Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L¹ ₀ intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.     

Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S₁). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north–south shortening (D₂), open to isoclinal south–southeast-oriented F₂ folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D₂ Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D₂ history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.     

Polyphase Deformation of the Weihai-Rongcheng UHP Unit Rocks, NE Sulu： Insights into the Tectonic Evolution of the Dabie-Sulu UHP and HP Belts, China

Different scales of structural data reveal a complex deformation history of ultrahigh- pressure （UHP） rocks exposed in the Weihai-Rongcbeng area, NE Sulu （northern Jiangsu-eastern Shandong）, eastern China. Excluding pre-UHP deformations, at least five major sequential deformational stages （D1-Ds） are recognized. The first deformation （DO produced a weak foliation and lineation in massive eclogites. The foliated eclogite with a dominant foliation containing a stretching and mineral lineation was developed during the I）2 deformation. Both the D1 and D2 deformations occurred under UHP metamorphic conditions, and are well preserved in the eclogite bodies. D3 structures which developed shortly after the formation of granulite/amphibolite facies symplectites are characterized by imbricated associations marked by a regional, steeply dipping foliation, compositional layering, eclogite boudinage, isoclinal folds and reverse ductile shear zones. The D3 deformation was accompanied by decompressional partial melting. A regional, gently dipping amphibolite facies foliation and stretching lineation, low-angle detachments, and dome- and arc-shaped structures formed during the D4 deformation stage dominate to some degree the map pattern of the Weihai-Rongcbeng UHP domain. The last stage of deformation （Ds） gave rise to the final exhumation of the UHP rocks. Ds is characterized by development of brittle-dominated high-angle faulting associated with emplacement of large volmnes of undeformed granite plutons and dykes dated at 134-100 Ma. The deformational and metamorphic sequence followed by the UHP rocks in the Weihai-Rongcheng area is similar to that studied in the entire Dabie-Sulu UHP and HP metamorphic belts from microscopic to mapping scale. Based on structural data, combined with available petrographic, metamorphic and geochronological data, a speculative tectonic evolutionary model for the Dabie-Sulu UHP and IIP belts is proposed, involving continental subduction/collision between the Sino-Korean and Yangtze cratons and subsequent polyphase exhumation histories of the UHP and IIP metamorphic rocks.     

Genetic types of tantalum ore deposits and their economic importance

Metasediments and meta-igneous rocks of the Willyama Supergroup in the Paleoproterozoic Olary Block of South Australia were deposited at ～1700 Ma. Intrusion by I-type granitoids at 1630 Ma was followed by the Olarian Orogeny, comprising two events of deformation and high-grade metamorphism at 1590 ± 20 Ma. Regional S-type granites and rare-metal pegmatites also formed during the Olarian Orogeny. The K-Ar isotopic system in primary pegmatitic muscovite closed at ～1505 ± 7 Ma, and the third event (regressive) of deformation and metamorphism together with minor granite emplacement, associated with the Olarian Orogeny, occurred at 1500 ± 20 Ma. A widespread thermal event occurred at 1100 to 1200 Ma and resulted from the Musgravian Orogeny. This was followed by crustal extension, tholeiitic dolerite dike intrusion, and rifting at 700 to 800 Ma. Cooling after the Delamerian Orogeny is recorded at ～466 to 475 Ma in the muscovite data. The ⁴⁰Ar/³⁹Ar data from many mica samples are complex because of multiple phases of thermal resetting and regression. This partial resetting of the K-Ar system is characterized by multiple age components and mixtures between them.     

