The push and pull of abandoned channels: How floodplain processes and healing affect avulsion dynamics and alluvial landscape evolution in foreland basins

River avulsions are an important mechanism by which sediment is routed and emplaced in foreland basins. However, because avulsions occur infrequently, we lack observational data that might inform where, when, and why avulsions occur and these questions are instead often investigated by rule-based numerical models. These models have historically simplified or neglected the effects of abandoned channels on avulsion dynamics, even though fluvial megafans in foreland basins are characteristically covered in abandoned channels. Here, we investigate the pervasiveness of abandoned channels on modern fluvial megafan surfaces. Then, we present a physically based cellular model that parameterizes interactions between a single avulsing river and abandoned channels in a foreland basin setting. We investigate how abandoned channels affect avulsion set-up, pathfinding, and landscape evolution. We demonstrate and discuss how the processes of abandoned channel inheritance and transient knickpoint propagation post-avulsion serve to shortcut the time necessary to set-up successive avulsions. Then, we address the idea that abandoned channels can both repel and attract future pathfinding flows under different conditions. By measuring the distance between the mountain-front and each avulsion over long (106 to 107 years) timescales, we show that increasing abandoned channel repulsion serves to push avulsions farther from the mountain-front, while increasing attraction pulls avulsions proximally. Abandoned channels do not persist forever, and we test possible channel healing scenarios (deposition-only, erosion-only, and far-field directed) and show that only the final scenario achieves dynamic equilibrium without completely filling accommodation space. We also observe megafan growth occurring via ~O:105 year lobe switching, but only in our runs that employ deposition-only or erosion-only healing modes. Finally, we highlight opportunities for future field work and remote sensing efforts to inform our understanding of the role that floodplain topography, including abandoned channels, plays on avulsion dynamics.

The Subhercynian Basin: an example of an intraplate foreland basin due to a broken plate

The Late Cretaceous intraplate shortening event in central western Europe is associated with a number of marine basins of relatively high amplitude and short wavelength (2–3 km depth and 20–100 km width). In particular, the Harz Mountains, a basement uplift on a single, relatively steeply dipping basement thrust, have filled the adjacent Subhercynian Cretaceous Basin with their erosive product, proving that the two were related and synchronous. The problem of generating subsidence of this general style and geometry in an intraplate setting is dealt with here by using an elastic flexural model conditioned to take account of basement thrusts as weak zones in the lithosphere. Using a relatively simple configuration of this kind, we reproduce many of the basic features of the Subhercynian Cretaceous Basin and related basement thrusts. As a result, we suggest that overall, it shares many characteristics with larger-scale foreland basins associated with collisional orogens on plate boundaries.

Unconformity development in retroarc foreland basins: Implications for the geodynamics of Andean-type margins

Unconformities in foreland basins may be generated by tectonic processes that operate in the basin, adjacent fold-thrust belt, and broader convergent margin. Foreland basin unconformities represent shifts from high accommodation to nondepositional or erosional conditions in which the interruption of subsidence precludes net sediment accumulation. This study explores the genesis of long duration (>1–20 Myr) unconformities and condensed stratigraphic sections by considering modern and ancient examples from the Andes. These cases highlight potential geodynamic mechanisms of accommodation reduction and hiatus development in Andean-type retroarc foreland settings, including: (a) shortening-induced uplift in the frontal thrust belt and proximal foreland; (b) growth and advance of a broad, low-relief flexural forebulge; (c) uplift of intraforeland basement blocks; (d) tectonic quiescence with regional isostatic rebound; (e) cessation of thrust loading and flexural subsidence during oblique convergence; (f) diminished accommodation or sediment supply due to changes in sea level, climate, erosion, or transport; (g) basinwide uplift during flat-slab subduction; and (h) dynamic uplift associated with slab window formation, slab breakoff, elevated intraplate (in-plane) stress, or related mantle process. These contrasting mechanisms can be distinguished on the basis of the spatial distribution, structural context, stratigraphic position, paleoenvironmental conditions, and duration of unconformities and condensed sections.Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

The interplay of Malm carbonate permeability, gravity-driven groundwater flow, and paleoclimate – implications for the geothermal field and potential in the Molasse Basin (Southern Germany), a foreland-basin play

