zircon e hf t from 9 8 to 16 0 and 18 o values from 2 41 to 4 72 these elemental and isotopic characters argue for parental magmas from partial melting of a newly trapped charnockitic lower crustal protolith and their subsequent fractional crystallization with an inherent hydrothermally altered oceanic crustal heritage and possible tectonic linkage to post collisional slab breakoff this kind of a type granites could not only serve as an insightful temporal milestone for monitoring the amalgamation between various constituent terranes in the caob but also present a spatial magmatic proxy for marking the maturation stage of juvenile crustal reworking during the final phase of a large orogenic event

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

monsoons and the dynamics of deserts the existence of subtropical deserts such as the sahara has often been attributed to the annual mean zonal mean hadley circulation which shows strong descent in the subtropics however the zonal mean hadley circulation shows considerable evolution over the course of the year with very strong subtropical descent during winter but practically no zonal mean subtropical descent during summer when rainfall over the eastern sahara and the mediterranean is least charney 1975 proposed a biosphere albedo feedback mechanism whereby local anthropogenic effects related to over grazing could affect the radiative balance enhancing summertime diabatic descent and leading to desertification of the subtropics in general the present study which uses an idealized model suggests a monsoon desert mechanism for desertification whereby remote diabatic heating in the asian monsoon region can induce a rossby wave pattern to the west integral with the rossby wave solution is a warm thermal structure that interacts with air on the southern flank of the mid latitude westerlies causing it to descend this adiabatic descent is localized over the eastern sahara and mediterranean and over the kyzylkum desert to the south east of the aral sea by the mountains of north africa and south west asia trajectories indicate that the monsoon desert mechanism does not represent a simple walker type overturning cell instead the descending air is seen to be mainly of mid latitude origin it is speculated that the monsoon forced adiabatic descent may result in clear air and therefore a local diabatic enhancement which effectively doubles the strength of descent with this mechanism desertification can be forced by remote changes in monsoon strength rather than by local effects this conclusion is supported by the observed dramatic strengthening of descent over the mediterranean and east sahara during the onset of the asian monsoon and on the longer timescale by relating prehistoric lake levels to milankovitch monsoon forcing the latter may help to explain the perceived discrepancies between the palaeoclimate of the eastern sahara and the strength of a tropic wide monsoon the monsoon desert mechanism may not be confined to the asian monsoon alone and the existence of other monsoon climate regions over the globe may in a similar way explain the observed summertime strengthening of the oceanic sub tropical anticyclones and the existence of western continental deserts and mediterranean type climate regions

impact of mongolian plateau versus tibetan plateau on the westerly jet over north pacific ocean mountains have long been considered to play an important role in the formation of modern climate particularly in the asia pacific sector the existence of tibetan plateau tp is believed to maintain the stationary planetary wave patterns and to intensify the westerly jet over north pacific however the potential role of mongolian plateau mp has often been neglected in these studies with an atmospheric general circulation model we show that the mp despite its smaller size exerts a great influence on the planetary scale circulation and the subtropical westerly jet the mp amplifies the meridional thermal gradient at the mid upper troposphere and thus strengthens the jet which is primarily facilitated by its high latitude location by blocking westerly winds the mp forces their northern branch further northward which allows the southward penetration of cold air at the lee side and intensifies the east asian trough in contrast the impact of the tp itself is not as large as expected hence one should not simply ascribe all the mountain induced climate change to the tp uplift because other smaller topography might be also very important

effect of marginal topography around the tibetan plateau on the evolution of central asian arid climate yunnan guizhou and mongolian plateaux as examples mountains are believed to have played an important role in the evolution of modern arid climate over central asia the main topography of the tibetan plateau tp suppresses the regional atmospheric rainfall by both the modulation of atmospheric circulation and blocking of water vapor transport from the ocean in this study the effect of yunnan guizhou and mongolian plateaux ygp and mp respectively two marginal topographies around the main tp on the central asian aridity are evaluated using general circulation model experiments the results show that the precipitation over central asia is significantly decreased by these two topographies compared to the whole tp induced annual precipitation decrease of 0 45 mm d the contributions of the ygp and mp reach 0 14 mm d and 0 08 mm d respectively these two marginal mountains occupy approximately one half of the total change by the tp although they are much smaller in heights and sizes the orographic forcing of the tp suppresses the precipitation significantly throughout the year while those of ygp and mp are mainly effective in boreal winter a moisture budget analysis shows that all the mountains examined drive increases in subsidence and resulting decreases in humidity over central asia with smaller or opposing roles for changes in horizontal winds these subsidence changes dominate the drying of central asia due to the tp and mp and are largely driven by the influences of the topography on stationary waves in contrast the ygp dries central asia primarily through altering transient eddies the forcing of ygp and mp originate from the mechanical blocking of the tropical easterly and mid latitude westerly respectively which exert significant changes in atmospheric circulation this implies that the effect of small scale mountains on arid climate over central asia might be underestimated and a considerable proportion of mechanical effect of the tp on the asian aridity actually comes from its margins

