middle miocene deduction and late miocene beginning of collision registered in the hengchun peninsula geodynamic implications for the evolution of taiwan abstract taiwan is located in the axis of the manila trench it results from an oblique collision between the northernmost part of the luzon arc and the chinese passive margin this active collision follows the subduction of the oligocene miocene oceanic crust of the south china sea along the manila trench the tectonized chinese margin emerged in the hengchun peninsula south taiwan gentle folds which are delineated by the quaternary reefal limestones demonstrate recent deformations these folds deformed a thick detrital sequence of miocene age ssuchung chi series which was previously strongly folded and thrust westward axis ns n20 upon the renting melange of latest miocene age these main deformations sealed by the middle pliocene are the evidence for the onset of collision in this part of taiwan at the end of the miocene because of its obliquity the collision started already in the northern part of taiwan during the late miocene 6 7 8 ma the ssuchung chi series a sequence of proximal turbidites has contained since the middle miocene nn 6 13 ma fragments of an oligocene to lower miocene oceanic crust this ophiolitic material is very similar to the east taiwan ophiolite of the coastal range it originated most probably from a slice of south china sea crust obducted in middle miocene times 13 14 ma upon the chinese margin north of the hengchun peninsula this obduction occurred 7 to 8 ma before the beginning of collision these results make it possible to propose an evolutionary model for taiwan from the oligocene to the recent with the different phases of a collision between a volcanic arc and a passive margin

variations in monsoonal rainfall over the last 21 kyr inferred from sedimentary organic matter in tung yuan pond southern taiwan abstract changes in paleorainfall intensity linked to the strength of the east asian ea summer monsoon since 21 cal kyr bp are inferred from the organic matter contents of a 15 m sediment core from tung yuan pond in southern taiwan high total organic carbon total nitrogen toc tn values in association with increased toc content suggest that more soil derived material containing terrestrial organic matter om was delivered to the lake during periods of increased runoff associated with extensive precipitation that resulted from intensified summer monsoons whereas low values indicate om possessing a dominant algal origin during weakened summer monsoons rainfall intensity in terms of the proportion of terrestrial om was high in four periods the last deglaciation 17 2 to 12 2 ka the early holocene 10 6 to 8 6 ka the middle holocene thermal optimum 7 7 to 5 ka and the late holocene 4 2 to 2 ka whereas it was low in the intervening time periods high toc tn values coincide with peak values of summer insolation and thus the strongest ea summer monsoon during the early and middle holocene small drops in these ratios correspond to increasing and decreasing solar radiation in the deglacial period and the late holocene respectively the four intervals with low toc tn ratios as well as episodic drops of the ratios during the deglaciation and the early and late holocene are concordant with the late last glacial 21 to 17 2 ka the oldest 14 8 ka the older 13 3 ka and the younger dryas 13 to 11 5 ka the 8 2 cold event 8 6 7 7 ka and a late holocene cold event 5 4 2 ka and suggest a weakened ea summer monsoon during these times moreover high frequency hydrological variability occurred during the early holocene heavy rainfall persisted during the middle holocene and precipitation intensity generally diminished after 5 ka the tung yuan pond sediment record indicates that the toc tn ratio can be used as a paleorainfall intensity proxy to trace variations in the ea summer monsoon strength in other small lakes

oxygen isotope studies of achondrites abstract oxygen isotope abundances provide a powerful tool for recognizing genetic relationships among meteorites among the differentiated achondrites three isotopic groups are recognized l snc mars 2 earth and moon and 3 hed howardites eucrites diogenites the hed group also contains the mesosiderites main group pallasites and silicates from iiiab irons the angrites may be marginally resolvable from the hed group within each of these groups internal geologic processes give rise to isotopic variations along a slope 1 2 fractionation line as is well known for terrestrial materials variations of 17o from one planet to another are inherited from the inhomogeneities in the solar nebula as illustrated by the isotopic compositions of chondrites and their constituents among the undifferentiated achondrites five isotopic groups are found 1 aubrites 2 winonaites and iab iiicd irons 3 brachinites 4 acapulcoites and lodranites and 5 ureilites the isotopic compositions of aubrites coincide with the earth and moon and also with the enstatite chondrites these bodies apparently were derived from a common reservoir the isotopic composition of which was established at the chondrule scale by nebular processes isotopic similarities between chondrites and achondrites are seen only for the following instances 1 enstatite chondrites and aubrites 2 h chondrites and he irons and 3 l or ll chondrites and iva irons the isotopic data also support the following genetic associations 1 winonaites and iab iiicd irons 2 acapulcoites and lodranites and 3 ureilites and dark inclusions of c3 chondrites an attempt to reconcile the whole planet isotopic compositions of earth mars and the eucrite parent body with mixing models of their chemical compositions failed it is not possible to satisfy both the chemical and isotopic compositions of the terrestrial planets using known primitive solar system components

