Christoph Glotzbach

Thermal history of the central Gotthard and Aar massifs, European Alps: Evidence for steady state, long‐term exhumation

[1] Quantifying long‐term exhumation rates is a prerequisite for understanding the geodynamic evolution of orogens and their exogenic and endogenic driving forces. Here we reconstruct the exhumation history of the central Aar and Gotthard external crystalline massifs in the European Alps using apatite and zircon fission track and apatite (U‐Th)/He data. Age‐elevation relationships and time‐temperature paths derived from thermal history modeling are interpreted to reflect nearly constant exhumation of ∼0.5 km/Ma since ∼14 Ma. A slightly accelerated rate (∼0.7 km/Ma) occurred from 16 to 14 Ma and again from 10 to 7 Ma. Faster exhumation between 16 and 14 Ma is most likely linked to indentation of the Adriatic wedge and related thrusting along the Alpine sole thrust, which, in turn, caused uplift and exhumation in the external crystalline massifs. The data suggest nearly steady, moderate exhumation rates since ∼14 Ma, regardless of major exogenic and endogenic forces such as a change to wetter climate conditions around 5 Ma or orogen‐perpendicular extension initiated in Pliocene times. Recent uplift and denudation rates, interpreted to be the result of climate fluctuations and associated increase in erosional efficiency, are nearly twice this ∼0.5 km/Ma paleoexhumation rate.

Detrital thermochronology records changing source areas and steady exhumation in the Western European Alps

A dramatic increase in sediment flux since ca. 5 Ma has been inferred for the European Alps and worldwide, and is interpreted to reflect a climatically driven increase in erosion rates. However, detrital thermochronology data from the Alpine forelands suggest steady Miocene–Pliocene exhumation, contradicting the sediment budget data. We present new detrital apatite fission-track (AFT) data from the foreland of the Western Alps to better resolve Miocene to present-day exhumation rates and to determine sediment provenance. A shift in AFT lag time from

Quantifying rates of detachment faulting and erosion in the central Menderes Massif (western Turkey) by thermochronology and cosmogenic 10Be

Exhumation of rocks in extensional tectonic settings results from a combination of normal faulting and erosion but the relative contribution of these processes has rarely been quantified. Here we present new low-temperature thermochronological data and the first 10Be-based catchment-wide erosion rates from the Boz Dağ region in the central Menderes Massif, which has experienced NNE–SSW extension since the Miocene. The slip rate of the shallow-dipping Gediz detachment fault, which defines the northern flank of the Boz Dağ block, is 4.3 (+3.0/−1.2) mm a−1, as constrained by zircon (U–Th)/He ages of c. 4–2 Ma in the footwall. Apatite and zircon (U–Th)/He and fission-track ages from the northern flank of the Boz Dağ block yield exhumation rates of 0.6–2 km Ma−1 beneath the Gediz detachment, whereas those on the southern flank are only 0.2–0.6 km Ma−1. Erosion of catchments on the northern and southern flanks proceeds at rates of 80–180 and 330–460 mm ka−1, respectively. This marked contrast is a combined effect of the topographic asymmetry of the Boz Dağ block and differences in rock erodibility. If these erosion rates persisted in the past, rock exhumation on the northern flank occurred predominantly by tectonic denudation, whereas rocks on the southern flank were mainly exhumed by erosion.

What perturbs isotherms? An assessment using fission-track thermochronology and thermal modelling along the Gotthard transect, Central Alps

