Andrés Folguera

Andean flat-slab subduction through time

Abstract The analysis of magmatic distribution, basin formation, tectonic evolution and structural styles of different segments of the Andes shows that most of the Andes have experienced a stage of flat subduction. Evidence is presented here for a wide range of regions throughout the Andes, including the three present flat-slab segments (Pampean, Peruvian, Bucaramanga), three incipient flat-slab segments (‘Carnegie’, Guañacos, ‘Tehuantepec’), three older and no longer active Cenozoic flat-slab segments (Altiplano, Puna, Payenia), and an inferred Palaeozoic flat-slab segment (Early Permian ‘San Rafael’). Based on the present characteristics of the Pampean flat slab, combined with the Peruvian and Bucaramanga segments, a pattern of geological processes can be attributed to slab shallowing and steepening. This pattern permits recognition of other older Cenozoic subhorizontal subduction zones throughout the Andes. Based on crustal thickness, two different settings of slab steepening are proposed. Slab steepening under thick crust leads to delamination, basaltic underplating, lower crustal melting, extension and widespread rhyolitic volcanism, as seen in the caldera formation and huge ignimbritic fields of the Altiplano and Puna segments. On the other hand, when steepening affects thin crust, extension and extensive within-plate basaltic flows reach the surface, forming large volcanic provinces, such as Payenia in the southern Andes. This last case has very limited crustal melt along the axial part of the Andean roots, which shows incipient delamination. Based on these cases, a Palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism. The geological evolution of the Andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system.

Tectonic evolution of the Andes of Neuquén: constraints derived from the magmatic arc and foreland deformation

Abstract The Andes of the Neuquén region (36° − 38°S latitude) of the Central Andes have distinctive characteristics that result from the alternation of periods of generalized extension followed by periods of compression. As a result of these processes the Loncopué trough is a unique long depression at the foothills parallel to the Principal Cordillera that consists of a complex half-graben system produced during Oligocene times and extensionally reactivated in the Pliocene-Pleistocene. Its northern sector represents the present contractional orogenic front. The nature and volume of arc-related igneous rocks, the location of the volcanic fronts, expansions and retreats of the magmatism, and the associated igneous activity in the foreland, together with the analyses of the superimposed structural styles, permit the constraint of the alternating tectonic regimes. On these bases, different stages from Jurassic to Present are correlated with changes in the geometry of the Benioff zone through time. Periods of subduction-zone steepening are associated with large volumes of poorly evolved magmas and generalized extension, while shallowing of the subduction zone is linked to foreland migration of more evolved magmas associated with contraction and uplift in the Principal Cordillera. The injection of hot asthenospheric material from the subcontinental mantle into the asthenospheric wedge during steepening of the subduction zone produced melting and poorly evolved magmas in an extensional setting. These periods are linked to oceanic plate reorganizations in the late Oligocene and in the early Pliocene.

Tectonic evolution of the Andean Fold and Thrust Belt of the southern Neuquén Basin, Argentina

Abstract The Andean Fold and Thrust belt between 36° and 39°S can be divided in two sectors. The Eastern Sector corresponds to the Agrio Fold and Thrust Belt (FTB) characterized by a major exhumation during the Late Cretaceous, and minor deformation during the late Eocene and Late Miocene. The Western Sector corresponds to the main cordillera and is characterized by a complex evolution that involves periods of out-of-sequence thrusting with respect to the previously deformed outer sector, and pulses of relaxation of the compressive structure. Cretaceous uplift constituted an orogenic wedge that extended to the inner sectors of the Agrio FTB. Eocene compression was mainly concentrated within the Western Sector but may have reactivated the pre-existing structures of the Agrio FTB, such as the Cordillera del Viento. Late Miocene minor compressional deformation occurred in the retro-arc area and extended into the foreland area. This deformation event produced the closure of a short-lived intra-arc basin (Cura Mallín Basin, 25-15 Ma) at the innermost sector of the FTB. The Pliocene and Quaternary, between 37°30′ and 39°S, have been periods of relaxation of the inner part of the FTB and fossilization of the Agrio Fold and Thrust Belt. Localization of episodic late Oligocene-Early Miocene and Pliocene to the present extensional structures in the intra- and inner retro-arc is controlled by pre-existing Jurassic half-grabens related to the formation of the Neuquén Basin. The Jurassic rift seems to be controlled by deep crustal-lithospheric discontinuities derived from a Proterozoic-Palaeozoic history of amalgamation in the area, now deeply buried under multiple episodes of Mesozoic-Tertiary synorogenic and synextensional sedimentation.

