Steep Belt Research Articles

Transpression and extensional collapse: Steep belts and flat belts in the Appalachian Central Mobile Belt, northern New Brunswick, Canada

Models of the Appalachian-Caledonian Orogen infer oblique collision and an enigmatic reversal from sinistral (Silurian) to dextral (Devonian) transpression. The Miramichi Highlands in Canada are relevant to such models because (1) they are a subductional thrust complex in which the inferred reversal coincided with recumbent folding, and (2) its kinematic indicators include rotated thrust structures and conjugate sets of coeval structures. Both are easily mistaken as evidence for sinistral and dextral transpression. The thrust complex is part of the Appalachian Central Mobile Belt and is partitioned into steep belts and flat belts according to the attitude of the schistosity. We argue that the steep belts are coeval with Late Silurian plutonism and low-pressure metamorphism, representing transpressional zones (D 2 ) in which the extension lineation of D 1 thrusting was tilted twice: first from gentle to steep, then back to a gentle plunge. The transpression was synchronous with exhumation as well as sedimentation. Flat belts represent a later flattening (D 3 ) that probably resulted from extensional collapse and preceded Early Devonian dextral transpression (D 4 ) along the Central Mobile Belt. Our model for the flat belts differs principally from models proposed for equivalent flat belts in Newfoundland.

The Kibaran of southern Africa: Tectonic evolution and metallogeny

The tectono-magmatic and metallogenic evolution of the ∼ 1100 Ma (Late Kibaran) high-grade Namaqua-Natal metamorphic belt is reviewed and compared with that of the Grenville Province and the Kibaran Belts of central and eastern Africa. In Namaqualand, 1300–1600 Ma supracrustal rocks are recognised, including arc-related sequences in the east and extensional foreland basin sequences in the west. These were laid down either on a heterogeneous basement of Eburnian (∼ 2000 Ma) age or on juvenile oceanic crust, prior to Late Kibaran deformation, metamorphism and multiple plutonic intrusion. The pre-Namaqua basement infrastructure has been recognised largely in the eastern and NW parts of the orogen, adjacent to areas of exposed Eburnian crust and probably only the Areachap Terrane in the east comprises juvenile Kibaran crust. In Natal, ∼1400 Ma, predominantly metavolcanic supracrustal gneisses were extensively intruded by voluminous plutonic batholiths during the ∼ 1050 Ma tectonothermal event. The northern terrane is interpreted as an ophiolitic assemblage, obducted to the NE onto the Kaapvaal Craton, whereas the southern domains are predominantly arc-related. No older basement is recognised and the entire belt comprises juvenile crustal material, none of which is apparently older than ∼ 1400 Ma. The kinematic evolution of the entire Namaqua-Natal Belt is comparable. In both areas, early SW- and NE- d directed thrusting and nappe emplacement during collision, were followed by pervasive steep transcurrent ductile shearing. In Natal. the later shearing event gave rise to major WNW-trending oblique sinistral transcurrent “steep belts”, whereas the same event in Namaqualand is manifested as major, subvertical, NW-trending dextral transcurrent shear zones. The difference in dominant sense of movement in the two regions can be attributed to changes in orientation of the adjacent Archaean craton margin, within a progressive, consistently oriented, NE-SW directed zone of plate convergence. In eastern Namaqualand and Namibia, the transcurrent shearing gave rise to a north-propagating stricke-slip rift system in which the volcano-sedimentary Koras-Sinclair-Ghansi sequences were deposited. Mineral cooling ages of ∼ 950 Ma, reported from throughout the belt, are considered to signal the end of the Kibaran cycle in southern Africa. The metallogenic evolution is discussed in the light of the tectono-magmatic model presented. Economically important stratiform volcanic and sedimentary exhalative base-metal occurrences are extensively associated with early supracrustal sequences throughout the belt. Other metal deposits and occurrences are predominantly related to the voluminous granitoid and more restricted metabasic suites, which characterise the younger rocks in both Natal and Namaqualand. The metallogenesis of the dominantly convergent-collisional Namaqua-Natal Belt is similar to that seen in the Grenville Province, but distinct from, and younger than, the lower grade, strike-slip basin-related Kibaran Belt of central and eastern Africa.

