Large Sill Research Articles

Acoustic mapping of the Ilulissat Ice Fjord mouth, West Greenland

A ship-based acoustic mapping campaign was conducted at the exit of Ilulissat Ice Fjord of West Greenland and in the sedimentary basin of Disko Bay west of the fjord mouth. Submarine landscape and sediment distribution patterns represented by five acoustic facies types represent glaciomarine sediment facies types that are related to variations in the past position and relative motion of the glacier front. Asymmetric ridges on the shelf that form a curved entity and a large sill at the fjord mouth represent moraines that depict at least two relatively stable positions of the ice front in the Disko Bay and at the fjord mouth. Comparable ice-end features are not observed seaward of the East Greenland Kangerdlugssuaq Glacier, although both glaciers are comparable in their latitudinal position, present size and present role for the ice discharge from the Inland Ice sheet. Apparently, the retreat of the Greenland Inland Ice after the last maximum expansion was a more discontinuous process on the West Greenland Shelf than on the East Greenland Shelf. The Iceberg Bank, a prominent sill at the fjord exit appears to play an important role for the sedimentation after the retreat of the ice front from the shelf was completed. The retreat of the glacier behind the Iceberg Bank into the inner fjord is marked by a reorganization of sediment delivery in Disko Bay, as most of the till is now deposited within the fjord. Two linear clusters of pockmarks in the center of the sedimentary basin seem to be linked to methane release due to dissociation of gas hydrates, a process driven by fast crustal uplift of the Greenland Shelf. The orientation of these clusters appears to reflect a migration path that is defined by a buried structure which we could not resolve.

The impact of host-rock composition on devolatilization of sedimentary rocks during contact metamorphism around mafic sheet intrusions

Sedimentary rocks represent vast reservoirs for hydrous and carbonaceous fluids (liquid or gas) that can be generated and released during contact metamorphism following the emplacement of igneous sill intrusions. A massive release of these fluids may impose perturbations in the global climate. In this study we assess the influence of varying host-rock compositions on the magnitude and type of fluids generated from thermal devolatilization, with particular emphasis on carbon and halogens released from heated limestone, coal and rock salt, and the different timescales of metamorphism. In limestones the generated fluids are dominated by H2O with limited CH4 and CO2 production on a time-scale of 600–3000 years. Cracking of organic matter and CO2 production (8000–28,000 years) dominates the fluid products from a coal sequence. In the case of evaporites, the presence of reactive organic matter or petroleum results in the generation of CH4 and CH3Cl (260–1000 years). In order to compare the basin scale impacts of the differing host-rocks, two plausible scenarios are explored in which a 100 m thick and 50 000 km2 large sill is emplaced into 1) organic-rich shale and coal, and 2) limestones and rock salt. The results show the formation of 1) >1600 Gt CH4, and 2) >700 Gt of CH3Cl, demonstrating that the sill emplacement environment (i.e., the composition of the host rocks) is of major importance for understanding both gas generation in sedimentary basins and the environmental impact of a Large Igneous Province. By evaluation of the isotopic signature of carbonaceous fluids from shales and coals, we show that intrusions into coal-rich sediments are potentially of much less importance for perturbing the atmospheric carbon isotope values than shales.

Spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 from InSAR and GPS observations

