testing the sensitivity of geomorphic indices in areas of low rate active folding eastern betic cordillera spain abstract active deformation structures have an incidence in topography that can be quantified by using geomorphic indices most of these indices have been checked in faulted regions with high deformation rates the application of several geomorphic indices hypsometric curve analysis normalized stream length gradient and valley width to valley height ratio to the drainage network of the southern limb of the sierra de las estancias antiform internal zones eastern betic cordillera where low rate active folding has been recognized allows us to investigate the suitability of these indices to identify active structures in such a scenario hypsometric curves clearly identify regions with recent uplift and young topography but they do not provide any constraint on the location of active folds local valley width to valley height index variations have been detected just coinciding whit the position of ene wsw active folds normalized stream length gradient index serves to locate active folds in areas of hard rock substratum but not in those areas with soft sediments neogene quaternary sedimentary basins this is most likely due to the fact that in the basins erosion is much more intense than in the hard rock sectors in view of these results we consider that geomorphic indices constitute a valuable tool for identifying sectors affected by low rate uplift related to active folding with the best results obtained in hard rock areas

age and origin of the ardal dike complex sw norway false isochrons incomplete mixing and the origin of caledonian granites in basement nappes 1 granitic dikes intruding the upper thrust sheets of the jotun nappe complex southwest norway previously assigned a sveconorwegian age based on rb sr data are redated to 427 1 ma by u pb the erroneous rb sr age can be explained by mixing of magmas with different isotopic compositions similar geochemical signatures evidence of sr mixing inherited proterozoic zircons and synkinematic modes of emplacement are also seen in silurian granites in the lifjorden complex as well as in the lindas nappe and the totland unit of the bergen arc complex we propose that these granites are related and derived from anatexis of rb poor sediments of baltica affinity overridden by the nappes during scandian thrusting this interpretation is compatible with the presence of the granites in nappes of both oceanic and of continental affinities and their absence in the autochthonous basement

fault generated mountain fronts in southeast spain geomorphologic assessment of tectonic and seismic activity abstract tectonic activity of se spain is evaluated through the application of a general morphometric analysis over 17 different mountain fronts the selected fronts are distributed along the two most prominent crustal scale structures of the mediterranean sector of spain the eastern betic shear zone ebsz and the valencia trough geomorphic indexes used in this study are mountain front sinuosity index smf and valley floor width ratio vf this work is largely based on the smf vf regression analysis of the studied fronts together with previous available data from sw usa the development of mountain fronts in both different lithologies and tectonic contexts enabled characteristic morphometric signatures for different activity scenarios tectonic activity classes to be obtained this study thus indicates that variations of mountain front geomorphology triggered by different styles of faulting can be detected by morphometric analysis it is demonstrated that discrete uplift rates of 0 15 0 08 m ka are sufficient to keep relatively low smf values down to 1 4 in active fronts class 1 taking into account the reported uplift rates and preliminary palaeoseismic data for some of the class 1 studied fronts recurrence periods for surface rupture events can be theoretically bracketed for se spain recurrence periods for class 1 fronts could theoretically range between 10 000 and 12 500 years for maximum slip conditions and 1750 and 2600 years for minimum slip conditions inactive fronts class 3 develop under uplift rates down to 0 03 m ka and tested recurrence intervals for rupture events range between 25 000 and 50 000 years class 2 fronts of intermediate characteristics and activity display uplift rates between 0 07 and 0 03 m ka

the influence of surface and tectonic processes on landscape evolution of the iberian chain spain quantitative geomorphological analysis and geochronology abstract in tectonically active areas the landscape response to tectonic forcing is described and possibly quantified by regional topographic and hydrographic features as well as by spatial variation in rates of surface processes we investigated the recent landscape evolution of the iberian chain ne spain an intraplate thrust belt formed in cenozoic times and characterized by a dome shaped topography in its central sector the landscape is dominated by low relief surfaces late neogene in age presently standing at an average altitude of 1300 m a recent regional uplift controlled the organization of the present fluvial network and dissection of the landscape in this framework we investigated the geomorphic responses to tectonic forcing by the calculation of morphometric parameters focusing on topography map of local relief swath profiles and hydrography basin hypsometric curve and integral basin asymmetry factor river longitudinal profiles and relative indices and using srtm dem the results of morphometric analysis have been coupled with radiometric uranium series dating of calcareous tufas lying on fluvial strath terraces the obtained ages allow the estimation of incision rate along the high tajo and martin rivers our results indicate that uplift and rock type erodibility are the main factors influencing landscape evolution of the study area the incision rates are very similar throughout the central sector of the range indicating that despite subtle local variation the rivers are responding to a main tectonic input the regional uplift in conclusion the iberian chain landscape is in a transient state in response to a recent dome like uplift indeed the fluvial processes that weakly incised this landscape at a rate of 0 6 mm yr are approaching a radial pattern on the basis of geological and geomorphic constraints we hypothesize that the uplift started around or after 3 ma

importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem 1 fluvial erosion of bedrock occurs during occasional flood events when boundary shear stress exceeds a critical threshold to initiate incision therefore efforts to model the evolution of topography over long timescales should include an erosion threshold and should be driven by a stochastic distribution of erosive events however most bedrock incision models ignore the threshold as a second order detail in addition climate is poorly represented in most landscape evolution models so the quantitative relationship between erosion rate and measurable climatic variables has been elusive here we show that the presence of an erosion threshold when combined with a well constrained probabilistic model of storm and flood occurrence has first order implications for the dynamics of river incision in tectonically active areas first we make a direct calculation of the critical shear stress required to pluck bedrock blocks for a field site in new york second we apply a recently proposed stochastic threshold bedrock incision model to a series of streams in california with known tectonic and climatic forcing previous work in the area has identified a weak relationship between channel gradient or relief and rock uplift rate that is not easily explained by simpler detachment limited models the results with the stochastic threshold model show that even low erosion thresholds which are exceeded in steep channels during high frequency flood events fundamentally affect the predicted relationship between gradient and uplift rate in steady state rivers in a manner consistent with the observed topography this correspondence between theory and data is however nonunique models in which a thin alluvial cover may act to inhibit channel incision in the low uplift rate zone also provide plausible explanations for the observed topography third we explore the broader implications of the stochastic threshold model to the development of fluvial topography in active tectonic settings we suggest that continued field applications of geomorphic models including physically meaningful thresholds and stochastic climate distributions are required to advance our knowledge of interactions among surficial climatic and crustal processes

geomorphic constraints on surface uplift exhumation and plateau growth in the red river region yunnan province china field observations digital elevation model dem data and longitudinal profi le analysis reveal a perched low relief upland landscape in the red river region yunnan province china which correlates to an uplifted regional low relief landscape preserved over the eastern margin of the tibetan plateau as with other major rivers of the plateau margin the red river has deeply incised the lowrelief upland landscape which we interpret to be the remnants of a pre uplift or relict landscape we examine longitudinal river profi les for 97 tributaries of the red river most profi les consist of three segments separated by sharp knickpoints an upper lowgradient channel segment a steeper middle channel segment and a very steep lower channel segment upper channel segments correspond to the relict landscape and have not yet experienced river incision steeper middle and lower segments indicate onset of rapid two phase river incision on the basis of which changes in external forcings such as climate or uplift can be inferred in terms of two end member scenarios two phase incision could be the result of pulsed plateau growth in which relatively slow uplift during the fi rst phase is followed by rapid uplift during the second phase or it could refl ect adjustments of the main channel to changing climate conditions against the backdrop of steady plateau growth reconstruction of the paleo red river indicates 1400 m river incision 1400 1500 m surface uplift and a maximum of 750 m vertical displacement across the northern red river fault elevating the northern ailao shan range above the surrounding relict landscape on the basis of stratigraphic constraints incision along the red river likely began in pliocene time

petrology and geochemistry of the lyngdal granodiorite southern norway and the role of fractional crystallisation in the genesis of proterozoic ferro potassic a type granites abstract in south western norway the sveconorwegian orogenic thickening 1024 970 ma is followed by an important post collisional magmatism 950 930 ma divided in two suites vander auwera et al 2003 the anorthosite mangerite charnockite suite amc suite and the hornblende biotite granitoids suite hbg suite the hbg suite displays a continuous trend from gabbronorites to granites this paper presents the petrography and geochemistry major and trace elements sr nd isotopes of the lyngdal granodiorite and associated massifs tranevag and red granite massifs which belong to the hbg suite although being very close to the anorthosite massifs mafic microgranular enclaves mme resulting from magma mingling can be abundant and probably correspond to the parent magma of the studied plutons the lyngdal granodiorite and associated massifs are subalkaline metaluminous a type granitoids with high feo t feo t mgo ratio and k 2 o content forming a ferro potassic a type continuous trend from quartz monzodiorite to granite 56 72 wt sio 2 in harker diagrams the lyngdal tranevag plutons share the proterozoic rapakivi granites trend that they extend to lower sio 2 content major and trace element modelling as well as sr nd isotopes show that fractional crystallisation without assimilation is the differentiation process for lyngdal tranevag fractionating minerals are clinopyroxene hornblende plagioclase oxides biotite apatite zircon and allanite the red granite 71 75 wt sio 2 does not belong to this trend probably due to a different initial magma composition this study shows that ferro potassic a type granitoids can be derived by fractional crystallisation from mafic magmas

quantifying differential rock uplift rates via stream profile analysis despite intensive research into the coupling between tectonics and surface processes our ability to obtain quantitative information on the rates of tectonic processes from topography remains limited due primarily to a dearth of data with which to test and calibrate process rate laws here we develop a simple theory for the impact of spatially variable rock uplift rate on the concavity of bedrock river profiles application of the analysis to the siwalik hills of central nepal demonstrates that systematic differences in the concavity of channels in this region match the predictions of a stream power incision model and depend on the position and direction of the channel relative to gradients in the vertical component of deformation rate across an active fault bend fold furthermore calibration of model parameters from channel profiles argued to be in steady state with the current climatic and tectonic regime indicates that 1 the ratio of exponents on channel drainage area and slope m n is 0 46 consistent with theoretical predictions 2 the slope exponent is consistent with incision either linearly proportional to shear stress or unit stream power n 0 66 or n 1 respectively and 3 the coefficient of erosion is within the range of previously published estimates mean k 4 3 10 4 m 0 2 yr application of these model parameters to other channels in the siwalik hills yields estimates of spatially variable erosion rates that mimic expected variations in rock uplift rate across a fault bend fold thus the sensitivity of channel gradient to rock uplift rate in this landscape allows us to derive quantitative estimates of spatial variations in erosion rate directly from topographic data

