Coordination between deformation, precipitation, and erosion during orogenic growth

Crustal thickening associated with orogenic growth elevates topography, causing orographic enhancement of precipitation, which in turn facilitates local erosion and possibly intensifies localization of deformation. How these three processes-deformation, precipitation, and erosion-coordinate during orogenic growth remains unknown. Here, we present a numerical model where tectonics, surface processes, and orographic precipitation are tightly coupled, and explore the impact on low, intermediate, and high erodibility orogens. We show that, for intermediate erodibility models, rock uplift rates and precipitation rates correlate well with erosion rates during the formation of orogenic plateaus with high correlation coefficients of ~0.9 between rock uplift and erosion rates, and ~0.8 between precipitation and erosion rates. We demonstrate a cyclicity of correlation evolution among uplift, precipitation, and erosion rates through the development of new faults propagating outward. These results shed insights into the relative tectonic or climatic control on erosion in active orogens (e.g., Himalayas, Central Andes, and Southern Alps of New Zealand), and provide a plausible explanation for several conflicting data and interpretations in the Himalayas, which depend on the stage of maturity of the newest fault and the relative locations to old faults.

Mountain building process of the Taiwan orogeny

The Taiwan orogeny, an example of arc-continental collision, exhibits complex geological structures and rapid exhumation. Many models have tried and failed to fully capture the dynamics of these processes. We developed a comprehensive thermomechanical model that considers the transition from brittle to ductile behavior with depth, lithology-dependent erosion and observed decollement and backstop geometries. This model successfully reproduces the intricate structures observed within the Taiwan orogeny, aligns with structural complexities, metamorphic temperature profiles, thermochronological records, strain distributions, and the rates of exhumation and cooling and elucidates the roles of ductile deformation and ramp structures in forming the Hsuehshan Range and the Western fold and thrust belt. The insights from this model offer potential applicability to other orogenic wedges worldwide.

Tectonic extension and paleoelevation influence mammalian diversity dynamics in the Basin and Range Province of western North America

Landscape properties have a profound influence on the diversity and distribution of biota, with present-day biodiversity hot spots occurring in topographically complex regions globally. Complex topography is created by tectonic processes and further shaped by interactions between climate and land-surface processes. These processes enrich diversity at the regional scale by promoting speciation and accommodating increased species richness along strong environmental gradients. Synthesis of the mammalian fossil record and a geophysical model of topographic evolution of the Basin and Range Province in western North America enable us to directly quantify relationships between mammal diversity and landscape dynamics over the past 30 million years. We analyze the covariation between tectonic history (extensional strain rates, paleotopography, and ruggedness), global temperature, and diversity dynamics. Mammal species richness and turnover exhibit stronger responses to rates of change in landscape properties than to the specific properties themselves, with peaks in diversity coinciding with high tectonic strain rates and large changes in elevation across spatial scales.

Self-similarity and vanishing diffusion in fluvial landscapes

Complex topographies exhibit universal properties when fluvial erosion dominates landscape evolution over other geomorphological processes. Similarly, we show that the solutions of a minimalist landscape evolution model display invariant behavior as the impact of soil diffusion diminishes compared to fluvial erosion at the landscape scale, yielding complete self-similarity with respect to a dimensionless channelization index. Approaching its zero limit, soil diffusion becomes confined to a region of vanishing area and large concavity or convexity, corresponding to the locus of the ridge and valley network. We demonstrate these results using one dimensional analytical solutions and two dimensional numerical simulations, supported by real-world topographic observations. Our findings on the landscape self-similarity and the localized diffusion resemble the self-similarity of turbulent flows and the role of viscous dissipation. Topographic singularities in the vanishing diffusion limit are suggestive of shock waves and singularities observed in nonlinear complex systems.

