late oligocene early miocene birth of the taklimakan desert as the world s second largest sand sea and one of the most important dust sources to the global aerosol system the formation of the taklimakan desert marks a major environmental event in central asia during the cenozoic determining when and how the desert formed holds the key to better understanding the tectonic climatic linkage in this critical region however the age of the taklimakan remains controversial with the dominant view being from 3 4 ma to 7 ma based on magnetostratigraphy of sedimentary sequences within and along the margins of the desert in this study we applied radioisotopic methods to precisely date a volcanic tuff preserved in the stratigraphy we constrained the initial desertification to be late oligocene to early miocene between 26 7 ma and 22 6 ma we suggest that the taklimakan desert was formed as a response to a combination of widespread regional aridification and increased erosion in the surrounding mountain fronts both of which are closely linked to the tectonic uplift of the tibetan pamir plateau and tian shan which had reached a climatically sensitive threshold at this time

dating the quaternary progress in luminescence dating of sediments abstract luminescence dating comprises a collection of numerical age techniques that are among the most significant chronological tools currently used in quaternary research this paper briefly reviews the key historical developments in luminescence dating from its roots in thermoluminescence dating of heated minerals to the development of optical dating methods for sunlight exposed sediments we describe the principles and practicalities of the various techniques commonly used in luminescence dating including multiple aliquot single aliquot and single grain procedures and we discuss some of the latest approaches to recognising and minimising potential errors in age estimation e g by means of component analysis and dose distribution methods in optical dating the overview also introduces the other papers in this special issue of quaternary science reviews which address a selection of important issues in basic research technique development and application of luminescence dating to critical questions in the geological and archaeological sciences illustrated with examples from around the world and the last million years of earth history

the paleoproterozoic magmatic metamorphic events and cover sediments of the tiekelik belt and their tectonic implications for the southern margin of the tarim craton northwestern china abstract the tiekelik belt in the southwestern part of the tarim craton nw china consists of archean paleoproterozoic orthogneisses and granitoids and younger sedimentary successions u pb dating of paleoproterozoic rocks in the tiekelik belt has provided new information on the southwestern margin of the tarim craton two orthogneisses from the heluositan complex yielded crystallization ages in the interval 2310 2260 ma and metamorphic ages of 2040 2007 ma and 1830 ma respectively two syenogranites from the buweituwei pluton yielded crystallization ages of 1900 and 1795 ma detrital zircon analyses from sedimentary rocks nonconformably overlying the buweituwei and akazi plutons indicate maximum depositional ages between 1950 ma and 1780 ma major detrital inputs are from the basement in the region and form prominent peaks at 1900 1800 ma and 2300 2200 ma our new data combined with previous results allow us to summarize the archean paleoproterozoic tectonic evolution of the tiekelik belt as follows 1 emplacement of archean aged ttg magmas during the 3 14 2 76 ga interval 2 a series of 2 41 2 26 ga magmatic events that represent an important period of crustal reworking within the tarim craton 3 2 03 1 80 ga magmatic and metamorphic events that include the emplacement of a 1 9 ga mafic dyke suite into the heluositan complex along with several episodes of intraplate magmatism the late magmatic events are coeval with metamorphism and may support an orogenic extensional model for the tiekelik belt during the 1 9 1 8 ga interval 4 post 1 8 ga subsidence and formation of a changcheng sequence in the tiekelik belt the paleoproterozoic ages of major tectonothermal events within the tiekelik north altyn dunhuang block and alashan block are almost identical and it is argued that they were connected at that time as north china and tarim were part of the larger columbia supercontinent

late quaternary arroyo formation and climate change in the american southwest arroyos entrenched ephemeral streams that form in desert environments first appeared in the arid and semiarid american southwest after 8000 14 c yr b p for at least 7 k y prior to that time climate vegetation and groundwater conditions were not conducive for arroyo formation along the floors of desert valleys after a hiatus in arroyo formation the frequency of arroyo cutting and filling increased dramatically after 4000 14 c yr b p the early holocene arroyos and increased frequency of arroyo incision after 4000 14 c yr b p are related to the establishment and changes in postglacial vegetation climate and groundwater conditions as a result arroyo sequences preserve a record of large scale climate change and small scale climatic perturbations that occurred during the holocene human modification of valley flood plains is an additional factor that contributed to mid nineteenth and early twentieth century arroyo cutting

valley fill alluviation during the little ice age ca a d 1400 1880 paria river basin and southern colorado plateau united states valley fill alluvium deposited from ca a d 1400 to 1880 is widespread in tributaries of the paria river and is largely coincident with the little ice age epoch of global climate variability previous work showed that alluvium of this age is a mappable stratigraphic unit in many of the larger alluvial valleys of the southern colorado plateau the alluvium is bounded by two disconformities resulting from prehistoric and historic arroyo cutting at ca a d 1200 1400 and 1860 1910 respectively the fill forms a terrace in the axial valleys of major through flowing streams this terrace and underlying deposits are continuous and interfinger with sediment in numerous small tributary valleys that head at the base of hillslopes of sparsely vegetated weakly consolidated bedrock suggesting that eroded bedrock was an important source of alluvium along with in channel and other sources paleoclimatic and high resolution paleoflood studies indicate that valley fill alluviation occurred during a long term decrease in the frequency of large destructive floods aggradation of the valleys ended about a d 1880 if not two decades earlier with the beginning of historic arroyo cutting this shift from deposition to valley entrenchment near the close of the little ice age generally coincided with the beginning of an episode of the largest floods in the preceding 400 500 yr which was probably caused by an increased recurrence and intensity of flood producing el nino events beginning at ca a d 1870

building the pamirs the view from the underside the pamir mountains are an outstanding example of extreme crustal shortening during continental collision that may have been accommodated by formation of a thick crust much thicker than is currently thought and or by continental subduction we pre sent new petrologic data and radiometric ages from xenoliths in miocene volcanic rocks in the southeastern pamir mountains that suggest that gondwanan igneous and sedimentary assemblages were underthrust northward buried to 50 80 km during the ear ly stage of the india asia collision and then heated and partly melted during subsequent thermal relaxation before finally being blasted to the surface these xenoliths the deepest crustal samples recovered from under any active collisional belt provide direct evidence for early cenozoic thickening of the pamirs and lower crustal melting during collision the xenoliths also suggest that the present mountain range was a steady state elevated plateau for most of the cenozoic

the influence of paleofloods on archaeological settlement patterns during a d 1050 1170 along the colorado river in the grand canyon arizona usa abstract excavations at four archaeological sites consisting of the material remains of a d 1050 1170 era farmers along the colorado river in grand canyon national park yield geomorphic information used to address questions related to settlement patterns archaeological excavation units test pits feature fill and natural exposures contain sediments used to interpret geomorphic history that can in turn shed light on archaeological site selection in a challenging environment the grand canyon experiences dramatic geomorphic events such as catastrophic floods and destructive debris flows that are preserved in the stratigraphic record and can be used to understand cultural landscape interactions by combining new geomorphic stratigraphic and archaeological data collected during recent excavations with results from previous geomorphic and sediment transport studies observed trends can be interpreted regarding the possible influence of paleofloods on past settlement patterns for example at each of the four sites reconstructed paleoflood elevations from existing hec ras virtual shorelines flood recurrence intervals site layout and site stratigraphy geomorphic setting suggests a temporal trend in site location the two early sites early pueblo ii period a d 1050 1080 contain habitation features located above the approximately 6 8 year high flood 3500 cubic meters per second cms recurrence interval larger floods 4800 to 5900 cms of a longer recurrence interval between 40 and 80 years inundate these features the two later sites in the sample late pueblo ii a d 1080 1170 contain habitation features located well above the 40 80 year recurrence high flows we suggest that early farmers early pueblo ii period a d 1050 1080 may not have had adequate experience with flood magnitudes and frequencies and therefore their habitation structures were located in risk prone areas relatively close to the river later habitations late pueblo ii a d 1080 1170 were positioned in more protected areas further from the river perhaps reflecting an acquired knowledge of river dynamics these trends although currently based on a limited data set provide insights into site selection decisions and settlement patterns of early farmers along the colorado river through grand canyon

exhumational history of the north central pamir 1 the pamir plateau forms a prominent tectonic salient along the western end of the tibet tarim margin despite its tectonic significance relatively little is known about the timing of major cenozoic tectonic events in the pamir here we present new apatite and zircon u th he ages bulk rock geochemistry and al in hornblende barometry results from the karakul graben a prominent north south oriented rift basin located 50 km south of the main pamir thrust although cooling ages do not record the onset of extension graben bounding normal faults provide exposures of otherwise slowly eroding rocks which record two cenozoic thermal events existing geochronology and new results suggest that granitic rocks in the karakul region were shallowly emplaced cooled very quickly through 300 c and have experienced less than 10 km of exhumation since the late triassic a long period of relatively slow exhumation throughout much of the late mesozoic and cenozoic was punctuated by two periods of accelerated exhumation during the middle eocene 50 40 ma and early miocene 25 16 ma we interpret the first period of accelerated exhumation as a result of tectonic uplift and subsequent erosion due to the northward propagation of the india asia collision we attribute the second period of rapid exhumation to a renewed phase of tectonism and plateau uplift in the pamir perhaps related to a break off event along the down going indian plate at 25 ma or to the onset of slip along the nascent karakoram fault

the rivers of civilization the hydromorphic regimes that underpinned old world river based civilizations are reviewed in light of recent research notable holocene climatic changes varied from region to region whilst the dynamics of floodplain environments were equally diverse with river channel changes significantly affecting human settlement there were longer term trends in holocene hydroclimate and multi centennial length flood rich and flood poor episodes these impacted on five identified flooding and settlement scenarios i alluvial fans and aprons ii laterally mobile rivers iii rivers with well developed levees and flood basins iv river systems characterised by avulsions and floodouts and v large river fed wetlands this gave a range of changes that were either more or less regular or incremental from year to year and thus potentially manageable or catastrophic the latter might be sudden during a flood event or a few seasons acute or over longer periods extending over many decades or even centuries chronic the geomorphic and environmental impacts of these events on riparian societies were very often irreversible contrasts are made between allogenic and autogenic mechanism for imposing environmental stress on riverine communities and a distinction is made between channel avulsion and contraction responses floods droughts and river channel changes can precondition as well as trigger environmental crises and societal collapse the nile system currently offers the best set of independently dated holocene fluvial and archaeological records and the contrasted effects of changing hydromorphological regimes on floodwater farming are examined the persistence of civilizations depended essentially on the societies that maintained them but they were also understandably resilient in some environments pharaonic egypt in the egyptian nile appear to have had more limited windows of opportunity in others the kerma kingdom in the nubian nile or required settlement mobility or exceptional engineering response huang he mesopotamia to accommodate problems such as river avulsion desiccation or local salinization

