Continental South American Plate Research Articles

Localization of Deformation in a Non‐Collisional Subduction Orogen: The Roles of Dip Geometry and Plate Strength on the Evolution of the Broken Andean Foreland, Sierras Pampeanas, Argentina

AbstractThe southern Central Andes (SCA, 27°–40°S) exhibit a complex deformation pattern that is influenced by multiple factors, including the present‐day dip angle of the subducting oceanic Nazca plate and the influence of inherited heterogeneities in the continental South American plate. This study employs a data‐driven geodynamic workflow to assess the role of various forcing factors in determining upper‐plate strain localization, both above the flat slab and the steeper segment to the south. These include the dip angle of the Nazca plate, the mechanically weak sedimentary basins, the thickness and composition of the continental crust, the strength of the subduction interface, and the plate velocities. Our modeling results predict two main deformation modes: (a) pure‐shear shortening in the broken foreland above the flat‐slab segment and eastward propagation of deformation, and (b) simple‐shear shortening restricted to the eastern margin of the Andean fold‐and‐thrust belt above the steep‐slab segment. While the convergence velocity and the frictional strength of the subduction interface primarily control the intensity of the deformation, inherited heterogeneities tend to localize deformation, and weak sediments leads to intensified surface deformation. Thicker crust and surface topography also influences strain localization by transferring stress to the eastern orogenic front. Above the flat‐slab segment deformation migrates eastward, which is facilitated by enhanced interface coupling. The transition between the steep and sub‐horizontal subduction segments is characterized by a diffuse transpressional shear zone, likely controlled by the change in dip geometry of the Nazca plate, and the presence of inherited faults and weak sedimentary basins.

Growth Rate Analysis Casuarina equisetifolia of The Case Study Pacitan and Samas Coast Area

The southern coast of Java is an area that is prone to earthquakes and tsunamis because the area is a meeting of the Pacific plate and the Indo-Australian plate meeting the South American continental plate, and the North American plate meeting the Philippine plate. The tsunami disaster can cause significant loss of life and material and cause damage to infrastructure in coastal areas. Casuarina equisetifolia is one type of vegetation that can grow in the southern coastal area of Java, where the coastal morphology is dominated by sand-type material used for protection from wind waves. The primary survey carried out in this study was to obtain the dimensions of coastal forest plants. The results of the Casuarina measurements were the age of 2 to 4 years at the Samas Coast area and 1 to 9 years were made at Teleng Ria Coast area. The analyzing show that increasing logarithmic function of Casuarina the trend of increasing growth of Casuarina tree height. It ages the older the tree height will increase, but this does not apply to the thickness of the tree canopy that grows to the side, because influenced by the position of the shoreline and the plant spacing factor.

Three-dimensional displacement field of the 2010 Mw 8.8 Maule earthquake from GPS and InSAR data with the improved ESISTEM-VCE method

The 2010 Mw 8.8 Maule earthquake occurred offshore central Chile and ruptured ∼500 km along the megathrust fault resulting from the oceanic Nazca plate subducting beneath the continental South American plate. The Maule earthquake produced remnant crustal displacements captured by a vast set of geodetic observations. However, given the nature of the observational techniques, it is challenging to extract its accurate three-dimensional coseismic deformation field with high spatial resolution. In this study, we modified the extended simultaneous and integrated strain tensor estimation from geodetic and satellite deformation measurements (ESISTEM) method with variance component estimation algorithm (ESISTEM-VCE) to retrieve the three-dimensional surface displacement field of this event by integrating the interferometric synthetic aperture radar (InSAR) and global positioning system (GPS) measurements. The ESISTEM-VCE method accounts for the spatial correlation of surface displacement among the adjacent points and determine the accurate weight ratios for different data sets, but also uses the uncertainties of GPS data and considers the different spatial scales from the different datasets. In the simulation experiments, the RMSEs of the ESISTEM-VCE method are smaller than those of the ESISTEM and ESISTEM-VCE (same d0) methods, and the improvements of 97.1%, 3.9%, and 84% are achieved in the east-west, north-south, and vertical components, respectively. Then, we apply the proposed methodology to the 2010 Mw 8.8 Maule earthquake, to obtain a three-dimensional displacement field that could provide fine deformation information. In the east-west component, the significant deformation is in the north of the epicenter, closed to the Constitución, with a maximum westward displacement of 495.5 cm. The displacement in the north-south component is relatively small compared to that in the east-west component. The maximum uplift reaches 211.8 cm, located at the southwest of the Concepción. Finally, the derived vertical displacements are also compared with field investigations, indicating that the ESISTEM-VCE method can obtain more accurate weight of different datasets and perform better than the ESISTEM method. The results highlight that the earthquake ruptured along the NE-SW direction, with a dominant thrust and a relatively small component of right-lateral strike-slip, coinciding with the characteristics of subduction and right-lateral shear. The experiments with the simulated and real data suggest that the improved ESISTEM-VCE method in this study is feasible and effective.

