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A revised Palaeozoic apparent polar wander path for Southern Britain (Eastern Avalonia)

A revised Palaeozoic apparent polar wander path for Southern Britain (Eastern Avalonia)
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  Geophys. J. Inr. (1991) 104, 227-233 RESEARCH NOTE A revised Palaeozoic apparent polar wander path for Southern Britain (Eastern Avalonia) Allan Trench and Trond H. Torsvik Deparhnenr of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Accepted 1990 September 1. Received 1990 August 20; in srcinal form 1990 May 31 SUMMARY A revised apparent polar wander path APWP) is presented for Palaeozoic Southern Britain. The new path, based on a structural reinterpretation of existing data combined with new data from Wales, differs significantly from previous estimates in the following ways: 1) the 1,ocus of the Ordovician path segment is extended by approximately 25 degrees of arc; (2) revised time-calibration of the APWP suggests more Ordovician APW than previously calculated; and 3) the new path implies Middle Ordovician separation across the British sector of Iapetus to be greater than previous palaeomagnetic estimates based on APWP analyses c. 3300 km; Northern Britain c. 133, Southern Britain c. 45s). Key words: apparent polar wander path, Eastern Avalonia, Iapetus Ocean, Palae- ozoic palaeomagnetism, Southern Britain. 1 INTRODUCTION Previous Palaeozoic palaeomagnetic data compilations for Britain have been made by Briden et al. (Briden, Morns & Piper 1973; Briden, Turnell & Watts 1984; Briden et al. 1988), Piper (1987) and Torsvik et al. (1990a). Of the major tectonic elements in Britain, i.e. i) North of the Great Glen Fault, (ii) South of the Great Glen Fault and North of the Iapetus suture, and (iii) South of the Iapetus suture (Southern Britain), the APWP for the latter block has been the least well defined. The present synthesis seeks to remedy this point, and in this account we present a revised APWP for Southern Britain. In Palaeozoic times, Southern Britain formed a constituent of the Avalonian terrane, which also comprised the Ardennes of Belgium and northern France, the Avalon peninsula of Newfoundland, much of Nova Scotia, southern New Brunswick and coastal New England (McKerrow 1988). In this paper we use the compilation by Torsvik et al. (1990a) as a starting point for discussion (Fig. la). Indeed, Fig. l(a) differs little from the assessments of Briden et al. (1984, 1988) due to a lack of new studies during the intervening period. For clarity, identical pole abbreviations to these earlier studies have been adopted in this study (Table 1, Fig. 1). Figure l(a) comprises three groups of poles which relate to the Ordovician, Silurian-Devonian and Carboniferous- Permian periods. The Ordovician poles plot close to 2S, 10E but we draw attention to three outlying poles from the Shelve Inlier intrusions SH), Carrock Fell Gabbro (CA) and Tramore Volcanics (TV). The palaeomagnetic pole subsequently shifted westwards by Silurian-Devonian times to a ÔcornerÕ position as indicated by determinations from the Old Red Sandstone of Wales ORS) and Silurian Somerset/Gloucester lavas SG). Finally, the south pole ÔbacktrackedÕ into southerly latitudes by Carboniferous-Permian times. The ÔcornerÕ and Ôback- trackÕ poles are also identified in Northern British APWs (Torsvik et al. 1989, 1990a; Trench ef af. 1989) relating to the coherent drift of the British Isles following closure of the Iapetus Ocean. We therefore confine our discussion primarily to the Ordovician period where the Southern British APWP is less clearly defined, notably due to the outlying poles. 2 PALAEOMAGNETIC DATA COMPILATION In our analysis, we re-examined the srcinal publications to cross-check all pole calculations and to assign a quality factor (QF) to each pole (Van der Voo 1988). QF depends on whether an individual study satisfies certain reliability 227  228 A. Trench and T. H. Torsvik Figure 1. (a) Ordovician to Permian poles from Southern Britain south of the Iapetus suture (After Torsvik depicted with the oval of 95 per cent confidence about the mean pole. Squares: Ordovician, diamonds: et al. 1990a). All poles are Silurian-Devonian, circles: Carboniferous-Permian. Poles are listed in Table 1. b) Revised poles from Southern Britain used in he computation of the new APWP. Amended Ordovician poles are indicated with stars. Equal-area projections. criteria (Table l), and can vary between 1 (poor reliability) and 7 (key pole). Our assessment of the poles formed the basis for the subsequent APW analysis. Our minimum requirements for pole consideration were (i) that poles were based on >25 samples, and (ii) that some demagnetization experiments had been performed. 