Ken McCaffrey

Granite magma formation, transport and emplacement in the Earth's crust

The origin of granites was once a question solely for petrologists and geochemists. But in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation, transport and emplacement of silica-rich melts in the continental crust. Accepted petrological models are being questioned, either because they require unrealistic rheological behaviours of rocks and magmas, or because they do not satisfactorily explain the available structural or geophysical data. Provided flow is continuous, mechanical considerations suggest that—far from being geologically sluggish—granite magmatism is a rapid, dynamic process operating at timescales of ≤100,000 years, irrespective of tectonic setting.

Are granitic intrusions scale invariant

Published length (L) and thickness (T) data on 135 laccolith and 21 granite intrusions define power-law relationships of the form L=kTa typical of systems exhibiting scale invariant (fractal) behaviour. Both data sets are characterised by an exponent a< 1 (0.88 ±0.1 for laccoliths and 0.80 ± 0.20 for plutons) that reflects an inherent preference for scale invariant tabular-sheet geometries. These power-law size relationships can be explained in mechanical terms by the need for an incoming magma sheet to travel laterally some distance before vertical thickening can occur. Sheet thickness is a function of available magma pressure which for an intrusion fed by a feeder dyke is proportional to the vertical magma transport distance.

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.

Growth of plutons by floor subsidence: implications for rates of emplacement, intrusion spacing and melt-extraction mechanisms

Abstract Geophysical and field-based studies indicate that granitic plutons occur as either tabular (disk) or wedge (funnel) shapes whose length ( L ) to thickness ( T ) ratio is controlled by the empirical power law, T = 0.6(±0.15) L 0.6(±0.1) . The dimensions of plutons are self-similar to other natural subsidence phenomena (calderas, ice cauldrons, sinkholes, ice pits) and it is proposed that they grow in a similar fashion by withdrawal of material (melt) from an underlying source, which is then transferred to the growing pluton within the crust. Experimental studies show that growth of subsidence structures occurs by vertical inflation ⪢ horizontal elongation of an initial depression with L ≈ width of the source region. If pluton growth is modelled in the same way, the empirical power law relating T and L defines limits for pluton growth that are imposed by the width, thickness and degree of partial melting from a lower crustal source. Several growth modes that predict testable internal structural patterns are identified for plutons, depending on whether they are tabular or wedge-shaped, grow by continuous or pulsed magma delivery and whether magma is accreted from bottom to top, or vice versa. Rates of pluton growth are geologically fast (hundreds to hundreds of thousands of years) if magma supply is effectively continuous, but can also take millions of years if the time between magma delivery events is much longer than magma injection events. Plutons formed by melt extraction from an area directly beneath require large degrees of partial melting and or very thick sources. Lower degrees of partial melting and thinner sources are permitted when melt extraction occurs over a larger region, which can lead to the formation of spaced plutons. Tabular pluton growth will tend to favour widely spaced plutons, unless degrees of partial melting in the source are high. Wedge-shaped plutons can form much closer together and require lower degrees of partial melting. These results are in general agreement with current geophysical, petrological and experimental estimates of partial melting in the lower continental crust.

Igneous emplacement in a transpressive shear zone: Ox Mountains igneous complex

The central and southwestern Ox Mountains Inlier in western Ireland, comprises amphibolite facies metasediments and metavolcanic rocks which have been deformed by a major sinistral shear zone. Granitoid emplacement occurred in two distinct styles in the Ox Mountains shear zone. The older pluton, the Ox Mountains granodiorite was emplaced syn-kinematically with respect to the main sinistral, transpressional events in its wall rocks. It was emplaced as a series of compositionally distinct sheets in the central part of the shear zone system thus suggesting the formation of transient, dilational sites occurred in response to the transpressive deformation. The younger, Lough Talt and Easky Lough adamellites were emplaced as individual intrusions in dilational cavities produced by reactivated movements on a discrete zone within the Ox Mountains inlier. The different intrusive styles are considered to be a product of the tectonic regimes operating during emplacement. Intrusion into transient dilational cavities may be one method by which granitoid magmas are emplaced into fault zones undergoing active contractional deformation.

Influence of layering on vein systematics in line samples

Abstract The thickness and spacing systematics of 30 line samples of vein arrays at ten localities have been analysed. The analytical methods were tested on four synthetic data sets. The synthetic and natural data sets were each analysed with respect to: cumulative thickness v. distance; thickness population; spacing population; the coefficient of variation of spacing, which measures clustering; the mass function, which measures the scaling of strain heterogeneity. A fundamental distinction is apparent between stratabound arrays, vein arrays in layered rocks where the veins are confined to individual mechanical units, and non-stratabound arrays which occur in rock volumes lacking persistent crack-stopping discontinuities. The stratabound arrays show regular spacing controlled by layer thickness and non-power-law thickness distributions. The non-stratabound arrays are clustered and have power-law thickness distributions resulting from the lack of a controlling length scale. Exponents of the thickness distributions have a modal value of 0.8 and do not vary significantly with lithology. Ore mineralization is favoured in non-stratabound arrays as they are more likely than stratabound arrays to form large clusters connected to a remote fluid source.

