D. H. W. Hutton

Granite emplacement mechanisms and tectonic controls: inferences from deformation studies

This paper is a structural and tectonic approach to the emplacement and deformation of granitoids. The main methods available in structural geology are briefly reviewed and this emphasises that (a) a wealth of data, particularly strain and shear sense, which pertain to these problems, can be determined in and around plutons; (b) given the nature, unlike many other crustal rock types, of granites to crystallise from isotropic through weakly anisotropic crystal suspension fluids, that deformation which has occurred in these states may not be well preserved; and (c) it is entirely possible, using this methodology, to separate deformation resulting from externally originating tectonic stresses from that which is associated with internal magma-related stresses. It is also recommended that the genetically-based Cloosian classification of granite fabrics and structures into “primary” (magmatic flow/magmatic flow current) and “secondary”, be abandoned and that a more observationally-based approach which classifies granite deformation fabrics and structures according to their time of occurrence relative to the crystallisation state of the congealing magma, be adopted (i.e. pre-full crystallisation deformation and crystal plastic strain deformation). Examples of recent, structurally based, studies of emplacement mechanisms of plutons within tectonic settings are described and these show that, in general, space for magma can be created by the combination of tectonically-created cavities and internal magma-related buoyancy. This occurs in both transcurrent and extensional tectonic settings and there is no reason to doubt that it can happen in compressive-contractional regimes. It is concluded that transient and permanent space creation, such as may be exploited by available magmas, is a typical feature of the tectonically stressed and deforming lithosphere and this, in combination with the natural buoyancy and ascending tendency of magmas, can generate the varied emplacement mechanisms of granites.

Strike-slip terranes and a model for the evolution of the British and Irish Caledonides

Evidence is presented that many of the major strike faults in the British and Irish Caledonides were active as sinistral strike-slip zones in the end-Silurian to pre-mid-Devonian period. Some, such as the Highland Boundary Fault, moved in this way at an earlier stage in the Ordovician. These data allow the Caledonian rocks lying between the Laurentian miogeocline (whose basement is represented by the Lewisian, Moine and possibly the Dalradian) and the Gondwanaland miogeocline (Midland Platform and Welsh Basin) to be re-analysed as a group of disorganized terranes which originated to the southwest in North America and southwest Europe/Africa prior to the Silurian. The Highland Border Terrane and Northern Belt Terrane are interpreted as duplicated pieces of a mid-Ordovician sequence which was a back are to northwest subduction. The Midland Valley Terrane is interpreted as a slice of Laurentian foreland onto which ophiolites were obducted in the lower Ordovician but which became the basement of a continental margin arc to northwest subduction in the mid-Ordovician. The Cockburnland Terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip. The Connemara Terrane is regarded as an allochthonous piece of the Dalradian miogeocline and the South Mayo Terrane as a remnant of an early Ordovician arc and fore arc which in mid-Ordovician times became a back arc/marginal basin to northwest subduction. The Lake District-Wexford Terrane is part of an arc to southeast subduction under Gondwanaland whose activity climaxed in the mid-Ordovician. The Central Terrane is interpreted as a Silurian overstep assemblage which blankets the junction between Laurentian- and Gondwanaland-derived oceanic terranes, and therefore Iapetus is regarded as an Ordovician ocean which closed prior to the Silurian. The model suggests that at the end of the Silurian, a clockwise-rotating Gondwanaland, having carried Laurentia into collision with Baltica, broke free and created a major sinistral strike-slip zone which disrupted the Ordovician palaeogeography in the British Isles/North American sector of Iapetus.

The Great Tonalite Sill: Emplacement into a contractional shear zone and implications for Late Cretaceous to early Eocene tectonics in southeastern Alaska and British Columbia

The Late Cretaceous to early Tertiary Great Tonalite Sill of southeast Alaska and British Columbia is a very long (∼1,000 km) and thin (<25 km), orogen-parallel, composite batholith, which lies between the Insular superter- rane (including the Alexander and Wrangellia terranes) and the Intermontane superterrane (including the Stikine and Cache Creek terranes). The batholith is delineated by many steep, sheet-like plutons, which are dominated by northwest- southeast-striking concordant fabrics with steep lineations that formed during deformation in a country-rock shear zone prior to the complete crystallization of the magmas. Deformation in this shear zone is dominated by northeast- southwest-directed contraction orthogonal to the orogenic strike, associated with a component of northeast over southwest, high-angle shear. The steep, multiple-dike-like nature of the body and its emplacement during orogenic contraction imply that ascent and emplacement have been achieved by dike-wedging mechanisms along the deep-reaching shear zone. The remarkable narrowness of the Great Tonalite Sill is probably the result of melting at the base of a very localized zone of thickened crust produced by the associated narrow contractional shear zone extending along the orogen length. Such a shear zone of Late Cretaceous to early Tertiary age, lying along 800 km of the possible boundary between the Insular and Intermontane superterranes, implies that it may represent the actual boundary between them. If this hypothesis is correct, it implies that the large-scale tectonic regime during emplacement of the Great Tonalite Sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data.

