Western Margin Of South America Research Articles

Alloys developed and computer-aided tomography used to study damage in aging aircraft: Mater. Eval. Feb. 1990 48, (2), 293–294

New apatite U–Pb and multiphase 40Ar/39Ar data constrain the high to medium temperature (~ 500 °C–~ 300 °C) thermal histories of igneous and metamorphic rocks exposed in the Mérida Andes of Venezuela, and new apatite and zircon fission track data constrain the ~ 500 °C–~ 60 °C thermal histories of pre-Jurassic igneous and metamorphic rocks of the adjacent Santander Massif of Colombia. Computed thermal history envelopes using apatite U–Pb dates and grain size information from an Early Palaeozoic granodiorite in the Mérida Andes suggest that it cooled from > 500 °C to < 350 °C between ~ 266 Ma and ~ 225 Ma. Late Permian to Triassic cooling is also recorded in Early Palaeozoic granitoids and metasedimentary rocks in the Mérida Andes by numerous new muscovite and biotite 40Ar/39Ar plateau dates spanning 257.1 ± 1.0 Ma to 205.1 ± 0.8 Ma. This episode of cooling is not recognised in the Santander Massif, where 40Ar/39Ar data suggest that some Early Palaeozoic rocks cooled below ~ 320 °C in the Early Palaeozoic. However, most data from pre-Jurassic rocks reveal a regional heat pulse at ~ 200 Ma during the intrusion of numerous shallow granitoids, resulting in temperatures in excess of ~ 520 °C, obscuring late Palaeozoic histories.The generally accepted timing of amalgamation of Pangaea along the Ouachita–Marathon suture pre-dates Late Permian to Triassic cooling recorded in basement rocks of the Mérida Andes by > 30 Ma, and its effect on rocks preserved in north-western South America is unknown. We interpret late Permian to Triassic cooling in the Mérida Andes to be driven by exhumation. Previous studies have suggested that a short phase of shortening and anatexis is recorded at ~ 253 Ma in the Maya Block, which may have been adjacent to the basement rocks of the Mérida Andes in the Late Permian. The coeval onset of exhumation in the Mérida Andes may be a result of increased coupling in the magmatic arc, which was located along the western margin of Pangaea. Triassic extension is documented in the Central Cordillera of Colombia and Ecuador between ~ 240 Ma and ~ 215 Ma, although extension at this time has not been clearly identified in the Mérida Andes or the Santander Massif. Permian to Triassic cooling is not recorded in the structurally isolated Caparo Block in the southern Mérida Andes, suggesting that it may have constituted a distinct fault block in the Triassic.New fission track data from the Santander Massif suggest that it started exhuming at ~ 40 Ma during a period of accelerated convergence between the Nazca/Farallòn Plate and the western margin of South America. Exhumation in the Santander Massif occurred diachronously since ~ 18 Ma in distinct fault blocks at rates of 0.5–1 km/Ma, and may have been driven by east–west compression as a result of the indentation of the Panama–Chocó terrane to western Colombia.

Volcanic facies, structure, and geochemistry of the marginal basin rocks of central Peru

The western part of the Mesozoic Peruvian Trough, the Huarmey Basin, has a fill of pillow lavas, sheet lavas, hyaloclastites, tuffs and minor cherts, siliceous and calcareous oozes. Maximum subsidence occurred in the Albian when 9000 meters of basin fill accumulated. This was later intruded by gabbros and dikes and then the Coastal Batholith. The basin is an extensional marginal basin continuous with other basins of similar age to the south. Facies analyses indicate that the basin was relatively deep, with no continental influx, and not dissimilar to spreading and off-axis systems on the ocean floor. Structures at the surface and at depth indicate the crust has split and the basin was floored by mantle material. The basin shows a marked polarity, with tholeiitic basaltic rocks at the center and high-K acid rocks at the eastern margin, with intermediate types between. These changes in petrology and chemistry relate to lateral changes in source composition. Secular variations are also present and indicate a calcalkali source giving way to a more MORB-like source with a variable continental component. The basin is part of the major rifting event in the Cretaceous which affected the whole western margin of South America and was an important and necessary precursor to major batholith intrusion.

