Importance of Hercynian tectonics within the framework of the Southern Alps

The Hercynian remnants present within the Alpidic structural zones of the Southern Alps are reviewed. The pattern of Hercynian metamorphism is zonal from granulites in the western area to anchimetamorphic facies in the eastern one. In the folded zone at the eastern margin a severe Hercynian folding phase took place during the Westphalian. Thrust sheets comprising sequences of different facies ranging in age from Caradocian to early Westphalian are sutured by late Westphalian molasse deposits. The assumption that the Southern Alps and the external Dinarides remained outside the Hercynian folding front is contradicted by field evidence.     

On the role of the Hercynian and Alpine thrusts in the Upper Paleozoic rocks of the Central and Eastern Pyrenees

Abstract

The structure of the Pyrenean pre-Hercynian rocks involved in the “Axial Zone” antiformal stack, results from the association of Hercynian cleavage-related folds and Hercynian and Alpine thrusts. Some of these Alpine and Hercynian thrusts separate thrust sheets in which Upper Paleozoic rocks, Devonian and pre-Hercynian Carboniferous, exhibit different lithostratigraphy and internal structure.

In order to know both, the original Devonian facies distribution and the structural characteristics, the effects of the Alpine and the Hercynian thrusts must be considered. If a conceptual restored cross-section is constructed taking into account both the Alpine and Hercynian thrusts, a different Devonian facies distribution is achieved. Devonian carbonatic successions were originally located in a northernmost position, whereas sequences made by alternations of slates and limestones lie in southernmost areas. Moreover, a N-S variation of the Hercynian structural style appears. In the northern units thrusts are synchronous to folding development and they are the most conspicuous structures. In the intermediate units, thrust postdate cleavage-related folds, and in the southernmost units several folding episodes, previous to the thrusts, are well developed.

We present some examples which enable us to discuss the importance of the Hercynian and Alpine thrusts in the reconstruction of the Pyrenean pre-Alpine geology.     

On the Structure and Metamorphism of the Roc de Frausa Massif (Eastern Pyrenees)

Abstract

The Roc de Frausa Massif, located at the Eastern Pyrenees, is formed by a stratoid Pre-Hercynian deformed granite (orthogneiss) interbedded with metasedimentary series. Hercynian granitoids (St. Llorenç — La Jonquera pluton) surround the southern and eastern part of the massif and Hercynian basic igneous rocks (Ceret stock) occupy the central part of it. The Pre-Hercynian granite and the sedimentary series were involved, during the Hercynian orogeny, in complex polyphasic tectonics and metamorphism. As a result, an ubiquitous penetrative foliation was developed during the earlier stages. This foliation was subsequently folded into a complex antiformal structural formed by a double dome : Roc de Frausa dome and Mas Blanc dome. Main lithological boundaries (gneiss — metasediments and metasediments — granitoids) are broadly parallel to the regional foliation, and they all display the dome geometry. Interference fold pattern between two late phases, an ealier one with NE-SW trending folds and a younger one with NW-SE trending folds is responsible for the dome geometry. Mylonitic deformation, with W-E to NW-SE orientations has been attributed to the last folding phase. Regional metamorphic climax and contact metamorphism, the last one resulting from Hercynian granitoid emplacement, preceeded the above mentioned late folding event, which developed under retrograde metamorphic conditions. Regional peak metamorphism is recognized by the static crystallization of cordierite + potassium feldspar. This paragenesis indicates pressure — temperature conditions of about 3.1 Kbar and 660 °C maximum. Contact metamorphism overprints the earlier regional metamorphism. Parageneses and thermal gradient of contact metamorphism around La Jonquera pluton are very similar to those related to regional metamorphism, whereas parageneses produced around Ceret stock present garnet + potassium feldspar. Geothermometry indicates metamorphic conditions locally higher for this paragenesis (around 700 °C).     

