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International audience; The northern part of South America has undergone several major changes in its tectonic setting since the Triassic, evolving from rifting to the development of a retroarc-foreland basin on its Andean margin, leading to margin extension and continental uplift following the breakup between Africa and South America. So far, it is unclear when and how these geodynamic events affected the paleo-Amazonian drainage. In this study, we investigate the provenance of Triassic–Eocene sedimentary rocks deposited in the northern Peruvian Amazonian basin, based on their Sr-Nd isotopic compositions and U-Pb zircon dating. The Triassic–Jurassic samples have εNd(0) values ranging from −7.9 to −10.0 and main U-Pb zircon peaks at 0.9–1.3 Ga (31–33%) and 0.5–0.7 Ga (21–28%) that suggest a mixed clastic supply from the Western and Eastern Cordillera or craton within a rift to post rift setting. Samples from the Albian–Maastrichtian interval yield much lower εNd(0) values (−16.8 to −18.6) and a dominance of zircon grains derived from terranes in the easternmost Brazilian Shield (Ventuari-Tapajos (2.0–1.82 Ga), Rio Negro-Jurena (1.82–1.54 Ga) and Rondonia San Ignacio (1.54–1.3 Ga), thus indicating a cratonic source for these sedimentary rocks. Finally, the early Paleocene–Eocene sedimentary rocks record the first arrival of Andean detritus in the Amazonian retroarc foreland, with εNd(0) values ranging between −5.6 to −12.0 and up to 16% of the zircon grains yielding ages younger than 120 Ma. Together with recently published studies, these provenance data document the existence of a long-lived Aptian–Maastrichtian continent-wide cratonic drainage in the northern part of South America. This cratonic drainage developed in response to the late Cretaceous uplift of the northeastern part of the South America craton likely driven by geodynamic processes related to post rift opening of the Atlantic Equatorial Ocean. The formation of a late Maastrichtian–early Paleocene mountain chain in the Peruvian Andes associated with the onset of provenance from the Andean orogenic belt led to a shift of the Amazon drainage pattern. Hence, the earliest Andean-Amazonian rivers are no younger than late Maastrichtian-early Paleocene.

International audience; The Huallaga Basin is a deformed foreland basin located in North Peru. The basin comprises several syntectonic depocenters. The most significant is the Biabo Syncline located at the back of the Chazuta Thrust, a long, flat-floored thrust detaching on an evaporitic décollement, which has accommodated more than 40 km of horizontal displacement. The hangingwall of the Chazuta Thrust has remained remarkably intact with little or no internal deformation and has incorporated a large volume of evaporites at its base.In order to unravel the formation and evolution of this thrust, we conducted a series of physical experiments that tested the role of various parameters. The goal is to investigate a system in which most of the deformation is accommodated in the frontal part of the chain (Chazuta Thrust), whereas deformation of the thrust sheet itself remains minor.Results from our experimental investigations suggest that the three key parameters that have allowed for such a long-lived, large-slip frontal thrust to operate are (1) the wedge-shaped syn-kinematic sedimentation, (2) the presence of the Biabo Syncline, which acted as a bulldozer pushing the evaporites forward, forcing their distal inflation and (3) the erosion at the front that favored farther advance of the frontal thrust, dragging passively large volumes of evaporites along with it.

Biostratigraphic, sedimentological and provenance analyses suggest that a proto-Andean Cordillera already existed in southern Peru by late Maastrichtian–late Palaeocene times. A 270-m-thick stratigraphic section shows changes in depositional environments from shallow marine (early Maastrichtian) to non-marine (late Maastrichtian) then back to estuarine (late Palaeocene) conditions. An erosional surface separates lower Maastrichtian from upper Maastrichtian deposits. Above this surface, the late Maastrichtian unit exhibits moderately developed palaeosols and syn-sedimentary normal faults. The sedimentary evolution is accompanied by a decrease in sedimentation rate and by changes in provenance. Shallow marine lower Maastrichtian deposits have a cratonic provenance as shown by their low eNd(0) values (-15 to -16) and the presence of Precambrian inherited zircon grains. The upper Maastrichtian deposits have a mixed Andean and cratonic origin with eNd(0) values of ~12.6 and yield the first Cretaceous and Permo-Triassic zircon grains. Estuarine to shallow marine upper Palaeocene deposits have an Andean dominant source as attested by higher eNd(0) values (-6 to -10) and by the presence of Palaeozoic and Late Cretaceous zircon grains. The changes in depositional environments and sedimentation rates, as well as the shift in detrital provenance, are consistent with a late Maastrichtian–late Palaeocene period of Andean mountain building. In agreement with recently published studies, our data suggest that an Andean retro-arc foreland basin was active by late Maastrichtian–late Palaeocene times. This article is protected by copyright. All rights reserved.

