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Radio‐echo sounding (RES) shows large‐scale englacial stratigraphic folds are ubiquitous in Greenland's ice sheet. However, there is no consensus yet on how these folds form. Here, we use the full‐Stokes code Underworld2 to simulate ice movements in three‐dimensional convergent flow, mainly considering ice anisotropy due to a crystallographic preferred orientation, vertical viscosity and density gradients in ice layers, and bedrock topography. Our simulated folds show complex patterns and are classified into: large‐scale folds (>100 m amplitude), small‐scale folds (<<100 m) and basal‐shear folds. The amplitudes of large‐scale folds tend to be at their maximum in the middle of the ice column or just below, in accordance with observations in RES data. We conclude that ice anisotropy amplifies the perturbations in ice layers (mainly due to bedrock topography) into large‐scale folds during flow. Density differences between the warm deep ice and cold ice above may enhance fold amplification. Plain Language Summary: Polar ice sheets are composed of compacted former snow layers deposited at the ice surface. If not distorted or deformed, these layers are flat or adapt to the underlying bedrock topography. However, vertical radar scans of Greenland's ice sheet show large‐scale folds of up to hundreds of meters in height. To investigate how these large‐scale folds form, we set up a three‐dimensional numerical ice‐sheet model and simulate fold growth. Our modeling emphasizes the distinctive physical properties of ice required for fold formation, notably its anisotropy (the direction dependency of the flow strength) and power‐law rheology (when ice becomes softer with increasing strain rate). These findings help to better explain ice flow dynamics. Key Points: Large‐scale fold formation in polar ice sheets is mainly controlled by ice anisotropy and bedrock topographyBuoyancy of deep warm ice can further enhance fold amplificationThe implementation of ice anisotropy should be included in large‐scale ice flow modeling [ABSTRACT FROM AUTHOR]

Numerical models have become an indispensable tool for understanding and predicting the flow of ice sheets and glaciers. Here we present the full-Stokes software package Underworld to the glaciological community. The code is already well established in simulating complex geodynamic systems. Advantages for glaciology are that it provides a full-Stokes solution for elastic–viscous–plastic materials and includes mechanical anisotropy. Underworld uses a material point method to track the full history information of Lagrangian material points, of stratigraphic layers and of free surfaces. We show that Underworld successfully reproduces the results of other full-Stokes models for the benchmark experiments of the Ice Sheet Model Intercomparison Project for Higher-Order Models (ISMIP-HOM). Furthermore, we test finite-element meshes with different geometries and highlight the need to be able to adapt the finite-element grid to discontinuous interfaces between materials with strongly different properties, such as the ice–bedrock boundary. [ABSTRACT FROM AUTHOR]

Hydrofractures, or hydraulic fractures, are fractures where a significantly elevated fluid pressure played a role in their formation. Natural hydrofractures are abundant in rocks and are often preserved as magmatic dykes or sills, and mineral-filled fractures or mineral veins. However, we focus on the formation and evolution of non-igneous hydrofractures. Here we review the basic theory of the role of fluid pressure in rock failure, showing that both Terzaghi's and Biot's theories can be reconciled if the appropriate boundary conditions are considered. We next discuss the propagation of hydrofractures after initial failure, where networks of hydrofractures may form or hydrofractures may ascend through the crust as mobile hydrofractures. As fractures can form as a result of both tectonic stresses and an elevated fluid pressure, we address the question of how to ascertain whether a fracture is a hydrofracture. We argue that extensional or dilational fractures that formed below c. 2–3 km depth are, under normal circumstances, hydrofractures, but at shallower depth they may, but must not be hydrofractures. Since veins and breccias are often the products of hydrofractures that are left in the geological record, we discuss these and critically assess which vein structures can, and which do not necessarily, indicate hydrofracturing. Hydrofracturing can suddenly and locally change the permeability in a rock by providing new fluid pathways. This can lead to highly dynamic self-organization of crustal-scale fluid flow. [ABSTRACT FROM AUTHOR]

Smith-Johnsen et al. (The Cryosphere, 14, 841–854, 10.5194/tc-14-841-2020, 2020) model the effect of a potential hotspot on the Northeast Greenland Ice Stream (NEGIS). They argue that a heat flux of at least 970 mW m -2 is required to have initiated or to control NEGIS. Such an exceptionally high heat flux would be unique in the world and is incompatible with known geological processes that can raise the heat flux. Fast flow at NEGIS must thus be possible without the extraordinary melt rates invoked in Smith-Johnsen et al. (2020). [ABSTRACT FROM AUTHOR]