geomorphic analysis in areas of low rate neotectonic deformation south epirus greece as a case study abstract river morphology and the distribution of river deposits reliably reflect the neotectonic activity of a region in relation to rock resistance and climatic changes geomorphic indices have been used to identify landscape evolution and active faults particularly in areas of high rate tectonic deformation in areas of low rate deformation the influence of neotectonic activity on landscape evolution is less apparent although it is important the aim of this study is to examine the region of south epirus greece a region where the rates of active tectonic processes are low because rock resistance is low and the sediment supply is intense in this region the fault surfaces are either not well preserved or they are covered for the purposes of this paper geomorphic analysis including the drainage basin asymmetry factor the basin hypsometric curve and integral the valley floor width to height ratio longitudinal river profiles the stream length gradient index normalized by the graded river gradient and the mountain front sinuosity index was used to evaluate neotectonic activity and to identify covered potentially active faults in places where fault surfaces could be observed geological mapping and tectonic analysis were applied to determine the fault characteristics the first result of this work is that important active or potentially active faults extend for a significant distance into the alluvial plains of south epirus in this case study we considered that geomorphic indices constitute a valuable tool for identifying neotectonic activity in regions of low rate deformation furthermore geomorphic analysis reveals the location of covered fault structures contributes to evaluating fault capability and therefore to estimating seismotectonic hazard

evolution and tectonic setting of the malani nagarparkar igneous suite a neoproterozoic silicic dominated large igneous province in nw india se pakistan abstract the neoproterozoic malani igneous suite mis in nw india along with analogous magmatic rocks from the adjoining nagarparkar region in se pakistan can be collectively classified as a silicic dominated large igneous province slip this magmatic event includes bimodal predominantly felsic volcanism granite emplacement and felsic and mafic dyke intrusions felsic rocks have typical a type affinity as indicated by high abundance of silica alkali high field strength and large ion lithophile element concentrations and low cao and mgo contents their nb negative anomalies and zr saturation parameters indicate significant crustal input and high temperature melting mafic volcanics and dykes show geochemical homogeneity and derivation from a depleted continental mantle source without any significant crustal contamination low u and th contents and no visible nb anomaly the region extending from the mount abu batholith in the east to jaswantpura in the west 2700 km2 representing a transition from the metamorphic sirohi terrane to the undeformed mis was evaluated through an integrated structural including satellite image analysis geochemical and geochronological study during the initial stage prior to 760 ma the granitic basement erinpura granites and overlying sirohi metasediments behaved in a brittle manner that led to development of linear fractures and nne trending rift structures and bimodal volcanic activity emplacement of voluminous granitic bodies in response to progressive extension of the crust is inferred during the more evolved second stage younger than 760 ma mirpur granite a representative of this younger granitic suite jalor type pink granite has yielded 753 9 ma zircon u pb crystallization age granitic plutons mark regions of crustal extension as seen in parallel alignment of plutonic bodies jaswantpura granitic belt and parallel mafic dyke swarms 340 transecting the granites structural analysis further identified an episode of crustal convergence which is documented in folding and faulting of the sindreth basin sequence and in tectonic overprint of early stage mafic rocks rifting and bimodal magmatic activity in mis is coeval with similar rock types in nagarparkar in se pakistan further traceable into the seychelles microplate and central madagascar considering the neoproterozoic paleogeography and our observations an extensional setting and an active continental margin position for mis is inferred

two phase growth of high topography in eastern tibet during the cenozoic high topography in eastern tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the indo asian collision implying that crustal flow alone could not have created the high topography

transient fluvial incision in the headwaters of the yellow river northeastern tibet china 1 we utilize topographic analysis of channel profiles combined with field measurements of erosion rates to explore the distribution of channel incision in the anyemaqen shan a broad mountainous region in the northeastern tibetan plateau tributary channels to the yellow river display systematic downstream increases in channel gradient associated with convex upward longitudinal profiles steep lower reaches of channels are associated with rapid 1 m ka incision rates along the yellow river while upstream reaches are associated with relatively slow 0 05 0 1 m ka erosion of soil mantled uplands covariance between erosion rates and channel steepness indices suggest that channels are adjusted to match long wavelength differential rock uplift across the range geologic constraints indicate that rapid incision downstream of the range is associated with excavation of basin fill driven by changes in relative base level farther downstream the upstream limit of this wave of transient incision is marked by a series of knickpoints that are found at nearly the same elevation throughout the watershed consistent with knickpoint migration as a kinematic rather than diffusional wave tributary channel gradients downstream of knickpoints however display a progressive adjustment to increased incision rates that may reflect the influence of increased sediment flux comparison of observed channel profiles to a stream power model of fluvial behavior reveals that the rate of knickpoint propagation can only be explained if the erosional efficiency coefficient k increases during incision our results thus highlight the utility of channel profile analysis to reconstruct the fluvial response to both active tectonism and external changes in base level

mesozoic and cenozoic tectonics of the northern edge of the tibetan plateau fission track constraints abstract fission track analysis on zircons and apatites yields new information about the timing of deformation of the northern tibetan plateau ages on zircons ranging from 221 22 to 96 4 ma are indicative of a general late triassic early jurassic cooling probably driven by the collision between the qiantang and kunlun blocks mid jurassic slow cooling is recorded also in the apatites in regions not affected by later cenozoic deformation this jurassic denudation was followed by a period of sedimentation during the cretaceous except along the altyn tagh fault atf zone and in some restricted areas of the western and eastern qilian shan this long and relatively quiet period ended at about 40 10 ma along the major altyn tagh and kunlun strike slip fault zones which were activated by the india asia collision this first movement along lithospheric faults resulted in the eastward extrusion of the tibet plateau which was followed in late oligocene miocene times by a major compression event initiating the formation of the high relief of north tibet a final compressional event took place at 9 5 ma and is well correlated with high sedimentation rates in the basins of this region this compression induced continental subduction in the kunlun ranges the altun shan belt and possibly the qilian shan belt

geochemistry of the late mesoproterozoic mount eve granite suite implications for late to post ottawan tectonics in the new jersey hudson highlands

relative tectonics and debris flow hazards in the beijing mountain area from dem derived geomorphic indices and drainage analysis abstract the geomorphic setting of the tectonically active area around beijing is a result of complex interactions involving yanshan neotectonic movements and processes of erosion and deposition the beijing mountain study area contains the junction of two mountain ranges the yanshan mountains and the taihang mountains tectonic activity has significantly influenced the drainage system and the geomorphic situation in the area leading to a high probability of the development of debris flows which is one of the major abrupt geological disasters in the region based on 30 m resolution aster gdem data a total of 752 drainage basins were extracted using arcgis software a total of 705 debris flow valleys were visually interpreted from alos satellite images and published documents seven geomorphic indices were calculated for each basin including the relief amplitude the hypsometric integral the stream length gradient the basin shape indices the fractal dimension the asymmetry factor and the ratio of the valley floor width to the height these geomorphic indices were divided into five classes and the ratio of the number of the debris flow valleys to the number of the drainage basins for each geomorphic index was computed and analyzed for every class average class values of the seven indices were used to derive an index of relative active tectonics irat the ratio of the number of the debris flow valleys to the number of the drainage basins was computed for every class of irat the degree of probable risk level was then defined from the irat classes finally the debris flow hazard was evaluated for each drainage basin based on the combined effect of probable risk level and occurrence frequency of the debris flows the result showed a good correspondence between irat classes and the ratio of the number of the debris flow valleys to the number of the drainage basins approximately 65 of the drainage basins with occurred debris flow valleys are at a high risk level while 43 of the drainage basins without occurred debris flow valleys are at a high risk level a comparison with results from past studies demonstrated that the accuracy of these findings is greater than 85 indicating that the basin topography created by rapid tectonic deformations is more favorable for debris flows

geomorphic indices and longitudinal profile of the daba shan northeastern sichuan basin evidence for the late cenozoic eastward growth of the tibetan plateau abstract the daba shan orocline is located in the northeastern sichuan basin and has undergone foreland propagation and distinct surface uplift owing to the eastward expansion of the tibetan plateau since the late cenozoic in order to describe and quantify the earth surface processes respond to deep geological mechanisms and to detect the kind of tectonic deformation in the daba shan area during the late cenozoic various geomorphic indices were analyzed for study area the obtained results are 1 the northeast of the hanzhong basin the hannan micang massifs along the chengkou fangxian fault and the shennongjia massif exhibit intense tectonic activity 2 the spatial patterns of the channel steepness indices indicate that the western segment of the northern daba shan fenghuang shan and paotaizi region have higher uplift rates than the eastern 3 the longitudinal profile analysis revealed that the landscape evolution of the shennongjia massif is in a transient state with an uplift amplitude of 1700 1900 m since the end of the paleogene 25 ma and an average uplift rate of 0 072 0 004 mm a the paleochannel reconstruction indicate an incision of 900 1000 m since the late neogene 3 0 3 4 ma with the average uplift rate being 0 3 0 03 mm a 4 three tectonic deformation zones can be distinguished in the daba shan from west to east the shortening deformation strike slip deformation and vertical uplift zones these observations suggest that the eastward growth of the tibetan plateau has affected the daba shan area and shaped the modern landscape

knickpoint retreat and transient bedrock channel morphology triggered by base level fall in small bedrock river catchments the case of the isle of jura scotland a sudden drop in river base level can trigger a knickpoint that propagates throughout the fluvial network causing a transient state in the landscape knickpoint retreat has been confirmed in large

petrogenesis of juvenile type birimian paleoproterozoic granitoids in central cote d ivoire west africa geochemistry and geochronology abstract the birimian paleoproterozoic granitoids of central cote d ivoire are associated either with greenstone belts or with basin sedimentary rocks field data indicate two stages of granitoid emplacement which according to singlezircon and monazite 207 pb 206 pb age determinations occurred within a short period of time the first generation of granitoids 2123 2108 ma intruded and metamorphosed the greenstone belts whereas the second generation 2108 2097 ma intruded and metamorphosed the sedimentary pile and subordinate andesitic volcanic rocks of the bandama basin unconformably overlying the greenstone belts geochemical and isotopic data show that the various granitoids belong to four magmatic groups 1 sodic calc alkaline nacg 2 potassic calc alkaline kcg 3 peraluminous alg and 4 alkaline alkg all the granitoids show juvenile features with mantle like low initial sr isotopic compositions 0 700 0 702 and positive epsilon nd values 1 5 2 7 these data indicate that a the nacg may have formed by partial melting of a garnet amphibolite crust b the kcg and alkg derived from partial melting of crustal mafic to intermediate metaigneous rocks and c the alg epsilon nd 1 5 represent infracrustal melting of young sialic igneous materials during a regional crustal shortening in a transcurrent tectonic context sm nd model ages 2 25 2 4 ga preclude the existence of an archean crust reworked in the birimian magmas an underplating model is discussed

