In its early decades, Antiquity regularly featured the subject of linear earthworks that criss-cross the British landscape. Subsequently, however, discussion has been largely relegated to period-specific and local journals. As a result, interpretations of these imposing but often poorly dated earthworks have been drawn in the contrasting research traditions of later prehistory and the early medieval period. Here, the authors propose a comparative dialogue as a means for reinterpreting these landscape features, and as a lens through which to explore social complexity. Combined with advances in archaeometrical dating, this new approach promises to reinvigorate the study of some of Britain's largest archaeological monuments.

The Shyok Suture Zone is an oceanic remnant of the Neo-Tethyan ocean sandwiched between the Ladakh Batholiths to the south and Karakoram Batholith to the north. The Tirit granitoids in this suture are dark-coloured, relatively rich in ferromagnesian minerals and range from granodiorite–tonalite to gabbro–diorite in composition. Mafic igneous enclaves are quite common and they are intruded by NW–SE parallel doleritic and aplitic dykes. The Tirit granitoids have a wide range of major oxide compositions (SiO2 = 52.1–72.11 wt %, TiO2 = 0.21–1.23 wt %, Al2O3 = 11.42–13.52 wt %, MgO = 1.69–10.69 wt % and CaO = 3.24–9.31 wt %) and show calc-alkaline, metaluminous, I-type characteristics, transitional between primitive and mature arc continental plutons. Rare earth elements (REE) show considerable enrichment in light REE (LREE) as compared to the heavy REE (HREE). Late Cretaceous U/Pb dates (74–68 Ma) show that they formed during the pre-collision northward movement of India. The Tirit dykes are only slightly younger and probably part of the same episode.

Fractures that form when fluid pressure ruptures the rock are referred to as fluid-driven fractures or hydrofractures. These include most dykes, inclined sheets and sills, but also many mineral veins and joints, as well as human-made hydraulic fractures. While considerable field and theoretical work has focused on the geometry and arrest of hydrofractures, how they select their propagation paths, particularly in layered and faulted rocks, has received less attention. Here I propose that of all the possible paths that a given hydrofracture may follow, it selects the path of least (minimum) action as determined by Hamilton’s principle. This means that the selected path is that along which the energy transformed (released) multiplied by the time taken for the propagation is a minimum. Hydrofractures advance their tips/fronts in steps, with a time lag between the fracture front and the fluid front. In the present framework, each step is then controlled by Hamilton’s principle. The results suggest that when the hosting rock body is regarded as homogeneous, isotropic and non-fractured, hydrofracture paths are everywhere perpendicular to the trajectories of the minimum compressive (maximum tensile) principal stress σ3 and follow the trajectories of the maximum principal compressive stress σ1. When applied to layered and faulted rock body, the results indicate that hydrofracture paths may follow existing faults for a while, depending primarily on (1) the dip of the fault (steep faults are the most likely to be used by vertically propagating hydrofractures), and (2) the tensile strength across the fault compared with the tensile strength of the host rock along a path following the direction of σ 1. The results suggest that hydrofractures may use faults as parts of their paths primarily if the fault is steeply dipping and with close to zero tensile strength.

In volcano-tectonic regions, dyke propagation from shallow magmatic chambers is often controlled by the interaction of the local and regional stress fields. The variations of the stress fields result from a combination of factors including the regional tectonic stress, the geometry of pressurized magma chambers, the layering and the pre-existing discontinuities (e.g. fractures). In this contribution, we describe and apply a new multiparametric inversion technique based on geomechanics that can invert for both the far field stress attributes and the internal pressure of magma chambers or stocks, constrained by observed dyke or eruptive fissure orientations. This technique is based on the superposition principle and uses linear elastic models that can be solved using many types of numerical methods. For practical reasons, we chose a 3D boundary element method (BEM) for a heterogeneous elastic half-space, where magma chambers are modelled as pressurized cavities. To verify this approach, the BEM solution has been validated against the known 3D analytical solution of a pressurized cylindrical cavity. Then the effectiveness of this technique and its practical use is demonstrated through application to natural examples of dyke network development around two different volcanic systems, the Spanish Peaks (USA) and the Galapagos Islands (Ecuador). Results demonstrate that regional stress characteristics as well as the internal pressure of magma chambers can be estimated from observed radial and circumferential dyke patterns and some knowledge of magma chamber geometry.

