Gabriel González

Tectonics of the Jurassic‐Early Cretaceous magmatic arc of the north Chilean Coastal Cordillera (22°–26°S): A story of crustal deformation along a convergent plate boundary

The tectonic evolution of a continental magmatic arc that was active in the north Chilean Coastal Cordillera in Jurassic-Early Cretaceous times is described in order to show the relationship between arc deformation and plate convergence. During stage I (circa 195–155 Ma) a variety of structures formed at deep to shallow crustal levels, indicating sinistral arc-parallel strike-slip movements. From deep crustal levels a sequence of structures is described, starting with the formation of a broad belt of plutonic rocks which were sheared under granulite to amphibolite facies conditions (Bolfin Complex). The high-grade deformation was followed by the formation of two sets of conjugate greenschist facies shear zones showing strike-slip and thrust kinematics with a NW–SE directed maximum horizontal shortening, i.e., parallel to the probable Late Jurassic vector of plate convergence. A kinematic pattern compatible to this plate convergence is displayed by nonmetamorphic folds, thrusts, and high-angle normal faults which formed during the same time interval as the discrete shear zones. During stage II (160–150 Ma), strong arc-normal extension is revealed by brittle low-angle normal faults at shallow levels and some ductile normal faults and the intrusion of extended plutons at deeper levels. During stage III (155–147 Ma), two reversals in the stress regime took place indicated by two generations of dikes, an older one trending NE–SW and a younger one trending NW–SE. Sinistral strike-slip movements also prevailed during stage IV (until ∼125 Ma) when the Atacama Fault Zone originated as a sinistral trench-linked strike-slip fault. The tectonic evolution of the magmatic arc is interpreted in terms of coupling and decoupling between the downgoing and overriding plates. The structures of stages I and IV suggest that stress transmission due to seismic coupling between the plates was probably responsible for these deformations. However, decoupling of the plates occurred possibly due to a decrease in convergence rate resulting in extension and the reversals of stages II and III.

Trench-parallel shortening in the Northern Chilean Forearc: Tectonic and climatic implications

In the Central Andes, the frictional coupling between South America and the subducting Nazca Plate occurs beneath the Coastal Cordillera of northern Chile. One of the most distinctive characteristics of the Coastal Cordillera is a suite of EW topographic scarps located between 19° and 21.6°S latitude. These scarps are associated with predominantly south dipping reverse faults that have almost pure dip slip and produce shortening parallel to the plate boundary. Limited geochronology as well as more regional studies indicate that the scarps formed during the late Miocene and early Pliocene, though some show activity extending into the Quaternary. In several areas, the scarps dammed local drainages, producing internally drained basins that accumulated evaporites. This relationship indicates that the Coastal Cordillera was probably moister during the Late Miocene and Pliocene than it is today and also indicates that the Coastal Cordillera has been significantly uplifted or that the Coastal Escarpment of northern Chile has advanced significantly eastward since the Pliocene. The limited latitudinal extent of the EW scarps and their location symmetrically about the axis of topographic and Wadati-Benioff zone symmetry suggest that they owe their origin to the concave seaward shape of the continental margin due to prior formation of the Bolivian orocline.

Pervasive cracking of the northern Chilean Coastal Cordillera: New evidence for forearc extension

Despite convergence across the strongly coupled seismogenic interface between the South American and Nazca plates, the dominant neotectonic signature in the forearc of northern Chile is arc-normal extension. We have used 1 m resolution IKONOS satellite imagery to map nearly 37,000 cracks over an area of 500 km2 near the Salar Grande (21°S). These features, which are best preserved in a ubiquitous gypcrete surface layer, have both nontectonic and tectonic origins. However, their strong preferred orientation perpendicular to the plate convergence vector suggests that the majority owe their formation to approximate east-west extension associated with plate boundary processes such as interseismic loading, coseismic and postseismic strain, and long-term instability resulting from subduction erosion. Similar structures were formed during or shortly after the 1995 Mw = 8.0 earthquake near the city of Antofagasta, south of Salar Grande, and in conjunction with the 2001 Mw = 8.2–8.4 Arequipa, Peru, event. Cracks such as these may form in other forearcs but remain largely unexposed because of vegetative cover or marked fluvial erosion—factors that are absent in northern Chile as a result of its hyperarid climate.

