Slaty Cleavage Research Articles

Deformation and regional carbonate metasomatism of turbidite-hosted Cretaceous alkaline lamprophyres (northwestern Papua New Guinea)

A suite of variably metasomatised camptonite dykes and rare stocks, named the Fu Intrusives, has been recognised within thrust plates of Mesozoic slates in a 3000-km 2 portion of the New Guinea Thrust Belt. The intrusions were emplaced prior to Oligo-Miocene fold deformation and low-grade regional metamorphism of their host, and consequently have foliated margins and are cross-cut by metamorphic quartz veins. Samples from the less altered stocks display primary cumulophyric aggregates of kaersutite + titansalite + titanbiotite, set in a matrix of albite + salite + apatite ± olivine ± quartz. Calcite ocelli are occasionally present. REE patterns ( La CN Lu CN ∼ 10 , La ∼ 100 × chondrite) and initial Sr ratios (∼0.7042) are typical of alkaline lamprophyres. Kaersutite and titanbiotite separates from such samples have yielded concordant KAr ages of ∼75 Ma (Campanian), which we accept as the age of intrusion. However, the dykes have suffered heavy texture-destructive carbonate-sericite alteration, though relict cumulophyric texture and globular structures are sometimes observed. Composition-volume calculations suggest that metasomatism was accompanied by loss of Na, Ti and P; and systematic addition of Ca, Fe and CO 2, as well as progressive LREE depletion and Fe reduction. Alteration carbonates exhibit a narrow range of δ 18O-values (+ 13.8 to + 14.9‰), indicating uniform conditions of metasomatism throughout the terrain. The calculated fluid δ 18O at a model temperature of 300 ± 50°C is +8±2‰, which corresponds with that deduced from the δ 18O-value (+ 12.3‰) of the cleanest sericite separate. The four analysed sericite separates have a mean δD of −64 ± 12‰, implying an hydrogen isotopic composition for the alteration fluid of −58 ± 30‰ at 300 ± 50°C. The calculated fluid composition therefore overlaps the normal range of metamorphic and magmatic waters. However, C and Sr isotopic ratios of the alteration carbonates ( δ 13 C − 7.3 to + 0.4‰; 87 Sr 86 Sr 0.7076–0.7126) strongly support a crustal source for the metasomatic fluids. This is corroborated by field evidence that country rock slates adjacent to the dykes are unaffected by carbonate metasomatism and hence were in equilibrium with the alteration fluid. We therefore conclude that regional carbonate metasomatism of the Fu Intrusive Suite occurred as a result of metamorphic dewatering of their host rocks during slaty cleavage development.

Decollements in slate belts, examples from the European variscides and the Qin Ling Belt of Central China

Decollements are conspicuous features of collision belts as a result of shortening and shearing of rocks of different competencies in response to intracontinental subduction and crustal stacking. When these decollements occur at upper levels, the classical thin skin tectonics of the foredeeps without important internal strain results; at mid crustal levels slaty cleavage and foliation development occurs. In the more internal parts of the chains, in the lower crust, near the crust mantle boundary, the more severe conditions (granulitic metamorphism) result in ductile deformation in the deepest parts of the mountain belts. Examples of middle and lower crust are well documented in Central China and the Variscan Belt of Europe from field evidence and deep seismic profiling. From these examples it appears more and more likely that most of the mid-crustal slate belts showing vertical slaty cleavage at the erosion surface overly flat decollements. At greater depth large decollements must occur as well in the lower continental crust and at crust-mantle boundary. Typical features of the deep seismic vertical profiles such as the strongly layered lower crust may represent the image of such deep decollements.

Grain growth and re-orientation of phyllosilicate minerals during the development of slaty cleavage in the South Kitakami Mountains, northeast Japan

Using X-ray diffraction analysis, the mutual relations among illite crystallinity, degree of preferred orientation of chlorite, grain size change of chlorite and illite during metamorphism and development of slaty cleavage have been investigated for argillaceous rocks in the South Kitakami Mountains, northeast Japan. The metamorphic grade of IC (illite crystallinity index) = 0.29 is a critical one, beyond which the homogenization of chlorite composition, coarsening of chlorite and illite grains and degree of preferred orientation of chlorite are abruptly advanced. Grain coarsening is also promoted by the development of slaty cleavage, especially in the range of coarser grain size. The oriented growth by the effects of both the anisotropy of intrinsic growth rate of mineral grains and that of the environment in which grains grow, is considered to bring about the preferred orientation of chlorite and illite.

