late cenozoic thrusting of major faults along the central segment of longmen shan eastern tibet evidence from low temperature thermochronology abstract the cenozoic orogenic process of the longmen shan lms and the kinematics of major faults along the lms are crucial for understanding the growth history and mechanism of the eastern tibetan plateau three major faults from west to east are present in the central segment of the lms the wenchuan maoxian fault wmf the beichuan yingxiu fault byf and the jiangyou guanxian fault jgf previous researchers have placed great impetus on the pengguan massif between the wmf and byf however limited low temperature thermochronology data coverage in other areas prevents us from fully delineating the tectonic history of the lms in this study we collect 22 samples from vertical profiles in the xuelongbao massif and the range frontal area located at the hanging walls of the wmf and jgf respectively and conduct apatite and zircon fission track analyses new fission track data reveal that the xuelongbao massif has been undergoing rapid exhumation with an average rate of 0 7 0 9 mm yr since 11 ma and the range frontal area began rapid exhumation at 7 5 ma with total exhumation of 2 5 4 5 km the exhumation histories indicate that the three major faults wmf byf and jgf in the central lms are all reverse faults and show a basinward in sequence propagation from middle miocene to present day such a pattern further implies that upper crustal shortening is the primary driver for the lms uplift during the late cenozoic nevertheless middle lower crustal deformation is difficult to be constrained by the exhumation histories and its contribution to lms uplift cannot be ruled out

late quaternary river incision rates and rock uplift pattern of the eastern qilian shan mountain china abstract the uplift rate and pattern of the qilian shan mountain which lies in the northeast margin of the tibetan plateau are key to understanding the dynamics of crustal deformation in the northeastern tibetan plateau in this work late quaternary terraces in seven river valley sections in the eastern qilian shan mountain are studied to determine terrace abandonment ages by osl optical simulating luminescence and terrace heights by gps global positioning system surveys river incision rates since 76 ka bp are calculated ranging from 0 37 mm a to 1 49 mm a distribution of river incision rates shows that higher river incision rates 0 85 1 49 mm a are located at the rivers upstream reaches where mean mountain elevation is 4000 m and relatively lower incision rates 0 32 0 62 mm a are located at the mountain front where the mean mountain elevation is

rise and fall of the southern santa cruz mountains california from fission tracks geomorphology and geodesy the santa cruz mountains are closely associated with a left bend along the right lateral san andreas fault the loma prieta area on the northeast side of the san andreas consists of fault bounded blocks that rise along active deeply rooted reverse and oblique slip faults six samples from a transect across this area yield concordant apatite fission track ages averaging 4 6 0 5 ma these ages date the time of cooling below 110 c and suggest that about 3 km of unroofing has occurred over the last 4 6 m y allowing for current elevations of about 1 km this suggests an average uplift rate of the order of 0 8 mm yr over the last 4 6 m y to further define the extent and distribution of this young uplift we used morphometric analyses of the youthful topography of the area steep drainage slopes and high local relief indicate that the area northeast of the san andreas forms a well defined zone of high uplift in contrast the region on the southwest side of the san andreas is characterized by broad upwarping and folding more subdued topography old fission track ages and mean quaternary uplift rates of 0 1 0 4 mm yr geodetic data 1906 san francisco earthquake and subsequent strain transients 20 years of interseismic deformation 1989 loma prieta earthquake and 2 years of post loma prieta earthquake strain show that the southern santa cruz mountains repeatedly rise and subside through a complex sequence of bay area deformation events an additional deformation element that involves reverse slip averaging 2 3 mm yr along the foothills thrust system must occur to explain the longer term uplift pattern in the loma prieta area since the late pliocene the asymmetry in deformation on opposite sides of the san andreas probably reflects the contrasting rock types on opposite sides of the fault the influence of preexisting structures and the interaction with neighboring faults of the san andreas system

effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains drummond c n wilkinson b h lohmann k c and smith g r 1993 effect of regional topography and hydrology on the lacustrine isotopic record of miocene paleoclimate in the rocky mountains palaeogeogr palaeoclimatol palaeoecol 101 67 79 oxygen isotopic compositions of upper miocene 9 2 m y lacustrine limestone from northwestern wyoming range from 21 5 to 35 2 0 pdb and are the lightest yet reported for a non marine carbonate sequence these values require that lake water was greatly depleted in 180 relative to modern meteoric precipitation and indicate that most inflow was sourced as meltwater from glaciers in the adjacent gros ventre range assessment of climatic factors influencing the isotopic composition of global meteoric water indicates either that nearly all of miocene precipitation in the southern rocky mountains was derived from large lake systems in the snake river plain and northern great basin to the west or that gros ventre catchment elevations were up to 2300 m higher than at present because erosion rates in modern alpine regions suggest that subaerial denudation could only account for up to 1200 m of post miocene elevation reduction any additional lowering must reflect the influence of post laramide epeirogeny during basin and range extensional tectonism lacustrine isotopic data therefore provide boundary conditions on the timing and magnitude of changes in late cenozoic paleoclimate topography hydrology and tectonism in the western wyoming portion of the rocky mountains

controls on erosional retreat of the uplifted rift flanks at the gulf of suez and northern red sea the gulf of suez and red sea rifts are currently bordered by large asymmetric uplifts that are undergoing erosion we find that the amount and timing of erosion vary systematically along the strike of the margin and have been controlled by variations in the prerift stratigraphy the prerift strata are composed of cliff forming eocene cretaceous carbonates overlying the easily eroded cretacous cambrian nubian sandstone this lithologic succession promotes scarp retreat of the sedimentary section followed by dissection of the underlying basement the prerift section thins from over 2000 rn at the northern end of the rift to less than 400 rn at its junction with the red sea thus at the northern part of the gulf of suez the nubian sandstone is minimallyexposed and the carbonates form a scarp at the rift border fault farther south undercutting of the carbonates by erosion of the sandstone has resulted in scarp retreat the escarpment cuts diagonally away from the border fault and is over 100 km inland from the border fault at the southernmost gulf of suez the amount of retreat varies inversely with the sediment thickness exposure and erosion of basement are initiated by the retreat of the escarp ment and the depth of erosion as indicated by fission track ages increases with distance from the escarpment these observations are explained by a model in which erosion along the gulf of suez is initiated as rift flank uplift becomes sufficiently large to expose the friable sandstones undercutting of the escarpment and exhumation of basement has been propagating northward and westward for at least 20 m y the average rate of scarp retreat has been 6 km m y and the along strike propagation of the erosion has been 12 km m y the diachronous erosion of the the rift flanks at the gulf of suez highlights the importance of distinguishing between the timing of uplift and of erosion both thermochronometric and stratigraphic data primarily indicate the timing of erosion which may differ significantly from the timing of the uplift that initiates it they must be interpreted carefully to avoid erroneous conclusions about rift tectonics

plutonic rock types series discrimination of various granitoid series and related rocks abstract the streckeisen qap diagram discriminates four characteristic type series among the large variety of granitoids associated in intrusions three of them integrate varied compositions from basic to acid terms 1 the calcalkaline series with its low k or trondhiemite medium k or granodioritic high k or monzonitic variants 2 the alkaline series with its characteristic syenitic members 3 the tholeiitic series deprived of k feldspars the fourth 4 corresponds to mobilizates associated with migmatites of varied composition but always quartz rich the generalization in the qapf system allowing the plotting of undersaturated alkaline series provides an excellent visual display of the major associations and trends for the plutonic rocks