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

the composition and petrogenesis of the lower crust a xenolith study granulite facies lower crustal xenoliths from a single basaltic vent hill 32 in the mcbride volcanic province north queensland australia illustrate the extreme lithologic diversity of the deep crust these xenoliths are dominantly mafic but intermediate and felsic granulites represent a significant proportion 20 of the xenolith population all xenoliths have high grade mineralogies and generally well equilibrated textures as well as decompression features which are indicative of derivation from deep crustal levels 0 7 1 0 gpa major and trace element chemistry for 12 xenoliths chosen to span the observed lithologic range are used to constrain the petrogenesis of these rocks in the smaller layered samples metamorphic differentiation may lead to nonrepresentative sampling however in most instances such secondary processes can be identified and the original chemical characteristics delineated the mafic xenoliths formed through a variety of processes including crystallization of mafic magma crystal accumulation from mafic and felsic magmas and partial melting of intermediate compositions to yield a mafic residuum the two analyzed intermediate xenoliths are metasediments based upon high alumina to alkali ratios and rare earth element patterns whereas the two felsic xenoliths have compositions similar to igneous rocks comparisons of noncumulate and or restite xenoliths with unmetamorphosed rock types show that k and rb are variably depleted whereas th and u are strongly depleted in all rock types the large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains suggesting similar processes were operative suggestions of an anomalously ba and sr rich lower crust are not supported by the data by using the observed lithologic proportions of xenoliths at hill 32 and the chemical analyses presented here a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions the weighted mean composition is mafic and formed through a combination of basaltic underplating crystal accumulation tectonic underplating and partial melting no simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in phanerozoic continental margin settings

active forearc shortening in tohoku japan constraints on fault geometry from erosion rates and fluvial longitudinal profiles abstract convexities in the longitudinal profiles of actively incising rivers are typically considered to represent the morphologic signal of a transient response to external perturbations in tectonic or climatic forcing distinguishing such knickzones from those that may be anchored to the channel network by spatial variations in rock uplift however can be challenging here we combine stream profile analysis 10be watershed averaged erosion rates and numerical modeling of stream profile evolution to evaluate whether knickzones in the abukuma massif of northeast japan represent a temporal or spatial change in rock uplift rate in relation to forearc shortening knickzones in channels that drain the eastern flank of the abukuma massif are characterized by breaks in slope area scaling and separate low gradient alluvial upper channel segments from high gradient deeply incised lower channel segments average erosion rates inferred from 10be concentrations in modern sediment below knickzones exceed erosion rates above knickzones by 20 50 although profile convexities could be interpreted as a transient response to an increase in rock uplift rate associated with slip on the range bounding fault geologic constraints on the initiation of fault slip and the magnitude of displacement cannot be reconciled with a recent spatially uniform increase in slip rate rather we find that knickzone position stream profile gradients and basin averaged erosion rates are best explained by a relatively abrupt spatial increase in uplift rate localized above a flat ramp transition in the fault system these analyses highlight the importance of considering spatially non uniform uplift in the interpretation of stream profile evolution and demonstrate that the adjustment of river profiles to fault displacement can provide constraints on fault geometry in actively eroding landscapes

active tectonics in the sierra nevada betic cordillera se spain insights from geomorphic indexes and drainage pattern analysis article i nfo the sierra nevada of the central betic cordillera is a 3000 m high mountain range surrounded by neogene quaternary sedimentary basins having been uplifted since late miocene times the southern and western mountain fronts of the sierra nevada are fault bounded while the northern one is an unconformity between the neogene quaternary sediments of the guadix baza basin and the metamorphic rocks of the nevado filabride complex we have carried out a geomorphic study by examining drainage patterns and characteristics of mountain fronts in order to reveal areal variations and styles of rock uplift mountain front sinuosity smf area altitude relations hypsometric curves and valley floor entrenchment differ significantly between the northern western and southern mountain fronts the lack of important faults along the northern sierra nevada mountain front together with the elevated topographic position of the guadix baza basin average altitude is around 1100 m points to similar uplift of both geomorphic units sierra and basin in a single large scale crustal block the asymmetry factors show systematic asymmetries at both sides of the lanjaron river probably due to the presence of an active nne ssw oriented antiform in the western sierra nevada finally river profiles indicate maximal river entrenchment in the western part of the sierra nevada probably related to the uplift of the footwall of the padul niguelas fault system therefore our geomorphic analysis suggests that the western part of the sierra nevada is tectonically active by means of a combination of normal faults along the mountain front and nne ssw oriented active folds which in turn likely have a gravitational origin related to the exhumation of the footwall of the normal fault system

mangerite magmatism associated with a probable late permian to triassic hongseong odesan collision belt in south korea abstract the odesan area in the eastern gyeonggi massif south korea consists principally of migmatitic and porphyroblastic gneisses intruded by mangerite mafic mangerites with sio 2 contents from 52 40 to 54 20 wt have higher feo mgo 14 98 18 28 wt and cao contents 5 80 7 84 wt but lower total alkali contents 4 74 2 o k 2 o 2 2 o k 2 o

experimental melting of crustal xenoliths from kilbourne hole new mexico and implications for the contamination and genesis of magmas experiments p 6 9 kb t 900 1000 c on four crustal xenoliths from kilbourne hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region granodioritic gneisses samples kh 8 and kh 11 yield little melt 900 ppm ba in high temperature melts coexisting with a k feldspar free restite although ree were not measured in either feldspar or melt the high kspar melt kds for eu suggests that the melts coexisting with k feldspar will have strong negative eu anomalies isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs for example the most radiogenic and incompatible element rich sample examined here the pelitic granulite 87sr 86sr 0 757 is refractory while samples with far less radiogenic sr 87sr 86sr 0 708 0 732 produced substantial melt this suggests that in this area the isotopic signature of contamination may be more subtle than expected the experimental results can be used to model the petrogenesis of oligocene volcanic rocks exposed 150 km to the nw of kilbourne hole in the black range in the mogollon datil volcanic field the experimental results suggest that a crustal melting origin for the kneeling nun and caballo blanco tuffs is unlikely even though such an interpretation is permitted by sr isotopes curstal contamination of a mantle derived magma best explains the chemical and isotopic characteristics of these tuffs both experimental and geochemical data suggest that the rhyolites of moccasin john canyon and diamond creek could represent direct melts of granodiorite basement similar but not identical to the kilbourne hole granodiorites perhaps slightly modified by crystal fractionation the absence of volcanic rocks having87sr 86sr 0 74 in the region is consistent with the refractory character of the pelitic granulite

characterizing the transient geomorphic response to base level fall in the northeastern tibetan plateau analysis of hillslope gradient landscape relief and channel steepness in the daxia river basin provides evidence of a transient geomorphic response to base level fall on the northeastern tibetan plateau low gradient channels and gentle hillslopes of the upper watershed are separated from a steeper high relief landscape by a series of convex knickzones along channel longitudinal profiles downstream projection of the relict portions of the profiles implies 800 850 m of incision consistent with geologic and geomorphic records of post 1 7 ma incision in the lower watershed we combine optically stimulated luminescence dating of fluvial terrace deposits to constrain incision rates downstream of knickpoints with catchment averaged 10be concentrations in modern sediment to estimate erosion rates in tributary basins both above and below knickpoints both sources of data imply landscape lowering rates of 300 m ma 1 below the knickpoint and 50 100 m ma 1 above field measurements of channel width n 48 and calculations of bankfull discharge n 9 allow determination of scaling relations among channel hydraulic geometry discharge and contributing area that we employ to estimate the patterns of basal shear stress unit stream power and bed load transport rate throughout the channel network our results imply a clear downstream increase of incision potential this result would appear to be consistent with a detachment limited response to imposed base level fall in which steepening of channels drives an increase in erosion rates in contrast however we do not observe apparent narrowing of channels across the transition from slowly eroding to rapidly eroding portions of the watershed rather the present day channel morphology as well as its scaling of hydraulic geometry imply that the river is primarily adjusted to transport its sediment load and suggest that channel morphology may not always reflect the presence of knickpoints and differences in landscape relief

partial melting of apatite bearing charnockite granulite and diorite melt compositions restite mineralogy and petrologic implications melting experiments p 6 9 kbar t 850 950 c nno 02 hm were done on mafic to felsic charnockites a dioritic gneiss and a felsic garnet granulite all common rock types in the grenville basement of eastern north america a graphite bearing granulite gneiss did not melt water h2o 0 60 to 2 0 wt is bound in low grade retrograde metamorphic minerals and is consumed during the earliest stages of melting most melts are water undersaturated melt compositions range from metaluminous silicic granodiorite diorite starting composition to peraluminous or weakly metaluminous granites all others in general liquids become more feldspathic less silicic and less peraluminous and are enriched in feo mgo and tio2 with increasing temperature residual feldspar mineralogy controls the cao k2o and na2o contents of the partial melts and the behavior of these elements can be used particularly if the degree of source melting can be ascertained to infer some aspects of the feldspar mineralogy of the source k feldspar a common restite phase in the charnockite and granulite but not the diorite should control the behavior of ba and possibly eu in these systems and yield signatures of these elements that can distinguish source regions and in some cases bulk versus melt assimilation apatite a common restite phase is enriched in rare earth elements ree especially middle ree retention of apatite in the restite will result in steep light ree enriched patterns for melts derived from the diorite and charnockites

