High-strain Zones Research Articles

Weakening of deforming granitic rocks with layer development at middle crust

In the middle crust, deformation of granitic rocks is characterized by crystal plastic deformation of quartz and cataclasis of feldspar. For common granitic rocks with textures of framework feldspar and interstice-filling quartz, brittle strength of feldspar is a close approximation of the bulk rock strength because feldspar forms the load-bearing framework. With deformation, these rocks tend to develop localized high strain zones. The high strain zones commonly consist of quartz-rich layers and feldspar-rich layers. We have examined how the strength of granitic rocks changes with the volume fraction of these layers by combining the flow laws developed by Handy et al. [Handy, M.R., Wissing, S.B., Streit, L.E., 1999. Frictional-viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength. Tectonophysics 303, 175–191]. Our results suggest a dramatic weakening of the bulk rock strength at an early stage of the layer development. We also found that formation of muscovite in the layer structures by breakdown of feldspar can result in further weakening in the layer structure. The bulk-rock strength with mica-bearing layers is reduced by a significant amount, compared with the mica-absent layers. Our analysis suggests that layer development with syntectonic formation of mica can be a major source of weakening for granitic rocks.

Mechanisms of weak phase interconnection and the effects of phase strength contrast on fabric development

To determine how deformation mechanisms and fabrics evolve during the strain weakening and localization that accompany the transition from a load-bearing framework to an interconnected weak phase, shear experiments were conducted on a fine-grained gneiss (58% quartz, interconnected; 28% plagioclase, dispersed; 13% biotite, aligned but dispersed) at 745 and 800 °C, 1500 MPa and two strain rates (2×10 −5 and 2×10 −6 s −1). In experiments with a high phase strength contrast (HPSC, 25× and 45×) between dispersed biotite and framework quartz grains, biotite grains become interconnected due to stress concentration at their tips that allow local semi-brittle flow of intervening quartz and cataclasis of intervening plagioclase. In the higher temperature HPSC samples, the biotite partially dehydrated in high strain zones; a single narrow shear zone formed because the PSC between the quartz and biotite/reaction product layers remained high. In the lower temperature HPSC samples there was no reaction, and a penetrative S–C′ fabric formed. The fabric is defined by the many multiply interconnected biotite strands that formed because the PSC between the quartz and biotite decreased as the biotites kinked. In slower strain rate experiments with a much lower PSC (∼10×), biotite interconnection occurs by shearing into quartz/quartz boundaries where new, weak strain-free recrystallized grains form. At low strain a weak S–C′ fabric forms, but it evolves to an S–C fabric as the PSC decreases with strain. Thus, the magnitude of the strength contrast between a weak phase and its matrix influences the mechanism of weak phase interconnection, the degree of strain localization and the fabric.

Post-collisional upper crustal faulting and deep crustal flow in the eastern Wabigoon subprovince of the Superior Province, Ontario: Evidence from structural and 40Ar/ 39Ar data from the Humboldt Bay High Strain Zone

The Humboldt Bay High Strain Zone (HBHSZ) is the most prominent Archean shear zone within the Onaman-Tashota greenstone belt of the eastern Wabigoon subprovince. The HBHSZ was initiated as a terrane boundary during assembly of the greenstone belt (D1) and displays several periods of reactivation (D2–D4) during post-assembly transpression. Each deformation phase was restricted to successively narrow zones and transcurrent shear sense changed with each phase. The boundary of the North Wind Pluton adjacent to the shear zone exhibits similar D2–D4 fabric and kinematics to the HBHSZ and the relations of plutonic phases to the boundary fabrics demonstrate that it was likely emplaced during D2 transpression. The occurrence of plutonism during transpression indicates the lithospheric scale of post-assembly phase D2. The best-defined 40Ar/ 39Ar plateau ages come from texturally simple, extensively recrystallized amphibolite that formed locally during D2 in the HBHSZ and throughout the margins of the North Wind Pluton. The average of these is ca. 2667 Ma, similar to the maximum mica age, and gives a minimum age for D2. Texturally complex amphibolite, formed by partial recrystallization during D2, contains D2 foliation-forming amphibole, and porphyroclasts of actinolite. These amphibolites yield discordant spectra with age gradients and, together with variably lower mica ages, may record later reactivation of the shear zone (D3–D4 and later events). By analogy with the linkage between the late, deep crustal flow and upper-crustal shear zones in the Wawa subprovince the structural and thermal reactivation may be the indicator in a stiff, old superstructure of deep-crustal flow beneath the eastern Wabigoon.