surface uplift history of the incipient banda arc continent collision geology and synorogenic foraminifera of rote and savu islands indonesia abstract field mapping and analysis of foraminifera from synorogenic pelagic units of rote and savu islands indonesia reveals high rates of surface uplift of the incipient banda arc continent collision during the past 1 8 myr new geological maps of these islands document accretion to the banda forearc of triassic through tertiary sedimentary cover units from the down going australian continental margin foraminifera rich synorogenic deposits of the batu putih formation unconformably overlie these accreted units we use paleodepth versus time estimates from benthic and planktic foraminifera s to measure long term surface uplift rates for the accretionary wedge although strong currents in the region cause some problems with reworking several distinctive species have been found synorogenic deposits in savu and rote yield foraminifer s of biozone neogene n 18 to n22 5 6 1 0 myr that were deposited at estimated depths of around 3000 m these deposits are unconfomrably overlain by uplifted coral terraces the highest coral terraces in savu are 300 m above sea level and perhaps as old as 0 8 myr in rote the highest coral terrace is 200 above sea level and 0 2 myr old these data indicate that collision of the australian continental margin with the banda arc which initiated much earlier in timor has propagated westward towards rote where it is in the initial stages of accretionary wedge emergence collision of the scott plateau propagated se from sumba 2 3 ma to savu 1 0 0 5 ma and then to rote 0 2 ma average rates of surface uplift of the batu putih formation pelagic deposits during the past 2 myr in rote and savu are 1 5 and 2 3 mm a respectively the rise of these islands is clogging the indo pacific seaway

neogene exhumation and relief evolution in the eastern betics se spain insights from the sierra de gador the betics are a key area to study an orogenic landscape disrupted by late orogenic extension new low temperature thermochronology ltt data ahe and aft coupled with geomorphic constraints in the sierra de gador alpujarride complex are used to reconstruct the cooling history and evolution of relief during the neogene we document three stages i a fast cooling event between 23 and 16 ma associated with the well known extensive tectonic exhumation of the alpujarride unit ii a period of slow cooling between 16 and 7 2 ma related to a planation event and iii a post 7 2 ma surface uplift associated with the inversion of the alboran domain undetected by ltt the planation event followed by this late uplift can explain the occurrence of inherited low relief surfaces overlain by tortonian messinian platform deposits at the top of the range finally we propose that the sierra de gador is a more transient landscape than the nearby sierra nevada this article is protected by copyright all rights reserved

late quaternary stream incision and uplift in the forearc of the cascadia subduction zone western oregon documentation of a latest pleistocene earliest holocene episode of strath formation and fluvial aggradation in the oregon coast range provides a datum from which long term bedrock stream incision rates are determined variations in long term incision rates probably reflect cumulative differential uplift in the forearc of the cascadia subduction zone although factors such as bedrock and climatic controls and isostatic adjustments to erosion obscure the precise relationship between surface uplift and stream incision patterns of differential incision are most striking near the latitude of newport where a steep gradient divides a region of higher rates 0 6 0 9 mm yr in the northern coast range from a region of lower rates 0 1 0 3 mm yr in the central coast range the steep incision gradient is nearly coincident with abrupt changes in marine terrace 80 125 kyr uplift rates the locations of quaternary faults and the southern flank of a saddle of low historic 40 70 years uplift the exact causes of these variable patterns of incision uplift are unknown analogies with uplift patterns in other subduction zones and comparisons with other neotectonic data in the region indicate that patterns of differential incision probably are caused by variations in permanent strain accumulation along the cascadia subduction zone such variations may be related to differences in seismic moment release during individual earthquakes to changes in plate geometry or rates of wedge accretion to segmentation of earthquake ruptures and or to deformation on active structures in the north american plate and accretionary wedge

quantification of tertiary erosion in the inner moray firth using sonic velocity data from the chalk and the kimmeridge clay abstract sonic velocities for the chalk and kimmeridge clay were analysed to determine the importance and magnitude of tertiary erosion for 26 wells in the inner moray firth apparent erosion height above maximum burial depth was derived by the displacement along the depth axis of a given velocity depth trend from the normal undisturbed trend the similarity in apparent erosion results derived from the cretaceous chalk and late jurassic kimmeridge clay successions of the inner moray firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and or diagenetic process resultant apparent erosion estimates are variable across the inner moray firth such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern inner moray firth however the actual magnitude of erosion equal to the sum of apparent erosion and post erosional tertiary burial was approximately 1 km throughout the whole inner moray firth area indicating that early tertiary erosion was regional and not simply restricted to the inverted basin margins however a well defined mechanism sufficient to have generated the regional event is still sought

response of glacial landscapes to spatial variations in rock uplift rate 1 the response of glaciated landscapes to rapid rock uplift driven by tectonic convergence is an important often neglected aspect of proposed interactions between plate tectonic processes and climate change rivers typically respond to more rapid rock uplift in part through increasing channel gradients in contrast the glacial buzzsaw hypothesis suggests that glaciers can erode as quickly as the fastest rock uplift rates 6 10 mm yr without any increase in mean elevations however it has not been established how this is achieved we examined moving window maps swath and longitudinal profiles hillslope relief and hypsometry for glacierized and formerly glacierized basins in areas of spatially variable rock uplift rate in the southern alps new zealand and around nanga parbat pakistan to determine whether glaciers have a specific response to rapid rock uplift the response of these glaciated landscapes to rapid rock uplift 6 10 mm yr comprises 1 modest steepening of the longitudinal profiles in smaller glaciated basins 2 maintenance of shallow downvalley slopes in larger glaciated basins 30 km2 southern alps 100 km2 nanga parbat 3 development of tall headwalls and 4 steepening of the basin as a whole dominated by hillslope lengthening around nanga parbat headwalls several kilometers high constitute 50 of the basin relief at rapid rock uplift rates although glaciers can incise the valley floor swiftly they cannot prevent headwalls from reaching exceptional heights the associated increase in mean distance between cirque heads i e a decrease in drainage density causes regional mean elevation to rise with increasing rock uplift rate however this is much less than the changes in elevation expected in unglaciated ranges

a thermochronological perspective on the morphotectonic evolution of the southeastern tibetan plateau a large part of the southeastern tibetan plateau tp is characterized by low relief surfaces at high elevations 3 5 km the origin of these landscapes and their geodynamic implications with regard to evolution of the southeastern tp has been the subject of considerable debate focusing on this topic this study utilizes fission track and u th he thermochronology to reconstruct the thermal history of the late triassic daocheng granite on which a low relief high elevation landscape has developed results suggest that the plateau surface experienced widespread late jurassic to early cretaceous cooling at a rate of 1 2 c ma followed by minimal regional cooling 0 3 c ma and early miocene enhanced cooling 2 3 c ma along deep valleys at the southern edge of the landscape given the subdued late jurassic to early cretaceous crustal deformation in the study area we relate the coeval cooling to crustal refrigeration and exhumation triggered by northward subduction of the meso tethys ocean along the bangong suture subsequent late cretaceous to cenozoic minimal cooling supports arguments for the formation of a low elevation landform prior to uplift of the southeastern tp the enhanced cooling commenced at the early miocene 15 22 ma along deep valleys and heralded the onset of river incision and surface uplift of the southeastern tp this finding does not support previous proposals claiming that high elevations of the southeastern tp were formed in late miocene time resulting from lower crustal flow but rather supports a model highlighting continental subduction and extrusion along the large tanggula yushu batang red river fault system

channel profiles around himalayan river anticlines constraints on their formation from digital elevation model analysis 1 we present a comparison between measured and numerically modeled channel profiles of rivers in two important drainage basins of central nepal the kali gandaki and the arun drainage basins modeled channel profiles are based on a simple stream power approach using best fit exponents defining the nonlinearities in the relative contributions of local channel gradient and water flux to erosion rate our analysis of the stream power in the whole river network confirms the work of other authors that a 50 to 80 km wide zone roughly corresponding to the high himalayan topography is subjected to rapid rock uplift we suggest a model where the uplift of this zone is driven by erosion and isostatic response so that centers of maximum uplift are located within the main channels of the north south draining rivers we also suggest that the rate of uplift slows down with increasing distance to the main channels such a spatial distribution of the uplift leads ultimately to the formation of river anticlines as observed along all major himalayan rivers we propose that the formation of river anticlines along south draining himalayan rivers was accelerated by a sudden increase of the drainage area and discharge when the rivers captured orogen parallel drainages on the north side of the range this may follow successive headward cutting into the tibetan plateau the model is confirmed by differences between main channels and east west running tributaries time dependent numerical models predict that capture events cause strongly elevated erosion rates in the main channel

