investigation of the relationships between basin morphology tectonic uplift and denudation from the study of an active fold belt in the siwalik hills central nepal the present study investigates correlations between an extensive range of geomorphic properties that can be estimated from a digital elevation model and the uplift rate on geological timescales the analysis focuses on an area in the siwalik hills central nepal where lithology and climate can be considered as uniform this area undergoes rapid tectonic uplift at rates of up to 15 mm yr 1 which are derived from the geometric pattern of a fault bend model of fold growth the selected geomorphic properties can be divided in two categories depending on whether or not the vertical dimension is taken into account none of the planar properties are significantly correlated to uplift rate unlike those that include the vertical dimension such as the mean elevation of basins hypsometric curve and hypsometric integral and relief defined by the amplitude factor of length scaling analysis correlation between relief and uplift rate is observed for all length scales of topography shorter than 600 m which suggests that all orders of the streams are able to adjust to the tectonic signal simple mass balance considerations imply that the average elevation is only 10 of surface uplift suggesting that a dynamic equilibrium has been reached quite rapidly using a simple two process model for erosion we find that fairly high diffusion coefficients order of 10 m 2 yr 1 and efficient transport of the material by rivers are required this unusually high value for mass diffusivity at small length scales may be obtained by either a very efficient linear diffusion or by landsliding actually both processes may be active which appears likely given the nature of the unconsolidated substratum and the favorable climatic conditions local relief in the study area may therefore be used to predict either uplift or denudation but the prediction is calibrated only for that specific climatic and lithologic conditions and cannot be systematically applied to other contexts

geochronology stratigraphy and geochemistry of cindery tuff in pliocene hominid bearing sediments of the middle awash ethiopia cindery tuff is a subalkaline rhyolitic air fall deposit that was probably produced by a mixed magma eruption it is a distinctive datable regional isochronous marker bed within the pliocene sediments of the middle awash district and is stratigraphically situated between two new fossil hominid discoveries based on 40ar 39ar analyses of plagioclase rhyolitic glass and basaltic glass as well as fission track analyses of zircons we estimate its age to be 3 8 4 0 myr this implies that associated hominid skull fragments are at least 3 9 myr old

dating crystalline groundmass separates of altered cretaceous seamount basalts by the 40ar 39ar incremental heating technique abstract alteration of submarine basalts compromises geochronology using conventional k ar and 40 ar 39 ar techniques to help overcome these problems we re evaluated the potential of groundmass dating techniques incremental heating on acid leached groundmass samples following an overnight bakeout at 200 c and using high resolution heating schedules eliminated most of the low temperature alteration effects in the submarine basalts studied more than 75 of the groundmass analyses n 32 display accurate age plateaus consisting of 30 70 of the total amount of 39 ar k released more than 50 of the analyses have plateau ages concordant with their total fusion ages implying minor or proportional loss of radiogenic 40 ar and 39 ar k overall we could show a high degree of coherence between ages of groundmass separates and comagmatic phenocrysts this suggests that the dating of aphyric basalts which previously has proven problematic can be accomplished with increasing confidence as well adding these rock types to the list of datable submarine basalts significantly enhances our ability to understand the eruptive history of linear submarine volcanic chains

40ar39ar and kar dating of altered glassy volcanic rocks the dabi volcanics p n g k ar and 40ar39ar ages have been determined for altered submarine tholeiitic and boninite high mg andesite lavas from the dabi volcanics cape vogel peninsula papua new guinea 40ar39ar whole rock total fusion and plateau ages identify a late paleocene age for the tholeiitic lavas 58 9 1 1 ma and also for the boninitic lavas 58 8 0 8 ma apparent k ar ages for the same samples range from 27 2 0 7 to 63 9 4 5 ma and young k ar ages for glassy boninites are probably due to variable radiogenic 40ar 40ar loss these new ages effectively reconcile previously ambiguous age data for the dabi volcanics and indicate contemporaneous tholeiitic and boninitic volcanism occurring in southeast png during the late paleocene smectites developed as alteration products after glass in oceanic lavas commonly do not retain 39ar during or subsequent to irradiation but in some cases may contain 40ar in the absence of other factors modifying k and ar contents samples which have not lost 40ar from smectite and suffer 39ar loss only are interpreted to have been altered immediately subsequent to the crystallization of the lava whereas samples which have lost 40ar as well as 39ar may be the result of either recent alteration or of continuous 40ar loss since the time of crystallization

high resolution 40ar 39ar dating of the oldest oceanic basement basalts in the western pacific basin 1 we report new 40ar 39ar ages for the oldest pacific oceanic floor at ocean drilling program site 801c in the pigafetta basin and site 1149d close to the izu bonin subduction zone in the nadezhda basin these ages were determined by applying high resolution incremental heating experiments including 15 30 heating steps to better resolve the primary argon signal from interfering alteration signatures in these low potassium ocean crust basalts combined with previous results from pringle 1992 for site 801b and 801c we arrive at a multistage history for the formation of the pigafetta ocean crust the oldest part of the pacific plate was formed at the spreading ridges at 167 4 1 4 3 4 ma n 2 2 internal absolute error offering an important calibration point on the geological reversal timescale grts since it represents the old end of the mesozoic magnetic anomalies this mid ocean ridge basalt sequence however is overlain by more tholeiites and alkali basalts that were formed 7 3 1 5 myr later around 160 1 0 6 ma n 7 2 internal error the older age group is confirmed independently by radiolarian ages ranging from late bajocian to middle bathonian 167 173 ma bartolini and larson 2001 and by profound differences in the structural characteristics of this basement section pockalny and larson 2003 thin layers comprising hydrothermal deposits separate these sequences which in addition to the difference in isotopic age show distinct major and trace element compositions this indicates that key volcanic and hydrothermal activity took place 400 600 km away from the spreading ridges on the basis of a jurassic 66 km myr half spreading rate in the pacific it remains unclear if these processes were active continuously after the initial formation of the pacific oceanic crust but all our observations seem to point to an episodic history site 1149d gives another important calibration point on the grts of 127 0 1 5 3 6 ma n 1 2 internal absolute error for anomaly m12 that is slightly younger when compared to current timescale compilations 134 2 2 1 ma gradstein et al 1995 this might suggest that the dated basalt from site 1149d does not represent the age of the ocean crust formed at its ridge axis it may also be part of the early cretaceous intraplate events that have produced dolerite sills in the pacific crust at sites 800 and 802 around 114 126 ma

