Florencia Bechis

Thrust belts of the southern Central Andes: Along-strike variations in shortening, topography, crustal geometry, and denudation

The Andean fold-and-thrust belts of west-central Argentina (33°S and 36°S), above the normal subduction segment, present important along-strike variations in mean topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. To analyze the controlling factors of these latitudinal changes, we compare these parameters and the chronology of deformation along 11 balanced crustal cross sections across the thrust belts between 70°W and 69°W, where the majority of the upper-crustal deformation is concentrated, and reconstruct the Moho geometry along the transects. We propose two models of crustal deformation: a 33°40′S model, where the locus of upper-crustal shortening is aligned with respect to the maximum crustal thickness, and a 35°40′S model, where the upper-crustal shortening is uncoupled from the lower-crustal deformation and thickening. This degree of coupling between brittle upper crust and ductile lower crust deformation has strong influence on mean topographic elevation. In the northern sector of the study area, an initial thick and felsic crust favors the coupling model, while in the southern sector, a thin and mafic lower crust allows the uncoupling model. Our results indicate that interplate dynamics may control the overall pattern of tectonic shortening; however, local variations in mean topographic elevation, deformation styles, and crustal root geometry are not fully explained and are more likely to be due to upper-plate lithospheric strength variations.

Eocene Patagonia Fossils of the Daisy Family

Fossil evidence suggests that daisies and sunflowers may have originated in South America more than 47 million years ago. Fossil capitula and pollen grains of Asteraceae from the Eocene of Patagonia, southern Argentina, exhibit morphological features recognized today in taxa, such as Mutisioideae and Carduoideae, that are phylogenetically close to the root of the asteracean tree. This fossil supports the hypothesis of a South American origin of Asteraceae and an Eocene age of divergence and suggests that an ancestral stock of Asteraceae may have formed part of a geoflora developed in southern Gondwana before the establishment of effective dispersal barriers within this landmass.

An extinct Eocene taxon of the daisy family (Asteraceae): evolutionary, ecological and biogeographical implications

BACKGROUND AND AIMS Morphological, molecular and biogeographical information bearing on early evolution of the sunflower alliance of families suggests that the clade containing the extant daisy family (Asteraceae) differentiated in South America during the Eocene, although palaeontological studies on this continent failed to reveal conclusive support for this hypothesis. Here we describe in detail Raiguenrayun cura gen. & sp. nov., an exceptionally well preserved capitulescence of Asteraceae recovered from Eocene deposits of northwestern Patagonia, Argentina. METHODS The fossil was collected from the 47·5 million-year-old Huitrera Formation at the Estancia Don Hipólito locality, Río Negro Province, Argentina. KEY RESULTS The arrangement of the capitula in a cymose capitulescence, the many-flowered capitula with multiseriate-imbricate involucral bracts and the pappus-like structures indicate a close morphological relationship with Asteraceae. Raiguenrayun cura and the associated pollen Mutisiapollis telleriae do not match exactly any living member of the family, and clearly represent extinct taxa. They share a mosaic of morphological features today recognized in taxa phylogenetically close to the root of Asteraceae, such as Stifftieae, Wunderlichioideae and Gochnatieae (Mutisioideae sensu lato) and Dicomeae and Oldenburgieae (Carduoideae), today endemic to or mainly distributed in South America and Africa, respectively. CONCLUSIONS This is the first fossil genus of Asteraceae based on an outstandingly preserved capitulescence that might represent the ancestor of Mutisioideae-Carduoideae. It might have evolved in southern South America some time during the early Palaeogene and subsequently entered Africa, before the biogeographical isolation of these continents became much more pronounced. The new fossil represents the first reliable point for calibration, favouring an earlier date to the split between Barnadesioideae and the rest of Asteraceae than previously thought, which can be traced back at least 47·5 million years. This is the oldest well dated member of Asteraceae and perhaps the earliest indirect evidence for bird pollination in the family.

Pre-Andean deformation of the Precordillera southern sector, southern Central Andes

This paper presents a detailed investigation of the structure and evolution of the Precordillera southern sector (Argentina). We document the development and successive reactivation of regional and discrete structural grain through time, and discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. Our kinematic studies indicate that the Permian orogeny generated a doubly vergent fold-and-thrust belt of transpressive deformation, where strain was partitioned into two different types of deformation domains. The west-vergent western domain was characterized by partitioned transpression with shortening dominating, and a strike-slip–dominated subdomain. The east-vergent eastern domain was characterized by pure contractional deformation. Our model for the Late Permian to Early Triassic evolution of the Precordillera involves a north-northwest–trending weakness zone affected by north-northeast–directed extension, generating an area with transtensional deformation during the Choiyoi volcanism development. Later, during the Triassic generation of the Cuyana rift basin, the northeast stretching direction was orthogonal to the rift trend, indicating pure extensional deformation. We propose a model where the clear parallelism between the distribution of an inferred early Paleozoic suture zone, a north-northwest–trending late Paleozoic belt, and Permian–Triassic rift-related magmatism indicates the reactivation of a north-northwest–trending long-lived lithospheric weakness zone.

