Margin Of South America Research Articles

Physical speciation of iron in the Atlantic sector of the Southern Ocean along a transect from the subtropical domain to the Weddell Sea Gyre

Distributions of total dissolvable iron (TDFe; unfiltered), dissolved iron (DFe; 0.2 μm filtered), and soluble iron (SFe; 0.02 μm filtered) were investigated during the BONUS‐GoodHope cruise in the Atlantic sector of the Southern Ocean (34°S/17°E–57°S/0°, February–March 2008). In the mixed layer, mean values of 0.43 ± 0.28 and 0.22 ± 0.18 nmol L−1 were measured for TDFe and DFe, respectively. In deeper waters, TDFe and DFe concentrations were 1.07 ± 0.68 and 0.52 ± 0.30 nmol L−1, respectively. DFe concentrations decreased from the north (subtropical waters) to the south (Weddell Sea Gyre). In the subtropical domain, dusts coming from Patagonia and southern Africa and inputs from the African continental margin may explain high DFe and TDFe concentrations in surface and intermediate waters. Results from numerical models gave support to these hypotheses. In the Antarctic Circumpolar Current domain, estimation of the median advective time of water masses suggests that sediment inputs from the Antarctic Peninsula, South America margin, and/or South Georgia Islands could be an important source of Fe. Except in the subtropical domain where 0.4–0.6 nmol L−1 of SFe were observed in the upper 1500 m, all stations exhibited values close to 0.1–0.2 nmol L−1 in surface and 0.3–0.5 nmol L−1 in deeper waters. For all stations, colloidal Fe (CFe) was a minor fraction of DFe in surface waters and increased with depth. Colloidal aggregation, sinking of CFe, and assimilation of SFe, followed by rapid exchange between the two fractions, are suspected to occur.

Volcanic Stratigraphy and Geochronology of the Cretaceous Lancones Basin, Northwestern Peru: Position and Timing of Giant VMS Deposits

A ~10-km-thick sequence of basaltic to rhyolitic volcanic rocks forms the arc component of the Cretaceous Lancones basin in northwestern Peru and underlies part of the Huancabamba deflection. The marine volcanic successions show markedly different compositional features and depositional facies consistent with a maturing arc within a shallowing marine basin. The earliest volcanism accompanying rifting was dominated by basaltic pillow lava and breccia with lesser aphyric to feldspar-quartz porphyritic felsic volcanic rocks. These volcanic successions filled the lowest exposed portion of the basin and were accompanied by volcanogenic massive sulfide (VMS) deposits, which are inferred to have formed in a localized but relatively deep marine setting. U-Pb zircon dating of felsic volcanic rocks associated with VMS deposits at Tambogrande indicates ages from 104.8 ± 1.3 to 100.2 ± 0.5 Ma for the ore-bearing volcanic sequence. The timing of onset of rift-related volcanism is not well constrained but is therefore of middle Albian age or older. Subsequent latest Albian to Turonian volcanism is composed of successions of relatively more felsic rich volcaniclastic rocks and yields U-Pb zircon ages of 99.3 ± 0.3 to 91.1 ± 1.0 Ma. These later volcanic successions are intercalated and overlain by siliciclastic and carbonate sedimentary sequences prevalent in the western forearc section of the Lancones basin. Finally, the basin was intruded by Late Cretaceous to Tertiary granitoids of the Coastal batholith. The genesis of the Cretaceous Lancones basin and other equivalent volcanic rift-related, marginal basins in western South America, including the western Peruvian trough, is related tectonically to the break-up of Gondwana. Early volcanism and associated VMS deposits formed in the Lancones basin during the Albian coincided with the initial rifting stage, prior to active oceanic spreading, between South America and Africa. During this time the relatively stationary western margin of continental South America was undergoing extension and rifting due to a westward and oceanward retreating arc, resembling a Mariana arc-type setting. The Mochica orogeny marks the termination of rifting, subsidence, and related volcanism along the western margin of South America. This orogenic event also broadly coincides with the onset of spreading of the South Atlantic and westward drift of the South American continent. Subsequent volcanism in the Lancones basin was more continental arclike under an Andean-type scenario.

