Glacial tectonics and deposition of stratified drift during formation of tills beneath an active glacier – examples from Skåne, southern Sweden     

MAX ÅMARK 《Boreas: An International Journal of Quaternary Research》1986,15(2):155-171

Åmark, Max 1986 06 01: Glacial tectonics and deposition of stratified drift during formation of tills beneath an active glacier – examples from Skåne, southern Sweden. Boreas , Vol. 15, pp. 155–171. Oslo. ISSN 0300–9483.
Three exposures in till have been investigated in Skåne, southern Sweden. The tills were deposited at the base of an active glacier, and are largely made up of subglacially deposited stratified drift. Deformation of the till took place syndepositionally, resulting in high-angle fractures, clastic dikes, shear planes and folds, and in irregular deformational structures. The stratified drift was transformed into a more or less diamicton-like material by the deformation. The orientation of clasts was influenced by the action of the subglacial meltwater and the shearing from the overriding glacier. The tills accumulated at most 100 km inside the margin of the receding glacier.  相似文献   

An Example of Island-Arc Petrogenesis: Geochemistry and Petrology of the Southern Luzon Arc, Philippines   总被引：7，自引：1，他引：6  

DEFANT  MARC J.; MAURY  REN? C.; RIPLEY  EDWARD M.; FEIGENSON  MARK D.; JACQUES  DARIO 《Journal of Petrology》1991,32(3):455-500

Volcanism throughout the Luzon arc is associated with eastwardsubduction of the South China Sea floor along the Manila Trench.The southern section of the arc, the focus of this study, extendsfrom the Lingayen-Dingalan fault to the small islands just southof Luzon. Two segments appear to exist along this section ofthe arc the northern Bataan and southern Mindoro segments whichare separated by the Macolod Corridor. The volcanic rocks have typical arc phenocryst mineralogies:olivine, clinopyroxene, plagioclase, and titanomagnetite inthe most mafic rocks and clinopyroxene, plagioclase, orthopyroxene,titanomagnetite, ? amphibole in the more felsic samples. Complexzoning, sieve textures, and decoupling of incompatible traceelements suggest that processes such as assimilation have takenplace. The rocks from the study area range from basalts to rhyolitesand show typical calc-alkaline features. The rocks of the MacolodCorridor and Mindoro segment are particularly enriched in largeion lithophile elements (LILE), light rare earth elements (LREE),and radiogenic Sr compared with the Bataan segment. The datafall within the mantle array on Sr-Nd isotopic diagrams andgrade toward higher Sr and lower Nd isotopic values from northto south. A likely source for the volcanics of this study is either amid-ocean ridge basalt (MORB)-type mantle that undergoes higherdegrees of partial melting than regions involved in MORB generationor a previously depleted source. We suggest that the high fieldstrength element (HFSE) anomalies have been derived throughdifferential element partitioning during fluid transport fromthe subducted lithosphere to the mantle wedge. Continental crustal material seems to play a significant roleparticularly in the Macolod Corridor and the Mindoro segment,based on the high LILE, La/Sm ratios, radiogenic Sr isotopes,and ¹⁸O values. The Macolod Corridor and the Mindoro segmenthave undergone source contamination by crustal material fromthe North Palawan-Mindoro crustal block either during the collisionof this block with the Manila Trench or by subduction of sedimentsrich in this crustal material. A similar component has alsobeen detected in the Bataan segment but in minor amounts. Thetrace element and isotopic differences between the northernand southern sections of the arc are interpreted in terms ofvariable composition (i. e., variable amounts of a crustal componentintroduced from the Palawan-Mindoro crustal terrain) of themetasomatic fluids released into the source.  相似文献   

Evaluating climate model simulations of tropical cloud   总被引：1，自引：0，他引：1  

MARK A. RINGER  RICHARD P. ALLAN 《地球，A辑:动力气象学与海洋学》2004,56(4):308-327

  相似文献   

Canada's environmental farm plans: transatlantic perspectives on agri-environmental schemes   总被引：2，自引：0，他引：2  

GUY M ROBINSON 《The Geographical journal》2006,172(3):206-218

Evaluation of Ontario's Environmental Farm Plan (EFP) scheme, launched in 1993, provides an opportunity for comparisons with agri-environmental measures instituted in the European Union and other parts of North America. The EFP has a strong 'bottom-up' dimension in that it is farmers' organizations that have been central both to the scheme's instigation and to its ongoing management. This has affected the nature of the actions taken by individual farmers participating in the scheme. These actions are reviewed, especially in terms of the participants' attitudes towards stewardship of the land, environmental outcomes, cross-compliance measures, barriers to participation and the role of statutory regulation. Some contrasts are drawn with the greater 'top-down' controls exerted in several EU agri-environment schemes, with the latter's promotion of extensification and the changing role of farmers as 'producers of countryside' in a multi-functional agricultural system. The diffusion of EFP schemes throughout Canada is noted and is cited as confirming the maintenance of fundamentally different attitudes to the development of farm-based environmental actions compared with those adopted in the EU.  相似文献   