印度-欧亚侧向碰撞带构造-岩浆演化的动力学背景与过程——以藏东三江地区构造演化为例

对于印度与欧亚板块的侧向碰撞带,即藏东三江地区的新生代构造分析揭示出三种不同性质的构造样式,它们形成于不同的地质时期,发育于不同的地壳层次：（1）区域规模至露头尺度上发育的具有薄皮属性的逆冲断层与推覆构造,它们广泛分布于三江地区,尤其是兰坪-思茅盆地内;（2）以红河-哀牢山断裂、澜沧江和怒江-高黎贡山断裂等为代表的区域高温型走滑韧性剪切带构造和局部发育的脆性走滑断裂构造,后者在中新生代盆地内部断裂更为发育;（3）遍布全区发育的变质核杂岩构造与地堑-半地堑盆地.区域岩浆活动性与区域构造事件的发生具有密切的时空联系.区域性递进收缩事件与走滑事件发生于碰撞过程的早期阶段,并随后伴随着早期具有岩石圈板块俯冲性质的碰撞弧高钾岩浆活动,而后期的递进伸展事件诱发了板内伸展环境中的晚期高钾岩浆活动.二者之间的碱性岩浆活动间歇期,对应着区域构造体制的转变与区域伸展作用的发生,变质核杂岩的发育与微弱的钙碱性岩浆活动是其最直接的表现.区域古地磁资料分析表明,印度-欧亚板块之间的板块相互作用、区域板块与地块的旋转以及由此所致的不同构造环境制约着各种地质事件的发生与发展.北向运动的印度板块的旋转致使三江地块在新生代演化中发生了两次规模与特点不一的地块旋转过程,即早期的大角度快速旋转和晚期的小角度慢速旋转事件.它们分别对应于早期的递进收缩变形、走滑事件和具有碰撞弧属性的碱性岩浆活动与中期的区域伸展、变质核杂岩的发育与微弱的钙碱性岩浆活动性,以及后期的递进伸展作用和晚期陆内碱性岩浆活动性.     

The Markoye Shear Zone in NE Burkina Faso

Birimian supracrustal sequences in NE Burkina Faso are dominated by meta-volcaniclastic greywacke, intercalated meta-conglomerate, siltstone and shale. The sequences where subjected to two phases of deformation and contact metamorphosed to hornblende–hornfels facies during emplacement of pyroxenite–gabbro–norite (Yacouba Mafic complex), granodiorite–tonalite (Tin Taradat granodiorite–tonalite) and dolerite dykes.Structural studies indicated that the NE-trending, first-order crustal-scale Markoye Shear Zone (MSZ; Markoye Fault of [Jeambrun, M., Delfour, J., Gravost, M., 1970. Carte géologique de L’Oudalan. Bureau De Recherches Geologiques et Miniéres, Burkina Faso.]) has undergone at least two phases of reactivation concomitant to two phases of regional deformation. The first phase of deformation, D1, resulted in the formation of NNW-NW trending folds and thrusts during dextral-reverse displacement on the MSZ. The deformation is termed the Tangaean Event and predates the Eburnean Orogeny. D2 phase involved a period of SE–NW crustal shortening and sinistral-reverse displacement on the MSZ, and is correlated to the Eburnean Orogeny 2.1 Ga. Deformation in D2 is characterised by NE-trending regional folds (F2) and a pervasive NE-trending foliation (S2-C to S2). Within the MSZ, deformation is characterised by NNE-trending zones of mylonite that are bordered in the hangingwall and footwall by pseudotachylite veins. Buck quartz-carbonate veins and quartz cataclasite veins crosscut the mylonite zones and are, in turn, crosscut by quartz–chlorite–(muscovite) shears that formed during reactivation of the MSZ late in D2. Several generations of veins are recognised at the Essakane main deposit (EMZ): Arsenopyrite–pyrite–gold mineralization in quartz veins formed in D1 during metasomatic alteration of the host rocks; Vein-stockwork gold mineralization is interpreted to have formed late in D2.     

