Strike-slip Shear Research Articles

The scale of ore-depositional systems: an important restraint on epigenetic vs remobilized syngenetic origins for Archaean mesothermal gold deposits

There is general consensus that the majority of gold deposits in the important group hosted by volcanic or intrusive host rocks within Archaean granitoid-greenstone terrains have an epigenetic origin. However, there is controversy surrounding those stratabound deposits hosted by Fe-rich sedimentary rocks, such as banded iron formation (BIF), for which epigenetic, syngenetic and remobilized syngenetic (lateral secretion) origins have all been proposed. In Western Australia, such controversial gold deposits in Fe-rich sedimentary units have similar ore mineralogies, ore-element ratios and, in places, alteration assemblages and, on the craton scale, show very similar structural controls to the other deposits: equivalent relationships are recorded for similar deposits in other cratons. All Western Australian deposits occur along kilometre-scale shear or fault zones linked to trans-craton, obliqueto strike-slip shear zones that were a focus for: i) carbonation with a mantle-like isotopic signature, and ii) emplacement of high-level A- and I-type granitoids, felsic porphyries and/or calc-alkaline lamprophyres. The scale of the mineralizing systems and the broad contemporaneity of mineralization, as indicated by radiogenic isotope studies, is much larger than that envisaged in conceptual secretion models which involve volumetrically small, locally-enriched source rocks. Instead, Archaean gold mineralization in Western Australia, at least, is probably the result of high fluid-flux during deformation, hightemperature metamorphism and magmatism which may be related to tectonism at convergent plate margins, as in modern examples. There is strong evidence for similar tectonic and mineralizing processes from the Canadian Shield.

Intraplate tectonics in Asia: A precise age for large-scale Miocene movement along the Ailao Shan-Red River shear zone, China

Tertiary left-lateral movement along the 1000 km long Ailao Shan-Red River shear zone appears to have played an important role in absorbing post-collisional northward penetration of India into Asia. Crustal strike-slip shear along this zone caused the formation of a gneiss belt, metamorphosed to amphibolite grade, including anatectic melting. Both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the Ailao Shan-Red River belt. 17 U-Pb isotope analyses were performed on small size-fractions of zircon, monazite and xenotime, extracted from two different leucogranitic layers. The two samples are located about 50 km apart in the central segment of Ailao Shan, in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation. All mineral U-Pb analyses had to be corrected for excess or deficit amounts of radiogenic 206Pb, originating from initial 230Th disequilibrium in the 238U decay series. In monazite, such disequilibrium 206Pb reaches 20% of total radiogenic 206Pb. The corrected U-Pb ages of monazite and xenotime lie between 22.1 and 23.9 Ma, whereas zircon yields significantly discordant U-Pb ages between 30.5 and 33.9 Ma pointing to Precambrian material in the magma source region. Inherited components could also be detected in monazite. The set of U-Pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early Miocene age of 23.0 ± 0.2Ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the Ailao Shan-Red River shear zone.

Ocmulgee fault: The Piedmont-Avalon terrane boundary in central Georgia

Research Article| August 01, 1990 Ocmulgee fault: The Piedmont-Avalon terrane boundary in central Georgia Robert J. Hooper; Robert J. Hooper 1Department of Geology, University of South Florida, Tampa, Florida 33620-5200 Search for other works by this author on: GSW Google Scholar Robert D. Hatcher, Jr. Robert D. Hatcher, Jr. 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 Search for other works by this author on: GSW Google Scholar Author and Article Information Robert J. Hooper 1Department of Geology, University of South Florida, Tampa, Florida 33620-5200 Robert D. Hatcher, Jr. 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (8): 708–711. https://doi.org/10.1130/0091-7613(1990)018<0708:OFTPAT>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert J. Hooper, Robert D. Hatcher; Ocmulgee fault: The Piedmont-Avalon terrane boundary in central Georgia. Geology 1990;; 18 (8): 708–711. doi: https://doi.org/10.1130/0091-7613(1990)018<0708:OFTPAT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Ocmulgee fault is a fundamental boundary within the Piedmont of central Georgia separating North American, Piedmont terrane rocks from exotic rocks of the Avalon (Carolina) terrane. Contrasts in stratigraphy, metamorphic grade, and aeromagnetic signature coincide with the fault trace. The fault comprises a broad zone (ca. 2 km) of foliation dipping steeply southeast and containing localized syn- and post-thermal-peak right-lateral strike-slip shear zones. Oblique dextral convergence between the Avalon and Piedmont terranes, most likely around 350 Ma, produced the steep zone and its attendant dextral shear zones. Steep foliation within the fault zone resulted in its predisposition to later faulting. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