In this study, the spatio-temporal evolution of Yellowstone deformation between 1992 and 2009 is monitored using interferometric synthetic aperture radar (InSAR) data acquired by the European Remote-Sensing Satellites (ERS-1 and ERS-2) and the Environmental Satellite (ENVISAT). These data are combined with continuous global positioning system (GPS) measurements to identify four discrete episodes of caldera subsidence and uplift, these episodes are: 1992–1995 (subsidence of 2.7 cm/year), 1996–2000 (subsidence of 0.5 cm/year, with local uplift of 1.7 cm/year at Norris), 2000–2004 (subsidence of 0.7 cm/year, with local uplift of 0.6 cm/year at Norris), and 2004–2009 (uplift of 3–8 cm/year, with local subsidence of 1–4 cm/year at Norris). We construct the full three-dimensional velocity field of Yellowstone deformation for 2005–2006 from ascending and descending ENVISAT orbits. The InSAR three-dimensional velocity field and three-component GPS measurements indicate that the majority of the observed deformation (3–8 cm/year) across the Yellowstone caldera and near Norris Geyser Basin (NGB) occurred in the vertical direction between the summers of 2005 and 2006. During this time, significant lateral displacements of 3–7 cm/year also occurred in the east–west direction at the southeastern and western rims of the Yellowstone caldera and in the area between Hebgen Lake and NGB. Minor north–south displacements of about 0.2 cm/year also occurred, however, in the southwestern section of the caldera and near Yellowstone Lake during the same period. The calculated three-dimensional velocity field for 2005–2006 implies the existence of two pressure-point sources, beneath the two structural resurgent domes in the Yellowstone caldera, connected by a planar conduit, rather than a single, large sill as proposed in previous studies. Furthermore, no measurable displacements occurred along any fault zone across the caldera during the entire period of observation (1992–2009). Therefore, we infer that magmatic and hydrothermal processes beneath the Yellowstone caldera and NGB were the main sources of deformation.

Modeling the growth of laccoliths and large mafic sills: Role of magma body forces

[1] The use of elastic plate theory to model the emplacement of laccoliths and large mafic sills has been debated for nearly 40 years. These intrusions typically attain a horizontal width that is large relative to the emplacement depth. Provided that large-scale plasticity and/or heterogeneity is not observed in the overlying host rock, it should then be valid to approximate its deformation based on analysis of a thin elastic plate with effective properties that are the consequence of interaction among heterogeneities that are small relative to the size of the intrusion. But the predictions that are usually cited from elastic plate theory are characterized by bell-shaped geometry, in contrast to the flat-topped, steep-sided geometry typical of many laccoliths or the nearly uniform thickness typical of large mafic sills. This fact has motivated several alternate explanations of laccolith and large mafic sill emplacement. Nonetheless, elastic plate theory should be revisited in light of the fact that previous elastic plate-based predictions have, in general, not taken into account an appropriate fracture propagation condition, fluid flow in the growing intrusion, and, importantly, the influence of the weight of the magma on intrusion growth. We present a model for the growth of circular intrusions that accounts for all of these factors. The model predicts the appropriate geometry for both laccoliths and large mafic sills. The predicted thickness to length relationships are also consistent with field data. Hence, while it may sometimes be appropriate, there is, in general, no fundamental need to appeal to large-scale rock plasticity in order to explain observed intrusion geometries, and it may, in fact, be appropriate to understand the growth of laccoliths and large sills in light of a single underlying mechanical model.

Early Cenozoic saucer-shaped sills of the Faroe Islands: an example of intrusive styles in basaltic lava piles

Abstract: The Early Cenozoic igneous activity of the North Atlantic Igneous Province that generated widespread sill complexes in sedimentary basins at the NW European margins also generated various intrusive systems in the contemporaneous basaltic lava pile of the Faroe Islands. The Faroe Island Basalt Group comprises seven formations with a total thickness of about 6.5 km, of which four major formations are built up of tholeiitic lava flows, each being several hundred metres thick, and three thinner formations are mostly built up of volcaniclastic lithologies, each being a few metres to a few tens of metres thick. The largest sills are exposed as partly saucer-shaped bodies in the three uppermost formations, where inner gently dipping basal sill sections gradually give way to more steeply inclined discordant outer rims that commonly cut several hundred metres into overlying lava flows. Numerous subvertical and moderately inclined dykes ranging in thickness from c . 0.5 to c . 4.0 m intersect the areas affected by sill intrusion, but only inclined dykes or sheets have been positively identified as sill feeders. Locally controlled rotations of least principal stress axes σ 3 during initial sill intrusion or propagation may have been an important contributing factor in determining the overall geometry of the investigated intrusions.

