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Geophysical framework and the Appalachian-Caledonide connection

R. T. Haworth, R. Hipkin, R. D. Jacobi, M. Kane, J. P. Lefort, M. D. Max, H. G. Miller & F. Wolff

SUMMARY: Gravity and magnetic data from the Appalachian-Caledonide area demonstrate the overall continuity of the orogen while identifying its segmentation into areas of contrasting structural style. The extension of this segmentation into 'drift'-covered areas (e.g. the southern U.S.A.) and offshore areas (e.g. around Newfoundland and the British Isles) provides an orogen-wide framework into which structural detail established at outcrop can be accommodated. Linear gravity and magnetic anomalies often reflect contrasts within the Precambrian basement that have controlled tectonic events to the present. Such basement structure has been investigated by deep seismic profiling, which has also identified large- scale thrusting throughout the Appalachians and the Caledonides with some thrusts even extending into the upper mantle. Speculation to greater depths based on conductivity and P- wave travel-time residuals suggests that traces of the early Palaeozoic collision zone may still exist in the lower crust and upper mantle beneath the northern Appalachians. The loading imposed by thrust sheets during that collision produced foreland basins in the eastern USA whose form and sedimentary record indicate the magnitude and duration of thrusting. Palaeomagnetic results suggest transcurrent movement in Devonian-Carboniferous time, but the early Palaeozoic collisional choreography has not yet been uniquely defined.

Participation by geophysicists in the work of the not coincide at all closely with a single bathy- International Geological Correlation Program metric contour and is generally far seaward of the Project 27 'The Appalachian-Caledonide Oro- 500 m contour chosen by Bullard et al. (1965). In gen' began in 1978, 4 years into the project. At addition to trying to fit the real outlines of that time it was recognized by all participants continental crust on both sides of the Atlantic, that many types of geophysical data available geophysicists have also attempted to match both onshore and offshore throughout the orogen specific features within the continental crust. could provide pictorial evidence for the continuity Continuity of features between the Appalachians of the orogen which was not available as easily and Caledonides is therefore both a consequence (or at all) with any of the geological data sets. of and an aid to making pre- (Mesozoic) drift Initially the efforts of the geophysicists were reconstructions of the N Atlantic. directed towards the compilation of gravity and Several reconstructions with little major differ- magnetic data in the Appalachians of the USA ence have been used to demonstrate the continuity and Canada (Haworth & MacIntyre 1975, Hood of geophysical lineations between Appalachia & Reveler 1977, Zietz & Gilbert 1980, 1981) and and Caledonia and to provide a framework composite maps on the same scale and projection within which the hypothesized continuity of as the Tectonic Lithofacies Map of the Appalachian structure could be examined (Lefort & Haworth Orogen were published (Williams 1978, Haworth 1978, Lefort 1980, 1983, 1984, Haworth 1981, et al. 1980, Zietz et al. 1980). These served both Jacobi & Kristoffersen 1981, Lefort & Van der to identify the geophysical characteristics of the Voo 1981, Haworth & Jacobi 1983). Interpreta- tectonic lithofacies units hypothesized by Wil- tion of these compilations suffered from three liams to be common along the length of the major problems: (i) the highly dissected nature of orogen and to demonstrate the extension of these the European continental margin across which it units beneath the coastal plain cover and to the was hoped to follow these lineations, and within edge of the continental shelf. which area the publicly available geophysical From the earliest days of speculation about data were considerably less abundant than on the continental drift, continuity between the Appa- N American margin; (ii) the uncertainties regard- lachians and Caledonides had been hypothesized, ing post-Caledonide, pre-Jurassic movements but only in 1965 did Bullard et al. (1965) attempt (e.g. Kent & Opdyke 1978, Swanson 1982); (iii) to demonstrate physically the degree of fit by the overlap and low-angle divergence of trends matching specified bathymetric contours from associated with post-Caledonide tectonism. The each side of the Atlantic. Unfortunately the edge latter problem is particularly severe when trying of the continental crust as we now know it does to use gravity and magnetic data for regional

From HARRIS,A. L. & FETTES,D. J. (eds), 1988, The Caledonian-Appalachian Orogen, Geological Society Special Publication No. 38, pp. 3-20. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

