Quaternary drainage network reorganization in the Colombian Eastern Cordillera plateau

Dramatic drainage reorganization from initial longitudinal to transversal domains has occurred in the Eastern Cordillera of Colombia. We perform a regional analysis of drainage basin geometry and transformed river profiles based on the integral form of the slope-area scaling, to investigate the dynamic state of drainage networks and to predict the degree of drainage reorganization in this region. We propose a new model of drainage rearrangement for the Eastern Cordillera, based on the analyses of knickpoint distribution, normalized river profiles, landforms characteristic of river capture, erosion rates and palaeodrainage data. We establish that the oldest longitudinal basin captured by the Magdalena River network was the Suárez Basin at ≈409 ka, inferring the timing of abandonment of a river terrace using in situ produced cosmogenic beryllium-10 (¹⁰Be) depth profiles and providing a first estimation of incision rate of 0.07 mm/yr. We integrate published geochronologic data and interpret the last capture of the Sabana de Bogotá, providing a minimum age of the basin opening to the Magdalena drainage at ≈38 ka. Our results suggest that the Magdalena basin Increased its drainage area by integrating the closed basins from the western flank of the Eastern Cordillera. Our study also suggests that the Magdalena basin is an aggressor compared to the basins located in the eastern flank of the orogen and provides a framework for examining drainage reorganization within the Eastern Cordillera and in similar orogenic settings. The results improve our understanding of headward integration of closed basins across orogenic plateaux. © 2020 John Wiley & Sons, Ltd.     

10Be‐derived assessment of accelerated erosion in a glacially conditioned inner gorge,Entlebuch, Central Alps of Switzerland

Inner gorges often result from the propagation of erosional waves related to glacial/interglacial climate shifts. However, only few studies have quantified the modern erosional response to this glacial conditioning. Here, we report in situ ¹⁰Be data from the 64 km² Entlen catchment (Swiss Alps). This basin hosts a 7 km long central inner gorge with two tributaries that are >100 m‐deeply incised into thick glacial till and bedrock. The ¹⁰Be concentrations measured at the downstream end of the gorge yield a catchment‐wide erosion rate of 0.42 ± 0.04 mm yr^‐1, while erosion rates are consistently lower upstream of the inner gorge, ranging from 0.14 ± 0.01 mm yr^‐1 to 0.23 ± 0.02 mm yr^‐1. However, ¹⁰Be‐based sediment budget calculations yield rates of ~1.3 mm yr^‐1 for the inner gorge of the trunk stream. Likewise, in the two incised tributary reaches, erosion rates are ~2.0 mm yr^‐1 and ~1.9 mm yr^‐1. Moreover, at the erosional front of the gorge, we measured bedrock incision rates ranging from ~2.5 mm yr^‐1 to ~3.8 mm yr^‐1. These rates, however, are too low to infer a post‐glacial age (15–20 ka) for the gorge initiation. This would require erosion rates that are between 2 and 6 times higher than present‐day estimates. However, the downcutting into unconsolidated glacial till favored high erosion rates through knickzone propagation immediately after the retreat of the LGM glaciers, and subsequent hillslope relaxation led to a progressive decrease in erosion rates. This hypothesis of a two‐ to sixfold decrease in erosion rates does not conflict with the ¹⁰Be‐based erosion rate budgets, because the modern erosional time scale recorded by ¹⁰Be cover the past 2–3 ka only. These results point to the acceleration of Holocene erosion in response to the glacial overprint of the landscape. Copyright © 2012 John Wiley & Sons, Ltd.     

Erosion history of the Tibetan Plateau since the last interglacial: constraints from the first studies of cosmogenic 10Be from Tibetan bedrock

The cosmogenic ¹⁰Be exposure histories of in situ bedrock surfaces from the Tibetan Plateau indicate low erosion rates of <30 mm/ka in southern and central Tibet during the last interglacial–glacial cycle that contrast strongly with unusually rapid erosion rates (60–2000 mm/ka) for Kunlun in northern Tibet during the Holocene, comparable with published values from the Himalaya. By comparing apatite fission-track ages with cosmogenic data, erosion rates in southern Tibet appear to be decelerating since the Miocene, whereas in the Kunlun, erosion rates have accelerated over the same timescale. Such secular changes suggest that the southern and central regions of the plateau had formed their present flat relief by the Pleistocene. Unusually high erosion rates along the northern margin of the plateau may reflect intense tectonic activity during the Holocene. These findings indicate that over much of the high plateau erosion rates are exceptionally low, and therefore the sources of detritus carried by the great Asian rivers that rise in Tibet lie overwhelmingly in bedrocks at lower altitudes. This study illustrates the potential of cosmogenic studies for unraveling the most recent phase of the erosion/exhumation history of orogenic belts that cannot be resolved by either Ar-isotope or fission-track thermochronometers.     