autopsy on a dead spreading center the phoenix ridge drake passage antarctica new bathymetric and magnetic anomaly data from the phoenix ridge antarctica show that extinction of all three remaining segments occurred at the time of magnetic chron c2a 3 3 0 2 ma synchronous with a ridge trench collision south of the hero fracture zone this implies that the ultimate cause of extinction was a change in plate boundary forces occasioned by this collision spreading rates slowed abruptly at the time of chron c4 7 8 0 3 ma probably as a result of extinction of the west scotia ridge which would have led to an increase in slip rate and transpressional stress across the shackleton fracture zone spectacular high relief ridges flanking the extinct spreading center mapped for the first time using multibeam swath bathymetry are interpreted as a consequence of a reduction in spreading rate involving a temporary magma oversupply immediately prior to extinction

thermal tectonic history of the baogutu porphyry cu deposit west junggar as constrained from zircon u pb biotite ar ar and zircon apatite u th he dating abstract understanding postmineralization tectonic movements in porphyry deposits is critical to interpreting the complete thermal tectonic history this study reports new zircon u pb ages hydrothermal biotite 39ar 40ar age and zircon and apatite u th he ages from the baogutu porphyry copper deposit which in conjunction with pre existing geochronology and thermochronology and inverse modeling simulations constrain the thermal tectonic history of the deposit zircon la icp ms u pb concordia ages indicate a late carboniferous age of 320 1 2 2 ma for the diorite complex and 309 8 2 2 ma for the mineralized granodiorite porphyry hydrothermal biotite selected from a quartz biotite chalcopyrite vein yields a plateau age of 311 0 1 8 ma which agrees with the age of the granodiorite porphyry and a previously reported molybdenite re os age and suggests that hydrothermal fluid circulated about 310 m y ago diorite and granodiorite porphyry yield weighted mean zircon u th he ages of 200 6 5 7 ma and 241 1 8 1 ma respectively with ages ranging from 221 0 ma to 174 3 ma and 225 9 to 261 6 ma weighted mean diorite and granodiorite porphyry apatite u th he ages of 87 4 2 3 ma and 120 0 4 2 ma were obtained with ages ranging from 68 9 ma to 100 8 ma and from 91 0 ma to 152 0 ma respectively the wide range of zircon u th he ages may be due to the combined effects of u and th zonation and radiation damage and radiation damage effect may also account for the wide range of apatite u th he ages the combined effects of depth and cooling due to meteoric water circulation contribute to an older u th he age for the granodiorite porphyry relative to the diorite a five episode cooling rate history for the diorite can be deduced by inverse model simulation fast cooling moderate fast cooling relatively slow cooling fast cooling again very slow cooling the thermal and tectonic history of the wall rock indicates that it suffered significant far field effects from the qiangtang eurasia and lhasa qiangtang collision however no visible thermal effect from the india asia collision is observed

tectonic development of the bransfield basin and its prolongation to the south scotia ridge northern antarctic peninsula abstract the bransfield basin is located off the northern tip of the antarctic peninsula the analysis of multichannel seismic reflection profiles allows the shallow structure of the bransfield basin and its eastwards prolongation through the scotia sea to be imaged the bransfield basin is asymmetrical with most of the sediment input coming from the passive antarctic peninsula margin which is deformed by normal faults generally dipping less than 45 northwestwards much less sediment comes from the conjugate south shetland islands margin which is dominated by high angle normal faults generally dipping between 45 and 60 constituting the inner boundary of the south shetland block tectonic horst the detachment and moving apart of the south shetland block from the antarctic peninsula led to the opening of the bransfield basin where incipient oceanic spreading has been reported previously the south shetland block outer boundary to the northwest is made of the south shetland trench which to the east passes into the sinistral transpressive scotia antarctic plate boundary marked by the prominent south scotia ridge following cessation of spreading in the phoenix antarctic ridge northwest of the south shetland block during middle pliocene roll back played a major role in the development of the bransfield basin with maximum extension in the central subbasin in addition the eastern bransfield basin accommodates most of the sinistral transtensional deformation along the south scotia ridge normal and reverse faults were simultaneously active along the inner and outer boundaries respectively of the continental crust fragment constituting the south shetland block the development of the bransfield basin offers a good example of the tectonic processes involved in continental fragmentation associated with backarc basin development

metamorphic evolution of a subduction complex south shetland islands antarctica a subduction complex composed of ocean floor material mixed with arc derived metasediments crops out in the elephant island group and at smith island south shetland islands antarctica with metamorphic ages of 120 80 ma and 58 47 ma respectively seven metamorphic zones i vii mapped on elephant island delineate a gradual increase in metamorphic grade from the pumpellyite actinolite facies through the crossite epidote blueschist facies to the lower amphibolite facies geothermometry in garnet amphibole and garnet biotite pairs yields temperatures of about 350 c in zone iii to about 525 c in zone vii pressures were estimated on the basis of si content in white mica al2o3 content in alkali amphibole nam4 aliv in sodic calcic and calcic amphibole alvi si in calcic amphibole and jadeite content in clinopyroxene mean values vary from about 6 7 5 kbar in zone ii to about 5 kbar in zone vii results from the other islands of the elephant island group are comparable to those from the main island smith island yielded slightly higher pressures up to 8 kbar with temperatures estimated between 300 and 350 c zoned minerals and other textural indications locally enable inference of p t t trajectories all with a clockwise evolution a reconstruction in space and time of these p t t paths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases d1 d2 that affected the area this thermal structure is in good agreement with the one expected for a subduction zone the arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex in this model d1 represents the subduction movements expressed by the first vector of the clockwise p t t path d2 reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure and d3 corresponds to isostatic uplift accompanied by erosion under circumstances of decreasing temperature and pressure

antarctic peninsula continental magnesian andesites indicators of ridge trench interaction during gondwana break up subduction of young hot oceanic lithosphere and a major episode of lithospheric extension have been considered as key tectonomagmatic factors in the genesis of setouchi type continental magnesian andesites we present data on a new discovery of such andesites from the northern part of the antarctic peninsula and propose that subduction of newly created oceanic lithosphere occurred along the proto pacific margin of antarctica during the middle jurassic continental magnesian andesite magmatism was contemporaneous with the early stages of gondwana break up and we suggest that subduction of young hot oceanic lithosphere and ridge trench interaction may have contributed to the disintegration of gondwana by thermally weakening the lithosphere and developing a broad linear extensional zone along the proto pacific margin

