Walls Boundary Fault Research Articles

Overprinting orogenic events, ductile extrusion and strain partitioning during Caledonian transpression, NW Mainland Shetland

A 3.6 km thick stack of mid-crustal deformed Precambrian rocks is associated with the North Roe Nappe (NRN) and Walls Boundary Fault in the northernmost Scottish Caledonides on NW Mainland Shetland. The greenschist- to amphibolite-facies rocks display unusually complex and heterogeneous combinations of coaxial and non-coaxial transpressional deformation. Previously published isotopic dating, together with new detailed field mapping and microstructural characterisation show that the NRN preserves a record of Neoarchaean, Neoproterozoic (Knoydartian) and Ordovician-Silurian (Caledonian) overprinting deformation and metamorphism. Neoarchaean events in the Uyea Gneiss Complex located in its footwall are reworked by younger events in the overlying nappe pile. The main ductile fabrics were formed during Caledonian top-to-the W/NW thrusting and top-to-the N sinistral shearing, with subordinate regions of top- to-the E extensional and NNE-SSW dextral shearing. In lower parts of the NRN, these different kinematic domains are texturally indistinguishable and overprinting relationships are absent. At higher levels, top-to-the-W/NW thrust-related fabrics are consistently overprinted by top-to-the-N/NE sinistral shearing. The highly partitioned transpressional deformation shows similarities with equivalent rocks of the Moine Nappe in NW Scotland.

Late Carboniferous dextral transpressional reactivation of the crustal-scale Walls Boundary Fault, Shetland: the role of pre-existing structures and lithological heterogeneities

The Walls Boundary Fault in Shetland, Scotland, formed during the Ordovician–Devonian Caledonian orogeny and underwent dextral reactivation in the Late Carboniferous. In a well-exposed section at Ollaberry, westerly verging, gently plunging regional folds in the Neoproterozoic Queyfirth Group on the western side of the Walls Boundary Fault are overprinted by faults and steeply plunging Z-shaped brittle–ductile folds that indicate contemporaneous right-lateral and top-to-the-west reverse displacement. East of the Walls Boundary Fault, the Early Silurian Graven granodiorite complex exhibits fault-parallel fractures with Riedel, P and conjugate shears indicating north–south-striking dextral deformation and an additional contemporaneous component of east–west shortening. In the Queyfirth Group, the structures are arranged in geometrically and kinematically distinct fault-bounded domains that are interpreted to result from two superimposed tectonic events, the youngest of which displays evidence for bulk dextral transpressional strain partitioning into end-member wrench and contractional strain domains. During dextral transpressional deformation, strain was focused into pelite horizons and favourably aligned pre-existing structures, leaving relicts of older deformation in more competent lithologies. This study highlights the importance of pre-existing structures and lithological heterogeneity during reactivation and suggests the development of a regional transpressional tectonic environment during the Late Carboniferous on the Shetland Platform.

Caledonian and Pre-Caledonian orogenic events in Shetland, Scotland: evidence from garnet Lu–Hf and Sm–Nd geochronology

Abstract Garnet Lu–Hf and Sm–Nd ages from the Shetland Caledonides provide evidence of a polyorogenic history as follows: (1) c. 1050 Ma Grenvillian reworking of Neoarchaean basement; (2) c. 910 Ma Renlandian metamorphism of the Westing Group; (3) c. 622–606 Ma metamorphism of the Walls Metamorphic Series but of uncertain significance because the eastern margin of Laurentia is thought to have been in extension at that time; (4) Grampian I ophiolite obduction at c. 491 Ma followed by crustal thickening and metamorphism between c. 485 and c. 466 Ma; (5) Grampian II metamorphism between c. 458 and c. 442 Ma that appears to have been focused in areas where pre-existing foliations were gently inclined and thus may have been relatively easily reworked; (6) Scandian metamorphism at c. 430 Ma, although the paucity of these ages suggests that much of Shetland did not attain temperatures for garnet growth. There is no significant difference in the timing of Caledonian orogenic events either side of the Walls Boundary Fault, although this need not preclude linkage with the Great Glen Fault. However, the incompatibility of Ediacaran events either side of the Walls Boundary Fault may indicate significant lateral displacement and requires further investigation.

