Peter Robert Cobbold

Université de Rennes 1 | UR1 · UMR CNRS 6118 - Géosciences Rennes

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Bedding-parallel veins of fibrous calcite (also called BPV or ‘beef’) occur in many sedimentary basins, especially those containing low-permeability strata with organic source material for petroleum. The formation of such veins is often linked with fluid overpressure in these source rocks. In this review, we demonstrate that beef veins are most com...

Abstract Hydrocarbons efficiently expel from fine-grained organic-rich source rocks upon thermal heating. Expulsion and primary migration through hydraulic fractures is a commonly cited process, but good outcrop examples are few. Occasionally, hydraulic fractures are permanently filled with, e.g., bitumen or fibrous carbonate cement, and we have st...

Bedding-parallel veins of fibrous calcite ('beef') are historical in the Wessex Basin. The veins are common in Mesozoic mudstones and shales, especially of Liassic to Mid-Cretaceous ages. Cone-in-cone structures, which consist of multiple nested cones, are also well developed within the 'beef'. To investigate the distribution and the context of for...

Résumé: Dans les bassins sédimentaires pétroliers, il est habituel de trouver des filons parallèles à la stratification et composés de fibres de calcite. Ces filons, les beefs, sont abondants dans les roches-mères, surtout quand elles ont atteint la fenêtre de génération de l'huile. C'est le cas notamment des bassins de Wessex (Angleterre) et Neuqu...

Geological evidence for overpressure is common worldwide, especially in petroleum-rich sedimentary basins. As a result of an increasing emphasis on unconventional resources, new data are becoming available for source rocks. Abnormally high values of pore fluid pressure are especially common within mature source rock, probably as a result of chemica...

We describe (1) bedding-parallel veins of fibrous calcite (beef) and (2) thrust detachments, which we believe provide good evidence for fluid overpressure in source rocks for petroleum. Our examples are from the surface or subsurface of the Magallanes-Austral Basin, which lies at the southern tip of South America. There, the best source rocks for p...

Since about 50 Ma, the ongoing continental collision of India and Asia has led to widespread deformation within Central Asia. A similar pattern results when a rigid indenter pushes into a deformable medium. Therefore, for simplicity, many models have assumed that continental India is rigid. However, in reality, its northern margin has deformed, pro...

So as to investigate the parameters influencing subduction and back-arc extension, we have done three series of laboratory experiments (32 in all) on physical models. Each model consisted of adjacent oceanic and continental plates, floating on an asthenosphere. In experiments of Series A, a wide rigid piston, moving horizontally, controlled the rat...

Bedding-parallel fibrous veins are common worldwide in sedimentary basins, especially within strata of low permeability. The term “beef” refers to bedding-parallel veins of fibrous minerals, where the fibres are mutually parallel and have formed quasi-vertically. More complex on a smaller scale are “cone-in-cone” structures, yet these are also comm...

The Great Glen Fault trends NNE-SSW across northern Scotland. According to previous studies, the Great Glen Fault developed as a left-lateral strike-slip fault during the Caledonian Orogeny (Ordovician to Early Devonian). However, it then reactivated right-laterally in the Tertiary. We discuss additional evidence for this later phase. At Eathie and...

We present quantitative laboratory models investigating the mechanics of sheath fold formation around a weak inclusion in simple shear. Sheath folds are intriguing, highly non-cylindrical structures that stimulated extensive field and experimental studies, leading to an ongoing debate concerning their formation and evolution. Through a parametric s...

The continental margin of southeast Brazil is elevated. Onshore Tertiary basins and Late Cretaceous/Paleogene intrusions are good evidence for post breakup tectono-magmatic activity. To constrain the impact of post-rift reactivation on the geological history of the area, we carried out a new thermochronological study. Apatite fission track ages ran...

The NE Atlantic Ocean opened progressively between Greenland and NW Europe during the Cenozoic. Seafloor spreading occurred along three ridge systems: the Reykjanes Ridge south of Iceland, the Mohns Ridge north of the Jan Mayen Fracture Zone (JMFZ), and the Aegir and Kolbeinsey Ridges between Iceland and the JMFZ. At the same time, compressional st...

It is a common assumption that elevated passive continental margins have remained high since rifting and breakup. Here, we show that the Atlantic margin of NE Brazil has undergone a more complex history. Our synthesis of geological data, landscape analysis, and paleothermal and paleoburial data reveals a four-stage history: (1) After Early Cretaceo...

Bedding-parallel veins of fibrous calcite (‘beef’) are common in sedimentary basins worldwide. We have studied two of them: (1) the Wessex Basin of SW England, and (2) the Neuquén Basin of western Argentina. Each contains at least three potential source rocks for oil: in (1), the Kimmeridge Clay, Oxford Clay and Lower Lias, and in (2), the Agrio, V...