pre metamorphic 40ar 39ar and u pb ages in hp metagranitoids from the hercynian belt france in most orogenic belts the age of hp metamorphism and subsequent exhumation events still remain open to debate since geochronology can yield results which appear to conflict with the closure temperature concept dodson m h 1973 contrib mineral petrol 40 259 274 and because the behaviour of daughter radiogenic isotopes under hp to uhp conditions is poorly constrained to obtain new data on isotope migration under high pressure conditions two undeformed hp metagranites with partially preserved magmatic assemblages from the french variscan belt were investigated using the 40ar 39ar laser probe and u pb ion probe methods in both cases 40ar 39ar biotite and u pb zircon ages are consistent and could be related to the emplacement of pre orogenic granites despite petrological evidence of a strong metamorphic overprint during the variscan orogeny temperatures experienced by the granites during subduction and exhumation events were more than 400 c above that normally estimated for argon retention in biotite but failed to cause significant resetting of the mica 40ar 39ar chronometer only a weak intragrain redistribution of argon is evidenced with the laser probe up to the contact with metamorphic garnet fringing biotite by contrast a complete resetting of biotite ages occurs in gneisses enclosing the metagranites these results suggest that in these dry undeformed hp metagranites the thermally activated diffusion was relatively ineffective and that recrystallisation is the main process which controlled isotopic exchanges of ar and pb it is likely that the absence of pervasive deformation and fluid circulation has also exercised some control on the preservation of pre metamorphic isotopic signature in the studied rocks the possible influence of several other parameters is also discussed this example reveals that recovering thermochronological information in high pressure metamorphic rocks is not straightforward and that great caution must be paid in the use of ages for the reconstruction of p t t paths

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

the age and stratigraphy of fore arc magmatism on alexander island antarctica fore arc magmatic sequences associated with high mg number andesite lavas unconformably overlie lemay group accretionary complex in alexander island high resolution 40ar 39ar u pb zircon fission track and k ar ages demonstrate that subduction related fore arc magmatism migrated northwards along the length of alexander island between c 80 ma and c 46 ma the magmatic rocks represent a third of the western margin of the antarctic peninsula magmatic arc and are critical to the understanding of the final phase of subduction along the southern antarctic peninsula margin the onset of late cretaceous magmatism is recorded by poorly exposed volcanic rocks on monteverdi peninsula 79 7 2 5 ma in central and northern alexander island the magmatic rocks can be distinguished by the proportion range and types of lithofacies present and by the periods of magmatism represented the volcanic rocks of the colbert mountains range in age from c 69 62 ma and are dominated by large volume dacitic and rhyolitic crystal rich ignimbrites interpreted as caldera fill deposits elgar uplands sequences range in age from c 55 50 ma and contain approximately equal proportions of pyroclastic deposits and less evolved basaltic andesite and andesite lavas including high mg number andesite lavas near the base of three sequences the volcanic rocks of finlandia foothills probably represent the youngest calc alkaline units on alexander island 48 2 ma the sequence is lithologically similar to the elgar uplands and also contains high mg number andesite lavas but it is dominated by polymict conglomerates with minor lavas which were deposited in a graben associated with regional extension plutonic rocks exposed in the rouen mountains adjacent to the elgar uplands yielded a u pb age of 56 3 ma which is in discordance with a previously published rb sr age 46 3 ma probably due to hydrothermal perturbation of the rb sr system northwards migration of magmatism was caused by the progressive collision and subduction of three ridge segments prior to the previously reported ridge crest trench collisions that occurred c 20 30 ma later and following which subduction ceased

continental erosion and large scale relief a worldwide investigation of continental erosion is carried out by the study of large drainage basins on the basis of hydrological data environmental factors and basin relief distribution inside each basin mean geochemical and mechanical denudation rates are defined a multicorrelation analysis shows that the mechanical denudation rates ds are uncorrelated with environmental factors and correlated with mean basin elevation h while chemical denudation rates dd are insensitive to relief but correlated with mean annual precipitation furthermore two linear relationships between h and ds are detected 1 ds m 10 yr 419 10 6 h m 0 245 with v explained variance 95 1 this law concerns basins related to orogenies younger than 250 ma the negative intercept is interpreted as a continental sedimentation rate of 245 m m y an alternative model in which one invokes a critical elevation separating erosion from sedimentation is equally successful and leads to lower sedimentation rates 60 110 m m y for both models one derives from the slope of the adjustments erosion time constants on the order of 2 5 m y 2 ds m 10 yr 61 10 6 h m with v 86 5 this law concerns basins related to older orogenies the null intercept suggests the lack of continental storage because of the more important dispersion of the data the erosion time constant is calculated separately for each basin it ranges from 15 to 360 m y the tectonic implications of these results are discussed in particular the short time constant 2 5 m y agrees with orogenic uplift rates on the order of 1 mm yr observed in active mountain chains

late cretaceous to early tertiary subduction history of the antarctic peninsula quantitative estimates of the rates and azimuths of phoenix plate convergence with the antarctic peninsula have been derived from plate rotation calculations for two periods in the late cretaceous and early tertiary 83 0 67 7 and 61 1 53 4 ma published marine magnetic anomaly identifications and flow lines interpreted from gravity anomaly maps were used in simple spherical geometry calculations to derive phoenix pacific stage rotations these were combined with published pacific antarctic rotation data to determine contemporaneous phoenix antarctic stage rotations the results indicate a significant change in azimuths of relative motion between the late cretaceous and early tertiary late cretaceous and perhaps earlier oblique subduction probably caused migration of fore arc slivers along the margin resulting in variations in width of the accretionary prism comparison between synthetic magnetic profiles and a 900 km long magnetic profile across ocean floor produced at the antarctic phoenix ridge during the early tertiary establishes the time of a major decrease in spreading rate and hence also in convergence rate as chron c23r 52 3 ma the associated change in subduction dynamics may have caused the dextral transtensional deformation observed in the george vi sound region and initiated uplift of blueschist facies rocks now exposed on smith island the calculated convergence history does not provide a simple explanation for the occurrence of high mg andesite lavas on alexander island which were erupted more than 20 ma before the antarctic phoenix ridge arrived at the margin the existence of these lavas implies either earlier subduction of another ridge or slab break up beneath alexander island

