We give an explanation for the polarity, localization, shape, size, and initiation of subduction zones on Earth. By considering a soft, thin, curved lithospheric cap with either elastic or viscous rheology supported by a thick, nearly incompressible mantle, we find two different characteristic subduction geometries arise depending on boundary conditions: (1) plate boundaries where subduction results primarily from the gravitational body force (free subduction) have characteristic plate lengths and form arc‐shaped dimpled segments resulting from the competition between bending and stretching in edge buckling modes of thin spherical shells, and (2) subduction zones due to localized applied loads that push one slab of thin, positively buoyant lithosphere beneath an overriding plate (forced subduction) form localized straight segments, consistent with the deformation of indented spherical shells. Both types of subduction are nonlinear subcritical instabilities, so small perturbations in the mechanical properties of the lithosphere have pronounced effects on subduction initiation and evolution. Yet in both cases, geometric relationships determined by the shape of the Earth itself play the most critical role in controlling the basic morphology and characteristic length scales of subduction zones.
Arc volcanism across Iran is dominated by a Paleogene pulse, despite protracted and presumably continuous subduction along the northern margin of the Neotethyan ocean for most of Mesozoic and Cenozoic time. New U‐Pb and 40Ar/39Ar data from volcanic arcs in central and northern Iran constrain the duration of the pulse to ∼17 Myr, roughly 10% of the total duration of arc magmatism. Late Paleocene‐Eocene volcanic rocks erupted during this flare‐up have major and trace element characteristics that are typical of continental arc magmatism, whereas the chemical composition of limited Oligocene basalts in the Urumieh‐Dokhtar belt and the Alborz Mountains which were erupted after the flare‐up ended are more consistent with derivation from the asthenosphere. Together with the recent recognition of Eocene metamorphic core complexes in central and east central Iran, stratigraphic evidence of Eocene subsidence, and descriptions of Paleogene normal faulting, these geochemical and geochronological data suggest that the late Paleocene‐Eocene magmatic flare‐up was extension related. We propose a tectonic model that attributes the flare‐up to decompression melting of lithospheric mantle hydrated by slab‐derived fluids, followed by Oligocene upwelling and melting of enriched mantle that was less extensively modified by hydrous fluids. We suggest that Paleogene magmatism and extension was driven by an episode of slab retreat or slab rollback following a Cretaceous period of flat slab subduction, analogous to the Laramide and post‐Laramide evolution of the western United States.
Iranian arc volcanism is dominated by a Paleogene flare‐up
The volcanic flare‐up overlaps in time with a phase of extensional tectonism
The extensional flare‐up is ascribed to Neotethyan slab rollback
Much remains to be understood about the links between regional vertical axis rotations, continental extension, and shortening. In western Turkey, Miocene vertical axis rotations have been reported that occur simultaneously with the extensional exhumation of the Menderes metamorphic core complex, which has been related to back‐arc extension in the eastern part of the Aegean back arc. In this paper we explore the spatial and temporal relationships between vertical axis rotations in southwestern Turkey and extensional unroofing of the Menderes Massif. To this end, we provide a large set of new paleomagnetic data from western Turkey, and integrate these with the regional structural evolution to test the causes and consequences of oroclinal bending in the Aegean region. The Lycian Nappes and Bey Dağları are shown to rotate ∼20° between 16 and 5 Ma, defining the eastern limb of the Aegean orocline. This occurred contemporaneously with the exhumation of the central Menderes Massif (along extensional detachments) and after the latest Oligocene to early Miocene exhumation of the northern and southern Menderes massifs. Exhumation of the latter two was not associated with vertical axis rotations. The lower Miocene volcanics in the region from Lesbos to Uşak, to the north of the central Menderes Massif underwent a small clockwise rotation, insignificant with respect to Eurasia. This shows that exhumation of the central Menderes Massif was associated with a vertical axis rotation difference between the northern and southern Menderes massifs of ∼25°–30°. This result is in excellent agreement with the angle defined by the trends of Büyük Menderes and Alaşehir detachments, as well as the angle defined by the regionally curving stretching lineation pattern across the central Menderes Massif. These structures define a pivot point (rotation pole) for the west Anatolian rotations. The rotation of the southern domain, including the southern Menderes Massif, the Lycian Nappes, and Bey Dağları, must have led to N–S contraction east of this pole. The eastern limit of the rotating domain is formed by the transpressional couple of the Aksu thrust and Kırkkavak Fault in the center of the Isparta angle. Previously reported clockwise rotations in the volcanic fields near Afyon may be the result of distributed N–S shortening east of the pivot point. The precise accommodation of this shortening history remains open for investigation. Late Oligocene to early Miocene extension in the eastern part of the Aegean back arc was NE–SW oriented and likely bounded by a discrete transform. This transform may be associated with an early evolution of the eastern Aegean subduction transform edge propagator fault. Oroclinal bending in the west Anatolian region is likely related to a reconnection of the eastern part of the Aegean orocline with the African northward moving plate in tandem with roll back in the Aegean back arc, comparable to a recently postulated scenario for western Greece.