Milankovitch-paced erosion in the southern Central Andes

It has long been hypothesized that climate can modify both the pattern and magnitude of erosion in mountainous landscapes, thereby controlling morphology, rates of deformation, and potentially modulating global carbon and nutrient cycles through weathering feedbacks. Although conceptually appealing, geologic evidence for a direct climatic control on erosion has remained ambiguous owing to a lack of high-resolution, long-term terrestrial records and suitable field sites. Here we provide direct terrestrial field evidence for long-term synchrony between erosion rates and Milankovitch-driven, 400-kyr eccentricity cycles using a Plio-Pleistocene cosmogenic radionuclide paleo-erosion rate record from the southern Central Andes. The observed climate-erosion coupling across multiple orbital cycles, when combined with results from the intermediate complexity climate model CLIMBER-2, are consistent with the hypothesis that relatively modest fluctuations in precipitation can cause synchronous and nonlinear responses in erosion rates as landscapes adjust to ever-evolving hydrologic boundary conditions imposed by oscillating climate regimes.

Climatic control on the location of continental volcanic arcs

Orogens and volcanic arcs at continental plate margins are primary surface expressions of convergent plate tectonics. Although it is established that climate affects the shape, size, and architecture of orogens via orographic erosion gradients, the ascent of magma through the crust and location of volcanoes along magmatic arcs have been considered insensitive to erosion. However, available data reveal westward migration of late-Cenozoic volcanic activity in the Southern Andes and Cascade Range where orography drives an eastward migration of the topographic water divide by increased precipitation and erosion along west-facing slopes. Thermomechanical numerical modeling shows that orographic erosion and the associated leeward topographic migration may entail asymmetric crustal structures that drive the magma ascent toward the region of enhanced erosion. Despite the different tectonic histories of the Southern Andes and the Cascade Range, orographic erosion is a shared causal mechanism that can explain the late-Cenozoic westward migration of the volcanic front along both magmatic arcs.

Late Cretaceous through Cenozoic Paleoenvironmental History of the Bagua Basin, Peru: Paleoelevation Comparisons with the Central Andean Plateau

Located in northern Peru, at the lowest segment of the Central Andes, the Bagua Basin contains a Campanian to Pleistocene sedimentary record that archives the local paleoenvironmental and tectonic history. We present new δ18O and δ13C signatures of pedogenic carbonate nodules from paleosols in the Campanian–Maastrichtian Fundo El Triunfo Formation and in the upper Eocene–middle Miocene Sambimera Formation to reconstruct the isotopic composition of paleo-meteoric water and the floristic biome. We compare these results to modern isotopic values from a newly obtained modern water transect to interpret the environmental evolution of this area and its relationship with the neighboring Eastern Cordillera. A ~2‰ δ18O depletion between the latest Cretaceous and the latest Eocene reflects a shift from a coastal to inland environment. A negative δ18O shift of ~3‰ from the middle Miocene to the present day reveals the establishment of the Eastern Cordillera as an orographic barrier for the moisture traveling westward, sometime after deposition of the top of the Sambimera Formation at ~13 Ma. A shift in the δ13C signature from ~−25‰ in the Campanian–Miocene deposits to ~−23‰ in modern–Holocene times suggests a change in biome from dominant C3 plants to a mixture of C3 and C4 plants. This environmental shift reflects both the late Miocene global C4 expansion and the transition to more arid conditions in the basin. The Campanian–middle Miocene environmental reconstruction of the Bagua Basin indicates a steady paleoelevation setting in the northernmost Central Andes during most of the Cenozoic and constrains the uplift of the Eastern Cordillera to the late Miocene–Pleistocene. This paleoelevation history contrasts with that of the Central Andean Plateau, which is characterized by two major episodes of surface uplift: early–middle Miocene and late Miocene–Pliocene. The contrasting modern topographic configuration of the Central Andean Plateau and the northernmost Central Andes gives rise to the question of what factors created such a dramatic difference in topographic evolution of the two regions that shared an overall common tectonic history. We discuss the possible factors responsible for this contrasting topographic configuration and suggest that the diachronous flat slab episodes are likely a major factor, resulting in greater shortening and crustal thickness and, ultimately, in earlier surface uplift episodes occurring in the Central Andean Plateau.