forearc tectonic evolution of the south shetland margin antarctic peninsula the main provinces of the south shetland margin antarctic peninsula characterized on the basis of multichannel seismic long range side scan sonar and swath bathymetry data include from northwest to southeast 1 the oceanic crust of the former phoenix plate flexed down toward the margin and affected by normal faulting in the upper crust 2 a narrow trench with a horizontally layered sediment fill which onlaps the thin sedimentary cover on the oceanic crust and shows incipient deformation near its landward edge 3 an accretionary prism with a complex internal structure the toe of which is overthrust above the youngest trench deposits 4 a midslope forearc basin with an asymmetric synformal structure and 5 the continental shelf which includes two distinct tectonic provinces calculations of the late cenozoic convergence history at the trench indicate a rapid decrease in convergence rate after 6 7 ma from about 60 mm yr resulting from the slowing and eventual cessation of spreading on the antarctic phoenix ridge once spreading had completely ceased 3 5 2 4 ma the convergence rate at the trench equalled the rate of extension in bransfield strait which was probably less than 10 mm yr at first but may have increased since 1 3 ma and this in turn would imply a corresponding increase in convergence rate above the basal detachment the toe of the accretionary prism is composed of a stack of thrust fault bounded wedges laterally and vertically segmented by normal faults most of the trench fill sediments are overthrust by the toe of the accretionary prism and subducted beneath it for as far as they can be traced on the seismic profiles this suggests that there may be tectonic erosion of the forearc since a large part of the trench sediment appears to be derived from erosion on the forearc slope the forearc basin records a long history of subsidence during which the depocenter migrated landward paralleling retrogradational erosion of the distal margin of the basin subduction of the hero fracture zone at approximately the same location since 10 ma may have enhanced tectonic erosion of the accretionary prism in the forearc basin a synform is coincident with a bathymetric reentrant but its axis is oblique to the trends of both the basin and the fracture zone the origin of this synform and the final phase of uplift of the blueschist facies terrane on which smith island stands are probably related to their position above the subducting fracture zone ridge the present elevation of smith island is thought to result from uplift which followed a ridge crest trench collision directly southwest of the hero fracture zone at 5 5 ma the northwestern edge of the mesozoic to early cenozoic arc batholith trending northeast southwest on the shelf is proposed to have acted as a backstop to the accretionary prism during cenozoic subduction

diffusion of ar in rhyolite orthoclase and albite composition glasses the diffusivity of ar has been determined in a natural rhyolite obsidian rhy and in glasses of naalsi3o8 ab and kalsi3o8 or composition at temperatures between 400 and 900 c at pressures from 1150 to 3700 bar the observed variations in the ar diffusion coefficient d with temperature can be described by the arrhenius relationshipd d0exp e rt with the following results for pressures from 1100 to 2500 bar d in cm2 s r in cal mole ab d 1 62 10 3 exp 32780rt or d 6 64 10 3 exp 29000rt rhy d 6 86 10 3 exp 34490rt the data although of limited extent also indicate a slight decrease ind with increasing pressure and the calculated activation volume of 36 8 cm3 mole for ar in rhy at 800 c agrees with the idea that the volume change associated with a diffusive jump is similar to the size of the diffusing species comparison of the ar results with cation tracer diffusivities indicates that ar diffusivity is similar to that of rubidium in rhy and ab argon diffusivity is several orders of magnitude slower than h2o att 900 c and exsolution of h2o to form bubbles might leave a residual rhyolitic melt enriched in ar and kr xe due to kinetic effects noble gases show lower activation energies for diffusion than are observed for tracer diffusion of cations with comparable radii a systematic increase in activation energy for diffusion with increasing size of the noble gas atoms and similarities in activation energies in rhyolitic through basaltic melts suggests that in basaltic melts he ne and probably ar can diffuse faster than co2 the major volatile component in basalts in contrast xe and possibly kr due to their estimated high activation energies may diffuse more slowly than co2 at temperatures below 1200 c this implies that kinetic factors may influence noble gas abundance patterns during separation of a co2 rich vapor phase from degasing basaltic magma during ascent

mafic dike swarms in the south shetland islands volcanic arc unravelling multiepisodic magmatism related to subduction and continental rifting eight groups of mafic dikes and related high level stocks cut triassic accretionary complex and mesozoic magmatic arc formations on livingston island some are affected by silicic sericitic alteration related to cretaceous hydrothermal activity and propylitic epidosite alteration analogous to that in ocean floor sheeted dikes alteration was accompanied by major and trace element metasomatism ar ar analysis of the freshest rocks indicates five intrusive events some of which are unexpectedly young groups 1 3 were intruded in the mid to late cretaceous 108 74 ma and were coeval with the calc alkaline arc between 70 and 50 ma relatively rapid and oblique plate convergence led to strike slip tectonism and a pause in magmatism at 52 ma orthogonal slow convergence resulted in extensional faulting and emplacement of calc alkaline group 2 and primitive tholeiitic dikes groups 4 6 between 51 and 45 ma extension of antarctic peninsula southern south american crust culminated in emplacement of mafic to intermediate medium grained plutons and group c porphyries between 44 and 36 ma localized hydrothermal flow along fault zones resulted in partial to complete argon loss from nearby cretaceous lavas and ar ar reset ages of 40 ma in mid cretaceous hydrothermal k feldspar primitive olivine basalts group d and epithermal carbonate veins 31 29 ma were emplaced during along arc extension accompanying the opening of drake passage and powell basin excess argon occurs in two forms strongly held in melt inclusions in the primitive tholeiites and weakly held in some secondary alteration there is no radiometric evidence in the area studied for magmatism related to late cenozoic subduction nor to the pleistocene recent opening of the back arc bransfield rift

inherited argon in a pleistocene andesite lava 40ar 39ar incremental heating and laser fusion analyses of plagioclase by using 40 ar 39 ar incremental heating and laser fusion techniques xenocrystic plagioclase was discovered in a late pleistocene andesitic lava that erupted through the andean cordillera inherited argon in the xenocrysts is as much as 450 times older than the host lava the age of which is independently known and is an obstacle to dating the lava by using a whole rock sample the xenocrysts are impossible to identify from petrography or chemical parameters such as their k ca ratios holocrystalline groundmass carefully separated from plagioclase and other phenocrysts gives an accurate 40 ar 39 ar age for the lava the xenocrysts could not have been degassed for more than several days in the magma and probably were assimilated from paleozoic rocks buried under kilometers of mesozoic and tertiary arc rocks composing the cordillera in central chile the required magma ascent velocity on the order of kilometers day is extraordinarily high compared to the 10 4 km day minimum implied by the 226 ra excess in continental arc lavas these data permit magma migration and storage in the mantle and lower crust for as much as thousands of years followed abruptly by rapid ascent to the surface

