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The Cenozoic Himalaya-Tibet orogen is generally regarded as the archetypal continental collision zone and is often used as an analogue for interpreting ancient orogenic events. However, given the wide diversity observed in present-day collisional mountain belts, the extent to which such inferences can be made remains debated. In this Review, we compare the metamorphic and magmatic record of the Himalaya-Tibet orogen to four ancient orogens — the Palaeozoic Caledonian orogen, the Meso-Neoproterozoic Grenville and Sveconorwegian orogens, and the Palaeoproterozoic Trans-Hudson orogen — to establish the controls on the underlying dynamics and the nature of the resulting rock record. The similarities in rock records, and, thus, thermal conditions, are interpreted to result from comparable foreland strengths, resulting in similar maximum crustal thicknesses. Apparent differences in the records are mainly attributed to variation in exposed structural level rather than fundamentally different tectonic processes. We, therefore, suggest that foreland rheology is a critical factor in determining the effectiveness of orogen comparisons. Future research is required to investigate the causes and consequences of lateral variability in mountain belts, in particular, focussing on the record of orogens smaller than those considered here, and to understand if and why mountain building processes have varied through Earth history. The links between modern collisional mountain belts and those preserved in the geological record are debated. This Review compares the Himalayan-Tibetan orogen with four ancient mountain belts and uses their similarities and differences to investigate the factors that control mountain building.

Quantifying natural processes that shape our planet is a key to understanding the geological observations. Many phenomena in the Earth are not in thermodynamic equilibrium. Cooling of the Earth, mantle convection, mountain building are examples of dynamic processes that evolve in time and space and are driven by gradients. During those irreversible processes, entropy is produced. In petrology, several thermodynamic approaches have been suggested to quantify systems under chemical and mechanical gradients. Yet, their thermodynamic admissibility has not been investigated in detail. Here, we focus on a fundamental, though not yet unequivocally answered, question: which thermodynamic formulation for petrological systems under gradients is appropriate—mass or molar? We provide a comparison of both thermodynamic formulations for chemical diffusion flux, applying the positive entropy production principle as a necessary admissibility condition. Furthermore, we show that the inappropriate solution has dramatic consequences for understanding the key processes in petrology, such as chemical diffusion in the presence of pressure gradients.

Earth’s plate-tectonic activity regulates the carbon cycle and, hence, climate, via volcanic outgassing and silicate-rock weathering. Mountain building, arc–continent collisions and clustering of continents in the tropics have all been invoked as controlling the weathering flux, with arcs also acting as a major contributor of carbon dioxide to the atmosphere. However, these processes have largely been considered in isolation when in reality they are all tightly coupled. To properly account for interactions among these processes, and the inherent multi-million-year time lags at play in the Earth system, we need to characterize their complex interdependencies. Here we analyse these interdependencies over the past 400 million years using a Bayesian network to identify primary relationships, time lags and drivers of the global chemical weathering signal. We find that the length of continental volcanic arcs—the fastest-eroding surface features on Earth—exerts the strongest control on global chemical weathering fluxes. We propose that the rapid drawdown of carbon dioxide tied to arc weathering stabilizes surface temperatures over geological time, contrary to the widely held view that this stability is achieved mainly by a delicate balance between weathering of the seafloor and the continental interiors. Earth’s surface temperature is stabilized by the drawdown of CO2 owing to weathering of continental arcs, whose length is shown to be a primary control on global weathering fluxes, according to a probabilistic analysis of interdependencies.

Establishing the timing of surface uplift in the Central Andes is essential for evaluating the geodynamic mechanisms responsible for mountain building and their role in the development of dry conditions along the western coasts of Peru and Chile. Here, we present new stable hydrogen isotopic values from stream waters and hydration water in volcanic glass from northern Chile (18.5–19.5°S) that show that the Western Cordillera was already elevated by the early Miocene. The hydrogen isotopic values of reconstructed surface waters obtained from ancient and modern volcanic glass indicate that the Western Cordillera in northern Chile attained modern elevations by at least 22.8 Ma. When combined with paleoaltimetric records from the Altiplano and northwestern Puna, these results demonstrate that surface uplift of the Andean plateau was a time-transgressive process that varied not just from west to east but also from north and south along the strike of the orogen. Our paleoaltimetry reconstruction also suggests that the Western Cordillera has blocked moisture coming from the east since at least the early Miocene, consistent with previously published evidence of arid-semiarid conditions in the Atacama Desert. However, hyperaridity on the western Andean slope developed later and appears to correspond with the timing of uplift in the Eastern Cordillera and Altiplano. Our results suggest that the growth of the Central Andean rain shadow relied not only on the elevation of the Western Cordillera but also on the widening of the plateau.

Convergence of the Indian Plate towards Eurasia has led to the building of the Himalaya, the highest mountain range on Earth. Active mountain building involves a complex interplay between permanent tectonic processes and transient seismic events, which remain poorly understood. In this Review, we examine the feedbacks between long-term tectonic deformation (over millions of years) and the seismic cycle (years to centuries) in the Himalaya. We discuss how surface morphology of the Himalaya indicates that the convergence is largely accommodated by slip on the Main Himalayan Thrust plate boundary fault, which developed in the roots of the mountain range over millions of years. At shorter (decadal) timescales, tectonic geodesy reveals that elastic strain is periodically released via earthquakes. We use examples from earthquake cycle models to suggest that partial ruptures could primarily occur in the downdip region of the Main Himalayan Thrust. Great (Mw 8+) Himalayan earthquakes are more commonly associated with complete megathrust ruptures, which release accumulated residual strain. By synthesizing numerous observations that co-vary along strike, we highlight that tectonic structures that developed over millions of years can influence stress accumulation, structural segmentation, earthquake rupture extent and location, and, consequently, the growth of the mountain range. Mountain building involves complex and debated feedback cycles between both permanent tectonic and transient seismic processes. This Review discusses how the fault architecture and tectonic segmentation of the Himalaya influence the seismic cycle, which in turn impacts the long-term growth of the mountain range.