Topography of mountain belts controlled by rheology and surface processes

It is widely recognized that collisional mountain belt topography is generated by crustal thickening and lowered by river bedrock erosion, linking climate and tectonics^1-4. However, whether surface processes or lithospheric strength control mountain belt height, shape and longevity remains uncertain. Additionally, how to reconcile high erosion rates in some active orogens with long-term survival of mountain belts for hundreds of millions of years remains enigmatic. Here we investigate mountain belt growth and decay using a new coupled surface process^5,6 and mantle-scale tectonic model⁷. End-member models and the new non-dimensional Beaumont number, Bm, quantify how surface processes and tectonics control the topographic evolution of mountain belts, and enable the definition of three end-member types of growing orogens: type 1, non-steady state, strength controlled (Bm > 0.5); type 2, flux steady state⁸, strength controlled (Bm ≈ 0.4-0.5); and type 3, flux steady state, erosion controlled (Bm < 0.4). Our results indicate that tectonics dominate in Himalaya-Tibet and the Central Andes (both type 1), efficient surface processes balance high convergence rates in Taiwan (probably type 2) and surface processes dominate in the Southern Alps of New Zealand (type 3). Orogenic decay is determined by erosional efficiency and can be subdivided into two phases with variable isostatic rebound characteristics and associated timescales. The results presented here provide a unified framework explaining how surface processes and lithospheric strength control the height, shape, and longevity of mountain belts.

Sensitivity of rift tectonics to global variability in the efficiency of river erosion

The efficiency of erosion in leveling relief mainly depends on climate and strength of exposed rocks. However, whether erosion is sufficiently efficient to influence the architecture of a tectonic plate boundary remains a topic of debate. Here, we analyze continental rift landscapes reworked by river incision to assess a globally representative range of fluvial erosion efficiency. We then simulate crustal extension exposed to surface processes acting within this documented range. We find that more efficient erosion favors the growth of half-grabens over horsts, which can explain contrasting tectonic styles across the Basin and Range province and the East African Rift. This suggests that variability in Earth’s geological structures partly reflects variability in hydrological conditions and associated surface processes.

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.

Badland landscape response to individual geomorphic events

Landscapes form by the erosion and deposition of sediment, driven by tectonic and climatic forcing. The principal geomorphic processes of badland – landsliding, debris flow and runoff erosion – are similar to those in full scale mountain topography, but operate faster. Here, we show that in the badlands of SW Taiwan, individual rainfall events cause quantifiable landscape change, distinct for the type of rainfall. Typhoon rain reduced hillslope gradients, while lower-intensity precipitation either steepened or flattened the landscape, depending on its initial topography. The steep topography observed in our first survey is inconsistent with the effects of any of the rainfall events. We suggest that it is due to the 2016 Mw 6.4 Meinong earthquake. The observed pattern in the badlands was mirrored in the response of the Taiwan mountain topography to typhoon Morakot in 2009, confirming that badlands offer special opportunities to quantify natural landscape dynamics on observational time scales.

The relative role of individual forcing events in long-term landscape evolution is challenging to measure in the field. Badlands offer special opportunities to quantify common, natural landscape dynamics on observational time scales.

Constraining tectonic uplift and advection from the main drainage divide of a mountain belt

One of the most conspicuous features of a mountain belt is the main drainage divide. Divide location is influenced by a number of parameters, including tectonic uplift and horizontal advection. Thus, the topography of mountain belts can be used as an archive to extract tectonic information. Here we combine numerical landscape evolution modelling and analytical solutions to demonstrate that mountain asymmetry, determined by the location of the main drainage divide, increases with increasing uplift gradient and advection velocity. Then, we provide a conceptual framework to constrain the present or previous tectonic uplift and advection of a mountain belt from the location and migration direction of its main drainage divide. Furthermore, we apply our model to Wula Shan horst, Northeastern Sicily, and Southern Taiwan.

Earthquake statistics changed by typhoon-driven erosion

Tectonics and climate-driven surface processes govern the evolution of Earth’s surface topography. Topographic change in turn influences lithospheric deformation, but the elementary scale at which this feedback can be effective is unclear. Here we show that it operates in a single weather-driven erosion event. In 2009, typhoon Morakot delivered ~ 3 m of precipitation in southern Taiwan, causing exceptional landsliding and erosion. This event was followed by a step increase in the shallow (< 15 km depth) earthquake frequency lasting at least 2.5 years. Also, the scaling of earthquake magnitude and frequency underwent a sudden increase in the area where mass wasting was most intense. These observations suggest that the progressive removal of landslide debris by rivers from southern Taiwan has acted to increase the crustal stress rate to the extent that earthquake activity was demonstrably affected. Our study offers the first evidence of the impact of a single weather-driven erosion event on tectonics.