Structural controls on Quaternary deepwater sedimentation, mud diapirism, and hydrocarbon distribution within the actively evolving Columbus foreland basin, eastern offshore Trinidad

Eastward migration of the Caribbean plate relative to the South American plate has caused lithospheric loading along the northern margin of South America, which is recorded by an 1100-km-long foreland basin which is oldest in the west (Maracaibo basin, 65–55 Ma) and youngest in the east (Columbus basin, eastern offshore Trinidad, 15–0 Ma). The Orinoco River has been the primary source of sediment for the basin since early Miocene. We have integrated approximately 775 km of deep-penetration 2D seismic lines acquired in the area of eastern offshore Trinidad as part of the 2004 “Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region” (BOLIVAR) project, 8000 km 2 of shallow industry 3D seismic data, and published industry well data from offshore eastern Trinidad. Active mud diapirism in the Columbus basin is widespread and is related to overthrusting and tectono-sedimentary loading of upper Miocene-lower Pliocene age mud. Analysis of the shallow 3D seismic data reveals the presence of extensive gravity-flow depositional elements on the Columbus basin slope and the deepwater area. These stacked gravity-flow deposits are characterized by mass-transport deposits at the base, turbidite frontal-splay deposits, leveed-channel deposits, and capped by fine-grained condensed-section deposits. Exploration targets in the deepwater area are located towards the center of the Columbus basin, where northeast-trending fault-propagation folds are important Plio-Pleistocene trap-forming elements. Deep basin wells drilled in recent years have proven that turbidites were transported into the deepwater Columbus basin during the Plio-Pleistocene. Analysis of these well results suggests that a deeper oil charge is present within the deepwater Columbus basin area. The primary uncertainty for this variable hydrocarbon system is whether fault or diapiric pathways connect or divert the petroleum charge at depth with shallower reservoir rocks.

Linking sedimentation in the northern Andes to basement configuration, Mesozoic extension, and Cenozoic shortening: Evidence from detrital zircon U-Pb ages, Eastern Cordillera, Colombia

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses of 29 samples from the Eastern Cordillera of Colombia reveal the origin of northern Andean basement and patterns of sedimentation during Paleozoic subsidence, Jurassic–Early Cretaceous extension, Late Cretaceous postrift subsidence, and Cenozoic shortening and foreland-basin evolution. U-Pb geochronological results indicate that presumed Precambrian basement is mainly a product of early Paleozoic magmatism (520–420 Ma) potentially linked to subduction and possible collision. Inherited zircons provide evidence for Mesoproterozoic tectonomagmatic events at 1200–1000 Ma during Grenville-age orogenesis. Detrital zircon U-Pb ages for Paleozoic strata show derivation from Andean basement, syn depositional magmatic sources (420–380 Ma), and distal sources of chiefl y Mesoproterozoic basement (1650–900 Ma) in the Amazonian craton (Guyana shield) to the east or in possible continental terranes along the western margin of South America. Sedimentation during Jurassic–Early Cretaceous rifting is expressed in detrital zircon age spectra as Andean basement sources, recycled Paleozoic contributions, and igneous sources of Carboniferous–Permian (310–250 Ma) and Late Triassic–Early Jurassic (220–180 Ma) origin. Detrital zircon provenance during continued Cretaceous extension and postrift thermal subsidence recorded the elimination of Andean basement sources and increased infl uence of craton-derived drainage systems providing mainly Paleoproterozoic and Mesoproterozoic (2050–950 Ma) grains. By Eocene time, zircons from the Guyana shield (1850–1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous–Paleocene (90–55 Ma), Jurassic (190–150 Ma), and limited Permian–Triassic (280–220 Ma) zircons recording initial uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Oligocene–Miocene sandstones of the proximal Llanos foreland basin document uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of the Paleogene cover succession rich in both arc-derived detritus (dominantly 180– 40 Ma) and shield-derived sediments (mostly 1850–950 Ma). Late Miocene–Pliocene erosion into the underlying Cretaceous section is evidenced by elimination of Mesozoic– Cenozoic zircons and increased proportions of 1650–900 Ma zircons emblematic of Cretaceous strata.

The North African Mosasaur Globidens phosphaticus from the Maastrichtian of Angola

New mosasaur fossils from Maastrichtian beds at Bentiaba, Angola, representing elements of the skull and postcranial axial skeleton from two individuals of the durophagous genus Globidens, are reported. Based on dental morphology, specifically the inflated posterior surface and vertical sulci, the Bentiaba specimens are identified as Globidens phosphaticus, a species defined by characters of a composite dentition from the Maastrichtian of Morocco. Comparisons indicate that G. phosphaticus is most closely related to G. schurmanni, from the late Campanian of South Dakota, the youngest north American Globidens species at about 72.5 Ma. The morphology of the premaxilla and its relationship with the maxillae is unique among mosasaurs, and supports the taxonomic validity of G. phosphaticus. In contrast with earlier species of the genus, G. phosphaticus is currently known from north and west Africa, the Middle East and the central eastern margin of South America, suggesting it may have been restricted to the Maastrichtian tropical zone as previously hypothesised.