Many mountain ranges survive in a phase of erosional decay for millions of years following the cessation of tectonic activity. Landscape dynamics in these post-orogenic settings have long puzzled geologists due to the expectation that topographic relief should decline with time. Our understanding of how denudation rates, crustal dynamics, bedrock erodibility, climate, and mantle-driven processes interact to dictate the persistence of relief in the absence of ongoing tectonics is incomplete. Here we explore how lateral variations in rock type, ranging from resistant quartzites to less resistant schists and phyllites, and up to the least resistant gneisses and granitic rocks, have affected rates and patterns of denudation and topographic forms in a humid subtropical, high-relief post-orogenic landscape in Brazil where active tectonics ended hundreds of millions of years ago. We show that catchment-averaged denudation rates are negatively correlated with mean values of topographic relief, channel steepness and modern precipitation rates. Denudation instead correlates with inferred bedrock strength, with resistant rocks denuding more slowly relative to more erodible rock units, and the efficiency of fluvial erosion varies primarily due to these bedrock differences. Variations in erodibility continue to drive contrasts in rates of denudation in a tectonically inactive landscape evolving for hundreds of millions of years, suggesting that equilibrium is not a natural attractor state and that relief continues to grow through time. Over the long timescales of post-orogenic development, exposure at the surface of rock types with differential erodibility can become a dominant control on landscape dynamics by producing spatial variations in geomorphic processes and rates, promoting the survival of relief and determining spatial differences in erosional response timescales long after cessation of mountain building.

The territory of Armenia, although relatively small, is geologically and tectonically complex. Its complexity is not only due to a dense network of faults. It results from a complicated history of tectonic development including several phases of mountain formation and planation, and the extensive development of fold-block, tectonic and magmatic processes. An important scientific task is identification of earthquake-prone structural blocks by analysing seismotectonic data on geotectonic zones in Armenia. This article describes the seismotectonic analysis of geological and geophysical data on the Viraayots-Karabakh zone.We used a wide spectrum of modern tectonic-geomorphological indices and GIS technologies in order to assess the neotectonic (Neogene – Quaternary) activity of the main block units of the study area and to classify the block units by their tectonic activity levels. Tectonics of the study area is contrasting, and many tectonically active blocks are in the immediate neighbourhood with passive blocks.Based on the records of seismic events of various magnitudes and historic earthquake data, we analysed modern seismicity of the block units. For each block, a quantitative analysis of its total seismic energy release was performed, and relationships between the released seismic energy values and the number of recorded earthquakes were analyzed. Based on such analysis, we identify a group of blocks wherein the total released seismic energy values are high, but the numbers of seismic events recorded in these blocks are rather limited. In the context of block tectonic activity, analysing these data makes it possible to detect the blocks with the highest probability of the occurrence of strong earthquakes.

Deformation in the orogen-foreland system of the southern Central Andes between 33° and 36° S varies in style, locus, and amount of shortening. The controls that determine these spatially variable characteristics have largely remained unknown, yet both the subduction of the oceanic Nazca plate and the strength of the South American plate have been invoked to play a major role. While the parameters governing the subduction processes are similar between 33° and 36° S, the lithospheric strength of the upper plate is spatially variable due to structures inherited from past geodynamic regimes and associated compositional differences in the South American plate. Regional Mesozoic crustal horizontal extension generated a < 40-km-thick crust with a more mafic composition in the lower crust south of 35°S; north of this latitude, however, a more felsic lower crust is inferred from geophysical data. To assess the influence of different structural and compositional heterogeneities on the style of deformation in the southern Central Andes, we developed a suite of geodynamic models of intraplate lithospheric shortening for two E–W transects (33° 40′ S and 36° S) across the Andes. The models are constrained by local geological and geophysical information. Our results demonstrate a decoupled shortening mode between the brittle upper crust and the ductile lower crust in those areas characterized by a mafic lower crust (36° S transect). In contrast, a more felsic lower crust, such as in the 33° 40′ S transect, results in a coupled shortening mode. Furthermore, we find that differences in lithospheric thickness and the asymmetry of the lithosphere–asthenosphere boundary may promote the formation of a crustal-scale, west-dipping detachment zone that drives the overall deformation and lateral expansion of the orogen. Our study represents the first geodynamic modeling effort in the southern Central Andes aimed at understanding the roles of heterogeneities (crustal composition and thickness) at the scale of the entire lithosphere as well as the geometry of the lithosphere–asthenosphere boundary with respect to mountain building. Fil: Barrionuevo, Matías. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Liu, Sibiao. Universitat Potsdam; Alemania Fil: Mescua, Jose Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Yagupsky, Daniel Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina Fil: Quinteros, Javier. German Research Centre for Geosciences; Alemania Fil: Giambiagi, Laura Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Sobolev, Stephan V.. Universitat Potsdam; Alemania Fil: Piceda, Constanza Rodríguez. Universitat Potsdam; Alemania Fil: Strecker, Manfred R.. German Research Centre for Geosciences; Alemania

Origins of an alpine flora The evolution of high mountain floras is strongly influenced by tectonic and climatic history. Ding et al. document the timing, tempo, and mode by which the world's most species-rich alpine flora, that of the Tibet-Himalaya-Hengduan region, was assembled. Alpine assemblages in the region are older than previously thought, with lineages tracing their alpine ancestry to the early Oligocene—older than any other modern alpine system. Alpine species diversified faster during periods of orogeny and intensification of the Asian monsoon, and the Hengduan Mountains—the most species-rich area in this region—played a key biogeographic role as the location of the earliest pulse of alpine diversification in the Oligocene. Science , this issue p. 578

Mountain building (orogeny) has a strong causal relationship with compression induced by plate drifting. It is generally accepted that orogenic belts are distributed perpendicular to the direction of plate movement. Extensive Late Mesozoic intra-continental deformations occurred around the North China Craton (NCC) in three different directions (i.e. EW-, WNW- and NNE-striking), which were previously attributed to three different geodynamic processes; the closure of the Mongolia-Okhotsk Ocean, convergence of the NCC and the South China block, and subduction of the paleo-Pacific plate. Remarkably, all these tectonic events were synchronised. Analogue modelling experiments were used to constrain far-field stress and basement shapes, to reconsider tectonic evolution around the NCC during this period. Results showed that all of these tectonic deformations could be plausibly interpreted as a unitary geodynamic regime governed by the westward indentation of the wedge-shaped NCC, driven by subduction of the paleo-Pacific plate. Mountain chains were formed along the edges of the wedge, indicating that the distribution of orogenic belts is not necessarily correlated with the direction of plate movement.

