Lidia Lonergan

Origin of the Betic‐Rif mountain belt

In recent years, the origin of the Betic-Rif orocline has been the subject of considerable debate. Much of this debate has focused on mechanisms required to generate rapid late-orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic-Rif mountain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present-day Calabrian Arc. The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic-Rif orocline during the early to middle Miocene.

Mechanisms and controls on the formation of sand intrusions

Sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep-water settings. They also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments. A variety of mechanisms have been cited as triggers for clastic intrusions. These include seismicity induced liquefaction, application of tectonic stresses, excess pore fluid pressures generated by deposition-related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body. The formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures. When the seal on an unconsolidated, overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize. The fluidized slurry can then inject along pre-existing or new fractures to form clastic intrusions. The scale and the geometry of an intrusive complex is governed by the stress state, depth and pre-existing joints or faults within the sedimentary succession, as well as the nature of the host sediments. For the simplest tectonic setting, where the maximum stress in a basin is vertical (gravitational loading), small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface, whereas at greater depth dykes and sills forming clastic intrusion networks are more typical. A simple relationship is derived to calculate the maximum burial depth at which a dyke–sill complex forms as a function of the source-bed to sill height, the bulk density of the surrounding sediments, and the ratio of the vertical to horizontal effective stresses, K0. When applied to three examples of large-scale dyke–sill complexes developed within Paleocene and Eocene deep-water reservoir sand bodies of the North Sea, maximum burial depths in a range of 375 to c. 500 m, 450–700 m and 550–850 m are estimated for intrusion of each of the three complexes.

The development of polygonal fault systems by syneresis of colloidal sediments

Abstract Polygonal fault systems occur in numerous sedimentary basins worldwide, are generally located on passive margins in onlap fill units and tend to comprise the finest grained sediments in this geological setting. These fault systems have been most thoroughly described in the central North Sea basin and the detailed structure shows a significant correlation with lithological variations, both vertically and laterally. Extension measured in stacked decoupled tiers of polygonal faults correlates positively with both clay fraction and smectite content. Lateral facies variations are also observed and indicate that time-equivalent sequences upslope from the smectite-rich polygonally faulted sediments are coarse-grained, clay-poor and undeformed. This leads us to believe that the structure and geometry of the fault system are controlled by the colloidal nature of the sediments, and that the volumetric contraction measured on seismic sections can be accounted for by syneresis of colloidal smectitic gels during early compaction. Syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of the constituent pore fluid. This process occurs due to the domination of interparticle attractive forces in marine clays, dependent on environment, and is governed by the change of gel permeability and viscosity with progressive compaction. The process of syneresis can account for a number of structural features observed within the fault systems, such as tiers of faults, the location of maximum fault throw and growth components at upper fault tips. As such, this paper represents the first attempt to correlate microscale properties of clay-rich sediments to their macroscale seismic character.

Exhumation of the Ronda peridotite and its crustal envelope: constraints from thermal modelling of a P–T–time array

The Ronda peridotite in the Betic Cordillera of southern Spain is a relic of the sub-orogenic lithospheric mantle that was exhumed during earliest Miocene time from about 66 km depth. Overlying crustal rocks show an apparently coherent metamorphic zonation from high-pressure granulite-facies rocks at the contact to unmetamorphosed rocks 5 km higher in the structural sequence, indicating drastic tectonic thinning of the whole orogenic crust during exhumation. P–T paths from the peridotite and its crustal envelope indicate decompression with rising temperature to shallow depths. U–Pb ion microprobe dating of zircon, Ar/Ar dating of hornblende, Ar/Ar laserprobe dating of muscovite and biotite, and fission-track analysis of zircon and apatite reveal that cooling was extremely rapid in the interval 21.2–20.4 Ma. One-dimensional thermal modelling of the array of P–T–time paths indicates that an asthenospheric heat source at an initial depth of about 67.5 km is required to explain heating during exhumation, and that the main period of exhumation lasted 5 Ma, starting at around 25 Ma. Exhumation must therefore have directly followed removal of most, but not all, of the lithospheric mantle beneath the Betic orogen, and was coeval with a period of late orogenic extension that profoundly modified the crustal structure and created the present-day Alboran Sea in the western Mediterranean.

