TY - JOUR A1 - Murray, Kendra E. A1 - Braun, Jean A1 - Reiners, Peter W. T1 - Toward Robust Interpretation of Low-Temperature Thermochronometers in Magmatic Terranes JF - Geochemistry, geophysics, geosystems N2 - Many regions central to our understanding of tectonics and landscape evolution are active or ancient magmatic terranes, and robust interpretation of low-temperature thermochronologic ages in these settings requires careful attention to the drivers of rock heating and cooling, including magmatism. However, we currently lack a quantitative framework for evaluating the potential role of magmatic coolingthat is, post-magmatic thermal relaxationin shaping cooling age patterns in regions with a history of intrusive magmatism. Here we use analytical approximations and numerical models to characterize how low-temperature thermochronometers document cooling inside and around plutons in steadily exhuming environments. Our models predict that the thermal field a pluton intrudes into, specifically the ambient temperatures relative to the closure temperature of a given thermochronometer, is as important as the pluton size and temperature in controlling the pattern and extent of thermochronometer resetting in the country rocks around a pluton. We identify one advective and several conductive timescales that govern the relationship between the crystallization and cooling ages inside a pluton. In synthetic vertical age-elevation relationships (AERs), resetting next to plutons results in changes in AER slope that could be misinterpreted as past changes in exhumation rate if the history of magmatism is not accounted for. Finally, we find that large midcrustal plutons, such as those emplaced at similar to 10-15-km depth, can reset the low-temperature thermochronometers far above them in the upper crusta result with considerable consequences for thermochronology in arcs and regions with a history of magmatic activity that may not have a surface expression. KW - He thermochronology KW - Peclet number KW - age-elevation relationships Y1 - 2018 U6 - https://doi.org/10.1029/2018GC007595 SN - 1525-2027 VL - 19 IS - 10 SP - 3739 EP - 3763 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Braun, Jean A1 - Gemignani, Lorenzo A1 - van der Beek, Peter T1 - Extracting information on the spatial variability in erosion rate stored in detrital cooling age distributions in river sands JF - Earth surface dynamics N2 - One of the main purposes of detrital thermochronology is to provide constraints on the regional-scale exhumation rate and its spatial variability in actively eroding mountain ranges. Procedures that use cooling age distributions coupled with hypsometry and thermal models have been developed in order to extract quantitative estimates of erosion rate and its spatial distribution, assuming steady state between tectonic uplift and erosion. This hypothesis precludes the use of these procedures to assess the likely transient response of mountain belts to changes in tectonic or climatic forcing. Other methods are based on an a priori knowledge of the in situ distribution of ages to interpret the detrital age distributions. In this paper, we describe a simple method that, using the observed detrital mineral age distributions collected along a river, allows us to extract information about the relative distribution of erosion rates in an eroding catchment without relying on a steady-state assumption, the value of thermal parameters or an a priori knowledge of in situ age distributions. The model is based on a relatively low number of parameters describing lithological variability among the various sub-catchments and their sizes and only uses the raw ages. The method we propose is tested against synthetic age distributions to demonstrate its accuracy and the optimum conditions for it use. In order to illustrate the method, we invert age distributions collected along the main trunk of the Tsangpo-Siang-Brahmaputra river system in the eastern Himalaya. From the inversion of the cooling age distributions we predict present-day erosion rates of the catchments along the Tsangpo-Siang-Brahmaputra river system, as well as some of its tributaries. We show that detrital age distributions contain dual information about present-day erosion rate, i. e., from the predicted distribution of surface ages within each catchment and from the relative contribution of any given catchment to the river distribution. The method additionally allows comparing modern erosion rates to long-term exhumation rates. We provide a simple implementation of the method in Python code within a Jupyter Notebook that includes the data used in this paper for illustration purposes. Y1 - 2018 U6 - https://doi.org/10.5194/esurf-6-257-2018 SN - 2196-6311 SN - 2196-632X VL - 6 IS - 1 SP - 257 EP - 270 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Herman, Frederic A1 - Braun, Jean A1 - Deal, Eric A1 - Prasicek, Gunther T1 - The response time of glacial erosion JF - Journal of geophysical research : Earth surface N2 - There has been recent progress in the understanding of the evolution of Quaternary climate. Simultaneously, there have been