Avalonian (Pan-African) mylonitic deformation west of Boston, U.S.A.

West of Boston, Mass., Castle and others (1976) recognized an up to 5km wide, possibly folded, NE-SW trending Burlington Mylonite Zone. We have extended mapping south into Natick and Framington quadrangles, and supplemented it by fixing local directions of tectonic motion, which are more variable than reported by Goldstein (1989). In Natick the mylonite zone is partly migmatized and converted into blastomylonites, forming the lithodemic Rice Gneiss and is intersected by the Dedham Granite dated ca 630 Ma. The granite also invades deformed, folded, and commonly mylonitized Westboro Quartzite. Thus mylonitization, folding, and formation of migmatitic blastomylonites are all earlier than ca 630 Ma, and can collectively be attributed to the main phase of the Avalonian orogeny that in Africa is referred to as the Pan-African I. The sense of movements in the Rice Gneiss is generally sinistral strike-slip with a NE-SW trend of foliation. Other local mylonites have more variable directions of motion.A narrower E-W zone of mylonitization has been recognized by Grimes (M.S. thesis 1993, Boston College) and named the Nobscot Shear Zone. It affects the Milford Granite, also about 630 Ma in age, while similar but narrow shear zones affect other local granites including the Dedham. These zones, dipping steeply north and including the Nobscot, are less intensely mylonitized and are not associated with migmatites. Their age is not known, but since they affect only Precambrian rocks, they are assumed to be late Proterozoic. We attribute these zones to the second stage of the Avalonian or the Pan-African II.The older rocks west of Boston are widely affected by numerous brittle faults. These are all of unknown age, but probably Phanerozoic. The most significant brittle fault in the Burlington area is the mid to late Paleozoic Bloody Bluff Fault. We do not associate large scale mylonitization with that fault, because the mylonites are commonly cut by undeformed or little deformed Siluro-Devonian gabbro-diorites.     

Mass-production of Cambro–Ordovician quartz-rich sandstone as a consequence of chemical weathering of Pan-African terranes: Environmental implications

A vast sheet of mature quartz sand blanketed north Africa and Arabia from the Atlantic coast to the Persian Gulf in Cambro–Ordovician times. U–Pb geochronology of a representative section of Cambrian sandstone in southern Israel shows that these sediments are dominated by 550–650 Ma detrital zircons derived from Neoproterozoic Pan-African basement. The short time lag between magmatic consolidation of a Pan-African source and deposition of its erosional products indicates that, despite their significant mineralogical maturity, the voluminous quartz-rich sandstones on the northern margin of Gondwana are essentially first-cycle sediments.

Mass production of these voluminous first-cycle quartz-rich sandstones resulted from widespread chemical weathering of the Pan-African continental basement. We suggest that conditions favoring silicate weathering, particularly a warm and humid climate, low relief and low sedimentation rates prevailed over large tracts of Gondwana in the aftermath of the Pan-African orogeny. An unusually corrosive Cambro–Ordovician atmosphere and humid climate enhanced chemical weathering on the vegetation-free landscape. We infer that late Neoproterozoic–Cambro–Ordovician atmospheric pCO₂ rose as a consequence of widespread late Neoproterozoic volcanism, followed by an uptake of CO₂ by chemical weathering to produce the Cambro–Ordovician sandstone as a negative feedback.     

Nick Rast and the recognition of the Avalonian Arc

Recognition of the eastern (Avalonian) margin of the northern Appalachian orogen as a Late Precambrian microcontinental arc terrane, rather than the opposing passive margin of the Proto-Atlantic (Iapetus) Ocean to that of eastern Laurentia, constituted a fundamental advance in Appalachian geology that profoundly influenced subsequent models for the orogen's plate tectonic evolution. This advance was first clearly articulated by Nick Rast and his students in 1976, who, by correlating rocks of the Avalon Platform with those of the British Midlands, established the Avalonian volcanic belt as a Japan-like microcontinent. Contrary to contemporary views of the Avalon Platform, which favored an extensional, Basin and Range-like setting for its volcanism, Rast argued on the basis of this correlation that the association of Avalonian volcanism with compressional orogeny, widespread calc-alkaline plutonism and, in Angelsey, with blueschists and ophiolitic rocks, indicated a convergent plate margin setting. Rast further proposed that the Avalonian volcanic belt was ensialic, and was bordered to the northwest and southeast by Precambrian oceanic domains. Contemporary reconstructions of the Avalonian and Cadomian belts as fragments of a Cordilleran-like accretionary orogen that developed along an active margin of Neoproterozoic Gondwana owe their origin to these early ideas and, while far removed from the tectonic model that Rast envisaged, are a direct heritage of his recognition of the Avalonian volcanic belt as a microcontinental arc terrane.     