Abstract Interpretation of low-temperature thermochronological data usually relies on assumptions on the shape of isotherms. Recently, a number of thermal modelling approaches investigate and predict the theoretical influence of topography on isotherms. The application and proof of these predictions is not well confirmed by measured data. Here we present apatite fission-track (AFT) data from samples collected along the Gotthard road tunnel and its corresponding surface line to test these predictions. AFT ages broadly cluster around 6 Ma along the tunnel. No correlation of tunnel ages with superimposed topography is seen, which means that topography-induced perturbation of isotherms under given boundary conditions (topographic wavelength 12 km; relief 1.5 km; exhumation rate 0.45 km Ma−1) can be neglected for the interpretation of AFT ages. Thus, in areas characterized by similar topographies and exhumation rates, apparent exhumation rates deduced from the age–elevation relationship (AER) of AFT data need no correction for topography-induced perturbation of isotherms. Three-dimensional (3D) numerical thermal modelling was carried out incorporating thermally relevant parameters and mechanisms, such as topography, geology, thermal conductivities and heat production. Modelling reveals a strong influence on the shape of isotherms caused by spatially variable thermal parameters, especially heat production rates. Therefore, not only topography has to be considered for interpreting low-temperature thermochronological data, but also other parameters like heat production rates. Supplementary material: 1. Electron microprobe analyses, 2. Topography and model extend, 3. Model parameters are all available online at http://www.geolsoc.org.uk/SUP18380.

Deriving rock uplift histories from data-driven inversion of river profiles

Reconstructing the evolution of Earth’s landscape is a key to understanding its future evolution and to identifying the driving forces that shape Earth’s surface. Cosmogenic nuclide and thermochronological investigations are routinely used to quantify Earth surface processes over 10 2 –10 4 yr and 10 6 –10 7 yr, respectively. A comparison of the rates of surface processes derived from these methods is, however, hampered by the large difference in their time scales. River profiles bridge this time gap and record the regional uplift history over 10 2 –10 7 yr. Here I present an integrative inverse modeling approach to simultaneously reconstruct river profiles, model thermochronological and cosmogenic nuclide data, and derive robust information about landscape evolution over thousands to millions of years. An efficient inversion routine is used to solve the forward problem and find the best uplift history and erosional parameters (such as the exponents of discharge [ m ] and slope [ n ] in the stream power equation) that reproduce the observed data. I test the performance of the algorithm by inverting a synthetic data set and a data set from the Sila massif (Italy). Results show that even complicated uplift histories can be reliably retrieved by the combined interpretation of river profiles and thermochronological and cosmogenic nuclide data.

Tectonic and glacial contributions to focused exhumation in the Olympic Mountains, Washington, USA

Tectonics and climate are major contributors to the topographic evolution of mountain ranges. Here, we investigate temporal variations in exhumation due to the onset of Pleistocene glaciation in the Olympic Mountains (Washington State, USA). We present 29 new apatite and zircon (U-Th)/He ages (AHe and ZHe), showing a decrease in ages toward the interior of the mountain range for both thermochronometric systems. Young AHe ages (<2 Ma) can be found on the western side and the interior of the mountain range. Thermokinematic modeling of sample cooling ages suggests, that ZHe ages can be explained by an ellipse-shaped exhumation pattern with lowest/ highest rates of ~0.25 and 0.9 km/m.y. These rates are interpreted as tectonically driven rock uplift, where the pattern of rates is governed by the shape of the subducted plate. However, the youngest AHe ages require a 50–150% increase in exhumation rates in the past 2–3 m.y. This increase in rates is contemporaneous with Pliocene-Pleistocene alpine glaciation of the orogen, indicating that tectonic rock uplift is perturbed by glacial erosion. INTRODUCTION The evolution of mountain topography (e.g., relief, mean elevation) is sensitive to variations in climate and tectonics that modulate the efficiency of various surface processes (e.g., Whipple, 2009). The onset of Pleistocene glaciation is hypothesized to have increased orogen exhumation rates, and significantly modified topography (e.g., Brocklehurst and Whipple, 2002; Ehlers et al., 2006; Valla et al., 2011; Glotzbach et al., 2013; Herman et al., 2013). Advances in low-temperature thermochronology and thermal modeling enable the quantification of spatial and temporal variations in exhumation (e.g., Braun, 2003). Here we test the hypothesis that enhanced Pleistocene glacial erosion can perturb the flux steady state of an orogen by increasing the erosional flux over million-year time scales. We evaluate this hypothesis through an application to the tectonically active and glaciated Olympic Mountains located in Washington State, USA (Fig. 1A). This orogen is the exhumed portion of the Cascadia Subduction zone accretionary wedge (Tabor and Cady, 1978). Previous studies have suggested that exhumation rates have been largely constant since ca. 14 Ma, and that the orogen is in flux steady state, where accretionary and erosional fluxes are balanced (Brandon et al., 1998; Batt et al., 2001; Pazzaglia and Brandon, 2001). Largely unexplored in previous work is the potential transient effect of Pleistocene glaciation on the orogen-wide exhumation. Here we complement previous work with new apatite and zircon (U-Th)/He ages (AHe and ZHe, respectively) from the Olympic Mountains (Fig. 1B) and compare them to predicted thermokinematic model ages to discriminate between different exhumation histories. BACKGROUND At the Cascadia subduction zone, the Juan de Fuca plate subducts beneath the North American plate and displays a three-dimensional (3-D) bend beneath the Olympic Mountains (Fig. DR1a in the GSA Data *E-mail: todd.ehlers@uni-tuebingen.de GEOLOGY, June 2018; v. 46; no. 6; p. 1–4 | GSA Data Repository item 2018161 | https://doi.org/10.1130/G39881.1 | Published online XX Month 2018

