ALBERT

Description: Focused, rapid exhumation of rocks is observed at some plate corners, but the driving mechanisms remain poorly understood and contested. In this study, we use a fully coupled thermo-mechanical numerical model to investigate the effect of slab advance and different erosion scenarios on overriding plate deformation. The subducting slab in the model is curved in 3D, analogous to the indenter geometry observed in seismic studies. We find that the amount of slab advance dramatically changes the orientation of major shear zones in the upper plate and the location of rock uplift zones. Shear along the subduction interface facilitates the formation of a basal detachment situated above the indenter, causing localized rock uplift there. Switching from flat (total erosion) to more realistic fluvial erosion leads to variation of rock uplift on the catchment-scale. Here, deepest exhumation again occurred above the indenter apex. We conclude that the change in orientation and dip angle set by the indenter geometry facilitates creation of localized uplift regions as long as subduction of the down-going plate is active. Tectonic uplift is modulated on even smaller scales by lithostatic pressure from the overburden of the growing orogen, and highest rock uplift can occur when a strong tectonic uplift field spatially coincides with large erosion potential. This implies that both the geometry of the subducting plate and the geomorphic and climatic conditions are important for the creation of focused, rapid exhumation.

Description: Assessing paleo-climatic changes across the Tibetan Plateau and the underlying driving mechanisms provides insights for the natural variability in the Earth's climate system in response to tectonic processes and global climate change. In this study, we use a high-resolution regional climate model to investigate various episodes of distinct climate states over the Tibetan Plateau region during the Cenozoic rise of the Plateau and Quaternary glacial/interglacial cycles. The main objective is to compare climate changes during the Miocene-Pliocene uplift period with climate anomalies during the last glacial maximum and the mid-Holocene optimum, based on a consistent modeling framework. Reduced plateau elevation leads to regionally differentiated patterns of higher temperature and lower precipitation amount on the plateau itself, whereas surrounding regions are subject to colder conditions. In particular, Central Asia receives much more precipitation prior to the uplift, mainly due to a shift of the stationary wave train over Eurasia. Cluster analysis indicates that the continental-desert type climate, which is widespread over Central Asia today, appears with the Tibetan Plateau reaching 50 % of its present-day elevation. The mid-Holocene is characterized by slightly colder temperatures, and the last glacial maximum by considerably colder conditions over most of central and southern Asia. Precipitation anomalies during these episodes are less pronounced and spatially heterogeneous over the Tibetan Plateau. The simulated changes are in good agreement with available paleo-climatic reconstructions from proxy data. The present-day climate classification is only slightly sensitive to the changed boundary conditions in the Quaternary Quaternary. It is shown that in some regions of the Tibetan Plateau the climate anomalies during the Quaternary Quaternary have been as strong as the changes occurring during the uplift period.

Description: The temporal and kinematic evolution of fold-thrust belts is a critical component for evaluating the viability of proposed plate tectonic, geodynamic and even climatic processes in regions of convergence. Thermochronometer data have the potential to provide temporal constraints, but interpretations of these data are sensitive to both exhumational and deformational processes. In this study, reconstructions of a balanced geologic cross section in the Himalayan fold-thrust belt of eastern Bhutan are used in a flexural and thermal-kinematic model to understand the sensitivity of predicted cooling ages to changes in fault kinematics, geometry and topography. We sequentially deform the cross section with ~ 10 km deformation steps and apply flexural loading and erosional unloading at each step to develop a high-resolution evolution of deformation, erosion, and burial over time. Comparison of model-predicted cooling ages to published thermochronometer data reveals that cooling ages are most sensitive to (1) location and magnitude of fault ramps, (2) variable shortening rates between 68-6.4 mm/yr, and (3) timing and magnitude of out-of-sequence faulting. The predicted ages are less sensitive to (4) radiogenic heat production, and (5) estimates of topographic evolution. We propose a revised cross section geometry that separates one large ramp previously proposed for the modern decollement into two smaller ramps. The revised cross section results in an improved fit to observed ages, particularly young AFT ages (2–6 Ma) located north of the Main Central Thrust.

Description: In this study, reconstructions of a balanced geologic cross section in the Himalayan fold–thrust belt of eastern Bhutan are used in flexural–kinematic and thermokinematic models to understand the sensitivity of predicted cooling ages to changes in fault kinematics, geometry, topography, and radiogenic heat production. The kinematics for each scenario are created by sequentially deforming the cross section with  ∼ 10 km deformation steps while applying flexural loading and erosional unloading at each step to develop a high-resolution evolution of deformation, erosion, and burial over time. By assigning ages to each increment of displacement, we create a suite of modeled scenarios that are input into a 2-D thermokinematic model to predict cooling ages. Comparison of model-predicted cooling ages to published thermochronometer data reveals that cooling ages are most sensitive to (1) the location and size of fault ramps, (2) the variable shortening rates between 68 and 6.4 mm yr−1, and (3) the timing and magnitude of out-of-sequence faulting. The predicted ages are less sensitive to (4) radiogenic heat production and (5) estimates of topographic evolution. We used the observed misfit of predicted to measured cooling ages to revise the cross section geometry and separate one large ramp previously proposed for the modern décollement into two smaller ramps. The revised geometry results in an improved fit to observed ages, particularly young AFT ages (2–6 Ma) located north of the Main Central Thrust. This study presents a successful approach for using thermochronometer data to test the viability of a proposed cross section geometry and kinematics and describes a viable approach to estimating the first-order topographic evolution of a compressional orogen.