Source mechanisms and rupture processes of the Jujuy seismic nest, Chile-Argentina border

The Altiplano-Puna plateau, in Central Andes, is the second-largest continental plateau on Earth, extending between 22° and 27°S at an average altitude of 4400 m. The Puna plateau has been formed in consequence of the subduction of the oceanic Nazca Plate beneath the continental South American plate, which has an average crustal thickness of 50 km at this location. A large seismicity cluster, the Jujuy cluster, is observed at depth of 150–250 km beneath the central region of the Puna plateau. The cluster is seismically very active, with hundreds of earthquakes reported and a peak magnitude MW 6.6 on 25th August 2006. The cluster is situated in one of three band of intermediate-depth focus seismicity, which extend parallel to the trench roughly North to South. It has been hypothesized that the Jujuy cluster could be a seismic nest, a compact seismogenic region characterized by a high stationary activity relative to its surroundings. In this study, we collected more than 40 years of data from different catalogs and proof that the cluster meets the three conditions of a seismic nest. Compared to other known intermediate depth nests at Hindu Kush (Afganisthan) or Bucaramanga (Colombia), the Jujuy nest presents an outstanding seismicity rate, with more than 100 M4+ earthquakes per year. We additionally performed a detailed analysis of the rupture process of some of the largest earthquakes in the nest, by means of moment tensor inversion and directivity analysis. We focused on the time period 2017–2018, where the seismic monitoring was the most extended. Our results show that earthquakes in the nest take place within the eastward subducting oceanic plate, but rupture along sub-horizontal planes dipping westward. We suggest that seismicity at Jujuy nest is controlled by dehydration processes, which are also responsible for the generation of fluids ascending to the crust beneath the Puna volcanic region. We use the rupture plane and nest geometry to provide a constraint to maximal expected magnitude, which we estimate as MW ∼6.7.

Deep Structure of the Continental Plate in the South‐Central Chilean Margin: Metamorphic Wedge and Implications for Megathrust Earthquakes

AbstractWe study the deep structure of the continental wedge along the south‐central Chilean convergent margin based on a joint interpretation of wide‐angle reflection and refraction seismic phases acquired between the southern part of the Maule 2010 (36°S) and the northern part of the Valdivia 1960 (41°S) earthquake rupture areas. When combined with results of previous seismic studies, our results provide new insights into the deep structure (to 15–20 km) of the overriding, continental South American plate. We observe a latitudinal variation of P wave velocity, interpreted in terms of the distribution of Permo‐Triassic metamorphic complex near the coast, and deep reflectors at the base of these metamorphic units. A change in the observed deep structural style near 38°S suggests a segmentation in the interplate frictional properties between the Maule and Valdivia earthquakes rupture zones. This regional change in the continental wedge structure indicates a spatial correlation between the coseismic ruptures of large earthquakes (e.g., the 1960 and 2010 megathrust earthquakes) and the rheology/lithology along the interplate boundary.

