Geophys. 1. Int. (1991) 105, 411-489 A palaeomagnetic study of the Builth Wells-Llandrindod Wells Ordovician Inlier, Wales: palaeogeographic and structural implications Allan Trench,'.2 Trond H. Tor~vik,'.~Mark A. Smethurst,2 Nigel H. Woodcock3 and Richard Metcalfe4 ' Department of Earth Sciences, University of Oxford, OX1 3PR, UK Norwegian Geological Survey, PB 3006, Lade N-7002, Trondheim, Norway Department of Earth Sciences, University of Cambridge, CB2 3EQ, UK Department of Earth Sciences, Uniuersily of Lee&, LS2 9JT, UK Accepted 1990 December 10. Received 1990 November 12; in original form 1990 August 22 SUMMARY Discrepancies exist in the Southern British Ordovician palaeomagnetic data set with individual studies indicating either high southerly or temperate southerly palaeolati- tudes. Although previous palaeomagnetic investigations from the Builth inlier have delineated a three-component remanence structure, doubts have been raised concerning (i) the magnetization age of the supposed 'primary' component (P); (ii) the structural setting of the sampling area (Llanelwedd quarry) within a zone of strike-slip deformation; and (iii) whether detailed step-wise demagnetization has fully separated the various magnetization components. In addressing these problems, we present new palaeomagnetic results as follows: (i) a positive palaeomagnetic conglomerate test establishing a pre-Late Llanvirn age for component (P); (ii) an enlarged geographic sampling embracing several tectonic domains indicating that relative rotations linked to strike-slip tectonism are minor within the Builth sampling area; and (iii) evidence that component (P) is uncontaminated by overprints at some sites suggesting the resulting palaeolatitude of 35"s to be representative for the Builth inlier. We also present evidence for local syn-volcanic deformation within Llanvirn volcanics and for intrusion-related deformation of Llanvirn shales. When the revised Llanvirn pole from the inlier (3"S, 4"E) is combined with other data from Southern Britain in a statistical spline analysis, a mean palaeolatitude of 44.5"s is calculated for Builth in Llanvirn times. A palaeoreconstruction for 468-474 Ma (c. Mid-Llanvirn) is presented utilizing this analysis. Key words: palaeogeography, palaeomagnetism, Southern Britain. enigmatic and contentious. This reflects a lack of 1 INTRODUCTION palaeomagnetic studies which employ modern detailed The Ordovician palaeopositions of Laurentia and Gond- demagnetization experiments in combination with appropri- wana are now reasonably constrained to equatorial-trop- ate field tests to constrain the relative age of magnetization. ical and high southerly latitudes respectively, based on Middle Ordovician palaeomagnetic data from Southern palaeomagnetic data (reviews by Torsvik et al. 1990a; Britain imply either mid-southerly palaeolatitudes (Briden, Bachtadse & Briden 1990). In addition, palaeomagnetic Turnell & Watts 1984; Briden et al. 1988) or somewhat data from Baltica suggest it moved northward from c. 60"s higher southerly latitudes (Piper et al. 1978; Thomas & to tropical southerly latitudes during Ordovician times Briden 1976; McCabe & Channel1 1990a). The dissimilar (Torsvik et al. 1990b; Torsvik & Trench 1990). The palaeolatitudes imply competing Middle Ordovician palaeo- latitudinal position of Southern Britain, however, is both geographic scenarios with Southern Britain either forming 477 478 A. Trench et al. an independent microplate in mid-latitudes, or lying have been linked to strike-slip tectonism on scales ranging proximal to Armorica/Gondwana in high latitudes. We refer from entire terranes to isolated fault-blocks (reviewed by to Armonca here as comprising only continental Europe (cf. Beck 1989). In this respect, Trench (1988) and McCabe & Van der Voo 1983) as we include Southern Britain in the Channell (1990a) noted that detailed palaeomagnetic Avalonian terrane (McKerrow 1988; Scotese & McKerrow analyses from the Builth inlier had been restricted to the 1990). fault-bounded Llanelwedd Quarry. We therefore aimed to In order to constrain the position of Southern Britain test for the presence of relative tectonic rotation between within the framework of the Iapetus bordering continents, proposed structural domains at the southern end of the we have initiated palaeomagnetic studies at several key Builth Wells inlier (Woodcock 1987). stratigraphic levels. The present account concerns volcanics Previous palaeomagnetic investigations of the inlier and intrusions of Llanvirn-Llandeilo age from the Builth undertaken by Nesbitt (1967), Piper & Bnden (1973) and Wells-Llandrindod Wells Ordovician Inlier, Mid-Wales Briden 8c Mullan (1984) resulted in the recognition of three (Fig. 1). magnetization components. Briden & Mullan (1984) dated In addition to the palaeogeographic enigmas, we aimed to these components as of Ordovician, Permo-Carboniferous elucidate whether strike-slip deformation of the Builth and Recent age on the basis of an agglomerate test and Wells inlier (Woodcock 1987) produced palaeomagnetically through comparison with reference poles. McCabe & detectable block rotations. Rotations about vertical axes Channell (1990a) questioned the agglomerate test suggesting C WALES So urn r ’ DOLERITE ..,> ...? 