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

geochronological and geochemical constraints on the petrogenesis of the early paleoproterozoic potassic granite in the lushan area southern margin of the north china craton abstract we conducted a geochronological and geochemical study on the paleoproterozoic potassic granites in the lushan area southern margin of the north china craton ncc to understand the tectonic regime of the ncc at 2 2 2 1 ga this rock suite formed at 2194 29 ma the rocks are rich in sio2 76 10 77 73 wt and k2o 5 94 6 90 wt with high k2o na2o contents from 7 56 wt to 8 48 wt but poor in cao 0 10 0 28 wt p2o5 0 02 0 05 wt and mgo 0 01 0 30 wt mg 1 08 27 3 indicating they experienced fractional crystallization major element compositions suggest the potassic granites share an affinity with high k calc alkaline granite even though the lushan potassic granitic rocks have high a cnk ratios 1 11 1 25 which can reach peraluminous feature the very low p2o5 contents and negative correlation of p2o5 and sio2 ruling out they are s type granites different from peralkaline a type granites the lushan potassic granites have variable zr concentrations 160 344 ppm 226 ppm on average and 10 000 ga al ratios 1 76 3 00 together with high zircon saturation temperatures tzr 826 885 c indicating they are fractionated aluminous a type granites enriched lree la yb n 9 72 81 8 negative eu anomalies and low sr y with no correlations in sr y and sr zr versus cao suggest the possible presence of ca rich plagioclase and absence of garnet in the residual magmatic zircon grains have variable ehf t values 2 4 to 7 3 with zircon two stage hf model ages tdmc varying from 2848 ma to 2306 ma mostly around ca 2 5 ga and are plotted in the evolution line of crustal felsic rock we propose that the rocks mainly formed by partial melting of ca 2 50 ga tonalitic granodioritic crust as a result of upwelling mantle derived magmas which provided thermal flux and source materials in an intra continent rifting the ca 2 2 ga magmatism suggests that intra continental rifting occurred at 2 35 1 97 ga at least in the southern margin of the ncc after its final cratonization in the late neoarchean

landscape response to tectonic forcing digital elevation model analysis of stream profiles in the mendocino triple junction region northern california the topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock channel incision quantitative field assessments of process based laws are needed to accurately describe landscape uplift and denudation in response to tectonics and climate we evaluate and calibrate the shear stress or similar unit stream power bedrock incision model by studying stream profiles in a tectonically active mountain range previous work on emergent marine terraces in the mendocino triple junction region of northern california provides spatial and temporal control on rock uplift rates digital elevation models and field data are used to quantify differences in landscape morphology associated with along strike northwest to southeast changes in tectonic and climatic conditions analysis of longitudinal profiles supports the hypothesis that the study area channels are in equilibrium with current uplift and climatic conditions consistent with theoretical calculations of system response time based on the shear stress model within uncertainty the profile concavity of the trunk streams is constant throughout the study area 0 43 as predicted by the model channel steepness correlates with uplift rate these data help constrain the two key unknown model parameters the coefficient of erosion k and the exponent associated with channel gradient n this analysis shows that k cannot be treated as a constant throughout the study area despite generally homogeneous substrate properties for a reasonable range of slope exponent values n best fit values of k are positively correlated with uplift rate this correlation has important implications for landscape evolution models and likely reflects dynamic adjustment of k to tectonic changes due to variations in orographic precipitation and perhaps channel width sediment load and frequency of debris flows the apparent variation in k makes a unique value of n impossible to constrain with present data

geochemistry nd and sr isotopes and u pb zircon ages of granitoid and metasedimentary xenoliths from the navajo volcanic field four corners area southwestern united states abstract proterozoic granitoid xenoliths from the navajo volcanic field nvf in the southwestern united states range in composition from tonalite diorite to granite with the latter greatly dominating there is no apparent relationship between chemical or isotopic composition and age geographic location or degree of alteration or deformation of the xenoliths two of the major ages of proterozoic plutonism recognized in the southwest 1 75 1 70 yavapai and 1 45 1 41 ga anorogenic are represented by the nvf xenoliths in some instances within a single diatreme we did not find any xenoliths in the 1 65 1 62 ma mazatzal range the xenoliths are dominantly i types and both nd and sr isotopes and trace element distributions indicate juvenile arc sources approximately 1 8 ga the granite magmas may have been produced by either vapor absent partial melting or fractional crystallization of a tonalite parent juvenile sources must have been available for melting in each of the three periods of magma production and hence some juvenile 1 8 ga crust in this region must have escaped melting in the first two episodes of magmatism and deformation metasedimentary xenoliths are chiefly biotite sillimanite schists garnet biotite schists and garnet paragneisses these three groups are strikingly similar in chemical and isotopic composition suggesting similar graywacke pelite protoliths the schists appear to be limited to the far northern part of the volcanic field consistent with a possible crustal boundary in the basement compositional and thermobarometric results suggest that the biotite schist xenoliths are not restites but that the paragneisses may be mixtures of melt and restite sedimentary protoliths may have been deposited in a forearc basin and later tectonically transported to the deep crust during one or more arc continent collisions in the late paleoproterozoic

petrology of the anorogenic oxidised jamon and musa granites amazonian craton implications for the genesis of proterozoic a type granites the 1 88 ga jamon and musa granites are magnetite bearing anorogenic a type granites of paleoproterozoic age they intrude the archaean rocks of the rio maria granite greenstone terrain in the eastern part of the amazonian craton in northern brazil a suite of biotite amphibole monzogranite to syenogranite with associated dacite porphyry dp and granite porphyry gp dykes dominates in these subalkaline granites that vary from metaluminous to peraluminous and show high feo feo mgo and k2o na2o in spite of their broad geochemical similarities the jamon and musa granites show some significant differences in their ree patterns and in the behaviour of y the jamon granites are related by fractional crystallisation of plagioclase potassium feldspar quartz biotite magnetite amphibole apatite ilmenite geochemical modelling and nd isotopic data indicate that the archaean granodiorites trondhjemites and tonalites of the rio maria region are not the source of the jamon granite and associated dyke magmas archaean quartz diorites differentiated from the mantle at least 1000 m y before the emplacement of the granites have a composition adequate to generate dp and the hornblende biotite monzogranite magmas by different degrees of partial melting a larger extent of amphibole fractionation during the evolution of the musa pluton can explain some of the observed differences between it and the jamon pluton the studied granites crystallised at relatively high fo2 and are anorogenic magnetite series granites in this aspect as well as concerning geochemical characteristics they display many affinities with the proterozoic a type granites of south western united states the jamon and musa granites differ from the anorthosite mangerite charnockite rapakivi granite suites of north eastern canada and from the reduced rapakivi granites of the fennoscandian shield in several aspects probably because of different magmatic sources

along strike variations of morphotectonic features in the western foothills of taiwan tectonic implications based on stream gradient and hypsometric analysis abstract the taiwan mountain belt provides a key example for analyzing the relation between morphology and tectonics in an active compressional environment there is a striking geometrical relationship between the location of salients and reentrants of the mountain belt and the presence of major crustal inhomogeneities such as structural highs in the foreland basin major along strike variations of structural styles occur in range front rock units of the fold thrust belt stream gradient and hypsometric analysis has been carried out to highlight the along strike variations of morphotectonic features and to illustrate the relative activities in different tectonic regimes of the western foothills the normalized stream length gradient index sl index and the hack profile are two of the stream gradient indices that effectively reflect the tectonic uplift in a region de trending and residual analysis of the hypsometric integral hi eliminates the spatial dependency of small drainage basins and the residual hypsometric integral corresponds to differential uplift activity integration of stream gradient indices and hypsometry provides more robust tectonic interpretation five morphotectonic provinces have been identified in the western foothills based on stream gradient indices and hypsometry each province is bounded by a series of n140 e trending transfer fault zones either inherited from the eurasian passive margin and or newly formed in the sedimentary cover in response to the presence of basement highs within the foreland basin the peikang and kuanyin highs the morphotectonic features as well as deformation styles vary within each province a high uplift but low shortening rate typifies the crustal deformation style in the transpressional regime north of the peikang basement high on the contrary a low uplift but high shortening rate characterizes the crustal deformation style in the transtensional regime south of the peikang basement high the higher geomorphic indices suggest that greater tectonic activity occurs in central than in southwestern taiwan this inference is also supported by the coseismic deformation of the 1999 chi chi earthquake

petrochemistry and petrology of i type granitoids in an arc setting the composite torul pluton eastern pontides ne turkey the upper cretaceous torul pluton located in the eastern pontides is of sub alkaline affinity and displays features typical of volcanic arc granitoids it is a composite pluton consisting of granodiorite biotite hornblende monzogranite quartz monzodiorite quartz monzonite and hornblende biotite monzogranite the oldest syenogranite 77 9 0 3 ma and the youngest quartz diorite form small stocks within the pluton samples from the granodiorites biotite hornblende monzogranites quartz monzodiorites quartz monzonites and hornblende biotite monzogranites have sio2 between 57 and 68 wt and display high k calc alkaline metaluminous to peraluminous characteristics chondrite normalized ree patterns are fractionated lacn lucn 6 0 14 2 with pronounced negative eu anomalies eu eu 0 59 0 84 initial nd i values vary between 3 1 and 4 1 initial 87sr 86sr values between 0 7058 and 0 7072 and 18o values between 4 4 and 7 3 the quartz diorites are characterized by relatively high mg number of 36 38 low contents of na2o 2 3 2 5 wt and sio2 52 55 wt and medium k calc alkaline metaluminous composition chondrite normalized ree patterns are relatively flat la yb cn 2 8 3 3 tb yb cn 1 2 and show small negative eu anomalies eu eu 0 74 0 76 compared to the other rock types radiogenic isotope signatures of the quartz diorites show higher 87sr 86sr 0 7075 0 7079 and lower nd i 4 5 to 5 3 the syenogranites have high sio2 70 74 wt and display high k calc alkaline peraluminous characteristics their ree patterns are characterized by higher lacn lucn 12 9 and eu eu 0 76 0 77 values compared to the quartz diorites isotopic signatures of these rocks nd i 4 0 to 3 3 87sr 86sr i 0 7034 0 7060 18 o 4 9 to 8 2 are largely similar to the other rock types but differ from that of the quartz diorites fractionation of plagioclase hornblende pyroxene and fe ti oxides played an important role in the evolution of torul granitoids the crystallization temperatures of the melts ranged from 800 to 900 c as determined from zircon and apatite saturation thermometry all these characteristics combined with low k2o na2o low al2o3 feot mgo tio2 and low na2o k2o feot mgo tio2 ratios suggest an origin through dehydration melting of mafic lower crustal source rocks