Earthquakes, Rivers and Ice: Scientific Research at the Laguna San Rafael, Southern Chile, 1986

The Laguna San Rafael, Aisen Province, southern Chile, is a tidal lagoon which receives the San Rafael glacier?the nearest tidewater glacier to the equator. The glacier is sustained by very high precipitation and pronounced topographic relief on the tectonically active western Andean margin. This area formed the focus for three research projects undertaken as part of an expedition mounted by Operation Raleigh in early 1986: investigating and dating subsidence events in association with past great subduction zone earthquakes; analysing the sedimentation of braided streams emerging from the North Patagonian ice field; and studying the tidal and meltwater dynamics of the lagoon. glaciers, southern Chile. THE WESTERN MARGIN of South America is underlain by a subduction zone, where Pacific Ocean crust passes down towards the east beneath the Americas. At the southern end of this subduction zone, in southern Chile, a mid-ocean spreading ridge intercepts the zone just offshore from the Taitao Peninsula (at around 47?S). This triple junction, where the Nazca, Antarctica and South American plates meet, is an area of massive earthquakes and very rapid crustal deformation (Forsythe and Nelson, 1985). This great tectonic activity is reflected by a correspondingly fast-evolving landscape with the steep fault-controlled margin of the Andes in southern Chile rising from sea level to over 1400 metres in less than 5 kilometres (Plate I). Where the Taitao Peninsula connects with the South American mainland, lies the Laguna San Rafael (46?40'S, 74?00'W) (Fig. 1), a 130 square kilometre tidal lagoon into which the San Rafael Glacier calves (Plate II). The glacier, classified by Lliboutry (1956) as the fifth largest in Chile, flows down the steep margin of the Andes from the North Patagonian Icefield at rates of up to 17 metres per day (Nakajima, 1985). This rapid velocity for a non-surging glacier is a product of the steep western slope of the tectonically active mountains and high snowfall in the accumulation area with annual precipitation at the Laguna San Rafael totalling more than 4000 mm (Enomoto and Nakajima, 1985). Although the San Rafael Glacier is the only branch of the North Patagonian Icefield to reach the sea, the meltwaters from other glaciers source large braided streams with high sediment loads and rapidly changing channel patterns. Various components of this dynamic landscape around the Laguna San Rafael provided the focus for scientific research on the Chile 4B phase of Operation Raleigh in January 1986. The Laguna San Rafael connects with the open fiord, Golfo Elefantes, to the north via a 30-metre deep channel, the Rio Tempanos. At present this is the only exit from the Laguna although in 1940 an attempt was made to open up a second exit via a 2-kilometre long canal through the moraine to the south-west, interconnecting with the Rio Negro. The Rio Negro flows from the bogs of the Taitao Peninsula and joins a series of sediment-laden rivers from the St Quintin Glacier before flowing into the Golfo de Penas. Completion of this canal would provide a sheltered route for boats, avoiding the exposed outer coast of the Taitao Peninsula. The canal was partially completed and there is, at present, renewed interest in finishing the project. The future construction of roads into the region is also planned in order to open up this remote area of the southern Andes to both settlers and tourists. In order for all these schemes to be successfully planned and their environmental impact monitored more information is needed on the morphological evolution of the region and possible local hazards.

MESOZOIC-CENOZOIC PALEOGEOGRAPHIC AND GEODYNAMIC EVOLUTION OF SOUTHERN SOUTH AMERICA

ln this presentation, we compare the paleogeographie evolution of continental South America with the established plate tectonic history of the oceanic crust of the South American and African Plates. Major paleogeographic changes that have occurred in the last 200 million years are illustrated by reconstructions at 200, 160, 120, 100, 70, 50, 20, and 5 Ma. The se reconstructions show location and type of magmatic activity, types and arnounts of subsidence, and transgressions and regressions of shorelines. The latter are eompared with proposed eustatie sea-Ievel ehanges. The sequence of events documented by the reconstructions begins in the mid-late Triassic with the development of successor troughs and intra-plate domes and sags that represent the prelude to the rupture of Gondwanaland. Extension eontinued through the Late Jurassie resulting in widespread faulting and intra-plate volcanism. Large-seale drift occurred by the Early Cretaceous (late Neocomian) accompanied by the formation of epicontinental rifts and sags, and a well-defined magmatic are along the western margin of South America. In the mid-Cretaceous the Atlantic Basin was well developed and rimmed by trailing-margin sedimentary prisms, and global changes in plate motions led to the reorganization of the subduction system along the Pacific margin of South America. The Late Cretaceous, a time of relative tectonic quiescence in southern South America, recorded the maximum flooding of the continent by Atlantic waters. Plate tectonic reorganization in the Paleogene introduced new patterns of Andean deformation and magmatism, and coincided with general marine retreat. Magmatic arc buildup and renewed marine flooding followed in the early Neogene, with the late Neogene being dominated by pronounced shortening, Cordilleran uplift, and marine withdrawal.