0.. : .. FELSPATHIC SANDSTONE .., . ., . .., . KERATOPHYRE 0., . SHALES 8 TUFFS OMITTED CARNEDDAU FAULT 0LLANDEILO - CARADOC SERIES ----_- SILURIAN COVER SEQUENCE CENTRAL DOMAIN UTHERN DOMAIN PARK WELLS FAULT 0 SO0 METRES 05 06 Figure 1. Location of palaeomagnetic sampling sites reported in this study. (a) Position of the Builth inlier, Mid-Wales. (b) Main structural elements of the Builth inlier. Labels are as follows; PF-Park Wells Fault, CFXarneddau Fault. CAF--Cwmamliw Fault, WF-WernTo Fault, ChfF--Cwm Mawr Fault, DOL-Blaenkerry Dolerite (Sites 25-28). Rocks of the Llanvirn series are unshaded. (c) Detailed geology of the Llanelwedd area (southern end of the inlier). Palaeomagnetic sampling sites are numbered. Palaeomagnetic study of the Builth Wells 479 it to contain some non-randomly dispersed directions. imparted with a Molspin pulse magnetizer. Component Subsequently, however, these authors present new evidence analysis of demagnetization data was performed using suggesting the test to be reliable (McCabe & Channel1 least-squares line-fitting algorithms (Kent, Briden & Mardia 1990b), but doubt that magnetization components have been 1983; Torsvik 1986) aided by visual inspection of orthogonal properly isolated in previous studies of the inlier. In order to and stereographic projections. address these uncertainties, this study presents the results of a more extensive sampling of the inlier including a 4 THERMAL DEMAGNETIZATION conglomerate test and two local fold tests. ANALYSIS 2 GEOLOGICAL BACKGROUND Demagnetization experiments identify three magnetization components distinguished by their contrasting directions and Llanvirn-Llandeilo rocks crop out over approximately 70 unblocking temperature spectra. For clarity, we adopt the square kilometres of the Builth Wells-Llandrindod Wells component nomenclature (R, S, P) used by Briden & inlier, Wales (Fig. lb). Llanvirn volcanics and sediments are Mullan (1984). These authors suggest the components to be overlain by shales now thought to be entirely Llandeilo in of Recent, Secondary, and Primary origin respectively. The age (Sheldon 1987). Dolerites and andesites form local, three components have discrete (non-overlapping) thermal mainly concordant, intrusions which were emplaced over a stability in most samples, but overlap of unblocking spectra considerable time period during the Mid-Ordovician (Jones occurs at some sites. The relative contribution of each & Pugh 1948; Piper & Briden 1973). The Ordovician rocks component to the total NRM varies between different are overlain with weak angular unconformity by patchy lithologies. Upper Llandovery sandstones followed by Wenlock Representative demagnetization data are shown in Fig. 2. mudstones and then Upper Silurian sedimentary rocks. Component site-mean data are listed in Table 1. Generally, Major steep faults striking mostly NE and E affect the the keratophyres display intensities of 20-60 mA m-', and Ordovician inlier as do local, open, NE-trending folds. The the dolerites and spilitic basalts <10 mA m-' predominant dip of the Ordovician rocks is to the west however. No cleavage is developed. Some of the faults do Component R (low blocking temperatures) not appear to cut the sub-Silurian unconformity (e.g. CAF and CF on Fig. lb), and an important phase of movement Maximum unblocking temperatures approach 350 "C in the on them must be pre-Llandovery (Jones & Pugh 1941). spilitic basalts but are generally less than 200 "C within the These faults show dextral strike-slip displacements of dolerites. This component is poorly developed or absent in hundreds of metres or more, inferred to be of Ashgill age the keratophyres. Component R is downwardly directed in (Woodcock 1987). Some NE-striking faults in the northwest situ with generally northward declinations (Fig. 2a, b) close corner of the inlier have been reactivated in post-Silurian to the Earth's present field. We interpret R as a viscous time as part of late Caledonian displacements along the remanent magnetization (VRM) of Recent age and Pontesford lineament. therefore do not discuss it further (see Briden & Mullan The area was mapped in detail by Jones & Pugh (1941, 1984). 1946, 1948, 1949) and is covered by a 1:250oO solid geological special sheet (Institute