Toward a stepwise Kwangsian Orogeny

The Kwangsian Orogeny originated along the southeast coast of China and stepwise developed in a northwest direction. It includes two stages, a long locally varying uplift from the Late Ordovician to the early Silurian and a finally tectonic movement near the Silurian and Devonian transition. The Kwangsian uplift event shows a stepwise delay northwestwards from the southeastern coast area in Nemagraptus gracilis Biozone (Sa1) to the south side of the Xuefeng Mountains in or later than Cystograptus vesiculosus Biozone (R3) to Coronograptus cyphus Biozone (R4). In the southern of Yangtze Platform, the Yichang Uplift was droved by the Kwangsian Orogeny forming a diachronous stratigraphical break through Rhuddanian and Aeronian. The distribution of the early Telychian lower marine red beds indicates a northwestward increase of the Cathaysian Oldland. Stratigraphical evidence may explain why the Kwangsian movement was marked by an angular disconformity during the Pridoli to earliest Devonian interval.     

Episodic gold mineralisation correlated with discrete structural events at Ballarat East,southeast Australia

New ⁴⁰Ar/³⁹Ar geochronological data suggest orogenic gold mineralisation at the Ballarat East deposit, southeast Australia, occurred in three main episodes at ca. 445–435 Ma, ca. 420–415 Ma and ca. 380–370 Ma. The gold mineralisation is localised in muscovite-bearing quartz and quartz-carbonate veins hosted in the steep faults (70–90°), on limbs of tight and isoclinal folds in an Ordovician turbidite sequence, and within west-dipping (≤45°) faults, historically known as leather jacket lodes. Initiation of the ≤45° faults that are confined to fold culminations, begins at ca. 445 Ma, with peak metamorphic conditions at 440 Ma. The earliest vein sets (V₁), were emplaced on limb thrusts at ca. 445–435 Ma and are characterised by arsenopyrite-dominated quartz veins. These V₁ veins parallel arsenopyrite-rich shale units, historically referred to as ‘indicator beds’. Both the steep and ≤45° faults were reactivated during fold amplification with deposition of the V₂ auriferous veins at ca. 420–415 Ma. A later set of auriferous veins (V₃–V₄) with ages of 380–370 Ma, dominated by pyrite-sphalerite-galena-white-mica quartz-(V₃) or carbonate-rich (V₄) veins are predominantly associated with reactivation of the ≤45° west-dipping faults. This new geochronological data constrains the local kinematic history of the Ballarat East deposit and has regional implications. The V₁–V₂ vein development appears to be synchronous across the entire western section of the Lachlan Orogen, where previous studies have suggested that initial gold mineralisation was linked to orogenesis at ∼440 Ma, as a result of metamorphic devolatilisation reactions in the lower crust. In contrast, a close spatial and temporal relationship exists between the felsic dykes and the mineralisation recognised in the V₃–V₄ veins. The deformation that accompanies V₃–V₄ vein development is attributed to small, localised events during east-west shortening, utilising pre-existing fold and fault structures. The origin of the fluids producing the V₃–V₄ veins may be metamorphic devolatilisation associated with widespread felsic magmatism that occurred at this time across central Victoria.     

四合堂剪切带活动时代及其对燕山运动B幕时间的限定

翁文灏以燕山为标准地区创名的中生代"燕山运动",现今人们将其分为A幕与B幕,各自对应着区域性角度不整合,但发生的准确时间一直无法确定。燕山构造带中段的云蒙山地区,发育了著名的四合堂逆冲型韧性剪切带,是燕山运动B幕的产物。该韧性剪切带出现在早白垩世初云蒙山复式岩体的北部边缘,上盘向南南西运动,周边的变质基底、中—新元古代盖层及晚侏罗世与早白垩世初期岩体一同卷入了变形。剪切带内及周边发育了大量同构造岩脉,可区分出早、中、晚3个阶段,对应着剪切带活动早、中、晚阶段。本次工作中分别从早、中、晚3阶段同构造岩脉中获得了锆石LA-ICP-MSU-Pb年龄,分别为143.0±2.1Ma、140.8±1.4Ma和137.5±2.4Ma。这可靠地指示了四合堂韧性剪切带的活动时间为143～138Ma。这也表明,以逆冲活动为特征的燕山运动B幕(尤其是燕山造山带中段)发生的时间就是143～138Ma。这一结果与近年来该B幕事件角度不整合之上、下地层中所获得火山岩锆石年龄相吻合。     

