Elongation Lineation Research Articles

The mechanism of ascent and emplacement of granite magma during transpression: a syntectonic granite paradigm

We propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern Appalachians. In the study area in western Maine, strain was distributed heterogeneously during Devonian Acadian transpression. Metasedimentary rocks (migmatites at high grades) record two contrasting types of finite strain in zones that alternate across strike. Rocks in both types of zones have a penetrative, moderately-to-steeply NE-plunging mineral elongation lineation defined by bladed muscovite (fibrolite/sillimanite at high grades). In `straight' belts of enhanced deformation rocks have S > L fabrics that record apparent flattening-to-plane strain (apparent flattening zones, AFZs), but rocks between these belts have L > S fabrics that record apparent constriction (apparent constriction zones, ACZs). At metamorphic grades above the contemporary solidus, rocks in AFZs developed stromatic structure in migmatite, which suggests that percolative flow of melt occurred along the evolving flattening fabric. Stromatic migmatites are intruded by concordant to weakly discordant, m-scale composite sheet-like bodies of granite to suggest magma transport in planar conduits through the AFZ rocks. Inhomogeneous migmatite is found in the intervening ACZs, which suggests migration of partially molten material through these zones en masse, probably by melt-assisted granular flow. Inhomogeneous migmatites are intruded by irregular m-scale bodies of granite that vary from elongate to sub-circular in plan view and seem cylindrical in three dimensions. These bodies apparently plunge to the northeast, parallel to the regional mineral elongation lineation, to suggest magma transport in pipe-like conduits through the ACZ rocks. We postulate that the form of magma ascent conduits was deformation-controlled, and was governed by the contemporaneous strain partitioning. Magma ascent in planar and pipe-like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform. Within this hot corridor, it is the difference in temperature between melt-producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing. Heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust. At the wet solidus, we suggest melt flows obliquely toward axial culminations in the thermal antiform, which are sites of melt accumulation and perturbations from which magma may escape to form plutons. Batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust, or ascent may become inhibited with decreasing depth by thermal arrest and solidification. If the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction, melt pressure ultimately may lead to (sub-)horizontal magma fracture, or viscous flow of wall rocks may allow lateral spreading. The resultant plutons have (sub-)horizontal tabular geometries with floors that slope down to the ascent conduit. As the thermal antiform decays, the height of sub-solidus crust that separates the deepest part of the pluton from the anatectic zone increases. Consequently, pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite, which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry.

Ductile and brittle extension in the southern Lofoten Archipelago, North Norway; implications for differences in tectonic style along an ancient collisional margin

The Lofoten archipelago, north Norway, occupies the most internal position of the Caledonian belt in northern Scandinavia, and rocks and structures exposed there are crucial to understanding processes of how the Baltic basement and its cover allochthons responded to continental lithospheric subduction and subsequent continental separation. Relatively little is published about the structural and metamorphic development and especially the timing of these events; consequently, it is unknown how features exposed on these spatially isolated islands relate to those of the adjacent mainland. Rocks in Lofoten were affected by Caledonian regional metamorphism, and structures record tops-east contraction and later extension related to late- to post-Caledonian basement exhumation. Tops-west extension is preferentially developed in meter to km-scale ductile shear zones containing west-dipping extensional shear bands, west-verging rootless folds, and asymmetric feldspar porphyroclasts. West-plunging, sinistral-oblique elongation lineations in the mylonitic foliation are interpreted to indicate the line of transport. Mesoscopic backfolds are locally developed within the tops-west shear zones and further document west-directed transport of structurally higher rocks. the ductile extensional shear zones locally are cross cut by low-angle, tops-west cataclastic normal faults, reflecting progressive unroofing of the shear zones to shallower crustal levels during late- and post-Caledonian extensional events. Northeast-striking, high-angle, brittle normal faults truncate all other fabrics and structures and juxtapose structurally deep undeformed Precambrian basement with the Caledonian nappe sequence. ^40^ Ar/ ^39^ Ar data indicate that ductile extension in Lofoten likely initiated soon after the Caledonian metamorphic peak (at approximately 430 Ma) and continued episodically until at least Permian time. Rocks passed through the brittle/ductile transition rapidly at approximately 275 Ma. Recent paleomagnetic data also point to later phases of brittle faulting in Lofoten, one in the Jurassic/Cretaceous and one related to Tertiary opening of the Norwegian Sea. The magnitude and style of late- to post-Caledonian extension and basement exhumation in Lofoten differs markedly from the formation of phenomenal Devonian basins and exposures of Caledonian ultra-high pressure assemblages in the Western Gneiss Region (WGR) of southern Norway. Differences in the styles of extension between northern and southern Norway are interpreted to reflect inherited differences in crustal architecture that may have been enhanced during the Caledonian orogeny.