In the Peruvian sub-Andean foreland basin system, the construction of serial balanced cross-sections from a good set of structural data and an extensive knowledge of the stratigraphy and geodynamic evolution allow a more refined definition of the unexplored plays, as subthrusts, duplexes, or pre-Andean structures. Sequential restorations are proposed by coupling thermochronologic analyses with growth strata studies. The results show significant north–south variations in geometry, timing and rates of deformation, and foreland sedimentation. These latitudinal variations are not only related to the pre-Andean basins’ inheritance but also to the interactions between thrusts propagation, erosion, and sedimentation. Thermochronologic ages correspond to the most recent thrust-related uplifts and are supplemented by the study of stratigraphic foreland basin records that can bring to light oldest tectonic events. North of the Peruvian sub-Andean zone, thrusts propagation is controlled by thick-skinned and thin-skinned salt tectonics. Northern thick-skinned tectonics has westward vergence and is inherited from a Middle Permian fold-and-thrust belt. To the south, thrusts deformation is largely controlled by the geometry of the preserved Paleozoic sedimentary wedge and becomes progressively thin skinned. Total sub-Andean shortening varies between 70 km (43 mi) in the north and 47 km (29 mi) in the south. Sub-Andean deformation started in the Late Cretaceous. After a period of quiescence during the middle Eocene, it reactivated and is still active. Three stages of sub-Andean deformation are clearly identified and help to define the preservation time in the suggested petroleum plays.

The western Amazon drainage basin, which extends from southern Colombia to northern Bolivia, comprises the Cordillera Oriental of the Andes and its adjacent foreland basin system. In northern Bolivia, the orogenic wedge of the eastern Andes is very large, and its forward propagation controls the morphology of the Madeira drainage basin. We consider here the erosion and sedimentation mass balance in this part of the Amazon Basin, estimated on the basis of recent sediment yield data, within the current tectonic and geomorphic framework. The total suspended sediment (TSS) flux exported from the present orogenic wedge of northern Bolivia has been estimated at 500–600 million t year � 1 . More than 50% of the total sediment load crossing the Madeira foreland basin system is deposited. The rest of the sediments (less than 46%) reaches the eastern Amazon Basin, bypassing the Brazilian craton to the north. The average mass of sediment that has been deposited from the late Miocene to the present in the Madeira foreland basin sedimentation system is less than that intercepted today, by a factor of about 2Ð4. These results can be interpreted as an increase in Bolivian foreland basin flexural subsidence over time, associated with crust thickening and orogenic loading, and accentuated by the growing mass of retained sediments. They are consistent with the uplift rates of the Cordillera Oriental, obtained from fission-track dating, which began increasing significantly around 10–15 Ma. Copyright  2009 John Wiley & Sons, Ltd.

International audience; Miocene and Pliocene deposits from the Ecuadorian, Bolivian and Peruvian Amazonian foreland basin (north and south Amazonian foreland basin, NAFB and SAFB) were analyzed for major and trace element geochemistry (97 and 104 samples respectively). Neogene deposits are well homogenized and have an overall PAAS-like composition. They are mainly derived from a granodioritic crust. In the NAFB, chemical weathering increases from Miocene to Pliocene to CIA values similar to 100. Physical sorting seems to have played a minor role in the chemical composition of these sediments. Chemical ratios such as Cr/Th and Th/U and Eu anomaly vary within each formation and from one formation to another. In the SAFB, sediments have a PAAS-like composition with recycling increasing toward the distal part of the foreland basin. In the NAFB, the Neogene formations have various provenances. Basic/andesitic sediments originated from the Ecuadorian Andes. These sediments were trapped in the subsiding Ecuadorian foredeep during the Miocene and reached the Iquitos forebulge to join the paleo-Amazon near Iquitos (Amazon formation) during the Pliocene. Other Miocene formations have PAAS-like affinities but are enriched in Zr and Hf relative to the PAAS suggesting that they are derived from a recycled and differentiated upper continental crust. Uplift of the Iquitos forebulge may have been responsible for dividing drainage networks originating from the Andes and the craton. The Quaternary floodplain deposits of the Amazon River resemble PAAS with depletion in trace elements and enrichment in Zr and Hf, which indicate that they are derived from a recycled differentiated upper continental crust. Overall, the transition from tidal sedimentation in the Miocene to fluvial sedimentation in the Late Miocene-Pliocene related to a change with foreland basin dynamics have caused increasing silicate weathering that could have had consequences on climate and global carbon cycle during the Neogene.

