South American Continental Margin Research Articles

Widespread relics of high-pressure metamorphism confirm major terrane accretion in Ecuador: a new example from the Northern Andes

Metamorphic rocks from two sections through the North Andean Cordillera Real in Ecuador contain phengites as relics of high-pressure (HP) metamorphism. Their maximum Si contents per formula unit are (1) 3.25 in Al-rich metapelites, (2) 3.35 in ordinary metapelites and garnet-rich metabasites, and (3) 3.45 in garnet-bearing orthogneisses. Detailed pressure–temperature paths were derived by calculating P–T pseudosections. Garnet-bearing metapelite and metabasite yielded a P–T path starting at 1.4 GPa and 525°C. Peak temperature conditions were 560°C at <1.4 GPa. The retrograde path passed through 0.7–0.8 GPa at 500°C. An adjacent metagranitoid experienced a similar P–T evolution at slightly lower temperatures of about 30–50°C. This is the first reported occurrence of HP metamorphism in the Cordillera Real; it is widespread and evidently was the result of continental collision. We hypothesize that a single microcontinent collided with the South American continental margin in Early Cretaceous time. In contrast to our findings, previous models have simply suggested the amalgamation of several terranes in the late Mesozoic or earlier in the region of Ecuador. Therefore, we propose that a widespread search for HP relics should be undertaken. Such relics provide a general criterion for defining subducted terranes and their respective boundaries.

Coupling of benthic oxygen uptake and silica release: implications for estimating biogenic particle fluxes to the seafloor

A new approach to predict biogenic particle fluxes to the seafloor is presented, which is based on diffusive oxygen uptake and, in particular, opal fluxes to the seafloor. For this purpose, we used a recently published empirical equation coupling benthic silica to oxygen fluxes, and showing a clear negative correlation between Si and O2 fluxes. Dissolution of biogenic silica mediated by aerobic microbial activity has been inferred at 24 sites along the African and South American continental margins. Based on the assumption that these findings hold essentially for the entire Southern Atlantic Ocean, we applied the silica to oxygen flux ratio to a basin-wide grid of diffusive oxygen uptake extracted from the literature. Assuming that the silica release across the sediment-water interface equals the particulate flux of biogenic opal to the seafloor, we estimated minimum opal rain rates. We combined these calculations with published relationships between aerobic organic carbon mineralization and dissolution rates of calcite above the hydrographical lysocline, thereby assessing the calcite rain rate and particulate organic carbon flux to the seafloor. The addition of the buried fraction completes our budget of biogenic particulate rain fluxes. The combination of such empirical equations provides a powerful and convenient tool which greatly facilitates future investigations of biogenic particle fluxes to the seafloor.

Tectonomagmatic setting and provenance of the Santa Marta Schists, northern Colombia: Insights on the growth and approach of Cretaceous Caribbean oceanic terranes to the South American continent

Metamorphosed volcano-sedimentary rocks accreted to the northern South American continental margin are major vestiges of the Caribbean oceanic plate evolution and its interactions with the continent. Selected whole rock geochemistry, Nd–Sr isotopes and detrital zircon geochronology were obtained in metabasic and metasedimentary rocks from the Santa Marta and San Lorenzo Schists in northernmost Colombia. Trace element patterns are characterized by primitive island arc and MORB signatures. Similarly initial 87Sr/86Sr-εNd isotopic relations correlate with oceanic arcs and MORB reservoirs, suggesting that the protoliths were formed within a back-arc setting or at the transition between the inta-oceanic arc and the Caribbean oceanic crust. Trace element trends from associated metasedimentary rocks show that the provenance was controlled by a volcanic arc and a sialic continental domain, whereas detrital U/Pb zircons from the Santa Marta Schists and adjacent southeastern metamorphic units show Late Cretaceous and older Mesozoic, Late Paleozoic and Mesoproterozoic sources. Comparison with continental inland basins suggests that this arc-basin is allocthonous to its current position, and was still active by ca. 82 Ma. The geological features are comparable to other arc remnants found in northeastern Colombia and the Netherland Antilles. The geochemical and U/Pb detrital signatures from the metasedimentary rocks suggest that this tectonic domain was already in proximity to the continental margin, in a configuration similar to the modern Antilles or the Kermadec arc in the Pacific. The older continental detritus were derived from the ongoing Andean uplift feeding the intra-oceanic tectonic environment. Cross-cutting relations with granitoids and metamorphic ages suggest that metamorphism was completed by ca. 65 Ma.