4 R.T. Haworth et al. extrapolation of Caledonide trends within the ides are therefore only preliminary in nature and UK. Hipkin & Hussain (1983) have used the can be expected to improve significantly in the available seismic reflection data offshore from next few years. Scotland to strip the effects of post-Caledonide As the geophysical contribution to Project 27 sedimentary basins and reveal Caledonide struc- has developed, so has recognition of the difference tural trends, the investigation of whose continuity between the geophysical and geological definition between Britain and Scandinavia must be a prime of suture zones and terrane boundaries. The target for future research. geophysical definitions are generally based upon Compilation of geological data in N America the integration of a physical response over the on which to demonstrate the continuity of thickness of the crust, whereas the geological Appalachian structure was no more easily accom- definition is almost always as a line or zone at the plished than for the Caledonides in Europe--the Earth's surface. The geophysical definition has number of U.S.A. state and Canadian provincial the advantage that it can often indicate the dip surveys to be coordinated were more numerous of the suture. However, the occasional 'disagree- than the national surveys in Europe. However, ment' between the geophysical identification of the N American geophysical programmes were the plan view of that dipping crustal zone and primarily national in origin so that compilation the geological identification of the outcrop of one of geophysical data was easier in N America than boundary within it has been known to hamper in Europe. Differing national specifications in communication between the disciplines. Greater Europe made the situation even more difficult. In emphasis will therefore be given in this paper to magnetics, for example, the aeromagnetic surveys interpretation of deeper structure and the overall of neighbouring countries in Europe have been geophysical recognition of terrane boundaries flown at different heights, at different times and within the orogen. Because deep seismic profiling with different regional fields removed, making it is dealt with in the companion paper by Cook et almost impossible to merge the predominantly al. (1988) we shall concentrate on techniques analogue data sets. Discontinuities along national other than seismic reflection. Most geophysical boundaries therefore compound the problems techniques (palaeomagnetism and geochronology associated with discontinuities onshore and are the prime exceptions) do not yield data that between offshore data sets. can be interpreted to give the timing of tectonic Major efforts within the U.K. have recently activity. Since palaeomagnetism is also dealt with produced a uniform file of gravity data (Hipkin in a companion paper (Briden et al. 1988) and the & Hussain 1983, Hipkin et al. 1986), and analogue chronological development of the orogen as a magnetic field data are being digitized. 1 : 250 000 whole is the objective of the rest of this volume, scale maps are available for all the surveyed land interpretative remarks on these subjects will be and marine areas of the UK except for a few kept to a minimum. coastal areas where merging of the two data sets The Caledonide-Appalachian and Hercynian has proved troublesome. Magnetic and gravity orogenies had two distinct phases, each with around Ireland, N of Scotland and in the North different temporal and geographical extents. The Sea is neither uniform nor universally releasable, 'sutures' representing closure of the Iapetus and covering as it does the hydrocarbon exploration Theic Oceans are parallel in the southern and areas of several countries. However, considerable eastern U.S.A. with consequent Hercynian reac- progress has recently been made with the compi- tivation of Appalachian structures. The 'sutures' lation and release of such data for Ireland and its diverge in southeastern Canada so that in neighbouring continental shelf (Max et al. 1982, Europe-Africa the two elements are distinct. The 1983). In Scandinavia, high-level reconnaissance southern (Appalachian-Hercynide-Mauritan- magnetic surveys (Geological Survey of Sweden ide) connection is discussed by Lefort et al. (1988) 1983) have been succeeded by low-level high- and will therefore be avoided in this paper except resolution surveys (Wolff 1981) directed towards where tectonic overprinting or rejuvenation ne- mineral and hydrocarbon exploration, and there- cessitates mention of it. Some alternatives to the fore restricted in their release. The efforts of the interpretations expressed in this paper about the Bureau Gravim6trique International (1982) in Avalon terrane in particular are to be found in assembling worldwide gravity data and the the paper by Lefort et al. (1988). objectives of the European Geotraverse Project (Mueller 1983) are additional stimuli for the Continuity of geophysical character preparation of uniform geophysical maps of along the orogen Europe which will have immediate benefits for Caledonide investigations. The compilations of The gravitational expression of the Appalachians data referred to here for the European Caledon- has long been recognized as primary evidence for Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