tectonic denudation and topographic development in the spanish sierra nevada 1 the denudation history of the rapidly uplifting western part of the spanish sierra nevada was assessed using apatite fission track aft ages and 10be analyses of bedrock and fluvial sediments major contrasts in the denudation history are recorded within the 27 km2 rio torrente catchment upland areas are characterized by low relief low slope angles and locally the preservation of shallow marine sediments which have experienced 2 km of denudation since circa 4 ma the minimum denudation rates of 0 4 mm yr 1 derived from aft also contrast with the slow medium term 104 years erosion rates 0 044 0 015 mm yr 1 estimated from 10be measurements at high elevations the local medium long term contrasts in denudation rates within the high sierra nevada indicate that much of the unroofing occurs by tectonic denudation on flat lying detachments in lower elevation parts of the catchment rapid river incision coupled to rock uplift has produced 1 6 km of relief implying that the rivers and adjacent hillslopes close to the edge of the orogen are sensitive to normal fault driven changes in base level however these changes are not transmitted into the low relief slowly eroding upland areas thus the core of the mountain range continues to increase in elevation until the limits of crustal strength are reached and denudation is initiated along planes of structural weakness we propose that this form of tectonic denudation provides an effective limit to relief in young orogens

a method for evaluating basement exhumation histories from closure age distributions of detrital minerals we have developed a two dimensional thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement using this model we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata at relatively high mean denudation rates of 0 5 km m y 1 sustained over millions of years magmatic heating events have minimal influence upon the age distributions of detrital minerals such as k feldspar that are moderately retentive of radiogenic ar at lower rates however the effects of batholith emplacement may be substantial we have applied the approach to detrital k feldspars from forearc strata derived from the deeply denuded peninsular ranges batholith prb agreement of the denudation history deduced from the detrital k feldspar data with thermochronologic constraints from exposed prb basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known c 1999 american geophysical union

polygenetic topography of the cascade range washington state usa we evaluate models for the cenozoic topographic evolution of the cascade range of washington state by analyzing the topography geology and exhumation patterns across the range the models for topographic evolution include 1 post miocene surface uplift of the range from initially low relief topography 2 post miocene surface uplift of the range superimposed on pre existing high relief topography and 3 post miocene surface uplift of high relief cascades in northern washington and low relief cascades in southern washington the third model a polygenetic topographic history is most consistent with our analysis of topographic data and the previously contradictory geologic paleontologic and isotopic lines of evidence regarding the cascades physiographic history between snoqualmie pass and the columbia river the east derived 15 my old columbia river basalt crb reaches nearly as far west as the modern drainage divide and is warped upward to the crest of the range these observations require subdued relief on the east flank of the southern cascades during the late miocene in the northern cascades the crb does not extend into the range and crb deformation only reflects 30 to 60 percent of the total relief thus the northern cascades were likely already a topographic barrier to the west flowing flood basalts moreover a polygenetic topographic history appears most consistent with the geologic data and the paleontologic and isotopic evidence for rainshadow formation during the middle miocene despite differences in topographic history and rock type the maximum and mean altitudes of the cascade range decrease smoothly from northern to southern washington we suggest that post uplift erosion particularly glacial erosion has reduced the topographic distinctions between the northern and southern cascades resulting in a single mountain range with a polygenetic topographic history

morphology and structure of a landslide complex in an active margin setting the waitawhiti complex north island new zealand article i nfo multi scale gravitational instabilities are widespread in the coastal ranges of the north island of new zealand we document here a detailed analysis of the waitawhiti landslide complex located in the core of the tawhero syncline and investigate the potential landslides triggering factors in the area four contiguous large slides form the waitawhiti complex these slides involve fine grained miocene sandstones and massive fractured siltstones sliding occurs mostly along nearly horizontal strata all slides are bounded laterally and or distally by deep incised valleys three gas seeps evidencing thermogenic gas release have been discovered in the vicinity of the slides we propose that river incision continuously removing distal buttresses is the main destabilizing factor in the area however additional factors such as tectonic activity and intense rainfall cannot be excluded we also propose that fluid overpressure reducing the effective shear strength at the base of low permeability layers may have influenced the triggering of landslides in the waitawhiti area

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

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

insights on the origin of the graciosa a type granites and syenites southern brazil from zircon u pb geochronology chemistry and hf and o isotope compositions abstract the neoproterozoic post collisional graciosa province southern brazil comprises several a type intrusions and related volcanics of alkaline and subalkaline or aluminous petrographic associations u pb age determinations trace element and hf and o isotopic compositions were obtained for zircons from six distinct a type granitic and syenitic plutons from both alkaline and subalkaline associations from the province geochronological results indicate that their emplacement petrogenetic evolution and crystallization spanned over a 9 million years interval with peak magmatism at 580 ma plutons were emplaced in a post collisional extensional regime related to the geodynamic evolution of the south southeastern part of the gondwana supercontinent hf isotopic compositions are highly variable with ehf t values between 1 4 to 31 whereas 18o signatures are mainly between 4 7 and 6 0 these features along with trace element signatures especially for hf u nb and rare earth elements suggest that generation of the graciosa rocks occurred via partial melting of a lithospheric mantle previously metasomatized by slab derived fluids followed by variable degrees of crustal contamination at depth

convective removal of thermal boundary layer of thickened continental lithosphere a brief summary of causes and consequences with special reference to the cenozoic tectonics of the tibetan plateau and surrounding regions england p 1993 convective removal of thermal boundary layer of thickened continental lithosphere a brief summary of causes and consequences with special reference to the cenozoic tectonics of the tibetan plateau and surrounding regions in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 67 73 the tectonics of tibet and its surroundings underwent marked changes at about 8 ma normal faulting of the plateau appears to have initiated at about this time widespread highly potassic volcanism interpreted as the result of rapid heating of the mechanical boundary layer began about 12 8 ma such rapid heating could be the consequence of convective removal of the lower lithosphere and its replacement by asthenosphere this process would have raised the surface height of the plateau and could have provided the driving force for extension the diffuse deformation in the indian ocean probably began at about 8 ma analysis of the buckling of a plastic lithosphere implies that the horizontal force per unit length required to drive this deformation is consistent with uplift of the tibetan plateau by 1 3 km the indian monsoon intensified at about 8 ma this intensification is consistent with the rapid passage of the surface height of the plateau across a threshold required to disrupt the pre existing atmospheric circulation of the region taken together these observations imply that the surface height of the tibetan plateau rose by between 1 and 3 km in a few million years around 8 ma such a rapid increase in surface height affecting an area the size of the tibetan plateau could not be achieved by crustal thickness changes but could have been caused by the convective removal of a substantial thickness of lower continental lithosphere of the region

geometry and kinematic evolution of the hotan tiklik segment of the western kunlun thrust belt constrained by structural analyses and apatite fission track thermochronology abstractthe western kunlun thrust belt defines the boundary between the stable tarim basin in the north and the intensely deformed cenozoic tibetan plateau in the south because of its important tectonic position understanding its tectonic evolution should have important implications for propagation of deformation from tibet to its neighboring cratonal regions during india eurasia convergence we here present new structural analyses based on field investigations and seismic reflection profiles across the hotan tiklik segment of the western kunlun thrust belt the results indicate that the structural section crosses two major thrust zones the tiklik zone in the hinterland to the south and the hotan zone in the foreland to the north within these the hotan thrust zone is thin skinned with its deformation characterized by fault bend folding and fault slipping along detachment layers whereas the tiklik thrust zone involves basement with its deformation driven by the currently steeply dipping tiklik faul

extensional arc setting and ages of middle jurassic eolianites cowhole mountains eastern mojave desert block california mesozoic strata in the cowhole mountains eastern mojave block california include 200 800 m of eolian quartz arenite aztec sandstone overlain by more than 575 m of silicic ignimbrites lava flows and minor sedimentary rocks cowhole volcanics u pb zircon geochronologic data on a crystal rich dacite lava flow in the aztec sandstone indicate that the sandstone is middle jurassic and is therefore age equivalent to backarc eolianites of the temple cap and carmel formations not the lower jurassic navajo formation as previously assumed our u pb zircon data on two ignimbrites of the cowhole volcanics indicate that they are the same age within error as the lava flow in the aztec sandstone at 170 3 ma new structural and stratigraphic data together with published data indicate syndepositional normal faulting and deposition of landslide blocks throughout deposition of this section these results are consistent with our previous volcanologic studies in middle jurassic rocks of southern arizona southeastern california and the central mojave block which document ignimbrite eruptions from calderas contemporaneous with deposition of craton derived eolian sands in extensional or transtensional intra arc basins busby c j schermer e r and mattinson j m 2002 extensional arc setting and ages of middle jurassic eolianites cowhole mountains eastern mojave desert block california in glazner a f walker j d and bartley j m eds geologic evolution of the mojave desert and southwestern basin and range boulder colorado geological society of america memoir 195 p 79 9