The topographically prominent Sierra Nevada de Santa Marta forms part of a faulted block of continental crust located along the northern boundary of the South American Plate, hosts the highest elevation in the world (∼5.75 km) whose local base is at sea level, and juxtaposes oceanic plateau rocks of the Caribbean Plate. Quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary, and the driving forces of rock uplift along the active margin. The Sierra Nevada Province of the southernmost Sierra Nevada de Santa Marta exhumed at elevated rates (≥0.2 Km/My) during 65–58 Ma in response to the collision of the Caribbean Plateau with northwestern South America. A second pulse of exhumation (≥0.32 Km/My) during 50–40 Ma was driven by underthrusting of the Caribbean Plate beneath northern South America. Subsequent exhumation at 40–25 Ma (≥0.15 Km/My) is recorded proximal to the Santa Marta‐Bucaramanga Fault. More northerly regions of the Sierra Nevada Province exhumed rapidly during 26–29 Ma (∼0.7 Km/My). Further northward, the Santa Marta Province exhumed at elevated rates during 30–25 Ma and 25–16 Ma. The highest exhumation rates within the Sierra Nevada de Santa Marta progressed toward the northwest via the propagation of NW verging thrusts. Exhumation is not recorded after ∼16 Ma, which is unexpected given the high elevation and high erosive power of the climate, implying that rock and surface uplift that gave rise to the current topography was very recent (i.e., ≤1 Ma?), and there has been insufficient time to expose the fossil apatite partial annealing zone.
Exhumation during 65–58 Ma was driven by the accretion of the Caribbean Plateau
The highest exhumation rates within the cordillera occurred during 26–29 Ma
Topography formed when the present surface was at temperatures lower than 60 C
This paper documents relationships between deformation and magmatic activity that occurred in the central part of eastern Mongolia during late Mesozoic continental‐scale NW‐SE extension. Two coarse‐grained, biotite‐bearing, syn‐tectonic intrusions are described. The Nartyn granite that extends over an area greater than 30 by 10 km was emplaced within low‐grade metasediments and shows a weak pervasive, magmatic fabric reworked by solid‐state deformation along its margins. The northwestern roof of the granite is marked by a normal shear zone, the Choyr Shear Zone, characterized by top‐to‐the‐NW motions. The shear zone is overlain by the Choyr Basin, which is filled with unmetamorphosed continental sedimentary rocks of early Cretaceous ages. From structural and geochronological data, we propose that the Nartyn massif was emplaced as a flat laccolith‐type intrusion at ca. 136–130 Ma during crustal thinning. The Altanshiree granite, located ∼140 km east of the Nartyn granite, is a syn‐kinematic pluton of similar age (134–128 Ma), also emplaced during crustal thinning. In the Nartyn and Altanshiree areas, extension implies pervasive crustal thinning, combined with limited exhumation. These areas are different from classical metamorphic core complexes, where strong strain localization along detachments induces exhumation of hot middle to lower crust. Results also suggest that early Cretaceous syn‐extensional intrusions are an important feature of the tectonic history of eastern Mongolia.