U–Pb, Rb–Sr, and K–Ar isotopic constraints for ductile deformation and related metamorphism in the Teslin suture zone, Yukon–Tanana terrane, south-central Yukon

The Teslin suture zone (TSZ) comprises a portion of the Yukon–Tanana terrane (YT), in the Yukon, formed by steeply dipping layering and L–S tectonite foliation. The TSZ forms the fundamental tectonic boundary between rocks deposited along the ancient margin of North America and allochthonous terranes to the west. TSZ tectonites evolved during initial penetrative dip-slip deformation (Dds) and later dextral strike-slip shear (Dss) along steep, 1–3 km wide shear zones. Several workers have speculated that the TSZ and related YT heterogeneous ductile deformation and associated metamorphism are Devonian to Mississippian in age and related to the intrusion of a similar-age orthogneiss throughout the Yukon and Alaska. However, recent structural and metamorphic studies of the TSZ provide evidence contradicting this view. New isotopic evidence, presented herein, indicates that TSZ dynamothermal metamorphism was cooled by Early Jurassic time, that it cannot be related to Devonian–Mississippian and Permian granitic intrusion, and that it predates Cretaceous plutonism.U–Pb zircon dating of peraluminous orthogneiss constrains primary peraluminous granite crystallization at 355 ± 25 Ma. Three Rb–Sr whole rock + muscovite and three K–Ar muscovite cooling dates of rocks containing Dds and Dss fabrics place a younger age limit of 182–213 Ma (latest Triassic to Early Jurassic) on Dds/Dss deformation. In addition, three Rb–Sr whole-rock + muscovite isochrons and one K–Ar date on biotite indicate peraluminous orthogneisses in the eastern portion of the study area were affected by a mid-Cretaceous thermal event. These data, togemer with structural and metamorphic relationships reported elsewhere, are summarized in pressure–temperature–time–displacement diagrams illustrating the evolution of TSZ and adjacent rocks from Devonian to Late Cretaceous time. Tectonites within the TSZ can be differentiated from peraluminous orthogneiss east of the d'Abbadie fault on the basis of their respective cooling histories.

Superimposed upper proterozoic collision-controlled orogenies in the Mozambique Orogenic Belt of Kenya

The geological history of the northernmost part of the Mozambique Orogenic Belt, exposed in north-central Kenya, is described. A basement of ∼ 1200-Ma (Kibaran) migmatites is overlain by altered sediments with lateral and vertical facies changes defining eight lithostratigraphic groups. There is no evidence for any Archaean-early Proterozoic crust within the mapped part of the orogenic belt. A tectonothermal (Samburuan-Sabachian) event in the amphibolitegranulite facies at ∼ 820 Ma produced major recumbent folds with ductile thrusting to interleave the basement, metasedimentary cover and slices of oceanic metavolcanic complexes. There was syntectonic emplacement of crustal melt granites and metabasic dykes. We interpret this tectonothermal event to be related to plate collision, oblique to the north-south orogenic strike, between the Archaean Tanzanian craton in the west and an eastern Kibaran craton. A one-dimensional manifestation of the suture between these cratonic areas is possibly exposed at West Pokot in western Kenya. Post-collisional greenschist-amphibolite facies (Baragoian-Barsaloian) deformation at between 620 and 570 Ma produced regional upright folds and vertical ductile strike-slip shear zones striking subparallel to the orogenic strike. The metamorphism culminated during the intrusion of syntectonic granites. High-level open folding and brittle shears record the final events of the orogeny. Uplift and cooling, which extended beyond the confines of the orogen, are dated by ubiquitous mineral ages of ∼ 500-480 Ma. Whereas the roughly contemporaneous (Pan-African) Nubian Shield to the north preserves large segments of island arc/ophiolitic material, in the Mozambique Orogenic Belt this volcanic material was mostly consumed during the Upper Proterozoic oblique continent-continent collision. The roots of the collision zone are now exposed in the highgrade Mozambique Orogenic Belt and contrast with the higher exposure levels seen in the Nubian Shield.