Two-sided Variscan thrust tectonics in the Vosges Mountains, northeastern France

AbstractVariscan convergence produced two-sided (bivergent) crustal-scale thrusting in the Vosges Mountains. In the northern Vosges the central polymetamorphic crystallines were thrust to the NW over Cambrian to Silurian low-grade and very low-grade metamorphic clastics. Synorogenic upper Devonian - lower Carboniferous turbidites and volcanics were folded into NW-vergent structures which display SE-dipping slaty cleavage. The entire sequence shows increasing metamorphism and deformation from NW to SE. Late right-lateral strike-slip faulting along the Lalaye-Lubine fault zone outlasted thrusting. In the southern Vosges a lower Carboniferous turbiditic basin that was fringed on the south by a volcanic arc was tectonically shortened by south-directed tectonic imbrication of slivers of varied rocks including ultramafics, gneissic basement, and synorogenic elastics. The increasing degree of deformation and metamorphism towards the north suggests a thrust contact with the polymetamorphic gneisses of the central...

Structure of the Abercrombie beds south of Reids flat, new South Wales

The Late Ordovician Abercrombie Beds, south of Reids Flat, New South Wales, and adjacent to the Wyangala Batholith, show evidence of three successive fold episodes. First generation folds are tight to isoclinal, with fold axes ranging from vertical to horizontal and north‐trending, and steep axial‐plane slaty cleavage. Second generation folds are steeply plunging, tight to open with north‐striking axial planes. In pelitic rocks the axial plane structure is a crenulation cleavage which overprints the slaty cleavage. The first two fold episodes were accompanied by greenschist‐facies metamorphism. Granite emplacement occurred prior to the second fold episode. A third deformation was of relatively mild intensity and produced open, north‐trending folds with axial planes dipping moderately to the east, and crenulation cleavage as the axial plane structure in pelitic rocks. These latest folds are correlated with the latest folds in the Abercrombie Beds north of the Abercrombie River. The mapped area has no apparent macroscopic structure and may be considered as a single domain.

Structural style and kinematics of an underplated slate belt, Kodiak and adjacent islands, Alaska

The Kodiak Formation, composed of coherent Maastrichtian turbidites, is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest Cretaceous. It consists of about 80% coherent landward-dipping thrust packets; zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder. About half of these disrupted sandstone zones are related to pre-accretion deformation, and the rest formed along late-stage, strike-slip faults that postdate development of slaty cleavage. The formation is divided into three structural belts. The landward and seaward belts include steeply dipping structures, and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain. The central belt probably acted as a low-angle, southeast-verging, floor thrust zone beneath the landward belt. The structural history of the Kodiak Formation includes (1) early soft-sediment disruption; (2) tectonic stratal disruption; (3) thrust faulting, slaty cleavage (S 1 ) development, and folding (F 1 ); (4) intrusion of granodioritic plutons, dikes, and sills and associated normal faulting; (5) development of crenulations (F 2 ) and crenulation cleavage (S 2 ); (6) thrust faulting; and (7) development of right-lateral, strike-slip faults. Event 3 produced the dominant northwest-dipping structural grain and caused the greatest amount of shortening; the timing of event 7 relative to events 5 and 6 is not certain. The dominant structures of the Kodiak Formation are interpreted as developing during underthrusting, underplating, and intra-wedge shortening during latest Cretaceous time within an accretionary wedge. The maximum time from deposition to emplacement was about 12 m.y., and the timing is constrained by the maximum depositional age (74 m.y.) and the age of plutonic rocks (62 m.y.) that crosscut the dominant fabric. Pre-cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting. Slaty cleavage, thrusts, and F 1 folds developed during underplating that also resulted in the formation of duplexes. Crenulations are probably related to post-underplating subhorizontal shortening of sediments within the accretionary wedge. Syn-deformation dynamic recrystallization of quartz, minimum syn-accretion metamorphic temperatures between 205 and 250 °C, and pressures of at least 2.65 kb recorded during the earliest stages of deformation suggest underplating at >10 km.