Pan-African and early Paleozoic tectonothermal events in the Nyainrong microcontinent: Constraints from geochronology and geochemistry

To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The ²⁰⁶Pb/²³⁸U concordant age of zircon is 505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P₂O₅ and Al₂O₃ content decreased with SiO₂ increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean-type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.     

Implications of island arc rotations to the studies of marginal terranes

Paleomagnetic studies of rocks from the Bonin and Mariana Island arcs indicate that these island arcs have undergone substantial rotation and northward translation since their formation 40 to 45 Ma. These rotations are inconsistent with existing models of marginal basin and island arc formation. The data presently available suggest to us that the Mariana and Bonin island arcs rotated as one tectonic element at the margin of the Philippine Sea and Pacific plate. These observations demonstrate that large vertical axis rotations are present in the island arc environment, prior to any interaction with a continental landmass. Since many circum-Pacific marginal terranes have been assigned island arc origins, these pre-accretion rotations should be considered when interpreting paleomagnetic results for possible microplate reconstructions. Knowledge of the structural and rotational styles of oceanic pre-accretionary deformations may prove useful for separating these pre-accretion effects from those imposed by the accretionary process in future studies aimed at deciphering the geologic history of island arc marginal terranes.     

Tectonic boundaries within the Grenville province of the Canadian shield

Recent work in a previously little-known part of the Central Gneiss Belt of the southwestern Grenville Province has outlined a regional structural pattern made by a number of geologically distinctive domains that are separated by broad ductile shear zones. Characteristics rock assemblages, metamorphism, geophysical signatures and structural trends are modified or truncated at domain margins. Bounding high strain zones contain mylonite and other forms of tectonically modified gneiss, formed under at least middle amphibolite facies conditions, within which kinematic indicators imply a northwesterly sense of overriding or lateral sliding between adjacent crustal masses. Similar kinematics also apply at the margin of the Central Metasedimentary Belt to the southeast and along the Grenville Front to the northwest. A scenario involving northwesterly stacking of large crustal blocks and slices at relatively deep level can account for the observed relationships and implies a period of crustal thickening that may represent the culmination of the Grenvillian Orogeny in this region.     

Plutonism in Pan-African belts and the geologic evolution of northeastern Africa

Plutonic igneous rocks of Pan-African belts (500–600 m.y. age) in Africa can be described in terms of three types of assemblages: (1) migmatites formed largely by remobilization of pre-existing sialic crust during Pan-African time; (2) calcalkaline batholithic suites formed in association with subduction of oceanic crust; and (3) alkali-rich, post-tectonic granites. Many suites cannot be placed precisely in one of these categories, either because of intermediate (gradational) characteristics or because of lack of adequate information. Distinction of rocks formed during Pan-African time from older ones whose radiometric clocks were reset at that time is also difficult.The rock suites are very unevenly distributed geographically. Migmatites formed by ensialic crustal remobilization occur mostly in southern and central Africa. Calcalkaline suites occur in north Africa, particularly the northeast. Alkali-rich granites are virtually restricted to the northeastern portion of the continent, the one major part of Africa not occupied by an Archean shield. Evidence for subduction of oceanic crust is also restricted to northeastern Africa.The geologic history proposed for northeastern Africa is as follows: During, and immediately preceding, Pan-African time an ocean basin of uncertain size and shape occurred between the West African and Nubian-Arabian shields. Subduction zones were active on the margins and within the basin. During, and at the end of, Pan-African time extensive development of alkali-rich granites was associated with cratonization of the basin. Paleozoic sediments were then deposited under platform conditions on the newly formed craton, which showed mild epeirogenic activity throughout the Phanerozoic.     

Age of Grenville Belt magnetisation: Rb-Sr and palaeomagnetic evidence from Swedish dolerites

Rb-Sr mineral ages and palaeomagnetic data are presented for nine dolerite dykes and sills in central and southern Sweden. The intrusions are representative of widespread dolerites, mainly dykes, which cut pre-Sveconorwegian (Svecofennian) crust and trend parallel to the Sveconorwegian Front Zone (or Schistosity Zone) over a band up to 150 km wide along most of its 700-km length. The Rb-Sr ages suggest emplacement through the approximate interval 1000?900 m.y. Cooling ages on country rocks show little evidence of significant Sveconorwegian (e.g. 1000 m.y.) heating effects, and the new isotopic and palaeomagnetic results are taken to represent the time of dolerite intrusion.The dolerites were emplaced in response to net tensional stresses in the Svecofennian crust: it is proposed that these were caused by marginal upwarping related to post-orogenic uplift of the adjacent Sveconorwegian region.The palaeomagnetic poles for the dolerites are similar to poles from the Sveconorwegian Province and the Front Zone, indicating that these also have a ca. 1000?900-m.y. age. The similar cooling histories of the Sveconorwegian Province and the Grenville Province of Canada allow this age to be assigned also to the Grenville poles. If so, it is not necessary to postulate plate motions during Grenville-Sveconorwegian magnetisation, and the agreement between dolerite poles and Sveconorwegian poles suggests that the orogenic belt cooled essentially in place against its bounding cratons. A possible continental fit shows Scandinavia and North America contiguous in late Grenville time, though relative motions before 1000 m.y. are not ruled out.     

