Mountain Building Processes Research Articles

Top-to-south shear at the base of the eastern Tethyan Himalayan Sequence during the Eocene-Oligocene Himalayan orogeny

The early mountain building processes in the Himalayan orogen are still not clear because of extensive deformation and metamorphism since the Miocene. A large gently dipping ductile shear zone, referred as the Tethyan Himalayan Décollement (THD), is defined here as the sole décollement of the south-verging Tethyan fold-and-thrust belt in the Lhozag-Cuona area of the eastern Himalayan orogen. The ~4 km-thick THD is characterized by a top-to-south shear sense, moderate T/P Barrovian to high T/P Buchan type metamorphism and Eocene-Miocene partial melting. Zircon UPb dating of metasedimentary rocks and granitic gneisses from the THD yields protolith ages of the Late Cambrian to Early Ordovician. Based on structural analysis, zircon UPb ages, monazite UPb ages and mica 40Ar/39Ar thermochronology, the THD was the boundary shear zone at the top of the Greater Himalayan Crystalline Complex (GHC) and accommodated the persistent north-south shortening in the Tethyan Himalayan Sequence (THS) from ~50 Ma to 20–17 Ma. From ~20–17 Ma, the top-to-north South Tibetan Detachment System (STDS) was predominantly activated to juxtapose the unmetamorphosed or low-grade THS over the GHC. This tectonic transition can be attributed to the roof collapse in the eastern Himalaya (younger than that of the central-western Himalaya), which triggered rapid exhumation of the GHC and the northern Tethyan Himalayan gneiss domes. Hence, the THD was the predecessor of the STDS and a prolonged pathway for leucogranitic melts from the Eocene to early Miocene. The transition from the THD-controlled crustal thickening in the Eocene and Oligocene to the STDS-controlled extrusion in the Miocene shed insights on a new synthesis of the tectonic wedging model for the Himalayan evolution.

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.

Phylogenomic insights into the historical biogeography, character-state evolution, and species diversification rates of Cypripedioideae (Orchidaceae)

Cypripedioideae (slipper orchids; Orchidaceae) currently consist of ∼200 herbaceous species with a strikingly disjunctive distribution in tropical and temperate regions of both hemispheres. In this study, an updated phylogeny with representatives from all five cypripedioid genera was presented based on maximum likelihood and Bayesian inference of plastome and low-copy nuclear genes. Phylogenomic analyses indicated that each genus is monophyletic, but some relationships (e.g., those among Cypripedium sects. Acaulia, Arietinum, Bifolia, Flabellinervia, Obtusipetala and Palangshanensia) conflict with those in previous studies based on Sanger data. Cypripedioideae appeared to have arisen in South America and/or the adjacent Qinghai-Tibet Plateau and Hengduan Mountains ∼35 Mya. We inferred multiple dispersal events between East Asia and North America in Cypripedium, and between mainland Southeast Asia and the Malay Archipelago in Paphiopedilum. In the Americas, divergences among four genera (except Cypripedium) occurred around 31–20 Mya, long before the closure of the Isthmus of Panama, indicating the importance of long-distance dispersal. Evolutionary patterns between morphological and plastome character evolution suggested several traits, genome size and NDH genes, which are likely to have contributed to the success of slipper orchids in alpine floras and low-elevation forests. Species diversification rates were notably higher in epiphytic clades of Paphiopedilum than in other, terrestrial cypripedioids, paralleling similar accelerations associated with epiphytism in other groups. This study also suggested that sea-level fluctuations and mountain-building processes promoted the diversification of the largest genera, Paphiopedilum and Cypripedium.

