R. E. Holdsworth

Transpression and transtension zones

Abstract Transpression and transtension are strike-slip deformations that deviate from simple shear because of a component of, respectively, shortening or extension orthogonal to the deformation zone. These three-dimensional non-coaxial strains develop principally in response to obliquely convergent or divergent relative motions across plate boundary and other crustal deformation zones at various scales. The basic constant-volume strain model with a vertical stretch can be modified to allow for volume change, lateral stretch, an oblique simple shear component, heterogeneous strain and steady-state transpression and transtension. The more sophisticated triclinic models may be more realistic but their mathematical complexity may limit their general application when interpreting geological examples. Most transpression zones generate flattening (k < 1) and transtension zones constrictional (k > 1) finite strains, although exceptions can occur in certain situations. Relative plate motion vectors, instantaneous strain (or stress) axes and finite strain axes are all oblique to one another in transpression and transtension zones. Kinematic partitioning of non-coaxial strike-slip and coaxial strains appears to be a characteristic feature of many such zones, especially where the far-field (plate) displacement direction is markedly oblique (<20°) to the plate or deformation zone boundary. Complex foliation, lineation and other structural patterns are also expected in such settings, resulting from switching or progressive rotation of finite strain axes. The variation in style and kinematic linkage of transpressional and transtensional structures at different crustal depths is poorly understood at present but may be of central importance to understanding the relationship between deformation in the lithospheric mantle and crust. Existing analyses of obliquely convergent and divergent zones highlight the importance of kinematic boundary conditions and imply that stress may be of secondary importance in controlling the dynamics of deformation in the crust and lithosphere.

Sinistral transpression and the Silurian closure of Iapetus

In recent years conflicting models have been proposed for the late Caledonian closure of the Iapetus ocean between Laurentia, Baltica and the Avalonian terranes. Recently published structural and stratigraphic evidence from Britain, Scandinavia, East Greenland and Newfoundland is reviewed and shows that Western Avalonia, Eastern Avalonia and Baltica all docked sinistrally against Laurentia in the Silurian. Western Avalonia collided sinistrally against the previously accreted Gander and Dunnage arc terranes on the Appalachian margin of Laurentia in mid-Silurian time and then shifted dextrally during the Acadian orogeny in the Devonian. Oblique collision of Baltica with the Greenland margin induced southeasterly crustal stacking in the Scandian orogen and sinistral transpression in the NE Greenland Caledonides, and was followed by more nearly orthogonal convergence. Eastern Avalonia underwent anticlockwise with the Scottish ‘corner’ of Laurentia, rotating into a re-entrant between Laurentia and Baltica. Some implications of this Silurian closure model are that convergence in the Tornquist zone was modest at that time, and that the Acadian (Devonian) deformation in the northern Appalahians and Britain had a subsequent external cause, most likely the impingement of Armorica and Iberia due to the northward drift of Gondwana.

The recognition of reactivation during continental deformation

Reactivation involves the accommodation of geologically separable displacement events (intervals >1 Ma) along pre-existing structures. The definition of a significant period of quiescence is central to this phenomenological definition and the duration of the interval chosen represents the resolution limit of reactivation criteria found in most ancient settings. In neotectonic environments, reactivation can be further defined as the accommodation of displacements along structures that formed prior to the onset of the current tectonic regime. This mechanistic definition cannot always be applied to ancient settings due to the uncertainties in constraining relative plate motion vectors. Four sets of criteria may be used to recognize reactivation in the geological record: stratigraphic, structural, geochronological and neotectonic. Some structural criteria may not be reliable if used in isolation to identify reactivated structures. Much of the previously published evidence cited to invoke structural inheritance is equivocal as it uses similarities in trend, dip or three-dimensional shape of structures. Numerous fault and shear zone processes can cause significant weakening both synchronously with deformation and in the long-term and may be invoked to explain reactivation. The collage of fault-bounded blocks forming most continents therefore carries a long-term architecture of inheritance which can explain much of the observed complexity of continental deformation zones.