Neoproterozoic and Cambrian successions deposited on the East European platform and Cadomian basement area in Poland

Summary A short review of the Neoproterozoic and Cambrian sedimentary and volcanogenic successions in Poland is concerned with depositional and tectonic developments. The successions accumulated in shallow shelf and deeper slope environments. They overlie Palaeoproterozoic crystalline basement of the Baltic Craton and Gondwana-derived Neoproterozoic Cadomian basement. This discrepancy in the nature of the basement is viewed as indicating distinct terranes. During the course of Palaeozoic time, the discrete terranes were assembled along the Teisseyre-Tornquist Lineament and the Trans-European Suture Zone adjacent to the Baltica palaeocontinent.     

The exhumation of high- and ultrahigh-pressure metamorphic terranes in subduction zone: Questions and discussions

Science China Earth Sciences - In terms of petrology, thermomechanical simulation is an important frontier to study the geodynamic process of the exhumation and uplift of high pressure (HP) to...     

福建省地体构造划分及构造演化

本文根据对福建省不同地区的地质特征进行详细的研究对比,划分出闽西北、闽西南、闽东和闽东南四个性质不同的地体,并研究了各地体的之间碰撞拼贴的历史,指出它们主要经历了三个阶段：1、晚元古代,闽西南地体和闽西北地体沿南平-宁化一线发生碰撞拼贴作用,同时伴随海底和陆上火山喷发活动;2、三叠纪,闽东地体沿政和-大埔一线和闽西南、闽西北地体发生碰撞拼贴作用,闽东南地体也随之拼贴在闽东地体之上;3、白垩纪,闽东南地体沿平潭一东山断裂带发生左行走滑运动。     

Paleomagnetic study of late Proterozoic and Early Cambrian rocks in terranes of the Amur plate

New paleomagnetic data are presented for Proterozoic metamorphic and Cambrian terrigenous-carbonate complexes of the southern Far East of Russia (Primor’e and the Amur and Trans-Baikal regions). Taking into account our results obtained previously, the paper presents revised positions of the paleomagnetic pole corresponding to the Riphean-Lower Paleozoic segment of the apparent polar wander path for terranes of the Amur plate (the Argun and Bureya-Khanka orogenic belts) in comparison with poles from the Siberian and North Chinese plates. It is shown that the paleolatitude positions of the Amur terranes were stable in space and time during the interval from the late Riphean to the end of the Early Cambrian: they were located in equatorial zones of the Northern and Southern hemispheres.     

Geochemistry of calc-alkaline volcanics in northwestern Nigeria,and a possible Pan-African suture zone

Major and trace element characteristics of dacites and rhyolites overlying and intruding basement rocks in northwestern Nigeria most closely resemble those of intracontinental orogenic volcanic associations. REE patterns point to a deep-seated source for the magmas, perhaps involving garnet fractionation at mantle depths and low-pressure plagioclase fractionation. The occurrence of calc-alkaline volcanics, small basic-ultrabasic complexes and major transcurrent faulting, is consistent with the presence of a Pan-African suture zone in northwestern Nigeria.     

Mesozoic terranes of the northwest Pacific continental margin (Russia): Radiolarian ages and sedimentary environments

