Report on the Third IGCP-649 International Workshop on the Mayarí-Baracoa Ophiolites and Chromitites, Cuba

<正>During April 3~(rd)–14~(th),the Third International Ophiolite Workshop of the International Geoscience Programme"Diamonds and Recycled Mantle"(IGCP-649)was successfully held in Havana,Cuba(Fig.1a).After the workshop,participants participated in a field trip to the Mayarí-Baracoa ophiolites and related chromitites in     

Seminar on geodynamics of the Himalayan region: Harsh. K. Gupta (Editor). National Geophysical Research Institute, India, 1973, 221 p., US $ 8.00

For a detailed palaeomagnetic research on Upper Permian red beds in the Wardha Valley (Central India) 265 samples from 47 sites at 6 localities were investigated.The samples from 3 localities (17 sites) appeared to be completely remagnetized during Early Tertiary times by the vast Deccan Trap flood basalts effusions. The samples from 22 sites of the other three localities (results from 8 sites rejected) could become cleaned from hard secondary Deccan Trap components by detailed thermal demagnetization.The resulting primary magnetization component reveals a mean direction (regardless of polarity, 7 sites normal, 15 sites reversed): D = 101.5°, I = +58.5°, α₉₅ = 6.5°, N = 3. This mean direction corresponds to a pole position at 129° W 4° N (dp = 7°, dm = 9.5°). This pole position fits well with other acceptable Late Permian—Early Triassic pole positions for the Indian subcontinent. From these acceptable results, a mean Permo-Triassic pole for the Indian subcontinent was computed at: 125° W 6°N. This Indian Permo-Triassic pole position, when compared with data from other Gondwanaland continents, suggests the hypothesis of an early movement between India and Africa before Permo-Triassic times.The partial or total remagnetization of some Indian red beds, mainly of Gondwana age, during Deccan Trap times is explained as acquisition of viscous Partial Thermoremanent Magnetization. This mechanism was advanced by Briden (1965), Chamalaun (1964) and Irving and Opdyke (1965).     

The origin of chloritoid – 3‐mica pseudomorph growth in staurolite–muscovite schist,Bangriposi (Eastern India)

At Bangriposi, variable stages in replacement of staurolite by chloritoid – Na–K–Ca mica shimmer aggregates in muscovite schists provides insight into the complex interplay between fluid flow, mass transfer, and dissolution–precipitation during pseudomorph growth. Idioblastic chloritoid growing into mica caps without causing visible deformation, and monomineralic chloritoid veins (up to 300 μm wide) within shimmer aggregates replacing staurolite attest to chloritoid nucleation in fluid‐filled conduits along staurolite grain boundaries and crystallographic planes. The growth of shimmer aggregates initiated along staurolite margins, and advanced inwards into decomposing staurolite along networks of crystallographically controlled fluid‐filled conduits. Coalescence among alteration zones adjacent to channel fills led to dismemberment and the eventual demise of staurolite. Mass balance calculation within a volume‐fixed, silica‐conserved reference frame indicate the shimmer aggregates grew via precipitation from fluids in response to mass transport that led to the addition of H₂O, K₂O, Na₂O and CaO in the reaction zone, and Al₂O₃ was transported outward from the inward‐retreating margin of decomposing staurolite. This aided precipitation of chloritoid in veins and in the outer collars, and as disseminated grains in the shimmer aggregates at mid‐crustal condition (~520 ± 20 °C, 5.5 ± 2.0 kbar). Computation using one‐dimensional transport equation suggests that staurolite decomposition involved advection dominating over diffusive transport; the permeation of externally derived H₂O caused flattening of chemical potential gradients in H₂O and aqueous species, for example, and , computed using the Gibbs method. This suggests that staurolite decomposition was promoted by the infiltration of a large volume of H₂O that flattened existing chemical potential gradients. In the initial stages of replacement, chloritoid super‐saturation in fluid caused preferential nucleation and growth of chloritoid at staurolite grain boundaries and in crystallographic planes. As reaction progressed, further chloritoid nucleation was halted, but chloritoid continued to grow as the 3‐mica aggregates continued to replace the remaining staurolite in situ, while the chloritoid‐compatible elements were transported in the water‐rich phase facilitating continued growth of the existing chloritoid grains.     