Megathrust shear force controls mountain height at convergent plate margins

The shear force along convergent plate boundary faults (megathrusts) determines the height of mountain ranges that can be mechanically sustained^1-4. However, whether the true height of mountain ranges corresponds to this tectonically supported elevation is debated^4-7. In particular, climate-dependent erosional processes are often assumed to exert a first-order control on mountain height^5-12, although this assumption has remained difficult to validate¹². Here we constrain the shear force along active megathrusts using their rheological properties and then determine the tectonically supported elevation using a force balance model. We show that the height of mountain ranges around the globe matches this elevation, irrespective of climatic conditions and the rate of erosion. This finding indicates that mountain ranges are close to force equilibrium and that their height is primarily controlled by the megathrust shear force. We conclude that temporal variations in mountain height reflect long-term changes in the force balance but are not indicative of a direct climate control on mountain elevation.

Fast long-term denudation rate of steep alpine headwalls inferred from cosmogenic 36Cl depth profiles

Quantifications of in-situ denudation rates on vertical headwalls, averaged over millennia, have been thwarted because of inaccessibility. Here, we benefit from a tunnel crossing a large and vertical headwall in the European Alps (Eiger), where we measured concentrations of in-situ cosmogenic ³⁶Cl along five depth profiles linking the tunnel with the headwall surface. Isotopic concentrations of ³⁶Cl are low in surface samples, but high at depth relative to expectance for their position. The results of Monte-Carlo modelling attribute this pattern to inherited nuclides, young minimum exposure ages and to fast average denudation rates during the last exposure. These rates are consistently high across the Eiger and range from 45 ± 9 cm kyr⁻¹ to 356 ± 137 cm kyr⁻¹ (1σ) for the last centuries to millennia. These high rates together with the large inheritance point to a mechanism where denudation has been accomplished by frequent, cm-scale rock fall paired with chemical dissolution of limestone.

Slow Long-Term Exhumation of the West Central Andean Plate Boundary, Chile

We present a regional analysis of new low-temperature thermochronometer ages from the Central Andean fore arc to provide insights into the exhumation history of the western Andean margin. To derive exhumation rates over 10 million-year timescales, 38 new apatite and zircon (U-Th)/He ages were analyzed along six ~500-km long near-equal-elevation, coast parallel, transects in the Coastal Cordillera (CC) and higher-elevation Precordillera (PC) of the northern Chilean Andes between latitudes 18.5°S and 22.5°S. These transects were augmented with age-elevation profiles where possible. Results are synthesized with previously published thermochronometric data, corroborating a previously observed trenchward increase in cooling ages in Peru and northern Chile. One-dimensional thermal-kinematic modeling of all available multichronometer equal-elevation samples reveals mean exhumation rates of <0.2 km/Myr since ~50 Ma in the PC and ~100 Ma in the CC. Regression of pseudovertical age-elevation transects in the CC yields comparable rates of ~0.05 to ~0.12 km/Myr between ~40 and 80 Ma. Differences between the long-term mean 1-D rates and shorter-term age-elevation-derived rates indicate low variability in the exhumation history. Modeling results suggest similar background exhumation rates in the CC and PC; younger ages in the PC are largely a function of increased heat flow and consequently an elevated geothermal gradient near the arc. Slow exhumation rates are suggestive of semiarid conditions across the region since at least the Eocene and deformation and development of the Andean fore arc around this time.

