ALBERT

Description: We investigate the feedbacks between surface processes and tectonics in an extensional setting by coupling a 2‐D geodynamical model with a landscape evolution law. Focusing on the evolution of a single normal fault, we show that surface processes significantly enhance the amount of horizontal extension a fault can accommodate before being abandoned in favor of a new fault. In simulations with very slow erosion rates, a 15 km thick brittle layer extends via a succession of crosscutting short‐lived faults (heave 〈 5 km). By contrast, when erosion rates are comparable to the regional extension velocity, deformation is accommodated on long‐lived faults (heave 〉10 km). Using simple scaling arguments, we quantify the effect of surface mass removal on the force balance acting on a growing normal fault. This leads us to propose that the major range‐bounding normal faults observed in many continental rifts owe their large offsets to erosional and depositional processes.

Description: Transient sediment storage and mixing of deposits of various ages during transport across alluvial piedmonts alter the clastic sedimentary record. We quantify buffering and mixing during cycles of aggradation–incision in the north piedmont of the Eastern Tian Shan. We complement existing chronologic data with 20 new luminescence ages and one cosmogenic radionuclide age of terrace abandonment and alluvial aggradation. Over the last 0.5 Myr, the piedmont deeply incised and aggraded many times per 100 kyr. Aggradation is driven by an increased flux of glacial sediment accumulated in the high range and flushed onto the piedmont by greater water discharge at stadial–interstadial transitions. After this sediment is evacuated from the high range, the reduced input sediment flux results in fluvial incision of the piedmont as fast as 9 cm year−1 and to depths up to 330 m. The timing of incision onset is different in each river and does not directly reflect climate forcing but the necessary time for the evacuation of glacial sediment from the high range. A significant fraction of sediments evacuated from the high range is temporarily stored on the piedmont before a later incision phase delivers it to the basin. Coarse sediments arrive in the basin with a lag of at least 7–14 kyrs between the first evacuation from the mountain and later basinward transport. The modern output flux of coarse sediments from the piedmont contains a significant amount of recycled material that was deposited on the piedmont as early as the Middle Pleistocene. Variations in temperature and moisture delivered by the Westerlies are the likely cause of repeated aggradation–incision cycles in the north piedmont instead of monsoonal precipitation. The arrival of the gravel front into the proximal basin is delayed relative to the fine‐grained load and both are separated by a hiatus. This work shows, based on field observations and data, how sedimentary systems respond to climatic perturbations, and how sediment recycling and mixing can ensue.

Description: Due to challenges involved in mapping the seafloor at high‐resolution (e.g., 〈 2 m), data are lacking to understand processes that control the evolution of submarine normal fault scarps, which cover large parts of the global seafloor. Here, we use data from autonomous deep‐sea vehicles to quantify local erosion and deposition associated with a pronounced tectonic surface scarp formed by slip on the submarine Roseau normal fault (Lesser Antilles). We use high‐resolution video imagery, photomosaics, and high‐resolution bathymetry data (0.1–10 m/pixel) to identify active erosional features on the scarp including channels, steep gullies, small scarps, and debris cones. We compare volumes of erosion and deposition and find that under certain depositional conditions, debris cones effectively record the erosion signal of mass wasting from the footwall with a ratio of hanging wall deposition to footwall erosion of 0.80. We use eroded volumes to estimate earthquake‐induced landslide erosion rates for the Roseau fault of 14–46 m Ma‐1. Assuming mass wasting of the Roseau fault scarp is mostly coseismic, the erosion rates for the Roseau fault imply that submarine earthquake induced mass‐wasting can occur at similar rates to various terrestrial lithological and tectonic settings. We present a process‐based model of submarine scarp degradation via retrogressive erosion in basement lithology where scarps have a gravitational stability threshold height of 20–40 m and a long‐term average slope of 30–40°. More generally, the results presented here may be applicable to develop models of submarine landscape evolution based on degradation of normal fault scarps on the seafloor.

Description: Erosion and sedimentation constantly rework topography created by tectonics but also modulate stresses in the underlying crust by redistributing surficial loads. Decades of numerical modeling further suggest that surface processes help focus deformation onto fewer, longer-lived faults at tectonic plate boundaries. However, because the surface evolution parameters used in these models are not quantitatively calibrated against real landscapes and because the history of fault activity can be difficult to infer from the geological record, the sensitivity of tectonic deformation to a realistic range of erosional efficiency remains unknown. Here, we model the growth of half-grabens, where slip on a master normal fault shapes an adjacent mountain range as it accommodates crustal stretching. We subject our simulations to fluvial incision acting at rates assessed by morphometric analysis of rivers draining natural rift systems. Increasing erosional efficiency within the geologically documented range alleviates the energy cost of topographic growth and increases the total extension that can be accommodated by half-graben master faults by as much as ∼50%. Efficient erosion favors an eventual basin-ward relocalization of strain, preventing the development of horst structures. This behavior is consistent with structural and morphometric observations across 12 normal fault-bounded ranges, suggesting that surface erodibility and climatic conditions have a measurable impact on the tectonic makeup of Earth’s plate boundaries.