Foraminiferal biostratigraphy and paleoenvironments of the Oligocene-Miocene carbonate succession in Campos Basin, southeastern Brazil

The present investigation of the Cenozoic carbonate succession of the Campos Basin, offshore southeastern Brazil, has resulted in a high resolution biostratigraphic profile of the region based on cores and cutting samples. In general, the studied carbonate successions evolved during three depositional stages: 1) In the Oligocene a warm period prevailed where temperature-sensitive organisms, such as larger benthic foraminifera, corals, algae and arthropods thrived in a reefal setting. Four different benthic foraminiferal biozones (BF1 – BF4) were defined based upon the occurrence of the following foraminiferal assemblages which were dominated by (BF1) lepidocyclinids, (BF2) miogypsinids, (BF3) Miogypsina-Amphistegina, and (BF4) Miogypsina and Heterostegina. These biozones, are equivalent to the P18-P22 planktonic foraminiferal biozones, and reflect temporal changes of environmental and ecological conditions associated with minor sea level fluctuation. 2) The warm period continued through the early Miocene, where corals, bryozoans, rare planktonic foraminifera and larger benthic foraminifera thrived in clear agitated waters of a forereefal setting. This period was represented by two benthic foraminiferal zones, BF5 - BF6, equivalent to the N4-N7 planktonic foraminiferal biozones. Miogypsinids, Lepidocyclina and Heterostegina dominate BF5, while attached forms growing in a massive branching structure, such as Sporadotrema are prominent in BF6. 3) The third depositional stage is reflected by a rapid deepening of the carbonate facies at the onset of the middle Miocene, coeval with a global warming event, the so-called mid-Miocene Climatic Maximum. This resulted in the replacement of the reefal organisms by pelagic sediments containing planktonic foraminifera. The planktonic foraminiferal biozones PF1 and PF2 are equivalent to N8-N12 planktonic foraminiferal biozones, and were present in all wells. This rapid sea level change is inferred to be related to a global warming event and related oceanographic changes. The high resolution depositional and biostratigraphic scheme put forward here for the Campos Basin will enable a more detailed understanding of the general regional evolution of the economically important Cenozoic carbonate facies that are found off the Atlantic margins of South America, the Caribbean and West Africa.

Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America

Abstract Accretionary orogens are considered major sites of formation of juvenile continental crust. In the central and southern Andes this is contradicted by two observations: siliciclastic fills of Paleozoic basins in the central Andean segment of the accretionary Terra Australis Orogen consist almost exclusively of shales and mature sandstones; and magmatic rocks connected to the Famatinian (Ordovician) and Late Paleozoic magmatic arcs are predominantly felsic and characterized by significant crustal contamination and strongly unradiogenic Nd isotope compositions. Evidence of juvenile crustal additions is scarce. We present laser ablation (LA)-ICPMS U–Pb ages and LA-MC-ICPMS Hf isotope data of detrital zircons from seven Devonian to Permian turbidite sandstones incorporated into a Late Paleozoic accretionary wedge at the western margin of Gondwana in northern Chile. The combination with Nd whole-rock isotope data permits us to trace the evolution of the South American continental crust through several Proterozoic and Paleozoic orogenic cycles. The analyzed detrital zircon spectra reflect all Proterozoic orogenic cycles representing the step-wise evolution of the accretionary SW Amazonia Orogenic System between 2.0 and 0.9 Ga, followed by the Terra Australis Orogen between 0.9 and 0.25 Ga. The zircon populations are characterized by two prominent maxima reflecting input from Sunsas (Grenville) age magmatic rocks (1.2–0.9 Ga) and from the Ordovician to Silurian Famatinian magmatic arc (0.52–0.42 Ga). Grains of Devonian age are scarce or absent from the analyzed zircon populations. The Hf isotopic compositions of selected dated zircons at the time of their crystallization (eHf(T); T = 3.3–0.25 Ga) vary between − 18 and + 11. All sandstones have a significant juvenile component; between 20 and 50% of the zircons from each sedimentary rock have positive eHf(T) and can be considered juvenile. The majority of the juvenile grains have Hf-depleted mantle model ages (Hf TDM) between 1.55 and 0.8 Ga, the time of the Rondonia–San Ignacio and Sunsas orogenic events on the Amazonia craton. The corresponding whole-rock eNd(T) values fot these same rocks are between − 8 and − 3 indicating a mixture of older evolved and juvenile sources. Nd-depleted mantle model ages (Nd TDM⁎) are between 1.5 and 1.2 Ga and coincide broadly with the zircon Hf model ages. Our data indicate that the Paleo- and Mesoproterozoic SW Amazonia Orogenic System, and the subsequent Neoproterozoic and Paleozoic Terra Australis Orogen in the region of the central and southern Andes, developed following two markedly different patterns of accretionary orogenic crustal evolution. The SW Amazonia Orogenic System developed by southwestward growth over approximately 1.1 Ga through a combination of accretion of juvenile material and crustal recycling typical of the extensional or retreating mode of accretionary orogens. In contrast, the central Andean segment of the Terra Australis Orogen evolved from 0.9 to 0.25 Ga in the compressional or advancing mode in a relatively fixed position without the accretion of oceanic crustal units or large scale input of juvenile material to the orogenic crust. Here, recycling mainly of Mesoproterozoic continental crust has been the dominant process of crustal evolution.