Polygonal Faults and Their Influence on Deep-Water Sandstone Reservoir Geometries, Alba Field, United Kingdom Central North Sea

Polygonal faults attributed to three-dimensional (3-D) volumetric contraction of muddy sediments during early burial are widespread within the Eocene-lower Miocene succession of the United Kingdom central North Sea. The analysis of a 3-D seismic survey encompassing the Eocene Alba field in the central North Sea has allowed us to investigate (1) the influence of polygonal faults in surrounding mudrocks on the geometry of the Alba deep-water sandstone reservoir and (2) how the presence of the reservoir sandstone influences the polygonal fault pattern above and below the reservoir. The main reservoir in the Alba field is an elongate (12 km long, 1-2 km wide, up to 90 m thick), fine-grained, massive sandstone body that may have been deposited in a deep-water channel or slope gully. Although depositional processes likely have been responsible for the dominantly linear sandstone distribution, the current reservoir geometry is largely controlled by the location of polygonal faults in the surrounding hemipelagic mudstones. Sandstones interpreted as injected along faulted margins of the Alba field indicate that faulting facilitated remobilization and sand injection during early burial, further modifying the reservoir shape. Unusual, isolated 1-km-wide subcircular mounds to the west of the main field also are attributed to sand withdrawal and remobilization during early burial. On a mapped marker horizon in the mudrocks 80-120 m above the reservoir there is a marked decrease in polygonal fault density compared to areas away from the reservoir. On a horizon in mudrocks within 5-50 m of the base of the reservoir there is an increase in horizon disruption due to small faults directly below the sand body. Changes in polygonal fault density and pattern thus may indicate the presence of sandstones and may be a useful exploration tool for explorationists searching for subtle Eocene deep-water sand bodies that typically are poorly imaged on seismic data in the North Sea.

THE GEOMETRY OF POLYGONAL FAULT SYSTEMS IN TERTIARY MUDROCKS OF THE NORTH SEA

Abstract Polygonal networks of extensional faults have recently been identified deforming large volumes of Tertiary mudrocks in the North Sea. The main distinctive feature of this fault system is a near equal distribution of fault strike orientations and polygonal organisation of the fault trace segments in plan view. Detailed fault mapping using three-dimensional seismic reflection datasets from the central North Sea show that the complex three-dimensional geometries are primarily a function of intersections between fault surfaces oriented in all directions in space. Within stratigraphic units or layers the deformation is distributed as opposed to the clustered deformation of more co-linear tectonic fault arrays where the strain is concentrated into zones or bands at a variety of scales. End member map patterns with curved, rectilinear or irregular polygonal geometries are identified that encompass the variability observed on over 30 fault maps made at different stratigraphic levels within the three-dimesional seismic datasets. Four main classes of intersections between fault segments are defined providing a descriptive geometric framework for polygonal fault systems. The geometry of the intersection types and their geometrical stability in three-dimensions is discussed with reference to examples from the seismic datasets. Fault shapes are irregular and, in detail, shape is largely a function of intersection relationships with neighbouring faults. Faults can be planar or listric, and can have triangular shapes that may taper upwards or downwards. They frequently exhibit complex irregular upper tips. Completely isolated non-connected faults are very rare within the three-dimensional datasets studied.

The control of stress history and flaw distribution on the evolution of polygonal fracture networks

Abstract We use boundary element models of fracture propagation and linkage to investigate the factors controlling the development of two-dimensional, multi-directional–polygonal fracture networks, characterised by a large number of abutting intersections between fractures. The position and orientation of a number of fracture seeds are prescribed in the model, which propagate when the applied stress reaches a critical value, according to linear elastic fracture mechanics theory. The applied boundary condition is a remote, isotropic, horizontal tension, where the stress is increased at a steady rate throughout each model to simulate continued fracture growth. Realistic polygonal systems are developed with the boundary element model simulations, which are comparable with those observed in natural systems (such as those found within Eocene and Oligocene mudrocks in the North Sea and on the surface of Mars). If conditions exist where a small number of fracture seeds propagate and develop significant structures before others, then these will dominate the resulting fracture network geometry. Not only do such early structures represent the largest fractures in the system, they also significantly modify the stress field around them preventing some other seeds from developing, and influencing the propagation paths of nearby fractures. Fracture seeding distribution and the rate at which the stress is increased are found to be the most significant parameters affecting the development of fracture network geometry. These results suggest that the geometry of evolving fracture networks should be considered not only in terms of the mechanical properties of the deforming material, but also in terms of the stress rate driving deformation.