Partición de la deformación en la zona del arco volcánico de los Andes neuquinos (36-39°S) en los últimos 30 millones de años

Los Andes neuquinos, entre los 36 y los 39°S, fueron construidos a traves de un amplio espectro de procesos durante los ultimos 30 millones de anos. Estos pueden ser resumidos en dos tipos alternantes: mecanicas de deformacion en las cuales se desarrollan amplias areas de cientos de kilometros de ancho sometidas a cizalla lateral homogenea paralela a los limites de placa y mecanicas con desarrollo de fallas transcurrentes en los dominios orogenicos internos y construccion de montanas en el retroarco generadas por el apilamiento de laminas de corrimiento de corteza superior. La corteza superior fue dominada por transtension entre 28 y 15 Ma, mientras la zona del arco volcanico concentro el area de desarrollo de pequenas cuencas volcanogenicas en un cinturon de mas de 100 kilometros de ancho. Luego, entre 12 y 10 (5) Ma, el arco concentro la intrusion de grandes volumenes de magma a lo largo de sistemas transcurrentes litosfericos, mientras la formacion de cuencas estaba inhibida y el retroarco se deformaba en un campo de cizalla pura perpendicular a los limites de placa, que invertia tectonicamente cuencas extensionales preexistentes. Finalmente luego de los 5 Ma, las condiciones llegaron a ser mas parecidas al lapso 28-15 Ma, en el cual una transtension homogenea afecto amplias fajas, de ancho superior a los 50 kilometros. La particion de la deformacion controla la forma en que es distribuida la cizalla lateral paralela a los limites de placas, la cual se relaciona con la componente de la convergencia paralela a la trinchera. Estas mecanicas de distribucion van de zonas discretas de cizalla, a amplias areas de deformacion por cizalla en las cuales existe un alto potencial de generacion de cuencas extensionales. El empinamiento y la somerizacion de la zona de Wadati-Benioff controlo localmente el grado de particion de la deformacion entre los 36 y 39°S, definiendo periodos discretos de acumulacion de grandes volumenes de materiales volcanicos alternados con periodos de intrusion de magmas a lo largo de fallas de rumbo en el arco volcanico

Multiple slope failures associated with neotectonic activity in the Southern Central Andes (37°–37°30′S), Patagonia, Argentina

Quaternary tectonic activity in the transition area between the Central and Patagonian Andes is closely associated with an anomalous cluster of rockslides: 19 rockslides with volumes up to 4 × 10 9 m 3 developed in plateau basalts. We divided them into two groups: (A) rockslides related to neotectonic activity and (B) rockslides not related to neotectonic activity. Thirteen rockslides, with a total volume of ∼10 km 3 , which lie on either folds or faults, have been displaced parallel to the structures and perpendicular to the valley axis, and they exhibit headscarps several kilometers away from the valley axis. Most of them are larger than 10 9 m 3 , and are generally of rock avalanche type with a high degree of crushing of rocks, although local relief in some cases does not exceed 200 m. Nine rockslides with a total volume of 8.9 km 3 are related to folds, while four with a total volume of 1.3 km 3 are related to faults. The six rockslides not related to neotectonic activity have a total volume of 0.25 km 3 (of which the largest one accounts for 0.17 km 3 ), and are rotational slides and block topples with a low degree of rock fragmentation, although local relief is up to 400 m. The 3 He and 21 Ne surface exposure ages for six of these slides, as well as relative age assessment based on stratigraphic relation with glacial deposits and the drainage development on the rockslide deposit, suggest that the rockslide ages spread rather randomly between pre-glacial and mid Holocene, discarding climatic conditions as a common triggering factor. The absence of structures that can represent ideal sliding planes shows that rock fracturing due to neotectonic activity is a major conditioning factor for failures and that the magnitude of landslides is strongly controlled by the type of deformation.

