Tectonic Rotation Within the British Paratectonic Caledonides and Early Palaeozoic Location of the Orogen

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

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. moved from the southerly shallow direction representative However, in the northern Lake District, Caradoc deformation of the Carboniferous-Permian field and applicable to the is identified by folding which occurred during emplacement of Variscan Episode (Table 1 and Fig. 2).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/1/9/4887813/gsjgs.154.1.0009.pdf by guest on 24 September 2021 TECTONIC ROTATION IN THE CALEDONIDES 11

A B ....'APETUS ...- Generalised ,/ /" Taconlc and ;u;r~~1'EA?r Forearc Acadlan Fofds Fig. 1. (a) Directions of normal polarity magnetizations of Late Ordovician (Caradocian--early Ashgill) age in Wales and the Lake District; the large and small arrows are normal polarity magnetizations in1erpreted as primary and secondary respectively. Regional fold trends in the Lower Palaeozoic rocks are also shown. (b) Palaeoreconstruction of the British para tectonic Caledonides south of the Iapetus Suture (Solway Line) following so km adjustment for 55 of Acadian rotation.

Whilst it is likely that rotation between Wales and the Lake Lower Devonian rocks distributed across Britain from District was produced by regional block rotations during Scotland to South Wales identify magnetizations rotated Acadian deformation. more studies will be required to delimit clockwise into a NE - /SW + field axis. Unfortunately most of the timing of this event. Acadian fold trends in the Silurian these results come from 'Old Red Sandstone' molassic deposits infill of the Welsh and Lake District Basins continue to and are difficult to date precisely; they cannot be reliably used highlight the contrast in directions of Late Ordovician mag to constrain the timing of rotation between Wales and the netization (Fig. 1a). In contrast, palaeomagnetic studies of Lake District. The mean Caradoc declination derived only from the pri mary magnetizations in Wales is 301 ± T. The comparable

0 primary field declination in the Lake District is 355 ± 8 • The c. 55" of implied rotation is much larger than uncertainties in the data; as noted above, it is supported by certain overprinted magnetizations and is recorded by change in the regional fold trends. Adjusting for this rotation brings the two volcanic arcs of Caradoc age into alignment. The detail of the reconstruc tion (see Fig. Ib) must account for the back-arc origin of the Snowdonian province (Kokelaar 1988) and the arc origin of the Borrowdale province (Millward et al. 1978); terranes including the Padarn ridge were presumably located in be tween. Collectively these provinces were sited at 30-400 S (Table 1) and produced by subduction of Iapetus oceanic crust close to the northern margin of A valonia. Following volcanic shutdown the Laurentian margin over ode the orogen to produce flexural deformation at the junction of the arcs and initiate a foreland basin; this was fed by axial turbidite input from the east in the Lake District commencing in Late Ordovician times. Flexural deformation occured later behind the arcs (Fig. I b) with turbidite input from the west commencing in Wales during Llandovery times but with major flexural subsidence concentrated in late Pridoli and early • ~~!J~~~~! Devonian times (Woodcock 1990; King 1994). • ~~~~nides Fig. 2. Mid-Ordovician-Early Carboniferous APW path derived Late Ordovician-Devonian Apparent Polar from the British paratectonic Caledonides following adjustment for Wander Path 5Y of tectonic rotation between the Welsh Basin and Lake District The time sequence of palaeomagnetic south poles (an apparent (Fig. I). This path becomes applicable to Britain as a whole polar wander or APW path) determined from the British following the culmination of the Acadian Episode in Late Silurian paratectonic Caledonides is summarized in Fig. 2 from the times. Poles plotted here are based on modern data from thermal data compilation of Table 1. It commences with the Upper demagnetization and component analysis as summarized and coded Ordovician results from the Lake District. Contemporaneous in Table I. Small circles are overprints on coded units. Reference pole positions from the Welsh Basin (Channell 1992; poles of Early Devonian and younger age not included in Table 1 et al. are: SL, Strathmore Lavas, Midland Valley of Scotland (Upper Piper 1995 and references cited therein) are recalculated and Silurian); ORSI, ORS2, 'Old Red Sandstone' (Upper plotted following adjustment of magnetic declinations for 55" Silurian-Lower Devonian), Anglo-Welsh Cuvette; DL, Derbyshire of anticlockwise rotation. This eliminates the difference in Lavas (Lower CClfboniferous); EL, Exeter Lavas (280 Ma). Note poles from the two regions evident in earlier compilations that for clarity only some representative post-Silurian poles are (Trench & Torsvik 1991; Channeli et al. 1992) and identifies plotted although the APW path would not be substantially altered a west to east trend in the Ordovician APW; the track by inclusion of other data. incorporates primary magnetizations assigned to this interval