Mafic lamprophyre dykes in northern England were emplaced soon after the final closure of the Iapetus Ocean at the end of the Caledonian orogeny. High Mg/(Mg+Fe) ratios and Cr and Ni contents are consistent with equilibrium with mantle peridotite. Incompatible trace element abundances suggest that the mantle sources were metasomatized prior to the melting events. Three components are recognized in the lamprophyre chemistry: (i) a depleted mantle source, taken to be that overlying the subducting lithosphere; (ii) a H2O-rich subduction zone component, related to the dehydration of the subducting oceanic crust; and (iii) a CO2-rich phase thought to result from the degassing of the mantle after ocean closure. This multi-component origin may be applicable to other lamprophyres of calc-alkaline affinity.

Metanorite dykes intrude the Banded Gneiss Complex at various places in Rajasthan, N.W. India. They show neither chilled margins nor gradational contacts with the country rock amphibolite or granulite facies gneisses. They have ophitic to subophitic texture with strongly zoned subcalcic clinopyroxene and orthopyroxene, olivine and plagioclase, with subsidiary biotite. During slow cooling a series of reaction coronas developed with garnet forming round biotite, ilmenite and orthopyroxene; hornblende round pyroxenes and orthopyroxene, hornblende ± spinel round olivine, which may be totally replaced. It is inferred that the dykes crystallised from a tholeiitic magma at about 1100-1150 °C and were intruded during the waning stages of granulite facies metamorphism. The corona minerals grew at about 650–700 °C. A series of reactions to account for the development of the coronas is proposed using measured mineral compositions. Although these reactions do not balance for individual corona formation, metamorphism was probably isochemical with Ca, Na, K, Ti, Si and H2O only mobile on the scale of a thin section. Si and H2O were possibly mobile on a larger scale.

Allan Hills nunatak, south Victoria Land, Antarctica, exposes an exceptional example of a shallow depth (< 500 m) intrusive complex formed during the evolution of the Ferrar large igneous province (LIP). Dyke distribution, geometries and relationships allow reconstruction of its history and mechanics of intrusion. Sills interconnect across host sedimentary layers, and a swarm of parallel inclined dolerite sheets is intersected by a radiating dyke-array associated with remnants of a phreatomagmatic vent, where the dolerite is locally quenched and mixed to form peperite. Intrusion geometries, and lack of dominant rift-related structures in the country rock indicate that magma overpressure, local stresses between mutually interacting dykes and vertical variations of host rock mechanical properties controlled the intrusive process throughout the thick and otherwise undeformed pile of sedimentary rocks (Victoria Group). Dolerite sills connected to one another by inclined sheets are inferred to record the preferred mode of propagation for magma-carrying cracks that represent the shallow portions of the Ferrar LIP plumbing system.

The large Variscan Karkonosze Granite in the West Sudetes, representative of the vast Variscan granite plutonism in Central Europe and located adjacent to regional tectonic suture and strike-slip-zones, has been difficult to date precisely; a range of published data varies between c. 304 and 328 Ma. However, the granite is cut by locally numerous lamprophyre and other dykes. Dating of the dyke rocks, emplaced shortly after the granite intrusion and cooled more rapidly, provides a promising tool for the verification of published SHRIMP results on the granite itself. SHRIMP zircon geochronology of a studied micromonzodiorite dyke indicates substantial admixture of inherited zircons of c. 2.0, 1.4 Ga (207Pb–206Pb minimum ages), and c. 570 (and 500?) Ma. The average concordia age of the main magmatic population of the zircons in the dyke is 313 ± 3 Ma (2σ); however, the true magmatic age might be older, around 318 Ma. This would constrain the age of the hypabyssal magmatism in the Karkonosze Massif and the minimum age of the host Karkonosze Granite. Thus, the Karkonosze Granite is confirmed as representative of an early phase of Variscan granite plutonic activity in the central-European Variscides.

