Metamorphic Terranes in NW Scotland

[Map from Baba et al., 1998]                                   [Map from Spear, 1993]

ITAR is undertaking a field program in western Scotland starting in 2012, as part of an ongoing research strategy to [1] investigate a broad range of ancient and modern terranes (i.e. oceanic plateau at Galapagos [1] and Andes magmatic arc) from a variety of tectonic settings for comparison with the orogenic systems being investigated in the Yilgarn Craton [2], Himalayas [3] and Pan-African Orogenic System [4] and [2] further develop the utilization of boudin structures as a tool kit to unravel the history of orogenic belts. Archaean gneisses of the Scourian Complex are compared with the high-grade gneissic and granulite terranes in the Southwest Terrane of the Yilgarn Craton. Thermobarometric and deformation effects on Lewisian basement due to reworking by the Caledonian Orogeny, can be compared with reworking of the Yilgarn Craton margins by the Gascoyne, Pinjarra and Albany Fraser Orogens. Metamorphic field gradients across the Caledonian Orogen can be compared with those in the Damara Orogen [5], for which the two orogenic systems superficially share many features in common.

Structural Style and metamorphism of the Lewisian Complex, NW Scotland

Ben Goscombe, March, 2014. Unpublished ITAR Report.

Introduction

The structural style of the Lewisian Complex in NW Scotland was investigated for comparison to the similarly high-grade gneisses of the Southwest Terrane in the Yilgarn Craton. The Southwest Terrane is very poorly exposed and consequently the kinematics, strain and stress history unknown and its structural style has not been characterized. The Southwest Terrane and Lewisian Complex share superficial similarities and because the Lewisian Complex is very well exposed it offers insights into a structural style that may be shared in common with other Neoarchaean high-grade gneissic terranes.

The superficial similarities between these two terranes are simply: Both are composed of predominantly magmatic protoliths of Neoarchaean age of approximately 2700-2800 Ma. The Isle of Lewis and North Harris are composed entirely of meta-igneous rocks. The Isles of South Harris and Uist are predominantly meta-igneous with paragneiss units in the Langavat and Leverburgh Belts, and a Mesoproterozoic (Laxfordian) igneous complex. Both share similar proportions of mafic (meta-dolerite, meta-gabbro, rare ultramafic) to felsic (granite, granodiorite, diorite, tonolite and anorthosite) magmatic protoliths, with approximately 10-30% mafics. This ratio of bimodal crust is common in Archaean granite-greenstone terranes worldwide and suggests that these terranes may have had origins as typical granite-greenstone terranes that were later highly modified by deformation and metamorphism to their current gneissic and strongly stretched nature. Both terranes have a highly distributed lithological pattern with mafic rocks widely distributed as isolated lenses and disaggregated layers. Any large-scale structures such as greenstone belts, if they ever existed, have been totally obliterated by the high strains experienced. This resulting in a seemingly homogeneous distribution of mafic rock units and lenses scattered throughout the entire rock mass. Both terranes are migmatitic, coarse-grained gneissic terranes of upper-amphibolite to granulite facies grades. Only moderate pressure parageneses are developed and the absence of garnet in mafic reworking parageneses indicate that current exposure levels in both terranes were buried no deeper than 8 kb. These simple constraints indicate that these terranes are of high-T / moderate-P type with high T/depth ratios >35 ºC/km. Consequently, these terranes did not experience continent-continent collision and crustal over-thickening and the levels exposed were never in the lower crust. Furthermore, the high proportion of granitoids (70-90%) that pool in the middle to upper crust also precludes these rocks being from the lower crust.