Kinematic modeling of neotectonic thrusts at the Andean orogenic front (southern Precordillera, Argentina)

The south-central Andes located between 32°-33° S are considered to be a transitional zone between the flat-lying and the normal subduction zones of the Nazca Plate. The active mountain-building processes are mainly concentrated between the Andean eastern foothills and the adjacent broken foreland, as highlighted by crustal seismicity, historic destructive earthquakes, and Quaternary-active contractional structures. Between 32° 10′ and 32° 40′ S, the Andean orogenic front is exposed at the Las Peñas-Las Higueras range, where east- and west-verging thrusts interact, resulting in an antithetic linkage transfer zone. We built kinematically-balanced structural sections through the integration of surface and subsurface data (2D seismic lines) to achieve a 3D characterization of the Quaternary-active thrusts and to estimate shortening rates of ∼1.8–3.4 mm/a during the last ∼8.5 Ma, thus extending the spatial and temporal penetration achieved by surface geology data. Our results indicate that the main east-verging thrusts detach at a depth of ∼10 km within Paleozoic basement rocks. This outcome implies larger potential rupture areas than if rooted at Neogene layers. West-verging thrusts present detachment levels at ∼5 km, also linked to Paleozoic basement rocks. The analysis of 2D seismic lines has also contributed to the recognition of blind thrusts without a morphologic signature, which could be regarded as relevant seismogenic structures.

Inverse magnetic fabric of remagnetized limestones in the Zaduo area, Eastern Qiangtang Terrane: Implications for oroclinal bending in the Eastern Himalayan Syntaxis

Magnetic fabric analysis is a common technique to assess the strain regime during mountain building processes. Here, we use this approach to evaluate the tectonic evolution of the Tibetan Plateau and the Eastern Himalayan Syntaxis by analyzing the limestones of the Jurassic Buqu Formation in the Zaduo area, Eastern Qiangtang Terrane (China). These limestones were chemically remagnetized during the Cenozoic. For a proper assessment, it is relevant to understand how the mineralogy of the remagnetized limestones affects their magnetic fabric and how the magnetic fabric can improve our understanding of the tectonic strain and regional deformation. The role of the authigenic magnetite in the development of the magnetic fabric should thus be explored. Comparison of the bulk susceptibility (Km) with various natural and laboratory rock magnetic properties (Km versus natural remanent magnetization, Km versus saturation isothermal remanent magnetization, and Km versus saturation magnetization) indicates that susceptibility and remanences are both carried by authigenic magnetite. Most of the magnetite grains show axial ratios <1.3:1 according to the Néel diagram, and fall within the single-domain range based on the mass magnetic susceptibility (χ) and DC field-normalized anhysteretic remanent magnetization (χARM) ratio, giving rise to the inverse magnetic fabric observed. Twelve sites (120 specimens) are divided into four groups based on the magnetic fabric and rock magnetic behaviors. Overall, there is a clear trend of decreasing Km, natural remanent magnetization, saturation isothermal remanent magnetization, ferromagnetic percentage and shape parameter from Group I to IV. The K1 axis of all four groups documents a NNE-SSW oriented compression during remagnetization, contrasting with the Eocene NE-SW compression in the Gongjue area farther east. This different compressional regime likely resulted in different rotations and structural trends surrounding the Eastern Himalayan Syntaxis.

Reconciling patterns of long-term topographic growth with coseismic uplift by synchronous duplex thrusting

How long-term changes in surface topography relate to coseismic uplift is key to understanding the creation of high elevations along active mountain fronts, and remains hotly debated. Here we investigate this link by modeling the development of growth strata and the folding of river terraces above the Pishan duplex system in the southern Tarim Basin. We show that synchronous duplex thrusting of two neighboring faults with varying slip rates, associated with in-sequence propagation of the Pishan thrust system, is required to explain the presence of opposite-dipping panels of growth strata on the duplex front, and basinward migration of terrace fold crests. Importantly, this process of synchronous thrusting within the duplex reconciles the discrepancy between the deformation of terrace folds at the 10−1–100 million-year timescale and the maximum coseismic uplift of the 2015 Mw 6.4 Pishan earthquake on the frontal thrust. These results suggest that topography mismatch at different time scales can reflect the long-term kinematic evolution of fault systems. Thus, our study highlights the importance of characterizing complex subsurface fault kinematics for studying topographic growth, and motivates rethinking of the mountain building process in worldwide active fold-and-thrust belts, from short-term to long-term timescales.