the ascent of felsic magmas in dykes abstract the results of a thermal and fluid dynamical analysis are presented that allow geologists to estimate the critical dyke or fracture width w c necessary for the transport of felsic magma through the crust without control by solidification the principal geological variables required to estimate w c are the magmatic viscosity the density and temperature contrasts and t between magma and country rock and the height of ascent h the results show that for typical values of and and for transport distances of 20 to 30 km the critical dyke width is constrained to lie between about 2 and 20 m

anabranching rivers their cause character and classification anabranching rivers consist of multiple channels separated by vegetated semi permanent alluvial islands excised from existing floodplain or formed by within channel or deltaic accretion these rivers occupy a wide range of environments from low to high energy however their existence has never been adequately explained they occur concurrently with other types of channel pattern although specific requirements include a flood dominated flow regime and banks that are resistant to erosion with some systems characterized by mechanisms to block or constrict channels thereby triggering avulsion the fundamental advantage of an anabranching river is that by constructing a semi permanent system of multiple channels it can concentrate stream flow and maximize bed sediment transport work per unit area of the bed under conditions where there is little or no opportunity to increase gradient on the basis of stream energy sediment size and morphological characteristics six types of anabranching river are recognized types 1 3 are lower energy and types 4 6 are higher energy systems type 1 are cohesive sediment rivers commonly termed anastomosing with low w d ratio channels that exhibit little or no lateral migration they are divisible into three subtypes based on vegetative and sedimentary environment type 2 are sand dominated island forming rivers and type 3 are mixed load laterally active meandering rivers type 4 are sand dominated ridge forming rivers characterized by long parallel channel dividing ridges type 5 are gravel dominated laterally active systems that interface between meandering and braiding in mountainous regions type 6 are gravel dominated stable systems that occur as non migrating channels in small relatively steep basins anabranching rivers represent a relatively uncommon but widespread and distinctive group that because of particular sedimentary energy gradient and other hydraulic conditions operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges

climatic geomorphic and archaeological implications of a late quaternary alluvial chronology for the lower salt river arizona usa abstract recent archaeological excavations along the lower salt river arizona resulted in the unexpected discovery of buried late pleistocene soils and cultural features dating 5800 7100 cal ybp early archaic the latter representing the earliest evidence of human activity in the lower salt river floodplain thus far identified because the lower salt river floodplain has been heavily impacted by recent agriculture and urbanization and contains few stratigraphic exposures our understanding of the river s geological history is limited here we present a late quaternary alluvial chronology for a segment of the lower salt river based on 19 accelerator mass spectrometry 14 c and four optically stimulated luminescence ages obtained during two previous geoarchaeological investigations deposits are organized into allostratigraphic units and reveal a buried late pleistocene terrace inset into middle to late pleistocene terrace deposits holocene terrace fill deposits unconformably cap the late pleistocene terrace tread in the site area and the lower portion of this fill contains the early archaic archaeological features channel entrenchment and widening 900 cal ybp eroded much of the older terrace deposits leaving only a remnant of fill containing the buried latest pleistocene and middle to late holocene deposits preserved in the site area subsequent overbank deposition and channel filling associated with a braided channel system resulted in the burial of the site by a thin layer of flood sediments our study confirms that the lower salt river is a complex mosaic of late quaternary alluvium formed through vertical and lateral accretion with isolated patches of buried soils preserved through channel avulsion although channel avulsion is linked to changes in sediment load and discharge and may have climatic linkages intrinsic geomorphic and local base level controls limit direct correlations of lower salt river stratigraphy to other large rivers in the north american southwest

direct stratigraphic dating of india asia collision onset at the selandian middle paleocene 59 1 ma the collision of india with asia had a profound influence on cenozoic topography oceanography climate and faunal turnover however estimates of the time of the initial collision when indian continental crust arrived at the transhimalayan trench remain highly controversial here we use radiolarian and nannofossil biostratigraphy coupled with detrital zircon geochronology to constrain firmly the time when asian derived detritus was first deposited onto india in the classical sangdanlin section of the central himalaya which preserves the best paleocene stratigraphic record of the distal edge of the indian continental rise deep sea turbidites of quartzarenite composition and indian provenance are replaced upsection by turbidites of volcano plutoniclastic composition and asian provenance this sharp transition occurs above abyssal cherts yielding radiolaria of paleogene radiolarian zones rp 4 6 and below abyssal cherts containing radiolaria of zone rp6 and calcareous shales with nannofossils of the paleocene calcareous nannofossil zone cnp 7 constraining the age of collision onset to within the middle paleocene selandian the youngest u pb ages yielded by detrital zircons in the oldest asia derived turbidites indicate a maximum depositional age of 58 1 0 9 ma collision onset is thus mutually constrained by biostratigraphy and detrital zircon chronostratigraphy as 59 1 ma this age is both more accurate and more precise than those previously obtained from the stratigraphic record of the northwestern himalaya and suggests that within the resolution power of current methods the india asia initial collision took place quasi synchronously in the western and central himalaya

channel instability in a braided sand bed river the gila river of central arizona is representative of braided sand bed rivers in alluvial valleys that have inherent unstable behavior and destructive channel migration the 112 year record of channel conditions along a portion of the gila river provides data for the construction of locational probability maps for main flow channels zones of stability and hazardous instability alternate with each other at 3 2 km 2 mi intervals during the past century the overall sinuosity of the main flow channel has remained close to 1 18 despite numerous changes in actual location spatial and temporal variation of sinuosity have occurred in subreaches as a result of sedimentation behind a dam and fluctuations in the density of phreatophyte growth which both affect the hydraulics of flood flows unstable zones of the channel correspond to the surface of the sediment wedge behind the dam and areas dense phreatophyte growth stable zones correspond to areas controlled by bedrock or man made structures as well as locations determined by these external factors plus the requirement to maintain a consistent sinuosity channels such as that of the gila river do not meet most assumptions of equilibrium and are best understood through probabilistic approaches with an assumption of catastrophic adjustment

paleocene initial indentation and early growth of the pamir as recorded in the western tarim basin abstract it is presently unclear as to when and how the pamir were uplifted to their current elevation because a cenozoic sedimentary record is lacking on the pamir plateau and the pamir domes only record metamorphism associated with crustal shortening since 37 ma studies of the tectonic and geomorphological evolution of the pamir in the early stages of the india asia continental collision are also lacking the western tarim basin at the northeastern margin of the pamir provides an ideal opportunity to investigate their formation and evolution four cretaceous paleogene stratigraphic sections were examined in this study from these we infer that the west kunlun mountains were topographic highs during the mesozoic that divided the adjacent central pamir tianshuihai terrane from the tarim basin this paleo topographic configuration lasted until a marine transgression occurred in the aksai chin and central pamir areas in the latest cretaceous provenance analysis indicates that sediments in the western tarim basin were derived solely from the west kunlun mountains until the paleocene the appearance of pamir derived detrital zircons in paleocene and eocene strata of the tarim basin suggests that the pamir was experiencing uplift and reached an elevation similar to or even higher than the west kunlun mountains at that time the sandstone composition documents a provenance change from a stable continental block to an active thrust belt of the proto west kunlun mountains indicating that the western tarim basin evolved from an intraplate to a foreland basin setting during the paleocene alluvial fan conglomerates were deposited along the northeastern margin of the pamir and a thick gypsum unit began to be deposited in the tarim basin all these lines of evidence indicate that the north verging thrusting foreland basin subsidence uplift and northward indentation of the pamir began in the paleocene 62 58 ma

cenozoic evolution of the eastern pamir implications for strain accommodation mechanisms at the western end of the himalayan tibetan orogen detailed field mapping geochronologic and thermochronologic analyses and petrologic investigations conducted along the southern segment of the late cenozoic kongur shan extensional system provide new information on the cenozoic tectonic evolution of the eastern pamir at the western end of the himalayan tibetan orogen field relations and cooling age patterns in the hanging wall and footwall of the active faults show a southward decrease in the magnitude of east west extension along the southern kongur shan extensional system from 20 km or less along the muztaghata massif in the north to l3 km along the tashkorgan fault in the south these results in conjunction with previously published work on the northern segment of the kongur shan extensional system show a general southward decrease in east west extension along the entire length of the extensional system consistent with models of extension primarily driven by oroclinal bending or radial thrusting of the pamir petrologic data 40 ar 39 ar cooling ages and monazite th pb ages from schists and gneisses in the footwall of the southern kongur shan normal fault along the muztaghata massif record two tectonic events that immediately preceded late miocene initiation of east west extension 1 high grade schists and gneisses experienced upper amphibolite facies metamorphic conditions 9 10 kbar 700 750 c dated as late oligocene to middle miocene by in situ ion microprobe analyses of monazite inclusions in garnet and 2 high grade schists and gneisses were subsequently rapidly exhumed to shallow crustal levels in the late miocene with 40 ar 39 ar biotite cooling ages of 7 5 9 ma rapid exhumation was accommodated in part by the east west striking south dipping shenti normal fault field relations and regional geologic correlations indicate that this exhumation event was related to the formation of the central pamir gneiss domes and the antiformal muztaghata massif is the eastward continuation of the sares dome of the central pamir these observations suggest that the antiformal gneiss domes of the central pamir have not been offset across the karakorum right slip fault from the qiangtang anticlinorium in tibet instead we propose that the development of the central pamir gneiss domes may have been related to oligocene miocene northward underthrusting and thickening of crust beneath the pamir

methods of determining the rheological properties of magmas at sub liquidus temperatures non vesicular basaltic melts behave as newtonian fluids at temperatures above their liquidus and their viscosities can be calculated using a method developed by shaw 1972 and bottinga and weill 1972 because many igneous processes involve the flow of silicates at sub liquidus temperatures numerous attempts have been made to calculate the interactive effects of the crystal phase however current methods are appropriate only for relatively low crystal concentrations and they assume newtonian behaviour we argue that this assumption is invalid when bubble or crystal concentrations exceed 30 at higher concentrations factors other than concentration become increasingly important these include particle shape and size distribution a method which takes these factors into account is tested on a range of suspensions including magmas at sub liquidus temperatures and rheological properties calculated using this method agree closely with the measured values we also demonstrate that an equation which was introduced to explain large differences in measured apparent viscosities during the cooling and crystallisation of mount st helens dacite murase et al 1985 and which is currently used to calculate the rheological properties of crystallising lavas generates viscosities which may be in error by several orders of magnitude this difference is argued to be caused by a combination of factors including the ten orders of magnitude range in the strain rates utilised during the mount st helens measurements causing orders of magnitude difference in the resulting apparent viscosities rheological data on crystallising silicic melts are reinterpreted taking into account the non newtonian rheology of the magmas and changes in activation energy of flow during crystallisation

