Long-Term Evolution of Fluid-Rock Interactions in Magmatic Arcs: Evidence from the Ritter Range Pendant, Sierra Nevada, California, and Numerical Modeling

A record of > 100 million years of fluid flow, alteration,and metamorphism in the evolving Sierra Nevada magmatic areis preserved in metavolcanic rocks of the Ritter Range pendantand surrounding granitoids. The metavolcanic rocks consist of:(1) a lower section of mostly marine volcaniclastic rocks, lavas,and intercalated carbonate rocks that is Triassic to Jurassicin age, and (2) an upper section comprising a subaerial caldera-fillcomplex of mid-Cretaceous age. Late Cretaceous high-temperaturecontact metamorphism (2 kbar, >450–500C) occurredafter renewed normal faulting along the caldera-bounding faultsystem juxtaposed the two sections. The style and degree of alteration and ¹⁸O values differ amongthe rocks of the upper and lower sections and the granitoids.Rocks of the lower section show pervasive lithologically controlledalkali alteration, local Mn and Mg enrichment, and oxidation.Some ash flow tuffs now contain up to 10% K²O by weight. Therocks of the upper section show lesser extents of alkali alteration.Granitoids that cut both sections are generally unaltered. Mostmetavolcanic rocks of the lower section have high ¹⁸O values(+ 11 to + 16%; whole rock and quartz phenocrysts); however,lower-section rocks within the caldera-bounding fault systemhave low ¹⁸O values of + 4 to +7. The metavolcanic rocks ofthe upper section also have low ¹⁸O values of + 2 to + 7. Granitoidshave ¹⁸O values of + 7 to + 10, typical of unaltered Sierrangranitoids. The lower section contains discontinuous veins ofhigh-temperature (450–500C) calc-silicate minerals. Theseveins are typically <5 m long, do not cross intrusive contacts,and postdate the pervasive alkali alteration. Late veins aretypically > 10 m long, formed at temperatures of less than450–500C, and cross intrusive contacts. Veins have similar¹⁸O values to those of the local host rocks. The nature of the alteration and the high oxygen isotopic valuesof the rocks of the lower section indicate that these rocksinteracted extensively with seawater at temperatures <300C,probably in superposed marine hydrothermal systems associatedwith coeval volcanic centers. Metavolcanic rocks of the uppersection evidently interacted with meteorie waters, probablyin a hydrothermal system associated with the Cretaceous caldera;rocks of the lower section that were adjacent to the calderawere also affected by this alteration. The preservation of thesignatures of these earlier events, the nature of the earlyveins, and results from numerical models of hydrothermal flowthat include fluid production indicate that during progradecontact metamorphism, the rocks of the pendant primarily interactedwith locally derived fluids. Fluid flow was predominantly upwardand away from intrusive contacts and down-temperature. Permeabilitiesare estimated to have been between 0•1 and 1µD, whichis that necessary for maintenance of lithostatic fluid pressures.In hydrothermal models with such permeabilities, large-scalecirculation of meteoric fluids develops after prograde metamorphismceases. The nature of the late veins in the Ritter Range pendantsuggests that such a flow pattern evolved only after the pendantand granitoids had cooled below 450–500C. The long-termhistory of alteration documented in the Ritter Range pendantis probably typical of wall rocks in most batholiths *Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721     

Re‐evaluation of the Last Glacial Maximum typesite at Dimlington,UK

Bateman, M. D., Buckland, P. C., Whyte, M. A., Ashurst, R. A., Boulter, C. & Panagiotakopulu, E. 2011: Re‐evaluation of the Last Glacial Maximum typesite at Dimlington, UK. Boreas, 10.1111/j.1502‐3885.2011.00204.x. ISSN 0300‐9483. Recent erosion has allowed re‐examination of the stratigraphy and sampling for both optically stimulated luminescence dating and palaeoecological analysis of the key sections in the Last Glacial Maximum deposits at Dimlington in East Yorkshire, England. Both stratigraphy and fossil insect evidence support a subaerial origin for laminated and cross‐bedded sediments between two diamictons previously interpreted as synchronous. The fossil biota indicates conditions similar to those of a pond on sandur in the high Arctic, with little or no vegetation cover. The existence of distinct oscillations of the ice front is indicated. The first, within the period 21.7–16.2 ka, appears coincident with climate warming, as deduced from Greenland ice‐core evidence, and is interpreted as an ice stream associated with changing flow patterns within the British–Irish Ice Sheet (BIIS). The second, dating between 16.2 and 15.5 ka, appears to coincide with a climatic cooling, although current models show that the BIIS had by this period already retreated back to ice centres. This new evidence supports the view that the eastern sector of the BIIS did not reach its maximal extent synchronously with other parts of the BIIS.     