Quaternary glaciolacustrine deposits around a Triple Junction site: Paleolakes at the foot of the Northern Patagonian Ice field (Argentina and Chile)

The area involved by the triple junction between the South American, Nazca and Antarctic plates activity was affected by Quaternary glaciations. Before 12,800 yrs BP an extended ice field occupied the top of the Patagonian Andes, irradiating glaciers towards the east and the west dominantly. Towards the east, the ice melted in piedmont lakes; towards the west, fjords melted into the Pacific Ocean. The Upper-Pleistocene climate amelioration caused the recession of those glaciers. Some piedmont lakes reversed their Atlantic outflow towards to the Pacific Ocean. The glaciers retreat caused the fluvial reactivations along crustal former faults that were located below the ice. The Patagonian ice field became therefore split into present Northern and Southern fields. At the second largest lake of South America, the Buenos Aires-General Carrera Lake, the water level dropped from about 500 m over present mean sea level to 230 m. Several glaciolacustrine deposits from this area are indicating significant variations caused by climatic changes, volcanism and tectonics, differing in spatial and temporal magnitudes. The triple junction activity involved subduction of the Chile Ridge below the continental South American plate, volcanic activity and faulting. During the glacier melting the Baker River captured three eastern-moving glacial systems towards the southwest, towards the Pacific Ocean. This rapid event is thought to occur 12,800 yrs BP. The lowering of these glaciolacustrine systems should be also interpreted in terms of the tectonic activity in the region and considering other processes operating in the lakes and within the watersheds.

Lithospheric density structure of the southern Central Andes constrained by 3D data-integrative gravity modelling

The southern Central Andes (SCA) (between 27° S and 40° S) is bordered to the west by the convergent margin between the continental South American Plate and the oceanic Nazca Plate. The subduction angle along this margin is variable, as is the deformation of the upper plate. Between 33° S and 35° S, the subduction angle of the Nazca plate increases from sub-horizontal (< 5°) in the north to relatively steep (~ 30°) in the south. The SCA contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated Cenozoic deformation within the Andean orogen. The distribution of the deformation within the SCA has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities. However, the possible influence that the thickness and composition of the continental crust have had on both short-term and long-term deformation of the SCA is yet to be thoroughly investigated. For our investigations, we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the Andean orogeny and, in particular, investigated the short- and long-term nature of the present-day deformation processes. We established a 3D model of lithosphere beneath the orogen and its foreland (29° S–39° S) that is consistent with currently available geological and geophysical data, including the gravity data. The modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains: the crystalline crust in the orogen (the magmatic arc and the main orogenic wedge) is thicker (~ 55 km) and less dense (~ 2900 kg/m3) than in the forearc (~ 35 km, ~ 2975 kg/m3) and foreland (~ 30 km, ~ 3000 kg/m3). Crustal thickening in the orogen probably occurred as a result of stacking of low-density domains, while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion. No clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early Paleozoic. Areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal-density contrast. This suggests that the short-term deformation within the interior of the Andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers. A thicker, denser, and potentially stronger lithosphere beneath the northern part of the SCA foreland is interpreted to have favoured a strong coupling between the Nazca and South American plates, facilitating the development of a sub-horizontal slab.

Segmentation in continental forearcs: Links between large-scale overriding plate structure and seismogenic behavior associated with the 2010 Mw 8.8 Maule, Chile earthquake

Subduction along the active margin of a continental plate occurs in a context where the overriding plate's crust and lithospheric mantle may contain inherited structures significantly predating the present tectonic conditions of the margin. These structures are persistent over very long-term time scales (>105 to >106 years) and are thought to play an important role in both seismogenic processes on the megathrust and development of topography along coastlines. We use receiver functions calculated from broadband seismic data collected along the Chilean forearc between ~33°S and 38.5°S in the vicinity of the 2010 Mw 8.8 Maule earthquake to determine the structure of the overriding South American continental plate and subducting Nazca oceanic plate along and inboard of the seismogenic portion of the megathrust. We show that the Chilean forearc is divided into three structurally distinct zones: a northern zone where the continental crust intersects the subducting plate well inboard of the coast at ~35–40 km depth, a central zone where the continental crust tapers to <20 km thickness at the coast before intersecting the subducting plate at 15–20 km depth, and a southern zone corresponding approximately to the rapidly uplifting Arauco Peninsula where we find that the continental crust forms a wedge-shaped feature ~35–40 km in thickness which intersects the subducting plate well inboard of the coast. The thin crust of the central zone is associated with a dense, high velocity mantle body (the Cobquecura anomaly) that may have played an important role in stabilizing this segment of the Chilean forearc since the late Paleozoic.