This chapter includes 11 cross-sections and one well log profile to show the depositional geometry and setting in specific areas around the basin: the Saudi Arabia outcrop belt; the Rimthan Arch; and the eastern and central areas of the intrashelf basin in Saudi Arabia, Bahrain, Qatar, Abu Dhabi and Oman. These cross-sections are used to demonstrate the similarity and degree of continuity of the upper Dhruma Formation, the Tuwaiq Mountain Formation, the source rock, the Hanifa, Jubaila–Arab and Arab–Hith formations and depositional sequences in these different locations in the basin. They show the manner in which the underlying platform formed, the rim developed, the source rock was deposited and the basin progressively filled. The blanket deposition of the Arab-D anhydrite was followed by the Arab-C to Arab-A and Hith carbonate and evaporite sequences. The cross-sections provide the framework used in subsequent chapters to make a series of facies maps and other interpretative diagrams and cross-sections that summarize and, for some intervals, revise the interpretation of the settings and geological events that formed the Arabian Intrashelf Basin.

One of the greatest strengths of the modern plate tectonics theory is its ability to explain the origin of virtually all of the present and most ancient mountain belts on Earth. In other words, mountain building (orogeny) stands in strong causal interrelation with the global plate drift pattern. The motor of orogeny is subduction. To enable subduction, a basin floored by oceanic crust must be present. The process of orogeny becomes initiated by subduction of ocean floor and finds its climax in the collision of continents and island arcs.

The geological record following the c. 2.3 billion years old Great Oxidation Event includes evidence for anomalously high burial of organic carbon and the emergence of widespread mountain building. Both carbon burial and orogeny occurred globally over the period 2.1 to 1.8 billion years ago. Prolific cyanobacteria were preserved as peak black shale sedimentation and abundant graphite. In numerous orogens, the exceptionally carbonaceous sediments were strongly deformed by thrusting, folding, and shearing. Here an assessment of the timing of Palaeoproterozoic carbon burial and peak deformation/metamorphism in 20 orogens shows that orogeny consistently occurred less than 200 million years after sedimentation, in a time frame comparable to that of orogens through the Phanerozoic. This implies that the high carbon burial played a critical role in reducing frictional strength and lubricating compressive deformation, which allowed crustal thickening to build Palaeoproterozoic mountain belts. Further, this episode left a legacy of weakening and deformation in 2 billion year-old crust which has supported subsequent orogenies up to the building of the Himalayas today. The link between Palaeoproterozoic biomass and long-term deformation of the Earth’s crust demonstrates the integral relationship between biosphere and lithosphere. High burial of organic carbon in sediments around 2 billion years ago acted to enhance crustal deformation and led to intensified mountain building both in the Paleoproterozoic and since, suggests an assessment of the timing of carbon burial and deformation

Arc-arc collision plays an important role in the formation and evolution of continents (e.g., Yamamoto et al., 2009; Tamura et al., 2010). The Izu collision zone central Japan, an active collision ...

This research is considered the first study to analyze the engineering properties of the carbonate rocks along the Najran-Sharourah District and aims to fill the study gap for the building properties of the limestone of Tuwaiq Mountain Formation, widely distributed along the Najran-Sharourah District. The engineering aspects of this limestone are investigated in field and experimental work. To evaluate the chemical, physical, and mechanical properties in the laboratory, samples collected during the fieldwork are tested. The mean physical properties of Tuwaiq Limestone Formation are characterized by a Ϭdry and Abs of 2.20 to 2.87 g/cm3 and 3.67% to 8.0%, respectively, where the mechanical properties are UCS of 309 to 689 MPa, Is50 of 6.48 to 14.32 MPa, a 4.75 to 11.48 MPa point load strength, 20–48 Schmidt hammer rebound number, impact strength index of 40–98%, and Bohme abrasion resistance from 13–25 (cm3/50 cm2). The quality index method and values obtained through laboratory tests confirm the enabled better specification of uses along the study area. The limestone of Tuwaiq Mountain Formation is highly valued for construction, as its rock characteristics and qualify are ideal for use as building stone, including the renovation of buildings to repair worn and fading pieces, as well as the cladding of new buildings.

Polyphase structural mapping and mineral age dating across the Salmon River suture zone in west-central Idaho (Riggins region; ∼45°30′N, ∼117°W–116°W) support a late Mesozoic history of penetrative deformation, dynamothermal metamorphism, and intermittent magmatism in response to right-oblique oceanic-continental plate convergence (Farallon–North America). High-strain linear-planar tectonite fabrics are recorded along an unbroken ∼48 km west-to-east transect extending from the Snake River (Wallowa intra-oceanic arc terrane; eastern Blue Mountains Province) over the northern Seven Devils Mountains into the lower Salmon River Canyon (ancestral North America; western Laurentia). Given the temporally overlapping nature (ca. 145–90 Ma) of east-west contraction in the Sevier fold-and-thrust belt (northern Utah–southeast Idaho–southwest Montana segment), we propose that long-term terrane accretion and margin-parallel northward translation in the Cordilleran hinterland (∼41°N–46°N latitude; modern coordinates) drove mid- to upper-crustal shortening >250 km eastward into the foreland region (∼115°W–113°W). During accretion and translation, the progressive transfer of arc assemblages from subducting (Farallon) to structurally overriding (North American) plates was accommodated by displacement along a shallow westward-dipping basal décollement system underlying the Cordilleran orogen. In this context, large-magnitude horizontal shortening of passive continental margin strata was balanced by the addition of buoyant oceanic crust—late Paleozoic to Mesozoic Blue Mountains Province—to the leading edge of western Laurentia. Consistent with orogenic float modeling (mass conservation, balance, and displacement compatibility), diffuse dextral-transpressional deformation across the accretionary boundary (Salmon River suture: Cordilleran hinterland) was kinematically linked to eastward-propagating structures on the continental interior (Sevier thrust belt; Cordilleran foreland). As an alternative to noncollisional convergent margin orogenesis, we propose a collision-related tectonic origin and contractional evolution for central portions of the Sevier belt. Our timing of terrane accretion supports correlation of the Wallowa terrane with Wrangellia (composite arc/plateau assemblage) and implies diachronous south-to-north suturing and basin closure between Idaho and Alaska.