MESOZOIC-CENOZOIC PALEOGEOGRAPHIC AND GEODYNAMIC EVOLUTION OF SOUTHERN SOUTH AMERICA

ln this presentation, we compare the paleogeographie evolution of continental South America with the established plate tectonic history of the oceanic crust of the South American and African Plates. Major paleogeographic changes that have occurred in the last 200 million years are illustrated by reconstructions at 200, 160, 120, 100, 70, 50, 20, and 5 Ma. The se reconstructions show location and type of magmatic activity, types and arnounts of subsidence, and transgressions and regressions of shorelines. The latter are eompared with proposed eustatie sea-Ievel ehanges. The sequence of events documented by the reconstructions begins in the mid-late Triassic with the development of successor troughs and intra-plate domes and sags that represent the prelude to the rupture of Gondwanaland. Extension eontinued through the Late Jurassie resulting in widespread faulting and intra-plate volcanism. Large-seale drift occurred by the Early Cretaceous (late Neocomian) accompanied by the formation of epicontinental rifts and sags, and a well-defined magmatic are along the western margin of South America. In the mid-Cretaceous the Atlantic Basin was well developed and rimmed by trailing-margin sedimentary prisms, and global changes in plate motions led to the reorganization of the subduction system along the Pacific margin of South America. The Late Cretaceous, a time of relative tectonic quiescence in southern South America, recorded the maximum flooding of the continent by Atlantic waters. Plate tectonic reorganization in the Paleogene introduced new patterns of Andean deformation and magmatism, and coincided with general marine retreat. Magmatic arc buildup and renewed marine flooding followed in the early Neogene, with the late Neogene being dominated by pronounced shortening, Cordilleran uplift, and marine withdrawal.

Cretaceous paleogeography and depositional cycles of western South America

The western margin of South America was encroached upon by a series of marine advances that increased in extent from the Early Cretaceous to a maximum in the early Late Cretaceous for northern South America (Venezuela to Peru). In southern South America, however, the area covered by the marine advances decreased from a maximum in the Early Cretaceous to a minimum during mid-Cretaceous time, followed by a widespread advance at the end of the period. A series of unconformity-bounded depositional cycles was recognized in these sequences: five cycles in northern South America, and six (but not exactly equivalent) cycles in the Cretaceous back-arc basins of southern South America (Neuquén and Austral, or Magallanes, Basins). Both widespread anoxic facies and maximum flooding of the continent in northern South America coincide in general terms with recognized global trends, but this is not the case in southern South America. Here, anoxic facies are restricted to the Lower Cretaceous and seem to be controlled by local aspects of the basin evolution and configuration. The contrasts observed between northern and southern South America can be explained by differences in tectonic setting and evolution. To the north, sediments were deposited around the tectonically stable Guayana-Brazilian Massifs, and thus registered global “signals” such as anoxic events and major eustatic changes. The southern portion of the continent, on the contrary, developed in an active tectonic setting. Here, the mid-Cretaceous Peruvian Orogeny overprinted, to a large extent, world-wide trends and only the earliest and latest Cretaceous conform to global depositional patterns.