Liquidity and Liquefaction

This article considers the collision of earthly and monetary phase shifts. It situates itself in Richmond, British Columbia, a seam where multiple and disparate processes of landing collaborate in the ongoing transformation and modulation of the earth’s surface. It poses a Chinese-funded construction boom on an island in western Canada as a geological formation, a physical outcropping at the collision of nineteenth-century Chinese terraforming labor and twenty-first-century flyaway Asian wealth. These collisions articulate fault lines in transpacific tectonic and geopolitical relations, even as the fluid dynamics of wealth and islands of impounded silt evince multiple figurations of Asian-ness. Through the juxtaposition of two permutations of land, economics, and racial formation across multiple centuries in the Fraser River Delta, I offer a notion of orogeny, the geological process of mountain building and crustal deformation, to attend to the earthmoving dynamisms of rivers, wealth, and Asian racialization.

Cretaceous mountain building processes triggered the aridification and drainage evolution in east Asia

Knowledge of the late Mesozoic topography and drainage system of the Tibetan Plateau is essential for understanding the Cenozoic tectonic dynamics of the plateau. However, systematic analyses of the pre-Cenozoic surface uplift history and sediment-routing systems of the Tibetan Plateau remain sparse. Here we present new results for paleocurrents and U-Pb detrital zircon geochronology from the Lanping Basin, a key junction in the southeastern (SE) Tibetan Plateau, and integrate multidisciplinary data sets to constrain sediment provenance and reconstruct paleotopography and its drainage system throughout the Cretaceous. Our results indicate that mid- to Late Cretaceous (ca. Albian−Santonian) tectonically induced surface uplift occurred in the SE Tibetan Plateau, leading to the build-up of an extensive topographic barrier, and resultant rain shadows in the interior of east Asia. Superimposition of this topographic pattern by uplands in the eastern margin of Asia meant that the Cretaceous topography of east Asia was characterized by an enclosed paleo-relief pattern that was high in both the east and west, with drainage from the east and west to the south, contrasting with previously proposed configurations. This topographic pattern interrupted the atmospheric circulation pattern and generated widespread intracontinental desertification and drainage network evolution in east Asia. Our study constrains a key part of the late Mesozoic growth of the Tibetan Plateau prior to the Cenozoic collision between India and Eurasia and will improve our understanding of the paleoclimate, atmospheric circulation, and modern drainage system evolution of the east Asian continent.

Sedimentary provenance constraints on the Cretaceous to Cenozoic palaeogeography of the western margin of the Jianghan Basin, South China

Provenance analysis of sedimentary basins is a direct way to understand the uplift of orogenic belts, basin deposition, and evolution of large rivers. A series of Mesozoic and Cenozoic strata from the western Jianghan Basin provides key sedimentary archives for investigating the growth of the surrounding orogenic belts and the evolution of the Yangtze drainage. However, the provenance of these sediments remains unclear. In this study, we report U–Pb data for 1098 detrital zircons from the western Jianghan Basin. We combine these data with published zircon U–Pb ages of potential source areas, paleocurrent data, and paleomagnetic age constraints to reconstruct provenance areas and establish related mountain-building processes. All Cretaceous and Early Cenozoic samples show five major age peaks at 131–210, 432–461, 720–986, 1836–1908, and 2470–2552 Ma. We infer that these sediments are primarily derived from recycled sediments associated with the uplift of the western Jiangnan Orogen. The tectonic uplift of the Jiangnan orogenic belt in the Cretaceous and Early Cenozoic was mainly controlled by the subduction of the Pacific and Indian plates underneath Eurasia. The detritus in the current upper reaches of the Yangtze River did not enter the Jianghan Basin before the Late Eocene (34 Ma). Early Pleistocene strata (1.2–0.7 Ma) show a distinct Cenozoic zircon U–Pb (41 Ma) peak age, which is mainly derived from the detrital sediments of the upper reaches of the Yangtze River. The combination of these data with published provenance tracing results for boreholes in the Jianghan Basin and Yangtze River Delta indicates that the Yangtze River was completely connected between 34 and 1.2 Ma. Such a long time interval may correspond to the tectonic uplift of the Tibetan Plateau and environmental changes during the Late Cenozoic.