Electrical conductivity beneath the Bolivian Orocline and its relation to subduction processes at the South American continental margin

A long‐period magnetotelluric data set was obtained during 2002 and 2004 in the central Andes to study the deep electrical conductivity structure in the region of the Bolivian Orocline between latitudes 17°S and 19°S. The profile extends from the Coastal Cordillera in northernmost Chile, crosses the volcanic arc and the Altiplano high plateau in central Bolivia, and ends in the Eastern Cordillera. Two‐dimensional inversion revealed several well‐defined conductivity anomalies: in upper crustal levels the conductive sedimentary basins of the central Altiplano and the resistive Arequipa block beneath the western Altiplano are imaged. Earlier seismological and magnetotelluric investigations on the southern Altiplano inferred a large, highly conductive (partially molten) body in the mid to deep crust. It was assumed that this structure would be underlying the entire plateau, but this is not the case according to the new models. Instead, the most prominent feature in the new investigation area is a high‐conductivity zone at upper mantle depths below the high plateau, which may be interpreted as an image of partial melts and fluids triggered by water supply from the subducting Nazca slab. This conductor would be in accordance with the standard subduction scenario; it is, however, laterally offset by almost 100 km from the volcanic arc. In contrast, the deep crust and upper mantle beneath the arc is moderately resistive. Both observations may hint at an emerging shift of the magmatic/fluid system in the central Andes.

The late quaternary paleoenvironment of Chile as seen from marine archives

Abstract. Many variables have been used to reconstruct Chilean paleoenvironmental changes during the late Quaternary. In this paper we present an overview of a number of these variables, so-called proxies, that have been inferred from marine sediments from the Chilean continental margin and summarise the results. In general, a glacial-interglacial pattern of climate changes can be recognised in the proxy records with high-frequency variabilities superposed. The synthesis shows that the records in the Southeast Pacific are clearly dominated by a high-latitude climate forcing mechanism and that there is a noticeable gradual increase of tropical forcing moving from south to north along the South American continental margin.

El Complejo Metamórfico Bahía Mansa en la cordillera de la Costa del centro-sur de Chile (39°30'-42°00'S): geocronología K-Ar, 40Ar/39Ar y U-Pb e implicancias en la evolución del margen sur-occidental de Gondwana

El Complejo Metamorfico Bahia Mansa (CMBM) esta formado por esquistos peliticos, metagrauvacas y esquistos maficos de afinidades oceanicas, con menor proporcion de cuerpos maficos y ultramaficos tectonicamente emplazados, milonitas a ultramilonitas y escasos cuerpos intrusivos traquiticos e intercalaciones de metaignimbritas. Las asociaciones de minerales metamorficos observados tanto en las secuencias metapeliticas como en las metavolcanicas indican que todo el conjunto litologico fue afectado por una fase de deformacion y metamorfismo principal (D2) en la facies de esquistos verdes, de caracter ductil y penetrativo, que ha obliterado completamente las caracteristicas originales de las rocas, exhibiendo ellas una tipica fabrica caracterizada por el desarrollo de foliacion (S2). La presencia local de anfibolas azules en esquistos maficos, de glaucofano en bloques de esquistos azules y de zussmanita en esquistos peliticos ricos en hierro, senalan la existencia de una fase de deformacion y metamorfismo previa (D1) en condiciones de alto gradiente P/T, en la facies de esquistos azules, actualmente evidenciada por texturas relictas (S1), tales como micropliegues y foliacion interna en albita, microfoliacion intrafolial y pliegues isoclinales desraizados de cuarzo. Las edades de enfriamiento 40Ar/39Ar y K-Ar en minerales indican que la deformacion y metamorfismo principal ocurrio durante el Permico-Triasico, en el lapso 260-220 Ma, mientras que la fase de alta presion-baja temperatura lo hizo, probablemente, durante el Carbonifero Superior, en el lapso 320-300 Ma. Una edad U-Pb en circones de 396 Ma obtenida en un cuerpo traquitico, que intruye a esquistos maficos, entrega indirectamente una edad minima devonica inferior para una porcion de estas rocas. Edades U-Pb en circones detriticos senalan edades maximas de deposicion en el Devonico Medio y el Permico Inferior para algunos componentes sedimentarios del complejo, lo cual implica que el metamorfismo y deformacion en la facies de esquistos azules habria afectado solo a las rocas con edades maximas de deposicion mas antiguas. Asi, en el CMBM se verifican, al menos, dos episodios de sedimentacion y similar numero de eventos de deformacion y de metamorfismo. El CMBM habria evolucionado durante el lapso Devonico-Triasico en el borde sur-occidental de Gondwana. Mas al sur, otros complejos metamorficos habrian evolucionado en el Triasico-Jurasico y en el Jurasico-Cretacico, lo cual implica un diacronismo en la acrecion en el borde continental de Gondwana y Sudamerica.