The structural character of the Southwest Terrane was developed in the Neoarchaean. Whereas deformation of the Lewisian Complex is less constrained and main phase deformation may have been in the late Neoarchaean or Mesoproterozoic. Metamorphism of the Neoarchaean Lewisian Complex has been dated by metamorphic zircons at 2485±5 Ma and mafic intrusives have zircon ages of 2418 Ma (Love et al., 2004; Hollis et al., 2006). Main phase reworking may have accompanied late Neoarchaean metamorphism or possibly during Mesoproterozoic Laxfordian Orogeny. The South Harris Igneous Complex was emplaced at 1950-2070 Ma, then metamorphosed and moderately deformed, indicating that the Laxfordian Orogeny was younger than 1950 Ma in the Mesoproterozoic. Deformation strain in the South Harris Igneous Complex was less than in the Lewisian rocks, indicating either strain partitioning or main phase deformation of the Lewisian Complex was older than 2070 Ma, possibly late Neoarchaean.

To further explore these commonalities, deformation style in the Lewisian Complex was characterized by structural mapping of the Isles of Lewis and Harris in July-October 2012. Structural mapping was undertaken by modification of the method developed by Blewett and Czarnota (2007c). In this extremely high strain and very ductile terrane, boudin structures have been primarily used to characterise deformation style. Including the following components: (1) the temporal history of multiple deformation events, (2) the orientation of the strain ellipsoid with comparison with the main phase L-S fabric, (3) the shape of the strain ellipsoid constraining the strain type as highly flattening, (4) the bulk strain experienced by both the degree of separation of boudins and the reduction in layer thickness due to ductile stretch, and (5) the kinematic shear sense indicated by asymmetric boudin structures (Goscombe and Passchier, 2003; Goscombe et al., 2004a).

A simple suite of only four linear components has been used to characterize deformation, and these are plotted on a map of the Isles of Lewis and Harris. These are: (1) long axis of boudin strips aligned parallel to the intermediate extension direction (Y) and orthogonal to the maximum extension direction (X), (2) stretching lineation direction indicated by aligned minerals and mineral aggregates defining an independent maximum extension direction (X), (3) The orientation of rare isolated lenticular boudin lenses that have been additionally stretched parallel to the maximum extension direction (X), and (4) the transport of the hanging wall indicated by domino and shearband boudinaged mafic layers. The map is also annotated with rough estimates of bulk strain utilizing boudin extension and layer thickness reduction by ductile drawn boudinage. This highly simplified rationalization of third-order structural complexities to a small number of linear features is considered suitable in this terrane. This is because the entire region represents a sub-horizontal to shallow south-dipping sheet that has been bucked by medium-scale upright folds (D4) that can be statistically ignored for the large-scale patterns. Furthermore, planar surfaces are highly influenced by the form of mesoscopic boudin structures, leading to third-order complexity that averages out to a near recumbent sheet. Across the 2,200 km² area documented there is remarkable consistency in structural style, strain type, bulk strain, kinematics and orientation of the strain ellipsoid. These structural patterns and the main phase deformation history is described below.

D1: Main Phase Extensional Stretching and Vertical Flattening

The already bimodal mafic-granitoid crust was over-printed by extremely high-strain deformation at regional scale at high metamorphic grades, producing a coarse-grained pervasive ductile L-S fabric. The foliation formed by considerable ductile grain refinement and flaser fabrics dominate in the granitoids producing augen enveloped by a quartz and feldspar aggregate ribbons. Mafic rocks are also strongly deformed and produced medium-grained grainshape L-S fabrics. These shear fabrics have been annealed at high-grade conditions to polygonal granoblastic textures. Quartzo-feldspathic orthogneiss was been migmatized producing stromatic segregations and developed gneissic banding prior to high-strain deformational reworking. Stretching lineations are defined by aligned mineral grainshapes (hornblende) and mineral aggregates (biotite streaks and quartz and feldspar aggregate ribbons). Stretching lineations trend NNW-SSE. Shear sense indicated by domino and shearband boudinage geometries indicate transport is consistently top to the NNW.