Influence of micaceous impurity on dynamically recrystallized quartz c-axis fabric in L- S tectonites from the Singhbhum Shear Zone and its footwall, Eastern India

When temperature and strain rate remain constant the quartz c-axis fabric in deformed pure quartz aggregates, is largely dependent on deformation kinematics. Asymmetry of the fabric, e.g. in type-I asymmetric crossed girdle pattern in natural quartz tectonites, simulated fabric or experimentally deformed quartz aggregates is generally related to sense of vorticity for a non-coaxial flow. Natural quartz tectonites, however, often contain micaceous impurities. Measurement on a sample of 59 quartz tectonites with mesoscopic L- S fabric and representing low T T m , deformation under non-coaxial flow, from the Singhbhum Shear Zone and Dhanjori quartzites. Eastern India provides the basic data to quantitatively assess the influence of mica on (i) asymmetry of quartx c-axis fabric and (ii) degree of crystallographic preferred orientation, i.e. fabric intensity, taking c-axes of dynamically recrystallized quartz grains as a fabric element. A fabric intensity parameter (κ) is defined as the ratio of the greatest eigenvalue to the least eigenvalue of the orientation tensor matrix corresponding to c-axis orientations in each measured specimen. The modal percent of mica (μ) in the sample varies from 2 to 35; that of recrystallized quartz grains (ν), as opposed to relict clasts. from 45 to 98. The asymmetry of the fabrics in the above sample, measured either as the Am statistic or as the angle between the central segment of the fabric skeleton and direction of mineral elongation lineation is independent of mica content. Correlation-regression analysis of the variables κ, μ, and ν. demonstrate a negative correlation between micaceous impurity and the fabric intensity parameter. The regression equation is of the form κ = 0.13 ν 1.11 μ −0.617.

Dextral Pan‐African Shear Along the Southwestern Edge of the Achankovil Shear Belt, South India: Constraints on Gondwana Reconstructions: A Discussion

Achankovil shear zone (ACSZ) (figure 1a), intering charnockites, garnet-biotite gneiss, biotite gneiss, calc-gneiss and granite, as revealed in the preted as a major NW-trending strike-slip shear zone of probable Proterozoic age (Drury et al. 1984), lithological mapping carried out as part of this study (figure 2). A preliminary structural assesshas prominently figured in Gondwana correlative studies (e.g., Windley et al. 1994). Although it is ment reveals the presence of a distinctive assemblage of asymmetric structures in the gneissic exclear that there is a major lithologic change across ACSZ (figure 1b), detailed studies indicating the posures indicating a consistent sinistral sense of shear. Gneissosity, forming the secondary compoprobable sense of shear are lacking across this 10– 20 km wide and .100 km long shear zone. Resitional banding, strikes WNW-ESE to NW-SE dipping moderately toward SW (with relatively rare cently, Sacks et al. (1997) reported field observations bearing on the probable sense of shear of the NE dip) and is conformable in all the rock types of the area, in conformity with the general NW-SE ACSZ. After combining field criteria and kinematic analysis of asymmetric features along the SW trend of the ACSZ. Structurally, the terrain has undergone four phases of deformation (summarized in edge of the ACSZ (known as the Tenmala shear zone), they concluded that the ACSZ indicated priRajesh and Santosh 1997). Open to isoclinal intrafolial folds encountered in marily dextral shear, with a minor component of reverse shear. The authors illustrate their discusthe study area are invariably asymmetric. Fold limbs are zones of transposed gneissosity that consion with examples of kinematic indicators, including shear bands, asymmetric folds, stretched and tain indicators of a probable sinistral sense of shear and well-developed mineral elongation lineation asymmetric garnet, feldspar and quartz porphyroclasts, hook folds, and lineations. Thus they appardefined by quartz ribbons, trails of xenoblastic garnet grains, and the alignment of the long axes of Kently question the correlation of the dextral Achankovil shear zone (rather Tenmala shear zone) with feldspar augen. Pinch-and-swell structures and boudins commonly show rotation with respect to the sinistral Bongolava-Ranotsara shear zone (BRSZ) in the India-Madagascar Gondwana enfoliation. Tight ‘‘S’’ asymmetric F2 folds and synkinematic pegmatite veins are seen in the western semble. We focus the present discussion on the probable part of the study area. The development of a refold structure by superposed buckling is illustrated in sense of shear of ACSZ in the light of new data. Our study area falls around Punalur and its adjoinfigure 3a. There are smaller asymmetric Sand Zfolds congruent with the larger F1, with one limb ing areas within the Quilon district, southern Kerala, South India and is located between longitudes striking E-W while the other strikes N50°E, showing negative extension. During the development of 76°55′ and 76°59′ E and latitudes 9°1′ and 9°3′ N (figure 2), represented in the Survey of India topo F2 on the traces of preserved axial planar cleavage the simple shear is dextral on the left limb and sisheet No. 58C/16. Outcrops within the study area expose a strongly deformed suite of rocks comprisnistral on the right limb. Asymmetrically folded veins (figure 3b), the bulk asymmetry of which represents a sinistral sense of shear, have been found 1 Manuscript received May 14, 1997. in a few locations. Elongate quartz lenticles and 2 Centre for Earth Science Studies, PB 7250, Trivandrum 695 031, India. garnet and biotite aggregates define penetrative lin-

