Southernmost Andes Research Articles

Geología de la zona del Cerro Fitz Roy, Provincia de Santa Cruz

Geology of the Cerro Fitz Roy area, province of Santa Cruz . The surveyed Cerro Fitz Roy area (73o15’-72o50’ W Long., 49o25’-49o17’ S Lat., southernmost Andes) is located between the Electrico River to the north and the Tunel River to the south, and limited to the east by the Las Vueltas River and to the west by the Campo Sur del Hielo Patagonico. The stratigraphy record starts with Devonian-Carboniferous leptometamorphic sedimentary rocks of the Bahia La Lancha Formation, which are unconformably overlayed by conglomerates of the Arroyo de la Mina Formation, a basal unit of the Jurassic volcano-sedimentary El Quemado Complex. A low angle unconformity separates this Complex from the Early Cretaceous black pelites of the Rio Mayer Formation. Miocene Fitz Roy Granite intrudes the whole column. Overthrusting of the Jurassic volcanics that acted like a rigid body over the Cretaceous pelites that deformed plastically with an intense folding, indicates a strong tectonic overprint. The later intrusion of the granites generates a contact aureole that attains amphibolite grade. A younger extensional tectonic is responsible for the gravitational faulting. An ill-defined sinistral E-W strike-slip faulting associated to the Mackena and Viedma megastructures is recognized.

Cenozoic crustal thickening, wrenching and rifting in the foothills of the southernmost Andes

The southernmost Andes form an orocline, between the Patagonian cordillera, trending N–S, and the Fueguian cordilleras, trending E–W. On the foreland side is the Magellan Basin. The area has a history of Paleozoic compression, Triassic to Early Cretaceous rifting and Late Cretaceous to Quaternary compression, in response to changing plate tectonics. Major structures of Late Cretaceous and Cenozoic age vary along the strike. In the Patagonian cordillera and foothills, folds and thrusts trend NNW, slightly oblique to the orogen, whereas strike–slip faults are parallel to the orogen and right-lateral. In the Fueguian cordilleras and foothills, folds and thrusts trend ESE, slightly oblique to the orogen, whereas strike–slip faults are parallel to the orogen and left-lateral. In the axial zone of the Magellan Basin, folds and thrusts are parallel to the orogen and rifts are sub-perpendicular to it. To a first approximation, the pattern of structures has mirror symmetry about the axis of the Magellan Basin. In detail, however, wrenching appears to be more prevalent in the Fueguian cordillera and foothills, than it is in the Patagonian cordillera and foothills. Minor faults of Cenozoic age are common in the foothills. From a kinematic analysis of fault–slip data: (1) shortening and stretching directions are mostly sub-horizontal; (2) shortening directions vary in trend, from ENE in the Patagonian foothills, to NE in the Fueguian foothills; and (3) stretching directions are sub-parallel to traces of major thrusts. In the Fueguian cordillera and foothills, strike–slip faulting is prevalent; in the Patagonian foothills, crustal thickening is prevalent over strike–slip faulting. The kinematics reflect a combination of thrusting and wrenching and they are consistent with the major structures. To investigate the origin of the Cenozoic structures, we used analogue models on a fully lithospheric scale, where an oceanic plate subducted beneath a continental corner. The corner was an area of transition, from frontal subduction, to transcurrent motion. The boundary conditions may not have been fully realistic, but the experiments did account for the major elements of the structural pattern in southernmost South America, including rifts that are perpendicular to the orogen and counterclockwise block rotations.