Igneous emplacement in a shear-zone termination: The biotite granite at Strontian, Scotland

Deformed xenolith (strain) data together with other structural information indicate that the biotite granite body (∼90 sq km) of the Strontian complex, Scotland, was emplaced in the extensional termination of a dextral transcurrent shear zone. This shear zone is a splay of the major Great Glen fault which lies along the southern boundary of the granite. Siting of the shear zone splay was probably controlled by (a) a slight releasing bend in the Great Glen fault in this area and (b) a large, pre-existing, asymetric synform in Proterozoic metasedimentary country rocks which intersects the Great Glen fault trace at a high angle. A model is proposed in which Moine Thrust (Caledonian) compression at ca. 435 Ma activated the Great Glen fault dextrally. Dextral movements around the releasing bend detached a flat segment from the inside fault wall, and the biotite granite was emplaced side-ways at depths of about 15 km as a sheet into this extensional, listric-fault-bounded, cavity.

The Silurian of the Southern Uplands and Ireland as a successor basin to the end-Ordovician closure of Iapetus

A suite of microconglomerates is recognized in Silurian rocks which occur on both sides of the proposed line of the Iapetus suture in Ireland. Clast composition and palaeocurrent data show that these conglomerates, which grade into the typical quartz-rich Silurian turbidites, were derived from two compositionally similar magmatic arc terranes which lay on either side of the present Silurian outcrop. In the Llandovery, derivation was from both the south and the north. In the Wenlock, derivation was from the north and sedimentation prograded southwards across the ‘suture’ and onto the southern margin. The source terrain in the south was probably the Ordovician Wexford–Lake District arc. We identify the northern source as another arc (Cockburnland) which has since been cut out by sinistral strike-slip against the Ordovician Northern Belt. These data imply that arc activity ceased synchronously on either side of Iapetus during the late Ordovician and this leads us to speculate that subduction of oceanic crust ended at that time. Closure was associated with deformation and uplift of the bounding Ordovician terrains. These rocks then contributed detritus to the Silurian infill of a successor basin. Regional sinistral transpression finally deformed and reorganized these units between the end Silurian and the early Devonian and led to the complete closure of the remaining Silurian seaway.

Syntectonic Granites and the Principle of Effective Stress: A General Solution to the Space Problem?

Syntectonic granite emplacement in dip slip and strike slip contractional shear zones is now well documented by a number of case histories including the spectacular 1200 km long, 20 km wide Great Tonalite Sill of North America. These examples show fundamentally that magma driving forces can overcome compressional tectonic stresses and suggest that in a contractional orogen it is not in general a necessity to make space for plutons by localised dilation along faults and shear zones. There are a number of other magma driving forces that are available, in addition to buoyancy, which can combine to exceed the tectonic compression, including one which is derived from the compression itself. These extra forces are optimised when ascent and emplacement is achieved along major transcrustal faults and lineaments. The principle of effective stress is applied to the general case of granitic magmas in crust undergoing tectonic horizontal compressive stress and it is argued that the magma pressure is an indistinguishable part of the regional (effective) stress field. This allows a general solution to the space problem in granite emplacement since in the lower crust, where the host rocks are weak, the emplacement strain will also become an indistinguishable part of the regional deformation field. This principle is also likely to underlie space creation mechanisms in the often highly heterogeneous middle crust, where it may be obscured by the observed opportunistic exploitation of dilation along tectonic structures and other weaknesses.

The Antrim-Galway Line; a resolution of the Highland Border Fault enigma of the Caledonides of Britain and Ireland

The westward continuation of the Highland Border fault of Scotland (HBFZ) into Ireland is problematic. It is widely thought to follow a pronounced magnetic and gravity lineament, the Fair Head-Clew Bay Line (FCL). The advantage of this interpretation is that it places all the Ordovician ophiolitic complexes and associated sedimentary basins to the south of the FCL, which would represent the contact between Laurentia and the outboard terranes. Its main shortcomings are that both the deep structure and timing of strike-slip are different on the HBFZ and FCL. In Ireland the FCL is a north-dipping feature that can be traced to the Moho on BIRPS profiles, while the HBFZ has no such signature. Terrane amalgamation in western Ireland was completed by the late Ordovician, while in Scotland the Midland Valley terrane did not finally dock until the early Devonian. These considerations suggest that in western Ireland a branch of the HBFZ exists, which was active in post-Ordovician time and must lie south of Connemara. An examination of Irish geological, geophysical and image-processed magnetic data shows that a profound lineament can be traced from Antrim to Galway Bay (the Antrim–Galway Line). Stitching plutons date movement on it as pre-405 Ma. We propose that the Antrim–Galway Line represents the continuation of the Scottish HBFZ, while the FCL is a preserved Ordovician splay of the HBFZ system whose northdipping geometry is a product of Ordovician collapse of the orogen in western Ireland.