Landslide Dams in the Central Andes of Argentina (Northern Patagonia and the Argentine Northwest)

Landslide dams are frequent phenomena in the Argentine Andes. We studied 20 landslide dams in NW Argentina and 41 landslide dams in northern Patagonia. These examples show that most of the landslide dams in both regions have longevity of several hundred to several tens of thousands of years. In those cases where the mode of dam erosion/breach was reconstructable it was either related to climatic variability influencing the inflow of water into the landslide-dammed lake or by landsliding into the landslide dammed lake causing a tsunami wave which overtopped the dam crown and caused its erosion. However such tsunami waves not always lead to dam failure. There is one case where flood deposits downriver a dam exist and the landslide dammed lake contains a voluminous landslide deposit, however the dam did not breach. Hence the flood deposits are related to the tsunami wave but not to a breach. In addition, our examples indicate the necessity of expanding the well established dam classification system used globally in the past 18 years. Here we define 4 further dam types which are related to (a) the diversion of the river away from the valley over bedrock (b) the diversion of the river into the neighbouring catchments (c) the deposition of the landslide in a drainage divide, and (d) the formation of multiple dams by the breach of a landslide dam itself. Furthermore, ponds on top of landslide deposits are frequent and depending on their size a catastrophic release may cause damage.

Late Oligocene–early Miocene submarine volcanism and deep-marine sedimentation in an extensional basin of southern Chile: Implications for the tectonic development of the North Patagonian Andes

The Chilean margin has been used as the model of an ocean-continent convergent system dominated by compression and active mountain building as a consequence of the strong mechanical coupling between the upper and the lower plates. The Andean Cordillera, however, shows evidence of alternating phases of compressional and extensional deformation. Volcano-sedimentary marine strata in the Aysen region of southern Chile contribute to an understanding of the causes of extensional tectonics and crustal thinning that occurred in the Andean orogeny because these deposits constitute the only reliable record of submarine suprasubduction volcanism during the Cenozoic in southern South America. In order to discern the age and tectono-sedimentary setting of these strata, referred to as the Traiguen Formation, we integrated sedimentology, ichnology, petrography, geochemistry, structural geology, foraminiferal micropaleontology, and U-Pb geochronology. Our results indicate that the Traiguen Formation was deposited in a deep-marine extensional basin during the late Oligocene–earliest Miocene. The geochemistry and petrography of the pillow basalts suggest that they formed in a convergent margin on a thinned crust rather than at an oceanic spreading center. We attribute the origin of the Traiguen Basin to a transient period of slab rollback and vigorous asthenospheric wedge circulation that was caused by an increase in trench-normal convergence rate at ca. 26–28 Ma and that resulted in a regional event of extension and widespread volcanism.