During the Middle–Late Miocene, tidal sedimentation was the rule in the Amazon foreland basin. The Amazon foreland basin became emerged in the Late Miocene–Pliocene times. In this study, trace element chemistry ( n = 104) and Nd–Sr isotopic systematic ( n = 30) are used to constrain the provenance of the Miocene tidal sediments and of the Late Miocene–Pliocene fluvial sediments. Neogene Amazonian foreland basin sediments are the result of a mixing between Andean andesitic volcanic rocks and cratonic shield rocks. The south Amazonian foreland basin (SAFB) sediments are the result of long-term weathering, recycling, and erosion of the Brazilian shield and Andean Paleozoic/Mesozoic rocks also Brazilian shield in provenance. Compared with north Amazonian foreland basin (NAFB) sediments, SAFB sediments are more felsic. NAFB sediments have 3 distinct signatures: i) an upper crust signature with trace element characteristics similar to PAAS and UCC and with ɛ Nd(0) values between − 8 and − 11.9; ii) an arc andesitic rock signature with high Cr / Th ratios, low Eu anomalies, low Th / Sc ratios and ɛ Nd(0) values between − 3 and − 5; and iii) a cratonic signature with high Eu anomalies and Zr / Sc ratios and with a very low ɛ Nd(0) value (− 15.5). Our data indicate that most of the basic detritus came from the Ecuadorian Andes. In the distal part of the NAFB, the successive uplifts of the Iquitos forebulge were responsible for Late Miocene to Pliocene divide between Andean and cratonic drainage systems. The modern “Amazon River” drainage network is no younger than the Pliocene.

International audience; Small volumes of Cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the Ecuadorian Oriente foreland basin. They are characterized by a restricted range of compositional variation, low LILE/HFSE ratios and Sr Nd isotope values within the range of oceanic island basalts (OIB). Reflection seismic data show that a pre-existing NNE SSW Triassic and Jurassic rift controls the location and occurrence of these alkaline eruptive sites. Radiometric ages (40Ar 39Ar, incremental heating method) and the biostratigraphic record of their surrounding sediments indicate a NNE SSW systematic age variation for the emplacement of this alkaline volcanism: from Albian (110 ± 5.2 Ma) in the northern part of the Oriente Basin, to Campanian (82.2 ± 2.0 Ma) in the west-central part. The geochemical, geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the SSW, into the region underlying the pre-existing Triassic and Jurassic rift (thin-spot?). We propose that subduction was abandoned, subsequent to the accretion of allochthonous terranes onto the Ecuadorian and Colombian margin in the latest Jurassic earliest Cretaceous, causing the relict slab material, corresponding to the eastwards-directed leading plate, to roll-back. Unmodified asthenospheric mantle migrated into the region previously occupied by the slab. This resulted in partial melting and the release of magmatic material to the surface in the northern part of the Oriente Basin since at least Aptian times. Then, magmatism migrated along the SSW-trending Central Wrench Corridor of the Oriente Basin during the Upper Cretaceous, probably as a consequence of the lateral propagation of the transpressive inversion of the Triassic Jurassic rift. Eventually, the Late Cretaceous east-dipping Andean subduction system was renewed farther west, and the development of the compressional retro-foreland Oriente Basin system halted the Cretaceous alkaline magmatic activity.

