Thierry Nalpas

Meso-Cenozoic geodynamic evolution of the Paris Basin: 3D stratigraphic constraints

Abstract 3D stratigraphic geometries of the intracratonic Meso-Cenozoic Paris Basin were obtained by sequence stratigraphic correlations of around 1 100 wells (well-logs). The basin records the major tectonic events of the western part of the Eurasian Plate, i.e. opening and closure of the Tethys and opening of the Atlantic. From earlier Triassic to Late Jurassic, the Paris Basin was a broad subsiding area in an extensional framework, with a larger size than the present-day basin. During the Aalenian time, the subsidence pattern changes drastically (early stage of the central Atlantic opening). Further steps of the opening of the Ligurian Tethys (base Hettangian, late Pliensbachian;...) and its evolution into an oceanic domain (passive margin, Callovian) are equally recorded in the tectono-sedimentary history. The Lower Cretaceous was characterized by NE–SW compressive medium wavelength unconformities (late Cimmerian–Jurassic/Cretaceous boundary and intra-Berriasian and late Aptian unconformities) coeval with opening of the Bay of Biscay. These unconformities are contemporaneous with a major decrease of the subsidence rate. After an extensional period of subsidence (Albian to Turonian), NE–SW compression started in late Turonian time with major folding during the Late Cretaceous. The Tertiary was a period of very low subsidence in a compressional framework. The second folding stage occurred from the Lutetian to the Lower Oligocene (N–S compression) partly coeval with the E–W extension of the Oligocene rifts. Further compression occurred in the early Burdigalian and the Late Miocene in response to NE–SW shortening. Overall uplift occurred, with erosion, around the Lower/Middle Pleistocene boundary.

Graben inversion in nature and experiments

The inversion of crustal-scale basement grabens is studied here through laboratory experiments on small-scale models and available oil industry seismic lines from the southern North Sea. Two basic configurations are considered. First, both the basement and the sedimentary cover are brittle, and inversion does not involve any potential decollement between them. Second, the basement and sedimentary cover are separated by a weak ductile layer (e.g., salt), which can allow decollement of the cover during both extension and later compression and inversion. The second configuration is more complicated and can lead to a large variety of geological structures. Laboratory experiments were carried out on brittle-ductile models built with sand to represent brittle layers (basement and sedimentary cover) and silicone putty to simulate the decollement layer between basement and cover. A mechanically based classification of inversion structures is proposed. The effects of some crucial parameters are investigated, including obliquity between the direction of shortening and normal faults, as well as strength profiles, and the presence or absence of salt diapirs. The experimental investigation leads to the following conclusions: (1) the inversion of the graben by reactivation of normal faults implies that the angle between the direction of compression and the graben is less than 45°, (2) if there is a superficial decollement (e.g., basement-cover interface), inversion initiates low dipping thrust faults in the cover, localized at graben borders, (3) salt diapirs or salt walls localized along the graben borders in the cover are preferential sites for the development of thrust faults, and (4) when the cover is decoupled from the basement by a decollement layer, inversion induced deformation in the cover which is partitioned between thrust faults along the graben borders and strike-slip faults within the graben trending oblique to the graben borders. Experimental results are compared with field examples, in particular from the southern North Sea.

Salt flow and diapirism related to extension at crustal scale

Abstract The analysis of the structure and history of salt diapirs in large ancient evaporitic basins such as the southeastern North Sea basin suggests that diapirism is controlled by crustal-scale extension. Laboratory experiments on small-scale models designed to simulate configurations comparable to the southeastern North Sea basin show that diapirs, which grow in a sedimentary cover during basement extension, can develop close to basement normal faults or at a significant distance from them, if the displacement on the basement faults is transfered horizontally along the salt layer. For given values of salt viscosity, strain rate and friction coefficient in brittle rocks, the pattern of salt diapirism in a particular area depends on the ratio of cover thickness to salt thickness, as well as the absolute thicknesses of cover and salt layers. The experiments give criteria for identifying different types of diapirs and provide models of horizontal flow in a salt layer undergoing extension and diapir development. Some dynamic implications of the experiments are applied to the southeastern North Sea basin.

The influence of pre-existing thrust faults on normal fault geometry in nature and in experiments

Relations between normal faults and pre-existing thrust faults are classically described in terms of three basic situations: normal faults can cross-cut thrust faults; they can branch out from thrust faults at depth on a de´collement level, or they can entirely reactivate thrust planes. The mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the ‘brittle’ rocks and silicone putty an interlayered de´collement. The models underwent compression, producing thrust faults with variable dips, followed by extension. Three possible ways of interaction are described here: (a)no interaction occurs in the case of low-dip thrust faults (<32 ° ± 1 °) and normal faults are developed independently, displaying a listric geometry; (b)branching at depth on the de´collement level occurs when dip of the thrust faults reaches 32 ° ± 1°. In this case, the dip of the normal faults, whose geometry becomes planar, decreases with increasing thrust dip. We suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre-existing fault zone, where a drop in the friction coefficient is likely to occur; (c)reactivation occurs in brittle material when dip of the pre-existing fault exceeds 41° ± 1°.