Scale-dependent erosional patterns in steady-state and transient-state landscapes

Landscape topography is the expression of the dynamic equilibrium between external forcings (for example, climate and tectonics) and the underlying lithology. The magnitude and spatial arrangement of erosional and depositional fluxes dictate the evolution of landforms during both statistical steady state (SS) and transient state (TS) of major landscape reorganization. For SS landscapes, the common expectation is that any point of the landscape has an equal chance to erode below or above the landscape median erosion rate. We show that this is not the case. Afforded by a unique experimental landscape that provided a detailed space-time recording of erosional fluxes and by defining the so-called E50-area curve, we reveal for the first time that there exists a hierarchical pattern of erosion. Specifically, hillslopes and fluvial channels erode more rapidly than the landscape median erosion rate, whereas intervening parts of the landscape in terms of upstream contributing areas (colluvial regime) erode more slowly. We explain this apparent paradox by documenting the dynamic nature of SS landscapes-landscape locations may transition from being a hillslope to being a valley and then to being a fluvial channel due to ridge migration, channel piracy, and small-scale landscape dynamics through time. Under TS conditions caused by increased precipitation, we show that the E50-area curve drastically changes shape during landscape reorganization. Scale-dependent erosional patterns, as observed in this study, suggest benchmarks in evaluating numerical models and interpreting the variability of sampled erosional rates in field landscapes.

Global drainage patterns and the origins of topographic relief on Earth, Mars, and Titan

Rivers have eroded the topography of Mars, Titan, and Earth, creating diverse landscapes. However, the dominant processes that generated topography on Titan (and to some extent on early Mars) are not well known. We analyzed drainage patterns on all three bodies and found that large drainages, which record interactions between deformation and erosional modification, conform much better to long-wavelength topography on Titan and Mars than on Earth. We use a numerical landscape evolution model to demonstrate that short-wavelength deformation causes drainage directions to diverge from long-wavelength topography, as observed on Earth. We attribute the observed differences to ancient long-wavelength topography on Mars, recent or ongoing generation of long-wavelength relief on Titan, and the creation of short-wavelength relief by plate tectonics on Earth.

Biodiversity and Topographic Complexity: Modern and Geohistorical Perspectives

Topographically complex regions on land and in the oceans feature hotspots of biodiversity that reflect geological influences on ecological and evolutionary processes. Over geologic time, topographic diversity gradients wax and wane over millions of years, tracking tectonic or climatic history. Topographic diversity gradients from the present day and the past can result from the generation of species by vicariance or from the accumulation of species from dispersal into a region with strong environmental gradients. Biological and geological approaches must be integrated to test alternative models of diversification along topographic gradients. Reciprocal illumination among phylogenetic, phylogeographic, ecological, paleontological, tectonic, and climatic perspectives is an emerging frontier of biogeographic research.

Chemical weathering as a mechanism for the climatic control of bedrock river incision

Feedbacks between climate, erosion and tectonics influence the rates of chemical weathering reactions, which can consume atmospheric CO2 and modulate global climate. However, quantitative predictions for the coupling of these feedbacks are limited because the specific mechanisms by which climate controls erosion are poorly understood. Here we show that climate-dependent chemical weathering controls the erodibility of bedrock-floored rivers across a rainfall gradient on the Big Island of Hawai'i. Field data demonstrate that the physical strength of bedrock in streambeds varies with the degree of chemical weathering, which increases systematically with local rainfall rate. We find that incorporating the quantified relationships between local rainfall and erodibility into a commonly used river incision model is necessary to predict the rates and patterns of downcutting of these rivers. In contrast to using only precipitation-dependent river discharge to explain the climatic control of bedrock river incision, the mechanism of chemical weathering can explain strong coupling between local climate and river incision.

Mid-Pleistocene climate transition drives net mass loss from rapidly uplifting St. Elias Mountains, Alaska

Erosion, sediment production, and routing on a tectonically active continental margin reflect both tectonic and climatic processes; partitioning the relative importance of these processes remains controversial. Gulf of Alaska contains a preserved sedimentary record of the Yakutat Terrane collision with North America. Because tectonic convergence in the coastal St. Elias orogen has been roughly constant for 6 My, variations in its eroded sediments preserved in the offshore Surveyor Fan constrain a budget of tectonic material influx, erosion, and sediment output. Seismically imaged sediment volumes calibrated with chronologies derived from Integrated Ocean Drilling Program boreholes show that erosion accelerated in response to Northern Hemisphere glacial intensification (∼ 2.7 Ma) and that the 900-km-long Surveyor Channel inception appears to correlate with this event. However, tectonic influx exceeded integrated sediment efflux over the interval 2.8-1.2 Ma. Volumetric erosion accelerated following the onset of quasi-periodic (∼ 100-ky) glacial cycles in the mid-Pleistocene climate transition (1.2-0.7 Ma). Since then, erosion and transport of material out of the orogen has outpaced tectonic influx by 50-80%. Such a rapid net mass loss explains apparent increases in exhumation rates inferred onshore from exposure dates and mapped out-of-sequence fault patterns. The 1.2-My mass budget imbalance must relax back toward equilibrium in balance with tectonic influx over the timescale of orogenic wedge response (millions of years). The St. Elias Range provides a key example of how active orogenic systems respond to transient mass fluxes, and of the possible influence of climate-driven erosive processes that diverge from equilibrium on the million-year scale.