Regional structure and tectonic history of the obliquely colliding Columbus foreland basin, offshore Trinidad and Venezuela

Cenozoic eastward migration of the Caribbean plate relative to the South American plate is recorded by an 1100-km-long Venezuela-Trinidad foreland basin which is oldest in western Venezuela (65–55 Ma), of intermediate age in eastern Venezuela (34–20 Ma) and youngest beneath the shelf and slope area of eastern offshore Trinidad (submarine Columbus basin, 15.0 Ma-Recent). In this study of the regional structure, fault families, and chronology of faulting and tectonic events affecting the hydrocarbon-rich Columbus foreland basin of eastern offshore Trinidad, we have integrated approximately 775 km of deep-penetration 2D seismic lines acquired by the 2004 Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region (BOLIVAR) survey, 325 km of vintage GULFREX seismic data collected by Gulf Oil Company in 1974, and published industry well data that can be tied to some of the seismic reflection lines. Top Cretaceous depth structure maps in the Columbus basin made from integration of all available seismic and well data define for the first time the elongate subsurface geometry of the 11–15 km thick and highly asymmetrical middle Miocene-Recent depocenter of the Columbus basin. The main depocenter located 150–200 km east of Trinidad and now the object of deepwater hydrocarbon exploration is completely filled by shelf and deepwater sediments derived mainly from the Orinoco delta. The submarine Darien ridge exhibits moderate (20–140 m) seafloor relief, forms the steep (12°–24°), northern structural boundary of the Columbus basin, and is known from industry wells to be composed of 0.5–4.5 km thick, folded and thrust-imbricated, hydrocarbon-bearing section of Cretaceous and early Tertiary limestones and clastic rocks. The eastern and southern boundaries of the basin are formed by the gently (1.7°–4.5°), northward-dipping Cretaceous–Paleogene passive margin of South America that is in turn underlain by Precambrian rocks of the Guyana shield. Interpretation of seismic sections tied to wells reveals the following fault chronology: (1) middle Miocene thrusting along the Darien ridge related to highly oblique convergence between the Caribbean plate and the passive margin of northern South America; continuing thrusting and transpression in an oblique foreland basin setting through the early Pleistocene; (2) early Pliocene-recent low-angle normal faults along the top of the Cretaceous passive margin; these faults were triggered by oversteepening related to formation of the downdip, structurally and bathymetrically deeper, and more seaward Columbus basin; large transfer faults with dominantly strike-slip displacements connect gravity-driven normal faults that cluster near the modern shelf-slope break and trend in the downslope direction; to the south no normal faults are present because the top Cretaceous horizon has not been oversteepened as it is adjacent to the foreland basin; (3) early Pliocene–Recent strike-slip faults parallel the trend of the Darien ridge and accommodate present-day plate motions.

Ultradeep continental roots and their oceanic remnants: A solution to the geochemical “mantle reservoir” problem?