Contrasting styles of superposed deformation in the southernmost Andes

Brittle fault arrays, regional structural patterns, and marine multichannel seismic reflection profiles across the Magallanes basin near 53°S latitude show contrasting styles of superposed extensional, contractional, and transtensional deformation. Below the northeastern end of a NE‐SW transect of the basin, seismic reflection profiles show deepening levels of pre‐Late Jurassic basement and Upper Jurassic to Lower Cretaceous volcanic horizons toward the southwest. This trend is associated with apparent normal fault offsets that may be attributed to a widespread Late Jurassic‐Early Cretaceous extensional event. Contrastingly, below the southwestern end of Magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest. Integration of seismic reflection data with published seismic refraction data, geologic mapping, and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton‐vergent, basement‐involved contractional deformation. This style of shortening contrasts with thin‐skinned styles of thrusting in the northern and central parts of the basin on Tierra del Fuego. Structural relationships, stratigraphic data, and previously published metamorphic data indicate that basement‐involved contraction occurred during latest Cretaceous to early Tertiary time. Overprinting all contractional structures, steep Tertiary strike‐slip and oblique‐slip faults in the central and southern parts of the Magallanes basin display oblique‐normal (mostly down‐to‐the‐southwest) offsets across WNW‐ESE trending segments of the South American‐Scotia transform plate boundary on the South American continent. Newly discovered features along the plate boundary segment include sediment‐filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the Magallanes fault zone. Kinematic analyses of fault‐slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the Straits of Magellan and central Tierra del Fuego. Together these data indicate that (1) the southernmost Andes were under compression during rapid cooling and exhumation of unique high‐pressure (∼8 kbars) basement rocks now exposed in the Darwin Metamorphic Complex (DMC) near 55°S latitude and (2) final exhumation of basement rocks in the DMC occurred during a shift away from compression toward transtension during late Tertiary crustal relaxation. We suggest that preexisting structural anisotropies formed during basement‐involved contractional deformation and the initiation of seafloor spreading south of South America during the Oligocene controlled late Tertiary sinistral transtensional patterns in the Magallanes fault zone.

Southernmost Andes and South Georgia Island, North Scotia Ridge: Zircon U-Pb and muscovite [formula omitted] age constraints on tectonic evolution of Southwestern Gondwanaland

Zircon U-Pb and muscovite 40Ar 39Ar isotopic ages have been determined on rocks from the southernmost Andes and South Georgia Island, North Scotia Ridge, to provide absolute time constraints on the kinematic evolution of southwestern Gondwanaland, until now known mainly from stratigraphic relations. The U-Pb systematics of four zircon fractions from one sample show that proto-marginal basin magmatism in the northern Scotia arc, creating the peraluminous Darwin granite suite and submarine rhyolite sequences of the Tobifera Formation, had begun by the Middle Jurassic (164.1 ± 1.7 Ma). Seven zircon fractions from two other Darwin granites are discordant with non-linear patterns, suggesting a complex history of inheritances and Pb loss. Reference lines drawn through these points on concordia diagrams give upper intercept ages of ca. 1500 Ma, interpreted as a minimum age for the inherited zircon component. This component is believed to have been derived from sedimentary rocks in the Gondwanaland margin accretionary wedge that forms the basement of the region, or else directly from the cratonic “back stop” of that wedge. Ophiolitic remnants of the Rocas Verdes marginal basin preserved in the Larsen Harbour complex on South Georgia yield the first clear evidence that Gondwanaland fragmentation had resulted in the formation of oceanic crust in the Weddell Sea region by the Late Jurassic (150 ± 1 Ma). The geographic pattern in the observed age range of 8 to 13 million years in these ophiolitic materials, while not definitive, is in keeping with propagation of the marginal basin floor northwestward from South Georgia Island to the Sarmiento Complex in southern Chile. Rocks of the Beagle granite suite, emplaced post-tectonically within the uplifted marginal basin floor, have complex zircon U-Pb systematics with gross discordances dominated by inheritances in some samples and Pb loss in others. Of eleven samples processed, only two had sufficient amounts of zircon for multiple fractions, and only one yielded colinear points. These points lie close to the lower concordia intercept for which the age is 68.9 ± 1.0 Ma, but their upper intercept is not well known. Inasmuch as this age is similar to the 40Ar 39Ar age of secondary muscovite growing in extensional fractures of pulled-apart feldspar phenocrysts in a Beagle suite granitic pluton (plateau age is 68.1 ± 0.4 Ma), we interpret the two dates as good time constraints for cooling following a period of extensional deformation probably related to the tectonic denudation of the highgrade metamorphic complex of Cordillera Darwin in Tierra del Fuego.