testing crustal melting models for the origin of flood rhyolites a nd pb sr isotopic study of the tertiary davis mountains volcanic field west texas flood rhyolites are lavas that have areal extents similar to large ignimbrites the davis mountains volcanic field differs from most other cenozoic volcanic fields in the southwestern united states in that a major volume of the silicic rocks erupted there are flood rhyolites rather than ignimbrites a comparative isotopic study was made of a major flood rhyolite unit the star mountain and mafic to silicic rocks from three other magma systems the entire range in nd isotopic compositions found in the volcanic field is relatively small with n4 values ranging from about 2 in a basalt to between 1 0 and 1 5 in the silicic rocks including the star mountain rhyolite pb isotope ratios of the star mountain lavas 206 pb 204 pb 17 9 are considerably higher than those of xenoliths of proterozoic deep crustal granulites from the region 17 4 two types of crustal sources appear viable for the origin of the flood rhyolites first if the star mountain samples represent near primary melts from a crustal source then that source must have been cenozoic in age and silicic in composition second if the star mountain samples represent highly fractionated magmas then the crustal source could have been mafic but it could not have experienced the proterozoic granulite facies metamorphism known to have affected the basement the overall isotopic trends of the volcanic rocks are qualitatively consistent with the hypothesis that all the silicic rocks including the flood rhyolites evolved from mantle derived basalts with variable but generally small amounts of crustal contamination

tectonomagmatic evolution of the sveconorwegian orogen recorded in the chemical and isotopic compositions of 1070 920 ma granitoids abstract the sveconorwegian province in southern norway and sweden hosts at least four granitoid suites representing apparently continuous magmatism at the sw margin of the fennoscandian shield between 1070 and 920 ma this study presents a compilation of published and new zircon la icp ms u pb geochronology whole rock and zircon geochemistry and sm nd isotope data for the granitoid suites and demonstrates the granitoids ability to record changes in the tectonomagmatic evolution of this orogenic province the sirdal magmatic belt smb ca 1070 1010 ma represents the earliest magmatism west in the province followed by two hornblende biotite granitoid suites hbg ca 1000 920 ma and the fla iddefjord bohus suite fib ca 925 ma in central and eastern parts of the province respectively the smb and the hbg bodies located outside of the smb referred to as hbgout are chemically similar whereas the hbg bodies located in the same region as the smb referred to as hbgin are more ferroan enriched in incompatible elements and have higher zircon saturation temperatures isotopically the smb and both hbg suites fall on an evolutionary trend from widespread 1 5 ga crust in the region suggesting this was the dominant crustal contribution to magmatism the fib suite is more peraluminous rich in inherited zircon and has isotopic compositions suggesting a more evolved source than both the hbg suites and the smb trace element modelling shows that the smb and hbgout suites could have formed by 50 partial melting of 1 5 ga crust whereas 5 10 remelting of the dehydrated and depleted smb residue accounts for the geochemical composition of the hbgin suite the available data suggest a scenario where the 1 5 ga lower crust underwent melting due to long lived mafic underplating giving rise to the smb suite after ca 1000 ma regional scale extension may have led to more widespread mafic underplating causing remelting of the residue following smb melt extraction forming the hbgin suite with lower crustal melting farther east forming the hbgout suite changes in melt composition over this 150 myr time interval may thus be ascribed to an evolving melt source rather than fundamental changes in tectonic regime deep continental subduction at ca 990 ma east in the orogen provided an isotopically evolved crustal source for the fib suite the data underline the difference in tectonic processes across the orogen with long lived high temperatures in the western and central parts and colder high pressure events in the eastern parts of the orogen

knickpoint recession rate and catchment area the case of uplifted rivers in eastern scotland knickpoint behaviour is a key to understanding both the landscape responses to a base level fall and the corresponding sediment fluxes from rejuvenated catchments and must be accommodated in numerical models of large scale landscape evolution knickpoint recession in streams draining to glacio isostatically uplifted shorelines in eastern scotland is used to assess whether knickpoint recession is a function of discharge here represented by its surrogate catchment area knickpoints are identified using ds plots log slope versus log downstream distance a statistically significant power relationship is found between distance of headward recession and catchment area such knickpoint recession data may be used to determine the values of m and n in the stream power law e kamsn the data have too many uncertainties however to judge definitively whether they are consistent with m n 1 bedrock erosion is proportional to stream power and kps should be maintained and propagate headwards or m 0 3 n 0 7 bedrock incision is proportional to shear stress and kps do not propagate but degrade in place by rotation or replacement nonetheless the e scotland m and n values point to the dominance of catchment area discharge in determining knickpoint retreat rates and are therefore more consistent with the stream power law formulation in which bedrock erosion is proportional to stream power copyright 2005 john wiley sons ltd

late cenozoic evolution of the eastern margin of the tibetan plateau inferences from 40ar 39ar and u th he thermochronology high topography in central asia is perhaps the most fundamental expression of the cenozoic indo asian collision yet an understanding of the timing and rates of development of the tibetan plateau remains elusive here we investigate the cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the sichuan basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed temperature time paths inferred from 40 ar 39 ar thermochronology of biotite multiple diffusion domain modeling of alkali feldspar 40 ar release spectra and u th he thermochronology of zircon and apatite imply that rocks at the present day topographic front of the plateau underwent slow cooling 30 50 c m y coincident with exhumation from inferred depths of 8 10 km at denudation rates of 1 2 mm yr samples from the interior of the plateau continued to cool relatively slowly during the same time period 3 c m y suggesting limited exhumation 1 2 km however these samples record a slight increase in cooling rate from 1 to 3 c m y at some time during the middle tertiary the tectonic significance of this change remains uncertain regardless late cenozoic denudation in this region appears to have been markedly heterogeneous with the highest rates of exhumation focused at the topographic front of the plateau margin we infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable sichuan basin and thus marks the onset of deformation related to the development of the tibetan plateau in this region the present margin of the plateau adjacent to and north of the sichuan basin is apparently no older than the late miocene or early pliocene 5 12 ma

stream profile analysis tectonic geomorphology and neotectonic activity of the damxung yangbajain rift in the south tibetan plateau the damxung yangbajain rift is one of the most active north south trending rifts in the south tibetan plateau and it has been playing an important role in accommodating the east west extension of the tibetan plateau both stream profiles on the nyainqentanglha range adjacent to the northwest part of the damxung yangbajain rift and tectonic geomorphology in the north of the rift are analyzed to assess the spatial pattern and intensity of rock uplift which is related to neotectonic activity a total of 85 stream profiles across the nyainqentanglha range are analyzed and 111 knickpoints are interpreted most of these stream profiles are characterized by prominent convexities with two or more knickpoints many of which are formed due to the strong rock uplift evidenced by abnormal concavity and extremely high steepness indices during the quaternary neotectonic activity in this region is well replicated in the stream profile indices and offset landforms tectono geomorphic analysis shows that the concavity and steepness indices correlate with the fault movements at many places the damxung yangbajain rift is characterized by left lateral strike slip in the north of damxung and by normal movement in middle and southern parts the middle and southern parts have been undergoing higher uplift than has the northern area it is most likely that the strong uplift is related to the heat flow under the crust earthquakes occurring in the damxung yangbajain rift including a m8 in 1411 and m6 6 in 2008 are thought to be related to heat flow activity all of the stream profile indices and tectonic geomorphology show that the damxung yangbajain rift is not in a stable state copyright 2016 john wiley sons ltd

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

spatial variation in meso cenozoic exhumation history of the longmen shan thrust belt eastern tibetan plateau and the adjacent western sichuan basin constraints from fission track thermochronology abstract the ne trending longmen shan thrust belt marking the eastern margin of the tibetan plateau adjacent to the sichuan basin is characterized by the paradox of high topographic gradients but low convergence rates and thus critical for understanding of geodynamic processes involved in the eastward growth of the plateau many low temperature thermochronological studies focused on the central and southern portions of the longmen shan and western sichuan basin to investigate the cenozoic exhumation history of the eastern tibetan plateau but little work has been done on their northern portions furthermore the spatial variation in both cenozoic and mesozoic thermal histories of this region has been poorly documented in this paper zircon zft and apatite aft fission track analyses for samples collected from three transects and three boreholes covering the northern central and southern longmen shan and the western sichuan basin provide new data demonstrating several cooling events in this region since the indosinian deformation they occurred during the late triassic 200 ma at the end of early cretaceous 100 ma during the early cenozoic 60 40 ma the early miocene 20 25 ma and the late miocene 9 14 ma respectively on regional scale the longmen shan experienced a slow cooling between the mesozoic and early cenozoic followed by a rapid cooling during the late cenozoic when the rapid regional exhumation began at 10 ma with a sharp increase in exhumation rate from 0 2 mm yr up to 0 9 mm yr locally comparison of zft and aft ages shows that during the mesozoic the northern segment of the longmen shan belt cooled more rapidly than the central and southern segments while younger aft ages indicate more rapid exhumation in central and southern segments of the belt during the late cenozoic similar trend is demonstrated within the western sichuan basin where the onset of exhumation changes from 45 ma in the northern portion to 20 25 ma in the central and southern portions of the basin the thermochronological data reveal that the yingxiu beichuan and guanxian anxian faults have accommodated significant differential exhumation by thrusting activity and associated denudation with a greater amount of differential exhumation from north to south we propose that the major cause for the rise in topography and rapid exhumation in the longmen shan thrust belt during the late cenozoic is the thrusting activities integrated with the regional uplift of the eastern tibetan plateau a process unrelated to the channel flow within the lower crust

introduction to special section mechanisms and consequences of melt segregation from crustal protoliths much of petrological effort has been concentrated on attempts to discern define and interpret regularly recurring patterns of igneous and metamorphic petrogenesis i do not wish to deny the existence of discernable order in petrological phenomena but every geological event is unique if we concentrate too heavily on discernment of order and pattern may we perhaps overlook or underrate the unique quality of each igneous or metamorphic episode possibly uniqueness may have as great significance as conformity to pattern when we attempt to fit the phenomena of petrology into the broad framework of geology francis j turner presidential address to the mineralogical society of america november 11 1969

