Any plate has a growth process from small to large. The micro-blocks or micro-plates are sometimes the precursors of large plates. The origin, growth, aborting, extinction and residual process of ...micro-blocks are of great significance for the study of plate tectonics and pre-plate tectonics. The micro-block can be divided into continental, oceanic and mantle micro-blocks according to their compositions. In this paper, the micro-blocks in the global oceans have been summarized according to the following five environments: mid-ocean ridge system, subduction system, transform fault system, deep-sea intraplate system and extension-rift system. We first propose a genetic classification of micro-blocks comprising: detachment-derived, rifting-derived, transform-derived, propagation-derived, ridge jumping-derived, subduction-derived, accretion-derived, collision-derived and delamination-derived micro-blocks. The different types of micro-block boundaries such as active or fossil detachment fault, subduction zone, mid-ocean ridge, transform fault, fracture zone, transfer fault, accommodation zone, lithosphere-scale strike-slip fault, pseudofault, intra-oceanic convergent zone, overlapping spreading centre, non-transform offset, rheological crustal or mantle discontinuity, are systematically discussed for different micro-blocks. Thus, the number of triple junctions will be more than the 16 in the traditional Plate Tectonic Theory. A stability analysis of these triple junctions is the key to understanding the causes of micro-blocks. However, the micro-block has no ultimate cause, so it is unnecessary to pursue one ultimate cause or initiation of plate tectonics. These micro-blocks within oceanic basins, along oceanic margins or within the deep mantle, can not only be used to develop deep ocean fine structural analysis and plate tectonic reconstruction, but also to explain the causes of micro-blocks in some orogens. This will enrich the research of more detailed pre-orogenic or syn-orogenic evolution of orogenic belts, and even extend to the study of early Precambrian pre-plate tectonic mechanisms. The micro-block can be a transition among microplate, plate and terrane under a plate tectonic regime. It can also be formed in inter-sphere tectonic processes. It helps better our understanding of regimes of cratonic basin formation and intra-continental deformation which are the difficulties faced by the Plate Tectonics Theory. We speculate that the Micro-block Tectonics Theory is a unified tectonic theory of cross-layer, cross-phase, cross-space-time scale and cross-planet.
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.
The cause of tectonic deformation in northwestern South America and its link to upper mantle structure and flow are debated. We use a combination of broadband and short period travel time seismic ...data for P-waves to show that observations are consistent with the presence of two subduction segments in Colombia and contrasting values of crustal thickness. In Northern Colombia, at latitudes greater than 6°N, most of the seismic stations are associated with negative teleseismic travel time residuals, relative to a regional mean, suggesting that the upper mantle is seismically faster than predicted from global models. In particular, for the Caribbean coastal plains there are no signs of significant anomalies in the upper mantle, evidenced by the small magnitude of the travel time delays and subdued Pn speeds (~7.97km/s). To the southeast of such plains there is an increase in magnitude of the negative travel time residuals, including the Northern Eastern Cordillera, the Perija Range and the Merida Andes. An analysis of non-isostatic residual topography, based on a model of crustal thickness in northwestern South America, is consistent with a slab-associated upper mantle flow beneath the region just east of the Bucaramanga Nest. We interpret these results to indicate the presence of a Caribbean slab, initially flat beneath the Caribbean coastal plains, and steepening sharply in the southeast, including the area of Bucaramanga. For most of the western Andean region and the Pacific coast, south of 6°N, teleseismic differential travel time residuals are predominantly positive, indicating that the upper mantle is in general seismically slower than the reference model. Beneath the Central Cordillera, just to the east of this area, the residuals become smaller and predominantly negative; residual non-isostatic topography is negative as well. These features are probably related to the effect of the Nazca subduction developing an asthenospheric wedge.
•We use teleseismic travel times to infer upper mantle average structure.•We correlate results with the presence of Caribbean and Nazca Slabs.•Results suggest a Caribbean slab with an initially flat subduction.•The Caribbean slab may steepen at around the location of the Bucaramanga Nest.•Results propose a Nazca plate that subducts “normally” beneath Colombia.
