The British Siluro-Devonian palaeofield, the Great Glen Fault and analytical methods in palaeomagnetism: comments on paper by K. M. Storetvedt<i>et al.</i>

S U M M A R YThe Dingle Group, a continental Upper Silurian redbed succession in SW Ireland, has yielded a multicomponent palaeomagnetic record. The oldest magnetization retrieved, the B component, with in situ mean direction of DIZ = 191/+9; K = 40; cyy5 = 3.6"; N = 41, post-dates the local late Silurian-mid-Devonian (Acadian) tectonic deformation and is in very good agreement with the prevailing remanences in other Caledonian rocks of western Ireland. An overprinted 'A' component with in situ mean direction D/Z = 194/-16; K = 53; cyy5 = 3.1"; N = 41, is about as prevalent as the B component and was probably acquired in late Palaeozoic (Kiaman) times. By comparison with other published data, including a positive fold test for corresponding magnetization of Upper Silurian redbeds in Clare Island, it is concluded that the flat-lying B field (relative to Britain and Ireland) was in existence at least from the late Silurian to late Middle Carboniferous, i.e. covering a minimum time span of 100 M.yr.

Section: N T E R P R E T a T I O Nmentioning

confidence: 99%

“…For more than 25 years the Siluro-Devonian palaeomagnetic field relative to the British Isles has been a matter of repeated debate (see Torsvik, Trench & Smethurst 1991;Storetvedt et al 1992 for recent contributions to this discussion). In short.…”

Section: N T E R P R E T a T I O Nmentioning

confidence: 99%

Magnetization history of the Upper Silurian Dingle Group, SW Ireland

Storetvedt

Murthy

Deutsch

1993

“…(1990), amongst others. Many of these postulated large-scale displacements have since been disproved, as some of the palaeomagnetic studies upon which they were based have been shown to be inadequate in resolving complex magnetization histories, leading to interpretations based on comparisons of components of different ages (Briden, Turnell & Watts 1984;Irving & Strong 1984, 1985Torsvik, Trench & Smethurst 1991a). We conclude, therefore, that there is no palaeomagnetic evidence for Devonian or late Palaeozoic tectonic discordance between Laurentia and its margins.…”

Section: The Tectonic Integrity O F Laurentja a N D Its Marginsmentioning

confidence: 83%

Palaeozoic palaeogeography of Laurentia and its margins: a reassessment of palaeomagnetic data

Niocaill

Smethurst

1994

S U M M A R YThe recognition of wide-spread remagnetization events during the Palaeozoic has greatly added to the problems of reconstructing the Palaeozoic drift history of Laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data, in an attempt to overcome some of these difficulties. In the light of these developments a revised apparent polar wander (APW) path has been constructed following a rigorous evaluation of all published Palaeozoic palaeomagnetic data from Laurentia and its Palaeozoic margins (i.e. North America, northern Britain, Northern Ireland and Greenland). The use of data from the Laurentian margins, when corrected for the opening of the North Atlantic, has resulted in a revision of previously published Palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for Laurentia are in good agreement with faunal and lithological data. A comparison of this new APW path with other APW paths from Avalonia and Baltica reveals a very close agreement between all three paths from mid-Silurian times onwards, indicating that the closure of the Iapetus ocean and the Tornquist sea was essentially complete by this time.Laurentia occupied equatorial latitudes from Cambrian to early Silurian times and its drift history was dominated by anti-clockwise rotation (up to 0.7" Ma-') and low latitudinal drift rates (up to 3 cm yr-l). Late Silurian and early Devonian times were characterized by increased rates of anti-clockwise rotation (up to 1.3" Ma-') and southerly directed latitudinal drift (rates up to 13 cm yr-l) of the continent, which place Laurentia in moderate southerly latitudes (30-40"s) by the early Devonian. These palaeolatitudes are comparable with those from Avalonia and Baltica for this time indicating that the amalgamation of Eurmerica had already taken place. By late Devonian time Laurentia had begun to drift north (rates up to 5 cm yr-') towards equatorial latitudes by mid-Carboniferous time.

“…(2) We recognized T as a trajectory on the vertical plot only with no corresponding branch in the horizontal plot (Storetvedt et al 1990, p. 160), and we argued that this is probably the result of partial spectra overlap of two components. With the new information on temperature steps and their sequential order (Torsvik et al 1991 (1989) claim that their high-temperature remanence is pre-tectonic, but the direction arrived at for this sample is anomalously steep even for the intermediately inclined palaeomagnetic axis they adhere to. They further argue that we applied improper analytical procedures in our analyses.…”

mentioning

confidence: 95%

“…With the new information on temperature steps and their sequential order (Torsvik et al 1991, this volume), we consider our original point as well emphasized. A perusal of fig.…”

mentioning

confidence: 99%

On the British Siluro-Devonian palaeomagnetic field problem: reply to comments by T. H. Torsviket al.

Storetvedt

Deutsch

Murthy

1991

We appreciate this opportunity to clarify our position on a few central issues of this debate. By our rejection of a number of Palaeozoic palaeomagnetic data from Scotland (Storetvedt et al. 1990), Torsvik et al. accuse us of disregarding all classical field tests. We have never disregarded any classical field test, but would argue that such tests have sometimes been misapplied. Also, we obviously have not rejected all earlier British 'SiluroDevonian' data, including Scottish results (e.g. table 3, Storetvedt et al. 1990).In our discussion of the data of Torsvik ef al. (1989), [see Storetvedt et al. 1990, fig. 10(b); Torsvik et al. 1991, this volume, fig. 4(b)], our objective was to point out the dangers of 'incomplete remanence analyses' in palaeomagnetism. Torsvik et al. now provide us with the order of demagnetization steps in their original data. With the information we had in 1989, we qualitatively suggested an alternate remanence model that fits the data. We wish to emphasize the following points. (1) We did not suggest at any time that T is representative of a real palaeomagnetic direction as attributed to us by Torsvik et al. (1991, this volume). (2) We recognized T as a trajectory on the vertical plot only with no corresponding branch in the horizontal plot (Storetvedt et al. 1990, p. 160), and we argued that this is probably the result of partial spectra overlap of two components. With the new information on temperature steps and their sequential order (Torsvik et al. 1991, this volume), we consider our original point as well emphasized. A perusal of fig. 4(a) (broken lines) of Torsvik et al. (1991, this volume) indicates that their interpretation of this sample's data is not sustainable: in computing the high-temperature component they admit that the line above 570 "C was origin-anchored, a procedure which forces the data into a pre-conceived frame. To support their original interpretation of sample L283 Torsvik et al. now submit results from two additional specimens ( fig. 2), but in cases of complex magnetization it is often possible to find individual examples to bolster any pre-conceived view. What really counts is a holistic understanding of the total database (aided by vector analysis). Torsvik et al. state that their 1989 data were in in situ coordinates and the regional bedding correction for the shallow SW directed B component inferred by our alternate interpretation would make the magnetization dip approximately 50" downwards, which they contend is an unacceptable result. But Torsvik et al.'s (1989) preferred hightemperature direction for this sample with Dec. /Znc. of 222/28 transforms to c. 190/80 after correction for bedding tilt and is no less unacceptable. Torsvik et al. (1989) claim that their high-temperature remanence is pre-tectonic, but the direction arrived at for this sample is anomalously steep even for the intermediately inclined palaeomagnetic axis they adhere to. They further argue that we applied improper analytical procedures in our analyses. First we fail to see the ...