mountain building in taiwan a thermokinematic model the taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening however a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge we propose here a new thermokinematic model of the taiwan mountain belt reconciling existing kinematic thermometric and thermochronological constraints in this model shortening across the orogen is absorbed by slip on the most frontal faults of the foothills crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge tananao complex tc where the uplift rate is estimated to 6 3 mm a 1 and beneath the westernmost internal region of the orogen hsueshan range units hr where the uplift rate is estimated to 4 2 mm a 1 our model suggests that the tc units experienced a synchronous evolution along strike despite the southward propagation of the collision it also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures exhumation of the hr units increases northward but has not yet reached an exhumational steady state presently frontal accretion accounts for less than 10 of the incoming flux of material into the orogen although there is indication that it was contributing substantially more 80 before 4 ma the incoming flux of material accreted beneath the tc significantly increased 1 5 ma ago our results also suggest that the flux of material accreted to the orogen corresponds to the top 7 km of the upper crust of the underthrust chinese margin this indicates that a significant amount 76 of the underthrust material has been subducted into the mantle probably because of the increase in density associated with metamorphism we also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field by combining available geological data on the thermal and kinematic evolution of the wedge our study sheds new light onto mountain building processes in taiwan and allows for reappraising the initial structural architecture of the passive margin

oxygen isotope relationships in iron meteorites iron meteorites with oxygen bearing phases can be classified in terms of their oxygen isotopic abundances these iron meteorite classes are isotopically similar to various stony meteorite classes which may indicate a common origin the group iab and iiicd irons may be related to the winonaites group iie irons may be related to h chondrites group iva irons may be related to l or ll chondrites

oxygen isotopes in asteroidal materials measurement of the variation of the 17o 16o and 18o 16o ratios of extraterrestrial materials has proven to be a key tool in understanding the formation and evolution of the solar system this chapter attempts to collate the huge data set of oxygen isotopic measurements of asteroidal material as sampled by meteorites in order to understand some of the processes involved in the formation of these primitive bodies and how such events have affected the oxygen isotopic ratios ultimately offering a window back to the very origin of solar system and its primordialoxygen isotopic heterogeneity oxygen is generally a major element in the gas liquid and solid phases that have interacted throughout the evolution of the solar system its chemical properties ensuring that there is abundant opportunity for reactions and exchange the details of which can be recorded in the isotopic ratios of the products such processes can be crudely subdivided into the following six categories early solar nebular isotopic heterogeneity high temperature modifi cation of nebular components metamorphism and melting in asteroids aqueous alteration in asteroids mixing of components brecciation terrestrial weathering it is clear from the isotopic variation in the earliest formed nebular components such as refractory inclusions and chondrules that there was considerable oxygen isotopic heterogeneity in the early solar nebula spanning over 50 and perhaps much greater in both 17o and 18o the origin of this variation is discussed elsewhere in this volume however isotopic exchange between the 16o rich solids and 17o and 18o rich gas phase s resulted in a wide range of isotopic variation in these early formed components offering insight into the conditions of these processes and the evolution of the reservoirs with time and or space once accreted onto the early planetisemals much of the primitive nebular components experienced considerable modification the metamorphism and melting experienced by some of these bodies most probably the result of heating from the decay of short lived radionuclides e g 26al resulted in progressive homogenization within individual bodies of any original isotopic heterogeneity complete homogenization appears to have been achieved in the hed parent body indicating very high levels of melting and the development of a large magma ocean the carbonaceous and ordinary chondrite parent bodies experienced more modest heating sufficient to mobilize water ice that accreted along with the more refractory materials the subsequent reactions between the liquid and solid phases at low temperature imparted large isotopic fractionations which record some of the conditions of this alteration such as water rock ratio temperature and isotopic signature and thus potentially the origin of the water ice the evolution of the asteroidal bodies has been far from simple and an important use of oxygen isotope measurements has been in the identifi cation of the components present in some meteorites the result of breakup reassembly and mixing of different bodies during impact processing the final oxygen isotopic signatures imparted into meteorite samples are those associated with terrestrial weathering the isotopic compositions of the weathering products depend on location primarily latitude while the magnitude of the weathering is dependent on many factors with some meteorites e g highly reduced enstatite chondrites and the matrix rich co3s particularly susceptible to such effects

the structural evolution of active fault and fold systems in central otago new zealand evidence revealed by drainage patterns abstract central otago in new zealand is an area of active continental shortening in which a peneplain surface cut into schist has been deformed by folds which are developed above buried reverse faults we use the drainage patterns in this region to demonstrate various processes in fold and fault growth and interaction that would be difficult to identify by other means in particular we show 1 how simple asymmetric folds can develop into box folds 2 how apparently continuous ridges were formed by the coalescing of quite separate propagating fold and fault segments 3 evidence for the relative ages or relative uplift rates of adjacent structures and 4 evidence for the propagation direction of folds or faults as they grow the few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10 50 m per earthquake on faults about 20 km long these estimates are very uncertain but are similar in magnitude to an estimate made in nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models they raise the question of whether fault growth earthquake recurrence rates and climate change can interact to produce semi regular discrete features in an active landscape

the pyroxene pallasites vermillion and yamato 8451 not quite a couple two pallasites vermillion and yamato y 8451 have been studied to obtain petrologic trace element and o isotopic data both meteorites contain low ca and high ca pyroxenes 2 by volume and have been dubbed pyroxene pallasites pyroxene occurs as large individual grains as inclusions in olivine and in other pyroxene and as grains along the edges of olivine symplectic overgrowths sometimes found in main group and eagle station pallasites are not seen in the pyroxene pallasites olivine compositions are fa10 12 similar to those of main group pallasites siderophile trace element data show that metal in the two meteorites have significantly differing compositions that are for many elements outside the range of the main group and eagle station pallasites these compositions also differ from those of iab and iiiab iron meteorites rare earth element ree patterns in merrillite are similar to those seen in other pallasites indicating formation by subsolidus reaction between metal and silicate with the merrillite inheriting its pattern from the surrounding silicates the o isotopic compositions of vermillion and y 8451 are similar but differ from main group or eagle station pallasites as well as other achondrite and primitive achondrite groups although vermillion and y 8451 have similar mineralogy pyroxene compositions ree patterns and o isotopic compositions there is sufficient evidence to resist formally grouping these two meteorites this evidence includes the texture of vermillion siderophile trace element data and the presence of cohenite in vermillion