U–Pb zircon geochronology and geodynamic significance of ‘Newer Granite’ plutons in Shetland, northernmost Scottish Caledonides

U–Pb zircon ages obtained from the late- to post-tectonic ‘Newer Granite’ suite in Shetland, northernmost Scottish Caledonides, indicate a significantly more protracted intrusion history than was inferred previously from K–Ar data. Emplacement of the Brae Complex ( c. 465 Ma), Graven Complex ( c. 440 Ma) and the Muckle Roe Granophyre ( c. 438 Ma) followed regional deformation and metamorphism of metasedimentary successions during the Grampian orogenic event, and is attributed to NW-directed subduction beneath Laurentia. The almost complete absence of plutons of this age along-strike in mainland Scotland suggests a change in subduction angle and/or the distance between the subduction zone and the Laurentian margin. Intrusion of the Ronas Hill Granite ( c. 427 Ma) was approximately coeval with displacement on the Moine Thrust in mainland Scotland, and therefore probably occurred during Baltica–Laurentia collision. A gap of c. 35 myr followed before emplacement of the Mangaster Voe Intrusion and Eastern Granophyre ( c. 390 Ma), and a further gap of c. 20 myr before emplacement of the Sandsting Complex ( c. 370 Ma). Both periods of magmatism are attributed to pulses of localized lithospheric melting in the vicinity of the Walls Boundary Fault during Devonian sinistral relative displacements between Laurentia and Baltica. Supplementary material: Petrological details, full analytical methods and data tables are available at https://doi.org/10.6084/m9.figshare.c.3655706

The Dalradian rocks of the Shetland Islands, Scotland

Metasedimentary and metavolcanic rocks to the east of the Walls Boundary Fault on Shetland have lithological similarities to those of the Dalradian Supergroup of the Scottish mainland. In particular, the middle part of the succession, termed the Whiteness Group, includes numerous metalimestones and associated pelites in a shallow-marine succession that recalls the upper parts of the Appin Group and the Argyll Group of mainland Scotland. Metavolcanic rocks within the deeper water turbiditic sequence of the succeeding Clift Hills Group might be broadly coeval with those of the Southern Highland Group of Scotland. Beyond that, correlations with the established Dalradian succession are tenuous and are not possible at formation level. A local succession immediately west of the Walls Boundary Fault is of even more-dubious Dalradian affinity.The dominant structure is the regional-scale, downward- and east-facing East Mainland Mega-monocline. This has a vertical western limb, which youngs to the east, and an eastern top limb that dips to the north-west at 20–30°. Strata on the eastern limb are inverted on Mainland, Whalsay and Out Skerries but are right way up on the west side of Unst, having been folded around the tight Valla Field Anticline. The Shetland Ophiolite-complex has been thrust over the inverted limb of the Valla Field Anticline on the east side of Unst. The regional monocline folds earlier small- to medium-scale, tight to isoclinal folds with associated planar and linear structures, which are all assigned to a single ‘Main Deformation’. It also post-dates the regional metamorphism, which ranges from chlorite to garnet grade, with localized development of staurolite-kyanite, gneissose fabrics, and the emplacement of schistose granitic sheets in the Colla Firth Permeation Belt.The GCR sites have been selected mainly to be representative of the East Mainland Succession with its associated structures and metamorphism. Highlights include well-preserved sedimentary structures, high-grade gneisses permeated by granitic material, basaltic pillow lavas and serpentinized ultramafic rocks. Some of the latter contain enigmatic skeletal pseudomorphs after olivine and have been interpreted as former high-magnesium lavas.

The movement history and fault rock evolution of a reactivated crustal-scale strike-slip fault: the Walls Boundary Fault Zone, Shetland

The Walls Boundary Fault Zone is a crustal-scale strike-slip fault that cuts Precambrian–Caledonian basement terranes in Shetland and has been interpreted as the northern continuation of the Great Glen Fault Zone in Scotland. This paper presents the first detailed account of the kinematic history and fault rock assemblages associated with the onshore evolution of the Walls Boundary Fault Zone. These observations suggest that it initiated as a major late Caledonian (Silurian–Devonian) sinistral strike-slip fault associated with the successive development of mylonites and cataclasites. These fault rocks are preserved only locally, and elsewhere are obscured by the effects of later brittle overprinting and dismemberment of the fault zone during dextral strike-slip reactivation, probably during late Carboniferous inversion of the Orcadian Basin. This led to the development of extensive cataclasite and fault gouge assemblages, which are widely preserved along the Walls Boundary Fault. Narrow zones of post-Triassic dip-slip, and finally sinistral strike-slip, reactivation are localized within earlier-formed gouge-filled fault cores. There are some similarities to the kinematic history of the Great Glen Fault Zone, most notably the recognition of late Caledonian sinistral shear and post-Devonian dextral reactivation, but the post-Triassic reactivation histories appear to differ significantly.

