Normal fault zone evolution and fault scarp degradation in the Aegean region   总被引：1，自引：0，他引：1  

IAIN S. Stewart  PAUL L. Hancock 《Basin Research》1988,1(3):139-153

Abstract The initial stages of rift-basin evolution are periods of great landform change. Fault scarps are newly created axes of erosion which, along with footwall uplands, act as sediment sources for subsiding hanging wall basins. Scarps formed during neotectonic normal faulting of Mesozoic carbonates in mainland Greece and western Turkey display a varied pattern of degradation related to the history of fault development and variations in fault zone architecture. Alternating zone-parallel compact breccia sheets and incohesive breccia belts, of contrasting resistance to erosion, underlie scarps. Meso-scale slip-plane phenomena, such as corrugations, gutters, comb fractures, and pluck holes, together with geomorphological features, such as subsurface solution pipes, and vegetation result in initial variations in the denudability of erosionally resistant compact breccia sheets. Migration with time of slip-plane activity within a fault zone into its hanging wall (i.e. intrafault-zone hanging wall collapse) adds to the structural heterogeneity of fault scarp footwalls. Quaternary talus, whether offset across a fault, banked unconformably against a slip plane, or faulted against a reactivated slip plane, has a dampening effect on degradation. The complexities of fault zone architecture combined with a history of hanging wall collapse lead, in the Aegean region, to non-uniform degradation and scarps which are commonly stepped and occasionally cavitated.  相似文献   

Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet): Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle   总被引：29，自引：0，他引：29  

ZHOU  MEI-FU; ROBINSON  PAUL T.; MALPAS  JOHN; LI  ZIJIN 《Journal of Petrology》1996,37(1):3-21

The Luobusa ophiolite in the Indus—Yarlung Zangbo sutureof southern Tibet hosts the largest known chromite deposit inChina. The podiform chromitites occur in a well-preserved mantlesequence consisting of harzburgite with abundant lenses of dunite.The harzburgites have relatively uniform bulk-rock compositionswith mg-numbers [100 Mg/(Mg + Fe)] ranging from 89 to 91 andshow flat, unfractionated, chondrite-normalized platinum groupelement (PGE) patterns. Their accessory chromite varies widelyin cr-number [100Cr/(Cr + Al)] (18–66). These rocks areessentially residua left after extraction of mid-ocean ridgebasalt (MORB)-type magmas. The podiform chromitites displaynodular, massive, disseminated and banded textures and typicallyhave dunite envelopes that grade into the surrounding harzburgiteand diopsidic harzburgite with increasing pyroxene contents.They consist of relatively uniform chromite with high cr-numbers(74–82), have strongly fractionated, chondrite-normalizedPGE patterns with enrichment in Os, Ir and Ru relative to Rh,Pt and Pt, and are believed to have formed from a boniniticmagma produced by a second stage of melting. Dunites containaccessory chromite intermediate in composition between thoseof harzburgite and chromitite and are believed to be the productsof reaction between new boninitic magmas and old MORB-type peridotites.The melt-rock reaction removed pyroxene from the peridotitesand precipitated oli-vine, forming dunite envelopes around thechromitite pods. The melts thus became more boninitic in compositionand chromite saturated, leading to precipitation of chromitealone. The interplay of melt-rock interaction, chromite fractionationand magma mixing should lead to many fluctuations in melt composition,producing both massive and disseminated chromitites and phaselayering within individual podiform bodies observed in the Luobusaophiolite. KEY WORDS: boninitic magmas; dunite envelope; melt—rock interaction; MORB peridotities; podiform chromitites ^*Corresponding author. Present address: Department of Geology, Laurentian University, Sudbury, Ont, Canada P3E 2C6.  相似文献   

Crustal Assimilation as a Major Petrogenetic Process in the East Carpathian Neogene and Quaternary Continental Margin Arc, Romania   总被引：5，自引：0，他引：5  

MASON  PAUL R. D.; DOWNES  HILARY; THIRLWALL  MATTHEW F.; SEGHEDI  IOAN; SZAKCS  ALEXANDRU; LOWRY  DAVID; MATTEY  DAVID 《Journal of Petrology》1996,37(4):927-959

