Time-clustering behavior of spreading-center seismicity between 15 and 35°N on the Mid-Atlantic Ridge: observations from hydroacoustic monitoring

An earthquake catalog derived from the detection of seismically-generated T-waves is used to study the time-clustering behavior of moderate-size (?3.0 M) earthquakes between 15 and 35°N along the Mid-Atlantic Ridge (MAR). Within this region, the distribution of inter-event times is consistent with a non-periodic, non-random, clustered process. The highest degrees of clustering are associated temporally with large mainshock-aftershock sequences; however, some swarm-like activity also is evident. Temporal fluctuations characterized by a power spectral density P(f) that decays as 1/f^α are present within the time sequence, with α ranging from 0.12 to 0.55 for different regions of the spreading axis. This behavior is negligible at time scales less than ∼5×10³ s, and earthquake occurrence becomes less clustered (smaller α) as increasing size thresholds are applied to the catalog. A power-law size-frequency scaling for Mid-Atlantic Ridge earthquakes also can be demonstrated using the distribution of acoustic magnitudes, or source levels. Although fractal seismic behavior has been linked to the structure of the underlying fault population in other environments, power-law fault size distributions have not been observed widely in the mid-ocean ridge setting.     

Environmental effects of river sand mining: a case from the river catchments of Vembanad lake,Southwest coast of India

Rivers in the southwest coast of India are under immense pressure due to various kinds of human activities among which indiscriminate extraction of construction grade sand is the most disastrous one. The situation is rather alarming in the rivers draining the Vembanad lake catchments as the area hosts one of the fast developing urban-cum-industrial centre, the Kochi city, otherwise called the Queen of Arabian Sea. The Vembanad lake catchments are drained by seven rivers whose length varies between 78 and 244 km and catchment area between 847 and 5,398 km². On an average, 11.73 million ty⁻¹ of sand and gravel are being extracted from the active channels and 0.414 million ty⁻¹ of sand from the river floodplains. The quantity of instream mining is about 40 times the higher than the sand input estimated in the gauging stations. As a result of indiscriminate sand mining, the riverbed in the storage zone is getting lowered at a rate of 7–15 cm y⁻¹ over the past two decades. This, in turn, imposes severe damages to the physical and biological environments of these river systems. The present paper deals with the environmental effects of indiscriminate sand mining from the small catchment rivers in the southwest coast of India, taking the case of the rivers draining the Vembanad lake catchments as an example.     

Shear-wave splitting and reservoir crack characterization: the Coso geothermal field

This paper aims to improve current understanding of the subsurface fracture system in the Coso geothermal field, located in east-central California. The Coso reservoir is in active economic development, so that knowledge of the subsurface fracture system is of vital importance for an accurate evaluation of its geothermal potential and day-to-day production. To detect the geometry and density of fracture systems we applied the shear-wave splitting technique to a large number of high-quality seismograms from local microearthquakes recorded by a permanent, 16-station, down-hole, 3-component seismic array running at 480 samples/s. The analysis of shear-wave splitting (seismic birefringence) provides parameters directly related to the strike of the subsurface fractures and their density (number of cracks per unit volume), and, consequently, is an important technique to outline zones of high permeability. Three major fracture directions N10–30W, N0–20E, and N40–50E, of which the first and the second are the most prominent, were identified from the seismograms recorded by the 16-station down-hole array. All orientations are consistent with the known strike of local sets of faults and fractures in local wells and at the surface, as well as with previous analyses of seismic anisotropy in the region. The high quality of the recordings has allowed us to launch an unprecedented investigation into the characteristics of the temporal variations in crack polarization and crack density in a producing geothermal environment. Preliminary results point to significant temporal changes in shear-wave time delays, probably influenced by temporal changes in crack density within a period of 5 years (1996–2000). They are tentatively interpreted as due to a local 3% increase in shear-wave velocity in the southwestern part of the field during 1999.     

Geochemistry of Cu, Co, Ni, Zn, Cd and Cr in the surficial sediments of a tropical estuary, southwest coast of India: a granulometric approach

 Geochemical characteristics of six trace metals – Cu, Co, Ni, Zn, Cd and Cr – in the bulk sediment and sand, silt and clay fractions of a tropical estuary on the southwest coast of India have been studied and discussed. In bulk sediment, the trace metal concentration is controlled mainly by the textural composition of the sample. Mud, sandy mud and sandy silt register higher concentrations of trace metals than that in sand-dominant sediments. The granulometric partitioning studies also re-affirmed the role of particle size in enriching the trace metals. The silt and clay fractions exhibit 7–8 times the enrichment of Cu and Cd compared to that in sand. The enrichment factors of Zn, Cr, Ni and Co in the silt and clay fractions, compared to that in sand, are 5–6, 4–5, 2–5 and 2–3 times, respectively. The trace metals in the sand fraction, particularly Ni and Cr, exhibit strong positive correlation with the heavy mineral content of the samples. It clearly indicates a heavy mineral pathway to the trace metals in the sand fraction. Cu and Co in silt and clay fractions exhibit a marked decrease towards the high saline zones of the estuary. This is attributed to the desorption of Cu and Co from particulate phases during estuarine mixing. Contrary to Cu and Co, the content of Zn in the clay fraction shows a marginal increase towards the estuarine mouth. This could be explained by the influx of Zn-rich contaminant discharges from Zn-smelting industries located slightly north of the estuarine mouth. The released Zn will effectively be held in the lattices of the clay mineral montmorillonite, which also exhibits a marked increase towards the estuarine mouth. The anomalously high values of Cd in some places of the Central Vembanad estuary is attributed to the local pollution. Received: 10 July 1995 · Accepted: 3 June 1996     