Evolution of shallow and deep structures along the Maipo-Tunuyán transect (33°40'S): From the Pacific coast to the Andean foreland

Abstract We propose an integrated kinematic model with mechanical constrains of the Maipo–Tunuyán transect (33°40′S) across the Andes. The model describes the relation between horizontal shortening, uplift, crustal thickening and activity of the magmatic arc, while accounting for the main deep processes that have shaped the Andes since Early Miocene time. We construct a conceptual model of the mechanical interplay between deep and shallow deformational processes, which considers a locked subduction interface cyclically released during megathrust earthquakes. During the coupling phase, long-term deformation is confined to the thermally and mechanically weakened Andean strip, where plastic deformation is achieved by movement along a main décollement located at the base of the upper brittle crust. The model proposes a passive surface uplift in the Coastal Range as the master décollement decreases its slip eastwards, transferring shortening to a broad area above a theoretical point S where the master detachment touches the Moho horizon. When the crustal root achieves its actual thickness of 50 km between 12 and 10 Ma, it resists further thickening and gravity-driven forces and thrusting shifts eastwards into the lowlands achieving a total Miocene–Holocene shortening of 71 km.

Cenozoic Orogenic Evolution of the Southern Central Andes (32–36°S)

This review explores the complex interactions of endogenic and exogenic processes in the segment of the Andes that straddle a transition from the Pampean flat slab to a normal subduction segment (32°–36°S). This segment shows remarkable along-strike variations in topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. In the flat-slab segment, high elevations, the development of several tectonic provinces (mountain systems) and the lack of active volcanism characterize the orogen. Deformation and uplift advanced to the east, together with arc-related magmatic activity, sequentially uplifting the Principal Cordillera (20 to ~8 Ma), the Frontal Cordillera (12–5 Ma), the Precordillera (<10 Ma) and the Sierras Pampeanas (<5 Ma). In the normal subduction segment to the south, the Andes are characterized by a decrease in elevation, with a big step in topography at ~35°S and the development of an active magmatic arc straddling the Argentina-Chile border. The Frontal Cordillera is only in the northern part of the normal subduction segment, disappearing at 34°S; only the Principal Cordillera remains south of this latitude. Similarly, deformation progressively advanced to the east, uplifting the Principal Cordillera (20–8 Ma), the Frontal Cordillera (<10 Ma) and the San Rafael basement block (<5 Ma). The amount of shortening systematically decreases from north to south along these two segments, but at the transitional zone between flat and normal subduction zones, there is a sharp decline from ~180 km of shortening (32°S) to ~70 km (33°40′S). South from this latitude, the amount of shortening lineally decreases until it reaches ~30 km at 35°S. Yet, interestingly, the amount of late Miocene surface uplift is opposite to the trend in crustal shortening . These along-strike variations are best explained by boundary conditions of the subduction system related to interplate dynamics controlling the overall pattern of tectonic shortening. However, local variations in mean topographic elevation, deformation styles and crustal root geometry are more likely to be due to upper-plate lithospheric strength variations. These strength variations have governed the coupling degree between brittle upper crust and ductile lower crust deformation. In the flat-slab segment, an initial thick and felsic crust favors the coupling model; while in the normal subduction segment, a thin and mafic lower crust allows the uncoupling model.

The Late Oligocene–Early Miocene Marine Transgression of Patagonia

The most important Cenozoic marine transgression in Patagonia occurred during the late Oligocene–early Miocene when marine waters of Pacific and Atlantic origin flooded most of southern South America including the present Patagonian Andes between ~41° and 47° S. The age, correlation, and tectonic setting of the different marine formations deposited during this period are debated. However, recent studies based principally on U–Pb geochronology and Sr isotope stratigraphy, indicate that all of these units had accumulated during the late Oligocene–early Miocene. The marine transgression flooded a vast part of southern South America and, according to paleontological data, probably allowed for the first time in the history of this area a transient connection between the Pacific and Atlantic oceans. Marine deposition started in the late Oligocene–earliest Miocene (~26–23 Ma) and was probably caused by a regional event of extension related to major plate reorganization in the Southeast Pacific. Progressive extension and crustal thinning allowed a generalized marine flooding of Patagonia that reached its maximum extension at ~20 Ma. It was followed by a phase of compressive tectonics that started around 19–16 Ma and led to the growth of the Patagonian Andes. The youngest (~19–15 Ma) marine deposits that accumulated in the eastern Andean Cordillera and the extra-Andean regions are coeval with fluvial synorogenic deposits and probably had accumulated under a compressive regime.