Fernando Hongn

Middle Eocene deformation and sedimentation in the Puna-Eastern Cordillera transition (23°-26°S): Control by preexisting heterogeneities on the pattern of initial Andean shortening

The Quebrada de los Colorados Formation, at the north end of Calchaqui Valley in Salta Province, northwest Argentina, preserves evidence of syndepositional deformation since the middle Eocene (ca. 40 Ma) that includes (1) an angular unconformity with the underlying Salta Group (Paleogene), and (2) internal unconformities and changes in vertical facies succession and provenance. Its fossil record [mammalian (notoungulates), middle Eocene] is correlatable to the Casa Grande Formation, which also unconformably overlies the Salta Group; both units record middle Eocene deformation along the eastern border of the Puna Plateau and outline previous fi rst-order mechanical heterogeneities related to the Cretaceous rift basin border. Along the western margin of the Puna, Eocene deformation coincides with thermal (magmatic arc) and mechanical (basin inversion) heterogeneities. Thus, the distribution of Eocene deformation followed an irregular pattern as a consequence of the heritage of preexisting heterogeneities.

The Time-Space Distribution of Cenozoic Volcanism in the South-Central Andes: a New Data Compilation and Some Tectonic Implications

The coincidence of late Paleogene to Neogene shortening and crustal thickening with vigorous volcanic activity in the central Andes has long invited speculation about a causal relationship between magmatism and deformation. In aid of understanding this and related issues, we present here a new compilation of radiometric ages, geographic location and dominant rock type for about 1450 Cenozoic volcanic and subvolcanic centers in the southcentral Andes (14–28° S). This paper describes variations in the timespace distribution of volcanism from 65 to 0 Ma, with emphasis on the post-30 Ma period where Andean-style shortening deformation and volcanism were most intense. The central Andes are unusual for the abundance of felsic ignimbrites and their distribution is shown separately from the intermediate to mafic volcanic centers which are here termed the “arc association”. Overall, the time-space patterns of volcanic activity for the ignimbrite and the arc association are similar but ignimbrite distribution is more patchy and more closely associated spatially with the plateau region.

The structure of the Precambrian and Lower Paleozoic basement of the Central Andes between 22° and 32°S. Lat.