High-resolution global seismic tomography (Vs) models reveal high-velocity domains beneath cratonic crust in Africa that extend to depths of 300–400 km. These high-velocity domains show a distinct contrast with the characteristics of “normal” asthenosphere and are interpreted as depleted, buoyant roots that formed in the Archean and have been metasomatised over time, but have remained attached to the overlying crust. Such deep roots are impediments to free horizontal convection in the upper mantle. The movement of magmas and other fluids in such regions may be more vertically constrained (a shallow lava lamp regime), creating a geodynamic environment conducive to interaction of such magmas with the boundaries of deep mantle domains that would carry old “crustal” geochemical signatures. The tomographic models and the new world magnetic-anomaly map show that these continental roots, overlain by thinned continental crust, locally extend well out under the deep Atlantic Ocean basin. However, such high-velocity domains are not confined to the basin margins, but are scattered randomly through the basin, some quite distant from the continental margins of South America and Africa. These high-velocity domains are interpreted to be remnant lithospheric fragments isolated by disruption of the ancient continental regions during rifting. This interpretation is supported by the old Os depletion ages of mantle peridotites from mid-ocean ridges and oceanic islands. Basaltic magmas near such high-velocity domains carry the geochemical signatures (EM1, EM2) of interaction with refertilised cratonic mantle. The interaction of rising magmas with fragments of ancient lithospheric mantle can explain such geochemical signatures and obviates the need for complex models in which these geochemical reservoirs are isolated and preserved in the convecting mantle.

Multiphase development of the Atacama Planation Surface recorded by cosmogenic 3He exposure ages: Implications for uplift and Cenozoic climate change in western South America

The Atacama Planation Surface is an extensive west-dipping surface developed between 16°S and 27°S along the Pacific margin of South America. It is considered to have formed between 16 and 7.5 Ma and to have important chronostratigraphic significance. Here we present new cosmogenic 3He exposure dates of boulders on the planation surface using pyroxene and amphibole. Exposure ages display good within- and between-site consistency and range from 22 to 1.2 Ma, with ages of ca. 14.6, 7, and 3 Ma recurring at more than one site on the planation surface. The 14.6 Ma peak records the cessation of the main period of planation surface development, but, contrary to popular opinion, the younger ages reflect subsequent modification of the planation surface by alluvial activity. Comparison with other climate proxies for western South America suggests that since 14.6 Ma, a predominantly hyperarid climate, interspersed with short-lived phases of more intense runoff driven by global climate change, has prevailed. The longevity and composite nature of the Atacama Planation Surface suggest that regionally extensive planation surfaces may have a multiphase history, are unlikely to have any chronostratigraphic significance, and cannot be used to reconstruct uplift histories.

Warm-water mollusc assemblages from northern Chile (Mejillones Peninsula): new evidence for permanent El Niño-like conditions during Pliocene warmth?

Although results have been controversial, understanding the tropical Pacific climatic state during the Pliocene warm interval ( c . 4.5–3.0 Ma) is crucial if insight is to be gained into the dynamic processes of present and future global warming. In the multi-proxy effort to reconstruct ancient climates, a critical role can be played by palaeoclimatic evidence provided by the spatial and temporal distribution of temperature-sensitive marine molluscs. Shallow-water strata of the Mejillones Peninsula, northern Chile (23°S), contain dense faunal assemblages in which molluscs exclusive to, or characteristic of, Pliocene deposits ( Chlamys simpsoni , Chlamys vidali , Chorus blainvillei , Concholepas nodosa , Fusinus remondi , Herminespina mirabilis ) coexist with surprisingly abundant and varied populations of extant warm-water species ( Bulla punctulata , Cerithium stercusmuscarum , Olivella sp., Turbo cf. fluctuosus , Anomia peruviana , Argopecten ventricosus , Donax peruvianus , Dosinia ponderosa , Mexicardia procera , Undulostrea megodon ), most of which have their current southern zoogeographical limit at 6°S. These tropical elements are reliable indicators of nearshore marine conditions and their abundant occurrence implies that sea surface temperatures (SST) along the northern Chile coast were at least 2 °C warmer in the mid-Pliocene than at present, and that these very different conditions lasted long enough to allow stable colonization of the area. Such a significantly warmer SST pattern strongly resembles general climatic conditions accompanying modern El Niño events, when warm tropical waters propagate southward along the western margin of South America; this supports the existence in this area of persistently El Niño-like conditions during the mid-Pliocene.

History of crustal growth and recycling at the Pacific convergent margin of South America at latitudes 29°–36° S revealed by a U–Pb and Lu–Hf isotope study of detrital zircon from late Paleozoic accretionary systems