Shallow intraplate seismicity related to the Illapel 2015 Mw 8.4 earthquake: Implications from the seismic source

The September 16, 2015, Mw 8.4 Illapel, Chile earthquake is the first large event occurring in north-central Chile after the 1943 earthquake, filling a known seismic gap in the region. The earthquake took place in a complex tectonic region, nearby an area where transition from erosive to accretionary margin occurs due to the collision of Juan Fernandez Ridge (JFR) along the Chilean margin. We inverted the kinematic rupture process of the 2015 Mw 8.4 Illapel earthquake from the joint inversion of teleseismic body waves and near-field data. The relative weighting between datasets and the weighting of spatial/temporal constraints are objectively estimated by applying the Akaike's Bayesian Information Criterion. The coseismic slip model yields a total seismic moment of 4.92 × 1021 Nm occurred over ~120 s. The rupture shows both downdip and updip propagation with slip extending along the thrust interface from ~50 km depth to shallow near-trench depths (<15 km), with maximum slip of ~9 m located at shallow depths, where low average rupture speeds ~1.8–2 km/s are estimated. Outer-rise events, triggered within the oceanic Nazca plate after the mainshock, did not penetrate into the mantle and are related to preexisting faults due to both bending of Nazca plate and JFR uplift, which promotes a tensional stress regime in the surrounding area. This seismicity was triggered by static stress transfer from near-trench slip revealed by our source rupture modelling, suggesting outer-rise seismicity as a proxy for near-trench coseismic slip. Crustal seismicity within continental South American plate is observed prior to, and after, the mainshock, mainly related to extensional faulting within eroded and fractured wedge due to tectonic processes along erosive margins. We also show evidence of shallow seismicity after the mainshock associated with a long-lived crustal fault, which can represent a high seismic hazard for La Serena-Coquimbo conurbation.

Dividing the South American continent to fit a Gondwana reconstruction: A model based on continental geology

The South American continental plate has been affected by intraplate deformation since the start of West Gondwana disruption in the Lower Cretaceous (about 140 Ma). That the present shape of South America is not precisely the same as its shape in reassembled Gondwana partly explains the imperfect fits of the conjugate margins of the South Atlantic proposed since the first reconstruction models of the early 20th century. Several attempts at defining subplates within South America have been published but not all take account of existing knowledge of its continental geology. Here a subdivision into eight rigid subplates is proposed, based primarily on geological and tectonic evidence. Our model is tested against three published models of a multi-subplate Africa, as re-shaped to the pre-breakup outline of that continent, by visual fitting and the use of piercing points. The South America blocks were rotated and the Euler poles calculated interactively in reconstruction software. All three proposed fits had overlapping block margins within South America, indicating post-breakup rifting, except for the Transbrasiliano lineament. This NNE-SSW crustal-scale shear zone was used as a boundary for seven of the eight blocks. It is clearly the main intraplate accommodation zone in South America and an important piercing point in relation to the Kandi lineament in West Africa. The other block boundaries correspond to narrow zones with sedimentary basins and/or dyke swarms that have developed since South Atlantic opening. Each fit required a different configuration of the South America subplates since the pre-rift disposition of the African subplates also differ from each other, contributing to the uncertainty in any detailed reassembly.

