The carbonate successions of the Cerro El Árbol and Cerros de Cuevitas formations in the central region of Antofagasta are herein studied. These stratigraphic units are characterized by an invertebrate marine fossil assemblage integrated by brachiopods, bivalves, gastropods, crinoids and bryozoans, in which the brachiopod Kochiproductus Dunbar is the dominant component. This genus, widely known in the Permian sequences of the world, can be considered a key to understand the possible connection between northern Chile and the central western Argentinean basins. Although the fauna studied shows clear affinities with those from Bolivia and Perú, it is notably less diverse and its composition appears to be not exactly equivalent. The taxonomic knowledge reached, indicates that this fauna, particularly brachiopods and bivalves, would represent a mixed-assemblage. From a biostratigraphic perspective, the fauna of northern Chile could be considered intermediate between the oldest Late Carboniferous-Early Permian faunas of central western Argentinian basins and the younger ones from Bolivia and Perú.
The central Andes of northern Chile and northwestern Argentina developed in a largely autochthonous, intracontinental setting during Proterozoic and Paleozoic time through a recurrent sequence of extensional and compressional tectonic regimes. The exposed pre-Andean crust consists of tectonically isolated outcrops of: (1) metamorphosed basement, (2) plutonic bodies, and (3) slightly metamorphosed volcanic and sedimentary strata resting upon and intruded by multiple plutonic units. U-Pb and Nd-Sm geochronology indicates the existence of a Precambrian foundation for the central Andes dating from at least middle Proterozoic time. Three episodes of Precambrian/Paleozoic deformation/magmatism are recognized: (1) the Panamerican at the Precambrian/Cambrian transition, (2)...
La interaccion entre magmas acidos y basicos ha sido ampliamente documentada en plutones y batolitos de arcos magmaticos. El Pluton Sierra de Varas (PSV), un cuerpo granitico eliptico zonado expuesto en el norte de Chile, y que se emplazo durante el limite Carbonifero-Permico (~300 Ma), muestra evidencias de interaccion sinmagmatica con magmas basicos durante el emplazamiento. Esta interaccion se manifiesta en la presencia de intrusiones de gabro, enclaves maficos y diques segmentados, los que exhiben bordes enfriados, contactos lobulados e intercambio de xenocristales con la roca granitica. Los datos termobarometricos en rocas del Pluton Sierra de Varas indican temperaturas de ~750°C para el equilibrio anfibola-plagioclasa y de 0,26 a 0,36 GPa (Al total en hornblenda), estimandose una profundidad de emplazamiento de entre 7 y 10 km. La termobarometria de anfibolas en diques diabasicos sinmagmaticos entrega temperaturas de 785 a 840°C y presiones de 0,20-0,45 GPa (barometro semicuantitativo de Al2O3-TiO2) para los centros de los cristales. El paso progresivo desde diques segmentados a enclaves elipticos se atribuye a un proceso de 'mingling' entre el magma acido y los magmas basicos que intruyeron episodicamente durante el enfriamiento del pluton, en el nivel de emplazamiento del Pluton Sierra de Varas en la corteza superior.
Brachiopods and graptolites are reported for the first time from the Quebrada
Grande Formation exposed at Cordon de Lila, south of the Salar de Atacama. The stratigraphic succession,
over 1000 m thick, consists of conglomerates interbedded with sandstones and shales. The calcareous microconglomerates
and calcarenites laying near the base of the unit have yielded the brachiopods
Paralenorthis sp., Monorthis transversa Benedetto, and Mollesella? sp. The three taxa have been recorded elsewhere
in the Sierra de Famatina, western Argentina, from beds of well-constrained Arenig age (O. evae
-B. navis conodont zones). The graptolite fauna recovered c. 300 m above the brachiopod levels consists of
Tetragraptus sp. and Cryptograptus? sp., suggesting a Yapeenian-early Darriwilian age (approximately late
Arenig-early Llanvirn of the British Series). This discovery is significant as it constitutes the westernmost
occurrence of Ordovician marine faunas in the Argentine-Chilean Puna region, and supports a paleogeographic
connection between the western Puna Basin and the Famatina volcaniclastic belt.