pinwarian to grenvillian magmatic evolution in the central grenville province new constraints from id tims u pb ages and coupled lu hf s mc icp ms data understanding the magmatic evolution of the rocks once comprising the hinterland of the grenville orogen through their mesoproterozoic formation is a key to understanding the grenvillian orogeny as a whole in this contribution we present high precision isotope dilution thermal ionization mass spectrometry id tims u pb and coupled solution multicollector inductively coupled plasma mass spectrometry s mc icp ms lu hf zircon data from magmatic rocks occurring in the allochthonous belt of the grenville orogen in the central part of the grenville province we document the presence of a large tract of pinwarian crust represented by a 1497 5 ma granitic gneiss as well as large late geon 14 to early geon 13 1434 7 11 1413 12 1393 8 1383 1 ma magmatic complexes one grenvillian plutonic suite of 1015 2 ma that cross cuts the host rock metamorphic fabric has also been dated this age provides a minimum age of ottawan metamorphism in the region the hf isotopic data show that the magmatic roc

from continental margin extension to collision orogen structural development and tectonic rotation of the hengchun peninsula southern taiwan as a result of oblique collision the taiwan orogen propagates southward the hengchun peninsula in the southern tip of the taiwan central range preserving the youngest the least deformed and the most complete accretionary prism sequences allows therefore better understanding of the tectonic evolution of taiwan orogen on the hengchun peninsula four main stages of paleostress can be recognized by the analysis of brittle tectonics after recording the first two stages of paleostress rocks of the hengchun peninsula the hengchun block have undergone both tilting and counterclockwise rotation of about 90j the structural boundaries of this rotated hengchun block are the kenting melange zone in the southwest the fongkang fault in the north and a submarine backthrust in the east the angle of this rotation is principally calculated by the paleomagnetic analysis data and a physical model experiment through a systematic back tilting and back rotating restoration the original orientations of the four paleostress stages of hengchun peninsula are recognized they are from the ancient to the recent a nw se extension a combination of nw se transtension and ne sw transpression a ne sw compression and finally a combination of ne sw transtension and nw se transpression this result can be explained by a phenomenon of stress axes permutation instead of a complex polyphase tectonism this stress axes permutation is caused by the horizontal compression increase accompanying the propagation of the accretionary prism combining the tectonic and paleomagnetic data with paleocurrent and stratigraphic data enables us to reconstruct the tectonic evolution of the hengchun peninsula this reconstruction corresponds to the deformation history of a continental margin basin from its opening to its intense deformation in the accretionary prism d 2002 elsevier science b v all rights reserved

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

a petrological and chemical reexamination of main group pallasite formation abstract the large number of pallasite parent bodies suggests that their formation was a common event in asteroid evolution new microprobe data has been generated for pallasite olivines and chromites that indicate subsolidus redox processes while literature data has been collated and correlated to form new results and interpretations the new data include information on metal compositions the metal cooling rates the formation of round and fragmental olivine the close packing of olivine the crystallization of the phosphates phosphoran olivine and orthopyroxene and the bulk pallasite p and s contents a model for main group pallasites has been developed using fractional melting of a chondritic precursor to make a multi layered parent body one portion of this contains a gradational metal olivine pallasite texture concentric downward crystallization of the molten metal is indicated based on cooling rates and metal compositions fractional crystallization of the residual silicate and metallic melts produces the minor phases on cooling comparisons to and a brief review of past models particularly those utilizing mixing have been made

knickpoint initiation and distribution within fluvial networks 236 waterfalls in the waipaoa river north island new zealand if knickpoints transmit signals of base level fall in river networks then improvements in our understanding of their retreat rate and basin wide distribution helps constrain the transient response following perturbation many studies of knickpoint retreat focus on the response of trunk streams to base level fall here we examine the response of an entire fluvial network as recorded by 236 active knickpoints distributed within the waipaoa river on the north island of new zealand base level fall within the waipaoa catchment initiated 18 000 years ago in response to a climatically triggered and tectonically exacerbated pulse of incision using observations from field work aerial photo analysis and a digital elevation model dem we study the knickpoint positions within the network we find that 70 of the knickpoints are located at drainage areas between 1 10 5 m 2 and 1 10 6 m 2 and more than half are b1 km upstream of a large change in drainage area for the knickpoints b1 km upstream of large tributary junctions we find that the retreat distances were well correlated with the tributaries drainage areas in order to determine how a pulse of incision distributes itself throughout a fluvial network we develop two simple end member models and compare their behavior to the observed knickpoint distribution in the waipaoa in the first model we propose that a knickpoint initiated at the basin outlet retreats upstream and distributes the signal throughout the network at a rate that is a power law function of drainage area in the second model we propose that knickpoints form near a threshold drainage area below which channels cannot incise with the same efficiency as possible in downstream reaches though neither model addresses the along stream variability in substrate or knickpoint form the misfit between the modeled and the observed knickpoints along stream positions are surprisingly low b1 km 1 25 of the total stream length for knickpoints with drainage areas b1 10 6 m 2 large misfits up to 3 5 km are observed for knickpoints with present day drainage areas greater than 1 10 6 m 2 the large single step in channel elevation that characterizes knickpoints presently observed in tributaries of the waipaoa river may not characterize the base level fall signal that propagated through the trunk streams evidence for progressive rather than instantaneous incision in the trunk streams the knickpoints vicinities to tributary junctions and the equivalent success of the two end member models lead us to conclude that the present positions of the 236 observed knickpoints are largely a consequence of thresholds in channel incision at low drainage areas