The Late Devonian contraction along the Precaspian Basin north margin: Transpressional range building in the intracratonic domain

The evolution of intraplate sedimentary basins located in the vicinity of an active convergent plate boundary is often controlled by the collisional dynamics of the adjacent orogen. The transfer of compression from the orogen to the platform's interior results in the formation of complex structural geometry and kinematics that often reactivate older crustal faults, focus far-field stresses and control the evolution of associated sedimentary basins. One place where this localisation can be optimally understood is the Precaspian Basin, situated at the SE periphery of the East European Craton and bordered to the east by the Uralian orogen. The Precaspian Basin and its northern margin experienced long-term extension and subsidence interrupted by several short-lived shortening episodes. To understand the impact and role of pre-existing basement structures on the geometry and kinematics of the subsequent deformation, we analysed subsurface data from the northern margin of the Precaspian Basin by the means of 3D seismic interpretation correlated with wells and structural modelling. The analysis results provide new insights into the kinematic effects of the Late Devonian contraction event. The superposition of stratigraphic units above and below the angular unconformity suggests an intra-Famennian age of the deformation event. Steeply dipping bi-verging reverse fault zones associated with variable amounts of sinistral strike-slip movement component display an arcuate geometry, trending from WSW-ENE to NW-SE. The distribution of deformation indicates that this complex kinematic pattern was driven by a NE-SW oriented contraction and transpression, where faults show the characteristics of a restraining bend area. This area is interpreted as part of a regional transcurrent fault system developed on the northern periphery of the Precaspian Basin. Furthermore, the study results suggest that intralithospheric stress localisation transmitted by the Paleouralian subduction zone resulted in the reactivation of pre-existing basement structures, propagation of faults, and localized short-term exhumation of the Precaspian Basin north margin. Generated by the collision of Magnitogorsk volcanic arc with East European Craton, far-field stress transfer produced a zone of oblique deformation. These data and interpretation demonstrate that the northern margin of the Precaspian Basin is an excellent natural setting to investigate and better understand mechanisms of far-field strain localisation and reactivation of deformational structures in stable platform areas resulting in the intracratonic mountain building process.

Large variations of crustal thickness across the Taiwan orogeny constrained by Moho-refraction recorded by the Formosa Array

The Taiwan orogenic belt is formed by the strong convergence between the Philippine Sea Plate and the Eurasian Plate. The detailed mountain building process is still under debated largely due to the poor constraint of deep crustal structures, particularly the geometry at the Moho-depth. Here the Moho-refracted P waves are identified from the seismic data recorded by a dense seismic array (Formosa Array) in northern Taiwan. Although the refracted seismic energy is often weak at each individual station, the waveform similarity recorded at the nearby stations provides a reliable constraint for estimating the apparent velocity recorded by the dense seismic array. The forward modeling of the observed Moho-refracted P waves shows a larger crustal thickness (~ 52 km) beneath the Backbone Ranges than beneath the adjacent Hsuehshan Ranges (~ 36 km). Such a result is not only confirming the Moho variations along a few of the NW-SE profiles from the previous studies, but also showing the strong Moho variation is well extended along the NE-SW direction. The large change in the crustal thickness across the Taiwan orogeny strongly indicate that the orogenic deformation in Taiwan might extend beyond the shallow crust, possibly involving in the deep crust and upper mantle. The Taiwan orogeny may not be reaching to the isostatic equilibrium yet.

Surface Uplift and Topographic Rejuvenation of a Tectonically Inactive Range: Insights From the Anti‐Atlas and the Siroua Massif (Morocco)

