Along-strike variations in the timing of exhumation in the eastern Peruvian Andes
University of Tübingen, Germany
Different controls and couplings between tectonics and climate-driven erosion have been suggested to explain the topographic and structural architecture of the Andes. One aspect of controversial discussions is the late Neogene incision of the Central Andean Plateau margin that has been interpreted as either the effect of climate change and increased river discharge on an already high plateau, or the effect of relatively young surface uplift. On the eastern flank of the Andean Plateau, ~4 Ma incision has been reported for two canyons in Bolivia and southeastern Peru (over a distance of >1250 km). However, there are still too many uncertainties concerning the spatio-temporal patterns of crustal shortening, erosion, and surface uplift to evaluate climatic and tectonic influences, especially in Peru. Studies that evaluate climate, erosion, and deformation histories in concert as well as along-strike continuations or variations are needed.
We contribute an extension of existing low-temperature thermochronometer data from three, up to 190-km-long transects spanning the Andean Plateau, the Eastern Cordillera including canyons and high-elevations, and the Subandean Zone in southeastern Peru. We present 46 new apatite (U-Th)/He (~1–41 Ma), 23 new zircon (U-Th)/He (~4–284 Ma), 21 new apatite fission-track (~3–63 Ma), and 11 new zircon fission-track dates (~14–37 Ma) from a total of 53 bedrock samples. We evaluate a representative set of samples' thermal histories with respects to deformation and incision of the plateau margin and discuss along-strike variations in the exhumation signal (onset and magnitude).
Exhumation and erosion rates in the flat-topped Nock Mountains in the Eastern Alps constrained by low-temperature and cosmogenic 10Be data
1Leibniz University Hannover, Institute for Geology, Callinstraße 30, 30167 Hannover, Germany; 2Eberhard Karls University Tübingen, Institute for Geology and Geodynamics, Schnarrenbergerstraße 94-96, 72076 Tübingen, Germany; 3University of Göttingen, Sedimentology & Environmental Geology, Geoscience Center, Goldschmidstraße 3, 37077 Göttingen, Germany
Constraining rates of landscape evolution is a necessary pre-requisite for reconstructing the spatiotemporal evolution of the Earth´s surface. In our study, we present new zircon and apatite fission track and (U-Th)/He ages as well as catchment-wide 10Be-derived erosion rates to determine exhumation and erosion rates in the Nock Mountains, an elevated-low relief landscape in the Eastern Alps. Zircon fission track and zircon (U-Th)/He analyses yielded cooling ages of ~90 and ~80 Ma, respectively, which we interpret to reflect late Cretaceous cooling after Eoalpine metamorphism. Apatite fission track and (U-Th)/He ages are significant younger and range from ca. 35-28 Ma. Time-temperature history modelling of the thermochronological ages suggests enhanced cooling in the Eocene followed by thermal stagnation. Our results show that amount of erosion since ~35 Ma does not exceed 2-3 km and that average erosion rates did not significantly change. Assuming that rock exhumation occurred by erosion only, the long-term erosion rate is ~50-100 mm//kyr and therefore of similar magnitude as the short-term 10Be erosion rates that range from ~90-200 mm/kyr, despite the different timescales of the methods. Our data support the notion that the Nock Mountains represent an area of long-lasting stability although tectonic and glacial-interglacial cycles affected the Eastern Alps since ~35 Ma.
sandbox – Creating and Analysing Synthetic Sediment Sections with R
1GFZ Potsdam, Germany; 2Geography & Earth Science, Aberystwyth University, Aberystwyth, Wales, United Kingdom; 3IRAMAT-CRP2A, UMR 5060, CNRS-Université Bordeaux Montaigne, Pessac, France; 4Helmholtz-Zentrum Dresden-Rossendorf, Helmholtz-Institut Freiberg for Resource Technology, Freiberg, Germany
Geoscientific concepts and hypotheses are usually formulated based on empirical data from the field or the laboratory (induction). After translation into models they can be applied to case study scenarios (deduction). However, the other way around – expressing hypotheses explicitly by models and test these by empiric data – is a rarely touched trail. There are several models tailored to investigate the boundary conditions and processes that generate, mobilise, route and eventually deposit sediment in a landscape. Thereby, the last part, sediment deposition, is usually omitted. Essentially, there is no model that explicitly focuses on mapping out the characteristics of sedimentary deposits – the material that is used by many disciplines to reconstruct landscape evolution. The R package sandbox is a model framework that allows creating and analysing virtual sediment sections for exploratory, explanatory, forecasting and inverse research questions. sandbox is a probabilistic and rule-based model framework for a wide range of possible applications. It has been advanced and linked to another model to allow the full work flow of modelling luminescence measurements. This contribution introduces news about recent developments and shows a set of applications.