Arcuate hinge cleavage associated with welded contacts: an example

Arcuate hinge cleavage (a.h.c.) shows a near bedding-parallel, concentric, arcuate development within the inner arcs of hinge zones. It is favoured by alternating layers of marked viscosity contrasts with little layer-parallel shortening prior to parallel folding. A field example of a.h.c. from a greywacke/pelite sequence of the Variscan of Central Europe is presented. The a.h.c. is developed as a slaty cleavage in the inner-arc hinge zones of pelite beds close to and welded to the outer arc of greywacke layers. Microscopically it is defined by the alignment of platy minerals and oblate quartz grains. The a.h.c. (S a) is cut by a divergently fanning crenulation cleavage (S b) which, in turn, is cut by slip surfaces parallel to bedding. The slip surfaces are cut by a fracture cleavage which is the macroscopically observed axial surface cleavage (S c). This sequence of deformational increments implies the onset of bedding slip after the formation of a.h.c. (S a) and (S b). We therefore suspect inhibited bedding slip by welded contacts to favour the development of a.h.c.

Regional variations in deformation mechanisms and structural styles in the central Appalachian orogenic belt

Orogenic belts, such as the central Appalachian fold and thrust belt, consist of distinct tectonic provinces characterized by different structural styles as a result of deformation under different physical conditions. A quantitative study of deformation mechanisms was undertaken for quartzites and sandstones of the central Appalachian orogenic belt in order to understand the controls of different physical parameters on the relative importance of these mechanisms and on the geometry and mechanics of formation of structures in the different provinces. In the Blue Ridge province, the dominant deformation mechanisms are pressure solution, dislocation creep, and extension fracturing, with some accompanying grain boundary sliding and recrystallization. The proportion of dislocation creep to pressure solution strain increases both with temperature and deviatoric stress. An axial plane slaty cleavage, which contains the maximum and intermediate principal directions of the strain ellipsoid, maintains a uniform orientation in both the Blue Ridge and the Great Valley provinces. Folds in the Blue Ridge province have a similar geometry, formed by progressive flattening of parallel folds due to internal deformation. Ductile deformation zones on the overturned limbs of folds acted as precursors for the propagation of thrust faults in the late stages of folding. In the Valley and Ridge and Massanutten synclinorium, important deformation mechanisms in the sandstone units include pressure solution, plastic deformation, cataclasis, brecciation, extension fracturing, and slip along minor faults and bedding planes. Major folds are strongly disharmonic and show variations between concentric and angular geometries. They are associated with thrust ramps that propagated through progressively developing folds. In the late stages of folding, brittle deformation zones characterized by high intensities of extension fractures and cataclastic bands formed on the steep forelimbs. These features are interpreted to be the result of layer-parallel extension and the propagation of splay faults through these limbs. Cleavage developed during layer parallel shortening prior to the onset of folding and was rotated to its present fanned geometry during folding. On the basis of the results of this study, the central Appalachian orogenic belt can be subdivided into two deformational provinces dominated by different deformation mechanisms and characterized by different geometries and mechanics of formation of the structures. For orthoquartzites, the tectonite front which separates the tectonites of the hinterland from the non-tectonites of the foreland is located along the Blue Ridge front. For carbonates and argillaceous sandstones, the tectonite front is located along the North Mountain fault.

Bundled slaty cleavage in laminated argillite, north-central minnesota

Exceptional bundled slaty cleavage (defined herein) has been found in drill cores of laminated, folded, weakly metamorphosed argillite at several localities in the early Proterozoic Animikie basin of north-central Minnesota. The cleavage domains are more closely spaced within the cleavage bundles than outside them, the mean tectosilicate grain size of siltstone layers, measured normal to cleavage, is less in the cleavage bundles than outside them, and the cleavage bundles are enriched in opaque phases and phyllosilicates relative to extra-bundle segments. These facts suggest that pressure solution was a major factor in bundle development. If it is assumed that opaque phases have been conserved during pressure solution, the modal differences in composition between intra-bundle and extra-bundle segments of beds provide a means for estimating bulk material shortening normal to cleavage. Argillite samples from the central part of the Animikie basin have been shortened a minimum of about 22%, as estimated by this method. These estimates are similar to the shortening values derived from other strain markers in other rock types interbedded with the argillite, and are also consistent with the regional pattern of deformation.

Sobre las deformaciones del paleozoico en el macizo del Desierto de las Palmas (Castellón)

The Paleozoic deposits in the Desierto de las Palmas massif are turbidites whose probable age is lower Carboniferous. They registered a first deformation phase giving rise to very flattened recumbent folds, with associated slaty cleavage and NE-SW axes. Afterwards two phases of buckling folds with small flattening and crenulation cleavage are developed: the second one is ENE and the third one is N to NNW. A last superposed ESE crenulation is probably of Alpine age.