tectono metamorphic evolution of the karakorum metamorphic complex dassu askole area ne pakistan exhumation of mid crustal ht mp gneisses in a convergent context the south karakorum margin east of the himalayan syntaxis consist of an e w elongated zone of young 10 3 ma high grade metamorphic rocks m2 and related migmatitic domes this late tectono metamorphic event post dates the palaeogene 55 37 ma phase of thickening of the belt featured by nw se structures and associated m1 amphibolite facies metamorphism 0 7 gpa 700 c this m2 metamorphism is characterised by low pressure high temperature conditions coeval with migmatite formation in response to a thermal increase of c 150 c compared to m1 culminating at a temperature of c 770 c and a pressure of 0 5 0 6 gpa rapid exhumation of migmatitic domes at a rate of 5 mm yr 1 was accommodated by vertical extrusion in the core of e w crustal scale folds these crustal scale folds formed in response to n s syn collisional shortening and were enhanced by thermal weakening of the migmatised continental crust m2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid crustal level such relationships between a mantle related magmatism and a high temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted indian slab since 20 ma

thermal and mechanical aspects of magma emplacement in giant dike swarms we consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts for driving pressure gradients inferred for giant dike swarms thicknesses of 10 m would enable dikes to transport magma laterally over the distances observed in the field up to thousands of kilometers without suffering thermal lock up using time dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions we investigate the possibility that the observed dike thicknesses of the order of 100 m result from thermal erosion of the country rocks during dike emplacement this implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure it is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts the presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement in this case measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width which implies that thicker dikes may have less tendency to produce consistent fabric alignment our results suggest that if the dike was propagating downslope off a plume related topographic swell the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse implosion of the shallow magma plumbing system feeding the intrusion radial dikes that erupted at the periphery of the topographic uplift might have increased rather than decreased extensional stresses in the crust within the topographic uplift upon their solidification

geoarchaeological dating of holocene stream terraces along the san pedro river southeastern arizona usa three pleistocene and four holocene floodplain terraces were differentiated along the san pedro river in southeastern arizona during a geological mapping project conducted by the arizona geological survey we used archaeological chronometric data to refine age estimates for the holocene surfaces that had been initially based on only geomorphic and pedologic criteria eight archaeological sites deeply buried in the fill of the most extensive holocene terrace map unit qy2r yielded 10 radiocarbon dates and temporally diagnostic san pedro complex 3150 1900 cal bp artifacts twenty sites recorded on the surface of qy2r terrace treads contained numerous temporally diagnostic hohokam 1950 500 cal bp ceramic sherds and features the age constraints provided by these archaeological materials suggest that the qy2r terrace fill was deposited primarily between 4500 and 1000 cal bp with only a thin veneer of flood deposits added between 1000 and 100 cal bp this late holocene to historical 4500 100 cal bp age estimate is consistent with other alluvial histories in the greater gila river drainage basin and supplants our initial geologically based latest holocene to historical 2000 cal bp qy2r age estimate the archaeologically derived maximum age estimate for qy2r also provides a minimum age for the topographically higher and older qy1r terrace prompting us to revise our original early to late holocene age estimate for qy1r to an early to middle holocene age range of 11 500 4500 cal bp the post a d 1850 historical age we originally proposed for the qy3r terrace deposits inset into qy2r however is consistent with the presence of four historical period surface sites and a concordant absence of surface and buried prehistoric sites

the effects of shape on crystal settling and on the rheology of magmas the effects of the shape of phenocrysts on their settling velocity and on the rheology of magmas are discussed spherical crystals fall at a velocity given by stokes law and need to be present in large concentrations before they have a marked effect on the viscosity of a suspension elongate crystals fall at a much smaller velocity which may be determined from the formulas and graphs given herein such elongate crystals can have a dramatic effect on the viscosity of a suspension even at low volume fractions suspended crystals and bubbles will not impart a yield stress to the magma unless they form a touching network across the entire suspension it is suggested therefore that most previous inferences of a yield stress in subliquidus magmas are erroneous measurements of the subliquidus rheology of magmas need to be accompanied by careful characterization of the microstructure and distribution of the solid phase on which such rheologies critically depend

universal and local controls of avulsions in southeast texas rivers this study addresses the relative importance of universal and local factors in river avulsions from the perspective of the configurational complexity of the fluvial system as it relates to the potential for avulsions because local factors cannot be addressed without a specific context several rivers of the southeast texas coastal plain brazos navasota trinity neches and sabine rivers are studied based on field observations and surveys digital elevation models and geographical information system gis analysis avulsions are influenced by a combination of universal factors relevant to any alluvial river and local factors at least partly contingent on the environmental setting and history of the study rivers the universal controls are factors that create conditions under which avulsions can occur channel aggradation banktop discharge and cross valley slope advantages a rate of channel aggradation greater than that of floodplain accretion is an important setup factor but superelevation channel bed elevation greater than or equal to floodplain elevation behind the natural levee is not required slope advantages are necessary but not sufficient for avulsions to occur local factors include abandoned channels on the floodplain and basins or depressions associated with higher discharge pleistocene conditions the local controls also include deflection factors that divert flow from the main channel during high discharges and levee weaknesses relationships among the universal and local factors were represented as directed graphs and the configurational complexity determined using connectance entropy results show that the relative importance of universal and local factors varies with the scale or scope of analysis in the broadest context universal factors contribute more than 75 of the connectance entropy signifying the importance of identifying the key setup factors within a given aggradation or valley filling context however local factors account for more than 80 of the connectance entropy indicating that knowledge of the trigger factors and local floodplain morphology is the key to understanding and predicting avulsions this study suggests a three stage process for predicting avulsions first the universal factors can be used to identify reaches with the potential to avulse second superelevation a sufficient but not necessary condition can be used to identify specific locations where avulsions are imminent and as a necessary but not sufficient condition the absence of cross valley slope advantages can be used to identify locations where avulsions cannot occur the final stage predicting avulsions at other locations within systems or reaches where the setup factors are present requires specific consideration of local factors

from static to dynamic provenance analysis sedimentary petrology upgraded abstract the classical approach to sandstone petrology established in the golden years of plate tectonics and based on the axiom that detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes has represented a benchmark for decades the composition of sand and sandstone however simply provides us with a distorted image of the lithological structure of source terranes and gives us little clue whether they are allochthonous or autochthonous orogenic or anorogenic young or old what we may able to see reflected in detrital modes is the nature of source terranes continental arc oceanic and the tectonostratigraphic level reached by erosion in space and time the proposed new approach to the petrology of sand and sandstone 1 starts with a simple classification scheme circulated since the 1960s which is purely descriptive objective and free of ill defined ambiguous terms and 2 focuses on the nature and tectonostratigraphic level of source terranes further steps are essential to upgrade provenance analysis acquiring knowledge from modern settings is needed to properly identify and wherever possible correct for physical and chemical processes introducing environmental and diagenetic bias and thus address nature s complexities with adequate conceptual tools equally important is the integration of multiple techniques ideally including bulk sediment multi mineral and single mineral methods bulk sediment petrography remains the fundamental approach that allows us to capture the most precious source of direct provenance information represented by the mineralogy and texture of rock fragments bulk sediment geochemistry applicable also to silt and clay carried in suspension is a superior method to check for hydraulic sorting chemical weathering and fertility of detrital minerals in different sediment sources detrital geochronology thermochronology and isotope geochemistry reveal the diverse time structures of source rocks and have become necessary complementary techniques in modern provenance analysis inferences on geodynamic processes need independent geological information and come last but if tackled properly they can lead us much farther than the standard label obtained by using triangular diagrams uncritically as if they were infallible oracles

cenozoic thermo tectonic evolution of the northeastern pamir revealed by zircon and apatite fission track thermochronology abstract the northeastern pamir is a key location to explore asian intracontinental tectonic processes during the cenozoic new zircon fission track zft data show a 20 to 50 km wide region of partially reset ages on the northeastern margin of the pamir salient interpreted as an exhumed and tilted partial annealing zone paz widespread zft age peaks at 50 ma within the zft paz likely date early motion of the kashgar yecheng transfer system kyts but suggest this fault system was narrower in the early cenozoic than it is today apatite fission track aft ages of 10 6 ma combined with field observations across the kyts hint at an episode of strong thrusting related exhumational cooling which indicates that the modern fault system probably formed at this time to the southwest of the kyts the combination of new fission track and existing thermochronology data allows establishing temperature time trajectories that present diachronous rapid cooling from 450 to 120 c in the sares 13 10 ma muztagata 10 7 ma and kongur shan 3 1 ma domes rapid cooling in the eastern sares and southern muztagata massifs is driven by doming as supported by kinematic analyses of the shen ti fault successive rapid cooling of these massifs confirms eastward propagation of doming processes shortly postdating magma emplacement at 11 ma we propose that the synchronicity of regional tectonism magmatism and metamorphism implies that strong crustal thickening and contraction occurred beneath the northeastern pamir during the middle late miocene possibly associated with initial collision between the pamir and tian shan

an island arc origin of plagiogranites at oytag western kunlun orogen northwest china shrimp zircon u pb chronology elemental and sr nd hf isotopic geochemistry and paleozoic tectonic implications abstract this paper presents detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope geochemistry of three intrusive plagiogranite bodies formed in an island arc setting within the oytag ophiolite suite of the oytag kuda suture zone shrimp zircon u pb dating of two of these intrusions shows that they were emplaced into the oytag volcanic rocks at 337 5 4 1 ma and 327 7 4 9 ma and that the younger intrusion contains inherited zircons ranging from 435 3 to 480 8 ma both intrusions are composed of low al trondhjemite and tonalite the rocks have low k 2 o and sr and high y contents with low sr y ratios they have relatively low rare earth element ree concentrations and are depleted in light ree lree and relatively enriched in heavy ree hree with marked negative eu anomalies in the chondrite normalized ree patterns they have initial 87 sr 86 sr ratios of 0 7048 to 0 7068 and e nd t values of 6 2 to 7 6 which are lower than those of typical morb in situ hf isotopic compositions of zircons with early carboniferous age from both intrusions are characterized by positive initial e hf values ranging from 12 5 to 19 5 similar to typical morb the inherited ordovician zircons in the younger intrusion have initial e hf values of 1 9 to 3 1 similar to zircons from ordovician granitoids in the region detailed elemental and isotopic data suggest that the oytag plagiogranites were formed in an oceanic island arc setting by differentiation of arc tholeiitic magmas that formed in response to the intra oceanic subduction the inherited zircons are interpreted as the xenocrysts captured from the incipient oceanic crust that contains relicts of discrete fragments of continental crust with ordovician zircons through which the plagiogranite was emplaced the oytag ophiolite suite might be the remnants of an ancient island arc the basement of which was the incipient oceanic crust the oytag kuda suture zone is suggested to be a multiple suture zone that represents the remnants of both the proto tethyan and paleo tethyan oceans the western kunlun orogen was therefore not a long lived active continental margin during the early and late paleozoic times it most likely underwent at least two orogenic cycles and thus might represent a multiple orogenic belt