petrogenesis of two granites from the nilgiri and madurai blocks southwestern india implications for charnockite calc alkaline granite and charnockite alkali a type granite link in high grade terrains abstract igneous charnockites share some of the characteristics of calc alkaline and alkali a type granites and together constitute important rock types in many high grade terrains the neoproterozoic kalpatta and munnar biotite hornblende granites intrude the upper amphibolite to granulite facies terrain of southwestern india geochemical characteristics of the kalpatta granite are similar to high k calc alkaline magnesian granitoids whereas those of the munnar granite are similar to alkali ferroan granitoids within the constraints imposed by the high temperature k rich nature of the magmas comparison with experimental studies on various granitoid source compositions and trace and rare earth element modeling the distinctive features of both granites reflect a source rock of charnockitic nature in this context the northern kerala nkm and cardamom hill cm charnockite massifs occurring near the granites were considered as probable source rock compositions both the nkm and cm charnockites consists of an intermediate low sio2 type and silicic high sio2 type with the intermediate type showing similarities to high ba sr granitoids with low k2o na2o ratios and the silicic type showing similarities to high ba sr granitoids with high k2o na2o ratios the proposed petrogenetic model involves partial melting of the intermediate nkm charnockite forming the kalpatta calc alkaline granite in contrast the munnar alkali a type granite was the product of melting followed by fractional crystallization of the intermediate cm charnockite it is suggested that granitoids that formed by melting of intermediate type charnockites show high k calc alkaline magnesian geochemical characteristics whereas those formed by intermediate type charnockite melting fractional crystallization show alkali ferroan geochemical characteristics

cenozoic exhumation and deformation of northeastern tibet and the qinling is tibetan lower crustal flow diverging around the sichuan basin apatite fission track thermochronology data elucidate the cooling exhumation history of the qinling qin mountains which contain a paleozoic mesozoic orogenic collage north of the sichuan basin and northeast of the tibetan plateau in particular we examine the extent to which the qinling were affected by the rising plateau the northern and eastern qinling show continuous cooling and slow exhumation since the cretaceous in contrast in the southwestern qinling rapid cooling initiated at 9 4 ma a few million years later than in the eastern tibetan plateau a compilation of major cenozoic faults in the eastern tibetan plateau and the qinling and their kinematic and dynamic characterization shows that deformation in the qinling has predominantly been strike slip active sinistral and dextral strike slip faults delineate an area of eastward rock flow and bound the area of rapid late cenozoic cooling outlined by apatite fission track thermochronology these data can be interpreted to indicate that lower crustal flow has been diverted around the longmen shan and beneath the southwestern qinling causing active plateau uplift in this area alternatively northeastern tibet may be growing eastward faster in the western qinling than the entire south china block is extruding to the east

petrogenesis of an alkali syenite granite rhyolite suite in the yanshan fold and thrust belt eastern north china craton geochronological geochemical and nd sr hf isotopic evidence for lithospheric thinning the yanshan fold and thrust belt in eastern china has been intruded by a series of alkalic igneous rocks ranging in composition from granite and rhyolite to syenite and trachyte laser ablation inductively coupled plasma mass spectrometry u pb analyses of zircon from three alkaline suites yield early cretaceous ages of 130 117ma three groups of rocks have been identified based on their mineralogical geochemical and sr nd hf isotope characteristics the alkali granites and rhyolites are ferroan and have low al2o3 mgo cao sr ba and eu concentrations and high sio2 total fe2o3 k2o nb ga ta th and heavy rare earth element abundances and ga al ratios geochemical data and sr ndand zircon hf isotopic compositions sr sr i1 4 0 7050 0 7164 end t 1 4 8 4 to 13 6 and ehf t 1 4 5 7 to 16 8 indicate that they were probably generated by shallow dehydration melting of biotiteor hornblende bearing granitoid crustal source rocks and then mixed with contemporaneous magma from a mantle and or lower crustal source ferroan syenites have distinct geochemical features from those of the alkaline granites and rhyolites suggesting that they were produced by clinopyroxene and plagioclase fractionation of melt derived from an enriched mantle source mixed with lower and upper crustal derived magmas the magnesian syenites and trachytes have sr ndand zircon hf isotopic compositions that are distinct from those of the ferroan syenites they were mainly derived from partial melting of lower crustal materials mixed with enriched mantle derived alkali basaltic magma the emplacement of an alkali syenite granite rhyolite suite coeval with the formation of metamorphic core complexes and pull apart basins in eastern china indicates they formed in an extensional setting possibly as a result of lithospheric thinning

expression of active tectonics in erosional landscapes understanding the manner and degree to which topography in active mountain ranges reflects deformation of the earth s surface remains a first order goal of tectonic geomorphology a substantial body of research in the past decade demonstrates that incising channel systems play a central role in setting relationships among topographic relief differential rock uplift rate and climatically modulated erosional efficiency this review provides an introduction to the analysis and interpretation of channel profiles in erosional mountain ranges we show that existing data support theoretical expectations of positive monotonic relationships between channel steepness index a measure of channel gradient normalized for downstream increases in drainage area and erosion rate at equilibrium and that the transient response to perturbations away from equilibrium engenders specific spatial patterns in channel profiles that can be used to infer aspects of the forcing these aspects of channel behavior lay the foundation for a series of case studies that we use to illustrate how focused quantitative analysis of channel morphology can provide insight into the spatial and temporal dynamics of active deformation although the complexities of river response to climate lithology and uplift patterns mean that multiple interpretations of topographic data alone will always possible we show that application of stream profile analysis can be a powerful reconnaissance tool with which to interrogate the rates and patterns of deformation in active mountain belts

probabilistic landslide hazard assessment at the basin scale abstract we propose a probabilistic model to determine landslide hazard at the basin scale the model predicts where landslides will occur how frequently they will occur and how large they will be we test the model in the staffora river basin in the northern apennines italy for the study area we prepare a multi temporal inventory map through the interpretation of multiple sets of aerial photographs taken between 1955 and 1999 we partition the basin into 2243 geo morpho hydrological units and obtain the probability of spatial occurrence of landslides by discriminant analysis of thematic variables including morphological lithological structural and land use for each mapping unit we obtain the landslide recurrence by dividing the total number of landslide events inventoried in the unit by the time span of the investigated period assuming that landslide recurrence will remain the same in the future and adopting a poisson probability model we determine the exceedance probability of having one or more landslides in each mapping unit for different periods we obtain the probability of landslide size by analysing the frequency area statistics of landslides obtained from the multi temporal inventory map assuming independence we obtain a quantitative estimate of landslide hazard for each mapping unit as the joint probability of landslide size of landslide temporal occurrence and of landslide spatial occurrence

apatite solubility in peraluminous liquids experimental data and an extension of the harrison watson model abstract new experimental data at 750 1000 c 2 5 kbar and for peraluminous melt compositions show a dramatic increase of apatite solubilities and phosphorus melt contents when compared to metaluminous or peralkaline compositions harrison and watson 1984 the formation of alumino phosphate units in peraluminous melts qualitatively explains our data a new apatite solubility model incorporating the data for the peraluminous region is presented application of this model to examples of peraluminous felsic suites is discussed

effect of drainage area on hypsometry from an analysis of small scale drainage basins in the siwalik hills central nepal hypsometry of drainage basins area elevation analysis has generally been used to infer the stage of geomorphic development and to study the influence of varying forcing factors i e tectonics climate lithology on topography however the scale dependence of hypsometry has generally been neglected in order to assess the scale dependence of hypsometry this study focuses on the sensitivity of hypsometry to different digital elevation model dem resolutions and on the influence of drainage area hypsometry inferred from different dems is shown to be robust against variations of their resolution however hypsometry appears to be dependent on drainage area we propose that this scale dependence may reflect the varying importance of river and hillslope processes with basin area copyright 1999 john wiley sons ltd

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

chemical diversity and origin of precambrian charnockitic rocks of the central pedlar massif grenvillian blue ridge terrane virginia abstract middle proterozoic rocks of the pedlar massif are comprised of varied charnockitic and granulitic lithologies representing 1150 1000 ma grenvillian metamorphic and igneous events in the central appalachians of virginia chemically and mineralogically diverse units include metaluminous to peraluminous charnockites enderbites jotunites and granulite gneisses ranging in sio2 from 47 to 79 wt relative to bulk continental crust typical charnockites are enriched by 3 6 in rb k ba lree zr and hf less enriched in ta sr p and hree and depleted in cs th ti and sc although low sio2 members have relatively high ti and p and some high sio2 units are depleted in k rb and ba the pedlar river charnockite suite prcs vesuvius megacrystic charnockite vmc nellysford granulite gneiss ngg and the lady slipper granulite gneiss lsgg exhibit common chemical signatures and trends notably evident in uniform lree enriched patterns with variable eu anomalies which reflect their derivation from a mixed protolith of volcanics and reworked volcaniclastic sediments dated at 1130 1150 ma pedlar river charnockites represent portions of deep seated granulite gneisses that were mobilized by granulite facies dehydration melting and emplaced en masse as plutons into overlying and surrounding gneisses during the grenville episode divergent ree patterns in the prcs are attributed to crystallization of quartz feldspar garnet or hree compatible mafic and accessory phases leading to complementary chemical signatures in mafic and felsic layers depletion of cs throughout the pedlar massif is attributed to its incompatibility with major mineral phases during either protolith evolution or granulite facies dehydration while dispersions in k rb sr ba and eu are due to the relative proportions of the major phases quartz k feldspar plagioclase and biotite mobility of insoluble trace elements zr hf ta and th require partial melting and crystallization of accessory mineral phases whereas sc cr and ree mobility depends on the proportions of pyroxene and garnet several leucocharnockites and other felsic rocks of the prcs show markedly elevated th values ranging from 10 to 83 ppm which complement depleted th in typical prcs units these charnockitic rocks represent low degree partial melts 1 5 from normal prcs or ngg protoliths to produce high silica magmas which separated from the main charnockitic body

