The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida,Kenya Rift Valley     

Robin W. Renaut  R. Bernhart Owen  Tim K. Lowenstein  Gijs De Cort  Emma Mcnulty  Jennifer J. Scott  Anthony Mbuthia 《Sedimentology》2021,68(1):108-134

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO₃) forms bladed crystals that nucleate on the lake floor. Trona (Na₂CO₃·NaHCO₃·2H₂O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO₂ is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.  相似文献   

Community-based responses to climate hazards: typology and global analysis     

Paterson  Barbara  Charles  Anthony 《Climatic change》2019,152(3-4):327-343

Climatic Change - The severity and frequency of climate change hazards are increasing around the world. Because the impacts are most acutely felt in local communities, it is critical to improve...  相似文献   

Kick‐and‐cook experiments on peridotite: simulating coseismic deformation and post‐seismic creep     

Anthony Druiventak  Agnes Matysiak  Jörg Renner  Claudia A. Trepmann 《地学学报》2012,24(1):62-69

  相似文献   

Anatomy of a continental subduction zone: The main mantle thrust in Northern Pakistan     

Robert W. H. Butler  David J. Prior 《International Journal of Earth Sciences》1988,77(1):239-255

The Himalayas form an ideal natural laboratory to study the deformation processes of continental crust during collision orogeny. New information is presented concerning the structural evolution of the Main Mantle thrust zone in the Himalayas of N Pakistan, in the region around Nanga Parbat. The hanging-wall lies at relatively high levels within the Kohistan arc terrane which has been emplaced onto Indian continental rocks. This thrust probably originated as a breakback structure in the hanging-wall to the pre-collisional (oceanic) subduction zone. The present hanging-wall contains a shear zone of about 1 km width developed under amphibolite facies conditions. Simple shear dominant strains have developed new fabrics parallel to the main thrust zone. However, these structures are redeformed by discrete extensional and compressional shears within about 100 m of the thrust contact, again developed under amphibolite facies conditions. The footwall consists of an old basement complex (the Nanga Parbat gneisses) overlain by a distinct suite of metasediments now at amphibolite facies. This cover assemblage of psammites, pelites and marbles with local metabasites consistently lies directly against rocks derived from the Kohistan arc along the MMT. The absence of structures suggestive of consistent rheological contrasts within the cover assemblages suggests that the vast majority of the deformation features they contain were developed only once they experienced substantial tectonic overburdens. Prior to this the Indian cover rocks will have been »passively« subducted beneath the Kohistan arc until into amphibolite facies. We discuss these inferences in terms of evolving shear zone width with time and comment on the implications for predicting the character of mid-deep crustal shear zones, particularly from seismic reflection profiles.

---

Zusammenfassung Der Himalaya bildet ein ideales, natrliches Laboratorium für Untersuchungen von Deformationsprozessen in kontinentalen Krustengesteinen während der Kollision bzw. Orogenese. Hier werden neue Daten vorgelegt, die sich mit der strukturellen Entwicklung der Hauptmantelüberschiebung im Himalaya von Nordpakistan im Gebiet um den Nanga Parbat befassen. Die Hangendeinheiten oberhalb der Störung liegen in einem relativ hohen Niveau innerhalb des »Kohistan arc terrane«, das auf die indischen Kontinentalgesteine überschoben wurde. Diese Überschiebung entstand wahrscheinlich als Rücküberschiebungsstruktur im Hangenden der Subduktionszone vor der Kollision. Im Hangenden befindet sich eine ca. 1 km breite Scherzone, die sich unter amphibolitfaziellen Bedingungen gebildet hat. Die durch »simple shear« erzeugten Deformationen sind mit ihren neuen Gefügen parallel zur Hauptüberschiebungszone ausgerichtet. Die Gefüge wurden nachfolgend von Extensionsund Kompressionsbewegungen im Bereich von ca. 100 m um den Überschiebungskontakt erneut unter amphibolitfaziellen Bedingungen erfaßt und deformiert. Das Liegende der Hauptüberschiebung besteht aus einem alten Basementkomplex (den Nanga Parbat Gneisen), die von deutlich abgesetzten, amphibolitfaziellen Metasedimenten überlagert werden. Diese Sedimenthülle bestehend aus Psammiten, Peliten und Marmoren mit lokalen Metabasiten stößt entlang der MMT direkt gegen die Gesteine des Kohistanbogens. Das Fehlen von Strukturen, die auf gleichbleibende rheologische Unterschiede hinweisen würde, läßt vermuten, daß der Großteil der in ihnen enthaltenen Deformationsgefüge auf einmal während beträchtlicher tektonischer Auflast entstanden ist. Vorher wurden die indischen Hüllgesteine »passiv« unter den Kohistanbogen bis in den Bereich der Amphibolitfazies subduziert. Die Folgerungen aus der sich über die Zeit entwickelnden Breite dieser Scherzone werden diskutiert und die Bedeutung für die Vorhersage der Charakteristik von mitteltiefen krustalen Scherzonen, insbesondere in Verbindung mit seismischen Reflektionsprofilen betont.