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

age and origin of high ba sr appinite granites at the northwestern margin of the tibet plateau implications for early paleozoic tectonic evolution of the western kunlun orogenic belt abstract the buya appinite granite is a typical high ba sr granite emplaced at the northern west kunlun orogenic belt along the northwestern margin of the tibetan plateau the granite is dated at ca 430 ma using the shrimp u pb zircon method it consists of alkaline feldspar granites with coeval appinite enclaves the granite possesses high sio2 69 77 72 69 k2o 4 44 5 10 and total alkalinity k2o na2o 8 80 9 92 sr 655 1100 ppm ba 1036 1433 ppm and lree and low hree and hfse contents and insignificant negative eu anomalies consequently the samples have very high sr y 74 141 and la yb n 37 96 ratios on the other hand they have low mgo or mg cr and ni contents and low radiogenic nd isotopes nd t 8 4 to 10 4 the high ba sr and other geochemical signatures of the granite also appear in the appinite enclaves except that the appinite enclaves have relatively higher abundances in these elements and higher nd t values 5 7 to 6 7 elemental and isotope compositions suggest that the appinites were derived from partial melting of an enriched lithospheric mantle source probably induced by upwelling of the asthenosphere due to the delamination of a subducted slab the granite was likely derived from partial melting of the mafic lower crust with residual garnet associated with involvement of minor lile enriched appinitic magma followed by crystal fractionation of hornblende biotite apatite and allanite in combination with previous investigations on the evolution of the western kunlun we suggest that the buya high ba sr plutons represent the end of an early paleozoic crust thickening event after a terrane accretion on southern tarim craton and the beginning of a post orogenic collapse phase in the paleozoic west kunlun orogenic belt

correcting temporal frequency distributions for taphonomic bias in this paper we expand upon a prior study surovell t a brantingham p j 2007 a note on the use of temporal frequency distributions in studies of prehistoric demography journal of archaeological science 34 1868 1877 that explored the problem of taphonomic bias taphonomic bias refers to the tendency for younger things to be over represented relative to older things in the archaeological record due to the operation of destructive processes like erosion and weathering using a database of radiocarbon dated volcanic deposits from bryson r u bryson r a ruter a 2006 a calibrated radiocarbon database of late quaternary volcanic eruptions earth discussions 1 123 124 we develop an empirical model of taphonomic bias in contrast to our prior study in which we modeled taphonomic bias as an exponential function wherein the likelihood of site loss remains constant through time we argue that the probability of site destruction actually decreases with site age we further demonstrate how this model can be used to correct temporal frequency distributions and extract demographic histories we illustrate this approach using databases of radiocarbon dates from rockshelter and open air sites in the bighorn basin wyoming usa and mammoths and humans in siberia russia

provenance analysis of the mesozoic hoh xil songpan ganzi turbidites in northern tibet implications for the tectonic evolution of the eastern paleo tethys ocean 1 mesozoic strata of the hoh xil songpan ganzi complex in northern tibet are exposed in a vast 370 000 km2 triangle shaped orogenic belt bound by the longmen shan thrust belt in the east the kunlun terrane and north china block in the north and the qiangtang terrane and yidun arc in the south these strata consist of middle upper triassic submarine fan and deep marine facies rocks that were deposited in the paleo tethys ocean late triassic early jurassic contractional deformation in the eastern hoh xil songpan ganzi complex marks the demise of the paleo tethys ocean basin and the accretion of the gondwana derived qiangtang terrane to eurasia we conducted geological mapping regional stratigraphic analyses and u pb geochronology of detrital zircons n 4128 on the mesozoic sequences exposed in the hoh xil songpan ganzi complex kunlun terrane and qiangtang terrane we identify for the first time marine silciclastic sandstone and shale of jurassic age in the northwestern hoh xil songpan ganzi complex that unconformably overlie upper triassic turbidites zircon age data indicate that the middle upper triassic marine gravity flow deposits of the hoh xil songpan ganzi complex were shed from the north and south china blocks and middle late triassic ultrahigh pressure qinling dabie orogenic belt as well as the kunlun and qiangtang terranes in addition the detrital zircon results suggest vast sediment source to sink distances 1500 km for the middle upper triassic hoh xil songpan ganzi strata which is consistent with tectonic models for the paleo tethys ocean basin that incorporate significant components of horizontal tectonic transport like opening of large back arc basins in response to oceanic slab rollback

origin of middle triassic high k calc alkaline granitoids and their potassic microgranular enclaves from the western kunlun orogen northwest china a record of the closure of paleo tethys abstract the western kunlun orogen the northwest tibet plateau occupies a key tectonic position at the junction between the pan asia and the tethyan tectonic domains it is related to subduction of proto and paleo tethys and terrane collision from early paleozoic to early mesozoic this paper presents the first detailed shrimp zircon u pb chronology mineral chemistry major and trace element and sr nd hf isotope geochemistry of three middle triassic plutons yuqikapa muztaga and taer and their microgranular enclaves in the western kunlun orogen shrimp zircon u pb dating shows that the yuqikapa pluton was emplaced in the anisian 243 ma and the muztaga and taer plutons that contain microgranular enclaves were emplaced in the ladinian 234 227 ma the yuqikapa pluton is composed of peraluminous high k calc alkaline two mica granites monzogranite syenogranite alkali feldspar granite with initial 87sr 86sr ratios of 0 7071 0 7077 end t of 4 4 to 6 3 and ehf t in situ zircon of 3 3 elemental and isotopic data suggest that they were formed by wet anatexis of the precambrian metasedimentary and metaigneous rocks in the continental collision overthrust zone the muztaga and taer plutons consist of metaluminous to weakly peraluminous high k calc alkaline granitoids granodiorite monzogranite with initial 87sr 86sr ratios of 0 7052 0 7104 end t of 3 6 to 7 3 and ehf t in situ zircon of 3 1 to 1 0 elemental and isotopic data suggest that they were formed by partial melting of the precambrian metasedimentary and metaigneous rocks in the normal lower crust

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

observations on the origins and ascent mechanisms of granitic magmas this review presents a summary of most current beliefs about the origins of granitic magmas together with calculations of some important properties of granitic melts and magmas turning attention to magma ascent mechanisms diapirism and dyking represent two competing end member models the differences in processes and products between the two are fundamental and the implications of the differences are profound for the evolution of the continental crust the weight of evidence favours dyke ascent for the majority of granitic magmas and modelling shows that dyke ascent is both rapid and thermally effcient furthermore the structural and petrological features expected in crust through which granitic diapirs have passed have not been discovered in nature granites in the upper crust commonly have forms and structures indicating that diapirism could not have been the emplacement mechanism in contrast granitic dykes are common suggesting that it is reasonable to consider the ascent of granitic magmas through fracture systems many granitic batholiths are tabular and some have been fed by large dykes the rate limit in granitic plutonism is not magma ascent but either the rate of melting and segregation near the magma source or the rate of deformation at the emplacement site

buried paleoindian age landscapes in stream valleys of the central plains usa abstract a systematic study of late quaternary landscape evolution in the central plains documented widespread deeply buried paleosols that represent paleoindian age landscapes in terrace fills of large streams 5th order in alluvial fans and in draws in areas of western kansas with a thick loess mantle alluvial stratigraphic sections were investigated along a steep bio climatic gradient extending from the moist subhumid forest prairie border of the east central plains to the dry subhumid and semi arid shortgrass prairie of the west central plains radiocarbon ages indicate that most large streams were characterized by slow aggradation accompanied by cumulic soil development from ca 11 500 to 10 000 14c yr b p in the valleys of some large streams such as the ninnescah and saline rivers these processes continued into the early holocene the soil stratigraphic record in the draws of western kansas indicates slow aggradation punctuated by episodes of landscape stability and pedogenesis beginning as early as ca 13 300 14c yr b p and spanning the pleistocene holocene boundary the development record of alluvial fans in western kansas is similar to the record in the draws slow aggradation was punctuated by multiple episodes of soil development between ca 13 000 and 9000 14c yr b p in eastern kansas and nebraska development of alluvial fans was common during the early and middle holocene but evidence shows fan development as early as ca 11 300 14c yr b p buried soils dating between ca 12 600 and 9000 14c yr b p were documented in fans throughout the region in stream valleys across the central plains rapid alluviation after ca 9000 14c yr b p resulted in deeply buried soils that may harbor paleoindian cultural deposits hence the paucity of recorded stratified paleoindian sites in the central plains is probably related to poor visibility i e deep burial in alluvial deposits instead of limited human occupation in the region during the terminal pleistocene and early holocene the thick dark cumulic a horizons of soils representing buried paleoindian age landscapes are targets for future archaeological surveys

the fluvial record of climate change fluvial landforms and sediments can be used to reconstruct past hydrological conditions over different time scales once allowance has been made for tectonic base level and human complications field stratigraphic evidence is explored here at three time scales the later pleistocene the holocene and the historical and instrumental period new data from a range of field studies demonstrate that croll milankovitch forcing dansgaard oeschger and heinrich events enhanced monsoon circulation millennial to centennial scale climate variability within the holocene probably associated with solar forcing and deep ocean circulation and flood event variability in recent centuries can all be discerned in the fluvial record although very significant advances have been made in river system and climate change research in recent years the potential of fluvial palaeohydrology has yet to be fully realized to the detriment of climatology public health resource management and river engineering

formation of braided river floodplains waimakariri river new zealand floodplains along the braided gravel bed waimakariri river are discontinuous although generally extensive landforms composed predominantly of gravel bars capped with vertically accreted fines in order of importance three mechanisms lead to floodplain formation river bed abandonment by lateral migration of the braid train initiates the formation of the largest floodpiains which usually occur downstream of tributary fans and valley bedrock spurs in the headwater reaches localized river bed aggradation during high magnitude events leads to floodplain formation by producing a surface resistant to erosion by lesser events due to either the coarse nature of the deposit or by its elevation over the river bed or both the least important mechanism initiating floodplain development is localized river or channel incision a six stage model is proposed for the sequential development of floodplains on the waimakariri river from i active river bed ii stabilizing bar iii incipient floodplain iv established floodplain v mature floodplain and to vi terrace two mechanisms lateral migration by the braid train and reactivation of abandoned channels within floodplains operating separately or in combination are responsible for floodplain reworking and their relatively young age 250 years clearly braided rivers can construct substantial areas of well developed floodplain

oceanic style subduction controls late cenozoic deformation of the northern pamir orogen abstract the northern part of the pamir orogen is the preeminent example of an active intracontinental subduction zone in the early stages of continent continent collision such zones are the least understood type of plate boundaries because modern examples are few and of limited access and ancient analogs have been extensively overprinted by subsequent tectonic and erosion processes in the pamir it has been assumed that most of the plate convergence was accommodated by overthrusting along the plate bounding main pamir thrust mpt which forms the principal northern mountain and deformation front of the pamir however the synopsis of our new and previously published thermochronologic data from this region shows that the hanging wall of the mpt experienced relatively minor amounts of late cenozoic exhumation the pamir orogen as a whole is an integral part of the overriding plate in a subduction system while the remnant basin to the north constitutes the downgoing plate with the bulk of the convergence accommodated by underthrusting herein we demonstrate that the observed deformation of the upper and lower plates within the pamir alai convergence zone resembles highly arcuate oceanic subduction systems characterized by slab rollback subduction erosion subduction accretion and marginal slab tear faults we suggest that the curvature of the north pamir is genetically linked to the short width and rollback of the south dipping alai slab northward motion indentation of the pamir is accommodated by crustal processes related to this rollback the onset of south dipping subduction is tentatively linked to intense pamir contraction following break off of the north dipping indian slab beneath the karakoram