Seismic investigation of thick evaporite deposits on the central and inner unit of the Mediterranean Ridge accretionary complex

Abstract Seismic reflection observations from the IMERSE cruise have confirmed the presence of at least two localised thick evaporite basins on the crest and Inner Plateau of the Mediterranean Ridge accretionary complex. The discovery is supported by interval velocity results from both near-normal incidence reflection data and wide-angle seismic data. These evaporite basins are located north of a deep trough known as the Cleft and at a currently active thrust zone south of the Cleft, on the crest of the Mediterranean Ridge. The polarity of the seismic reflections associated with the base of the evaporite shows a phase reversal relative to the seafloor. This high amplitude phase reversed reflection indicates a decrease in velocity between the evaporite and the underlying sediment. The existence of these thick evaporite deposits suggests that the distribution of Messinian salt is much thicker (∼1.8–2 km) on parts of the Ridge crest than initially perceived by previous investigations. We propose that these thick evaporite deposits were deposited in localised basins on the Ridge, formed in response to pre- and syn-Messinian tectonics. The evaporites on the Inner Plateau basin most likely represent the infill of a Messinian forearc basin. The existence of local deep evaporite basins on the crest of the Ridge at depths of 2.7–3.3 km below sea level at the present day supports the hypothesis that the Mediterranean Sea level must have dropped by around 3 km below its present sea level during the Messinian salinity crisis.

The role of thermal conductivity measurements in modelling thermal histories in sedimentary basins

Thermal conductivity is an important parameter in thermal maturity modelling, yet is often poorly constrained. Here we examine the consequences for thermal history models of incorporating imprecise and inaccurate thermal conductivity data. We adopt a formal inversion scheme to determine the heat flow history of Inigok 1, Colville Basin, Alaska, using an extensive suite of vitrinite reflectance measurements as calibration data. The initial model incorporates lithology-dependent matrix thermal conductivity values based on published data. Subsequently, we reran the inversion 100 times, using different matrix thermal conductivities generated by Monte Carlo sampling of normal distributions specified for each lithology. The mean of each distribution was set to the initial model value and standard deviation was set at 20% of the mean. The results show that the relative variation in palaeotemperatures associated with the inversion-generated thermal histories is considerably less than that used to generate the thermal conductivity samples. This is because the heat flow history compensates for the variations in thermal conductivity to satisfy the constraints afforded by downhole temperature measurements and downhole thermal indicator data. These results show that thermal histories modelled with the inversion scheme are robust to imprecise and inaccurate thermal conductivity data. However, if a meaningful estimate of the actual heat flow is required, then accurate thermal conductivity data are necessary.

Polygonal faulting in the Tertiary of the central North Sea: implications for reservoir geology

Abstract Interpretation of regional two-dimensional seismic surveys and three-dimensional seismic surveys in the central North Sea has demonstrated the existence of a pervasive polygonal network of normal faults affecting Tertiary shale-dominated slope and basin-floor depositional systems. The area affected by the faulting encompasses most areas of hydrocarbon production from Tertiary sandstone reservoirs. The polygonal fault networks were active during sedimentation and early burial. Throws measured on individual faults range from 8 to 100m, with average fault plane dips of 45°. Lengths of individual fault segments range from 80 to 1400m, and average fault spacings range from 100 to 500m. The high density and the polygonal geometry of the faults make seismic interpretation of the Lower Tertiary interval problematic, and can lead to misinterpretation of faults as apparent seismic-stratigraphic features. The recognition of this fault system has consequences for the development of of fields in Tertiary reservoirs. The remaining hydrocarbon potential in the Lower Tertiary could be considered predominantly to be stratigraphic plays, but the presence of faults active during sedimentation and early burial implies that this play concept is too simplistic, particularly for Eocene reservoirs. The role of both syn-depositional and early-burial normal faulting on original reservoir distribution and post-depositional modification should be considered further. The presence of this fault system may also be important for secondary migration into Lower Tertiary reservoirs from Kimmeridge Clay Formation source rocks. The existence of an interconnected fault and fracture network in the low-permeability mudrocks may have provided an efficient vertical migration pathway for charging isolated lower Tertiary sandstone reservoirs. Finally, the maximum fault throws of between 50 and 100m are large enough to represent potential barriers for lateral communication in sandstone reservoirs where individual sand bodies are commonly 25–100m thick.