Influence of anthropogenic land‐use change on hillslope erosion in the Waipaoa River Basin,New Zealand

European settlement of the Poverty Bay Region resulted in deforestation and conversion of > 90% of the landscape to pastureland. The resulting loss of vegetation triggered a rapid increase in hillslope erosion as widespread landslide complexes and gully systems developed on weak lithologic units in the Waipaoa Basin. To quantify the rate and volume of historic hillslope degradation, we used a 1956–2010 sequence of aerial photographs for a ~16 km² catchment to map temporal changes in the spatial extent of active landslides. Then we created a ‘turf index’ based on the extent and style of pastoral ground disruption, which correlates with downslope velocity. Based on the movement of trees and other features, we assigned average velocities to the turf classes as follows: (1) minimal disrupted ground: 0.6 m/yr, (2) a mix of disrupted ground and intact blocks: 3.4 m/yr, and (3) no intact blocks or vegetation: > 6 m/yr. We then calculated the average annual sediment flux using these turf‐derived velocities, the width of the landslide‐channel intersection, and an average toe depth of 4.4 ± 1.3 m (mean ± standard deviation [SD]) from 37 field measurements. The resulting catchment averaged erosion rates are (mean ± SD): 29.9 ± 12.9 mm/yr (1956), 28.8 ± 13.7 mm/yr (1969), 13.4 ± 4.9 mm/yr (1979), 17.0 ± 6.2 mm/yr (1988), and 9.9 ± 3.6 mm/yr (2010). Compared with long‐term (post‐18 ka) erosion rates (1.6 mm/yr) and the long‐term uplift rate (~1 mm/yr) for this site, the 50‐year anthropogenically‐driven rate is an order of magnitude larger (~20 mm/yr). Previously, we measured an increase in erosion over the past 3.4 kyr (2.2 mm/yr), and here, we demonstrate this increase could be primarily due to human land‐use change – showing that a century of rapid erosion superimposed on the background geologic rate can profoundly skew the interpretation of erosion rates. Copyright © 2016 John Wiley & Sons, Ltd.     

Very slow erosion rates and landscape preservation across the southwestern slope of the Ladakh Range,India

Erosion rates are key to quantifying the timescales over which different topographic and geomorphic domains develop in mountain landscapes. Geomorphic and terrestrial cosmogenic nuclide (TCN) methods were used to determine erosion rates of the arid, tectonically quiescent Ladakh Range, northern India. Five different geomorphic domains are identified and erosion rates are determined for three of the domains using TCN ¹⁰Be concentrations. Along the range divide between 5600 and 5700 m above sea level (asl), bedrock tors in the periglacial domain are eroding at 5.0 ± 0.5 to 13.1 ± 1.2 meters per million years (m/m.y.)., principally by frost shattering. At lower elevation in the unglaciated domain, erosion rates for tributary catchments vary between 0.8 ± 0.1 and 2.0 ± 0.3 m/m.y. Bedrock along interfluvial ridge crests between 3900 and 5100 m asl that separate these tributary catchments yield erosion rates <0.7 ± 0.1 m/m.y. and the dominant form of bedrock erosion is chemical weathering and grusification. Erosion rates are fastest where glaciers conditioned hillslopes above 5100 m asl by over‐steepening slopes and glacial debris is being evacuated by the fluvial network. For range divide tors, the long‐term duration of the erosion rate is considered to be 40–120 ky. By evaluating measured ¹⁰Be concentrations in tors along a model ¹⁰Be production curve, an average of ~24 cm is lost instantaneously every ~40 ky. Small (<4 km²) unglaciated tributary catchments and their interfluve bedrock have received very little precipitation since ~300 ka and the long‐term duration of their erosion rates is 300–750 ky and >850 ky, respectively. These results highlight the persistence of very slow erosion in different geomorphic domains across the southwestern slope of the Ladakh Range, which on the scale of the orogen records spatial changes in the locus of deformation and the development of an orogenic rain shadow north of the Greater Himalaya. Copyright © 2014 John Wiley & Sons, Ltd.     