an 40ar 39ar geochronology on a mid eocene igneous event on the barton and weaver peninsulas implications for the dynamic setting of the antarctic peninsula the genesis of basaltic to andesitic lavas mafic dikes and granitoid plutons composing the subaerial cover on the barton and weaver peninsulas antarctica is related to arc formation and subduction processes precise dating of these polar rocks using conventional 40ar 39ar techniques is compromised by the high degree of alteration with loss on ignition as high as 8 in order to minimize the alteration effects we have followed a sample preparation process that includes repeated acid leaching acetone washing and hand picking followed by an overnight bake at 250 c after this procedure groundmass samples can yield accurate age plateaus consisting of 70 100 of the total 39ark released using high resolution heating schedules the different rock types studied on the barton and weaver peninsulas yielded almost coeval ages suggesting a giant igneous event in the weaver and barton peninsulas at 44 5 ma a compilation of newly published ages indicate that this event took place throughout the whole south shetland islands suggesting a dynamic incident occurred at this stage during the arc evolution history we related this igneous event to a mantle delamination mechanism during eocene times the delamination process began at 52 ma and the resultant upwelling of asthenosphere baffled the subduction of phoenix plate causing an abrupt decrease in convergence rate then multiple magmatic sources were triggered resulting in a culminating igneous activity during 50 40 ma with a peak at 45 ma along the archipelago the delamination also caused the extension regime indicated by the dike swarm plugs and sills all over the archipelago and the uplift of smith metamorphic complex and livingston island delamination process may have finished at some time during 40 30 ma leaving a weak igneous activity at that stage and thereafter the convergence rate then recovered gradually as indicated by the magnetic anomaly identifications this model is supported by seismic observation of deep velocity anomalies beneath the antarctic peninsula

potassium argon dating of fine grained basalts with massive ar loss application of the 40ar39ar technique to plagioclase and glass from the kirkpatrick basalt antarctica 40ar39ar incremental heating measurements are reported for separated plagioclase and matrix fractions and a glass sample of the kirkpatrick basalt from victoria land antarctica these results are used to address the limitations of whole rock analyses of glassy or very fine grained basalts the use of plagioclase for determining ages of such rocks the ar behavior of matrix glass and feldspar components the use of k cl ratios in interpretation of step heating results and the age of the kirkpatrick basalt the precise age of the kirkpatrick basalt has been a long standing problem the wide range of reported k ar dates has been attributed to loss of ar despite massive ar loss from the matrix plagioclase is shown to yield good 40ar39ar plateaus plateaus are observed for increments above 700 c and are defined not only by consistent apparent ages but also appropriately low k ca and high k cl ratios cl relations help identify ar released from small amounts of matrix which can seriously contaminate a plagioclase separate an age of 176 8 1 8 ma defines the time of formation of the youngest lava at mt frustum in north victoria land a glass sample from south victoria land gives the same age indicating that terrestrial basaltic glasses as old as jurassic do not inherently lose ar the step heating profile of glass with very fine crystals shows discordance which is an artifact of ar recoil during irradiation

the exhumation history of collision related mineralizing systems in tibet insights from thermal studies of the sharang and yaguila deposits central lhasa abstract the large newly discovered sharang porphyry mo deposit and nearby yaguila skarn pb zn ag mo deposit reside in the central lhasa terrane northern gangdese metallogenic belt tibet multiple mineral chronometers zircon u pb sericite 40 ar 39 ar and zircon and apatite u th he reveal that ore forming porphyritic intrusions experienced rapid cooling 100 c ma during a monotonic magmatic hydrothermal evolution the magmatic hydrothermal ore forming event at sharang lasted 6 0 myr 1 8 myr for cooling from 900 to 350 c and 4 0 myr for cooling from 350 to 200 c whereas cooling was more prolonged during ore formation at yaguila 1 8 myr from 900 to 500 c and a maximum of 16 myr from 900 to 350 c all porphyritic intrusions in the ore district experienced exhumation at a rate of 0 07 0 09 mm yr apatite he ages between 37 and 30 ma combined with previous studies this work implies that uplift of the eastern section of the lhasa terrane expanded from central lhasa 37 30 ma to southern lhasa 15 12 ma at an increasing exhumation rate all available geochronologic data reveal that magmatic hydrothermal exhumation activities in the sharang yaguila ore district occurred within four periods of magmatism with related mineralization significant porphyry type mo mineralization was associated with late cretaceous eocene felsic porphyritic intrusions in the central lhasa terrane resulting from neotethyan oceanic subduction and india asia continental collision

phanerozoic polyorogenic deformation in southern jiuling massif northern south china block constraints from structural analysis and geochronology abstract the structure of the jiuling massif has been investigated in order to delineate the polyorogenic deformation and discuss its geodynamic evolution and orogenic mechanisms detailed structural analysis indicates that the d1 event is characterized by top to the nnw ductile shearing with pervasive foliation and mineral and stretching lineation developed in the entire region compared with the d1 deformation d2 structures are localized in ductile shear zones with subvertical foliation and subhorizontal e w trending lineation indicating a dextral ductile shearing the d3 event marked by folds and thrusts mainly in a brittle domain modified the d1 structures by asymmetrical folds the dominant d4 structures are gravitational folds and normal faults corresponding to a later extension our new geochronological data suggest that the d1 event occurred between 465 and 380 ma with d2 dextral shearing at the end of this early paleozoic orogen and the d3 event has been constrained at 245 215 ma the final uplift of the jiuling massif by the d4 event can be correlated with the late mesozoic extension across the eastern south china block along with previous studies in the south china block the structural pattern of the jiuling massif elucidates the influence of the early paleozoic and early mesozoic intracontinental belts triggered by repeated reactivation of the jiangshan shaoxing fault combined with deformation to the south the early paleozoic belt shows a positive flower pattern with opposing kinematics rooted in the jiangshan shaoxing fault during the early mesozoic a general intracontinental belt was developed with uniform kinematics in both the jiuling massif and the xuefengshan belt possibly resulted from the far field effect of the triassic nw directed paleo pacific subduction