crustal evolution of the saykhandulaan inlier mongolia implications for palaeozoic arc magmatism polyphase deformation and terrane accretion in the southeast gobi mineral belt abstract the saykhandulaan inlier in south east mongolia lies within the central asian orogenic belt caob and records a complex history of palaeozoic tectonism and magmatism associated with terrane accretion on the northern margin of the palaeo asian ocean the inlier spans the boundary between the gobi altai back arc basin terrane in the north and the mandalovoo and gurvansayhan island arc terranes in the south which are notable for their many mineralised intrusions including the oyu tolgoi gold rich copper porphyry deposit results from cross strike transects within the saykhandulaan inlier reveal that it can be subdivided into five parallel east west striking litho tectonic domains 1 the northern slate belt comprising devonian greenschist grade pelites and psammites with deep marine to coastal siliciclastic protoliths 2 the saykhandulaan valley lineament zone svlz a tectonised zone of faulted and lithologically altered volcanic rocks 3 the high strain belt consisting of tightly folded and flattened metamorphosed clastic sedimentary rocks 4 the molasse succession consisting of relatively undeformed coarse conglomerates and sandstones and 5 the oyut ulaan volcanic group a nearly 5 km thick folded carboniferous volcanic succession that hosts the mid carboniferous oyut ulaan mineralised granite the northern slate belt metasedimentary rocks record a northerly cratonic provenance whereas all rocks to the south of the svlz have arc affinities the svlz is thus interpreted to be the boundary between the gobi altai and mandalovoo terranes two major deformation events are documented 1 back arc basin closure and inversion involving regional scale folding and greenschist grade metamorphism in the northern slate and high strain belts 2 contraction associated with mandalovoo terrane accretion and final closure of the palaeo asian ocean to the south following terrane accretion and cessation of subduction crustal extension and strike slip faulting further modified the crustal architecture of the inlier the results presented here provide a useful framework for understanding the crustal evolution of adjacent regions within the southeast gobi mineral belt

sensitivity of eurasian climate to surface uplift of the tibetan plateau experiments with climate models are used to illustrate how earth s climate could be expected to change if surface uplift occurred the experiments contrast two climate simulations one with all continents at sea level and one with present day mountains and plateaus the simulations in the eurasia sector show that surface uplift increased elevation produces marked increases in the intensity of summer and winter monsoons of asia with development of wetter climates to the south and east of the tibetan plateau and drier climates to the north and west of the plateau there is a general cooling in northern polar latitudes comparison of simulations from two different models shows that the magnitude of the simulated changes depends upon how the models calculate the land surface hydrology and the ocean temperature the simulated changes produced by surface uplift agree in a general way with many geologic indicators of late cenozoic climate change the results suggest that surface uplift of mountains and platea

late triassic intrusive complex in the jidong region jiamusi khanka block ne china geochemistry zircon u pb ages lu hf isotopes and implications for magma mingling and mixing abstract whole rock major and trace element geochemistry together with zircon u pb ages and lu hf isotope compositions are reported for a late triassic intrusive complex in the jidong region jiamusi khanka block ne china zircon u pb dating yields ages between 211 and 208 ma for enclaves of microgranular diorite and quartz diorite and between 211 and 209 ma for the host granitoids these ages correlate with a previously established intensive late triassic magmatic event along the eastern asian margin field observations together with petrographic features geochemistry and zircon hf isotope data preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex the syenogranite suite has high sio 2 75 5 76 3 wt and low mgo 0 15 0 19 wt and yields enriched lile and lree patterns most of the zircons in the syenogranites have two stage model ages of 766 and 1461 ma together with positive e hf t values of 0 6 to 9 1 these results indicate that the granitoid magmas were generated by partial melting of meso to neoproterozoic lower crust the gabbro suite has a restricted range of sio 2 46 1 51 9 wt together with high mg values 49 70 and high concentrations of ni co and cr zircons from two diorite samples have single stage hf model ages of 557 787 ma and e hf t values of 1 9 to 8 3 that are consistent with the coeval gabbros previously studied in the jidong region these features together with the observation that all the gabbros are enriched in lree and lile suggest that the mafic magmas were derived from melting of depleted neoproterozoic lithospheric mantle that had been metasomatized by slab derived fluids it is concluded that the dominant igneous suites within the late triassic intrusive complex formed by mingling mixing of felsic and mafic magmas the geochemical data combined with regional geological investigations indicate that the late triassic intrusive complex formed during lithospheric extension caused by slab break off of the yuejinshan oceanic plate that was subducting westward beneath the jiamusi khanka block

petrogenesis and tectonic implications of late carboniferous continental arc high k granites in the dongwuqi area central inner mongolia north china abstract central inner mongolia is located in the southeastern segment of the central asian orogenic belt caob and controversy has long surrounded its late paleozoic tectonic evolution here we present an integrated study of zircon u pb ages whole rock major and trace elements and zircon hf isotopes for five less studied alkali feldspar granitic plutons in the dongwuqi area to the north of the erenhot hegenshan heihe suture zone zircon u pb dating by laser ablation inductively coupled plasma mass spectrometry la icp ms indicates that these granitic plutons were emplaced in the late carboniferous 315 300 ma the late carboniferous alkali feldspar granites belong to high k calc alkaline series and are evolved i type granites they are high in sio2 73 22 77 36 and low in mgo 0 07 0 49 with enrichments in light rare earth elements lrees zr hf rb th and u and depletions in ba nb ta sr and eu the dongwuqi granitic rocks are characterized by high positive zircon ehf t values 7 6 to 15 1 and relatively young two stage hf model tdm2 ages 364 835 ma these geochemical data together the common miarolitic texture indicate that they were derived from partial melting of juvenile arc related mafic to intermediate rocks in the lower crust and eventually emplaced in low pressure conditions or at shallow crustal levels with involvement of extensive fractional crystallization during magma evolution our new results along with the published regional data delineate a ne trending carboniferous continental arc belt more than 1700 km in the northern accretionary zone of the erenhot hegenshan heihe suture accordingly we invoke that the dongwuqi late carboniferous high k granites were emplaced in an active continental margin setting which was probably related to the northwestward subduction of the hegenshan oceanic plate beneath the xing an uliastai nuhetdavaa block