Unlocking the spatial dimension: digital technologies and the future of geoscience fieldwork

The development of affordable digital technologies that allow the collection and analysis of georeferenced field data represents one of the most significant changes in field-based geoscientific study since the invention of the geological map. Digital methods make it easier to re-use pre-existing data (e.g. previous field data, geophysical survey, satellite images) during renewed phases of fieldwork. Increased spatial accuracy from satellite and laser positioning systems provides access to geostatistical and geospatial analyses that can inform hypothesis testing during fieldwork. High-resolution geomatic surveys, including laser scanning methods, allow 3D photorealistic outcrop images to be captured and interpreted using novel visualization and analysis methods. In addition, better data management on projects is possible using geospatially referenced databases that match agreed international data standards. Collectively, the new techniques allow 3D models of geological architectures to be constructed directly from field data in ways that are more robust compared with the abstract models constructed traditionally by geoscientists. This development will permit explicit information on uncertainty to be carried forward from field data to the final product. Current work is focused upon the development and implementation of a more streamlined digital workflow from the initial data acquisition stage to the final project output.

Fault zone weakening and character of slip along low-angle normal faults: insights from the Zuccale fault, Elba, Italy

A seismically active low-angle normal fault is recognized at depth in the Northern Apennines, Italy, where recent exhumation has also exposed ancient examples at the surface, notably the Zuccale fault on Elba. Field-based and microstructural studies of the Zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability, promoting influx of CO2-rich hydrous fluids. This triggered low-grade alteration and the onset of stress-induced dissolution–precipitation processes (e.g. pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit ductile deformation textures very similar to those formed during pressure-solution-accommodated ‘frictional–viscous’ creep in experimental fault rock analogues. The presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core, which significantly enhanced the sealing capacity of the fault zone. A slip model for low-angle normal faults in the Apennines is proposed in which aseismic frictional–viscous creep occurs on a weak, slow-moving (slip rate <1 mm a−1) fault, interspersed with small seismic ruptures caused by cyclic hydrofracturing events. Our findings are potentially applicable to other examples of low-angle normal faults in many tectonic settings.

Lateral extrusion in transpression zones: the importance of boundary conditions

The homogeneous transpression strain model formulated by Sanderson and Marchini (Journal of Structural Geology 6, 449–458, 1984) has proved to be a useful tool in the analysis of complex three-dimensional deformation patterns. However, some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive. In this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically, but also horizontally in and out of the deforming zone parallel to its length—‘lateral extrusion’. Three end-member plane-strain components are defined: wrench simple shear; pure shear in XY (lateral stretch); and pure shear in YZ (vertical stretch). These biaxial strains can be viewed as the apices of a ‘strain triangle’ for unconfined transpression or transtension. The edges of the triangle correspond to: triaxial pure shear; non-coaxial, biaxial lateral extrusion; and the triaxial confined transpressional or transtensional strain of Sanderson and Marchini. During unconfined transpression, the orientation and, in particular, the geometry (k-value) of the finite-strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike-slip parallel to the zone, but also upon the ratio of vertical to lateral stretch. This can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone-boundary displacements. Examples of transpression zones in which there is evidence of a component of lateral extrusion are described from SW Cyprus and central Scotland. These examples illustrate that antithetic strike-slip shearing is a kinematic requirement of laterally unconfined transpression, implying that synchronous shear-sense indicators may give opposite senses of movement in shear zones. Specific geometric and mechanical boundary conditions, together with internal fault-zone rheologies, may favour the lateral extrusion of material. Kinematic partitioning can occur to form fault-bounded domains in which end-member biaxial and/or non-coaxial strains are developed. Analysis of such domains can give a clearer understanding of regional-scale triaxial deformations. These findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones.

A reappraisal of the Sibson-Scholz fault zone model : the nature of the frictional to viscous ("brittle-ductile") transition along a long-lived, crustal-scale fault, Outer Hebrides, Scotland.

