Journal of'the (ieulogical Society, London, Vo], 154, 1997, pp. 9--13, 3 figs. I table. Printed in Great Britain Tectonic rotation within the British paratectonic Caledonides and Early Palaeozoic location of the orogen J. D. A. PIPER Geomagnetism Laboratory, Department of Earth Sciences, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK ( e-mail: [email protected]. uk) Abstract: Magnetizations of Late Ordovician (Caradoc-early Ashgill) age in the paratectonic Caledonides of Wales and the Lake District are directed westerly and northerly respectively. They identify c. 55' of relative rotation of probable Acadian age. Southward subduction of Japetus ocean crust occurred in Ordovician times beneath a latitudinally oriented orogen sited in mid-southerly latitudes with the Borrowdale and Snowdonia Volcanic provinces forming parallel fore-arc and back-arc lineaments respectively. Mid-Ordovician to mid-Devonian palaeomagnetic poles from the orogen correlate with contemporaneous Gondwana data to identify a former location sited west of the South American perimeter and not near to North Africa as assumed in previous reconstructions. Keywords: Caledonides, Lower Palaeozoic, Gondwana, plate tectonics, rotation. Recent palaeomagnetic studies of British Lower Palaeozoic - 69'). Although the significance of palaeomagnetic data rocks (Trench & Torsvik 1991; Channell et al. 1992 and from intrusions emplaced late in the history of this province references contained therein) have sought to quantify the width (Thomas & Briden 1976) has yet to be resolved, adjustment of of the Iapetus Ocean and document its demise during Caledo­ the mean direction (D/I= 143/ - 71 ') for tilt on the northern nian orogenesis. In these assessments the two outcrops of the limb of the Llyn Synform from where these data were derived paratectonic Caledonides south of the Solway Line (Wales and yields a direction (c. 280/ - 68°) comparable with results cited the Lake District) have been considered as an single entity above. integral with the Neoproterozoic basement of England and In contrast Late Ordovician (early-mid-Caradoc) magneti­ Wales, and by implication a peripheral part of the Eastern zations from the Lake District are rotated clockwise by c. 60' Avalonian microcontinent. Extension of palaeomagnetic from the contemporaneous results in Wales (Fig. la). Palaeo­ studies into Upper Ordovician (Caradoc-Iower Ashgill) rocks magnetic field tests on autobrecciated lava, lava c1asts in tuff permits a comparison of primary magnetizations of the same horizons, and on caldera collapse structures show that lavas age from Wale, and the Lake District. It is now possible to and sheets of the Borrowdale Volcanic Group retain primary evaluate the palaeogeography of this segment of the Caledo­ remanence. The volcanoclastic strata, however, have a nian Orogen a::ld unify aspects of volcanism, sedimentation volcano-tectonic overprint postdating downsag that produced and tectonics e:nbraced by the Late Ordovician and Acadian the Scafell Syncline late in the volcanic cycle (Channell & tectonic episodes. McCabe 1992). Lavas and sheets in the Eycott Volcanic Group (mean direction of magnetization DII= 5/ - 43°) and Lower Caradoc-Early Ashgill palaeofields (DII=347/ - 48') and Upper (D/I=342/ - 51 ') divisions of the Borrowdale Volcanic Group record normal polarity through­ In mid-Ordovician (L1anvirn) times dual polarity palaeofields out. This observation constrains emplacement of the Lake in the Welsh sector were close to north-south in direction District volcanic successions to the single long episode of (Trench et af. 1991). By Late Ordovician (mid-Caradoc, Soud­ normal polarity embracing latest L1andeilo to mid-Caradoc leyan) times this palaeofield had rotated to a WNW - /ESE+ times (Nemagraptus gracilis and the earlier part of the diplo­ direction of predominant normal polarity. This is recorded by graptus multidens biozones, Torsvik & Trench 1991); it appears the Moel-y-Golfa intrusive andesite (declination/inclination to exclude emplacement during the preceeding Llanvirn and (D/J) =294/ - SW) and the slightly younger Shelve (D/I=292/ L1andeilo epochs of frequent reversal. - 59') and Breidden (D/I=314/ -73') dolerites (Piper 1995). Migration of the orogen into higher southerly palaeolati­ Remanence in the former unit is demonstrably primary and tudes apparent in the Welsh data noted above (see palaeolati­ magnetizations in the dolerite suites predate late Caradoc­ tude, le in Table 1) is also recognized from the tectono-thermal early Ashgill regional folding. The Stapely Hill Volcanic Mem­ overprint pervasive in the volcanoclastic formations (D/I=347/ ber (early Llanvirn) in the Shelve Inlier records pre-folding but - 62" Channell & McCabe 1992) which, however, retains the apparently secondary remanence (McCabe & Channell 1990) typical northerly declination (Fig. I). This regional Caradoc close to the pervasive regional dolerite suite (DII=296/ - 68'); orientation