Mid-crustal flow during Tertiary extension in the Ruby Mountains core complex, Nevada

Structural analysis and geochronologic data indicate a nearly orthogonal, late Eocene–Oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer-thick, extensional, mylonitic shear zone of the Ruby Mountains metamorphic core complex, Nevada. New U-Pb radiometric dating indicates that the development of a northward-trending lineation in the infrastructure is partly coeval with the development of a pervasive, west-northwest–trending lineation in the mylonitic shear zone. U-Pb monazite data from the leucogranite orthogneiss of Thorpe Creek indicate a crystallization age of ca. 36–39 Ma. Zircon fractions from a biotite monzogranite dike yield an age of ca. 29 Ma. The three dated samples from these units exhibit a penetrative, approximately north-south–trending elongation lineation. This lineation is commonly defined by oriented bundles of sillimanite and/or elongated aggregates of quartz and feldspar, indicating a synmetamorphic and syndeformational origin. The elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic, nonmylonitic infrastructural core of the northern Ruby Mountains. A portion of this midcrustal flow is coeval with the well-documented, west-northwest sense of slip in the structurally overlying kilometer-thick, mid-Tertiary mylonitic shear zone. Lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure, suggesting fundamental plastic decoupling between structural levels in this core complex. Furthermore, the infrastructure is characterized by overlapping, oppositely verging fold nappes, which are rooted to the east and west. One of the nappes may be synkinematic with the intrusion of the late Eocene orthogneiss of Thorpe Creek. In addition, the penetrative, elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes, suggesting a kinematically related evolution. The area is evaluated in terms of a whole-crust extension model. Magmatic underplating in the lower crust stimulated the production of late Eocene–early Oligocene granitic magmas, which invaded metasedimentary and Mesozoic granitic rocks of the middle crust. The midcrustal rocks, weakened by the magmatic heat influx, acted as a low-viscosity compensating material, decoupled from an extending upper crust. The fold nappes and lineation trends suggest large-scale flow of the weakened crust into the study area. The inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks. At present the inferred Eocene–early Oligocene phase of upper-crust extension remains unknown, but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large-scale upper-crustal extension.

Misalignment of quartz c-axis fabrics and lineations due to oblique final strain increments in the Ruby Mountains core complex, Nevada

To interpret c-axis stereonet patterns of deformed quartz the patterns must be viewed in the XZ strain plane, which is usually assumed to contain the elongation lineation. This study describes several c-axis patterns from a region of complex, amphibolite facies deformation in the northern Ruby Mountains where the late XZ plane is oblique to most of the lineations. Without the lineation as a guide, the proper orientation for interpretation of each pattern is determined by rotating the c-axis data around the normal to the foliation until the c-axes within the foliation plane lie at the center of the stereonet. This new view is inferred to be the XZ plane for the final increments of strain. The rotation produces an internal symmetry in the patterns. In addition, the rotated views across the area show a more consistent geographic orientation than the lineations. This geographic orientation is aligned with strain in a mylonitic shear zone which represents the latest macroscopic evidence for plastic strain along the edge of the study area. The complete reorientation of the quartz textures during the final strain increments implies that they record very little of the finite strain history. Quartz is the only mineral recrystallized during the late strain thereby providing insight into the geometry of strain increments that cannot be determined from other minerals.

