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J. Kjøde K.M. Storetvedt D. Roberts A. Gidskehaug 《Physics of the Earth and Planetary Interiors》1978,16(2):132-144
Palaeomagnetic data from Late Precambrian dykes from the northern part of Varanger peninsula, north Norway, suggest a two-axis magnetization structure. The dominant component is considered to be syn- to late-tectonic and probably acquired at around 640 m.y. B.P. Superposed on this magnetization is a minor component which is compatible with the relative Lower-Middle Palaeozoic field; i.e. it was most likely imposed during the climax of the Caledonian orogenic movements in north Norway. The estimated relative Late Precambrian palaeopole cannot easily be reconciled with the European Late Precambrian polar path. This disagreement can be accounted for by assuming a post-magnetization dextral megashear, of the order of 500–1000 km, along the Trollfjord-Komagelv fracture zone. This type of displacement is in line with geological evidence and the palaeomagnetic reconstruction supports the long-held view of there having been continuity between the depositional environments of the Varanger Peninsula Barents Sea Group, the Eleonore Bay Group of east Greenland and the Hecla Hoek Formation of east Spitsbergen. The character and age of the horizontal displacement, post-640 to pre-500 m.y. B.P., is seen in conjunction with the opening up of the lapetus Ocean and reactivation of ancient deep-seated fractures during both the spreading and the contraction phases of ocean development. 相似文献
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Summary. The remanent magnetization of intrusive/extrusive rocks of the 'basement' complex of East Maio constitutes four components that define two different axes of magnetization, at around dec. 328, inc. 12 and dec. 007, inc. 14 respectively. In general, two or more components co-exist in separate specimens or sites but both axes are present most frequently in the normal sense. The NNW-striking axis, the B-axis, fits very well with the Upper Cretaceous polar wander path for Africa. It is consequently inferred that the major phase of sheet intrusions in Maio dates from this time, probably from the interval 90–70 Myr bp. Comparisons of the directional dispersions in the folded and unfolded states suggest that this injection phase post-dates the uplift of the Central Igneous Complex of the island. The second axis of magnetization, the A -axis, agrees very well with late Teritary—Quaternary palaeomagnetic data for Africa and the Canary Islands. The A -axis is therefore regarded as of secondary origin, being the consequence of a thermal/ chemical overprint during the Miocene—Pliocene volcanism on the island. The occurrence of a 50–70 Myr long period of volcanic quiescence and erosion, between the termination of the early igneous activity (Upper Cretaceous) and the rejuvenated magmatism in Miocene/Pliocene time, is compatible with similar observations in the Canary Islands. In contrast to the palaeomagnetic conclusions, the K/Ar data only give ages around 10 Myr. The unusually young isotope dates are regarded as being due to an almost complete age resetting and are seen in conjuction with the overprinted magnetization. This explanation is further supported by the fact that K/Ar results of pillow lavas underlying Upper Jurassic limestones only give Tertiary ages. 相似文献
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K.M. Storetvedt C.M. Carmichael A. Hayatsu H.C. Palmer 《Physics of the Earth and Planetary Interiors》1978,16(4):379-392
The Duncansby volcanic neck, intruding the Middle Devonian red beds of north Caithness, Scotland, has revealed two significantly different axes of magnetization, yielding pole positions at 149°E, 24°N and 126.5°E, 60°N, respectively. The first pole, which is interpreted as corresponding to the oldest magnetization, is in perfect agreement with Devonian polar estimates from west of the Great Glen Fault. It is tentatively suggested therefore that the Duncansby neck correlates with the Late Devonian volcanism in the nearby Orkney Islands though palaeomagnetism allows an upper age estimate of around Middle Carboniferous. The data support an earlier proposition of there being a palaeomagnetic discordance across the Great Glen Fault that can be interpreted in terms of a large-scale late- or post-Devonian transcurrent movement along this fracture zone. The original (? Late Devonian) magnetization has been nearly completely erased by the second phase of magnetization which, according to its pole position, most likely dates from about the Middle Jurassic. The latter magnetization is thought to be a consequence of burial, the coastal districts of Caithness having participated in the general subsidence of the North Sea area in late Palaeozoic and Mesozoic times. The burial magnetization, involving VPTRM and or TCRM processes, is considered to have been “frozen-in” as a result of uplift in connection with the well-documented mid-Jurassic tectonic phase that affected the northern North Sea basin, including the adjacent Moray Firth area. K/Ar analyses of the Duncansby intrusion give apparent ages ranging from 258 to 239 Ma. These dates, which lie between the two geological events inferred from palaeomagnetism, are not seen as true rock ages but rather as the result of a partial Ar loss during burial reheating. 相似文献
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K.M. Storetvedt 《Physics of the Earth and Planetary Interiors》1978,16(1):45-58
In the British Tertiary igneous province one commonly observes reversed magnetizations with an abnormally large range of inclinations. Two of the Skye lava sequences which are of Early Eocene age have been chosen to investigate why this range of inclinations exists. Various laboratory studies of the natural remanence reveal a composite palaeomagnetic record. There are two axes of magnetization present: on steeply inclined (～ 75°) and one with an intermediate inclination (～35°). There is an excess of reversed polarity components in the bulk fossil remanence of most lavas and the inclination spread seems basically caused by superposition of these components. The experimental problem of splitting the polyphase magnetization into its separate sub-components is demonstrated by many examples. It is concluded that processes of low-temperature mineral alteration (which strongly overprints the high-temperature exsolution structures) and remagnetization must have been active for a minimum time span of 20 m.y. after the original cooling of the lavas, involving both polarity inversions and a major geomagnetic axis shift in mid-Tertiary times. As a conseqence, the original TRM has probably been erased to a major extent and replaced by CRM's in subsequent times. The polar estimate based on the shallow magnetization group agree well with suggested Lower Tertiary palaeopoles from Northern Ireland and from the Faeroe Irelands. The multivectoral nature of the remanent magnetization in the Skye lavas signifies that even for geologically very young rocks it is necessary to employ much more rigorous analysis techniques than are currently being used in many palaeomagnetic laboratories. 相似文献
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A palaeomagnetic re-examination of the basal strata of the Caithness Old Red Sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region. After structural correction the dominant remanence component has D = 205°, I = +3°, α95 = 6.4° (N = 27). The existence of this shallow inclined magnetization in the Middle Devonian strata of Caithness invalidates the model, proposed by Van der Voo and Scotese (1981), involving a ca. 2000 km sinistral offset along the Great Glen Fault in the Carboniferous. However, the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone. However, the possibility of there having been a much larger transcurrent shift between Europe and North America in late/post-Devonian times, accumulated along various fracture zones within the Caledonian fold belt, is discussed. On the basis of an inferred overprinted magnetization, it is tentatively concluded that the tectonic deformation of the Old Red Sandstone of Caithness has a mid-Jurassic or younger age. 相似文献
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