Miocene to Pleistocene calc-alkaline volcanism in the East Carpathianarc of Romania was related to the subduction of a small oceanbasin beneath the continental Tisza–Dacia microlate. Volcanicproducts are predominantly andesitic to dadtic in composition,with rare basalts and rhyodacites (51–l71% SiO₂; mg-number0.65–0.26) and have medium- to high-K calcalkaline andshoshonitic affinities. Mg, Cr and Ni are low in all rock-types,indicating the absence of primary erupted compositions. Detailedtrace element and Sr, Nd, Pb and 0 isotope data suggest thatmagmas were strongly crustally contaminated. Assimilation andfractional crystallization (AFC) calculations predict the consumptionof 5–35% local upper-crustal metasediments or sedimentsfrom the palaeo-accretionary wedge. Variations in the isotopiccomposition of the contaminants and parental magmas caused variationsin the mixing trajectories in different parts of the arc Themost primitive isotopic compositions are found in low-K dacitesof the northern Cdlimani volcanic centre and are interpretedas largely mantle derived. A second possible mantle reservoirof lower ¹⁴⁹ Nd/¹⁴⁴ Nd and lower ²⁰⁶ Pb/²⁰⁴ Pb is identifiedfrom back-arc basic calc-alkaline rocks in the south of thearc Both magmatic reservoirs have elevated isotopic characteristics,owing either to source bulk mixing (between depleted or enrichedasthenosphere and <1% average subducted local sediment) orlower-crustal contamination. KEY WORDS: Carpathians; assimilation; calc-alkaline; Sr-Nd-Pb-0 isotopes; laser flurination  相似文献   

Flow and sediment transport over large subaqueous dunes: Fraser River, Canada   总被引：2，自引：0，他引：2  

RAY KOSTASCHUK  PAUL VILLARD 《Sedimentology》1996,43(5):849-863

Large symmetric and asymmetric dunes occur in the Fraser River, Canada. Symmetric dunes have stoss and lee sides of similar length, stoss and lee slope angles <8°, and rounded crests. Asymmetric dunes have superimposed small dunes on stoss sides, sharp crests, stoss sides longer than lee sides, stoss side slopes <3° and straight lee side slopes up to 19°. There is no evidence for lee side flow separation, although intermittent separated flow is possible, especially over asymmetric dunes. Dune symmetry and crest rounding of symmetric dunes are associated with high sediment transport rates. High near-bed velocity and bed load transport near dune crests result in crest rounding. Long, low-angle lee sides are produced by deposition of suspended sediment in dune troughs. Asymmetric dunes appear to be transitional features between large symmetric dunes and smaller dunes adjusted to lower flow velocity and sediment transport conditions. Small dunes on stoss sides reduce near-bed flow velocity and bed load transport, causing a sharper dune crest. Reduced deposition of suspended sediment in troughs results in a short, steep lee slope. Dunes in the Fraser River fall into upper plane bed or antidune stability fields on flume-based bedform phase diagrams. These diagrams are probably not applicable to large dunes in deep natural flows and care must be taken in modelling procedures that use phase diagram relations to predict bed configuration in such flows.  相似文献   

The relationship between basin and margin thermal evolution assessed by fission track thermochronology: an application to offshore southern Norway     

MAX Rohrman  PAUL Andriessen  PETER van der  Beek 《Basin Research》1996,8(1):45-63

Abstract We use a quantitative model of apatite fission track (AFT) annealing to constrain the thermal evolution of a sedimentary basin and its margin. Apatites deposited in a basin contain several types of information. Provided temperatures remained below ?70°C, they retain much of their provenance thermal signatures and mainly record the thermal evolution of their source area. Above 70°C, the fission tracks in apatite rapidly fade, reflecting the thermal evolution of the basin. Therefore, downhole AFT dates in a well section can in principle be used to assess both the provenance detail (from shallow/cool samples) and the subsequent thermal history in the basin (from the deeper samples). We apply this concept to the south Norwegian offshore and onshore using AFT and ZFT (zircon fission track) data; the latter constrain maximum palaeotemperatures and provide additional provenance information. AFT and ZFT data from three offshore wells in the northern North Sea are shown to contain a record of palaeogeographical and tectonic evolution, closely associated with the Norwegian basement. ZFT data from Middle Triassic sediments suggest a Permian volcanic source. Modelling of AFT data from Jurassic sediments presently residing at temperatures below 70°C indicate rapid cooling during the Late Triassic to Early Jurassic, similar to onshore AFT data. During the Cretaceous minor sediment supply was derived from the Norwegian basement, as evidenced by ZFT ages that do not correlate to the onshore, suggesting that parts of southern Norway were covered with sediments at this time. At the end of the Palaeogene and during the Neogene, the south Norwegian basement again became a major source of elastics. AFT and ZFT data indicate that all wells are presently at maximum temperatures. No significant (> 500 m) erosion events are indicated in the three wells since the Jurassic. AFT data have not yet effectively equilibrated to present-day temperatures as nonzero fission track ages are maintained in sediments currently at temperatures of > 120°C. This implies that the present-day thermal regime has only recently been installed. Probable causes include rapid subsidence and an increase in the geothermal gradient during the last 5 Myr.  相似文献   

VLF RESISTIVITY MAPPING AND VERTICALIZATION OF THE ELECTRIC FIELD1     

ALAIN TABBAGH  YVES BENDERITTER  PIERRE ANDRIEUX  JEAN PAUL DECRIAUD  ROGER GUERIN 《Geophysical Prospecting》1991,39(8):1083-1097