Southernmost South America‐Antarctic Peninsula relative plate motions since 84 Ma: Implications for the tectonic evolution of the Scotia Arc region

We have attempted to quantify the relative motion history between southernmost South America (SSA) and the Antarctic Peninsula (AP) by calculating and comparing SSA‐Africa, AP‐Africa and SSA‐AP synthetic flow lines for 84–0 Ma. The flow lines were created using published poles of rotation and plate reconstruction software. The results indicate that since 84 Ma, SSA and AP have moved approximately westward relative to a fixed Africa; however, SSA's rate of westerly motion in that reference frame has been significantly more rapid than AP's rate. Approximately 1320 km of east‐west, left‐lateral strike‐slip displacement and 490 km of north‐south, divergent displacement have occurred between the southern tip of SSA and the northern tip of AP since 84 Ma. Increased rates of SSA‐AP interplate separation and a change in the angle of plate divergence at approximately 55–40 Ma marked the onset of accelerated continental separation that eventually led to seafloor spreading in the western Scotia Sea at 30 Ma and the development of the Scotia Arc. Increased separation rates between SSA and AP at 55–40 Ma may be related to a global Eocene plate reorganization event. The northeast‐southwest oriented western Scotia Sea spreading centers appear to have accommodated all of the SSA‐AP interplate motion between 30 and 9 Ma. We suggest that prior to 30 Ma and the opening of Drake Passage, components of interplate strike‐slip and divergent motion were accommodated by intracontinental deformation that included strike‐slip faulting, counterclockwise tectonic rotation, and continental extension in the southernmost Andes. The results indicate that the opening of the Scotia Sea was caused by plate‐scale motions as SSA and AP drifted away from Africa at different velocities along different, nonparallel trajectories. Subduction retreat along the South Scotia Ridge and South Sandwich arc and back arc spreading in the Scotia Sea contributed to the width of separation between SSA and AP across Drake Passage. The results place limits on how SSA‐AP relative motion has been temporally and spatially partitioned in the Scotia Arc region.

Relationship between uplift of the metamorphic core of the southernmost Andes and shortening in the Magallanes foreland fold and thrust belt, Tierra del Fuego, Chile

Rocks containing upper amphibolite facies assemblages reflecting Mesozoic‐Cenozoic metamorphism form the 5000 km2 core of the southernmost Andes in Cordillera Darwin (approximately 55°S latitude). Results from mapping along a 50‐km, continuous transect across‐ the southernmost Andes on the main island of Tierra del Fuego show structural relationships between the Late Cretaceous to early Tertiary uplift of the metamorphic core in northern Cordillera Darwin and mid‐Cretaceous to Paleogene shortening in the adjacent Magallanes foreland fold and thrust belt. New data indicate that pre‐Late Jurassic basement schists exposed along the northeastern edge of Cordillera Darwin were uplifted along a discrete, brittle, continent‐vergent thrust fault. This thrust overlies a series of thin‐skinned thrusts and back thrusts that affect Mesozoic‐Tertiary rocks of the Magallanes foreland basin. The thrust that uplifts basement schists is interpreted to have formed out of sequence with respect to the thin‐skinned structures because it placed the schists over previously shortened Lower Cretaceous sedimentary strata of the Magallanes basin. The geometry of thrusts observed across the transect and their inferred sequence of deformation indicate a thick‐skinned, contractional mechanism for uplift of the pre‐Late Jurassic schists of northern Cordillera Darwin relative to the structural level of metamorphic basement in the Magallanes foreland. The relative time of out of sequence thrusting and basement uplift inferred from thrust geometries in this paper is consistent with the latest Cretaceous to early Tertiary episode of rapid exhumation and cooling of the metamorphic core of the southern Andean orogen determined in previous structural, metamorphic, and geochronological studies.