Syn-convergence exhumation of the Central Alps

The exhumation of rocks in a plate convergence setting is commonly related to erosion and/or tectonic denudation accompanied by isostatic adjustment. Isostatic compensation is the physical response to denudation. It leads to unroofing of deep levels of the crust. A new model for producing topographic relief is proposed which explains well the rapid exhumation of high-temperature rocks in the Central Alps via erosion and tectonic denudation (i.e. gravitational collapse and normal faulting). It is shown that the forward motion of the cold and rigid Adriatic indenter into the European crust is twofold. Firstly, horizontal compression led to the vertical extrusion of the deepest ductile European basement into shallower levels. This tectonic process induced heat transfer through the southern steep belt as well as heat advection together with the extruded material, resulting in the metamorphic aureole observed in the Central Alps. Secondly, the lower part of the Adriatic crust protruded into the warm European crust as a result of continuous forward motion. Geophysical data suggest that the isostatic response to indentation (i.e. deepening of the alpine root) has been inhibited by the mechanical strength of the cold and rigid Adriatic crust. Then, the indentation process induced a deviation from isostatic equilibrium by creating a tremendous topographic relief. This relief disappeared rapidly, possibly as fast as it forms, by enhanced erosion and tectonic denudation leading to rapid exhumation of the metamorphic dome.

Neogene lateral extension during convergence in the Central Alps: Evidence from interrelated faulting and backfolding around the Simplonpass (Switzerland)

The Simplon Fault Zone occurs at the western end of the Lepontine metamorphic dome in the Central Alps and represents a major, low-angle (25° dip) normal fault associated with WSW-ENE-directed lateral extension. Movement began in the early Miocene and continued until at least 3 Ma, accumulating a total vertical throw of around 10 km, corresponding to 24 km displacement parallel to the fault. This displacement is partitioned between a broad zone of ductile mylonites in the footwall and a more discrete cataclastic “detachment fault” (the Simplon Line). In the central region (Simplon village to Zwischbergen), some 3 km of the total vertical displacement is accommodated by the discrete cataclastic zone. Due to the higher exposed structural level, the importance of the cataclastic component increases towards the northwest. In the northern “steep belt” around the Simplonpass, retrograde greenschist-facies mylonite formation and younger cataclastic faulting associated with lateral extension occurred contemporaneously with SSE-vergent, large-scale backfolding, representing NNW-SSE shortening and crustal thickening. The combined effect was an overall shortening across the orogen accommodated by an extension parallel to the orogen, allowing lateral escape of continental material. Normal faulting and the resultant lateral escape was not just superficial. It extended to depths of at least 15 km, and probably to depths exceeding 25 km. The overall geometry and kinematics of the Simplon Fault Zone, and in particular the interplay between folding and faulting, is very similar to that described from the metamorphic core complexes and detachment faults of the Cordillera of the southwestern USA. Within the Central Alps, extension was associated with rapid exhumation of the Lepontine metamorphic dome, as evidenced by very rapid cooling rates determined from mineral ages in the footwall. Similar to the Cenozoic tectonics of the Himalayas and Tibet, orogen-parallel extension in the mountain belt may have been promoted by an overall reduction in the horizontal compressive stress in this direction. More limited extension of the same age and orientation is known from a large area of the European foreland. Greater extension in the mountain belt compared with the foreland was accommodated by major strike-slip transfer fault systems and was linked to coeval overthrusting of the lowlands towards the southwest in the external southwestern Alps of Haute Provence (France). Coeval folding and orogen-parallel extension, such as at the Simplonpass, were developed in a dextral transpressive regime due to oblique WNW-ESE convergence across the Alps during the Neogene.