Abstract Geological mapping using detailed tectonic and complex radiolarian analysis revealed significant northward displacement of a number of Russian Far and Northeast Asia terranes. It was recorded that some terranes possibly crossed the equator. Terranes of north-east Russia were composed of different allochthonous formations, ranging in age from Middle Triassic to Maestrichtian-Paleocene and accumulated from the margin to oceanic basins. The Middle to Upper Triassic interval included two formations: (i) volcanogenic, consisting of typical volcanic rocks of the island arcs (up to 800 m thick); and (ii) a chert-limestone-terrigenous one composed of marginal sandstone, siltstone, limestone and tuffic chert (about 400 m). Lower Jurassic allochthonous formations are represented by chert-terrigenous (about 300 m) and jasper-alkaline-basaltic (WPB-type) seamount deposits (about 100 m). Middle Jurassic to Hauterivian allochthonous terranes from the northern part of the Koryak-Kamchatka region include five formations: jasper (bedding jaspers with condensed limestone lenses with Buchias, 80 m), jasper-basalt (with MORB, 100-150 m), ferrotitanic basalt (WPB with lenses of jasper mainly composed of genus Parvicingula, about 75%, 150 m), terrigenous-volcanic (with MORB, IAT, CA basalts and olistostrome, 600 m), tuffic-jasper-basalt (MORB and deposits of arc-trench system, about 500 m) with the same age according to radiolarian data. Aptian? Albian-Maestrichtian ones are predominantly terrigenous-tuffaceous-siliceous. Moreover, the Early and Middle Jurassic faunas of the northwest Pacific margin contain many boreal elements similar to those of New Zealand (Southern Hemisphere), Japan, ODP Site 801. The Late Jurassic faunas of the Koryak and Kamchatka region are mainly North Tethyan and seldom Central Tethyan and are very closely related to those of the Americas. The Tithonian to Early Cretaceous radiolarian are predominantly Central Tethyan and Equatorial in contrast to Boreal Late Cretaceous. The combining in the same region at 60°N Pacific margin of the formations accumulated in different tectonic paleoenvironments and paleoclimatic provinces, is good evidence for the possible significant northward displacement of some terranes in the northwestern Pacific.     

四川盆地—大巴山结合带地壳构造特征:深反射地震约束的重磁解释

大巴山位于四川盆地北部、秦岭南缘,以发育大规模的中生代逆冲推覆构造为世人瞩目,其地壳尺度结构与构造特征对于理解扬子克拉通和华北克拉通的碰撞过程有重要意义.本文基于深反射地震剖面和宽角反射与折射地震数据的约束,结合地质与钻井资料,通过重磁场分析拟合解释,对四川盆地北部—大巴山地壳尺度的构造进行综合研究,建立了地壳断面结构与构造模型.模型显示,四川盆地北部基底和沉积盖层变形差异较大,且上下地壳具有解耦性,上地壳向北延伸至紫阳断裂一带;而下地壳与上地壳拆离,向北延伸远至安康断裂一带.研究区莫霍面起伏较大,自南向北先加深后抬升,在大巴山下出现Moho的构造叠置,这种现象源于大陆地块(扬子克拉通)下地壳向大巴山—秦岭造山带下的俯冲.类似的现象也出现在西昆仑山下,即塔里木盆地下地壳向西昆仑山下俯冲,俯冲板片前缘出现Moho的叠置.     

Observations of EM signals prior to earthquake events

Conclusion The results described above were obtained in the frequency band below about 20 Hz within 200 km of the observation station. therefore, they had the characteristies of quasi — static field. The amplitude of these signals was much higher than the environment noise level. The EM signals prior to earthquakes have good correlation with the seismic events. From the records of 16 signal groups received in 5 years, it can be seen that the correlation reached 75%. So the EM signals observed are possibly precursors useful for the prediction of the earthquake events. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 121–124, 1991.     

Relationship between zonal position of the North Atlantic Oscillation and Euro-Atlantic blocking events and its possible effect on the weather over Europe

The North Atlantic Oscillation(NAO) exhibited a marked eastward shift in the mid-1970 s. Observations show that the extreme weather events in Europe have emerged frequently in the past decades. In this paper, based upon the daily NAO index, we have calculated the frequency of in-situ NAO events in winter during 1950-2011 by defining the Eastern-type NAO(ENAO) and Western-type NAO(WNAO) events according to its position at the east(west) of 10°W. Then, the composites of the blocking frequency, temperature and precipitation anomalies for different types of NAO events are performed. Results show that the frequency of Euro-Atlantic blocking events is distributed along the northwest-southeast(southwest-northeast) direction for the negative(positive) phase. Two blocking action centers in Greenland and European continent are observed during the negative phase while one blocking action center over south Europe is seen for the positive phase. The action center of blocking events tends to shift eastward as the NAO is shifted toward the European continent. Moreover, the eastern-type negative phase(ENAO) events are followed by a sharp decline of surface air temperature over Europe(especially in central, east, and south Europe), which have a wider and stronger impact on the weather over European continent than the western-type negative phase(WNAO) events do. A double- branched structure of positive precipitation anomalies is seen for the negative phase event, besides strong positive precipitation anomalies over south Europe for ENAO event. The eastern-type and western-type positive phase(ENAO+ and WNAO+) can lead to warming over Europe. A single-branched positive precipitation anomaly dominant in central and north Europe is seen for positive phase events.