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

wave train model for knickpoint migration rivers respond to a drop in their base level by incising the topography the upstream propagation of an incision as usually depicted by a knickpoint migration is thought to depend on several parameters such as the drainage area lithology and the amplitude of the base level drop we first investigate the case of the messinian salinity crisis that was characterized by the extreme base level fall 1500 m of the mediterranean sea at the end of the miocene the response of drainage areas of three orders of magnitude 103 to 106 km2 highlights the dominant role of the drainage area with a square root relationship in controlling the knickpoint migration after a base level fall a compilation of mean rates of knickpoint propagation for time durations ranging from 102 to 107 years displays a similar relationship indicating that successive wave trains of knickpoint can migrate in a river first wave trains linked to the release of the alluvial cover and then wave trains related to the bedrock incision which correspond to the real time response of rivers wave trains with very low retreat rates long lived knickpoints 1 my rather correspond to the response time of regional landscape

mesoscale fluvial erosion parameters deduced from modeling the mediterranean sea level drop during the messinian late miocene 1 after a base level drop rivers are the first components of the landscape to respond by incising into topography a base level drop results in a knickpoint in the downstream part of river longitudinal profiles whether knickpoints are preserved or erased during the upstream propagation of incision is still debated preservation and erasure of knickpoints are two end member processes that work in natural systems at different timescales different length scales and different places the huge 1500 m and fast tens of kiloyears sea level drop in the mediterranean during the messinian resulted in the fast propagation of incision far inland this is especially the case in the rhone valley southern france where the knickpoint is 300 km from the mediterranean coast numerical modeling of this event has been performed using the ros model which simulates both erosional and depositional processes in rivers the best fit between numerical results and geological data is obtained for a nonlinear relation between incision and drainage area and for a small transport length of sediment this small transport length at least 2 orders of magnitude lower than the length of the rhone suggests that the longitudinal profile relaxed in a diffusive way so that the initial knickpoint was not preserved finally after a base level fall the propagation of fluvial incision is very fast at geological timescales hundreds of kiloyears despite this the diffusive response implies that the time required for restoration of an equilibrium profile is very long

geomorphic evidence for ancestral drainage patterns in the zagros simple folded zone and growth of the iranian plateau we describe the geomorphology of a large 10000 km 2 internally draining region within the zagros fold and thrust belt of fars province iran a series of wind gaps through fold crests and a continuous line of low slope pixels in digital elevation models indicate the presence of an older and now abandoned through going river system we suggest from the presence of the wind gaps that the original through going river system was abandoned as a direct result of fold growth at present through going drainage in fars is restricted to only two major rivers the kul and the mand which bound the margins of the internally drained region the low gradients of the kul and the mand rivers are similar to those in topographic profiles drawn along the course of the abandoned drainage the mand and kul rivers may be defeated in the future causing an expansion of the internally drained region and resulting in a profound change in the distribution of sediment and surface elevations within the zagros the internally draining part of the zagros resembles the central iranian plateau both in its geomorphology and in the apparently slow rates of deformation within it we speculate that the development of internally drained basins and distribution of shortening within the range may be causally linked the geomorphology that we describe might therefore record a stage in the southward expansion of the non deforming and topographically high central iranian plateau

partitioning in ree saturating minerals theory experiment and modelling of whitlockite apatite and evolution of lunar residual magmas we present compositions including rees determined by ion microprobe of apatite and whitlockite in lunar rock assemblages rich in incompatible trace elements total concentrations of ree oxides in whitlockites range from 9 13 wt and those in apatites range from 0 15 to 1 wt ratios of ree concentrations in whitlockite to those in coexisting apatite range from 10 to 60 the distribution of mg and fe between apatite and whitlockite is correlated to that of coexisting mafic silicates magnesium is strongly preferred by whitlockite and fe is preferred by apatite incorporation of rees in whitlockite is dominated by the coupled substitution of 2ree3 in ca b sites vacancy in ca iia for 2ca 2 in ca b sites and ca2 na in ca iia other substitutions account for only a small portion of the rees in whitlockite over the observed concentration range thus ree concentrations become partially saturated as the primary substitution approaches its stoichiometric limit of two rees per fifty six oxygens leading to reduced whitlockite melt distribution coefficients e g decreasing from twenty five to ten for nd the ree concentrations of lunar residual melts are not depleted by whitlockite crystallization in assemblages consisting mainly of other minerals in typical proportions distribution coefficients for the rees in lunar apatite appear to be low and variable e g 0 2 0 8 for nd variations in the modal ratio of whitlockite to apatite specifically the abundance of whitlockite lead to a range of ree concentrations in the phosphates and variations in the magnitude of ree concentration ratios between whitlockite and apatite if apatite crystallizes before whitlockite or in the absence of whitlockite as textures in several samples indicate then apatite zoned in rees and apatite crystals of different ree concentrations may occur in a given sample provided there is some amount of fractional crystallization and apatite does not later equilibrate this may occur because in the absence of whitlockite in the crystallizing assemblage the rees are highly incompatible relative to the crystalline assemblage so ree concentrations in lunar residual melts increase strongly during small increments of late stage crystallization once whitlockite begins to crystallize bulk distribution coefficients for the rees although still 1 are only mildly incompatible so the change in ree concentrations of residual melts with further crystallization is small consistent with the lack of ree zoning in whitlockite the ree concentrations in lunar whitlockites are modelled as resulting mainly from equilibrium crystallization of the assemblages in which they occur metasomatism or other secondary metamorphic processes are not indicated local melt pocket equilibrium at advanced stages of crystallization may lead to variable ree concentrations and variable whitlockite apatite concentration ratios within the same sample parent melts with extremely high ree concentrations are not required in order to crystallize ree rich lunar whitlockite if modal proportions of whitlockite are low

