The Andean subduction zone is an excellent place to study deformation within a subducting slab as a function of depth, owing to the varying and well‐resolved geometry of the subducting Nazca slab ...beneath South America. Here we combine the results of source‐side shear wave splitting with the latest regional tomography model to isolate intraslab raypaths and determine the spatial distribution of anisotropy within the Nazca slab. We observe that in the upper mantle, the intraslab anisotropy appears strongest where the slab is most contorted, suggesting a strong link between anisotropy and subduction‐related slab deformation. We identify a second source of anisotropy (δt∼ 1 s) within the subducting slab at lower mantle depths (660–800 km). The surrounding mantle and transition zone appear largely isotropic, with deep anisotropy concentrated within the slab as it deforms while entering the higher‐viscosity lower mantle.
Plain Language Summary
Few observations exist of how a tectonic plate deforms as it descends deep into the Earth's interior at a subduction zone. Carefully selected seismic waves that mostly travel through this subducting plate, or slab, provide some of the most direct measurements of how the slab behaves as it sinks through the upper mantle (0–410 km) and the mantle transition zone (410–660 km). Studying the polarization of seismic waves allows us to detect and infer the pattern of deformation within the Earth's interior. Using this technique, we find that the Nazca slab in the Andean subduction zone in South America has undergone internal deformation during the process of subduction, in particular where the slab's 3‐D shape changes. Furthermore, we find that the deeper Nazca slab (≥660 km) appears to undergo further deformation as it interacts with the stiffer uppermost lower mantle.
Key Points
We find a notable presence of anisotropy within the Nazca slab in both the upper and lower mantle
Upper mantle slab anisotropy is strongest where the slab geometry is most contorted and deformed
Widespread anisotropy is observed at the base of Nazca slab where it penetrates the higher‐viscosity lower mantle
NNR‐MORVEL56, which is a set of angular velocities of 56 plates relative to the unique reference frame in which there is no net rotation of the lithosphere, is determined. The relative angular ...velocities of 25 plates constitute the MORVEL set of geologically current relative plate angular velocities; the relative angular velocities of the other 31 plates are adapted from Bird (2003). NNR‐MORVEL, a set of angular velocities of the 25 MORVEL plates relative to the no‐net rotation reference frame, is also determined. Incorporating the 31 plates from Bird (2003), which constitute 2.8% of Earth's surface, changes the angular velocities of the MORVEL plates in the no‐net‐rotation frame only insignificantly, but provides a more complete description of globally distributed deformation and strain rate. NNR‐MORVEL56 differs significantly from, and improves upon, NNR‐NUVEL1A, our prior set of angular velocities of the plates relative to the no‐net‐rotation reference frame, partly due to differences in angular velocity at two essential links of the MORVEL plate circuit, Antarctica‐Pacific and Nubia‐Antarctica, and partly due to differences in the angular velocities of the Philippine Sea, Nazca, and Cocos plates relative to the Pacific plate. For example, the NNR‐MORVEL56 Pacific angular velocity differs from the NNR‐NUVEL1A angular velocity by a vector of length 0.039 ± 0.011° a−1 (95% confidence limits), resulting in a root‐mean‐square difference in velocity of 2.8 mm a−1. All 56 plates in NNR‐MORVEL56 move significantly relative to the no‐net‐rotation reference frame with rotation rates ranging from 0.107° a−1 to 51.569° a−1.
Key Points
31 plates are added to MORVEL to describe geologically current plate motion
The no‐net‐rotation frame for these plates, NNR‐MORVEL56, is determined
NNR‐MORVEL56 differs significantly from NNR‐NUVEL1A and other realizations
•We reconstruct the evolution of the Nazca slab using plate kinematics and seismic tomography.•We run numerical models of subduction of oceanic lithosphere into lower mantle.•The onset of Andean ...orogeny is controlled by the penetration of slab into the lower mantle.