ZusammenfassungIm Andenbasement dieser Region lassen sich drei diastrophische Hauptzyklen, die durch ihren strukturellen Baustil und ihre geographische Verbreitung definiert sind, unterscheiden. Die ältesten Strukturen sind verknüpft mit den Bewegungen der Panamerikanischen Orogenese (500–700 m. a.), welche im Zentralkraton einen polydeformierten Komplex von Schiefern, Gneissen und Graniten erzeugte. Diesem lagern diskordant kambrische und ordovizische Sequenzen ohne Metamorphose und präandiner Deformation auf. Nach Westen zu gibt es einen Gürtel stark deformierter ordovizischer Sedimente, die einen einfacheren Baustil als das präkambrische Basement aufweisen, und diskordant von undeformierten devonischen Abfolgen bedeckt sind. Die Faltung des Ordoviziums wird der Ocloyischen Phase der Kaledonischen Bewegung zugerechnet.Weiter westwärts tritt ein jüngerer Faltengürtel aus devonischen Sedimenten auf, die während der Chanischen Phase der Herzynischen Bewegung intensiv deformiert wurden, und von karbonischen und permischen Sequenzen diskordant überlagert sind.Der Ocloyische Gürtel ist von postordovizischen Graniten die von devonischen Schichten überlagert sind, und der Chanische Gürtel von karbonischen und permischen Graniten intrudiert. Im präkambrischen Basement des Zentralkratons treten einerseits präkambrische Granite auf, denen diskordant kambrische Schichten auflagern. Andererseits findet man Granite mit paläozoischen Isotopenaltern, letztere aber in Gebieten, wo wegen des Fehlens von Deckschichten eine stratigraphische Kontrolle unmöglich ist.Die gemachten Beobachtungen scheinen darauf hinzudeuten, daß die paläozoischen Faltengürtel intrakratonisch sind. Ob es allochtone »terrains« in der pazifischen Küstenkordillere gibt, ist Gegenstand der Kontroverse.AbstractThree major diastrophic cycles, defined by their structural style and their spatial distribution are recognized in the Andean Basement of this region. The oldest structures are related to the Panamerican Orogeny (500 to 700 m. a.) which produced in the Central Craton multiply deformed complexes of schists, gneisses and granites, that are covered discordantly by unmetamorphosed Cambrian and Ordovician beds. West of the Central Craton Ordovician sedimentary beds are folded with a simple structural style and intruded by granites. Both the sedimentry beds and the granites are covered discordantly by undeformed Devonian sequences. The folding of the Ordovician is attributed to the ocloyic phase of the Caledonian movements. West of the ocloyic belt is another foldbelt consisting of strongly folded Devonian beds attributed to the chanic phase (hercynian). The chanic belt is intruded by carboniferous and permian granites and covered discordantly by Carboniferous and Permian sequences.The features observed in the eastern slope of the Andes suggest that the Paleozoic foldbelts are intracratonic. Whether there are accreted terrains in the Pacific Coastal Cordillera is matter of controversy.ResumenEn el basamento de los Andes de esta región se distinguen tres ciclos diastróficos mayores definidos por sus estilos estructurales y su distributión espacial. Las estructuras mas antiguas responden a movimientos que culminaron con la Orogénesis Panamericana (500–700 m. a.) que produjo en el Cratógeno Central un complejo de esquistos, gneises y granitos polideformados, al que se le superponen discordantemente secuencias cámbricas y ordovicicas sin metamorfismo ni déformatión preandina. Hacia el oeste hay un cinturón de sedimentos ordovícicos intensamente deformados con un estilo de plegamiento mas simple que el del basamento precámbrico y cubierto discordantemente por secuencias marinas devónicas sin deformatión. Los movimientos responsables de este plegamiento se atribuyen a la fase oclóyica de los movimientos caledónicos. Más hacia el oeste se manifiesta una franja de deformatión mas jóven atribuída a la Fase chánica de los movimientos hercínicos, en ella se encuentran sedimentitas devónicas intensamente plegadas cubiertas discordantemente por secuencias carbónicas y pérmicas.El cinturón oclóyico esta acompanãdo por granitos postordovícicos cubiertos por Devónico. Asimismo, el cinturón chánico (hercínico) esta intruido por granitos probablemente carbónicos. Con respecta a los granitos intruidos en el zócalo antiguo, hay algunos indudablemente precámbricos por estar cubiertos discordantemente por secuencias cámbricas sin metamorfismo, además hay granitos de edad dudosa que han dado valores isotópicos paleozoicos pero en lugares donde el control estratigráfico no es posible por ausencia de la cobertura paleozoica. De acuerdo a los hechos observados en la ladera oriental de los Andes surge la idea de que los cinturones de plegamiento paleozoicos son intracratónicos, aun es materia de controversia si en la Cordillera de la Costa junto al Pacifico hay terrenos alóctonos acrecionados.Краткое содержаниеВ названном регионе о снования Анд различа ют три больших диастроф ических цикла, отличающихся своими структурными стилям и и их географическим р асположением. Более древние структуры со ответствуют Панамер иканскому орогенезу (500–700 миллион ов лет), при котором они образ овались в центрально м кратогенном комплек се из кристаллическо го сланца, гнейсов и гран итов, на которые отлож ились несогласно кембрийс кие и ордовицкие седименты, не претерп евшие ни метаморфизм а, ни до-андскую деформа цию. На запад от центрального кратон а отмечен пояс сильно деформированных седиментов ордовицк ого возраста с более простым типом складок, чем в докембрийском основ ании, и с интрузиями гр анитов. Они покрыты несоглас но слоями морских девонских отложений, также не претерпевши х деформацию.За складкообразован ие в ордовике считают ответственным Оклож скую фазу каледонски х сдвигов. Далее на запа д от окложского пояса появляется регион се диментов девонского возраста, которые во в ремя чанской фазы герцинского горообр азовательного проце сса подверглись сильном у смятию в складки; их п ерекрывают свиты карбонового и п ермского возраста. Пояс Чана со держит интрузии гран итов карбонового и пермсо го возраста и покрыт несогласно свитами с едиментов того же возраста. В докембрий ском основании центр ального кратона находят с одн ой стороны докембрий ские граниты, на которых за легают несогласно отложени я кембрийского возра ста, а с другой стороны — гр аниты, соотношения изотопов в которых ук азывает на палеозойс кий возраст. Эти граниты н айдены также и в областях, где провест и точное стратиграфи ческое подразделение из-за о тсутствия покровных слоев невозможно.Отмеченные схемы на в осточном склоне Анд разрешают предпо лагать, что пояса скла док палеозоя являются ин тракратоновыми. Спорным остается воп рос существуют-ли зде сь участки аккреционно го материала в Кордил ьерах у берегов Тихого океа на.