Regionally extensive emplacement of sandstone intrusions: a brief review

ABSTRACTLarge sandstone intrusion provinces (LSIPs) are described from three basins with contrasting tectonic settings, the North Sea Basin, the Faeroe‐Shetland Basin and the San Joaquin Basin. Hundreds of reservoir‐scale sandstone intrusions have been identified in each of these basins over areal extents of >500 km2, and with vertical extents of >1 km from their underlying source units. It is likely that these large intrusion networks were emplaced over geologically short time periods i.e. as a single hydrodynamic event, because of their highly interconnected geometry. Large sills are extensively intruded into the host claystones in each basin, and the presence and distribution of these intrusions demonstrates that pore fluid pressures in the source units may have been in excess of lithostatic at the time of remobilization and intrusion. Mechanisms that could bring source sand bodies of large volume and areal extent to a condition of supralithostatic pore fluid pressure are not well understood in the context of sandstone intrusion. Of the possible candidate mechanisms, shear‐induced liquefaction resulting from a large earthquake or bolide impact and geologically rapid pressure transfer from higher pressure cells are the most likely. The relative rarity of LSIPs worldwide is either a result of undersampling at outcrop or perhaps signifies that a rare combination of factors is required to form them. Once formed, they disrupt the sealing properties of the sequences they intrude, and this has major implications for fluid migration in these areas.

Thermal evolution of the northeastern Siberian platform in the light of apatite fission-track dating of the deep drill core

Thermal evolution of the continental crust beneath the northeastern Siberian craton was studied based on the interpretation of apatite fission-track ages. The samples selected for AFT dating were collected from depths between 2 and 3 km along a 1000-km-long profile, from the crystalline basement of the Siberian platform. The AFT ages range from 185 to 222 Ma, indicating that in the late Triassic-early Cretaceous, the top of the crystalline basement was cooled below ~100 ° C. Once the apatite cooled below this temperature, it began to accumulate and preserve tracks produced by spontaneous fissioning of 238 U, and the number of tracks pre- served is effective in determining the ages of events using the apatite fission-track method (AFT). The study showed that the apatite from Archean rocks was largely formed at 1.8-1.9 Ga as a result of a Pale- oproterozoic metamorphic overprinting during the terrane collision and the subsequent accretion of the Siberian craton. The last thermal event, the self-heating of the collision prism, was terminated by cooling at ~1.3 Ga. At that time, the Rb-Sr isotopic system became closed and the upper crust passed the ~300°e isograd. The calcu- lation results showed that on further cooling, the ~100°e isograd was passed at 1143 Ma. This age estimate could be obtained using AFT dating if the above event had been the last one in the thermal history of the Sibe- rian craton. The obtained track ages indicate the existence of a repeated, significantly younger, heating of the crystalline crust due to some local reason. The present-day temperature at the sampling depth is 29°e based on the geothermal gradient curve, which was derived from the surface heat flow rate of 25 mW/m 2 and the crustal heat production value calculated from the concentrations of radioactive elements in the crust. In the near geological past, the maximum heat flow reaching 45 mW/m 2 was determined from the mineral equilibria in the mantle xenoliths from the 245-135 Ma kimber- lites. Based on the respective geotherm, the crustal temperature in the study area did not exceed 53°e . It is obvi- ous that an enhanced heat flux during kimberlite magmatism would not cause additional heating of the crust in the Mesozoic above ~ 100°e . The emplacement of flood basalts at 250 Ma was undoubtedly most important but underestimated the thermal event in the Mesozoic history of the Siberian platform. Numerous dolerite dykes were intruded at a great dis- tance, far beyond the limits of the volcanic facies of the Putorana plateau, extending onto the study area. They were probably sourced from sills, which were emplaced along the base of the crust during basaltic underplating. The calculation results suggest that the reason of the crust's heating in the Mesozoic (222-185 Ma) might have been the emplacement of a large sill (or sills) into the base of the crust at 250 Ma. This heating of the top of the crystalline basement of the Siberian platform in the Mesozoic above ~100°e suggests that the respective heating event may have occurred at the base of the sedimentary cover and, thus may help in obtaining the age dates of the processes of the transformation of organic matter and oil generation.