Direct 40Ar/39Ar dating of Late Ordovician and Silurian brittle faulting in the southwestern Norwegian Caledonides

Structural data as well as U–Pb zircon and ⁴⁰Ar/³⁹Ar biotite and muscovite ages were collected from the Rolvsnes granodiorite in western Norway. The granodiorite intruded at c. 466 Ma, cooled quickly and escaped later viscous deformation. Brittle top‐to‐the‐NNW thrust faults (Set I) and WNW–ESE striking dextral strike‐slip faults (Set II) formed in a NNW–SSE transpressional regime. ⁴⁰Ar/³⁹Ar dating of synkinematic mica from both sets reveals a c. 450 Ma (Late Ordovician) age of faulting, which constrains early‐Caledonian brittle deformation. Set I and II faults are overprinted by a set of lower‐grade, variably oriented chlorite‐ and epidote‐coated faults (Set III) constraining WNW–ESE shortening. A lamprophyric dyke oriented compatibly with this stress field intruded at c. 435 Ma (Silurian), indicating that Set III formed at the onset of the Scandian Baltica–Laurentia collision. The preservation of Caledonian brittle structures indicates that the Rolvsnes granodiorite occupied a high tectonic level throughout the Caledonian orogeny.     

Spatial variations in chemical compositions along Langtang–Narayani river system in central Nepal

Surface water samples were collected from Langtang Lirung glacier outlet point to the Narayani river system in central Nepal in order to investigate the role of elevation in the variation of chemistry along the drainage networks. The chemistry of Langtang–Narayani river system was dominated by sulfide oxidation coupled with carbonate dissolution and weathering of silicate minerals. Calcium and magnesium concentrations were relatively higher than other cations and the sum of both species strongly correlated with alkalinity, supporting the dissolution of carbonate and dolomite as the dominant source for these ions. Aluminosilicate minerals primarily as albite and anorthite appeared as dominant silicate minerals within the drainage basin. Bisiallitization was the dominant type of weathering within the entire drainage system. Hydrogen ion concentration was lower in the low elevation sites than in high elevation sites reflecting the more consumption of carbon dioxide in the low elevation sites due to enhanced chemical weathering rates. Furthermore, major solutes like sum of base cations, silicon as well as alkalinity increased in concentration in the lower elevation sites. All regulating factors appeared to be directly related to elevation and hence elevation appeared to be the prime factor for the variation in chemical species along the Langtang–Narayani river system. Toshiyuki Masuzawa: deceased.     

Palynological characteristics of Kazakhstan Province of Carboniferous Euramerican Region

The paper presents results of palynological studies of the coal-bearing Carboniferous in Central Ka-zakhstan.A quantitative dynamics of major genera of miospores reflects an overall succession of the Carbonifer-ous miospore assemblages.Viséan Serpukhovian assemblages are dominanted by Lophozonotriletes fuscatus,Ac-anthozonotriletes cristifer,A.inclusus,Punctatisporites glabratus,Crassispora ignorata,C.scrupea,Cyclobacu-lisporites trichacanthus,Lycospora breviapiculata,L.echinata and Vallatisporites decrescens.Rare Punctatis-porites bertschoguriensis,Stenozonotriletes marginellus,Vallatisporites appilicatus,Verrucosisporites scrobiculatus,Reticulatisporites cancellatus,Granulatisporites arcuatus are recorded,which are not found in assemblages from overlying sediments.Bashkirian--Moscovian assemblages are dominated by Lycospora subtriquetra,L.pusilla,Vallatisporites ciliaris,Apiculatisporis mollis,Cyclogranisporites testiculatus,Calamospora microrugosa,Schopfi-pollenites principalis,Psilohymena psiloptera,Florinites grandis and Granulatisporites piroformis miospores.Cris-tatisporites latispinus,Angulisporites didymus,Thymospora obscura and Torispora securis are also present,as well as single Apiculatisporis curvispinus,Raistrickia inordinata,Marsupipollenites retroflexus,Entylissa caperata,Potonieisporites grandis and Cordaitina sp.A similar succession of miospore assemblages is recorded in the Carboniferous deposits of Europe and North America,which allows carrying out correlations,despite an endemic characters of many species of Paleozoic miospores from the coal-bearing Carboniferous of the Kazakh-stan Province in the Euramerian Paleofloristic Region.     

Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars

Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013 ) and a ¹⁴⁷Sm‐¹⁴³Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016 ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ¹⁷O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014 ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015 ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015 ).