Quaternary Timescales Research Articles

River terraces and alluvial fans: The case for an integrated Quaternary fluvial archive

The fluvial archive literature is dominated by research on river terraces with appropriate mention of adjacent environments such as lakes. Despite modern sedimentary basins comprising a significant (>88%) volume of distributive fluvial systems, of which alluvial fans (>1 km, <30 km in scale) are a significant part, interaction with these environments tends to be neglected and discussed in separate literature. This paper examines the dynamic role of alluvial fans within the fluvial landscape and their interaction with river systems, highlighting the potential value of alluvial fans to the wider fluvial archive community. Published literature is used to examine both thematic and geographical based benefits of alluvial fan research that can assist understanding of Quaternary fluvial archives. 3 regional case studies are presented that illustrate the interaction between alluvial fan and river terrace archives at Quaternary time-scales at 3 different stages of landscape evolution. These are i) continuous mountain front alluvial fans interacting with a non incising but laterally eroding axial fluvial system; ii) alluvial fans which transition into fluvial terraces as sedimentary basins shift from net aggradation to net incision and iii) tributary-junction alluvial fans that develop predominantly within incising river valley systems. A simple conceptual model is proposed to summarise the dynamic role of alluvial fans within this landscape context. The alluvial fans act as potential ‘buffers’ between hillslopes and river terrace records under ‘top down’ climate-driven high sediment supply and alluvial fan aggradation, and ‘couplers’ during periods of less sediment (in relation to water) discharge and alluvial fan incision. These dynamics will change with the addition of ‘bottom up’ controls such as main river incision, which will typically enhance the coupling effect of both systems.

Violet stimulated luminescence dating of quartz from Luochuan (Chinese loess plateau): Agreement with independent chronology up to ∼600 ka

Luminescence dating at the Luochuan loess type (China) section is at present limited to ∼0.1 Ma using quartz blue light stimulated luminescence (BLSL), but can be extended back in time to ∼0.5 Ma by resorting to the more developmental post-infrared infrared stimulated luminescence (post-IR IRSL) and thermally transferred OSL (TT-OSL) signals. Since both the latter are associated with systematic uncertainties due to the potential (a)-thermal instability of these signals, a search continues for alternative, and demonstrably stable luminescence signals that can cover the entire Quaternary timescale. Here we explore the violet stimulated luminescence (VSL) signal at the Luochuan section, which provides a continuous archive of homogenous sediment with favourable luminescence characteristics and a solid independent age framework. By testing several VSL protocols and their associated performance, we demonstrate that the Multi-Aliquot Additive-Dose (MAAD) protocol produces a VSL chronology at Luochuan which is in agreement with independent ages up to ∼0.6 Ma. For a more representative environmental dose rate of ∼2 Gy/ka (∼35% lower than at Luochuan), the documented range of MAAD-VSL sensitivity (200–1800 Gy) would correspond to the ability to date sediment up to ∼1 Ma back in time, offering a remarkable advance over existing methods.

Multi-OSL-thermochronometry of feldspar

Optically stimulated luminescence (OSL)-thermochronometry has recently been proposed as a tool capable of resolving cooling histories from the top 1–2 km of the Earth's crust. This is beyond the resolution of most low-temperature thermochronometric systems, and it offers a new opportunity to investigate the interactions between climate, tectonics and surface processes over Quaternary timescales. Here we present a multi-OSL-thermochronometer which exploits the different thermal stabilities of different temperature infra-red stimulated luminescence (IRSL) signals from K- and Na-rich K-feldspar extracts, utilising the established multi-elevated-temperature (MET) measurement protocol (Li and Li, 2011. Luminescence dating of K-feldspar from sediments: A protocol without anomalous fading correction. Quaternary Geochronology 6, 468–479). The theoretical aspects of multi-OSL-thermochronometry are discussed, prior to validation with an example from the eastern Himalayan syntaxis, one of the most rapidly exhuming settings on Earth. Our results show that multi-OSL-thermochronometry of feldspar is able to resolve rock cooling histories over timescales ≤0.2 Ma and provides much tighter constraint on late-stage cooling histories than single-system OSL-thermochronometry.

Frequent but asymmetric niche shifts in Bulbophyllum orchids support environmental and climatic instability in Madagascar over Quaternary time scales.