rapid fluvial incision along the yellow river during headward basin integration the onset of fluvial erosion in an area of tectonic uplift is thought to reflect the timing of the uplift geomorphological data from the yellow river in tibet indicate that the rapid incision of this river channel occurred as a result of climate change at least six million years after the onset of plateau uplift

holocene mean uplift rates across an active plate collision boundary in taiwan samples of holocene fossil coral from uplifted reefs of three tectonically distinct yet geographically proximal regions of taiwan have been dated by uranium series and 14c isotopes applying corrections for altitude change caused by sea level fluctuations enables evaluation of long term average holocene uplift rates for three areas across an active convergent margin i the hengchun peninsula of the eurasian tectonic plate ii the eastern coastal range of taiwan a plate boundary and iii two offshore islands lanyu and lutao both situated on the leading edge of the adjoining philippine plate the data indicate that while all three areas have experienced uplift through the holocene plate collision has caused significantly higher uplift rates in the region directly along the plate boundary

hanging valleys in fluvial systems controls on occurrence and implications for landscape evolution 1 we document and characterize hanging valleys in a fluvially eroded landscape in eastern taiwan our conceptual model for the initiation of hanging valleys builds on a recently proposed model of bedrock incision in which erosion actually becomes less efficient on very steep channel gradients if a pulse of incision in the main stem outpaces the tributary response the gradients at tributary mouths may therefore pass a threshold value beyond which erosional efficiency declines giving rise to a mismatch between trunk and tributary erosion rates this mismatch is expected at junctions with small tributaries where a step function decrease in drainage area also leads to sharp contrasts in water and sediment flux between trunk and tributary channels the occurrence of hanging valleys in actively uplifting fluvial landscapes such as the central range of taiwan suggests that the most common parameterizations of bedrock erosion which typically assume a monotonic positive correlation between channel gradient and incision rate may be violated in very steep channels in addition hanging valleys could greatly increase the response time of landscapes to tectonic perturbations since catchments above these tributary mouths will be insulated from these perturbations until a new suite of processes e g weathering and rock mass failure wear through the hanging valley lip the results of this study underscore the need for a more complete understanding of bedrock erosion processes and the incorporation of process transitions and threshold conditions into landscape evolution models

characterization of topographic steady state in taiwan abstract the arc continent collision in taiwan has resulted in surface uplift high relief and high rates of erosion because the collision is older in the north and has propagated southward taiwan offers the opportunity to study the evolution of topography as it evolves towards a large scale steady state in this paper we analyze a 40 m digital elevation model of taiwan to determine how topographic steady state is expressed in the landscape particular attention is paid to the eastern central range where major drainage basins are approximately aligned in the direction of the collision propagation analyses of orogen size regional topographic slope drainage basin hypsometry and local topographic slope reveal a geomorphological transition 100 125 km north of the southern tip of the island to the south of this transition topographic characteristics change with distance along the island to the north these characteristics are relatively constant which we interpret as representing large scale topographic steady state assuming that the collision has propagated southward at a rate of 55 mm yr the time to topographic steady state is 1 8 2 3 myr after emergence above sea level within the steady state region we evaluate regional variability in the topography and find that the most prominent deviations from a steady form are associated with large scale structures though our analyses of local topographic slope and river profile form suggest that spatial variations in rock strength and rock uplift rate also contribute to regional topographic variability

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

transient fluvial landscape and preservation of low relief terrains in an emerging orogen example from hengchun peninsula taiwan the oblique collision of taiwan orogen led to a progressive southward uplift and emergence of a submarine accretionary wedge the hengchun peninsula located at the southernmost tip of taiwan island represents the most recently emerged landform of an antecedent submarine surface in this study we examine the geomorphic evolution of this newly emerged and uplifted landscape the extraction of geomorphic parameters knickpoint distribution steepness index of rivers indicates a transient nature of the landscape but shows that a simple model of regressive erosion consecutive to sudden uplift is unsatisfactory knickpoint migration modeling shows that only the upstream sizhong basin can be explained by this single process on the other hand the occurrence of a relict landscape mutan ponds low relief developed at low elevations and today uplifted in the core of the peninsula has strongly influenced the drainage evolution numerous knickpoints are linked to capture processes around the mutan ponds area indicating a dynamic reorganization of the drainage system we propose that the landscape of the hengchun peninsula is in a transient stage and results from a combination between rejuvenation and drainage rearrangement caused by the late stage of emergence of the south of taiwan