These migmatized felsic rocks developed biotite-quartz-feldspar assemblages devoid of muscovite, garnet, alumino-silicates, cordierite and orthopyroxene and so do not constrain metamorphic conditions apart from forming at temperatures greater than 750 ºC. Mafic rocks develop grainshape gneissic fabrics with medium to coarse-grained biotite-amphibolite-plagioclase assemblages, and pyroxene-hornblende-plagioclase-biotite mafic assemblages are less common. Garnet has not been identified in the reworking parageneses, indicating pressures <8kb at temperatures >750 ºC. Partial melt is rarely developed in mafic gneiss where boudin lenses are internally stretched producing small-scale irregular dilations filled with quartz-plagioclase segregations. Anticlockwise P-T paths have been described in literature, suggesting reworking was extensional. Early garnet porphyroclasts in sheared amphibolite are extremely rare, these are 1-2 cm diameter pre-kinematic porphyroclasts that have been rounded and enveloped by the proto-mylonitic fabric. Garnets in low-strain domains are enclosed by thin plagioclase coronas indicating decompression before main phase reworking.

PT conditions reported in literature from Lewisian Gneiss on South Harris are 790±40 ºC, 10.8±1.0 kb and 21.0±2 ºC/km (Love et al., 2004), 825 ºC, 13.0 kb and 18.1 ºC/km (Cliff et al., 1983) and 955 ºC, 10.0 kb and 27.3 ºC/km (Friend and Kinny, 2001; Mason et al., 2004; Hollis et al., 2006). Lewisian Gneiss in NW Scotland formed at 840 ºC, 8.5 kb and 28.2 ºC/km at Scourie (Sills and Rollinson, 1987) and 1068 ºC, 13.0 kb and 23.8 ºC/km in the Assynt Terrane (Love et al., 2004; Hollis et al., 2006). Peak metamorphic conditions and semi-quantitative P-T paths for garnet-hornblende-plagioclase-ilmenite±clinopyroxene±phlogopite amphibolite samples BG12-54 and BG12-56 from North Harris, have been determined by ITAR using Thermocalc v3.5 (Powell & Holland, 1988). Peak metamorphic conditions are 785±87 ºC, 9.0±1.4 kb and 684±122 ºC, 9.3±1.7 kb for the two samples, indicating T/depth ratios of 21 to 25 ºC/km. Post-peak conditions calculated from plagioclase moats and mineral rims indicate decompression through approximately 666 ºC and 7.3 kb. These PT determinations and the sequence of mineral growth interpreted within the Mahan (2008) NCFMAST PT pseudosection, suggest both anticlockwise and clockwise P-T paths.

At both outcrop and larger scales the entire terrane appears to be transposed with no original layering or primary intrusive contacts preserved. Early folding associated with the penetrative L-S fabric are small rootless and wispy isoclines and rare sheaths. All mafic rock units are highly dismembered and are now predominantly widely separated boudin strips with lensoid cross-sections, three-dimensional lenticular pods and a smaller proportion that are boudinaged near contiguous sheets. Mafic layers and lenses range in thickness from cm-scales to at least 10 m thickness, with most 1-4 m thick. These geometries indicate that original layering in the terrane has been strongly dismembered and the homogenous distribution of mafics throughout the granitoid rock mass indicates that the terrane has been strongly transposed. Large map-scale units of mafic rock do not occur and any large-scale lithological structure such as greenstone belts have not been preserved. Ultramafic boudin lenses are rare and form boudins and pods with more circular to oval shapes.