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

ophiolites in the xing an inner mongolia accretionary belt of the caob implications for two cycles of seafloor spreading and accretionary orogenic events the xing an inner mongolia accretionary belt in the southeastern segment of the central asian orogenic belt caob was produced by the long lived subduction and eventual closure of the paleo asian ocean and by the convergence between the north china craton and the mongolian microcontinent two ophiolite belts have been recognized the northern erenhot hegenshan xi ujimqin ophiolite belt and the southern solonker linxi ophiolite belt most basalts in the northern ophiolite belt exhibit characteristics of normal type to enriched type mid ocean ridge basalt affinities with depleted nd isotopic composition epsilon nd t 5 comparable to modern eastern pacific mid ocean ridge basalts most basaltic rocks in the southern belt show clear geochemical features of suprasubduction zone type oceanic crust probably formed in an arc back arc environment the inferred back arc extension along the solonker linxi belt started at circa 280 ma statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading subduction which gave rise to two main epochs of magmatic activity at 500 410 ma and 360 220 ma respectively with a gap of similar to 50 million years myr the spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction toward the two continental margins in the convergence history with final collision at similar to 230 220 ma in the whole belt signals of continental collision and himalayan style mountain building are lacking we thus conclude that the xing an inner mongolia segment of the caob experienced two cycles of seafloor subduction back arc extension and final appalachian type soft collision

evolution of a permian intraoceanic arc trench system in the solonker suture zone central asian orogenic belt china and mongolia abstract the identification of a fossil arc trench system from the ophiolite decorated solonker suture zone in the southernmost central asian orogenic belt caob enables us to constrain the timing of pre subduction extension ca 299 290 ma subduction initiation ca 294 280 ma ridge trench collision ca 281 273 ma and slab break off ca 255 248 ma in the permian a fraction of proto arc crust ca 45 km long up to 8 km wide is preserved as a volcanic plutonic sequence and is juxtaposed against a wide ca 30 80 km forearc melange this proto arc crust comprises two distinct magma series island arc tholeiite iat and mid ocean ridge basalt morb both of which have strong supra subduction zone ssz geochemical signatures zircons from a gabbro and a plagiogranite yielded weighted mean 206pb 238u ages of 284 0 4 0 and 288 0 6 0 ma the forearc melange consists of numerous ophiolite fragments and continental margin derived olistoliths blocks that predate the ophiolite the olistoliths are best represented by a gabbroic block 291 8 2 3 ma that contains granite xenoliths 312 6 1 8 and 313 6 3 1 ma other dated blocks include a trondhjemite 323 9 2 7 ma a gabbro 296 6 1 7 ma and a tonalite 294 9 2 4 ma small bodies of diabase andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures we dated zircons from an n morb like diabase 274 4 2 5 ma an e morb like diabase 252 5 2 3 ma a transitional sanukitoid adakite andesite 250 2 2 4 ma a sanukitoid high mg diorite 251 8 1 1 ma and an anorthosite 252 2 1 7 ma the n morb like diabase contains ca 301 394 ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench the other dated rocks simultaneously formed near the permian triassic boundary and captured abundant zircon xenocrysts ca 269 295 ma ca 301 495 ma and ca 923 2501 ma our new formation ages constrain a magmatic episode in response to slab break off beneath a fossil forearc in a young post collisional setting and the youngest xenocryst ages ca 269 273 ma may define the maximum depositional age of trench sediments

dominant control of the south asian monsoon by orographic insulation versus plateau heating heat emitted from the tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the south asian summer monsoon but new work suggests that in fact it is the neighbouring mountains that are the major influence william boos and zhiming kuang use an atmospheric model to show that flattening the tibetan plateau has little effect on the monsoon so long as the himalayas and surrounding mountain ranges remain the plateau does boost rainfall locally along its southern edge but it is the build up of hot moist air over india insulated from colder drier air by the himalayas that drives large scale monsoon circulation the elevation of the tibetan plateau is thought to cause its surface to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales here however an atmospheric model is used to show that flattening of the tibetan plateau has little effect on the monsoon provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved the tibetan plateau like any landmass emits energy into the atmosphere in the form of dry heat and water vapour but its mean surface elevation is more than 5 km above sea level this elevation is widely held to cause the plateau to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales1 2 3 4 5 observations of the present climate however do not clearly establish the tibetan plateau as the dominant thermal forcing in the region peak upper tropospheric temperatures during boreal summer are located over continental india south of the plateau here we show that although tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model the large scale south asian summer monsoon circulation is otherwise unaffected by removal of the plateau provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm moist air over continental india from the cold and dry extratropics these results call for both a reinterpretation of how south asian climate may have responded to orographic uplift and a re evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades

relative roles of land sea distribution and orography in asian monsoon intensity abstract the relative impacts of various land sea distributions lsds and mountains on asian monsoon extent and intensity are assessed using a series of agcm simulations the air sea coupling effects are not considered in this study all simulations were integrated with zonal mean sst globally uniform vegetation soil color and except several simulations soil texture the results show that the lsd plays a more fundamental role than orography in determining the extent of asian and african monsoons the tropical zonal lsd and asian mountains both play a crucial role for establishing summer monsoon convection over the south asian region the monsoon circulation index mci1 defined by the difference of zonal wind between 850 and 200 hpa is used to measure the intensity of the south asian summer monsoon the large scale meridional land sea thermal contrast between the eurasian continent and the indian ocean only induces a 1 8 m s 1 increase of mci1 the presence of the indian subcontinent and indochina p