BackgroundSpecies or clades may retain or shift their environmental niche space over evolutionary time. Understanding these processes offers insights into the environmental processes fuelling lineage diversification and might also provide information on past range dynamics of ecosystems. However, little is known about the relative contributions of niche conservatism versus niche divergence to species diversification in the tropics. Here, we examined broad-scale patterns of niche evolution within a Pliocene–Pleistocene clade of epiphytic Bulbophyllum orchids (30 spp.) whose collective distribution covers the northwest and eastern forest ecosystems of Madagascar.ResultsUsing species occurrence data, ecological niche models, and multivariate analyses of contributing variables, we identified a three-state niche distribution character for the entire clade, coinciding with three major forest biomes viz. phytogeographical provinces in Madagascar: A, Northwest ‘Sambirano’; B, ‘Eastern Lowlands’; and C, ‘Central Highlands’. A time-calibrated phylogeny and Bayesian models of niche evolution were then used to detect general trends in the direction of niche change over the clade’s history (≤5.3 Ma). We found highest transitions rates between lowlands (A and B) and (mostly from B) into the highland (C), with extremely low rates out of the latter. Lowland-to-highland transitions occurred frequently during the Quaternary, suggesting that climate-induced vegetational shifts promoted niche transitions and ecological speciation at this time.ConclusionsOur results reveal that niche transitions occurred frequently and asymmetrically within this Madagascan orchid clade, and in particular over Quaternary time scales. Intrinsic features germane to Bulbophyllum (e.g., high dispersal ability, drought tolerance, multiple photosynthetic pathways) as well as extrinsic factors (ecological, historical) likely interacted to generate the niche transition patterns observed. In sum, our results support the emerging idea of dramatic environmental and climatic fluctuations in Madagascar during the recent geological past, which overturns the long-held paradigm of long-term stability in tropical forest settings. The generality of the patterns and timings reported here awaits the availability of additional comparative studies in other Madagascan endemics.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0586-3) contains supplementary material, which is available to authorized users.

Orbital forcing of glacial/interglacial variations in chemical weathering and silicon cycling within the upper White Nile basin, East Africa: Stable-isotope and biomarker evidence from Lakes Victoria and Edward

On Quaternary time scales, the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae (notably diatoms, for which Si is an essential nutrient). Large tropical river systems dominate the export of Si from the continents to the oceans. Here, we investigate variations in Si cycling in the upper White Nile basin over the last 15 ka, using sediment cores from Lakes Victoria and Edward. Coupled measurements of stable O and Si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and Si cycling, while the abundances of lipid biomarkers characteristic of terrestrial/emergent higher plants, submerged/floating aquatic macrophytes and freshwater algae document past ecosystem changes. During the late-glacial to mid-Holocene, 15–5.5 ka BP, orbital forcing greatly enhanced monsoon rainfall, forest cover and chemical weathering. Riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower Si-isotope values. Since 5.5 ka BP, increasingly dry climates and more open vegetation, reinforced by the spread of agricultural cropland over the last 3–4 ka, have reduced dissolved silica inputs into the lakes. Centennial-to millennial-scale dry episodes are also evident in the isotopic records and merit further investigation.

An integrated field and numerical modelling study of controls on Late Quaternary fluvial landscape development (Tabernas, southeast Spain)

AbstractThe variability of Quaternary landforms preserved in the Tabernas basin of southeast (SE) Spain raises numerous questions concerning the roles of external forcing mechanisms (e.g. tectonics and/or climate) and internal landscape properties (e.g. lithological controls) in the evolution of the basin‐wide fluvial system over Late Quaternary timescales. In this study, we apply the FLUVER2 numerical model to investigate the significance of these landscape controls upon patterns of landscape evolution. We highlight the complications of generating realistic input datasets for use in the modelling of long‐term landscape evolution (e.g. discharge and runoff datasets). Model outputs are compared to extensive field mapping of fluvial terraces, their sedimentary architecture and optically stimulated luminescence dating results of the terraces. The results demonstrate the significance of non‐linear rates of flexural tectonic uplift towards the west of the Tabernas Basin which have controlled base levels throughout the Quaternary and promoted the formation of a series of diverging fluvial terraces. Our numerical model results further highlight the importance of climate cycles upon river terrace formation. Basin‐wide aggradation events were modelled during the transition from Marine Isotope Stage (MIS) 6 to 5 and the Last Glacial Maximum (LGM) as supported by field evidence. This aggradational pattern supports the regional hypothesis of terrace formation during global glacial cycles and cold‐to‐warm stage transitions and supports the use of sea surface temperature climate proxy data in the modelling exercise. The availability of sediments derived from the surrounding hillslopes and adjacent alluvial fans explains the generation of substantial terrace aggradations. Copyright © 2015 John Wiley &amp; Sons, Ltd.