The Langavat and Leverburgh Belts are thin 1-2 km wide slivers of Lewisian Gneiss on the margin and interleaved with the Laxfordian Harris Igneous Complex in SW South Harris. Both belts have a similar suite of meta-igneous rocks to elsewhere in the Lewisian Gneiss in Harris and Lewis Isles, such as dolerite, gabbro, granite, granodiorite and granite. However, unlike Lewisian Gneiss elsewhere, the Leverburgh Belt also contains thin dm- to m-scale supracrustal layers. Para-amphibolites have garnet-hornblende-biotite-plagioclase assemblages, impure marbles contain probable wollastonite and grossular and metapelites are coarse garnet-biotite±kyanite schists. Most significantly, this belt contrasts with all Lewisian Gneiss elsewhere on Harris and Lewis Isles by having developed metamorphic parageneses indicating deep burial into the eclogite field. Meta-gabbros have mineral textures preserving the eclogitization process at different stages. These gabbros contain irregular veins and segregation consisting of garnet-plagioclase assemblages. Where proportion of veining is high, these represent an irregular network of bleaching very similar to charnockite networks in Southern India. The gabbros also have late garnet porphyroblasts growing throughout the rock mass. These garnets are irregularly distributed and independent of primary rock variation. Furthermore, these garnets range up to 4 cm diameter and grow across the earlier gneissic fabric. In very rare cases true eclogite is developed in small irregular shaped dm-scale patches. These eclogites are medium to coarse-grained, devoid of plagioclase, predominantly pale green clinopyroxene with ~20-30% garnet up to 1cm across.

The whole SW margin of South Harris is a 7 km wide sub-vertical zone that contains the Langavat and Leverburgh Belts and Harris Igneous Complex. Deformation fabric in the Langavat Belt trends NW-SE, dips ~80º to the SW and oblique stretching lineations plunge steeply to the west. The Leverburgh Belt trends NW-SE, is sub-vertical and stretching lineations are sub-vertical. Boudin neck axes are sub-horizontal and trend NW-SE. Both belts develop deformation fabrics identical to elsewhere in the Lewisian Gneiss, with strongly sheared L-S gneissic fabrics. The eclogitic metamorphic parageneses indicate that the SW South Harris zone was buried very deeply and later exhumed in this probable sub-vertical upthrust zone.

There are three styles of boudinaged mafic layers developed during main phase deformation, accompanying the development of the regional pervasive L-S gneissic fabric and peak metamorphic parageneses.

[1] Boudin Style 1

The predominant boudin style is highly ductile drawn boudin strips with tapered lentil shaped cross-sections. These boudin strips are strongly tapered with very thin and long tails and the boudin strips are widely separated. The maximum extension direction of the strain ellipsoid (X) is approximately parallel to the stretching lineation, and both are orthogonal to the length of the boudin strips within the plane of the gneissic fabric. Throughout the whole mapped area the trend of boudin strips is predominantly E-W and all lie between ENE-WSW and ESE-WNW. The tapered shapes and wide separation indicate extremely high strains at ductile high temperature conditions.

The degree of separation is mostly beyond the scale of outcrops and hard to document. Where documented, boudin separation (M) with respect to boudin length (L) indicates layer stretch [ X=(M+L)/L ] ranging 260% to 1430% (n=23) and most averaging 477%. Separation of large-scale boudins indicate strain ratios [ R=X/Z=((M+L)/L)/(1/((M+L)/L)) ] ranging 6.8 to 205 and most averaging 25.5. In addition, the thinning of layers is extreme; from near original layer thickness preserved by the boudin block (W) to very thin layer thickness in the highly stretched tails (W'). Outcrops indicate that layer thinning [ 1/Z=((W-W')+W')/W'=W/W' ] ranges 500% to 7500% (n=13) and averages 3000%, though many layers are thinned to no connecting tail at all.

Boudin strips are also boudinaged along their length indicating ductile "chocolate block" boudinage, forming lentil-shaped sub-boudin blocks. Boudinage in this second direction indicates stretch along the intermediate extension direction (Y) and flattening rather than constrictional strain type. Extension along the Y-axis is also mostly beyond the scale of outcrop and individual structures possibly show similar orders of stretch and layer thinning as documented along the X-axis. Extreme stretch along both the X-axis and Y-axis, and extreme shortening along the Z-axis, indicate very high flattening strains and elongate pancake shaped strain ellipsoids. The documented layer stretch parallel to X-axis and layer thinning parallel to Z-axis can be used to calculate the maximum Flynn ratio [ k=(X/Y)/(Y/Z) ], assuming Y=1. Maximum Flynn ratios range 0.10 to 0.67 and average 0.26, indicting highly oblate strain ellipsoids.