Does the Great Glen fault really disrupt Moho and upper mantle structure?

Many previous interpretations of deep seismic reflection profiles across the Great Glen strike‐slip fault system have postulated that the fault penetrates the crust and upper mantle as a vertical plane that sharply offsets the Moho discontinuity. Such an interpretation has become a general feature of more recent regional syntheses of the deep geology of the British Isles. Reprocessing of portions of four profiles across the fault improves the resolution of lower crustal and upper mantle structure and calls into question the “Moho step” interpretation. Diffraction analysis and seismic migration applied to reflection data north of Shetland across the northward continuation of the Great Glen fault (Walls Boundary fault) indicate a narrow synform or “keel” developed on the Moho directly beneath the fault. This keel is itself underlain by a possible mantle reflection that is cut off by the downward projection of the fault. North and west of Ireland, analyses of amplitudes, frequencies, and the geometrical behavior of reflections upon migration show that structures previously interpreted as Moho steps may be better explained as distinct packages of reflectivity that are uppermost mantle or possibly out of the plane in origin. In one location north of Ireland where some three‐dimensional control exists, the vertically downward projection of the Great Glen fault intersects, without disrupting, dipping structure in the upper mantle. This observation leads to a model for displacement on the fault system in which motion is laterally transferred along a dipping ramp (or blind thrust) in the uppermost mantle, somewhat analogous to models developed for the San Andreas fault that indicate displacement along the fault to be laterally offset within the middle crust. One of the principal conclusions of this study, that major vertical steps on the Moho beneath the Great Glen fault are difficult to support from the available seismic data, is consistent with Theologically based studies which predict that Moho “topography” such as vertical steps is unlikely to be preserved over long periods of geologic time.

Structure of a continental strike‐slip fault from deep seismic reflection: Walls Boundary fault, northern British Caledonides

Reprocessing of four offshore deep reflection profiles across the Walls Boundary strike‐slip fault, postulated as the northward continuation of the Great Glen fault over the Shetland platform, provides useful constraints on the lithospheric structure of a major continental transform and some insight into the manner in which strike‐slip faults deform. The reprocessing was aimed at delineating complex structure and reducing noise contamination and consisted of analyses of diffractions and amplitude variations and experimentation with seismic migration. The results demonstrate a highly variable crustal and uppermost mantle structure for the Walls Boundary fault along more than 150 km of its length. Six key observations can be made from this study: (1) as middle and lower crustal reflections approach the fault, they are truncated by, and do not continue across, the downward projection of the fault; (2) trends in reflection structure of the crust and/or uppermost mantle differ markedly from one side of the fault to the other; (3) diffractions typically are concentrated at the level of the Moho discontinuity directly beneath the fault; (4) after seismic migration, steeply dipping reflectors are imaged in places directly beneath the fault in the lower crust; (5) in one location, the Moho reflection is deflected across the fault into a narrow synform or “keel” structure which is exactly bisected by the downward projection of the fault; and (6) in a separate location, a prominent upper mantle reflector is truncated across the fault. A vertical or steeply dipping structure is deduced for the fault in much of the crust and perhaps parts of the uppermost mantle; however, more diverse reflecting structures are interpreted for the lower crust, such as the Moho deflection and steeply dipping reflectors directly beneath the fault that may have been related to a localized component of transpression and resultant crustal thickening. The presence of diffractions at the level of the Moho discontinuity where the downward projection of the fault intersects it suggests points of structural “roughness” that may be related to deformation of the Moho by late motion along the fault. Integrating the interpretation of the reflection data with previous geological studies implies that the age of the Moho deflection is post Early Cretaceous but that the dipping structure adjacent to and cut off by the fault is probably Caledonian (Silurian‐Early Devonian). Although the disparate structure of the crust on opposite sides of the fault supports geologic interpretations of large‐scale displacement, a major step in the Moho discontinuity directly beneath the Walls Boundary fault cannot be substantiated from the seismic data.

