Basin-scale architecture of deeply emplaced sill complexes: Jameson Land, East Greenland
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Abstract:
Igneous sills are common components in rifted sedimentary basins globally. Much work has focused on intrusions emplaced at relatively shallow palaeodepths (0 – 1.5 km). However, owing to constraints of seismic reflection imaging and limited field exposures, intrusions emplaced at deeper palaeodepths (>1.5 km) within sedimentary basins are not as well understood in regard to their emplacement mechanisms and host-rock interactions. Results from a world-class, seismic-scale outcrop of intruded Jurassic sedimentary rocks in East Greenland are presented here. Igneous intrusions and their host rocks have been studied in the field and utilizing a 22 km long ‘virtual outcrop’ acquired using helicopter-mounted lidar. The results suggest that the geometries of the deeply emplaced sills ( c. 3 km) are dominantly controlled by host-rock lithology, sedimentology and cementation state. Sills favour mudstones and even exploit centimetre-scale mudstone-draped dune-foresets in otherwise homogeneous sandstones. Sills in poorly cemented intervals show clear ductile structures, in contrast to sills in cemented units, which show only brittle emplacement structures. The studied host rock is remarkably undeformed despite intrusion. Volumetric expansion caused by the intrusions is almost exclusively accommodated by vertical jack-up of the overburden, on a 1:1 ratio, implying that intrusions may play a significant role in uplift of a basin if emplaced at deep basinal levels. Supplementary materials: Uninterpreted versions of Figures 7, 8 and 11 are available at http://doi.org/10.6084/m9.figshare.c.3281882Keywords:
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Dike
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Inflow
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A large-scale palaeomagnetic study (125 specimens from 88 sampling localities) has been carried out on the Whin Sill complex and its associated dykes in northern England. The dominant carrier of remanent magnetization of the doleritic rocks is relatively pure magnetite of pseudo-single domain size, and the characteristic remanent magnetizations are typically well defined and unidirectional. Regionally consistent palaeomagnetic directions are obtained from three geographically distinct parts of the Whin Sill complex, here named as the Holy Island Sill (Declination/Inclination ( D / I )=189.5/−2.8, α 95 =6.3), the Alnwick Sill ( D / I =194.6/−25.6, α 95 =8.1) and the Hadrian's Wall–Pennines Sill ( D / I =189.2/3.3, α 95 =3.5). Although the Holy Island Sill and the Hadrian's Wall–Pennines Sill are of similar age ( c . 296 Ma) and indistinguishable on palaeomagnetic grounds, the difference in palaeomagnetic directions between them and the Alnwick Sill demonstrates the multi-component nature of the complex; magma-flow indicators suggest that the Holy Island Sill and the Hadrian's Wall–Pennines Sill may also represent separate intrusion events. Dykes local to the three sills have remanence directions corresponding to their respective sills and can tentatively be identified as their feeder dykes. Virtual geomagnetic poles for the three palaeomagnetic sites are: Holy Island Sill and Holy Island Dyke, 346.8E, 35.4S; Alnwick Sill, High Green Dyke and St. Oswald's Chapel Dyke, 337.1E, 47.1S; Hadrian's Wall–Pennines Sill and Hett Dyke, 347.1E, 32.9S.
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Large igneous province
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Abstract Magmatic differentiation is demonstrated in two epidiorite sills from the Grampian Highlands of Scotland, and the distribution of major and minor elements compared with data given for other intrusions, and for trace elements in the Whin Sill at High Cup Gill, Westmorland. The latter sill has Sr and Ba enriched towards the top. The element distribution for the epidiorite sills indicates that they are overturned. The data are conclusive for one sill but merely suggestive for the other, since such phenomena as multiple injection may also have affected the element distribution. The structural significance is briefly discussed.
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Fjord
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This thesis reports on over 27 sills and 213 associated vents. The sills and
vents were investigated using 3D seismic data, in a ~1000 km2 area,
offshore Norway between the More and Voring Basins (the Edvarda
survey).
A wide range of sill geometries are observed which are interpreted to be the
result of five different processes acting on the sills. Three of these
processes relate to how the host deforms. If sill intrusion causes
deformation of the seafloor, creating folds, or the sills interact with folds
created by neighbouring sills, sills are found to cross bedding (transgress)
abruptly. Alternatively, if deformation is interpreted to be local, then
continuously increasing Young’s Modulus with depth is interpreted to
result in sills which transgress continuously upwards, akin to smooth
‘bowls’. At shallow depths the host is interpreted to fluidise, leading to
limited transgression or in some cases multiple bowls. The seismic
amplitude responses of shallow sills include flow related features such as
channels and lobes. The other two processes interpreted to affect sill
propagation stem from structures in the host: abrupt changes in lithology
and pre-existing faults. Multiple sills are found to terminate, and in some
cases form, at sand rich units in the otherwise mudstone dominated host.
Additionally, some sills are interpreted to have intruded into a host with
pre-existing polygonal faults, which led to angular sill geometries.
Vents are found to occur directly above sills, often along the margins of
sills, but in some cases over sill interiors, especially where the sills are
locally shallower. Additionally, a cluster of 98, relatively small vents occur
above the shallowest sill. Differential compaction and slumping are found
to affect some larger vent morphologies. Overall, vent size is found to
closely follow a power-law such that smaller vents are significantly more
numerous than larger vents.
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Lithology
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Igneous sills are common features in tectonically active regions, acting as nascent magma storage systems, or feeding eruptions at large lateral distances from the magma source. Sills reaching a critical radius, rc, relative to their initial depth, H, interact with Earth’s surface, leading to mild discordant growth at angle [theta] typically <10[deg] forming saucer-shaped sills; commonly rc>3H for natural sills. Analogue and numerical models for sills produce saucer shapes where [theta] >10[deg] and rc
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Abstract In this paper, we address sill emplacement mechanisms through the three-dimensional relationships between sills and their potential feeders (dykes or sills) in the well-exposed Golden Valley Sill Complex (GVSC), Karoo Basin, South Africa. New field observations combined with existing chemical analyses show that: 1) the contacts between sills in the GVSC are not sill-feeding-sill relationships, and 2) there are, however, close structural and geochemical relationships between one elliptical sill, the Golden Valley Sill (GVS), and a small dyke (d4). Such relationships suggest that GVS is fed by d4 and that the linear shape of d4 may have controlled the elliptical development of the GVS. To test this hypothesis, we present preliminary results of experimental modelling of sill emplacement, in which we vary the shape of the feeder. In the first experiment (E1) with a punctual feeder the sill develops a sub-circular geometry, whereas in the second experiment (E2) with a long linear feeder the sill develops an elliptical geometry. The geometrical relationships in E2 show that the elliptical shape of the sill is controlled by the linear shape and the length of the linear feeder. The experiment E2 presents strong similarities with the GVS–d4 relationships and thus supports the proposition that d4 is the feeder of the GVS. Our experimental results also indicate that the feeders of the other elliptical sills of the GVSC may be dykes.
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