Sequence Architecture and Lithofacies Assemblages of Submarine Fan Deposits in Los Molles Formation (Jurassic), Neuquen Basin, Argentina: ABSTRACT

The Neuquen basin is a remnant of the Mesozoic back-arc basin trend that developed along the western margin of South America. It contains a thick, diverse sequence of Jurassic sedimentary strata, whose facies distribution was strongly influenced by syndepositional tectonism. The predominantly dark, laminated shales and siltstones of the Los Molles Formation range from Pliensbachian to Callovian in age and record the progradation of the outer shelf, slope, and basin-plain sediments deposited during the shoaling phase of the lowermost, or Cuyan, Jurassic cycle. Based on outcrop, well, and seismic data, several thick packages of sandstone and conglomerate within the Los Molles are interpreted to represent submarine fan deposits that developed during periods of relative sea level lowstand. Sea level falls were probably related to local synchronous tectonic pulses rather than true eustatic fluctuations. The distribution of coarse-grained fan deposits was apparently strongly controlled by the location of major Jurassic fault trends that stabilized the position of the shelf-slope break through time. Based on the geometry and sequence architecture, two distinct styles of fan development are recognized in the Los Molles sandstones. The most common style (type A) is characterized by sequences that have poorly defined, sand-poor lobes with well-developed channel-leveemore » complexes. Some channels exhibit large-scale accretion surfaces, probably resulting from lateral migration. Thick-bedded arenite facies are limited to amalgamated channel fills, whereas thin-bedded classical turbidites are present as overbank deposits. Type A fans were built by turbidity and fluxoturbidity currents from submarine canyon point sources. The less common fan sequences (type B) lack channeling; they are dominated by thick, massive beds of internally featureless sandstone that are bounded by chaotic slump deposits.« less

Spreading-subsidence and generation of ensialic marginal basins: an example from the early Cretaceous of central Chile

Summary During the early Cretaceous, intracontinental rifting, spreading and subsidence led to the formation of an ensialic trough in central Chile. This trough is interpreted as an aborted marginal basin since no oceanic crust was generated. However, mantle-derived material, represented by flood basalts with low initial 87 Sr/ 86 Sr ratio and evolved geochemical characteristics, was deposited in the basin. Rb-Sr dating shows that the lavas suffered burial metamorphism shortly after their extrusion. The flood basalts were extruded during an episode of slow oceanic spreading in the Pacific, and subsequent folding, granitoid intrusion and uplift are correlated with an episode of fast oceanic spreading and plate subduction. The intracontinental events related to these two episodes constitute a cycle, repetition of which might account for the geological evolution of central Chile during the Mesozoic and the Palaeogene. The western margin of South America, during the Cretaceous, was characterized by basin formation with eruption of mantle-derived basalts. Some basins developed into marginal basins proper, with generation of oceanic crust, others were aborted. The type of basin was determined by the rate and volume of upwelling mantle material, and continental crust thickness. Rapid upwelling gave rise to back-arc spreading and marginal basins with primitive basalts and ocean-floor metamorphism or burial metamorphism reflecting steep thermal gradients. Slow upwelling produced ensialic basins with evolved basalts affected by burial metamorphism with less steep thermal gradients, as in central Chile during the early Cretaceous.

Tectonic and magmatic evolution of the Andes of northern Argentina and Chile

Abstract Two orogenic cycles, both with different evolution, are developed in the western margin of the South American continent in northern Argentina and Chile: the Paleozoic “Hercynic” cycle and the Meso-Cenozoic “Andean” cycle. The Hercynic cycle. A wide marine basin extending westward of the Cordillera Oriental which developed in Cambrian-Ordovician times marks the beginning of this cycle. In contrast to Peru and Bolivia where this basin developed between two Precambrian blocks, the western margin of this basin in northern Argentina and Chile is still unknown. The Ordovician sedimentation and accompanied volcanism ends with the Ocloyic deformation phase and its synkinematic granitic plutonism. Two basins were developed in the Silurian-Devonian separated by the Puna arc, uplifted during this ocloyic phase. The shallow-water marine terrigeneous sediments which were deposited in them were deformed by a new tectono-magmatic associated phase (Chanic phase). Carboniferous to Lower Permian marine carbonates were deposited west of the Puna arc and red beds east of it. Later on, during the Permian to Triassic, a magmatic belt was developed along the Cordillera Occidental. The rhyolites, ignimbrites and the granitic to granodioritic related intrusives are well represented in Chile. Although the overall geologic history of this period is known, many problems concerning its origin and relations to plate tectonics are still unsolved. The Andean cycle. During this cycle, a series of magmatic-arc systems related to the subduction of the Pacific crust was built up along the western margin of South America. Huge volumes of calc-alkaline lavas and related plutons were emplaced since the Jurassic, along belts parallel to the present coastline, showing a general eastward migration trend. Up to the Lower Cretaceous, an ensialic back-arc basin was formed east of this magmatic arc. Thousands of meters of marine and continental sediments were deposited in it. This basin disappeared during the Middle Cretaceous, probably as a result of the final opening of the Atlantic and the active westward movement of the South American Plate. Since Middle Cretaceous times, the magmatic-arc has been the fundamental paleogeographic element. The arc progressively migrated stepwise eastward, each step marked by a tectonic phase. Subduction-related crustal erosion could explain the lack of fore-arc petrotectonic assemblages. The eastward magmatic polarity which characterizes this section of the Andes, could also be explained by such a mechanism.