constraints on the long term colluvial erosion law by analyzing slope area relationships at various tectonic uplift rates in the siwaliks hills nepal 1 the large database of topographic form and uplift rates that exists for the siwaliks hills central nepal makes possible a thorough analysis of the long term erosion model the study especially focuses on drainage areas larger than 5 10 3 km2 fixed by the database resolution and smaller than 1 km2 above which a fluvial signature is recorded this area range corresponds to colluvial valleys in which the dominant erosion process is likely debris flow we evaluate a phenomenological model wherein erosion is considered to depend on drainage area and slope we test this model by assuming that the uplift rate is in approximate equilibrium with erosion the stream power law model formulated by analogy to river incision and transport problems is found to be consistent with data since an inverse power law relationship between slope and drainage area is systematically observed between 7 10 3 and 1 km2 with little variability on the exponent 0 24 thanks to the range of uplift rates we obtain constraints on the slope dependency of erosion law which appears linear and which predicts a significant erosion threshold the linear dependence on slope in the debris flow zone is consistent with findings by kirby and whipple 2001 in the fluvial downstream zone and with the linear relationship between local relief and uplift rate documented by hurtrez et al 1999 the transition between this colluvial channel regime and the fluvial regime appears quite sharp in contrast with recent studies but the latter regime is not sufficiently documented to derive definite conclusions

pollen records from southern taiwan implications for east asian summer monsoon variation during the holocene pollen records from two cores retrieved from a valley basin in southern taiwan show major changes in vegetation and climate occurring during the holocene between 11 400 and 8200 cal yr bp the up

main group pallasites thermal history relationship to iiiab irons and origin we have determined metallographic cooling rates below 975 k for eight main group mg pallasites from ni profiles across taenite lamellae of known crystallographic orientation in metallic regions with widmanstatten patterns comparison with profiles generated by modeling kamacite growth gave cooling rates ranging from 2 5 to 18 k myr relative cooling rates were also inferred from the sizes of cloudy zone particles in 28 mg pallasites 86 170 nm and tetrataenite bandwidths in 20 mg pallasites 1050 2170 nm as these parameters are positively correlated with each other and negatively correlated with the metallographic cooling rates these three different techniques show that mg pallasites cooled below 975 k at significantly diverse rates since samples from the core mantle boundary should have indistinguishable cooling rates mg pallasites could not have cooled at this location group iiiab irons which were previously thought to be core samples from the mg pallasite body have faster cooling rates 50 350 k myr and smaller cloudy zone particle sizes and tetrataenite bandwidths this shows that iiiab irons cooled faster than mg pallasites and could not plausibly be from the same body the absence of related iron meteorites and achondrites and our thermal constraints suggest that mg pallasites cooled at diverse depths in a pallasitic body consisting of well mixed olivine and metallic fe ni such a body may have formed during an impact on a differentiated asteroid or protoplanet that mixed olivine mantle fragments with residual ir poor molten metal from the out ermost part of a core that chemically resembled the iiiab core and was 80 fractionally crystallized separation of the solid core and most of the associated mantle may have resulted from a grazing hit and run impact with a larger protoplanet or asteroid thermal calculations suggest that the radius of the pallasitic body was 400 km but the likely presence of a regolith would reduce this estimate considerably 2010 elsevier ltd all rights reserved

chemical zoning of olivine in several pallasites chemical zoning profiles of olivines in the esquel imilac and yamato 8451 pallasites were measured by using an electron probe microanalyzer in order to study their cooling histories pallasite olivines are considered to be compositionally uniform but show faint zoning for fe mg cao al2o3 cr2o3 and mno within a few hundred micrometers from the rim the cao al2o3 and cr2o3 contents of olivines in pallasites decrease gradually toward the rim the fe mg zoning and the mno enrichment in olivine rims of pallasite olivines may reflect a redox process on the basis of diffusion calculations to estimate the cooling rate by using chemical zoning of pallasite olivine the formation process of chemical zoning was discussed similar cooling rates obtained for the fe mg cao and mno profiles of an olivine imply that these zoning patterns formed during the same event

from submarine continental accretion to arc continent orogenic evolution the thermal record in southern taiwan constraining the timing of initial collision is critical for understanding how long term plate convergence is accommodated at collisional plate boundaries reevaluation of the age of collision from orogenic thermal evolution requires distinguishing the onset of submarine continental accretion from earlier oceanic subduction and subsequent subaerial orogenic evolution we present new thermochronological constraints from the first age elevation relationship transect in taiwan and zircon and apatite fission track ages from sediments in the western foreland our data reveal the onset of cooling from at least 7 1 1 3 ma at a minimum rate of 21 c m y in the submarine sedimentary wedge followed by a marked acceleration of subaerial exhumation after ca 3 2 0 6 ma at an average rate of 1 7 km m y our data reflect the effect of margin architecture as taiwan evolved from submarine accretion of the distal extremely thinned continental margin to crustal thickening of the proximal margin and orogenic development

modeling of knickpoint retreat on the roan plateau western colorado 1 the roan plateau in western colorado constitutes a natural experiment for studying landscape response to a drop in base level late cenozoic incision of the upper colorado river led to elevational isolation of the plateau and initiation of a wave of incision into its southern edge knickpoints oversteepened reaches that contain waterfalls 60 110 m in height mark the upstream extent of this headward propagating wave that this incision has occurred in a laterally extensive well stratified and essentially flat lying bedrock and in an area with relatively uniform climate implies that it should serve as a good test of existing knickpoint propagation models we predict the locations of knickpoints by using a stream power based celerity model in which knickpoint recession rate is a power function of drainage area and is proportional to rock susceptibility to erosion models of the parachute and roan drainages 17 and 16 knickpoints respectively show expected rapid initial knickpoint propagation rates which decline as drainage area decreases stepwise at tributary junctions the modeled positions of knickpoints match well with observed features using a single combination of parameters to model retreat in both drainage basins we compare our celerity model results with past studies and explore how longitudinal profile analysis may be used to derive independently the exponent on drainage area in the celerity model