Timing and origin of deformation along the Patagonian fold and thrust belt

The Andean orogeny in the Patagonian Cordillera of southern South America reflects the consequences of the Mesozoic and Cenozoic subduction of an oceanic plate beneath the South American continental margin. The geological evolution of the region has been influenced by the Eocene collision and subduction of the Farallon–Aluk Ridge and the Miocene–Recent subduction of the Chile Ridge. Another aspect of plate interaction during this period was two intervals of rapid plate convergence, one at 50–42 Ma, and the other at 25–10 Ma, between the South American and the oceanic plates. It has been proposed that the collision of the Chile Ridge with the trench was responsible for the development, at least in part, of the Patagonian fold and thrust belt. This belt extends for more than 1000 km along the eastern foothills of the southern Andes between 46° and 54° S along the southwestern rim of the Austral Basin. The interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during Middle and Late Miocene times. The main Tertiary contractional events in the Patagonian fold and thrust belt took place during latest Cretaceous–Palaeocene–Eocene and during Miocene times. Although the timing of deformation is still poorly constrained, the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments. Most if not all of the contractional tectonism pre-dated the latest episodes of ridge collision. Collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years. Contrary to the suggestion of a relationship between ridge subduction and compression, the main result of this collision has been fast uplift and extensional tectonism. The initiation of the Patagonian fold and thrust belt in latest Cretaceous or early Tertiary times coincided with a fundamental change in the tectonic evolution of the Austral Basin. Throughout the Cretaceous most of this basin subsided as a broad backarc continental shelf. Only in latest Cretaceous times, and coinciding with the initiation of the fold and thrust belt, the basin underwent a transition to a retro-arc foreland basin. This change to an asymmetrically subsiding foreland basin, with an associated foreland fold and thrust belt, was related to uplift of the Andean orogenic belt in the west.

Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data

Magnetotelluric and geomagnetic deep soudings in northern Chile revealed a pronounced high conductivity zone (HCZ). Below the Western Cordillera, which constitutes the present magmatic arc with active volcanism of the South American continental margin, conductivities in the range of 1 S/m are observed. The anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km, are interpreted in terms of partial melting. Other geophysical observations, such as a zone of low seismic velocities (LVZ) at similar depths, high heat flow values (> 100 mW/m 2) and a pronounced negative anomaly in the residual gravity field, are also considered. Impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities. At sub-solidus temperatures, electrical behavior is described by defect electrons with an activation energy of 1.34 eV and a conductivity of 2.5 mS/m at 900°C. Model calculations using a modified-brick-layer model (MBL) were compared with experimental observations. A good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 S/m at 1250°C assuming an activation energy of 1 eV. The same MBL model is used to calculate melt proportions beneath the Western Cordillera. Between 14 and 27 vol.% of interconnected melt are necessary to explain the observed HCZ. The stability of the melt rich crust is explained by a dynamic melting-crystallisation behavior during crustal anatexis and by magma filled dikes.

