Patrice Baby

Geometry and kinematic evolution of passive roof duplexes deduced from cross section balancing: Example from the foreland thrust system of the southern Bolivian Subandean Zone

The Subandean Zone of Bolivia is a foreland fold and thrust belt which forms the eastern edge of the central Andes mountains. Between 19°S and 22°S latitude, the construction of five balanced cross sections shows that the N-S trending Subandean Zone is characterized by the existence of passive roof duplexes. These complex structures can be distinguished by the lithotectonic unit within which duplexing occurs. The five balanced cross sections permit the geometric and kinematic analyses of these passive roof duplexes. The sequential restorations of certain cross sections reveal a possible development of a piggy back sequence of three passive roof duplexes. Apparently, these passive roof duplexes propagated toward the foreland from deeper and deeper lithotectonic units. While a passive roof duplex was developing, the sole thrust stuck and the major horizontal displacements were then transferred either to out-of-sequence thrusts or to a new sole thrust, in a deeper detachment horizon. Therefore each passive roof duplex would correspond to the orogenic front of the Andean range at one very particular time in the history of the Subandean Zone of southern Bolivia. From south to north, the quantitative analysis by cross section balancing shows a transfer of displacement from the hinterland structures to the passive roof duplex that forms the present orogenic front. Available data do not permit us to explain completely this phenomenom.

Fossil Evidence for Evolution of the Shape and Color of Penguin Feathers

Feather of the Penguin Penguins are highly adapted for their cold, aquatic environment. Changes in their wings and feathers have allowed rapid swimming and protection from the near-freezing water. Clarke et al. (p. 954, published online 30 September; see the cover) describe an early penguin, dating to about 35 million years ago, that includes well-preserved feathers. The melanosomes in the feathers, which influence their strength, as well as their color, are like those of many other aquatic birds and unlike those of present-day penguins, even though the morphology of the wings and feathers had already been modified. Thus, in penguins, the shape and form of the feather evolved before microstructural changes occurred. The melanosome arrangement also suggests that the penguin was mostly gray-brown. A fossil penguin shows that the wing and feather form evolved before distinctive microstructural changes in the feathers. Penguin feathers are highly modified in form and function, but there have been no fossils to inform their evolution. A giant penguin with feathers was recovered from the late Eocene (~36 million years ago) of Peru. The fossil reveals that key feathering features, including undifferentiated primary wing feathers and broad body contour feather shafts, evolved early in the penguin lineage. Analyses of fossilized color-imparting melanosomes reveal that their dimensions were similar to those of non-penguin avian taxa and that the feathering may have been predominantly gray and reddish-brown. In contrast, the dark black-brown color of extant penguin feathers is generated by large, ellipsoidal melanosomes previously unknown for birds. The nanostructure of penguin feathers was thus modified after earlier macrostructural modifications of feather shape linked to aquatic flight.

Piggyback basin development above a thin-skinned thrust belt with two detachment levels as a function of interactions between tectonic and superficial mass transfer: the case of the Subandean Zone (Bolivia)

Abstract The Subandean fold and thrust belt of Bolivia is characterised by two major detachment levels and large piggyback basins. ‘Sand-box’ and numerical models have been used to study sedimentation and erosion control on thrust belt evolution and to study the retroactive effects of tectonics on piggyback development in thin-skinned thrust belts with two detachment levels. Analogue models show that surface processes play a dominant role in controlling wedge evolutions: erosion promotes fault reactivation and tectonic delamination (passive roof duplex) while sedimentation promotes forward shifting of the frontal thrust and consequently piggyback basin development. Numerical models were used to understand the development of the Subandean fold and thrust belt of Bolivia. Numerical experiments show that the simultaneity of basement tilting and high sedimentation rates promotes the formation of a stable tectonic wedge. Outer and inner faults are alternately active during the beginning of deformation, a kinematic evolution that favours the development of piggyback basins between. The step-by-step history of the thrust belt predicts that each change in tectonic location is recorded with large unconformities in basins, but these unconformities are not well preserved from progressive erosion in the final geometry.

Amber from western Amazonia reveals Neotropical diversity during the middle Miocene

Tertiary insects and arachnids have been virtually unknown from the vast western Amazonian basin. We report here the discovery of amber from this region containing a diverse fossil arthropod fauna (13 hexapod families and 3 arachnid species) and abundant microfossil inclusions (pollen, spores, algae, and cyanophyceae). This unique fossil assemblage, recovered from middle Miocene deposits of northeastern Peru, greatly increases the known diversity of Cenozoic tropical–equatorial arthropods and microorganisms and provides insights into the biogeography and evolutionary history of modern Neotropical biota. It also strengthens evidence for the presence of more modern, high-diversity tropical rainforest ecosystems during the middle Miocene in western Amazonia.

Stratigraphic responses to a major tectonic event in a foreland basin: the Ecuadorian Oriente Basin from Eocene to Oligocene times

The Eocene to Oligocene sediments of the Ecuadorian Oriente Basin record two kinds of second-order stratigraphic response to the tectonic evolution. Lower Eocene shows evidences of local scale syntectonic deposits. This tectonic activity can be related to right lateral convergent movements inverting pre-cretaceous extensional structures. Upper Eocene and Oligocene sediments are integrated as the expression of an isostatic rebound characterizing a basin scale syntectonic deposition. This response is evidenced by a reciprocal architecture of the depositional sequences identified in the sedimentary formations. These data have allowed us to propose a new geodynamic model for the Paleogene evolution of the Oriente Basin.