late cenozoic metamorphism and mountain building in taiwan a review abstract the orogenic wedge of taiwan results from the late miocene collision of the chinese passive margin and the luzon volcanic arc because of the southward propagation of the collision over time and because of its extreme rates of deformation and erosion it has appeared as an ideal place to investigate mountain building processes here we review existing metamorphic and thermochronological data on late cenozoic burial and exhumation most of the island has been extensively investigated in particular in terms of zircon or apatite fission track and u th he thermochronology peak metamorphic temperatures have also been quantified from the raman spectroscopy of carbonaceous material along three transects in central taiwan the different datasets appear consistent and indicate that cooling rates and exhumation are more intense within the hsueshan range and within the tananao complex lateral variations from south to north are also observed and they can be interpreted in terms of different stages of exhumation and mountain building existing models provide a framework to analyze the data and discuss the limits of some previous interpretations in particular in terms of exhumational steady state or of recent acceleration of uplift and exhumation these models are also discussed in light of available data it appears that all of them fit thermochronological data within the tananao complex whatever the kinematics the depth of the wedge or the mechanism responsible for exhumation and that exhumation of the hsueshan range has been ignored in most models future models need therefore to consider the taiwan wedge as a whole additional constraints on peak metamorphic conditions on kinematics of shortening and exhumation or on the geometry of the wedge at depth need to be gathered to further unravel mountain building processes at the scale of the whole orogen

queen alexandra range 93148 a new type of pyroxene pallasite trace elements including the rare earth elements were measured in olivine and orthopyroxene from queen alexandra range que 93148 and in olivine from two main group pallasites springwater and mount vernon although que 93148 was originally classified as a lodranite a variety of data including oxygen isotopic compositions goodrich and righter 2000 preclude a genetic relationship with the acapulcoites lodranites incompatible trace element e g ti zr distributions in orthopyroxene do indicate large amounts of melting and are consistent with the ultramafic assemblage observed in this meteorite trace element abundances in olivine are consistent with suggestions that que 93148 may be related to the main group pallasites goodrich and righter 2000 although there are some inconsistencies its trace element distributions are most like those of the pyroxene pallasites suggesting that it may have formed in a similar manner que 93148 may represent a new type of pyroxene pallasite with links to the main group pallasites

a ab complex iron meteorite containing low ca clinopyroxene abstract northwest africa 468 nwa 468 is a new ungrouped silicate rich member of the iab complex of nonmagmatic iron meteorites the silicates contain relatively coarse 300 m grains of low ca clinopyroxene with polysynthetic twinning and inclined extinction low ca clinopyroxene is indicative of quenching from high temperatures either from protoenstatite in a few seconds or high temperature clinoenstatite in a few hours it seems likely that nwa 468 formed by impact melting followed by rapid cooling to 660 c after the loss of a metal sulfide melt from the silicates sulfide was reintroduced either from impact mobilized fes or as an s 2 vapor that combined with metallic fe to produce fes the o isotopic composition 17 o 1 39 indicates that the precursor material of nwa 468 was a metal rich e g cr carbonaceous chondrite lodranites are similar in bulk chemical and o isotopic composition to the silicates in nwa 468 the mac 88177 lodranite which also contains low ca clinopyroxene is close in bulk chemical composition both nwa 468 and mac 88177 have relatively low abundances of rare earth and plagiophile elements siderophiles in the metal rich areas of nwa 468 are similar to those in the mac 88177 whole rock both samples contain low ir and relatively high fe cu and se most unweathered lodranites contain 20 to 38 wt metallic fe ni these rocks may have formed in an analogous manner to nwa 468 i e by impact melting of metal rich carbonaceous chondrite precursors but with less separation of metal rich melts from silicates

refractory inclusions in the prototypical cm chondrite mighei abstract refractory inclusions in the mighei cm2 meteorite are very abundant 35 being found in three thin sections of the meteorite the most common are pyroxene rimmed spinel rich inclusions that occur in a range of structural and textural types some inclusions are simple aggregates of crystals or rimmed crystals ranging from loose clusters of spinel grains to distended chains of pyroxene rimmed spinel knots others are more compact and may have experienced recrystallization or melting these include nodular spinel inclusions with polygonal granular textured spinel interiors and pyroxene rims ribbons of spinel in some cases with hibonite and or perovskite that are again rimmed by pyroxene and refractory spherules four inclusions are complex aggregates of several texturally distinct objects each collectively mighei inclusions are similar to those in murchison but appear to be somewhat more altered volatility fractionated and unfractionated trace element patterns similar to those found in refractory inclusions in other carbonaceous chondrites are represented in mighei inclusions and in addition some mighei inclusions have volatility fractionated pattern types not previously found in refractory inclusions in three of the complex aggregates fundamentally different trace element patterns are present within different portions of each inclusion meaning that these objects accreted from genetically different components that never equilibrated with one another subsequent to accretion bulk inclusion magnesium isotopic compositions show minor mass fractionation effects fmg values range from 2 3 to 7 5 but the average fmg value is indistinguishable from normal 1 8 1 2 there are no correlations between magnesium isotopic composition and textural type or trace element pattern type the diverse inclusion types in mighei represent a variety of origins and histories textures suggest that the nodular and banded inclusions may have experienced solid state recrystallization whereas the porous and chain like varieties and complex aggregates have not magnesium isotopic compositions and trace element abundances suggest that the mighei inclusions are unlikely to be evaporation residues and are more likely to be the products of condensation or recondensation processes