Structural and Geochronological Evidence for Multiple Episodes of Tertiary Deformation along the Ailaoshan-Red River Shear Zone, Southeastern Asia, Since the Paleocene

Abstract Structural analyses show that the Ailaoshan-Red River shear zone (ASRRSZ) in Ailao Mountain is composed of three different deformational domains. These domains may represent three episodes of left-lateral slip experienced by the ASRRSZ. The first episode of such deformation occurred throughout the eastern high-grade belt of the ASRRSZ under a transtensional regime and produced L-type tectonites of amphibolite grade. The second episode of left-lateral slip formed high strain zones overprinting the high-grade belt. Its deformational mechanism is similar to simple shear and the deformed rocks are L-S mylonites of greenschist grade. The third episode of left-lateral slip took place chiefly in a western low-grade belt of the ASRRSZ. This deformation occurred in a transpressional regime, formed an overall structure pattern of a sinistral thrust system and produced phyllonites of low-greenschist grade. Geochronological data indicated that the three episodes of left-lateral slip happened before ∼58–56 Ma, at least from ∼27 Ma to 22 Ma and at ∼13–12 Ma respectively. The first episode of slip in the ASRRSZ appeared to correspond to the initial collision of India and Asia at −60 Ma. The second episode took place almost at the same time as the most intensive compression and uplift in Tibet. The latest event might represent a further eastward material flow in Tibet after ∼16–13 Ma. Thus, the ASRRSZ of southeastern Asia probably experienced three main episodes of Tertiary left-lateral slip in the course of intracontinental convergence since the India-Asia collision.

An argument for channel flow in the southern Canadian Cordillera and comparison with Himalayan tectonics

Abstract Crustal thickening in excess of 55 km, and high heat flow, suggest that a high-standing plateau region in the Cordilleran hinterland was present in the Late Cretaceous. A low strength middle crust developed beneath the plateau, and parts of this layer were exhumed to upper crustal levels in Late Cretaceous to Eocene time. During Late Cretaceous time, structures in the hinterland were reactivated. Strata, buried to mid-crustal depths since the Jurassic, began to flow upward to higher levels; earlier structures were refolded and tightened, and a new transposition fabric developed. Some 10–20 km of the middle crust was involved in high temperature ductile flow. The lower boundary of the ductile zone lies with thrust sense on top of Precambrian rocks of Canadian Shield affinity, and splays upwards to the NE where it closely coincides with highly strained rocks in the hanging wall of the Purcell Thrust Fault. The upper boundary is marked by a normal-sense high strain zone, above which only minor Cretaceous deformation occurred. The boundaries were reactivated at upper crustal levels after cessation of flow in the mid-crustal channel. This reactivation resulted in formation of ductile to brittle extension faults such as the Okanagan Fault System. During final stages of flow, the Precambrian basement gneisses at the base of the channel became domed and exhumed to upper crustal levels. Comparisons with Himalayan tectonics are clearly drawn, but there are significant contrasts such as the long residence time of the proposed Cordilleran channel, and the nature of the channel boundaries.

The orogenic superstructure-infrastructure concept: Revisited, quantified, and revived

The historical superstructure-infrastructure concept (S-I) expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels. Two-dimensional thermal-mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening (phase 1), thermal relaxation and rheological weakening (phase 2), and ductile flow at depth (phase 3). Results predict an upper-crustal superstructure, dominated by early steep structures, separated across a subhorizontal high-strain zone from a ductile infrastructure with late gently dipping structures; this is consistent with observations from the western Superior Province. These models can account for contrasts in structural style, metamorphic grade, seismic reflectivity, and age between upper- and lower-crustal levels. In contrast to conventional thrust-tectonics models, the revived S-I model shows how young structures can form beneath older ones during progressive convergence, thereby encouraging reassessment of standard seismic reflection interpretations.