Early Andean tectonomagmatic stages in north Patagonia: insights from field and geochemical data

The Andes in northern Patagonia are mainly formed by Mesozoic magmatic units: the mostly Jurassic–Cretaceous North Patagonian Batholith and volcanism of the Jurassic Lago La Plata (Ibáñez) Formation as well as the mid-Cretaceous Divisadero Group. These rocks represent the development of a magmatic belt through Jurassic–mid-Cretaceous time, during a switch of the tectonic regime from extension to compression. To study arc evolution during this transition, we carried out fieldwork and geochemical sampling at c. 43°S, clarifying structural relationships and characterizing the magmatic sources. Multi-element diagrams for both volcanic units suggest a slab-derived signature, whereas isotopic ratios (Sr–Nd–Pb) indicate parental melts sourced from the subduction-modified asthenospheric mantle interacting with crustal sources during their emplacement. An angular unconformity is identified between the synextensional Jurassic volcanic rocks and Lower Cretaceous sedimentary rocks beneath the mid-Cretaceous sequences. Although this deformational event was simultaneous with generalized overriding plate compression, geochemical ratios indicate an immature Aptian–Albian arc with no associated crustal thickening. Late Jurassic to mid-Cretaceous arc settlement after a trenchward retraction of magmatism from the foreland between c. 41 and 45°S, with an associated increase in slab dip angle, may have provoked crustal softening facilitating the subsequent initial fold–thrust belt growth. Supplementary material: Petrographic descriptions and geochemical–isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3677974

Collision of the Mocha fracture zone and a <4 Ma old wave of orogenic uplift in the Andes (36°–38°S)

The southern Central and northern Patagonian Andes (34°–45°S) are characterized by low to no crustal seismicity at the retroarc fold and thrust belt, in contrast to the Pampean flat subduction zone located immediately to the north (27°–33°30′S). Detailed examination of this area shows no indication of contractional neotectonics with the exception of the segment located between 36° and 38°S. There, out-of-sequence transpressional deformation, initially developed in the 1.7–1.4 Ma interval, affects the western retroarc between 36° and 38°S next to the arc zone. It is between these latitudes that contractional deformation

A Review of the Geology, Structural Controls, and Tectonic Setting of Copahue Volcano, Southern Volcanic Zone, Andes, Argentina

Copahue Volcano lies in the Southern Volcanic Zone of the Andes Mountains, although its geology and local structural controls differ from nearby active volcanic centers. Most of its geology is substantially older than active volcanoes at these latitudes, as the postglacial component is relatively minor. The basement of Copahue Volcano, represented by the Agrio Caldera products and its basal sections, accumulated in extensional depocenters when the arc narrowed from a broad geometry on both sides of the Andes to its present configuration. Initial stages comprise early Pliocene basaltic-andesitic eruptions associated with extensional (transtensional?) processes that ended with the formation of a series of rhombohedral calderas that emitted important amounts of ignimbrites in latest Pliocene-early Pleistocene time. Copahue Volcano concentrates the Pleistocene activity of one of these calderas, the Agrio Caldera, before the emplacement and development of the Present arc front to the west. Volcano morphology reflects this particular evolution, looking more degraded than Antuco, Callaqui and Lonquimay volcanoes located immediately to the west in the arc front. Most of Copahue’s volume is early Pleistocene in age, showing a thin resurfacing cover in synglacial (>27 ka) and postglacial times. A synglacial stage occurred mainly to the east of Copahue Volcano toward the caldera interior in a series of independent, mostly monogenetic centers. Postglacial eruptions occurred as both central and fissural emissions reactivating the old Pleistocene conduits. Its particular geological record and eastern longitudinal position indicate that Copahue was probably part of the late Pliocene-Pleistocene arc mostly developed in the axial and eastern Andes. Narrowing and westward retraction of the arc front, proposed in previous works for the last 5 Ma at 38°S, could have been the result of the eastward migration of the asthenospheric wedge during slab steepening. Reasons for this long-lived eruptive history at Copahue volcano could be related to the particular geometry of the active Liquine-Ofqui dextral strike-slip fault system that runs through the arc front from south to north when penetrates the retroarc area at the latitude of Copahue volcano. This behavior could be due to the collision of the oceanic Mocha plateau at these latitudes, as recently proposed. This jump and related deflection would have produced local transtensional deformation associated with abundant emissions of syn- and post-glacial products that could have partially resurfaced this long-lived center.