Mountains are topographic features caused by erosion after vertical uplift or mountain building. Mountain building is often confused with orogeny, which today means the formation of structures in fold belts. The common assumption that folding and mountain building go together is generally untrue. Many mountains occur in unfolded rocks, granites and volcanic rocks, so there is no direct association of folding and mountain building. In those places where mountains are underlain by folded rocks the folding pre-dates planation and uplift. The age of mountains is therefore not the age of the last folding (if any) but the age of vertical uplift. Since mountains are not restricted to folded rocks, lateral compression is not required to explain the uplift. A compilation of times of uplift of mountains around the world shows that a major phase of tectonic uplift started about 6 Ma, and much uplift occurred in the last 2 Ma. This period is known as the Neotectonic Period. It is a global phenomenon including mountains on passive continental margins, and those in deep continental interiors. Several hypotheses of mountain building have problems with this timing. Some fail by being only able to make mountains out of folded rock at continental margins. Many translate the vertical uplift into lateral compression, but vertical uplift alone can create mountains. The Neotectonic Period has important implications for geomorphology, climate and global tectonics. In geomorphology it does not fit into conventional theories of geomorphology such as Davisian or King cycles of erosion. Neotectonic uplift might initiate several cycles of erosion, but most planation surfaces are much older than the Neotectonic Period. The increasing relief associated with Neotectonic uplift affected rates of erosion and sedimentation, and also late Cenozoic climate. The Neotectonic Period does not fit within plate tectonics theory, in which mountains are explained as a result of compression at active margins: mountains in other locations are said to have been caused by the same process but further back in time. This is disproved by the young age of uplift of mountains in intercontinental and passive margin positions. Subduction is supposed to have been continuous for hundreds of millions of years, so fails to explain the world-wide uplifts in just a few million years. Geomorphologists should be guided by their own findings, and refrain from theory-driven hypotheses of plate collision or landscape evolution.

The Middle-Late Jurassic mountain building process in the Western Tethyan realm was triggered by west- to northwestward-directed ophiolite obduction onto the wider Adriatic shelf. This southeastern to eastern Adriatic shelf was the former passive continental margin of the Neo-Tethys, which started to open in the Middle Triassic. Its western parts closed from around the Early/Middle Jurassic boundary with the onset of east-dipping intra-oceanic subduction. Ongoing contraction led to ophiolite obduction onto the former continental margin since the Bajocian. Trench-like basins formed concomitantly within the evolving thin-skinned orogen in a lower plate situation. Deep-water basins formed in sequence with the northwest-/westward propagating nappe fronts, which served as source areas of the basin fills. Basin deposition was characterized by coarsening-upward cycles, i.e. sedimentary melanges as synorogenic sediments. The basin fills became sheared successively by ongoing contractional tectonics with features of typical melanges. Analyses of ancient Neo-Tethys melanges along the Eastern Mediterranean mountain ranges allow both, a facies reconstruction of the outer western passive margin of the Neo-Tethys and conclusions on the processes and timing of Jurassic orogenesis. Comparison of melanges identical in age and component spectrum in different mountain belts figured out one Neo-Tethys Ocean in the Western Tethyan realm, instead of multi-ocean and multi-continent scenarios.

Species distribution and richness ultimately result from complex interactions between biological, physical, and environmental factors. It has been recently shown for a static natural landscape that the elevational connectivity, which measures the proximity of a site to others with similar habitats, is a key physical driver of local species richness. Here we examine changes in elevational connectivity during mountain building using a landscape evolution model. We find that under uniform tectonic and variable climatic forcing, connectivity peaks at mid-elevations when the landscape reaches its geomorphic steady state and that the orographic effect on geomorphic evolution tends to favour lower connectivity on leeward-facing catchments. Statistical comparisons between connectivity distribution and results from a metacommunity model confirm that to the 1st order, landscape elevation connectivity explains species richness in simulated mountainous regions. Our results also predict that low-connectivity areas which favour isolation, a driver for in situ speciation, are distributed across the entire elevational range for simulated orogenic cycles. Adjustments of catchment morphology after the cessation of tectonic activity should reduce speciation by decreasing the number of isolated regions.

The nature of metamorphism, magma compositions, the spatial distribution of plutons, and foreland sediments reflect, in part, the character and thickness of continental crust. We utilized metamorphic pressure-temperature-time (P-T-t) paths, garnet Sm-Nd ages, zircon U-Pb ages, and pluton compositions to estimate paleocrustal thickness and temporal changes in crustal magma sources in the Blue Ridge of the southernmost Appalachians. Garnet Sm-Nd ages for amphibolite-facies metamorphic rocks range from 331 ± 4 to 320 ± 3 Ma. Low- and high-Sr/Y plutons that intruded these metamorphic rocks have zircon U-Pb ages of 390 ± 1 to 365 ± 1 Ma and 349 ± 2 to 335 ± 1 Ma, respectively. Therefore, garnet growth began during regional metamorphism synchronous with or shortly after intrusion of the youngest high-Sr/Y trondhjemite plutons. Phase diagram sections and thermobarometry indicate that garnet growth initiated at ∼5.8 kbar and 540 °C and grew during temperature increases of 60–100 °C and pressure increases of 2–3 kbar. The older, low-Sr/Y magmas are inferred to have been sourced in the crust at depths 30 km depths where garnet was stable. Hafnium isotopic compositions for all the plutons, but one, exhibit a range from negative initial εHf(i) to weakly positive initial εHf(i), indicating incomplete mixing of dominantly crustal sources. Our data require minimum crustal thicknesses of ∼33 km at 331 Ma; however, Alleghanian crustal thicknesses must have locally reached 39 km, based on crustal reconstruction adding the Alleghanian thrust sheet beneath the eastern Blue Ridge. We infer the presence of hot, tectonically thickened crust during intrusion of the early Alleghanian high-Sr/Y plutons and conclude that garnet growth and plutonism reflect a progressive increase in crustal thickness and depth of magma generation. The crustal thickening was synchronous with deposition of Mississippian to early Pennsylvanian sediments in the foreland basin of the Appalachian orogen between 350 and 320 Ma. This crustal thickening may have preceded emplacement of the Alleghanian thrust sheets onto the North American craton.

In this paper the recent evolution of marginal segment of the North-Western Caucasus orogen in its junction to Kerch-Taman periclinal trough was studied. Geomorphological analysis included geological, tectonophysics data and digital elevation models was carried out. The North-Western Caucasus torn-folded morphostructures that had their extension in intermediate Kerch-Taman region westward to North-Western Caucasus mountain structure are both established to be developed in uniform regional compression settings. The prevailing horizontal compression direction changed from N–E in the North-Western Caucasus to the meridional one in Taman region, which was expressed in the reorientation of the axes of the folded morphostructures and geophysical anomalies from N–W to the latitudinal direction. The separation of the compressional vectors occurred in Pliocene-Quaternary period. Pre-Pliocene fault structures of N–E strike in this area were replaced with meridional Anapa-Dzhiginka and Abrau active fault zones.We compared Abrau zone with Anapa-Dzhiginka zone and established the faulting in Abrau zone became more dynamic in Pliocene-Quaternary stage, we revealed clear signs of vertical amplitude droppable-sliding kinematics of 500–600 m of normal fault deformation in Abrau zone during this stage. Abrau zone is the Greater Caucasus western boundary zone.The possible reasons for the restructuring of regional structural geometry and lateral transition activity of the faults transverse to the Northern-Western Caucasus orogen, are considered.