contrasting channel response to floods on the middle gila river arizona floods of january and february 1993 in arizona resulted in the most dramatic channel widening on the middle gila river since 1905 an earlier flood in october 1983 had a larger instantaneous discharge but resulted in little channel change the 1993 flood was of greater volume and duration factors important in destabilizing flood plain vegetation and eroding bank material the 1983 flood was produced by a dissipating eastern pacific tropical storm whereas the 1993 flood was produced by a series of cold fronts from the northern pacific ocean supplied with subtropical moisture from a split jet stream meridional global circulation patterns enhance the frequency of winter storms that produce sustained flooding in arizona and are more likely to result in channel widening and flood plain instability on main trunk streams like the gila river

mesozoic tectonics of the gondwanan terranes of the pamir plateau abstract the central and southern pamir terranes represent gondwanan crustal fragments within the pamir plateau and are bounded by the tanymas rushan pshart and wakhan tbz suture zones from north to south stratigraphic and geochronologic data suggest that collision of the central and southern pamir terranes with the southern margin of asia was broadly coeval during the latest triassic earliest jurassic cimmerian orogeny 200 ma the cimmerian orogeny in the pamir region does not appear to have resulted in significant crustal thickening of the central and southern pamir as both terranes are overlain by lower to middle jurassic carbonate platform deposits post suturing latest triassic earliest jurassic sandstones with a strong arc signature in the southern pamir suggest derivation from the karakul mazar terrane to the north implying the northern pamir were a region of elevated topography relative to the gondwanan terranes immediately after closure of the palaeotethys and rushan ocean basins a key question in the post cimmerian tectonic evolution of the pamir is the extent and magnitude of mesozoic deformation of the central and southern pamir three lines of evidence suggest that much of the post jurassic crustal shortening documented in the central and southern pamir is related to mesozoic tectonic activity rather than the cenozoic india asia collision as generally interpreted 1 the southern pamir does not record post jurassic marine deposition indicating the terrane elevated above sea level by the early cretaceous most likely due to crustal shortening and thickening 2 coarse terrigenous deposits distributed throughout the central and southern pamir interpreted to be syn tectonic with regional folding and thrust faulting are poorly dated with many studies interpreting them to be jurassic to cretaceous rather than paleogene 3 strong cretaceous crustal thickening and shortening documented to the south in the karakoram terrane and to the north in the northern pamir suggest that the central and southern pamir were similarly affected these relationships suggest that most of the upper crustal shortening documented in the central and southern pamir is related to retroarc deformation north of the jurassic cretaceous andean margin that developed within the karakoram and southern pamir terranes during subduction of the neotethys ocean thus the pamir plateau may have been a westward continuation of the pre cenozoic lhasa plano of the tibetan plateau

the cimmerian geopuzzle new data from south pamir based on its permian triassic stratigraphic and biotic evolution we correlate the se pamir to the karakoram terrane and we consider them equivalent along with the central pamir to the qiantang terrane of tibet all of palaeozoic gondwanan ancestry we prove the occurrence of a marked cimmerian unconformity documented by lowermost jurassic deposits suturing intensively faulted and folded permian and triassic units which suggests that the south pamir collided around the t j boundary with the central pamir along the rushan pshart suture collision of the karakoram to the south pamir happened slightly later along the wakhan tirich boundary zone progressive time shifting of deformation can be related to the complex setting of the cimmerian belt which was subdivided into minor blocks by incipient oceanic basins providing strong crustal mobility terra nova 25 352 360 2013

the north karakorum side of the central asia geopuzzle an italian geological team visited a remote and in part never studied area in the northern hunza region pakistan which represents the link between the karakorum and pamir ranges the north karakorum sequence commences in the permian with terrigenous sediments followed by shallow to deep marine carbonates deposited on a newly formed passive margin deep water sedimentation continued till the end of the middle triassic when carbonate platform conditions resumed an episode of deltaic red sandstones with orogenic provenance is interbedded in the liassic and it is transgressed by a middle to upper jurassic shallow water marine unit eventually all of the sequence was faulted and folded with weak metamorphic imprint before fluviatile red polygenic conglomerates sealed the succession in a spectacular unconformity the north karakorum provides an example of a microplate that rifted away from gondwana in the permian reached deep marine conditions in the early triassic and marginally recorded compressive movements in the liassic a subsequent orogenic episode points to a reorganization of the southern asian margin possibly around middle cretaceous time finally the north karakorum was affected by strong fold thrust deformation and low to very low grade metamorphism in the cainozoic related to the india asia collision

basin analysis of the jurassic lower cretaceous southwest tarim basin northwest china the jurassic through cretaceous southwest tarim basin northwest china contains more than 6 km of fluvial and lacustrine strata deposited in a foreland setting during the successive collisions with eurasia of the changtang block during late triassic early jurassic time and with the mega lhasa block during late jurassic early cretaceous time this tectonism is chronologically linked with the creation of a narrow lower middle jurassic transtensional basin with thick sedimentary infill succeeded by a broader upper jurassic lower cretaceous compressional basin with thinner sedimentary infill the older basin formed between a north northwest striking dextral fault on the eastern side of the southwest tarim basin in the tian shan and a postulated strike slip or normal fault on the western margin of the basin along the kunlun shan the former fault is now the talas ferghana fault the latter may be a predecessor to the main pamir thrust subsidence analysis of the thickest sedimentary section suggests thermal subsidence interpreted as the result of transtension between the two basin bounding faults the younger basin extends farther east and west and does not preserve evidence of activity along the talas ferghana fault the change in basin style between these two episodes of basin development likely reflects either a small counterclockwise rotation of basin bounding structures during the first episode or a small clockwise rotation of the maximum compressive stress between the two episodes

temporal frequency distributions of alluvium in the american southwest taphonomic paleohydraulic and demographic implications abstract the use of radiocarbon frequency distributions to reconstruct prehistoric human and animal populations must account for taphonomic loss and other factors that affect the archaeological and paleontological records surovell et al jas 36 1715 1724 have recently proposed a correction for taphonomic bias that is based on the radiocarbon frequency of a global sample of volcanic deposits analysis of 717 radiocarbon dates sampled from the alluvium of the san pedro and santa cruz rivers and their tributaries in southeastern arizona shows that discovery and scientific biases also play an important role in the creation of radiocarbon frequency distributions and that the rate of taphonomic bias in prehistory is not predicted by the radiocarbon frequency of volcanic deposits the latter principle is further argued using a sample of 123 pliocene to clovis age proboscideans from the san pedro valley we propose an alternative model that is based on the nature of the stratigraphic record with discovery bias scientific bias taphonomic loss and the shape of the calibration curve all operating to influence the temporal frequency distribution of radiocarbon dated prehistoric phenomena

application of geoarchaeological evolutionary models for the interpretation of complex archaeological structures in the central ebro basin spain in landscapes that are extremely sensitive to small environmental changes like some mediterranean areas archaeological contexts could present some problems to their interpretation and evolutionary reconstruction this is the case of los pedregales archaeological site located in the hoya de huesca ne of spain where several stone mounds of unknown age and function were found in an extensive badland the geomorphological edaphological and archaeological study together with radiocarbon and osl datings and contextualization into the geomorphological regional framework made it possible to interpret these structures and to establish an evolutionary model it was determined that they were silos excavated in pleistocene sands and silts unit 1 during the visigothic and early muslim epochs 6th 7th centuries and that they were later filled with limestone blocks successive aggradational and degradational stages units 2 3 and 4 during mca medieval climatic anomaly and lia little ice age periods covered and incised the area leaving the shape of positive structures included in the badland

flood related channel change in an arid region river a review of 112 years of change in the channel of the salt river central arizona u s a shows that this arid region river has a main flow channel that has migrated laterally up to 1 6 km 1 mi in response to floor events maps showing locational probabilities indicate that along the channel zones of relative locational stability alternate with zones of relative instability at a 3 2 km 2 mi interval construction of upstream reservoirs has reduced sediment input into the main river but has not controlled floods the channel width has not changed except for moderate fluctuations around mean values the main flow channel has incised approximately 6 m 20 ft over most of the 48 km 30 mi study reach during six recent floods gradient has remained unchanged during floods bed material was mobilized to a depth below the original bed level that was greater than the height of the water surface above the original bed calculations based on tractive force indicate a threshold discharge of instability that is equal to the flow with a five year return interval the river exhibits remarkable stability with respect to gradient and sinuosity irrespective of water and sediment discharges but horizontal channel location exhibited selective instability over the record period of more than a century the channel appears not to have been in equilibrium considering geometry discharge and sediment

cenozoic evolution of the pamir plateau based on stratigraphy zircon provenance and stable isotopes of foreland basin sediments at oytag wuyitake in the tarim basin west china abstract the pamir salient is the western expression of mountain growth related to indo eurasian convergence though a rough framework has emerged describing the tectonic evolution of the pamir detailed knowledge of the spatial and temporal evolution of cenozoic deformation is necessary to determine how strain progressed through the orogenic belt here we present new stratigraphic zircon provenance and stable isotope data from jurassic to miocene strata along the pamir s northeastern margin near the town of oytag wuyitake in the tarim basin west china prominent 40 ma peaks in oligocene to early miocene detrital zircon grains record the erosion of an eocene belt of shoshonitic rocks in the central to southeastern pamir this is roughly coincident with an 4 shift in the oxygen isotopic composition 18o of carbonates during the eocene and or oligocene from an average of 8 7 to 12 6 suggesting a reorganization of atmospheric circulation during that time this could have been caused by uplift of tarim basin bounding ranges and or retreat of the paratethys sea a subsequent change from eocene to jurassic aged detrital zircon grains in the early to middle miocene indicates provenance shifted from source rocks in the central and or se pamir to the hanging wall of the main pamir thrust mpt coincident with prograding facies at that time this suggests deformation progressed outward toward the northeast margin of the pamir plateau in the early to middle miocene our results corroborate outward advancement of himalayan deformation affecting all margins of the tarim basin by the middle miocene