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/1/9/4887813/gsjgs.154.1.0009.pdf by guest on 24 September 2021 12 J. D. A. PIPER

and is recognized in both the Wclsh and Lake District Caledo nides. With this refinement the succeeding east to west trend of poles (Fig. 2) is essentially the same as that invoked by previous authors with the inclusion of an excursion into higher + + southerly latitudes during Late Ordovician times identified by the data from both regions (Table I). Owing to paucity of igneous rocks, the Silurian APW is less well defined, but appears to show motion of Britain back into lower southerly latitudes (Torsvik et al. 1991). The depth of the loop in this part of the path is subjective because this region is also occupied by Lower Carboniferous poles (Fig. 2, pole DL). The subsequent path (Fig. 2) embraces a wide range of poles from Upper Silurian-Lower Devonian rocks including a NE - /SW + dipolar axis predating Acadian deformation. The succeeding westerly migration is resolved by a shallow E-W palaeofield axis assigned to late Early Devonian times (c. 395 Ma) and identified in surface expressions of the Lake District batholith (Shap and Skiddaw Granites); positive con tact tests and dual polarity imply a primary cooling-related origin whilst a wider record is present within the Lake District as a regional partial overprinting by the batholith (Piper, in press). This axis also postdates all but late brittle phases of • Africa Acadian deformation. ... Australia A closed APW loop is therefore traced out by return to the • E. Antarctica t;:,. Tasman Geosyncline. Early Carboniferous pole position and by subsequent motion SE Australia towards the Late Carboniferous-Early Permian position; the o Patagonia latter is extensively recognized as a major overprinting event Fig. 3. The Mid-Ordovician-Carboniferous APW path derived from bordering the Variscan-Appalachian orogen in both western the paratectonic Caledonides with key ages in Ma (see Fig. 2) Europe and North America. The summary APW path (Fig. 2) rotated into a Gondwana reference frame and compared with thus comprises two major loops and (although some latitudi selected palaeopoles from this supercontinent covering the same time nal motions are involved as documented in Table I) records interval. Stratigraphic ages assigned to the Gondwana poles are predominantly rotational movements of a continental block assigned suffixes: 0, Ordovician, S, Silurian and D, Devonian, lying in mid-southerly latitudes. The senses of the rotations C, Carboniferous and P, Permian with I, m, u indicating Lower, are: clockwise (mid- to Late Ordovician), anticlockwise (Late Middle and Upper chronostratigraphic divisions respectively. The Ordovician to late Lower Devonian) and clockwise (mid Gondwana poles are coded: AFRICA: DG, Damara Granites (458, Devonian to Permo-Carboniferous). The hairpins in the APW Rb/Sr); NT, Ntonya Ring structure (522 K-Ar); GR, Graafwater path correlate with the Late Ordovician and Acadian episodes Formation (01); BL, Blaubeker Formation (471, Rb/Sf); Sa, Salala thus linking changes in the direction of APW to changes Ring Complex (460, Rb/Sf); PAC, Pakjuis and Cedarberg in plate geometry documented by the progressive suturing Formations (Ou/SI); AIR, Air intrusive rocks (480-400 K-Ar, 435 Ar39/Ar40); BA, Bayuda Ring Complex (377, Rb/Sr); BV, Bokkeveld of Laurentia, Baltica, Avalonia and Gondwana (Scotese & Group (Dm); DV, Dwyka Varves (Cu); TR, Tanzanian redbeds McKerrow 1990). (P-C). AUSTRALIA: JF, Jinduckin Formation (01); SS, Stairway Sandstone (Om); MS, Mereenie Sandstone (D?); CB, Canning Basin Comparison with Gondwana rocks (Du); HG, Hervey Group (Du); MP, Mugga Mugga porphyry (414, Rb/Sr); SV, Snowy River Volcanics (D!); CV, Comerong An understanding of the Palaeozoic motions of Gondwana has volcanic rocks (Dm). EASTERN ANTARCTICA: SR, S

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/1/9/4887813/gsjgs.154.1.0009.pdf by guest on 24 September 2021 TECTONIC ROTATION IN THE CALEDONIDES