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

petrology of the hegenshan ophiolite and its implication for the tectonic evolution of northern china abstract petrographic observations and mineralogical and geochemical analyses have revealed that the hegenshan ophiolite is of mid ocean ridge origin and has been subjected to dynamothermal metamorphism at medium p t conditions the metamorphism is characterized by a prograde change in paragenesis from the greenschist to epidote amphibolite facies with peak temperature conditions of 570 640 c at pressures of 4 10 kbar the amphiboles formed by this metamorphism show k ar ages of 110 130 ma the metamorphic conditions and k ar ages suggest that the hegenshan ophiolite is located at the suture between the siberian and north china continental blocks where the continental collision in this area took place in middle mesozoic time given the temporal and spatial distribution of the igneous activity around the da hinggan ling mountains it is suggested that the extensive yanshanian magmatism in this region resulted predominantly from a southward subduction of an oceanic plate prior to collision alternatively it may possibly have resulted from the collision itself at the final stage

late permian tectonic evolution at the southeastern margin of the songnen zhangguangcai range massif ne china constraints from geochronology and geochemistry of granitoids abstract this paper reports the la icp ms u pb zircon dating and geochemical data for the huangqigou xiaobeihu and lalagou granitic plutons at the southeastern margin of the songnen zhangguangcai range massif which will not only place important constraints on the rock forming ages source characteristics and tectonic setting of these granitic plutons but will also provide insights into understanding the tectonic evolution of the eastern segment of the central asian orogenic belt zircons from three plutons have distinct oscillatory zoning structures in their cathodoluminescene cl images which combined with the th u ratios of 0 28 1 06 indicate their magmatic origins zircon u pb dating results for these granitic plutons indicate that the granitic magmatism occurred mainly in the late permian 256 252 ma the granites have sio2 of 58 0 74 3 na2o of 2 74 3 76 k2o of 2 75 3 95 na2o k2o of 0 77 1 13 mg 100 mg mg fe2 of 21 8 39 9 and a cnk of 0 88 1 08 with ree patterns characterized by lree hree 7 75 12 82 la yb n 7 88 14 9 and eu 0 56 0 87 the trace element spider diagrams indicate that these granites are enriched in large ion lithosphile elements rb ba th u etc and relatively depleted in high field strength elements ti nb ta with sr and pb ranging from 205 to 350 ppm and from 2 02 to 3 60 ppm respectively the in situ hf isotope analysis reveals that ehf t values range from 2 22 to 11 65 and the two stage hf model ages tdm2 range between 641 and 1274 ma these geochemical characteristics indicate that the late permian granitic plutons at the se margin of the songnen zhangguangcai range massif can be assigned to metaluminous and weakly peraluminous high potassium calc alkaline i type granites with low sr and high yb features and that the magmas to have formed these granites were originated from the partial melting of mafic igneous rocks in the lower crust the source rocks are composed mainly of the juvenile crust with small contributions of ancient continental crust considering the existence of coeval mafic intrusive rocks and sedimentary basins we propose that the late permian magmatism in the study area occurred in a post collisional extensional setting following the collision of the xingkai block and the united jiamusi and songnen zhangguangcai range massifs

juvenile crustal recycling in an accretionary orogen insights from contrasting early permian granites from central inner mongolia north china abstract coeval high k calc alkaline to alkaline granites constitute important components of post collisional to post orogenic igneous suites in most orogenic belts of various ages on earth and their genesis harbors a key to ascertaining critical geodynamic controls on continental crustal formation and differentiation this zircon u pb dating and geochemical study documents three contrasting early permian granites from erenhot of central inner mongolia eastern central asian orogenic belt caob and reveals concurrent high k calc alkaline to alkaline granite association derived from successive partial melting of distinct protoliths the ca 280 ma gancihuduge gcg pluton shows a calc alkaline i type character with initial 87 sr 86 sr ratios of 0 7035 to 0 7039 e nd t of 1 87 to 4 70 zircon e hf t of 8 0 to 13 2 and 18 o from 7 4 to 8 7 the ca 276 ma cailiwusu cls pluton is magnesian and peraluminous with initial 87 sr 86 sr ratios of 0 7036 to 0 7040 e nd t of 1 9 to 2 4 zircon e hf t of 6 5 to 12 1 and 18 o from 9 7 to 10 9 these features are consistent with partial melts of mixed sources composed of newly underplated meta basaltic to andesitic protoliths and variable supracrustal components with distinctively higher proportion of the latter in the cls pluton by contrast the ca 279 ma kunduleng kdl suite exhibits an a type magmatic affinity with typical enrichment in alkalis ga zr nb and y e nd t of 2 39 to 3 55 zircon e hf t from 8 3 to 12 3 and 18 o values from 6 8 to 7 5 these features suggest that they stem from high temperature fusion of dehydrated k rich mafic to intermediate protoliths besides presenting a snapshot into a stratified crustal architecture in 18 o these contrasting granites could not only serve as a temporal marker for monitoring post collisional extension in the aftermath of a retreating subduction zone but also present spatial magmatic proxy for tracing crustal formation and differentiation within back arc basin environments in the caob

the emplacement time of the hegenshan ophiolite constraints from the unconformably overlying paleozoic strata abstract controversy has long surrounded the emplacement time of the hegenshan ophiolite that is considered to mark a suture zone called the hegenshan heihe suture resulting from the closure of a back arc basin in the paleo asian ocean the hegenshan ophiolite in the xiaobaliang area is unconformably overlain by a sequence of paleozoic strata called the zhesi formation that consists of conglomerate sandstone siltstone and limestone some of which contain permian marine fossils of brachiopods therefore the ages of these paleozoic strata can be used to constrain the emplacement time of the hegenshan ophiolite four samples of the zhesi formation collected in the xiaobaliang area yield the detrital zircon u pb ages of 285 272 ma with the peak at 279 ma 315 288 ma with a peak at 300 ma 320 358 ma with a peak at 336 ma and 406 3 ma of which the 280 and 300 ma age groups are remarkably similar to the ages of latest carboniferous early permian gegenaobao dashizai formation or a type granites which formed under a post collisional setting however the age groups of 320 to 358 ma with a peak at 336 ma show the features of mafic ultramafic zircons in cl image most likely derived from local mafic ultramafic rocks of the hegenshan ophiolite in the xiaobaliang area which is supported by the fact of the ophiolite unconformably overlain by the middle permian zhesi formation therefore we propose that the emplacement time of the hegenshan ophiolite must have happened at some time before the middle permian 280 ma most likely between 300 and 335 ma not in the silurian devonian or mesozoic as previously considered