paleovalley morphology and fluvial system evolution of eocene oligocene sediments auriferous gravels northern sierra nevada california implications for climate tectonics and topography in the northern sierra nevada california eocene oligocene fluvial sediments the auriferous gravels rest unconformably in well defined basement incised paleovalleys providing a record of paleolandscape geomorphology and drainage system evolution paleovalley surface mapping shows that local topography and bedrock lithology controlled the location of broad terraced paleovalley segments which formed in the central and eastern metamorphic bedrock belts and subsequently became areas of significant fluvial deposition high gradient high energy deposits are present near the edges of these areas suggesting that tributaries and outlets resided in steeper narrow paleovalleys multiple strath terraces are common in broad paleovalleys indicating that incision events occurred both before deposition and during paleovalley widening followed by periods of aggradation broad paleovalley deposits contain predominantly braided stream facies deposited in an energetically variable system multiple upward fining cycles bounded by erosional surfaces are composed of alternating coarse grained channel belt and fine grained floodplain packages an overall upward fining succession suggests that the locus of fluvial deposition shifted and depositional energy waned over time as the system backfilled successive broad paleovalleys paleochannel slope estimates of 0 004 0 055 were obtained based on average grain sizes of coarse channel fills anomalously thick up to 11 m laterally continuous 1 3 km floodplain units containing kaolinite rich clay and oxisol intervals record high fine grained sediment supply driven by high rates of chemical weathering in the warm eocene climate based on sedimentary characteristics combined with floral and detrital zircon data braided stream deposition within the basin occurred from the late early eocene through late eocene and aggradation primarily occurred in broad lower gradient reaches behind a major bedrock knickpoint created in the late mesozoic early cenozoic possibly due to asymmetric batholith unroofing the diachronous localized nature of paleovalley incision and braided stream aggradation within the northern sierra nevada invalidates previous estimates of the timing and amount of range uplift based on paleochannel reconstructions and gradients both internal and external drivers influenced the evolution of the fluvial system through 1 the pattern of basin filling as the locus of deposition migrated east to west 2 the warm eocene climate which increased chemical weathering and contributed a high sediment supply and 3 changes in base level due to eustasy and or laramide tectonism this record of laramide tectonics in the sierra nevada and basin and range is important for the timing of sierra nevada uplift for tectonic reconstructions of western north america and for studies of landscape evolution in tectonically active orogens

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

stable isotope constraints on the elevation history of the sierra nevada mountains california research on the uplift history of the sierra nevada mountain range has yielded seemingly conflicting results some studies argue for substantial uplift within the past 3 5 m y others suggest that high elevations may have existed since the cretaceous the rain shadow across the sierra nevada is associated with a strong isotopic gradient with lower 18o values in precipitation on the leeward side of the range reconstruction of the 18o value of meteoric water as a monitor of paleoelevation has focused mainly on the leeward side of the sierras but interpretation of the results of these studies may be complicated by shifts in global climate and regional moisture sources we address these concerns by analyzing the 18o value of tooth enamel bioapatite from contemporaneous mammalian fossils on either side of the present sierra range by sampling across the range 18o differences induced by a rain shadow can be isolated from other complicating factors our results indicate that the sierra rain shadow has existed since at least 16 ma which is an important constraint on models for the tectonic evolution of the western united states unfortunately temporal resolution for localities is too coarse to differentiate between glacial and interglacial localities during the past 2 m y so we cannot evaluate if there was a latest cenozoic pulse of uplift or elevation loss

uplift shortening and steady state topography in active mountain belts we present a tectonic surface process model used to investigate the role of horizontal shortening in convergent orogens and the effects on steady state topography the tectonic model consists of a specified velocity field for the earth9s surface and includes a constant uplift rate and a constant horizontal strain rate which varies to reflect the relative importance of frontal accretion and underplating in an orogenic wedge the surface process model includes incision of a network of rivers formed by collection of applied precipitation and diffusive hillslope mass transfer three non dimensional parameters describe this model a ratio of the maximum horizontal velocity to the vertical velocity a peclet number expressing the efficiency of the hillslope diffusion relative to the uplift rate and a fluvial erosion number reflecting the fluvial incision efficiency relative to the uplift rate a series of models are presented demonstrating the resultant steady state landforms parameterized by these three numbers a finite velocity ratio results in an asymmetric form to the model mountain range although the magnitude of the asymmetry also depends on the peclet number topographic steady state is achieved faster for models with no horizontal component to the velocity field with finite horizontal velocity topographic steady state is achieved only at the scale of the entire mountain range even the first order drainage basins are unstable with time in the presence of horizontal shortening we compare our model results to topographic profiles from active mountain ranges in taiwan new zealand and the olympic mountains of washington state all these examples exhibit asymmetric topographic form with the asymmetry consistent with the polarity of subduction suggesting that horizontal tectonic motion is affecting the macro geomorphic form of these ranges

deep crustal break back stacking and slow exhumation of the continental footwall beneath a thrusted marginal basin grenville orogen canada abstract granulite to upper amphibolite facies ductile thrusting in the central gneiss belt grenville orogen ontario represents the tectonic shortening of a continental scale footwall beneath the thrust emplaced central metasedimentary belt during the closure of a postulated back arc basin ca 1 19 1 18 ga break back stacking in the footwall occurred at mid to deep crustal depths ca 35 km within tectonically thickened ca 70 km continental crust and culminated with renewed thrusting at the base of the overlying central metasedimentary belt ca 1 08 1 05 ga the individual mylonite belts which constitute the ductile thrust zones and the scale of penetrative deformation of the intervening crystalline thrust sheets are comparable with the largest known examples of high grade thrust belts elsewhere they reflect the large scale thermal and theological boundary conditions of the deformation flow within individual thrust zones may reflect local boundary conditions such as the rheological behaviour of older thrust sheets and the geometry of interfaces within the thrust stack restoration of the thickness of erosionally removed crustal overburden by break back thrusting may retard the rates of exhumation and cooling of a mid to deep crustal thrust stack

gold butte crustal section south virgin mountains nevada the gold butte block consists of a complex of proterozoic medium to high grade metamorphic and plutonic rocks unconformably overlain by steeply east dipping paleozoic sedimentary rocks on its east side this complex is one of the largest continuous exposures of crystalline rocks in the southwestern united states that lie structurally beneath a steeply tilted section of phanerozoic strata and it may represent part of an intact proterozoic miocene crustal section reaching paleodepths of approximately 15 km the section was exposed during tectonic denudation in miocene time at about 15 ma and lies directly adjacent to the colorado plateau to the east proterozoic and tertiary geological features support the hypothesis that the block represents a relatively intact crustal section a large exposure of proterozoic rapakivi granite the gold butte granite circa 1 45 ga crops out over most of the crystalline complex and displays strong zoning in phenocryst abundance which increases toward the top of a 10 to 12 km thick magma chamber the wallrocks of the granite are predominantly garnet bearing gneisses circa 1 7 ga which display increasing pressures of final equilibration westward away from the basal phanerozoic unconformity thermobarometry of gneisses immediately beneath the unconformity indicate pressures of 2 3 kbar whereas those 12 13 km west of the unconformity yield pressures of 5 6 kbar suggesting the crustal section was established by 1 7 ga the style of deformation displayed by denudation structures also varies from east to west with brittle faulting only in eastern portions of the block and retrograde metamorphism mylonitization and chlorite brecciation of the garnet gneiss observed in western areas