The kinematic history of the Pennine Nappes east of the Lepontine Dome: Implications for the tectonic evolution of the Central Alps

The Pennine zone of the eastern Central Alps (South, Middle, and North Pennine from top to bottom of sequence) underwent a complex structural and strain history. A first phase (D1) is characterized by E‐W oriented stretching and is of Cretaceous/lower Tertiary age. D1 structures affected most, but not all, of the South and Middle Pennine units, were probably obliterated in the North Pennine Adula nappe, and are absent in the basement nappes underneath the Adula nappe. Further deformation led to N/NW directed nappe movement (D2), followed by incipient strike‐slip motion along the Periadriatic line (late D2). Subsequently, large‐scale folding (D3) affected mainly the nappe fronts. In the Adula nappe F3 folds verge towards the foreland, whereas the overlying units were backfolded towards the hinterland. The D2/D3 events were accompanied by a complex strain pattern that is best seen in the Adula nappe and includes plane to prolate strains in the upper frontal part and oblate strains in both the lower frontal part and the interior of the nappe. D3 backfolding was associated with the beginning of backthrusting at the Periadriatic line and was accompanied and/or followed by rapid uplift in the Lepontine dome and subsequent unroofing of the latter which led to E/NE directed extensional movements (D4) in the eastern Central Alps. The D1 event is related to the anticlockwise rotation of the Adriatic plate relative to Europe. This caused dextral transpression between the Adriatic and the European plates and partitioning of the deformation into convergence and strike‐slip. It is suggested that the North Pennine nappes below the Adula nappe were situated along a major strike‐slip fault and escaped this event. D2 resulted from continental collision between Adria and Europe. D2 displacement directions are subperpendicular to the Alpine arc and are best approximated by WNW/NW oriented motion of Adria. The structures and the strain pattern of D3 are interpreted as the result of gravitational force. Late D2 strike‐slip deformation at the Periadriatic line, D3 backthrusting at the southern steep belt, and the unroofing stage (D4) are generally attributed to dextral transpression between Adria and Europe. This approach implies that the Alpine evolution of the Central Alps was largely governed by tangential convergence between the Adriatic and the European plates.

Chitinozoan reflectance in the Lower Palaeozoic of the Welsh Basin

ABSTRACTVitrinite (woody matter) and spores are generally uncommon or absent in pre‐Devonian sediments. To determine organic maturity in the Lower Palaeozoic alternative methods, such as chitinozoan reflectance, must be employed. Chitinozoa are marine, vase‐shaped organic‐walled microfossils of unknown affinity. The reflectance of incident light from polished chitinozoa walls can provide reliable and accurate maturity data and, due to their relative facies‐independency, a high degree of resolution to regional maturity studies. Calibration of the chitinozoan reflectance scale with vitrinite reflectance enables palaeotemperature estimates to be made from pre‐Devonian sediments.The range of chitinozoan reflectance for the Welsh Basin is from under 1% to 8%. This corresponds to estimated palaeotemperatures of under 100°C to over 300°C. The regional spatial distribution of organic maturation recorded by chitinozoan reflectance in the Welsh Basin is attributed to palaeobasin form. The increase in reflectance with age in the rocks of the Myddfai Steep Belt and associated areas allows an estimation of a palaeogeothermal gradient. In the Silurian sequences around Llandovery the gradient is calculated to be in the region of 50°C km−1. Similar gradients are derived from other studies (clay minerals, metabasite mineral assemblages, conodont colours). The results from Myddfai Steep Belt suggest the thermal peak occurred during the Acadian event.Chitinozoan reflectivity demonstrates that, with respect to hydrocarbon generation, mature conditions occur on the palaeoplatform, where the sedimentary sequences are relatively thin. Overmature conditions are prevalent in the palaeobasin where there is a thick sedimentary pile which has been subsequently deformed.