Sediment production in the Coastal Cordillera of Chile from detrital apatite geochemistry and thermochronology
Universität Tübingen, Germany
We study spatial patterns of sediment production in two catchments of the Coastal Cordillera (Chile) situated in semi-arid and mediterranean bioclimates. To do so, we measure bedrock and detrital apatite trace elements as well as apatite cooling ages with the U/Pb, fission track, U-Th(-Sm)/He thermochronometric systems. The compositional and geochronologic data measured in bedrock are run through a Principal Component Analysis and a Support Vector Machine clustering algorithm to find the parameters that are best suited to trace sediment provenance at sub-catchment scale. Next, we analyse the distribution of the same parameters within the detritus to infer the relative contribution of different areas of the catchments. We find that spatial variations of bedrock cooling age and geochemical composition are significant even within small-scale (10-100 km2) monolithological catchments. Therefore, the combination of detrital apatite geochronology and geochemistry allows discrimination among source areas with acceptable confidence. This methodology has the potential to identify the main drivers of centennial to millennial sediment production, which will be tested in the following research steps.
Timing of the post-LGM retreat of the Iller Piedmont Glacier (Southern Germany) based on in-situ 36Cl exposure dating of glacial erratics
1Department of Earth and Environmental Sciences, Ludwig Maximilian University Munich, Germany; 2Department Accelerator Mass Spectrometry and Isotope Research, Helmholtz-Zentrum Dresden-Rossendorf, Germany; 3Present address: Isotope Physics, University of Vienna, Austria
The dynamic behavior of glacial retreat following the Last Glacial Maximum (LGM), which is globally diachronous, is poorly understood. Along strike of the northern Alpine margin, multiple lobes of large foreland glaciers left a complex morpho-sedimentary record. While the reconstructed LGM ice extent is laterally constant in the west, it shows significant variations in the central and eastern part. We use these local geologic variations to explore how regional climatic conditions relate to global climate during this period of rapid late Pleistocene climate change. The chronology for this interval has been well-constrained in Switzerland, but radiometric ages have only been reported for a few locations along the Alpine Foreland in Germany. In this study, we employ cosmogenic 36Cl in limestone to constrain the in-situ exposure age of glacial erratics situated on moraine walls of the Iller Piedmont Glacier. We sampled erratic boulders along a transect perpendicular to three moraine ridges previously interpreted to represent the LGM along with two post-LGM retreat stands. Our preliminary raw data show that the sampled lithology provides internally consistent, reproducible, and geologically meaningful dates. We discuss our results taking into account limestone weathering, erosion and local postglacial landscape stabilization, and apply appropriate correction factors to obtain more accurate ages. This exposure age data gives first insights into the spatio-temporal patterns of glacial retreat in the northern Alpine Foreland, which can be used to reconstruct Central European paleoclimate in the late Pleistocene.
Contribution of the extreme events to the surface transformation in proglacial areas (the Djankuat River catchment case study, Caucasus)
1Faculty of Geography, Lomonosov Moscow State University, Moscow, Russian Federation; 2Institute of Geography, Russian Academy of Sciences, Moscow, Russian Federation
This study is dedicated to a quantitative assessment of two different-scale impulsive geomorphic events in proglacial areas. The study site is the Djankuat River catchment in the Caucuses. The first event is an ice-wall collapse of a buried ice outcrop. We conducted a repeated UAV survey with an annual interval (September 2019 – August 2020) and a two-day interval (August 18 – 20, 2020). For a 3,950 m2 area, the defined material (both sediment and melting ice) transfer in two days was 1,880 m3, while for the annual interval was 6,330 m3. The event contribution is almost 1/3 of the total annual change. The second event is the lateral moraine breakthrough that happened in July 2015 due to the preceding 227 mm precipitation per week. The evaluation was based on DEMs obtained from satellite image stereo pairs. About 134 thousand m3 of material was removed with the formation a cone in the valley bottom (volume ≈ 101 thousand m3). Another 81 thousand m3 was transported to the Djankuat River during the following 2 years due to active erosion of the new-formed channel. The volume of material removed from the Djankuat River catchment due to the event is at least 20 times higher than the average annual sediment runoff in years with the usual formation of sediment runoff. This research illustrates the importance of the individual extreme events assessment since they determine the primary changes in the proglacial terrain.
The study was supported by the Russian Science Foundation (project No. 19-17-00181).