Rupture process of the April 24, 2017, Mw 6.9 Valparaíso earthquake from the joint inversion of teleseismic body waves and near-field data

The central Chilean margin (32°–33°S) is characterized by the subduction of the Juan Fernández Ridge (JFR) beneath the continental South American plate. The JFR corresponds to a hotspot track composed by seamounts typically 3–3.5 km high above the surrounding seafloor, with a ridge-trench collision zone underlying the prominent Valparaiso Forearc Basin (VFB). This region has been affected by several large and mega earthquakes, where the last event corresponds to a complex seismic sequence that took place at the southern edge of VFB in April 2017. The spatio/temporal distribution of the seismic events is characterized by a predominant southeast migration of the seismicity. An Mw 6.9 earthquake triggered two days after the sequence started and occurred at the northern end of the rupture area of the 1985 Mw 8.0 Valparaiso earthquake. We compute the kinematic rupture process of the 2017 Mw 6.9 Valparaiso earthquake from the joint inversion of teleseismic body waves and near-field data. The Akaike’s Bayesian Information Criterion was used to objectively estimate both, the relative weighting between datasets and the weighting of spatial and temporal constraints used as a priori information. The coseismic slip is distributed over an area of dimensions ∼35 × 10 km2, with a maximum slip of 1.5 m. The rupture propagated unilaterally downdip. The source duration from the moment-rate solution is ∼20 s, with a total seismic moment of 3.05 × 1019 Nm (Mw 6.9). The analysis of the seismicity shows that most of the events occurred along the plate interface, foreshock clustered northern from the mainshock epicenter and the aftershocks occurred to the southeast, at a deeper location. The inverted regional moment tensors show similar faulting mechanism than the mainshock. The seismic sequence started two days before the mainshock and lasted for about two weeks, and a migration pattern of the seismicity was observed. The rupture of the 2017 Mw 6.9 earthquake nucleated where the San Antonio seamount (belonging to the JFR) is subducting, and propagated downwards along a zone that presents high interseismic coupling. The complex seismic sequence might be explained by an aseismic slip transient in the zone and the influence of the downdip migration of fluids from the accretionary prism along the subduction channel. The erosive and tunneling effect left by the sudden slip of the subducting seamount might provide the cavity for downdip migration of fluids and subsequent swarm seismicity.

Geomorfología de los fondos submarinos del Parque Nacional Natural Corales de Profundidad, mar Caribe colombiano

Detailed geomorphological mapping allows us to understand how the key ecological processes are influenced by the submarine relief, in addition to identifying sites of particular interest where benthic habitats develop in tropical areas such as the central Caribbean. The Corales de Profundidad Natural National Park was declared in 2013 as a Marine Protected Area in the Colombian Caribbean Sea. It is located in the Sinu Fold Belt tectonic terrain formed by the interaction between the Caribbean oceanic and the South American continental plates. The geological characteristics contribute to the understanding of the origin of the seabed geomorphology associated with different ecosystems in which deep-water corals are found. Due to their ecological importance, the benthic communities are to be conserved. The digital elevation model, developed with the bathymetric information acquired with multibeam echosounders, allowed the updating and interpretation of geomorphological units and features based on the hillshade model to visualize abrupt changes and slopes as descriptive attributes of the relief units. This work describes 16 geomorphological units based on the digital terrain model, which were associated with possible geological processes such as uplift, erosion, sedimentation and mass movements, as well as geomorphological and geological features and alignments. This information contributes to broadening the knowledge of the Colombian seabed for which little research of this type has been conducted

Migmatization and low-pressure overprinting metamorphism as record of two pre-Cretaceous tectonic episodes in the Santander Massif of the Andean basement in northern Colombia (NW South America)