Biotite porphyroblast nucleation and growth: control by microfracture of pre-existing foliations in schists in the Robertson River Metamorphics, Australia

AbstractA macroscopic fold formed during the fourth deformation, D4, in the Robertson River Metamorphics, north Queensland, Australia. Growth of biotite, garnet, staurolite and andalusite porphyroblasts also occurred synchronous with D4. Only biotite porphyroblasts have formed preferred alignments across the fold, and they define two dominant orientations. The more common is parallel to D4 fold axes, which is also parallel to D1 D2 and D4 intersection lineations. The other varies up to 90° from this within the D4 axial plane. Mineral elongation lineations do not reflect the extension direction of the deformation in which they formed (D4), but are a function of pre-existing anisotropics in the rock parallel to intersection lin-eations. Porphyroblast growth is inferred to be a result of microfracturing along favourably oriented linear anisotropics formed by multiple intersection of foliations, as well as parting of the the S1, and S2 foliations.

Synkinematic(?) intrusion of the “anorogenic” 1425 Ma Beer Bottle Pass pluton, southern Nevada

The tectonic setting of a transcontinental belt of 1500–1300 Ma granitic intrusions that extends from southern California to Labrador is controversial; however, the granites are conventionally considered to be “anorogenic.” Detailed field, microstructural, and geochronologic data from the 1425 Ma Beer Bottle Pass pluton, southern Nevada, indicate that major mylonite zones recording dextral‐contractile strains were probably active during and/or shortly after pluton emplacement and suggest that the anorogenic interpretation for this pluton requires reevaluation. Mylonite zones up to 100 m thick strike northeast, dip moderately northwest, and contain a consistent west plunging elongation lineation. Mylonites occur along 15% of the exposed granite‐wall rock contact and extend into both the pluton and the wall rock. Mesoscopic and microscopic kinematic indicators record an oblique, dextral/reverse (pluton side down) movement sense. Synkinematic mineral assemblages of hornblende and biotite and dynamic recrystallization of feldspars suggest that deformation occurred minimally under amphibolite facies conditions. A K/Ar biotite date of 1399±32 Ma, obtained from a sample of mylonitic granite, suggests that deformation took place during or soon following pluton crystallization. We reject forcible emplacement of the Beer Bottle Pass pluton as a mechanism for formation of the mylonite zones because (1) rocks near the granite‐wall rock contact are largely unstrained, (2) the mylonite zones conform only locally to the pluton‐wall rock contact, (3) mylonite zones strike at high angles to, and truncate, the intrusive pluton‐wall rock contact, (4) the pluton‐side‐down shear sense is more compatible with a uniform sense simple shear zone than a forcibly intruding pluton, and (5) fabrics indicative of noncoaxial deformation dominate over flattening fabrics. We suggest that the Beer Bottle Pass pluton is fundamentally synkinematic with respect to either (1) a local, contractile deformational event or (2) regional strains produced by distant plate tectonic processes operative between 1500 and 1300 Ma as suggested by Nyman and Karlstrom (1994). The timing and kinematics of shear zones associated with other circa 1400 Ma plutons in the western United States (e.g., Graubard and Mattinson, 1990; Nyman et al., 1994; Kirby et al., this issue) support the latter interpretation. Our kinematic data are more consistent with a contractile or transpressive tectonic setting at circa 1400 Ma (Nyman and Karlstrom, 1994; Nyman et al, 1994) than with models involving regional extension (e.g., Windley, 1993).

Extensional origin of ductile fabrics in the Schist Belt, Central Brooks Range, Alaska—II. Microstructural and petrofabric evidence

