Subduction zones megathrust faults constitute a considerable hazard as they produce most of the world's largest earthquakes. However, the role in megathrust earthquake generation exerted by deeper ...subduction processes remains poorly understood. Here, we analyze the 2003 – 2014 space-time variations of the Earth's gravity gradients derived from three datasets of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 Maule earthquake. In all these datasets, our analysis reveals a large-amplitude gravity gradient signal, progressively increasing in the three months before the earthquake, North of the epicentral area. We show that such signals are equivalent to a 60km3 water storage decrease over 2 months and cannot be explained by hydrological sources nor artefacts, but rather find origin from mass redistributions within the solid Earth on the continental side of the subduction zone. These gravity gradient variations could be explained by an extensional deformation of the slab around 150-km depth along the Nazca Plate subduction direction, associated with large-scale fluid release. Furthermore, the lateral migration of the gravity signal towards the surface from a low coupling segment around −32.5∘ North to the high coupling one in the South suggests that the Mw 8.8 earthquake may have originated from the propagation up to the trench of this deeper slab deformation. Our results highlight the importance of observations of the Earth's time-varying gravity field from satellites in order to probe slow mass redistributions in-depth major plate boundaries and provide new information on dynamic processes in the subduction system, essential to better understand the seismic cycle as a whole.
•An anomalous GRACE gravity gradient signal is detected prior to the Maule earthquake.•This signal cannot be explained by hydrological mass redistributions nor artefacts.•It is consistent with extensional deformation of the subducted slab near 150 km depth.•The earthquake may have originated from the propagation of this deeper deformation.
The Andean Cordillera has evolved since the Late Cretaceous in the context of subduction of oceanic lithosphere beneath continental lithosphere, making the kinematics between South America and its ...adjacent oceanic plates in the Pacific basin valuable to analyze the development of the Andean orogen. The latest Cretaceous–Cenozoic convergence history in western South America may be divided into three stages. The youngest Stage 1 (25–0Ma) is characterized by ENE directed convergence of the Nazca plate toward most of South America, and by ~E–W subduction of the Antarctic plate beneath southern Patagonia. The Nazca–South America convergence rate in Stage 1 shows a continuous decrease from the highest values in the Cenozoic (~15cm/yr) to the present day values from GPS measurements (~7cm/yr). Stage 2 (47–28Ma) is characterized by NE directed subduction of Farallon with the convergence rate remaining almost constant during the entire interval. In those times obliquity was dextral in Chile, sinistral in southern Peru, while almost head-on convergence occurred in central and northern Peru. During latest Cretaceous to Early Eocene times (Stage 3) the Farallon plate was subducted beneath Perú and the Phoenix plate was subducted farther south, where a triple junction migrated southward along the Chilean margin. The subduction of the Farallon plate was rather slow with variable direction imposed by the position of the triple junction, whereas subduction of the Phoenix plate was rapid (>10cm/yr) and ESE directed. We present a working hypothesis suggesting no major changes in the age of subducted lithosphere in the Chile trench from Middle Eocene to Late Oligocene, followed by subduction of progressively older oceanic lithosphere in the early Neogene and progressively younger lithosphere during the late Neogene and the Quaternary. In addition, it is shown that South American motion as predicted by available hotspot models has insufficient resolution to be applied to the analysis of Cenozoic Andean deformation.
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► We model convergence in western South America since Late Cretaceous. ► Farallon–South America convergence rate between 47 and 28Ma was almost constant. ► Phoenix plate was likely subducted beneath most Chile in Late Cretaceous–Paleocene. ► We present an exercise on age of subducted lithosphere beneath Chile since Eocene. ► Hotspot models do not work for analyzing the Cenozoic evolution of Andes.
Transient slab flattening beneath Colombia Wagner, L. S.; Jaramillo, J. S.; Ramírez‐Hoyos, L. F. ...
Geophysical research letters,
16 July 2017, Letnik:
44, Številka:
13
Journal Article
Recenzirano
Subduction of the Nazca and Caribbean Plates beneath northwestern Colombia is seen in two distinct Wadati Benioff Zones, one associated with a flat slab to the north and one associated with normal ...subduction south of 5.5°N. The normal subduction region is characterized by an active arc, whereas the flat slab region has no known Holocene volcanism. We analyze volcanic patterns over the past 14 Ma to show that in the mid‐Miocene a continuous arc extended up to 7°N, indicating normal subduction of the Nazca Plate all along Colombia's Pacific margin. However, by ~6 Ma, we find a complete cessation of this arc north of 3°N, indicating the presence of a far more laterally extensive flat slab than at present. Volcanism did not resume between 3°N and 6°N until after 4 Ma, consistent with lateral tearing and resteepening of the southern portion of the Colombian flat slab at that time.