This article presents new biostratigraphic dating, facies analysis, organic geochemical data and Nd-Sr isotopic provenance from five outcrops of southern Amazonia (MD-85, MD-177 MD-184, MD-255 and MD-256) to document for the first time the presence of a shallow marine ingression in the Paleocene of southern Amazonia basin. The co-occurrence of a selachian assemblage encompassing Potobatis sp., Ouledia sp., and Pristidae indet with the ostracod Protobuntonia sp. and the charophytes Peckichara cf. varians meridionalis, Platychara perlata, and Feistiella cf. gildemeisteri suggests a Paleocene age for the studied deposits (most likely Thanetian but potentially Danian). Fifteen fades have been recognized and have been grouped into three facies assemblages. Facies association A corresponds to the sedimentary filling of a tide-influenced meandering channel formed in the fluvial-tidal transition zone. Facies association B is related to more distal tidal-flats, little channelized tidal inlets and saltmarsh deposits. Facies association C corresponds to a stressed shallow marine environment such as a bay or a lagoon. The delta C-13(TOC) value (- 23.4%) of MD-184 is enriched in C-13 compared to the other samples suggesting the presence of substantial amounts of marine organic matter in MD-184. The delta C-13(TOC) values of samples from other outcrops ( - 27.3 to - 29.8%) indicate a mixed organic matter origin, from terrestrial to brackish environments. The analyzed sediments have similar Nd-Sr isotopic compositions as those of the Cenozoic sediments of the Altiplano (epsilon Nd(0) values from - 6.2 to - 10.7 and Sr-87/Sr-86 compositions from 0.712024 to 0.719026) indicating a similar volcanic source. This multidisciplinary dataset documents the presence of a tide-dominated estuary sourced by the proto-Westem Cordillera debouching into a shallow marine bay during Paleocene times. This transgression might be explained by subsidence created in response to the proto-Western Cordillera loading. Similar to Miocene marine incursions affecting the Pebas megawetland, Paleogene marine incursions in the Amazonian foreland basin associated with Andean uplift may have played a role in the Neotropical biodiversity dynamics in favoring biogeographical isolation and promoting allopatric speciation for terrestrial organisms.

International audience; Although the structure of the central Peruvian Subandean zone is well defined, the timing of thrust-related exhumation and Cenozoic sedimentation remain poorly constrained. In this study, we report new apatite (U-Th)/He (AHe) and fission track (AFT) ages from thrust-belt and foreland strata along three published balanced cross sections. AHe data from the northern, thick-skinned domain (i.e. Shira Mountain, Otishi Cordillera and Ucayali Basin) show young AHe ages (ranging from 2.6 ± 0.2 to 13.1 ± 0.8 Ma) compared with AFT ages (ranging from 101 ± 5 to 133 ± 11 Ma). In the southern Camisea Basin, where deformation is mainly thin-skinned, AHe and AFT ages have been both reset and show young cooling ages (3.7 ± 0.8 Ma and 8 ± 2 Ma respectively). Using low-temperature thermochronology data and the latest fission track annealing and He diffusion codes, the thermal history of the study area has been reconstructed using inverse modelling. This history includes two steps of erosion: Early Cretaceous and late Neogene, but only Neogene sedimentation and exhumation varies in the different sectors of the study area. From a methodological point of view, large AHe data dispersion point to the need for refinement of AHe damage and annealing models. The influence of grain chemistry on damage annealing, multiple age components and the possibility of fission tracks as traps for He need further consideration. For the central Peruvian Subandes, AHe and AFT ages combined with balanced cross sections emphasize the dominant control of Paleozoic inheritance rather than climate on Cenozoic infilling and exhumation histories. Finally, our data provide the first field example of how thick-skinned thrust-related deformation and exhumation in the Subandes can be directly dated through AHe thermochronology.

Formation of the Bolivian eastern cordillera started at the end of the Oligocene and continues to the present day with thrusting affecting the Sub Andean Zone since late Miocene. The 9classical9 source rock is the Devonian, but the Silurian interval has some potential. Their initial original potential is not high but they are thick (700 m, Los Monos Fm) and may be thicker still due to stacking in thrust duplexes. The three phases of sedimentation during the Tertiary each start with conglomeratic deposits and are discordant over the previous one. Seismic data allow us to correlate these steps with three periods of tectonic accretion: Late Miocene (Tariquia Fm), Pliocene (Guandacay Fm) and Plio-Quaternary (Emborozu Fm). Modelling demonstrates two phases of hydrocarbon generation. A first phase from Devonian to Carboniferous is due to the deepening of the Palaeozoic basin. The second phase of maturation is due to the Tertiary deposits in the foreland and under the piggyback basins. The central area (Santa Cruz) has never been more deeply buried than during the Palaeozoic and has been a high during the Tertiary.