Tertiary kinematics in the Lepontine dome

Summary In the Pennine zone of the central (Lepontine) Alps, the mineral-zone boundaries of Tertiary metamorphism and the regional attitudes of foliation and thrust sheets define a dome. We investigate the Tertiary kinematic history of the dome using structural, metamorphic and geochronological data. Thus we distinguish two main ductile deformations, that occurred under differing metamorphic conditions. The first high-temperature deformation (HTD) developed under amphibolite facies conditions. It is responsible for (1) a radiating pattern of stretching lineation trends at the scale of the Lepontine area, and (2) structures indicating overthrusting towards the European foreland in the lineation direction (i.e. N and NW). The HTD event is attributable to NW-SE Tertiary convergence between Europe and the Adriatic sub-plate. The second retrograde deformation (RD) is responsible for differential uplift of the Pennine zone, with formation of an earlier subdome to the E (Ticino subdome, 25-20 Ma) and a later one to the W (Simplon subdome, 15-10 Ma). In the Ticino subdome, kinematic indicators reveal a horizontal shearing top W. Some major structures such as the Maggia steep zone and the vertical duplication of the Antigorio basement nappe (Bosco-Gurin area) may be related to this westward phase. Dextral transcurrent shearing along the southern steep belt and backthrusting of the central Alps over the southern Alps along the N dipping Insubric mylonite belt are probably coeval with the retrograde deformation recorded in the Ticino subdome. In the Simplon subdome, kinematic indicators reveal a radial motion parallel to the lineation, which plunges towards the SSW in the Verampio window and towards the WSW at the Simplon Line. Serial uplift of Ticino and Simplon subdomes is attributed to continuing dextral transpression at the boundary between Europe and the Adriatic sub-plate. Finally, the back-folding event located at the northern limit of the Pennine zone postdates both HTD and RD deformational events described above. It is part of a later pop-up responsible for uplifting the external crystalline massifs.

Dalradian slides and basin development: a radical interpretation of stratigraphy and structure in the SW and Central Highlands of Scotland

Movements on syndepositional listric faults are responsible for many of the lateral variations in Dalradian stratigraphy. Sedimentary facies and thickness variations show that the surface traces of these faults correspond, in many cases, to what have previously been interpreted as tectonic slides, although normal faults, thrusts and sedimentary contacts may also mark their position. Three main episodes of faulting are described in the Islay to Lochaber area. Appin Group sedimentation was controlled by a fault now represented by the Loch Skerrols Thrust-Fort William Slide-Ossian steep belt. In the Islay-Jura area an anomalously thick Islay Subgroup succession accumulated in a half-graben bounded by a listric normal fault to the NW and terminated by transfer faults at each end. The deposition of a thick Easdale Subgroup section was controlled by movements on what is now the Benderloch-Ballachulish Slide. This listric fault terminates southwards in the Scarba Fault and probably extends north into the Ossian steep belt. The thinness of the Appin and lower Argyll Group succession between the Ericht-Laidon and Bridge of Balgie Faults is an original feature. The stratigraphic gap represented by the Boundary Slide here is reinterpreted as a disconformity. However, further east the Boundary Slide may well be a listric fault, movement on which accommodated the thick succession around Schiehallion. The development of the major structure was influenced by the geometry of the sedimentary basins. The Schiehallion-Tummel area is reinterpreted as lying on the lower limb of the Ben Lui fold. This resolves the problem of the emplacement of the Tay nappe which is likely to be a gravity structure derived from above the Ossian steep belt.

Late Ordovician shallow marine environments due to glacio-eustatic regression: Scrach Formation, Mid-Wales

An Ashgill to Llandovery offshore marine sequence in Mid-Wales is punctuated by a shallow marine sequence (the Scrach Formation) attributed to Late Ordovician glacio-eustatic regression. The base of the regressive sequence is gradational and suggests progressive fall in sealevel during mid-Hirnantian (late Ashgill) time. An abrupt top to the Scrach Formation marks a rapid sealevel rise, probably within the persculptus Zone (late Hirnantian). Interbedded lenticular thin sands and muds dominate the Scrach Formation and represent shallow shelf environments, with evidence of NW-directed tidal ebb flow and storm currents. Sporadic medium-bedded sand sheets represent the strom events. Locally, lenticular thick-bedded sand units embedded in the fine facies probably represent barrier facies in an area of high clastic input. The Scrach Formation thins SE into the present NE-striking Myddfai steep belt. This is underlain by NE-striking faults that actively controlled late Ordovician to early Silurian sedimentation. The faults probably defined an emergent but low relief coast during the glacial regression. Offshore, to the NW, the Scrach passed into parallel laminated mudstones, then across a fault-influenced slope (the present Tywi lineament) into coarse clastic sediments deposited by gravity flows.