Metamorphic fluids and transtension in the Cantabrian Mountains of northern Spain: an application of the conodont colour alteration index

AbstractConodont colour alteration index (CAI) values from Upper Paleozoic rocks in the Cantabrian zone of northern Spain show that temperatures during Hercynian metamorphism locally exceeded 300 °C. Various temperature domains have been defined, which are generally separated by fundamental structures. These domains do not correspond with the tripartite subdivision based on stratigraphic analysis.The observed CAI values of conodonts are in general agreement with the mineral paragenesis. Areas with high CAI values display extensive alteration and mineralization, and where CAI values exceed 4–4.5 (>200 °C) slaty cleavage has developed.The Cantabrian zone is an area of very low grade metamorphism, where peak conditions were reached in Upper Carboniferous to Lower Permian times. The characteristics of the metamorphism and its spatial relationship with major faults suggest that fluids were the main source for regional heating and that fluid transport was focussed along crustal-scale structural features.The overall deformation regime in this part of the Variscan orogen of western Europe is interpreted to be large-scale transtension. This is in agreement with earlier proposed models for the formation of Upper Palaeozoic basins in this area.

A paleomagnetic study of tectonically deformed red beds of the Lower Glarus Nappe Complex, eastern Switzerland

A paleomagnetic study at 15 sites (315 samples) in Permo‐Triassic red beds of the Helvetic nappes of eastern Switzerland isolated stable characteristic and secondary remanent directions. Thermal demagnetization and isothermal remanent magnetization experiments suggest pigmentary hematite carries the characteristic direction. Secondary directions are scattered but may have been acquired during the Alpine deformation in the Oligocene and Miocene. The characteristic directions also show large between‐site scatter. Anisotropy of magnetic susceptibility indicates that the hematite grains are flattened within the slaty cleavage, and that there is a north‐south lineation. This anisotropy pattern implies that the magnetic grains were realigned by the Alpine deformation. It is not possible to explain the characteristic remanent magnetization deviations on a site‐by‐site basis, but the site mean directions are streaked along a great circle, the down dip direction of which is approximately parallel to the regional fold axes (60°/20°). This spread could be explained by a penetrative simple shear deformation associated with the nappe‐internal deformation.

Variscan strain pattern in the Paleozoic series at the Lizard Front, southwest England

A structural analysis of the Devonian sedimentary Gramscatho formations, South Cornwall, allows us to specify the Variscan strain pattern at the Lizard front. This deformation is consistent with a NNW verging tangential shear which first induces a synmetamorphic folding phase (F1S1) and whose noncoaxial character is emphasized, on different scales, by the existence of a positive deformation gradient toward narrow shear zones, the development of syncleavage noncylindrical folds whose axes tend to be reoriented, in the S1 slaty cleavage plane, toward a pervasive stretching lineation (L1) which trends N160°E and by the existence of NNW verging sheath folds. This folding phase evolves finally to tangential structures which induce, over the parautochthonous Gramscatho formations, subhorizontal shear planes bearing strong slickenside lineations trending parallel to L1, lineations which thus represent, on a regional scale, the “tectonic transport” trend of allachthonous units migrating northwards. The existence of such allochthonous units in the South Cornubian area is now well established by the discovery, at the Lizard front, of a Paleozoic (Carboniferous ?) limestone lens wedged under the Lizard hornblendeschist. The structural evolution of the South Cornubian domain is consistent with the “scheme” of a continental platform progressively implicated in a tangential deformation propagating northward. This compression leads, in the internal southern zones, to an earlier (Mid‐Upper Devonian) crustal tangential tectonic which later (Carboniferous) reaches the external domain of England, and it expresses in the Devono‐Carboniferous cover by a tegumentary tectonic. This northward verging tangential pattern is now testified by SWAT (South West Approaches Traverses) profile interpretation.