International audience; Avulsion is the main process at the origin of anastomosing rivers. This study illustrates 3 examples of avulsions resulting from crevasse-splays evolving in anastomosed channels along the Rio Pastaza, a tropical humid river sourced in the Ecuadorian Andean Cordillera and flowing into the Amazonian foreland. The Lower Pastaza flows in an alluvial plain, with no tectonic influence and an average monthly rainfall equally distributed throughout the year. Based on the analysis of satellite image recorded over the period 1977-2008, three cases have been studied. The first one began in 1990 with crevassing of natural levees of the right bank of the Pastaza main channel and the formation of a small stream travelling toward a pre-existing tributary to this main channel. A splay formed at the confluence beheaded the tributary which became an anabranch of the main river. Downstream, two other avulsions developed from ancient crevasse-splays on a low gradient floodplain. In both cases, capture of one of the distributary channels flowing on the splay by a pre-existing drain of the floodplain and consecutive headward erosion arrives to disconnect the other drains and capture their flow into a single thread channel. As this channel rejoins the Pastaza main channel downstream, this process gives rise to the larger-scale anastomosing system which characterizes the lower Pastaza. Although the precipitation regime and discharges curves are rather regular, levees breaching and crevasse splay formations is likely to be a result of annual floods which are present in this area as in the rest of Amazonia.

Oligocene to Quaternary deposits from the Oriente Amazonian foreland basin (Ecuador and Peru) were analyzed for major and trace element geochemistry (46 and 32 samples respectively) and Nd–Sr isotopic systematics (n = 10). Chemical Index of Alteration values lower than those of other Amazonian foreland basin sediments and scattering along the AK join in the A–CN–K diagram indicate that the Oriente foreland basin has been continuously fed by poorly to moderately weathered sediments having an overall Andesitic composition since the Oligocene. Chemical ratios such as Cr/Th and Th/Sc as well as Eu anomaly and Nd–Sr isotopic compositions indicate that most of the analyzed sediments contained a greater proportion of volcanic arc rock material than the other Amazonian foreland basin sediments. When compared with the older sediments The Quaternary sediments are characterized by a greater contribution of the volcanic arc source. The composition of the sediments deposited in the Ecuadorian Amazonian foreland basin is mainly controlled by geodynamic processes. We suspect the Late Pliocene-Pleistocene subduction of the Carnegie ridge to be responsible for the back arc volcanism feeding the Amazonian foreland with more basic materials. Input of young Ecuadorian volcanic rocks may explain the difference in Sr and Nd isotopic ratios of suspended sediments between the Solimoes and the Madeira rivers.

[1] Structural analysis of the southern Tunisian Atlas was carried out using field observation, seismic interpretation, and cross section balancing. It shows a mix of thick-skinned and thin-skinned tectonics with lateral variations in regional structural geometry and amounts of shortening controlled by NW-SE oblique ramps and tear faults. It confirms the role of the Late Triassic–Early Jurassic rifting inheritance in the structuring of the active foreland fold and thrust belt of the southern Tunisian Atlas, in particular in the development of NW-SE oblique structures such as the Gafsa fault. The Late Triassic–Early Jurassic structural pattern is characterized by a family of first-order NW-SE trending normal faults dipping to the east and by second-order E-W trending normal faults limiting a complex system of grabens and horsts. These faults have been inverted during two contractional tectonic events. The first event occurred between the middle Turonian and the late Maastrichtian and can be correlated with the onset of the convergence between Africa and Eurasia. The second event corresponding to the principal shortening tectonic event in the southern Atlas started in the Serravalian-Tortonian and is still active. During the Neogene, the southern Atlas foreland fold and thrust belt propagated on the evaporitic decollement level infilling the Late Triassic–Early Jurassic rift. The major Eocene “Atlas event,” described in hinterland domains and in eastern Tunisia, did not deform significantly the southern Tunisian Atlas, which corresponded in this period to a backbulge broad depozone.