late cenozoic eolian deposition in the north pacific asian drying tibetan uplift and cooling of the northern hemisphere a newly constructed record of eolian dust accumulation from the central north pacific shows that dust deposition increased by an order of magnitude quite rapidly at 3 6 ma we associate this sudden drying with the uplift of at least the northern portion of the tibetan plateau shutting off the indian ocean moisture source to central and western china this ten fold increase in atmospheric dust loading appears to be associated with the 1 m y long shift toward heavy 18o values that occurred at 3 6 2 6 ma the dust grain size record of wind intensity begins its late cenozoic coarsening a million years before the drying event at 4 5 ma the northern hemisphere cooling that results in intensification of the subpolar westerly winds may have as its ultimate cause the drawdown of atmospheric co2 in the latest miocene and the early pliocene closing of the panamanian seaway

late paleozoic rift related basalts from central inner mongolia china abstract late paleozoic basaltic volcanism widely occurred in central inner mongolia the southeastern part of the central asia orogenic blet caob three volcanic pulses are identified by ion microprobe shrimp ii zircon u pb analysis which are late carboniferous ca 315 ma the latest carboniferous ca 303 ma and early permian ca 284 ma basalts in the earliest stage are characterized by flat hree depletion in nb and ta morb like th yb ratios and depleted mantle dm like hf isotopic ratios these are similar to forearc basalts that have been generated by melting of a fluxed depleted mantle in an extensional environment the latest carboniferous basalts sample d1363 have no zr hf anomalies relatively radiogenic hf isotopic ratios ehf t 12 1 to 19 7 and high 18o values up to 8 5 vsmow these are likely to be magmas originated from decompressional melting of a depleted mantle that interacted with ocean floor pillow basalts during their ascending through the crust the early permian sample d1380 have variable 206pb 238u zircon ages with an interpreted magma crystallization age of 284 4 2 5 ma these zircons have scattered hf isotope ehf t 5 to 14 and 18o 1 8 11 0 vsmow which suggest the involvement of a relatively enriched mantle source overall the temporal evolution of the late paleozoic basaltic volcanism from central inner mongolia suggests a rift related tectonic setting

provenance analysis of the late paleozoic sedimentary rocks in the xilinhot terrane ne china and their tectonic implications abstract the xilinhot terrane is located in the eastern segment of the central asian orogenic belt in ne china and is a key to a hotly debated issue on the paleozoic tectonic evolution of this giant progenic belt to constrain the tectonic evolution of the xilinhot terrane in the late paleozoic we undertook zircon u pb dating and geochemical analyses of the zhesi and benbatu formations in the suolun and xi ujimqin areas in the xilinhot terrane samples of the benbatu formation yield detrital zircon u pb ages ranging from 2659 ma to 316 ma with four age populations at 2659 1826 ma 1719 963 ma 590 402 ma and 396 316 ma whereas samples from the zhesi formation yield detrital zircon u pb ages ranging from 1967 ma to 250 ma with four age populations at 1967 1278 ma 971 693 ma 561 403 ma and 399 250 ma the age groups of both the benbatu and zhesi formations in the xilinhot terrane are similar to those in other parts of the central asian orogenic belt caob this evidence indicates that the xilinhot terrane is a microcontinent and not an accretionary complex as previously thought furthermore the youngest zircon grains in the benbatu and zhesi formations yield weighted mean 206 pb 238 u ages of 322 12 ma msdw 0 12 n 4 and 257 2 8 ma msdw 1 6 n 8 respectively combined with fossil data our new data suggest that the benbatu and zhesi formations in the xilinhot terrane were possibly deposited at 322 ma and 257 ma respectively based on the provenance of the carboniferous permian sandstones came from the blocks of ne china we speculate that the xilinhot terrane is the western part of the songliao block

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

the pre devonian tectonic framework of xing an mongolia orogenic belt xmob in north china abstract a new tectonic division of the xing an mongolia orogenic belt xmob in north china has been presented according to our research and a lot of new data of tectonics geochronology and geochemistry four blocks and four sutures have been recognized in the xmob including the erguna eb xing an airgin sum xab songliao hunshandake shb and jiamusi jb and xinlin xiguitu xxs xilinhot heihe xhs mudanjiang ms and ondor sum yongji sutures oys the framework of the xmob is characterized by a tectonic collage of the blocks and orogenic belts between them different precambrian basements have been found in the blocks including the neoproterozoic metamorphic rocks and plutons in the eb the neoproterozoic metamorphic rocks in western and eastern of segments of the xab mesoproterozoic and neoproterozoic metamorphic rocks in middle segments of the xab respectively the neoproterozoic metamorphic rocks and mesoproterozoic volcanic rocks and plutons in the shb and neoproterozoic metamorphic rocks in the jb the xxs resulted from a northwestward subduction of the xab beneath the eb during the cambrian which was followed by the forming of the xhs and oys in the northwest and south margins of the shb in the silurian respectively the ms was caused by a westward subduction of the jb beneath the east margin of the shb during the middle devonian the three cambrian silurian and middle devonian events indicate that the xmob belongs to a pre middle devonian multiple orogenic belt in the central asian orogenic belt caob forming of the xmob suggests that the southeast part of the paleo asian ocean closed before the middle devonian