Boudinage was an ongoing process throughout the main phase deformational period. Earlier gneissic layering is disrupted and rotated by boudin blocks, which are themselves enveloped by the main phase gneissic fabrics. Boudinage, with no perceptual change in style, continues after the D2 episode of mafic sill intrusions, and accompanies further flattening and stretch during D3 deformation period (see below). Where documented, stretch during D3 deformation is lower than during D2, with maximum stretch of 400% and maximum thinning of 2000% documented. Extension of boudin strips along the Y-axis did not occur during D3.

[2] Boudin Style 2

Extension in two directions has resulted in discrete sub-boudin blocks (lenses) with three-dimensional tapered lenticular shapes. In rare cases, where outcrop allows, these sub-boudin lenses are internally deformed and stretched in the direction of the X-axis leading to elongate lenticular shapes aligned with the regional stretching lineation. Strain is so high that the regionally pervasive stretching lineation is also developed within boudin bocks of all types, indicating some degree of internal ductile deformation and stretch of boudins along the maximum extension direction.

[3] Boudin Style 3

Lower strain boudinage structures are developed where mafic sheets are better preserved, typically where layers are thinner (typically <1 m). Boudinage of near contiguous layers give a wide range of different boudinage types. Most are ductile necked or drawn boudins. Less common are torn boudins with varying degrees of barrelling of the boudin face ranging from slightly concave with melt infill to fish-mouth boudins (Goscombe et al., 2004a). A variety of different domino boudins are moderately common and shearband boudins are less common. These two asymmetric boudin-types allow the determination of shear sense in this terrane.

D2: Extension and Mafic Intrusives

Mafic sills that in part crosscut the penetrative L-S fabric are also metamorphosed, boudinaged and folded, indicating that they are also early formed, but post-date the initial episodes of main phase reworking. These mafic intrusives indicate an episode of extension or relaxation closely associated with high-grade reworking. The sill geometries imply an extensional regime giving the horizontal magma flow. Consequently, the presence of sill geometries in the middle crust, along with the anticlockwise P-T path, recumbent fabrics, high stretching strains and very little folding all indicate that D1-D2 deformations were in a lithospheric extensional setting. These early mafic intrusives are also boudinaged, folded and metamorphosed in ongoing progressive main phase orogenesis. These mafics also produce polygonal granoblastic high-grade gneissic fabrics, though the intensity of the L-S fabric is lower than the host gneisses. Mafic intrusives in the Lewisian Complex in the Assynt Terrane have zircon ages of 2418 Ma (Hollis et al., 2006). If these are correlated with the mafic sills on the Isle of Lewis, it possibly indicates two distinct deformation events; main phase D1 fabrics accompanying metamorphism at 2485±5 Ma and later D2 deformation after 2418 Ma.

D3: Ongoing Vertical Flattening and Horizontal Shortening

The main phase penetrative L-S fabric is folded by tight to isoclinal recumbent folds with little to no axial planar fabric developed. These folds appear to be closely associated with boudin structures and may have been nucleated at these heterogeneities. Furthermore, the inter-boudin region contains irregular scar folds formed during earlier boudinage, which are further exaggerated by ongoing shearing during and after boudinage. Most of the folding in outcrop is irregular, disharmonic scar folds associated with the inter-boudin region. These scar folds are atypically irregular and chaotic indicating that a superimposed stress field modified these folds forms. For example the isoclinal mesoscopic folds indicate a recumbent shear deformation was superimposed onto earlier structures, both modifying the developed scar folds into more chaotic forms and nucleating new folds due to perturbation of the stress field around boudin structures. Apart from scar folds associated with boudinage, almost all fold structures appear to have formed after the penetrative L-S fabrics and because boudin trains are folded, after most main phase boudinage also. Furthermore, the discordant mafic sills and dykes are also isoclinally folded boudin trains, indicating that much of the mesoscopic folding post-dates both these intrusions and their boudinage. Folding results in both the folding of these two generations of boudin trains and also shortens boudin trains resulting in the modification of boudin block shapes and the ramp tiling of boudin blocks back over each other (Goscombe et al., 2004a). Folds have shallow axial planes sub-parallel to the folded penetrative foliation and fold axes orthogonal to the stretching lineation and parallel to boudin strips. Folds are typically asymmetric with predominantly top to the north vergence consistent with shear sense during main phase D1 deformation. Consequently, these fold structures are interpreted to have formed in the same stress field in a progressive continuum with main phase D1 deformation, which was however apparently punctuated by an episode of extension (or relaxation) and mafic intrusions.