The 400 000 km 2 -wide Fitzcarrald Arch constitutes a wide topographic high of the Amazon Basin against the central Andes. In order to constrain its formation mechanisms and in particular to test its relationships to the Nazca ridge subduction, a quantitative geomorphology analysis of the Arch is performed using hypsometric integrals, elongation and azimuths of 7th- and 5th-order catchments. They all express a trend from high maturity to low maturity from NW towards SE. This maturity gradient coupled with the local drainage direction demonstrate that the Fitzcarrald Arch is not a ‘classical’ alluvial fan, since its apex is located 100 km east to the Subandean Thrust Front and the corresponding sedimentary pile is lacking. Nor is the Arch the superficial expression of an inherited transfer zone, because its geomorphic shape is radial and it does not diverge from a symmetry axis; moreover, such a reactivated structure is not found at depth on seismic profiles. In addition, our data show that underlying geomorphic control on catchment initiation and development has progressed from NW to SE, which in combination with the observation of crustal doming by Espurt et al. [Espurt, N., Baby, P., Brusset, S., Roddaz, M., Hermoza, W., Regard, V., Antoine, P.O., Salas-Gismondi, R., Bolanos, R., 2007. How does the Nazca Ridge subduction influence the modern Amazonian foreland basin? Geology 35, 515–518.] suggests that this relief is caused by the eastward sliding of the buoyant Nazca ridge beneath the South American lithosphere.

[1] The Neogene evolution of the Ene and southern Ucayali basins of the Subandes has been controlled by two stacked thrust wedges that differ in terms of tectonic styles. The lower thrust wedge is formed by deep-seated decollements within the basement related to thick-skinned foreland structures inherited from an Early Carboniferous thrust system. Seismic reflection data show that this Paleozoic compressional system has been eroded and unconformably covered by Late Carboniferous clastic sediments. It generated an irregular Paleozoic sedimentary architecture controlling the Neogene thrust propagation. The upper thin-skinned thrust wedge developed within this Paleozoic sedimentary series and constitutes the Subandean zone. Cross-section balancing shows an along-strike homogenous horizontal shortening of ∼56 km (∼30%) across the Ene−southern Ucayali thrust system. This amount of shortening was vertically partitioned onto the two stacked thrust wedges. The N-S thickness variations of the Paleozoic sedimentary prism controlled the eastward propagation of the upper thrust wedge. The southern thickening of the Paleozoic series generated major decollements and the shortening excess is of 7 km (16%) in comparison to the north. Consequently, the northern lack of shortening onto the upper thrust wedge was transferred to the Early Carboniferous compressional structures of the lower thrust wedge. We suggest that this vertical partitioning of the shortening was accommodated by a regional oblique ramp: the Tambo transfer zone. This geometrical analysis of the Ene−southern Ucayali thrust system provides new perspectives for future hydrocarbon exploration in this region.

International audience; The subduction of an aseismic ridge has important consequences on the dynamics of the overriding upper plate. In the central Andes, the Nazca Ridge subduction imprint can be tracked on the eastern side of the Andes. The Fitzcarrald arch is the long-wavelength topography response of the Nazca Ridge flat subduction, 750 km inboard of the trench. This uplift is responsible for the atypical three-dimensional shape of the Amazonian forelland basin. The Fitzearrald arch uplift is no older than Pliocene as constrained by the study of Neogene sediments and geomorphic markers, according to the kinematics of the Nazca Ridge subduction.

This article presents new biostratigraphic dating, facies analysis, organic geochemical data and Nd–Sr isotopic provenance from five outcrops of southern Amazonia (MD-85, MD-177 MD-184, MD-255 and MD-256) to document for the first time the presence of a shallow marine ingression in the Paleocene of southern Amazonia basin. The co-occurrence of a selachian assemblage encompassing Potobatis sp., Ouledia sp., and Pristidae indet. with the ostracod Protobuntonia sp. and the charophytes Peckichara cf. varians meridionalis, Platychara perlata, and Feistiella cf. gildemeisteri suggests a Paleocene age for the studied deposits (most likely Thanetian but potentially Danian). Fifteen facies have been recognized and have been grouped into three facies assemblages. Facies association A corresponds to the sedimentary filling of a tide-influenced meandering channel formed in the fluvial–tidal transition zone. Facies association B is related to more distal tidal-flats, little channelized tidal inlets and saltmarsh deposits. Facies association C corresponds to a stressed shallow marine environment such as a bay or a lagoon. The δ13CTOC value (− 23.4‰) of MD-184 is enriched in 13C compared to the other samples suggesting the presence of substantial amounts of marine organic matter in MD-184. The δ13CTOC values of samples from other outcrops (− 27.3 to − 29.8‰) indicate a mixed organic matter origin, from terrestrial to brackish environments. The analyzed sediments have similar Nd–Sr isotopic compositions as those of the Cenozoic sediments of the Altiplano (eNd(0) values from − 6.2 to − 10.7 and 87Sr/86Sr compositions from 0.712024 to 0.719026) indicating a similar volcanic source. This multidisciplinary dataset documents the presence of a tide-dominated estuary sourced by the proto-Western Cordillera debouching into a shallow marine bay during Paleocene times. This transgression might be explained by subsidence created in response to the proto-Western Cordillera loading. Similar to Miocene marine incursions affecting the Pebas megawetland, Paleogene marine incursions in the Amazonian foreland basin associated with Andean uplift may have played a role in the Neotropical biodiversity dynamics in favoring biogeographical isolation and promoting allopatric speciation for terrestrial organisms.