Over geological timescales, mountain building or orogenesis is associated with increased weathering, the drawdown of atmospheric CO2, and global cooling. However, a multimillion‐year delay appears to exist between peaks in low‐latitude mountain uplift and the maximum extent of Phanerozoic glaciation, implying a more complex causal relationship between the two. Here we show that global silicate weathering can be modulated by orogeny in three distinct phases. High, young mountain belts experience preferential precipitation and the highest erosion. As mountains are denuded, precipitation decreases, but runoff temperature rises, sharply increasing chemical weathering potential and CO2 drawdown. In the final phase, erosion and weathering are throttled by flatter topography. We conclude that orogeny acts as a capacitor in the climate system, granting the potential for intense transient CO2 drawdown when mountain ranges are denuded; the mechanism suggests such a scenario potentially happening 10‐50 million years in the future.

AIM: The summits of mountain ranges at mid‐latitude in the Northern Hemisphere share many ecological properties with the Arctic, including comparable climates and similar flora. We hypothesize that the orogeny during the Oligocene‐Miocene combined with global cooling led to the origin and early diversification of cold‐adapted plant lineages in these regions. Before the establishment of the Arctic cryosphere, adaptation and speciation in high elevation areas of these mountain ranges may have led to higher species richness compared to the Arctic. Subsequent colonization from mid‐latitude mountain ranges to the Arctic may explain similar but poorer flora. LOCATION: Arctic‐Alpine regions of the Northern Hemisphere. METHODS: We mapped the cold climate in the Northern Hemisphere for most of the Cenozoic (60 Ma until present) based on paleoclimate proxies coupled with paleoelevations. We generated species distribution maps from occurrences and regional atlases for 5,464 plant species from 756 genera occupying cold climates. We fitted a generalized linear model to evaluate the association between cold‐adapted plant species richness and environmental, as well as geographic variables. Finally, we performed a meta‐analysis of studies which inferred and dated the ancestral geographic origin of cold‐adapted lineages using phylogenies. RESULTS: We found that the subalpine‐alpine areas of the mid‐latitude mountain ranges comprise higher cold‐adapted plant species richness than the Palearctic and Nearctic polar regions. The topo‐climatic reconstructions indicate that the cold climatic niche appeared in mid‐latitude mountain ranges (42–38 Ma), specifically in the Himalayan region, and only later in the Arctic (22–18 Ma). The meta‐analysis of the dating of the origin of cold‐adapted lineages indicates that most clades originated in central Asia between 39–7 Ma. MAIN CONCLUSIONS: Our results support the hypothesis that the orogeny and progressive cooling in the Oligocene‐Miocene generated cold climates in mid‐latitude mountain ranges before the appearance of cold climates in most of the Arctic. Early cold mountainous regions likely allowed for the evolution and diversification of cold‐adapted plant lineages followed by the subsequent colonization of the Arctic. Our results follow Humboldt's vision of integrating biological and geological context in order to better understand the processes underlying the origin of arctic‐alpine plant assemblages.

Due to oblique collision of the Luzon Volcanic arc and Eurasian passive margin, in Taiwan we observe southward progression of mountain building processes. Consequently, the southernmost tip of Taiwan represents the most recently emerged part of the orogen - the Hengchun Peninsula. The area shows evidence of a trench-slope basin that emerged in the Holocene with landforms that bear submarine characteristics. In the Late Pleistocene, lagoonal sediments blanket the basin, however the deeper deposits (Maanshan Formation) can be found on the surface in several areas. We document that the Maanshan Formation was brought to the surface through upwelling structures—mud diapirs—as a consequence of fast and overpressured sedimentation from the emerging orogen. Although fast erosional processes continuously reorganize the new landscape, we are able to capture relicts of fast eroding inactive mud volcanoes. These unique features are preserved on the surface due to shielding by coral terraces during the shallowing and the lagoonal period of the basin. Stable carbon δ13C and oxygen δ18O isotope analysis, performed on samples of carbonate blocks of cold vents collected on the outcrops of the Maanshan Formation, confirm classic seep carbonates origins. We also identify a mud-core anticline below the study area acting as the main source for the mud volcanoes and a possible cause of the uplift of the trench-slope basin. Two multichannel seismic profiles from marine surveys along with a synthesis of several previous surveys helped construct a model of the characteristics of mud diapirism in southeastern offshore Taiwan.

Although the development of Gobi Desert in central and eastern Asia has greatly affected the regional and even the global climate, its precise origin and evolution have yet to be determined. The three preconditions for the formation of Gobi Desert are: i) a dry climate, ii) basin landforms and iii) abundant sediment production. In this study, we present a synthesis of both new and published data on the formation and evolution of Gobi Desert in central and eastern Asia. We conclude that the combined effects of mountain building, the mid-latitude westerly circulation and changes in the Asian monsoon, accompanied by global cooling, were principally responsible for the formation of modern Gobi Desert landscapes in central and eastern Asia during the late Pliocene. The arid climate in central and parts of eastern Asia probably developed in the early Cenozoic, from ~50 Ma. Related events included the collision of the Indian and Asian plates, the closure and complete retreat of the Paratethys Ocean from central Asia, and the growth of the Himalayas and the Tibetan Plateau in the Eocene through late Miocene, which blocked the water vapor supply and intensified the aridification of the Asian interior. Superimposed on the topographic changes was the process of stepwise global cooling since the early Oligocene, and in particular since the late Miocene, which controlled the formation and evolution of the Gobi Desert landscape. Global cooling weakened the Asian monsoon circulation, strengthened the westerly circulation and enhanced physical weathering processes in mountain areas, which together promoted both the aridification of the Asian interior and sediment production. These processes finally resulted in the establishment of the modern Gobi Desert landscape in the late Pliocene. We estimate that the modern Gobi Desert landscape was formed at ~2.6 Ma and was the result of the stepwise evolution of Asian topography and climate during the Cenozoic, dominated by Asian tectonic deformation and uplift, and the evolution of Asian monsoon climate and the westerly circulation, forced by global temperature change.