Rheological Properties of Partially Molten Lherzolite

Lherzolite samples synthesized from fine-grained powders preparedfrom a natural xenolith were deformed at P = 300 MPa and 1373     

Four barriers to the development of effective exploratory visualisation tools for the geosciences

This paper outlines four specific problems that appear to represent considerable obstacles to the development of visualisation strategies for use within the domain of geography and the Earth sciences. These are: (1) the speed of graphical rendering, (2) the management of perceptual anomalies due to visual combination effects, (3) the vast range of potential approaches and mappings (the complexity of the visual assignment process), and (4) the orientation of the user into an artificial or virtual reality. Each problem is discussed in terms of the visualisation of geographical data for the purpose of exploratory visual analysis. The specific underlying research issues and questions are described, with particular emphasis to how these relate to the geographical domain. Where possible, some potential solutions are suggested. Specific examples of geographical data visualisation are given to substantiate the arguments presented. The discussion highlights the need for further research in a number of key areas, and stresses the weaknesses of current visualisation theory and technology when applied to non-trivial geographical datasets.     

The type section of the Vikinghôgda Formation: a new Lower Triassic unit in central and eastern Svalbard

The Vikinghøgda Formation (250 m) is defined with a stratotype in Deltadalen-Vikinghøgda in central Spitsbergen. The Vikinghøgda Formation replaces the Vardebukta and Sticky Keep Formations of Buchan et al. (1965) and the lower part of the Barentsøya Formation of Lock et al. (1978) as extended geographically by Mørk, Knarud et al. (1982) in central Spitsbergen, Barentsøya and Edgeøya. The formation consists of three member: the Deltadalen Member (composed of mudstones with sandstones and siltstones), the Lusitaniadalen Member (dominated by mudstones with thin siltstone beds and some limestone concretions) and the Vendomdalen Member (composed of dark shales with dolomite interbeds and nodules). The Lusitaniadalen and Vendomdalen members replace the former Sticky Keep Formation/ Member in the siirne areu. The Vikinghøda Formation can be followed through central and eastern Spitsbergen to Barentøya and Edgeøya and includes all sediments between the chert-rich Kapp Starostin Formation (Permian) and the organic-rich shales of the Botneheia Formation (Middle Triassic). The subdivision into three members is also reflected in the organic carbon content and palynofacies. Upwards. each succeeding member becomes more distal, organic-rich and oil-prone than the one below.
The Vikinghøda Formation is well-dated by six ammonoid zones. although the transitional beds between the Deltadalen and Lusitaniadalen members lack age diagnostic macrofossils. Corresponding palynozonation and magnetustratigraphy have also been determined. The overall stratigraphical development correlates well with other key Triassic areas in the Arctic, although intervals in the late Dienerian and early Smithian may be condensed or missing.     

Map and GIS database of glacial landforms and features related to the last British Ice Sheet

A review of the academic literature and British Geological Survey mapping is employed to produce a 'Glacial Map', and accompanying geographic information system (GIS) database, of features related to the last (Devensian) British Ice Sheet. The map (1:625 000) is included in a folder and GIS data are freely available by web download (http://www.shef.ac.uk/geography/staff/clark_chris/britice.html). Emphasis is on information that constrains the last ice sheet. The following are included: moraines, eskers, drumlins, meltwater channels, tunnel valleys, trimlines, limit of key glacigenic deposits, glaciolacustrine deposits, ice-dammed lakes, erratic dispersal patterns, shelf-edge fans and the Loch Lomond Readvance limit of the main ice cap. The GIS contains over 20 000 features split into thematic layers (as above). Individual features are attributed such that they can be traced back to their published sources. Given that the published sources of information that underpin this work were derived by piecemeal effort over 150 years, then our main caveat is of data consistency and reliability. It is hoped that this compilation will stimulate greater scrutiny of published data, assist in palaeoglaciological reconstructions and facilitate use of field evidence in numerical ice-sheet modelling. It may also help direct field workers in their future investigations.     