International audience; Mio-Pliocene deposits of the forebulge backbulge depozones of the Beni-Mamore foreland Basin indicate tidally to fluvially dominated sedimentation. Seven facies assemblages have been recognized: FAA FAG. FAA represents a distal bottom lake assemblage, FAB and FAD are interpreted as tidal flat deposits, FAC and FAG are interpreted as fluvial systems, FAE sediments are deposited in a subtidal/shoreface setting, and FAG represents a meandering fluvial system. The identification of stratigraphic surfaces (SU, MFS, and MRS) and the relationship among the facies assemblages permit the characterization of several systems tracts: a falling-stage systems tract (FSST) followed by a lowstand systems tract (LST), a transgressive systems tract (TST), and a highstand systems tract (HST). The FSST and LST may have been controlled by the uplift of the Beni-Mamore forebulge, whereas TST may result from a quiescent stage in the forebulge. Subaerial unconformity two (SU2) records the passage from a tide-influenced depositional system to a fully continental depositional system. The Miocene tidal-influenced deposits in the Beni Mamore Basin suggest that it experienced a connection, either with the South Atlantic Ocean or the Caribbean Sea or both.

The Amazonian foreland basin system of Peru has been partitioned in seven Subandean basins by Mio-Pliocene Andean tectonics.

International audience; The sub-Andean Huallaga basin is part of the modern retroforeland basin system of Peru. It corresponds to a thrust-and-fold belt superimposed on inverted and halokinetic structures and is characterized by Eocene Pliocene, thick synorogenic series that have controlled the burial history of petroleum systems. Sedimentological analysis and a sequentially restored cross-section based on seismic data and new field studies show three sequences of synorogenic deposits. The Eocene (Lower Pozo member) developed in shoreface environments, when the basin morphology corresponded to a foresag depozone linked to an orogenic unloading period. The Middle Eocene sequence (Upper Pozo member) developed in shallow marine environments and recorded a change in Andean geodynamics and the retroforeland basin system. The basin morphology corresponded to a foredeep depozone linked to an orogenic loading period. This configuration remained until the Middle Miocene (Chambira Formation). The Middle Miocene Pliocene sequence recorded the onset of the modern sub-Andean Huallaga basin that became a wedge-top depozone. Thrust propagation occurred in a deltaic environment, which evolved progressively to an alluvial system linked to the modern Amazon River.

International audience; The Iquitos Arch corresponds to a broad topographic high in the Western Amazonia. Morphostructural and geophysical data and flexural modeling show that the Iquitos Arch is the present-day forebulge of the Northwestern Amazonian foreland basin. A detailed tectono-sedimentary study of the Neogene and Quaternary deposits of the Iquitos area has been carried out in order to circumscribe the timing of the forebulge uplift and its environmental consequences. The Neogene and Quaternary sedimentary succession of the Iquitos Arch consists of six formations that evolved from tidal to fluvial environments. The first three formations exhibit Late Miocene gliding features and synsedimentary normal faults. Such soft-sediment deformations bear witness to tectonic activity ascribed to the growth of the forebulge. Regional erosive surfaces that separate the Neogene and Quaternary formations recorded the progressive forebulge emersion and the evolution of Amazonian drainage system. This uplift is related to an increase in tectonic activity within the Andes, which has provoked the eastern propagation of the orogenic wedge and caused an orogenic loading stage in the Amazonian foreland basin system. The emersion of the forebulge induced the retreat of the Pebas ?marine megalake? nearby the Iquitos area and consequently caused important environmental changes in the Amazonian basin. From the end of the Late Miocene to the Pliocene, the forebulge acted as a barrier inducing the deposition of fluvial deposits in the forebulge depozone and the deposition of the ?White Sand? deposits in the backbulge depozone. Since about 6 Ma, the forebulge is incised and crossed over by the modern Amazon River. The Iquitos forebulge is still growing as shown by the faulted Holocene terrace deposits.