interplay between geochemistry and magma dynamics during magma interaction an example from the sithonia plutonic complex ne greece orogenic granitoids often display mineralogical and geochemical features suggesting that open system magmatic processes played a key role in their evolution this is testified by the presence of enclaves of more mafic magmas dispersed into the granitoid mass the occurrence of strong disequilibrium textures in mineralogical phases and or extreme geochemical and isotopic variability in this contribution intrusive rocks constituting the sithonia plutonic complex northern greece are studied on the basis of mineral chemistry whole rock major trace element geochemistry and sr and nd isotopic composition sithonia rocks can be divided into a basic group bearing macroscopic mafic enclaves microscopic disequilibrium textures geochemical and isotopic evidence of magma interaction and an acid group in which most geochemical and isotopic features are consistent with a magma mixing process but macroscopic and microscopic features are lacking a two step mixing plus fractional crystallization mfc process is considered responsible for the evolution of the basic group the firststepexplainsthechemicalvariationinthemaficenclavegroup abasicmagma representedbytheleastevolvedenclaves interacted

age and origin of anorthosites charnockites and granulites in the central virginia blue ridge nd and sr isotopic evidence rb sr isotopic data for anorthosites charnockites ferrodioritic to quartz monzonitic plutons and high grade gneisses of the blue ridge of central virginia show evidence of post emplacement metamorphism but in some cases retain grenville ages the pedlar river charnockite suite yields an isochron age of 1021 36 ma initial 87sr 86sr ratio of 0 7047 6 which agrees with published u pb zircon ages five samples of that unit which contain paleozoic mylonitic fabrics define a regression line of 683 ma interpreted as a mixing line with no age significance samples of the roseland anorthosite complex show excessive scatter on a rb sr evolution diagram probably due to paleozoic 475 m y metamorphism data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 26 ma inital ratio 0 7058 4 which is in the range of the u pb zircon ages of 1000 1100 ma the stage road layered gneiss yields an age of 1147 34 ma initial ratio of 0 7047 5 sm nd data for the pedlar river charnockite suite reflect a pre grenville age of 1489 118 ma nd 6 7 1 2 data for the roseland anorthosite complex and the ferrodioritic to quartz monzonitic plutons yield grenville isochron ages of 1045 44 ma nd 1 0 0 3 and 1027 101 ma nd 1 4 1 0 respectively two roseland anorthosite samples plot far above the isochron demonstrating the effects of post emplacement disturbance of sm nd systematics while mylonitized pedlar river charnockite suite samples show no evidence of sm nd redistribution the disparity of the sm nd age and other isotopic ages for the pedlar river charnockite suite probably reflects a sm nd source age suggesting the presence of an older crust within this portion of the ca 1 ga old basement

geomorphologic proxies for bedrock rivers a case study from the rwenzori mountains east african rift system geomorphic proxies yield useful insights into understanding long term endogenic and exogenic response to erosion and or rock uplift rates by evaluating areal proxies including asymmetry factor af mountain front sinuosity smf hypsometric integral hi geophysical relief and shape factor shp and linear proxies including normalized steepness index ksn length gradient index slk and chi gradient m the erosion and or rock uplift rates can be quantified we carried out morphotectonic analysis in the rwenzori mountains which represents an anomalously uplifted precambrian horst within the western branch of the east african rift system ears this study aims to 1 evaluate the relationship between geomorphic proxies and drainage basin s maturity 2 evaluate the usefulness of geomorphic proxies as recorders of erosion and or rock uplift rates 3 evaluate the sensitivity of each geomorphic proxy to the drainage basin size and geometry stream order glaciers extent and local structures 4 explore internal correlation within the geomorphic proxies and 5 contribute to the understanding of morphotectonic evolution of the rwenzori mountains for this we computed the stream s good of fitness r2 an indicator of the drainage basin s maturity and geomorphic proxies for the drainage basins and their streams in the rwenzori mountains from the advanced spaceborne thermal emission and reflection radiometer aster digital elevation model dem subsequently we correlated the areal geomorphic proxies with each other and with r2 also we correlated the linear geomorphic proxies with each other and with published erosion rates obtained from cosmogenic 10be analysis our results show that the areal geomorphic proxies af smf hi relief and shp considering the drainage basin size and geometry stream order glacier extent and local structures can be applied to locally evaluate the maturity of the drainage basin we also found that the linear geomorphic proxies ksn slk and m have strong positive correlation with the erosion rates they are effective in detecting fault activities and some of them ksn and m are highly correlated with each other additionally our geomorphic proxy results suggest that the north and south sectors of the rwenzori mountains are in relative tectonic quiescence but that the central sector is experiencing higher levels of erosion and rock uplift

mesozoic deformations of the dabashan in the southern qinling orogen central china abstract the dabashan a curved tectonic belt in the southern qinling orogen was formed as a result of the collision of the qinling micro plate and the yangtze plate in the late triassic this study shows that the dabashan experienced multistage structural deformations two main deformational episodes are recognized 1 foreland folding and thrusting due to the late triassic collision and 2 structural superposition during the late jurassic to early cretaceous the northern dabashan is typified by imbricate thrust sheets whereas the southern dabashan is characterized by occurrence of widespread superimposed folds the early fold axis is orientated almost due north or ne sw the folding propagated southward and southwestward into the sichuan basin from the chengkou fangxian fault which separates the southern from the northern dabashan belts it is concluded that the southern and northern dabashan were two distinct domains at least in early mesozoic this interpretation is important for understanding the tectonic evolution of the qinling orogen and the collisional process of the north and south china plates

evolution of vertical knickpoints waterfalls with resistant caprock insights from numerical modeling 1 vertical knickpoints waterfalls mark a prominent process transition zone whose governing mechanics is not represented by conventional stream power incision models we examine the evolution of vertical knickpoints with resistant caprock utilizing numerical simulations that explicitly represent 1 face failure mechanisms 2 flow acceleration and amplified erosion above a knickpoint lip 3 deposition and removal of coarse debris below the knickpoint and 4 base level lowering or tectonic uplift rates our model demonstrates that knickpoint retreat rate where the subcaprock is weak or vertically jointed and base level fall rates are steady is likely to become tied to downstream conditions and equal to the downstream incision rate divided by channel gradient mechanically this coupling occurs where the subcaprock reaches a threshold height for failure in shear or buckling or where the weathering rate of the subcaprock is higher than the downstream incision wave velocity vi ds the height of the subcaprock face can influence its gravitational stability and the knickpoint lateral erosion rate and lead to a feedback between downstream incision and retreat rate retreat rate can be lower than vi ds during transients which could be long 106 years and set by the weathering rate of the subcaprock or influenced by lag debris evacuation key variables other than discharge can be important in setting retreat rates these include base level lowering rate the rock strength of stratigraphic units downstream of the knickpoint and the size and flux of sediment contributed from above the knickpoint or from the canyon walls two types of oversteepened reaches can form in association with a vertical knickpoint 1 an upstream free fall induced oversteepened reach whose length is longer than the flow acceleration zone and 2 a downstream coarse debris induced oversteepened reach although the model was constructed with caprock type knickpoints in mind some of its elements and insights are also relevant to homogenous substrates

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

from sea level to high elevation in 15 million years uplift history of the northern tibetan plateau margin in the altun shan approximately 1300 in of oligocene miocene clastic strata are exposed along the miran river in the southeastern tarim basin where the adjacent altun shan form the topographic escarpment of the northern tibetan plateau the sedimentary section is faulted against proterozoic rocks of the altun shan in the footwall of the south dipping oblique reverse northern altyn tagh fault oligocene lower miocene strata consist of fine grained rocks that record low gradient depositional systems mid miocene and younger rocks consist of coarse conglomerate derived from the altun shan and deposited by high gradient depositional systems the change to coarse high gradient depositional systems with detrital source areas coincident with the modern miran river drainage is interpreted to mark the onset of uplift of the altun shan on the northern altyn tagh fault and its erosional exhumation the age of the change from pre orogenic to synorogenic sedimentation is constrained by a foraminifera assemblage at the base of the conglomeratic section that includes early middle miocene planktonic foraminifera this interpretation is also supported by apatite fission track and u th he ages and thermal models that indicate rapid miocene cooling and hence rapid exhumation of the altun shan in addition to defining the age of the synorogenic section the foraminifera assemblage contains planktonic taxa indicating a connection to open marine waters and benthic assemblages typical of brackish to near sea level paleobathymetry thus micropaleontologic evidence demonstrates that the miran river locality now at 1400 in elevation was at sea level approximately 15 million years ago thus in addition to constraining the age of surface uplift and exhumation of the altun shan the principal mountain range of the tibetan plateau in this region as 15 to 16 ma the foraminifera assemblage indicates that the se tarim basin off the northern edge of the plateau had an average surface uplift rate of nearly 100 m m y for the past 15 million years these results suggest that shortening in the altun shan and uplift of the range significantly post dated the initiation of large scale strike slip on the altyn tagh fault and that regional surface uplift mechanisms operated in the tarim basin beyond the margins of the tibetan plateau

a quantitative evaluation of playfair s law and its use in testing long term stream erosion models playfair s law j playfair illustrations of the huttonian theory of the earth 1802 requires any two tributaries in a river network to lower at the same rate near their junction although this law holds exactly at the junction it is unclear how well it holds in the vicinity of the junction this issue has practical importance because playfair s law has been used to estimate parameters for detachment limited models of erosion if the incision rate of a stream is modelled as amsn where is an erodibility parameter a is the area drained by the stream and s is the local gradient of the channel then the ratio of the parameters m n can be estimated from junctions by assuming that playfair s law holds over the distance used to determine s for each tributary in this paper playfair s law and associated m n estimates are evaluated for simulated basins with constant and temporally varying uplift rates or baselevel lowering rates the results demonstrate that estimates of m n may be biased for basins with upward concave stream profiles because the local slope must be approximated with an average upstream slope in addition when uplift rate varies temporally knickpoints are shown to travel through the basins with constant vertical velocity because incision rates vary within the basin playfair s law only holds exactly at the junctions these effects are more important when slopes are measured over longer distances finally measurement techniques are presented which address these potential biases copyright 2001 john wiley sons ltd