The role of Pan-African structures in intraplate seismicity near the termination of the Romanche fracture zone, West Africa

The distribution of epicenters of both historic earthquakes and recent seismic events in southeastern Ghana, compiled from local and teleseismic networks, show strong correlation with the Pan-African structures onshore and indicate an alignment with disruptions on seismic sections offshore. The seismic reflection sections reveal basement structures of the external zone of the Pan-African Dahomeyide orogen and these structures can be traced to offsets of shelf strata and seabottom reflectors, providing direct evidence, for the first time, for neotectonic activity that may be responsible for seismicity in the area. The deep structure of the external zone consists of moderately-dipping reflectors inferred to represent high-strain zones in the variably deformed margin of the West African craton. Taken together, the available data suggest that active tectonics in this intraplate environment may involve inversion of the Pan-African thrust structures but that this activity is apparently not related to reactivation of the nearby Romanche Fracture Zone.

The Chako antiform: A folded segment of the Greater Himalayan sequence, Nar valley, Central Nepal Himalaya

Recent and previously published mapping in the Nar valley, north of the Annapurna massif in central Nepal, recognised an enigmatic metamorphic culmination of the Greater Himalayan sequence, surrounded by rocks correlated with the Tethyan sedimentary sequence. The map area is re-interpreted as two structural levels within the Greater Himalayan sequence. The Lower Level is correlated to the well-studied Greater Himalayan sequence of the Annapurna region, based on characteristic rock types, high-strain zones with south-verging shear-sense indicators, and high-grade metamorphism. The rocks of the Upper Level, previously mapped as the sub-greenschist or zeolite facies Tethyan sedimentary sequence, are garnet-bearing schists. Petrography and garnet-biotite thermometry suggest the Upper Level equilibrated at amphibolite facies (500–650 °C). These results support the recent contention that the Upper Level rocks are a component of the Greater Himalayan sequence, and thus indicate that components of the Himalayan metamorphic core vary considerably along strike. Structural, metamorphic and age constraints are integrated into a cohesive regional tectonometamorphic model. The Lower and Upper Levels both experienced D 1 deformation and peak metamorphism before ∼20 Ma. The Lower and Upper Levels were juxtaposed along the synmetamorphic Chame detachment after 20 Ma during retrograde metamorphism. After ∼19 Ma, the Phu detachment placed the unmetamorphosed Tethyan sedimentary sequence above the Lower and Upper Levels. The entire package was then subsequently folded, after 19 Ma, by a non-cylindrical antiform-synform pair with a ∼25 km wavelength creating an apparent dome. The proposed tectonometamorphic model reconciles previously contradictory interpretations of the transition between metamorphic core and overlying sediments in the Nar valley.

Structural testing of tectonic hypotheses by field-based analysis of distributed tangential shear: examples from major high-strain zones in the Grenville Province and other parts of the southern Canadian Shield

The Grenville Province and other parts of the Canadian Shield contain major (>100 km long) high-strain zones, also called shear belts or ductile shear zones, that are hosted by heterogeneously deformed gneisses and schists. In well-exposed segments of three nontabular zones whose dip angle is known locally, at the erosion level and (or) in the shallow subsurface, we investigate the tangential shear strain (better called the tangential unit shear or TUS) without assuming that mineral-shape lineations, common varieties of stretching lineations, are effectively parallel to the local TUS direction. Employing a graphic technique that copes with the geometric conditions of general triaxial strain, we approximate the actual direction and find the sense of local TUS in parts of (i) the Parry Sound shear zone, Grenville Province; (ii) the South Range shear zone, Southern Province; and (iii) the Uchi – English River subprovince boundary zone, Superior Province. Information thus obtained for individual high-strain zones in Ontario confirms the validity of published hypotheses: (i) 1020–970 Ma, normal-sense distributed shearing in the Grenvillian thrust stack; (ii) northwest- directed thrusting of Huronian rocks over Archean basement; and (iii) north-directed thrusting of English River metasediments and associated migmatites onto the Uchi granite–greenstone terrain, under peak metamorphic conditions.