Discussion on the location and history of the Walls Boundary fault and Moine thrust north and south of Shetland

J. D. Ritchie & K. Hitchen write: Flinn (1992) provides a comprehensive review of the published information on the location and history of the Walls Boundary Fault and its relationship to the Caledonian Front, which he equates with the Moine Thrust. However there are serious flaws and omissions in his arguments which invalidate his conclusions about the location of the Walls Boundary fault and Moine thrust north of Shetland. Walls Boundary fault. Flinn (1992) proposes that the fault is associated with Bouguer gravity anomaly ‘lows’ and magnetic negative anomaly ‘valleys’ and hence, north of Shetland, must trend northeastwards. This argument is unsound for the following reasons. (1) Gravity ‘lows’ do not necessarily mark the position of faults. If rocks of equal density are juxtaposed across a fault then no anomaly will be caused. (2) If, according to Flinn’s logic, the fault is aligned through gravity ‘lows’, why does this not apply south of Shetland where his fig. 2A shows the fault to miss completely a large –20 mgal low west of the southern tip of the Shetland mainland? This low is actually caused by the West Fair Isle Basin, whose eastern margin is the Walls Boundary fault (BGS Shetland solid geology map). (3) Flinn (1992, his fig. 2A) uses the published British Geological Survey (BGS) Bouguer gravity anomaly map, with selectively chosen contours, to trace the Walls Boundary fault north of Shetland. However, the ‘lows’ which Flinn claims to mark the location of the fault are actually caused by the

The history of the Walls Boundary fault, Shetland: the northward continuation of the Great Glen fault from Scotland

Interpretations of the Walls Boundary fault usually take into account only a small part of the previously published evidence, and some even present unsupported speculative interpretations. In particular, geological accounts rarely mention the geophysical evidence available, and geophysical papers take little account of the geology of the area. This paper reviews all published evidence. Geological and marine geophysical evidence show that the fault passes from close to Fair Isle, through Shetland and continues to the northeast; that a 65 km dextral offset occurred in Jurassic times on a gouge-filled fracture; and that prior to this a net sinistral displacement of the order of 100 km had occurred, including a mylonite-producing offset of unknown sense and distance in Carboniferous times.

On the origin and the tectonic implications of magnetic overprinting of the Old Red Sandstone, Shetland

SUMMARY The magnetic signature of the Old Red Sandstone (ORS) and post-orogenic plutonics on Shetland is influenced by three principal components, A (pole: S51,E003), B (pole: S24,E340) and C (pole: SlO,E003). As evidenced from negative fold tests, A is secondary, and most likely thermochemical in origin, presumably associated with hydrothermal fluids circulating in faults and crush zones subsequent to Permian-Early Triassic extensional reactivation of older Caledonian fault structures. From the ORS, the B component also can be proven to represent a magnetic overprint. The secondary nature of component B, and the fact that it reasonably can be correlated on both sides of the Walls Boundary Fault, commonly assumed to be the continuation of the Great Glen Fault, argues against recent suggestions of mega-shearing within the Great Glen Fault system. The precise time of acquisition of B is uncertain, but we consider a lower Carboniferous age synchronous with late-post orogenic plutonic activity (334-358 Ma) to be most likely. This implies that the B component carried by the plutonic rocks may represent a primary cooling event. C is exclusively carried out by Middle Devonian andesites and basalts from the Esha Ness Peninsula. It is evidently of post-fold origin, and we relate this earliest magnetic overprinting to Middle-Upper Devonian (Svalbardian) tectonism which affected the North Atlantic domain during this

Permo-Carboniferous mylonite formation in the Walls Boundary Fault system, Shetland

SUMMARY The Walls Boundary Fault system of Shetland is a major zone of transcurrent faulting which has been active on more than one occasion. It may include a northward extension of the Great Glen Fault. Evidence suggests that the latest major episode of movement, which resulted in the formation of a gouge filled fracture, is of Lower Jurassic age. Earlier movements suggested by lithological offsets across the fault are supported by the presence of bands and slices of mylonite within the fault zone. A mineralogical investigation of the mylonites shows that growth and recrystallisation of K-feldspar and white mica occurred during mylonitisation, and is responsible for partial overprinting of the potassium-argon age of the host rock. 40 Ar/ 39 Ar stepheating spectra from two samples suggest that the phase of movement associated with the development of the observed ductile mylonites occurred during the latest Carboniferous or early Permian, although earlier movements are not ruled out.