Laccoliths emplaced during regional‐scale lower Cretaceous extension
Crustal extension started at least at 138 Ma
Extension has affected a previously hot crustal domain
Both the timing and mechanism for the removal of a ∼150–250 km wide forearc block from southern Mexico during the Cenozoic are controversial. Principal competing hypotheses are (1) removal due to sinistral strike‐slip shear, in which slow, diachronous removal of the Chortis Block throughout the Cenozoic is inferred, and (2) removal due to subduction erosion, in which rapid removal of a large forearc block during the late Oligocene/early Miocene is inferred to be synchronous with the rapid landward migration of the southern Mexican arc. New data indicate northeast‐directed back‐thrusting in (1) the Chacalapa shear zone west of −96.5°E, with the timing of shear deformation bracketed by a 25.5 ± 0.5 Ma U/Pb zircon age and a 20.7 ± 0.6 Ma Ar/Ar biotite age, and (2) in an unnamed shear zone to the south, with the timing of deformation bracketed by a 27.5 ± 0.5 Ma U/Pb zircon age and a 25.1 ± 0.2 Ma Ar/Ar biotite age. Zircon and biotite ages date the emplacement and cooling of deformed plutons, respectively. The observed back‐thrusting is consistent with a model of forearc removal due to subduction‐erosion processes because it is evidence for subduction‐orthogonal shortening occurring within the upper plate just before the landward migration of the southern Mexican arc. Rapid subduction of the southern Mexican forearc could have recycled continental lithosphere into the upper mantle at a rate up to half the global average rate of subduction erosion during the late Oligocene/early Miocene.
Late Oligocene/early Miocene back‐thrusting is preserved in southern Mexico
Back‐thrusting is synchronous with rapid landward arc migration
Forearc removal due to subduction erosion may have caused the back‐thrusting
The mechanics of low‐angle normal faulting and metamorphic core complexes continue to be a subject of debate. We investigate the conditions, timing, and kinematics of slip in the late, upper‐crustal stages of core complex evolution of the Ruby Mountains detachment fault at the well‐exposed Secret Pass locality with an X‐ray diffraction (XRD) and Ar‐Ar study of clay gouge samples from three separate faults, two from the low‐angle detachment system and one from a high‐angle normal fault that soles into the main detachment fault system. XRD analysis and modeling of XRD analysis show that authigenic illite‐rich illite/smectite (I/S) in gouge at Secret Pass is distinguishable from clay phases in hanging wall rocks because the I/S in the gouges contains only one‐water layer as opposed to the more common two‐water I/S phases found in both the hanging wall and footwall. Ar‐Ar ages for the monomineralic one‐water I/S found in the hanging wall high‐angle fault, the main detachment, and a low‐angle normal fault structurally above the main detachment are 11.6 ± 0.1 Ma, 12.3 ± 0.1 Ma, and <13.8 ± 0.2 Ma, respectively. The not‐quite‐flat Ar‐Ar spectra indicate the gouge illites grew over some interval of time and not in discrete events. The nearly overlapping ages indicate that gouge formation and thus the last major period of activity on the detachment were at 11–13 Ma and were active coevally as part of a kinematically linked fault system with the main detachment active at dips <45° and possibly as low as 22°.
To elucidate factors controlling the geometry of, and kinematics associated with, prominent upper‐crustal structures in the Eastern Cordillera of NW Argentina, structural analyses of key areas were complemented by fault slip analysis, remote sensing and 3D representation of prominent faults. The analyses revealed that deformation during Tertiary to Quaternary times was accomplished mostly by prominent orogen‐parallel reverse faults, the geometry of which was significantly influenced by Paleozoic and Cretaceous planar structures. Locally, this deformation was preceded by kilometer‐scale doming of upper‐crustal rocks. Analysis of 767 brittle faults at 67 stations in the studied areas indicates that local upper‐crustal doming and orogen‐parallel reverse faults formed chiefly under NW‐SE and E‐W shortening. Shortening directions inferred from fault slip data portray the kinematics of first‐order faults and folds. More specifically, shortening directions are mostly uniform with respect to first‐order structures. Age and kinematics of deformation inferred from brittle fault analysis in the study areas is consistent with equivalent data compiled from other parts of the Eastern Cordillera, Puna and Pampean Ranges. Collectively, the data is at variance with hypotheses relating late Tertiary to Quaternary deformation in the Eastern Cordillera to plate (boundary) kinematics. The kinematics of intracontinental deformation in the southern Central Andes points to deformation partitioning of upper crust as a consequence of bulk E‐W shortening. We, therefore, caution the usage of brittle fault‐kinematics in continental interiors as an indication for plate‐kinematic changes.