Buckle folding and deep-crustal shearing of high-grade gneisses at the junction of two major high-strain zones, Central Gneiss Belt, Grenville Province, Ontario

Low-plunging, transport-parallel F3 folds are common at all scales in the Central Gneiss Belt of the Grenville Province, but few of these folds are sheath folds. Where the D1–D2 Parry Sound shear zone intersects the D3 Shawanaga shear zone (SSZ) at a high angle, F3 folds formed at several scales (centimetre to greater than outcrop scale) in layered D1–D2 "straight" gneisses. At the start of their evolution, the F3 folds formed just beyond the SSZ with hinges near orthogonal to the D3 shear direction and with typical buckle features, e.g., wavelengths vary with layer thickness, and hinges are discontinuous and bifurcate. The buckle folds evolved within the SSZ by rotation of hinges towards the shear direction. Even though hinges initiated at a high angle to the shear direction, sheath folds were not produced. In addition to tightening the buckles, the ductile reorientation produced thin–thick (extended–shortened) limb pairs and very straight, ridge-like fold hinges and removed small folds from the extended limbs of larger folds. Such features may serve as criteria to distinguish transport-parallel folds that initiated in layering at high angles to the shear direction from those formed in layers containing the shear direction. A general shear parallel to the SSZ can reproduce several features inferred to mark stages in the progressive reorientation of the folds; the pure shear component of the general shear is inferred to have had a positive stretch direction down the dip of the shear zone, at a high angle to the transport (simple shear) direction. The interplay of buckling and shearing in the study area is, plausibly, the expression of deformation at the upper boundary of a channel-like flow that succeeded initial crustal thickening.

Folding of the Rustenburg layered suite adjacent to the Steelpoort pericline: implications for syn-Bushveld tectonism in the eastern Bushveld Complex

Research Article| September 01, 2005 Folding of the Rustenburg layered suite adjacent to the Steelpoort pericline: implications for syn-Bushveld tectonism in the eastern Bushveld Complex Brendan M. Clarke; Brendan M. Clarke KwaZulu-Natal Unit, Council for Geoscience, P.O. Box 900, Pietermaritzburg, 3200, South Africa, School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: bmclarke@mweb.co.za Search for other works by this author on: GSW Google Scholar Ron Uken; Ron Uken School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: uken@ukzn.ac.za; watkeys@ukzn.ac.za; reinhardtj@ukzn.ac.za Search for other works by this author on: GSW Google Scholar Mike K. Watkeys; Mike K. Watkeys School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: uken@ukzn.ac.za; watkeys@ukzn.ac.za; reinhardtj@ukzn.ac.za Search for other works by this author on: GSW Google Scholar Jürgen Reinhardt Jürgen Reinhardt School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: uken@ukzn.ac.za; watkeys@ukzn.ac.za; reinhardtj@ukzn.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information Brendan M. Clarke KwaZulu-Natal Unit, Council for Geoscience, P.O. Box 900, Pietermaritzburg, 3200, South Africa, School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: bmclarke@mweb.co.za Ron Uken School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: uken@ukzn.ac.za; watkeys@ukzn.ac.za; reinhardtj@ukzn.ac.za Mike K. Watkeys School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: uken@ukzn.ac.za; watkeys@ukzn.ac.za; reinhardtj@ukzn.ac.za Jürgen Reinhardt School of Geological Sciences, University of KwaZulu-Natal, Durban, 4041, South Africa, e-mail: uken@ukzn.ac.za; watkeys@ukzn.ac.za; reinhardtj@ukzn.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2005 Geological Society of South Africa South African Journal of Geology (2005) 108 (3): 397–412. https://doi.org/10.2113/108.3.397 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brendan M. Clarke, Ron Uken, Mike K. Watkeys, Jürgen Reinhardt; Folding of the Rustenburg layered suite adjacent to the Steelpoort pericline: implications for syn-Bushveld tectonism in the eastern Bushveld Complex. South African Journal of Geology 2005;; 108 (3): 397–412. doi: https://doi.org/10.2113/108.3.397 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 SocietySouth African Journal of Geology Search Advanced Search Abstract The northeastern Bushveld Complex contact aureole is characterised by numerous, subequally-spaced, kilometre-scale metasedimentary Pretoria Group floor rock domes that penetrate the overlying Rustenburg Layered Suite to varying stratigraphic levels. The origin of these floor domes has been the subject of much debate, with both regional folding and diapiric models invoked as causal processes. Of late, however, the diapiric model of deformation has gained support and has been successfully thermomechanically modelled. Here the Rustenburg Layered Suite adjacent to one of these floor domes, the Steelpoort pericline, is examined and high-strain zones in the magmatic rocks in close spatial proximity to the pericline are described. On the northeastern flank of the pericline, the lower part of the Rustenburg Layered Suite is gently folded but discordant to the floor rock structure against which it abuts. The folds are characterised by their curvilinear axial traces that are parallel to the contact between the metasediments of the floor dome and the encircling igneous rocks. On the southwestern flank, high strain shear zones, characterised by sheath folds and strong tectonic fabrics, indicate shedding of igneous material off the flanks of the dome. These deformation zones adjacent to the Steelpoort pericline add independent support to the diapiric model of deformation proposed for the domal structures. The folding on the northeastern flank is readily attributable to diapir-induced drag and compression associated with diapiric growth. The sheath folds on the southwestern flank formed by the gravity-induced shedding of partially crystallised igneous material off the dome flanks during diapiric uplift. In addition, the numerous transgressive and onlapping relationships observed between units of the Rustenburg Layered Suite adjacent to the floor domes is compatible with diapiric deformation of the floor rocks. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Strain state and kinematic evolution of a fold-nappe beneath the Samail Ophiolite, Oman