Description: Compilation of data used to support the article: "Co-location of the downdip end of seismic coupling and the continental shelf break" (Malatesta et al., 2020). Along subduction margins, the morphology of the near shore domain records the combined action of erosion from ocean waves and permanent tectonic deformation from the convergence of plates. We observe that at subduction margins around the globe, the edge of continental shelves tends to be located above the downdip end of seismic coupling on the megathrust. Coastlines lie farther landward at variable distances. This observation stems from a compilation of well-resolved coseismic and interseismic coupling datasets. The permanent interseismic uplift component of the total tectonic deformation can explain the localization of the shelf break. It contributes a short wave-length gradient in vertical deformation on top of the structural and isostatic deformation of the margin. This places a hinge line between seaward subsidence and landward uplift above the downdip end of high coupling. Landward of the hinge line, rocks are uplifted in the domain of wave-base erosion and a shelf is maintained by the competition of rock uplift and wave erosion. wave erosion then sets the coastline back from the tectonically meaningful shelf break. We combine a wave erosion model with an elastic deformation model to illustrate how the downdip end of high coupling pins the location of the shelf break. In areas where the shelf is wide, onshore geodetic constraints on seismic coupling is limited and could be advantageously complemented by considering the location of the shelf break. Subduction margin morphology integrates hundreds of seismic cycles and could inform seismic coupling stability through time.

Description: The abandonment of terraces in incising alluvial rivers can be used to infer tectonic and climatic histories. A river incising into alluvium erodes both vertically and laterally as it abandons fill‐cut terraces. We argue that the input of sediment from the valley walls during entrenchment can alter the incision dynamics of a stream by promoting vertical incision over lateral erosion. Using a numerical model, we investigate how valley wall feedbacks may affect incision rates and terrace abandonment as the channel becomes progressively more entrenched in its valley. We postulate that erosion of taller valley walls delivers large pulses of sediment to the incising channel, potentially overwhelming the local sediment transport capacity. Based on field observations, we propose that these pulses of sediment can form talus piles that shield the valley wall from subsequent erosion and potentially force progressive channel narrowing. Our model shows that this positive feedback mechanism can enhance vertical incision relative to 1‐D predictions that ignore lateral erosion. We find that incision is most significantly enhanced when sediment transport rates are low relative to the typical volume of material collapsed from the valley walls. The model also shows a systematic erosion of the youngest terraces when river incision slows down. The autogenic entrenchment due to lateral feedbacks with valley walls should be taken into account in the interpretation of complex‐response terraces.

Description: Fluvial terraces carved by incising rivers are widely used to investigate external forcing by climate and tectonics. In the Eastern Tian Shan (Central Asia), the north piedmont rivers are much more deeply incised (mean: 124 m) compared to the south piedmont (mean: 17 m) despite very similar tectonic and climatic settings. We attribute the incision contrast to a difference in glacial imprint between southern and northern catchments. Whereas the upper halves of the valleys in the northern higher subrange are formerly glaciated, wide and gently sloping, U-shaped valleys flowing into V-shaped valleys, the valleys of the lower southern subrange are entirely V-shaped. The glacially widened valleys act as capacitors that accumulate and release glacial and periglacial sediment onto the piedmont. The resulting discrete pulse in sediment flux, Qs, forces aggradation and steepening, followed by incision and gentler slopes of the piedmont rivers. The fluvial valleys do not accumulate sediment, and changes in water discharge primarily control the slope of the piedmont rivers. Today, incision in the north is associated with the drop in Qs that occurred after depletion of the upstream reservoir, while aggradation in the south is due to Central Asian aridity. The same climatic forcing can have strikingly distinct morphological expressions downstream of catchments with different glacial imprints.

Description: Marine terraces are a cornerstone for the study of paleo sea level and crustal deformation. Commonly, individual erosive marine terraces are attributed to unique sea-level high stands based on the reasoning that marine platforms could only be significantly widened at the beginning of an interglacial. However, this logic implies that wave erosion is insignificant at other times. We postulate that the erosion potential at a given bedrock elevation datum is proportional to the total duration of sea-level occupation at that datum. The total duration of sea-level occupation depends strongly on rock uplift rate. Certain rock uplift rates may promote the generation and preservation of particular terraces while others prevent them. For example, at rock uplift of ~1.2 mm/yr, the Marine Isotope Stage (MIS) 5e (ca. 120 ka) high stand reoccupies the elevation of the MIS 6d–e mid-stand, favoring creation of a wider terrace than at higher or lower rock uplift rates. Thus, misidentification of terraces can occur if each terrace in a sequence is assumed to form uniquely at successive interglacial high stands and to reflect their relative elevations. Developing a graphical proxy for the entire erosion potential of sea-level history allows us to address creation and preservation biases at different rock uplift rates.