The Cordillera of the Andes is a double-vergent orogenic belt built up by thickening of South American plate crust. Several models provide plausible explanations for the evolution of the Andes, but the reason why shortening started at ∼50 Ma is still unclear. We explore the evolution of the subduction zone through time by restoring the position of the Nazca trench in an absolute reference frame, comparing its position with seismic tomography models and balancing the evolution of the subducting slab. Reconstructions show that the slab enters into the lower mantle at ∼50±10 Ma, and then progressed, moving horizontally at shallow lower mantle depth while thickening and folding in the transition zone. We test this evolutionary scenario by numerical models, which illustrate that compression in the upper plate intensifies once the slab is anchored in the lower mantle. We conclude that onset of significant shortening and crustal thickening in the Andes and its sustained action over tens of million years is related to the penetration of the slab into the lower mantle, producing a slowdown of lateral slab migration, and dragging the upper plate against the subduction zone by large-scale return flow.
Calculated Bouguer gravity anomalies from the Andean orogenic belt interpreted as derived from regional gravity data to aid understanding of the lithospheric structure and tectonic evolution of the ...belt. These anomalies reveal lithospheric structures distributed throughout the belt, including linear and circular structures. NE‐trending structures reflect sinistral transpression across the northern part of the belt, and NW‐trending structures represent dextral transtension in the southern part. These results are supported by gravity‐anomaly patterns that demonstrate mantle flow in a trench‐parallel direction both northward and southward away from the stagnation band that is beneath the subducting Nazca slab. This mantle flow has served as an important driving force in the evolution of the Andean orogenic belt. Features of the modified tectonic model of the Andean orogenic belt are consistent with the spatial variation in and interpretation of Bouguer gravity anomalies.
Flat-slab subduction occurs when the descending plate becomes horizontal at some depth before resuming its descent into the mantle. It is often proposed as a mechanism for the uplifting of deep ...crustal rocks ('thick-skinned' deformation) far from plate boundaries, and for causing unusual patterns of volcanism, as far back as the Proterozoic eon. For example, the formation of the expansive Rocky Mountains and the subsequent voluminous volcanism across much of the western USA has been attributed to a broad region of flat-slab subduction beneath North America that occurred during the Laramide orogeny (80-55 million years ago). Here we study the largest modern flat slab, located in Peru, to better understand the processes controlling the formation and extent of flat slabs. We present new data that indicate that the subducting Nazca Ridge is necessary for the development and continued support of the horizontal plate at a depth of about 90 kilometres. By combining constraints from Rayleigh wave phase velocities with improved earthquake locations, we find that the flat slab is shallowest along the ridge, while to the northwest of the ridge, the slab is sagging, tearing, and re-initiating normal subduction. On the basis of our observations, we propose a conceptual model for the temporal evolution of the Peruvian flat slab in which the flat slab forms because of the combined effects of trench retreat along the Peruvian plate boundary, suction, and ridge subduction. We find that while the ridge is necessary but not sufficient for the formation of the flat slab, its removal is sufficient for the flat slab to fail. This provides new constraints on our understanding of the processes controlling the beginning and end of the Laramide orogeny and other putative episodes of flat-slab subduction.
The largest tectonic relief breaking the Earth's surface (13km vertically) is at the subduction margin of the Andes, which generates routinely megathrust earthquakes (Mw>8.5) and drives the ...paradigmatic Andean orogen. Here we present key geologic evidence to reassess first-order features of geomorphology and tectonics across the Central Andes, where the orogen includes the Altiplano Plateau and attains its maximum integrated height and width. The Andean subduction margin has a stepped morphology dominated by the low-relief Atacama Bench, which is similar to a giant uplifted terrace, slopes gently over a width of 60–100km from the Andes to the Pacific, and runs over more than 1000km of coastal length. We find that the genesis of stepped morphology at the Andean seaboard is due to concomitant development of large west-vergent thrusts parallel to the subduction interface and increasing aridity in the Atacama Desert, which keeps an unprecedented large-scale record of interplaying tectonics and Cenozoic climate change. Incorporating our results with published geological knowledge demonstrates that Andean orogeny is characterized by trench-perpendicular (bivergent) and trench-parallel (bilateral) growth over the past 50Myr, associated with positive trench velocity toward the continent (trench advance) and subduction of a wide slab under South America. We hypothesize that a global plate tectonic reorganization involving long-lasting viscous mantle flow has probably forced both, Andean orogeny and global climate cooling since ~50Ma. In contrast, two important stepwise pulses of increasing aridity and trench-perpendicular Andean growth appear to be results of changes in erosion rates due to global Late Eocene and Middle Miocene cooling events.