SILURIAN DEPOSITS OF THE NORTHERN SLOPE OF THE DZHUNGARIAN ALATAU

Silurian deposits were first identified on the northern slope of the Dzhungarian Alatau by F. A. Makarenko on the basis of fauna collected in the Aksu river basin. In 1938, Silurian deposits, 400 kilometers in length, were mapped by M. M. Yudichev. Later investigations have shown that these beds are Devonian in age. It seems obvious that Paleozoic sediments uncomformably overlying the Precambrian (?) crystalline foundation began accumulating in the Lower Devonian. The principal Paleozoic structures are the result of Hercynian folding, not the final phase of Caledonian folding, as assumed by Makarenko.     

Mise en évidence d''une tectonique tangentielle antérieure au plissenent majeur dans la chaîne hercynienne de l''Anti-Atlas occidental, Maroc

Variscan deformation in the Palæozoic foreland of the Moroccan Anti-Atlas is characterised by its heterogeneous character and its dampening effect towards the Tindouf Basin. Structural analyses shows three sequential episodes of deformation, D1, D2 and D3, displayed at different levels of observation. The first episode D1, which was observed for the first time in the western Anti-Atlas Hercynian Belt provides clear evidence for tangential tectonic deformation operating before the major phase of folding. It corresponds to an early event of ‘thin-skin tectonics’, which develops a composite schistosity related to ductile horizontal pure shearing. It is considered that D1 was responsible for the first folding of the sedimentary sequences associated with several imbricated thrust sheets with vergence to the east. The second episode, D2, represents the major phase of north-northeast-southwest Variscan folding. The D3 episode was more discrete, of brittle type and marked by subequatorial sinistral kink-bands.This D1 tangential event, in the western Anti-Atlas, Morocco, represents a typical structural domain as part of the tectonic evolution during Variscan-Appalachian collision.     

Distribution patterns of lead-zinc mineralization in Gorno-Altay

The map and chart of correlations of the region's polymetallic ores with geologic, tectonic, and other variables, on the geochronological scale, indicate the end of the Hercynian geologic-tectonic cycle as the time of the regional appearance of the lead-zinc mineralization and also, locally of some deposits of copper ores. However, many of the Zmeyinogorsk (Middle Carboniferous), formed during the main stage of the Hercynian folding, carry ores only in the inherited Hercynian synclinoria or in zones of deep ruptures. Their polymetallic and other ores are of the skarn type, as a rule. -- AGI Staff.     

华北地洼区北缘东部及邻区活化构造和铀成矿作用

该区处于欧亚东西向海西铀成矿带与环太平洋成矿带相交的有利部位。铀矿化产于地槽褶皱阶段、反射活化阶段和地洼阶段,具有铀成矿作用时间长、矿化类型多的特点。该区中新生代岩浆岩,特别是火山岩十分发育,与此相应地洼期铀成矿作用以热液型为主,特别是与火山作用有关的成矿作用具有十分重要的意义。     

Les plis hercyniens kilométriques couchés vers l'ouest-sud-ouest dans la région du pic du Midi d'Ossau–col du Somport (zone axiale des Pyrénées occidentales)Kilometric westward Hercynian recumbent folds in the western Pyrenees (Pic du Midi d'Ossau area)

The Palaeozoic of the western Pyrenees shows two superposed tectonics easily defined by their different geometry and the major unconformity of the Permian sediments and volcanics on the Devono-Carboniferous series: an Hercynian tectonic found only in the pre-Permian series, characterised by kilometric westward recumbent folds with a weak cleavage; a Pyrenean tectonic, characterised by tight east–west folds, upright to overturned to the south with slaty cleavage, which is the only deformation found in the Permian and Mesozoic series and the second deformation in the pre-Permian Palaeozoic. The Hercynian folding, roughly perpendicular to the trend of the Pyrenees characterises the northern branch of the Ibero-Armorican virgation. To cite this article: P. Matte, C. R. Geoscience 334 (2002) 773–779.     