Analogue models of transpressive systems

This paper presents three series of analogue models of transpressional deformation in a brittle/ductile system: (1) simple transpression, (2) transpression combined with erosion of uplifted areas of the hanging wall above the deformation front, and (3) transpression combined with erosion of the hanging wall and sedimentation at the foot of uplifted zones. In each series of experiments, different convergence angles α, from 0° (pure wrenching) to 90° (pure thrusting) were applied to the models. Results show a sharp contrast between structures formed at α≤15° (wrench-dominated transpression) and α≥30° (thrust-dominated transpression). For a low convergence/strike-slip ratio (0°≤α≤15°), the deformation is localized and structures are typical of a strike-slip regime (R and Y faults). For higher convergence angles (30°≤α≤90°), the deformation is similar for all models, with an elongate asymmetric uplift showing fault-propagation-fold geometries and flanked by thrust-wrench faults. Fault dips also show a significant change from more than 70° for α≤15° to less than 40° for α>30°. For α≥30°, the geometry of the main faults at the borders of the uplift zone is modified by P faults. In experiments with erosion and sedimentation, and thrust-dominated transpression, new faults with increasing dips form during progressive deformation, branching on the main fault at the base of the model.

Influence of syntectonic sedimentation on thrust geometry. Field examples from the Iberian Chain (Spain) and analogue modelling

Abstract Steep thrusts are usually interpreted as oblique-slip thrusts or inverted normal faults. However, recent analogical and numerical models have emphasised the influence of surface mass-transfer phenomena on the structural evolution of compressive systems. This research points to sedimentation and erosion during deformation as an additional explanation for the origin of steeply dipping thrusts. The present study uses both field observations and analogue modelling to attempt to isolate critical parameters of syntectonic sedimentation that might control the geometry of the upper part of thrust systems. A field study of thrust systems bounding two compressive intermountain Tertiary basins of the Iberian Chain is first briefly presented. We describe the surface geometry of thrusts surrounding the Montalban Basin and the Alto Tajo Syncline at the vicinity of deposits of Oligocene–Early Miocene alluvial fans at the footwall of faults. Field observations suggest that synthrusting sedimentation should influence the structure of thrusts. Indeed, the faults are steeper and splitted at the edge of the syntectonic deposits. Effects of sedimentation rate on footwall of thrusts, and of its change along fault strike are further investigated on two-layer brittle-ductile analogue models submitted to compression and syntectonic sediment supply. Two series of experiments were made corresponding to two end-members of depositional geometries. In the first series, the sedimentation was homogeneously distributed on both sides of the relief developed above the thrust front. In the second series, deposits were localised on a particular area of the footwall of thrust front. In all experiments, the sedimentation rate controls the number and the dip of faults. For low sedimentation rates, a single low-angle thrust develops; whereas for high sedimentation rates, a series of steeper dipping thrust is observed. In experiments with changing sedimentation rate along fault strike, the thrust geometry varies behind the areas with the thickest sediment pile.

Role of synkinematic ductile levels on the evolution of compressive zones – analogue modelling

In foldbelt faults, layers with ductile behaviour can form levels of decollement [Byerlee, 1978]. When these levels are prekinematic, they play a significant role in the genesis, evolution and final geometry of the foldbelt faults, as, for example in the Appalachian Mountains [Davis and Engelder, 1985], the Jura [Sommaruga, 1999], or the Pyrenees [Verges et al., 1992]. Previous studies based on analogue modelling have shown how a prekinematic decollement level can influence the geometry of foldbelt faults and structures [Ballard, 1989; Colletta et al., 1991; Letouzey et al., 1995; Merle et Abidi, 1995]. However, no study has yet described the influence of synkinematic sedimentation of incompetent levels on the genesis and evolution of compressive structures. The laboratory experiments presented here are designed to explore some of the mechanisms of formation of synsedimentary thrust faults, in relation with the occurrence of a decollement layer during syntectonic sedimentation.

Hydrocarbon Prospectivity in Mesozoic and Early–Middle Cenozoic Rift Basins of Central and Northern Kenya, Eastern Africa

The northern (NKR) and central (CKR) segments of the Kenya Rift are among the most important areas of the East African rift system for hydrocarbon prospecting because they offer the oldest and longest lived sedimentary basins and they are a crossover area between Cenozoic and Cretaceous rifts. During the 1970s and 1980s, the interest of oil companies focused in the Turkana depression and the northeastern region of Kenya. Seismic reflection surveys and several exploration wells enabled the identification of several deeply buried basins: (1) In the NKR, three strings of north–south-oriented half grabens, the oldest known basins being of Cretaceous?–Paleogene to middle Miocene age; (2) In the CKR, two north–south half grabens, the Baringo-Bogoria Basin (Paleogene–Present Term), and the Kerio Basin (Paleogene–upper Miocene). All basins are filled by up to 8 km (5 mi) thick sediments of alluvial, fluviodeltaic, or lacustrine origin and volcanics of late Eocene to Neogene age. New studies have focused on reservoir and/or source rock quality in several of these basins. In terms of hydrocarbon potential, arkosic sandstones in CKR or NKR demonstrate a fair to good reservoir quality, with porosity up to 25%. Strong changes in terms of diagenetic alteration relate to deformation events or change in sediment source as a result of tectonic activity and hydrothermal fluid circulation associated with volcanism. High-quality source rocks were deposited in freshwater lake environments under a tropical climate. Such environments have been identified during the Paleogene in the NKR and lower Neogene in the CKR. The combination of reservoir and source rock characteristics results in a provisional classification of each studied basin, in terms of very high to medium potential for hydrocarbons.