Climate and topography control the size and flux of sediment produced on steep mountain slopes

Weathering on mountain slopes converts rock to sediment that erodes into channels and thus provides streams with tools for incision into bedrock. Both the size and flux of sediment from slopes can influence channel incision, making sediment production and erosion central to the interplay of climate and tectonics in landscape evolution. Although erosion rates are commonly measured using cosmogenic nuclides, there has been no complementary way to quantify how sediment size varies across slopes where the sediment is produced. Here we show how this limitation can be overcome using a combination of apatite helium ages and cosmogenic nuclides measured in multiple sizes of stream sediment. We applied the approach to a catchment underlain by granodiorite bedrock on the eastern flanks of the High Sierra, in California. Our results show that higher-elevation slopes, which are steeper, colder, and less vegetated, are producing coarser sediment that erodes faster into the channel network. This suggests that both the size and flux of sediment from slopes to channels are governed by altitudinal variations in climate, vegetation, and topography across the catchment. By quantifying spatial variations in the sizes of sediment produced by weathering, this analysis enables new understanding of sediment supply in feedbacks between climate, tectonics, and mountain landscape evolution.

Erosion in southern Tibet shut down at ∼10 Ma due to enhanced rock uplift within the Himalaya

Exhumation of the southern Tibetan plateau margin reflects interplay between surface and lithospheric dynamics within the Himalaya-Tibet orogen. We report thermochronometric data from a 1.2-km elevation transect within granitoids of the eastern Lhasa terrane, southern Tibet, which indicate rapid exhumation exceeding 1 km/Ma from 17-16 to 12-11 Ma followed by very slow exhumation to the present. We hypothesize that these changes in exhumation occurred in response to changes in the loci and rate of rock uplift and the resulting southward shift of the main topographic and drainage divides from within the Lhasa terrane to their current positions within the Himalaya. At ∼17 Ma, steep erosive drainage networks would have flowed across the Himalaya and greater amounts of moisture would have advected into the Lhasa terrane to drive large-scale erosional exhumation. As convergence thickened and widened the Himalaya, the orographic barrier to precipitation in southern Tibet terrane would have strengthened. Previously documented midcrustal duplexing around 10 Ma generated a zone of high rock uplift within the Himalaya. We use numerical simulations as a conceptual tool to highlight how a zone of high rock uplift could have defeated transverse drainage networks, resulting in substantial drainage reorganization. When combined with a strengthening orographic barrier to precipitation, this drainage reorganization would have driven the sharp reduction in exhumation rate we observe in southern Tibet.

Topographic Evolution and Climate Aridification during Continental Collision: Insights from Computer Simulations

How do the feedbacks between tectonics, sediment transport and climate work to shape the topographic evolution of the Earth? This question has been widely addressed via numerical models constrained with thermochronological and geomorphological data at scales ranging from local to orogenic. Here we present a novel numerical model that aims at reproducing the interaction between these processes at the continental scale. For this purpose, we combine in a single computer program: 1) a thin-sheet viscous model of continental deformation; 2) a stream-power surface-transport approach; 3) flexural isostasy allowing for the formation of large sedimentary foreland basins; and 4) an orographic precipitation model that reproduces basic climatic effects such as continentality and rain shadow. We quantify the feedbacks between these processes in a synthetic scenario inspired by the India-Asia collision and the growth of the Tibetan Plateau. We identify a feedback between erosion and crustal thickening leading locally to a <50% increase in deformation rates in places where orographic precipitation is concentrated. This climatically-enhanced deformation takes place preferentially at the upwind flank of the growing plateau, specially at the corners of the indenter (syntaxes). We hypothesize that this may provide clues for better understanding the mechanisms underlying the intriguing tectonic aneurisms documented in the Himalayas. At the continental scale, however, the overall distribution of topographic basins and ranges seems insensitive to climatic factors, despite these do have important, sometimes counterintuitive effects on the amount of sediments trapped within the continent. The dry climatic conditions that naturally develop in the interior of the continent, for example, trigger large intra-continental sediment trapping at basins similar to the Tarim Basin because they determine its endorheic/exorheic drainage. These complex climatic-drainage-tectonic interactions make the development of steady-state topography at the continental scale unlikely.