evolution of the solonker suture zone constraints from zircon u pb ages hf isotopic ratios and whole rock nd sr isotope compositions of subduction and collision related magmas and forearc sediments abstract the solonker zone in northern inner mongolia china is considered as the suture between the north china craton and the south mongolian microcontinent two magmatic belts are recognized along the suture zone a subduction related magmatic belt represented by the baolidao arc rocks and a younger collision related granite belt represented by the halatu granites we use zircon u pb ages zircon in situ hf isotopic analyses and whole rock nd sr isotopic data of the two magmatic belts and related forearc sediments the xilinhot metamorphic complex to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks a gabbroic diorite bld 1 of the baolidao arc was dated at 310 5 ma by shrimp this sample shows an e nd t value of 2 5 and i sr of 0 7052 hf isotopic analyses on 25 zircons from the same sample show e hf t 5 4 to 11 5 another diorite sample xh 2 of the same arc from south of xilinhot displays even more depleted isotopic compositions with e nd t 5 6 and i sr 0 7037 the main population of zircons from this sample have highly variable and depleted hf isotopic compositions e hf t 0 18 3 the large variation in hf isotopic composition of zircons with largely the same crystallization age from a single pluton is explained by a mixing process between depleted mantle derived magma and continental crust in an active continental arc setting the halatu granite hlt 2 was dated at 234 7 ma by shrimp zircons from the granite also show a large variation of e hf t values 9 1 to 26 despite most samples having whole rock e hf t 2 the large variation in e hf t values suggests that the granite formed probably by partial melting of two source regions a dominant juvenile crust and a subordinate old continental crust most zircons from the xilinhot metamorphic complex show ages comparable with those of the baolidao arc rocks suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism some zircons however show precambrian ages that fall into two groups one with ages of 780 900 ma resembling those from the south mongolian microcontinent and the other with ages of 1524 2900 ma similar to those of the north china craton thus the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents and metamorphosed subsequently during collision in the late paleozoic our zircon age data thus constrain timing of collision between the south mongolian microcontinent and the north china craton to have been between 296 and 234 ma

geochronology and geochemistry of late carboniferous middle permian i and a type granites and gabbro diorites in the eastern jiamusi massif ne china implications for petrogenesis and tectonic setting abstract late carboniferous middle permian magmatism in the jiamusi massif of northeast china in the eastern segment of the central asian orogenic belt caob provides critical evidence regarding the tectonic history and geodynamic processes in the region the gabbro diorites of the longtouqiao pluton and two groups of coeval granite in the study area comprise a bimodal magmatic suite precise la icp ms u pb zircon ages indicate that the granitoids and gabbro diorites were emplaced in the late carboniferous middle permian 302 267 ma group i granites have high sio 2 70 75 77 04 wt and k 2 o 3 65 5 89 wt contents are enriched in liles e g rb th and u relative to hfses and lrees and have negative nb ta p and ti anomalies which collectively indicate affinities with subduction related magmas group ii granites are weakly peraluminous a cnk 1 03 1 07 and are characterized by enrichment in alkalis na 2 o k 2 o 8 22 8 90 wt low mgo 0 04 0 09 wt and p 2 o 5 0 01 0 04 wt contents high zr and nb contents high 10 000 ga al ratios and they are geochemically similar to aluminous a type granites all the magmatic zircons in these granitoids have great variations of e hf t 7 89 to 5 60 and two stage hf model ages t dm2 of 0 8 1 7 ga which suggest that the precursor magmas originated from a heterogeneous source that involved juvenile components derived from a depleted mantle source during magma generation the aluminous a type granite magmas were probably derived by high temperature partial melting of a felsic crustal source whereas the other granite magmas probably resulted from partial melting of a mafic lower crust the gabbro diorites of the longtouqiao pluton are depleted in nb ta p and ti and show flat distributions of most liles and hfses except for large positive anomalies in ba k and pb these features reflect a limited degree of crustal contamination associated with the subduction related magmatic processes these data together with previously reported data and the nature of various rock types along the eastern part of the jiamusi massif suggest that the intrusive rocks were formed in a geodynamic regime that changed from compression to extension during the westwards subduction of the paleo pacific oceanic lithosphere probably as a result of slab break off of the subducting plate

permian geodynamic setting of northeast china and adjacent regions closure of the paleo asian ocean and subduction of the paleo pacific plate abstract northeast china and adjacent regions are located in the central east asian continent and consist tectonically of both the paleo asian and paleo pacific orogens between the siberian platform and sino korean north china block this paper discusses some hotly debated issues concerning the permian geodynamic setting of these regions based on a comprehensive analysis of available geological geochemical paleobiogeographical and paleomagnetic data spatial and temporal distribution of ophiolites and associated continental marginal sequences permian sedimentary sequences spatial distribution and geochemistry of permian magmatic rocks and the evolution of paleobiogeographical realms imply 1 that the permian marine basins in northeast china and adjacent regions include remnants of the paleo asian ocean in southeastern inner mongolia and central jilin province and active continental margins of the paleo pacific ocean 2 that the suture between the siberian and sino korean paleoplates was finally emplaced in the permian and is located in areas from suolunshan solonker eastwards through regions north to the xar moron river in southeastern inner mongolia and then central jilin province to the yanji area and 3 that the permian crustal evolution of northeast china and adjacent regions as well as parts of the siberian paleoplate was influenced by subduction of the paleo pacific oceanic plate finally the permian tectonic framework and paleogeography of northeast china and adjacent regions in central east asia are discussed briefly and early and late permian palinspastic reconstruction maps are provided

reconstructing tsunami run up from the characteristics of tsunami deposits on the thai andaman coast abstract tsunamis can leave deposits on the land surface they inundate the characteristics of tsunami deposits can be used to calculate tsunami run up height and velocity this paper presents a reconstruction of tsunami run up from tsunami deposit characteristics in a simple mathematical model the model is modified and applied to reconstruct tsunami run ups at ao kheuy beach and khuk khak beach phangnga province thailand the input parameters are grain size and maximum run up distance of the sediment the reconstructed run up heights are 4 16 4 91 m at ao kheuy beach and 5 43 9 46 m at khuk khak beach the estimated run up velocities maximum velocity at the still water level are 12 78 19 21 m s in the area located 70 140 m inland to the end of run up inundation estimated mean run up velocities decrease from approximately 1 93 m s to 0 m s reasonably good agreements are found between reconstructed and observed run up heights the tsunami run up height and velocity can be used for risk assessment and coastal development programs in the tsunami affected area the results show that the area from 0 to 140 m inland was flooded by high velocity run ups and those run up energies were dissipated mainly in this area the area should be designated as either an area where settlement is not permitted or an area where effective protection is provided for example with flood barriers or forest