The Appalachian-Caledonide connection 5

ABBREVIATIONS

Ad ...... Adirondaks AI ...... Alabama Ar ...... Ardennes BS ...... Brendan Seamount CFZ ..... Chariie Fracture Zone CH ...... Cape Hatteras CS ...... Clare Seamount DB ...... Dingle Bay EA ...... East Anolia F ...... Froyabanken FI ...... Florida G ...... Gander Ga ...... Georgia GHB ..... Gael Hankers Bay GId ..... Greenland GM ..... Gulf of Maine GRF ..... Gilbert River Fault HB ...... Hare Bay I ...... Ireland JMFZ .... Jan Mayen Fracture Zone KMB .... Kings Mountain Belt KOF ..... Kong Oscars Fjord ...... ,, ::~..-... / KRL ..... Knud Rasmussens Land KS ...... Kelvin Seamounts L ...... Labrador / Lfl ...... Lofoten Islands LI ...... Long Island Md ..... Maryland Me ...... Maine MS ..... Menai Straights MT ..... Moine Trust NB ...... New Brunswick Nfld ..... Newfoundland NfS ..... Newfoundland Seamount~ .....i~i!ii},, NS ...... Nova Scotia NY ...... New York Oy ...... Orkney Pa ...... Pennsylvania PL ...... Protogine Line

...... PP ...... Porcupine Promontory ,~iiiil;i!i!i?i~ii!~ iiii!{i!iiii!¸¸ ~: PS ...... Porcupine Seabight iii~ii : R ...... Rosslare RT ...... Rockall Trough S ...... Soroy Sc ...... Scotland ~,i~i~i~ii,~iiiiiiii~,iill~ iii!~~!ii~i~ ¸ ~iiiiii!i ¸ Sf ...... Sogne~orden Sh ...... Shetlands St. L ..... St. Lawrence T ...... Trondheim Tx ...... Texas Va ...... Virginia Ves ..... Veslerolen VP ...... Voring Plateau

FIG. 1. Features of the N Atlantic borderlands in their early Mesozoic pre-drift positions. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

6 R.T. Haworth et al.

I' / 7" m °%~o

ANOMALIES Magnetic Highs ...... Gravity Highs ....~iiii~iiiiiiii~iii~iii~iii~i~ii

ABBREVIATIONS AA ...... Assembler Anomaly CA ...... Collector Anomaly KS ...... Kelvin Seamounts NfS ...... Newfoundland Seamoonts

~ ,!I!!I!G~~'

FIG. 2. Prominent gravity and magnetic anomalies of the N Atlantic borderlands (see text for details). Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