Emplacement mechanisms of sill complexes: Information from the geochemical architecture of the Golden Valley Sill Complex, South Africa

The well-exposed Golden Valley Sill Complex, Karoo basin, South Africa, consists of four large sills (ca. 100 m thick; long axes: 13–24 km), one small sill (55–80 m thick; long axis: 4 km; forming an appendix to one of the large sills), and two large dykes (15–20 m thick; 25 and 70 km long), plus some minor intrusions. Field mapping shows physical connections between the small sill and one of the large sills, but no other connection between the large sills, or connections between the sills and the large dykes. Geochemical variation diagrams for 327 dolerite samples from the different sills and dykes in the Golden Valley Sill Complex showed similar compositional ranges for the dolerite bodies, but some differences in ratios between strongly incompatible elements. These differences were probed by Forward Stepwise-Discriminant Function Analysis using all major and trace elements as variables, and the height major sills and dykes as populations. The Forward Stepwise-Discriminant Function Analysis identified six different geochemical signatures among the sill-dyke populations. The six geochemical signatures were interpreted as representing six magma batches of distinct chemical characteristics that were involved in the formation of the Golden Valley Sill Complex. Four different magma batches gave rise to two of the large saucer-shaped sills and the two dykes. The exposed large dykes thus do not represent feeders to the main sills. A fifth magma batch gave rise to the two other large saucer-shaped sills as well as the smaller one. The statistical analysis also gave information about the connection between different dolerite exposures that could not be obtained by field, and made it possible to reconstruct the different sills through complex areas where several sills meet, and through widely spaced exposures. On the basis the field mapping and statistical analysis of geochemical data the geochemical architecture of the complex was established. Our study gives support to both the main models proposed earlier to explain the emplacement of sill complexes: (1) each sill in a sill complex is fed by different magma batches, and (2) the different sills in a sill complex feed one another, forming a group of nested sills generated by one single batch of magma. Furthermore, our study indicates that the interconnected sills in the Golden Valley Sill Complex are connected along their outermost edges through lateral overflow. The statistical analysis establishes a link between minor dolerite bodies and the main intrusion, thereby improving the geological mapping of the sill complex. The different units in the Golden Valley Sill Complex show strong geochemical affinity to lavas in the Lesotho Formation, the younger of the two formations in the Lesotho lava remnant. This suggests that these sills were emplaced at the same time as the Lesotho Formation, and that magmas with the same chemical characteristics as the Lesotho Formation were emplaced over a significantly larger area than covered by the remnants of the Lesotho Formation.

Silurian/Ordovician asymmetrical sill-like bodies from La Codosera syncline, W Spain: A case of tholeiitic partial melts emplaced in a single magma pulse and derived from a metasomatized mantle source

A Silurian/Ordovician extensional event in the southernmost sectors of the Central Iberian Zone is inferred from the Sm/Nd isochron obtained (436 ± 17 Ma) after the diabase sills from the La Codosera syncline. From the geochemical and mineralogical points of view, the diabase sills are subalkaline and range between high-Mg tholeiite diabases to tholeiite andesites. LREE enrichment, an Nb negative anomaly, the absence of a Ta trough and a high Nd isotope signature ( ε Nd t = + 6) are the most relevant geochemical features. The diabase bodies are up to 330 m in thickness and were sampled from bottom to top along several different sections, permitting the definition of an accumulation of clinopyroxene, olivine and plagioclase close to chilled margins at the bottom, and abundant pegmatoid layers at the top. Chemical profiles and mass-balance modelling suggest that the bulk rock and chilled margin compositions are not dissimilar, defining an unusual S-type vertical compositional profile for large (> 50 m thick) sills, which in turn strongly suggests a single magma pulse and a probable gravitational settling. Assuming chilled margin samples as the parental magma, as well as Cr-enriched samples as cumulate layers, a two-stage liquid line of descent has been established, the first one consisting of a clinopyroxene-plagioclase-olivine cumulate assemblage. A second stage in relation to the depletion in Ti, Fe and V is accounted for by ilmenite fractionation, along with that of clinopyroxene, plagioclase and olivine fractionation. Thermobarometric estimations reveal that the clinopyroxene (around 1100 °C and 197 MPa) was a late mineral phase, whereas the plagioclase (around 1200 °C) was pre- to syn-emplacement, in agreement with the presence only of plagioclase phenocrysts in the chilled margins and the very abundant positive Eu anomaly. The energy constraint modelling is consistent with the lack of a significant assimilation process owing to the high temperature contrast between the country rock and the magma itself. The compositional characteristics of chilled margins enable them to be ruled out as primary melts in equilibrium with mantle olivine (Fo 91), a certain amount of olivine fractionation being required, which might have occurred in magma conduits en route to shallow emplacement levels in the crust. Spinel lherzolitic xenoliths from the European Cenozoic alkaline magmatism appear to be unsuitable protoliths to account for the chilled margin compositions. Instead, a hybrid mantle source consisting of a small amount of OIB-mantle component (5 wt.%) and a depleted end-member mantle component seems to be a plausible protolith, resulting in a good fit with the fractionation-corrected chilled margin trends for 10% of partial melting.