Fractal geometries of vein systems and the variation of scalingrelationships with mechanism

Abstract Investigation of vein geometries has identified empirical relationships which allow extrapolation of outcrop-scale observation to larger scales. Measurements of vein shapes and aspect ratios in a number of different environments demonstrate that veins follow similar empirical scaling laws to those of faults. Geometries of veins are easier to characterise accurately than faults because their thickness is related to the variation in displacement from zero at their tips to a maximum at their thickest point. Vein sizes obey the empirical relationship: L = kTa where L = length in mm, T = thickness in mm, 20 1. This suggests that veins amplify initially by inflation and subsequently by elongation.

Domainal deformation patterns and strain partitioning during transpression: an example from the Southern Uplands terrane, Scotland

The partitioning of deformation into wrench- and contraction-dominated deformation domains is a widely reported but poorly described phenomenon in ancient transpression zones. This paper documents spectacularly exposed examples of such partitioning from the Southern Uplands terrane in SE Scotland (Berwickshire), which was deformed during late Llandovery to Wenlock time. A well-exposed coastal section from Eyemouth to Burnmouth preserves a broadly homoclinal sequence in which a highly heterogeneous array of contemporaneous structures formed during regional triclinic transpression. The deformation involved components of NW–SE contraction with subvertical extension, top-to-the-SE thrusting and top-to-the-SW sinistral shear. In the northern third of the section studied these components are partitioned into a series of fault-bounded, metre- to kilometre-scale structural domains that contain geometrically and kinematically distinct assemblages of variably curvilinear folds, strike-slip detachments and locally transecting cleavages. The structures are all broadly contemporaneous and, in individual domains, record either non-coaxial contractional- or sinistral wrench-dominated strains. Similar highly heterogeneous, domainal structural patterns are likely to be found in other regions of oblique convergence in both ancient and modern settings.

Surface Faulting of the 6 April 2009 Mw 6.3 L’Aquila Earthquake in Central Italy

This paper documents evidence of surface faulting associated with the 6 April 2009 moderate-sized earthquake (ML 5.8, Mw 6.3) in the central Apennines of Italy, which caused major damage to the town of L’Aquila and its surroundings. Coseismic surface ruptures were mapped for a minimum distance of 2.6 km along the Paganica fault, a fault still poorly investigated relative to the other active faults nearby, which bound much wider range fronts. Surface rupture length (SRL) and maximum displacement parameters (2.6 km minimum and 10–15 cm, respectively) are in agreement with what is expected for an Mw 6.3 event in the Italian Apennines tectonic environment. Different viewpoints exist on the amount of SRL and the number of activated faults. We propose a pattern of sympathetic and secondary slip on an array of faults around the master seismogenic structure. Past seismicity and evidence for larger Holocene offsets on this and other capable faults nearby prove that the 2009 event is not a good reference event for assessing the seismic hazard of the region. Nevertheless, the 2009 L’Aquila earthquake once more confirmed the importance of detailed geological studies for a proper seismic hazard assessment, and it clearly illustrates the need to pay attention to moderate events and supposedly minor active faults. Indeed, this type of earthquake is rather frequent in the whole Mediterranean region and is potentially much more destructive than in the past, due to the expanding urban centers and infrastructures inside their epicentral regions and even right above the traces of capable faults.

Vertical coupling and decoupling in the lithosphere

Continental tectonics, and the formation of mountain belts, do not adhere to the plate tectonic paradigm (Molnar 1988). Mountain belts at plate boundaries are areas of diffuse deformation in which geologists have recognized that not only are the plates not rigid (Gordon 1998), but parts of the lithosphere (e.g. upper crust) are moving laterally with respect to other parts (e.g. lower crust), such as in thrust belts (Bally et al. 1966). An exciting development in tectonics is the detailed investigation of the behaviour of continental crust during orogenesis. In particular, the role of coupling (attachment) or decoupling (detachment) of the lithospheric layers during continental deformation has significant implications for all aspects of modern and ancient tectonics. The recognition of regional detachments or décollements, an idea developed in foreland fold and thrust belts, was the first major contribution to our understanding of the vertical stratification in orogenic belts. Elucidation of the structure of foreland fold and thrust belts in the external parts of orogens by the development of the techniques of balanced cross-section construction (Price 1981, 1986) and deep seismic reflection profiling (e.g. Mueller et al. 1980) showed that foreland thrust systems are typically think-skinned and bounded at depth by a basal detachment, décollement or sole thrust. Shortening in the external parts of cordilleran and collisional orogens was taken up by folding and thrusting above a basement which remained essentially undeformed and part of the foreland (e.g. Bally et al. 1966). Balanced cross-section techniques were developed and refined in the Alberta