Haakon Fossen

Deformation bands in sandstone: a review

Deformation bands are the most common strain localization feature found in deformed porous sandstones and sediments, including Quaternary deposits, soft gravity slides and tectonically affected sandstones in hydrocarbon reservoirs and aquifers. They occur as various types of tabular deformation zones where grain reorganization occurs by grain sliding, rotation and/or fracture during overall dilation, shearing, and/or compaction. Deformation bands with a component of shear are most common and typically accommodate shear offsets of millimetres to centimetres. They can occur as single structures or cluster zones, and are the main deformation element of fault damage zones in porous rocks. Factors such as porosity, mineralogy, grain size and shape, lithification, state of stress and burial depth control the type of deformation band formed. Of the different types, phyllosilicate bands and most notably cataclastic deformation bands show the largest reduction in permeability, and thus have the greatest potential to influence fluid flow. Disaggregation bands, where non-cataclastic, granular flow is the dominant mechanism, show little influence on fluid flow unless assisted by chemical compaction or cementation.

Extended models of transpression and transtension, and application to tectonic settings

Abstract We introduce a spectrum of transpressional and transtensional deformations that potentially result from oblique plate interaction. Five separate types of deformation are designated, in which a simple shear deformation is combined with an orthogonal coaxial deformation. The types vary in the amount of extension v. contraction, both parallel to the margin and vertically. The interaction between the angle of convergence, kinematic vorticity, infinitesimal strain axes, finite strain, and rotation of material lines and planes is investigated. Quantification of the finite strain indicates that the orientation, magnitude, and geometry (flattening, constriction, etc.) change continually during steady-state transpression. These results are then applied to the cases of transpression, particularly resulting from oblique plate convergence of terranes. The obliquity of plate motion and the geometry of the plate margin determine which of the types of transpression or transtension is favoured. A component of margin-parallel stretching also potentially causes terrane motion to locally exceed oblique plate motion, or move opposite to the general direction of movement between the converging plate boundaries. The kinematic models also suggest that the boundaries between converging terranes are likely to exhibit vertical foliation, but either vertical or horizontal lineation. Finally, narrow transpressional zones between colliding blocks may have very high uplift rates, resulting in exhumation of high-grade metamorphic fabrics.

Geometric analysis and scaling relations of deformation bands in porous sandstone

Displacement, length and linkage of deformation bands have been studied in Jurassic sandstones in southeastern Utah. Isolated deformation bands with lengths (L) that span more than three orders of magnitude show similar displacement (D) profiles with more or less centrally located maxima and gently increasing gradient toward the tips. Soft- and hard-linked examples exhibit steeper displacement gradients near overlap zones and immature hard links, similar to previously described fault populations. The deformation band population shows power-law length and displacement distributions, but with lower exponents than commonly observed for populations of larger faults or small faults with distinct slip surfaces. Similarly, the Dmax-L relationship of the deformation bands shows a well-defined exponent of ca 0.5, whereas the general disagreement for other fault populations is whether the exponent is 1 or 1.5. We suggest that this important difference in scaling law between deformation bands and other faults has to do with the lack of well-developed slip surfaces in deformation bands. During growth, deformation bands link to form zones of densely spaced bands, and a slip surface is eventually formed (when 100 m < L < 1 km). The growth and scaling relationship for the resulting populations of faults (slip surfaces) is expected to be similar to ‘ordinary’ fault populations. A change in the Dmax-L scaling relationship at the point when zones of deformation bands develop slip surfaces is expected to be a general feature in porous sandstones where faults with slip surfaces develop from deformation bands. Down-scaling of ordinary fault populations into the size domain of deformation bands in porous sandstones is therefore potentially dangerous.