An experimental investigation of sill formation and propagation in layered elastic media

A series of experiments are described where dyed water (a magma analogue) was intruded into solid gelatine (a crustal analogue) to investigate the formation of sills. We considered a layered gelatine system with contrasting adjacent layers. By varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer, with intrusion occurring in a plane directly below the interface. Experimental dykes were observed to propagate to the surface when the Young's Modulus ratio of upper to lower gelatine layers was less than one. Experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong. Two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak. The form of the intrusion depends on the balance of driving pressures and the Young's Modulus ratio of contrasting adjacent layers. When the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill. However, when the rigidity ratio and driving pressure are both close to one a dyke–sill hybrid forms. Under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer. During sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer. Experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill; causing acceleration with increasing length. Our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes. In ancient cratonic areas the Moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle. Mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts. The boundary between the upper and lower crust (Conrad discontinuity) may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration.

Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills

Major and trace element data for the Tertiary, Shiant Isles Main Sill, NW Scotland, are used to discuss its complex internal differentiation. Vertical sections through the sill exhibit sharp breaks in chemistry that coincide with changes in texture, grain size and mineralogy. These breaks are paired, top and bottom, and correspond to the boundaries of intrusive units, confirming a four-phase multiple-intrusion model based on field relations, petrography, mineralogy and isotopes. Whole-rock chemistry is consistent with this model and necessitates only minor revisions to the intrusive and differentiation mechanisms previously proposed. The rocks contain strongly zoned minerals (e.g. olivine Fo70–5, clinopyroxene Mg# = 75–5, plagioclase An75–5) indicating almost perfect fractional crystallization, but whole-rock compositions do not show such extreme variations. Thus, while residual liquids became highly evolved in situ, they mainly became trapped within the crystal network and did not undergo wholesale inward migration. Some inward (mainly upward) concentration of residual liquids did occur to form a ‘sandwich horizon’, but the more volatile-rich, late-stage liquids that did not crystallize in situ appear to have migrated to higher levels in the sill to form pegmatitic horizons. Parental liquid compositions are modelled for the intrusive units and it is concluded that the original parent magma formed by partial melting of upper mantle that was more depleted in LREE than the sources of most Scottish Tertiary basaltic rocks. Incompatible trace elements in the picrodolerite–crinanite intrusive unit support isotope evidence that its parent magma was contaminated by crustal material. Attempts to reconcile the chemical characteristics of the sill with a recently proposed petrogenetic model based on a single intrusion of magma differentiated by novel, but controversial, processes fail comprehensively. It is predicted that the complex petrogenetic history of the Shiant Isles sill is not unusual and could become the model for other large (>50 m thick) sills.

Geochemistry of North Shore Hypabyssal dikes and sills in the Midcontinent Rift of Minnesota: an example — the 47th Avenue sill

This study presents geochemical data for several of the numerous small to large dikes and sills, including the 47th Avenue sill, exposed along the shore of Lake Superior in and north of Duluth, Minnesota. These intrusions are late magmatic features of the Proterozoic Midcontinent Rift System and together form the North Shore Hypabyssal Group. The dikes are geochemically distinct from the sills, and, when the two are exposed together, the younger dike intrudes the older sill. Dikes are primitive with Mg# up to 68, have positive εNd values, and are oriented approximately north–south with steep westerly or near vertical dips. The older sills are more evolved, usually have εNd values near or below 0, and have the same gentle easterly dip as the thick sequence of North Shore Volcanic Group flows they intrude. Dike compositions correlate best with a mixture of widespread basalt compositions types 4 and 5, with primitive geochemistry and positive εNd values, whereas sill compositions are similar to widespread basalt composition type 4 typical of most North Shore Volcanic Group flows. The 47th Avenue sill in Duluth is an evolved single intrusion North Shore Hypabyssal Group diabase sill with trough banding, sharp lower and upper contacts, and a spectacular fractured and undulating roof zone containing blocks of the overlying ferroandesite flow.