Reflection seismic evidence for a Moho offset beneath the Walls Boundary strike-slip fault

BIRPS has acquired two deep seismic reflection profiles across the Walls Boundary Fault, a major late Caledonian strike-slip fault that is probably the northern continuation of the Great Glen Fault of Scotland. Where crossed by the SHET deep seismic survey north of the Shetlands, the Walls Boundary Fault appears to be a narrow vertical structure which penetrates the entire crustal thickness and juxtaposes crusts of different thicknesses. The seismic data show a Moho offset in travel-time of 1.0–1.5 seconds and two high-amplitude diffractions originating at Moho travel-times. These diffractions are, as yet, unique among seismic data across strike-slip faults and can be used to argue that the Moho is offset in depth by 2–3 km over a lateral distance of less than 6 km. Seismic modelling is used to constrain this interpretation. The preservation of this Moho topography suggests that the structure and rheology of the lower crust of the Shetland Platform was not significantly modified during post-Devonian extension of the North Sea and Atlantic margin.

The offshore continuation of the Moine Thrust north of Shetland as deduced from basement isotopic ages

Synopsis Caledonian isotopic ages from the southern section of the Scotland–Norway Basement Ridge effectively separated the Archaean–Proterozoic Lewisian Complex and the Lofoten-Vesterålen Complex forming the SW and NE ends of the ridge. The distribution of Caledonian and Lewisian age data to the north and west of Shetland confirm suggestions that the Moine Thrust merges generally within the region of the west margin of the Caledonian mobile belt, as seen on east Greenland. The Moine Thrust between Orkney and Shetland is offset in a dextral sense by c. 25 km. The Walls Boundary Fault intersects the West Shetland Platform edge to the north of Shetland, with the basement displaying an apparent c. 15 km sinistral offset.

Nontypical BIRPS on the margin of the northern North Sea: The SHET Survey

Summary. Striking similarities in the reflectivity of the crust and upper mantle on BIRPS profiles has led to the development of the BIRP, a model seismic section for the British continental lithosphere. The SHET survey, collected in the region of the Shetland Islands and the northern North Sea, fits the general pattern to a certain extent. Caledonian structures and Devonian or younger basins are imaged in the otherwise acoustically transparent upper crust. An unexpected and exciting feature imaged on SHET is a short wavelength structure on the Moho or abrupt Moho offset beneath the strike-slip Walls Boundary Fault. SHET differs markedly from the SWAT typical BIRP, however, by showing a poorly reflective lower crust. Only a narrow zone (-1 s) at the base of the crust contains high-amplitude reflections. The SHET survey therefore highlights the wide variation in lower crustal reflectivity within the total BIRPS data set rather than the similarities.

The deep structure of the Moine Thrust, southwest of Shetland

The Caledonian orogenic front (the Moine thrust zone) has been described in detail from outcrops in the NW Highlands and Shetland (see [Flinn et al. 1979][1]; [McClay and Coward 1981][2]). Recently however, the use of deep seismic reflection profiling on the adjacent continental shelf has provided new information on the deep structure of the orogen ([Smythe et al. 1982][3]; [Brewer et al. 1983][4]). This note presents further evidence for the offshore position and orientation of the thrust plane as interpreted from seismic profiles in the vicinity of Foula. In the extreme NW of Shetland, Lewisian orthogneisses which crop out on North Roe and Ve Skerries belong to the Caledonian foreland (Fig. 1). These are separated from the younger metasediments of Shetland and Foula by a steeply (60°) east-dipping Caledonian thrust (the Wester Keolka shear), which is generally considered to be the northward extension of the Moine Thrust ([Pringle 1970][5]; [Flinn et al. 1979][1]). Further west, an area of strongly developed magnetic and gravity anomalies has been attributed by [Bott and Watts (1970)][6] to basement gneiss at or near the seabed (Fig. 1). A further prominent seabed feature is the Foula Ridge, which connects Foula and Ve Skerries (Fig. 1). This is composed of metamorphic rocks and has yielded samples of banded hornblendic gneisses and granulite ([May et al. 1984][7]). The youngest tectonic feature in the area is the Permo-Triassic West Fair Isle Basin which is preserved on the downthrown side of the Walls Boundary Fault (Fig. 1; Bott and Browitt . . . [1]: #ref-8 [2]: #ref-11 [3]: #ref-15 [4]: #ref-4 [5]: #ref-13 [6]: #ref-2 [7]: #ref-12