The Saih Hatat domal culmination of the Oman Mountains, Arabian Peninsula contains a major, refolded, NE-facing, recumbent, anticlinal fold-nappe within Pre-Ordovician sedimentary cover (autochthon) and Mesozoic platform carbonates that underlie the Samail and Hawasina nappes. Tracts of overturned stratigraphy covering over a 1000 km 2 are associated with a marked strain gradient towards sheath-like, a-type folds that define the fold-nappe lower limb transitional into an elongated SW-facing recumbent syncline and the underlying upper plate–lower plate shear zone. Directions of stretching recorded by fibrous, pressure shadow fringes on pyrite show a 20–30° counterclockwise rotation in the foliation plane. Strain and kinematic constraints require the fold-nappe to form progressively during top-to-the-NE shearing of the Arabian margin with (1) development of a high-strain zone, and (2) subsequent folding of the strongly deformed to transposed stratigraphy in the hanging wall to the shear zone. Rolling of the autochthonous stratigraphic layering of the upper limb through the hinge accompanied by differential shear strain produced the regional scale, highly attenuated a-type folds at the structurally lowest levels. This occurred by simple underthrusting of the footwall (lower plate), as the upper limb of the fold-nappe has to be pinned to the autochthon.

Synchronous vertical and horizontal tectonism in the Neoarchean: Kinematic evidence from a synclinal keel in the northwestern Superior craton, Canada

The long and narrow Carrot River greenstone belt in the northwestern Superior craton is a synclinal keel between two broad granitoid domes. Its unique structural geometry and kinematics place significant constraints on the timing relationship between vertical and horizontal tectonism. A high-strain zone spatially coincident with the belt records evidence for synchronous greenstone-down/granitoid-up dip-slip and dextral strike-slip components. Kinematic and regional considerations lead to the conclusion that the dip-slip component was a result of vertical tectonism and the strike-slip component was a result of horizontal tectonism, and therefore that the vertical and horizontal tectonism were synchronous. It is emphasized that vertical and horizontal tectonism do not have to act exclusively of each other. It is not necessary to downplay the significance of vertical tectonism to validate horizontal tectonism, and vice versa. Evidence for synchronous vertical and horizontal tectonism reported here makes it necessary that both processes (as opposed to either one or the other) and their potential interactions are considered in interpretations of Archean terrains. It is suggested that synchronous vertical and horizontal tectonism was a common process in the Neoarchean and might represent a transition from dominant vertical tectonism in the Mesoarchean (and Paleoarchean?) to dominant horizontal tectonism in the Proterozoic and Phanerozoic. It might be an important process for gold mineralization at the late stages of Archean cratonization.

Granite emplacement and its relation with regional deformation in the Aravalli Mountain Belt (India)—inferences from magnetic fabric