Magnetic properties of Argentine basin project MUDWAVE samples

The magnetic properties of samples from nine box cores and three piston cores recovered from three giant mudwaves in the Argentine Basin, South Atlantic Ocean, have been measured. Measurements on mudwave samples include magnetic susceptibility, natural remanent moment and anhysteretic remanent moment. Anisotropy of magnetic susceptibility was also measured on selected box core samples. The results have been compared to magnetic data already available from similar measurements on Argentine Basin core-top samples. Magnetic characteristics for the core-top samples reflect broad changes in magnetic mineral concentration rather than substantial changes in magnetic grain size across the basin. Changes in this pattern are caused by multiple sources of terrigenous sediment influx from the South American continental margin. The mudwave fields in the center of the basin act as a sediment sink for these materials and are generally characterized by uniform magnetic-grain concentrations, compositions and small grain-sizes. Between and within the mudwaves sampled there are systematic differences as well as similarities in the magnetic properties that appear to be related, at least in part, to sample location with respect to depositional or erosional slopes within each mudwave. The magnetic results are interpreted to indicate that clear changes in bottom water flow are represented in samples recovered from the mudwaves.

Structure of the Cordillera de la Costa Belt, North-Central Venezuela: Implications for Plate Tectonic Models

Preliminary results of an on-going study of the Cordillera de la Costa belt between Puerto Cabello and Choroni, north-central Venezuela, indicate that the deformational history is far more complicated than expected from simple plate-tectonic models. The Cordillera de la Costa belt consists of oceanic rocks (e.g., serpentinites, amphibilites, with lenses of eclogite and blueschist) intimately intermixed with metamorphosed continental margin deposits (e.g., mica and graphite schist, quartzite, marble). Locally, large granitic (basement ) complexes of Lower Paleozoic age are included as well. In late Cretaceous time, the entire belt was involved in four synmetamorphic deformations phases (D[sub 1a] to D[sub 1d]); the first (D[sub 1a]) occurred at depths of at 35-40 km and the later ones at successively shallower depths. This deformation occurred in a subduction zone, related to right-oblique convergence of the Farallon and Atlantic plates. The most penetrative structures resulted from (all in present coordinates) north-south contraction and east-west dextral simple shear (D[sub 1b]). During an Early Tertiary ( ) event (D[sub 2]), the belt was emplaced southward onto the South American continental margin. Subsequent deformational structures (D[sub 3]) resulted in cross folds and faults (with small pull-apart basins) which are consistent with the eastward passage of themore » Caribbean past the South American plate.« less

EXPLORATION POTENTIAL OF TRINIDAD AND TOBAGO

The Maturin Basin of eastern Venezuela, including Trinidad, has produced approximately 9.5 billion brls of oil, and is the site of significant new discoveries at El Furrial and El Carito. The present structural basin was formed by oblique compression during Oligocene to Miocene time, with the northern flank being a folded and thrusted terrane over-riding the South American continental margin. This deformed northern flank extends into the subsurface to the basin axis, and includes the southern half of the onshore and offshore territory of the Republic of Trinidad and Tobago. The basin was infilled from west to east by deep-marine shales, turbidites, pelagic oozes and deltaic sediments of Neogene age. Deltaic sands form the reservoirs of all the important oilfields in the basin. Oil occurrence on the northern flank of the basin is associated with thrust-related structures in Venezuela and wrench-related structures in Trinidad.

Alkaline magmatism within the segment 38°–39°S of the Plio‐Quaternary Volcanic Belt of the southern South American Continental Margin

In the segment 38°–39°S of the southern South American volcanic belt, the subalkaline central‐vent volcanoes that form the late Quaternary volcanic front of the Andean orogenic arc are located in the Main Andean Cordillera 30–80 km west of the centers that formed the Pliocene/early Pleistocene volcanic front. The Pliocene/early Pleistocene volcanic front was located along a precordilleran uplift that diverges southeast from the Main Andean Cordillera. During the Pliocene the centers along this uplift erupted subalkaline magmas similar to those currently being erupted along the late Quaternary front. In the early Pleistocene the centers at the southeast end of this uplift began to erupt alkaline magmas. By the late Pleistocene, magmatic activity in these centers had diminished, and the volcanic front had migrated westward to its current location, but alkaline back arc basaltic volcanism continued in the late Quaternary within the valleys flanking the precordilleran uplift on which the Pliocene/early Pleistocene arc had been situated. The early Pleistocene alkaline orogenic arc lavas and the late Quaternary alkaline back arc basalts have La/Yb higher than, but Ba/La and 87Sr/86Sr similar to, the subalkaline magmas erupted from centers of both the Pliocene and late Quaternary volcanic fronts. Their TiO2 and La/Nb are intermediate between subalkaline magmas erupted from both the Pliocene and late Quaternary volcanic fronts and alkaline basalts of the Patagonian plateau lavas south of 39°S which have intraplate oceanic island petrochemical characteristics. Compared with the subalkaline lavas of the current volcanic front, Quaternary alkaline arc and back arc lavas between 38° and 39°S formed by lower degrees of partial melting of a similar mantle source. This mantle had been modified by source region contamination processes related to dehydration of subducted oceanic lithosphere such as occur below the current volcanic front in the southern Andes.