The Appalachian-Caledonide connection 7 continuity of structure along the orogen. Recent continental margins of the Labrador Sea and the maps for the entire conterminous USA (Simpson Rockall plateau in the vicinity of which it should et al. 1982) show the Appalachian gravity high as have its landfall. A series of gravity and magnetic the longest most continuous anomaly in the highs in northwestern Britain appear to be related country and second only in prominence to the to the northwestern (Grenville) continental mar- mid-continent gravity high. Despite such promi- gin. The most prominent of these highs is a nence and its more detailed presentation (Ha- lineation running SSW immediately to the W of worth et al. 1980) for direct correlation with the the Shetlands (Tully 1983). This anomaly would tectonic lithofacies map of the Appalachians at first sight appear to be a prolongation of the (Williams 1978), the origin of this gravity anom- line of the Moine thrust, although the latter has aly remains unclear. The positive anomaly is little gravitational expression on land. However, paired with a negative anomaly on its western the higher anomalies might indicate Mesozoic margin whose origin has been attributed amongst intrusions along this old line of weakness. Where other things to a sedimentary basin covered by this anomaly dies away at the shore it is replaced over-thrust crystalline rocks (Woollard 1939) or farther W by an equally large anomaly correlative to a depression of the crust into the mantle with the Outer Isles thrust (McQuillin & Watson (Diment 1968). The positive anomaly was corre- 1973) which continues SSW to become tangen- spondingly attributed to the uplift of dense lower- tial with the continental margin on the southeast- crustal rocks (Diment 1968). Thomas (1983), by ern margin of the Rockall trough. This two- comparison with the paired anomalies observed element lineation is the most prominent of the over suture zones in the Canadian Shield (Gibb linear highs on the NW British continental shelf. & Thomas 1976), suggested that it marked a Approximately 60 km to the SW of each element collisional suture zone dipping southeastward is another linear high of about half the amplitude. beneath the inner Piedmont. Despite the non- In Shetland this high correlates with the zone of uniqueness of gravity modelling, Hutchinson et greenstones and serpentinized rocks (McQuillin al. (1983) also support this interpretation. Follow- & Brooks 1967) which is interpreted to form part ing the interpretation of dipping seismic reflectors of a post-Cambrian ophiolite suite (Flinn et al. identified by deep seismic reflection programmes 1979) and is bounded on its western margin by a in the eastern USA and Canada as thrust planes thrust fault (Flinn 1977). On the mainland the (Cook et al. 1979, Granger et al. 1980), gravity correlative high lies over Lewisian basement interpretation has proceeded to identify the line rocks W of the Moine thrust. This double band of maximum gravity gradient with the eastern of anomalies continues northeastward from the edge of Precambrian continental crust onto which Shetlands merging with a gravity and magnetic the allochthonous sedimentary sequences were anomaly apparently associated with the continen- thrust (Cook 1984a, b). This is consistent with tal margin. This anomaly continues northeast- the situation in western Newfoundland where ward (Talwani & Eldholm 1972) until it is the gravity gradient follows the eastern edge of truncated at a point that lies approximately on a the Precambrian outcrop in the northern penin- line that is the extension of the trend of the Jan sula (Weaver 1967, Haworth 1975). However, Mayen fracture zone near Froyabanken. gravity models for the central part of Newfound- The merging of the 'Shetland anomaly' with land (Haworth & Miller 1982) have been inter- the 'edge anomaly' at 62°N, 0°W, the northeast- preted as indicating that that part of the island is ward continuation of the 'merged' high along the entirely allochthonous above the continental crust trend of the 'edge anomaly' rather than following (Karlstrom 1983) which would require Precam- the Scottish trend, the disturbance of the merged brian basement to extend E of the line of gravity anomaly by an ESE-trending low towards Sog- gradient, and this interpretation is supported by nefjorden and the location of the Brendan deep seismic reflection data (Keen et al. 1986). seamount (Smythe et al. 1983) suggest that this Miller (1984) disputes this, indicating that the point marks the northeastern limit of the western gravity and refraction data for central Newfound- edge of the Scottish Caledonides. Several recon- land are consistent with a thicker crust in that structions of the northern N Atlantic fit the Knud area indicative of a rooted or autochthonous Rasmussens Land promontory into the embay- 'oceanic' zone. We can therefore say that through- ment of the Norwegian margin offTrondheim. If out eastern N America the line of maximum this is correct there is excellent parallelism or gravity gradient is the minimum eastward extent continuity between the Outer Isles thrust trend, of Grenville crust. the Shetland trend and the western thrust margin N of Newfoundland the line of maximum of the Caledonian orogen in Greenland. Gravity gravity gradient swings sharply offshore and data for eastern Greenland are unfortunately not merges with anomalies associated with the available and the aeromagnetic data have not yet Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