Complex exposure histories of chert clasts in the late Pleistocene shorelines of Lake Lisan,southern Israel

Activities of ²⁶Al and ¹⁰Be in five chert clasts sampled from two beach ridges of late Pleistocene Lake Lisan, precursor of the Dead Sea in southern Israel, indicate low rates of chert bedrock erosion and complex exposure, burial, and by inference, transport histories. The chert clasts were derived from the Senonian Mishash Formation, a chert‐bearing chalk, which is widely exposed in the Nahal Zin drainage basin, the drainage system that supplied most of the material to the beach ridges. Simple exposure ages, assuming only exposure at the beach ridge sampling sites, range from 35 to 354 ky; using the ratio ²⁶Al/¹⁰Be, total clast histories range from 0·46 to 4·3 My, unrelated to the clasts' current position and exposure period on the late Pleistocene beach ridges, 160–177 m below sea level. Optically stimulated luminescence dating of fine sediments from the same and nearby beach ridges yielded ages of 20·0 ± 1·4 ka and 36·1 ± 3·3 ka. These ages are supported by the degree of soil development on the beach ridges and correspond well with previously determined ages of Lake Lisan, which suggest that the lake reached its highest stand around 27 000 cal. years BP . If the clasts were exposed only once and than buried beyond the range of significant cosmogenic nuclide production, then the minimum initial exposure and the total burial times before delivery to the beach ridge are in the ranges 50–1300 ky and 390–3130 ky respectively. Alternatively, the initial cosmogenic dosing could have occurred during steady erosion of the source bedrock. Back calculating such rates of rock erosion suggests values between 0·4 and 12 m My^?1. The relatively long burial periods indicate extended sediment storage as colluvium on slopes and/or as alluvial deposits in river terraces. Some clasts may have been stored for long periods in abandoned Pliocene and early Pleistocene routes of Nahal Zin to the Mediterranean before being transported again back into the Nahal Zin drainage system and washed on to the shores of Lake Lisan during the late Pleistocene. Copyright © 2003 John Wiley & Sons, Ltd.     

Quantifying hillslope erosion rates and processes for a coastal California landscape over varying timescales

The Earth's surface erodes by processes that occur over different spatial and temporal scales. Both continuous, low‐magnitude processes as well as infrequent, high‐magnitude events drive erosion of hilly soil‐mantled landscapes. To determine the potential variability of erosion rates we applied three independent, field‐based methods to a well‐studied catchment in the Marin Headlands of northern California. We present short‐term, basin‐wide erosion rates determined by measuring pond sediment volume (40 years) and measured activities of the fallout nuclides ¹³⁷Cs and ²¹⁰Pb (40–50 years) for comparison with long‐term (>10 ka) rates previously determined from in situ‐produced cosmogenic ¹⁰Be and ²⁶Al analyses. In addition to determining basin‐averaged rates, ¹³⁷Cs and ²¹⁰Pb enable us to calculate point‐specific erosion rates and use these rates to infer dominant erosion processes across the landscape. When examined in the context of established geomorphic transport laws, the correlations between point rates of soil loss from ¹³⁷Cs and ²¹⁰Pb inventories and landscape morphometry (i.e. topographic curvature and upslope drainage area) demonstrate that slope‐driven processes dominate on convex areas while overland flow processes dominate in concave hollows and channels. We show a good agreement in erosion rates determined by three independent methods: equivalent denudation rates of 143 ± 41 m Ma^?1 from pond sediment volume, 136 ± 36 m Ma^?1 from the combination of ¹³⁷Cs and ²¹⁰Pb, and 102 ± 25 m Ma^?1 from ¹⁰Be and ²⁶Al. Such agreement suggests that erosion of this landscape is not dominated by extreme events; rather, the rates and processes observed today are indicative of those operating for at least the past 10 000 years. Copyright © 2006 John Wiley & Sons, Ltd.     