Uplifted ophiolitic rocks on Isla Gordon, southernmost Chile: implications for the closure history of the Rocas Verdes marginal basin and the tectonic evolution of the Beagle Channel region

A succession of mafic rocks that includes gabbro, sheeted dikes and deformed pillow basalts has been mapped in detail on Isla Gordon, southernmost Chile and is identified as an upper ophiolitic complex representing the uplifted floor of the Late Jurassic-Early Cretaceous Rocas Verdes marginal basin. The complex was uplifted, deformed, and regionally metamorphosed prior to the intrusion of an undeformed 90 Ma granodiorite that cuts the complex. The complex appears para-autochthonous, is gently tilted to the northeast and is internally sheared by near-vertical foliation zones. No evidence for obduction was observed although the base of the complex is not exposed. The ophiolitic rocks have been regionally metamorphosed to mid-upper greenschist levels. Isla Gordon is bounded by the northwest and southwest arms of the Beagle Channel, two important structural boundaries in the southernmost Andes that are interpreted to have accommodated north-side-up and left-lateral displacements. Directly north of Isla Gordon is the Cordillera Darwin metamorphic complex that exposes the highest grade metamorphic rocks in the Andes south of Peru. On the north coast of Isla Gordon a volcaniclastic turbidite sequence that is interpreted to have been deposited above the mafic floor is metamorphosed to lower greenschist levels in strong metamorphic contrast to amphibolite-grade othogneisses exposed in Cordillera Darwin only 2 km away across the northwest arm of the Beagle Channel. The profound metamorphic break across the northwest arm of the Beagle Channel and the regional northeast tilt of the ophiolitic complex are consistent with the previously proposed hypothesis that Isla Gordon represents the upper plate to an extensional fault that accommodated tectonic unroofing of Cordillera Darwin. However, limited structural evidence for extension was identified in this study to support the model and further work is needed to determine the relative importance of contractional, extensional and strike-slip displacements during the closure of the Rocas Verdes marginal basin and uplift of Cordillera Darwin. The Isla Gordon ophiolitic complex is correlative with other regional occurrences of ophiolitic rocks including the previously studied Tortuga, Sarmiento and Larsen Harbour complexes. The existence of the Isla Gordon ophiolitic complex helps link the known occurrences of the marginal basin floor into a semi-continuous belt that sheds light on the original continuity of the basin.

Strike‐slip faults in the southernmost andes and the development of the Patagonian orocline

The Patagonian orocline is the 90° bend in the southernmost Andes between 50°S and 56°S. Paleomagnetic and structural data indicate that the orocline is, at least in part, the product of tectonic rotation. Recent field work in the Beagle Channel region of southernmost Chile provides evidence for widespread left‐lateral strike‐slip faulting in the internal zones of the mountain belt. Both arms of the Beagle Channel are interpreted to be left‐lateral strike‐slip faults based on detailed study of mesoscale strike‐slip faults (Riedel shears) observed in coastal outcrops. Although much of the evidence indicates Cenozoic brittle strike‐slip faulting, other fabric data, including vertical foliation zones containing horizontal quartz stretching lineations and ductile left‐lateral kinematic indicators, suggest that Mesozoic ductile strike‐slip or oblique‐slip shearing also occurred. The implication is that the mid‐Cretaceous Andean orogeny involved the transpressional inversion of the Rocas Verdes marginal basin and that transpression has been the dominant deformational regime in the region for the last 120 Ma. Regional left‐lateral strike‐slip faults are now recognized in all lithotectonic provinces of the southernmost Andes. A statistical study of regional lineament trends using aerial photographs and satellite imagery suggests that many unstudied lineaments are also strike‐slip faults. A new model is proposed that integrates the development of strike‐slip faulting and the structural evolution and uplift of the southernmost Andes with the rotational development of the orocline. The Patagonian orocline appears to be the product of broad interplate shearing accommodated by strike‐slip faulting, block rotation, and contraction and is probably continuing to evolve today.

Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: New data from the Antarctic Peninsula and southernmost Andes

Subduction complexes along the Andean margin in central and southern Chile yield mid-Paleozoic to lower Mesozoic ages, yet they crop out within 100 km of the modern trench that shows evidence of active accretion along much of its length. The scarcity of uplifted subduction-complex rocks younger than mid-Mesozoic along the Chilean margin and in parts of the Scotia Arc suggests to us that these old, crystalline rocks, uplifted in the Triassic and Jurassic, represent a boundary in the forearc beyond which tectonic erosion does not easily occur. Greenschist-, blueschist-, and amphibolite-facies subduction-complex rocks from the Scotia Arc were originally thought to be a simple continuation of the subduction complexes in Chile. Based on new 40 Ar/ 39 Ar ages, the Scotia Arc subduction complexes reveal a complex history related to distinct local tectonic events and are not a simple continuation of the old accretionary prism in Chile. Structural and metamorphic analysis indicates the earliest and most penetrative deformation in the subduction complexes around the Scotia Arc occurred at some depth in a subduction zone or zones, certainly below the brittle-ductile transition, and in some cases under blueschist-facies conditions. We believe that the early subduction-related deformation and metamorphism in the greenschist- and blueschist-facies rocks of the Scotia Arc to be overprinted by mid-Cretaceous transpression along the South America-Antarctica plate boundary in the case of Elephant Island, transpression and subsequent localized transtension in the earliest Cenozoic in the case of the Darwin Complex, a mid-Cenozoic spreading-rate change in the case of Smith Island, and early Neogene initiation of Drake Passage opening/Shackleton Fracture Zone formation in the case of the Gibbs Island subgroup.

Intrusion of basaltic magma into a crystallizing granitic magma chamber: The Cordillera del Paine pluton in southern Chile

The Cordillera del Paine pluton in the southernmost Andes of Chile represents a deeply dissected magma chamber where mafic magma intruded into crystallizing granitic magma. Throughout much of the 10x15 km pluton, there is a sharp and continuous boundary at a remarkably constant elevation of 1,100 m that separates granitic rocks (Cordillera del Paine or CP granite: 69–77% SiO2) which make up the upper levels of the pluton from mafic and comingled rocks (Paine Mafic Complex or PMC: 45–60% SiO2) which dominate the lower exposures of the pluton. Chilled, crenulate, disrupted contacts of mafic rock against granite demonstrate that partly crystallized granite was intruded by mafic magma which solidified prior to complete crystallization of the granitic magma. The boundary at 1,100 m was a large and stable density contrast between the denser, hotter mafic magma and cooler granitic magma. The granitic magma was more solidified near the margins of the chamber when mafic intrusion occurred, and the PMC is less disrupted by granites there. Near the pluton margins, the PMC grades upward irregularly from cumulate gabbros to monzodiorites. Mafic magma differentiated largely by fractional crystallization as indicated by the presence of cumulate rocks and by the low levels of compatible elements in most PMC rocks. The compositional gap between the PMC and CP granite indicates that mixing (blending) of granitic magma into the mafic magma was less important, although it is apparent from mineral assemblages in mafic rocks. Granitic magma may have incorporated small amounts of mafic liquid that had evolved to >60% SiO2 by crystallization. Mixing was inhibited by the extent of crystallization of the granite, and by the thermal contrast and the stable density contrast between the magmas. PMC gabbros display disequilibrium mineral assemblages including early formed zoned olivine (with orthopyroxene coronas), clinopyroxene, calcic plagioclase and paragasite and later-formed amphibole, sodic plagioclase, mica and quartz. The early formed gabbroic minerals (and their coronas) are very similar to phenocrysts in late basaltic dikes that cut the upper levels of the CP granite. The inferred parental magmas of both dikes and gabbros were very similar to subalkaline basalts of the Patagonian Plateau that erupted at about the same time, 35 km to the east. Mafic and silicic magmas at Cordillera del Paine are consanguineous, as demonstrated by alkalinity and trace-element ratios. However, the contemporaneity of mafic and silicic magmas precludes a parent-daughter relationship. The granitic magma most likely was derived by differentiation of mafic magmas that were similar to those that later intruded it. Or, the granitic magma may have been contaminated by mafic magmas similar to the PMC magmas before its shallow emplacement. Mixing would be favored at deeper levels when the cooling rate was lower and the granitic magma was less solidified.

Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile

Research Article| January 01, 1991 Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile TERRY J. WILSON TERRY J. WILSON 1Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 Search for other works by this author on: GSW Google Scholar Author and Article Information TERRY J. WILSON 1Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (1): 98–111. https://doi.org/10.1130/0016-7606(1991)103<0098:TFBATF>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation TERRY J. WILSON; Transition from back-arc to foreland basin development in the southernmost Andes: Stratigraphic record from the Ultima Esperanza District, Chile. GSA Bulletin 1991;; 103 (1): 98–111. doi: https://doi.org/10.1130/0016-7606(1991)103<0098:TFBATF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract During the Mesozoic evolution of the southernmost Andes, back-arc basin formation in an extensional setting was followed by foreland basin development in a compressional setting. A stratigraphic record of this tectonic transition is described from Jurassic and Cretaceous rock units exposed in the Patagonian fold-thrust belt at 51°S latitude. Initiation of an extensional deep-marine trough in Late Jurassic time is documented by interstratified rhyolites and marine mudstones in the Tobifera Formation. Development of the Early Cretaceous Rocas Verdes back-arc basin is recorded by Zapata Formation submarine-slope deposits, which draped the west-facing, passively subsiding cratonic margin of the basin. The onset of Andean compressional orogenesis and creation of the Magallanes foreland basin is marked by the influx of coarse-grained sandstone turbidites of the Albian-Cenomanian Punta Barrosa Formation. During Late Cretaceous time, the foreland basin configuration consisted of a narrow foredeep trough bounded by a gently sloping foreland ramp on the craton to the east. This geometry reflects greater flexural subsidence of the stretched and thermally weakened passive back-arc basin margin beneath the load of the obducted Rocas Verdes basin floor. Changes in depositional regime and sediment dispersal patterns in the latest Cretaceous, together with an eastward shift in the Magallanes basin depocenter, record cratonward migration of deformation in the Patagonian fold-thrust belt. Loading of the thicker, cratonic lithosphere caused subsidence of the foreland ramp to form a wide flexural basin. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Isotopic composition of Late Jurassic and Early Cretaceous mafic igneous rocks from the southernmost Andes: impllcations for Sub-Andean mantle

ABSTRACT. A basaltic dike and minerals within a gabbro from the ophiolitic Tortuga Complex, a portion of the mafic floor of the Late Jurassic Rocas Verdes back-arc basin in southernmost South America, have initial 87Sr/86Sr ratios of 0.70323 to 0.70429. This range is due to variations in the amount of seawater which interacted with different pseudostratigraphic levels of the Tortuga Complex during the 'ocean floor' hydrothermal metamorphism that affected the Rocas Verdes. An original magmatic 87Sr/86Sr of 0.7031 is estimated for the mafic igneous rocks of the Tortuga Complex, which along with ENdi values of +6.8 to +7.6, is within the range of Late Jurassic depleted asthenospheric mantle. This is consistent with the previous documented oceanic petrochemical and structural character of the Tortuga Complex. Early Cretaceous Barros Arana basalts, which were erupted on the continental platform along the eastern edge of the Rocas Verdes Basin, have similar initial 87Sr/86Sr of 0.7030, but lower ENdi value of +5.0 to +5.2. This is interpreted as reflecting a contribution from the Mesozoic subcontinental lithospheric mantle, which like the current lithospheric mantle below southernmost South America, has isotopic characteristics indicating time-integrated depletion of large-ion-lithophile elements. The data support the suggestion that, by contrast, even the most mafic gabbros of the Patagonian batholith have incorporated crustal components. RESUMEN. Un dique basaltico y minerales de un gabro del Complejo ofiolitico Tortuga, pertenecientes al suelo malico igneo de la cuenca trasarco Rocas Verdes del Jurasico Superior en el extremo sur de Sudamerica, tienen razones iniciales de 87Sr/86Sr desde 0,70323 hasta 0,70429. Este intervalo se debio a las variaciones en la cantidad de agua oceanica que interactuo con los diferentes niveles seudoestratigraficos del Complejo Tortuga, durante el episodio de metamorfismo hidrotermal de 'fondo oceanico', que afecto a las Rocas Verdes. Se ha estimado una razon magmatica original de 87Sr/86Sr de 0,7031 para las rocas igneas maficas del Complejo Tortuga, la cual, junto a su ENdi de +6,8 a +7,6, se encuentra dentro del intervalo de valores del manto astenosferico empobrecido del Jurasico Superior. Esto es consistente con el caracter oceanico, petroquimico y estructural, previamente documentado para el Complejo Tortuga. Los basaltos de la Formacion Barros Arana (Cretacico Inferior), productos de erupcion en la plataforma continental, a lo largo del borde oriental de la Cuenca Rocas Verdes, tienen una razon inicial 87Sr/86Sr similar (0,7030), pero ENdi mas bajo (+5,0 a +5,2). Esto se interpreta como un reflejo de la contribucion del manto litosferico subcontinental, mesozoico, que, como el actual manto litosferico bajo el extremo sur de Sudamerica, tiene caracteristicas isotopicas indicativas de un empobrecimiento integrado a traves del tiempo por elementos litofilos de iones grandes. La informacion disponible apoya la idea de que, por el contrario, hasta los gabros mas maficos del Batolito Patagonico han incorporado componentes corticales.