Geomagnetic Field Disturbances from the Fall of the Lipetsk (June 21, 2018) and Chelyabinsk (February 15, 2013) Meteorites

Doklady Earth Sciences - Based on the Chelyabinsk (February 13, 2013) and Lipetsk (June 21, 2018) events, disturbances in the Earth’s geomagnetic field, which were induced by the fall of...     

Research on the February 18, 1996 earthquake in the caves of Saint-Paul-de-Fenouillet area, (eastern Pyrenees,France)

Abstract

Eight caves have been investigated near Saint-Paul- de-Fenouillet after the earthquake of 5.2 magnitude of February 1996 which occurred in the eastern Pyrenees (France) and caused moderate damage at the ground surface. The earthquake has been associated with the movement of an E-W fault. The caves had not been visited since the earthquake. Some damage, mainly collapses of soda straws and small rocks, could be attributed to this earthquake. The most interesting cave in the epicentral area is the Paradet cave which is situated on a recently activated fault plane. In this cave, soda straw falls could be attributed to the earthquake, but other more ancient damage was also observed. Analysis of the azimuth of fallen speleothems, which are natural pendulums, may indicate the directions, and an estimation of their mechanical properties gives the threshold of the seismic ground motion amplitude responsible for their collapse, thus supplying information to calibrate damage due to past earthquakes. A statistical study indicates that the main direction of the collapsed soda straws is E–W. Numerical simulations confirm that soda straws are relatively strong objects that may break under certain conditions during earthquakes. © Elsevier, Paris     

Research on the February 18, 1996 earthquake in the caves of Saint-Paul-de-Fenouillet area, (eastern Pyrenees,France)

Eight caves have been investigated near Saint-Paul-de-Fenouillet after the earthquake of 5.2 magnitude of February 1996 which occurred in the eastern Pyrenees (France) and caused moderate damage at the ground surface. The earthquake has been associated with the movement of an E-W fault. The caves had not been visited since the earthquake. Some damage, mainly collapses of soda straws and small rocks, could be attributed to this earthquake. The most interesting cave in the epicentral area is the Paradet cave which is situated on a recently activated fault plane. In this cave, soda straw falls could be attributed to the earthquake, but other more ancient damage was also observed. Analysis of the azimuth of fallen speleothems, which are natural pendulums, may indicate the directions, and an estimation of their mechanical properties gives the threshold of the seismic ground motion amplitude responsible for their collapse, thus supplying information to calibrate damage due to past earthquakes. A statistical study indicates that the main direction of the collapsed soda straws is E-W. Numerical simulations confirm that soda straws are relatively strong objects that may break under certain conditions during earthquakes.     

Proterozoic–Paleozoic development of the basement of the Central Andes (18–26°S) — a mobile belt of the South American craton