Quantifying periglacial erosion: insights on a glacial sediment budget,Matanuska Glacier,Alaska

Glacial erosion rates are estimated to be among the highest in the world. Few studies have attempted, however, to quantify the flux of sediment from the periglacial landscape to a glacier. Here, erosion rates from the nonglacial landscape above the Matanuska Glacier, Alaska are presented and compare with an 8‐yr record of proglacial suspended sediment yield. Non‐glacial lowering rates range from 1·8 ± 0·5 mm yr^?1 to 8·5 ± 3·4 mm yr^?1 from estimates of rock fall and debris‐flow fan volumes. An average erosion rate of 0·08 ± 0·04 mm yr^?1 from eight convex‐up ridge crests was determined using in situ produced cosmogenic ¹⁰Be. Extrapolating these rates, based on landscape morphometry, to the Matanuska basin (58% ice‐cover), it was found that nonglacial processes account for an annual sediment flux of 2·3 ± 1·0 × 10⁶ t. Suspended sediment data for 8 years and an assumed bedload to estimate the annual sediment yield at the Matanuska terminus to be 2·9 ± 1·0 × 10⁶ t, corresponding to an erosion rate of 1·8 ± 0·6 mm yr^?1: nonglacial sources therefore account for 80 ± 45% of the proglacial yield. A similar set of analyses were used for a small tributary sub‐basin (32% ice‐cover) to determine an erosion rate of 12·1 ± 6·9 mm yr^?1, based on proglacial sediment yield, with the nonglacial sediment flux equal to 10 ± 7% of the proglacial yield. It is suggested that erosion rates by nonglacial processes are similar to inferred subglacial rates, such that the ice‐free regions of a glaciated landscape contribute significantly to the glacial sediment budget. The similar magnitude of nonglacial and glacial rates implies that partially glaciated landscapes will respond rapidly to changes in climate and base level through a rapid nonglacial response to glacially driven incision. Copyright © 2009 John Wiley & Sons, Ltd.     

Variability in erosion rates related to the state of landscape transience in the semi‐arid Chilean Andes

We quantify erosion rates in the higher sectors of the Huasco Valley, in the Main Cordillera of the semi‐arid Andes of Chile, using elevation differences between three successive geomorphic markers (pediments and paleo‐valleys) and the present day valley. Available Ar‐Ar ages of Neogene pediments are used to estimate mean erosion rates for the three periods (16 to 13 My, 13 to 8 My, and following 8 My). The landscape of the Huasco Valley is in a transient state, as indicated by well‐preserved pediment surfaces in interfluves, valleys deeply incised by fluvial and glacial erosion and scarped head‐valleys that represent the current knickzones. Higher erosion rates (45–75 m/My) are calculated for the more recent period (< 8 My) during which deep incision developed compared to previous periods (6–31 m/My). Quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the Main Cordillera. Comparison with erosion rates calculated in other drainage basins of the Chilean Andes suggests that the variability of erosion rates depends on the landscape's transient erosive state. The landscape's geomorphologic response to the uplift of the Main Cordillera results in the retreat of a knickzone, for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity. Copyright © 2011 John Wiley & Sons, Ltd.     

Non-steady long-term uplift rates and Pleistocene marine terrace development along the Andean margin of Chile (31°S) inferred from 10Be dating

Pleistocene uplift of the Chilean coast is recorded by the formation of wave-cut platforms resulting from marine erosion during sea-level highstands. In the Altos de Talinay area (~ 31°S), we have identified a sequence of 5 wave-cut platforms. Using in situ produced ¹⁰Be exposure ages we show that these platforms were formed during interglacial periods at 6, 122, 232, 321 and 690 ka. These ages correspond to marine isotopic stages (MIS) or substages (MISS) 1, 5e, 7e, 9c and 17. Shoreline angle elevations used in conjunction with our chronology of wave-cut platform formation, illustrate that surface uplift rates vary from 103 ± 69 mm/ka between 122 and 6 ka, to 1158 ± 416 mm/ka between 321 and 232 ka. The absence of preserved platforms related to the MIS 11, 13 and 15 highstands likely reflects slow uplift rates during these times. We suggest that since 700 ka, the Altos de Talinay area was predominantly uplifted during 2 short periods following MIS 17 and MISS 9c. This episodic uplift of the Chilean coast in the Pleistocene may result from subduction related processes, such as pulses of tectonic accretion at the base of the forearc wedge.     