This contribution presents field, microstructural and anisotropy of magnetic susceptibility (AMS) data from the Godhra Granite, a ca. 1 Ga pluton at the southern margin of the Aravalli Mountain Belt (western India) that lies near the Central Indian Tectonic Zone (CITZ). The northern and southern margins of the granite are high strain zones with north dipping, E–W striking foliations, and NW plunging stretching lineations. The presence of preferentially oriented feldspars suggests that the granite was subjected to magmatic deformation during its emplacement. High temperature solid-state deformation fabrics are recorded from microstructural studies, which imply that the granite is deformed. AMS data show magnetic foliations and magnetic lineations in the granite with orientations similar to the field data. The magnetic foliation in the granite is parallel to the D 3 foliation in the gneissic host-rock. Both the stretching lineations and the magnetic lineations at the granite margin high strain zones are parallel with the axes of D 3 reclined folds in the gneiss. Therefore, the emplacement of the Godhra Granite is inferred to have taken place syntectonically, while the gneiss was undergoing the D 3 phase of deformation. The orientation and character of these lineations and the vergence of the D 3 folds imply oblique simple shear, which is inferred to have controlled the emplacement of the granite. The magnetic fabric data are compared with the regional structural trends of the metasedimentary rocks of the southern Aravalli region and the available geochronological data from rocks of the southern Aravalli region are summarised. Based on this information, it is suggested that the emplacement of Godhra Granite, deformation of the southern parts of Aravalli Mountain Belt and, tectonothermal events along the CITZ and Rodinia Supercontinent assembly were related events.

Degree of magnetic anisotropy as a strain intensity gauge in ferromagnetic granites

Anisotropy of magnetic susceptibility data for a ferromagnetic granite (Godhra Granite, NW India) are presented and it is shown that the degree of magnetic anisotropy ( P ′) is not controlled by the mean susceptibility ( K m ). Analyses carried out across a high-strain zone lying between granite and adjacent gneiss show that P ′ values are highest in samples that lie close to the contact and decrease away from it. Based on these results it is concluded that if P ′ is not controlled by K m , then the former can be used to gauge strain-intensity variations in ferromagnetic granites.

An investigation of lower crustal deformation: Evidence for channel flow and its implications for tectonics and structural studies

High-grade metamorphic terranes generally have similar deformation fabrics characterised by a shallowly dipping transposition foliation and recumbent folds. Deformation paths are typically non-coaxial and strain magnitudes extreme. We refer to this association of structures and metamorphism as the high-grade nappe association (HGNA), and argue that it is common, and represents crustal-scale (kilometres thick) shear zones. The structure of such rocks is generally interpreted as comprising stacked thrust sheets separated by thrusts. We concluded, however, that the ‘thrusts’, which are ubiquitously parallel to the transposition foliation, are not thrusts in the normal sense, but are various discontinuities, rotated by large pervasive shear strains. The thrust-like appearance may be further enhanced by late localised shear strain. The sense of shear may be constant across an HGNA shear zone, giving it the geometry and kinematics of a crustal-scale (kilometres thick) detachment zone. Alternatively it may reverse across the body, consistent with channel flow. Structural evidence therefore supports current ideas on the behaviour of the middle to lower crust during orogeny, and illustrates the deformation mechanisms involved. We describe the association with special reference to the Monashee complex, and discuss the implications of our interpretation for the kinematics of high-strain zones, palinspastic reconstruction, and interpretation of deformation fabrics at various scales.

Neoarchean decoupling of upper- and mid-crustal tectonothermal domains in the southeast Slave Province: evidence from the Walmsley Lake area

The Walmsley Lake area of the southeastern Slave Province, Northwest Territories, exposes a crustal transect from greenschist through to upper amphibolite facies. Two tectonothermal crustal domains are proposed based on disparate tectonic histories and rock types. Within the upper tectonothermal domain, D1 deformation reached lower amphibolite facies prior to 2614 Ma and associated M1 metamorphic conditions outlasted D1. Peak M2 conditions reached middle amphibolite facies prior to 2603 Ma and outlasted D2 deformation. Deformation style and metamorphic sequencing during these two pre-2600 Ma events are consistent with regional crustal shortening and thickening in the upper tectonothermal domain. At mid-crustal levels in the lower tectonothermal domain, a third tectonothermal event (D3–M3) produced uppermost amphibolite-facies peak metamorphic conditions, transposed preexisting fabrics to shallow dips, and produced a subhorizontal foliation and recumbent folds at ca. 2583 Ma. D3 structures are not present in the upper tectono thermal domain, however, evidence of M3 heating is locally displayed in rocks at the base of the upper tectonothermal domain. Upper and lower domains were structurally decoupled during the D3–M3 event. In rare places where the zone between the two domains outcrops, it comprises a narrow high-strain zone. The metamorphic and structural topology of the Walmsley Lake area is consistent with the topology predicted from thermomechanical modeling and modern observations of gravitational accommodation in structurally thickened crust.