The Late Precambrian–Early Paleozoic metamorphic basement forms a volumetrically important part of the Andean crust. We investigated its evolution in order to subdivide the area between 18 and 26°S into crustal domains by means of petrological and age data (Sm–Nd isochrons, K–Ar). The metamorphic crystallization ages and t_DM ages are not consistent with growth of the Pacific margin north of the Argentine Precordillera by accretion of exotic terranes, but favor a model of a mobile belt of the Pampean Cycle. Peak metamorphic conditions in all scattered outcrop areas between 18 and 26°S are similar and reached the upper amphibolite facies conditions indicated by mineral paragensis and the occurrence of migmatite. Sm–Nd mineral isochrons yielded 525±10, 505±6 and 509±1 Ma for the Chilean Coast Range, the Chilean Precordillera and the Argentine Puna, and 442±9 and 412±18 Ma for the Sierras Pampeanas. Conventional K–Ar cooling age data of amphibole and mica cluster around 400 Ma, but are frequently reset by Late Paleozoic and Jurassic magmatism. Final exhumation of the Early Paleozoic orogen is confirmed by Devonian erosional unconformities. Sm–Nd depleted mantle model ages of felsic rocks from the metamorphic basement range from 1.4 to 2.2 Ga, in northern Chile the average is 1.65±0.16 Ga (1σ; n=12), average t_DM of both gneiss and metabasite in NW Argentina is 1.76±0.4 Ga (1σ; n=22), and the isotopic composition excludes major addition of juvenile mantle derived material during the Early Paleozoic metamorphic and magmatic cycle. These new data indicate a largely similar development of the metamorphic basement south of the Arequipa Massif at 18°S and north of the Argentine Precordillera at 28°S. Variations of metamorphic grade and of ages of peak metamorphism are of local importance. The protolith was derived from Early to Middle Proterozoic cratonic areas, similar to the Proterozoic rocks from the Arequipa Massif, which had undergone Grenvillian metamorphism at ca. 1.0 Ga.     

Tectonic features of the Lipari–Vulcano complex (Aeolian archipelago,Italy) from 10 years (1996–2006) of GPS data

The tectonic deformation of the Lipari–Vulcano complex, one of the most important active volcanic areas of the Mediterranean region, is studied here through the analysis of 10 years (1996–2006) of GPS data from both three permanent and 13 non‐permanent stations. This area can be considered crucial for the understanding of the interaction between the Eurasian and African plates in the Mediterranean area, and, in general, this work emphasizes a methodological approach, already applied in other areas worldwide ( J. Geophys. Res., 1996, 101 , 27 957 ; J. Geodyn., 1999, 27 , 213 ) where geodetic data and strain parameters maps of critical areas can help to improve our understanding of their geodynamical aspects. In this framework, this study is aimed at providing a kinematic deformation model on the basis of the dense geodetically estimated velocities of the Lipari–Vulcano complex. In particular, the observed deformation pattern can be described by a combination of (1) the main N–S regional compression and (2) a NNE–SSW compression with a small right‐lateral strike slip component acting along a tectonic structure trending N°40W between the two islands. This pattern was inspected through a simplified synthetic model.     

In situ monazite (U–Th)–Pb ages from the Southern Brasília Belt,Brazil: constraints on the high‐temperature retrograde evolution of HP granulites