Subduction beneath the northernmost Andes in Colombia is complex. Based on seismicity distributions, multiple segments of slab appear to be subducting, and arc volcanism ceases north of 5° N. Here, ...we illuminate the subduction system through hypocentral relocations and Vp and Vs models resulting from the joint inversion of local body wave arrivals, surface wave dispersion measurements, and gravity data. The simultaneous use of multiple data types takes advantage of the differing sensitivities of each data type, resulting in velocity models that have improved resolution at both shallower and deeper depths than would result from traditional travel time tomography alone. The relocated earthquake dataset and velocity model clearly indicate a tear in the Nazca slab at 5° N, corresponding to a 250-km shift in slab seismicity and the termination of arc volcanism. North of this tear, the slab is flat, and it comprises slabs of two sources: the Nazca and Caribbean plates. The Bucaramanga nest, a small region of among the most intense intermediate-depth seismicity globally, is associated with the boundary between these two plates and possibly with a zone of melting or elevated water content, based on reduced Vp and increased Vp/Vs. We also use relocated seismicity to identify two new faults in the South American plate, one related to plate convergence and one highlighted by induced seismicity.
•Joint inversion provides a clearer image of subduction than body-wave-only tomography.•Normal-dip subduction of Nazca beneath volcanogenic southern Colombia.•Non-volcanogenic flat-slab subduction of Nazca and Caribbean plates further north.•Slab tear between normally dipping and flat sections of Nazca plate.
Nazca subduction beneath South America is one of our best modern examples of long‐lived ocean‐continent subduction on the planet, serving as a foundation for our understanding of subduction ...processes. Within that framework, persistent heterogeneities at a range of scales in both the South America and Nazca plates is difficult to reconcile without detailed knowledge of the subducted Nazca slab structure. Here we use teleseismic travel time residuals from >1,000 broadband and short‐period seismic stations across South America in a single tomographic inversion to produce the highest‐resolution contiguous P wave tomography model of the subducting slab and surrounding mantle beneath South America to date. Our model reveals a continuous trench‐parallel fast seismic velocity anomaly across the majority of South America that is consistent with the subducting Nazca slab. The imaged anomaly indicates a number of robust features of the subducted slab, including variable slab dip, extensive lower mantle penetration, slab stagnation in the lower mantle, and variable slab amplitude, that are incorporated into a new, comprehensive model of the geometry of the Nazca slab surface to ~1,100 km depth. Lower mantle slab penetration along the entire margin suggests that lower mantle slab anchoring is insufficient to explain along strike upper plate variability while slab stagnation in the lower mantle indicates that the 1,000 km discontinuity is dominant beneath South America.
Key Points
High‐resolution teleseismic P wave tomography reveals the detailed structure of the subducted Nazca slab to lower mantle depths
A new, comprehensive Nazca slab model is produced from the trench to 1,100 km depth
Lower mantle slab penetration occurs along the entire South American margin, inconsistent with variable upper plate shortening
Several central Andean pre-Columbian sites struck by hydrogeological disasters due to El Niño-Southern Oscillation (ENSO) events are reported in the literature. The mainstream explanation for the ...decline and demise of Cahuachi (pampa of Nazca, south Peru) implies the damage and burial of such a ceremonial center as a consequence of two catastrophic river floods, which occurred around 600 CE and 1000 CE, respectively. Therefore, geological studies at Cahuachi are mandatory with regard to both the correlations of ENSO-related deposits (“event-strata”) among different Peruvian sites and the assessment of the millennium-scale climate variability. In particular, the latter is crucial to evaluate the environmental and economic consequences due to the incoming fluctuations of ENSO. In this paper, stratigraphic, grain-size distribution, and petrographic investigations on a sedimentary section exposed close to one of the main temples of Cahuachi are reported. They represent the first test for the current mainstream explanation. The preliminary finding indicates that the studied stratigraphic interval may belong to the common regional succession of the pampa of Nazca rather than the ENSO-related deposits described in the literature. However, further geological research will be necessary to unravel this issue in more detail.