alkali calcic and alkaline post orogenic po granite magmatism petrologic constraints and geodynamic settings abstract the end of an orogenic wilson cycle corresponds to amalgamation of terranes into a pangaea and is marked by widespread magmatism dominated by granitoids the post collision event starts with magmatic processes still influenced by subducted crustal materials the dominantly calc alkaline suites show a shift from normal to high k to very high k associations source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle affected by dehydration melting and generating more and more k and lile rich magmas in the vicinity of intra crustal magma chambers anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves the post collision event ends with emplacement of bimodal post orogenic po suites along transcurrent fault zones two suites are defined i the alkali calcic monzonite monzogranite syenogranite alkali feldspar granite association characterised by biotite plagioclase fractionation and moderate lile hfse enrichments and ii the alkaline monzonite syenite alkali feldspar granite association characterised by amphibole alkali feldspar fractionation and displaying two evolutionary trends one peralkaline with sodic mafic mineralogy and higher enrichments in hfse than in lile and the other aluminous biotite bearing marked by hfse depletion relative to lile due to accessory mineral precipitation alkali calcic and alkaline suites differ essentially in the amounts of water present within intra crustal magma chambers promoting crystallisation of various mineral assemblages the ultimate enriched and not depleted mantle source is identical for the two po suites the more primitive lile and hfse rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere present rock compositions are a mixture of major mantle contribution and various crustal components carried by f rich aqueous fluids circulating within convective cells created around magma chambers in favourable areas po suites pre date a new orogenic wilson cycle

u pb 39ar 40ar geochronology of the metamorphosed volcanic rocks of the bainaimiao group in central inner mongolia and its implications for ore genesis and geodynamic setting abstract the greenschist facies volcano sedimentary sequence of the bainaimiao group in central inner mongolia hosts the bainaimiao cu mo au deposit and the bainaimiao lode gold deposit but the ore classification and genetic process of the bainaimiao cu mo au deposit is still disputed in this study we have selected the interbedded metamorphosed volcanic rocks of the bainaimiao group for la icp ms zircon u pb analysis 17 analyses form a tight cluster on the concordia and yield a weighted mean 206 pb 238 u age of 465 1 ma 95 confidence level mswd 0 95 n 17 which represents the best estimate of the crystallization age of the volcanic rocks of the bainaimiao group biotite from the greenschist of the bainaimiao group yielded a well defined plateau with an age of 429 2 4 1 ma the inverse isochron age is 429 1 4 2 ma with an elevated 40 ar 36 ar i ratio of 299 9 22 7 which represents the time of regional greenschist facies metamorphism previous studies showed that the re os isochron age of the molybdenite from the bainaimiao cu mo au deposit is 445 0 3 4 ma and the mineralized granodiorite porphyry yielded a weighted mean 206 pb 238 u age of 445 6 ma these data indicate that the bainaimiao volcanic rocks were formed at about 465 ma and the porphyry cu mo au mineralization took place at about 445 ma volcanic rock emplacement is 20 ma older than the cu mo au mineralization indicates it is a porphyry deposit rather than vms deposit the dynamic setting for the porphyry mineralization is the paleo asian oceanic plate subduction along the northern margin of the north china craton in early paleozoic subsequently the collision between the bainaimiao arc and the north china craton led to regional metamorphism and deformation at about 430 ma and the old porphyry system may have been overprinted during this metamorphic process

geochronology petrogenesis and tectonic implication of late paleozoic volcanic rocks from the dashizhai formation in inner mongolia ne china abstract in this paper we present geochemical zircon u pb and hf isotopic data on the late paleozoic volcanic rocks of the dashizhai formation which are exposed along the northwestern margin of the songnen terrane in eastern inner mongolia our aim is to constrain the petrogenesis and tectonic setting of the volcanic rocks and to unravel the late paleozoic tectonic evolution of the northwestern part of the songnen terrane along the eastern segment of the central asian orogenic belt lithologically the dashizhai formation is composed mainly of rhyolitic tuff rhyolite dacite andesite basaltic andesite and basalt with minor basaltic trachyandesite the zircons separated from these rocks are euhedral subhedral have high th u ratios 0 2 1 6 and display broad oscillatory growth zoning indicating a magmatic origin the results of zircon u pb dating indicate the volcanic rocks formed during the early permian 295 283 ma geochemically these volcanic rocks belong to the mid k to high k calc alkaline series and are characterized by an enrichment in large ion lithophile elements liles and a depletion in high field strength elements hfses such as nb ta and ti similar to igneous rocks that form in active continental margin settings most magmatic zircons of the rhyolites show positive e hf t values 3 65 to 13 0 and two stage model ages t dm2 of 1396 551 ma these geochemical characteristics indicate that the acidic volcanic rocks of the dashizhai formation were most likely derived from the partial melting of dominantly juvenile crustal components with a possible addition of old materials in contrast the basic to intermediate volcanic rocks were derived from the partial melting of a depleted lithospheric mantle that had been metasomatized by fluids derived from a subducted slab these data together with regional geological investigations suggest that the generation of the early permian volcanic rocks of the dashizhai formation was related to the southward subduction of the paleo asian oceanic plate beneath the songnen terrane this also implies that the terminal collision between the songnen and xing an terranes did not occur before the early permian

pliocene uplift of the northern tibetan plateau neogene redbeds passing upward into upward coarsening conglomerate and debris flow deposits at the foot of the kunlun mountains record the change in paleoslope related to uplift of the surface of the northern tibetan plateau detailed magnetostratigraphy of a 4 5 km section near yecheng in the western kunlun mountains shows that the change from deposition on distal alluvial plains to proximal alluvial fans occurred during the gilbert reversed chron 4 5 3 5 ma the change in depositional facies was accompanied by an increase in sedimentation rate from an average 0 15 mm yr between the earliest oligocene and the earliest pliocene to 1 4 mm yr in the gauss normal chron 3 6 2 6 ma we interpret the change in depositional facies and increase in sedimentation rate as indicating that the main uplift of the northwestern tibetan plateau began ca 4 5 ma