Detrital zircon provides a powerful archive of continental growth and recycling processes. We have tested this by a combined laser ablation ICP-MS U–Pb and Lu–Hf analysis of homogeneous growth domains in detrital zircon from late Paleozoic coastal accretionary systems in central Chile and the collisional Guarguaráz Complex in W Argentina. Because detritus from a large part of W Gondwana is present here, the data delineate the crustal evolution of southern South America at its Paleopacific margin, consistent with known data in the source regions. Zircon in the Guarguaráz Complex mainly displays an U–Pb age cluster at 0.93–1.46 Ga, similar to zircon in sediments of the adjacent allochthonous Cuyania Terrane. By contrast, zircon from the coastal accretionary systems shows a mixed provenance: Age clusters at 363–722 Ma are typical for zircon grown during the Braziliano, Pampean, Famatinian and post-Famatinian orogenic episodes east of Cuyania. An age spectrum at 1.00–1.39 Ga is interpreted as a mixture of zircon from Cuyania and several sources further east. Minor age clusters between 1.46 and 3.20 Ga suggest recycling of material from cratons within W Gondwana. The youngest age cluster (294–346 Ma) in the coastal accretionary prisms reflects a so far unknown local magmatic event, also represented by rhyolite and leucogranite pebbles. It sets time marks for the accretion history: Maximum depositional ages of most accreted metasediments are Middle to Upper Carboniferous. A change of the accretion mode occurred before 308 Ma, when also a concomitant retrowedge basin formed. Initial Hf-isotope compositions reveal at least three juvenile crust-forming periods in southern South America characterised by three major periods of juvenile magma production at 2.7–3.4 Ga, 1.9–2.3 Ga and 0.8–1.5 Ga. The 176Hf/ 177Hf of Mesoproterozoic zircon from the coastal accretionary systems is consistent with extensive crustal recycling and addition of some juvenile, mantle-derived magma, while that of zircon from the Guarguaráz Complex has a largely juvenile crustal signature. Zircon with Pampean, Famatinian and Braziliano ages (< 660 Ma) originated from recycled crust of variable age, which is, however, mainly Mesoproterozoic. By contrast, the Carboniferous magmatic event shows less variable and more radiogenic 176Hf/ 177Hf, pointing to a mean early Neoproterozoic crustal residence. This zircon is unlikely to have crystallized from melts of metasediments of the accretionary systems, but probably derived from a more juvenile crust in their backstop system.

Pericontinental petroliferous basins of the South Atlantic Ocean

Sedimentary basins located at Atlantic margins of South America and Africa (Brazil, Congo, and Angola) are composed of thick Cretaceous-Pleistocene rocks (7–9 km). The principal oil-bearing complexes of both regions are dominated by terrigenous rocks. Their accumulation took place at the rifting stage, ended with precipitation of evaporites in the Aptian at the oceanic stage of evolution. Starting with Coniacian and Campanian, the accumulation of terrigenous clastic formations in the Santos and Campos basins was governed by main phases of orogenesis in the Peruvian Andes. In the Angola and Lower Congo basins, the sedimentation was governed by movements related to the development of the East African Rift System. The formation of carbonate and carbonate-clayey sediments prevailed here at early (Albian-Late Eocene) stages of the South Atlantic opening.

The metamorphic complexes of the Patagonian and Fuegian Andes

The Patagonian and Fuegian Andes are made up in part by late Paleozoic to Mesozoic metamorphic complexes. The mostly low grade late Paleozoic Eastern Andes Metamorphic Complex (EAMC) crops out to the East of the Meso-Cenozoic South Patagonian batholith (SPB), which intruded the metamorphic complexes. The protholit of the EAMC was likely deposited in a passive margin setting and at the Puerto Eden area underwent Late Jurassic sillimanite grade and migmatite local metamorphic conditions. It is suspected, but not proven, that the Cordillera Darwin Metamorphic Complex is a higher grade metamorphic complex equivalent of the EAMC. West of the SPB, paleo subduction complexes occur and are represented by the allochtonous Madre de Dios terrane. This terrane is composed of the ocean floor lithologies of the Denaro Complex topped by the Tarlton limestones that represent a guyot assemblage. The low grade continent derived Duque de York complex was deposited down top of the ocean floor lithologies. Further west, the blueschist bearing Middle Jurassic Diego de Almagro Complex, with psammopelitic, mafic and siliceous volcanic rock protoliths, evolved deep in a subduction zone during the Cretaceous. The possibility that the Antarctic Peninsula was located west of the present margin of South America is discussed.