Silurian and Early Devonian sinistral deformation of the Ratagain granite, Scotland: constraints on the age of Caledonian movements on the Great Glen fault system

The Ratagain granite complex (425 ± 3 Ma) was emplaced adjacent to the Strathconan Fault, an important member of the Great Glen fault system, and was deformed by sinistrpl shear associated with this fault just prior to its complete crystallization. It was deformed again by low temperature sinistrral mylonite and breccia zones penecontemporaneously with the regional minette dyke swam (410-395 Ma). Very little evidence is seen in the complex of deformation in the high temperature/post-crystsllization intend, and it is concluded that Caledonian sinistral motion on the Strathconan Fault occurred in two distinct phases: Mid-Siurian (Wenlock) and Early Devonian.

The Caledonian strike-swing and associated lineaments in NW Ireland and adjacent areas : sedimentation, deformation and igneous intrusion patterns

Part of the major swing in strike associated with the Greenland-Labrador Promontory of the North Atlantic Caledonides is exposed in considerable detail in the metasedimentary and meta-igneous rocks of the Neo-Proterozoic Dalradian Supergroup in northwest Ireland. We show that the strike swing controlled Dalradian stratigraphy and sedimentation patterns and is therefore at least c. 600 Ma in age. Our interpretation of the stratigraphy allows a reconstruction of this promontory in the Laurentian continental margin and shows that it separated a NE-trending. gently inclined and broad continental shelf in the north from an E-trending. more steeply inclined and narrower shelf to the south. Once established in the geometry of the Dalradian sequence, the strike swing and basement promontory had a major influence on Caledonian deformation patterns. It controlled: the nucleation of major folds: the geometry of the regionally developed primary cleavages; the facing direction of the early folds: and it also acted as a major buttress in perturbing and deflecting. on a regional scale, the orogenic transport vector. Accurate restoration of the sinistral displacement on a late Caledonian fault system shows that the apices of the strike swing at different stratigraphic levels, and their associated sedimentary facies changes, define a line trending approximately N12ºE. This line is coincident with (again, after fault restoration): six out of the eight Devonian Donegal granites: the vast majority of the mantle-related, ultrabasic ‘Appinite’ suite (with most of these bodies concentrated at the intersection of this line and the NE-trending Main Donegal Granite shear zone): and the highest density of the Neo-Proterozoic basic tholeiite suite. Thes e and other data are used to suggest that the Donegal Lineament is the expression of a major steeply inclined fault in the basement beneath the Dalradian which is intimately related to the morphology of the promontory. The fault, which might be as old as 1800 Ma, probably reached down into the lithospheric mantle. controlling igneous activity, including the ascent, emplacement site and perhaps sources of the magmas. Another complementary strike swing and parallel lineament is described from the Dalradian rocks of the adjacent part of Scotland. It is suggested that these and other major lineaments (i.e old pre-Caledonian faults) which have either NNE. NE, or ESE trends reflect not so much the location of the northern British Isles at a 120º triple junction during Iapetus rifting. but rather the exploitation of these pre-existing trends in the pre-Caledonian Atlantic borderlands during Iapetus opening.

Major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones

The dominant structure controlling the disposition of Dalradian stratigraphy in mid-Ulster has hitherto been regarded as a southeast-facing gently inclined F 1 anticline, a gross geometry modelled on, and thought to be a possible correlative of, the Tay Nappe in Scotland. Remapping of the supposedly inverted southern limb of this major fold reveals that much of it is in fact the correct way up. However, a stratigraphie repetition coupled with a reversal in younging does occur in the Sperrin Mountains, much further south than previously realized. This hitherto unrecognized upward southeast-facing isoclinal Sperrin Nappe is, however, a D 2 structure, traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km 2 . Following D 2 , a major 10 km thick D 3 ductile shear zone resulted in translation towards the east-southeast. In the south, this deformation carried the Dalradian over Ordovician volcanics of the Tyrone Igneous Complex along the Omagh Thrust. Penecontemporaneity of magmatism with deformation clearly demonstrates that D 3 is Caledonian (Arenig-Llanvirn). This deformation correlates with similar southeast-directed Caledonian thrusting in southern Donegal and Connemara. The apparent absence of Dalradian deformation of this age in southwestern Scotland may imply that Caledonian collision of outboard terranes with the miogeoclinal margin was initiated in Ireland and/or subsequent strike-slip has removed the evidence for deformation of this age from southwestern Scotland. The D 3 shear zone in the Sperrin Mountains affects a very large volume of psammitic rocks. Within this shear zone the strain is not markedly higher than surrounding areas; however, its existence is demonstrated by the reorientation of mineral lineations over a large area. Such broad zones of only moderate strain may, we believe, be typical of translatory tectonics in areas of the mid-crust where there is little lithological diversity.