Erosion influences the seismicity of active thrust faults

Assessing seismic hazards remains one of the most challenging scientific issues in Earth sciences. Deep tectonic processes are classically considered as the only persistent mechanism driving the stress loading of active faults over a seismic cycle. Here we show via a mechanical model that erosion also significantly influences the stress loading of thrust faults at the timescale of a seismic cycle. Indeed, erosion rates of about ~0.1-20 mm yr(-1), as documented in Taiwan and in other active compressional orogens, can raise the Coulomb stress by ~0.1-10 bar on the nearby thrust faults over the inter-seismic phase. Mass transfers induced by surface processes in general, during continuous or short-lived and intense events, represent a prominent mechanism for inter-seismic stress loading of faults near the surface. Such stresses are probably sufficient to trigger shallow seismicity or promote the rupture of deep continental earthquakes up to the surface.

Feedbacks in human-landscape systems

This article identifies key questions and challenges for geomorphologists in investigating coupled feedbacks in human-landscape systems. While feedbacks occur in the absence of human influences, they are also altered by human activity. Feedbacks are a key element to understanding human-influenced geomorphic systems in ways that extend our traditional approach of considering humans as unidirectional drivers of change. Feedbacks have been increasingly identified in Earth-environmental systems, with studies of coupled human-natural systems emphasizing ecological phenomena in producing emerging concepts for social-ecological systems. Enormous gaps or uncertainties in knowledge remain with respect to understanding impact-feedback loops within geomorphic systems with significant human alterations, where the impacted geomorphic systems in turn affect humans. Geomorphology should play an important role in public policy by identifying the many diffuse and subtle feedbacks of both local- and global-scale processes. This role is urgent, while time may still be available to mitigate the impacts that limit the sustainability of human societies. Challenges for geomorphology include identification of the often weak feedbacks that occur over varied time and space scales ranging from geologic time to single isolated events and very short time periods, the lack of available data linking impact with response, the identification of multiple thresholds that trigger feedback mechanisms, the varied tools and metrics needed to represent both physical and human processes, and the need to collaborate with social scientists with expertise in the human causes of geomorphic change, as well as the human responses to such change.

Climatic control of bedrock river incision

Bedrock river incision drives the development of much of Earth's surface topography, and thereby shapes the structure of mountain belts and modulates Earth's habitability through its effects on soil erosion, nutrient fluxes and global climate. Although it has long been expected that river incision rates should depend strongly on precipitation rates, quantifying the effects of precipitation rates on bedrock river incision rates has proved difficult, partly because river incision rates are difficult to measure and partly because non-climatic factors can obscure climatic effects at sites where river incision rates have been measured. Here we present measurements of river incision rates across one of Earth's steepest rainfall gradients, which show that precipitation rates do indeed influence long-term bedrock river incision rates. We apply a widely used empirical law for bedrock river incision to a series of rivers on the Hawaiian island of Kaua'i, where mean annual precipitation ranges from 0.5 metres to 9.5 metres (ref. 12)-over 70 per cent of the global range-and river incision rates averaged over millions of years can be inferred from the depth of river canyons and the age of the volcanic bedrock. Both a time-averaged analysis and numerical modelling of transient river incision reveal that the long-term efficiency of bedrock river incision across Kaua'i is positively correlated with upstream-averaged mean annual precipitation rates. We provide theoretical context for this result by demonstrating that our measurements are consistent with a linear dependence of river incision rates on stream power, the rate of energy expenditure by the flow on the riverbed. These observations provide rare empirical evidence for the long-proposed coupling between climate and river incision, suggesting that previously proposed feedbacks among topography, climate and tectonics may occur.