---

Résumé L'Himalaya constitue un laboratoire naturel idéal pour l'étude des processus de déformation de la croûte continentale au cours d'une orogenèse de collision. Les auteurs présentent des informations nouvelles relatives à l'évolution structurale de la zone du Main Mantle Thrust dans la région du Nanga Parbat au nord du Pakistan. Le toit de cet accident occupe un niveau assez élevé dans le «Kohistan arc Terrane» qui a été charrié sur les roches du continent indien. Le charriage doit probablement son origine à une structure en retour apparue au-dessus de la zone de subduction pré-collisionnelle (océanique). Le toit actuel de l'accident contient une zone de glissement (shear zone) épaisse d'environ 1 km et formée dans les conditions du facies des amphibolites. Les déformations engendrées par glissement simple (simple shear) ont développé de nouvelles fabriques parallèles à la surface de charriage. Cependant, dans une tranche d'une centaine de mètres à partir du contact du charriage, ces structures ont été reprises par des cisaillements extensionnels ou compressionnels, toujours dans les conditions du facies des amphibolites.Le mur de l'accident est formé d'un complexe ancien (le gneiss du Nanga Parbat) surmonté d'une série de métasédiments distincts qui présentent aujourd'hui le facies des amphibolites. Cette couverture de psammites, de pélites, de marbres et de métabasites locales est directement en contact le long du MMT avec l'arc du Kohistan. L'absence, dans cette couverture, de structures témoignant de contrastes rhéologiques marqués, suggère que la grande majorité des structures déformatives n'y ont été développées qu'après un enfouissement tectonique important. Avant cela, les roches de la couverture indienne ont dû être subductées passivement sous l'arc du Kohistan, jusqu'au facies des amphibolites. Les auteurs discutent ces conclusions en termes d'évolution temporelle d'une shear zone et en commentant les implications dans le domaine de la prédiction du caractère des shear zones de profondeur crustale moyenne, en particulier à partir des profils de sismique réflexion.

---

, , . -. Kohistan arc terrane —, . , , . 1 , . , , . 100 . — - —, . , , , . , , , , , . . .

  相似文献   

Effect of chemical affinity on feldspar hydrolysis rates in two natural weathering systems     

Michael Anthony Velbel 《Chemical Geology》1989,78(3-4):245-253

Recent studies of alkali-feldspar hydrolysis kinetics have shown that, in the near-neutral pH range, weathering rates in natural systems are up to three orders of magnitude slower than laboratory rates. It has been hypothesized that decelerated rates may result from lower thermodynamic affinities for the hydrolysis reaction in natural systems than in laboratory systems. However, the chemical affinities for the feldspar hydrolysis reaction in two well-constrained natural systems are significantly higher than the threshold value at which affinity would exert detectable influences on the reaction rates. Thus, the hypothesis is rejected, and closer proximity to thermodynamic equilibrium in natural weathering systems does not account for the observed discrepancy between natural and laboratory rates of feldspar hydrolysis. Differences in feldspar weathering rates between natural and laboratory systems are most likely due to a combination of experimental preparation artifacts, loss of reactive surface to the formation of etch pits in naturally weathered feldspars, and inhomogeneous access of reactive fluids to those surfaces.  相似文献   

Two-dimensional kinematic and rheological modeling of the 1912 pyroclastic flow,Katmai, Alaska     