Structural setting and genesis of ore bodies in carbonate rocks in the M.te Atzei-M.te Ega mining area (Sulcis,SW Sardinia)

At least seven different types of ore bodies, with an ore stock of Ba, F, Fe, Pb, Zn, Cu and Sb occur in Paleozoic rocks (Lower-Middle Cambrian and Ordovician) in an ?15 km² area north of the Narcao village (SW Sardinia, Italy). The ore bodies are related both to syngenetic accumulations, and later epigenetic recirculation of the primary ores. In the Lower Cambrian the area studied shows evidence of synsedimentary tectonic instability, probably related to carbonate shelf margin features. The Upper Cambrian is lacking and the transgressive Ordovician sediments cover large areas of the previously gently folded Cambrian rocks (Sardic phase of Caledonian orogenesis). The main Hercynian tectonic phases had a much stronger compressive folding that is mirrored in a sequence of tight folds and upthrusts. The geometric setting resulting from the tectonic frame of the Cambrian and Ordovician sediments must be taken into account to understand the distribution and the genesis of most of the ore bodies.     

Multiple crustal sources for post-tectonic I-type granites in the Hercynian Iberian Belt

A post-tectonic plutonic array of felsic I-type granites crops out in the western Hercynian Iberian Belt. Isotope (Sr, Nd, Pb) data favour the absence of an important input of juvenile magmas in late- to post- tectonic Hercynian felsic magmatism in western Iberia, but suggest a reworking of different crustal protoliths, including oceanic metabasic rocks accreted to mid-to-lower crustal levels during the early stages of the collision. I-type granites were derived from different meta-igneous protoliths ranging from metabasic to felsic compositions depending on their geographical position from the external (e.g. Galicia—N Portugal, GNP) to the innermost continental areas (Spanish Central System and Los Pedroches Batholiths). The GNP I-type plutons related to eo-Hercynian accretional terranes have lower initial ⁸⁷Sr/⁸⁶Sr ratios, lower negative εNd values, and higher ²⁰⁶Pb/²⁰⁴Pb ratios than other I-type granites of the Central Iberian zone. These more isotopically primitive Hercynian I-type granites are important in tracking pre-Hercynian accreted oceanic lithosphere terranes.     

TECTONIC DEVELOPMENT OF THE YARKAND SECTOR OF THE KUN LUN SHAN

The Yarkand sector of the Kun Lun Shan, western Sinkiang, China, was studied jointly with Chinese geologists in 1957. During the Paleozoic, this area represented the junction between the Kun Lun geosyncline and the Tarim platform. The principal events in the development of the marginal part of the geosyncline were its initiation in the Sinian; a temporary stabilization of conditions common with the platform in the Cambrian‐Devonian; pre‐Upper Devonian folding; expansion of the geosyncline at the expense of the platform in early Hercynian; and a final folding at the close of the Paleozoic. The junction was effected by means of marginal uplift and deep‐seated faults. The Alpine Kun Lun presented an epi‐Hercynian platform developed as a low‐angle uplift, adjacent to the subsidence in the margin of the Tarim platform. The most recent uplift in this region involved also the pre‐Paleozoic platform margin. The pre‐Paleozoic stage culminated in an intense folding as evidenced by a sharp unconformity between the Proterozoic and lower Paleozoic rocks. A neotectonic stage caused uplift of a lower Quaternary erosion surface to an absolute elevation over 3,000 meters and a relative uplift of up to 800 meters. Relation of older structural trends to later ones is considered. Structural elevation of the area is summarized under six conclusions.‐‐W. D.     

Relationship between tectonic structures and generation of anatectic melt,central Spain

This study relates the anatectic generation of in situ granites to deformation in the Spanish Central System. Regionally, the principal rocks of the anatectic complexes in central Iberia are migmatites, and interlayered with them are anatectic granites. The second phase of the Hercynian orogeny is characterized by subhorizontal structures (e.g. shears, folding) due to a contractional deformation, followed by extensional collapse of the orogen. Melting and the generation of in situ granites occurred in fertile rocks, but was concentrated in subhorizontal structures formed during the contractional deformation. Accumulation of volatile elements, such as boron, in shear structures may have triggered the anatexis, and enabled local migration of the melt fraction. This process, plus the existence of a fertile source, are determining factors in the generation of different melt-fraction granitoids.     