The widely cited Sibson-Scholz conceptual fault zone model suggests that seismically active, upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10–15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep. The crustal-scale Outer Hebrides Fault Zone (OHFZ) in NW Scotland has been described as the type example of such a continental fault zone. It cuts Precambrian basement gneisses and is deeply exhumed, allowing direct study of the deformation products and processes that occur across a wide range of crustal depths. A number of fault rock assemblages are recognized to have formed during a long-lived displacement history lasting in excess of 1000 Myr. During Caledonian movements that are recognized along much of the 190 km onshore fault trace, brittle, cataclasite-bearing faults in the west of the OHFZ are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones. Field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust-related crustal thickening or shear heating. Microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids. This led to retrogression, with the widespread development of new finegrained phyllosilicate-bearing fault rocks (“phyllonites”), and the onset of fluid-assisted, grain size-sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal-scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal-scale faults containing interconnected networks of phyllosilicate-bearing fault rocks will be characterized by long-term relative weakness and shallow (∼5 km) frictional-viscous transition zones. Similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the San Andreas Fault.

Intraplate strike-slip deformation belts

Abstract Intraplate strike-slip deformation belts are typically steeply-dipping structures that develop in both oceanic and continental lithosphere where they form some of the largest and most spectacular discontinuities found on Earth. In both modern and ancient continental settings, intraplate strike slip deformation belts are of major importance in accommodating horizontal displacements where they additionally form very persistent zones of weakness that substantially influence the rheological behaviour of the lithosphere over very long time periods (up to 1 Ga or more). These deformation zones provide a fundamental geometric, kinematic and dynamic link between the more rigid plate-dominated tectonics of the oceans and the non-rigid, complex behaviour of the continents. During convergence, they help to transfer major displacements deep into the plate interiors. During divergence, they act as transfer zones that segment rifts, passive continental margins and, ultimately, oceanic spreading ridges. Such belts are also of great economic importance, controlling the location of many destructive earthquakes, offshore and onshore hydrocarbon deposits and metalliferous ore deposits. In the oceans, intraplate strike-slip movements are relatively minor along transform-related fracture zones, but there are an increasing number of documented examples that may reflect spatial and temporal variations in spreading rate along individual active ridge segments.

Three-dimensional brittle shear fracturing by tensile crack interaction

Faults in brittle rock are shear fractures formed through the interaction and coalescence of many tensile microcracks. The geometry of these microcracks and their surrounding elastic stress fields control the orientation of the final shear fracture surfaces. The classic Coulomb–Mohr failure criterion predicts the development of two conjugate (bimodal) shear planes that are inclined at an acute angle to the axis of maximum compressive stress. This criterion, however, is incapable of explaining the three-dimensional polymodal fault patterns that are widely observed in rocks. Here we show that the elastic stress around tensile microcracks in three dimensions promotes a mutual interaction that produces brittle shear planes oriented obliquely to the remote principal stresses, and can therefore account for observed polymodal fault patterns. Our microcrack interaction model is based on the three-dimensional solution of Eshelby, unlike previous models that employed two-dimensional approximations. Our model predicts that shear fractures formed by the coalescence of interacting mode I cracks will be inclined at a maximum of 26° to the axes of remote maximum and intermediate compression. An improved understanding of brittle shear failure in three dimensions has important implications for earthquake seismology and rock-mass stability, as well as fluid migration in fractured rocks.

Development of interconnected talc networks and weakening of continental low-angle normal faults

Fault zones that slip when oriented at large angles to the maximum compressive stress, i.e., weak faults, represent a significant mechanical problem. Here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite + abundant talc along a low-angle normal fault. Within the fault core, talc forms an interconnected foliated network that deforms by frictional sliding along 50–200-nm-thick talc lamellae. The low frictional strength of talc, combined with dissolution-precipitation creep, can explain slip on low-angle normal faults. In addition, the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures. The development of phyllosilicates such as talc by fluid-assisted processes within fault zones cutting Mg-rich carbonate sequences may be widespread, leading to profound and long-term fault weakness.