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

the role of mass movements on landscape evolution in the central karakoram discussion and speculation abstract mass movement constitutes an important process in the evolution of landscapes in mountain regions however the role of massive slope failures in denudational unloading and landscape evolution has not been extensively studied large scale mass movements in one of the greatest mountain ranges on earth the central karakoram in pakistan were therefore examined to help evaluate their role in landscape evolution in high mountains specifically four major mass movement complexes ghoro choh rock avalanche busper sackung and slope failure gomboro slope failure and urdokas rockslide each comprising 10 6 m 3 of debris were assessed and mapped in detail two of these mass movement complexes the ghoro choh rock avalanche and gomboro slope failure were dated using terrestrial cosmogenic nuclides the ages of occurrence of the mass movement complexes studied in the central karakoram date from the late pleistocene to the holocene the four major mass movement complexes all involved the removal of mass from the tops of mountain ridges and peaks that failed and were subsequently transported towards the bottom of their respective valleys such massive movement of mass is anomalous compared to other forms of mass movement and is generally spatially coincident with exposed deeply buried gneiss dome structures these large scale movements appear to be part of a coupled system involving river incision and glacial debuttrussing although earthquakes might have triggered these mass movements this study illustrates the role of climate forcing which is part of a coupled system of denudational unloading but it is unclear whether high magnitude low frequency events such as these initiate the isostatic and tectonic influx of mass or if sustained high magnitude denudation resulting from a coupled system is responsible for the exhumation of buried structures

high cooling and denudation rates at kongur shan eastern pamir xinjiang china revealed by 40ar 39ar alkali feldspar thermochronology orthogneiss samples taken from the kongur antiform show ages varying from 2 ma to 1 ma for 40ar 39ar ages of biotites and muscovites and fission tracks on apatites leading to cooling rates of 150 c m y modeling of k feldspars highlights the effect of a range of diffusion domains with contrasting diffusion characteristics yielding closure temperatures from 400 to 150 c the feldspar data document the cooling history since 5 ma and indicate a sudden change in cooling rates of the antiform at 2 ma at that time cooling increases by a factor of 5 from an average of 20 c m y to a minimum of 150 c m y consideration of the regional thermal history ongoing uplift and erosional history of the antiform during the quaternary suggests that denudation rates have been of the order of 5 7 km m y since 2 m y ago and could be associated with significant upward surface movement triggered by major normal faulting the antiform is interpreted to have formed during thrusting at the pamir front as a result of the development of thrust ramps and normal faulting at the crustal scale ramp stacking is an important process of mountain building and normal faulting in this context must be regarded as a very efficient way of building high relief

the late paleoproterozoic orogeny in the north china craton abstract granulite ttg and khondalite formations are the two typical types of precambrian metamorphic basement in the north china craton the former belongs to the basement of the archean craton while the latter is a paleoproterozoic metasedimentary rock series the formation of the ttg rock series took place during the period of 2500 to 2450 ma near the end of the archean and it has undergone multiple times of metamorphism the cooling rate of the ttg lower crust is quite low less than 0 3 ma developed during slow crustal cooling in a rather stable tectonic environment the khondalite rock series underwent amphibolite to granulite facies metamorphism that is isotopically constrained to 2000 to 1800 ma followed by fast crustal cooling from 2 4 to 3 4 ma based on the thermal and dynamic features of different types of metamorphic rocks the tectonic state of the crust and the obvious change of crustal cooling rate from neo archean to paleoproterozoic it is clear that continental orogeny existed in the north china craton between 2000 ma and 1800 ma

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

late cretaceous early eocene laramide uplift exhumation and basin subsidence in wyoming crustal responses to flat slab subduction low angle subduction of the farallon oceanic plate during the late cretaceous early eocene is generally considered as the main driver forming the high rocky mountains in wyoming and nearby areas how the deformation was transferred from mantle to upper crust over the great duration of deformation 40 myr is still debated here we reconstruct basin subsidence and compile paleoelevation thermochronology and provenance data to assess the timing magnitude and rates of rock uplift during the laramide deformation we reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information the amount and rate of rock uplift of individual laramide ranges was less than 2 4 4 8 km 0 21 0 32 mm yr during the early maastrichtian paleocene stage 1 and increased to more than 3 km 0 38 0 60 mm yr during the late paleocene early eocene stage 2 our quantitative constraints reveal a two stage development of the laramide deformation in wyoming and an increase of rock uplift during stage 2 associated with enhanced intermontane basin subsidence exhumation and uplift during stage 1 is consistent with eastward migration of cordilleran deformation associated with low angle subduction whereas the change in exhumation during stage 2 seems to follow a southwestward trend which requires an alternative explanation we here suggest that the increase of rock uplift rate during the late paleocene early eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the farallon oceanic slab

imaging of the moho depth in madagascar through the inversion of gravity data geodynamic implications several e w profiles of bouguer anomalies across the precambrian basement were inverted by applying a linearized inversion procedure in the spectral domain and using a simplified two layer model with a crust upper mantle interface of constant density contrast from these inverted profiles a contour map of moho topography has been constructed which covers the area of exposed precambrian basement in the eastern and central parts of the island such imaging of the moho depth in particular exhibits a n20 e trending zone of substantially thinned crust along the axis of the island this parallels the east coast margin which resulted from the northward motion of india relative to madagascar in the upper cretaceous from combined analysis of geological tectonic and metamorphic observations this prominent crustal feature in the precambrian basement of madagascar is interpreted as the relic of continental lithospheric extension and thinning possibly related to panafrican collapse tectonics on the eastern edge of the mozambique belt this extensional feature would have been reactivated only in part during the rifting and dislocation stages of the madagascar and india continental blocks as evidenced by an axis of cretaceous volcanism emphasized by the ankaratra and itasy massifs final separation nevertheless occurred further east at the site of the east coast margin

modern vertical deformation rates and mountain building in taiwan from precise leveling and continuous gps observations 2000 2008 1 to characterize the present day vertical displacement field in the active taiwan orogenic belt 1843 precise leveling and 199 continuous gps measurements from 2000 to 2008 are collected and analyzed in this study vertical velocities derived from the leveling data are placed in a reference frame of the chinese continental margin using continuous gps observations at nearby sites the leveling and gps vertical velocities generally reveal a dome shaped pattern with uplift of 0 2 18 5 mm yr in the interior of the mountain range and subsidence on the flanks of the mountains and coastal plains modern uplift rates in the active fold and thrust belt are generally consistent with geologic uplift rates however present day uplift rates in the central range are faster than the million years averaged exhumation rates the modern subsidence rates are generally consistent with geologic rates except for the rates in western coastal areas due to groundwater pumping present day subsidence in the southern central range and northern coastal range is however inconsistent with long term uplift which may reflect interseismic elastic strain accumulation across faults present day subsidence in northern taiwan occurs in a region of postcollisional orogenic collapse we model the present day and geologic vertical velocities and published gps horizontal velocity data across southern taiwan using a 2 d lithospheric model the model suggests a combined slip rate of 40 mm yr on the frontal thrusts and 45 mm yr on the longitudinal valley fault the model requires an additional source of crustal thickening under the central range to match the observed present day uplift rates

impact of wind erosion on detecting active tectonics from geomorphic indexes in extremely arid areas a case study from the hero range qaidam basin nw china abstract geomorphologic analysis has been used widely to detect active tectonics in regions where fluvial incision is the major erosional process in this paper however we assess the feasibility of utilizing these frequently used geomorphic indexes e g hypsometric curves longitudinal channel profiles normalized stream length gradient slk index to determine active tectonics in extremely arid areas where wind erosion also plays an important role the case study is developed on the hero range in the western qaidam basin one of the driest regions on earth with severe wind erosion since late pliocene the result shows that in the west and south sectors as well as the western part of the east sector of the hero range where fluvial incision prevails these geomorphic indexes are good indicators of active faulting and consistent with the geological result based on study of fault traces scarps faulted holocene fans and historical seismicity within the past four decades in contrast along the northeastern margin the ne and the se parts of the east sector of the range where wind erosion is also important the results from the geomorphic indexes show quite active tectonics contrary with the geological evidence favoring weakly active tectonics moreover the positive slk anomaly lies oblique to the fault trace and the anticline axis but parallel to the wind direction to reconcile the contradiction we propose that wind erosion caused by northwestern winds has a tendency to make geomorphic indexes exhibit anomalous values that indicate higher activities by way of 1 lowering the base level to generate knickpoints on the longitudinal channel profiles and therefore positive slk anomalies and 2 lateral erosion of the mountain front making the hypsometric curves and even the longitudinal channel profiles more convex and producing obvious slope breaks