Structure of some Lower Old Red Sandstone conglomerates, Kincardineshire, Scotland: deposition from late-orogenic antecedent streams?

A thick succession of early Lower Old Red Sandstone (ORS) conglomerates and subordinate sandstones are preserved in a small fault-bounded basin in the NE Midland Valley of Scotland. The conglomerates associated with the northern margin of this basin are distinguished by a complex provenance which involved both reworking of existing gravels and addition of contemporary volcanic and plutonic detritus. This may explain their characteristic bimodal texture. Deposition was in large, probably permanent, low sinuosity channels with evidence for both periods of uniform high stage flow and repeated flow stage fluctuations. Variable palaeoflows in the basal part of the succession suggest deposition on wet-type alluvial fans supplied by feeder channels with a high sediment and water discharge. The thickness of the succession, the poorly developed cyclothems (despite active syndepositional faulting), evidence for rapid aggradation, and the inferred scale of channel bars and coarsening-upward units produced by bar migration suggest the basin may have been externally drained. Transverse antecedent streams are invoked to account for the structure of alluvium. During the early Devonian, central Scotland lay close to the boundary between regions suffering Scandian orthogonal shortening and Acadian strike-slip shear. The tectonic controls which governed the preservation of early Lower ORS sediments may have been largely uncoupled from those determining the supply and dispersal of the sediment.

Post-Collisional Alkaline Granites

Alkaline (A-type and highly fractionated felsic I-type) granites that may have formed in post-collisional plate tectonic environments are present in the Alaskan Cordillera, the New England and Lachlan Fold Belts of Australia, and the Arabian-Nubian Shield. Their emplacement is associated with diffuse extension and/or strike-slip shear movements that follow the proposed collisional events by ~25 to 75 Ma. The post-collisional alkaline granites are compositionally most similar to within-plate (anorogenic) alkaline granites, but also show affinities with volcanic arc and syn-collisional granites. They may easily be mistaken for anorogenic granites in complexly deformed or poorly exposed terranes. Most of the alkaline granites probably originate by middle or lower crustal anatexis, but the origin of the necessarily high melting temperatures remains unresolved. Their source rocks are just as likely to be previously unmelted mafic meta-tonalites as they are to be restite from which I-type granitic melt had been extracted earlier.

Late Mesozoic strike slip movement on the Border Ranges Fault System in the Eastern Chugach Mountains, southern Alaska

Preliminary studies of mylonitic rocks from the Haley Creek metamorphic assemblage in south‐central Alaska suggest that the Border Ranges fault system was a strike slip, ductile‐shear zone during part of its late Mesozoic history. Evidence for this strike slip event is limited to the hanging wall of the Border Ranges fault system, where deformed crystalline rocks were apparently subjected to a retrograde, mylonitic deformation prior to subduction‐related underthrusting of the latest Cretaceous Valdez Group, but subsequent to a Late Jurassic plutonic event. The strike slip interpretation contrasts with previous interpretations, which related mylonitic fabric development to multiple deformational events during underthrusting of the Chugach terrane, a conflict resulting from the interpretation of lineation as an intersection lineation parallel to the Y axis of strain versus an extension lineation. Evidence for horizontal stretching on a vertical flattening plane includes: (1) a parallelism between all lineations, including intersection lineations, and unequivocal extension lineations developed within systems of strike slip, ductile‐shear zones, (2) extensional fragmentation of feldspars parallel to lineation, (3) local rodding and development of L‐tectonites, (4) boudin axes at a high angle to lineation, (5) crystallographic preferred orientations of quartz, and (6) syn‐kinematic crack‐seal veins with fibers parallel to lineation. The local parallelism between intersection lineations and extension lineation is interpreted as a consequence of sheath folding in a high‐strain zone, and we suggest that it is this complication that is largely responsible for confusion on the kinematic history. Despite evidence for a strain pattern consistent with a strike slip shear zone, evidence for noncoaxial strain and a clear sense of shear is elusive. Nonetheless, coaxial deformation seems unlikely given the structural position of the mylonitic rocks along an ancient plate boundary. Regional correlation of rock types is suggestive of a dextral offset of at least 50 km, although this offset could be due to younger faulting. Thus further work will be needed to clearly distinguish a dextral versus sinistral sense of shear. Important absolute age problems still remain, and thus the tectonic significance of the strike slip event is uncertain. However, the general late Mesozoic age places the event within an important time period in the history of the northern Cordillera. Thus the strike slip event may have (1) played a role in a transform history preceding Early Cretaceous thrusting along the Border Ranges fault system, (2) been a complication from collision of Wrangellia with North America, and (3) been important in the northward transport of outer Cordilleran terranes that is implied by paleomagnetism.