Intermittent folding and faulting in the Lake Moondarra area, Mount Isa, Queensland

Abstract Faulting and folding are interspersed and locally interrelated in the Lake Moondarra area. Minor faulting occurred during deposition, after which the region was deformed by three folding events, developing weak slaty cleavages and regional folds. Geometrical constraints and overprinting relations between associated quartz veins have established the relative timing of cleavages. Following F1 folding, major E‐W trending faults formed with dip‐slip motion followed by an interval of strike‐slip movement. The second deformation produced N‐S folds in S0 and S1, and a slaty cleavage, S2. Some faults were dilated and infilled locally with quartz and country rock fragments during the second deformation or slightly earlier. The fault‐filling material contains a foliation, with about the same orientation as S2 throughout the region, and also a mineral‐elongation lineation of aligned quartz fibres and deformed elongate quartz grains. There was a further component of strike‐slip movement, the sense of which changes across and was apparently controlled by F2 folds. The third deformation caused NNW‐SSE folds and an associated cleavage. During D3 some faults underwent further extension and dilation, parallel and perpendicular to the maximum finite extension direction (i.e. L33), after which another group of faults formed at a low angle to S3 and subsequently underwent a component of sinistral strike‐slip displacement.

Structural history of continental volcanic arc rocks, eastern Sierra Nevada, California: A case for extensional tectonics

Mesozoic metavolcanic rocks forming part of the continental volcanic arc along the eastern Sierra Nevada near Mt. Goddard and in the Ritter Range show a complex history related to extensional tectonics. The rocks comprise a thick section of tuffs, breccias, lava flows, sills, and ash‐flow tuffs deposited in a subaerial to subaqueous environment, with some subvolcanic sill‐like plutons. Pb/U ages of the rocks in the Mt, Goddard area range from ca. 130–160 Ma, while rocks in the Ritter Range have a somewhat wider age range as reported previously. Repetition of the section occurs by faulting, and with the exception of parts of the mid‐Cretaceous Minarets Caldera, all the volcanic rocks show a regional slaty cleavage which was subsequently crenulated and/or folded locally. The first cleavage has well‐developed stretching lineations, and does not appear to have been associated with significant folding. Finite strain measurements show considerable variation both in magnitude and symmetry. The Mt. Goddard rocks, however, tend to show slightly higher overall strain magnitude and greater constrictional component than the Ritter Range for rocks of comparable age. Calculations based on the strain data suggest the Mt. Goddard section has been thinned by about 50% normal to bedding, much as that documented previously for rocks in the Ritter Range. Deformation within this part of the continental arc was originally thought to have formed by regional compression during the late Jurassic (Nevadan) orogeny. However, our study indicates that (1) parts of the deformed volcanic section are younger than late Jurassic, (2) Nevadan‐age breaks in deposition are not present, (3) large‐scale folds expected during a regional compression event are not common, and (4) the beds were tilted to a high dip prior to internal deformation. An extensional model is proposed in which beds were rotated to high tilts early in the deformation as a result of listric normal faulting. This normal faulting is thought to have occurred above a regional tumescence related to voluminous magmatism at depth, with preservation of the steeply tilted Goddard and Ritter sections being facilitated by their downward transport along the margins of rising plutons. Flattening and steeply plunging constrictional fabrics superimposed on the tilted sections are related to strain induced by high‐level inflation of magma chambers and downward return flow of the keellike pendants. The main tectonic fabric shown by the continental volcanic arc rocks in the eastern Sierra Nevada is largely of Cretaceous age, rather than Jurassic (Nevadan) as originally supposed. In addition, the deformation, both rotation of beds and subsequent tectonite fabric, appears to be genetically related to the dynamic evolution of the magmatic arc, and not the result of an externally imposed tectonic event.

The stratigraphy and structure of the Devonian rocks around Liskeard, east Cornwall, with regional implications

A formal stratigraphy based on new macrofaunal evidence is proposed for a little-known area of slates, thin limestones, and volcaniclastic rocks of Lower Devonian to early Upper Devonian age around Liskeard, between South Devon and north Cornwall. Open shallow marine shelf conditions persisted from the Emsian until the onset of basinal sedimentation in the Frasnian, with the local development of pillow lavas and pillow fragment breccias during Eifelian and Givetian times. Regional stratigraphic correlations identify a Middle Devonian axial ridge which runs from Tor Bay to Bodmin, and separates a redefined Trevone (north Cornwall) Basin from the south Cornish Basin. A moderate to steeply dipping slaty cleavage (Dl) dips southward throughout the area and is associated with early E–W major folds, inferred to be north- or upward-facing, which plunge gently east. A penetrative, closely spaced crenulation cleavage (D2) is developed in a zone, >0.7 km thick, which trends E–W and dips at 15º to 30º south. This fabric, previously misidentified as the D1 slaty cleavage, is associated with major D2 folds which have near horizontal, east–west trending axes; structures of this age and geometry have not been reported from elsewhere in the region. The Portwrinkle Fault has been traced NW–SE across the area and passes west of Liskeard; it shows a large dextral strike-slip displacement and may have developed early in the history of the thrust belt.