Slow Long‐Term Exhumation of the West Central Andean Plate Boundary, Chile

Abstract We present a regional analysis of new low‐temperature thermochronometer ages from the Central Andean fore arc to provide insights into the exhumation history of the western Andean margin. To derive exhumation rates over 10 million‐year timescales, 38 new apatite and zircon (U‐Th)/He ages were analyzed along six ~500‐km long near‐equal‐elevation, coast parallel, transects in the Coastal Cordillera (CC) and higher‐elevation Precordillera (PC) of the northern Chilean Andes between latitudes 18.5°S and 22.5°S. These transects were augmented with age‐elevation profiles where possible. Results are synthesized with previously published thermochronometric data, corroborating a previously observed trenchward increase in cooling ages in Peru and northern Chile. One‐dimensional thermal‐kinematic modeling of all available multichronometer equal‐elevation samples reveals mean exhumation rates of <0.2 km/Myr since ~50 Ma in the PC and ~100 Ma in the CC. Regression of pseudovertical age‐elevation transects in the CC yields comparable rates of ~0.05 to ~0.12 km/Myr between ~40 and 80 Ma. Differences between the long‐term mean 1‐D rates and shorter‐term age‐elevation‐derived rates indicate low variability in the exhumation history. Modeling results suggest similar background exhumation rates in the CC and PC; younger ages in the PC are largely a function of increased heat flow and consequently an elevated geothermal gradient near the arc. Slow exhumation rates are suggestive of semiarid conditions across the region since at least the Eocene and deformation and development of the Andean fore arc around this time.

A new Poissonian algorithm for the determination of fission-track ages

Fission-track (FT) thermochronology has been widely used since the 1970s to constrain the low-temperature thermal history of rocks in numerous geological settings. Statistical problems associated with this method have been addressed since the 1980s. Today, with the improvements brought to the technique, new questions arise which require the adaptation of previously established statistical methods. Samples used for FT thermochronology are typically composed of minerals (e.g. apatite, zircon, titanite, etc.) with a wide variability in uranium (U) concentration. In this work we propose a new statistical methodology, called p-partition, that even assuming grains of a same sample could have different uranium concentrations. We apply our methodology to fission-track ages standards, in-situ and detrital samples. For standards we found that the new estimated age is closer to the independently determined K-Ar age, for detrital samples we applied our methodology to existent and published data for the case of the Venezuelan Andes. Our results were compared with other decomposition methods for the fission-track ages distributions, they show robustness, and in some particular case our method has more resolution in comparison with conventional methods. In this work we present a methodology and algorithms that attempt to solve mathematically the problem of decomposition of data with Poisson mixtures. A new estimator for the fission-track age based in maximum likelihood is presented.Applications leads naturally to solve a problem of mixed Poisson distributions.We proposed the p-partition algorithm for decompose such mixtures.Our algorithm is applied to real standard, in-situ and detrital fission-track data.Comparison with others decomposition methods for simulated data are presented here.