Timing and kinematics of Caledonian thrusting and extensional collapse, southern Norway: evidence from 40Ar/39Ar thermochronology

40Ar/39Ar dating methods have been applied to rocks across the Caledonian orogen in southwestern Norway. In the eastern nappe area, K-feldspar thermochronological modeling, microfabric characteristics and conodont color alteration together indicate that temperatures were not above the closure temperature of muscovite for any significant period of time. Two groups of muscovite and biotite ages from this area (415–408 for most samples with top-to-the-SE fabrics and 402–394 for samples with top-to-the-NW fabrics) are therefore interpreted as ages of contractional (thrusting) and extensional (hinterland-directed nappe translation) deformation, respectively. In the west (hinterland), peak Caledonian temperatures were higher, and 40Ar/39Ar plateau ages are generally interpreted as cooling ages. The western basement and lower nappes cooled rapidly through ∼500°C (basement) at ∼404 Ma and 350°C (basement and lower nappes) shortly after, i.e. during extensional top-to-the-NW transport of the orogenic wedge. In addition, tectonostratigraphically higher nappes in the hinterland show evidence of earlier cooling, probably following Ordovician orogenic activity prior to the main collisional stage. The new 40Ar/39Ar data conform to kinematic observations that contraction and extension in the Caledonian nappe region were sequential, and that the change from contraction (convergence) to extension (divergence) was quick (between 408 and 402 Ma).

Deformation bands and their influence on fluid flow

Deformation bands represent a common type of strain localization in deformed porous sandstones and occur as single structures, as clusters, and in fault damage zones. They show from zero to six orders of magnitude reduction in permeability and may therefore potentially affect fluid flow. We here present mathematical calculations indicating that uncommonly high permeability contrasts and/or exceptionally high band concentrations are required for deformation bands to significantly affect production rate. We also present field observations showing rapid variations in porosity and permeability along deformation bands and deformation-band zones alike. Furthermore, many paleofluid fronts seen in the field are unaffected or only gently affected by deformation bands. Together, these calculations and observations suggest that their function during reservoir production is small or negligible in most cases. Structural complications caused by subseismic faulting and complex fault anatomy are more likely to cause production problems, in addition to stratigraphic and diagenetic effects. Nevertheless, the arrangement and orientation of deformation bands may have an effect on the flow pattern and reservoir sweep. In cases where deformation bands do cause production problems, it may be possible to resolve these by means of hydraulic fracturing.

Extensional tectonics in the Caledonides: Synorogenic or postorogenic?

Extensional tectonism may form during as well as after the contractional history of a collisional orogeny. Kinematic studies combined with various age constraints in the southern Scandinavian Caledonides have revealed an extensional history which started by hinterland-directed transport of the orogenic wedge above the basal decollement zone (Mode I extension) and proceeded by the development of hinterland-dipping shear zones (Mode II) and subsequent brittle faults (Mode III). The top-to-hinterland kinematics of the basal decollement zone during Mode I extension indicate that this extensional history was entirely postorogenic already from the start. Any synorogenic extensional deformation must have occurred prior to the onset of this extensional history, which is dated to circa 405 Ma. Although synorogenic extension is likely to have occurred, large-scale synorogenic extensional collapse models in the Caledonides are at present difficult to prove, whereas impressive postorogenic multi-stage extension is beautifully portrayed.

Displacement-length scaling in three dimensions: the importance of aspect ratio and application to deformation bands

Deformation bands confined to a 9-m-thick layer of the Entrada Sandstone in Utah accumulate less displacement per unit length than fractures that are not stratigraphically confined. This difference in displacement–length (D–L) scaling is related to the increasing length-to-height (aspect) ratio of the bands. Here we derive new expressions for displacement–length scaling and fracture strain for three-dimensional (3-D) elliptical fractures. The maximum (relative) displacement Dmax on an elliptical fracture surface depends on the fracture geometry (both length L and height H), the end-zone length (through driving stress and the rocks yield strength), and the properties of the surrounding rock (modulus, Poissons ratio). A given elliptical fracture will show different values of Dmax/L in horizontal and vertical sections due to differences in fracture dimension (length vs. height) and end-zone length. A population of elliptical fractures can accommodate less displacement or strain if fracture aspect ratios increase with L than a population of fractures having constant aspect ratios. These relationships reveal how 3-D fracture geometries systematically influence the population statistics. The magnitude of horizontal (extensional) fracture strain accommodated by the population of deformation bands predicted by the analysis is consistent with that obtained independently from traverse measurements on the outcrop (0.12%). The 3-D fracture geometry can contribute at least an order of magnitude in displacement deficit (or excess) relative to tall 2-D fractures and comparable scatter on maximum displacement vs. length (D–L) diagrams. In general, fractures confined to stratigraphic layers grow nonproportionally (L/H≠constant for L>the layer thickness), leading to reduced capacity to accommodate displacement and a shallower slope on the D–L diagram. Similarly, fractures that grow by segment linkage (preferentially down-dip or along-strike) scale as nonproportional 3-D fractures. A unit slope on D–L diagrams implies proportional growth (L/H=constant). Faults with slip surfaces and other fractures with nonpreferred growth directions can produce unit slopes, so that particular trajectories on D–L diagrams can reveal physical controls on fracture growth, such as stratigraphic confinement or segment interaction and linkage.

