Bertrand Nivière

Geomorphic evidence for Plio-Quaternary compression in the Andean foothills of the southern Neuquén Basin, Argentina

The southern central Andes foothills are made of imbricate thick- and thin-skinned fold and thrust belts. They were formed during Cretaceous and Miocene compressive periods by successive eastward propagation episodes of the orogenic front. We analyze remnant terraces and alluvial deposits of the Neuqun River and its tributaries to study the Plio-Quaternary tectonic regime in the southern Neuqun Basin. Topographic profiles of remnant terraces show a crustal-scale (tens of kilometers) anomaly above the Chihuido South anticline. This anomaly is accompanied by sedimentary aggradations downstream and upstream of the anticline. Another anomaly in the terrace profiles, correlated with a knickpoint in the current Neuqun River, is observed farther east, near Aelo. We interpret these anomalies to be the consequence of a Plio-Quaternary uplift of the Chihuidos and Aelo anticlines. Folding and surface faulting of Plio-Quaternary alluvial deposits show that this uplift is concomitant with the reactivation of the Miocene orogenic front and of the predominantly thin-skinned Agrio fold-and-thrust belt. From these data we infer that the southern Neuqun Basin is still shortening under a compressive stress regime and that the Andean orogenic front is migrating farther to the east. Extensional features in the study area correspond mainly to thin-skinned gravitational gliding, resulting from crustal uplift. Thus, we argue that the tectonic compressive regime is preserved in the southern Neuqun Basin throughout the Plio-Quaternary, though the slab may have steepened during this period.

Extensional forced folding and décollement of the pre-rift series along the Rhine graben and their influence on the geometry of the syn-rift sequences

Abstract On the western margin of the Rhine graben, forced folding and décollement of competent Dogger strata occurred during the latest Eocene-Early Oligocene (Priabonian) as a result of normal faulting in the basement. The Dogger series are represented by NNE-striking thick layers of oolitic limestones strongly disrupted by extensional structures and incipient boudinage. To the east, the overlying Priabonian (syn-rift) sequences exhibit divergent onlaps, steeply dipping intraformational unconformities and a general drag syncline geometry. These syn-depositional structures attest to a close control by the progressive tilting and normal dip-slip sliding of the Dogger strata. Interpretation of a shallow seismic reflection profile reveals that this tilting is related to an underlying master normal fault which dips 60–70° to the east. This fault offsets the Variscan crystalline basement and the gently dipping Triassic cover by about 2 km. To be balanced, the structural arrangement requires the presence of a décollement layer located between the top of the Triassic series and the Dogger series, corresponding to the Late Triassic incompetent gypsiferous clays and marls. The response of the competent Dogger to the imposed forced folding is through the development of a large-scale extensional syncline in the hangingwall of the master detachment, compensated by the development of a piggy-back half-graben in the footwall. In the light of this new interpretation, a mechanical hypothesis is proposed including a two-stage evolution model of extension and subsidence for the Rhine graben.

Do river profiles record along-stream variations of low uplift rate?

environments (� 1m m yr � 1 ) where climate changes may have erased tectonic features. We evaluate the respective contribution of low uplift rate (� 0.1 mm yr � 1 ) and Pleistocene climate oscillations on gradient variations of two comparable river profiles crossing different uplift zones in the southern Upper Rhine Graben. We compare the observed points of discontinuity in river profile (knickpoints) and convex portions (knickzones) with those predicted by a detachment-limited model that includes stochastic short-term and cyclic long-term variations in climate, a bedrock detachment threshold and rock uplift. The detachment-limited model is chosen as it predicts the development of persistent knickpoints. Differing values of the shear stress exponent, erosion threshold, climate variability and uplift pattern have been checked. Our modeling suggests that climate changes had no significant effects on profiles and that anomalies are more likely due to anticline growth. This surprising result arises from the combination of a very low regional uplift rate and the detachment-limited assumption. The detachment-limited model implies an upstream propagation of knickpoints and knickzones generated by uplift at the outlet during dry climate periods of low erosion. The greater the uplift rate, the larger the variations in river bed elevation. Thus, for high uplift rate, knickpoints and knickzones generated by climate oscillations are more likely to hide tectonic features. This result seems counterintuitive because it suggests that tectonic knickzones will be better preserved in low uplift rate environments, provided that the lithology is homogeneous.

The fracture network, a proxy for mesoscale deformation: Constraints on layer parallel shortening history from the Malargüe fold and thrust belt, Argentina

An analysis was performed of the fracture networks in the N-S trending thick-skinned Malargue fold and thrust belt (MFTB). A total of 2000 planar structures including joints and veins were measured in different structural domains ranging from surficial thin-skinned systems detached in the cover to large-scale structures such as basement-cored folds. The investigated stratigraphic section ranges from the Middle Jurassic (Cuyo Group) to the Paleocene (Malargue Group), including sandstones, siltstones, shales, and limestones. Four main fracture sets are identified trending, E-W, NW-SE, NE-SW, and N-S. The abutting relationships provide a reliable chronology between the four fracture sets which are ubiquitously found in the MFTB throughout the various structural domains. Due to this observation, we assume the fracture signal to be regional and developed in response to both large-scale processes and folding. In particular, based on a fold test and the characteristics of data dispersion, the fracture sets I, II, and III exhibit a prefolding origin, while set IV shows a synfolding origin. A regional interpretation of the various fractures is proposed, involving several stages of fracture formation from compaction to folding, including prefolding layer parallel shortening. The fracture signal yields useful insights about the structural history of the MFTB and the spatiotemporal evolution of the foreland tectonic regime since Late Cretaceous times. We then place the various identified fracture sets into the known pattern of geodynamic evolution since the Late Cretaceous.