Turbiditic trench deposits at the South-Chilean active margin: A Pleistocene–Holocene record of climate and tectonics

The active plate margin of South America is characterized by a frequent occurrence of large and devastating subduction earthquakes. Here we focus on marine sedimentary records off Southern Chile that are archiving the regional paleoseismic history over the Holocene and Late Pleistocene. The investigated records – Ocean Drilling Program (ODP) Site 1232 and SONNE core 50SL – are located at ~ 40°S and ~ 38°S, within the Perú–Chile trench, and are characterized by frequent interbedded strata of turbiditic and hemipelagic origin. On the basis of the sedimentological characteristics and the association with the active margin of Southern Chile, we assume that the turbidites are mainly seismically triggered, and may be considered as paleo-megaearthquake indicators. However, the long-term changes in turbidite recurrence times appear to be strongly influenced by climate and sea level changes as well. During sea level highstands in the Holocene and Marine Isotope Stage (MIS) 5, recurrence times of turbiditic layers are substantially higher, primarily reflecting a climate-induced reduction of sediment availability and enhanced slope stability. In addition, segmented tectonic uplift changes and related drainage inversions likely influenced the postglacial decrease in turbidite frequencies. Glacial turbidite recurrence times (including MIS 2, MIS 3, cold substages of MIS 5, and MIS 6), on the other hand, are within the same order of magnitude as earthquake recurrence times derived from the historical record and other terrestrial paleoseismic archives of the region. Only during these cold stages sediment availability and slope instability were high enough to enable recording of the complete sequence of large earthquakes in Southern Chile. Our data thus suggest that earthquake recurrence times on the order of 100 to 200 years are a persistent feature at least during the last glacial period.

Tectonic implications of a paleomagnetic study of the Sarmiento Ophiolitic Complex, southern Chile

A paleomagnetic study was carried out on the Late Jurassic Sarmiento Ophiolitic Complex (SOC) exposed in the Magallanes fold and thrust belt in the southern Patagonian Andes (southern Chile). This complex, mainly consisting of a thick succession of pillow-lavas, sheeted dikes and gabbros, is a seafloor remnant of the Late Jurassic to Early Cretaceous Rocas Verdes basin that developed along the south-western margin of South America. Stepwise thermal and alternating field demagnetization permitted the isolation of a post-folding characteristic remanence, apparently carried by fine grain (SD?) magnetite, both in the pillow-lavas and dikes. The mean “in situ” direction for the SOC is Dec: 286.9°, Inc: − 58.5°, α95: 6.9°, N: 11 (sites). Rock magnetic properties, petrography and whole-rock K–Ar ages in the same rocks are interpreted as evidence of correlation between remanence acquisition and a greenschist facies metamorphic overprint that must have occurred during latest stages or after closure and tectonic inversion of the basin in the Late Cretaceous. The mean remanence direction is anomalous relative to the expected Late Cretaceous direction from stable South America. Particularly, a declination anomaly over 50° is suggestively similar to paleomagnetically interpreted counter clockwise rotations found in thrust slices of the Jurassic El Quemado Fm. located over 100 km north of the study area in Argentina. Nevertheless, a significant ccw rotation of the whole SOC is difficult to reconcile with geologic evidence and paleogeographic models that suggest a narrow back-arc basin sub-parallel to the continental margin. A rigid-body 30° westward tilting of the SOC block around a horizontal axis trending NNW, is considered a much simpler explanation, being consistent with geologic evidence. This may have occurred as a consequence of inverse reactivation of old normal faults, which limit both the SOC exposures and the Cordillera Sarmiento to the East. The age of tilting is unknown but it must postdate remanence acquisition in the Late Cretaceous. Two major orogenic events of the southern Patagonian Andes, in the Eocene (ca. 42 Ma) and Middle Miocene (ca. 12 Ma), respectively, could have caused the proposed tilting.

Strong plate coupling along the Nazca–South America convergent margin

The force balance in plate tectonics is fundamentally important but poorly known. Here, we show that two prominent and seemingly unrelated observations—trench-parallel gravity anomalies along the Nazca–South America margin that coincide with the rupture zones of great earthquakes, and a rapid slowdown of Nazca–South America convergence over the past 10 m.y.—provide key insights. Both result from rapid Miocene-Pliocene uplift of the Andes and provide quantitative measures of the magnitude and distribution of plate coupling along the Nazca–South America margin. We compute the plate-tectonic force budget using global models of the faulted lithosphere coupled to high-resolution mantle circulation models and find that Andean-related plate-margin forces are comparable to plate-driving forces from the mantle, and they have sufficient magnitude to account for pronounced bathymetry variations along the trench. Our results suggest that plate coupling, gravity anomalies, and bathymetry variations along a given trench are all controlled by long-term stress variations in the upper portion of plate boundaries and that an explicit budget of driving and resisting forces in plate tectonics can be obtained. For the convergent margin considered here, spatial variations in the effective coefficient of friction associated with the distribution of lubricating sediments entering the trench are, by comparison, of minor importance.