The Sierra de Varas Norte pluton is a tonalitic intrusive body with an elliptical shape on the horizontal, it has a 12 km long axis by a recostructed 8 km width. It has a funnel shape in vertical section and is calcalkaline in composition. It was emplaced during the Late Carboniferous into a stratigraphic succession of Pennsylvanian to Permian? age which belongs to the Sierra de Varas Formation. The pluton contains a concentric, steeply inclined magmatic foliation enhanced by concordant 'schlieren', strangled synplutonic microdioritic dykes and microdioritic enclaves. The enclaves are more abundant towards the pluton margin and they are probably related to the dykes. Ellipticity measurements on the enclaves yield an oblated strain ellipsoid with Flinn parameter k=0,5, characterized by an X axis longer than the Y axes, with the XY plane contained in the magmatic foliation. In the deformed country rock an oblate strain ellipsoid also with Flinn parameter k=0,5 was obtained from deformed oncolites. The folding, axial plane cleavage and contact metamorphism in the country rock are more intense towards the contact with the pluton. This strongly suggests a mostly lateral flattening. The pluton and the country rock fabrics are interpreted to be concurrent and derived from a ballooning forceful emplacement of the pluton in shallow levels of the continental crust. The stretched oblate form of the strain ellipsoid (X longer than Y), and also the plunge towards the centre of the pluton of the stretching lineation in the country rock , suggest that the pluton-country rock system retained the memory of a diapiric ascent.
RESUMEN. El margen continental en las inmediaciones de Antofagasta puede subdividirse en tres dominios morfoestructurales: Cordillera de la Costa, plataforma costera y talud continental. La Cordillera de la Costa se encuentra separada de la plataforma costera por el acantilado costero, de cerca de 1.000 m de altura. En la Cordillera de la Costa, fallas norte-sur limitan una morfologia de bloques alzados y deprimidos, asimetricos. Las fallas mas recientes (cuaternarias). Exhiben indicadores cinematicos de movimientos transcurrentes sinistrales. La plataforma costera, la peninsula de Mejillones y el talud continental corresponden a una zona afectada por extension desde el Mioceno al Reciente. Las estructuras de esta zona consisten en cuencas tectonicas asimetricas limitadas por fallas normales en su margen occidental. Movimientos episodicos a lo largo de estas fallas han acomodado la rotacion de bloques segun un eje horizontal. Esta rotacion marca un colapso asimetrico y progresivo de bloques tectonicos hacia el eje de la fosa oceanica. El inicio del colapso esta marcado por la transgresion marina del Mioceno. La existencia del acantilado costero se explica por una fuerte erosion marina retrocedente, favorecida por el clima arido del norte de Chile. La deformacion extensional se explica por erosion tectonica prolongada, que habria producido, a partir del Mioceno, un colapso de la region de antearco proxima al frente de subduccion. En dicho colapso toma un papel importante la Falla Antofagasta, la cual marca el desacople de gran parte del talud continental. Se concluye que los movimientos transcurrentes cuaternarios en la Cordillera de la Costa solo obedecen a un fenomeno local, no ligado al colapso del margen continental, ni tampoco a la subduccion oblicua. Esto, porque el vector de convergencia apunta hacia el este noreste en la actualidad, y por lo tanto, es incompatible con el sentido sinistral determinado para las fallas. ABSTRACT. Cenozoic tectonic evolution of the active continental margin of Northern Antofagasta, Chile. The active continental margin of Antofagasta, in Northern Chile, can be divided into three morphostructural domains. These are from east to west: Coastal Cordillera, coastal platform and Continental slope. The boundary between the Coastal Cordillera and coastal platform is the coastal scarp ca. 1.000 m high. The origin of this remarkable morphologic feature is the result of strong marine erosion that was enhanced by the extreme aridity of Northern Chile. In the Coastal Cordillera a set of north-south trending faults controls geometry of asymmetric tectonic blocks and basins. The younger faults exhibit kinematic indicators for sinistral transcurrence. The coastal platform, Mejillones Peninsula and continental slope show the effects of an extensional tectonics since the Miocene. In all three main domains, the extensional tectonics has controlled the sedimentation in the finger print of tectonic history of the continental margin. The extensional tectonics is here interpreteted as the result of tectonic erosion that acted under the South American plate during most of the Mesozoic and Cenozoico This tectonic erosion is responsible for the collapse of the Continental margin towards the Chile-Peruvian Trench. It is believed that the start of this massive collapse is contemporaneous with the Miocene marine transgression. An important part in this collapse is taken by the Antofagasta Fault, which is the material fracture feature against which the collapse took place. It is concluded that the youngest transcurrent sinistral displacements along longitudinal faults in the Coastal Cordillera are only local phenomena, and cannot be related to the massive collapse of the Continental margin, nor to the oblique subduction vector, which at present, has an east northeast direction.