seismotectonics and relative plate motions in the scotia sea region source parameter inversion of p and sh waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the scotia sea region the derived focal mechanisms show both thrust and strike slip faulting along the north scotia ridge and normal and strike slip faulting along the south scotia ridge two thrust faulting mechanisms south of tierra del fuego indicate diffuse convergence of the antarctic plate along the western margin of the scotia plate earthquakes are consistent with active subduction and back arc spreading along the south shetland trench and bransfield strait the depths of two earthquakes 35 km and 55 km and one shallow thrust faulting event suggest continued subduction along the south shetland trench but the underthrusting is largely aseismic due to the young plate age and slow subduction rate the magnitude and depth of normal faulting earthquakes along bransfield strait and the south scotia ridge are suggestive of diffuse extension rather than typical organized mid ocean spreading which is generally associated with smaller and shallower earthquakes eight strike slip and thrust faulting events provide well constrained slip vectors showing scotia plate motion relative to the surrounding larger plates using a four plate model scotia sc south sandwich ss antarctica an and south america sa and a fixed sa an euler vector we inverted these data together with published centroid moment tensor slip vectors along the south sandwich trench and published marine magnetic data from the ss back arc spreading center to derive the first quantitative plate motion model for the scotia region the sc sa euler pole is located in the south atlantic and shows large latitudinal uncertainties the sc an pole is better constrained and located in the weddell sea these results predict left lateral strike slip motion with a component of compression along the north scotia ridge left lateral strike slip with a component of extension along the south scotia ridge and east west compression in drake passage although no rate data have been published connecting the scotia plate with the global plate system euler vector magnitudes are obtained through closure the results though poorly constrained suggest relative motion rates of 0 5 cm yr along the north scotia ridge and 1 0 cm yr along the south scotia ridge and in drake passage

geochronology and geochemistry of the northern scotia sea a revised interpretation of the north and west scotia ridge junction abstract understanding the tectonic evolution of the scotia sea is critical to interpreting how ocean gateways developed during the cenozoic and their influence on ocean circulation patterns and water exchange between the atlantic and southern oceans we examine the geochronology and detrital age history of lithologies from the prominent submerged barker plateau of the north scotia ridge metasedimentary rocks of the north scotia ridge share a strong geological affinity with the fuegian andes and south georgia indicating a common geological history and no direct affinity to the antarctic peninsula the detrital zircon geochronology indicates that deposition was likely to have taken place during the mid late cretaceous a tonalite intrusion from the barker plateau has been dated at 49 6 0 3 ma and indicates that magmatism of the patagonian fuegian batholith continued into the eocene this was coincident with the very early stages of drake passage opening the expansion of the proto scotia sea and reorganization of the fuegian andes the west scotia ridge is an extinct spreading center that shaped the scotia sea and consists of seven spreading segments separated by prominent transform faults spreading was active from 30 6 ma and ceased with activity on the w7 segment at the junction with the north scotia ridge reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the w7 spreading segment is distinct to the other segments of the west scotia ridge basaltic lava samples from the eastern flank of the w7 segment have been dated as early mid cretaceous in age 137 93 ma and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the w7 segment instead the w7 segment is likely to represent a downfaulted block of the north scotia ridge of the fuegian andes continental margin arc or is potentially related to the putative cretaceous central scotia sea

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

metamorphic evolution and geochronology of the dunhuang orogenic belt in the hongliuxia area northwestern china abstract garnet bearing mafic granulites and amphibolites from the hongliuxia area of the southern dunhuang orogenic belt northwestern china commonly occur as lenses or boudinages enclosed within metapelite or marble which represent the block in matrix feature typical of orogenic melange three to four generations of metamorphic mineral assemblages are preserved in these rocks in the high pressure amphibolites prograde mineral assemblages m1 occur as inclusions hornblende plagioclase quartz chlorite epidote ilmenite preserved within garnet porphyroblasts and formed at 550 590 c and 7 7 9 2 kbar based on geothermobarometry the metamorphic peak mineral assemblages m2 are composed of garnet hornblende plagioclase quartz clinopyroxene as well as titanite zircon rutile apatite as accessory minerals in the matrix and are estimated to have formed at 640 720 c and 14 1 16 0 kbar the first retrograde assemblages m3 are characterized by white eye socket symplectites hornblende plagioclase quartz biotite epidote magnetite rimming garnet porphyroblasts which formed at the expense of the garnet rims and adjacent matrix minerals during the decompression stage under p t conditions of 610 630 c and 5 6 11 8 kbar the second retrograde assemblages m4 are intergrowths of actinolite and worm like quartz produced by the breakdown of the matrix hornblendes and formed under p t conditions of 490 c and 2 8 kbar for the high pressure mafic granulites the prograde assemblages m1 are represented by plagioclase quartz preserved within the garnet porphyroblasts the metamorphic peak assemblages m2 are garnet matrix minerals clinopyroxene plagioclase quartz hornblende rutile zircon and were estimated to have formed at 680 c and 15 4 kbar the retrograde assemblages m3 are characterized by fine grained patches of hornblende plagioclase quartz rimming the garnet porphyroblasts as well as hornblende rimming clinopyroxene in the matrix and were inferred to have formed at 620 c and 4 2 kbar for the metapelitic gneiss the metamorphic peak assemblages are the garnet porphyroblasts plus the matrix minerals biotite plagioclase quartz ilmenite zircon which were estimated to have formed at 630 c and 8 9 kbar the mafic granulites and amphibolites record fairly similar clockwise p t paths that include nearly isothermal decompression processes which suggest that they experienced subduction and subsequent rapid tectonic exhumation sims and la icp ms u pb dating of zircons and 40ar 39ar dating of hornblende suggest that the metamorphism occurred at 430 390 ma field occurrences different protolith ages of the mafic granulites and amphibolites and the considerable gap in peak p t conditions between the amphibolite and mafic granulite boudinages and their country rock may suggest a melange accumulation process during the paleozoic caused by the silurian devonian orogeny which is possibly associated with the closure of the liuyuan ocean a branch of the paleo asian ocean near the southern central asian orogenic belt

evolution of the pacific margin of the northern antarctic peninsula an overview the sector of the northern antarctic peninsula between the tula and shackleton fracture zones provides evidence for the subduction of south east pacific oceanic crust under antarctic continental crust during late mesozoic through miocene times the pre subduction depositional history of this sector includes the formation of a marine siliciclastic turbidite wedge permian triassic deposited in a marginal basin setting it was folded and thrust retroarc before the middle jurassic to form the trinity accretion foldbelt which extended for several hundred kilometres along the pacific margin of gondwanaland the foldbelt was deeply eroded and levelled under subaerial conditions then unconformably covered either by middle upper jurassic alluvial to lacustrine deposits in the north or by early cretaceous basic lavas in the south