Possible climatic controls on the accumulation of Peru's most prominent alluvial fan: The Lima Conglomerate

Sediment accumulation can occur in response to a change in either tectonic or climatic driving forces. Here, we explore these controls on the deposition of the Lima Conglomerate, Peru. We use a combination of quantitative methods to explore the age of sediment accumulation, the provenance of the material and the paleo-erosion rates recorded by these deposits. Isochron burial dating with cosmogenic ¹⁰Be and ²⁶Al yield an age of c. 500 ka for the base (490 ± 70 ka) and the uppermost sample situated c. 30 m higher upsection (490 ± 80 ka). Results of paleo-erosion rate estimates with concentrations of in situ ¹⁰Be show a c. 60% increase from 105 ± 10 mm ka^-1 for the base to 169 ± 14 mm ka^-1 for the uppermost sample. Finally, provenance tracing with in situ U/Pb ages on detrital zircon implies that the material has been derived from the entire drainage basin. The combination of results suggests that sediment accumulation occurred in response to an erosional pulse, which affected the entire basin within a short time interval. Because ¹⁰Be data represents a large spatial record of erosion, we exclude the possibility where a breakout of a lake or a focused release of material in response to earthquakes, were responsible for the large material flux. Instead, the erosional pulse was likely to have occurred at the scale of the entire basin, supporting the idea of a larger-scale, most likely climate driven control. In this context, the accumulation age of c. 500 ka falls into an orbital cycle fostering the emerging picture in the literature that sediment routing in the Andes have most likely been driven by climate and cyclic changes. We suggest that the Andean mountain range offers an ideal laboratory to explore the erosional history in relation to climate patterns, at least in Peru. © 2018 John Wiley & Sons, Ltd.     

Multitemporal ALSM change detection,sediment delivery,and process mapping at an active earthflow

Remote mapping and measurement of surface processes at high spatial resolution is among the frontiers in Earth surface process research. Remote measurements that allow meter‐scale mapping of landforms and quantification of landscape change can revolutionize the study of landscape evolution on human timescales. At Mill Gulch in northern California, USA, an active earthflow was surveyed in 2003 and 2007 by airborne laser swath mapping (ALSM), enabling meter‐scale quantification of landscape change. We calculate four‐year volumetric flux from the earthflow and compare it to long‐term catchment average erosion rates from cosmogenic radionuclide inventories from adjacent watersheds. We also present detailed maps of changing features on the earthflow, from which we can derive velocity estimates and infer dominant process. These measurements rely on proper digital elevation model (DEM) generation and a simple surface‐matching technique to align the multitemporal data in a manner that eliminates systematic error in either dataset. The mean surface elevation of the earthflow and an opposite slope that was directly influenced by the earthflow decreased 14 ± 1 mm/yr from 2003 to 2007. By making the conservative assumption that these features were the dominant contributor of sediment flux from the entire Mill Gulch drainage basin during this time interval, we calculate a minimum catchment‐averaged erosion rate of 0·30 ± 0·02 mm/yr. Analysis of beryllium‐10 (¹⁰Be) concentrations in fluvial sand from nearby Russian Gulch and the South Fork Gualala River provide catchment averaged erosion rates of 0·21 ± 0·04 and 0·23 ± 0·03 mm/yr respectively. From translated landscape features, we can infer surface velocities ranging from 0·5 m/yr in the wide upper ‘source’ portion of the flow to 5 m/yr in the narrow middle ‘transport’ portion of the flow. This study re‐affirms the importance of mass wasting processes in the sediment budgets of uplifting weak lithologies. Copyright © 2011 John Wiley & Sons, Ltd.     

Erosion in northwest Tibet from in‐situ‐produced cosmogenic 10Be and 26Al in bedrock

Concentrations of in‐situ‐produced cosmogenic nuclides ¹⁰Be and ²⁶Al in quartz were measured by accelerator mass spectrometry for bedrock basalts and sandstones located in northwest Tibet. The effective exposure ages range between 23 and 134 ka (¹⁰Be) and erosion rates between 4·0 and 24 mm ka^?1. The erosion rates are significantly higher than those in similarly arid Antarctica and Australia, ranging between 0·1 and 1 mm ka^?1, suggesting that precipitation is not the major control of erosion of landforms. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role in controlling long‐term erosion rates. The obtained erosion rates are, however, significantly lower than the denudation rate of 3000–6000 mm ka^?1 beginning at c. 5‐3 Ma in the nearby Godwin Austen (K2) determined by apatite fission‐track thermochronology. It appears that the difference in erosion rates within different time intervals is indicative of increased tectonic activity at c. 5–3 Ma in northwest Tibet. We explain the low erosion rates determined in this study as reflecting reduced tectonic activity in the last million years. A model of localized thinning of the mantle beneath northwest Tibet may account for the sudden increased tectonic activity at c. 5–3 Ma and the later decrease. Copyright © 2006 John Wiley & Sons, Ltd.     