An astronomical age for the Bishop Tuff and concordance with radioisotopic dates

AbstractThe Bishop Tuff forms a key stratigraphic horizon for synchronization of Quaternary sedimentary records in North America. The unit stratigraphically overlies the Matuyama‐Brunhes geomagnetic polarity reversal by several thousand years; high‐precision dating of this tuff may be valuable for regional and global correlation of records. The Quaternary time scale is anchored by 40Ar/39Ar ages on lava flows and ash layers where available, with stage boundaries and geomagnetic reversals including astronomically tuned records. However, astronomical dating has not yet validated the high‐precision 238U/206Pb zircon and 40Ar/39Ar sanidine ages of the Bishop Tuff. We have identified the Bishop Tuff within the marine sedimentary record and derived an astronomical age of 0.765 ± 0.008 Ma by correlation to the LR04 δ18O global benthic stack and its age model. This age is consistent with Bishop Tuff radioisotopic ages, including new single crystal 40Ar/39Ar sanidine fusion analyses presented here, which demonstrates that concordance through multiple dating techniques is achievable within the Quaternary.

Effective discharge for suspended sediment transport of the Ganga River and its geomorphic implication

Effective discharge (Qe) for suspended sediment transport in the alluvial reaches of the Ganga River in the western Ganga plains (WGP) has been computed using ‘analytical’ and an alternative ‘magnitude–frequency’ approach. Thirty years of mean monthly discharge data from various sites of the Ganga River have been assessed, and the abundance of discharge occurrence has been determined. Our analysis shows that less than 40% of the flow causes effective sediment transport in the Ganga, and this can be considered as the effective discharge for suspended sediment transport. Alternatively, 50% of the sediment load for all studied sites is moved by a discharge varying between 14 and 40% of the total discharge. Effective discharges calculated over the period of record are well below the bankfull discharges (Qb). A few events are close to the bankfull level, but with a high return period (RI>40years), and therefore, not effective to transport most of the available sediments. Our computation shows that the mean annual discharge (RI=2.33yrs) can transport only 0 to 10% of the total available sediments. The computation of effective discharge also provided important insights to understand the linkage between hydrology and channel morphology. Sediment storage and removal processes, which are reflected in sediment budget, cause changes in cross-sectional area/channel bathymetry at various sites but the channel margins are not affected. A high ratio of bankfull to effective discharge (Qb/Qe) forces the flow lines to be concentrated to the thalweg position and channels are incised. Our study also implies that incision and aggradation of the river valley during a relatively long period are caused by changes in effective discharge. We argue that the valley incision and filling episodes in the western Ganga plains at Late Quaternary timescales in response to monsoonal fluctuations were primarily affected by changes in the effective discharges of the rivers.

Shallow subsurface stratigraphy and alluvial architecture of the Kosi and Gandak megafans in the Himalayan foreland basin, India