The core of the Santander Massif in the northern Andes of Colombia is dominated by migmatitic gneisses with a <1.71Ga protolith and was affected by continuous interactions of oceanic plates to the west and the northwestern corner of the South American continental plate. The exposed metamorphic core of the massif offers a unique opportunity to understand the tectonic evolution of northwestern South America. We present new metamorphic petrology and geochemistry data from the Bucaramanga Gneiss in the Santander Massif to document part of this tectonic evolution from late Proterozoic to Jurassic times.Metapelitic migmatite gneiss, quartz-feldspathic gneiss, and amphibolite from the Bucaramanga Gneiss recorded metamorphic peak conditions in the range of 660–850°C at pressures of >7.5kbar. Lithologies are overprinted by low-pressure metamorphism, related to extensive Jurassic intrusions and linked with growth of cordierite and equilibration of low-pressure mineral assemblages, recorded metamorphic conditions are <750°C and <6.5kbar.Observed leucosomes display significant compositional variations and can be grouped in three groups: i) Group One leucosomes with high total REE content, high LREE/HREE, and negative Eu anomaly, ii) Group Two leucosomes with low total REE, low LREE/HREE, and positive Eu anomalies, and iii) Group Three leucosomes with relatively low LREE/HREE and strong positive Eu anomaly. Geochemical data support the interpretation that Group Two leucosomes crystallized from melts originated in a partial melting event affecting mostly pelitic and quartz-feldspathic lithologies with fluid-present melting reactions. The evaluation of mesosomes (amphibolite, pelitic and quartz-feldspathic rocks) as potential protoliths or restites indicates that at least two pelitic samples of the analyzed lithologies have characteristics consistent with the occurrence of fluid-present melting reactions involving quartz and feldspar. The leucosomes produced by crystallization of modified partial melts contrast with several other leucosomes that were injected; however, in some cases the melts crystallized as injected leucosomes show consistent geochemistry with partial melting of lithologies geochemically similar to the ones observed in the unit.The migmatization and the low pressure metamorphic overprint are related here to two main tectonic events: an early Paleozoic tectonic pulse produced by subduction of the oceanic crust of the Iapetus Ocean beneath northwestern Gondwana, and an Upper Triassic to Lower Jurassic tectonic pulse produced by subduction of oceanic crust of the proto-Pacific ocean beneath western Pangaea.

Hydrogeochemistry Characterization of Hot Springs Located in The Andes of Ecuador

The formation of several sources of hot springs in the Andes from Ecuador is the result of an intense volcanic activity due to the subduction of the Nazca oceanic plate under the South American continental plate. The aim of this study was to describe the hydrogeochemistry water geothermal origin, its chemical classification and relationship with the complex geology of Ecuador, using different hydro chemical diagrams (Stiff, Piper and Schoeller-Berkaloff). Geothermal waters can be divided into two groups. The first group, associated with an extinct volcanic activity produced in the Cenozoic, represents the Na+-Cl- type. The second group is associated with a young Quaternary volcanic activity and its types of water are Mg2+-HCO3-, Na+-HCO3-, Na+-SO42-, Mg2+-SO42-.

Hydrogeochemical analysis of volcanic and geothermal fluids in the Andes from Ecuador using hydrochemical plots (Stiff, Piper and Schoeller-Berkaloff diagrams)

The formation of several sources of hot springs in the Andes from Ecuador was the result of intense volcanic activity due to the subduction of the Nazca oceanic plate under the South American continental plate. The aims of this study include the presentation of chemical analysis in graphical form in order to describe the hidrogeochemistry water geothermal origins, their chemical classification and their relationship to the complex geology of Ecuador using different hydro chemical plots such as Stiff's polygonal diagram, Piper's trilinear diagram and Schoeller-Berkaloff's logarithmic vertical columns diagram. Geothermal waters can be divided into two groups. The first group was associated with an extinct volcanic activity produced in the Cenozoic and were qualified based on the type of water Na+-Cl-, while the second group was associated with young Quaternary volcanic activity, and the types of water were Mg2+- HCO3-, Na+-HCO3-, Na+-SO42-,Mg2+-SO42-.