The Miocene igneous rocks in the Basal Unit of the Lavrion area form part of the granitoid province of the central Aegean. Undeformed, subvertical dykes of quartz-syenite to granodiorite and granite porphyries, and a little deformed but variably altered granodiorite stock intrude metamorphic rocks of the Basal Unit. A 9.4 ± 0.3 Ma K–Ar age on feldspar for a dyke rock provides a minimum age for the igneous activity in the Basal Unit. East–west orientation of porphyry dykes is indicative of a regional extensional stress field with roughly north–south direction. Substantial extension in the Basal Unit after granodiorite emplacement is evident from widespread quartz veining associated with hydrothermal alteration of the granodiorite and the occurrence of mineralized tension gashes cutting the hydrothermally altered hornfelses. Final emplacement of the Blueschist Unit over the Basal Unit by extensional detachment post-dates contact metamorphism of the rocks surrounding the granodiorite. Geochemical diagrams show a continuous range of compositions from the dykes to the granodiorite. Radiogenic isotope compositions are compatible with a common magmatic source for the two lithologies. Elemental variations, as well as the considerable geochemical similarity of the dyke rocks to the Hercynian paragneiss of the central Cyclades, indicate that crustal melts were significant components during the evolution of the igneous rocks with fractional crystallization as an important process during later stages of evolution. The granodiorite displays geochemical signatures indicative of a significant mafic mantle-derived magma component.

In one of the higher nappes of the Caledonides of Troms, northern Norway, rare thin dykes of calcite marble lie subparallel to the axial surfaces of folds in multilayered marble beds. The dykes emanate from pure calcite marble layers mostly in or close to the hinge zones of these flat-lying, tight to sub-isoclinal, F3 folds. In many cases, they appear to penetrate along the axial surfaces of these same folds, and there is an up to 10° angular disparity between dyke orientation and axial surfaces. The dykes are considered to have originated, and been expelled, from parent calcite marble layers either at or shortly after the peak of amphibolite-facies metamorphism between the D2 and D3 deformation phases of the Scandian orogeny. The dykes display a dyke-parallel, dark grey to white, colour banding, the origin of which is uncertain at present. In thin-section, an equigranular texture is dominated by 2–3 mm, equant, unstrained, calcite grains with straight boundaries and low-T twin lamellae, denoting a late-stage recrystallization. A weak, oblique, calcite grain-shape orientation fabric curves into a dyke-parallel alignment along the finer-grained margins, reflecting the simple-shear deformation imposed on the host rock. Regarding the initiation of these marble dykes, it seems likely that a combination of P–T conditions and possibly partial melting processes was conducive to generating extreme ductility/superplasticity, at least locally, and that this led to the sudden expulsion of a partially molten carbonate aggregate which had momentarily attained a critical pressure and intruded preferentially along available, transient fractures prior to the imposition of the D3 simple-shear regime.

The metamorphic sole rocks observed between the Pozantı–Karsantı ophiolite and the melange unit are located on the eastern part of the Tauride carbonate platform. They consist of ortho-amphibolites at the top and metasedimentary lithologies at the base. Amphibolites from the metamorphic sole rocks are represented by OIB, MORB and IAT based on their major, trace and REE compositions. The isolated dolerite dykes intrude both the metamorphic sole rocks and the ophiolitic unit at different structural levels. The dolerite dykes cutting the metamorphic sole rocks are enriched in LILE and depleted in HFSE. Enrichment in LILE such as Th, relative to HFSE, is indicative of the presence of a subduction component. Flat-lying REE patterns of the dolerite dykes also confirm an IAT source. Pyroxenite and albitite dykes also cut the metamorphic sole rocks. REE patterns of pyroxenite dykes show prominent LREE enrichment, similar to that observed in within-plate alkaline basalts. The alkaline isolated pyroxenite dykes were probably the result of late-stage magmatism fed by melts that originated from an OIB source, shortly before the emplacement of the Pozantı–Karsantı ophiolite onto the Tauride carbonate platform. A hydrothermal alteration stage is characterized by albitite formation in the joints of the metamorphic sole rocks and by secondary mineralization along the contact zones of dolerite dykes. Mineral parageneses in the metamorphic sole rocks exhibit amphibolite and greenschist-facies assemblages. Geothermobarometric studies based on a newly recognized mineral assemblage (e.g. kyanite) and chemical compositions of minerals in the metamorphic sole rocks indicate that the metamorphic temperature during the metamorphism was 570–580°C and the pressure was around 5–6 kbar.