Estimating the sediment trap efficiency of intermittently dry reservoirs: lessons from the Kruger National Park, South Africa

The assessment of sediment yield from reservoir siltation requires knowledge of the reservoirs sediment trap efficiency (TE). Widely used approaches for the estimation of the long-term mean TE rely on the ratio of the reservoirs storage capacity (C) to its catchment size (A) or mean annual inflow (I). These approaches have been developed from a limited number of reservoirs (N ≤ 40), most of them located in temperate climate regions. Their general applicability to reservoirs receiving highly variable runoff such as in semi-arid areas has been questioned. Here, we examine the effect of ephemeral inflow on the TE of ten small (≤280 × 103 m3), intermittently dry reservoirs located in the Kruger National Park. Field work was conducted to determine the storage capacity of the reservoir basins. The frequency and magnitude of spillage events was simulated with the daily time step Pitman rainfall-runoff model. Different runoff scenarios were established to cope with uncertainties arising from the lack of runoff records and imperfect input data. Scenarios for the relationship between water and sediment discharge were created based on sediment rating curves. Taking into account uncertainties in hydrological modelling, uncertainties of mean TE estimates, calculated from all scenarios (N = 9), are moderate, ranging from ±6% to ±11% at the 95% confidence level. By comparison, estimating TE from the storage capacity to catchment area (C/A) ratio induces high uncertainty (ranges of 35% to 65%), but this uncertainty can be confined (15% to 33%) when the latter approach is combined with hydrological modelling. Established methods relying on the storage capacity to mean annual inflow (C/I) ratio most probably lead to an overestimation of the TE for the investigated reservoirs. The approach presented here may be used instead to estimate the TE of small, intermittently dry reservoirs in semi-arid climate regions.

How steady are steady-state mountain belts? – a re-examination of the Olympic Mountains (Washington State, USA)

The Olympic Mountains of Washington State (USA) represent the aerially exposed accretionary wedge of the Cascadia Subduction Zone and are thought to be in flux steady-state, whereby the mass outflux (denudation) and influx (tectonic accretion) into the mountain range are balanced. We use a multi-method approach to investigate how temporal variations in the influx and outflux could affect 10 previous interpretations of flux steady-state. This includes analysis of published and new thermochronometric ages for (U-Th)/He dating of apatite and zircon (AHe and ZHe, respectively) and fission track dating of apatite and zircon (AFT and ZFT, respectively), 1D thermo-kinematic modelling of thermochronometric data, and independent estimates of outflux and influx. In total, we present 61 new AHe, ZHe, AFT, and ZFT thermochronometric ages from 21 new 15 samples. AHe ages are generally young (<4 Ma), and, in some samples, AFT ages (5–8 Ma) overlap with ZHe ages (7–9 Ma) within uncertainties. Thermo-kinematic modelling shows that exhumation rates are temporally variable, with rates decreasing from >2 km/Myr to <0.3 km/Myr around 5–7 Ma. With the onset of Plio-Pleistocene glaciation, exhumation rates increased to values >1 km/Myr. This demonstrates that the material outflux is varying through time, requiring a commensurate variation in influx to maintain 20 flux steady-state. Evaluation of the offshore and onshore sediment record shows that the material influx is also variable through time and that the amount of accreted sediment in the wedge is spatially variable. This qualitatively suggests that significant perturbations of steady-state occur on shorter timescales (10– 10 yr), like those created by Plio-Pleistocene glaciation. Our quantitative assessment of influx and outflux indicates that the Olympic Mountains could be in flux steady-state on long timescales (10 yr). 25