From mass‐wasting to slope stabilization – putting constrains on a tectonically induced transition in slope erosion mode: a case study in the Judea Hills,Israel

Calcrete‐coated remnants of landslide debris and alluvial deposits are exposed along the presently stable hillslopes of the Soreq drainage, Judea Hills, Israel. These remnants indicate that a transition from landslide‐dominated terrain to dissolution‐controlled hillslope erosion had occurred. This transition possibly occurred due to the significant decrease in tectonic uplift during the late Cenozoic. The study area is characterized by sub‐humid Mediterranean climate. The drainage hillslopes are typically mantled by thick calcrete crusts overlying Upper Cretaceous marine carbonate rocks. Using TT‐OSL dating of aeolian quartz grains incorporated in the calcrete which cements an ancient landslide deposit, we conclude that incision of ~100 m occurred from 1056 ± 262 to 688 ± 86 ka due to ~0·3° westward tilt of the region; such incision invoked high frequency of landslide activity in the drainage. The ages of a younger landslide remnant, alluvial terrace, and alluvial fan, all situated only a few meters above the present level of the active streambed, range between 688 ± 86 ka and 244 ± 25 ka and indicate that since 688 ± 86 the Soreq base level had stabilized and that landslide activity decreased significantly by the middle Pleistocene. Copyright © 2012 John Wiley & Sons, Ltd.     

Cosmogenic isotope burial dating of fluvial sediments from the Lower Rhine Embayment,Germany

Cosmogenic isotope burial dating, using ¹⁰Be and ²⁶Al, was applied to Plio–Pleistocene fluvial successions from the Lower Rhine Embayment, Germany. The approach consists of three principal steps: (1) measurement of cosmogenic nuclides in depth profiles, (2) modelling of hypothetical nuclide concentrations based on a first-order conceptualisation of the geological context and the principal succession of depositions and subsequent erosional and burial phases, and (3) using parameter estimation to identify values for the a priori unknown model parameters (burial age, initial nuclide concentrations, terrace erosion rates) that result in minimal disagreement between hypothetical and measured nuclide concentrations.The Late Pliocene Kieseloolite Formation was dated to 3650 ± 1490 ka and the Early Pleistocene Waalre Formation to 900 ± 280 ka. The unconformably overlying Upper Terrace Formation revealed ages of 740 ± 210 ka and 750 ± 250 ka for the two different sites. These findings are in good agreement with independent age control derived by bio-, magneto-, and litho-stratigraphy. Furthermore, identifiability and uncertainty analysis reveal a relationship between burial depth and sensitivity of isotope concentrations to burial age and erosion rate. Our results indicate that using shallower buried samples would enable a considerably more robust estimation of the burial age and the terrace erosion rate. Uncertainties arose mainly from nuclide measurements, and not from the uncertainties derived from modelling or insufficient knowledge of nuclide production and decay properties.     

Fuerteventura – Assessment of a calibration site for cosmogenic 3He exposure dating with the 40Ar/39Ar incremental heating method

In situ Terrestrial Cosmogenic Nuclides (hereafter TCNs) are increasingly important for absolutely dating terrestrial events and processes. This study aimed at improving our knowledge of the production rate of Terrestrial Cosmogenic ³He formed in situ in rock surfaces at low latitude and sea level as well as re-evaluation of the Canary Islands as a calibration site for TCNs. For this purpose, we sampled basaltic lava flows from some of the youngest and yet undated volcanic sites and used the ⁴⁰Ar/³⁹Ar incremental heating method on groundmass samples and in situ cosmogenic ³He on olivine and clinopyroxene phenocrysts. ⁴⁰Ar/³⁹Ar analysis was done on a Hiden HAL Series 1000 triple filter quadrupole mass spectrometer with extraction furnace. Incremental heating data shows ages in the Late Pleistocene from 52.7 ± 21.6 ka to 398.6 ± 27.6 ka.We measured cosmogenic ³He concentrations in olivine and clinopyroxene phenocrysts from flow top samples on a MAP 215-50 sector mass spectrometer with a crushing device and a diode laser extraction system. Exposure age calculations yielded ages in the range 38.9 ± 4.0 ka to 62.3 ± 6.7 ka for the youngest lava flow and the data series is in broad agreement with the argon data up to 250 ka and reveals a more continuous time line of volcanism during the late Pleistocene on the island. However, the dataset was not sufficient for calculation of production rates for in situ Terrestrial Cosmogenic ³He as many samples showed signs of erosion. Calculated erosion rates range from none to as high as 7.3 mm/kyr assuming a rock density of 2.9 g/cm². This finding puts a constraint on the use of Fuerteventura as a calibration site for exposure histories older than 50–100 ka. A comparison with cosmogenic ³⁶Cl data supports these findings and indicates substantial weathering.     