2.1 Revision and assessment of Ordovician poles During the compilation of results, we found that the positions of the three outlying Ordovician poles (SH, CA and TV) ll required substantial revision. The pole obtained from dolerite/andesite intrusions of the Shelve Ordovician inlier, mid-Wales SH), was srcinally reported in in situ coordinates i.e. uncorrected for regional Ashgill (late Ordovician) deformation of the inlier. In fact, many of the intrusions pre-date this folding (Lynas, Rundle & Sanderson 1985) and should be structurally corrected. Reanalysis of the srcinal palaeomagnetic data (Piper 1978) for both andesites and dolerites, yields a positive fold test at the 95 per cent confidence level (Trench & Torsvik 1990) producing an amended pole at 10N 46E (Fig. lb). The Carrock Fell Gabbro pole (CA) was srcinally reported assuming (i) near-vertical intrusion of the gabbro, and, (ii) a lack of major subsequent structural rotation (Briden & Morris 1973). As radiometric studies indicate a Llanvirn age for the gabbro (Rundle 1981; Thirlwall & Fitton 1983), post-Llanvirn folding must be considered if the gabbro has retained an srcinal remanence. A revised structural interpretation for the area (Harris & Dagger 1987) places the Carrock Fell gabbro on the northern limb of a major antiform correlated with Silurian (F2) tructures of Roberts (1971). Correction for this antiform produces an amended pole at 17N 31E (Table 1, Fig. lb). This rotation also restores igneous layering within the gabbro to a near-horizontal attitude (Harris & Dagger 1987). The F2 structure also affects the Binsey Formation (BN) and High Ireby Formation (HI) lavas (srcinally combined in the  230 A Trench and T. H. Torsvik Eycott Group pole; EG) and tectonic correction produces revised poles at 3s 345E and 12s 357E respectively (Table 1). Incidentally, recalculation of the CA pole removes the last conflicting data set to the palaeomagnetic recognition of the Iapetus Ocean in Britain (see Briden & Mullen 1984). A pole quoted by Deutsch (1980) from the Ordovician Tramore Volcanics of SE Ireland proves incorrect when recalculated [the srcinally published pole was plotted by Torsvik et al. (1990a) (Fig. la)]. The discrepancy is most likely a typographic error as the published declination/ inclination is the correct mean of the listed site mean data. The amended pole (11N, 18E is therefore plotted in Fig. We rejected the results of only two palaeomagnetic studies after compiling all the available data. We contend the results of Thomas Briden (1976) from the North Wales intrusions to either record an anomalous field (as suggested by the srcinal authors) or unresolved multicom- ponents. Notably, data from the Cader Idris Basalts (Thomas & Briden 1976) plot close to a Permo- Carboniferous direction in situ and may therefore represent an overprint magnetization. We consider the high inclination results reported by Piper et al. (1978) from Ordovician minor intrusions in W England to be based on inadequate demagnetization studies (as approximately 90 and 40 per cent of remanence remains following ÔdemagnetizationÕ in the published examples). 2.2 New Middle-Ordovician poles Recent palaeomagnetic studies from Southern Britain by McCabe & Channell (1990a) and Trench et al. (1991) further contribute to the available palaeomagnetic data set. McCabe & Channell (1990a) report a pole from Llanvirn (Mid-Ordovician) Stapely volcanics of the Shelve inlier at 27N, 36E (Fig. lb). The Stapely volcanics are intruded by the Shelve dolerites (pole SH above). Although these authors initially favoured a late Ordovician magnetization age, further deliberation suggests a primary Llanvirn srcin (Trench & Torsvik 1990; McCabe & Channell 1990b). A reinvestigation of Llanvirn volcanics from the Builth inlier, mid-Wales, (Trench et al 1990, 1991) confirms a previously reported pole (Briden & Mullen 1984; BU, srcinally 3s 3558). The reliability of the pole, however, is increased as the data set and sampling area are substantially enlarged. Furthermore, additional field tests now unam- biguously establish the magnetization age as pre-Llandeilo (Mid-Ordovician) (Trench et al. 1991; McCabe & Channell 1990b). The poles from the Builth (BU, BI) and Shelve inliers (SV, SH) do not overlap at the 95 per cent confidence level (Fig. lb) which