Holocene to Pliocene tectonic evolution of the region offshore of the Los Angeles urban corridor, southern California

Quaternary tectonism in the coastal belt of the Los Angeles urban corridor is diverse. In this paper we report the results of studies of multibeam bathymetry and a network of seismic reflection profiles that have been aimed at deciphering the diverse tectonism and at evaluating the relevance of published explanations of the region's tectonic history. Rapid uplift, subsidence in basins, folds and thrusts, extensional faulting, and strike‐slip faulting have all been active at one place or another throughout the Quaternary Period. The tectonic strain is reflected in the modern physiography at all scales. Los Angeles (LA) Basin has filled from a deep submarine basin to its present condition with sediment impounded behind a large sill formed behind uplifts near the present shoreline. Newport trough to the south‐southeast of LA Basin also accumulated a large volume of sediment, but remained at midbathyal depths throughout the Period. There is little or no evidence of Quaternary extensional tectonism in either basin although as much as 6 km of subsidence, which mainly occurred by sagging, has been recorded in places since the middle Miocene. The uplifts include folded and thrust faulted terranes in the Palos Verdes Hills and the shelves of Santa Monica and San Pedro Bays. The uplifted areas have been shortened in a southwest‐northeast direction by 10% or slightly more, and some folds are reflected in the bathymetry. Two large adjacent midbathyal basins, Santa Monica and San Pedro, show strong evidence of subsidence and slight west‐northwest extension (10%) during the same time folding was taking place in the uplifts. The tectonic boundaries between uplifts and basins are folded, normal faulted, reverse‐faulted, and strike‐slip faulted depending on location. The rapid Quaternary uplift and subsidence, along with the filling of LA Basin, have produced a reversal in the regional physiography. In the early Pliocene, LA Basin was a submarine deep, Palos Verdes and the shelves comprised a northeast basin slope, and the present offshore basins and Catalina Island formed an emergent or shallowly submerged shelf. Since extensional, compressional, and lateral strains are all locally in evidence, simple notions that this part of southern California underwent a change from Miocene transtension to Quaternary transpression fail to explain our observations.

Extensional and compressional faults in the Everest–Lhotse massif, Khumbu Himalaya, Nepal

Two large-scale north-dipping, low-angle normal faults cut the Everest massif at the top of the High Himalayan slab in Nepal. The upper fault the Qomolangma Detachment, follows the north slope of Everest from above the 'Yellow Band' at c. 8500 m on the Southwest Face down to the Rongbuk glacier. On the south side of Everest this fault places unmetamorphosed Ordovician mudstones and limestones above biotite-grade marbles, calc-silicates and greenschists (Everest series pelites). The lower normal fault, the Lhotse Detachment, places greenschist grade calc-silicates and pelites above sillimanite–K-feldspar-grade gneisses (quartz+biotite+garnet + K-feldspar+plagioclase+sillimanite ± cordierite assemblages) formed at temperatures above 650°C. Abundant sheet intrusions, sills and dykes of leucogranite containing tourmaline+muscovite ± biotite ± garnet are restricted to the footwall of the Lhotse Detachment. The mid–upper greenschist grade Everest pelites, some 2000 m thick on the SW face of Everest, form a northward tapering wedge bounded by normal faults below and above. A major compressional fault, the Khumbu Thrust, bounds the base of a c . 3–6 km thick sheet which consists of a series of flat-lying leucogranite sills or sheets extending more than 25 km south of Everest. The leucogranite peaks of Ama Dablam, Kangteiga and Tamserku are all part of the same sheet, which, prior to erosion, was probably originally continuous. The main granite emplacement mechanism was by syn-tectonic magma injection by hydraulic fracture propagation during simple shear, along a series of large sills from their source region at depth to the north. Granite emplacement occurred during upper crustal extension along the top of the High Himalayan slab, although final motion on the Lhotse Detachment post-dated granite emplacement. Both the normal faults at the top of the slab and the Khumbu thrust at the base of the leucogranite sheets were mechanically linked, resulting in the southward extrusion of rocks formed at c. 12–30 km depth within the High Himalayan slab.