Controls on timing of exhumation and deformation in the northern Peruvian eastern Andean wedge as inferred from low‐temperature thermochronology and balanced cross section

In northern Peru, a 500 km long regional balanced section has been constructed across the eastern Andean wedge, using fieldwork, industrial seismic sections, and wells. The structure is characterized by a thin-skinned thrust system involving the Eastern Cordillera (EC), the sub-Andean zone (SAZ), and the Maranon foredeep. In the SAZ and the easternmost foredeep the development of the thrust system has been driven by the combination of two structural events. Permian thrust faults had been reactivated to form a basement duplex underlying the SAZ and the foredeep. At the same time a Triassic-Jurassic extensional basin has been transported as a crustal ramp anticline on to the duplex roof fault, giving rise to the EC. The impingement of the EC was responsible for the deformation of the SAZ and the propagation of the thrust wedge. The minimum shortening calculated is 142 km, representing a shortening strain of ~ −28%. A sequential restoration calibrated by (U-Th)/He and Fission Track dating on apatites and vitrinite reflectance values shows that shortening rates vary from 7.1 mm yr−1 between 17 and 8 Ma to 3.6 mm yr−1 between 8 Ma and today and suggests that the thrust wedge commenced propagation between 30 and 24 Ma. When compared with other Andean thrust wedges, we suggest that the timing of the thrust wedge propagation is not a simple function of the distance to the hinge of the Bolivian orocline and the propagation is not controlled by the precipitation regime. We rather suggest that reactivated basement faults favored thrust wedge propagation.

Evidence for major shortening on the Eastern edge of the Bolivian altiplano : the Calazaya nappe

Abstract For the first time, evidence is presented of major shortening on the eastern edge of the Altiplano, in Bolivia. A tectonic nappe showing at least 40 km of horizontal displacement towards the east is revealed at the Altiplano-Cordillera Oriental transition (20°S). Analysis of the differences in stratigraphy, sedimentology and structure between allochthonous and autochthonous units confirms the 40 km of overthrust. A geometric analysis of the thrust structures has been made to interpret the three-dimensional geometry of the nappe, and a deformation sequence is suggested. The nappe emplacement is related to the eastward transport of a large allochthonous domain during the Late Oligocene-Early Miocene. The sole thrust of this allocthtonous domain is located in a stratigraphie unit whose three-dimensional basin geometry controlled the deformation. The existence of the Calazaya nappe allows us to explain the exceptional thickness of the Altiplano (55–70 km) as mainly resulting from the Neogene shortening that occurred between the Altiplano and the Cordillera Oriental.

Structure et contrôle paléogéographique de la zone subandine péruvienne

Abstract The north–south structural evolution in the Subandean zone (eastern side of the Central Andes) reflects the complexity of the orogenic front that propagates on an heterogeneous substratum inherited from pre-Andean palaeogeographies. The construction of balanced cross-sections shows that this north–south evolution corresponds to a progressive transformation from thick-skin transpressive deformation to thin-skin compressive deformation. The thrust front propagation is controlled, in the north, by NNE–SSW to north–south rift systems Permo-Triassic to Jurassic in age, oblique to the chain, and in the south, by the relatively isopach series of the preserved Palaeozoic silico-clastic platform.

Ouverture et évolution tectono-sédimentaire du golfe de guayaquil : bassin d'avant-arc néogène et quaternaire du sud des andes équatoriennes

The analysis of industrial seismic and well data of the Gulf of Guayaquil allows us to constrain the structure of the Gulf of Guayaquil. The main opening phase of the deepest South Ecuadorian Neogene and Quaternary fore-arc basin occurred during the Lower Pleistocene. It is located at the southern tip of the Dolores-Guayaquil megashear zone, between WNW-ESE normal faults and north-south transpressional structures. The processes from which it originated are similar to those of a pull-apart basin. It developed between the Dolores-Guayaquil megashear to the east, and the oblique convergent Nazca-South America plate boundary to the west. During the Upper Pleistocene, the sedimentation rate decreases drastically in relation to both the glaciation-related sea level falls and a transpressional event.

Les ensembles fluviatiles néogènes du bassin subandin d'Équateur et implications dynamiques

Abstract A sedimentological study of the Neogene continental infill of the Subandean foreland basin of Ecuador led us to define an evolution of the fluvial system from an alluvial plain to an alluvial fan with an increasing slope in the same time as the drainage changed from mostly longitudinal to transverse. Combined with the data presently available on palaeotopography, exhumation, tectonic evolution and geomorphology, these results enable us to infer that, in contrast with the other Subandean foreland basins of Bolivia and Peru, the progradation of the Neogene alluvial fans proceeded by an overall expansion, associated with a relatively small tectonic shortening and not as a result of the development of successive thrust-related depocentres. This also indicates that the surrection of the Cordillera progressed in Ecuador throughout the Neogene. To cite this article: F. Christophoul et al., C. R. Geoscience 334 (2002) 1029–1037.