Aeromagnetic signature of an exhumed double dome system in the SW Grenville Province (Canada)

Analysis of aeromagnetic data in the Grenville Province reveals the presence of two regional‐scale unmapped structural domes (the Morin and Mékinac‐Taureau domes) with an oval‐shaped magnetic pattern bounded by regional‐scale shear zones and a geometry that is similar to that produced in crustal flow models under extension, which predict two upright domes of foliation (double dome) separated by a steep shear zone. The Mékinac‐Taureau dome, a metamorphic core complex, and the Morin dome may have been exhumed by channel flow. Exhumation occurred by a combination of thrust, normal‐sense and wrench shear zones. The preservation of paragneisses in the Morin dome suggests that it underwent a lesser degree of exhumation than did the Mékinac‐Taureau dome. This study shows how the integration of local field information with magnetic data in a regional tectonic setting can reveal and delineate concealed gneiss domes and highlights a role for strike‐slip tectonics in the creation of regional structures involving the exhumation of deep crust.     

Superposition de la tectonique éburnéenne et panafricaine dans les granitoïdes de la bordure nord du craton ouest africain, boutonniére de Zenaga, Anti-Atlas central, Maroc

The Zenaga Inlier shows a comprehensive record of the Eburnian and Pan-African Orogenies. The Eburnian is characterised by high-temperature regional metamorphism and complex magmatism. The early (Azguemerzi) granodiorite has an isotopic mantle signature and was emplaced diapirically during the Eburnian Orogeny causing local thermal metamorphism. The foliation observed in this granitoid is a result of the interference between its primary syn-emplacement foliation and the regional foliation under amphibolite-facies conditions. The northern part of Zenaga has been intruded by the leucocratic granites of Tazenakht. These granites are cut by mylonites and phyllonites, corresponding to the Pan-African shear zones and accompanied with sub-greenschist-facies metamorphism during the Pan-African Orogeny. The deformation was the result of a regional sinistral transpressive event. This study in the northern part of the West African Craton shows the superposition of the Pan-African on the Eburnian Orogeny and the presence of a major fault in the Anti-Atlas.     

Polystage deformation of the Gaoligong metamorphic zone: Structures, 40Ar/39Ar mica ages,and tectonic implications

The Gaoligong metamorphic zone is located southeast of the Eastern Himalayan Syntaxis in western Yunnan, China. The zone is characterized by four stages of deformation (D1–D4). D1 structures record early compressive deformation during the Indosinian orogeny, which formed tight to isoclinal F1 folds of bedding with a penetrative S1 foliation developed parallel to fold axial planes. Mid-crustal horizontal shearing during D2 resulted in overprinting of D1 structures. D1 and D2 structures are associated with granulite facies metamorphism. D3 doming resulted in late crustal thickening and the development of a regional NW–SE trending F3 antiform. Synchronous with or slightly subsequent to D3 deformation, the zone experienced D4 ductile strike-slip shearing, resulting in its exhumation to shallow crustal levels and retrograde metamorphism. Granitic D4 mylonites predominantly yield ⁴⁰Ar/³⁹Ar mica ages of 15–16 Ma, indicating that D4 dextral strike-slip shearing occurred in the Miocene. Weakly deformed leucogranite and protomylonite yield ⁴⁰Ar/³⁹Ar ages of 10–11 Ma, suggesting that ductile strike-slip shearing continued to the Late Miocene. The new ⁴⁰Ar/³⁹Ar data indicate that escape-related deformation along the Gaoligong strike-slip shear zone occurred in the Miocene. In association with recent geophysical studies, and on the basis of the structural, crystal preferred orientation (CPO), and geochronological data presented in this paper, we suggest that the Gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of Tibet, characterized as intense deformation and metamorphism at middle–upper crustal levels.     