Changing techniques and paradigms for the evaluation of magmatic processes

The study of magmatic processes is at a juncture. New continuum models for the complex physics of crystallization and melting with convection and crystal‐melt segregation are now available, and the models are amenable to exploration by numerical methods. These models, which were largely developed in the fields of material science and mechanical engineering, will allow petrologists to consider a wide range of transport phenomena: crystal settling with reaction as fully multiphase flow, combined thermal and compositional buoyancy with realistic material property variations, open system and eruptive behavior and the appropriate system geometry and boundary conditions. The traditional notions of magma chambers as large vats of near liquidus material is being replaced by models where a substantial portion of the chamber may consist of a crystal‐melt mush, subject to reintrusion or mobilization. Percolative flow and reaction in this mush, including contaminants, may be as important as near‐liquidus processes in driving petrological diversity. Thus simple dimensionless numbers are of little utility in describing the vigor or style of magmatic processes. Melting of the crust following intrusion by basalt can be considered in light of two end‐members: the large sill or tabular mafic magma body and intrusion of basalt as sequences of spatially and temporally over lapping dikes. Although more commonly cartooned, the large sill type may not be nearly as efficient as repeated diking in driving crustal melting, magma mixing, and mingling. The greatest challenge for those studying magma dynamics will be to key the models to well‐constrained geological examples and to raise the sophistication of the content of these models in light of well‐defined tests that have geological significance.

Geostatistical analysis of overbank deposits of anastomosing and meandering fluvial systems; Rhine-Meuse delta, The Netherlands

The architecture of overbank deposits of anastomosing and meandering rivers in the central Rhine-Meuse delta (The Netherlands) is analysed by geostatistical investigation of three detailed cross sections in two study areas. The thickness of the overbank deposits in the cross sections were examined by semivariogram analysis. The cross sections show that two different types of overbank deposits are found at different depths at the same location. At greater depths, lithologically complex overbank deposits with many crevasse splays and a varying thickness are found. At shallow depths lithologically uniform overbank deposits with a more or less constant thickness are found. The semivariogram analysis serves as a tool to distinguish between these two types of overbank deposits. Semivariograms of the lithologically complex overbank thicknesses strongly differ from the semivariograms of the more uniform overbank deposits. The former show large nugget values combined with large sill values and a small range. The latter show small nugget values and a clear trend and no sill is reached. The complex overbank deposits are thought to be deposited by anastomosing rivers, the uniform overbank deposits by meandering rivers. Maximum boring intervals for mapping of the overbank deposits can be derived from the semivariograms. If a complete understanding of the fluvial architecture in the study areas is needed, e.g. for reservoir characterization, a sampling grid of 25 m perpendicular and 100 m parallel to main flow direction is necessary.

Solidification Front Fractionation in Phenocryst-Free Sheet-Like Magma Bodies

Basal cumulate zones are common in sills formed of magma-carrying phenocrysts upon emplacement, and the process leading to these cumulates and the corresponding roofward depleted zones are well understood. Many large sills which seem to have been emplaced with phenocryst-free magma, are nearly uniform in composition and show little tendency to differentiate by crystal settling. We examine whether, for an initially crystal-free sill solidifying inwardly from top and bottom, crystals nucleated and grown along the roof can escape incorporation, survive resorption in settling through the hotter central region, and reach the basal solidification front to form a cumulate. We call this process solidification front fractionation and build a relatively simple model to examine the rheologic, dynamic, and crystal growth rate conditions leading to the downward escape of newly grown, single crystals from the roof of a sheet-like body of mafic magma cooling by conduction. Our results suggest that, whereas a thin micro-...