Crustal scale ductile fault systems in the Arunta Inlier, central Australia

Detailed geologic observations along three 100 km long traverses across the Early Proterozoic Arunta Inlier, central Australia, indicate that Paleozoic ductile shear zones are organized into crustal scale structural systems which control the map pattern and distribution of metamorphic zones. Two provinces can be distinguished: 1. (1) a southern one where shear zones form a series of hinterland dipping and south verging, imbricate reverse faults in the west (Redbank area), grading eastward into a foreland dipping duplex in the Arltunga Nappe Complex. Directly north of this complex, flat lying Paleozoic south-directed, ductile shear affects a large portion of the crust, a point which has been underestimated so far. 2. (2) The northern province is characterized by both south and north dipping shear zones showing reverse motion to produce a crustal-scale “pop-up” structure, presumably resulting from a more coaxial deformation history. Strike-slip shear zones become predominant in the eastern part of this province. Stratigraphic and cross-cutting relationships indicate that the southern systems developed first; the northern systems probably developed in response to the lock-up stages of overthrusting in the southern province. One important result of this work is that the distribution of granulite facies rocks and major geophysical anomalies throughout the Arunta Inlier is best explained by their position relative to the Paleozoic ductile fault systems described above. There is no need to invoke extensive Proterozoic uplift of these lower crustal materials.

Structural and kinematic evolution of the teslin suture zone, Yukon: record of an ancient transpressional margin

The Teslin suture zone (TSZ), Yukon, which forms the fundamental boundary between rocks deposited along the ancient margin of North America, and allochthonous terranes to the west, preserves a complex history of pre-mid-Jurassic convergence and dextral strike-slip translation which is overprinted locally by late Cretaceous dextral strike-slip shear. The TSZ, comprised of a 15–20 km thick structural sequence of ductilely deformed sedimentary and volcanic strata, basalt, peridotite and granitoids, is divisible into distinct elongate structural domains based on the distribution of two well-defined but differently oriented stretching lineations, L ds and L ss, L ds and L ss both formed during non-coaxial ductile deformation; L ds trends E-W and plunges down-dip, whereas L ss trends NNW-SSE and plunges shallowly. Two 1–2 km wide, NNW-trending anastomosing shear zones of L ss cross-cut L ds structures, indicating L ss is younger. Other field relations indicate further that L ds began forming earlier than L ss, followed by a period of coeval L ds and L ss formation; latest motion was dominantly parallel to L ss. Synkinematic mineral assemblages associated with both L ds- and L ss-related structures record greenschist to albite-epidote amphibolite facies conditions. L ds tectonites record a complex movement history: to the west textural asymmetries indicate west-side-down, or normal shear, whereas in eastern domains top-to-the-east thrust-style asymmetries dominate. L ss tectonites consistently record dextral, or top-to-the-north, shear parallel to L ss. Tectonic motion began as chiefly penetrative dip-slip shear ( L ds) at a high angle to the present trend of the TSZ, and evolved to dominantly dextral strike-slip shear ( L ss) parallel to the trend of the zone. The geometry, kinematics and sequence of deformation in TSZ tectonites evoke a general model of terrane accretion during oblique plate convergence involving initial shortening at a high angle to the convergent margin and progression to margin-parallel translation.