In the southern sector of the Southern Brasília Belt, late Neoproterozoic arc–passive margin collision resulted in juxtaposition of an arc‐derived nappe (the Socorro–Guaxupé Nappe) over a stack of passive margin‐derived nappes (the Andrelândia Nappe Complex) that lies on top of autochthonous basement of the São Francisco Craton. (U–Th)–Pb monazite ages are reported from the high‐grade nappes of the Andrelândia Nappe Complex to better constrain the high‐temperature retrograde evolution. For residual HP granulites from the uppermost Três Pontas–Varginha Nappe, (U–Th)–Pb ages of c. 662 and 655 Ma from low yttrium monazite inclusions in the rims of, or associated with garnet are interpreted to date the late‐stage close‐to‐peak prograde evolution, whereas an age of c. 648 Ma from a similar low yttrium monazite inclusion is interpreted to record post‐peak recrystallization with melt via factures in garnet. For the same nappe, ages of 640–631 Ma retrieved from higher yttrium areas or cores in monazite grains that occur both as inclusions in garnet and in the matrix are interpreted to record growth of monazite either by local breakdown of garnet (±older monazite) and mass exchange with a matrix melt reservoir along cracks or growth from residual melt in the matrix as it crystallized during high‐pressure, close‐to‐isobaric cooling close to the solidus, the temperature of which, at a given pressure, varies with bulk composition of the residual granulites. (U–Th)–Pb ages in the range 620–588 Ma from lower yttrium areas in these monazite grains and from matrix‐hosted patchy monazite are interpreted to date exhumation, as recorded by close‐to‐isothermal decompression and subsequent close‐to‐isobaric cooling. Older monazite ages in this group are interpreted to record late‐stage interaction with melt close to the solidus whereas younger monazite ages are interpreted to record recrystallization of monazite by dissolution–reprecipitation owing to ingress of alkali fluid from the Carmo da Cachoeira Nappe beneath as fluid was released by crystallization of in‐source melt at the solidus. In the underlying Carmo da Cachoeira Nappe, higher yttrium areas in monazite and one single domain monazite yield chemical ages of 619–616 Ma, which are interpreted to date growth as in‐source melt crystallized close to the solidus along the high‐pressure, close‐to‐isobaric segment of the retrograde P–T evolution. Younger (U–Th)–Pb ages of 600–595 Ma retrieved from lower yttrium areas and one single domain monazite are interpreted to record recrystallization of monazite by dissolution–reprecipitation owing to release of fluid at the solidus during exhumation of this nappe. Monazite from the Carvalhos Klippe, interpreted to be correlative with the uppermost nappe, yields a wide range of (U–Th)–Pb ages: for two zoned grains, c. 619 and c. 614 Ma from higher yttrium cores, and c. 583 and c. 595 Ma from lower yttrium rims; and, 592–580 Ma from single domain grains in one sample, and ages of c. 593 and c. 563 Ma from monazite in a second sample. Ages younger than 605 Ma are interpreted to date a fluid‐induced response to the early stages of orogenic loading associated with terrane accretion in the Ribeira Belt to the southeast. The results reported here demonstrate that ages retrieved from monazite that grew close to the solidus in residual granulites from a single tectonic unit will vary from sample to sample according to differences in the solidus temperatures. Further, we show that monazite inclusions may yield ages that are younger than the host mineral and confirm the propensity of monazite to record evidence of tectonic events that are not always registered by other high‐temperature mineral chronometers.     

Biostratigraphy of the Upper Pleistocene (Upper Neopleistocene)–Holocene deposits of the Lemeza River valley of the Southern Urals region (Russia)

Numerous caves and terraces with late Late Pleistocene (Upper Neopleistocene according to the Russian stratigraphic scale)–Holocene deposits are located in the Lemeza River valley in the surroundings of the Atysh waterfall, the native reserve territory of the Bashkortostan Republic. Lemeza River runs in the southern part of the western slope of the Urals and belongs to the Belaya River valley system (Russian Federation). A summary of the biostratigraphical investigations between 1992 and 2007 in this area is given. Deposits of cave and fluvial origin are characterized in the framework of the regional stratigraphy. The results of mammalian investigations and radiocarbon dating provide the basis for the stratigraphical subdivision. Palynology, mollusca, fishes, amphibian and reptiles are used for the reconstruction the palaeoenvironments. The Southern Urals stratigraphic subdivisions are correlated with Western European (Weichselian-Holocene), Eastern European (Russia) (Leningrad–Ostashkov–Shuvalov) and Uralian (Nevyansk–Polar Urals–Gorbunovsky) stratigraphic schemes.     

The Last Permafrost Maximum (LPM) map of the Northern Hemisphere: permafrost extent and mean annual air temperatures, 25–17 ka BP

This paper accompanies a map that shows the extent of permafrost in the Northern Hemisphere between 25 and 17 thousand years ago. The map is based upon existing archival data, common throughout the Northern Hemisphere, that include ice‐wedge pseudomorphs, sand wedges and large cryoturbations. Where possible, a distinction is made between areas with continuous permafrost and areas where permafrost is either spatially discontinuous or sporadic. The associated mean annual palaeo‐temperatures that are inferred on the basis of present‐day analogues increase understanding of the possible changes in permafrost extent that might accompany current global warming trends. Areas with relict permafrost and areas that were formerly exposed due to lower sea level (submarine permafrost) are also mapped. Mapping is mostly limited to lowland regions (areas approximately <1000 m a.s.l.). Striking features that appear from the map are (i) the narrow permafrost zone in North America, which contrasts with the broader LPM permafrost zone in Eurasia (that may be related to different snow thickness or vegetation cover), (ii) the zonal extent of former LPM permafrost (that may reflect sea‐ice distribution), which contrasts with the present‐day pattern of permafrost extent (especially in Eurasia) and (iii) the relatively narrow zones of LPM discontinuous permafrost (that may indicate strong temperature gradients).     