The Molasse Basin in Southern Germany is part of the North Alpine Foreland Basin and hosts the largest accumulation of deep geothermal production fields in Central Europe. Despite the vast development of geothermal energy utilization projects especially in the Munich metropolitan region, the evolution of and control factors on the natural geothermal field are still debated. Especially seismic and deep well data from extensive oil and gas exploration in the Molasse Basin led to conceptual hydrogeological and thermal-hydraulic models. Corrected borehole-temperature data helped to constrain subsurface temperatures by geostatistical interpolation and facilitated the set-up of 3D temperature models. However, within the geothermally used Upper Jurassic (Malm) carbonate aquifer, temperature anomalies such as the Wasserburg Trough anomaly to the east of Munich and their underlying physical processes are yet poorly understood. From other foreland basins like the Alberta Basin in Western Canada, it is known that climate during the last ice age has a considerable effect even on subsurface temperatures up to two kilometres depth. Therefore, we study the impact of paleoclimatic changes on the Molasse Basin during the last 130 ka including the Würm glaciation. We consider the hydraulic and thermal effects of periglacial conditions like permafrost formation and the impact of the numerous glacial advances onto the Molasse Basin. The major difference between the thermal-hydraulic regime in the western and eastern parts of the Southern German Molasse Basin are delineated by calculating two contrasting permeability scenarios of the heterogeneously karstified Malm carbonate aquifer. Thermal-hydraulic modelling reveals the effect of recurrent glacial periods on the geothermally drillable subsurface, which is minor compared to the effect of permeability-related, continuous gravity-driven groundwater flow as a major heat transport mechanism. Practically, the results might help to reduce the exploration risk for geothermal energy projects in the Molasse Basin. More importantly, this study serves as a reference for the comparison and understanding of the interplay of high permeability aquifers, gravity-driven groundwater flow and paleoclimate in other orogenic foreland basins worldwide.

Magnetostratigraphic study of a Late Cretaceous−Paleogene succession in the eastern Xining basin, NE Tibet: Constraint on the timing of major tectonic events in response to the India-Eurasia collision

Cretaceous-Cenozoic basins developed in the NE Tibetan Plateau contain key archives to unravel the growth history of the plateau in response to the India-Eurasia collision. Here we present magnetostratigraphic results of a Late Cretaceous to Paleogene succession of the Zhongba section outcropping at the southern margin of the eastern Xining basin. This succession consists of three lithological units punctuated by two stratigraphic unconformities, which best recorded the deformation history of this foreland basin. Detailed magnetostratigraphic investigation show that the lower terrestrial sedimentary rock unit, the Minhe Group, was deposited in latest Cretaceous in the time span of ca. 74.5−69.2 Ma; the middle unit was deposited in Paleogene in the time span of ca. 49.3−22 Ma; and the upper conglomeratic unit, not dated, possibly was deposited in early Miocene. Accordingly, the Cretaceous−Paleogene unconformity, widely observed in the foreland basins of NE Tibet, represents a sedimentary hiatus duration of ∼19.9 m.y., from ca. 69.2 Ma to ca. 49.3 Ma, which possibly recorded the far-field response to the tectonic transition from Neo-Tethys oceanic plate subduction to the India-Eurasia collision in southern Tibet. Changes in provenance, sedimentary accumulation rate, and mean susceptibility value at ca. 33−30 Ma, and the total prolate anisotropy of magnetic susceptibility (AMS) ellipsoids and provenance shifting since ca. 23−19 Ma, point to the pulsed growth of West Qinling, and rapid uplift of Laji Shan, respectively, indicating an enhanced effect of the India-Eurasia collision in Oligocene and early Miocene. AMS results show a clockwise rotation of the shortening direction from NEN-SWS in latest Cretaceous to NE-SW in Paleogene.

Burial-deformation History of Folded Rocks Unraveled by Fracture Analysis, Stylolite Paleopiezometry and Vein Cement Geochemistry: A Case Study in the Cingoli Anticline (Umbria-Marche, Northern Apennines)

Unravelling the burial-deformation history of sedimentary rocks is prerequisite information to understand the regional tectonic, sedimentary, thermal, and fluid-flow evolution of foreland basins. We use a combination of microstructural analysis, stylolites paleopiezometry, and paleofluid geochemistry to reconstruct the burial-deformation history of the Meso-Cenozoic carbonate sequence of the Cingoli Anticline (Northern Apennines, central Italy). Four major sets of mesostructures were linked to the regional deformation sequence: (i) pre-folding foreland flexure/forebulge; (ii) fold-scale layer-parallel shortening under a N045 σ1; (iii) syn-folding curvature of which the variable trend between the north and the south of the anticline is consistent with the arcuate shape of the anticline; (iv) the late stage of fold tightening. The maximum depth experienced by the strata prior to contraction, up to 1850 m, was quantified by sedimentary stylolite paleopiezometry and projected on the reconstructed burial curve to assess the timing of the contraction. As isotope geochemistry points towards fluid precipitation at thermal equilibrium, the carbonate clumped isotope thermometry (Δ47) considered for each fracture set yields the absolute timing of the development and exhumation of the Cingoli Anticline: layer-parallel shortening occurred from ~6.3 to 5.8 Ma, followed by fold growth that lasted from ~5.8 to 3.9 Ma.