Description: Focused, rapid exhumation of rocks is observed at some orogen syntaxes, but the driving mechanisms remain poorly understood and contested. In this study, we use a fully coupled thermomechanical numerical model to investigate the effect of upper-plate advance and different erosion scenarios on overriding plate deformation. The subducting slab in the model is curved in 3-D, analogous to the indenter geometry observed in seismic studies. We find that the amount of upper-plate advance toward the trench dramatically changes the orientation of major shear zones in the upper plate and the location of rock uplift. Shear along the subduction interface facilitates the formation of a basal detachment situated above the indenter, causing localized rock uplift there. We conclude that the change in orientation and dip angle set by the indenter geometry creates a region of localized uplift as long as subduction of the down-going plate is active. Switching from flat (total) erosion to more realistic fluvial erosion using a landscape evolution model leads to variations in rock uplift at the scale of large catchments. In this case, deepest exhumation again occurs above the indenter apex, but tectonic uplift is modulated on even smaller scales by lithostatic pressure from the overburden of the growing orogen. Highest rock uplift can occur when a strong tectonic uplift field spatially coincides with large erosion potential. This implies that both the geometry of the subducting plate and the geomorphic and climatic conditions are important for the creation of focused, rapid exhumation.

Description: The majority of Antarctic ice shelves are bounded by grounded ice rises. These ice rises exhibit local flow fields that partially oppose the flow of the surrounding ice shelves. Formation of ice rises is accompanied by a characteristic upward-arching internal stratigraphy (“Raymond arches”), whose geometry can be analysed to infer information about past ice-sheet changes in areas where other archives such as rock outcrops are missing. Here we present an improved modelling framework to study ice-rise evolution using a satellite-velocity calibrated, isothermal, and isotropic 3-D full-Stokes model including grounding-line dynamics at the required mesh resolution (2.0 m yr−1) of up to 3.5 km. In contrast, instantaneous ice-shelf disintegration causes a short-lived and delayed (by 60–100 years) response of smaller magnitude (

Description: Detecting and explaining differences between palaeoclimates can provide valuable insight into climatic tipping points and a useful framework of information for Earth scientists investigating processes that are affected by climate change over geological time. We apply a combination of multivariate cluster- and discriminant analysis techniques to a set of consistently set-up high-resolution palaeoclimate simulations conducted with the ECHAM5 climate model. A pre-industrial (PI) climate simulation serves as the control experiment, which is compared to a suite of simulations of Late Cenozoic climates, namely a Mid-Holocene (MH, ca. 6.5 ka), Last Glacial Maximum (LGM, ca. 21 ka) and Pliocene (PLIO, ca. 3 Ma) climate. For each of the study regions (Western South America, Europe and Himalaya–Tibet and South Alaska), differences in climate are subjected to geographical clustering to identify dominant modes of climate change and their spatial extent for each time slice comparison (PI-MH, PI-LGM and PI-PLIO). The selection of climate variables for the cluster analysis is made on the basis of their relevance to Earth surface processes and includes 2 m air temperature, 2 m air temperature amplitude, consecutive freezing days, freeze-thaw days, maximum precipitation, consecutive wet days, consecutive dry days, zonal wind speed and meridional wind speed. We then apply a two-class multivariate discriminant analysis to simulation pairs PI-MH, PI-LGM and PI-PLIO to evaluate and explain the discriminability between climates within each of the anomaly clusters. Changes in ice cover create the most distinct and stable patterns of climate change, and create the best discriminability between climates in western Patagonia. The distinct nature of European palaeoclimates is mostly explained by changes in 2 m air temperature (MH, LGM, PLIO), consecutive freezing days (LGM) and consecutive wet days (PLIO). These factors typically contribute 30 %–50 %, 10 %–40 % and 10 %–30 % respectively to climate discriminability. Finally, our results identify regions particularly prone to changes in precipitation-induced erosion and temperature-dependent physical weathering.