Hunter–Bowen deformation in South Percy Island,northeastern Australia

South Percy Island is located approximately 50 km off the central Queensland coast and comprises a disrupted ophiolite mass alongside a diverse array of metamorphosed felsic and mafic rocks that record several episodes of magmatism, volcanism and deformation from the Permian to Early Cretaceous. This paper aims to constrain the age, affinity and deformation history of these units, as well as to establish the tectonic significance of the terrane. The trace-element compositions of mafic and felsic meta-igneous rocks record a change from MORB-like prior to ca 277 Ma to subduction-related by ca 258 Ma. Overprinting relationships between intrusive phases and deformation features reveal a relative chronology for the tectonothermal evolution of the area, while U–Pb and ⁴⁰Ar/³⁹Ar geochronology provides absolute age constraints. Deformation is localised around a NNE-striking tectonic contact that separates serpentinised ultramafic rocks from metamorphosed pillow lavas. Early formed ductile fabrics associated with the main episode of deformation (D₁) preserve bulk flattening strains at greenschist-facies conditions. Emplacement and post-kinematic cooling ages of a pre-D₁ quartz-monzonite dyke constrain the age of D₁/M₁ deformation and metamorphism to the period between ca 258 and ca 248 Ma. Minor brittle deformation (D₂) occurred at ca 230 Ma, based on U–Pb dating of a syn-D₂ diorite dyke (ca 231 ± 10 Ma) and several ca 230 Ma ⁴⁰Ar/³⁹Ar cooling ages. The deformation, metamorphism, and supra-subduction zone magmatism preserved on South Percy Island is correlated with the nearby Marlborough Terrane and more broadly with the second pulse of the Hunter–Bowen Orogeny, which affected much of the central and northern parts of eastern Australia in the late Permian and Early Triassic. Our results support previous suggestions that the second pulse of the Hunter–Bowen Orogeny involved coeval thrust systems in both the inboard and outboard parts of the orogen.     

Caledonide influences on the Old Red Sandstone fluvial systems of the Oslo Region,Norway

Red or buff‐coloured sandstones and siltstones of fluvial origin comprise approximately 80% of the Ringerike Group, a late Silurian Old Red Sandstone (ORS) sequence that crops out extensively in the Oslo Region of southern Norway. These fluvial sediments are lithostratigraphically ascribed to two laterally equivalent formations—the Stubdal Formation (to the north of Oslo) and the Skien Formation (to the south of Oslo). The fluvial strata of each of the two formations have a distinct style of sandbody geometry, facies, provenance, and palaeocurrent direction. Within the Stubdal Formation, shallow channelized sandbodies, low‐ to upper‐flow regime sedimentary structures, a Caledonide provenance and a palaeoflow toward the southeast are evident. Within the Skien Formation, sandbody geometry is entirely sheet form, with upper‐flow regime sedimentary structures, a provenance from Precambrian rocks to the northern and local parts of the Oslo Region and a palaeoflow toward the east. No stratal contact can be seen between the two fluvial formations, due to a 15 km break in exposure between the southernmost Stubdal Formation and the northernmost Skien Formation. Relationships with adjacent formations indicate that they are diachronous, lateral equivalents. Given the abrupt change in sedimentary style between the two formations, it is proposed that a barrier had developed within the foreland basin, diverting the ORS fluvial systems in southern Norway, from a southward (north of Oslo), to an eastward direction (south of Oslo). This diversion had implications for depositional gradient, fluvial regime and provenance, resulting in the differences visible in the deposits of those rivers. The barrier invoked is arguably a Caledonide blind thrust fault that developed a topographic high, running east–west through the vicinity of Oslo, during the late Silurian. Copyright © 2004 John Wiley & Sons, Ltd.     

