Tectonic Speleogenesis of Devils Hole, Nevada, and Implications for Hydrogeology and the Development of Long, Continuous Paleoenvironmental Records

Devils Hole, in southern Nevada, is a surface collapse into a deep, planar, steeply dipping fault-controlled fissure in Cambrian limestone and dolostone. The collapse intersects the water table about 15 m below land surface and the fissure extends at least 130 m deeper. Below water, most of the fissure is lined with a >30-cm-thick layer of dense maxillary calcite that precipitated continuously from groundwater for >500,000 yr. The thick mammillary calcite coat implies a long history of calcite-supersaturated groundwaters, which, combined with the absence of dissolutional morphologies, suggests that Devils Hole was not formed by karst processes. Devils Hole is located in a region of active extension; its tectonic origin is shown by evidence of spreading of its planar opening along a fault and by the orientation of its opening and others nearby, perpendicular to the northwest-southeast minimum principal stress direction of the region. Most Quaternary tectonic activity in the area, including seismicity and Quaternary faults and fractures, occurs on or parallel to northeast-striking structures. The hydrogeologic implications of this primarily structural origin are that fracture networks and caves opened by extensional tectonism can act as groundwater flowpaths functionally similar to those developed by karst processes and that, during active extension, transmissivity can be maintained despite infilling by mineral precipitation. Such extensional environments can provide conditions favorable for accumulation of deposits preserving long, continuous paleoenvironmental records. The precipitates in Devils Hole store chronologies of flow system water-level fluctuations, hydrochemistry, a half-million-yr proxy paleoclimate record, evidence of Devils Hole's tectonic origin, and probably atmospheric circulation.     

The source of Central American lavas: inferences from geochemical inverse modeling

The rate earth element chemistry of a large suite of samples from the Central American volcanic front has been determined to understand their petrogenesis. Different segments of the volcanic front are compared on the basis of their calculated source rare earth patterns as deduced from inverse modeling. The results yield a range in the extent of light rare earth enrichment of the source, as well as in source mineralogy. Moderateto-strong LREE enrichment and high modal garnet contents are observed for the sources of both Guatemala and central Costa Rica, whereas slight LREE depletion and little or no garnet occurs in the Nicaraguan source region. Although distinct source regions beneath each Central American segment are permitted by the modeling, it is more likely that the mantle wedge is broadly homogeneous but locally heterogeneous. Small volume, relatively enriched, garnet-bearing veins surrounded by a matrix of isotopically depleted mantle periodotite could exist throughout the mantle wedge. Apparently distinct sources occur due to the variation in partial melting beneath the different segments, controlled by the amount of subduction-generated flux per unit wedge volume, which in turn is a function of the dip of the subducted lithosphere.     

Tidal triggering of seismic and volcanic phenomena during the 1879–1880 eruption of Islas Quemadas volcano in El Salvador, Central America

The Islas Quemadas dome in Lake Ilopango, El Salvador, was extruded during December 1879 to March 1880. The eruption took place in six distinct periods, five of which began near fortnightly minima in the amplitude of the semidiurnal solid earth tide. An earthquake swarm preceded dome extrusion. Felt earthquakes were carefully recorded during a two-day period preceding a destructive main shock that occurred on December 27, 1879. During this foreshock sequence the periods of highest earthquake activity occurred at times of maxima and minima in semidiurnal solid-earth tide. The four largest earthquakes, including the main shock, occurred at maxima in the semidiurnal tide.     