Constant slip rate during the late Quaternary along the Sulu He segment of the Altyn Tagh Fault near Changma,Gansu, China

The question of whether millennial‐scale geological slip rates are consistent with decade‐scale geodetic slip rates is of great importance in evaluating the nature of continental deformation within the Tibetan Plateau. We determined the time‐averaged slip rate of the Sulu He segment of the Altyn Tagh Fault, near Changma in Gansu Province, China, based on geomorphic analysis, remote sensing data, and cosmogenic ¹⁰Be surface‐exposure age dating. Quaternary alluvial fan deposits in the study area (Qf1, Qf2, Qf3) are displaced by left‐lateral movement along the Altyn Tagh Fault. Because of the large accumulated displacement of these fans, some of them have become disconnected from the fan apexes that are directly linked to the debris‐source areas in the piedmont of the Qilian Shan to the south. The total minimum offsets are estimated to be about 429 ± 41 m for Qf1, about 130 ± 10 m for Qf2, and 32 ± 1 m for Qf3. The ¹⁰Be surface‐exposure ages obtained for Qf1 and Qf2 are 100–112 ka and 31–43 ka, respectively. Accordingly, the slip rate since the period of Qf1 and Qf2 depositions is calculated to have been about 3.7 mm/yr.     

Late-stage volcano geomorphic evolution of the Pleistocene San Francisco Mountain, Arizona (USA), based on high-resolution DEM analysis and 40Ar/39Ar chronology

The cone-building volcanic activity and subsequent erosion of San Francisco Mountain, AZ, USA, were studied by using high-resolution digital elevation model (DEM) analysis and new ⁴⁰Ar/³⁹Ar dating. By defining remnants or planèzes of the volcano flanks in DEM-derived images, the original edifice can be reconstructed. We propose a two-cone model with adjacent summit vents which were active in different times. The reconstructed cones were 4,460 and 4,350 m high a.s.l., corresponding to ∼2,160 and 2,050 m relative height, respectively. New ⁴⁰Ar/³⁹Ar data allow us to decipher the chronological details of the cone-building activity. We dated the Older and Younger Andesites of the volcano that, according to previous mapping, built the stage 2 and stage 3 stratocones, respectively. The new ⁴⁰Ar/³⁹Ar plateau ages yielded 589–556 ka for the Older and 514–505 ka for the Younger Andesites, supporting their distinct nature with a possible dormant period between. The obtained ages imply an intense final (≤100 ka long) cone-building activity, terminating ∼100 ka earlier than indicated by previous K-Ar ages. Moreover, ⁴⁰Ar/³⁹Ar dating constrains the formation of the Inner Basin, an elliptical depression in the center of the volcano initially created by flank collapse. A 530 ka age (with a ±58.4 ka 2σ error) for a post-depression dacite suggests that the collapse event is geochronologically indistinguishable from the termination of the andesitic cone-building activity. According to our DEM analysis, the original cone of San Francisco Mountain had a volume of about 80 km³. Of this volume, ∼7.5 km³ was removed by the flank collapse and subsequent glacial erosion, creating the present-day enlarged Inner Basin, and ∼2 km³ was removed from the outer valleys by erosion. Based on volumetric analysis and previous and new radiometric ages, the average long-term eruption rate of San Francisco Mountain was ∼0.2 km³/ka, which is a medium rate for long-lived stratovolcanoes. However, according to the new ⁴⁰Ar/³⁹Ar dates for the last ≤100 ka period, the final stratovolcanic activity was characterized by a greater ∼0.3 km³/ka rate.     