is supported by pre-folding remanence in the it therefore probably also records a palaeofield of Caradoc age. Carrock Fell layered gabbros (DII= 17/ - 58°) genetically Since Britain then lay in mid-southerly latitudes, directions linked to the Eycott Group (Piper in press). referred to above and plotted in Fig. la are of normal polarity. In North Wales comparable declinations are identified in the Upper Ordovician igneous province (Fig. I a). They include the The paratectonic Caledonian margin Tan-y-Grisiau granite (D/I=305/ - 60', Piper et al. 1995) and a In the Welsh Borderlands the Late Ordovician magnetizations regional overprint in the Snowdon Volcanic Group (DII=299/ are orthogonal (Piper 1995; Fig. 1) to the Late Ordovician fold 9 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/1/9/4887813/gsjgs.154.1.0009.pdf by guest on 24 September 2021 10 J. D. A. PIPER Table 1. Summary of mid-Ordovician··Early Devonian palaeomagnetic results ji-om Caledonian terranes of Wales and the Lake District Pole dp/dm Code Rock unit Age 'N 'E n Q (a) Lake District terrane EVGI Binsey Volcanic Group Om-u -5 347 7110 30'S 6 EVG2 High Ireby Vole. Group Om-u -14 359 5/8 21'S 6 GCP Great Cockup Picrite OU,458 -9 315 4/8 17'S 4 BVGI Borrowdale Vole. Group OU,457 -5 14 7/9 28'S 6 CFGI Carrock Fell Gabbros Ou 5 344 3/5 39'S 6 BVG2 Borrowdale Group, o/p Ou* 8 6 8/11 43'S 4 CFG2 Carrock Granophyre Ou 20 24 11113 50'S 5 TSJG St John's Granite OU,438 20 318 6/7 45'S 6 CFG3 Round Knott Dolerite Ou -4 339 5/9 29'S 4 HF Howgill Mudstones SI -14 310 7113 14'S 3 BIE Eskdale Granite o/p 429* - 22 2 5110 12'S 4 CFG5 NE-SW Dykes, Eycott >CI -14 327 13/22 ITS 4 SHI Skiddaw Hornfels 399* 9 259 7114 3'S 5 SGI Shap Adamellite DI,394 7 264 2/5 9'S 7 (b) North-central Wales terrane Rotated polet 'N 'E BVI Builth Vole. Group, south Om -3 4 7110 35'S 5 6 318 BV2 Builth Vole. Group, south Om - 16 4 4/6 25'S 5 5 301 BV3 Builth Vole. Group, north Om -3 18 14117 54'S 6 19 337 MYG Moel-y-Golfa Andesite Ou II 50 5/8 31 'S 5 -6 4 SVM Stapely Volcanic Member Om* 27 36 7/8 51 'S 4 14 I SD Shelve Dolerites Ou 20 46 7/9 58'S 6 3 5 TYG Tan y Grisiau Granite Ou 15 37 5/7 40'S 5 3 355 BDC Breidden Dolerites Ou 27 22 19/22 41 'S 6 21 350 o/p, overprint. *Units recognized as regional overprints. tPole positions adjusted for 55' of counterclockwise rotation about a local vertical axis. 0, S, D and C in the age column refer to Ordovician, Silurian, Devonian and Carboniferous assigned ages respectively and I, m, u refer to Lower, Middle and Upper divisions. le is the palaeolatitude. Q is the quality factor and is assigned from seven criteria comprising: I, well-determined magnetization age with no reason to believe that it is significantly different from the rock age; 2, sufficient number of samples (> 25) and high enough precision (k> 10); 3, demagnetization undertaken; 4, positive field tests; 5, sufficient structural control with no suspicision of local rotations; 6, presence of reversals; 7, lack of similarity with younger palaeopoles. Pole positions are summarized in Channell et al. (1992), Trench & Torsvik (1991) and Piper (1995 and in press). structures (Cave & Dixon 1993). The influences of Late the Carrock Fell Complex and prior to deposition of the Ordovician and Acadian deformation are readily differentiated mid-Caradoc (Longvillian) Drygill Shales. in this region by proximity of the lower Ashgill-middle Llan­ Thus a corollary of the palaeomagnetic evidence is that the dovery unconformity. Late Ordovician magnetizations are also influence of Acadian folding in the paratectonic Caledonides orthogonal to the large scale regional buckle folds in North needs to be distinguished from deformation during final ocean Wales and the Lake District. However, in these latter regions closure in Late Ordovician times. The latter event is recognized the age of deformation is contentious. Although majority as the cause of prominent overprinting because post-folding opinion regards folding in North Wales as Acadian, this is by magnetizations with similar directions to the Caradoc primary no means certain because (i) post-folding magnetizations in the magnetizations are found in the Welsh Borderlands (McCabe Snowdon Volcanic Group have typical Upper Ordovician & Channell 1990), in North Wales, and in the Lake District declinations and (ii) younger members of the dolerite suite, (Channell & McCabe 1992). These overprints do not corre­ which are probably a late pulse of the Late Ordovician activity, spond to any known younger palaeofield directions. Most postdate regional cleavage. Within the Lake District deforma­ significantly, they are rotated by nearly 90° from typical tion in the Borrowdale Volcanic Group has most recently been Acadian field directions (which identify a northeast negative/ attributed to volcano-tectonics late in the igneous episode southwest positive dipolar axis in Britain); they are far re­ (Branney & Soper 1988) and to subsequent Acadian folding.