Deformation mechanisms in the mylonite/ultramylonite transition

Abstract The deformation mechanisms and controls that operate in the mylonite/ultramylonite transition are interpreted from microstructural observation. The investigated mylonites and ultramylonites were derived from a granitic protolith which was deformed under greenschist facies conditions, and in the presence of fluid, in a regional-scale shear zone from northwest Argentina. Several deformation mechanisms were recognized to operate simultaneously in different domains of the microstructure at each particular stage of the microstructural evolution. This continuously mobile deformation partitioning, present throughout the microstructural evolution, ceases abruptly in the ultramylonite stage, where a stable-state microstructure is achieved. Domainal quartz c-axis fabrics indicate that quartz deforms by crystal-plastic processes at the initial and intermediate stages of deformation, but solution-transfer processes become predominant in the ultramylonite stage. Plagioclase is progressively transformed into muscovite through retrograde softening reactions. K-feldspar is progressively transformed into fine-grade aggregates via cataclastic flow and incipient recrystallization. Mica deforms by kinking and basal slip, with progressive development of fine-grained, morphologically oriented aggregates. Plagioclase disappearance as well as the development of intrafolial microfolds characterize the transition between the mylonitic and ultramylonitic domains. Disruption of these microfolds is interpreted to represent the ultimate control on the localization of the ultramylonite bands.

Tectonic Evolution of the Western Margin of Gondwana Inferred from Syntectonic Emplacement of Paleozoic Granitoid Plutons in Northwest Argentina

The Cerro Durazno, Brealito, and Cobres granitoid plutons belong to the eastern Ordovician magmatic belt of northwest Argentina and are associated with a network of NW‐ and N‐striking ductile shear zones. A structural analysis of these plutons and their metamorphic host rocks was conducted to elucidate mid‐ to upper‐crustal magmatism and tectonism at the western margin of Gondwana. Pluton emplacement took place at 480–455 Ma, toward the end of a ca. 75‐Ma‐long episode of regional deformation and high‐temperature metamorphism. The Cerro Durazno and Brealito plutons were probably emplaced as NE‐SW‐striking, vertical magma sheets that coalesced into large magma bodies under overall E‐W shortening that generated SW‐dipping metamorphic foliations at a later stage of pluton formation. These fabrics were superimposed to the magmatic structures in the granite plutons and to the high‐grade metamorphic structures of the host rocks. At the northern pluton margins, ductile high‐strain zones, notably the Agua Rosada Shear Zone north of the Cerro Durazno pluton, further localized ascent of granitoid magma. Similarly, emplacement of the Brealito and Cobres plutons is genetically related to prominent shear zones. Geometry and metamorphic character of the shear zone network in the eastern Ordovician magmatic belt point to orogen‐parallel extension and transverse shortening of the belt under variable metamorphic conditions. The width as well as protracted deformation, high‐temperature metamorphism, anatexis, and granitoid magmatism of the magmatic belt are key characteristics of a hot, wide orogen that evolved during the lower Paleozoic at the western margin of Gondwana. Remarkable geotectonic similarities with respect to the modern Andes point to comparable plate‐tectonic conditions at the lower Paleozoic and the Cenozoic convergent plate margins.

Quartz crystallographic and morphologic fabrics during folding/transposition in mylonites

Abstract Quartz grain shape fabrics (GSF) and c-axis lattice preferred orientation (LPO) were investigated in folded quartz ribbons with variable fold geometry in low metamorphic-grade granitic mylonites from northwestern Argentina. The folds, which vary from open to isoclinal, show an asymmetric short limb–long limb geometry developed simultaneously with the quartz fabrics during progressive simple shear. GSF shows a consistent obliquity relative to the bulk shear plane throughout the folded ribbons. GSF inclination varies between 25° and 45° oblique to the shear plane, indicating that relatively small finite strain increments (γ∼1.2) are attained before the GSF regeneration. In contrast, the c-axis preferred orientation varies throughout the folded ribbons. Unfolded ribbons and long limbs of folds show a typical c-axis maximum near the shear plane pole, while short limbs of tight and isoclinal folds show a c-axis maximum nearly orthogonal to that of long limbs. This pattern of domainal LPO is maintained in thickened ribbons derived from folding and transposition. This indicates that the GSF regeneration is comparatively much faster than a cycle of folding/transposition. Conversely, the LPO regeneration is much slower. We conclude that folding/transposition is a mechanism for development of domainal quartz LPO in kinematic frameworks with dominant simple shear.

Upper-Crustal Structure of the Central Andes Inferred from Dip Curvature Analysis of Isostatic Residual Gravity