Morphologic dating of slowly evolving scarps using a diffusive analogue

Morphologic dating techniques that have been applied successfully in arid and semi-arid areas are also suitable for slowly evolving scarps that are usually found in temperate climate environments. Two approaches, based on diffusion, to relate the present-day shape of a scarp to its age are used here. The first assumes a model of scarp degradation based on a diffusive process (D method). The second evaluates the state of scarp degradation using the slope distribution along a topographic profile (SD method). Based on an anthropic scarp of known age, we have obtained a mass diffusivity close to 1.5 m²/ka, a reasonable value for areas characterized by a continuous vegetation cover and a temperate climate. We show that the SD method requires an age correction, since it does not take into account the earliest stage for which the mid-height point is not yet affected by erosion. However only the SD method reveals that at several profiles a later scarp reactivation event has occurred. Such information can be important when trying to identity the recent geological or environmental events that have affected a region.

Evidence of active shortening along the eastern border of the San Rafael basement block: characterization of the seismic source of the Villa Atuel earthquake (1929), Mendoza province, Argentina

On the 30 May 1929, a massive earthquake occurred in the San Rafael area (southern Mendoza province) leading to the destruction of the Villa Atuel and Las Malvinas towns. The region affected by the ground shaking covers a large part of southern South America. Although no surface breaks have been detected on the surface, several authors have pointed out active faults that could be related to the event of 1929. Using satellite imagery and field observations, we investigated two active faults situated on the eastern border of the San Rafael Block (SRB) close to or within the epicentral area. The most prominent faults are the c. 40 km long Las Malvinas and c. 30 km long Cerro Negro reverse faults which are located near the epicentral area. Geological and morphological observations allow us to describe late Pleistocene activity and estimate the long-term slip rates of these faults. Possible ruptures that match our observations and which are compatible with the cartographic length of these faults would account for a seismic moment magnitude of M 0 = 2.8×10 19 N m and a moment magnitude of M W = 6.9. The morphological signatures of these fault segments and the occurrence of the San Rafael earthquake suggests that the southern Mendoza Province is still currently submitted to shortening.

Mechanisms of basin contraction and reactivation in the basement-involved Malargüe fold-and-thrust belt, Central Andes (34–36°S)

We have conducted a structural study of both the basement-involved Malargue fold-and-thrust belt (MFTB) and the active San Rafael Block (SRB), which are located in the Central Andes at latitude 34–36°S. Based on several field examples located both in the inner and frontal part of belt and from the distal foreland zone, we focus on the relationships between basement and cover deformation with respect to the known palaeogeography and structural inheritance. In several zones, we point out similarities in the structural and sedimentary responses to Andean shortening. The recent morphologic response has also been investigated through the analysis of active deformation along the eastern border of the SRB. We show that these structural and sedimentary processes are continuous in time and space since they can be applied in the various parts of the fold belt and also at different stages of fold-and-thrust-belt building as well. Finally, we propose the illustration of those mechanisms by complete cross-section along the Rio Grande valley and a possible kinematic scenario of deformation propagation.

Mechanical analysis of a natural example of onland gravity gliding: The role of river incision and deposition

Gravity gliding implies rigid translation of a body down a slope where displacements are parallel to a tilted detachment plane. Although large-scale gravity gliding is commonly observed offshore, under conditions of high fluid overpressure and abundant upslope sedimentary supply, its occurrence on land is debated. We investigate the mechanical feasibility of such a process as well as the role of fluvial incision and sedimentation down the slope in the initiation of the gliding. We use a two-dimensional (2-D) finite element model combined with a 2-D failure analysis approach. The numerical models simulate the deformation and provide quantitative estimates of the failure criteria at the head and toe of the overburden. Analytical solutions approximate the numerical results by taking into account the fluvial incision and sedimentation, the internal friction angle, and the thickness and length of the overburden. Our models are based on a field example in the Andean foothills of Argentina, where gravity gliding of a 1000 m thick section is suspected above a crustal-scale anticline. The incision and sedimentation reduce and strengthen, respectively, the downslope resistance to contractional failure. The critical slope at which the gliding is initiated is reduced by fluvial incision and increased by sedimentation. We show that tectonic uplift may lead to large-scale gravity gliding on land where the overburden thickness is less than 2000 m. Incision facilitates and localizes the frontal shortening. Incision greater than 1000 m may trigger gliding for overburden up to 4000 m thick, while sedimentation thicker than 1000 m inhibits gliding. These results show that thin-skinned onland gravity gliding could be common in tectonically active regions where incision is important.