Subduction-related Late Permian shoshonites of the Sydney Basin, Australia

Summary The Late Permian shoshonitic province of the southern Sydney Basin consists of lavas and intrusions confined to a 140 km long coastal belt, but geophysical and sedimentological data indicate that the province once had a length of at least 340 km. Both petrographic and geochemical data indicate shoshonitic affinities for these Late Permian rocks and, although the compositional range is from 48.4 to 60.6% SiO₂, all units except an andesite lava have <55% SiO₂. Subduction-related attributes of the province include enrichment in Al₂O₃ and LILE (Rb, K and LREE), depletion in HFSE (Nb, Ta, Zr, Hf and Ti), low Nb/U, and Sr/Nd of 30–35. Initial (at 250 Ma)⁸⁷Sr/⁸⁶Sr ranges from 0.70294 to 0.70440 whereas _Nd values range from +5.11 to +2.14 and plot almost exclusively in the mantle array. Low MgO and mg-numbers (maximum 7.37% and 64.5, respectively) demonstrate that none of the shoshonites represent primary magmas in equilibrium with mantle peridotite. Isotopic data and elemental contents are not supportive of a model in which crustal contamination is the dominant process in magma petrogenesis, but do not exclude contamination with Sr-rich material having a low⁸⁷Sr/⁸⁶Sr and high _Nd values.A temporal correlation between igneous rocks from the southern Sydney Basin, Dampier Ridge, New England and Tasmania indicates a widespread magmatic event which was, at least in part, shoshonitic in character and which developed in response to subduction along the east coast of Gondwanaland. The geochemical similarity between the Dampier Ridge and Sydney Basin samples does not support a model for spatial compositional variation across a subduction-related magmatic belt but, coupled with the variations in isotopic ratios, implies heterogeneity in the source.

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Spät-permische Schoschonite des Sidney Beckens in Australien

Zusammenfassung Die spät-permische Schoschonit Provinz des südlichen Sidney Beckens besteht aus Laven und Intrusionen, die in einem 140 km langen Gürtel längs der Küste auftreten. Geophysikalische und sedimentologische Daten weisen darauf hin, daß diese Provinz ursprünglich eine Länge von mindestens 340 km hatte. Sowohl petrographische wie geochemische Daten weisen auf schoschonitische Affinität dieser spät-permischen Gesteine hin. Obwohl die SiO₂ Gehalte von 48.4 bis 60.6% schwanken, haben alle Einheiten mit Ausnahme einer andesitischen Lava < 55%. Auf Subduktion weisen u.a. Anreicherungen an Al₂O₃ und LILE (Rb, K and LREE), die Verarmung an HFSE (Nb, Ta, Zr, Hf and Ti) niedrige Nb/U und Sr/Nd von ungefähr 30–35 hin. Die⁸⁷Sr/⁸⁶Sr Initiale, berechnet für 250 Ma, liegen zwischen 0.70294 und 0.70440 und die _Nd-Werte zwischen +5,11 und +2,14 und fallen fast ausschließlich in den Mantel-Bereich. Niedrige MgO und mg-Zahlen (Maximal 7,37% bzw. 64,5) zeigen, daß die Schoschonite nicht primäre Magmen, die mit Mantelperidotiten im Gleichgewicht sind, darstellen. Isotopendaten und Elementgehalte weisen nicht auf ein Modell hin, bei dem Krusten-Kontamination bei der Magmen-Genese eine wichtige Rolle spielte, aber eine Nichtkontamination mit Material, das niedrige⁸⁷Sr/⁸⁶Sr Werte und hohe _Nd Werte hat, ist nicht auszuschließen. Eine zeitliche Beziehung zwischen den magmatischen Gesteinen des südlichen Sydney-Beckens, dem Dampier Ridge, Neuengland und Tasmanien weist auf ein weit verbreitetetes magmatisches Ereignis hin, das zumindest teilweise schoschonitischen Charakter hatte und das auf Subduktion längs der Ostküste von Gondwanaland zurückführbar ist. Die geochemische Ähnlichkeit von Proben des Dampier Ridge und des Sydney Beckens weist nicht auf räumliche Variationen in der Zusammensetzung über einen subduktionsbezogenen magmatischen Gürtel hin, sondern läßt eher, zusammen mit den Variationen der Isotopendaten, heterogene Quellen für diese Gesteine vermuten.