James E. Beget  Anthony J. Limke 《Bulletin of Volcanology》1988,50(3):148-160

Pyroclastic flow emplacement is strongly influenced by eruption column height. A surface along which kinetic energy is zero theoretically connects the loci of eruption column collapse with all coeval ignimbrite termini. This surface is reconstructed as a two-dimensional energy line for the 1912 Katmai pyroclastic flow in the Valley of Ten Thousand Smokes from mapped flow termini and the runup of the ignimbrite onto obstructions and through passes. Extrapolation of the energy line to the vicinity of the source vent at Novarupta suggests the eruption column which generated the ignimbrite eruption was approximately 425 m high. The 1912 pyroclastic flow travelled about 25 km downvalley. Empirical velocity data calculated from runup elevations and surveyed centrifugal superelevations indicate initial velocities near Novarupta were greater than 79–88 m s^–1. The flow progressively decelerated and was travelling only 2–8 m s^–1 when it crossed a moraine 16 km downvalley. The constant slope of the energy line away from Novarupta suggests the flow was systematically slowed by internal and basal friction. Using a simple physical model to calculate flow velocities and a constant kinetic friction coefficient (Heim coefficient) of 0.04 derived from the reconstructed energy line, the flow is estimated to have decelerated at an average rate of –0.16 m s^–2 and to have taken approximately 9.5 minutes to travel 25 km down the Valley of Ten Thousand Smokes. The shear strength of the flowing ignimbrite at the moraine was approximately 0.5 kPa, and its Bingham viscosity when it crossed the moraine was 3.5 × 10³ P. If the flow was Newtonian, its viscosity was 4.2 × 10³ P. Reynolds and Froude numbers at the moraine were only 41–62 and 0.84–1.04, respectively, indicating laminar, subcritical flow.  相似文献   

Book reviews     

F. Alejandro Nava  Manfred Koch  Andrzej Hanyga  Jacek Leliwa-Kopystyński  Frazer Tabeart  Sueo Veno  Andrzej W. Wernik  Yu. N. Korenkov  Kenji Satake  Carlos Lozano  Aysel Karafistan  Ian N. James  Anthony Dore  Jacek Wałczewski  Krzysztof Haman  Brian F. Farrell 《Pure and Applied Geophysics》1994,143(4):753-780

  相似文献   

The Wavelength-Smoothing Method For Approximating Broad-Band Wave Propagation Through Complicated Velocity Structures     

Anthony Lomax 《Geophysical Journal International》1994,117(2):313-334

  相似文献   

Chemistry of alkali extraction of brown coals—I. Kinetics, characterisation and implications to coalification     

Louis S.K. Pang  Anthony M. Vassallo  Michael A. Wilson   《Organic Geochemistry》1990,16(4-6)

Three Australian brown coals have been separated into humin and humic acid fractions and studied by high resolution solid state ¹³C NMR spectroscopy and Fourier transform IR spectroscopy. The aromatic rings of the humic acids are highly substituted showing that extensive cross linking must have occurred during formation from wood lignin and tannin. However, the humins contain more aliphatic carbon and hydrogen than the corresponding humic acids. This shows that little cross linking has occurred with other components of the brown coal such as resins, waxes cutin and algal detritus, and cross linking has not rendered the aromatics alkali insoluble. The kinetics of extraction are complex and not simple first order. This is reflected in the chemical composition of the humic acid which is extraction temperature dependent. We also observed that there is a conversion of aromatic carbon to aliphatic carbon and gas during extraction, probably by alkaline oxidation, resulting in ring opening. A range of suitable model compounds have been studied to confirm this finding. Such a mechanism may account for the modification of lignin in oxidising environments such as those occurring in the initial stages of coalification (lignite or brown coal formation) and in soils.  相似文献   

Fouling of floating plastic debris under Biscayne Bay exposure conditions     

Song Ye  Anthony L Andrady 《Marine pollution bulletin》1991,22(12):608-613

Six plastic/rubber materials commonly encountered in marine debris and beach litter were studied under Biscayne Bay (Florida, USA) exposure conditions to determine the effect of fouling on buoyancy. Studies under restricted floating, and restricted submerged exposure conditions suggest that most plastic samples undergo fouling to an extent to cause the sample to be negatively buoyant in sea water. Rapid defouling of the submerged fouled samples was observed. The findings suggest that free-floating plastics at sea may, under certain conditions, undergo fouling-induced sinking followed by resurfacing as floating debris.  相似文献