A regional system of S-dipping faults is exposed in the Florence and Fall Creeks area of the south-central Brooks Range. This fault system has previously been mapped as the ·root zone’ of a N-vergent fold-and-thrust belt of Jurassic-Cretaceous age, although individual faults juxtapose lower grade on higher grade metamorphic rocks suggesting apparent extensional geometries. The structurally highest of these faults places rocks of the oceanic Paleozoic-Mesozoic Angayucham terrane, together with unconformably overlying Cretaceous clastic rocks, on Devonian metagreywacke and phyllite. This metagreywacke-phyllite (MP) unit in turn structurally overlies Devonian (?) and older basement rocks of the Brooks Range Schist Belt along a S-dipping structural contact previously mapped as the Florence Creek fault. The Schist Belt and MP units are both characterized by a regionally developed, S-dipping greenschist facies foliation that displays a pronounced N-S-trending, down-dip elongation lineation which earlier workers regarded as being associated with N-vergent thrusting. While only one foliation ( S d) is recognized in the MP unit, in the underlying Schist Belt this foliation ( S 2) overprints an earlier blueschist facies mineral assemblage and associated foliation ( S 1). Mylonites with strong asymmetric crystal fabrics are well developed in quartz stringers in the MP unit and in Schist Belt rocks located at less than 250 m beneath the Florence Creek fault. These asymmetric single- and cross-girdle c-axis fabrics indicate a top down-to-the-south shear sense. This shear sense is confirmed in XZ sections by the presence of elongate, dynamically recrystallized quartz grains oblique to the S d − S 2 mylonitic foliation. Quartzose and quartzo-feldspathic lithologies become coarser grained in the deeper levels of the Schist Belt, and a greater proportion of the quartz grains are equant in outline suggesting the increasing importance of temperature-sensitive recovery processes. These deeper level quartz tectonites display less strongly asymmetric cross-girdle c-axis fabrics and oblique grain shape alignments for at least a further 7.5 km beneath the fault, the asymmetry still indicating a top-down-to-the-south shear sense. Of the 35 quartz-rich samples selected for detailed microstructural and crystal fabric analysis, only two contain c-axis fabrics and oblique grain shape alignments indicating a top-up-to-the-north shear sense. This pervasive top-down-to-the-south shear sense is inconsistent with tectonite fabrics in the Schist Belt being related to N-vergent thrust faulting, but accords well with an extensional model for exhumation of high- P rocks of the Schist Belt. Extension-related high strain fabrics persist for a structural thickness of at least 7.7 km beneath the Florence Creek Fault, indicating that the Florence and Fall Creeks area may be situated over a large-scale (at least 8.0 km thick) zone of extensional shear dipping towards the south. How far this zone of penetrative deformation can be traced along strike remains unclear.

Bedding parallel veins and their relationship to folding

Laminated bedding parallel veins hosted in turbiditic sandstone shale sequences from central Victoria, Australia, consist of stacked, millimetre thick, sub-parallel sheets of quartz separated by micaceous layers, wall rock slivers and pressure solution seams. They have very high length to thickness ratios, are laterally continuous over tens to hundreds of metres, and have relatively uniform thickness compared to other vein types. They are intimately associated with and folded by chevron folds, and the quartz grain shape elongation lineation is commonly orthogonal to mesoscopic and macroscopic fold hinge lines. The bedding parallel veins have two morphological forms. Type I are thin (commonly 5–10 cm) laminated veins which have complex microstructures dominated by phyllosilicate inclusion surfaces, related to oblique opening along bedding with varying rates of deposition (opening) relative to shear displacement (slip) along the bedding surfaces. More common are Type II, thicker (generally <20 cm), banded veins of alternating milky-white quartz with wall rock inclusion laminae (formerly fragments) bounded by stylolitic partings parallel to both bedding and the vein margins. The inclusion surfaces in Type I veins track the opening direction during vein formation. Vein opening-sense criteria suggest cyclical pore fluid pressure fluctuations which predate the amplification and propagation of the host chevron folds; i.e. prior to attainment of significant limb dip. Different layer parallel shortening and amplification rates for individual layers within the sedimentary sequence may lead to bedding parallel veins with an opening sense unrelated to the flexural slip folds which eventually follow.

Southern Menderes Massif: an incipient metamorphic core complex in western Anatolia, Turkey

In the southern sector of the Menderes Massif, north of Selimiye (Milas) augen gneisses interpreted as a deformed peraluminous granite have been dynamothermally metamorphosed and are surrounded by and intrude a regionally metamorphosed Palaeozoic ‘envelope’. The granitic rocks exhibit a moderately-dipping mylonitic foliation and NNE-SSW- trending mineral elongation lineation. The progressive deformation of the granitic rocks produces a structural sequence typical of an extensional shear zone marked, from bottom to top, by a very thick extensive zone of mylonites followed in turn by brecciated mylonite and cataclasite. The kinematic indicators exhibit a top-to-the south, down-dip sense of shear. These structures are attributed to exhumation of the granitic rocks of the massif along a major, south-dipping, normal-sense shear zone that accommodated crustal extension during Late Oligocene collapse of the orogen in western Turkey. Thus, the southern Menderes Massif may be interpreted as the exhumed footwall of a major extensional shear zone, and possibly as an incipient core complex.