8 R.T. Haworth et al. been published and so the geophysical character fracture zone NW of the Voring plateau and, of that margin and its northward extension according to the reconstruction by Talwani & cannot be examined. Eldholm (1977), would have a landfall in Green- The western edge of the Appalachian-Cale- land between Kong Oscars Fjord and Gael donide orogen therefore has a geophysical expres- Hankes Bay. Offsets are indeed recognizable as a sion that can be followed as a more or less change in the trend of the Greenland Caledonides continuous feature from the emergence of the and by contrasts within the Scandinavian Cale- Appalachians from beneath the coastal plain donides (Stromberg 1976) as discussed later. cover in Alabama to eastern Greenland. Between southern New England and New- The eastern margin of the orogen is by no foundland the eastern continental margin of means so clearly marked geologically nor so Appalachia is the Avalon terrane (Williams & directly identifiable geophysically. It is known Hatcher 1982, Skehan & Rast 1983). This terrane geologically in only scattered areas throughout has a clear geophysical signature in Newfound- the orogen: Virginia?, southern New England, land which can be used to define its extent eastern New Brunswick and Newfoundland, SE throughout Atlantic Canada (Haworth & Lefort Ireland ?, Wales ? and then clearly in Scandinavia. 1979). Such characteristics can also be used to The base of the thrust sheet in Scandinavia can interpret its extension into the USA, although be seen along the coastal zone and this base is Lefort et al. (1988) have inferred a history for the part of the 'southeastern' Caledonian continental USA elements of the Avalon terrane that is an margin. Inshore of the interpreted northwestern alternative to that described below. escarpment of the Voring plateau and seaward of The continental shelf S of Long Island has a the Lofoten Islands and Vesterolen lies a linear geophysical character that is almost identical band of correlative magnetic and gravity anom- with that of the Avalon terrane of Newfoundland, alies, both positive and negative. The positive including lineations that continue onshore into anomaly coincides approximately with what the Avalon terrane of southern New England Talwani & Eldholm (1972) identify as the shelf (Haworth 1979). In both these areas the north- edge at a water depth of approximately 200 m. western limit of the Avalon zone is marked by Inshore of these linear anomalies is a second truncation of a series of linear sub-parallel band of intense positive anomalies associated positive magnetic anomalies indicating Precam- with the Proterozoic granulites in the Lofoten brian volcanic highs. In some areas where the archipelago (Wolff 1983). Positive anomalies lineation is not oblique to the terrane boundary, over Soroy (Brooks 1970) are of similar amplitude the minimum westward extent of the terrane is to those over the Lofoten and Vesterolen islands. marked by the westernmost anomaly band. In Talwani & Eldholm (1972) show that there is no Newfoundland truncation of the anomalies is direct continuity between the anomalies, but it relatively clear and is emphasized by its juxtapo- seems likely that they have a common source in sition against the Gander terrane with which a the basement rocks or the ultramafic elements of pronounced gravity and magnetic low is associ- the allochthon thrust over them. The interpreted ated. This northwestern margin of the Avalon cross-sections of S~roy (Brooks 1970) are extre- terrane is clearly marked geophysically to the mely similar to those of the eastern margin of the edge of the NE Newfoundland shelf at the northern peninsula of Newfoundland onto which inshore edge of the Charlie fracture zone (Ha- the Hare Bay ophioliticcomplex was thrust (Sriva- worth 1977). Its southeastern margin is just as stava et al. 1977). It might therefore be implied clearly followed by means of a major magnetic that this anomaly marks the northwestern edge of anomaly (the Collector anomaly (Haworth 1975)) the eastern continental margin of Caledonia. trending E from New Brunswick, across Nova Talwani & Eldholm (1972) suggested that the Scotia and the southern Grand Banks to the Lofoten-Vesterolen anomaly has a similar intra- inshore end of the Newfoundland seamounts. A basement source to that of the linear anomaly similar anomaly occurs at the southern edge of running from N of Shetlands to Froyabanken. the geophysically defined Avalon terrane S of They infer that the intra-basement structure has Long Island where the equivalent of the Collector controlled the subsequent development of the anomaly, denoted here the Assembler anomaly, shelf edge. If this is the situation and there is an follows the edge of the continental shelf. Haworth offset between the two linear anomalies along a (1979) infers that this 'edge anomaly' is therefore line trending NW from Trondheim, a similar a precursor, rather than a consequence, of the offset might be anticipated in other elements of development of the continental margin at a the Proterozoic basement of Scandinavia and crustal discontinuity, similar to the interpretation Greenland. Such a line would be a landward by Talwani & Eldholm (1982) of Scandinavian extrapolation of the trend of the Jan Mayen 'edge anomalies'. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