Palaeomagnetism of the middle-upper Devonian Esha Ness ignimbrite, W. Shetland

The ignimbrite division of the Esha Ness volcanics of W. Shetland has been studied palaeomagnetically. The major blocking temperature spectrum ranges between 650 and 680°C, implying that haematite is the principal remanence carrier. The magnetization build-up is simple: a minor low stability magnetization aligned along the direction of the present Earth's field is superimposed on a well-defined and highly stable palaeomagnetic component with Dec. = 220 and Inc. = + 3. The Esha Ness volcanic sequence is folded into a gentle syncline trending NNE. The rock collection concerned covers 8 sites: 7 from the western flank and 1 from the eastern flank of the syncline. The 7 sites on the west flank are consistently reversely magnetized while the site on the east flank has a normally directed magnetization. After structural unfolding the normal and reverse directions are almost perfectly antiparallel, i.e., the stable magnetization most likely predates the tilting which is probably of Devonian age. Esha Ness is located some 12 km to the west of the Walls Boundary Fault (WBF) which is considered as the northward continuation of the Great Glen Fault (GGF). The shallow palaeomagnetic inclination found in Old Red Sandstone rocks to the west of GGF/WBF corresponds to that encountered in most other European Old Red formations, discounting therefore megascale (> 1000 km) late-post Devonian movements along this dislocation.

Transcurrent faults and associated cataclasis in Shetland

A series of transcurrent faults with northerly trend is well exposed in the cliffs of Shetland. The Walls Boundary Fault which cuts through the middle of Shetland has a continuously changing trend so that it forms a very flattened S. Dextral movement along the fault seems to have created further dextral transcurrent faults, including the Nesting Fault, across the eastern concavity in the trend. These main faults lie within broad zones of cataclasis, subsidiary faulting and local folding. The offsets on the subsidiary faults are very much less than on the main faults, and the crushed rocks have isotropic fabrics. The zones probably arose during faulting from varying local stresses caused by the interaction of the unevennesses on the two sides of the non-planar main faults. Gouge in the Walls Boundary Fault contains analcite. Gouge-like laumontite in subsidiary shears was probably formed by mechanochemical reactions. The occurrence of these minerals and of blastomylonitic scapolite veins near the Walls Boundary Fault may indicate up to three phases of movement taking place between Cretaceous and Devonian times under different depths of overburden. Slices of secondarily cataclastic mylonite occurring along the Walls Boundary Fault are probably relics of the Great Glen Fault now partially cut out by the nearly coincident Walls Boundary Fault.

Interpretation of geophysical observations between the Orkney and Shetland Islands

A N.–S. orientated region of low gravity adjacent to the W. coast of the S. Shetland mainland is partly caused by a basin of probable Mesozoic age, as indicated by seismic refraction, sparker and bathymetric observations. The floor of the basin is estimated to be up to 2.3 km deep. The sediments of the western part of the basin are thin and of variable dip, but the eastern part takes the form of an eastward dipping sediment wedge or synclinal structure truncated on the east side by a normal boundary fault of contemporaneous or later age. The basin divides at its southern end into two tongues. The gravity low is also partly attributed to low density basement rocks such as granite, both at its north end and in the vicinity of Fair Isle, and to westward thickening of upper Palaeozoic (O.R.S.?) strata beneath the basin. The postulated Mesozoic basin appears to be a subsidence structure along a major underlying dislocation approximately continuing the line of the Walls boundary fault. The new evidence is consistent with this being the Great Glen fault or a splay of it and interpretation of the linear magnetic gradient coinciding with the eastern tongue supports this hypothesis. The eastern boundary fault of the basin, however, appears to be normal. Tertiary transcurrent movement along the line appears to be improbable.

A survey of the age relations of Shetland Rocks

AbstractThe survey is based on field work by Flinn, on forty‐two K‐Ar age determinations by Miller, and on previously published work on Shetland. Most of the metamorphic rocks give K‐Ar ages of about 420 m.y. It is clear from petrological and stratigraphical evidence that this age is not the age of the metamorphisms and migmatizations responsible for the more obvious features of the rocks, and also that the metamorphic rocks in different areas have had different histories of development. Ages up to 515 m.y. have been found in various areas, and these may be more closely related to the main metamorphisms than the more common 420 m.y. ages. In the Mainland the 420 m.y. age may be related to a late porphyroblast metamorphism: in Unst and Fetlar it seems to be the age of Read's second metamorphism which accompanied the emplacement of nappes and the formation of orogenic sediments. About 400 m.y. ago a series of postorogenic granitic and appinitic complexes were emplaced in the southern part of Shetland. By 380 m.y. ago erosion had reached migmatitic rocks and they were being buried again beneath Old Red sediments and contemporaneous volcanics. Later still, possibly 350 m.y. ago, in Upper Devonian times, granites were emplaced in the west of Shetland cutting the Old Red rocks. Finally the Walls Boundary Fault (Great Glen Fault ?) cut one of these late granites.