Key Points
The flat slab beneath Colombia originally extended much farther south than it does today
Volcanic ages suggest that flattening began at ~9 Ma, with full arc cessation by ~6 Ma
Modern arc in the south was reestablished after 4 Ma during slab tearing and resteepening
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.
We present a velocity field for northwestern South America and the southwest Caribbean based on GPS Continuously Operating Reference Stations in Colombia, Panama, Ecuador and Venezuela. This paper ...presents the first comprehensive model of North Andean block (NAB) motion. We estimate that the NAB is moving to the northeast (060°) at a rate of 8.6 mm/yr relative to the South America plate. The NAB vector can be resolved into a margin-parallel (035°) component of 8.1 mm/yr rigid block motion and a margin-normal (125°) component of 4.3 mm/yr. This present-day margin-normal shortening rate across the Eastern Cordillera (EC) of Colombia is surprising in view of paleobotanical, fission-track, and seismic reflection data that suggest rapid uplift (7 km) and shortening (120 km) in the last 10 Ma. We propose a “broken indenter” model for the Panama-Choco arc, in which the Choco arc has been recently accreted to the NAB, resulting in a rapid decrease in shortening in the EC. The Panama arc is colliding eastward with the NAB at approximately 15–18 mm/yr, and the Panama-Choco collision may have been responsible for much of the uplift of the EC. The present on-going collision poses a major earthquake hazard in northwestern Colombia from the Panama border to Medellin area. Since the northeastward margin-parallel motion of the NAB is now greater than the rate of shortening in the EC, northeast trending right-lateral strike-slip faulting is the primary seismic hazard for the 8 million inhabitants of Bogota, the capital city of Colombia. There continues to be a high risk of a great megathrust earthquakes in southern Colombia along the Ecuador-Colombia trench. Trench earthquakes have only released a fraction of the energy accumulated in the Ecuador-Colombia trench since the 1906 Ecuador earthquake, and interseismic strain is accumulating rapidly at least as far north as Tumaco, the rupture area of the 1979 earthquake.
•New velocity field for NW South America and SW Caribbean based on GPS stations in Colombia, Panama, Ecuador and Venezuela.•First comprehensive model of North Andean block (NAB) motion.•A broken indenter model for the Panamá-Choco arc collision is consistent with GPS data, paleobotanical, and geologic evidence.
Closure of the Central American seaway was a local tectonic event with potentially global biotic and environmental repercussions. We report geochronological (six U/Pb LA‐ICP‐MS zircon ages) and ...geochemical (19 XRF and ICP‐MS analyses) data from the Isthmus of Panama that allow definition of a distinctive succession of plateau sequences to subduction‐related protoarc to arc volcaniclastic rocks intruded by Late Cretaceous to middle Eocene intermediate plutonic rocks (67.6 ± 1.4 Ma to 41.1 ± 0.7 Ma). Paleomagnetic analyses (24 sites, 192 cores) in this same belt reveal large counterclockwise vertical‐axis rotations (70.9° ± 6.7°), and moderate clockwise rotations (between 40° ± 4.1° and 56.2° ± 11.1°) on either side of an east‐west trending fault at the apex of the Isthmus (Rio Gatun Fault), consistent with Isthmus curvature. An Oligocene‐Miocene arc crosscuts the older, deformed and segmented arc sequences, and shows no significant vertical‐axis rotation or deformation. There are three main stages of deformation: 1) left‐lateral, strike‐slip offset of the arc (∼100 km), and counterclockwise vertical‐axis rotation of western arc segments between 38 and 28 Ma; 2) clockwise rotation of central arc segments between 28 and 25 Ma; and 3) orocline tightening after 25 Ma. When this reconstruction is placed in a global plate tectonic framework, and published exhumation data is added, the Central American seaway disappears at 15 Ma, suggesting that by the time of northern hemisphere glaciation, deep‐water circulation had long been severed in Central America.