Structural geology of the Llandovery Series in the type area, Dyfed, Wales

The type area of the Llandovery Series lies structurally between the Ordovician core of the Tywi ‘anticline’ to the northwest and the higher Silurian rocks of the Myddfai steep belt to the southeast. Major SE-verging anticline–syncline pairs dominate the structure in the northeast and southwest of the area but they are not continuous with each other along strike. Their non-cylindricity results from nucleation on lenticular sand-rich units. The steep belts are more laterally continuous and are thought to be localized above major basement faults. Fault tips have propagated up into the steep belts. Components of SE-down dip-slip and sinistral strike-slip operated during fold and fault development. Accompanying NW–SE shortening is recorded by a steep NW-dipping cleavage, here axial planar to the major folds but clockwise transecting them further northwest. This transpressive event climaxed in Early to early Mid Devonian time. Relict structures, basin-down faults and growth folds remain from earlier syndepositional extension or transtension.

Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps

Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross‐section in the central Lepontine Alps in the Nufenen Pass area, Switzerland.Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone.The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther‐mobarometers for the maximum post‐D2 por‐phyroblast stage are from North to South: 500° C at 5‐6 kbar and 600° C at 7‐8 kbar.Detailed thermobarometry of garnet por‐phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P‐T‐path, post‐D2 rims grew with increasing temperature but decreasing pressure, and finally post‐D3 garnet formed along a retrograde P‐T‐path.It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post‐D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel.Much of the observed metamorphic pattern can be explained as the result of a significant post‐D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got‐thard massif with its cover. Whereas, in the Pennine metasediments, post‐D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure.The interpretation given here for the Nufenen Pass area may also apply to the Luk‐manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic‐Penninic contact zone.

Discussion on the Caledonian metamorphic core: an Alpine model

D r . P. A. R. N ell writes: Bradbury (1985) presented a brave attempt at a crustal-scale model of the Dalradian. New work in the critical area of Glen Lyon ( Nell 1984 ) suggests that Dr Bradbury’s model of Dalradian structure needs revision. The concept of a Knapdale–Tummel downbend as an ‘antithetic rootless rotation zone’ is fundamental to Bradbury’s interpretation (e.g. Bradbury 1979a , Bradbury 1985 ; Bradbury et al. 1979 ); however, the evidence from Glen Lyon shows that this structure is not of regional importance. The Tay Nappe, Bradbury et al. 1979 proposed, was extruded by Dl–D3 pure shear in the Tummel steep belt accommodated by northwesterly-directed movement on the Boundary and Killiecrankie Slides, with rolling round the Tummel downbend. The major problems with this interpretation are: 1. The Boundary Slide cuts across folds associated with the early Tay Nappe. The D2 fold equivalent to the Ben Lui fold in Glen Lyon (the ‘net F2 antiform’, Fig. 1) folds a stack of F1 folds. These folds face southwards into the ‘flat belt’ of the Tay Nappe on the upper, gently dipping limb of the D2 antiform, but face upwards to the north-west on the lower limb. As the Boundary Slide is not folded along with these F1 folds but remains parallel to the S2 cleavage, and is folded by later folds, movement on the slide in the central Highlands is confined to D2 ‘times’. It is clear that on a larger scale, from Dalmally to Schichallion, and at Braemar, the Boundary Slide continues as an integral

Sedimentation across the Tywi Lineament, mid Wales

Along the southeast edge of the Welsh Basin, faults control sedimentation throughout the Lower Palaeozoic. Two important fault zones in the Llandovery area are the Myddfai Steep Belt and the Tywi Lineament, both of which show several phases of movement that are reflected by subtle changes of sedimentary facies. The Myddfai Steep Belt consists of a relatively simple fault system that defined a palaeoshoreline in Ashgill time, only becoming fully overstepped by a basal Wenlock transgression. The Tywi Lineament, which lies further offshore, is a more complex structure. Facies variations across the lineament were controlled by a fine balance between the sedimentation and fault displacement rates, with the fault tip lines concealed beneath a blanket of shelf sediment. Coarse elastics were resedimented locally in response to rapid displacement on fault strands. Less dram atic fault movements are shown by in situ liquefaction of mudstones, together with a variety of small-scale soft sediment deformation features. The Tywi Lineament was active from at least Ashgill to Pridoli time, and was probably originally an extensional fault, reactivated in a transpressive stress regime. Evidence for possible oblique slip during Ashgill time is found elsewhere along the Lineament.

Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and an age of vertical movement on the Insubric fault line

96 new fission track (FT) apatite and zircon, K/Ar and Rb/Sr biotite and muscovite ages are presented for 19 samples (mainly acid gneisses) from a 40 km traverse through the Lepontine Alps in the Maggia Valley, South Central Switzerland. Plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample. The entire profile shows a fairly uniform Late Neogene-Recent mean uplift rate of ∼0.5 mm/a, confirmed by a gradient of FT apatite age with elevation. Cooling from higher temperatures occurred earlier in the south, where uplift rates of ∼2.2 mm/a in the Steep Belt (root zone) indicate >9 km Early Miocene uplift of the northern Pennine block. This uplift started before 23 Ma and is interpreted as resulting from a major phase of backthrusting along the Insubric Line, and as dating the formation of the mylonite belt. Estimated cooling rates constrain the timing of Lepontine Mid-Tertiary metamorphism: 3 schematic models are proposed which also consider published Rb/Sr, K/Ar mica and hornblende and U/Pb monazite ages. Slow cooling, differential initial heating and subsequent cooling of different parts of the Central Alps and post-38 Ma cooling with syntectonic metamorphism at ∼27 Ma are postulated as alternative interpretations of isotopic data and geologic evidence. From extrapolation between K/Ar and Rb/Sr mica ages and apatite FT ages, 240±50° C is proposed as the closure temperature for the retention of fission tracks in zircon.

Polymetamorphism in the Dalradian of the central Scottish Highlands

AbstractSystematic variations in both timing and conditions of metamorphism are observed in the Dalradian rocks of Central Perthshire, Scotland. Early (D2–D3 and syn-D3) relatively low-pressure metamorphism (550 °C, 7 kb) occurs in the Flat belt rocks of the Tay Nappe to the south, and later (post-D3) higher-pressure conditions (550 °C, 9 kb) dominate the Steep belt to the north. The latter appear to be superimposed on earlier low pressure assemblages (chloritoid–biotite) with the pressure increase probably relating to formation of the Tummel Steep Belt, possibly near 460 Ma. This polymetamorphism, in addition to the generally high pressures of Barrovian metamorphism throughout Perthshire, may account for the poor development of the metamorphic zonal pattern in this region compared to the eastern Dalradian.

Discussion of the structural setting and tectonic significance of the Glen Dessary Syenite, Inverness-shire

M r A. W. B aird writes: Roberts, Smith & Harris (1984) have produced a significant piece of structural mapping in Glen Dessary but I cannot agree with their interpretation of its regional significance. I propose an alternative which does not involve the unlikely situation of regional scale vertical extension, and is more in keeping with the structural history obtained from studies nearby (e.g. Baird 1982; Powell et al. 1981). Roberts et al. describe a series of D 3 major folds with sub-vertical axial plane traces. The D 3 Glen Dessary Synform has an intensely curvilinear hinge line with a sub-vertical extension (X) direction within its fold axial plane. All the D 3 major folds east of the Glen Dessary Synform have sub-horizontal fold hinge lines. They attribute the change of hinge line orientation and development of curvilinear folds to a rapid increase in the amount of D 3 strain westwards, and they link this strain variation to the generation of the ‘steep belt’, with the Loch Quoich line representing the eastern limit of intense D 3 strain and upright reworking. The post-D 2 Glen Dessary Syenite was intruded at 456 ± 5 Ma. (van Breemen et al. 1979). Elsewhere in the Western Moine schists, D 2 is regarded as a Precambrian (Grenville) tectonic event (Brook et al. 1976; Brook et al. 1977; Brewer et al. 1979; Powell et al. 1983). Therefore Roberts et al. argue that the production of the steep belt during D 3 deformation must represent the first Lower Palaeozoic structural reworking of the Moine rocks. However