Structural control on gold mineralization at Walhalla,Victoria

Gold mineralization associated with quartz reefs is related to the structural history of the Early Devonian, Walhalla Group. These reefs are situated in the Walhalla Synclinorium, developed during the Middle to Late Devonian Tabberabberan Orogeny. A pervasive north‐south‐trending axial planar cleavage and two styles of folding were produced during regional east‐west compression. The first are upright, open to close folds with sub‐horizontal fold axes. The second are plunging inclined, close to tight folds with fold axes that plunge steeply to the north and south. An extensional event is associated with the emplacement of the Woods Point Dyke swarm and a set of normal faults that offset all earlier structures. High‐angle reverse faults, which post‐date the folding and the emplacement of the dykes, were utilized as conduits for hydrothermal fluids and preferentially localize mineralization to laminated quartz veins. En echelon vein arrays formed during initial stages of reverse faulting became deformed during prolonged shearing to produce ptygmatic veins. Laminated quartz veins within high‐angle reverse faults contain arsenopyrite and pyrite in vein margins and gold in fractures that cross‐cut continuous quartz crystals. Gold, galena, chalcopyrite and sphalerite may also be deposited adjacent to and within fractured arsenopyrite and pyrite. Late‐stage, cross faults developed in a regime of north‐south compression and post‐date the laminated quartz veins and mineralization.     

Regional high-pressure metamorphism during intracratonic deformation: the Petermann Orogeny, central Australia

The Petermann Orogeny is a late Neoproterozoic to Cambrian ( c . 560–520  Ma) intracratonic event that affected the Musgrave Block and south-western Amadeus Basin in central Australia. In the Mann Ranges, within the central Musgrave Block, Mesoproterozoic granulite facies gneisses, granites and mafic dykes have been substantially reworked by deep crustal non-coaxial strain of late Neoproterozoic to early Cambrian age. Dolerite dykes have recrystallized to garnet granulite facies assemblages, associated with the development of a mylonitic fabric at P =12–13  kbar and T  =700–750 °C. Migmatization is restricted to discrete shear zones, which represent conduits for hydrous fluids during metamorphism. Peak metamorphism was followed by decompression to c . 7  kbar, reflecting exhumation of the terrane along the south-dipping Woodroffe Thrust. In scattered outcrops north of the Mann Ranges, peak metamorphism occurred at P =9–10  kbar and T  = c . 700 °C. The Woodroffe Thrust separates these deep crustal mylonites from granites that were metamorphosed during the Petermann Orogeny at P = c . 6–7  kbar and T  = c . 650 °C. The similarity in peak temperatures at different crustal levels implies an unusual thermal regime during this event. The existence of a relatively elevated geotherm corresponding with Th- and K-enriched granites that were in the mid-crust during the Petermann Orogeny suggests that radiogenic heat production may have substantially contributed to the thermal regime during metamorphism. This potentially has implications for the mechanisms by which intra-plate strain was localized during this event.     

Genesis of the Jurassic Carbonate‐Hosted Pb–Zn Deposits of Jebel Ressas (North‐Eastern Tunisia): Evidence from Mineralogy,Petrography and Trace Metal Contents and Isotope (O,C, S,Pb) Geochemistry