The Kosi and the Gandak are two major Himalayan tributaries of the Ganga River in the north Bihar plains India. With a large hinterland in the Nepal Himalaya, both these rivers have generated megafans in the plains over the Quaternary time scale. Both these rivers are known to be highly dynamic and sediment-charged. A few conceptual models and limited field data suggested that these megafans have produced thick sand sheets over Late Quaternary period but these ideas have remained speculative and there is no data on the size and dimension of these sand bodies. This paper attempts to reconstruct the subsurface stratigraphy and alluvial architecture for the upper ~100m of the megafans based on electrical resistivity soundings, borehole data and drill cores. Alluvial architecture of the Kosi megafan shows significant variability from proximal to medial parts of the fan in terms of sediment grain size and layer thicknesses. While the medial part shows ~20–30m thick medium to coarse sand sheets which are laterally stacked, the proximal part of the fan has a dominantly gravel unit below ~15m depth that is underlain and overlain by medium to coarse sand units. Further, the medial fan also shows significant vertical and lateral variability in alluvial stratigraphy. The near-surface (<20m depth) deposits from the Kosi megafan have pockets of clay and silt within large amalgamated sand bodies whereas the shallow sub-surface (50–100m depth) sediments are largely sandy and devoid of clay and silt pockets. Alluvial architecture of the Gandak megafan shows two major lithounits; the upper fan succession has a higher stacking density of smaller sand bodies perhaps reflecting the migratory behavior of the river whereas the lower succession shows narrow but thick sand fills reflecting incised channels. The western part of the Gandak megafan has more abundant sand bodies compared to the eastern side of the river along both transects. There are no significant differences between proximal and medial transects across the Gandak megafan. The absence of gravel deposits in the shallow subsurface of the Gandak megafan may be attributed to the presence of a prominent intermontane valley in the hinterland of the Gandak river which has acted as a ‘sediment filter’ thereby trapping most of the coarser fraction. On the other hand, the Kosi river exits directly through the mountain front, and therefore, has been able to transport gravels into the plains in pre-historic times. Our study thus suggests significant variability in subsurface stratigraphy of the Kosi and the Gandak megafans even though they are located in similar geographic region. Such differences are attributed to the geomorphic diversity of the mountain exits of these megafans and their sediment transport history.

Volcanic disruption and drainage diversion of the palaeo-Hudut River, a tributary of the Early Pleistocene Gediz River, Western Turkey

The importance of extrinsic drivers of fluvial system behaviour (climate, tectonics, eustatic sea level) over Quaternary timescales is well documented. However, comparatively fewer studies have been reported concerning the significance of more localised changes at reach to sub-catchment scale, over these extended (104–106years) timescales. In this paper we examine the Early Pleistocene sedimentary record of the palaeo-Hudut River and compare it with the record from the trunk river into which it drains, the Gediz River of Western Turkey. Both the Gediz River and the Hudut River were subjected to major localised disruption during the Early Pleistocene as a consequence of volcanism but their respective responses to these events appear to differ. Observations are reported from the sedimentary sequence buried beneath the lavas which cap the Burgaz plateau. These sediments record a remarkable amount of detail for a significant period of the Early Pleistocene. These suggest that the palaeo-Hudut system responded largely to the creation and failure of downstream lava dams, both through channel incision and subsequent filling, and via route diversions around lava dams and their associated lakes. In contrast, the Gediz terrace record appears to demonstrate a river which was able to accommodate these changes more readily and hence, continue to undergo sedimentation–incision cycles consistent with a climate forcing.

Specialization in plant-hummingbird networks is associated with species richness, contemporary precipitation and quaternary climate-change velocity.

Large-scale geographical patterns of biotic specialization and the underlying drivers are poorly understood, but it is widely believed that climate plays an important role in determining specialization. As climate-driven range dynamics should diminish local adaptations and favor generalization, one hypothesis is that contemporary biotic specialization is determined by the degree of past climatic instability, primarily Quaternary climate-change velocity. Other prominent hypotheses predict that either contemporary climate or species richness affect biotic specialization. To gain insight into geographical patterns of contemporary biotic specialization and its drivers, we use network analysis to determine the degree of specialization in plant-hummingbird mutualistic networks sampled at 31 localities, spanning a wide range of climate regimes across the Americas. We found greater biotic specialization at lower latitudes, with latitude explaining 20–22% of the spatial variation in plant-hummingbird specialization. Potential drivers of specialization - contemporary climate, Quaternary climate-change velocity, and species richness - had superior explanatory power, together explaining 53–64% of the variation in specialization. Notably, our data provides empirical evidence for the hypothesized roles of species richness, contemporary precipitation and Quaternary climate-change velocity as key predictors of biotic specialization, whereas contemporary temperature and seasonality seem unimportant in determining specialization. These results suggest that both ecological and evolutionary processes at Quaternary time scales can be important in driving large-scale geographical patterns of contemporary biotic specialization, at least for co-evolved systems such as plant-hummingbird networks.