Deformational Styles in the Proterozoic Pinal Schist, Pinal Peak, Arizona: Microstructural and Quartz cAxis Analyses

A >5 km thick, north-dipping shear zone in the Pinal Peak map area, near Globe, Arizona, reveals contact metamorphism of the Pinal Schist through intrusion of the Proterozoic Madera Diorite. In Pinal Schist, sillimanite and rare andalusite occur in the metamorphic aureole around the diorite, but not elsewhere in this or adjoining map areas, where metamorphic grade is interpreted to be low greenschist facies. Deformation was synchronous in the schist and the diorite and is recorded by a well-developed, generally N- and NW-trending elongation lineation. Field interpretations of ductile deformation fabrics generally indicate top-to-the-south or reverse (present day coordinates) displacement. Microstructures in the Pinal Schist, the Madera Diorite, and the Pinal-Madera contact zone include S-C fabrics, mica fish, asymmetric σ-type tails on porphyroclasts, and grain-shape and lattice-preferred mineral orientation, most of which indicate top-to-the-south (locally SE to SW) displacement, consistent with field ...

A shear zone origin for Alleghanian (Permian) multiple deformation in eastern Massachusetts

The area around Worcester, Massachusetts, has been used to determine which deformational and metamorphic features are due to the Alleghanian (Permian) orogeny and which are pre‐Alleghanian. Isolated, fault‐bounded inliers of Carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages. The first cleavage formed synchronously with metamorphism. Pre‐Carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages. The initial cleavage formed during the first metamorphism and the second cleavage formed during the second, retrogressive metamorphism. Thus the second two cleavages and the second metamorphism in pre‐Carboniferous rocks are interpreted as Alleghanian. Normal displacements on two distinct faults are bracketed by the formation of the first and second Alleghanian cleavages. The initial faulting, along the newly defined Wachusett mylonite zone (WMZ), formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest. The second faulting, along the Clinton‐Newbury fault (CNF) occurred along a steeply inclined plane which cuts the WMZ mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west. Alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined. This zone is interpreted as a ductile shear zone which moved twice, forming the first and second Alleghanian cleavages. Early‐formed, pre‐Alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates, and new hydrous minerals formed in the shear zone, indicating that it was a pathway for fluid flow. The initial motion was left‐lateral with a thrust component. The second cleavage formed during top‐to‐the‐northwest normal displacements. Thus the history of Alleghanian tectonism in this area began with sinistral faulting and then included three displacements along normal faults or shear zones. This suggests that the strain history included two distinct episodes, with the second experiencing slightly different strain orientations at different times, resulting in first WMZ normal motion, then CNF normal motion and finally normal displacements related to the second Alleghanian cleavages. This history agrees well with other work on the nature of the Alleghanian orogeny in the northeast United States.

Kinematic evolution of the Miller Range Shear Zone, Central Transantarctic Mountains, Antarctica, and implications for Neoproterozoic to Early Paleozoic tectonics of the East Antarctic Margin of Gondwana

High‐grade ductile tectonites of the Precambrian Nimrod Group in the central Transantarctic Mountains form the Miller Range shear zone (MRSZ). With no exposed boundaries, this zone has a minimum structural thickness of 12–15 km. Shear‐sense indicators record consistent top‐to‐the‐SE, or left‐lateral, shear within the NW striking, moderately SW dipping zone. Cylindrical folds with axes normal to elongation lineation (Le) are kinematically consistent with other shear indicators. They may represent early stages in the development of subordinate noncylindrical sheath folds, which indicate locally high bulk ductile strain and a moderate strain gradient. Pervasive, open to tight cylindrical folds with axes parallel to Le formed during shear and may reflect a component of constrictional strain. Quartz c axis fabrics from micaceous quartzites show asymmetric single girdles evident of dominantly rhombohedral slip, with limited basal‐plane slip, affirming both the consistency of shear sense and high‐grade syn‐kinematic conditions. Deformation did not persist during subamphibolite facies cooling, as shown by (1) a lack of basal‐plane slip in ductilely deformed quartz, (2) a lack of quartz subgrains and grain shape‐preferred orientation, and (3) the presence of oriented muscovite “fish” included within polygonal quartz grains, which show that quartz grain boundaries migrated and annealed under static conditions following ductile shear. From the uniform Le orientation and consistent shear sense, we interpret that ductile deformation resulted from a single, kinematically simple, left‐lateral (top‐to‐the‐SE) shear event. Together, the scale, high total strains (γ ≥ 5), fabric uniformity, and the widespread presence of asymmetric microstructures formed at high temperatures, all indicate that strain rates within the MRSZ were high and that it represents a major crustal structure. Orogen‐parallel displacements within this zone during the latest Neoproterozoic to Early Cambrian were at a high angle to penecontemporaneous orogen‐normal contraction in outboard supracrustal rocks, suggesting that the Neoproterozoic to early Paleozoic plate margin of Antarctica was characterized by left‐oblique convergence in which strain within the orogen was partitioned into deep‐level strike slip and shallow‐level contraction.