Kinematics of intracontinental deformation is unrelated to plate motion
Intracontinental deformation is controlled by pre‐Neogene structures
Kinematics of brittle shear faults portrays first‐order structures
Map‐scale curvature is a fundamental feature of most contractional orogenic belts and is central to understanding the kinematic and dynamic evolution of mountain systems. Paleomagnetic analysis, combined with detailed structural studies, is the most robust means of quantifying vertical axis rotations that produce curvature over a range of temporal and spatial scales. This paper explores how vertical axis rotations can best be evaluated for multiple data sets by applying a weighted least squares method to the classic strike test. This refined method provides measures of best fit slope, confidence interval, and goodness of fit between map‐scale structural trend, paleomagnetic rotations, and deformation fabric orientations. Structural trend is estimated by averaging fold axial trace, formation contact, and bed strike data over kilometer‐scale areas. Paleomagnetic and deformation fabric site‐mean orientations and measurement uncertainties are estimated using vector and bootstrap statistics. Weighting factors are estimated from combined measurement uncertainty, structural noise related to small‐scale block rotation and stress/strain refraction, and variations in restoration paths. The number of sites needed to obtain a significant confidence interval in strike test slope is a function of combined uncertainty in paleomagnetic or deformation fabric directions and the total range in structural trend around a curved orogen. Improved estimates of strike test slope and rotation thus require systematic sampling with a wide distribution of sites, evaluation of appropriate weighting factors, and statistical analysis. A case study is presented that highlights application of this refined method to paleomagnetic and deformation fabric data sets from the Wyoming salient of the Sevier thrust belt. Paleomagnetic data yield a strike test slope of 0.76 ± 0.11, indicating that the Wyoming salient is a progressive arc, with ∼3/4 secondary curvature related to vertical axis rotation synchronous with large‐scale thrusting and ∼1/4 initial curvature. Finite strain, anisotropy of magnetic susceptibility, and mesoscopic structural orientations, which are related to early layer‐parallel shortening, all yield strike test slopes of ∼0.9 ± 0.1. Comparing these slopes with paleomagnetic results indicates that deformation fabrics had initial curvature and thus cannot be used alone to accurately estimate rotations. By integrating systematic paleomagnetic and structural data using statistical analysis, curvature models for the Wyoming salient are closely constrained.
In Central Asia's Tien Shan, deformation is distributed across the wide orogen, a characteristic of intracontinental mountain building. Active faults are commonly found within intramontane basins that separate its constituent ranges. In order to explore the controls on this intramontane basin deformation, we study the Naryn Basin of south‐central Kyrgyzstan. A series of five balanced cross‐sections reveals a transition in patterns of faulting from faults confined to basin margins to faults focused within the basin center. The 20‐km‐wide eastern Naryn Basin displays deformation attributed to low‐angle splays of the northern, basin‐bounding fault. In the 40‐km‐wide western Naryn Basin, the pattern of deformation linked to the northern range remains, but is accompanied by steeper faults that dip both south and north without being directly linked to the basin‐bounding fault. We compare these cross‐sections to synthetic aperture radar interferometry (InSAR) measurements of surface deformation. Profiles of InSAR‐derived surface deformation rates across the Naryn Basin reveal that in the west, deformation is distributed across the broad basin interior, whereas in the east, rapid uplift is concentrated at the margin of the narrower basin. From the geodetic and structural data, we infer that in the western Naryn Basin, deformation has migrated away from the northern basin margin and into the interior. Deformation of the eastern basin interior, however, remains linked to the basin‐bounding fault. A simple mechanical model demonstrates that basin width may control basin deformation whereby basin‐interior faulting in the narrow, eastern Naryn Basin is inhibited by the overburden of adjacent ranges.
The Naryn Basin is actively deforming
InSAR and balanced cross sections show a structural transition
Basin width may control the style of basin‐interior faulting
Tectonic interpretation of images and models of the subsurface derived from geophysical data must be based on a firm foundation of the following four precepts; (1) the data must be shown to be internally consistent, (2) the dimensionality assumed for modeling must be shown to be valid, and if 2D then the adopted strike direction must be shown to be consistent with the data, (3) the final model must fit the data to within their statistical limits, and (4) resolution of interpreted features must be demonstrated. Particularly in a highly nonlinear problem, such as electromagnetic geophysics, without examining model resolution it is impossible to know the veracity of the interpretation.