Surface cracks record long-term seismic segmentation of the Andean margin

Understanding the long-term patterns of great earthquake rupture along a subduction zone provides a framework for assessing modern seismic hazard. However, evidence that can be used to infer the size and location of past earthquakes is typically erased by erosion after a few thousand years. Meter-scale cracks that cut the surface of coastal areas in northern Chile and southern Peru preserve a record of earthquakes spanning several hundred thousand years owing to the hyperarid climate of the region. These cracks have been observed to form during and/or shortly after strong subduction earthquakes, are preserved for long time periods throughout the Atacama Desert, demonstrate evidence for multiple episodes of reactivation, and show changes in orientation over spatial scales similar to the size of earthquake segments. Our observations and models show that crack orientations are consistent with dynamic and static stress fi elds generated by recent earthquakes. While localized structural and topographic processes infl uence some cracks, the strong preferred orientation over large regions indicates that cracks are primarily formed by plate boundary‐scale stresses, namely repeated earthquakes. We invert the crack-based strain data for slip along the well-known Iquique seismic gap segment of the margin and fi nd consistency with gravity anomaly‐based inferences of long-term earthquake slip patterns, as well as the magnitude and location of the November 2007 Tocopilla earthquake. We suggest that the meter-scale cracks can be used to map characteristic earthquake rupture segments that persist over many seismic cycles, which encourages future study of cracks and other small-scale structures to better constrain the persistence of asperities in other arid, tectonically active regions.

Alzamiento litoral Pleistoceno del norte de Chile: edades 21Ne de la terraza costera más alta del área deCaldera-Bahía Inglesa

En este trabajo se presenta la edad de la terraza costera emergida mas alta del area de Caldera-Bahia Inglesa, localizada a una altura de 224±6 m s.n.m. La metodologia empleada consiste en el uso de edades de exposicion de clastos de cuarzo mediante 21Ne de origen cosmogenico. La edad obtenida fue de 0,86 ± 0,11 Ma. Dentro de este rango de edad, se desarrollan tres interestadiales fuertes correspondientes a los estadios isotopicos MIS 19 (780 ka), MIS 21 (860 ka) y MIS 25 (950 ka) en los cuales se pudo formar esta terraza costera. La tasa de alzamiento promedio considerando estos tres casos es de 0,28±0,02 mm/a, cuyo valor es menor que el determinado por autores previos para los ultimos 400 ka. Entre 750 y 400 ka no se desarrollaron terrazas costeras en el area de estudio. Esto seria un indicador de tasas de alzamiento bajas e interestadiales debiles durante ese periodo, y la fortaleza del interestadial desarrollado durante el estadio MIS 11 (400 ka) que provoco una importante erosion de la topografia litoral incluyendo las terrazas costeras que pudieron haberse formado entre 750 y 400 ka. El alzamiento litoral en el area de estudio es de caracter regional y estaria relacionado con procesos generados como consecuencia de la convergencia de las placas de Nazca y Sudamericana; e.g., los terremotos de subduccion

Coeval compressional deformation and volcanism in the central Andes, case studies from northern Chile (23°S–24°S)

Received 22 May 2009; revised 6 August 2009; accepted 24 August 2009; published 1 December 2009. [1] In this contribution we examine the relationship between active compression and construction of Pleistocene volcanoes in the present-day magmatic arc of the central Andes (23S–24S). Deformation producedseveralN–Sstriking, � 40kmlongsubparallel ridges. These ridges formed by folding of Pliocene ignimbrites and upper Pliocene and Pleistocene lavas; they are asymmetrical in profile and have a gentle back limb and steeper frontal limb. Andesitic monogenetic volcanoes show a close spatial relationship with the ridges; some volcanoes are on the hinge zone, whereas others lay on the sides of the ridges. We interpret this spatial pattern as a result of magma storage and migration along a system of subhorizontal reservoirs and reverse faults. Magma reservoirs probably formed along flat portions of reverse faults between ramp structures that serve as episodic magma transport

Neogene constriction in the northern Chilean Coastal Cordillera: Neotectonics and surface dating using cosmogenic 21Ne