Lithology and structure of the Wandilla terrane, Gladstone‐Yeppoon district, central Queensland, and an overview of the Palaeozoic subduction complex of the New England Fold Belt

The Wandilla terrane consists of, from west to east, the Doonside Formation and the Wandilla Formation. The Doonside Formation is dominated by chert and mudstone. Conodonts from a single locality (Devils Bend) indicate an age in the interval Late Silurian to Middle Devonian. The Wandilla Formation consists of mudstone with greywacke, tuff, chert and greenstone. The greywacke is dominated by volcanic detritus with sparse but persistent ooliths that suggest contemporaneity with Early Carboniferous strata of the Yarrol shelf to the west. The Wandilla terrane is structurally complex with two main deformation events. The first deformation (D1) formed widespread lenticular melange during offscraping at the toe of a subduction complex. The second deformation (D2) formed a moderately to shallowly east‐dipping cleavage that typically dips less steeply than the north‐northwest‐trending D1 structures, and contains a down‐dip elongation lineation. D2 structures were produced by the Late Permian to Triassic Hunter‐Bow...

The geometrical relationship between the stretching lineation and the movement direction of shear zones: Discussion

IN n recent paper, Lin & Williams (1992) emphasized the difference between the plunge of stretching lineations on S-surfaces and that of the movement direction in shear zones, and discussed its implications for interpretation of shear zone kinematics. They further showed that, knowing the strike of a shear zone, the orientation of the S-surface and of the stretching lineation, it is possible to determine the dip of the shear zone and the movement direction along it. They applied this technique to the D1 deformation increment documented by Robert (1989) along the eastern segment of the Cadillac tectonic zone (CTZ) and they proposed an alternative interpretation. This major fault zone extends over 200 km of strike length in an E-W direction, parallel to contacts between major lithologic units. The D1 increment of deformation along the CTZ was interpreted by Robert (1989) as transpressional, involving a significant component of shortening across the CTZ, accompanied by localized dip-slip movements, and a minor component of dextral transcurrent shear. This interpretation was proposed to reconcile the obliquity of $1 to the shear zone boundaries and other asymmetric features, observed in plan but not in cross-section, and the contained steeply E-plunging elongation lineation. Lin & Williams (1992), considering that the obliquity of foliation to shear zones boundaries observed in plan view is apparent, used the orientation of $1 and the stretching lineation to infer a southern dip of 60 °. With a note of caution, they proposed that D~ deformation along the CTZ recorded reverse oblique movements (south over north) with a minor dextral component. Alternative interpretations of the deformational history of the CTZ are welcome because the internal structure of the CTZ is very complex and challenges conventional shear zone analysis. However, as will be discussed below, the interpretation proposed by Lin & Williams (1992) is not consistent with the available data. I also take this opportunity to present additional observations that bear on the interpretation of the CTZ. BOUNDARIES OF THE CTZ AND OBLIQUITY OF $1

Progressive deformation and the rotation of contemporary fold axes in the Ballybofey Nappe, north‐west Ireland

AbstractInternal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north‐west Ireland, shows that an intense S3 schistosity is developed axial planar to mesoscopic and minor F3 folds. In areas of low D3 strain, F3 fold axes plunge gently towards the north‐east, whereas in regions of greater strain plunges are towards the south‐east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non‐coaxial deformation oblique to the original mean F3 orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.