Effectiveness of combined unspiked K–Ar and 40Ar/ 39Ar dating methods in the 14C age range

In this study, the effectiveness of combined unspiked K–Ar and 40Ar/ 39Ar dating methods as currently applied now at LSCE in the 14C age range was evaluated by studying two 30,000 years old phonolites from Tenerife (Canary Islands). New 40Ar/ 39Ar ages obtained in this work are comparable with the K–Ar ages, and of similar precision. This remarkable agreement obtained between the two dating methods in this young age range, validates the new LSCE 40Ar/ 39Ar facility and provides a very promising approach to calibrate the Quaternary timescale and to date key events such as geomagnetic field instabilities and climatic changes. The approach combining the 40Ar/ 39Ar and the unspiked K–Ar methods has the advantage that all the basic assumptions, which rule the K–Ar clock, are checked via the isochron approach. In the event of positive check, the 40Ar/ 39Ar and unspiked K–Ar ages can be pooled to produce very precise ages.

Assessment of sediment delivery from successive erosion on stream-coupled hillslopes via a time series of topographic surveys in the central high mountain range of Taiwan

Landslides play an important role in the evolution of mountain topography and are a major source of sediment in tectonically active mountain belts. For the active orogeny of Taiwan, erosion rates have been assessed by various methods. The discrepancies observed between erosion rates integrated over modern and Quaternary time scales may result from various effects related to surface processes.More process-oriented research on the spatio-temporal distribution of hillslope processes and coupling relationships with the fluvial system is needed to reconcile the temporally heterogeneous data in the future. This paper assesses and describes the processes of subsequent erosion on hillslopes via a time series of topographic surveys of hillslopes after the initial failure in order to better understand the temporal and spatial variability of landslide erosion, sediment fluxes, and storage of landslide sediment within channels.The results demonstrate that hillslope-channel coupling is controlled by the relative frequency of erosive flooding events and the magnitude of rainfall-driven hillslope processes. The latter is assumed to be disproportionately high on larger failures in periods with less rainfall.Mass wasting may only affect part of the total failure’s area. Volumes of eroded sediment from gullies on the landslide scar may be as high as the volume estimated for the initial failure. The geometry of the sliding masses eroded after the initial failure appears to be uniform, and their depths average 6.1 ± 2.2 m.

Partitioning of localized and diffuse deformation in the Tibetan Plateau from joint inversions of geologic and geodetic observations

The spatial complexity of continental deformation in the greater Tibetan Plateau region can be defined as the extent to which relative motion of the Indian and Asian plates is partitioned between localized slip on major faults and distributed deformation. Potency rates provide a quantitative metric for determining the magnitudes of diffuse and on-fault crustal deformation, which are proportional to strain rates within crustal micro-plates and fault slip rates, respectively. We simultaneously estimate micro-plate rotation rates, interseismic elastic strain accumulation, fault slip rates on major structures, and strain rates within 24 tectonic micro-plates inferred from active fault maps in the greater Tibetan Plateau region using quasi-static block models constrained by interseismic surface velocities at 731 GPS sites and 9 Holocene–Late Quaternary geologic fault slip rates. The joint geodetic–geologic inversion indicates that geologic slip rates are kinematically consistent with differential micro-plate motions. Estimated left-lateral slip rates on the Altyn Tagh, west-central Kunlun, and Xianshuihe faults are relatively homogeneous along strike (~ 11.5, 10.5, and 11 mm/yr, respectively) while segmentation of the eastern Kunlun fault by the intersecting Elashan and Riyueshan faults results in a decreased slip rate, consistent with geologic observations. The fraction, ϕ, of total potency rate associated with intrablock strain, uncorrected for observational noise, ranges from 0.27 in the Himalayan Range block to 0.91 in the Aksai Chin block. Monte Carlo simulations are used to quantify the likelihood that internal deformation is statistically distinguishable from the uncertainties in geodetic velocities. These simulations indicate that internal block deformation is statistically significant only within the Himalayan Range Front (where internal deformation accounts for ϕ ID = 0.11 of block potency rate budget), west-central plateau ( ϕ ID = 0.68), Ganzi-Yushu/Xianshuihe (0.58), Burma (0.09), Jiali (0.38), and Aksai Chin (0.68) blocks. In the other 18 tectonic micro-plates within the plateau region, estimated internal block potency is not currently distinguishable from the expected contribution of observational noise to residual velocities. Of the total potency budget within the Tibetan Plateau, 85% is taken up by slip on major faults, with the remaining 15% accommodated by internal processes at sub-block scale distinguishable from observational noise. The localization of the majority of plate boundary activity is also supported by the spatial distribution of modern and historical crustal earthquakes. Sixty-seven percent of the total moment released by earthquakes in the CMT catalog and 90% of historical moment since 1900 have been released within 25 km of the major faults included in the block model, representing only 12% of the characteristic half-block length scale of ~ 215 km. The localization of deformation inferred from geologic, geodetic, and seismic observations suggests that forces applied to tectonic micro-plates drive fault system activity at the India–Asia collision zone over decadal to Quaternary time scales.