kinematic evolution of a large offset continental normal fault system south virgin mountains nevada the south virgin mountains and grand wash trough comprise a mid miocene normal fault system that defines the boundary between the unextended colorado plateau to the east and highly extended crust of the central basin and range province to the west in the upper 3 km of the crust the system developed in subhorizontal cratonic strata in the foreland of the cordilleran fold and thrust system the rugged topography and lack of vegetation of the area afford exceptional three dimensional exposures compact stratigraphy and well defined prefaulting configuration of the rocks permitted a detailed reconstruction of the system reconstruction of cross sections based on more than 300 km2 of detailed mapping at a scale of 1 12 000 shows that the fault system accommodated more than 15 km of roughly east west directed miocene extension extension was initially accommodated on moderately to steeply dipping listric normal faults as the early faults and fault blocks tilted steeply to moderately dipping faults initiated within the fault blocks soling into the early faults some of the early faults were active at dips of 20 isostatically driven tilting is superimposed on tilting due to active slip and domino style rotation of the fault blocks collectively these processes rotated originally steeply dipping faults to horizontal orientations the kinematics are inconsistent with the widely accepted view that many near horizontal normal faults were rotated to their present orientations by later crosscutting normal faults however reexamination of other areas suggests that the evolutionary sequence seen in the south virgin mountains may in fact be widely applicable

quantitative constraints on the rate of landform evolution derived from low temperature thermochronology in recent years the nature of the complex interactions between the solid earth and the overlying atmosphere has been the subject of much debate england and molnar 1990 whipple et al 1999 braun et al 1999 does erosion play an important role in the dynamics of mountain building is there a strong feedback into the balance of forces at the crustal scale from mass transport by erosional and depositional processes at the earth s surface or does erosion passively react to changes in the earth s surface topography to shape mountains with tectonic forces originating in the underlying convecting mantle dominating the dynamics of plate interactions and crustal deformation at convergent plate margins the answers to these fundamental questions lie in our ability to determine the rate at which surface processes can react to tectonic forcing whipple et al 1999 where erosion is efficient the coupling will be strong as the rate of change of surface loading by mass transport will be similar to the rate of creation of topography by crustal deformation this may lead to a concentration of deformation and thus exhumation in regions of intense erosion koons 1990 willett et al 1993 beaumont et al 2001 koons et al 2002 2003 braun and pauselli 2004 where erosion is relatively inefficient mountain building is the result of a balance between internal driving forces and those originating from the deflection of an upper free surface there is no feedback from erosional processes low temperature thermochronology such as u th he he or fission track ft dating in apatite and zircon provides information on the rate at which rocks cool as they are exhumed towards the cold earth surface duddy et al 1988 warnock et al 1997 farley 2000 reiners and farley 1999 because the

detecting rock uplift across southern taiwan mountain belt by integrated gps and leveling data abstract rock uplift on the earth surface is a key observation for studies of tectonics and geodynamic processes geodetic measurements from global positioning system gps and leveling are able to track vertical deformation over a wide range of spatial and temporal scale taiwan mountain belt is subject to a rapid uplift rate of 20 mm yr as revealed by gps and leveling measurements in previous studies the extremely high rates motivate us to analyze a set of newly processed gps and leveling data along a nw se transect across southern taiwan by considering the effects of the earthquake related deformation as well as hydrological and surface processes our estimates of rock uplift rates from gps and leveling are from 12 to 14 mm yr across the southern central range of taiwan vertical velocity changes on the small spatial scale are primary related to fault locking in the seismic cycle the large spatial feature showing the mean uplift rate of 6 mm yr between the eastern central range and the frontal thrust faults can be explained by a large scale pop up structure assuming the depth of 15 20 km in the orogenic wedge and the horizontal shortening of 30 mm yr our results draw attention to carefully study geodetic vertical deformation in the mountain belts and seek relevant causes

post orogenic tectonic and thermal history of a rifted continental margin the scoresby sund area east greenland abstract fission track ft analysis of zircon sphene and apatite from crystalline basement in the scoresby sund area east greenland and ft modelling reveal information about the cooling and uplift paths for individual tectonic blocks since the caledonian orogeneny up to c 325 ma the area cooled and behaved as a single unit however it subsequently broke into several fault bounded blocks each block moving independently of adjacent blocks net uplift since 325 ma was of a similar magnitude 8 km assuming a geothermal gradient of 30 c km along the crest of the devonian fault zone for the southern blocks but about 9 km for the stauning alper and these blocks were probably tilted away from the rifted continental margin the relief evolution accompanying uplift in the caledonian basement is closely related to mesozoic basin formation subsidence and infilling in the area ft modelling reveals fast cooling from c 70 c to surface temperatures during the last 55 m y probably as a result of tertiary rifting and magmatic activity in the area

origin of the highly elevated pyrenean peneplain 1 peneplanation of mountain ranges is generally considered the result of long term erosional processes that smooth relief and lower elevation near sea level therefore peneplain remnants at high elevation in mountain ranges are used to infer posttectonic surface uplift such an interpretation has been proposed for the pyrenees where high elevation low relief erosional surfaces rose up to more than 2000 m because the pyrenean foreland basins are filled with very thick continental deposits which have buried the early jagged landscape we challenge this hypothesis by pointing out that relief applanation does not necessarily require elevation lowering we propose an alternative interpretation in which piedmont aggradation of detrital sediment that comes from erosion of the high chain induces the rise of the base level of the range therefore reducing strongly the erosive efficiency of the drainage system and resulting in the progressive smoothing of the relief such a process allows a high elevation low relief erosional surface to develop at the scale of the range in the pyrenees occurrence of high elevation low relief erosional surface remnants does not imply a posttectonic uplift but is instead due to the dissection of the initial miocene high elevation low relief surface by the recent drainage system the erosive activity of which has been enhanced by global climate change from the late pliocene onward

crustal tomographic imaging and geodynamic implications toward south of southern granulite terrain sgt india abstract the crustal structure toward southern part of sgt is poorly defined leaving an opportunity to understand the tectonic and geodynamic evolution of this high grade granulite terrain surrounded by major shear and tectonically disturbed zones like achankovil shear zone aksz and palghat cauvery shear zone pcsz to develop a geologically plausible crustal tectonic model depicting major structural elements a comprehensive tomographic image was derived using deep seismic sounding data corroborated by bouguer gravity modeling coincident reflection seismic heat flow and available geological geochronological informations along the n s trending vattalkundu kanyakumari geotransect the final tectonic model represents large compositional changes of subsurface rocks accompanied by velocity heterogeneities with crustal thinning 44 36 km and moho upwarping from north to south this study also reveals and successfully imaged anomalous zone of exhumation near aksz having transpression of exhumed rocks at mid to lower crustal level 20 30 km with significant underplating and mantle upwelling forming a complex metamorphic province the presence of shear zones with high grade charnockite massifs in the upper crust exposed in several places reveal large scale exhumation of granulites during the pan african rifting 550 ma and provide important insights of plume continental lithosphere interaction with reconstruction of the gondwanaland

retroarc basin reorganization and aridification during paleogene uplift of the southern central andes u s national science foundation ear 1049605 robert r shrock foundation at indiana university nsf ear award 1338583 project pdts unsj e985

surface uplift uplift of rocks and exhumation of rocks uplift of the surface of mountain belts requires forces that are comparable in magnitude to those associated with plate motion and therefore determination of rates of surface uplift could provide important information on the dynamics of mountain ranges rates of uplift of the surfaces of mountain ranges have not however been quantified sufficiently well that they provide useful constraints on those processes many reports off surface uplift in mountain ranges are based on mistaking exhumation of rocks or uplift of rocks for surface uplift and provide no information whatsoever on the rates of surface uplift