The active plate boundary of the California margin includes the active San Andreas Fault System and the active mountain building of the California Coast Ranges where ultramafic rocks are also commonly found. There are numerous hypotheses for the origin and the timing and emplacement mechanisms of such rocks that ultimately came from Earth's mantle, but typically the mineralogical, structural, and geochemical features of fresh serpentinite, and their reaction and deformation histories were unknown due to heavy weathering of most of the exposures. We investigated a block-and-matrix serpentinite melange in Redwood City, California on the San Francisco Peninsula that is proximal to an active strand of the San Andreas Fault. The melange is composed of 6 m- to cm-sized serpentinite blocks surrounded by fine-grained serpentine matrix. The blocks are polygonal in shape and show a remarkable internal mineralogical structure that has recorded at least three stages of serpentinization of a clinopyroxene-bearing harzburgite protolith. The largest, freshest polygonal serpentinite blocks contain a core comprised of surviving peridotite minerals plus lizardite (lz) ± antigorite (atg) + brucite (brc) + magnetite (mag) domains. The cores are surrounded by a ~10 cm thick peripheral rim of lizardite + magnetite. The reaction boundaries between rims and cores are parallel to the external polygonal surfaces of the blocks. These blocks are also sheathed by thin skins of sheared lizardite + chrysotile (ctl). The higher-temperature lz/atg + brc + mag serpentinization represented by the core mineralogy (Stage A) probably occurred on a large scale in the mantle and the hydration metamorphism producing the lz + mag mineralogy in the rims (Stage B) was likely triggered by the growth of a system of approximately planar fractures under crustal conditions that was enabled by the ingress of water at high pressure and that defined the polygonal block shapes as well as the Stage-A to Stage-B reaction boundaries. Stage-A serpentinization reactions are internally balanced except for addition of H2O, Cs and Rb as indicated by core mineral composition. The Stage-B serpentinization was open system with CaO, Cs and Rb migrating outside the blocks with addition of H2O, U, Pb ± LREE. The sheared lizardite skin probably formed during the ascent of the RCS through the crust. Several classes of mineral-filled veins were found in both cores and rims and indicate fluid pressures approaching lithostatic pressures that enabled these opening- mode fractures to occur. We posit that such high-pressure hydrothermal conditions were important in promoting the serpentinization reactions as well as enabling a weak rheology consistent with a cold-intrusion/diapiric emplacement mechanism. These factors give insights into where these mantle rocks came from, the sources of the water causing serpentinization, and the deformation mechanisms by which they come to Earth's surface. Similarities of the structures of serpentinite blocks reported in this paper with those in blocks from other serpentinite bodies in the San Francisco Bay Area ( Lewis and Kirby, 2015 AGU Abstract T13H-05) suggests that the processes inferred from our investigation are regional in spatial extent.

The recent evolution of the marginal segment of the Northwestern Caucasus orogen at its junction with the Kerch–Taman periclinal trough has been studied. Geomorphological analysis included geological and tectonophysical data and digital elevation models. The Northwestern Caucasus fault–fold morphostructures extend westward to the Crimean mountain structure, in the intermediate Kerch–Taman region, developing in a uniform regional compression setting. The prevailing direction of horizontal compression changed from NE in the Northwestern Caucasus to N–S in the Taman region, resulting in reorientation of the axes of fold morphostructures and geophysical anomalies from NW to EW. The compression vectors separated in the Pliocene–Quaternary. The NE-trending pre-Pliocene fault structures in this area were replaced by the N–S-trending Anapa–Dzhiginka and Abrau active fault zones. As follows from comparison of the Abrau and Anapa–Dzhiginka zones, faulting in Abrau zone became more dynamic in the Pliocene–Quaternary; we revealed clear signs of normal faulting with a vertical offset of 500‒600 m in the Abrau zone during this stage. The Abrau zone is the western boundary zone of the Greater Caucasus. The possible reasons for the restructuring of the regional structural geometry and lateral transition activity of faults transverse to the Northwestern Caucasus orogen are considered.

Taiwan is widely considered to be a typical example of an arc-continent collision surrounded by two opposite dipping subduction zones. The manner by which the interaction of the two neighboring slabs caused plate collision and mountain building is insufficiently understood. Various hypotheses have been proposed, but the geodynamic feasibility of those remains to be tested. Here we present 3-D thermomechanical models to study the geodynamic evolution process of a Taiwan-like setting after an initial transform fault was consumed. In our model setup, the boundary between the Eurasian plate and the South China Sea is northeast trending. The results show that all simulations result in toroidal mantle flow around the slab edges and that slab breakoff as well as a small-scale mountain belt with high topography and crustal exhumation occurs in most cases. The Eurasian continental crust is exhumed in a dome-like manner exposing higher-grade metamorphic rocks, facilitated by high erosion rates and a weak continental lower crust rheology, but inhibited by the presence of a weak arc. A high topography within the orogen, as well as continental slab detachment, can develop for the convergence direction of N307 degrees and large convergence rates. Our modeling results are thus generally consistent with the Eurasian slab-tearing model proposed for Taiwan based on seismic tomographic studies, and we suggest that the main characteristic features in Taiwan can be explained by the combined effects of fast erosion, a weak lower crust, fast convergence, and a small convergence azimuth.

Since the Late Cenozoic, the central-southern Longmen Shan (LMS) region is characterized by a two-decollement thrust-fold system with a localized upper evaporite weaker layer just beneath the Sichuan Basin and the lower stronger decollement beneath the LMS. To date, the role of upper decollement and erosion in shaping the structural deformation of the LMS remains unknown. The investigation of this question may shed lights on the assessment of the structural evolution and the further discrimination of mountain building mechanism. In this work, a 2D finite difference method is applied with regards to the variations of viscosity/friction angle, and lengths of upper decollement under different erosion rates. Two types of numerical models are setup, with purely lower-upper frictional (FF) decollements and lower-frictional upper-viscous (FV) decollements. Tectonic evolution with FF models exhibits large similarity with that of FV models. Meanwhile, our results underline the presence of in-sequence fault propagation procedures during most of the stages. The development of the boundary ramp, the Range Frontal Blind fault, is strongly induced by a prerequisite with upper decollement of necessary length. The activation of Beichuan/Pengguan faults or blind ramp piercing to the surface is extremely mechanically sensitive to the viscosity of the salt decollement, erosion rate, and the length of the upper decollement. Out-of-sequence thrusting, shallow and possible basal accretion processes beneath the Sichuan Basin are found when deformation transfers from the thick-skinned LMS to the thin-skinned Sichuan Basin at the very late stage, supporting upper crustal shortening dominates the mountain building mechanism.