The term 'sheath fold' describes a cone-shaped structure with a rounded apex. Such folds can occur in all rock-types and range in size from sub-millimeters to kilometers. They are in many cases associated with shear zones and shear deformation. Even though sheath folds are three-dimensional structures they are rarely exposed as such. In the field t...

Sheath folds are cone-shaped structures that can be found in different rock types. They are mostly associated with shear zones. Even though they are three-dimensional structures they are most commonly recognized in nature in cross sections perpendicular to their stretching direction. These cross-sections exhibit so called eye-structures. The geomet...

The continental margin of SE Brazil shows good evidence for tectonic activity well after the break-up of Western Gondwana (see Cobbold et al., 2001 for a review). Additionally, SE Brazil ranks as an HEPM (high elevation passive margin), summits reaching 2800 m. To constrain the onshore evolution of the margin, especially during the Tertiary, we did...

We performed a new thermochronological study (fission track analysis and (U-Th)/He dating on apatite) in SE Brazil and integrate those data with inverse and forward modelling via QTQt software (Gallagher, 2012) to obtain thermal histories. The inversion results were used to characterize the general thermal histories and the associated uncertainties...

The NE Atlantic Ocean opened progressively between Greenland and NW Europe during the Cenozoic. Sea-floor spreading occurred along three ridge systems: the Reykjanes Ridge south of Iceland, the Mohns Ridge north of the Jan Mayen Fracture Zone (JMFZ), and the Aegir and Kolbeinsey ridges between Iceland and the JMFZ. At the same time, compressional s...

The origin of the forces that produce elevated, passive continental margins (EPCMs) has been a hot topic in geoscience for many years. Studies of individual margins have led to models, which explain high elevations by invoking specific conditions for each margin in question. We have studied the uplift history of several margins and have found some...

During intrusion of buoyant magma into a rift zone, it is a common belief that the magmatic flux will be dominantly vertical and therefore will lead rather readily to volcanic eruptions. Nevertheless, many dykes in active rift zones (such as those in Hawaii, Iceland or the Afar) are blade-shaped (i.e. horizontal length, L, versus vertical height, H...

In contrast to the classical concept of magma ascent in extensional settings, recent studies show that volcanism also occurs in compressional settings. The nature of the interplay between magmatism and tectonics in fold-and-thrust belts however, remains a major question, notably in active margins. The mechanisms of magma transport in such settings...

The term 'sheath fold' describes a cone-shaped fold with a rounded apex. Such folds can occur in metamorphic rocks, sediments, ignimbrites, evaporites, and ice, ranging in size from sub-millimeters to kilometers. They are in many cases associated with simple shear and high strain. How sheath folds form and how they evolve with increasing strain is...

On the continental margin of southeast Brazil there is good geological evidence for post-rift tectonic reactivation, in the form of onshore Tertiary basins up to 800m deep and Cretaceous to Paleocene alkaline intrusive bodies at the surface. With the aim of constraining the Mesozoic and Cenozoic exhumation history of this region, we present new apa...

Steinmann (1929) reconoció en la Cordillera del Perú tres fases orogénicas (Peruana, Incaica y Quechua). Hoy, a raíz de numerosos estudios de superficie y subsuelo, se admiten estas tres fases y otras más antiguas, en múltiples sectores de la cordillera. Hasta dónde alcanzan las deformaciones andinas a través del antepaís? Cobbold et al. (2007) com...

RESUMEN Donde la Cordillera incide en Cuenca Neuquina, afloran centenares de vetas de hidrocarburo sólido (asfaltita). Muchas han sido objetos de trabajos mineros. De común acuerdo, la asfaltita es producto de maduración de lutitas, ricas en materia orgánica, en particular de la Fm. Vaca Muerta, de edad Jurásica tardía. En la provincia del Neuquén,...

The growing interest in quantification of vertical ground motion stems from the need to understand in detail how the Earth's crust behaves, for both scientific and social reasons. However, only recently has the refinement of dating techniques made possible the use of paleoshorelines as reliable tools for tectonic studies. Although there are many lo...

Bedding parallels veins of fibrous calcite ('beef') are common in the Wessex Basin of SW England, as in other basins worldwide. The Wessex Basin contains at least three potential source rocks for oil: the Kimmeridge Clay, the Oxford Clay and the Lower Lias. We have studied classical outcrops of (1) the Blue Lias, between Lyme Regis and Charmouth, a...