Exhumation of the lower crust during crustal shortening: an Alice Springs (380 Ma) age for a prograde amphibolite facies shear zone in the Strangways Metamorphic Complex (central Australia)

Foliated garnet-bearing amphibolites occur within the West Bore Shear Zone, cutting through granulite facies gneisses of the Strangways Metamorphic Complex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs within fine-grained recrystallized leucocratic diffusion haloes of plagioclase–quartz. The garnet and their haloes include a well-developed vertical foliation, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. The textures indicate syn- to late kinematic growth of the amphibolite facies mineral assemblages.
All mineral assemblages record an arrested prograde reaction history. Noteworthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite–hornblende to give plagioclase–gedrite. These dehydration reactions indicate increasing P – T  conditions during metamorphism, and suggest heating towards the end of a period of intense deformation. Temperature estimates for the garnet–amphibolite and related staurolite–hornblende assemblages from the shear zone are about 600 °C. Pressure is estimated at about 5 kbar.
An Sm–Nd isochron gives an age of 381±7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite facies conditions prevailed during shear zone development within the Strangways Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assuming a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite facies, Palaeoproterozoic, lower crust. Along-strike variations of the tectonic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny.     

Isostatic constraints on the central Victorian lower crust: Implications for the tectonic evolution of the Lachlan Fold Belt

Interpretation and 2‐D forward modelling of aeromagnetic datasets from the Olary Domain to the north of the outcropping Kalabity Inlier, South Australia, is consistent with a buried structural architecture characterised by isolated anticlines (also referred to as growth anticlines) bounded by steeply dipping reverse faults. The isolated anticlines are interpreted to have formed by half‐graben inversion during crustal shortening associated with the ca 1600–1580 Ma Olarian Orogeny. We interpret the bounding reverse faults as reactivated high‐angle normal faults, originating from a listric extensional fault architecture. As shortening increased, ‘break‐back bypass’ and ‘short‐cut‘ thrusts developed because of buttressing of the hangingwall successions against the footwall. The resulting architecture resembles a combination of a thrust‐related imbricate fan and an accumulation of inverted basins. Using this structural architecture, synrift sediments proximal to interpreted normal faults were identified as prospective for sediment‐hosted massive sulfide Pb–Zn–Ag mineralisation.     

Structural and metamorphic evolution of the phengite‐bearing schists of the northern Adula Nappe (Central Alps,Switzerland)

Phengite‐bearing schists of the northern Adula Nappe experienced a polymetamorphic and polycyclic evolution that was associated with five deformation episodes. Evidence of a pre‐Alpine metamorphic event is preserved within garnet cores of some amphibole‐bearing schists. The D1 and D2 deformation episodes are recorded by S1 and S2 foliations preserved only within metre‐scale domains of low‐D3 strain. S1 is a relict foliation. Blueschist‐facies conditions at 565 ± 10°C and 11.5 ± 1.5 kbar were attained during D2 and were associated with the development of isoclinal folding and an S2 foliation. The D3 episode took place at 665 ± 50°C and 11.5 ± 2.1 kbar and was responsible for the development of a transpositive S3 foliation. The D4 episode took place at T < 550 ± 10°C and was associated with the development of a discrete S4 foliation and S‐C structures. The D5 episode is recorded by sub‐vertical metre‐scale open folds or centimetre‐scale kinks. The structural and metamorphic evolution described here indicates that the northern and central parts of the Adula Nappe were distinct continental crustal fragments and were brought together under amphibolite‐facies conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Character and kinematics of faults within the turbidite-dominated Lachlan Orogen: implications for tectonic evolution of eastern Australia