Neogene constriction in the northern chilean Coastal Cordillera: Neotectonics and surface dating using cosmogenic 21 Ne. This work documents fault activity and the Neogenes strain fi eld in northern Chilean Coastal Cordillera. Fault activity is expressed as a group of fault scarps and fault-bend fold scarps whose orientation defi nes three main domains WNW-ESE, N-S and NNW-SSE. The WNW-ESE and N-S faults show reverse kinematics, and NNW-SSE faults shows dextral-reverse kinematics. Exposure ages using cosmogenic 21 Ne show that the faults disrupt an Oligocene-Miocene landscape preserved at the Coastal Cordillera. Inactive valleys incised in this landscape are offset by the faults showing that faults were active after 4 and 2 Ma. 40 Ar/ 39 Ar chronology of displaced volcanic tuffs and the deformation of Late Pleistocene sediments indicate that fault activity remain still active during the Quaternary. The deformation regime is constrictional and characterized by subhorizontal shortening in all directions, that is explained by oblique convergence along an active curved continental margin.

The 1 April 2014 Pisagua tsunami: Observations and modeling

On 1 April 2014, an earthquake with moment magnitude Mw 8.2 occurred off the coast of northern Chile, generating a tsunami that prompted evacuation along the Chilean coast. Here tsunami characteristics are analyzed through a combination of field data and numerical modeling. Despite the earthquake magnitude, the tsunami was moderate, with a relatively uniform distribution of runup, which peaked at 4.6 m. This is explained by a concentrated maximal slip at intermediate depth on the megathrust, resulting in a rapid decay of tsunami energy. The tsunami temporal evolution varied, with locations showing sustained tsunami energy, while others showed increased tsunami energy at different times after the earthquake. These are the result of the interaction of long period standing oscillations and trapped edge wave activity controlled by inner shelf slopes. Understanding these processes is relevant for the region, which still posses a significant tsunamigenic potential.

Upper Plate Reverse Fault Reactivation and the Unclamping of the Megathrust During the 2014 Northern Chile Earthquake Sequence

After 137 years without a great earthquake, the Mw 8.1 Pisagua event of 1 April 2014 occurred in the central portion of the southern Peru–northern Chile subduction zone. This megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating ∼3.5 km/day toward the mainshock hypocenter. This foreshock sequence was triggered by an Mw 6.7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench, similar to well-documented reverse faults onshore. These margin-oblique reverse faults accommodate north-south shortening resulting from subduction across a plate boundary that is curved in map view. Reverse slip on the crustal fault unclamped the subduction interface, precipitating the subsequent megathrust foreshock activity that culminated in the great Pisagua earthquake. The combination of crustal reverse faults and a curved subduction margin also occurs in Cascadia and northeastern Japan, indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity.

Segmentación, cinemática y cronología relativa de la deformación tardía de la Falla Salar del Carmen, Sistema de Fallas de Atacama, (23°40'S), norte de Chile

La Falla Salar del Carmen es una de las estructuras principales del Sistema de Fallas de Atacama, que se ubica al este de Antofagasta en el borde oriental de Sierra del Ancla. El evento de deformacion mas reciente a lo largo de esta falla dio origen a la formacion de siete segmentos de falla de orientacion submeridiana, cuya longitud promedio es de 8 km. Los segmentos muestran un escalonamiento lateral izquierdo, cuyos extremos estan ligados por fallas de transferencia. La deformacion formo escarpes de 0,2 a 9 m de altura en depositos aluviales pliocenicos. Los escarpes mas antiguos estan caracterizados por un talud de detritos, en tanto que en los mas jovenes se conserva aun la cara libre. El proceso de formacion de escarpes estuvo controlado por deslizamientos normales a lo largo de fallas subverticales. El estado de deformacion esta dado por un eje de extension buzante 33° en la direccion N90E y una direccion de acortamiento buzante 56&° en la direccion N87W. La estimacion de la edad mediante datacion morfologica de escarpes, indica que las fallas no serian mas antiguas que el Pleistoceno tardio (< 400 Ka). Grietas con desplazamiento vertical centimetrico, formadas durante el terremoto de Antofagasta del 30 de Julio de 1995 (Mw=8,1), evidencian que este sistema experimenta reactivacion cosismica asociada a sismos de subduccion. Los deslizamientos verticales 1 m, medidos en los segmentos estudiados, son probablemente inducidos por sismos de subduccion con Mw8,0