As an example of microstructural and magnetic fabric evolution, and geochemistry of mafic dykes during a subsequent orogenic overprint, a major Mesoproterozoic dyke complex in Scandinavia, the Västerbotten complex of the Central Scandinavian Dolerite Group, is traced westwards into the crystalline nappes of the early Phanerozoic Caledonian orogen. Using geophysical, field, microscopic, magnetic and geochemical information, dykes and sills are characterized, and their overprint during Caledonian orogeny documented. The Västerbotten complex is composed of sets of dykes, trending NE–SW, NW–SE and WNW–ESE, respectively. Similar dykes are exposed in allochthonous positions (Lower and Middle allochthons) in the Caledonian fold-and-thrust belt. The autochthonous dykes are generally undeformed and retain both their primary texture and mineralogy. Chilled margins are well preserved. In the Caledonian Lower and Middle allochthons, similar dykes in crystalline basement rocks are progressively faulted and sheared when proceeding from the marginal to the interior parts of the orogen. Dyke margins are more likely to be sheared than the interior parts of dykes. In the Lower Allochthon, under very low- and low-grade metamorphic conditions, dykes are distinctly less competent than granitic host rocks. Thick dykes are more competent than gneisses; thin dykes do not show such competence contrasts. In the Middle Allochthon, metre-scale dykes with patches of altered plagioclase phenocrysts can still be discerned in low-strain domains. Highly sheared dykes are drawn out to thin layers of centimetre thickness. Dykes are deformed together with the crystalline country rocks under greenschist-grade metamorphic conditions without major competence contrasts. Magnetic fabrics show an evolution similar to the silicate mineral fabrics. The magnetic fabrics in the dykes are transformed successively from ferromagnetic–magmatic in the Autochthon to ferromagnetic deformational in the Lower Allochthon and, finally, paramagnetic deformational in the Middle Allochthon. As a consequence, the magnetic susceptibility decreases for several orders of magnitude. Geochemically, the dykes are dominantly sub-alkaline basalts typical for continental tholeiites and can be distinguished from the Neoproterozoic dykes in the Särv-Nappe equivalents (highest part of the Middle Allochthon), which show a more MORB-like (E-MORB) magmatic signature. Preliminary age information from a dyke in the Lower Allochthon of the Børgefjell area and the Middle Allochthon is consistent with the assumption that these dykes are time equivalent with the Central Scandinavian Dolerite Group. Therefore, the studied dykes may represent an extension of the Västerbotten complex or a new complex of the Central Scandinavian Dolerite Group. According to section restorations, the Caledonian allochthons were situated further WNW relative to their present position, and, originally, the mafic dykes cut across all of the Fennoscandian lithosphere, at least to the present Atlantic margin and the earlier passive margin of the Baltica terrane. As a consequence, these dykes may provide a link for pre-Caledonian and pre-Grenvillian plate reconstructions.

Diapirism has been discredited as a transport mechanism for magmas partly because diapirs seem to be unable to bring magmas to shallow crustal levels (<10km) and partly because recent developments in the theory of dyke propagation have shown that sufficiently wide dykes are able to efficiently transport felsic magmas through the crust. However, it is still unclear how felsic dykes grow to widths that allow them to propagate faster than they close by magma freezing. Ultimately, it may be the ability of felsic dykes to grow within the source that controls which mechanism dominates ascent.

The ability of dykes to propagate, from the top of rising diapirs depends among other factors on the changing temperature gradient of the wall rocks. The steep gradient around rapidly rising diapirs in the low viscosity lower crust will cause dykes to freeze. As diapirs rise to colder stiffer crust and decelerate, heat diffuses further from the diapir, resulting in shallower temperature gradients that favour dyke propagation. The mechanism may thus swap, during ascent, from diapirism to dyking. Calculations of the thermal evolution of diapirs and their surroundings show that basaltic diapirs may never form because they would be drained by dykes at a very early stage; felsic diapirs may be unable to give rise to successful dykes, whereas diapirs of intermediate magmas may propagate dykes during ascent.

A review of granite emplacement mechanisms in transcurrent, extensional and contractional (thrust sense) shear zones reveals that in all three tectonic settings the plutons have been constructed by multiple granite sheeting parallel to the shear zone walls and deformation fabrics. The sheets and plutons are non-Andersonian in type and were emplaced obliquely to the principal stress directions. Their shape and orientation is more likely to reflect the exploitation of faults and shear zones which were active during emplacement. Sheeting (dyking) is therefore also likely to be the mechanism of ascent along fault zones in the crust.