Fault zones within turbidite-dominated orogenic systems, typified by the Lachlan Orogen of eastern Australia, are characterised by higher than average strain and intense mica fabrics, transposition foliation and isoclinal folds, poly-deformation with overprinting crenulation cleavages, and steeply to moderately plunging meso- and micro-folds. They have a different character compared to the brittle–ductile fault zones of classic foreland fold-and-thrust belts such as the Appalachians and the Canadian Rocky Mountains. Multiple cleavages and transposition layering record a progressive shear-related deformation history. An intense mica fabric evolves initially during shortening of the overlying sedimentary wedge, but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults. The deformed wedge outside the fault zones generally undergoes one phase of deformation, shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright, subhorizontally plunging chevron-folds. Other faults within the turbidites of the Lachlan Orogen include the steep zones of ‘ductile’ strike-slip deformation that bound a centrally located, high T/low P metamorphic complex. Characterised by S–C mylonites, these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge, with emplacement to higher structural levels along a leading-edge, ductile thrust-fault. Ar–Ar dating constrains the timing of regional deformation to be mostly Late Ordovician through Silurian across the Lachlan Orogen. Faults in the low grade turbidite sequences record the kinematic evolution of the developing Lachlan Orogen and indicate progressive deformation associated with simultaneous, eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt. These deformation fronts are related to ‘accretionary style’ deformation at the leading edges of overriding plates, in an inferred southwest Pacific-type subduction setting from the Late Ordovician to the mid-Devonian, along the former Gondwana margin. The fault zones effectively accommodate and preserve movements within the structurally thickening, migrating and prograding accretionary wedge.     

Temporal constraints on the timing of high-grade metamorphism in the northern Gawler Craton: implications for assembly of the Australian Proterozoic

LA-ICPMS U–Pb data from metamorphic monazite in upper amphibolite and granulite-grade metasedimentary rocks indicate that the Nawa Domain of the northern Gawler Craton in southern Australia underwent multiple high-grade metamorphic events in the Late Paleoproterozoic and Early Mesoproterozoic. Five of the six samples investigated here record metamorphic monazite growth during the period 1730–1690 Ma, coincident with the Kimban Orogeny, which shaped the crustal architecture of the southeastern Gawler Craton. Combined with existing detrital zircon U–Pb data, the metamorphic monazite ages constrain deposition of the northern Gawler metasedimentary protoliths to the interval ca 1750–1720 Ma. The new age data highlight the craton-wide nature of the 1730–1690 Ma Kimban Orogeny in the Gawler Craton. In the Mabel Creek Ridge region of the Nawa Domain, rocks metamorphosed during the Kimban Orogeny were reworked during the Kararan Orogeny (1570–1555 Ma). The obtained Kararan Orogeny monazite ages are within uncertainty of ca 1590–1575 Ma zircon U–Pb metamorphic ages from the Mt Woods Domain in the central-eastern Gawler Craton, which indicate that high-grade metamorphism and associated deformation were coeval with the craton-scale Hiltaba magmatic event. The timing of this deformation, and the implied compressional vector, is similar to the latter stages of the Olarian Orogeny in the adjacent Curnamona Province and appears to be part of a westward migration in the timing of deformation and metamorphism in the southern Australian Proterozoic over the interval 1600–1545 Ma. This pattern of westward-shifting tectonism is defined by the Olarian Orogeny (1600–1585 Ma, Curnamona Province), Mt Woods deformation (1590–1575 Ma), Mabel Creek Ridge deformation (1570–1555 Ma, Kararan Orogeny) and Fowler Domain deformation (1555–1545 Ma, Kararan Orogeny). This westward migration of deformation suggests the existence of a large evolving tectonic system that encompassed the emplacement of the voluminous Hiltaba Suite and associated volcanic and mineral systems.     

Crustal lineament control on magmatism and mineralization in northwestern Argentina: geological, geophysical, and remote sensing evidence