The Tay nappe and associated folding in the Ben Ledi–Loch Lomond area

Synopsis Recent mapping in the Ben Ledi—Loch Lomond area has shown the southern closure of the Tay nappe to consist of three major folds, the Aberfoyle anticline, Ben Ledi antiform and Benvane synform. The D 1 age of these folds is clearly demonstrated by cleavage-bedding relationships and younging reversals. The previously unrecognised Benvane synform, the structurally lowest major fold, is responsible for the regional inversion of the flat belt. It is defined both on structural and stratigraphical grounds. The D 2 event is dominantly a product of a deformation event in which simple shear increases in intensity downwards. The limit of significant D 2 deformation cuts the major D 1 fold hinges. D 4 is responsible for the Highland Border downbend, the major monoformal structure which divides the area into a flat belt and a steep belt. The steep belt lies adjacent to the Highland Boundary fault and provides a section through the structurally high levels of the Tay nappe.

Metamorphic conditions in central Connemara, Ireland

Metamorphic conditions have been estimated for an area in central Connemara that includes Cashel and Lissoughter Hill. Pressures lie in the range 3.0–6.0 kbar. Temperatures increase from 625–675°C on the N slopes of Lissoughter Hill to 650–700°C on the S slopes, to 700–750°C in the migmatized metasediments N of the Cashel ultrabasic intrusion before increasing sharply towards the contact. Away from the area adjacent to the intrusion a horizontal thermal gradient of between 30°C and 40°C/km may be inferred. Differences in the relative spatial position with increasing temperature of the staurolite-out, muscovite and quartz-out and beginning of partial melting isograds imply that the rocks exposed at the eastern end of the steep belt at Maam Cross were metamorphosed at a lower pressure than those farther W at Recess. Similarly, the stable co-existence of muscovite and quartz in the Recess area may indicate that the second sillimanite isograd dips towards the S, a condition that would be consistent with sub-vertical isotherms associated with a thermal anticline centred on the Cashel volcanic arc.

The structural setting and tectonic significance of the Glen Dessary Syenite, Inverness-shire

The Glen Dessary Syenite crops out in the steep belt of the Northern Highland Moine. It is enclosed within a psammite envelope which was previously assigned to the Glenfinnan Division, but which is here correlated with the psammites of the Loch Eil Division exposed to the E of the Loch Quoich Line. Three episodes of deformation have been identified in the Moine metasediments of the Glen Dessary area, but only the latest (D 3 ) is thought to have deformed the syenite, imposing on it a strong, steeply plunging mineral lineation. The syenite is thought to occupy the core of a large, intensely curvilinear, D 3 synform. The D 3 structures which deform the syenite can be correlated with upright structures, which reworked previously flat-lying, already crystalline rocks, recognized at Loch Quoich. Upright reworking must be younger than the Glen Dessary Syenite, dated by U-Pb methods at 456 ± 5 Ma. Two, hitherto unrecorded, small bodies of granitic gneiss, similar to the c. 1020 Ma Ardgour Gneiss, have been identified in psammites of the Loch Eil Division near the W end of Loch Arkaig. Their presence suggests that the Loch Eil Division has a Grenville history, and is part of the 'Old Moine'.

The Monian ‘Penmynydd Zone of Metamorphism’ in Llŷn, North Wales

AbstractIn the Mona Complex of Llŷn, a narrow, steep belt of fine grained schists known as the Penmynydd Zone of Metamorphism lies between a deformed melange (Gwna Group) and a unit of gneisses and plutonic rocks (Sarn Complex). Rocks within and adjacent to this Zone are characterized by mylonitic textures. The highest grade Penmynydd rocks on Llŷn reached greenschist and, locally, glaucophanitic greenschist fades conditions. The schist belt is interpreted as a major shear zone formed as the Sarn Complex moved up against the lower grade Gwna Group. A previous interpretation of the schists (Shackleton 1956) as being the result of a prograde metamorphism from sub‐greenschist facies Gwna Group to anatectic granitic and gneissic rocks of the Sarn Complex is rejected.