indicates either substantial APW or structural rotation to have occurred. Trench et al. (1991) favour a predominantly structural srcin for the difference in poles noting their similar magnetization ages. As neither pole can be unambiguously established as ÔunrotatedÕ however, no rotation correction about a local vertical axis has been applied prior to APWP calculation. Accordingly, neither study meets the criterion for Ôsufficient structural controlÕ in the assessment of a quality factor (criteria 5; Table 1). This criterion is generally not met for poles of pre-Carboniferous age. 3 APWP ANALYSIS To produce a time-calibrated APWP, we used the method outlined by Jupp & Kent (1987) which aims to fit a spherical smoothed spline to a given data set on a sphere. In order to achieve this, a working age was assigned to each pole (Table 1). This was generally based upon geological information pertaining to the rock age for each respective study. Stratigraphic ages were converted to approximate absolute ages using the time-scale of Harland et al. (1982). In cases where no independent magnetic age constraint was available, and the palaeomagnetic pole falls on a demonstrably younger part of the APWP between well-dated poles, a secondary Ômagnetic ageÕ was assigned. Pole positions were ÔweightedÕ in the spline analysis using their individual error parameters (a95 per cent, Table 1). No key poles were assigned cf. Torsvik et al. 1990a) as no poles achieved a quality factor of 7 (Table 1). A moderate smoothing parameter (200) was assigned in our analysis (see program description detailed in Torsvik et af. 1990b). 4 THE REVISED SOUTHERN BRITISH APWP AND ITS TECTONIC IMPLICATIONS The revised APWP for Southern Britain is compared with its predecessor and with paths for Palaeozoic Northern Britain in Fig. 2(a and b). The revision of poles CA, SH, TV nd BU together with the addition of pole SV yields a significantly extended Ordovician spline which can be traced back into the northern hemisphere. The time-calibration of the path is also affected, notably indicating more substantial Ordovician APW. We note however that unaccounted block rotations within the data set (e.g. poles SV and SH versus BU and BI) may produce an exaggerated APW along a small circle centred on Southern Britain and also distort the absolute timing of the path. Indeed, the raw pole data do not display a smooth east-west migration with decreasing magnetic age indicating some influence of local structural effects (Table 1). We do consider remagnetization of the westerly Ordovician poles as a possibility however as these data are substantiated by positive field tests (criterion 4, Table 1). The Devonian-Permian segments of the old and new paths differ slightly due to minor revisions of magnetic age and smoothing parameter cf. ables 2 and 3 of Torsvik et al. 1990a). Siluro-Devonian time calibration should be treated cautiously given that the APW segment is presently constrained by only two poles ORS, SG). The new path (Table 2) has significant implications for the closure history of the British sector of the Iapetus Ocean, given that oceanic separation can be directly inferred through comparison of the British paths (Fig. 2b). We now estimate a Middle Ordovician width for the British sector of Iapetus to be in the order of 3300 km [comparing the revised Southern British path with the respective Northern British paths of Torsvik et al. (1990a)l. This estimate is significantly greater than previous calculations based upon a comparison of APW paths cf. Briden et al. 1984, 1988; Torsvik et al. 1990a). In conclusion, the revised path reconciles a number of previously anomalous poles in the Southern British data set. We submit that the trend of this path remains better  .. . . Figure 2. (a) A comparison of the old (1) and revised 2) APWPs fo alaeozoic Southern Britain. Equal-area projection. Geological periods are shaded. Paths widths do not represent formal confidence limits. (b) Comparison of the revised Southern Britain APWP with paths from Northern Britain (Torsvik et al. 1990a). N.GGF: North of the Great Glen Fault, S.GGF: South of the Great Glen Fault and North of the Iapetus suture. Tnble 2. Iistine of revised APWP. Poles are auoted at 10 Myr 470-400 Ma) and 20 Myr intervals 400-280 Ma). HACNEI'IC AGE LATITUDE IBNGITUDE 280 -47 343 300 -46 347 320 340 360 380 400 410 420 430 440 450 460 470 -40 -30 -21 -12 -6 -5 -4 -3 -2 -0.5 5 12 341 328 317 309 308 311 318 328 340 354 008 023
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