COSEISMIC DEFORMATION FIELD AND FAULT SLIP DISTRIBUTION OF THE 2015 CHILE Mw8.3 EARTHQUAKE

On September 16, 2015, a magnitude 8.3 earthquake struck west of Illapel, Chile. We analyzed Sentinel-1A/IW InSAR data on the descending track acquired before and after the Chile Mw8.3 earthquake of 16 September 2015. We found that the coseismic deformation field of this event consists of many semi circular fringes protruding to east in an approximately 300km long and 190km wide region. The maximum coseismic displacement is about 1.33m in LOS direction corresponding to subsidence or westward shift of the ground. We inverted the coseismic fault slip based on a small-dip single plane fault model in a homogeneous elastic half space. The inverted coseismic slip mainly concentrates at shallow depth above the hypocenter with a symmetry shape. The rupture length along strike is about 340 km with maximum slip of about 8.16m near the trench. The estimated moment is 3.126×1021 N.m (Mw8.27)，the maximum depth of coseismic slip near zero appears to 50km. We also analyzed the postseismic deformation fields using four interferograms with different time intervals. The results show that postseismic deformation occurred in a narrow area of approximately 65km wide with maximum slip 11cm, and its predominant motion changes from uplift to subsidence with time. that is to say, at first, the postseismic deformation direction is opposite to that of coseismic deformation, then it tends to be consistent with coseismic deformation.It maybe indicates the differences and changes in the velocity between the Nazca oceanic plate and the South American continental plate.

COSEISMIC DEFORMATION FIELD AND FAULT SLIP DISTRIBUTION OF THE 2015 CHILE Mw8.3 EARTHQUAKE

Abstract. On September 16, 2015, a magnitude 8.3 earthquake struck west of Illapel, Chile. We analyzed Sentinel-1A/IW InSAR data on the descending track acquired before and after the Chile Mw8.3 earthquake of 16 September 2015. We found that the coseismic deformation field of this event consists of many semi circular fringes protruding to east in an approximately 300km long and 190km wide region. The maximum coseismic displacement is about 1.33m in LOS direction corresponding to subsidence or westward shift of the ground. We inverted the coseismic fault slip based on a small-dip single plane fault model in a homogeneous elastic half space. The inverted coseismic slip mainly concentrates at shallow depth above the hypocenter with a symmetry shape. The rupture length along strike is about 340 km with maximum slip of about 8.16m near the trench. The estimated moment is 3.126×1021 N.m (Mw8.27)，the maximum depth of coseismic slip near zero appears to 50km. We also analyzed the postseismic deformation fields using four interferograms with different time intervals. The results show that postseismic deformation occurred in a narrow area of approximately 65km wide with maximum slip 11cm, and its predominant motion changes from uplift to subsidence with time. that is to say, at first, the postseismic deformation direction is opposite to that of coseismic deformation, then it tends to be consistent with coseismic deformation.It maybe indicates the differences and changes in the velocity between the Nazca oceanic plate and the South American continental plate.

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This paper analyzes the focal mechanism of 49 earthquakes with magnitude Mw ≥ 4.8 in the region of Valle del Cauca and surrounding areas during the period 1978-2010. The solutions were taken from the catalogs of the International Seismological Centre (ISC) and The Global Centroid-Moment-Tensor (GCMT) Project. The depth of the events is distributed between surface and intermediate about 215 km, associated with the subduction of the Nazca plate, and a few superficial events that correspond to the activity of cortical sources in the South American continental plate. In accord with model environments strong regimes associated with the “static” deformation of the subducting plate, four seismotectonic environments, are distinguished by its own seismic, tectonic and strong regime characteristics. The seismotectonic deformation rate is presented for each of these environments. The modeling of the Coulomb effort field shows that the shear creates a barrier that prevents the compressive effort is projected into the continent in SW-NE direction, interrupted in the Colombo-Ecuadorian trench. While thetensile effort, although is also interrupted, it manages to project the continent in NW-SE direction, perpendicular to compressive effort.Keywords: Seismotectonic, focal mechanisms, earthquakes, stress field.