a classification of metamorphic grains in sands based on their composition and grade abstract an operational classification of metamorphic grains in sands and sandstones is proposed with the aim of enhancing data reproducibility among operators and the potential of high resolution bulk petrography in provenance studies for each of four protolith compositions metapelite metapsammite metafelsite metacarbonate metabasite six archetype grains displaying increasing degree of recrystallization and foliation development are illustrated such a classification grid is specifically devised as a subsidiary tool for point counting with the gazzi dickinson method traditional qfr parameters can also be easily recalculated from the data set obtained thus meeting all possible needs decker and helmold 1985 suttner and basu 1985 an experiment shows that usage of visual comparison standards effectively minimizes operator variation and allows retrieval of crucial information during point counting in a reproducible way a petrogenetic grid is presented as a subsidiary tool for classifying grains that include index minerals and to help correlation with dense mineral data the metamorphic index mi is introduced as an estimator of average metamorphic grade of source rocks our classification an extension of concepts used first in the study of arc continent collision in taiwan dorsey 1988 and successfully expanded to interpret the evolution of continent continent collision in the himalayas najman and garzanti 2000 white et al 2002 proves to be fruitful in provenance analysis of foreland basin sediments shed from alpine type thick skinned collision orogens particularly when integrated with dense mineral geochemical and geochronological data

early evolution of the pamir deep crust from lu hf and u pb geochronology and garnet thermometry determining early orogenic processes within the pamir tibet orogen represents a critical step toward constructing a comprehensive model on the tectonic evolution of the region here we investigate the timing and cause of prograde metamorphism of cenozoic metamorphic rocks from the pamir plateau through lu hf geochronology u pb rutile thermochronology and garnet thermometry regional prograde metamorphism and heating to 750 830 c as constrained by thermometry occurred between 37 and 27 ma prograde growth of garnet first occurred in the south pamir and spread to the central pamir during the following 10 m y the early metamorphism is attributed to high mantle heat flow following the ca 45 ma break off of the indian slab south of the pamir our investigation confirms a long lived thermal history of the pamir deep crust before the miocene and provides a causal link between break off enhanced mantle heat flow and prograde heating of the subduction hanging wall

millennial scale dynamics of valley fills over the past 12 000 14c yr in northeastern new mexico usa we studied the alluvial histories of eight small 2 watersheds in the uplands of northeastern new mexico the data come from radiocarbon dated sections exposed in the banks of arroyos and permanent channel streams results show that nine periods of valley aggradation separated by incision episodes occurred over the past 12 000 14 c yr these millennial scale cycles occurred in rough synchrony within different drainages parts of some cycles coincided with well known climatic fluctuations for example aggradation occurred during the younger dryas chronozone 11 000 10 000 14 c yr b p and valley fills persisted in incised states during the early holocene peak in milanko vitch summer insolation incision occurred early in the medieval warm period ca a d 1000 1300 aggradation during the little ice age a d 1300 1880 and incision during the last century changes in precipitation tied to the strength of the north american monsoon system probably drive these cycles of aggradation and incision when the system is strong more frequent summer thunderstorms cause increased flooding in these small watersheds wetter summers over periods of decades to centuries allow forest vegetation to spread which reduces sediment input from hillslopes at the same time floods are eroding valley fills aggradation of valley fills probably occurs when summer thunderstorms are less frequent and large floods are correspondingly rare prolonged summer drought reduces forest cover which increases erosion from slopes and causes sediment to accumulate in valleys in turn the strength of the north american monsoon system is probably controlled by fluctuating sea surface temperatures the geomorphic cycles we describe may reflect a previously unrecognized millennial scale climate oscillation that is important in shaping the landscapes of the southern great plains

on the relationship between dike width and magma viscosity on the basis of measurements of bulk rock and phenocryst chemical compositions phenocryst content and estimation of water content and magma temperature the magma viscosity of 44 dikes from oki dozen and tango southwest japan and ocros peruvian andes was estimated to examine a relationship between viscosity and dike width the results show that magma viscosity increases with dike width mafic magmas with viscosities of 101 102 pa s form dikes l m wide while felsic magmas with viscosities of 106 107 pa s form dikes 100 m wide this indicates that magma viscosity as well as magma driving pressure and host rock stiffness are important factors in explaining the variation of dike width because magma viscosity is a measure of internal friction the magmatic force required to widen the fracture during hydrofracturing is proportional to viscosity on the basis of the range of chemical composition of the studied samples dike widths formed by highly viscous felsic magma should be of the order of 100 m maximum furthermore the observed basaltic dike width variations from various tectonic settings indicate that the effective factor on driving pressure of dike magma may be not magma pressure but compressive stress on the other hand dikes derived from low viscosity basaltic magma and exceeding 100 m width may be formed by extremely high magma pressure associated with flood basalt volcanism this is supported by 1 the observation that a rifting event with decreased compressive stress was not always associated with the volcanism and 2 the inferred effusion rates for this type of volcanism are anomalously high suggesting increased magma pressure in addition the active magma generation and the decreased host rock stiffness by locally anomalous thermal conditions like a hotspot phenomenon may assist flood basalt volcanism or emplacement of anomalously wide mafic dikes

zircon u pb and hf isotopic constraints on petrogenesis of the cretaceous tertiary granites in eastern karakoram and ladakh india abstract multiple subduction events occurred preceding the collision of greater india with the karakoram terrane that consumed the neotethyan ocean between early cretaceous and eocene time zircon u pb dating and hf isotope analyses of granitic rocks collected from the karakoram terrane and ladakh batholith have been conducted to constrain these geological processes undeformed ladakh batholith granodiorite yielded zircon u pb ages of 68 and 64 ma with ehf t values of 6 4 and 10 3 and three diorites near leh and darbuk gave zircon u pb ages of 50 4 51 0 ma and ehf t values of 7 4 to 8 9 indicating that the paleocene eocene was the most important period of granitic magmatism that derived from a juvenile source three tonalite granodiorite enclaves at skyangpoche within the karakoram batholith have zircon u pb ages of 101 103 ma with ehf t values of 2 3 to 3 7 indicating generation of granitic rocks by partial melting of the karakoram basement three samples of the miocene karakoram batholith from around darbuk yielded ages of 16 6 19 1 ma with ehf t values of 8 5 and 10 5 that indicate an input of ancient crust from the indian plate granodiorite within the karakoram metasedimentary sequence zircon u pb age 74 ma ehf t of 5 4 from the tangtse gorge and granite around muglib zircon u pb age of 63 and 56 ma ehf t of 6 8 and 6 0 indicate similar geochronological and isotopic characteristics to those of the ladakh batholith thus the time resolved hf isotopic compositions of magmatic zircons from the granites in the karakoram and ladakh batholith show a large variation that indicate their derivation from both ancient and juvenile continental crusts in active continental margin since at least the mid cretaceous

exhumation history of the west kunlun mountains northwestern tibet evidence for a long lived rejuvenated orogen abstract how and when the northwestern tibetan plateau originated and developed upon pre existing crustal and topographic features is not well understood to address this question we present an integrated analysis of detrital zircon u pb and fission track double dating of cenozoic synorogenic sediments from the kekeya and sanju sections in the southwestern tarim basin these data help establishing a new chronostratigraphic framework for the sanju section and confirm a recent revision of the chronostratigraphy at kekeya detrital zircon fission track ages present prominent triassic early jurassic 250 170 ma and early cretaceous 130 100 ma static age peaks and paleocene early miocene 60 21 ma to eocene late miocene 39 7 ma moving age peaks representing source exhumation triassic early jurassic static peak ages document unroofing of the kunlun terrane probably related to the subduction of paleotethys oceanic lithosphere in combination with the occurrence of synorogenic sediments on both flanks of the kunlun terrane these data suggest that an ancient west kunlun range had emerged above sea level by triassic early jurassic times early cretaceous fission track peak ages are interpreted to document exhumation related to thrusting along the tam karaul fault kinematically correlated to the main pamir thrust further west widespread middle late mesozoic crustal shortening and thickening likely enhanced the early mesozoic topography paleocene early eocene fission track peak ages are presumably partially reset limited regional exhumation indicates that the early cenozoic topographic and crustal pattern of the west kunlun may be largely preserved from the middle late mesozoic the main pamir tam karaul thrust belt could be a first order tectonic feature bounding the northwestern margin of the middle late mesozoic to early cenozoic tibetan plateau toward the tarim basin late oligocene early miocene steady exhumation at a rate of 0 9 km myr is likely related to initial thrusting of the tiklik fault and reactivation of the tam karaul thrust thrusting together with upper crustal shortening in the mountain front indicates basinward expansion of the west kunlun orogen at this time this episode of exhumation and uplift associated with magmatism across western tibet is compatible with a double sided lithospheric wedge model primarily driven by breakoff of the indian crustal slab accelerated exhumation of the mountain front at a rate of 1 1 km myr since 15 ma supports active compressional deformation at the margins of the northwestern tibetan plateau we thus propose that the west kunlun mountains are a long lived topographic unit dating back to triassic early jurassic times and have experienced middle late mesozoic to early cenozoic rejuvenation and late oligocene miocene expansion

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

dynamics of magmatic intrusions in the upper crust theory and applications to laccoliths on earth and the moon 1 to understand the dynamics of shallow magmatic intrusions i propose a theoretical model of magma spreading laterally below an elastic crust nondimensionalization of the flow equation leads to the identification of characteristic scales for the intrusion while the characteristic intrusion length is controlled by the elastic response of the crust its characteristic thickness primarily depends on magma properties and injection rate three spreading regimes are identified and characterized by different morphologies as well as by scaling laws for thickness versus length and thickness versus time the first spreading regime is controlled by the elastic response of the crust and the shape of the flow is self similar when the time length and thickness become larger than an elastic time length and thickness scale the edges of the flow become steeper and the system transitions to a gravity current regime when the intrusion is thick enough to accommodate the pressure head the flow enters a regime of lateral propagation and keeps a constant thickness the intrusion shape in the elastic regime fits the observed shape of terrestrial laccoliths the elastic scaling law for intrusion thickness versus length fits observations of laccoliths at elba island italy and provides for a physical explanation for the observed relationship between length and thickness on terrestrial laccoliths laccoliths are predicted to form over short time scales depending on magma viscosity that vary between approximately a month to several years for felsic magmas on earth on the moon several elongated low slope domes have recently been identified as possibly formed by laccolith intrusions at depth although they are much larger than terrestrial laccoliths because the moon has a smaller gravity than on the earth a deeper magma source and a more mafic magma composition than for terrestrial laccoliths implying smaller pressure gradient and dike width lunar intrusions have a larger characteristic length and a smaller characteristic thickness after nondimensionalization the morphologies length versus thickness of terrestrial and inferred lunar laccoliths follow the same curve and are well fitted by the elastic scaling law this model thus explains the size discrepancy between terrestrial laccoliths and lunar low slope domes therefore low slope domes identified on the moon are good candidates for laccolith type intrusions at depth

global warming and fluvial geomorphology future global warming has a number of implications for fluvial geomorphology because of changes in such phenomena as rates of evapotranspiration precipitation characteristics plant distributions plant stomatal closure sea levels glacier and permafrost melting and human responses potential changes in rivers are outlined in this review in the context of changes in the intensity of rainfall the activity of tropical cyclones runoff response including that of europe dry lands and high latitude environments and geomorphological reactions including rates of soil erosion in general however much work remains to be done to establish the full range of geomorphological responses that may take place in fluvial systems