AbstractThe Atlas‐Meseta intracontinental orographic system of Morocco experienced recent, large‐scale surface uplift as documented by elevated late Miocene, shallow‐water marine deposits exposed in the Middle Atlas Mountains. The Anti‐Atlas Mountains do not present any stratigraphic records that document regional vertical movements, however, the presence of a high‐standing, erosional surface, and the transient state of the river network, provides insights into the uplift history of the belt and the mechanisms that drove it. Here, we combine geomorphic and stream profiles analyses, celerity of knickpoints and linear inverse landscape modeling with available geological evidence, to decipher the spatial and temporal variations of surface uplift in the Anti‐Atlas and the Siroua Massif. Our results highlight the presence of a transient landscape and document a long wave‐length topographic swell (∼100 × 600 km) with a maximum surface uplift of ∼1,500 m in the Siroua Massif and ∼1,100 m in the central Anti‐Atlas most likely starting from ∼14 to 10 Ma. Surface uplift occurred in association with the onset of late Miocene magmatism in the Siroua and Saghro Massif and contractional deformation in the High Atlas. Regional surface uplift was most likely due to deep‐seated mechanism, such as asthenospheric upwelling. Additional processes such as magma injection and faulting contributed to the surface uplift of the Siroua Massif. Overall, our approach allows to quantitatively constrain the transient state of the landscape and the contribution of regional surface uplift on mountain building processes.

Sunda subduction drives ongoing India-Asia convergence

The Himalaya and the Tibetan plateau, the highest mountain range on Earth, have been growing continuously for the last 55 Myr since India collided with Eurasia. The forces driving this protracted mountain building process are still not fully understood. Although subduction zones are considered the main driving force for plate tectonics, mantle flow and plate boundary migration, their role in driving the Indian indentation and the northward movement of the collisional plate boundary is yet to be tested with geodynamic models. Here, we use four-dimensional geodynamic physical models to show that active subduction of the Indo-Australian plate along the Sunda subduction zone is probably the main driver of the India-Asia convergence, Indian indentation, and the consequent growth of the Himalaya-Tibet mountains, and also the present-day eastward crustal displacement of southeast Asia. Our experiments show that at least 880 km of northward indentation of India would not have ensued in the absence of the lateral subduction zones. Our experiments with lateral subduction zones show that subduction of the Indian continental lithosphere is maximum close to the eastern and western syntaxes, which ranges between 450 and 500 km. Based on our model results we propose that the protracted growth of collisional mountains on Earth, like the Himalaya, is highly dependent on nearby active subduction zones.

Geodetic modelling of the 2022Mw 6.6 Menyuan earthquake: insight into the strain-partitioned northern Qilian Shan fault system and implications for regional tectonics and seismic hazards

SUMMARYStrain partitioning between strike-slip faults in mountains and thrust faults in the foreland is a typical mountain building process to accommodate oblique plate convergence. Studying the geometry and movement of such strain-partitioned fault systems is key to understanding the mountain building process and related seismic hazards. The 2022 Mw 6.6 Menyuan earthquake is the largest strike-slip earthquake to have ruptured the northern Qilian Shan fault system in the modern geodetic era. We combined satellite and field observations to determine the fault geometry and coseismic slip distribution in the Menyuan earthquake, and link the distribution of coseismic slip with the pattern of interseismic strain accumulation within the northern Qilian Shan from our geodetic slip model. We find that the Menyuan earthquake ruptured a 25 km-long section of the left-lateral Longlongling Fault between the surface and 7 km depth. The maximum slip was 4 m at 3–4 km depth. Damage to a high-speed railway tunnel recorded a fault offset of 2.7 m at a depth of 200 m compared to 2.5–3.0 m on the surface, suggesting that dispersion of the rupture through unconsolidated shallow sediments was limited, at least at the tunnel site. We also determined the pattern of interseismic deformation prior to the earthquake using Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data. We found the interseismic geodetic data can be explained by the oblique movement of a low-angle décollement beneath the Qilian Shan rather than a strain partitioning fault system. We suggested that the strike-slip faults and foreland thrusts are separated by a creeping décollement, which would act as a barrier to stop the cascading rupture of the strike-slip and thrust fault.