The relationship between long-lived deep crustal lineaments and the locations of magmatic centers and associated mineral deposits has been investigated in the Puna region of northwestern Argentina, through the analysis of regional aeromagnetic surveys, Landsat images, and geological information. The good exposure and excellent preservation of basement and supracrustal geology in this region makes it particularly suitable for such a study. At a regional scale, several contrasting magnetic domains are recognized, which correlate with crustal geology. Two basement domains are separated by a NNE-trending boundary, which is believed to correlate with a Paleozoic suture zone between the Pampia (to the southeast) and Arequipa–Antofalla terranes (to the northwest). Locally overlying these basement terranes is the Cenozoic magmatic domain, which is best developed in the N–S-trending volcanic arc at the western edge of the Puna (the Cordillera Occidental). In addition, four southeast-trending volcanic zones extend for several hundred kilometers across the Puna. Many important mineral deposits and areas of hydrothermal alteration are associated with these volcanic breakouts, and we have selected three such areas for more detailed study: Bajo de la Alumbrera (Argentina's largest porphyry copper deposit), Cerro Galán (the largest ignimbrite caldera in Argentina, with associated hydrothermal alteration zones), and El Queva (a historic polymetallic district located within a major volcanic range). A comparison of lineament maps generated from aeromagnetic and Landsat TM images reveals broad correlation between these different remote sensing techniques, which respectively highlight subsurface magnetic and surface geological features. In addition, the locations of magmatic and hydrothermal centers can be related to the interpreted structural framework, and are seen to occur near the intersections of major lineament zones. It is suggested that in three dimensions, such intersection zones form trans-lithospheric columns of low strength and high permeability during transpressional or transtensional tectonic strain, and may thereby serve as conduits for magma ascent to the shallow crust. Pooling of large volumes of deeply derived magma in shallow crustal magma chambers may then result in voluminous devolatilization and the formation of hydrothermal mineral deposits. It is important to note that in this model, structural intersections serve as facilitators for magma ascent and volatile exsolution, but do not in themselves cause this process—other factors such as magma supply rate and tectonic stress are essential primary ingredients, and local magmatic and volcanic processes affect the ultimate potential for ore formation. Nevertheless, we suggest that lineament analysis provides a valuable framework for guiding the early stages of mineral exploration; other regional and local geological considerations must then be applied to identify priority targets within this framework.     

Relationships between magmatism and deformation in northern Yorke Peninsula and southeastern Proterozoic Australia

The ca 1600–1580 Ma time interval is recognised as a significant period of magmatism, deformation and mineralisation throughout eastern Proterozoic Australia. Within the northern Yorke Peninsula in South Australia, this period was associated with the emplacement of multiple phases of the Tickera Granite, an intensely foliated quartz alkali-feldspar syenite, a leucotonalite and an alkali-feldspar granite. These granites belong to the broader Hiltaba Suite that was emplaced at shallow crustal levels throughout the Gawler Craton. Geochemical and isotopic analysis suggests these granite phases were derived from a heterogeneous source region. The syenite and alkali-feldspar granite were derived from similar source regions, likely the underlying ca 1850 Ma Donington Suite and/or the ca 1750 Ma Wallaroo Group metasediments with some contamination from an Archean basement. The leucotonalite is sourced from a similar but more mafic/lower crustal source. Phases of the Tickera Granite were emplaced synchronously with deformation that resulted in development of a prominent northeast-trending structural grain throughout the Yorke Peninsula region. This fabric is associated with composite events resulting from folding, shearing and faulting within the region. The intense deformation and intrusion of granites within this period resulted in mineralisation throughout the region, as seen in Wheal Hughes and Poona mines. The Yorke Peninsula shares a common geological history with the Curnamona Province, which was deformed during the ca 1600–1585 Ma Olarian Orogeny, and resulted in development of early isoclinal and recumbent folds overprinted by an upright fold generation, a dominant northeast-trending structural grain, mineralisation, and spatially and temporally related intrusions. This suggests correlation of parts of the Gawler Craton with the Curnamona Province, and that the Olarian Orogeny also affected the southeastern Gawler Craton.     

The development of orientated symplectites during deformation

Abstract Orientated symplectites have been observed in deformed granulite facies metabasic rocks from the Ivrea-Verbano zone in northern Italy. The area underwent lower crustal extension, accommodated by movement on localized high- T shear zones. In areas of relatively low strain, such as at the margins of shear zones, symplectites of orthopyroxene, plagioclase and spinel have formed. The symplectites are vermiform and orientated parallel to the main foliation and in the regional stretching direction. The reaction was synkinematic with the deformation, and only developed in potentially dilatant grain boundaries in the rock. It was presumably inhibited in grain boundaries subjected to higher normal stress due to the relatively large volume increase involved in the reaction.
The observations support the interpretation that the deformation was related to regional extension under high- T granulite facies conditions, the symplectites forming as a result of decreasing pressure.     

