The nature and age of the Cenozoic compressional/transpressional deformation within the NE Faroe–Shetland Basin, the Wyville–Thomson Ridge and Hatton Bank areas have been investigated, primarily using seismic reflection data. In all three areas, the folds reach approximately 2 to 4k min amplitude and 40k min wavelength. Early and mid-Eocene compressional/transpressional deformation affected the Hatton Bank and Wyville–Thomson Ridge areas, and folding was locally active even earlier, during Paleocene/Cretaceous times. However, the main Cenozoic compressional/transpressional tectonism that affected the Hatton Bank area was coeval with development of the regional Late Eocene Unconformity (C30), and with changes in spreading geometries and a phase of accelerated subsidence in the Rockall Basin. Within the NE Atlantic margin, WNW-to NW-trending lineaments/transfer zones and associated oceanic fracture zones facilitate significant structural segmentation. Offsets in the continent–ocean boundary along Hatton Bank probably reflect inherited basin architecture, and many Cenozoic folds in the Hatton Bank, Wyville–Thomson Ridge and NE Faroe–Shetland Basin areas are considered to mainly reflect compressional buttressing against pre-existing structures. However, relatively small lateral displacements probably occurred along some reactivated transfer zones following continental break-up. Paleocene–Eocene compressional/transpressional deformation may have affected parts of the Faroe–Shetland Basin, but seismic resolution of this is largely masked by pervasive polygonal faulting. Significant, early to mid-Miocene compressional/transpressional deformation is recorded in the NE Faroe–Shetland Basin, and may also have exerted a major influence on the Wyville–Thomson Ridge and surrounding area. In particular, mid-Miocene growth of the Faroe Bank Channel syncline may have resulted in major changes in northern hemisphere deep-ocean circulation with associated impact on global climate. Compressional/transpressional deformation appears to have continued into Pliocene– ?Recent times and resulted in the development of features such as the Pilot Whale Anticline and associated mud volcanoes/diapirs.
Induced polarisation (IP) surveys were carried out at four
localities in the Halkyn - Minera area, all sited mainly
on rocks of the Cefn-y-Fedw Sandstone Group north of
the Llanelidan Fault. The first survey covered the
Llandegla Moor area but failed to produce any evidence
for a north-westward continuation of the rich Minera
lode system. Regional and detailed gravity data
subsequently demonstrated the existence of a
pronounced Bouguer anomaly low over the western
margin of the Cefn-y-Fedw Sandstone Group, including
the Llandegla Moor area, suggesting that these rocks
thickened rapidly eastwards, perhaps along a concealed
northerly-trending fault. In an attempt to locate this
fault and to test its mineral potential, trial IP surveys
were carried out at three sites covering the boundary
between the Cefn-Fedw Sandstone Group and the
underlying Vi&an limestones. No strong chargeability
anomalies were discovered and the overlap and
variability of the resistivities of the sandstones and the
limestones prevented recognition of the boundary
between these two rock types. The chargeability values
recorded over sandstones seem to be largely dependent
upon the resistivity values, suggesting that variations in
the porosities of the rocks are mainly responsible.
The Bouguer low is interpreted as probably being due
to the presence of high-porosity and rapidly-thickening
sandstones in the Cefn-y-Fedw Sandstone Group. For
these reasons the area where the low was discovered is
not strongly recommended for any future exploration for
extensions of the mineral veins.
Synopsis A marked magnetic anomaly at Sandhead, 11 km SSE o f Stranraer on the Rhins of Galloway, has been modelled in terms of an intrusive body which rises to within 50 m of the ground surface. Two culminations are apparent but both remain concealed beneath a cover of hornfelsed Ordovician and Silurian grey-wackes. The intrusion seems most likely to be a late Caledonian diorite. The controlling influence on its location and shape was possibly a late phase of dextral shear on the Cairngarroch (Orlock Bridge) Fault.
Chalcopyrite-pyrite-molybdenite mineralisation, in
disseminated, veinlet and fracture-filling forms, is
developed in adamellite and microadamellite in the
Ballachulish igneous complex. Minor scheelite is
associated with the sulphides, but is mostly confined
to the adamellite. The mineralisation occurs
sporadically over an area of at least 1800 x 800 m.
It is best developed in and around the eastern part
of the microadamellite over an area of about
250 x 450 m, where it was observed over a.vertical
interval of 250 m from the highest exposure to the
base of a borehole. An IP survey showed that chargeability
values are slightly higher in this area. The
grade is variable. In 10 ft (3 m) lengths of core, the
maximum Cu content was 264 ppm and the maximum
molybdenum content 501 ppm, but the average
tenor over the (250 x 450 m) mineralised area is
not more than 50-100 ppm Cu and lo-30 ppm MO.
Selected mineralised outcrop samples gave values of
up to 2386 ppm Cu, 9257 ppm MO, 2434 ppm W,
0.31 ppm Au and 8 ppm Ag. Rb-Sr isotopic studies
indicate that the ore minerals were deposited shortly
after emplacement of the host rocks, and it is
considered that they were introduced by a hydrothermal
system which, compared with those of
classic porphyry models, was small in extent and
weak in intensity. Sericitic alteration is generally
associated with the mineralisation, but there is no
potassic alteration evident and the standard zonation
of porphyry copper deposits is absent. There is
very little K or Rb metasomatism, the best defined
chemical change being a loss of Sr in altered rocks.