clinopyroxene ree distribution coefficients for shergottites the ree content of the shergotty melt abstract ree distribution coefficients were measured between synthetic pyroxenes and melts similar in composition to the inferred shergotty intercumulus liquid by in situ analysis with the electron microprobe using samples doped to per cent concentration levels these coefficients exhibit a strong positive correlation with pyroxene wollastonite content recommended values for augites similar in composition to the shergotty cumulus augite cores wo32en48 are d la 0 023 d ce 0 039 d nd 0 10 d sm 0 17 d eu 0 16 d gd 0 20 d yb 0 29 and d lu 0 30 recommended values for pigeonites similar in composition to the shergotty cumulus pigeonite cores wo12en58 are d la 0 002 d ce 0 004 d nd 0 019 d sm 0 031 d yb 0 13 and d lu 0 13 these values resemble distribution coefficients measured for terrestrial basalts of higher mg fe than the shergotty melt rather than those of more evolved siliceous rocks of similar mg fe to the shergotty melt thus it appears that mg fe is considerably less important than other factors in determining distribution coefficient values for pyroxenes ree abundances were computed for the shergotty intercumulus melt using distribution coefficients measured for the natural phase compositions these computed abundances display lree depletion such lree depleted abundance patterns cannot be generated by simple partial melting of a mafic mineral assemblage having the lree enriched patterns required for a two stage sm nd evolution history this result implies that either 1 the melting process was complex e g continuous melting 2 the source region had a more complex history than that of the simple 2 stage model or 3 shergotty does not satisfy the closed system assumption of the calculation and has suffered post magmatic lree enrichment

the iab iron meteorite complex a group five subgroups numerous grouplets closely related mainly formed by crystal segregation in rapidly cooling melts abstract we present new data for iron meteorites that are members of group iab or are closely related to this large group and we have also reevaluated some of our earlier data for these irons in the past it was not possible to distinguish iab and iiicd irons on the basis of their positions on element ni diagrams but we now show that plotting the new and revised data yields six sets of compact fields on element au diagrams each set corresponding to a compositional group the largest set includes the majority 70 of irons previously designated ia we christened this set the iab main group the remaining five sets we designate subgroups within the iab complex three of these subgroups have au contents similar to the main group and form parallel trends on most element ni diagrams the groups originally designated iiic and iiid are two of these subgroups they are now well resolved from each other and from the main group the other low au subgroup has ni contents just above the main group two other iab subgroups have appreciably higher au contents than the main group and show weaker compositional links to it we have named these five subgroups on the basis of their au and ni contents the three subgroups having au contents similar to the main group are the low au l subgroups the two others the high au h subgroups the ni contents are designated high h medium m or low l thus the old group iiid is now the slh subgroup the old group iiic is the slm subgroup in addition eight irons assigned to two grouplets plot between sll and slm on most element au diagrams a large number 27 of related irons plot outside these compact fields but nonetheless appear to be sufficiently related to also be included in the iab complex many of these irons contain coarse silicates having similar properties most are roughly chondritic in composition the mafic silicates show evidence of reduction during metamorphism in each case the silicate o isotopic composition is within the carbonaceous chondrite range 17o 0 3 in all but four cases these are within the so called iab range 0 30 17o 0 68 fine silicates appear to be ubiquitous in the main group and low au subgroups this requires that viscosities in the parental melt reached high values before buoyancy could separate these the well defined main group trends on element au diagrams provide constraints for evaluating possible models we find the evidence to be most consistent with a crystal segregation model in which solid and melt are essentially at equilibrium the main arguments against the main group having formed by fractional crystallization are a the small range in ir and b the evidence for rapid crystallization and a high cooling rate through the iron stability field the evidence for the latter are the small sizes of the iron crystals parental to the widmanstatten pattern and the limited thermal effects recorded in the silicates including retention of albitic plagioclase and abundant primordial rare gases in contrast crystal segregation in a cooling metallic melt and related processes such as incomplete melting and melt migration can produce the observed trends in the main group we infer that this melt was formed by impact heating on a porous chondritic body and that the melt was initially hotter than the combined mix of silicates and metal in the local region the melt cooled rapidly by heat conduction into the cooler surroundings mainly silicates we suggest that the close compositional relationships between the main group and the low au subgroups are the result of similar processes instigated by independent impact events that occurred either at separate locations on the same asteroid or on separate but compositionally similar asteroids

olivine dominated asteroids and meteorites distinguishing nebular and igneous histories melting models indicate that the composition and abundance of olivine systematically co vary and are therefore excellent petrologic indicators however heliocentric distance and thus surface temperature has a significant effect on the spectra of olivine rich asteroids we show that composition and temperature complexly interact spectrally and must be simultaneously taken into account in order to infer olivine composition accurately we find that most 7 9 of the olivine dominated asteroids are magnesian and thus likely sampled mantles differentiated from ordinary chondrite sources e g pallasites however two other olivine rich asteroids 289 nenetta and 246 asporina are found to be more ferroan melting models show that partial melting cannot produce olivine rich residues that are more ferroan than the chondrite precursor from which they formed thus even moderately ferroan olivine must have non ordinary chondrite origins and therefore likely originate from oxidized r chondrites or melts thereof which reflect variations in nebular composition within the asteroid belt this is consistent with the meteoritic record in which r chondrites and brachinites are rare relative to pallasites