Three-dimensional reference deformations and strain facies

Abstract In an attempt to categorize three-dimensional deformations, the concepts of kinematic axes, three-dimensional reference deformations, and strain facies are utilized. We have chosen 12 reference deformations, each being an idealized end-member of deformation involving a simultaneous combination of a three-dimensional coaxial component (constriction, flattening, or pure shear) and an orthogonal simple shear component. Velocity and displacement fields, infinitesimal deformation parameters, and finite deformation parameters can be calculated for each reference deformation, assuming steady-state deformation. There are three possibilities for the orientation of foliation and three possibilities for the orientation of lineation, depending on the relative contributions of the coaxial and non-coaxial components. The six emerging combinations of foliation and lineation orientations each give rise to a characteristic strain facies. Each of the strain facies is correlated to the reference deformations, and thus deformation parameters, which caused its formation. However, since the coaxial deformation component accumulates more effectively than the non-coaxial component, a change from one strain facies to another (i.e. a change in the orientation of lineation or foliation) is possible during steady-state deformation. The strain facies emphasize the boundary conditions of deformation and, together with the reference deformations, provide a framework for three-dimensional deformations.

Possible absence of small faults in the Gullfaks Field, northern North Sea: implications for downscaling of faults in some porous sandstones

Faults and fractures have been studied in more than 6 km of cores from the Gullfaks Field, northern North Sea, and compared to fault populations determined by stratigraphic correlation of well logs and seismic data. Statistical analysis indicates a power-law correlation between displacement and fault frequency for displacements down to 5‐10 m. Observations of deformation bands, with displacements ranging from 1 mm to 10 cm, perfectly fit the low-end extension of this power-law model. However, integrated use of well-log correlation data and core data indicates that few faults exist with displacement between 020 cm and 5 m. If this data gap is real and representative for other sandstones, uncritical downscaling of seismic-scale fault sizes for use in oil reservoir description or strain estimation may yield erroneous results. The possible gap may be related to the high density and coalescent nature of (larger) faults, excluding the tip portions of many faults and thus the likelihood for wells to intersect low-displacement tip regions. Deformation bands, on the other hand, seldom develop displacements much in excess of 010 cm, thus providing a distinct population of displacement data in the lower end of the displacement scale which, by coincidence, falls close to the extrapolated trend of the well data in log‐log space. 7 2000 Elsevier Science Ltd. All rights reserved.

Extensional tectonics in the North Atlantic Caledonides: a regional view

Abstract Extensional structures characterize significant parts of the North Atlantic Caledonides. Silurian extensional deformation took place, particularly in the heated crust in the southern Greenland Caledonides, but the majority of the mapped extensional structures are Devonian (403–380 Ma). They formed by reactivation of low-angle Caledonian thrusts and by the formation of hinterland-dipping shear zones, of which the largest system is located in SW Norway and related to exhumation of the subducted margin of Baltica. The Devonian extension was concentrated to the central and southern part of the Caledonides, with maximum extension occurring in the area between the Western Gneiss Region of SW Norway and the Fjord Region of East Greenland. Kinematic data indicate that the main tectonic transport direction was toward the hinterland, and this pattern suggests that the main Devonian extension/transtension in the southern part of the North Atlantic region was postcontractional while strike-slip motions and possibly transpression occurred farther north. Late Devonian to enigmatic Early Carboniferous ages from UHP metamorphic assemblages in NE Greenland suggest that intracontinental subduction was going on in NE Greenland at a time when extensional deformation governed the rest of the orogenic belt.