A palaeomagnetic investigation of the Malvernian and Old Radnor Precambrian,Welsh Borderlands

Alternating field and thermal demagnetization of igneous rocks of the Malvern Hills identifies a number of magnetite-held components which are characterized by a high blocking temperature (M2) component D = 283°, I = 47°, and lower blocking temperature (M3) component D = 269°, I = −43° which is of complex origin or more than one age. Two subordinate components are (M1) D = 7°, I = 56° and (M4) D = 174°, I = 51° in later dolerites. A pervasive hematite-held remanence with a mean D = 186°, I = −5° is linked to Hercynian palaeofield directions and the uplift/folding of the Malvernian axis. The similarity of the magnetization directions in the Stanner–Hanter (702 Ma) and Malvernian (681 Ma) rocks suggests that folding of the Palaeozoic rocks in the Malvern Hills was achieved by upthrust of the basement and involved little folding of the latter. The Old Radnor sediments possess a post-folding remanence D = 117°, I = −13° of probable Cambrian age and a subordinate remanence which may be Hercynian in age. The late Precambrian–Cambrian palaeomagnetic record (ca. 700–500 Ma) of England and Wales is compared with data from the Armorican Massif. Although the apparent polar wander (a.p.w.) paths are widely dissimilar prior to 550 Ma, the two regions had similar latitudes and went through similar palaeolatitudinal movements throughout this interval. The palaeomagnetic data support models involving tectonic rotations but little closure across this part of the Hercynian Belt.     

长江中下游成矿带九瑞矿集区叠合断裂系统和叠加成矿作用

通过矿坑和地表勘察并结合区域构造分析,初步阐述九瑞矿集区叠合断裂系统和叠加成矿作用及其相互匹配关系。与矿有关的断裂梳理成海西期陆缘同生断裂,印支期褶皱断裂和燕山晚期负反转断裂三个系统,并以武山铜矿的F1断层对其在三期断裂系统中的交接复合和成矿作用进行了个例描述,综合三个时期的地质勘探和野外及室内观测资料提出一个三阶段的成因模式。即:第一阶段为海西期,此时陆缘伸展、掀斜和裂陷活动引发张性同生断裂,顺层-切层(阶梯状)生长断裂发育。此阶段晚期通过与深构造位的隐伏断裂交接复合可形成海底火山和热液通道,喷流沉积形成层(块)状含铜黄铁矿。第二阶段为印支期,此阶段发育背驼式或叠瓦式构造类型,且逆掩-逆冲构造很可能将第一阶段形成的层状矿体逆向移位上推。第三阶段为燕山晚期,此时发生高角度冲断和区域应力场变更,有利于断裂由压性转为张性,形成负反转构造,断裂扩张,与燕山期岩浆热液有关的成矿溶液上升,再次叠加成矿。     

Der Laurier Belt,Eine Rotationszone Zwischen Kern-Orogen und Saumsenke (Kanadische Rocky Mountains)

Between Lat. 56° N and 57° 30′ N, the Central Ranges of the Canadian Rocky Mountains contain a belt of slaty-cleaved vertical or overturned east-dipping beds which is considered as the west edge of the thrust-faulted area. Its antithetic rotation is accompanied by small-scale folding parallel to the plane of greatest shear stress, by synthetic and later antithetic movements along the plane of slaty cleavage. Reversal of movement on older reverse faults and a discordant, post-Laramide phase of synthetic normal faulting terminate the movements. Folding and rotation are contemporaneous with the thrusting to the east, and the synthetic normal faulting belongs to the grabening of the Rocky Mountain Trench adjacent to the east. Since rotation and thrusting above the uninvolved basement are contemporaneous, the rotation cannot be powered by differential movement along a stationary lineament within the basement. Its cause is suspected in the west-dipping clinoforms of the Palaeozoic sediments whose inclination placed them within the antithetic quadrant of the Laramide stress field, forcing them to continue rotation and to develop compressional folding instead of bedding-parallel elongated thrust sheets. One of two structurally analogous cases, the Swiss Massif Zone with the Helvetic nappes, shows an antithetically inclined gneissic foliation which might be a mechanical equivalent of the clinoforms in the Rocky Mountains. In the other example, from the north flank of the Hercynian Mosel syncline, a remarkably similar tectonic history and a close geographic coincidence between clinoformic sedimentation and development of an antithetic rotation zone is known.     