The Geology of Chile.: TERESA MORENO and WES GIBBONS, Editors. 424 Pp. Geological Society of London. 2007. Hardcover. ISBN 1-86239-219-6. Hardcover Price Members, 42.50. Other Societies, 51.00, Nonmembers, 85.00. Paperback Price Members, 27.50. Other Societies, 27.50, Nonmembers, 35.00.

More than 70 authors contributed to this English language volume, which handles the very broad geologic settings of Chile in able fashion, with approximately one-half of the text dedicated to the tectonostratigraphic and chronologic development of a country more than 4,000-km long, yet no more than about 200-km wide. Beginning with discussions of Chilean basement complexes and tectonostratigraphy, this book provides a very good general understanding of the geologic setting of the western margin of South America and gives a background on which essentially all other discussions of Chilean geology follow. The Andean orogen is highlighted by substantial detail and recent investigations concerning Andean magmatism (chapter 4). The authors subdivide Chile’s Andean magmatism into four distinct segments, in addition to Quaternary volcanism (chapter 5). The treatment of both the tectonic and geographic differences in Chilean magmatic activity gives readers an excellent …

Tectonic rotations in the Late Palaeozoic continental margin of southern South America determined and dated by palaeomagnetism

SUMMARY Detailed palaeomagnetic studies of Late Palaeozoic rocks exposed in the UspallataCalingasta Valley region, located at about 32s 69.5W in the Argentine Andean Chain, have been recently accomplished. Previous results are discussed in Valencio & Vilas (1985) and Rapalini et al. (1989). The most recent studies were carried out mainly on the thick sequences of late Early Permian to Late Permian rhyolites and ignimbrites assigned to the Tambillos and Horcajo Formations. Both formations yielded palaeomagnetic pole positions (P15: 319.6E, 78.9S7 N = 16, k = 33.5, AgS = 6.5 and P17: 264.8E, 72.4S, N = 26, k = 6.4, Ag5 = 12, respectively) concordant with the Late-Palaeozoic path of South America. Preliminary palaeomagnetic results from the Middle Carboniferous Hoyada Verde Formation, exposed in the same region, suggest that these rocks carry a synfolding magnetization acquired during the Late Carboniferous, as the palaeomagnetic pole position computed from partially corrected remanence directions (C6: 356.2E, 41.9S, D1 = 8.3, 02 = 6.0) agrees with the Late Carboniferous poles from cratonic areas of South America. This concordant position of C6 definitely rules out the possibility of a Late Palaeozoic allochthony of this section of the Argentine Andean Chain. The discordant positions previously found in Late Carboniferous and early Early Permian rocks of this region are interpreted as caused by large clockwise crustal block rotations that occurred not later than the late Early Permian. A tectonic model is proposed to explain these rotations which suggests that they were associated with strike-slip displacements parallel to the western continental margin of South America caused by an oblique subduction of the Proto-Pacific Plate during the Early Permian. Many tectonic and geologic observations fit this model. The concordant position of C6 also suggests that a complex pattern of crustal block rotations should have developed in this continental margin. Other palaeomagnetic data suggest that similar rotations may have taken place in other areas of the Argentine Andean Chain in the Early Permian.

Insights in the Neoproterozoic–Early Cambrian transition of NW Argentina: facies, environments and fossils in the proto-margin of western Gondwana

Abstract In NW Argentina, over 3000 metres of highly tectonized and metamorphosed siliciclastics of the Puncoviscana Formation underlie the more fossiliferous Cambro–Ordovician strata of the Mesón and Santa Victoria groups. Historically regarded as the non-fossiliferous ‘basal Precambrian shield’ of the region, its age was later found to be in part Phanerozoic, with the discovery of Early Cambrian trace fossils, and more recently refined with geochronological data. Widespread siliciclastic deposits characterize this sequence. A review of trace fossils in the Puncoviscana Formation has added new taxa. Trace fossil assemblages denote? geographical belts, indicating shallower marine conditions to the east ( Nereites association) and deeper to the west ( Oldhamia association). Such assemblages related to the palaeomorphology of the basin also represent different temporal levels, and in so doing reflect the overall evolution of the Puncoviscana Sea through time. The use of taxa that characterize Siberian stages in order to date parts of the Puncoviscana Formation should be abandoned as Siberian stages are defined by a specific set of fossils that are absent in the strata of NW Argentina. Thus, careful systematic studies and re-evaluation of the strata and fauna in the Puncoviscana Basin are the only way to improve understanding of the Neoproterozoic–Cambrian transition in the Andean margin of South America.