Multi‐method (14C, 36Cl, 234U/230Th) age bracketing of the Tschirgant rock avalanche (Eastern Alps): implications for absolute dating of catastrophic mass‐wasting

Correct and precise age determination of prehistorical catastrophic rock‐slope failures prerequisites any hypotheses relating this type of mass wasting to past climatic regimes or palaeo‐seismic records. Despite good exposure, easy accessibility and a long tradition of absolute dating, the age of the 230 million m³ carbonate‐lithic Tschirgant rock avalanche event of the Eastern Alps (Austria) still is relatively poorly constrained. We herein review the age of mass‐wasting based on a total of 17 absolute ages produced with three different methods (¹⁴C, ³⁶Cl, ²³⁴U/²³⁰Th). Chlorine‐36 (³⁶Cl) cosmogenic surface exposure dating of five boulders of the rock avalanche deposit indicates a mean event age of 3.06 ± 0.62 ka. Uranium‐234/thorium‐230 (²³⁴U/²³⁰Th) dating of soda‐straw stalactites formed in microcaves beneath boulders indicate mean precipitation ages of three individual soda straws at 3.20 ± 0.26 ka, 3.04 ± 0.10 ka and 2.81 ± 0.15 ka; notwithstanding potential internal errors, these ages provide an ‘older‐than’ (ante quam) proxy for mass‐wasting. Based on radiocarbon ages (nine sites) only, it was previously suggested that the present rock avalanche deposit represents two successive failures (3.75 ± 0.19 ka bp , 3.15 ± 0.19 ka bp ). There is, however, no evidence for two events neither in surface outcrops nor in LiDAR derived imagery and drill logs. The temporal distribution of all absolute ages (¹⁴C, ³⁶Cl, ²³⁴U/²³⁰Th) also does not necessarily indicate two successive events but suggest that a single catastrophic mass‐wasting took place between 3.4 and 2.4 ka bp . Taking into account the maximum age boundary given by reinterpreted radiocarbon datings and the minimum U/Th‐ages of calcite precipitations within the rock avalanche deposits, a most probable event age of 3.01 ± 0.10 ka bp can be proposed. Our results underscore the difficulty to accurately date catastrophic rock slope failures, but also the potential to increase the accuracy of age determination by combining methods. Copyright © 2016 John Wiley & Sons, Ltd.     

A locally calibrated,late glacial 10Be production rate from a low-latitude,high-altitude site in the Peruvian Andes

Well-dated records of tropical glacier fluctuations are essential for developing hypotheses and testing proposed mechanisms for past climate changes. Since organic material for radiocarbon dating is typically scarce in low-latitude, high-altitude environments, surface exposure-age dating, based on the measurement of in situ produced cosmogenic nuclides, provides much of the chronologic information on tropical glacier moraines. Here, we present a locally calibrated ¹⁰Be production rate for a low-latitude, high-altitude site near Quelccaya Ice Cap (∼13.95°S, 70.89°W, 4857 m asl) in the southeastern Peruvian Andes. Using an independent age (12.35 +0.2, −0.02 ka) of the late glacial Huancané IIa moraines based on thirty-four bracketing radiocarbon ages and twelve ¹⁰Be concentrations of boulders on the moraines, we determine a local production rate of 43.28 ± 2.69 atoms gram⁻¹ year⁻¹ (at g⁻¹ yr⁻¹). Reference ¹⁰Be production rates (i.e., production rates by neutron spallation appropriate for sea-level, high-latitude sites) range from 3.97 ± 0.09 to 3.78 ± 0.09 at g⁻¹ yr⁻¹, determined using scaling after Lal (1991) and Stone (2000) and depending on our assumed boulder surface erosion rate. Since our boulder surface erosion rate estimate is a minimum value, these reference production rates are also minimum values. A secondary control site on the Huancané IIIb moraines suggests that the ¹⁰Be production rates are at least as low as, or possibly lower than, those derived from the Huancané IIa moraines. These sea-level, high-latitude production rates are at least 11–15% lower than values derived using the traditional global calibration dataset, and they are also lower than those derived from the late glacial Breque moraine in the Cordillera Blanca of Peru. However, our sea-level, high-latitude production rates agree well with recently published, locally calibrated production rates from the Arctic, New Zealand, and Patagonia. The production rates presented here should be used to calculate ¹⁰Be exposure ages in low-latitude, high-altitude locations, particularly in the tropical Andes, and should improve the ability to compare the results of studies using ¹⁰Be exposure-age dating with other chronological data.