The relationship between Bouguer gravity, isostatic residual gravity and its dip curvature, first-order structural elements and distribution of Neogene volcanic rocks was examined in the southern Altiplano and Puna Plateau. In the southern Altiplano, strong positive Bouguer gravity corresponds to areas affected by late Cenozoic faulting and large-scale folding of upper crustal rocks. Dip curvature analysis of isostatic residual gravity shows that elongate zones of maximum curvature correspond remarkably well with the structural grain defined by first-order folds and faults. Similarly, isostatic residual gravity in the Puna is largely controlled by prominent, upper-crustal structures and also by the distribution of Miocene and younger volcanic rocks. In particular, the Central Andean Gravity High, one of the most prominent features of the residual gravity field, corresponds with domains of low topography, i.e., internally- drained basins, which are surrounded by zones of Neogene faults and abundant felsic volcanic rocks. Dip curvature analysis of the isostatic residual gravity field shows that elongate zones of maximal curvature correlate with the strike of prominent Neogene faults. Our study suggests that such analysis constitutes an important tool for imaging upper-crustal structures, even those that are not readily apparent at surface. For example, upper-crustal faults in the Salar de Atacama area, the presence of which is suggested by the dip curvature of residual gravity, offers a plausible explanation for the pronounced angular departure of the volcanic belt from its overall meridional trend and its narrowing south of the salar. In contrast to previous interpretations, our study suggests that gravity anomalies of the Central Andes are largely controlled by the distribution of late Cenozoic volcanism and tectonism. Dip curvature analysis of gravity fields bear great potential for elucidating first-order structural elements of deformed, upper-crustal terrains such as the modern Andes.

Construction and degradation of a broad volcanic massif: The Vicuña Pampa volcanic complex, southern Central Andes, NW Argentina

The Vicuna Pampa volcanic complex, at the SE edge of the arid Puna Plateau of the Central Andes, records the interplay between volcanic construction and degradational processes. The low-sloping Vicuna Pampa volcanic complex, with a 1200-m-deep, southeastward-opening depression, was previously interpreted as a collapse caldera based on morphological considerations. However, characteristic features associated with collapse calderas do not exist, and close inspection instead suggests that the Vicuna Pampa volcanic complex is a strongly eroded, broad, massif-type composite volcano of mainly basaltic to trachyandesitic composition. Construction of the Vicuna Pampa volcanic complex occurred during two distinct cycles separated by the development of the depression. The first and main cycle took place at ca. 12 Ma and was dominated by lava flows and subordinate scoria cones and domes. The second cycle, possibly late Miocene in age, affected the SW portion of the depression with the emplacement of domes. We interpret the central depression as the result of a possible sector collapse and subsequent intense fluvial erosion during middle to late Miocene time, facilitated by faulting, steepened topography, and wetter climate conditions compared to today. We estimate that ∼65% of the initial edifice of ∼240 km 3 was degraded. The efficiency of degradation processes for removing mass from the Vicuna Pampa volcanic complex is surprising, considering that today the region is arid, and the stream channels within the complex are predominantly transport limited, forming a series of coalesced, aggraded alluvial fans and eolian infill. Hence, the Vicuna Pampa volcanic complex records the effects of past degradation efficiency that differs substantially from that of today.

The Pre-Andean Phases of Construction of the Southern Andes Basement in Neoproterozoic–Paleozoic Times

During the late Neoproterozoic and Paleozoic times, the southern Andes of Argentina and Chile (21o–55o S) formed part of the southwestern margin of Gondwana. During this period of time, a set of continental fragments of variable extent and allochtony was successively accreted to that margin, resulting in six Paleozoic orogenies of different temporal and spatial extension: Pampean (Ediacaran–early Cambrian), Famatinian (Middle Ordovician–Silurian), Ocloyic (Middle Ordovician–Devonian), Chanic (Middle Devonian–early Carboniferous), Gondwanan (Middle Devonian–middle Permian), and Tabarin (late Permian–Triassic). All these orogenies culminate with collisional events, with the exception of the Tabarin and a part of the Gondwanan orogenies that are subduction-related.

SEM and TEM evidence of mixed-layer illite-smectite formed by dissolution-crystallization processes in continental Paleogene sequences in northwestern Argentina

Abstract In the northernmost Calchaquí Valley (Salta, Argentina), the Paleogene continental sediments show a transition from smectite, at the top, to R3 I-S (>90% illite) through R1 I-S (65-80% illite), in contrast to the remaining sectors, containing smectite up to the bottom. Samples at the base of the succession were characterized by high-quality step-scan X-ray diffraction (XRD), scanning electron microscopy (SEM) and analytical high-resolution transmission electron microscopy (HRTEM). Analysis by SEM demonstrated dissolution of primary phases (feldspars, micas and quartz) and crystallization of illite, I-S and kaolinite. As this alteration is not pervasive, an intermediate fluid/rock ratio could be inferred. The lattice-fringe images of the samples from upper parts of the sequence show abundant I1-rich areas, whereas in the lower parts of the sequence, illite packets and I3 I-S coexist and compositions evolve towards muscovite (tetrahedral-charge increase, principally compensated by Mg-by- Al substitution in octahedral sites and by a slight decrease in Ca in interlayer sites). As burial temperatures were probably similar in all the samples, depth was not responsible for the illite formation at the bottom. The TEM textures suggest that illitization proceeded mainly by dissolution-crystallization. The active faults close to the northern Calchaquí Valley probably promoted the circulation of hot, deep fluids, favouring illitization.