Most natural deformations are variable in intensity throughout a given region. If the variations are evenly distributed and sufficiently numerous, it is useful to consider a measure of average strain for the region. In this paper, the average deformation and average strain for a region are defined in terms of the local deformation gradients at each...

This paper offers a generalized mechanical explanation for the origin and development of bandlike deformation structures such as shear zones, mylonite zones, kink bands, 'pressure-solution' seams, extension gashes, and similar folds.Methods of continuum mechanics are used to examine permissible variations in strain rate, stress, and rheological pro...

The northern North Atlantic and its adjacent continental margins have specific and uncommon features: location of the ridge on a major mantle plume, a history of ridge jump and extinction, and intraplate deformation on the margins (inverted basins and compressional domes). Reconstructions of the opening of the North Atlantic on the basis of two rig...

In structural geology and tectonics, most fundamental advances have come through observation. However, mechanical and kinematical modelling has provided an understanding of physical processes, as well as ideas for further investigation. As in other sciences, the main techniques in tectonics have been three: analytical, physical and numerical. For p...

Chemical compaction, overpressure development and hydraulic fracturing are common in source rocks for petroleum, yet there is no consensus as to the underlying physical processes. We have investigated them by physical modelling. In our experiments, mixtures of coloured silica powder and beeswax microspheres (50% by volume) formed two basal layers o...

One of the main assumptions of the theory of plate tectonics is that all plates are rigid. However, in some plate reconstructions, the fits improve if the continents deform. Moreover, along parts of the North Atlantic continental margins, there is good evidence for post-rift deformation, in the form of inverted basins and compressional domes. Possi...

We have undertaken a regional study of landscape development and thermo-tectonic evo-lution of NE Brazil. Our results reveal a long history of post-Devonian burial and exhuma-tion across NE Brazil. Uplift movements just prior to and during Early Cretaceous rifting led to further regional denudation, to filling of rift basins and finally to formatio...

In sedimentary basins, the emplacement of magmatic sills tends to occur within rock of low mechanical competence and permeability, such as shale. This often contains pore fluids at abnormally high pressure. We first theoretically show that, in anisotropic media, the higher the pore fluid pressure, the deeper the sill emplacement. Then we introduce...

We describe a set of thin-skinned structures from the offshore Camamu Basin. The margin is about 40 km wide and the sea floor has a slope of up to 10%. The sedimentary cover is up to 7 kin thick. Neocomian strata accumulated during continental rifting. Aptian strata consist mainly of coarse clastic sediment and evaporite. Late Cretaceous strata are...

For the conjugate margins of the South Atlantic (West Africa and Brazil), we compare and contrast the structural styles, using data from the subsurface and surface. Our aim is to explain the similarities and differences for both syn-rift and post-rift structures. The area of interest spans the Lower Congo, Kwanza and Benguela basins of West Africa,...

Seepage forces result from fluid flow through a porous medium, in response to an overpressure gradient, according to Darcy's Law. As the fluid flows past each grain or element of the solid framework, frictional drag and a local difference in pore pressure impart a seepage force to the grain. This contributes to the local balance of forces and modif...

Regional seismic reflection profiles and potential field data across the conjugate magma-poor Camamu/Almada-Gabon margins, complemented by crustal-scale gravity modelling and plate reconstructions, are used to reveal and illustrate the relationship of crustal structure to along-margin variation of potential field anomalies, to refine and constrain...

Sand injectites are structures that result from intrusion of fluidized sand into fractures. We have studied them in the Tampen Spur area of the North Sea, and have reproduced them experimentally, by driving compressed air through layers of sand, glass microspheres, and silica powder. The silica powder was cohesive and capable of hydraulic fracturin...

The evolution of the landscape of north-east Brazil in relation to the burial and exhumation history of both onshore and offshore areas is the focus of a research project carried out for StatoilHydro do Brasil and Petrobras from 2007 to 2009 by the Geological Survey of Denmark and Greenland in collaboration with Geotrack International. In hydrocarb...

In the Neuquen Basin of Argentina, 'beef' (bedding-parallel veins of fibrous calcite) is widespread within Late Jurassic black mudstones. A typical vein consists of two grey inner zones and two white outer zones. The inner zones contain inclusions of wall rock and hydrocarbons. Calcite fibres are perpendicular to the margins. In the outer zones, th...

In the Niger Delta, which has prograded onto the African continental shelf and neighbouring oceanic crust since the Eocene, abundant thin-skinned structures provide evidence for slope instability. The structures have detached on shale of the Akata Fm. At the toe-of-slope, the outer thrust belts contain compressional structures. The surface slopes a...

Using available information on the magnitude and age of tectonic shortening, as well as paleomagnetically determined tectonic rotations, we have run a series of 2-D map view restorations of the central Andes. Neogene shortening in the foreland belt induced only slight orogenic curvature of the central Andes. The constraints on the ages of the large...