Hydrogen isotopic composition (δD) and 40Ar/39Ar geochronological data of neomineralized clay minerals from fault rocks and cleavage seams in the Sevier belt record primary involvement of surface-sourced fluids during Cretaceous–Paleogene mountain building events. δD values of authigenic clays range from − 78.8 ± 1.7 ‰ to − 97.2 ± 3.2 ‰ . Given temperature constraints of mineral formation, we calculate δD values of mineralizing fluids as ∼−63 to −96‰, which fall within the range of δD values of Cretaceous–Paleogene surface waters in the region. The potential infiltration and presence of surface fluids at depths of several kilometers during deformation implies that pore spaces and fractures acted as conduits for surface water. Uplift of thrust sheets generated topography that favored meteoric recharge and flow of fluids into margins of the foreland basin, which were subsequently incorporated into the propagating thrust wedge. This is in contrast to the hypothesis that the release of overpressured metamorphic fluids dominates during fold–thrust belt evolution. As a result of shortening and tectonic uplift, fluid flows through basin sediments toward the foreland basin as meteoric waters recharge sediments at the basin margins. This study shows that stable isotopic compositions of shallow, deformation-related clay minerals are robust and may be used alongside other surface fluid proxies to probe the origin and composition of ancient fluids in orogenic systems.

If a community and its substrate are raised by tectonic uplift, the species present can either die out in the area, survive in situ unchanged, or survive in situ with adaptation and differentiation. The large-scale passive uplift of plant and animal populations during mountain-building is accepted in a growing number of studies, but the idea has seldom been examined critically. If passive uplift does occur, it has implications for interpreting community structure and speciation in some of the most biodiverse places on Earth, tropical mountains. It would also provide a simple explanation for many altitudinal anomalies, such as the occurrence of typical coastal elements at unusually high altitudes in certain localities. Examples include the coastal saltmarsh plant Salicornia at 4200 m altitude in the rapidly uplifted Andes, coastal frogs and ferns in African mountains, and inland mangroves in New Guinea. The first aim of this paper is to review previous work on passive uplift worldwide and the main ideas that have been discussed. A second goal is to discuss possible tests of passive uplift.

Late Cenozoic orogen-derived sediments recorded in the pro- and retro-foreland basins on both sides of the Taiwan orogenic wedge provide important constraints on sediment provenance evolution, and further outline the exhumation and erosion histories of the orogenic belt. We present 172 sandstone petrographic data from seven representative regions on the pro- and retro-foreland basins, which we coupled with previous thermochronological data from the pro-wedge of the Hsuehshan Range and the retro-wedge of the Backbone Range to elucidate the source-sink relationships between a source orogenic wedge and its associated foreland basin. Our data analysis and estimation provide evidence that the initial depositional age of orogen-derived sediments in pro- and retro-foreland basins is consistent with the exhumation and erosion histories of the adjacent orogen, indicating that the Backbone and Hsuehshan Ranges started to build up at ~12 and 5–6 Ma, respectively. The diachronous onset of orogenic sediment deposition and orogenic uplift also suggest a tendency for progressively southward and westward migration of the foreland basin, and asymmetric exhumation and erosion of the orogenic wedge over time due to oblique collision. The timing of the onset of the Taiwan orogeny formed the retro-wedge of the Backbone Range at ~12 Ma, which is much older than the previously determined onset age of Taiwan mountain building of ~6 Ma. We divide the Taiwan orogenic evolution into two stages: the first stage comprises the extrusion and exhumation of the subduction wedge formed as the Yuli Belt of the Backbone Range prior to the collision during the middle Miocene. The second stage comprises the arc-continent collision, producing doubly vergent orogenic wedge extrusion, leading to a diachronous uplift history of the Hsuehshan and Backbone Ranges since late Miocene.

The utilization of thermal-chronological data to constrain mountain building processes exploits the links among rock uplift, exhumation, and cooling during orogenesis. Conceptually, periods of rapid uplift and associated denudation will lead to cooling of rocks as they approach Earth’s surface. The linkage between uplift and exhumation can be complex, but in practice exhumation is often assumed to directly track uplift. The reconstruction of temperature-time histories via thermochronologic systems provides a proxy method to relate the cooling of rock as it is exhumed toward the surface to orogenesis. For the rapid exhumation rates that can occur in active orogenic systems the thermal history will be complex as a result of heat advection, rates of propagation of thermal perturbations, and other processes that affect the cooling behavior. These effects become amplified as exhumation rates increase, and in regions experiencing exhumation rates greater than ∼0.2–0.3 mm/yr (0.2–0.3 km/Ma) simple assumptions of cooling through a constant geotherm will bias the subsequent interpretation. Here we explore, through a suite of generalized models, the impact of exhumation rate and duration on the resulting thermal history and apparent age results. We then apply lessons from these simple exhumation systems to data sets from the high-relief ranges along the eastern margin of the Tibetan Plateau to determine exhumation histories constrained by those data. The resulting exhumation histories provide constraints on the onset of Cenozoic exhumation, the subsequent pace of exhumation, and on the tectonic history of one of the major fault systems in the central Longmen Shan.

The subsurface fault geometry is the base for understanding a process of crust deformation and mountain building. Based on kinematic models for fault-related folds, a geomorphic method is recently applied to estimate the subsurface fault geometry, while the validation on its reliability is lacking. In this study, we surveyed a suit of river terrace surfaces across an active fold at the north front of the Qilian Shan. According to the deformation geometry of the terraces, the fold deformation is interpreted by a listric fault fold model, and based on this kinematic model, the fault geometry underlying the fold is estimated. In comparison between the estimated fault geometry and a seismic reflection profile, we found that the decollement depth and the back thrust are highly consistent with each other. Although some small fault bends or internal shearing cannot be estimated solely by the terrace deformation, the overall fault geometry is successfully revealed by the terrace deformation. Using this fault geometry and the terrace dating results, the region deformation kinematics are re-evaluated, which suggest that the dip slip (in a rate of 1.8 ± 0.4 mm/a) along the decollement is mainly accommodated by two structures, one is the blind-back-thrust fault within the piggy basin in a dip-slip rate of 0.9 ± 0.3 mm/a and another is the thrust and fold at the west portion of the Yumu Shan range.