We analyzed the structure and evolution of the external Calabrian Arc (CA) subduction complex through an integrated geophysical approach involving multichannel and single‐channel seismic data at different scales. Pre‐stack depth migrated crustal‐scale seismic profiles have been used to reconstruct the overall geometry of the subduction complex, i.e., depth of the basal detachment, geometry and structural style of different tectonic domains, and location and geometry of major faults. High‐resolution multichannel seismic (MCS) and sub‐bottom CHIRP profiles acquired in key areas during a recent cruise, as well as multibeam data, integrate deep data and constrain the fine structure of the accretionary wedge as well as the activity of individual fault strands. We identified four main morpho‐structural domains in the subduction complex: 1) the post‐Messinian accretionary wedge; 2) a slope terrace; 3) the pre‐Messinian accretionary wedge and 4) the inner plateau. Variation of structural style and seafloor morphology in these domains are related to different tectonic processes, such as frontal accretion, out‐of‐sequence thrusting, underplating and complex faulting. The CA subduction complex is segmented longitudinally into two different lobes characterized by different structural style, deformation rates and basal detachment depths. They are delimited by a NW/SE deformation zone that accommodates differential movements of the Calabrian and the Peloritan portions of CA and represent a recent phase of plate re‐organization in the central Mediterranean. Although shallow thrust‐type seismicity along the CA is lacking, we identified active deformation of the shallowest sedimentary units at the wedge front and in the inner portions of the subduction complex. This implies that subduction could be active but aseismic or with a locked fault plane. On the other hand, if underthrusting of the African plate has stopped recently, active shortening may be accommodated through more distributed deformation. Our findings have consequences on seismic hazard, since we identified tectonic structures likely to have caused large earthquakes in the past and to be the source regions for future events.
Overall geometry, tectonic processes, and kinematics of the subduction complex
Segmentation of the continental margin in two different lobes
Location and geometry of active faults absorbing plate motion
The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian‐Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Béarn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian‐early Albian. They are unconformably overlain by the Albian‐Cenomanian flysch formations and have experienced high temperature‐low pressure mid‐Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust‐mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low‐angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm.
New detrital zircon and isotopic (Nd and Sr) analyses from the eastern Pamir provide information on the depositional age and tectonic terrane affiliation of regional metamorphic terranes. Our results show the following. First, detrital zircon analyses from metasedimentary units along the Kongur Shan extensional system dominantly yield Triassic maximum depositional ages, with a similar age distribution to the Tibetan Songpan‐Ganzi terrane. Further, zircon analyses from quartzofeldspathic gneisses in the core of the Muztaghata massif show the protoliths are Triassic granites. These units are interpreted to be part of the Permian‐Triassic Karakul‐Mazar arc‐accretionary complex terrane. Second, εNd(0) compositions of Triassic granites overlap with other metasedimentary units not analyzed for detrital zircons and are also interpreted to be part of the Karakul‐Mazar terrane. Third, schists in the Sares‐Muztaghata gneiss dome structurally above Triassic orthogneisses yield an Ordovician maximum depositional age with a distinct detrital age distribution similar to the Tibetan Qiangtang terrane and are interpreted to be part of the Central Pamir terrane. Finally, Triassic and Ordovician schists along the Muztaghata massif record an Early Jurassic metamorphic event interpreted to date south‐directed subduction of the Karakul‐Mazar terrane beneath the Central Pamir during final closure of the Paleo‐Tethys. These results, integrated with previously published results and field relations, reveal a complex Mesozoic to Cenozoic interleaving of tectonic terranes in the eastern Pamir with emplacement of the Karakul Mazar terrane both above and below the Kunlun and Central Pamir terranes to the north and south, respectively.
Metamorphic lithologies in the northeast Pamir are Triassic in age
Most of the northeast Pamir is part of the Karakul‐Mazar terrane
The central Pamir are correlative to the Qiangtang terrane of Tibet
A high‐temperature shear zone, Toijem shear zone, with a top‐to‐the‐SW sense of shear affects the core of the Higher Himalayan Crystallines (HHC) in western Nepal. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high‐pressure metamorphism deformed under the kyanite stability field. PT conditions indicate that the footwall experienced higher pressure (∼9 kbar) than the hanging wall (∼7 kbar) and similar temperatures (675°–700°C). Monazite growth constrains the initial activity of the shear zone at 25.8 ± 0.3 Ma, before the onset of the Main Central Thrust zone, whereas the late intrusion of a crosscutting granitic dike at 17 ± 0.2 Ma limits its final activity. Monazites in kyanite‐bearing gneisses from the footwall record prograde metamorphism in the HHC from ∼43 to 33 Ma. The new data confirm that exhumation of the HHC started earlier in western Nepal than in other portions of the belt and before the activity of both the South Tibetan Detachment System (STDS) and Main Central Thrust (MCT) zones. As a consequence, western Nepal represents a key area where the channel‐flow‐driven mechanism of exhumation, supposed to be active from Bhutan to central‐eastern Nepal, does terminate. In this area, exhumation of crystalline units occurred by foreland propagation of ductile and, subsequently, brittle deformation.