Glaciation as a destructive and constructive control on mountain building

Theoretical analysis predicts that enhanced erosion related to late Cenozoic global cooling can act as a first-order influence on the internal dynamics of mountain building, leading to a reduction in orogen width and height. The strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion, producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude, where long-term glacier mass gain equals mass loss. However, acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive. Here we present an extensive new low-temperature thermochronologic data set from the Patagonian Andes, a high-latitude active orogen with a well-documented late Cenozoic tectonic, climatic and glacial history. Data from 38° S to 49° S record a marked acceleration in erosion 7 to 5 Myr ago coeval with the onset of major Patagonian glaciation and retreat of deformation from the easternmost thrust front. The highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen, as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity. In contrast, towards higher latitudes (49° S to 56° S) a transition to older bedrock cooling ages signifies much reduced late Cenozoic erosion despite dominantly glacial conditions here since the latest Miocene. The increased height of the orogenic divide at these latitudes (well above the equilibrium line altitude) leads us to conclude that the southernmost Patagonian Andes represent the first recognized example of regional glacial protection of an active orogen from erosion, leading to constructive growth in orogen height and width.

Apatite thermochronology in modern geology

Fission-track and (U–Th–Sm)/He thermochronology on apatites are radiometric dating methods that refer to thermal histories of rocks within the temperature range of 40°–125 °C. Their introduction into geological research contributed to the development of new concepts to interpreting time-temperature constraints and substantially improved the understanding of cooling processes within the uppermost crust. Present geological applications of apatite thermochronological methods include absolute dating of rocks and tectonic processes, investigation of denudation histories and long-term landscape evolution of various geological settings, and basin analysis.

Time scales of tectonic landscapes and their sediment routing systems

In regions undergoing active tectonics, the coupling between the tectonic displacement field, the overlying landscape and the redistribution of mass at the Earth's surface in the form of sediment routing systems, is particularly marked and variable. Coupling between deformation and surface processes takes place at a range of scales, from the whole orogen to individual extensional fault blocks or contractional anticlines. At the large scale, the attainment of a steady-state between the overlying topography and the prevailing tectonic conditions in active contractional orogens requires an efficient erosional system, with a time scale dependent on the vigour of the erosional system, generally in the range 106–107 years. The catchment–fan systems associated with extensional fault blocks and basins of the western USA are valuable natural examples to study the coupling between tectonic deformation, landscape and sediment routing systems. Even relatively simple coupled systems such as an extensional fault block and its associated basin margin fans have a range of time scales in response to a tectonic perturbation. These response times originate from the development of uniform (steady-state) relief during the accumulation of displacement on a normal fault (c. 106 years), the upstream propagation of a bedrock knickpoint in transverse catchments following a change in tectonic uplift rate (c. 106 years), or the relaxation times of the integrated catchment–fan system in response to changes in climatic and tectonic boundary conditions (105–106 years). The presence of extensive bedrock or alluvial piedmonts increases response times significantly. The sediment efflux of a mountain catchment is a boundary condition for far-field fluvial transport, but the fluvial system is much more than a simple transmitter of the sediment supply signal to a neighbouring depocentre. Fluvial systems appear to act as buffers to incoming sediment supply signals, with a diffusive time scale (c. 105–106 years) dependent on the length of the system and the extent of its floodplains, stream channels and proximal gravel fans. The vocabulary for explaining landscapes would benefit from a greater recognition of the importance of the repeat time and magnitude of perturbations in relation to the response and relaxation times of the landscape and its sediment routing systems. Landscapes are best differentiated as ‘buffered’ or ‘reactive’ depending on the ratio of the response time to the repeat time of the perturbation. Furthermore, landscapes may be regarded as ‘steady’ or ‘transient’ depending on the ratio of the response time to the time elapsed since the most recent change in boundary conditions. The response of tectonically and climatically perturbed landscapes has profound implications for the interpretation of stratigraphic architecture.