The burial and exhumation history of NE Brazil, including onshore and offshore areas, is the focus of an ongoing research project carried out for Statoil do Brasil by GEUS in collaboration with Geotrack International. Early Cretaceous rift systems are prominent along the Atlantic margin where the intracontinental Recôncavo-Tucano-Jatobá (RTJ) Rift...

Magmatic activity tends to concentrate at tectonic plate boundaries. At rapidly convergent margins, such as the Andes, intense magmatic activity is coeval with strong tectonic shortening, and some volcanoes and magmatic intrusions have been emplaced near active compressional structures, usually major thrust faults. in order to understand the links...

Did the Pharaohs of Egypt make their tombs in solid rock on the Theban Plateau?

By scaled physical modelling, we have investigated the mechanical response to gravitational forces in an oceanic lithosphere, overlying a less dense asthenosphere. In the models, an upper wedge-shaped layer of sand represented an oceanic lithosphere (0 35 Ma old, with a half-spreading velocity of 3 cm/yr), and a lower layer of polydimethylsiloxane...

We document evidence for growth of an active volcano in a compressional Andean setting. Our data are surface structures and 39Ar-40Ar ages of volcanic products on Tromen volcano. Tromen is an active back-arc volcano in the Andean foothills of Neuquén province, Argentina. Its volcanic products are unconformable upon Mesozoic strata of the Neuquén ba...

Bedding-parallel fibrous veins ('beef' and 'cone-in-cone') are common to a number of sedimentary basins, especially those containing black shale. The type locality is SW England. The commonest mineral in the fibres is calcite. The fibres indicate vertical opening, against the force of gravity. In the past, this has been attributed to fluid overpres...

1] Magmatic activity tends to concentrate at plate margins. At divergent margins, extensional tectonics provide steep conduits for magma to reach the surface. At rapidly convergent margins, such as the Andes, one might imagine that horizontal compression prevents the rise of magma. Nevertheless, volcanoes are also common. In order to study the mech...

The Andean Orogeny in South America has lasted over 100 Ma. It comprises the Peruvian, Incaic and Quechuan phases. The Nazca and South American plates have been converging at varying rates since the Palaeocene. The active tectonics of South America are relatively clear, from seismological and Global Positioning System (GPS) data. Horizontal shorten...

Geographic Information Systems (GISs) are very useful tools for managing, checking, and organizing spatial information–from many sources and of many types–in thematic layers. Processing of these data enables exploration-oriented GISs to produce potential and predictive maps for a given commodity, which constitute documents of real use in decision-m...

We have used analogue experiments to investigate the effects of surface topography on the curvature of fold-and-thrust belts, under conditions of (1) initial relief, but no erosion, and (2) no initial relief, but differential erosion, sedimentation and transport.In experiments where a 2-layer model lithosphere shortened and thickened in front of an...

The well-known model for the critical taper of an accretionary wedge includes overpressure as a first-order parameter. Fluid overpressures reduce frictional resistance at the base of a wedge but they also act as body forces on all material particles of the wedge, in addition to that of gravity. By means of sandbox modeling, many workers have tried...

Whereas in previous analogue experiments on gravitational spreading and gliding, detachment occurred on a ductile layer, we have used a relatively new technique of injecting compressed air into sand packs so as to simulate the effects of fluid overpressures in sedimentary strata and to trigger slope instabilities. In our experiments, the governing...

In the upper crust, intrusive bodies adopt different configurations, depending on the tectonic setting. This paper describes a new technique for analogue modelling of such intrusions, specifically of low-viscosity magma into a deforming brittle crust. Proper dynamic scaling is an important consideration. If the crust has a Coulomb failure envelope,...

In the upper crust, intrusive bodies adopt different configurations, depending on the tectonic setting. This paper describes a new technique for analogue modelling of such intrusions, specifically of low-viscosity magma into a deforming brittle crust. Proper dynamic scaling is an important consideration. If the crust has a Coulomb failure envelope,...

We describe 18 experiments on the formation of strike-slip fault systems in sand. All models were in a rectangular box. A piston imparted strike-slip motion along a basal cut. In some experiments, uplifted areas underwent erosion. In others, all areas were subject to sedimentation. In experiments without erosion or sedimentation, first to develop w...

The Salar de Atacama basin lies in the inner fore arc of northern Chile. Topographically and structurally, it is a first-order feature of the central Andes. The sedimentary fill of the basin constrains the timing and extent of crustal deformation since the mid-Cretaceous. We have studied good exposures along the western edge of the basin and have c...