The continental collision process has made a large contribution to continental growth and reconfiguration of cratons throughout Earth history. Many of the mountain belts present today are the product of continental collision such as the Appalachians, the Alps, the Cordillera, the Himalaya, the Zagros, and the Papuan Fold and Thrust Belt. Though collisional mountain belts are generally elongate and laterally continuous, close inspection reveals disruptions and variations in thrust geometry and kinematics along the strike of the range. These lateral variations typically coincide with cross structures and have been documented in thrust fault systems with a variety of geometries and kinematic interpretations. In the Himalaya, cross faults provide segment boundaries that, in some cases separate zones of differing thrust geometry and may even localize microseismicity or limit areas of active seismicity on adjacent thrust systems. By compiling data on structural segmentation along the length of the Himalayan range, we find lateral variations at all levels within the Himalaya. Along the Gish fault of the eastern Indian Himalaya, there is evidence in the foreland for changes in thrust-belt geometry across the fault. The Gish, the Ganga, and the Yamuna faults all mark boundaries of salients and recesses at the mountain front. The Benkar fault in the Greater Himalayan sequence of eastern Nepal exhibits a brittle-ductile style of deformation with fabric that crosscuts the older thrust-sense foliation. Microseismicity data from several regions in Nepal shows linear, northeast-striking clusters of epicenters sub-parallel to cross faults. The map pattern of aftershock data from the 2015 Nepal earthquakes has an abrupt northeast-trending termination on its eastern side suggesting the presence of a structure of that orientation that limited slip. The orientations of the recognized cross faults and seismic patterns also align with the extensional zones to the north on the Tibetan Plateau and the Indian basement structures to the south. Results from multiple studies are consistent with a link between cross faults and either of these structural trends to the north or south and suggest that cross faults may play a role in segmenting deformation style and seismic activity along the length of the Himalaya.

Increased glaciation in the North Patagonian Andes may have influenced tectonic dynamics over the past 7 million years, suggesting a connection between climate change and mountain-building processes.

The heterogeneous continental lithosphere of Europe inherits billion of years of tectonic evolution, mineral transformation and magmatic addition. Though there is now an extensive body of studies on the long-term geological, geochronological and geochemical evolution of the continental crust and lithospheric mantle available in Europe, yet this knowledge has not been linked to the understanding of tectonic evolution of Cenozoic Alpine mountain building. In this aim, we review geophysical, geological, petrographical, geochemical, and thermochronological constraints to infer a kinematically coherent time-integrated tectonic model for the evolution of mountain building in Western Europe, along a 4000 km long lithospheric transect from Africa to the East European Craton. We show that the key drivers of plate-scale processes related to Alpine orogenic and topographic evolution reflect three main ingredients : 1) a protracted magmatic and tectono-thermal transformation of Africa (Gondwana) and North Europea (Baltica) cratonic mantle lithosphere since the Neoproterozoic, 2) an overall limited Mesozoic Tethyan extension of the weak Variscan lithosphere characterized by the lack of wide, thermally relaxed, oceanic lithosphere, 3) a relatively slow Cenozoic convergence between Africa and Europe, preserving initial stages of distributed tectonic inversion of rifted continental blocks throughout Europe, and partial subduction and delamination in the Mediterranean region of the most evolved lithospheric domains.

Spatial and temporal patterns of mountain building in the northeastern Tibetan Plateau provide important constraints on competing plateau growth models. We focus on the East Kunlun Shan (EKLS), where the timing of Cenozoic deformation remains controversial. Seven apatite (U‐Th)/He samples were collected around a tilted erosion surface in the middle segment of the EKLS. A break in slope at ∼25 Ma is identified along the paleodepth below the erosion surface, which we interpret to represent the onset of thrust faulting at northern margins of the EKLS. Published thermochronologic data from the South Qilian Shan and North Qilian Shan reveal tectonic uplift at 15‐18 and 8‐10 Ma respectively, obviously later than thrust faulting in the EKLS. Our study supports that the northeastern margin of the Tibetan Plateau has migrated from the EKLS to North Qilian Shan since the late Oligocene.

Mountain height at convergent plate margins is limited by the megathrust shear force, but it remains unclear how this constraint affects the topographic evolution and mountain building at the transition from subduction to collision. Generally, mountain height increases during the subduction-collision transition in response to crustal thickening or processes like mantle delamination and slab breakoff, but the main parameters controlling how much mountain height increases remain poorly understood. Here we show, based on analytical and finite-element force-balance models, that the increase in mountain height depends on the magnitude of the megathrust shear force and the reduction of submarine margin relief. During the subduction stage, the shear force is balanced by the gravitational effect of the margin relief and the deviatoric stresses in the upper plate are low. When the submarine margin relief is reduced during the closure of the ocean basin, the effect of the gravitational force decreases and the upper plate experiences enhanced deviatoric compression, which allows the mountain height to increase until a near-neutral stress state beneath the high mountains is restored. If the increase in mountain height cannot keep pace with the submarine relief reduction, the compression of the upper plate increases by a few tens of MPa, which promotes tectonic shortening and mountain building. Our analysis implies that mountain height can increase by hundreds of meters to a few kilometers during collision, depending primarily on the trench depth during the subduction stage and possible syncollisional changes of the megathrust shear force.

The Upper Oligocene Macigno Formation cropping out in the coastal section of SW Tuscany represents the sedimentary fill of a foredeep basin developed during the collisional stages that led to the construction of the Apennine mountain belt of peninsular Italy. The stratigraphic sequence consists of alternating sandstones and siltstones, that are affected by km-scale contractional structures, namely SW-dipping thrusts and related NE-verging folds. An original field survey carried out along a superb coastal exposure, integrated with analysis of mesoscopic fabrics and their overprinting relationships, makes it possible to unravel a complex deformation history. Four main deformation stages are recognized: i) layer-parallel shortening A; ii) top-to-the-foreland shear B; iii) folding C, comprising the fold nucleation sub-stage C1 and the fold amplification sub-stage C2; and iv) thrust propagation D. The sequence of recognized stages indicates a progressive deformation history of the Macigno Formation as it was incorporated within the evolving Apennine orogenic system. The kinematic history inferred from deformation fabrics and their overprinting relationships within Macigno Fm. deposits in SW Tuscany shows remarkable analogies with the structural evolution of foredeep sediments from other fold-and-thrust belts, yet with little deviations mainly consisting of pureshear vs. simple shear fluctuations in the early history of accretion. These provide original information on the modes of accretion of foredeep deposits within evolving collisional belts, thus contributing to an enhanced understanding of orogenic dynamics during mountain building with a classical example whose evolution is unravelled in detail.