The hydrothermal fluids, as seen in fluid inclusions,
were of moderate salinity, unlike the high salinity
fluids usually characteristic of porphyry copper
deposits. Anomalously low Rb and high K/Rb values
in the unaltered microadamellite are attributed
to the separation of a Rb-rich aqueous fluid from
the microadamellite before or at the time of consolidation
of the rock. The mineralised area lies adjacent
to and northwest of a NNE-trending shatter
belt, which may have provided structural control at
depth, although at the present level of exposure the
microadamellite body appears to be the structural
control.
Abstract: A high-resolution, airborne conductivity survey has proved spectacularly successful in delineating the zones of carbonaceous mudstone (Moffat Shale Group) that form the structural and stratigraphical base of various sandstone-dominated tracts within the Northern Ireland sector of the Southern Uplands–Down–Longford Ordovician–Silurian accretionary terrane. The anomalies associated with mudstone both at outcrop and concealed allow the major tract boundary faults to be plotted across large areas of poor exposure to reveal a large-scale regional swing in the strike of these faults from their prevalent ENE–WSW trend into a NE–SW orientation. The fault traces defined by the geophysical anomalies cut across those, deduced by extrapolation into an area of sparse bedrock exposure, that are illustrated on the current geological map. A substantial revision of the regional fault pattern is thus required. In places the major tract boundary faults appear to anastomose into strike-slip duplexes, suggesting transpression in the accretionary regime. However, the wholesale realignment of the strike trend is likely to have had a later, post-accretion origin, perhaps involving a releasing bend on a major, strike-parallel fault that controlled emplacement of the Newry granitoid pluton (425 Ma), and might also be associated with Au mineralization in Armagh and Monaghan.
A gravity investigation of the Orcadian Basin area has been conducted which involved the following stages:
- compilation, imaging and qualitative interpretation of BGS gravity and magnetic data from the region;
- compilation of rock densities from geophysical well logs and modelling of density variations within the sedimentary sequence;
- construction of a structural model of the cover sequence down to the top of the Permian, based on depth-converted seismic interpretation;
- calculation of the gravity effect of the sequence to top Permian using the structural and density models;
- removal of this calculated gravity effect and a regional background field from the observations to leave a residual stripped gravity anomaly;
- analysis of the signatures within the residual stripped gravity anomaly map, integrated with seismic evidence of Upper Palaeozoic structure and magnetic imaging.
The residual stripped gravity anomaly map reveals features that can be correlated with the West Bank Basin and the eastern end of the Caithness Graben of Arsenikos et al. (2016), and with the thickened Upper Palaeozoic sequence in the East Orkney Basin inferred by those authors. Gravity signatures indicative of a thickening of the Upper Palaeozoic sedimentary rocks are also identified in the Dutch Bank Basin and the South Buchan Basin, areas in which seismic interpretation of Palaeozoic structure was difficult because of problems with data quality and line spacing.
The influence of granitic intrusions is seen in a belt that extends in a north-north-east direction from Quadrant 19 into Quadrant 13, although the magnetic characteristics of the bodies might indicate separate post-tectonic and late-tectonic suites of Caledonian intrusions. Further granites are inferred in the Inner Moray Firth, in Quadrants 12 and 17. A Caledonian age is possible for these but an alternative interpretation invokes Palaeoproterozoic calc-alkaline basement, at least for the more magnetic component. Gravity signatures in the Inner Moray Firth are also influenced by low density Dalradian (Grampian Group) basement and Devonian sedimentary rocks, making it difficult to partition the response accurately between the different sources.
Positive gravity signatures are associated with the Buchan Block and its offshore extension, and with Jurassic intrusives beneath the Forties Volcanic Province. Dense/shallow basement extends in a west-north-west direction from the Forties area in a broad axis, and this is an important component of the long-lived structural configuration of the region that may be linked to an early transform offset in the Laurentian margin.
Recommendations for further work include more detailed and extensive gravity modelling, quantitative magnetic modelling and a geochemical/isotopic study of the samples available from offshore granite well penetrations.
Abstract Quantitative modelling of potential field data has been used to test and extend geological sections constructed for the new British Geological Survey Tectonic Map of Britain and Ireland. Three of the profiles cross part of the Anglo-Brabant Massif and provide new information on the nature of the pre-Mesozoic basement. A profile across southern England (passing just to the west of London) suggests that a significant contribution to observed gravity variations in the region results from changes in the thickness of relatively low density Lower Palaeozoic rocks. It also identifies a major deep-seated body with relatively high magnetic susceptibility and low density which is interpreted as a Precambrian cratonic core underlying the southeastern part of the Midlands Microcraton. Profiles across central and eastern England reveal major boundaries within the Precambrian basement, some of which coincide with structures mapped at surface. A number of intrusions of probable Caledonian age have been recognized, including bodies beneath the Widmerpool Gulf and The Wash. Those beneath The Wash appear to lie in a discrete basement region which separates belts of more magnetic basement lying to the northwest and southeast.
Abstract The distribution of Cenozoic compressional structures along the NW European margin has been compared with maps of the thickness of the crystalline crust derived from a compilation of seismic refraction interpretations and gravity modelling, and with the distribution of high-velocity lower crust and/or partially serpentinized upper mantle detected by seismic experiments. Only a subset of the mapped compressional structures coincide with areas susceptible to lithospheric weakening as a result of crustal hyperextension and partial serpentinization of the upper mantle. Notably, partially serpentinized upper mantle is well documented beneath the central part of the southern Rockall Basin, but compressional features are sparse in that area. Where compressional structures have formed but the upper mantle is not serpentinized, simple rheological modelling suggests an alternative weakening mechanism involving ductile lower crust and lithospheric decoupling. The presence of pre-existing weak zones (associated with the properties of the gouge and overpressure in fault zones) and local stress magnitude and orientation are important contributing factors.
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