Proterozoic polydeformation in basement rocks of the Needle Mountains, Colorado

Proterozoic rocks in the Needle Mountains include ∼1,755-m.y. old amphibolite-grade gneisses and ∼1,690-m.y.-old granites that comprise basement to the siliciclastic Uncompahgre Group. The mafic and felsic gneisses, which contain primary structures indicative of volcanic and plutonic protoliths, under-went two phases of isoclinal folding and foliation development prior to emplacement of the ∼1,690-m.y.-old plutons. Synkinematic garnet porphyroblast textures suggest that these fabrics developed during a progressive deformation event, D B . Subsequent D BC deformation caused folding of D B fabrics in the gneisses; development of a subvertical, east-striking foliation in the granites; and generation of a macroscopic sigmoidal foliation pattern throughout the area. D BC structures in the basement are correlated with structures in the Uncompahgre Group and formed prior to pluton emplacement at ca. 1440 Ma. Gently east-plunging mineral lineations in the granites and quartz c -axis fabrics of both the gneisses and granites indicate subhorizontal extension on steeply dipping foliation surfaces during D BC . Asymmetric c -axis fabrics and rare mesoscopic shear sense indicators in the gneisses record a component of dextral shear in domains of east-striking foliation and sinistral shear in areas of northeast-striking foliation. In contrast, symmetric c -axis fabrics in the granites suggest nearly coaxial extension. A model for D BC involving the development of conjugate strike-slip shear zones in response to north-northwest shortening is most consistent with the kinematic and fabric-orientation data. Partitioning of deformation into subparallel zones of noncoaxial and coaxial deformation was controlled by the presence of a strong pre-existing anisotropy in the gneiss complex. The shortening direction during D BC is consistent with that related to north-northwest-directed thrusting in rocks of similar age in central Arizona and northern New Mexico. Therefore, D BC structures are tentatively interpreted to have accommodated transverse shortening and orogen-parallel extension within a foreland during regional crustal shortening between ca. 1690 and 1400 Ma.

Transpression in an Archean greenstone belt, northern Minnesota

Weakly metamorphosed Archean sedimentary and volcanic rocks of the Vermilion district, northern Minnesota, occupy an east–west-trending belt between gneisses of the Vermilion granitic complex to the north and the Giants Range batholith to the south. All the measured strain, a foliation, and a mineral lineation in this belt are attributed to the "main" phase of deformation (D2). Foliation strikes parallel to the belt and dips steeply, and the mineral lineation plunges moderately to steeply east or west and is parallel to the maximum stretching direction, X, and subparallel to fold hinges. An earlier, possibly nappe-forming, event (D1) left little evidence of fabric in the Vermilion district.A number of features indicate that the D2 deformation involved a significant component of dextral strike-slip shear in addition to north–south compression. They include ductile shear zones with sigmoidal foliation patterns, shear bands, asymmetric pressure shadows, and the fact that the asymmetry of the F2 folds is predominantly Z. Other features are more simply explained by a deformation involving simple shear. The S2 cleavage is locally folded, and a new spaced cleavage developed in an orientation similar to that of the old cleavage away from the folds. We consider this the result of a process of continuous shear, with perturbations of flow resulting in folding of S2 and the development of a new foliation axial planar to the folds. The same type of perturbation can lead to the juxtaposition of zones of constrictional and flattening strains, a distinctive feature of the rocks of the Vermilion district otherwise hard to account for. The strain pattern requires a north–south component of shortening in addition to shear. The D2 deformation in the Vermilion district can therefore be characterized as one of transpression: oblique compression between two more rigid lithospheric blocks to the north and south.

Interaction between Strike-Slip and Thrust-Shear: Deformation of the Bullbreen Group, Central-Western Spitsbergen

Field mapping and strain measurement data are used to infer a deformation history for synorogenic, subgreenschist facies sedimentary rocks of the Ordovician Bullbreen Group in central-western Spitsbergen. Evidence for true extension parallel to fold axes, and comparison of incremental strains preserved by fiber-filled extension veins with numerical deformation models, suggest that strike-slip shear was a significant component in what was generally a thrust-shear deformation regime. Initial dextral strike-slip shear parallel to the fault-controlled Bullbreen sedimentary basin subsequently decreased in importance relative to thrusting perpendicular to the basin margin. Although the presumed Caledonian-aged dextral strike-slip motion described here may have been a local phenomenon, it is difficult to reconcile with existing models of N-S orientated, large-scale, sinistral strike-slip in western Spitsbergen.