International audience; 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.

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.

In the Guateque–Medina area, Paleozoic basement and Mesozoic rift basins have been uplifted and exhumed during the Andean orogeny (12 Ma to present). Surface exposures and subsurface data constrain the deformation style and the rift geometry. We have mapped a regional transect and restored a cross section. We have also reconciled existing stratigraphic data, from cordillera, foothills and foreland basin, and have added new data of our own. In Early Cretaceous shales, there is evidence for fault-controlled sedimentation. A brecciated evaporitic layer, which is locally emerald bearing, has acted as a regional detachment. The underlying basement, composed of Paleozoic sediments, crops out as the Quetame Massif. It was uplifted during the Andean orogeny on a series of high-angle reverse faults. The main SE-verging Tesalia fault has resulted from Andean reactivation of an Early Cretaceous normal fault, which bounded a half-rift. A series of NW-verging back-thrusts may have resulted from Andean reactivation of Paleozoic faults. Between the back-thrusts and the Tesalia fault is a basement pop-up. It may be part of a flower structure, because components of right-lateral slip have been identified. These are attributed to eastward subduction of the Nazca plate beneath South America. In general, the style and timing of Andean deformation in the Guateque–Medina area are compatible with the plate tectonic setting of the northern Andes.

A coupled study of the kinematics of thrusting and hydrocarbon maturation has been carried out in the northern and central sub-Andean belt of Bolivia to define the petroleum potential of the area. In addition to the classic Devonian source rock (Tomachi-Tequeje formations to the north and Iquiri-Limoncito formations in the central area), two other source rock intervals are recognized: the Retama Formation (Upper Devonian-Lower Carboniferous) and the Copacabana Formation (Upper Carboniferous-Lower Permian). These are the most prospective units in northern Bolivia and are of marine origin. The structural style varies from north to south due to variations in the sedimentary column involved in the thrusts. The orogenic front was guided by the northern boundary of a Paleozoic sedimentary wedge. In the Boomerang area, this boundary is oriented obliquely to the regional shortening and controlled the development of a prominent transfer zone. To the north, the thrusts are wider and the amount of shortening increases. The western part of the northern sub-Andean zone is characterized by a very thick Tertiary piggyback basin fill. Two phases of hydrocarbon maturation are recognized. The first began in Early Carboniferous and affected mostly Devonian strata. Formation of structural traps during this period occurred rarely. The entire basin was then deeply eroded in Permian-Jurassic time, causing any hydrocarbons that may have formed to be lost. The second phase of maturation was contemporaneous with Andean deformation and with the resulting burial under the Tertiary cover in the foreland basin and in piggyback basins on thrust structures. The hydrocarbon expelled during this period may fill the Andean anticlines. The known source rocks are not proven to be gas prone, but current discoveries indicate a high gas to oil ratio that may be due to secondary cracking in the source rock. Because the initial potential of the source rocks is low, expulsion of heavy compounds is expected to be weak.

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.

In their Comment of our Geology paper ([Espurt et al., 2007][1]), [Clift and Ruiz (2008)][2] argue that: 1) the flat-slab subduction of the Nazca Ridge is unlikely to have produced uplift of the Fitzcarrald Arch in the Amazonian retroforeland basin (using geologic data from the forearc area); 2)

In the North Ucayali basin, HC reserves have been already found but the structural style is rather complex due to the variability of the sedimentary covers and the presence of inherited structures. The risk of undercharge of the prospects due to a timing problem exists. A calibration of the erosion at the top of the various structures published by Bertolotti and Moretti (2009) indicated that the tertiary structures have different ages. Although the importance of the Paleozoic depot centres and faults in localizing has been largely published, a major role of the early Mesozoic evaporitic pillows was only recently proposed although seismic data evidences these pillows that influence the thrust geometry. Such behaviour of the evaporites is similar with the one observed in other compressive fronts. However, in the classical example of compressive front above salt layer, the evaporite forms either the main decollement level (as in the Zagros). In the north Ucayali basin, the salt pillows are isolated rather flat bodies, there are also deeper decollement levels. We have designed analogue models to discuss the influence of the depth and continuity of the salt pillows on the structure style.