The Physico-Chemical Properties of a Subducted Slab from Garnet Zonation Patterns (Sesia Zone, Western Alps)   总被引：3，自引：0，他引：3  

KONRAD-SCHMOLKE  MATTHIAS; BABIST  JOCHEN; HANDY  MARK R.; O'BRIEN  PATRICK J. 《Journal of Petrology》2006,47(11):2123-2148

Garnets in continentally derived high-pressure (HP) rocks ofthe Sesia Zone (Western Alps) exhibit three different chemicalzonation patterns, depending on sample locality. Comparisonof observed garnet zonation patterns with thermodynamicallymodelled patterns shows that the different patterns are causedby differences in the water content of the subducted protolithsduring prograde metamorphism. Zonation patterns of garnets inwater-saturated host rocks show typical prograde chemical zonationswith steadily increasing pyrope content and increasing XMg,together with bell-shaped spessartine patterns. In contrast,garnets in water-undersaturated rocks have more complex zonationpatterns with a characteristic decrease in pyrope and XMg betweencore and inner rim. In some cases, garnets show an abrupt compositionalchange in core-to-rim profiles, possibly due to water-undersaturationprior to HP metamorphism. Garnets from both water-saturatedand water-undersaturated rocks show signs of intervening growthinterruptions and core resorption. This growth interruptionresults from bulk-rock depletion caused by fractional garnetcrystallization. The water content during burial influences significantly thephysical properties of the subducted rocks. Due to enhancedgarnet crystallization, water-undersaturated rocks, i.e. thoselacking a free fluid phase, become denser than their water-saturatedequivalents, facilitating the subduction of continental material.Although water-bearing phases such as phengite and epidote arestable up to eclogite-facies conditions in these rocks, dehydrationreactions during subduction are lacking in water-undersaturatedrocks up to the transition to the eclogite facies, due to thethermodynamic stability of such hydrous phases at high P–Tconditions. Our calculations show that garnet zonation patternsstrongly depend on the mineral parageneses stable during garnetgrowth and that certain co-genetic mineral assemblages causedistinct garnet zonation patterns. This observation enablesinterpretation of complex garnet growth zonation patterns interms of garnet-forming reactions and water content during HPmetamorphism, as well determination of detailed P–T paths. KEY WORDS: dehydration; high-pressure metamorphism; Sesia Zone; subduction; thermodynamic modelling  相似文献   

Tectonic versus chemical-weathering controls on the composition of fluvial sands in tropical environments   总被引：2，自引：0，他引：2  

MARK J. JOHNSSON 《Sedimentology》1990,37(4):713-726

The compositions of fluvial sands collected in mountainous regions along the volcanic front in Central America and along the Andean front in Venezuela and Colombia correlate with the tectonic setting of their source terrains. The sands derived from the Central American volcanic arc are compositionally distinct from those derived from the Andean terrains, and minor variations in tectonic style can also be discriminated. Subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight; they are strongest in sands from low-order streams draining mountainous areas with modest alluvial-plain development. Sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases. With increased weathering, sands from continental terrains converge toward a supermature quartz arenite end member, and the imprint left by source rock composition and tectonic setting is obscured. In intraoceanic terrains, a similar but much less pronounced enrichment in stable phases relative to less-stable phases is observed, similarly overprinting the tectonic signal. Although detailed tectonic information is easily lost, sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical-weathering conditions.  相似文献   

The shelf sand-plume model: a critique     

MARK H. SCHEIHING  GERARD C. GAYNOR 《Sedimentology》1991,38(3):433-444

In recent years, a new model for deposition of sand bodies in a shelf environment has appeared. This model, known as the shelf sand-plume model, is hypothesized to result from storm-driven currents that are deflected around a deltaic headland, stripping sand from the headland and redepositing it in a downcurrent ‘plume’ on the inner shelf. The modern analogue for this model is considered to be an arcuate shelf sand body located off the Damietta branch of the Nile Delta. However, the distribution of older deltaic and shoreline sands probably controls the arcuate outline of the sand body. The present current system has certainly reworked these sands into ridges and large-scale bedforms but is not responsible for the overall outline of the sand body. Grain-size range and distribution of sand on the shelf demonstrate that the source of sand in the Nile shelf sand body is not the modern Damietta headland as postulated by the shelf sandplume model. In our view, the shelf sand-plume model is presently unsubstantiated and has orginated as a misapplication of the original Nile example. As a geological model, the shelf sand-plume model lacks a set of observable, consistently applicable criteria. The only common denominator to the model is the ‘plume’ geometry of a sand body located off a deltaic promontory. However, workers postulating the existence of shelf sand-plumes have neither clearly established a ‘plume’ geometry nor shown the juxtaposition of these bodies with respect to coeval deltaic headlands in their outcrop or subsurface examples. The model does not provide criteria to distinguish a ‘shelf sand-plume’ from other classes of shelf sand bodies, notably sand ridges and storm-generated sheet-like sands. Its application to the rock record should be re-evaluated.  相似文献   