kinematics and age constraints of deformation in a late carboniferous accretionary complex in western junggar nw china abstract the karamay area situated in the eastern part of western junggar southern altaids contains an ophiolitic melange with ultramafic rocks gabbro basalt chert and limestone which show typical block in matrix structures and coherent turbidites and tuffs these lithological associations are interpreted as incoherent and coherent series formed in an accretionary complex on the basis of detailed field mapping and analyses of the asymmetry of imbricate thrusts duplexes tilted structures shear band cleavages and the nw verging inclined to overturned folds we conclude that the overall movement in the accretionary complex was top to the nw the youngest tuff involved in the deformation contains detrital zircons that have a u pb age la mc icp ms of 308 7 ma 39ar 40ar resistance furnace step heating of amphibole separates from a diorite dike which cuts the folded and imbricated rocks in the accretionary prism yielded a plateau age of 307 2 ma consequently the age of the deformation in the prism is tightly constrained at 307 308 ma implying that the deformation occurred in an extremely short time span during se ward subduction combined contemporaneous occurrence of baogutu adakite high mg sr enriched and y poor dioritic dikes miaoergou charnockite and maliya mafic rocks we further suggest the accretionary complex was cut by near trench volcanic rocks and plutons possibly due to interaction with a spreading ridge

an assessment of 40ar39ar dating for the whole rock volcanic samples from the luzon arc near taiwan from electron microprobe petrographic and image analyses of samples from the northern tip of the luzon arc the effects of various alteration reactions phenocryst and groundmass assemblages on the whole rock 40ar39ar step heating spectra have been delineated the alteration of primary hornblende to an assemblage of zoisite chlorite sphene the palagonitization of glass and the crystallization of phillipsite tends to cause discordant 40ar39ar dates in the low temperature steps of the age spectra discordant 40ar39ar dates in the high temperature steps appear to result from degassing of inherited or trapped 40ar from coarse grained phenocrysts samples in which most of the k resides in groundmass plagioclase tend to yield meaningful plateau dates whereas samples in which most of the k resides primarily in palagonitized glass tend to yield meaningless plateau and isochron dates substantial loss of 39ar 13 40 occurred from irradiated samples prior to 40ar39ar step heating analysis which appears to be correlated with the abundance of groundmass and the degree of alteration this argon loss is attributed to thermally induced argon diffusion from the fine grained groundmass plagioclase glass and alteration phases during the pre step heating bakeout

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

sediment flux from a mountain belt derived by landslide mapping in humid uplands landsliding is the dominant mass wasting process in the western southern alps of new zealand landslides are scale invariant and have a power law magnitude frequency distribution independent studies from other regions suggest that this is a general property of landsliding this observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution it is particularly useful when estimating regional geomorphic rates because it constrains the frequency and overall significance of extreme events which cannot otherwise be evaluated by integrating the complete response of the system we estimate the regional denudation rate due to landsliding to be 9 4 mm yr 1 sediment discharge from the western southern alps is dominated by landslide derived material

new paleomagnetic and 40ar 39ar geochronological results for the south shetland islands west antarctica and their tectonic implications

active margin processes along the antarctic peninsula abstract the antarctic peninsula has a remarkable record of active margin processes which include subduction with progressive ridge trench collisions margin segmentation by major fracture zones rifting in a hybrid back arc and sheared plate margin context fore arc basin development and glacial marine controlled trench fill processes several facets of these active margin processes both of internal crustal dynamic and external origin climate controlled have been documented by a geophysical survey during the antarktis vi 2 cruise of r v polarstem october december 1987 reflection seismic profiles have been shot over the rift basin of bransfield strait over an elongated sediment filled trough interpreted as a fore arc basin over accretional and progradational slopes over recent and ancient trench environments and over the facing oceanic domain in this oceanic domain different fracture zones have highly contrasting morphological and geophysical expressions the subduction of a fracture zone like hero f z characterized by a significant relief possibly related to the presence of buoyant serpentinite ridges may have been a factor of subduction termination for the last segment of the aluk drake plate it may also have played a role in the separation of a blueschist bearing fragment smith island from the base of the accretionary plate margin and in its lift to the surface the magnetic anomaly pattern of the oceanic slabs facing the northwestern peninsula margin shows evidence of an intriguing spreading acceleration which apparently preceded ridge trench collision the same anomaly pattern provides a clue to the stratigraphie interpretation of the oceanic sediment cover and of the frontal part of the prograding now passive margin south of the south shetland island arc an apparently broken and tilted oceanic plate fragment squeezed between the south shetland trench and shackleton fracture zone may argue for the role of transpression associated with the oblique convergence of the antarctic and scotia plates

k ar geochronology of the south shetland islands lesser antarctica apparent lateral migration of jurassic to quaternary island arc volcanism abstract approximately 70 k ar whole rock ages for low k tholeiitic and andesitic volcanic and intrusive rocks from the south shetland islands antarctica including about 50 not previously published are reviewed activity mainly spanned the range 130 to 30 ma jurassic cretaceous to oligocene miocene with a very recent 2 ma more alkaline renewal throughout the main period of activity magmatism or perhaps its cessation migrated continuously northeastwards along the length of the island chain

relief history and denudation evolution of the northern tibet margin constraints from 40ar 39ar and u th he dating and implications for far field effect of rising plateau abstract how does the rising tibetan plateau affect its peripheral region the current understanding of the mechanism of orogenic plateau development is incomplete and thus no consensus yet exists in this regard however our new 40ar 39ar and u th he dataset presented in this study may shed some light on this issue 40ar 39ar dating on two vertical transects from the massif between nuomuhong and golmud indicates that the eastern kunlun range was built up and exhumated during the later triassic initially and a minimum overburden of 11 7 14 0 km has been eroded since 220 ma u th he age elevation relationships aers indicate a rapid exhumation event at 40 ma following a long period of slow exhumation phase from late mesozoic to early eocene time in this study two scenarios one assuming a single stage and the other assuming multiple stages of evolution history are modeled modeling of a multiple stage scenario is reasonable and is able to reflect the actual situation which reveals the entire denudation and relief history of the northern tibet from late mesozoic to the present time after prolonged denudation before 50 ma a low topography 0 17 times the relief of the present developed by 50 ma with an erosion rate of 0 013 0 013 0 025 mm yr the highest relief 1 82 times the relief of the present of the cenozoic time came into being at 40 ma with an erosion rate of 0 052 0 025 mm yr which was possibly a result of the collision between india and eurasia subsequently the relief steadily decreased to the present level due to continued denudation this suggests that deformation propagation from the continued convergence boundary between india and eurasia was insignificant after the construction of the highest relief this observation is broadly consistent with published accounts on the stratigraphic cooling and faulting histories of the northern tibet margin