The preservation of naturally mummified bodies in the Nazca drainage and Yauca Valley provided an opportunity to analyse for the first time which of the psychoactive plants were used on the southern ...Peruvian coast. Toxicological analysis allows us to better understand the ancient medicine, trade network and religiosity of the region of interest. Hair samples of 22 individuals (including four trophy heads) were examined using LC-MS/MS for the presence of coca alkaloids and metabolites (cocaine, benzoylecgonine, cocaethylene), mescaline, tryptamine, harmaline, and harmine. LC–MS/MS was performed using electrospray ionization (ESI) in the positive mode, multiple reaction monitoring, and a deuterated internal standard (Diazepam-D5). The limits of quantification achieved for analytes were from 1 to 5 ng/g. Recoveries ranged from 91,6 to 113,7%. The method demonstrated an intraday and interday precision CV of <15%.
The results of the study show that coca leaves were present on the southern Peruvian coast since the Early Nazca Period (100 BCE - 450 CE). The Nazca inhabitants were also positive tested for the presence of harmine and harmaline coming probably form Banisteriopsis caapi (the main compound of the hallucinogenic ayahuasca beverage), and the San Pedro cactus, a source of mescaline. This is the oldest archaeological evidence of the consumption of these two plants. In modern medicine, the properties of harmine have led to its use in anti-depression and anti-addiction treatment. Banisteriopsis caapi is native to the Amazonian rainforest and had to be the object of long-distance trade, which showed its important role in ancient medicine and rituals. San Pedro cactus is taken for its strong hallucinogenic properties and was detected in hair belonging to a child victim whose head was transformed into a trophy head. This is the first proof that some of the victims transformed into trophy heads were given stimulants prior to their death.
•Coca leaves have been consumed on the southern Peruvian coast since the Early Nazca Period (100 BCE-450 CE).•The study reveals the oldest case of the consumption of San Pedro and probably Banisteriopsis caapi.•The child victim whose head was transformed into a trophy head was intoxicated by San Pedro cactus.
We present analogue models simulating the subduction of a buoyant ridge oriented perpendicularly or obliquely with respect to the trench, beneath an advancing overriding plate. The convergence ...velocity is imposed by lateral boundary conditions in this experimental set. We analyze the three-dimensional geometry of the slab, the deformation and topography of the overriding plate. Experiments suggest that ridge subduction diminishes the dip of the slab, eventually leading to the appearance of a horizontal slab segment in case boundary conditions impose a rapid convergence. This result contrasts with that obtained in free subduction experiments, in which ridge subduction diminishes the convergence velocity which, in turn, increases the dip of the slab beneath the ridge. The slab dip decrease is accompanied by the indentation of the overriding plate by the ridge, resulting in arc curvature. Experiments suggest that indentation is larger for small convergence velocity and large slab dip. Ridge subduction also uplifts the overriding plate. Uplift first occurs close to the trench (~fore-arc area) and is accompanied by the flexural subsidence of the overriding plate behind the uplifted area (~back-arc subsidence). The uplifted area migrates within the overriding plate interiors following the appearance of a horizontal slab segment. These results are compared with natural examples of ridge subduction in the circum-Pacific area. They explain why ridge subduction may have contrasted effects on the overriding plate dynamics depending on the global conditions that constrain the converging system.
► Complete set of analogue models simulating oceanic ridge subduction. ► Ridge subduction favors slab flattening if the convergence velocity is imposed. ► A horizontal slab segment appears for large overriding plate trenchward velocities. ► The indentation of the overriding plate is moderate when the dip of the slab is small. ► Ridge subduction initially results in fore-arc uplift and back-arc subsidence.
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