moisture variability in the southwestern united states linked to abrupt glacial climate change the atmospheric response to millennial scale circulation changes in the north atlantic ocean during the last glacial period has been difficult to constrain cave deposits from southwestern north america reveal that atmospheric moisture in this region increased in response to slowdowns of the atlantic meridional overturning circulation

impact of drainage integration on basin geomorphology and landform evolution a case study along the salt and verde rivers sonoran desert usa abstract the salt and verde rivers sonoran desert usa integrated multiple endorheic extensional basins near the start of the quaternary integration began via spillover on both rivers spillover resulted in headward erosion in upstream basins leading to basin wide incision and excavation of large volumes of sediment transported into downstream basins as a result the verde river aggraded to what is now the highest terrace in the lower verde river valley lvrv the lousley hills terrace evidence suggests deposition of the lousley hills terrace fill led to aggradation and backfilling of adjacent pediments ancestral salt river deposits asrd of the combined salt and verde rivers deposited an extensive alluvial fan in the higley basin raising local base level for over 2 million years eventually this led to aggradational piracy of the salt river 0 46 ma that integrated the higley and luke basins consequently this shortened and steepened the salt river resulting in 30 m of aggradation downstream and headward incision upstream headward incision abandoned the asrd and created the sawik stream terrace piedmont pediment and alluvial fan systems that were formerly adjusted to the asrd incised in response since abandonment the asrd floodplain has accumulated a sheet of silt and fine sand from aeolian and local fluvial processes sedimentary metamorphic and granitic pediments that developed in slowly aggrading endorheic basins display evidence of response to base level adjustments resulting from drainage integration processes classic ballena landforms eroding alluvial fans began to form in the lvrv only after drainage integration providing the first known maximum age for the ballena form drainage integration of the salt and verde rivers clearly demonstrates the impact of base level fluctuations on basin scale geomorphology however integration led to very different geomorphic responses in different extensional basins revealing the difficulty of a one size fits all conceptual model of geomorphic response drainage integration

multiple mantle sources during island arc magmatism u pb and hf isotopic evidence from the kohistan arc complex pakistan this paper presents high precision u pb ages and initial hf isotopic compositions of zircon from mafic to felsic rocks of the kohistan arc complex pakistan three magmatic pulses tapping geochemically different reservoirs are distinguished partial melting of mantle with morb type isotopic characteristics generated 99 92 ma old magmas plutonism around 85 ma tapped a more fertile mantle source most likely consisting of a 600 ma old metasomatically enriched mantle or of mantle contaminated by an old sedimentary component 82 ma old felsic peraluminous dykes have morb type isotopic compositions considered to be inherited from remelting earlier magmas in the deep base of the arc the isotopic results demonstrate several and rather rapid changes in melt source region during arc development they also show that there was subordinate continental influence and negligible importance of slab components for the hf budget during the generation of the kohistan arc complex

magmatic intrusions and deglaciation at mid latitude in the northern plains of mars abstract on earth igneous rocks are generally affected by magmatic differentiation and the volume of intrusive bodies is considered to be 5 10 times larger than the volume of volcanic rocks in contrast in the case of mars the occurrence of olivine phyric basalts at gusev crater mcsween h y 2006 j geophys res planets 111 e10 e02s10 the primitive nature of at least four picritic shergottites grott m 2013 planet space sci 174 49 111 and the chemical composition of major volcanic provinces baratoux d toplis m j monnereau m gasnault o 2011 nature 472 338 341 suggest that primary melts of the mantle may have commonly reached the surface these observations together with sparse morphologic evidence of intrusions in comparison with other terrestrial planets raise the issue of the conditions for the formation and occurrence of sub surface zones of magma storage on mars in this respect a series of two types of young amazonian domes located at mid latitudes in western arcadia planitia east of phlegra montes previously interpreted as surface expressions of intrusive bodies farrand w h lane m d edwards b r yingst r a 2011 icarus 211 139 156 are examined here a fraction of the domes consist of upraised plain material whereas others distinguished by a massive core and a large aspect ratio could be partially exhumed intrusions or extrusions of viscous differentiated magma the latter interpretation is however not compatible with the mafic compositions inferred from orbital visible and near infrared spectroscopic observations the thicknesses and radii of both types of domes appear to be related through a power law relationship with exponents close to 1 or 5 4 these exponent values are not compatible with extrusive morphologies but are characteristic of intrusions magma below an elastic layer michaut c 2011 j geophys res 10 1029 2010jb008108 we conclude that all types of domes are intrusive bodies the large aspect ratios of some of these features may be reconciled with a mafic composition and a shallow emplacement a few hundred meters as well as a relatively large injection rate magma emplacement below an ice rich horizon that was subsequently removed offers a plausible scenario for their partial exhumation this scenario is supported by independent signs of ice removal and deflation events in the northern plains the formation of these intrusions appear therefore to be related to an unusual context of magma ascent below a thick and low density ice rich horizon

late quaternary geology of the middle gila river gila river indian reservation arizona abstract stratigraphy and dating provide an 18 000 yr record of erosion and deposition by the middle gila river prior to 18 000 cal yr b p the gila river cut into its floodplain and created a deep wide channel sand and gravel accumulated within this channel until 4250 4400 cal yr b p by this time the channel of the gila river was narrow and deep around 5000 cal yr b p fine grained sediments began to accumulate on the floodplain at 800 950 cal yr b p there was a major period of channel widening after this erosional episode the channel again narrowed as it filled with sand and gravel until around 200 cal yr b p on the floodplain deposition continued from about 5000 cal yr b p to 500 cal yr b p a period of stability and soil formation occurred on the floodplain between 500 and 200 cal yr b p after which overbank deposition resumed and buried the soil channel widening again occurred in the late 19th century the changes observed in the sedimentologic history of the gila river show that during the late pleistocene the gila river was a competent stream capable of carrying and depositing coarse sediment loads later deposition of sand and gravel during the first half of the holocene implies an increase in sediment yield from upstream watersheds changes during the last 4000 years reflect the response of the river to climatic perturbations the timing of large floods internal landscape thresholds and human impacts

holocene flood histories in south western usa harden t macklin m g baker v r 2010 holocene flood histories in south western usa earth surface processes and landforms 35 6 707 716 sponsorship aberystwyth university university of arizona

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

granites of western karakorum and northern kohistan pakistan a composite mid cretaceous to upper cenozoic magmatism abstract the petrography the chemical mineralogical typology and the ages of six plutonic units four from the karakorum axial batholith darkot pass ghamu bar batura and hunza units and two of northern kohistan gindai and nomal plutons are defined and compared based on field data and analyses of up to 78 samples major elements ree rb s sbnd sr and k ar isotopes the karakorum axial batholith is a composite body three major intrusive stages occurred 1 around mid cretaceous times ca 110 95 ma with the emplacement of subalkaline i e monzonitic darkot pass and calc alkaline hunza ghamu bar units 2 during palaeogene maybe up to 43 ma batura subalkaline unit a strong tectonommetamorphic event recorded in the gneissification of the cretaceous intrusives occurred between these two stages it may be of palaeocene age p t estimates of the highest metamorphic grade rocks of the hunza unit have yielded values of 580 640 c and 5 0 5 kbar and 3 during upper miocene ca 9 ma baltoro subalkaline unit debon et al 1986c in addition a conspicuous network of aplo pegmatitic dykes emplaced into the hunza area possibly from the eocene up to the upper cenozoic with a maximum during the middle miocene ca 15 ma most of these major magmatic stages are met again among the acidic intrusives and dykes of northern kohistan the first one as blastomylonitic tholeiitic plutons nomal ca 102 ma petterson and windley 1985 the second one as subalkaline plutons gindai ca 59 ma and a third one as leucocratic dykes of oligocene age ca 30 ma petterson and windley 1985 these data may be related to the geodynamic evolution of the nw part of the india eurasia suture zone thus allowing better constraints on the major steps of this evolution the partly synchronous closures by n dipping subduction of the two tethys branches which are assumed to have encircled the kohistan arc in upper mesozoic times may have generated both the karakorum and the kohistan intrusives of cretaceous and palaeogene ages the northern branch very likely closed before the southern one at the time of the second intrusive stage palaeogene a kohistan and karakorum had already collided were welded and had suffered the same major tectonometamorphic event b subduction of the southern tethys floor beneath the welded kohistan karakorum was still active c however collision between india and kohistan karakorum may have already begun particularly at the level of the nanga parbat promontory finally it is emphasized that the intrusive processes continued in the karakorum long after the collision e g baltoro granite

experimental study of avulsion frequency and rate of deposition in existing models of alluvial architecture it is typically assumed that mean avulsion frequency is independent of sedimentation rate however if avulsion is driven by superelevation of a river bed above its surrounding flood plain one might expect avulsion rate to increase with sedimentation rate we have carried out a series of experiments with laboratory scale fluvial fans in which we measured the frequency of apical avulsions as a function of mean sedimentation rate on the fan avulsion frequency increased strongly with increasing sedimentation rate and then stabilized as mass flows began to influence deposition in the regime of increasing avulsion frequency the added volume of sediment needed to trigger an avulsion decreased with increasing sedimentation rate although our experimental results cannot simply be scaled up to natural rivers they suggest the possibility of coupling between avulsion frequency and sedimentation rate that would be strong enough to change qualitatively the results of existing models of alluvial architecture these models should be applied with caution until avulsion mechanics are better understood

holocene alluvial geology and geoarchaeology of the san xavier reach of the santa cruz river arizona the 15 km segment of the santa cruz river traversing the san xavier indian reservation near tucson arizona has undergone major environmental changes during the past 8 000 yr the holocene alluvial sequence for this segment called the san xavier reach of the santa cruz river shows three major episodes aggradation until 8000 yr b p by a braided stream channel erosion from 8000 to 5500 yr b p and subsequent vertical aggradation of the flood plain punctuated by short periods of arroyo cutting the major changes in the fluvial regime were probably the result of climatic changes but cycles of arroyo cutting and filling during the past 2 500 yr were probably the result of unstable internal geomorphic conditions flooding and human impacts on the flood plain the alluvial history of the san xavier reach had a pronounced effect on the preservation of archaeological remains environmental changes on the flood plain including arroyo cutting and filling cienega formation and destruction and sand dune development led to the disruption and reorganization of hohokam settlement and subsistence patterns between 1150 and 500 yr b p

reconciling arroyo cycle and paleoflood approaches to late holocene alluvial records in dryland streams abstract a century of research into the environmental records of alluvial valley fills in drylands has led to new theories about landscape response to climate change and cultural evolution over the holocene in a largely separate line of inquiry paleoflood hydrologists have scoured bedrock canyons for slackwater deposits extending the flood record and exploring their climatic significance both approaches rely on the analysis and dating of late holocene alluvium sometimes along the same drainages yet they differ in the geomorphic setting in which they should be applied studies of arroyo cut and fill cycles are focused on broad alluvial valleys whereas paleoflood hydrology has been focused mainly in narrow bedrock canyons with a focus on the southwestern u s we review and compare the fundamentals of these two approaches and their paradigmatic disconnect and then discuss potential linkages that could lead to a more complete understanding of how dryland streams adjust to external stimuli recent regional compilations provide insight into the broader relation between the two record types over entire physiographic provinces meanwhile new tools such as osl dating short lived isotopes and improved hydraulic modeling are paving the way for refinement and reconciliation of the two approaches within individual drainages the relation between past arroyo cut and fill cycles and paleofloods must be thoroughly explored if we are to fully understand how drainages will respond to a changing climate over the coming decades to centuries