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

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

geochronology geochemistry and deformation history of late jurassic early cretaceous intrusive rocks in the erguna massif ne china constraints on the late mesozoic tectonic evolution of the mongol okhotsk orogenic belt abstract this paper presents new zircon and sphene u pb ages biotite and hornblende 40 ar 39 ar ages hf isotopic data and geochemical data for five mesozoic plutons in the erguna massif of ne china these data are used to constrain the late mesozoic tectonic evolution of the mongol okhotsk orogenic belt this new dating when combined with previously published ages indicates that the late jurassic early cretaceous j 3 k 1 intrusive rocks can be subdivided into three stages that represent periods of magmatism during the late jurassic 155 ma early early cretaceous 137 ma and late early cretaceous 123 ma in addition the rocks have undergone later deformation recorded by peak ages of 137 and 123 ma the late jurassic and early early cretaceous intrusive rocks in the study area are dominantly syenogranites and are either a type granites or are classified as alkaline series suggesting that they formed in an extensional environment the late early cretaceous intrusive rocks in this area are generally monzogranitic and were emplaced as dikes in an extensional environment along with coeval bimodal volcanics these data combined with the presence of regional unconformities in the northern part of hebei province and western part of liaoning province and the spatial distribution of coeval volcanic rocks in ne china suggest the late jurassic and early early cretaceous magmatisms and the early early cretaceous deformation in this area occurred in an extensional environment related to the delamination of a thickened part of the crust after closure of the mongol okhotsk ocean in comparison the late early cretaceous deformation and magmatism occurred in an extensional environment related to either delamination of the previously thickened crust related to the mongol okhotsk tectonic regime or the subduction of the paleo pacific plate or the combined influence of these two tectonic regimes

analytic solution of the mean elevation of a watershed dominated by fluvial incision and hillslope landslides n active mountainous landscapes dominated by detachment limited fluvial incision and hillslope landslides the scaling relations that characterize both river profiles and planimetric organization of the river network can be linked to provide an analytic solution to describe the mean elevation of a watershed this new analytical approach directly links mountain elevation precipitation bedrock erodibility and uplift rate and yields results that are consistent with those obtained through classic 2 d surface process model when applied to a growing linear mountain range the approach provides an approximate but much faster solution for 2 d or 3 d numerical models of crustal deformation that explicitly include surface processes

numerical modeling of mountain building interplay between erosion law and crustal rheology 1 coupling between erosion and tectonics is thought to play a determinant role in orogenic evolution here we investigate the interplay in this coupling between the assumed erosion law and the crustal rheology at the margin of a collisional plateau like the himalaya of central nepal lithospheric deformation is calculated over a time scale of 100 kyr by a 2d finite element model that incorporates the rheological layering of the crust and the main features of the convergence across the range for the upper boundary condition two surface processes were tested a linear diffusion model and a 1d1 2 integrative model including fluvial incision along the fluvial network and hillslope erosion by landsliding model results and their sensitivity to the chosen combinations of erosion law and crustal properties are discussed in light of the constraining geologic and geomorphologic observations in contrast with the conclusions of cattin and avouac 2000 where a compliant quartz rich crustal rheology and diffusion law were required we combine a composite quartz diabase rheology for the crust with fluvial incision erosion law to account for erosion and elevation profiles across the himalaya of central nepal more generally it is proposed that because of the interplay between the dominant denudation conditions and the rheology of the crust both well documented erosion rates and processes can provide significant constraints on crustal properties within an active orogen

geochemistry of mafic dykes in the antarctic peninsula continental margin batholith a record of arc evolution mafic dykes of the antarctic peninsula continental margin arc are compositionally diverse comprising calc alkaline dominant shoshonite tholeiite and oib like varieties their compositions give information about different mafic magma sources tapped during arc evolution the compositional groups represent partial melts of at least five distinct mantle sources a low nd subduction modified garnet bearing lithospheric mantle older calc alkaline a high nd subduction modified garnet bearing lithospheric mantle shoshonites a high nd subduction modified spinel bearing asthenospheric mantle younger calc alkaline e morb like spinel bearing asthenosphere depleted by a previous melting event tholeiites and within plate non subduction modified garnet and spinel bearing asthenosphere oib like slab derived fluids subducted sediment and arc crust also contributed to the magmas consideration of previous work in the light of our new compositional and geochronological data enables presentation of a summary of arc evolution for most of the cretaceous and tertiary the tectonic regime of the antarctic peninsula arc was transtensional and calc alkaline magmas intruded an oceanic spreading centre collided with the trench during the late cretaceous and induced tectonic changes which caused tapping of different magma sources a pulse of shoshonitic tholeiitic and oib like mafic magmatism resulted three ridge trench collisions are now recognized during the history of the arc in mid late jurassic late cretaceous and early mid tertiary times

thermal history of the giant qulong cu mo deposit gangdese metallogenic belt tibet constraints on magmatic hydrothermal evolution and exhumation abstract a complete thermal history for the qulong porphyry cu mo deposit tibet is presented zircon u pb geochronology indicates that the mineralization at qulong resulted from brecciation veining events associated with the emplacement of a series of intermediate felsic intrusions combined with previously published ages our results reveal a whole intrusive history of the qulong composite pluton causative porphyries were emplaced at 16 0 ma as revealed by 40ar 39ar dating of hydrothermal biotite 15 7 0 2 ma and sericite 15 7 0 2 ma zircon and apatite u th he zhe and ahe dating of qulong revealed that both followed similar monotonic thermal trajectories from 900 c u pb ages 17 5 15 9 ma to 200 c zhe 15 7 14 0 ma and that the causative porphyries experienced faster cooling at a maximum rate of greater than 200 c myr the qulong deposit was exhumed between 13 6 ma and 12 4 ma ahe at an estimated rate of 0 16 0 24 mm y which is consistent with previous estimates for other gangdese miocene porphyry deposits our ahe thermochronology results suggest that neither the gangdese thrust system nor the yadong gulu graben affected or accelerated exhumation at the qulong deposit

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

the effects of remote internal tides on continental slope internal tide generation abstractinternal tide generation at sloping topography is nominally determined by the local slope geometry density stratification and tidal forcing recent global ocean models have revealed that