the serra da graciosa a type granites and syenites southern brazil part 2 petrographic and mineralogical evolution of the alkaline and aluminous associations abstract the serra da graciosa granites and syenites comprise five distinct plutons in the brasiliano pan african graciosa a type province southern brazil six petrographic series can be identified in these plutons 1 alkaline series 1 composed of amphibole bearing alkali feldspar syenites with varied mafic mineralogy and quartz contents from alkali feldspar syenites with calcic amphibole clinopyroxene olivine and allanite to alkali feldspar quartz syenites with sodic calcic amphibole and chevkinite perrierite and to alkali feldspar granites with sodic amphibole 2 alkaline series 2 characterized by amphibole bearing alkali feldspar granites with limited modal variations but amphibole compositions also varying from calcic to sodic 3 alkaline series 3 of limited occurrence which includes alkali feldspar syenites with olivine and clinopyroxene and no amphibole 4 aluminous series 1 of widespread occurrence with various petrographic facies of biotite granites with amphibole 5 aluminous series 2 characterized by alkali feldspar granites with biotite and only minor amphibole 6 monzodiorites typically with biotite calcic amphibole and augitic clinopyroxene partially mingled with granitic magmas the mafic minerals present are in general fe rich with correspondingly low mg and al contents in alkaline series 1 amphiboles crystallized in progressively more oxidizing and alkaline conditions while in alkaline series 2 the initial conditions were somewhat more oxidizing and shifted to reducing in the final stages in aluminous series 1 and aluminous series 2 amphiboles are calcic and comparatively homogeneous the amphiboles in the monzodioritic rocks while also homogeneous are more mg rich and show compositions quite distinct from the calcic varieties in the other associations and this is also the case for clinopyroxene mg in biotite decreases from the monzodioritic rocks to aluminous series 1 and further to aluminous series 2 contrasting evolution of the various associations suggests that several coeval magmatic series are present in the serra da graciosa granites

quaternary hinterland evolution of the active banda arc surface uplift and neotectonic deformation recorded by coral terraces at kisar indonesia abstract coral terrace surveys and u series ages of coral yield a surface uplift rate of 0 5 m ka for kisar island which is an emergent island in the hinterland of the active banda arc continent collision based on this rate kisar first emerged from the ocean as recently as 450 ka these uplifted terraces are gently warped in a pattern of east west striking folds these folds are strike parallel to more developed thrust related folds of similar wavelength imaged by a seismic reflection profile just offshore this deformation shows that the emergence of kisar is influenced by forearc closure along the south dipping kisar thrust however the pinnacle shape of kisar and the protrusion of its metamorphic rocks through the forearc basin sediments also suggest a component of extrusion along shear zones or active doming coral encrusts the island coast in many locations over 100 m above sea level terrace morphology and coral ages are best explained by recognizing major surfaces as mostly growth terraces and minor terraces as mostly erosional into older terraces all reliable and referable coral u series ages determined by mc icp ms correlate with marine isotope stage mis 5e 118 128 ka the only unaltered coral samples are found below 6 m elevation however an unaltered tridacna giant clam shell in growth position at 95 m elevation yields a u series age of 195 31 ka which corresponds to mis 7 this age agrees with the best fit uplift model for the island loose deposits of unaltered coral fragments found at elevations between 8 and 20 m yield u series ages of

the serra da graciosa a type granites and syenites southern brazil part 3 magmatic evolution and post magmatic breakdown of amphiboles of the alkaline association amphiboles are the main mafic minerals in most metaluminous to peralkaline alkali feldspar granites and syenites and they usually preserve an important record of the compositional evolution of the melts from which they crystallize in the alkaline association of the serra da graciosa a type granites and syenites southern brazil amphibole compositions span a large range including calcic sodic calcic and sodic amphiboles calcic amphiboles are typically observed in the metaluminous rocks while sodic amphiboles are characteristic of the more strongly peralkaline rocks sodic calcic amphiboles are found in intermediate varieties compositional variations record the differentiation trends within two petrographic series of the alkaline association the overall evolution of amphibole compositions is similar in both they reveal a progressive increase in na and fe3 with differentiation increase in alkalinity of the magmas a characteristic shared by undersaturated peralkaline or agpaitic differentiation trends in detail however the evolutions of the amphibole compositions in the two series are distinct in alkaline series 1 the cores of the crystals form a continuum from calcic to sodic compositions with the exception of a small compositional gap within the sodic calcic amphiboles the rims however show compositions that diverge from this main trend this divergence results from increasing amounts of the oxy amphibole component and reflects more oxidizing conditions at the final stages of magmatic crystallization in alkaline series 2 these oxidation trends are much more subtle and a reverse trend is observed in the sodic amphiboles sodic calcic amphiboles are in several cases replaced by intergrowths of post magmatic sodic amphibole and al poor tetrasilicic biotite

rates of continental erosion and mountain building the first objective of this work was to obtain values for the rates at which continental erosion can smooth out or remove the topographic expression produced by orogeny the dominant part is played by mechanical erosion which acts most strongly in regions of large topographic expression chemical erosion depends strongly on precipitation or runoff in individual river drainage basins but because most continents have very similar average rainfall chemical erosion is fairly uniform for continental sized areas and will succeed in planing down all continents to a level peneplain if given enough time the exception to this rule is australia which has a very low chemical erosion rate because of its dryness the time constants for mechanical and chemical erosion so obtained vary between about 30 and 300 my depending on the continent and the assumptions made mountain building occurs throughout the geological time scale but at a non uniform rate although there will not be a balance between erosion and mountain building over a short time scale due to the non uniform rate of mountain building the long term situation must be that the two phenomena should balance out it is shown that the freeboard of continents will respond to the long term balance between mountain building and erosion an expression has been derived for the average continental elevation in which the rate of mountain building depends on the rate of radiogenic heat production within the earth it is shown that relatively small changes in average elevation above sea level of a few hundred metres are predicted to have occurred since the beginning of the proterozoic as mountain building is predicted to decrease on average with time because of the reduction in internal heat generation and as erosion is dependent on the average elevation this average elevation will decrease slowly through time the opposite of what some workers have predicted a more complicated model of mountain building is then investigated in which one component of mountain building has a sinusoidal signal the oscillations in average elevation depend on the period of the sinusoid being smaller for shorter periods finally an average continental elevation is derived using a list of real orogenic events although this list of orogenies is incomplete there is some indication that the actual continental elevation as seen in the flooding history of the continents is similar to that derived in this paper

evolution of magmatic afm mineral assemblages in granitoid rocks the hornblende melt biotite reaction in the liberty hill pluton south carolina

systematic mineralogical diversity in a type granitic intrusions control of magmatic source and geological processes the origin and causes of mineralogical diversity of a type granites are debated the series of a type granite plutons with distinct mineralogical differences emplaced along an upper paleozoic crustal scale shear zone in the cobequid highlands nova scotia provide an opportunity to examine the origin of different a type plutons in a similar tectonic setting based on the ferromagnesian minerals present the plutons are classified into sodic granites with sodic amphibole calcic granites with calcic amphibole and biotite granites sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone whereas plutons in the western shear zone with lesser gabbro are solely biotite granites trace elements and radiogenic isotopes show that the three granite types have different sources intensive parameters including temperature pressure and water in melt contents were estimated from mineralogical and geochemical data modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20 40 partial melting of intracrustal feldspathic rocks whereas the sodic granites are extreme fractionates 90 of coeval mafic magma we propose that supply of upper paleozoic mafic magma probably related to regional extension and decompression melting beneath the magdalen basin created a deep crustal hot zone in the eastern cobequid highlands and extreme fractionation of underplated mafic sills produced the sodic granites heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust creating batches of biotite and calcic granitic melts in different depths fractionated and crustally derived melts segregated along crustal scale faults constructing the complex plutons in the east melting of the crust was further facilitated by the release of water from the crustal rocks upon heating in the eastern shear zone water was released predominantly by magmatic rocks and in lesser amounts compared to the west where neoproterozoic sedimentary and volcaniclastic rocks are more abundant the volatile rich granitic melts in the western part of the shear zone were crystallized rapidly stabilizing only biotite this study demonstrates that the mineralogical variations in a type granites arise from rather similar magma compositions but they are important petrogenetic indicators of varying sources specific magmatic processes and emplacement conditions