Petrogenesis of Andean andesites from combined O–Sr isotope relationships

The descent of the oceanic Nazca plate below the western margin of South America is responsible for the seismicity, tectonism and magmatism characteristic of Andean-type continental margins. Within this geological setting there is much debate concerning the petrogenesis of the calc-alkaline volcanic association. A thermal model for the descent of oceanic below continental lithosphere suggests that melting of subducted oceanic crust, the asthenospheric mantle wedge, and the lower crust might occur1, and hence calc-alkaline volcanic rocks erupted at the surface in such regions might be derived from one or more of these sources. In the Andes, for example, Pichler and Zeil2 have invoked partial melting of lower continental crust, while others3–5 envisage petrogenesis of andesites from sub-crustal mantle sources possibly in conjunction with crustal contamination6–9. We present here new O-isotope data for volcanic rocks from three contrasted volcanic settings in Ecuador, north Chile, and north-west Argentina. We then consider the significance of observed O–Sr isotope relationships, and attempt to evaluate the important factors in the petrogenesis of Andean andesitic lavas.

Lithofacies and Depositional Environments of Monterey Shale, California: ABSTRACT

The Miocene Monterey Formation records the deep basinal phase of a major late Tertiary cycle of basin formation and filling associated with wrench fault tectonism. Over much of coastal California, Neogene facies show a progression from Oligocene nonmarine and neritic rocks to Miocene basinal shales and siliceous rocks and then to upper Miocene to Pliocene turbiditic and neritic clastic rocks. In many places the Monterey consists of a basal calcareous facies, a middle transitional phosphatic facies, and an upper thick siliceous facies composed of diatomaceous rocks and their diagnetic equivalents (cherts, porcelanites, etc). Hydrocarbon-producing turbidite sandstones occur in these basinal rocks in the southern San Joaquin Valley and in redeposited pelletal phosphorites in the central Coast Ranges. By analogy with modern environments of phosphate formation off South America and Africa, Monterey phosphatic shales probably represent phosphatization of shelf-slope-basin muds near the boundaries of the oxygen minimum zone. Nodular and pelletal phosphorites may also have formed near the boundaries of the oxygen minimum zone, but on sediment-starved bank tops and shelves. The widespread siliceous facies represents rapidly deposited diatom ooze that records high plankton productivity spawned by late Miocene climatic deterioration and intensified upwelling. Abundant organic matter, ecologic inferences from faunal data, and sedimentary structures such as alternations of massive and laminated cycles indicate that these siliceous rocks also formed as basin, slope, and shelf deposits within or near the fluctuating boundaries of a well-developed oxygen minimum zone--a depositional environment similar to the present Gulf of California and western margin of South America. End_of_Article - Last_Page 929------------

Late Paleozoic K/Ar ages of blueschists from Pichilemu, Central Chile

Two crossite concentrates and one blueschist whole rock were analyzed by the K/Ar method. These samples belong to the high/intermediate pressure Western Series of the Chilean metamorphic basement and, in this area, are intruded by a small monzonite body. Ages obtained were 211 m.y. and 329 m.y. for the mineral concentrates and 211 m.y. for the whole rock. Discussion based on crystal size as a factor for retention of 40Ar during localized re-heating of the metamorphic rocks due to the monzonitic intrusion leads to the acceptance of 329 m.y. as the minimum age of crossite crystallization. This age agrees with the whole rock Rb/Sr limiting reference isochrons (273–342 m.y.) previously obtained for the metamorphic basement of Central Chile which did not include samples of the present area. This age provides the first evidence of a Paleozoic blueschist assemblage in the eastern Pacific border and would suggest the existence of a Late Paleozoic subduction zone along the western margin of South America.