improvements in the understanding of glacial erosion processes, with better parameter constraints. Despite this, there remains much debate about whether or not the observed cooling over the Quaternary has driven an increase in glacial erosion rates. Most studies agree that the erosional response to climate change must be transient; therefore, the time scale of the climatic change and the response time of glacial erosion must be accounted for. Here we analyze the equations governing glacial erosion in a steadily uplifting landscape with variable climatic forcing and derive expressions for two fundamental response time scales. The first time scale describes the response of the glacier and the second one the glacial erosion response. We find that glaciers have characteristic time scales of the order of 10 to 10,000 years, while the characteristic time scale for glacial erosion is of the order of a few tens of thousands to a few million years. We then use a numerical model to validate the approximations made to derive the analytical solutions. The solutions show that short period forcing is dampened by the glacier response time, and long period forcing (>1 Myr) may be dampened by erosional response of glaciers when the rock uplift rates are high. In most tectonic and climatic conditions, we expect to see the strongest response of glacial erosion to periodic climatic forcing corresponding to Plio-Pleistocene climatic cycles. Finally, we use the numerical model to predict the response of glacial systems to the observed climatic forcing of the Quaternary, including, but not limited to, the Milankovich periods and the long-term secular cooling trend. We conclude that an increase of glacial erosion in response to Quaternary cooling is physically plausible, and we show that the magnitude of the increase depends on rock uplift and ice accumulation rates. Y1 - 2018 U6 - https://doi.org/10.1002/2017JF004586 SN - 2169-9003 SN - 2169-9011 VL - 123 IS - 4 SP - 801 EP - 817 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Pico, T. A1 - Mitrovica, J. X. A1 - Braun, Jean A1 - Ferrier, K. L. T1 - Glacial isostatic adjustment deflects the path of the ancestral Hudson River JF - Geology N2 - Quantifying the pace of ice-sheet growth is critical to understanding ice-age climate and dynamics. Here, we show that the diversion of the Hudson River (northeastern North America) late in the last glaciation phase (ca. 30 ka), which some previous studies have speculated was due to glacial isostatic adjustment (GIA), can be used to infer the timing of the Laurentide Ice Sheet’s growth to its maximum extent. Landscapes in the vicinity of glaciated regions have likely responded to crustal deformation produced by ice-sheet growth and decay through river drainage reorganization, given that rates of uplift and subsidence are on the order of tens of meters per thousand years. We perform global, gravitationally self-consistent simulations of GIA and input the predicted crustal deformation field into a landscape evolution model. Our calculations indicate that the eastward diversion of the Hudson River at 30 ka is consistent with exceptionally rapid growth of the Laurentide Ice Sheet late in the glaciation phase, beginning at 50–35 ka. Y1 - 2018 U6 - https://doi.org/10.1130/G40221.1 SN - 0091-7613 SN - 1943-2682 VL - 46 IS - 7 SP - 591 EP - 594 PB - American Institute of Physics CY - Boulder ER - TY - JOUR A1 - Prasicek, Günther A1 - Herman, Frederic A1 - Robl, Jörg A1 - Braun, Jean T1 - Glacial steady state topography controlled by the coupled influence of tectonics and climate JF - Journal of geophysical research : Earth surface N2 - Glaciers and rivers are the main agents of mountain erosion. While in the fluvial realm empirical relationships and their mathematical description, such as the stream power law, improved the understanding of fundamental controls on landscape evolution, simple constraints on glacial topography and governing scaling relations are widely lacking. We present a steady state solution for longitudinal profiles along eroding glaciers in a coupled system that includes tectonics and climate. We combined the shallow ice approximation and a glacial erosion rule to calculate ice surface and bed topography from prescribed glacier mass balance gradient and rock uplift rate. Our approach is inspired by the classic application of the stream power law for describing a fluvial steady state but with the striking difference that, in the glacial realm, glacier mass balance is added as an altitude-dependent variable. From our analyses we find that ice surface slope and glacial relief scale with uplift rate with scaling exponents indicating that glacial relief is less sensitive to uplift rate than relief in most fluvial landscapes. Basic scaling relations controlled by either basal sliding or internal deformation follow a power law with the exponent depending on the exponents for the glacial erosion rule and Glen's flow law. In a mixed scenario of sliding and deformation, complicated scaling relations with variable exponents emerge. Furthermore, a cutoff in glacier mass balance or cold ice in high elevations can lead