D5: Laxfordian South Harris Igneous Complex and Granitic Intrusions

The South Harris Igneous Complex on the SW side of South Harris is a sub-vertical complex consisting of anorthosite, diorite, gabbro and norite of Mesoproterozoic age. These rocks have not been strongly grain refined and anorthosite in the core of the complex experienced very low strains. The meta-anorthosite contains magmatic fabrics defined by flattened garnet-hornblende lenses and scleren. Most of the rest of the complex is meta-diorite and meta-gabbros that developed a gneissic fabric with moderate grainshape L-S fabric. The deformation fabrics developed in the igneous complex are of lower strains, but otherwise similar to the L-S sheared gneissic fabric in the Lewisian Gneiss. Consequently, it is possible that the flattening fabric in the Lewisian Gneiss formed in the Laxfordian Orogeny in the Mesoproterozoic. The layering and gneissic fabric is sub-vertical and NW-trending throughout the complex.

The Harris Igneous Complex has been metamorphosed at high pressures. All gneissic fabrics and relict igneous textures have been annealed to coarse polygonal granoblastic matrix. The SW margin of the complex contains garnetized meta-gabbro and meta-gabbros very similar to the metamorphic parageneses developed in the adjacent Leverburg Belt (see above). The Leverburgh and Langavat Belts and Harris Igneous Complex all experienced the same deformation events as the Lewisian Gneiss throughout the rest of Harris and Lewis Isles. However, they contrast strongly by being rotated 90º into vertical orientation and preserve evidence for deep burial into the eclogite field. Metamorphism in the South Harris  Lewisian Complex has been dated at ~1880 Ma (Friend and Kinny, 2001). The steep orientations and high-P parageneses indicate that the SW margin of South Harris is an upthrust zone. A similar steep Laxfordian zone near Stoer on the NW Scotland coast experienced granulite facies metamorphism at conditions of 13-16 kb and >900 ºC. Granulite metamorphism possibly occurred in the Neoarchaean at 2700-2800 Ma, with retrogression at 2500-2600 Ma (Zirkler et al., 2012). Peak metamorphic conditions and semi-quantitative P-T paths for garnet-clinopyroxene-orthopyroxene-phlogopite-rutile-ilmenite-plagioclase eclogite sample BG12-57b from the Leverburg Belt, has been determined by ITAR using Thermocalc v3.25 (Powell & Holland, 1988). Peak pressure conditions are approximately 825 ºC and 14.1 kb. Followed by peak metamorphic conditions at 849±35 ºC and 12.4±1.2 kb, indicating T/depth ratios of 19.6±2.0 ºC/km. Retrogarde garnet-hornblende-plagioclase symplectites forming at approximately 780 ºC and 7.2 kb. These PT determinations and the sequence of mineral growth interpreted within the Mahan (2008) NCFMAST PT pseudosection, indicate a clockwise P-T path with near isothermal decompression from very high pressures.