Rocks metamorphosed to high temperatures and/or high pressures are rare across the Himalayan orogen, where peak metamorphic conditions recorded in the exposed metamorphic core, the Greater Himalayan Sequence (GHS), are generally at middle to upper amphibolite facies. However, mafic garnet‐clinopyroxene assemblages exposed at the highest structural levels in Bhutan, eastern Himalaya, preserve patchy textural evidence for early eclogite‐facies conditions, overprinted by granulite‐facies conditions. Monazite hosted within the leucosome of neighboring granulite‐facies orthopyroxene‐bearing felsic gneiss yields LA‐MC‐ICP‐MS U‐Th‐Pb ages of 13.9 ± 0.3 Ma. Monazite associated with sillimanite‐grade metamorphism in granulite‐hosting migmatitic gneisses yields U‐Th‐Pb rim ages between 15.4 ± 0.8 Ma and 13.4 ± 0.5 Ma. Monazite associated with sillimanite‐grade metamorphism in gneiss at structurally lower levels yields U‐Pb rim ages of 21–17 Ma. These data are consistent with Miocene exhumation of GHS material from a variety of crustal depths at different times along the Himalayan orogen. We propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming Indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh‐pressure eclogites in the western Himalaya. Their formation and exhumation in the Miocene therefore do not require diachroneity in the timing of the initial India‐Asia collision.
We document a crustal‐scale structural model for the central Chile Andes based on seismicity and surface geology, which consists in a major east verging ramp‐detachment structure connecting the subduction zone with the cordillera. The ramp rises from the subducting slab at ∼60 km depth to 15–20 km below the western edge of the cordillera, extending eastward as a 10 km depth flat detachment. This structure plays a fundamental role in the Andean orogenesis because most of the shortening has been accommodated by structures rooted in it and allows the distribution of crustal thickening in a “simple shear deformation mode.” Indeed, despite shortening distribution being very asymmetric (∼16 km versus ∼70 km in the western and eastern side, respectively), the western side is higher and thicker than what is expected. Yield strength envelopes show strong rheological control on this structure. Vp and Vp/Vs variations in the upper mantle and in the deepest limit of the seismogenic interplate contact mark the intersection of the ramp with the slab, which coincides with the blueschist‐eclogite transition. Therefore, subduction processes would control the depth where the major east verging structure may merge with the slab. Such a ramp‐flat structure is observed in other parts of the Chilean margin; hence, it seems to be a first‐order feature in the Andean subduction zone. This structure delimitates upward the rocks, transmitting part of the plate convergence stress from the plate interface, and controls mountain‐building tectonics, thus playing a key role in the Andean orogeny.
The Sila Massif in the Calabrian Arc (southern Italy) is a key site to study the response of a landscape to rock uplift. Here an uplift rate of ∼1 mm/yr has imparted a deep imprint on the Sila landscape recorded by a high‐standing low‐relief surface on top of the massif, deeply incised fluvial valleys along its flanks, and flights of marine terraces in the coastal belt. In this framework, we combined river longitudinal profile analysis with hillslope erosion rates calculated by 10Be content in modern fluvial sediments to reconstruct the long‐term uplift history of the massif. Cosmogenic data show a large variation in erosion rates, marking two main domains. The samples collected in the high‐standing low‐relief surface atop Sila provide low erosion rates (from 0.09 ± 0.01 to 0.13 ± 0.01 mm/yr). Conversely, high values of erosion rate (up to 0.92 ± 0.08 mm/yr) characterize the incised fluvial valleys on the massif flanks. The analyzed river profiles exhibit a wide range of shapes diverging from the commonly accepted equilibrium concave‐up form. Generally, the studied river profiles show two or, more frequently, three concave‐up segments bounded by knickpoints and characterized by different values of concavity and steepness indices. The wide variation in cosmogenic erosion rates and the non‐equilibrated river profiles indicate that the Sila landscape is in a transient state of disequilibrium in response to a strong and unsteady uplift not yet counterbalanced by erosion.
Transient landscape evolution
Recent (300‐400 ka) increasing of the uplift rates