Description: The denudation history of active orogens is often interpreted in the context of modern climate and vegetation gradients. Here we address the validity of this approach and ask the question: what are the spatial and temporal variations in paleo-climate for a latitudinally diverse range of active orogens? We do this using high-resolution (T159, ca. 80 × 80 km at the equator) paleo-climate simulations from the ECHAM5 global Atmospheric General Circulation Model and a statistical cluster analysis of climate over different orogens (Andes, Himalaya, SE Alaska, Pacific NW USA). Time periods and boundary conditions considered include the Pliocene (PLIO, ~ 3 Ma), the Last Glacial Maximum (LGM, ~ 21 ka), Mid Holocene (MH, ~ 6 ka) and Pre-Industrial (PI, reference year 1850). The regional simulated climates of each orogen are described by means of cluster analyses based on the variability of precipitation, 2 m air temperature, the intra-annual amplitude of these values, and monsoonal wind speeds where appropriate. Results indicate the largest differences to the PI climate are observed for the LGM and PLIO climates in the form of widespread cooling and reduced precipitation in the LGM and warming and enhanced precipitation during the PLIO. The LGM climate shows the largest deviation in annual precipitation from the PI climate, and shows enhanced precipitation in the temperate Andes, and coastal regions for both SE Alaska and the US Pacific Northwest Pacific. Furthermore, LGM precipitation is reduced in the western Himalayas and enhanced in the eastern Himalayas, resulting in a shift of the wettest regional climates eastward along the orogen. The cluster-analysis results also suggest more climatic variability across latitudes east of the Andes in the PLIO climate than in other time-slice experiments conducted here. Taken together, these results highlight significant changes in Late Cenozoic regional climatology over the last ~ 3 Ma. Finally, we document regions where the largest magnitudes of Late Cenozoic changes in precipitation and temperature occur and offer the highest potential for future observational studies interested in quantifying the impact of climate change on denudation and weathering rates.

Description: The majority of Antarctic ice shelves are bounded by grounded ice rises. These ice rises exhibit local flow regimes that partially oppose the flow of the surrounding ice shelves. Formation of such ice rises is accompanied by a characteristic upward arching internal stratigraphy (Raymond arches), archiving potential past divide migration and the onset of divide flow. Information about past ice-sheet conditions can therefore be retrieved in areas where other archives are missing. However, the quantitative interpretation of the stratigraphy requires modelling and radar observations. Hitherto, ice-rise modelling has been restricted to 2D and excluded the coupling between ice shelf and ice rise. This presents a major limitation for the interpretation of ice rises as ice-dynamic archive. Here we present an improved modelling framework to study ice-rise evolution using a calibrated, isothermal, and isotropic 3D Full-Stokes model including grounding-line dynamics at the required mesh resolution (〈500 m). We apply the model to the Ekström Ice Shelf catchment containing two ice rises. Our results show that changes in the surface mass balance result in immediate and sustained divide migration (〉2.0 m/yr) of up to 3.5 km. In contrast, instantaneous ice-shelf disintegration causes a short-lived and delayed (by 60–100 years) response of smaller magnitude (〈0.75 m/yr). The model tracks migration of a triple junction and synchronous ice-divide migration in both ice rises with similar magnitude but differing rates. The model is suitable for glacial/interglacial simulations on the catchment scale, providing the next step forward to unravel the ice-dynamic history stored in ice rises all around Antarctica.

Description: Vegetation is crucial for modulating rates of denudation and landscape evolution as it stabilizes and protects hillslopes and intercepts rainfall. Climate conditions and atmospheric CO2 concentration ([CO2]) influence the establishment and performance of plants and thus have a direct influence on vegetation cover. In addition, vegetation dynamics (competition for space, light, nutrients and water) and stochastic events (mortality and fires) determine the state of vegetation, response times to environmental perturbations, and the successional development. In spite of this, state-of-art reconstructions of past transient vegetation changes have not been accounted for in landscape evolution models. Here, a widely used dynamic vegetation model (LPJ-GUESS) was used to simulate vegetation composition/ cover and surface runoff in Chile for the Last Glacial Maximum (LGM), Mid Holocene (MH) and present day (PD). In addition, we conducted transient vegetation simulations from LGM to PD for four sites of the Coastal Cordillera of Chile at a spatial and temporal resolution adequate for coupling with landscape evolution models. Using a regionally-adapted parametrization, LPJ-GUESS was capable of reproducing present day potential natural vegetation along the strong climatic gradients of Chile and simulated vegetation cover was also in line with satellite-based observations. Simulated vegetation during the LGM differed markedly from PD conditions. Coastal cold temperate rainforests where displaced northward by about 5° and the tree line and vegetation zones were at lower elevations than at PD. Transient vegetation simulations indicate a marked shift in vegetation composition starting with the past-glacial warming that coincides with a rise in [CO2]. Vegetation cover between the sites ranged from 13% (LGM: 8%) to 81% (LGM: 73%) for the northern Pan de Azúcar and southern Nahuelbuta sites, respectively, but did not vary by more than 10% over the 21,000yr simulation. A sensitivity study suggests that [CO2] is an important driver of vegetation changes and, thereby, potentially landscape evolution. Comparisons with other paleoclimate model driver highlight the importance of model input on simulated vegetation. In the near future, we will directly couple LPJ-GUESS to a landscape evolution model (see companion paper) to build a fully-coupled dynamic-vegetation/ landscape evolution model that is forced with paleoclimate data from atmospheric general circulation models.