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

phanerozoic evolution of the inner mongolia daxinganling orogenic belt in north china constraints from geochronology of ophiolites and associated formations

the oyut ulaan volcanic group stratigraphy magmatic evolution and timing of carboniferous arc development in se mongolia abstract the palaeozoic mesozoic tectonic evolution of central asia including the vast terrane collage that makes up mongolia has been a topic of considerable debate the oyut ulaan volcanic group is a sequence of volcanic and sedimentary rocks in se mongolia that forms the southern part of the devonian permian saykhandulaan inlier fieldwork traverses and mapping have established four distinct formations in the oyut ulaan volcanic group that record the nature of arc activity in part of the central asian orogenic belt during the carboniferous physical volcanological and sedimentological characteristics of the four formations suggest three clear eruptive styles 1 periodic andesite volcanism in an actively eroding arc setting that also contained large rivers and swamps 2 highly effusive plateau andesite volcanism 3 explosive rhyolitic effusion geochemical analyses of volcanic lithologies suggest that the group represents subduction related mature continental arc volcanism geochemical results document an evolving magma system to which surface processes of the volcano sedimentary model may be linked magma pulses and replenishments are identified from variations in chemostratigraphy newly obtained zircon ages from the volcanic succession fix its emplacement eruption at 323 0 0 7 ma mid carboniferous or late mississippian a granite cobble from the lower part of the oyut ulaan volcanic group gives a u pb zircon age of 338 9 0 4 ma indicating that arc plutons were emplaced 10 ma prior to the oyut ulaan volcanism and were eroded soon after our work provides timing constraints for final accretion and continental assembly in se mongolia and also sheds light on the petrological development of a magmatic arc system within an evolving accretionary orogen

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

permian alkaline granites in the erenhot hegenshan belt northern inner mongolia china model of generation time of emplacement and regional tectonic significance abstract the central asian orogenic belt caob is known to be the earth s largest phanerozoic accretionary orogenic belt its southern margin particularly in the southern mongolia and inner mongolia smim shows an extensive distribution of alkaline granites study of these granites could shed light on long debated hypotheses on the late paleozoic tectonic evolution of this region in this work we performed a detailed zircon age determination and whole rock geochemical analysis on alkaline granites from four granitic plutons in northern inner mongolia hongol saiyinwusu baolag and baiyinwula u pb zircon dating yielded early permian ages ca 280 ma for the four plutons whole rock geochemical analyses show chemical characteristics typical of alkaline granites coeval alkaline granites from southern mongolia to northern inner mongolia constitute a gigantic 900 km permian 292 275 ma alkaline granite belt in the southern caob furthermore these alkaline granites have whole rock e nd t varying between 3 6 and 6 4 and zircon e hf t from 4 9 to 20 3 the geochemical and nd hf isotopic signatures suggest derivation by partial melting of a protolith assemblage dominated by metasomatised mantle and followed by fractional crystallization in a post collisional extensional environment

use of a regional relict landscape to measure vertical deformation of the eastern tibetan plateau 1 field work and topography analysis show that remnant local areas of a low relief landscape or erosion surfaces are geographically continuous across the southeastern tibetan plateau margin we correlate these remnant surfaces as a paleolandscape that formed at low elevation remnants of this paleolandscape are preserved because incision of the fluvial system has been largely limited to major rivers and principal tributaries and has not yet progressed throughout the entire fluvial network the incomplete adjustment of the fluvial system signals initiation of rapid bedrock incision into a developing plateau margin and erosional denudation is concentrated in the major river channels this interpretation contradicts earlier notions that low gradient regional topography is the product of regional elevation reduction by intense landscape dissection due to the presence of several large southeast flowing rivers the modern altitude of the reconstructed paleolandscape or relict landscape constrains the vertical displacement of the plateau surface in response to crustal thickening and subsequent erosion during the lateral growth of the tibetan plateau regional preservation of the relict landscape is consistent with minor surface disruption by late cenozoic thrust faults and folds and supports a model of distributed lower crustal thickening because significant erosion is limited to narrow river gorges an increase in the plateau elevation due to isostatic rebound is minor therefore we propose that the modern elevation of the relict landscape reflects isostatically compensated thickening of the lower crust

how does the tibetan plateau affect the transition of indian monsoon rainfall abstract the roles of the tibetan plateau tp upon the transition of precipitation in the south asian summer monsoon are investigated using a simplified regional climate model before the onset of the south asian monsoon descending flow in the midtroposphere which can be considered as a suppressor against precipitation prevails over northern india as revealed by the ncep ncar reanalysis data the descending motion gradually weakens and retreats from this region before july consistent with the northwestward migration of the monsoon rainfall to examine a hypothesis that the dynamical and thermal effects of tp cause the midtropospheric subsidence and its seasonal variation a series of numerical experiments are conducted using a simplified regional climate model the mechanical effect of the tp generates robust descending flow over northern india during winter and spring when the zonal westerly flow is relatively strong but the effect becomes weaker after april as the westerly flow tends to be weaker