Controls on magnetic grain-size variations and concentration in the Argentine Basin, South Atlantic Ocean

Measurement of two magnetic parameters, magnetic susceptibility (ξ) and anhysteretic remanent moment (ARM), was performed on 192 core-top samples from the Argentine Basin of the South Atlantic Ocean. Susceptibility indicates the concentration of magnetic grains in the sediment sample, while ARM estimates changes in magnetic mineral grain size: large ARM values indicate finer grain sizes. ARM normalized for magnetic concentration variation (ARM/ξ) increases precision in magnetic grain-size estimation. In the Argentine Basin, the magnetic characteristics reflect broad changes in magnetic mineral concentration rather than a substantial change in magnetic grain size across the basin. Data variations are consistent with deep-water circulation and sediment sources in the basin. The center of the basin acts as a sediment sink and is characterized by uniform magnetic grain concentrations and small grain sizes. Localized sediment input to abyssal depths from the South American continental margin dramatically modifies the magnetic pattern along the western margin of the basin. The data also show a marked downstream decrease in bulk magnetic grain content of the sediment as Antarctic Bottom Water enters the basin to the southeast and moves first westward and then northward along the margin of the basin.

The quaternary volcanic belt of the southern continental margin of South America: Transverse structural and petrochemical variations across the segment between 38°S and 39°S

In the segment between 38°S and 39°S of the southern South American continental margin volcanic belt, the volcanoes of the Quaternary “orogenic arc” may be divided into two groups: the first forming the volcanic front, which is located within the Cordillera Principal along a trend oriented 15° east of north; the second east of the volcanic front, occurring upon an uplifted precordilleran block that is east of the Bío-Bío/Aluminé fault system, trends west of north, and merges with the Cordillera Principal near 38°S. Quaternary “back-arc” plateau lavas, consisting of basalts erupted from small cones, occur in the valley east of this precordilleran block. Quaternary intra-arc and back-arc extension appear to be occurring in this section (38–39°S) of the southern South American volcanic belt and perhaps as far north as 34°S. South of 39°S, intra-arc extension is absent and the orogenic arc is restricted to the Cordillera Principal. Differences in the structural characteristics of the arc north and south of 39°S may be due to the change in age from older to younger oceanic lithosphere being subducted beneath, respectively, the two regions. Basalts erupted from both Quaternary arc and back-arc centers located east of the volcanic front between 38°S and 39°S have higher K 20, Rb, Ba, and LREE contents, and higher La/Yb but lower Ba/La ratios, than basalts erupted along the volcanic front. 87Sr/ 86Sr ratios for arc lavas from volcanic centers both within and east of the volcanic front range from 0.7038 to 0.7041 independent of their Si0 2 content, suggesting that more silicic compositions developed from basalts by crystal-liquid fractionation without significant crustal assimilation. Back-arc plateau basalts from the region 38–39°S have Sr isotopic compositions similar to those of the arc volcanoes. Basalts from both the arc and the back-arc centers between 38°S and 39°S may form by partial melting of a large-ion-lithophile-element-enriched mantle source similar to the source of mantle-xenolith-bearing Quaternary alkali plateau basalts from southern Patagonia (south of 39°S), further modified by the addition of alkali elements derived from subducted oceanic lithosphere.

Plate kinematic implications of Atlantic equatorial fracture zone trends

We present a plate kinematic evolution of the South Atlantic which is based largely on the determination of the equatorial fracture zone trends between the African and South American continental margins. Four main opening phases are dated by oceanic magnetic anomalies, notably MO, A34, and A13, and are correlated with volcanism and tectonic events on land around the South Atlantic Ocean. The Ceara and Sierra Leone rises are probably of oceanic origin and were created 80 m.y. ago or later in their present‐day positions with respect to South America and Africa.