The constant pre-mid-Devonian configuration implied by CHANNELL, l.E.T. & MCCABE, c. 1992. Palaeomagnctic data from the the correlation in Fig. 3 suggests that the Caledonides were Borrowdale Volcanic Group: volcano-tectonics and Late Ordovician palaeolatitudes. Journal of the Geological Society. London, 149,881-888. separated by a wide continental block (presumably a segment --, --, TORSVIK, T.H., TRENCH, A. & WOODCOCK, N.H. 1992. Palaco of the Euro-Siberian landmass) from Gondwana. The match magnetic studies in the Welsh Basin-recent advances. Geological Magazine, ing between the mid-Ordovician ..... Silurian ..... Early Devonian 129, 533-542. poles from the para tectonic Caledonides (Fig. 2) with the KING, L.M. 1994. Subsidence analysis of Eastern Avalonian sequences: implica contemporaneous Gondwana poles (Fig. 3) identifies the tions for lapetlls closure. Journal 0/ the Geolocial Society, London, 151, orogen as a peripheral part of a large supercontinent. Since 647-657. the arc distance between the Late Ordovician and late Lower KOKELAAR, P. 1988. Tectonic controls of Ordovician arc marginal basin volcan Devonian hairpins in the Caledonide path (Fig. 2) is the same ism in Wales. Journal 0/ the Geological Society, London, 145, 759-775. MCCABE, C. & CHANNEl.l., l.E.T. 1990. Palaeomagnetic results from volcanic as that between Gondwana poles of the same age (cf. Siluro rocks of the Shelve Inlier, Wales: evidence for a wide Late Ordovician Devonian poles from SE Australia and Patagonia in Fig. 3), no lapetus Ocean in Britain. Earth and Planetary Science Letters, 96, 58-68. major relative movements are recognized between them prior MILLWARD, D., MOSELEY, F. & SOPER, N.l. 1978. The Eycott and Borrowdale to mid-Devonian times. After this (and therefore following the volcanic rocks. In: MOSELEY, F. (ed.) The Geology 0/ the Lake District. Acadian episod,~) poles from the two regions diverge widely Yorkshire Geological Society Occasional Publications, 3, 99-120. (see Figs 2 & .3). Hence the familiar Pangaean location of PERROUD, H. & VAN DER VOO, R. 1985. Palaeomagnetism of the Late Ordovician Britain appears to have been achieved by relative movements Thouars Massif, Vendee Province, France. lournal of Geophysical Re search. 90,4611-4625. between mid-D(~vonian and Late Carboniferous times. Thus a PIPER, l.D.A. 1995. Palaeomagnetism of Late Ordovician igneous intrusions corollary of this palaeomagnetic correlation is that a regime from the northern Welsh Borderlands: implications to motion of Eastern of large scale dextral strike slip (c. 5500 km) occurred between Avalonia and regional rotations. Geological Magazine, 132,65-80. the Eurasian and Gondwanan wings of Pangaea during the ---, NowELL, D.A.G. & CRIMES, T.P. 1995. Palaeomagnetism of the Tan y ensuing Variscan Orogeny to achieve the symmetrical and Grisiau granite, North Wales; evidence for a subvolcanic origin in Late hemispheric geometry of the supercontinent by late Palaeozoic Ordovician times. Geological Journal, 30, 39-47. times. The geological case for this dextral interaction has been SCOTESE, C.R. & McKERROW, S.W. 1990. Revised World Maps and introduc tion. In: McKERROW, W.S. & SCOTESE, C.R. (eds) Palaeozoic Palaeogeog evaluated elsewhere (Badham 1982). raphy and Biogeography. Geological Society, London, Memoirs, 12, 1-21. THOMAS, C. & BRlDEN, l.C. 1976. Anomalous geomagnetic field during the late I am grateful to R. Cave and N.R. Woodcock for suggesting many Ordovician. Nature, 259, 380··382. improvements to earlier versions of this paper and to N.R. TRENCH, A. & TORSVIK, T.H. 1991. A revised Palaeozoic apparent polar wander Woodcock for roting essential revisions to Fig. I. K. Lancaster path for Southern Britain (Eastern Avalonia). Geophysical Journal Inter kindly drafted the figures. national, 104, 227-233. --, --, SMETHURST, M.A., WOODCOCK, N.H. & METCALFE, R. 1991. A palaeomagnetic study of the Builth Wells-Llandrindod Wells Inlier, Wales; References palaeogeographic and structural implications. Geophysical Journal Inter national, 105,477-489. BADHAM, l.P.N. 1982. Strike-slip orogens-an explanation for the Hercynides. TORSVIK, T.H. & TRENCH, A. 1991. Ordovician magnetostratigraphy: Llanvirn Journal of the Geological Society. London. 139, 493-504. Caradoc limestones of the Baltic platform. Geophysical Journal inter BACHTADSE, V. & BRIDEN, l.C. 1990. Palaeomagnetic constraints on the position national, 107, 171-184. of Gondwana during Ordovician to Devonian times. In: McKERROW, W.S. --, SMETHURST, M., BRlDEN, l.C. & STURT, B.A. 1990. A review ofPalaeozoic & SCOTESE, CR. (eds) Palaeozoic Palaeogeography and Biogeography. paleomagnetic data from Europe and their palaeogeographical implica Geological So,:iety, London, Memoirs 12,43-48. tions. In: McKERROW, W.S. & SCOTESE, C.R. (eds) Palacozoic Palaeogeog BRANNEY, MJ. & SCWER, N.1. 1988. Ordovician volcano-tectonics in the English raphy and Biogeography. Geological Society, London, Memoirs, 12,25-41. Lake District. Journal of the Geological Society, London, 145, 367-376. WOODCOCK, N.H. 1990. Sequence stratigraphy of the Palaeozoic Welsh Basin. CAVE, R. & DIXON, R.l. 1993. The Ordovician and Silurian of the Welshpool Journal 0/ the Geological Society, 147, 537-547. Area. In: WOODCOCK, N.H. & BASSETT, M.G. (eds) Geological Excursions in Powys, Centrd Wales. University of Wales Press, Cardiff, 51-84.

Received 20 July 1996; revised typescript accepted 12 August 1996. Scientific editing by Gordon Taylor.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/1/9/4887813/gsjgs.154.1.0009.pdf by guest on 24 September 2021