El Sistema de Fallas de la Cordillera de Domeyko, de rumbo esencialmente norte-sur, es paralelo a la fosa chileno-peruana en el norte de Chile. Se analizaron los desplazamientos en el rumbo y en la vertical de una de sus fallas maestras: la Falla Sierra de Varas (FS V). Esta se estudio en el segmento entre Aguada del Hornito y Aguada del Cerro Alto de Varas, sobre la base del desplazamiento en el rumbo de dos porciones de la unidad de Granitoides paleozoicos con identica petrografia, estructura interna y edad. Se determino una separacion sinistral horizontal de 15,6±1 km con una componente vertical de 4,9±0,1 km, lo cual implica un rechazo neto sinistral-inverso de 16,4±1 km. El alzamiento vertical esta de acuerdo con el espesor estratigrafico erosionado del bloque oriental. Se investigo, ademas, la dinamica y cinematica de la FS V mediante el analisis de mesofallas asociadas a sus desplazamientos. Se determino asi un sistema estructural sinistral-inverso, que tuvo lugar durante el Eoceno medio tardio. Un segundo sistema estructural dextral, con desplazamiento de 0,6 km se sobrepuso sobre el anterior y ocurrio con posterioridad al Mioceno. Los desplazamientos simultaneos, tanto en el rumbo como segun fallas inversas, dados por la existencia de una 'flor inversa en el segmento estudiado, indican que este fue afectado primeramente por una transpresion. El desplazamiento sinistral demostrado paralaFSVy su cambio de rumbo hacia el SE, inmediatamente al sur de Aguada de Alto de Varas, son compatibles con la vergencia hacia el oeste de pliegues y fallas inversas asociadas del cinturon plegado y corrido de El Profeta, el cual tambien habria resultado de la transpresion.
Two 'conjugate' systems of subvertical ordovician cataclastic bands of metric wide and whose strike are northeast and northwest are studied in the eastern border of Tambillo Monzogranite, also of Ordovician age. The northeastern system is the most conspicuous and its kinematic analysis shows sinistral displacement. The northwestern system is dextral and offsets the northeastern system. The two directions of bands form approximately right angle suggesting that they formed from joints after the Tambillo Monzogranite cooled. The spatial pattern of the cataclastic bands is fractal. It is suggested a north-south orientation for σ 1 . Cataclasis occurred during the Late Tremadocian and could attain the Darriwillian and it is assigned to an early subphase of the Ocloyic movements. A number of granodioritic dykes intruded along the bands that were weak zones used for their emplacement.
Thee trace fossils here studied belong to beds located at the top of the basal section of the Llandoverian (Lower Silurian) Quebrada Ancha Formation in the Cordon de Lila, Antofagasta Region, northern Chile. The measured succession is 275 m thick and is formed by very fine to coarse-grained quartzarenite s with abundant planar and trough cross bedding. At the top of the studied succession beds with moderate to intense bioturbation are exposed in which trace fossils were identified: Arthrophycus linearis, Skolithos linearis, Arthrophycus alleghaniensis, Planolites beverleyensis, Palaeophycus tubularis, Zoophycos circinnatus, Monocraterion tentaculatum and Polykladichnus aragonensis. A. alleghaniensis and A. linearis are frequent in Silurian strata of different parts of South America and other gondwanan and laurentian localities. The stratigraphical and sedimentological analysis of the succession show that it was deposited in a shoreface environme nt, with paleocurrents directed towards the northwest. Specially Arthrophycus has been reported before as a form characteristic of siliciclastic facies of shallow waters in a high en ergy environment, strong tidal currents and sporadic storms. The succession is the first outcro p of Silurian reported in Chile. It formed part of a Chilean-Bolivian basin intermedious between the Arequipa Terrane and the Central argentinian Ridge and is a new fact for the interpretation of the evolution of the southwestern border of Gondwana during the Early Silurian (Llandoverian).
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