SUMMARY
We present an upgraded version of a previously published 3‐D density model of the Andean subduction zone between 18°S and 45°S. This model consists of 3‐D bodies of constant density, which ...geometry is constrained by independent seismic data and is triangulated from vertical cross‐sections. These bodies define the first‐order morphology and internal structure of the subducted Nazca slab and South American Plate. The new version of the density model results after forward modelling the Bouguer anomaly as computed from the most recent version of the Earth Gravitational Model (EGM2008). The 3‐D density model incorporates new seismic information to better constrain the geometry of the subducted slab and continental Moho (CMH) and has a trench‐parallel resolution doubling the resolution of the previous model. As an example of the potential utility of our model, we compare the geometry of the subducted slab and CMH against the corresponding global models Slab1.0 and Crust2.0, respectively. This exercise demonstrates that, although global models provide a good first‐order representation of the slab and upper‐plate crustal geometries, they show large discrepancies (up to ±40 km) with our upgraded model for some well‐constrained areas. The geometries of the slab, lithosphere–asthenosphere boundary below the continent, CMH and intracrustal density discontinuity that we present here as Supporting Information can be used to study Andean geodynamic processes from a wide range of quantitative approaches.
Subducting plates around the globe display a large variability in terms of slab geometry, including regions where smooth and little variation in subduction parameters is observed. While the vast ...majority of subduction slabs plunge into the mantle at different, but positive dip angles, the end-member case of flat-slab subduction seems to strongly defy this rule and move horizontally several hundreds of kilometers before diving into the surrounding hotter mantle. By employing a comparative assessment for the Mexican, Peruvian and Chilean flat-slab subduction zones we find a series of parameters that apparently facilitate slab flattening. Among them, trench roll-back, as well as strong variations and discontinuities in the structure of oceanic and overriding plates seem to be the most important. However, we were not able to find the necessary and sufficient conditions that provide an explanation for the formation of flat slabs in all three subduction zones. In order to unravel the origin of flat-slab subduction, it is probably necessary a numerical approach that considers also the influence of surrounding plates, and their corresponding geometries, on 3D subduction dynamics.
We present a new seismic tomography model for the crust and upper mantle beneath the Central Andes based on multi‐scale full seismic waveform inversion, proceeding from long periods (40–80 s) over ...several steps down to 12–60 s. The spatial resolution and trade‐offs among parameters are estimated through the multi‐parameter point‐spread functions. P‐ and S‐wave velocity structures with spatial resolution of 30–40 km for the upper mantle and 20–25 km for the crust could be resolved in the central study region. In our study, the subducting Nazca slab is clearly imaged in the upper mantle, with dip‐angle variations from the north to the south. Bands of low velocities in the crust and mantle wedge indicate intense crustal partial melting and hydration of the mantle wedge beneath the frontal volcanic arc, respectively, and they are linked to the vigorous dehydration from the subducting Nazca plate and intermediate depth seismicity within the slab. These low‐velocity bands are interrupted at 19.8°–21°S, both in the crust and uppermost mantle, hinting at the lower extent of crustal partial melting and hydration of the mantle wedge. The variation of lithospheric high‐velocity anomalies below the back‐arc from north to south allows insight into the evolutionary foundering stages of the Central Andean margin. A high‐velocity layer beneath the southern Altiplano suggests underthrusting of the leading edge of the Brazilian Shield. In contrast, a steeply westward dipping high‐velocity block and low‐velocity lithospheric uppermost mantle beneath the southern Puna plateau hint at the ongoing lithospheric delamination.
Key Points
Normal dip subduction of the Nazca plate beneath the Central Andes
Dehydration of the subducted Nazca plate, hydration of the mantle wedge and partial melting of the continental crust
Underthrusting of the Brazilian Shield beneath the southern Altiplano and delamination beneath the southern Puna
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