Miocene rapid strike-slip faulting along the Altyn Tagh Fault, North Tibet: Insight from sedimentology records in the Tula and Qaidam basins

A better understanding of the kinematics of strike-slip faults in northern Tibet greatly affects our knowledge of the Cenozoic growth of the Tibetan plateau. Despite recent achievements in geometry and slip rate of the modern Altyn Tagh fault as well as the exhumation history of the basement rocks along the fault, how did the Altyn Tagh fault construct through time remains controversial. Here we combined sedimentological and geochronological datasets from the Anxi section in the remote Tula basin to reveal the source to sink system across the western segment of the Altyn Tagh fault through time. Compared with sedimentary records from other two sections (namely Caishiling and Eboliang sections) at the central segment of the Altyn Tagh fault, our study reveals that the Altyn Tagh Range consistently served as the major source of clastic materials for the Tula basin from the Early Jurassic to the Early Miocene, suggesting negligible strike-slip motion along the western segment of the Altyn Tagh fault before Miocene. However, the sediments in both Caishiling and Anxi sections show a consistent variation of composition through time, likely corresponding to the pre-Early Miocene strike-slip faulting along the central segment of the Altyn Tagh fault. Integrated with existing evidence of multistage mountain building process in orogenic belts in the northern Tibet, our inference of the differential initial rupture on different parts of the Altyn Tagh fault highlights a Middle Miocene reorganization of the deformation in the northern Tibetan plateau.

Suprasubduction ophiolite (SSZ) components in a middle to lower upper Jurassic Hallstatt Mélange in the Northern Calcareous Alps (Raucherschober/Schafkogel area)

The Northern Calcareous Alps in the Western Tethys realm were affected in Middle to Late Jurassic times by a mountain building process triggered by ophiolite obduction similar to that in the Inner Western Carpathians or Inner Dinarides. In contrast to these other mountain ranges, in the Northern Calcareous Alps the obducted ophiolites or ophiolite derived components in the Bathonian-Oxfordian mélanges are missing. Cr-spinels in Kimmeridgian basinal deposits are the oldest known relics of a Jurassic ophiolite obduction. This study reveals new data from a newly detected Hallstatt Mélange below the Late Jurassic Plassen Platform in the southeastern Northern Calcareous Alps (Raucherschober/Schafkogel area). Upper Triassic Hallstatt Limestone blocks from the former distal northwestern continental margin of the Neo-Tethys Ocean, as well as ophiolite and radiolarite blocks from the Neo-Tethys Ocean floor rest within an upper Middle to lower Upper Jurassic radiolaritic-argillaceous matrix. Ophiolitic blocks show calc-alkaline volcanic arc affinity, defining the rocks as the product of intra-oceanic subduction and the formation of an early arc during stacking of the oceanic crust. Resedimented ribbon radiolarite blocks deposited above the newly formed suprasubduction (SSZ) ophiolites in the Neo-Tethys Ocean east of the island arc have a Middle Jurassic age. Later, at a time of decreasing tectonic activity, the Hallstatt Mélange was sealed by the Kimmeridgian-Tithonian Plassen Carbonate Platform, showing a shallowing-upward trend.

Middle Miocene paleoenvironmental change and paleoelevation of the Lunpola Basin, Central Tibet

The Tibetan region underwent diachronous differential rises during the Cenozoic era. Its elevation history is critical for understanding the processes of mountain building, and for evaluating its climatic effects on atmospheric circulations. However, knowledge of paleoenvironment and paleoelevation in different geological domains of the Tibetan Plateau is still limited. Here we provide chronology of lacustrine deposits of the Upper Dingqing Formation in the Lunpola Basin in Central Tibet, which yields a middle Miocene age (15–12 Ma). Both stable isotope and palynological records indicate a climatic transition at ∼13.8 Ma marked by a change towards a cooler and drier climate after the Middle Miocene Climate Transition driven by global climatic cooling. Pollen and spore assemblages suggest that the vegetation was dominated by mixed coniferous–broadleaved forest with some grasslands. New palynological elevation reconstructions constrained by the co-existence approach yield a maximum paleo-elevation of 3100–3600 m (average 3350 m) of the Lunpola Basin, which was consistent with a maximum elevation of 3400 m based on floating-leaf plants. Our new data do not support previous views that the Lunpola Basin achieved its current elevation as early as the late Eocene or early Oligocene, but instead we suggest there has been a rise of up to 1200 m since 12 million years ago. Our results bring new light on both the paleoenvironmental reconstructions and the paleo-elevation estimates of the Lunpola Basin, Central Tibet.