Evidence for ascent of differentiated liquids in a silicic magma chamber found in a granitic pluton

ABSTRACTFluid dynamic modelling of crystallising calc-alkalic magma bodies has predicted that differentiated liquids will ascend as boundary layers and that accumulation of these buoyant liquids near chamber roofs will result in compositionally stratified magma chambers. This paper reports physical features in La Gloria Pluton that can be interpreted as trapped ascending differentiated liquids. Leucogranitic layers decimetres thick, which are locally stratified, are trapped beneath overhanging wall contacts. The same felsic magmas were also preserved where they were injected into the wall rocks as dykes and as large sill complexes. These rocks do not represent differentiated magmas produced by crystallisation along the exposed walls because the felsic layers occur at the wall rock contact, not inboard of it. Rather, we speculate that evolved felsic liquids are generated by crystallisation all across the deep levels of chambers and that initial melt segregation occurs by flowage of melt into tension fractures. Melt bodies so formed may be large enough to have significant ascent velocities as diapirs and/or dykes. The other way in which the leucogranite occurrence is at variance with the convective fractionation model is that the ascending liquids did not feed a highly differentiated cap to the chamber, as the composition at the roof, although the most felsic in this vertically and concentrically zoned pluton, is considerably more mafic than the trapped leucogranitic liquids. This suggests that these evolved liquids were usually mixed back into the main body of the chamber. Backmixing may be general in continental-margin calc-alkalic magmatic systems, which, in contrast to those in intracontinental settings, rarely produce volcanic rocks more silicic than rhyodacite. That the highly differentiated liquids are preserved at all at La Gloria is a result of the unusual stepped nature of the contact and the entirely passive mode of emplacement of the pluton, which, in contrast to ballooning in place, does not result in wall zones being “scoured”.

Evidence of relief inversion from the rim-syncline of Bannock Basin

The last steps in the geological exploration of Bannock Basin (cruises BAN-88 and BAN-89 of R.V. Bannock) provided direct evidence of an inversion of relief that was long suspected but hitherto never recorded. Dolostones and dolomitic mudstones of probable Messinian age subcropping beneath the brine in the northern part of Bannock Basin and Zanclean oozes recorded on the sill separating two narrow and elongate satellite basins along the eastern part of the rim-syncline record a relief inversion during their evolution from cylindric folds to collapse structures. In particular, the core from the eastern intrabasin sill contains two major unconformities: one separates early Zanclean oozes from late Piacenzian pelagic sediments characterized by winnowed layers, inclined bedding and incipient hardgrounds, and the second separates the latter from Middle Pleistocene pelagic sediments. The deepening is certainly post-late Piacenzian (post-M Pl 6 biozone) because sediments of that age record an actively uplifting structure. Another argument in favour of the relief inversion, and of the youthful age of the deepening, is derived from a core raised from the northern part of the major western basins (large sill separating Maestro from Borea basins) which contains a coarse turbidite of North African origin representing an upflow turbidite similar to others recorded on the outer slopes of the Mediterranean Ridge. This unique finding in the area of Bannock Basin is preserved in the central part of the collapse basin, whereas in other settings an erosional gap is noticed at the equivalent stratigraphic position.

Geodynamic implications of geochemical data for the Pyrenean ophites (Spain-France)

Field observations and radiometric data (K/Ar) of Pyrenean tholeiites issued from numerous large sills in Mesozoic basins indicate a Liassic intrusion. Variations in the chemical composition of these tholeiitic rocks show that the magmatic liquids are slightly differentiated via fractional crystallization and/or in situ density fractionation. Data on the highly hygromagmaphile (HYG) elements exhibit a noteworthy homogeneity of compositions and display, relative to chondritenormalized abundances, a distinctly negative Ta anomaly and a slight enrichment of highly HYG elements. These geochemical features are significantly different from ocean-type tholeiites due to the geochemical and mineralogical properties of their mantle source. The peculiar composition of the Pyrenean mantle source can be interpreted as the result of the Hercynian convergence process in which the dehydration of the subducted slab induced hydrothermal transformation and high- P- T hydrous pyroxenite segregation. The opening of the North Atlantic Ocean could have produced the ophites by inducing melting in this subcontinental mantle; with the enlargement of the new ocean basin, such a magma source would become progressively less available.