Subsidence of the Thessaloniki (northern Greece) coastal plain, 1960–1999

Comparison of aerial photos, maps and triangulation data reveal that in the last 40 yr a part of the Thessaloniki coastal plain, a delta formed in the last 2500 yr, subsided at a rate of up to 10 cm/yr. As a consequence the sea invaded up to 2 km inland; precious land in the suburbs of the city was lost, while a village and major industrial plants are in risk of flooding. Part of the land was reclaimed thanks to barriers, pumping and artificial raising of the land surface. Yet, the situation is unstable and flooding is not unusual. Ground water withdrawal for the needs of the Thessaloniki metropolitan complex has initially been regarded as the cause of the subsidence. However, the lack of correlation in space and in time between fluctuations of piezometric levels, topographic changes and pumping indicates that the observed subsidence should be regarded as the cumulative effect of several factors, including consolidation of near-surface sediments due to the decline of the piezometric level and the partial abandonment of the delta, oxidation of peat soils in the vadose zone, synsedimentary deformation (faulting and flow) and loading-induced consolidation of deeper sediments.     

Dolomite characteristics and diagenetic model of the Calcari Grigi Group (Asiago Plateau,Southern Alps – Italy): an example of multiphase dolomitization

A multidisciplinary study, conducted over the carbonate platform deposits of the Liassic Calcari Grigi Group (Southern Alps), highlighted how the use of outcrop analogues can contribute to better define the distribution of dolomitic bodies related to fault networks, to characterize the petrophysical properties of the dolomitic sequence and unravel a complex diagenetic history. This study was carried out in the Asiago Plateau (southernmost part of the eastern Southern Alps, northern Italy) which provides excellent outcrops of the Jurassic Calcari Grigi Group. The dolomitization of the Jurassic sequence is variable in terms of stratigraphic extension and geographic distribution. In the studied localities the dolomitization is generally limited to the Mount Zugna Formation and is characterized by an undulatory front, with ‘sub‐vertical dolomitic chimneys’ along the major faults. Within this unit, and often associated with faults, stacked high‐porosity and permeability bed‐parallel dolomitic bodies are developed that show excellent petrophysical properties. The dolomitic intervals are characterized by pervasive unimodal and patchy polymodal dolomite crystals. Thin section, cathodoluminescence, isotopic and fluid inclusion analyses were used to constrain the paragenetic evolution of the sequence which is similar in all the studied localities. The first dolomitization stage is marked by zoned dolomite crystals with a dull luminescent core. The porosity is thought to have increased after this stage, with dark blue luminescent dolomite accompanied by the corrosion of older crystals. The appearance of saddle dolomite marks the onset of the porosity reduction stage, ending with the infilling of vugs and the remaining open pores with calcite cement. The diagenetic evolution locally stopped at the saddle dolomite stage with the complete occlusion of the remaining pores. Paragenetic and fluid‐inclusion data suggest an evolutionary trend of increasing temperatures and decreasing salinity toward brackish fluids responsible for dolomite and calcite precipitation. The integration of the available data seem to indicate that the diagenetic evolution of the study area is related to: (i) the interplay between evolving fluids (from marine to brackish); (ii) the burial of the sequence (increasing temperature); and (iii) the evolution of the hydrogeological system (fault and fracture network, fluid mixing). This complex paragenetic evolution is strongly linked to the evolution of the porosity framework that evolved from a good, widespread network in the early stages of the burial history to a confined system in the later stages due to reduction of porosity by the deposition of late calcite and dolomite cements.