to substantially larger scaling exponents which may provide an explanation for high relief in high latitudes. KW - glacial equilibrium KW - steady state topography KW - glacial erosion KW - glacial buzzsaw KW - rock uplift-relief scaling KW - scaling relation Y1 - 2018 U6 - https://doi.org/10.1029/2017JF004559 SN - 2169-9003 SN - 2169-9011 VL - 123 IS - 6 SP - 1344 EP - 1362 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Biswas, R. H. A1 - Herman, F. A1 - King, G. E. A1 - Braun, Jean T1 - Thermoluminescence of feldspar as a multi-thermochronometer to constrain the temporal variation of rock exhumation in the recent past JF - Earth & planetary science letters N2 - Natural thermoluminescence (TL) in rocks reflects a dynamic equilibrium between radiation-induced TL growth and decay via thermal and athermal pathways. When rocks exhume through Earth's crust and cool from high to low temperature, this equilibrium level increases as the temperature dependent thermal decay decreases. This phenomenon can be exploited to extract thermal histories of rocks. The main advantage of TL is that a single TL glow curve has a wide range of thermal stabilities (lifetime 100 °C/Ma, whereas deeper traps, i.e. with higher activation energies, provide constraints on thermal histories for higher cooling rates (>300 °C/Ma). Finally, we show how the path of rock exhumation (i.e., depth vs. time) can be constrained using an inverse approach. The newly developed methodology is applied to rapidly cooled samples from the Namche Barwa massif, eastern Himalaya to suggest a trend in exhumation rate with time that follows an inverse correlation with global temperature and glaciers equilibrium altitude line (ELA). KW - TL of feldspar KW - TL-thermochronology KW - multi-thermochronometer KW - rock exhumation KW - Namche Barwa Y1 - 2018 U6 - https://doi.org/10.1016/j.epsl.2018.04.030 SN - 0012-821X SN - 1385-013X VL - 495 SP - 56 EP - 68 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Deal, Eric A1 - Braun, Jean A1 - Botter, Gianluca T1 - Understanding the role of rainfall and hydrology in determining fluvial erosion efficiency JF - Journal of geophysical research : Earth surface N2 - Due to the challenges in upscaling daily climatic forcing to geological time, physically realistic models describing how rainfall drives fluvial erosion are lacking. To bridge this gap between short-term hydrology and long-term geomorphology, we derive a theoretical framework for long-term fluvial erosion rates driven by realistic climate by integrating an established stochastic-mechanistic model of hydrology into a threshold-stochastic formulation of stream power. The hydrological theory provides equations for the daily streamflow probability distribution as a function of climatic boundary conditions. The new parameters introduced are rooted firmly in established climatic and hydrological theory. This allows us to account for how fluvial erosion rates respond to changes in rainfall intensity, frequency, evapotranspiration rates, and soil moisture dynamics in a way that is consistent with existing theories. We use this framework to demonstrate how hydroclimatic conditions and erosion threshold magnitude control the degree of nonlinearity between steepness index and erosion rate. We find that hydrological processes can have a significant influence on how erosive a particular climatic forcing will be. By accounting for the influence of hydrology on fluvial erosion, we conclude that climate is an important control on erosion rates and long-term landscape evolution. Y1 - 2108 U6 - https://doi.org/10.1002/2017JF004393 SN - 2169-9003 SN - 2169-9011 VL - 123 IS - 4 SP - 744 EP - 778 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Pico, Tamara A1 - Mitrovica, Jerry X. A1 - Perron, J. Taylor A1 - Ferrier, Ken L. A1 - Braun, Jean T1 - Influence of glacial isostatic adjustment on river evolution along the US mid-Atlantic coast JF - Earth & planetary science letters N2 - Long-term river evolution depends partly on crustal deformation, which shapes the topography crossed by rivers. On glacial timescales, ice-sheet growth and decay can produce crustal vertical motion of ∼10 mm/yr resulting from the solid Earth's adjustment to variations in ice and water loads, comparable to tectonically-driven rates in the most rapidly uplifting mountains on Earth. This process of glacial isostatic adjustment (GIA) can influence river courses and drainage basins substantially, particularly near former ice margins. We explore the extent to which GIA influenced the evolution of rivers along the United States east coast during the last glacial cycle. We compute gravitationally self-consistent GIA responses that incorporate recent constraints on the Laurentide Ice Sheet history through the last glacial build-up phase, and we connect the predicted variations in topography to abrupt changes in river dynamics recorded in the Hudson, Delaware, Susquehanna, and Potomac Rivers from 40 ka to present. To the extent that increases in sediment transport capacity imply increases in river incision rate, the GIA-driven changes in slope and drainage area are consistent