Collision tectonics in the late Precambrian Eastern Ghats Mobile Belt: mesoscopic to satellite-scale structural observations

Regional mapping of a section across the Eastern Ghats Mobile Belt (EGMB) north of the Godavari graben in Eastern Peninsular India by using Landsat Thematic Mapper data enables recognition of a number of shear zones, lineaments, and structural domes and basins. A conspicuous megashear occurs at the western boundary of the granulite facies rocks of the EGMB adjacent to the Archean granite-greenstone craton. The confinement of a suite of alkaline igneous rocks to this shear zone is a notable feature. The strike extensions of this shear belt extend through to the Elchuru alkaline complex, Prakasam District, Andhra Pradesh, and the syenite plutons of Koraput district, Orissa. The contrasting lithologies, metamorphism and structural history on either side of the shear zone suggests that it might be a Precambrian suture zone. The mesoscopic structural features in the EGMB include prominent foliation with moderate to steep dips, folds, faults/shears, S-C fabrics, pinch and swell structures and other linear fabric elements. These observations favour the consideration of drastic crustal shortening and thickening and a complex deformational sequence. The major rock units in this part of EGMB comprise garnetiferous sillimanite gneisses, quartzites and calc-granulites forming the khondalitic suite of rocks and a wide variety of charnockitic rocks. The contact of the two rock units is generally sheared and often migmatised. The structural fabric suggests two major tectonic events: an essentially horizontal tectonic regime resulting in thrust systems and associated structures, subsequently followed by strike-slip tectonics characterized by high shear strains. Features such as westward-verging thrusts, large-scale recumbent folds, major shear zones, structural domes and basins, indications of tectonic crustal shortening, extensive calc-alkali magmatism and widespread migmatization in the region are attributed to collisional processes during Proterozoic times. The spatial disposition of the EGMB and its linkage with the distribution of similar rock units during the late Precambrian time in a global tectonic scenario are discussed.     

Archaean tectonics in the Agnew supracrustal belt,Western Australia

The Agnew supracrustal belt consists of a greenstone sequence (interlayered metabasalt, differentiated gabbroic sills, ultramafic bodies, and black volcanogenic sediment) unconformably overlain by granitoid-clast conglomerate and meta-arkose. The base of the preserved sequence is intruded by grey tonalite with a crudely concordant upper contact, and by small discordant bodies of leucogranite.An early deformation (D1) produced isoclinal folds and a regional penetrative foliation. These structures were probably gently dipping when formed. D2 produced large-scale NNW-trending upright folds, a regional foliation, and a vertical N-trending ductile fault on the west side of the belt. D2 structures indicate a combination of ENE-WSW shortening, and right-lateral shear along the ductile fault. Both D1 and D2 were accompanied by metamorphism under upper greenschist to lower amphibolite facies conditions.The interpreted sequence of tectonic events is (1) deposition of the greenstone sequence on an unknown basement; (2) intrusion of large volumes of tonalite, separating the supracrustal rocks from their basement; (3) erosion of mafic rocks and tonalite to produce the clastic sedimentary sequence; (4) the first deformation; (5) intrusion of small volumes of leucogranite; (6) the second deformation.The bulk of the granitoid rocks were emplaced before the first recognisable deformation. Thus the granitoid magma cannot have been produced by partial melting of previously downbuckled ‘greenstone belt’ rocks, nor can the large-scale upright folds (D2) be a result of forceful emplacement of the magma — two common postulates for Archaean terrains. The D2 folds are closely related to the ductile fault bounding the zone: these structures, which give the present N-trending tectonic belt its form, are the youngest features in the terrain.