Arctic amplification: can the past constrain the future?

Arctic amplification, the observation that surface air temperature changes in the Arctic exceed those of the Northern Hemisphere as a whole, is a pervasive feature of climate models, and has recently emerged in observational data relative to the warming trend of the past century. The magnitude of Arctic amplification is an important, but poorly constrained variable necessary to estimate global average temperature change over the next century. Here we evaluate the mechanisms responsible for Arctic amplification on Quaternary timescales, and review evidence from four intervals in the past 3 Ma for which sufficient paleoclimate data and model simulations are available to estimate the magnitude of Arctic amplification under climate states both warmer and colder than present. Despite differences in forcings and feedbacks for these reconstructions compared to today, the Arctic temperature change consistently exceeds the Northern Hemisphere average by a factor of 3–4, suggesting that Arctic warming will continue to greatly exceed the global average over the coming century, with concomitant reductions in terrestrial ice masses and, consequently, an increasing rate of sea level rise.

Late Quaternary paleosols and their paleoenvironmental significance along the Andean piedmont, Eastern Bolivia

The Andean piedmont of eastern Bolivia is situated at the southern margin of Amazonia characterized by an overall humid climate regime with a marked contrast between the rainy and dry seasons. The nearby Subandean foothills deliver abundant sandy sediments to the piedmont, leading to a complex array of sediments and paleosol horizons. Within this setting, the presented study analyzes four profiles of paleosol-sediment-sequences along incised ephemeral streams near Santa Cruz de la Sierra with a focus on past pedogenic variability in the context of the regional late Quaternary geomorphic and environmental evolution. Based on field observations, micromorphological analysis, geochemical and clay mineralogical data five classes of paleosol horizons could be distinguished. The individual paleosol horizons as well as the sediments, in which they developed, were interpreted regarding their paleoenvironmental significance, taking into consideration the various controls on soil formation with particular focus on changes of local environmental conditions through time. Thus, three different pathways of soil formation were established. On the late Quaternary timescale, the results suggest a strong relation between paleoenvironmental conditions (climate, vegetation etc.), soil environment (soil water flow, micro-environment) and the type of paleosol horizons developed in the study area. The formation of “red beds” (Bw horizons) implies very dry soil environments under dominantly dry conditions, which seem to have prevailed in the study area some time before ∼ 18 cal ka BP. Moderately dry but markedly seasonal environmental conditions with a long dry season and strong seasonal contrasts in soil water flow could explain the formation of moderately developed Bwt horizons around ∼ 18 cal ka BP and much of the mid-Holocene. The formation of Bt horizons and/or clay lamellae in relation to intense neoformation of clay and dominant clay illuviation by soil water points to wet conditions similar to today, which have probably prevailed in the study area before ∼ 8 cal ka BP and since ∼ 5 cal ka BP.