with episodes of erosion and sedimentation observed in the Hudson, Delaware, and Potomac Rivers, but inconsistent with the observed accelerated river incision in the Susquehanna River at 30-14 ka. These analyses add to a growing body of evidence showing that GIA strongly influences river evolution over millennial timescales. KW - glacial-isostatic adjustment KW - US east coast river geomorphology KW - river dynamics on glacial timescales Y1 - 2019 U6 - https://doi.org/10.1016/j.epsl.2019.06.026 SN - 0012-821X SN - 1385-013X VL - 522 SP - 176 EP - 185 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Margirier, Audrey A1 - Braun, Jean A1 - Gautheron, Cecile A1 - Carcaillet, Julien A1 - Schwartz, Stephane A1 - Jamme, Rosella Pinna A1 - Stanley, Jessica T1 - Climate control on Early Cenozoic denudation of the Namibian margin as deduced from new thermochronological constraints JF - Earth & planetary science letters N2 - The processes that control long term landscape evolution in continental interiors and, in particular, along passive margins such as in southern Africa, are still the subject of much debate (e.g. Braun, 2018). Although today the Namibian margin is characterized by an arid climate, it has experienced climatic fluctuations during the Cenozoic and, yet, to date no study has documented the potential role of climate on its erosion history. In western Namibia, the Brandberg Massif, an erosional remnant or inselberg, provides a good opportunity to document the Cenozoic denudation history of the margin using the relationship between rock cooling or exhumation ages and their elevation. Here we provide new apatite (UThSm)/He dates on the Brandberg Inselberg that range from 151 +/- 12 to 30 +/- 2 Ma. Combined with existing apatite fission track data, they yield new constraints on the denudation history of the margin. These data document two main cooling phases since continental break-up 130 Myr ago, a rapid one (similar to 10 degrees C/Myr) following break-up and a slower one (similar to 12 degrees C/Myr) between 65 and 35 Ma. We interpret them respectively to be related to escarpment erosion following rifting and continental break-up and as a phase of enhanced denudation during the Early Eocene Climatic Optimum. We propose that during the Early Eocene Climatic Optimum chemical weathering was important and contributed significantly to the denudation of the Namibian margin and the formation of a pediplain around the Brandberg and enhanced valley incision within the massif. Additionally, aridification of the region since 35 Ma has resulted in negligible denudation rates since that time. (C) 2019 Elsevier B.V. All rights reserved. KW - climate KW - Early Eocene Climatic Optimum KW - apatite (U-Th-Sm)/He thermochronology KW - denudation KW - weathering KW - Namibian passive margin Y1 - 2019 U6 - https://doi.org/10.1016/j.epsl.2019.115779 SN - 0012-821X SN - 1385-013X VL - 527 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Yuan, Xiaoping P. A1 - Braun, Jean A1 - Guerit, Laure A1 - Rouby, D. A1 - Cordonnier, G. T1 - A New Efficient Method to Solve the Stream Power Law Model Taking Into Account Sediment Deposition JF - Journal of geophysical research : Earth surface N2 - The stream power law model has been widely used to represent erosion by rivers but does not take into account the role played by sediment in modulating erosion and deposition rates. Davy and Lague (2009, ) provide an approach to address this issue, but it is computationally demanding because the local balance between erosion and deposition depends on sediment flux resulting from net upstream erosion. Here, we propose an efficient (i.e., O(N) and implicit) method to solve their equation. This means that, unlike other methods used to study the complete dynamics of fluvial systems (e.g., including the transition from detachment-limited to transport-limited behavior), our method is unconditionally stable even when large time steps are used. We demonstrate its applicability by performing a range of simulations based on a simple setup composed of an uplifting region adjacent to a stable foreland basin. As uplift and erosion progress, the mean elevations of the uplifting relief and the foreland increase, together with the average slope in the foreland. Sediments aggrade in the foreland and prograde to reach the base level where sediments are allowed to leave the system. We show how the topography of the uplifting relief and the stratigraphy of the foreland basin are controlled by the efficiency of river erosion and the efficiency of sediment transport by rivers. We observe the formation of a steady-state geometry in the uplifting region, and a dynamic steady state (i.e., autocyclic aggradation and incision) in the foreland, with aggradation and incision thicknesses up to tens of meters. KW - stream power law model KW - efficient method KW - sediment transport and deposition KW - river erosion KW - dynamic steady state KW - aggradation and incision cycles Y1 - 2019 U6 - https://doi.org/10.1029/2018JF004867 SN - 2169-9003 SN - 2169-9011 VL - 124 IS - 6 SP - 1346 EP - 1365 PB - American Geophysical Union CY - Washington ER -