Stratal geometries, facies and sea-floor erosion in Upper Cretaceous Chalk, Normandy, France   总被引：2，自引：0，他引：2  

MARK QUINE  DAN BOSENCE 《Sedimentology》1991,38(6):1113-1152

Unusual lenticular stratal geometries and facies of the Upper Cretaceous Chalk of coastal Haute Normandie, France, are described and interpreted. Thirteen facies within these chalks are described and illustrated on the basis of field, thin-section and SEM investigations: nannofossil mudstones, nannofossil hardgrounds, echinoderm wackestones and packstones, echinoderm hardgrounds, bryozoan mudstones and wackestones, bryozoan hardgrounds, bryozoan packstones and wackestones, inoceramid wackestones, inoceramid hardgrounds, sponge hardgrounds, marly chalks, conglomeratic chalks and debris flow chalks. These facies occur within lenticular bedded structures with both concave-up and concave-down geometries which have been previously interpreted as megaripples, mud mounds or tectonic structures. Detailed examination of the structures and the associated facies indicates that the concave-up geometries were formed from submarine erosion, and redeposition in NW-SE longitudinal channels. The concave-down geometries developed between adjacent channels. Assessment of the regional and temporal setting indicates that the erosion occurred in the Armorican-Cornubian straits of the Anglo-Paris Basin during sea-level lowstands. Within these straits channelling is preferentially developed on the positive, south-western block to the Lillebonne-Fécamp-Cotentin Fault.  相似文献   

Variscan very low-grade metamorphism in southwest England: a diastathermal and thrust-related origin   总被引：2，自引：0，他引：2  

L. N. WARR  T. J. PRIMMER  D. ROBINSON 《Journal of Metamorphic Geology》1991,9(6):751-764

Abstract Regional metamorphism in the external Variscides of southwest England varied from diagenetic level to greenschist facies. There is a fundamental difference in the metamorphic character between the northern and southern regions of the area. In the north, M1 metamorphism is of a sedimentary burial character associated with high heat flow, whilst to the south it is related to tectonic burial during thrust thickening processes, with lower geothermal gradients. This pattern appears to be related to the character of basin development and its subsequent tectonic evolution. The northern region has features that accord with a diastathermal (extensional) origin for the very low-grade metamorphism whilst in the southern region the very low-grade metamorphism is linked to thrusting as a consequence of Variscan compression. The Tintagel High-Strain Zone presents an anomaly in this regional pattern where an M2 metamorphic phase is attributed to localized D2 thrust stacking along the southern margin of the Culm Basin.
There is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region. However, there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith (excluding the North Devon area). This link is attributed largely to the late-stage structural up-doming of the higher grade areas over the roof of the batholith.  相似文献   

Composition and genesis of glacial hummocks, western Wisconsin, USA   总被引：1，自引：0，他引：1  

MARK D. JOHNSON  DAVID M. MICKELSON  LEE CLAYTON  JOHN W. ATTIG 《Boreas: An International Journal of Quaternary Research》1995,24(2):97-116

Glacial hummocks associated with the Superior Lobe in western Wisconsin are stagnant-ice features composed of melt-out till, meltwater-stream sediment, and flow till. The greater proportion of melt-out till in these hummocks than in hummocks described elsewhere suggests that a model of extensive, supraglacial reworking of supraglacially released debris does not apply to the western Wisconsin hummocks. Interpretation of melt-out till in hummock exposures is based on its strong fabric oriented parallel to regional ice-flow direction. Other features of this melt-out till include poorly developed stratification (color banding and discontinuous thin sandy lenses), and minor faulting, both of which support a melt-out origin. We suggest that as stagnant, debris-rich ice began to melt, supraglacially released debris was deposited as flow till and meltwater-stream sediment (with some debris-flow sediment and lake sediment), but as the thickness of supraglacial debris increased, debris melting out at depth was stabilized, allowing features characteristic of melt-out till to be retained. Because the supraglacial debris was sandy and the stagnant ice was likely at the pressure-melting point, the supraglacial debris was well drained and did not readily fail and flow. Debris volume in the glacier generally was greater at the glacier margin, but lateral and longitudinal variations within this zone were caused by thrusting, freezing-on, or ice-margin fluctuations, which in turn resulted in variations in hummock relief. Ice-walled-lake plains are commonly associated with the hummocks and developed where debris volume was small.  相似文献