4D reconstruction of the Doldenhorn nappe-basement system in the Aar massif: Insights into late-stage continent-continent collision in the Swiss Alps

The inversion of passive margins and their transportation into fold-and-thrust belts is a critical stage of mountain-building processes. In this study, we selected the Doldenhorn Nappe and Aar/Gastern Massifs (Central Swiss Alps) system as an ideal laboratory to document the impact of inherited structures, their along-strike variations, and basement tectonics on the evolution of a fold-and-thrust belt. Three-dimensional geological modelling and cross-section restoration allowed us to reconstruct the 4D evolution of the investigated area during the late-stage Alpine orogeny (30 to 0 Ma). Our results demonstrate that: (i) the Doldenhorn Nappe is the product of the inversion of an asymmetric half-graben basin; (ii) variations in incipient basin sediment thicknesses correlate directly with the along-strike variation of the deformation of the Doldenhorn Nappe; and (iii) the multiphase thick-skinned deformation that overprinted the Doldenhorn Nappe from 22 Ma until today changed the shape of the Doldenhorn Nappe and Aar/Gastern Massifs. This reconstruction shows how thin-skinned nappe formation mechanisms and the nappe geometries are controlled by the initial basement geometry and by the rheological strength contrasts between basement and cover sediments. Basement-involved uplift and shortening controls then the late-stage collisional 3D overprint and mechanics of the fold-and-thrust belts.

Extensional mountain building along convergent plate boundary: Insights from the active Taiwan mountain belt

Abstract Late brittle extension is a common feature in orogenic belts, and its role in mountain building processes is still the subject of debate. Its timing relationship with crustal thickening, the building of topography, basin infill, and rock exhumation are of key importance in determining whether it is a major factor in orogenic development or merely causes near-surface secondary effects. We examined this question in relation to the active arc-continent collision of Taiwan, studying its structural evolution by integrating new and critical geochronological results for tensile vein filling of hinterland metamorphic terrane with syn-collision deposition records. Acceleration of rock exhumation and molasse deposition was found to be coeval with the initiation of brittle tensile structures at ca. 1.6 Ma, which was long overdue as continental subduction started well before 6.5 Ma in central to northern Taiwan. The topographic mountain of Taiwan was thus constructed when the upper crust of the thickened orogenic prism turned extensional, as orographic elevation and relief are prerequisites for molasses production. Syn-collisional brittle extension is therefore proposed as a possible facilitator of both augmented extrusive exhumation and the formation of orography.

High pressure-temperature phase equilibrium studies on Martian basalts: Implications for the failure of plate tectonics on Mars

Mars lacks ongoing tectonic activities such as volcanism and mountain-building processes. Modern plate tectonic movements on the Earth's surface are driven primarily by the descent of subducted slabs into the mantle. Slab crust made of dense eclogite metamorphosed from the Mid-Ocean Ridge Basalt provides an important driving force for slab subduction. Thus, mantle convection inside Mars can be hindered if the density contrast between Martian slab crust and the ambient Martian mantle is sufficiently smaller than that of Earth. To evaluate this hypothesis, we carried out high pressure-temperature phase equilibrium experiments on three different Martian basalts: Yamato 980459, NWA 8159, and GUSEV basalt (Humphrey). The GUSEV basalt and NWA 8159 undergo partial or complete melting along the Martian areotherm due to their high Fe content, suggesting that both compositions are geochemically evolved. Yamato 980459, the nearly primitive Martian basalt, on the other hand, would transform to a low-density eclogite at a depth of ∼250 km. The density contrast between a Martian crustal slab made of Yamato 980459, and the ambient Martian mantle is much smaller than that for Earth's mantle. Calculated slab sinking torques and velocities further suggest that sustained buoyancy-driven subduction of thin slabs was unlikely early in Martian history. Additional experiments exploring wider composition and pressure-temperature ranges are needed to fully understand the consequences of Martian mantle compositions and cooling history for the tectonic history of Mars.