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With 10 Figures     

Book Review Introduction to Geostatistics: Applications in Hydrogeology By Peter K. Kitanidis,Cambridge University Press, 1997, 249 p + xx, $74.95 (U.S.), hardback,ISBN 0-521-58312-8; $29.95 (U.S.), paperback,ISBN 0-521-58747-6

Mathematical Geosciences -     

Large pseudomorphous trapezohedra of analcime and pumpellyite after leucite,Aghda area,Central Iran

Summary Spectacular large (3 cm) euhedral trapezohedra composed of single crystals of analcime or aggregates of pumpellyite occur in three stratigraphically adjacent phonolitic lavas from Aghda. The trapezohedra contain concentrically arranged inclusions of plagioclase, pyroxene, titanomagnetite and apatite which are fresh in the analcime but extensively altered in the pumpellyite. The analcime crystals are remarkably homogeneous in composition and are interpreted as having formed by ion-exchange pseudomorphous replacement of primary leucite. In the lower phonolite, however, the analcime became unstable and was replaced by pumpellyite which has a wide compositional range, reflecting variable input of Fe, Mg, Ca and Al from the precursor analcime and alteration of inclusions. The occurrence of analcime and pumpellyite at Aghda is indicative of zeolite facies conditions with low f_{CO 2}and H₂O activity and probably high P_fluid.

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Große pseudomorphe Trapezoeder von Analcim im Pumpellyit nach Leuzit, Aghda-Gebiet Zentraliran

Zusammenfassung Ungewöhnlich große idiomorphe Trapezoeder, die aus einzelnen Kristallen von Analcim oder aus Aggregaten von Pumpellyit bestehen, kommen in drei stratigraphisch benachbarten phonolitischen Lagen von Aghda vor. Die Trapezoeder enthalten konzentrisch verteilte Einschlüsse von Plagioklas, Pyroxen, Titanomagnetit und Apatit, die im Analcim frisch, im Pumpellyit jedoch intensiv umgewandelt sind. Die Analcim-Kristalle sind von bemerkenswerter Homogenität in ihrer Zusammensetzung und werden auf pseudomorphe Verdrängung von primärem Leucit durch Ionenaustausch zurück-geführt. Im unteren Phonolit ist der Analcim jedoch instabil geworden, und wurde durch Pumpellyit ersetzt, der ein weites Spektrum von Zusammensetzungen erkennen läßt und auf variierende Zufuhr von Fe, Mg, Ca, and Al vom Vorläuferanalcim und auf Umwandlung von Einschlüssen hinweist. Das Vorkommen von Analcim und Pumpellyit bei Aghda ist ein Hinweis auf Zeolithfazies-Bedingungen mit niedrigen f_{CO 2} und H₂O und hohem P_fluid.

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With 5 Figures     

Structural,stratigraphic, and petrologic aspects of the Arenal-Chato volcanic system,Costa Rica: Evolution of a young stratovolcanic complex

Geologic mapping on a scale of 1:10000 and detailed stratigraphic studies of lava flows and tephra deposits of the Arenal-Chato volcanic system reveal a complex and cyclic volcanic history. This cyclicity provides insight into the evolution of magma batches during the growth of the andesitic volcanic system. The Arenal and Chato volcanoes have a central zone comprised of a lava armor and a distal zone comprised of a tephra apron. During Arenal's last two eruptive periods major craters formed near intersections of regional fractures at the lava armortephra apron transition. We suggest that such intersections are potential sites for future major explosions. The earliest rocks, i.e., the Chato lava flows, range in composition from basaltic andesite to andesite. These rocks, except for the andesitic domes of Chatito and La Espina, appear to have evolved from a common parental magma. The last active period of Chato volcano occurred 3550 B. P. The earliest known activity of Arenal volcano is 2900 B. P. Arenal lava flows have 54–56 wt% SiO₂ and may be subdivided into a high-alumina group (HAG, Al₂O₃ = 20 wt%) and a low-alumina group (LAG, Al₂O₃ = 19 wt%). Compared to the HAG, the LAG also has smaller amounts of incompatible elements and higher amounts of FeO and MgO. Arenal tephra deposits were emplaced by Plinian-Sub-Plinian explosions occurring at 300±150-yr intervals. These deposits are compositionally zoned and alternate between dacite and basalt. The stratigraphy reveals an apparent magmatic cycle consisting of (a) dacitic-andesitic tephra, (b) HAG lava flows, (c) LAG lava flows, and (d) andesitic-basaltic tephra. This magmatic cycle is repeated four times during Arenal's history and is interpreted to have developed by the crystal fractionation and crystal redistribution of a single magma batch. The period of this cycle, and consequently the life of a magma batch, is about 800 years. If the cyclic pattern continues, a basaltic explosive phase may occur in the next 250 years.