Occurrence of a skarn-type mineralogy found in Ciénaga Marbles, located in the NW foothills of the Santa Marta Massif (Colombia)

&lt;p&gt;The early Cretaceous Ciénaga Marbles that crop out in the NW foothills of the Santa Marta Massif (Colombian Caribbean region) present an epigenetic mineral assemblage (skarn-type), overprinting the metamorphic mineral assemblage previously developed along the regional metamorphic history that affected this unit. The skarn-type mineralogy allows at least three paragenetic contexts to be distinguished, which are represented by the following neoformed minerals: (a) garnet, forsterite, diopside, titanite, wollastonite and calcite (early anhydrous metamorphic stage), (b) actinolite, tremolite, allanite and clinohumite (metasomatic or hydrated stage), and (c) chlorite, serpentine, sepiolite and quartz (late low temperature retrograde stage, probably due to infiltration of descending meteoric waters). The skarn-type mineralogy is observed as alteration halos developed around porphyritic granodiorites emplaced as sills between anisotropy planes related to metamorphic regional foliation of rock that are considered to be the causative bodies of the skarntype mineralogy. Zircon U-Pb ages obtained from granodioritic bodies yielded an age of 55.5±0.7 Ma (Ypresian, Early Eocene). The formation of the skarn-type mineralogy in the Ciénaga Marbles is temporarily related to the formation and emplacement of hydrated silicate masses that were generated at the beginning of the subduction polarity change (i.e. when the Caribbean oceanic plate began to subduct beneath South American continental plate).&lt;/p&gt;

Subduction consequences along the Andean margin: thermal and topographic signature of an ancient ridge subduction in the Marañón Basin of Perú

All along the eastern border of the Andes lie foreland basins that are among the most prolific hydrocarbon provinces of the world. Their Cenozoic evolution was controlled by the Andean uplift and its consequences on deformation and sedimentation. In turn, the Andean uplift results from the interplay between the subducting Nazca oceanic plate and the South American continental plate. Although the process exists all along the margin, the subducting plate is not regular including bathymetric anomalies and segments that result in different response in the deformation and active volcanism of the overriding plate. In the Maranon Basin of Peru, evidences allow documenting the consequences of a topographic anomaly subduction on the thermal regime and deformation of the Andean foreland during the Neogene. In this basin, a maturity anomaly is difficult to explain by considering only the present day thermal regime. However, it spatially coincides with the trace of a lost subducting ridge, the Inca Plateau. Other features like differential uplift and erosion can be related to the same event in the area. We review the consequences of oceanic ridge subduction along the Andean margin. Their effects on the deformation and volcanism of the forearc and arc regions have been extensively described. Their influence on the present day foreland topography is testified by the existence of giant alluvial fans and displaced terraces. Their effect on magmatism and ore deposits formation has also been demonstrated. The example illustrated here shows that their influence on thermal regime, deformation, erosion and ultimately on petroleum systems must also be taken into account in the search for hydrocarbons in subduction related basins. All along the eastern border of the Andes lie foreland basins that are among the most prolific hydrocarbon provinces of the world. Their Cenozoic evolution was controlled by the Andean uplift and its consequences on deformation and sedimentation. In turn, the Andean uplift results from the interplay between the subducting Nazca oceanic plate and the South American continental plate. Although the process exists all along the margin, the subducting plate is not regular including bathymetric anomalies and segments that result in different response in the deformation and active volcanism of the overriding plate. In the Maranon Basin of Peru, evidences allow documenting the consequences of a topographic anomaly subduction on the thermal regime and deformation of the Andean foreland during the Neogene. In this basin, a maturity anomaly is difficult to explain by considering only the present day thermal regime. However, it spatially coincides with the trace of a lost subducting ridge, the Inca Plateau. Other features like differential uplift and erosion can be related to the same event in the area. We review the consequences of oceanic ridge subduction along the Andean margin. Their effects on the deformation and volcanism of the forearc and arc regions have been extensively described. Their influence on the present day foreland topography is testified by the existence of giant alluvial fans and displaced terraces. Their effect on magmatism and ore deposits formation has also been demonstrated. This example illustrated here shows that their influence on thermal regime, deformation, erosion and ultimately on petroleum systems, must also be taken into account in the search for hydrocarbons in subduction related basins.