quaternary rupture behavior of the karakoram fault and its relation to the dynamics of the continental lithosphere nw himalaya western tibet abstract the karakoram fault is a major strike slip system that partitions strain between the compressional regime of the nw himalaya and the translational and extensional fault systems of western tibet although seismic and geodetic observations suggest that the fault is currently locked a high millennial slip rate infers a dominant role in the deformation of western tibet we examine all quaternary data available for the karakoram fault and attempt to explain the temporal and spatial variability of fault slip rate we show that the karakoram fault stress level could be modulated by coulomb stress changes associated with varying seismicities on the subducting indian plate beneath western tibet our model has implications for the understanding of stress transfer between large fault systems and their interaction with lithosphere deformation we suggest that while the long term deformation of tibet may be modeled as a partially viscous medium the slip rates of major faults may vary by one order of magnitude during the quaternary our analysis may resolve the frequently observed discrepancy between millennial and decadal records of fault slip rates

lapse rates and other variables applied to estimating paleoaltitudes from fossil floras abstract a previous method for determining paleoaltitude from fossil floras has used the difference between sea level and upland paleotemperatures which is then multiplied by the reciprocal of a normal lapse rate the present study examines other variables that can significantly influence such results terrestrial lapse rates have been calculated for 39 areas of the world based upon mean annual temperature mat mean warm month temperature mat and mean cold month ttemperature cmt these show regional and seasonal variability that is explained by factors including continentality elevation of large massifs or high base levels cold air drainage fog mountain barrier effects and the location of the subtropical high pressure zone lapse rates and projected sea level temperatures based on data from the western united states show distinct regional patterns these indicate that within this region the highest lapse rate and temperature values are correlated with 1 continental climates in the subtropical high pressure zone and 2 areas having relief where the lowest elevation land surfaces base levels occur at high altitude the data are applied to evaluate and revise the methodology for estimating paleoaltitudes variables of climatic change sea level fluctuation and paleogeography are also considered limitations in application of the method are emphasized the method is exemplified by application to the latest eocene florissant flora of colorado u s a although the results are conjectural they strongly suggest that the late eocene erosion surface in the southern rocky mountains occurred at a much higher paleoaltitude than previously thought this high elevation may have resulted from earlier laramide crustal thickening

combined paleomagnetic and geochronological study on cretaceous strata of the qiangtang terrane central tibet abstract a combined paleomagnetic and geochronological investigation has been performed on cretaceous rocks in southern qiangtang terrane 32 5 n 84 3 e near gerze central tibetan plateau a total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled our new u pb geochronologic study of zircons dates the volcanic rocks at 103 8 0 46 ma early cretaceous while the red beds belong to the late cretaceous rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers after removing a low temperature component of viscous magnetic remanence stable characteristic remanent magnetization chrm was isolated successfully from all the sites by stepwise thermal demagnetization the tilt corrected mean direction from the 14 lava sites is d 348 0 i 47 3 k 51 0 95 5 6 corresponding to a paleopole at 79 3 n 339 8 e a 95 5 7 and yielding a paleolatitude of 29 3 5 7 n for the study area the chrm directions isolated from the volcanic rocks pass a fold test at 95 confidence suggesting a primary origin the volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole vgp scatter the mean inclination from the late cretaceous red beds however is 13 1 shallower than that of the 100 ma volcanic rocks after performing an elongation inclination analysis on 174 samples of the red beds a mean inclination of 47 9 with 95 confidence limits between 41 9 and 54 3 is obtained which is consistent with the mean inclination of the volcanic rocks the site mean direction of the late cretaceous red beds after tilt correction and inclination shallowing correction is d 312 6 i 47 7 k 109 7 95 3 0 n 22 sites corresponding to a paleopole at 49 2 n 1 9 e a 95 3 2 yielding a paleolatitude of 28 7 3 2 n for the study area the chrm of the red beds also passes a fold test at 99 confidence indicating a primary origin comparing the paleolatitude of the qiangtang terrane with the stable asia there is no significant difference between our sampling location in the southern qiangtang terrane and the stable asia during 100 ma and late cretaceous our results together with the high quality data previously published suggest that an 550 km n s convergence between the qiangtang and lhasa terranes happened after 100 ma comparison of the mean directions with expected directions from the stable asia indicates that the gerze area had experienced a significant counterclockwise rotation after 100 ma which is most likely caused by the india asia collision

pliocene and pleistocene exhumation and uplift of two key areas of the northern apennines application of different thermochronological methods to two areas of pronounced relief apuane alps and mt falterona of the northern apennines chain documents an average exhumation rate of about 0 7m m yr this result suggests that the general exhumation of the chain is driven mainly by erosion nevertheless the long term exhumation trend seems to be perturbed by shortterm events at higher exhumation rates the time of increased exhumation rates coincides with onset of intramontane sedimentation documenting a contribution of tectonics local normal faulting to the surface uplift the data indicate an eastward shifting of the onset of topographic relief early pliocene in the apuane alps and early pleistocene in the mt falterona area r 2002 elsevier science ltd and inqua all rights reserved

landscape response to climate change insights from experimental modeling and implications for tectonic versus climatic uplift of topography we present the results of an experimental investigation of the concurrent action of tectonic uplift and climate variation on relief evolution we designed an experimental apparatus that allows the study of erosion of laboratory scale topographies that evolve under given uplift and rainfall rates for constant uplift and rainfall rates the experimental topography evolves toward a statistical steady state defined by a mean elevation constant with time starting from such a steady state and keeping the input uplift rate constant a subsequent change in the rainfall rate yields a change in the mean elevation of the landscape to a new equilibrium elevation an increase in precipitation yields a lower mean steady state elevation whereas for a decrease in precipitation the surface is uplifted we define this phenomenon as a climatically induced surface uplift as opposed to a tectonically induced surface uplift the climatically and tectonically induced surface uplifts correspond to different dynamics of denudation so that it is theoretically possible to differentiate between the climatic or tectonic causes of surface uplift from records of output sediment fluxes

allanite and chevkinite in a type granites and syenites of the graciosa province southern brazil abstract allanite and chevkinite perrierite are important accessory phases in the granitic and syenitic a type rocks of the graciosa province southern brazil allanite ce is the main ree phase in the granites of the aluminous association which includes metaluminous to weakly peraluminous granites while chevkinite ce is the typical ree phase in the metaluminous syenites and alkali feldspar syenites and the peralkaline alkali feldspar granites of the alkaline association allanite shows complex zoning patterns but is always rich in the ferriallanite molecule and in tio 2 a compositional trait that distinguishes allanite in a type aluminous associations from allanite in calcic alkali calcic and calc alkaline granitoids normalized ree patterns show strong fractionation of heavy over light ree with la n nd n ratios between 3 and 5 and la n y n ratios between 200 and 400 primary allanite is variably affected by post magmatic transformations due to the interaction with a fluid phase and metamictization caused by radioactive decay chevkinite ce is always primary and is only present in the alkaline association the early crystallization of chevkinite indicates that the parental magmas were probably close to ree saturation the ree y abundance in chevkinite 43 49 wt is almost twice that observed in primary allanite yet the normalized patterns are similar in terms of fractionation between light and heavy ree as indicated by similar la n nd n ratios in chevkinite and allanite la n y n ratios are on average lower in chevkinite 90 300 chevkinite compositions were altered by interaction with post magmatic fluids which led to marked increase in tio 2 and a reduction in ree and fe t post magmatic allanite is present in both the alkaline and aluminous associations and corresponds to ferriallanite ce in the alkaline association and allanite y in the aluminous association compared to primary allanite post magmatic allanite ce shows smaller ree fractionation 2 n nd n 5 times chondrite the mineralogy of the rocks indicates crystallization conditions close to the titanite magnetite quartz amphibole ilmenite tmqai buffer for the aluminous association and close to the fayalite quartz magnetite fqm buffer for the alkaline association and temperatures between 800 and 900 c liquidus and ca 750 c solidus the experimentally determined stability fields for chevkinite and allanite are in agreement with the textural evidence for early chevkinite crystallization and late allanite crystallization even though allanite and chevkinite only rarely coexist in the alkaline association wherever allanite and chevkinite coexist they reveal reaction textures such that it is possible that only allanite or chevkinite is stable at any given condition the distribution of allanite and chevkinite in the alkaline association suggests that the composition of the magmas is an important control on the stability fields of these minerals a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing and mingling of magmas in this sense the importance of composition on the stability of allanite and chevkinite may be larger than previously suggested