A Crustal Progenitor for the Intrusive Anorthosite--Charnockite Kindred of the Cupriferous Koperberg Suite, O'okiep District, Namaqualand, South Africa; New Isotope Data for the Country Rocks and the Intrusives

The Koperberg Suite comprises some 1700 small bodies of intrusiverocks largely composed of andesine anorthosite, biotite diorite,and leuconorite, norite and melanorite-hypersthenite; 30 mineshave been established in the O'okiep District in the cupriferousrocks of this anorthosite-charnockite kindred. The suite isintrusive into a sequence of granite gneiss and metavolcanicand metasedimentary rocks, and intrusive granite, that wereelevated to the granulite fades of regional metamorphism.TheSm-Nd model ages for the country rocks and the Koperberg Suiteare all 1700 Ma (T_CHUR) and 2000 Ma (T_DM) supporting a majorcrustforming event in this portion of Namaqualand at the endof Lower Proterozoic times. The granulite fades metamorphismin the O'okiep District is recorded by a Rb-Sr isochron ageof 1223 48 Ma on the Nababeep Granite Gneiss, and by (1197 15)-Ma-old inherited cores of zircons in the Koperberg Suite.The time of intrusion of the Concordia and Rietberg Granitesis believed to be reflected by their Rb-Sr whole-rock age of1105 24 Ma. The mean U-Pb age of 1029 10 Ma on individualzircon grains and zircon rims from the Koperberg Suite recordsthe time of its intrusion, and this is supported by the Sm-Ndwhole-rock age of 1022 42 Ma for the suite. Subsequent coolingand reheating events are recorded by the Ar-Ar ages of 800–850Ma for the Koperberg Suite, and of 500–550 Ma for thesuite and certain country rocks, respectively.An Nd value of-7,and its volume and composition, suggest a crustal-melt sourcefor the intrusive Concordia Granite. Moreover, the age-correctedhigh l_Sr (07061-07272) and low Nd (-9), and the high µ₂(101), that characterize the Koperberg Suite also imply a crustalsource, and a model is presented for the generation of the majorpart of the suite by partial melting of granulites of overallintermediate (diorite) composition in the lower crust. Corresponding author     

Integrating outcrop data and forward computer modelling to unravel the development of a Messinian carbonate platform in SE Spain (Sorbas Basin)

Geological mapping, definition of facies distributions and reconstruction of platform‐interior growth geometries of the Messinian Cariatiz carbonate platform (Sorbas basin, South Spain), were performed to evaluate the controlling factors in platform growth and to test a 3‐D computer simulation program. For the simulation with the program REPRO, five platform‐related facies were modelled: (1) the reef crest facies by the numerical solution of a Fisher equation; (2) the lagoonal facies by a function of water depth‐dependent carbonate production; (3) the proximal and middle slope facies (breccia and block facies, calcarenite facies) by a subroutine simulating gravity‐driven particle export from the reef crest; (4) a distal slope; and (5) a basinal facies by a pelagic rain function. Development of a fan delta conglomeratic system is simulated by using a siliciclastic point source and gravity‐driven particle redistribution. A best fit between the observed platform growth geometries and modelling results is achieved by assuming that high‐frequency sea‐level changes superimposed onto a longer term sea‐level fall controlled platform growth. For the modelling, a relative sea‐level curve was reconstructed, which is based on a deep‐sea benthic foraminiferal stable oxygen isotope record at ODP Site 926 with a 45 m eustatic sea‐level fall, and a tectonic uplift component of 20 m. The consistency of 3‐D simulation results is corroborated by the coral growth rates provided by the Fisher‐equation subroutine. These rates of 2–8 mm year⁻¹ compare well to the coral growth rates in Recent fringing reefs. We propose that during the early stage of platform evolution the high‐frequency fluctuations were obliquity‐modulated precessional cycles, whereas precessional cycles control later stages of platform growth. REPRO provides a separate visualization of the different facies bodies as a function of time and space, showing the intrinsic pattern of facies distribution in the platform. This is the result of a combination of platform growth and syndepositional subaerial erosion. For example, only the youngest stages of reef framework facies in the development of the Cariatiz carbonate platform are preserved.     

LAND–USE CHANGES AND CONSERVATION OF THE HAWAI‘I ‘AMAKIHI

Hawai‘ian honeycreepers have undergone widespread extinction and population declines due to human disturbances, including habitat fragmentation, introduced predatory mammals, alien competitors, and introduced avian diseases. The Hawai‘i ‘amakihi (Hemignathus virens) is one of seven extant Hawai‘ian honeycreepers that, like all other native honey‐creepers, vanished from the low‐elevation native forests on the island of Hawai‘i due to these disturbances. But recent observations indicate that ‘amakihi have begun to recolonize low‐elevation forests in eastern Hawai‘i. In this article we discuss the current abundance of Hawai‘ian ‘amakihi in a suburban habitat on the island of Hawai‘i. We also examine the ‘amakihi's relative preference for native or exotic vegetation. Recolonization in low‐elevation habitats underscores the importance of the remaining native forests. However, concurrent with this recolonization, eastern Hawai‘i is undergoing a residential building boom that has resulted in increased deforestation and forest fragmentation. Thus the future of honeycreepers is uncertain, given the widespread environmental changes taking place in eastern Hawai‘i.