Tectonics of the Neoproterozoic Southern Granulite Terrain, South India

Geological correlations between South India and adjacent Gondwana fragments are hampered by the lack of a coherent structural framework for India's Southern Granulite Terrain (SGT). This paper is an attempt at delineating its large-scale geometry and Neoproterozoic kinematics and tectonics. The basic regional pattern is that of a high-grade gneissic layering produced during several deformation stages (grouped as D 1), which became refolded during two regionally separated phases of upright folding (D 2a: NNW trending; D 2b: NE trending) and reworked in E- to ESE-trending zones of increased strain (D 3). Of these, the Palghat Cauvery Shear Zone (PCSZ), which marks the boundary between Archaean terrains in the north and Proterozoic belts in the south, may represent a major crustal shear zone, whereas the Achankovil Zone (AZ) is better considered as a high-strain zone characterized by flattening with a reverse component. D 1 structures developed close to the peak of Pan-African high-grade metamorphism. D 2a and D 2b occurred on the retrograde path, partly at supersolidus and partly at subsolidus conditions. D 3 high-strain zones show subsolidus textures. In view of the general clockwise nature with a clear decompressional component of all published P– T paths, we propose that D 2–D 3 occurred during partial exhumation of the terrain. Correlations with Madagascar are weakened as the AZ is no longer considered a major crustal shear zone, whereas correlations with Sri Lanka are strengthened by the presence of a similar syntactical bend possibly produced by an indentor.

RELATIVE AND ABSOLUTE TIMING OF GOLD MINERALIZATION ALONG THE MELIADINETREND, NUNAVUT, CANADA: EVIDENCE FOR PALEOPROTEROZOIC GOLD HOSTED IN AN ARCHEANGREENSTONE BELT

The Meliadine area of the Neoarchean Rankin Inlet greenstone belt is located on the western shore of Hudson Bay, Canada, and is host to several important orogenic or mesothermal gold deposits. Supracrustal rocks comprise alternating southeast-striking panels of mafic volcanic and clastic sedimentary rocks, both of which are host to banded iron formation. The dominant structural element is the Pyke Break, a several-kilometer-wide, high-strain zone characterized by multiple foliations and regionally important shear zones. Gold mineralization is spatially associated with these shear zones. The maximum age of mineralization is constrained by the observation that auriferous veins crosscut ca. 2450 Ma mafic dikes, and a minimum age is constrained by undeformed, postore lamprophyre dikes emplaced at ca. 1830 Ma. Highly concordant SHRIMP-determined ages on hydrothermal monazite indicate auriferous veining occurred at ca. 1850 Ma. These relative and absolute timing constraints suggest gold was concentrated in Neoarchean supracrustal rocks during Paleoproterozoic tectonothermal activity that was widespread in the western Churchill province at ca. 1850 Ma. These observations do not preclude the presence of Archean gold occurrences in the Rankin Inlet greenstone belt, but no early deformed ores can be linked to auriferous veins introduced at ca. 1850 Ma, suggesting remobilization from earlier formed deposits was not a factor in concentrating gold at Meliadine.

Constraining kinematic rotation axes in high-strain zones: a potential microstructural method?

Abstract The correct determination of the kinematic rotation axis in high-strain zones is essential to the study of the tectonic evolution of the Earth’s crust. However, the common assumption that the kinematic rotation axis lies orthogonal to the XZ plane of the finite strain ellipse may be invalid in the case of general shear. Orientation data obtained by electron backscatter diffraction from calcite, deformed in the high-strain Gressoney Shear Zone of the Western Alps, has been investigated using orientation maps, bulk sample crystallographic orientation and misorientation analyses, and detailed intragrain misorientation and crystallographic dispersion analysis. The results demonstrate a strong geometrical coincidence amongst (1) the bulk macroscopic kinematic rotation axis, (2) the orientation of misorientation axes associated with low-angle boundaries, and (3) rotation axes associated with crystallographic dispersion at the intragrain scale. This coincidence is interpreted to reflect a geometric control of the kinematic framework of the high-strain zone on the activity of crystal slip systems. It is proposed that this relationship may be exploited as a new microstructural tool to determine the orientation of bulk kinematic rotation axes in high-strain zones without assuming a geometric link between kinematic rotation and XZ sections. Although further testing is required, application of the approach may lead to a significant advance in our understanding of natural general shear deformation.