Key Points
A strain marker was used to reconstruct the Panama Isthmus as an orocline
This reconstruction suggests that deep‐water seaway closure is 15 Ma old
Central American seaway closure did not trigger northern hemisphere glaciation
Over 100 GPS sites measured in 2008–2013 in Peru provide new insights into the present‐day crustal deformation of the 2200 km long Peruvian margin. This margin is squeezed between the eastward ...subduction of the oceanic Nazca Plate at the South America trench axis and the westward continental subduction of the South American Plate beneath the Eastern Cordillera and Subandean orogenic wedge. Continental active faults and GPS data reveal the rigid motion of a Peruvian Forearc Sliver that extends from the oceanic trench axis to the Western‐Eastern Cordilleras boundary and moves southeastward at 4–5 mm/yr relative to a stable South America reference frame. GPS data indicate that the Subandean shortening increases southward by 2 to 4 mm/yr. In a Peruvian Sliver reference frame, the residual GPS data indicate that the interseismic coupling along the Nazca megathrust is highly heterogeneous. Coupling in northern Peru is shallow and coincides with the site of previous moderate‐sized and shallow tsunami‐earthquakes. Deep coupling occurs in central and southern Peru, where repeated large and great megathrust earthquakes have occurred. The strong correlation between highly coupled areas and large ruptures suggests that seismic asperities are persistent features of the megathrust. Creeping segments appear at the extremities of great ruptures and where oceanic fracture zones and ridges enter the subduction zone, suggesting that these subducting structures play a major role in the seismic segmentation of the Peruvian margin. In central Peru, we estimate a recurrence time of 305 ± 40 years to reproduce the great 1746 Mw~8.8 Lima‐Callao earthquake.
Key Points
New GPS data provide insights on the present‐day crustal deformation of the Peruvian Andes
We describe the kinematics and boundaries of the Peruvian Forearc Sliver and Subandean retro‐shortening rates
We map a highly heterogeneous interseismic coupling pattern of the Nazca megathrust at the scale of the 2200 km of the Peruvian margin
A large number of ore deposits that formed in the Peruvian Andes during the Miocene (15–5 Ma) are related to the subduction of the Nazca plate beneath the South American plate. Here we show that the ...spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies, namely the Nazca Ridge in central Peru and the now-consumed Inca Plateau in northern Peru, at the subduction zone. Plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench. This is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction. It is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments.
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
A new seismic model for crust and upper mantle of the south Central Andes is derived from full waveform inversion, covering the Pampean flat subduction and adjacent Payenia steep subduction segments. ...Focused crustal low‐velocity anomalies indicate partial melts in the Payenia segment along the volcanic arc, whereas weaker low‐velocity anomalies covering a wide zone in the Pampean segment are interpreted as remnant partial melts. Thinning and tearing of the flat Nazca slab is inferred from gaps in the slab along the inland projection of the Juan Fernandez Ridge. A high‐velocity anomaly in the mantle below the flat slab is interpreted as relic Nazca slab segment, which indicates an earlier slab break‐off triggered by the buoyancy of the Juan Fernandez Ridge during the flattening process. In Payenia, large‐scale low‐velocity anomalies atop and below the re‐steepened Nazca slab are associated with the re‐opening of the mantle wedge and sub‐slab asthenospheric flow, respectively.
Plain Language Summary
Taking advantage of the abundant information recorded in seismic waveforms, we imaged the seismic structure of the crust and upper mantle beneath central Chile and western Argentina, where the oceanic Nazca slab is subducting beneath the South American plate. The subducted Nazca slab is almost flat at a depth of 100–150 km in the north of the study area below the Pampean region, where the Juan Fernandez seamount ridge is subducting as part of the Nazca slab. The slab steepens again in the south in the Payenia region. Our model reveals pronounced low‐velocity anomalies within the Pampean flat slab along the inland projection of the Juan Fernandez Ridge, indicating that the Pampean flat slab is thinned or even torn apart. A high‐velocity anomaly is imaged beneath the flat slab, representing a former slab segment that was broken off during the slab flattening process and was overridden by the advancing young slab. Our model suggests a causal relationship between the oceanic ridge subduction and the flat slab formation. In the Payenia region, the slab re‐steepening resulted in the re‐establishment of the mantle wedge and induced hot mantle flow below the slab, which are characterized by low‐velocity anomalies in the model.
Key Points
A new seismic model for the crust and upper mantle beneath central Chile and western Argentina is presented
Thinning and tearing within the Pampean flat slab is detected along the inland projection of the Juan Fernandez Ridge
A relic slab is imaged beneath the Pampean flat slab, reflecting slab break‐off during the flattening process
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