Calcite mylonites in the Central Alpine “root zone”

Abstract North of the Insubric line, in the Central Alpine “root zone”, carbonate rocks are concentrated in very narrow zones and have been metamorphosed under amphibolite facies conditions by the Tertiary Lepontine metamorphism (grain size ~1 mm). Post-metamorphic deformation under greenschist facies conditions produced calcite mylonite bands a few millimeters to tens of meters wide in these marble zones. Microstructural development begins with twin formation, bending of twin boundaries, grain and twin boundary migration and recrystallization in high stress regions. Progressive mylonitization—by dynamic recrystallization—results in a microstructure with elongated calcite crystals (long axis 20–50 μm, axial ration 1:4). In this fine-grained matrix, porphyroclasts of calcite, quartz, white mica, biotite, diopside, tremolite, scapolite and plagioclase are preserved. Ultra-mylonite bands in pure calcite rocks show an even finer grain size of 5–10 μm. Lattice preferred orientation is not present in the undeformed marbles, but it develops during mylonitization. The c-axis orientation in the mylonites forms an asymmetric point maximum. In the ultra-mylonite no preferred orientation is left. It is concluded from microstructural and textural aspects, that during mylonitization, dislocation creep accompanied by dynamic recrystallization were the most important processes, whilst grain-boundary sliding was the dominant mechanism during the formation of the ultra-mylonites. Shear-sense determinations indicate a horizontal right-lateral strike-slip shear system. This is in good agreement with evidence regarding other movements along the Insubric line which can be observed in ductile and brittle shear zones.

Ductile shear zones in the northern Red Sea Hills, Sudan and their implication for crustal collision

In the northern Red Sea Hills, of Sudan, deformation is largely concentrated into a network of shear zones. Several of these zones strike north–south, but the important Nakasib and Sol Hamid zones are northeast-trending, and there are also minor shear zones which strike east–west. We distinguish between massive shear zones, such as that of Nakasib, and more diffuse braided shear zones, typified by the Oko zone. The massive type owes its character to an origin as a reactivated oceanic suture, whereas the braided type was characterized by strike-slip shearing from its inception. Most of the shear zone rocks are mylonites formed under greenschist facies conditions, but early shearing along the Oko zone took place at higher temperature and resulted in gneissose mylonites with amphibolite facies mineralogy. The northeast-trending, Nakasib shearing appears to have preceded a phase of batholithic intrusion, whereas north–south shearing in the Oko and Abirkitib zones is younger than the batholithic intrusions and is in turn post-dated by emplacement of bimodal granite–gabbro complexes. These events cannot be dated precisely as yet, but took place between 99 Ma and 700 Ma, and probably towards the end of that interval. The pattern of shear and suture zones in northeast Sudan suggests that the northeast-trending Hijaz and Asir arcs, recognized in Saudi Arabia, terminate to the west against a north–south suture which was later rejuvenated to form the Abirkitib shear zone. West of this suture was an immature volcanic arc which may have lain along the margin of the African continent. The rejuvenation which caused northeast and north–south strike-slip shear was probably a consequence of soft collisions in East Arabia.

The East Greenland Nagssugtoqidian mobile belt compared with the Lewisian complex

Summary The rocks, structures and geological history of the Lewisian complex are similar to those of the Nagssugtoqidian mobile belt in East Greenland. Both regions comprise a heterogeneous, mainly tonalitic, gneiss complex which formed 2900–2700 Ma. Both gneiss complexes were intruded by basic dykes and intensely deformed and metamorphosed during two major episodes. The first episode, called Inverian in Scotland, is broadly equivalent to the Nag. 1 episode in Greenland dominated by major dextral strike-slip shear zones about 2600–2300 Ma. The second episode, the Laxfordian of Scotland, is broadly equivalent to the Nag. 2 episode of Greenland associated with the southward translation of sheets of rock along northward-dipping thrusts and shear zones about 1900–1700 Ma. Both gneiss complexes were raised to a high crustal level by 1600–1400 Ma and extensively eroded.

Proterozoic cuspate basement-cover structure, Needle Mountains, Colorado

Deformation zones along basement-cover contacts, which are commonly loci of significant displacement, may also be zones of limited movement when cover and basement are folded together. Proterozoic basement gneisses and siliciclastic cover rocks of the Uncompahgre Group in the Needle Mountains, Colorado, are in contact along steeply dipping deformation zones. Analysis of structures in the basement and cover, and along their contacts, provides evidence for (1) deformation of the basement prior to deposition of the Uncompahgre Group, (2) north-directed, thin-skinned thrusting, and (3) cuspate infolding of cover into basement accompanied by upward and outward movement of the cover. Opposed senses of movement along basement-cover contacts on opposite limbs of the synclinorium generated during cuspate infolding imply that the Uncompahgre Group is parautochthonous on basement. Conjugate strike-slip shear zones in both units accommodated north-northwest shortening. Deformation of basement and cover into cuspate structures may explain the presence of some cover sequences as isolated remnants within gneissic basement.