The Jebel Ressas Pb–Zn deposits in North‐Eastern Tunisia occur mainly as open‐space fillings (lodes, tectonic breccia cements) in bioclastic limestones of the Upper Jurassic Ressas Formation and along the contact of this formation with Triassic rocks. The galena–sphalerite association and their alteration products (cerussite, hemimorphite, hydrozincite) are set within a calcite gangue. The Triassic rocks exhibit enrichments in trace metals, namely Pb, Co and Cd enrichment in clays and Pb, Zn, Cd, Co and Cr enrichment in carbonates, suggesting that the Triassic rocks have interacted with the ore‐bearing fluids associated with the Jebel Ressas Pb–Zn deposits. The δ¹⁸O content of calcite associated with the Pb–Zn mineralization suggests that it is likely to have precipitated from a fluid that was in equilibrium with the Triassic dolostones. The δ³⁴S values in galenas from the Pb–Zn deposits range from ?1.5 to +11.4‰, with an average of 5.9‰ and standard deviation of 3.9‰. These data imply mixing of thermochemically‐reduced heavy sulfur carried in geothermal‐ and fault‐stress‐driven deep‐seated source fluid with bacterially‐reduced light sulfur carried in topography‐driven meteoric fluid. Lead isotope ratios in galenas from the Pb–Zn deposits are homogenous and indicate a single upper crustal source of base‐metals for these deposits. Synthesis of the geochemical data with geological data suggests that the base‐metal mineralization at Jebel Ressas was formed during the Serravallian–Tortonian (or Middle–Late Miocene) Alpine compressional tectonics.     

Fold-interference patterns in the Bowen Basin,northeastern Australia

Deformation patterns of Paleozoic and Mesozoic strata in eastern Australia are evidence of a structural and tectonic history that included multiple periods of deformation with variable strain intensities and orientations. Detailed analysis of structural data from the Bowen Basin in northeastern Australia reveals previously undescribed, north–south elongate, Type-1 fold-interference patterns. The Bowen Basin structures have similar orientations to previously described interference patterns of equivalent scale in upper Paleozoic strata of the New England Orogen and Sydney Basin of eastern Australia. The east Australian folds with north–south-trending axes most likely formed during late stages of the Permian–Triassic Hunter–Bowen Orogeny, and they were subsequently refolded around east–west axes during post 30 Ma collision of the Indo-Australian plate with the Eurasian and Pacific plates. The younger, east–west-trending folds have orientations that are well aligned with the present-day horizontal stress field of much of eastern Australia, raising the possibility that they are active structures.     

Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere

The evolution of the European Cenozoic Rift System (ECRIS) and the Alpine orogen is discussed on the base of a set of palaeotectonic maps and two retro-deformed lithospheric transects which extend across the Western and Central Alps and the Massif Central and the Rhenish Massif, respectively.During the Paleocene, compressional stresses exerted on continental Europe by the evolving Alps and Pyrenees caused lithospheric buckling and basin inversion up to 1700 km to the north of the Alpine and Pyrenean deformation fronts. This deformation was accompanied by the injection of melilite dykes, reflecting a plume-related increase in the temperature of the asthenosphere beneath the European foreland. At the Paleocene–Eocene transition, compressional stresses relaxed in the Alpine foreland, whereas collisional interaction of the Pyrenees with their foreland persisted. In the Alps, major Eocene north-directed lithospheric shortening was followed by mid-Eocene slab- and thrust-loaded subsidence of the Dauphinois and Helvetic shelves. During the late Eocene, north-directed compressional intraplate stresses originating in the Alpine and Pyrenean collision zones built up and activated ECRIS.At the Eocene–Oligocene transition, the subducted Central Alpine slab was detached, whereas the West-Alpine slab remained attached to the lithosphere. Subsequently, the Alpine orogenic wedge converged northwestward with its foreland. The Oligocene main rifting phase of ECRIS was controlled by north-directed compressional stresses originating in the Pyrenean and Alpine collision zones.Following early Miocene termination of crustal shortening in the Pyrenees and opening of the oceanic Provençal Basin, the evolution of ECRIS was exclusively controlled by west- and northwest-directed compressional stresses emanating from the Alps during imbrication of their external massifs. Whereas the grabens of the Massif Central and the Rhône Valley became inactive during the early Miocene, the Rhine Rift System remained active until the present. Lithospheric folding controlled mid-Miocene and Pliocene uplift of the Vosges-Black Forest Arch. Progressive uplift of the Rhenish Massif and Massif Central is mainly attributed to plume-related thermal thinning of the mantle-lithosphere.ECRIS evolved by passive rifting in response to the build-up of Pyrenean and Alpine collision-related compressional intraplate stresses. Mantle-plume-type upwelling of the asthenosphere caused thermal weakening of the foreland lithosphere, rendering it prone to deformation.