Evidence for syntectonic crystallization for the mudstone to slate transition at Lehigh gap, Pennsylvania, U.S.A.

Data primarily for phyllosilicates have been obtained for the continuous transitional sequence from mudstone to slate with well-developed slaty cleavage at Lehigh Gap, Pennsylvania, and for slates from quarries in the same area. Samples were studied by optical microscopy, powder X-ray diffraction and electron microprobe analysis, with emphasis on transmission and analytical electron microscopy. Mineral grains are virtually free of deformation-induced strain. Concomitant with the gradual development of cleavage normal to bedding the following changes are observed or confirmed: (1) the orientation of phyllosilicate grains changes discontinuously from being preferentially parallel to bedding to being parallel to cleavage; (2) crystal imperfections as expressed in layer terminations, low angle grain boundary-like features and other defects decrease in density; (3) complex mixed layering is replaced by homogeneous packets of layers of single phases and (4) illite transforms to muscovite, with increase in K + Al and change from a 1M d to 2M polytype. Slaty cleavage apparently develops due in part to pressure solution of phyllosilicates oriented parallel to bedding, mass transport of components, and crystallization to form new grains parallel to cleavage. It reflects transitions from imperfect, metastable phases toward ordered stable phases in a low temperature (∼225°C) metamorphic environment.

Development of slaty cleavage in a mudstone unit from the Cantabrian Mountains, northern Spain

A 200 m thick mudstone unit in the Carboniferous of the Cantabrian Mountains, northern Spain, exhibits an increase in intensity of the slaty cleavage from top to bottom which appears to be correlated with a decrease in the mean grain size, average bed thickness and quartz: mica ratio. Anastomosing cleavage domains, formed by pressure solution and by kinking and rotation of the detrital micas, become closer spaced, wider and more continuous towards the finer-grained base of the unit. Growth of strongly oriented new micas within the cleavage domains also appears to be correlated with the intensity of the domain development and hence with the initial lithology. Clay minerals have been completely replaced throughout the mudstone by muscovite, paragonite and pyrophyllite during low-grade metamorphism. The growth of an oriented mica fabric, however, is restricted to samples with well-developed cleavage domains.

Strain paths during slaty cleavage formation-the role of volume loss: Discussion

Strain paths during slaty cleavage formation-the role of volume loss: Discussion

East-directed imbrication and oblique-slip faulting in the Humber Arm Allochthon of western Newfoundland: structural and tectonic significance

Many of the dominant outcrop-scale structural features in the lower, clastic thrust sheets of the Humber Arm Allochthon were not generated during the westerly emplacement of the allochthonous terranes of western Newfoundland. Two general groups of structures are abundant in the Humber Arm rocks: (1) east-verging folds accompanied by a weakly to moderately developed slaty cleavage and cut by west-dipping thrust faults; and (2) northeast–southwest-striking high-angle faults, with predominantly normal oblique-slip motion and with larger faults down-stepping to the northwest. Evidence of the earlier, west-directed thrusting (refolded and downward-facing folds, folded thrusts, etc.) is uncommon in the Humber Arm area. Slaty cleavage-generation structures, however, appear to overprint the phacoidal fabrics of the mélange zones that exist between and within thrust slices of the allochthon, making the mélange fabrics the most readily identified features associated with the initial east over west imbrication and emplacement of the allochthon.These observations suggest that the original detachment of the rocks of the Humber Arm Supergroup from their basement (early Taconian deformation) occurred with only limited internal deformation. Mélange zones presently define some or all of the early surfaces of movement. The fully assembled and emplaced allochthonous terrane was subsequently reimbricated on a smaller scale through east-directed thrusting, at which time the allochthon was more pervasively deformed (regional slaty cleavage and fold formation). This may represent late Taconian back thrusting or Acadian shortening. The youngest deformation of the Humber Arm region appears to have been a regional extensional event, with a significant northeast–southwest strike-slip component of movement. This may correlate with the development of Carboniferous strike-slip basins in the present Gulf of St. Lawrence and western Newfoundland. Much of the present structural geometry in the Humber Arm region, including the contacts between ophiolitic and clastic thrust sheets, may have originated during these later two deformational sequences, rather than as a consequence of the initial emplacement history.