chemical and sr nd isotopic compositions and zircon u pb ages of the birimian granitoids from ne burkina faso west african craton implications on the geodynamic setting and crustal evolution abstract the west african craton was the site of massive juvenile crustal addition in the early proterozoic during the eburnean orogeny 2 1 ga in this study we determined chemical and isotopic compositions as well as zircon u pb ages of granitoid samples from ne burkina faso these data were then used to constrain the chemical characteristics emplacement age and tectonic environment of these rocks in the context of the west african craton evolution birimian paleoproterozoic granitoids of ne burkina faso are associated with volcanic and volcano sedimentary sequences of the greenstone belts analyses indicate that the na rich birimian granitoids of tonalite trondhjemite granodiorite ttg composition were generated by partial melting of a garnet bearing amphibolite crust granite intrusions derived from partial re melting of the deep basement of felsic calc alkaline nature with contribution of metasediments zircon u pb isotopic analyses yielded ages between 2122 15 ma and 2181 7 ma for the birimian na rich granitoids and 2151 10 ma for a biotite granite ij10 sr nd isotopic data show very low initial sr composition i sr t 0 7007 positive epsilon neodymium values nd t 0 7 to 2 5 and sm nd model ages t dm of 2363 2558 ma these data are similar to those of mafic rocks of the pissila greenstone belt i sr t 0 7003 0 7015 nd t 3 2 to 4 1 t dm 2336 2552 ma and further suggest that the na rich granitoids are of juvenile nature the present study together with published data on the birimian volcanics and meta sedimentary rocks of the west african craton preclude contamination of the juvenile birimian rocks of ne burkina faso by reworked archean materials the birimian granitoids of ne burkina faso were mostly emplaced during the early stage 2 19 2 15 ga of a large scale crustal growth event 2 2 2 0 ga where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs

characterization and origin of a compositionally zoned aluminous a type granite from south india the pan african ambalavayal granite intrudes the high grade metamorphic terrain of northern kerala south india and is spatially associated with the moyar and calicut lineaments the pluton was aligned nearly parallel to the northeast southwest and east west faults in the basement consistent with magma ascent along pre existing deep crustal lineaments in an extensional tectonic regime the pluton is characterized by the presence of iron rich hydrous mafic minerals primary magnetite f o2 above the ni nio buffer and high initial emplacement temperatures near 1000 c modal and textural analyses reveal two probable compositional zones within the pluton outer and inner major element variations support this zoning and point to a peralkaline to metaluminous outer zone and a metaluminous to slightly peraluminous inner zone both zones exhibit major and trace element characteristics of the a type granites with the outer zone belonging to the a1 subtype and the inner zone to the a2 subtype of eby the trace element trends observed from outer zone to the inner zone suggests that crystal fractionation may have been the dominant process in the generation of high levels of the incompatible elements in the case of inner zone samples the high initial 87sr 86sr ratio 0 7135 and high y nb ratios y nb 1 2 are in the range expected for rocks derived from crustal protoliths a petrogenetic model involving partial melting of a charnockitic mafic to intermediate lower crust followed by limited fractional crystallization of the magma in a high level magma chamber is proposed the enrichment of hfse and ree except eu in the inner zone is considered the ultimate product of crystal melt and volatile activity during the final stage of crystallization in a highly silicic sio2 74 magma chamber

tectonic climatic and lithologic influences on landscape fractal dimension and hypsometry implications for landscape evolution in the san gabriel mountains california east west variation in tectonic activity and strong north south climatic gradients provide a unique opportunity to study tectonic climatic and lithologic influences on landscape evolution in the san gabriel mountains california the competing tendencies of constructive tectonic and degradational climatic effects act against lithologic resistance to influence fluvial systems and thus the nature of adjacent and nested drainage basins landscape fractal dimension d a measure of surface roughness over a variety of scales and the hypsometric integral i a measure of the distribution of landmass volume above a reference plane are useful measures of altitudinal variation with scale as such they may provide clues as to the relative influences of tectonism climate and rock type topographic analyses of the san gabriel mountains clearly indicate that tectonism strongly influences d at range wavelength scales while rock type variation apparently influences d and i at smaller scales tectonism is also shown to influence i across the mountain piedmont junction at all scales investigated tectonic activity shows strong negative correlation with both i and d because tectonically active portions of the mountain front do not allow time for much landscape dissection by lower order streams three dimensional topographic modeling suggests climatic parameters exert a stronger influence on d than does tectonism this modeling also suggests an inverse correlation between range scale d and i with varying climate and uplift rate a positive correlation is observed in the san gabriel mountains we suggest this difference results from 1 differences in uplift style between the san gabriel mountains and the models and or 2 variation in rock type erodibility present in the san gabriel mountains but which was not modeled we postulate that the interaction of tectonic climatic and lithologic parameters influences the stable i and d to which a landscape evolves key questions remaining include 1 the time required to reach a stable i or d after a climatic or tectonic change 2 what does the fractal nature of topography tell us about the scaling characteristics of major landforming processes 3 how does climate influence range scale i and both range and small scale d and 4 what are the effects on i and d of range scale lithologic variation both in the models and in real landscapes

trace element sanidine glass distribution coefficients for peralkaline silicic rocks and their implications to peralkaline petrogenesis abstract sanidine glass distribution coefficients for 11 trace elements have been determined on six peralkaline and two subalkaline silicic rocks distribution coefficients for na sc fe cs la ce sm tb and lu from this study and the literature show little variation within analytical uncertainty for silicic rocks of peralkaline and subalkaline affinity distribution coefficients for eu and rb show a marked decrease with increasing peralkalinity this variation may be the result of the decrease in the degree of polymerization from subalkaline to peralkaline silicic melts previous studies involving modelling of peralkaline rocks have selected incorrectly eu and rb sanidine glass distribution coefficients determined from subalkaline silicic rocks

dem and gis analysis of geomorphic indices for evaluating recent uplift of the northeastern margin of the tibetan plateau china the northeastern margin of the tibetan plateau is a tectonically active region consisting of a series of faults with bounded intermountain basins and is located in the transition zone between the tibetan plateau and the loess plateau active deformation that may affect the topography in this region can be quantified using geomorphic indices therefore we applied geomorphic indices such as the hypsometric integral and the stream length gradient index to infer neo tectonics in the northeastern margin of the tibetan plateau different time scaled geodetic leveling data and river incision rates were also integrated into the investigation the results show that the hypsometric integrals are not significantly affected by lithology but spatially correspond to the hanging walls of thrust faults the hypsometric integrals are also positively correlated with the leveling data although the stream length gradient index is influenced by lithology its most pronounced anomalies of the stream length gradient are associated with the thrust faults consequently the uplift in the northeast margin of the tibetan plateau appeared to be concentrated along the hanging walls of the thrust faults

assessment of relative active tectonics southwest border of the sierra nevada southern spain abstract we present a new method for evaluating relative active tectonics based on geomorphic indices useful in evaluating morphology and topography indices used include stream length gradient index sl drainage basin asymmetry af hypsometric integral hi ratio of valley floor width to valley height vf index of drainage basin shape bs and index of mountain front sinuosity smf results from the analysis are accumulated and expressed as an index of relative active tectonics iat which we divide into four classes from relatively low to highest tectonic activity the study area along the southwest border of the sierra nevada in southern spain is an ideal location to test the concept of an index to predict relative tectonic activity on a basis of area rather than a single valley or mountain front the study area has variable rates of active tectonics resulting from the collision of africa with europe that has produced linear east west anticlinal forms as well as extension with variable vertical rates of normal faulting to about 0 5 m ky we test the hypothesis that areas of known relatively high rates of active tectonics are associated with indicatives values of iat

the nagercoil charnockite a magnesian calcic to calc alkalic granitoid dehydrated during a granulite facies metamorphic event the paleoproterozoic nagercoil charnockite massif in southern india consists of garnet absent and garnet bearing varieties with garnet occurring only in those areas where the charnockite incorporates supracrustal lithologies there is a late neoproterozoic cambrian granulite facies metamorphic overprint on the charnockite massif this study examines whether orthopyroxene in the charnockites crystallized during the original magmatic stage or formed during the later high grade metamorphic event the textural association of orthopyroxene and relic garnet grains and reaction intergrowths of garnet and or biotite with orthopyroxene similar to those reported in dehydration zones within the gneisses surrounding the charnockite massif show that orthopyroxene in the charnockite massif formed during the late high grade metamorphic event from dehydration reactions and is not part of the original igneous assemblage metasomatic textures such as thin k feldspar rims together with the presence of co2 rich fluid inclusions in these rocks suggest that co2 metasomatism widely prevalent in the gneisses surrounding the massif probably resulted in the low water activity necessary to form orthopyroxene during the metamorphic overprint an oxygen isotope traverse along a metapelite enclave charnockite contact within the massif shows a distinct variation in d 18 o values with the charnockite away from the contact having the lowest d 18 o value 8 9o similar to that of typical metaluminous granitic rocks the charnockite at the contact shows an intermediate d 18 o value 10 3o and the surrounding metapelite has the highest d 18 o value 12o the intermediate d 18 o value indicates partial assimilation of metapelite with a high d 18 o value into the charnockite with a low d 18 o value both the garnet absent and garnet bearing charnockites have similar compositional ranges to tonalite granodiorite granite ttg suites irrespective of whether garnet is present or not the sr contents of the charnockites show a bimodal distribution a high sr group which is magnesian calcic to calc alkalic metaluminous showing ttg affinities and a low sr group which is dominantly ferroan calcic to calc alkalic metaluminous showing similarities to the orthopyroxene bearing dehydration zones petrogenetic characterization suggests that the protoliths of the nagercoil charnockites were high temperature melts of hydrous basalt

distribution of active rock uplift along the eastern margin of the tibetan plateau inferences from bedrock channel longitudinal profiles 1 current models of long term river incision into bedrock suggest that the local rate of differential rock uplift should exert a primary control on the gradient of channel longitudinal profiles however discrimination of this effect from the influence of variations in substrate erodibility sediment flux precipitation and transient changes in profile shape has proved difficult in practice here we investigate the controls on the spatial distribution of bedrock channel gradients adjacent to the sichuan basin in an effort to assess the degree and nature of active deformation along this margin of the tibetan plateau analysis of river longitudinal profiles utilizing a channel steepness index a measure of profile gradient normalized for drainage area reveals a zone of anomalously steep channels adjacent to the topographic front of the plateau margin channel profiles are systematically less steep in their headwater reaches on the plateau and in their lower reaches east of the plateau margin comparison of steepness indices to mapped lithologic variations reveals that lithology has only a limited influence on channel gradient in this field area we observe no systematic relationship between steepness indices and upstream drainage area channels of all size are steeper near the plateau margin we argue that these systematic changes are not readily explained as a consequence of increased sediment flux or of orographic precipitation we are led to conclude that steep channel profiles along the topographic front of the plateau reflect active differential rock uplift between this region and the foreland