Holocene erosion and pedogenesis on watersheds of the Central Russian Plain
1Lomonosov Moscow State University, Russian Federation; 2Institute of Geography, Russian Academy of Sciences, Russian Federation
Watershed landscapes of the Borisoglebsk Upland and Suzdal Opolie are characterized by absence of the well-developed early Holocene paleosols suggesting dominantly negative sediment budget. Detailed study of Retisols revealed polycyclic type of pedogenesis during the Early Holocene. Discontinuous deposition with certain interruptions (but without distinct buried soil formation) occurred only within closed depressions and gully fans. The second part of the Holocene prior to the widespread human settlement left more substantial traces in soil and sediment record. It is identified both at locations dominated by denudation (evidences of multiple topsoil truncation in Atlantic and Subatlantic) and at zones of alternating incision and infill of small linear erosion features. Such extremes were most likely associated with combination of several triggers including natural forest fires and high-magnitude rainfall or snowmelt runoff events. The last phases of increased hillslope and fluvial activity within the study area can be related to increased human interference, starting about 1600-900 years ago. Latest period of intensive gully growth can most likely be attributed to the XIXth Century land tenure reform and peak tillage cultivation after the World War II when most of the study area gullies experienced significant linear growth, bottom incisions and appearance of several new gully branches. Rates of the most recent soil redistribution on slopes decreased significantly over the last several decades due to combination of natural and anthropogenic impacts.
Late Pleistocene chronostratigraphy of infills and incisions based on 3D-modelling of a local watershed sediment sink structure (Borisoglebsk Upland, Central European Russia)
1Lomonosov Moscow State University, Russian Federation; 2Institute of Geography, Russian Academy of Sciences, Russian Federation
3D-modelling based on a series of geological exposures and cores up to 7 m deep allowed to reveal the structure, stratigraphy and chronology of a local sediment sink on the watershed of one of the Late Saalian glacial uplands at the center of the Russian Plain. This local section representing an infill of a kettle hole was initially occupied by the shallow stagnant water body during the Eemian up to the Late Weichselian, which had periodically dried and transformed into the forested bog. The Late Pleistocene to Holocene transition was associated with most dramatic environmental changes and abrupt fluctuations. Response in the local geomorphodynamics first involved activation of mass movements followed by the gully incision reaching the site by regressive head knickpoint retreat. The observed sediment record provides evidences of at least 4 linear erosion incision-infill cycles. The first incision into the gradually undulating surface composed by lacustrine and colluvial deposits and its infill by stratified colluvial silts occurred not later than 6.5 cal. ka BP. It is fixed by the next incision phase infilled by pedo-sediments of reworked humic, eluvial and sub-eluvial horizons of the upper slope soils enriched by pyrogenic charcoal. The third incision stage can be linked to regressive growth of the main gully head and its branches (ca. 1.4 cal. ka BP) triggered by the onset of cut-and-burn agriculture practices determining conditions favorable for active deposition of agrogenic colluvium. Modern gully incision was preceded at least by another infill phase and occurred between 1941-1968 AD.
Rock alteration at the post-Variscan nonconformity: implications for Permo-Carboniferous surface weathering versus burial diagenesis
1Material and Geosciences, Institute of Applied Geosciences, Technical University of Darmstadt, Darmstadt, 64287, Germany; 2Faculty of Geosciences/ Geography, Goethe-University, Frankfurt, 60438, Germany
Nonconformity surfaces are key features to understand the relations among climate, lithosphere and tectonic movements in Earth history. One of the most prominent stratigraphic surfaces in Central European is the post-Variscan nonconformity separating the metamorphic Variscan basement from non-metamorphic platform deposits starting in the Upper Carboniferous, but more widely distributed since the Permian. This study aimed to analyze and distinguish the physical and chemical processes happening during exposure of the surface as well as during subsequent burial diagenesis on a drill core located on the eastern graben shoulder of the Upper Rhine Graben. The core penetrates through a Lower Permian volcano-sedimentary succession into the crystalline basement which here consists of Cadomian plutonic rocks. Samples were analyzed across the nonconformity by means of polarization microscopy, environmental scanning electron microscope, X-Ray diffraction, X-ray fluorescence and Inductively Coupled Plasma Mass Spectrometry. In both segments, secondary minerals are dominated by illite and a mix-layer phase of illite and smectite (I/S). The corrected weathering indices indicate an intermediate to unweathered degree in the gabbroic diorite and an extreme to unweathered degree in the basaltic andesite. The τ value for both parts indicates an abnormal enrichment of K, Rb, and Cs. Accompanying minerals such as adularia suggest subsequent overprint by (K-rich) fluids during burial diagenesis which promoted the conversion from smectite to illite. Overall, a new workflow to eliminate distractions for paleoclimate evaluation and evolution has been developed. Our study shows that features of supergene physical and chemical paleo-weathering can be disentangled by a multi-proxy approach.