INHERITED TRENDS IN THE DEVELOPMENT OF THE PALEOZOIC STRUCTURES OF THE SARYSU-TENIZ UPLIFT (CENTRAL KAZAKHSTAN)

The history of the development of the structural forms in the Sarysu-Teniz uplift, central Kazakhstan, is expressed in the lower Caledonian, late Caledonian, and upper Hercynian. The first period is characterized by meridional folding which resulted in the formation of the Paleozoic geosyncline. In the second period, development of the geosyncline ceased and volcanic activity, both effusive and intrusive, was intense. The principal tectonic elements of the Devonian (late Caledonian) were inherited from the lower Paleozoic. In Middle Devonian, extensive faulting occurred. These "newborn" Hercynian structures, developed on older Caledonian structures, were not determinative for this region. During the third period of tectonic deformation, large west and northwest-trending fold-blocks, called graben-synclines and horst-anticlines, were formed. Because the structural features of the first two periods are structurally similar and trend in the same direction, the differing third period is superimposed disconcordantly.     

Geological features and origin of gold deposits occurring in the Baotou–Bayan Obo district, south-central Inner Mongolia, People's Republic of China

Located at western portion of northern margin of North China craton, the Baotou–Bayan Obo district is one of the most important Fe–REE–Nb and Au metallogenic provinces in China. Presently, about 52 gold deposits and prospects have been discovered, explored and mined, among which Shibaqinhao, Laoyanghao, Houshihua, Saiyinwusu, Wulashan and Donghuofang are the most important ones. All these gold occurrences can be subdivided into three groups (or types) according to its host rocks: (1) hosted by Archean high-grade metamorphic rocks; (2) hosted by Proterozoic sedimentary rocks; (3) hosted by or related to Hercynian alkaline intrusive rocks. The first group contains the Shibaqinhao, Laoyanghao and Houshihua gold deposits. Gold mineralization at these three deposits occurs within Archean amphibolite, gneiss and granulite as gold-bearing quartz veins and veinlet groups containing native gold, electrum, pyrite and chalcopyrite. The Saiyinwusu deposit belongs to the second group, and occurs within Proterozoic sandstone, quartzite and carbonaceous slate as quartz veins and replacement bodies along the fracture zones. Pyrite, marcasite, arsenopyrite, native gold and electrum are identified. The third group includes the Wulashan, Donghuofang and Luchang deposits. Gold mineralization at these three deposits occurs predominantly within the Hercynian alkaline syenite or melagabbro stocks and dyke swarms or along their contacts with Archean metamorphic wall rocks as K-feldspar–quartz veins, dissemination and veinlets. Pyrite, galena, chalcopyrite, native gold and calaverite are major metallic minerals.δ³⁴S value of sulfides (pyrite, galena and pyrrhotite) separates from groups 1 and 2 varies from −4.01‰ to −0.10‰ and −3.01‰ to 2.32‰, respectively. δ³⁴S values of Archean and Proterozoic metamorphic wall rocks for groups 1 and 2 deposits range from −20.2‰ to −17.0‰ and −15.8‰ to −16.2‰, respectively. The values are much lower than their hosted gold deposits. All these pyrite separates from Hercynian alkaline intrusions associated with the gold deposits show positive δ³⁴S values of 1.3‰ to 4.8‰, which is higher than those Precambrian metamorphic wall rocks and their hosted gold deposits. δ³⁴S values of the sulfides (pyrite and galena) from the Donghuofang and Wulashan deposits (group 3) increase systematically from veins (−14.8‰ to −2.4‰) to the Hercynian alkaline igneous wall rocks (2.8‰ to 4.8 ‰). All of these deposits in groups 1, 2 and 3 show relatively radiogenic lead isotopic compositions compared to mantle or lower crust curves. Most lead isotope data of sulfides from the gold ores plot between the Hercynian alkaline intrusions and Precambrian metamorphic wall rocks. Data are interpreted as indicative of a mixing of lead from mantle-derived alkaline magma with lead from Precambrian metamorphic wall rocks.Isotopic age data, geological and geochemical evidence suggest that the ore fluids for the groups 1 and 2 deposits were generated during the emplacement of the Hercynian alkaline syenite and mafic intrusions. The Hercynian alkaline magma may provide heat, volatiles and metals for these groups 1 and 2 deposits. Evolved metamorphic fluids produced by the devolatilization, which circulated the wall rocks, were also progressively involved in the alkaline magmatic hydrothermal system, and may have dominate the ore fluids during late stage of ore-forming processes. Most of these gold deposits hosted by Archean high-grade metamorphic rocks occur at or near the intersections of the NE- and E–W-trending fracture systems. The ore fluid of the group 3 deposits may have resulted from the mixing of Hercynian alkaline magmatic fluids and evolved meteoric waters. The deposits are believed to be products of Hercynian alkaline igneous processes along deep-seated fault zones within Archean terrain.     