New Species and Varieties of Passiflora (Passifloraceae) from the Andes of South America

Three new species and two new varieties of Passiflora in the subgenera Tacsonia and Rathea are described. All are endemic to Andean habitats at altitudes above 2600 meters, with Passiflora huamachucoensis, P. runa, and P. trifoliata var. tarmensis known only from Peru. Passiflora rugosa var. venezolana is found in the Cordillera de Merida, Venezuela and northeastern Colombia while P. colombiana is confined to the southernmost Andes of Colombia.

Tectonic Relations of South Georgia Island to the Southernmost Andes

Research Article| July 01, 1975 Tectonic Relations of South Georgia Island to the Southernmost Andes I.W.D DALZIEL; I.W.D DALZIEL 1Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar R. H. DOTT, JR.; R. H. DOTT, JR. 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar R. D. WINN, JR.; R. D. WINN, JR. 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar R. L. BRUHN R. L. BRUHN 3Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964 Search for other works by this author on: GSW Google Scholar Author and Article Information I.W.D DALZIEL 1Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964 R. H. DOTT, JR. 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 R. D. WINN, JR. 2Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 R. L. BRUHN 3Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (7): 1034–1040. https://doi.org/10.1130/0016-7606(1975)86<1034:TROSGI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation I.W.D DALZIEL, R. H. DOTT, R. D. WINN, R. L. BRUHN; Tectonic Relations of South Georgia Island to the Southernmost Andes. GSA Bulletin 1975;; 86 (7): 1034–1040. doi: https://doi.org/10.1130/0016-7606(1975)86<1034:TROSGI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Rocks on South Georgia Island at the eastern end of the North Scotia Ridge are no older than late Mesozoic. The Cumberland Bay and Sandebugten graywacke and mudstone sequences there are comparable in general lithology and structural style to the Lower Cretaceous Yahgan Formation of the Beagle Channel area in southernmost South America. The Cumberland Bay rocks, which form most of South Georgia Island, were thrust northeastward over the Sandebugten sequence. The Cumberland Bay and Yahgan sequences contain Cretaceous fossils, whereas the Sandebugten rocks are unfossiliferous.The dominant dispersal of Cumberland Bay detritus was toward the northwest. The Sandebugten dispersal pattern was more complex but was dominated by a south-directed component. In Early Cretaceous time, however, the South Georgia microcontinent apparently was attached to South America along the present southern margin of the Burdwood Bank. The Cumberland Bay, Sandebugten, and farther westward along strike, the Yahgan, apparently were deposited in a marginal small ocean basin between a calc-alkalic volcanic arc built on a sliver of old South American continental crust and the main part of the South American continent from which the sliver moved away. According to this interpretation, deformation of the sediments occurred when the arc moved back toward the continent in middle Cretaceous time, and the basin was closed and uplifted with the arc. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.