Riser diachroneity, lateral erosion, and uncertainty in rates of strike‐slip faulting: A case study from Tuzidun along the Altyn Tagh Fault, NW China

At late Quaternary timescales, offset fluvial terrace risers are among the most common landforms used to determine rates of strike‐slip faulting. Although diachroneity in the age of riser segments on opposite sides of the fault has been noted previously, an unexplored source of uncertainty associated with deriving slip rates from these markers centers on quantifying the size of the displacement uncertainty such diachroneity introduces. To evaluate the impact of riser diachroneity, we investigated the Tuzidun site (37.73°N, 86.72°E) along the Cherchen He reach of the active central Altyn Tagh Fault. The east bank of the channel is flanked by a left‐laterally offset terrace riser. While the measured offset is 54 ± 3 m, geochronologic measurements and analysis of riser topography indicate that the downstream riser segment formed between 6.0 ± 0.8 ka and 5.7 ± 0.4 ka, while the upstream riser segment may have been laterally refreshed as recently as 0.5 ± 0.2 ka. A valley wall on the west bank of the channel places a maximum limit of 38 ± 6 m on the amount of possible lateral erosion of the upstream riser. This bound, in turn, limits the total offset since formation of the downstream riser to range from 54 ± 3 to 89 ± 7 m. Together, these observations bracket the millennial Altyn Tagh Fault slip rate to range from 9.0 ± 1.3 to 15.5 ± 1.7 mm a−1. More generally, this investigation shows that the observed riser displacement does not necessarily correlate with the age of either riser segment (downstream or upstream of the fault) in cases where one segment is displaced while the other is subjected to lateral erosion. If this diachroneity goes undetected, erroneous slip rate measurements are likely to result.

ESF EuroCLIMATE Spring School: Late Quaternary timescales and chronology

ESF EuroCLIMATE Spring School: Late Quaternary timescales and chronology

Distinguishing allogenic from autogenic causes of bed elevation change in late Quaternary alluvial stratigraphic records

Allogenic and autogenic mechanisms both cause changes in the bed elevation of rivers and thereby influence the characteristics of alluvial stratigraphic records (ASRs). Allogenic forcing mechanisms can be grouped into five categories whose relative influence varies with timescale: climate, tectonism, base level, land use/land cover and direct human modification of channels. Late Quaternary ASRs are influenced by the greatest range of allogenic forcing variables with climate among the most important. Autogenic mechanisms of bed elevation change are ubiquitous throughout fluvial systems and are always time-transgressive. Autogenic bed elevation change propagates through drainage networks at predictable rates and results in a time-space envelope within which its effects are capable of operating. ASRs that can be correlated over geographical areas large enough and time intervals small enough to exist outside this envelope most likely result from allogenic forcing. This formulation represents a quantitative and geographic set of threshold criteria for distinguishing between autogenic and allogenic mechanisms. Over late Quaternary timescales (10 2 to 10 5 years) in tectonically stable regions climate change is the dominant allogenic mechanism and, therefore, the first-order control on the morphology, sedimentology, pedologic characteristics and chronology of alluvial stratigraphic records that meet or exceed the threshold criteria for demonstrating allogenic causality.

Preservation patterns of Late Cenozoic fluvial deposits and their implications: Results from IGCP 449

IGCP 449 (2000–2004) amassed data on fluvial systems world-wide, concentrating on key sequences, especially those benefiting from multiple lines of dating evidence. These archives are preserved either in terraced or stacked sequences, the latter confined to areas of subsidence. Terrace staircases record repeated incision (in response to progressive surface uplift), which, alternately with aggradation, is thought to have been climatically triggered. This triggering can be attributed to Quaternary (Milankovitch) climatic fluctuation, but the number of terraces produced in each Middle–Late Pleistocene 100 ka climatic cycle varies significantly between different systems. An unexpected result of this data collection has been the recognition of differing patterns of fluvial sedimentation and valley evolution over Neogene and Quaternary timescales, apparently related to different types of continental crust with different uplift/subsidence histories. These fall into three groups: (1) a typical uplifting pattern, with extensive terrace staircases of the type that dominate the global fluvial archive; (2) a subsiding pattern, with stacked fluvial sediments, usually coinciding with major depocentres; and (3) a stable pattern, with preservation of deposits related to channel diversion rather than significant incision. The third type, in which Early and pre-Quaternary deposits occur within a few metres of modern river level, is generally restricted to ancient cratonic or shield areas, which have apparently experienced minimal Late Cenozoic uplift, in marked contrast with most continental areas on more recently formed crust. There is also an intermediate situation, in Early Proterozoic crust, involving alternations of uplift and subsidence, with little resultant net vertical motion.