Plate Tectonics and Origin of Caribbean Sea and Gulf of Mexico: ABSTRACT

Previously published reconstructions of the late Paleozoic fit of crustal plates and continents fail to explain many geologic features present in the southwestern U.S., Mexico, Central America, and northern South America. In particular, they fail to consider major geologic and tectonic continuities of Paleozoic age observable in the Southern Appalachians, the Ouachita and Marathon fold belts, the fold belts of southern Mexico and Central America, and the eastern Andean mountain belt of northern South America, as well as the significance of many major transcurrent fault systems or megashears that cross these regions. With the well-documented joining of Africa-North America as a control for the positioning of South America relative to North America, this report suggests a somewhat different fit than any heretofore proposed. Instead of truncating North America in northern Mexico and filling in the Gulf of Mexico with fragments as is most commonly done, this reconstruction wraps Mexico and Central America around the western margin of South America, thus placing in juxtaposition the major tectonic belts of both continents. There is evidence that indicates that the Late Ordovician Taconic orogeny was an arc-continent collision rather than a continent-continent collision as has been End_Page 1904------------------------------ suggested previously. Similar evidence indicates that the late Paleozoic Ouachita and Marathon orogenies were arc-continent collisions. Correlative periods of deformation for both of these orogenies have been documented from many places in northern and northwestern South America. The early Paleozoic history of the Cordilleran mobile belt appears to have been independent from that of the eastern mobile belt. In the late Paleozoic, however, these mobile belts seem to have become coupled tectonically to produce regional stresses that were released along several major megashears. In southern and southwestern North America these include the Wichita and Texas megashears; a third megashear is probably present in northern Mexico. Late Paleozoic movement on these fault zones produced numerous basins and uplifts throughout these regions. Modifications of the model proposed by Malfait and Dinkleman for the origin of the Caribbean region include the opening of a sphenochasm in the Gulf of Honduras, and regional tensional and compressional stresses resulting from the clockwise rotation of North America. The Gulf of Mexico and the present dislocated positions of the Ouachita and Marathon fold belts are the result of an opening sphenochasm under the present Mississippi embayment and the westward displacement of the Ouachita and Marathon fold belts by left lateral movement on the Wichita and Texas megashears. End_of_Article - Last_Page 1905------------

Petroleum Geology of Coastal Peru and Ecuador: ABSTRACT

Oil fields of coastal Peru and Ecuador lie within the only significant coastal plain along the western margin of South America. This plain is underlain by chiefly marine lower Tertiary and variably marine to continental upper Tertiary sediments, and is capped by a spectacular, but locally heavily dissected, elevated Quaternary marine terrace, or Tablazo. Although evidence is meager, the marine lower Tertiary sediments are considered not to have been deposited as in a basin, but more likely as in a half-basin or marine bank on a narrow continental shelf, facing the open ocean on the west. The principal producing areas in this belt, from north to south are (Ecuador) Ancon, and (Peru) El Alto, Lobitos, Talara-Negritos, and Portachuelo. Estimated ultimate recovery from established reserves are: Ecuador, possibly in excess of 125 million barrels of 37° gravity oil; and Peru, possibly up to 1 billion barrels of 37° gravity oil. Production comes chiefly from Eocene sandstones and conglomerates, with fracturing commonly playing a major part in reservoir permeability. The Tertiary coastal belt is one of both common stratigraphic complications and a rather unique structural arrangement. Much of the marine Tertiary sedimentary sequence demonstrates rapid facies change and evidence of large-scale submarine landslides, resulting in common turbidity deposits and recurrent faunal zones. The structural situation is dominated by a myriad of normal faults having no regular pattern, but predominantly following a rule of faulting which favors downward movement of the updip block (in a regional sense), which is interpreted as dislocation resulting from a westerly spreading or creep of the sediments of this ancient marine bank during periodic recurrent elevations of the ancestral Amotape mountains on the east, with no massive basement buttress on the west to co tain this semi-plastic mass. Because of both the restricted remaining undeveloped area and vertical sedimentary column in the onshore, particularly in Peru, development of additional major reserves may be expected to come chiefly from the submerged continental margin adjacent to existing production. End_of_Article - Last_Page 279------------