Structure and late cenozoic evolution of the upper lithosphere in Southwest Tuscany (Italy)

Geological and geophysical data on southwest Tuscany are reviewed in order to define the structure and evolution of the upper lithosphere from the Miocene to the Quaternary. Petrologic studies reveal the existence, below all of Tuscany, of Hercynian and older polyphased metamorphic rocks and of Hercynian granite, whose top is an important seismic reflecting horizon. The basement is characterized by NE-SW trending structures, in contrast with the main NW-SE “Alpine” structures of the uppermost levels. The heat flow map shows two broad areas with values higher than 80 mW/m², reaching maximum values of 10.5 and 15 H.F.U. in the geothermal areas, which are also characterized by negative Bouguer anomalies. A Landsat study revealed a NE-SW band of subcircular structures passing through Larderello and coinciding with a regional fault system and a steep rise in the Moho. Petrologic, geochemical and radiometric data on the Tuscan igneous rocks show that partial melting took place in the Tuscan crust at different levels and to varying degrees from the Miocene to Quaternary, producing a continuous “Alpine” granitic layer. The known Tuscan intrusive bodies and two batholiths below the Larderello and Mt. Amiata geothermal fields represent culminations of the “Alpine” granite. The rise of the Tuscan magmas was closely correlated to a post-Tortonian tensional tectonics and followed its N-E migration. Tensional tectonics started after the last compressional phase (10–11 Ma B.P.) as a consequence of the anticlockwise rotation of Italy, the opening of the Tyrrhenian Sea and the swelling of the mantle below southwest Tuscany.     

新疆诺尔特地区岩浆岩形成的构造环境及其意义

诺尔特地区位于阿尔泰北部山区,西伯利亚板块西南缘。花岗岩广泛分布,主要为加里东晚期、华力西中、晚期花岗岩,存在燕山期花岗岩。区内火山岩主要是泥盆纪及石炭纪火山岩,泥盆纪火山岩赋存于上泥盆统忙代恰组中,石炭纪火山岩赋存于下石炭统红山嘴组中,主要为中-酸性火山岩,属钙碱性系列。区内岩浆岩构造环境的研究表明,加里东晚期花岗岩形成于汇聚阶段,属挤压环境;华力西中期花岗岩形成于新陆壳阶段弛张期向活化期过渡的时期,应力体制由拉张向挤压过渡;华力西晚期花岗岩形成于新陆壳阶段活化期,属挤压环境;燕山期花岗岩形成于古生代新陆壳在中生代的发展阶段,属挤压环境;泥盆纪火山岩是新陆壳发展阶段固结期的产物,石炭纪火山岩为弛张期及活化期演化过程中的产物。