Crustal-scale fluid flow can be regarded as a bimodal transport mechanism. At low hydraulic head gradients, fluid flow through rock porosity is slow and can be described as diffusional. Structures such as hydraulic breccias and hydrothermal veins both form when fluid velocities and pressures are high, which can be achieved by localized fluid transport in space and time, via hydrofractures. Hydrofracture propagation and simultaneous fluid flow can be regarded as a "ballistic" transport mechanism, which is activated when transport by diffusion alone is insufficient to release the local fluid overpressure. The activation of a ballistic system locally reduces the driving force, through allowing the escape of fluid. We use a numerical model to investigate the properties of the two transport modes in general and the transition between them in particular. We developed a numerical model in order to study patterns that result from bimodal transport. When hydrofractures are activated due to low permeability relative to fluid flux, many hydrofractures form that do not extend through the whole system. These abundant hydrofractures follow a power-law size distribution. A Hurst factor of ~0.9 indicates that the system self-organizes. The abundant small-scale hydrofractures organize the formation of large-scale hydrofractures that ascend through the whole system and drain fluids in large bursts. As the relative contribution of porous flow increases, escaping fluid bursts become less frequent, but more regular in time and larger in volume. We propose that metamorphic rocks with abundant veins, such as in the Kodiak accretionary prism (Alaska) and Otago schists (New Zealand), represent regions with abundant hydrofractures near the fluid source, while hydrothermal breccias are formed by the large fluid bursts that can ascend the crust to shallower levels. [ABSTRACT FROM AUTHOR]

At the time of energy transition, it is important to be able to predict the effects of fluid overpressures in different geological scenarios as these can lead to the development of hydrofractures and dilating high-porosity zones. In order to develop an understanding of the complexity of the resulting effective stress fields, fracture and failure patterns, and potential fluid drainage, we study the process with a dynamic hydromechanical numerical model. The model simulates the evolution of fluid pressure buildup, fracturing, and the dynamic interaction between solid and fluid. Three different scenarios are explored: fluid pressure buildup in a sedimentary basin, in a vertical zone, and in a horizontal layer that may be partly offset by a fault. Our results show that the geometry of the area where fluid pressure is successively increased has a first-order control on the developing pattern of porosity changes, on fracturing, and on the absolute fluid pressures that sustained without failure. If the fluid overpressure develops in the whole model, the effective differential and mean stress approach zero and the vertical and horizontal effective principal stresses flip in orientation. The resulting fractures develop under high lithostatic fluid overpressure and are aligned semihorizontally, and consequently, a hydraulic breccia forms. If the area of high fluid pressure buildup is confined in a vertical zone, the effective mean stress decreases while the differential stress remains almost constant and failure takes place in extensional and shear modes at a much lower fluid overpressure. A horizontal fluid pressurized layer that is offset shows a complex system of effective stress evolution with the layer fracturing initially at the location of the offset followed by hydraulic breccia development within the layer. All simulations show a phase transition in the porosity where an initially random porosity reduces its symmetry and forms a static porosity wave with an internal dilating zone and the presence of dynamic porosity channels within this zone. Our results show that patterns of fractures, hence fluid release, that form due to high fluid overpressures can only be successfully predicted if the geometry of the geological system is known, including the fluid overpressure source and the position of seals and faults that offset source layers and seals. [ABSTRACT FROM AUTHOR]

A mechanism is presented which explains how intra-continental rifting can cause large topographic uplift. The effect is sufficient to account for the uplift of rift flanks and the very high and strongly localized uplift of the Rwenzori horst in the Western Branch of the East African Rift System. We propose that the uplift is generated by crustal bending, which is caused by a misfit of the lateral tensile stress between the upper and middle crust. The misfit is a function of different yield mechanisms when the upper crust breaks whereas the middle crust flows. Two independent numerical schemes confirm the suggested uplift mechanism. Both models--a 2 and 2.5 D elastoplastic lattice-particle model and a multilayer beam model were used to calculate the surface topography as a result of lateral uniaxial extension. Using the fault geometry of the Rwenzori area, we find that the amount of topographic uplift is controlled by the viscosity and elasticity of the crust. The extreme uplift of the Rwenzori horst is--at least to some extent--a function of its considerably high elastic stiffness. The stiffness unites the two rifts that bound the Rwenzori horst and leads to an extremely high topography and a high Moho uplift in the center of the two rifts where the Rwenzori mountains sit. [ABSTRACT FROM AUTHOR]

The two branches of the East African Rift System (EARS) are believed to have initiated diachronously. However, a growing body of work continues to suggest the onset of rifting in the Western Branch occurred in the Paleogene, coeval to the Eastern Branch. Due to a lack of pre-Miocene stratigraphy, attempts to resolve the geological history of the Western Branch must study the uplift and erosional histories of the modern rift topography. In this study, the rock uplift history of the Rwenzori Mountains, Western Uganda, is resolved to better our understanding of the tectonic history of the Western Branch of the EAR. Through the application of low-temperature thermochronology, χ -mapping and the modelling of river profiles, we show that rock uplift of the Rwenzori dates back to the Oligocene, with thermal history models suggesting uplift induced exhumation may date back as far as the Eocene. This provides tangible evidence that extension began in the region in the Paleogene, coeval with the Eastern Branch, and not the late Neogene. These results have broad implications for the tectonic evolution of the entire East African Rift System and suggest our current understanding of the region's rift history remains incomplete. • The Rwenzori Mountains began uplifting in the Paleogene. • Uplift occurred in two phases Eocene-Oligocene and late Miocene-Pliocene. • This work suggests Western Branch of the East African Rift is older than thought. [ABSTRACT FROM AUTHOR]