The structural development of the early Proterozoic Birimian and tarkwaian rocks of southwest Ghana, West Africa

Rocks of early Proterozoic age belonging to the Birimian Supergroup and Tarkwaian Group make up a large part of the West African Craton. In southwest Ghana, the Birimian comprises a sequence of mostly fine-grained sedimentary/volcanoclastic rocks, which separate a series of four, northeast-trending volcanic belts. The Tarkwaian consists of coarse clastic sediments derived from Birimian and granitoid source rocks. Early formed structures in Birimian and Tarkwaian rocks include upright sub-horizontal to gently plunging folds with ongoing deformation resulting in the formation of higher strain zones located at the northwest margins of the volcanic belts. Deformation was principally one of flattering as revealed by the tight to isoclinal nature of the folds and absence of a pronounced mineral elongation lineation. Non-coaxial deformation is most clearly evident along the northwest margin of the Ashanti Belt where oblique thrusting of Birimian onto Tarkwaian rocks is observed. Deformation is attributed to a single progressive deformation event involving compression along a northeast-southwest directed axis that affected both Birimian and Tarkwaian rocks. Birimian sedimentary and volcanic rocks are intruded by the geochemically and texturally distinct Cape Coast and Dixcove granitoids. Field, petrographic and isotopic data have shown, contrary to previous interpretations, that the Dixcove granitoids within the volcanic belts were emplaced prior to regional deformation. A new model for the evolution of the Birimian and Tarkwaian is proposed and its implications are discussed.

Late Miocene ductile extension and detachment faulting, Mykonos, Greece

The geometry and style of structural relations at map to microscopic scale on Mykonos, Cyclades, Aegean Sea, Greece, suggests ductile extension and detachment faulting during the late Miocene. This island is dominated by granitic rocks that have a well-developed, shallowly dipping mylonitic foliation and a northeast-southwest-trending mineral elongation lineation. Kinematic indicators in these rocks exhibit a top-to-the-northeast, down-dip sense of shear. Textural relations suggest that conditions during mylonitic deformation were greenschist to amphibolite facies, and published K/Ar geochronology suggests a late Miocene age for this deformation event. Mylonitic rocks lie in the footwall of a low-angle fault, and fault striae parallel to the elongation lineation suggest kinematic coordination between mylonitic deformation and movement along the fault. Translation along the fault juxtaposes younger tilted sedimentary rocks of low metamorphic grade upon the once deeper mylonitic rocks. The style and geometry of deformational fabrics and map relations on this island are strikingly similar to those observed in metamorphic core complexes of the western United States.

“Pseudostratigraphy” and thrusting in relation to the structural evolution of the Joma ore-body, North Trøndelag, Norway

Joma Mine exploits a stratiform CuZn volcanic-associated massive sulphide deposit within the Norwegian Caledonides. The ore layer overlies an inferred feeder system hosted by mafic metavolcanites within the grossly inverted stratigraphy of the Leipik Nappe. Mine-scale inversion is supported by orebody zonation and the relative position of ore layer and feeder system. Mesoscale structural data from regionally distributed quartz-rich phyllites require two major periods (D 2 and D 3) and one minor period (D 4) of deformation. D 2 generated a transposition foliation containing an elongation lineation essentially co-linear with F 2 hinge lines. D 3 formed a NW -dipping crenulation foliation as hinge surface to SW -plunging F 3 folds. Before F 3 , D 2 linear data probably trended NW in the shallowly W -dipping transposition foliation. D 4 formed minor kinkbands. Mesoscale data from the ore layer affirm the deformation sequence, but also suggest a D 1 thrusting and folding event. F 2 vergence and facing at regional and mine scales further support D 1. The ore-layer sequence varies between and, less commonly, within layers, and lacks consistent polarity. This variable sequence is ascribed to D 1 and D 2 thrusting, folding and sliding, and is herein termed “pseudostratigraphy”. The existence of level-scale pseudostratigraphy contrasts with preservation of gross relationships at mine scale, and places limitations on the dimensions and consequences of D 1 and D 2 structures. At the end of D 1, pseudostratigraphy of the thrust-disrupted ore system occupied the lower limb of an E-closing regionally developed antiformal anticline. Folding and sliding during D 2 resulted in the further disrupted ore system being inverted and occupying the lower limb of a large-scale, E-closing, F 2 antiform. In D 3, the inverted ore layer occupied and was slightly S of the broad hinge zone of the regionally developed NE-closing Joma Synform; its present arcuate geometry reflects this. D 4 had little influence on the structural evolution of the orebody.