The Mylonite zones in the Pyrenees and their place in the Alpine tectonic evolution

In the Pyrenees, the development of mylonites zones is one of the most striking structural features. Two sets of mylonites of regional extent have been recognized: large longitudinal E-W to N110°E trending zones (e.g. Mérens fault and North Pyrenean fault) and oblique NW-SE trending zones cross-cutting both the Hercynian and the post-Hercynian terrains. The longitudinal zones limit the major structural zones of the Pyrenees and are associated with NW-SE “en échelons” folds in the Mesozoic terrains and rotations of rootless plutonic or gneissic massifs, acting as competent inclusions in a more ductile matrix, in the Hercynian basement. The oblique mylonite zones limit map-scale fold-bands and appear as the sheared limbs of these folds. The age of the oblique zones and of the major movements along the longitudinal zones is clearly Alpine and the “en échelons” folds seem to have controlled the sedimentation during the Upper Albian and possibly during the Upper Cretaceous. Early movements along the longitudinal zones may have been Hercynian. The analysis of the structures at all scales leads us to interpret these mylonite zones and associated structures as the ultimate result of a transcurrent simple shear acting during the whole Mesozoic period. This strike-slip shearing was probably associated with an extension perpendicular to it from the Permian to the Upper Cretaceous and then to a shortening component also perpendicular to it from the Late Cretaceous to the Eocene. The development of the mylonite zones appears to have predated the major Alpine thrusting but to have been reactivated during this thrusting, acting as initiation sites for the thrusts or as oblique ramps in the case of the oblique mylonite zones.

Hercynian-thrust related shear zones and deformation of the Varied Group on the contact of granulites/Southern Moldanubian, Bohemian Massif/

The three deformation phases inferred from the detailed structural analysis of the Cesky Krumlov Varied Group record the Hercynian development of the Southern Moldanubian of the Bohemian Massif. The deformation is related to the NW-SE thrusting of the large crustal units including granulites. The formation of the NS and NW-SE trending shear zones is connected with the thrust movement. The structural development begins with F1 isoclinal fold formation, that could originate in unmetamorphosed sediments. In the final stage, they were strongly flattened and B1 boudinage developed in the rocks. Aplite dykes and migmatitization of paragneisses occurred at the same time the rocks were metamorphosed. The D2 deformational phase was produced by the simple shear deformation of the unit and folds of various styles around rigid inclusions and the strike-slip shear zone near the boundary of the granulite and the Varied Group were formed. The F1 and F2 folds are parallel with the stretching and mineral lineation indicating a NS to NW-SE direction of the thrusting. The youngest deformation is characterized by spectacular boudinage and by folding of the vertically oriented planes.

Kinematics of Austro-Alpine cover nappes: Changing translation path due to transpression

Upper and Middle Austro-Alpine nappes of the Eastern Alps, the structural uppermost subunits of the Alps, suffered two major superposed deformations (D 1, D 2) during the eo-Alpine (Cretaceous) orogeny. D 1 is characterized by NW- to W-directed thrusting and nappe-internal rock flow, D 2 by large-scale folding and imbrication toward the north and northeast. Structures, strain, metamorphic history, microfabrics, and stretching trajectories indicate that the superposition of D 2 over D 1 structures can be interpreted as a progressive deformation. It is concluded that the emplacement of the nappes follows a curved translation path. A model which emphasizes non-coaxial deformation structures explains the detachment of the nappes, the configuration of the separating thrust planes, and the distribution of intense deformation zones. The overall kinematic history of the Austro-Alpine nappes fits a model in which strike-slip shearing, horizontal shortening, and vertical lengthening act together continuously over a finite deformation zone (transpressional model). This model, emphasizing a gradually changing strain regime, eliminates the space problems encountered in models which consider simple south over north imbrication during the Alpine orogeny. The study stresses two interpretations: 1. (1) stretching lineations indicate nappe-transport directions. 2. (2) transpression may be a common feature of orogenic belts.