Networks of massive microgranite dykes, irregular steep intrusive bodies and rare pegmatite dykes of Mesoproterozoic age occur in discrete domains. Granite pervaded domains merge into large batholiths up to 10 km long. These late-stage granites are undeformed and develop no penetrative fabric and interpreted to have been placed late in the Laxfordian Orogeny. Pegmatite dykes are rarely pinched into pseudo-boudin trains that developed by differential collapse of the dyke while still in a supra-solid state.

Archaean Gneissic Terranes Summary

There are broadly two models for the origin of Archaean high-grade gneissic terranes. Either they are highly reworked granite-greenstone terranes or formed in tectonic settings distinct from and independent of granite-greenstone geodynamics. Determining granite-greenstone origins in gneissic terranes is probably not possible in most cases unless the two terranes are still contiguous and reworking transition can be documented. A long implied generalization is that Archaean gneissic terranes are lower crust of a tectonic environment distinct from granite-greenstones. There is however indications that this generalization cannot always be true and that some Archaean gneissic terranes have reworked granite-greenstone origins. Both the Lewisian Complex in Scotland and Southwest Terrane in the Yilgarn Craton are archetypal Archaean gneissic terranes and neither shows any metamorphic evidence for widespread deep burial, residence in the lower crust or crustal over-thickening typical of continent-continent collisions. Furthermore, the extremely high proportions of granitoids (~70-90%) in both these terranes indicate that these terranes were too buoyant to have been lower crust. The vast majority of Archaean gneissic terranes formed at pressures typically lower than 8 kb and experienced granulite metamorphism at middle to upper crustal levels. Both terranes consist of both the same suite and same proportions, of magmatic protoliths that are typical of granite-greenstone terranes. Thus, to apply Occam's razor, they are most probably highly reworked granite-greenstone terranes.

It is inferred that these two terranes were possibly typical granite-greenstone terranes that have been extensively reworked by extreme stretching and flattening strains at mid-crustal levels. This reworking destroyed any original large-scale structure such as greenstone belts and producing a homogeneous lithological distribution through a transposed crust. Reworking occurred in the late Neoarchaean during possible lithospheric extension, crustal delamination and high heat flow in the Southwest Terrane. Reworking of the Lewisian Complex was also in a vertical maximum compressive stress setting giving extreme flattening of a recumbent middle crust with elongate pancake strain ellipsoids, while accompanying top to the north shear giving a dominant extension direction. It is unknown if the recumbent shear was convergent or extensional, though the dominance of layer disruption and boudinage and relatively minor folding are more indicative of an extensional environment. By comparison, the structural style in the Southwest Terrane is essentially unknown, apart from layering being also highly disrupted. However, we do know that the Southwest Terrane experienced high heat flow, moderate pressure metamorphism and anticlockwise P-T paths that are indicative of lithospheric extension environments. Consequently, since both terranes show evidence (structural and metamorphic) for reworking with vertical flattening and lithospheric extension, we speculate that the Southwest Terrane may also share a similar structural style to that well illustrated in the Lewisian Complex.

Kinematic and strain-field map of the Neoarchaean Lewisian Complex, Isle of Lewis and Harris. The Lewisian Complex is transitional granulite facies orthogneisses of 2700-2800 Ma age, with regional geometry of a strongly flattened recumbent sheet that has been buckled by multiple upright fold events. Map summarizes the linear structural elements that constrain kinematics and shape and orientation of the strain ellipsoid during main phase high-grade and high-strain deformation. Solid arrow: indicates transport direction of the hanging wall, typically indicated by asymmetric boudinage of mafic layers. Red line: indicates trace of inter-boudin neckline or length of boudin strip, a vector orthogonal to the maximum extension direction. Boudin strips are typically sub-horizontal to shallow plunging. Thin arrow: indicates plunge direction of stretching direction defined by aligned minerals and streaks of mineral aggregate. Red ellipse: indicates alignment of a stretched sub-boudin block formed by Y-direction ductile stretch and boudinage of the boudin strip into an elongate lentil shape. Maximum strain ratios from large-scale boudin separation are indicated. Garnet and eclogite localities are indicated.