For over 20 years, powerful VLF transmitters have been used as electromagnetic sources for subsurface investigations in mining exploration. Measurements initially concerned the vertical component of the magnetic field or the inclination of the field and were later extended to measurement of the horizontal electric field in the direction of the transmitter, to determine the resistivity of the terrain. Measurement of the electric field is usually performed with electric lines, grounded or not, with lengths of at least 5 m. This paper presents the concept of a VLF resistivity meter with a very short electric sensor (1 m) and the results obtained with it. This technique improves the measurement of the electric field, which is in principle a point value. It also permits a higher spatial sampling rate and, by closely linking the electric sensor with the magnetic sensor on a lightweight mount, makes it possible for the instrument to be used by a single operator. In addition, transformation of the electric field data, analogous to reduction to the pole in magnetism, is proposed to correct the horizontal deformation of the anomalies created by polarization of the primary field. Comparison with direct current electrical measurements shows highly satisfactory correlations. This transformation, validated for VLF, can be extended to any electrical or electromagnetic method using a uniform primary field, i.e. gradient array in direct current or low-frequency magnetotellurics. We call this verticalization of the electric field. Resistivity measurements and mapping using the VLF frequency range can be applied not only to mining but also to a wide range of shallow geophysical studies (hydrology, civil engineering, etc.) and are not limited to problems concerning the location of conductive targets  相似文献   

Sedimentary model for intertidal mudflat channels, with examples from the Solway Firth, Scotland   总被引：1，自引：0，他引：1  

PAUL H. BRIDGES  MICHAEL R. LEEDER 《Sedimentology》1976,23(4):533-552

Intertidal mudflat channels (gullies) in the Solway Firth, Scotland possess width/depth ratios similar to meandering rivers. Most channels deeper than 1 m show cut-bank slides, but narrow, deep channels also have rotational slides on the point-bar slopes. The channels display two types of point-bar. The first, associated with gently curved meanders, is sigmoidal in profile. The second, associated with tight meander bends, possesses a pronounced lower platform. The onset of flow separation in meander bends, a phenomenon which enhances cut-bank erosion and point-bar deposition, is a direct function of meander-bend tightness and Froude number. The effects of flow separation are greatest on tight meander bends at times of high velocity during late spring ebb and also during rainfall run-off at low tide. These events appear to be responsible for the growth of the point-bar platforms. A model, predicting the type of point-bar development to be expected in different channel meanders, is used to reconstruct the sedimentary history of active and fossil point-bars.  相似文献   

Gabbroic Pegmatite Intrusions, Iberia Abyssal Plain, ODP Leg 173, Site 1070: Magmatism during a Transition from Non-volcanic Rifting to Sea-floor Spreading   总被引：1，自引：1，他引：1  

BEARD  JAMES S.; FULLAGAR  PAUL D.; KRISHA SINHA  A. 《Journal of Petrology》2002,43(5):885-905

On the Iberia Abyssal Plain (Ocean Drilling Program Site 1070),gabbroic pegmatites and related rocks (127 ± 4 Ma, U–Pbzircon) intrude upper mantle that was subsequently exposed andserpentinized during Early Cretaceous non-volcanic rifting.The pegmatites include a 3–4 m dike or sill (the ‘main’pegmatite), numerous dikelets of 1–5 cm thickness, andclasts within the overlying ophicalcite breccia. Exclusive ofrodingitization, the main pegmatite contains 40–70% calcicandesine, 25–35% kaersutitic amphibole (Mg# 60–70),5–25% augite (Mg# 70–80) and 1–2% ilmenite.The dikelets are more magnesian (Mg# up to 82 in kaersutiteand 88 in augite). Most indications are that the high Mg#s inthe dikelets reflect igneous compositions. Isotopic and elementalchemistry indicate that the pegmatite-forming melt was enrichedin incompatible elements relative to normal mid-ocean ridgebasalt, but not as enriched as Azores basalts. The amphibole-bearingplagioclase peridotites of the Iberia Abyssal Plain are an appropriatesource for the pegmatite melts. A combination of decompressionaccompanying unroofing and heating from the upwelling asthenospherebeneath the developing rift caused P–T conditions in theamphibole-bearing lithosphere to exceed the dehydration-meltingsolidus (  相似文献   

OPINIONOpen‐minded Geographers: Their Potential Role in Integrated Adaptive Environmental Management     

GRAEME APLIN  PAUL BATTEN 《The Australian geographer》2004,35(3):355-363

There are three key factors to be considered in comprehensive environmental management: (a) integration across disciplines and interests; (b) flexibility and adaptability based on feedback; and (c) interactions between policy and implementation at different scales. Successful management is both integrated and inclusive, and also adaptive and flexible. It involves a synthesis of work, requiring empathy among contributors and active synthesis. Furthermore, there is a need for a dialectical consideration of processes acting at various scales. Geographers who are not too narrowly specialised have a key role to play, and, collectively, might have been more centrally involved in environmental management than they have been.  相似文献   

CULTURE AND POLITICS OF INVASIVE SPECIES     

PAUL ROBBINS 《Geographical review》2004,94(2):iii-iv

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