floodplain stages in the braided ngaruroro river new zealand this study investigates floodplain deposits in the braided ngaruroro river new zealand to characterize and quantify the spatial extent of deposits at different stages of development using a sequence of aerial photography depositional surfaces were mapped and then assigned a floodplain development stage based on age the discharge causing inundation and field observations of sediment and vegetation characteristics the ensemble of floodplain deposits makes up 40 of the river valley the remainder being the active river bed stabilizing bars and mature floodplains dominate the floodplain ensemble while the existence of stabilizing bars points to a recent shift or fluctuation in controlling conditions the extent and location of mature units suggest an earlier change in hydrologic and sediment supply conditions that limited braidtrain mobility mosaics of different aged deposits range from simple two stage mosaics to more complex arrangements of three and four stages in the rapid evolution from stabilized bars to mature floodplains a large amount of fine grained sediment is stored in the ngaruroro river valley highlighting the possibility of finding extensive fine grained facies in the ancient sedimentary record of braided systems understanding floodplain ensembles and their controls will contribute to effective restoration of braided rivers

origin of late triassic high k calc alkaline granitoids and their potassic microgranular enclaves from the western tibet plateau northwest china implications for paleo tethys evolution the western tibet plateau comprises a series of crustal terranes that were successively accreted to the southern margin of eurasia associated with the evolution of paleo and neo tethys this paper presents the first detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope geochemistry of three mesozoic plutons south kudi arkarz and mazha and their microgranular enclaves in the western kunlun and tianshuihai terranes shrimp zircon u pb dating shows that the three plutons were emplaced in the late triassic 215 209 ma and show a southward younging trend the south kudi pluton 215 ma is composed of high k calc alkaline granodiorite and monzogranite with initial 87sr 86sr ratios of 0 7093 0 7099 end t of 4 9 to 5 4 and ehf t in situ zircon of 0 3 elemental and isotopic data suggest that the granitoids were generated by partial melting of the precambrian metasedimentary igneous basement in the normal lower crust 40 km of the western kunlun terrane triggered by underplating of basaltic magma the arkarz pluton 213 ma consists of high k calc alkaline monzogranite syenogranite and alkali feldspar granite and contains abundant microgranular enclaves the host granites have initial 87sr 86sr ratios of 0 7071 0 7085 end t of 3 7 to 4 8 and ehf t of 0 7 and were also generated by partial melting of the precambrian metasedimentary igneous basement in the normal lower crust 40 km of the western kunlun terrane triggered by underplating of enclave forming potassic magma fractional crystallization of these pure crustal melts may explain the more felsic end member granite the enclaves are mainly basic sio2 48 0 54 9 wt with high k2o 1 4 3 8 wt they have initial 87sr 86sr ratios of 0 7071 0 7080 and end t of 2 6 to 4 5 we interpret the enclave magmas as having been derived by partial melting of the metasomatized mantle wedge in the spinel garnet transition zone 60 80 km the mazha pluton 209 ma is composed of high k calc alkaline tonalite and granodiorite with initial 87sr 86sr ratios of 0 7087 0 7097 end t of 6 3 to 6 5 and ehf t of 2 8 elemental and isotopic data suggest that the granitoids were generated by mixing of mantle derived basaltic magma with felsic melt derived by partial melting of lower crust of the tianshuihai terrane our new data suggest that during the late triassic 215 209 ma the tianshuihai terrane and the southern part of the western kunlun terrane were a continental arc in response to the northward subduction of the paleo tethys between the tianshuihai and karakorum terranes the collision of the karakorum terrane with the tianshuihai terrane is not earlier than the age of 209 ma and most likely occurred during the early jurassic

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

assembly of the pamirs age and origin of magmatic belts from the southern tien shan to the southern pamirs and their relation to tibet 1 magmatic rocks and depositional setting of associated volcaniclastic strata along a north south traverse spanning the southern tien shan and eastern pamirs of kyrgyzstan and tajikistan constrain the tectonics of the pamirs and tibet the northern pamirs and northwestern tibet contain the north facing kunlun suture the south facing jinsha suture and the intervening carboniferous to triassic karakul mazar subduction accretion system the latter is correlated with the songpan garze hoh xi system of tibet the kunlun arc is a composite early paleozoic to late paleozoic triassic arc arc formation in the pamirs is characterized by 370 320 ma volcanism that probably continued until the triassic the cryptic tanymas suture of the southern northern pamirs is part of the jinsha suture a massive 227 ma batholith stitches the karakul mazar complex in the pamirs there are striking similarities between the qiangtang block in the pamirs and tibet like tibet the regional structure of the pamirs is an anticlinorium that includes the muskol and sares domes like tibet the metamorphic rocks in these domes are equivalents to the karakul mazar songpan garze system granitoids intruding the qiangtang block yield 200 230 ma ages in the pamirs and in central tibet the stratigraphy of the eastern pshart area in the pamirs is similar to the bangong nujiang suture zone in the amdo region of eastern central tibet but a triassic ocean basin sequence is preserved in the pamirs arc type granitoids that intruded into the eastern pshart oceanic basin arc sequence 190 160 ma and granitoids that cut the southern qiangtang block 170 160 ma constitute the rushan pshart arc cretaceous plutons that intruded the central and southern pamirs record a long lasting magmatic history their zircons and those from late miocene xenoliths show that the most distinct magmatic events were cambro ordovician 410 575 ma triassic 210 250 ma likely due to subduction along the jinsha suture middle jurassic 147 195 ma subduction along rushan pshart suture and mainly cretaceous middle and late cretaceous magmatism may reflect arc activity in asia prior to the accretion of the karakoram block and flat slab subduction along the shyok suture north of the kohistan ladakh arc respectively before india and asia collided the pamir region from the indus yarlung to the jinsha suture was an andean style plate margin our analysis suggests a relatively simple crustal structure for the pamirs and tibet from the kunlun arc in the north to the southern qiangtang block in the south the pamirs and tibet likely have a dominantly sedimentary crust characterized by karakul mazar songpan garze accretionary wedge rocks the crust south of the southern qiangtang block is likely of granodioritic composition reflecting long lived subduction arc formation and cretaceous cenozoic underthrusting

alluvial chronologies and archaeology of the gila river drainage basin arizona abstract late quaternary alluvial chronologies are established for five streams gila river salt river tonto creek santa cruz river and san pedro river in the gila basin of southern arizona each streams has a complex history of deposition erosion and landscape stability that structured and fragmented the archaeological record over the last 15 000 years the limitations that geologic processes imposed on the archaeological record of these alluvial environments must be recognized before meaningful interpretations of prehistory can be made these stratigraphic sequences also provide the basis for reconstructing changes to the alluvial landscape of each valley over time all five streams were intensively utilized during the late prehistoric period a d 300 1450 by the hohokam the hohokam were irrigation agriculturalists who were dependent upon these streams for survival thus the regional stability and instability of the floodplain environments of southern arizona influenced the expansion contraction reorganization and collapse of the hohokam

a new quaternary stratigraphy of the kallang river basin singapore implications for urban development and geotechnical engineering in singapore abstract the quaternary stratigraphy of many coastal areas in southeast asia is poorly understood developing a detailed framework is important as many coastal cities are built on these unconsolidated coastal marine sediments this study reviews the current understanding of quaternary deposits in the kallang river basin singapore using 161 boreholes to create 14 cross sections and a 3d geological model the dataset is augmented with a 38 5 m long sediment core obtained from marina south 1 2726 n 103 8653 e and a previous record from geylang 1 3137 n 103 8917 e to provide age constraints and stratigraphic reference a new quaternary stratigraphic framework for singapore is presented here constrained by new radiocarbon and optically stimulated luminescence osl dating and identify palaeo features that record the geomorphic and sedimentary evolution of the basin fluvial deposits of pleistocene age are uncomfortably overlain by littoral tidal sands and subsequently marine clay possibly during marine isotope stage mis 5e 125 ka bp subsequent subaerial exposure and weathering of the marine clay during the last interglacial produced a stiff clay layer the stiff clay is unconformably overlain by holocene transgressive sands and peats from 9 5 ka bp followed by marine clays with a maximum basal age of 9 2 ka bp regressive sandy peat units were subsequently deposited beginning 6 ka bp here the new quaternary stratigraphic framework for singapore is presented as well as provide important constraints on the regional sea level history and geomorphological evolution of singapore s southern coast from mis 5e to present the work also highlights the complexities of geoengineering work in such subsurface terranes

geomorphological dynamic changes during the holocene through ephemeral stream analyses from northwest argentina abstract this paper studies the changes in the geomorphological dynamics recorded in 18 ephemeral streams located on the east side of loma pelada tafi valley a part of the sierras subandinas northwest argentina the applied methodology is based on photointerpretation field survey and descriptions of the holocene alluvial fillings dated by tephras archaeological artefacts and absolute datings the records show a coupled system of slope terrace alluvial fans resulting from the environmental changes of four alternating aggradation degradation stages the oldest stage 1 was generated by climatic causes and covers early to mid holocene ca 13 000 10 000 bp to ca 4200 bp after the incision of these accumulations a new aggradation stage 2 was triggered by anthropogenic activity occurring from right before 2500 bp to the 15th century during that time human occupations of the tafi valley were intense two wetter events occurred around 4200 and 2800 bp in the area related to global cooling phases lastly two more recent phases 3 and 4 are associated with the climatic variability of the lia and the present warm period the results highlight the sensitivity of headwater catchments of dry subtropical mountainous areas to climate changes anthropogenic impact and their relationship with global climatic data

dike transport of granitoid magmas thermal and fluid dynamical analyses suggest that for viscosities and density contrasts spanning the range considered typical for many calc alkalic granitoids dike ascent is a viable mechanism for the transport of large volumes of granitoid melt through the continental crust we present calculations showing that a granitoid melt with calculated viscosity of the order of 10 6 pa s and a density contrast between magma and crust of 200 kg m 3 can be transported 30 km through the crust in 1 month corresponding to a mean ascent velocity of 1 cm s using analysis modified from numerical studies of the flow of basaltic magmas in dikes we also present an expression that allows the calculation of the critical minimum dike or fault width required for granitic magma to ascend without freezing for all reasonable estimates of cordilleran granitoid viscosity and density contrast the critical dike width is determined to be between 2 and 7 m calculated peak batholith filling rates are orders of magnitude greater than mean cavity opening rates based on estimated fault slippage which is consistent with chemical evidence for intermittent supply of magma pulses