The Appalachian-Caledonide connection 9

S of about 39°N the location of the Avalon 1981) renders interpretation of upstanding base- zone is not clear on geophysical grounds. Mag- ment uncertain. This zone extends across the netic and gravity anomalies in the Carolina Slate Irish Sea to the Lleyn Peninsula and N Wales, Belt are not atypical of those in the Avalon NE of which its continuity is concealed by the terrane of New England and Canada, but they effects of the Palaeozoic cover, particularly the tend to be lineated in far narrower zones and cut Cheshire basin and its northwestward extension. by N- and NW-trending dykes. The undoubted By comparison with the structure of eastern change in geophysical character at the Kings Canada this line could be the northeastward limit Mountain belt is perhaps the most likely correla- of the stable craton of 'Avalonia'. Max et al. tive with that at the northwestern margin of the (1983) have proposed that such a boundary to an Avalon terrane farther N. Whereas on-land imbricate tectonic zone involving slices of Ava- exposure of the Avalon terrane is widest in lon-related rocks lies 10-30 km farther N along a Newfoundland (approximately 150 km), its geo- line from Dingle Bay to Rosslare. The deep physically inferred width is considerably more seismic reflection profile WINCH (Brewer et al. (over 500 kin). Extension of the Avalon terrane 1983) shows a series of reflectors with a north- in the southern Appalachians eastwards to the westward component of dip whose uppermost 'shelf-edge' anomaly S of Cape Hatteras and surface reaches the sea-floor at the southernmost southwards to the possible equivalent of the of the two suggested boundaries. The Menai Collector anomaly that skirts the northern state Straits line (Barber & Max 1979), which is boundary and northwestern coast of Florida regarded as the boundary between the stable would therefore seem possible. However, Wil- craton margin and the imbricated marginal zone, liams & Hatcher (1982) have subdivided that may be the onshore continuation of the WINCH area into two terranes: the Avalon terrane and reflector. the Brunswick terrane. Gravity and magnetic data are unable to The eastern margin of Appalachia, presumably demonstrate the continuity of the Avalon terrane the western margin of the Avalon terrane, farther to the E. Thorpe et al. (1984) report little therefore has a more variable and less distinct geological or geochemical evidence for basement geophysical character with which to trace its older than 900 Ma in this area. Skehan & Rast continuity or extrapolation. Fortunately, NE of (1983) propose an extension of this Avalon terrane Newfoundland the character is most pronounced, beneath E Anglia into the Ardennes, and al- and since the boundary correlates with the though regional trends in the gravity and mag- western end of the Charlie fracture zone (Ha- netic data do follow such an ESE line there is worth 1977) the eastern end of the latter should little evidence to suggest that their sub-surface mark its European landfall. The Clare lineament origin is within Avalonian terrane. (Dingle et al. 1982, Megson 1983) is the most E of or overlying the Avalon terrane in N prominent feature at the eastern end that might America is the Meguma 'terrane' (Schenk 1983) be correlative with the northwestern edge of the whose lower Palaeozoic sequence may be equiv- Avalon terrane. The lineament has an associated alent, in the British Isles, to the Welsh basin magnetic high that can be traced ESE across the (Kennedy 1979). Meguma and Avalon rocks in Porcupine Bank and the Porcupine Seabight to Nova Scotia are in fault contact, with several reach the 'stable' margin as a series of high- hundred kilometres of transcurrent movement amplitude short-wavelength anomalies near postulated to have occurred along the contact in 51 °N 11 °W. These anomalies have been regarded Devonian time (Keppie 1982). Continuity of by Lefort & Max (1984) as an extension of the magnetic and gravity anomalies associated with Mesozoic volcanics E of the elbow of the both the Meguma and Avalon terranes in the Porcupine Seabight or as representing upstanding Gulf of Maine strongly suggests that the Meguma magnetic basement. On the margin S and E of group overlies Avalonian basement. However, Ireland the gravity anomalies in particular are this could be the result of thrusting, the inferred well lineated and trend northeastwards similar to transcurrent movement therefore being compat- those in the Avalon terrane of Canada. Although ible with observations at the edge of the thrust its continuity from 51°N 11°W is not as good as sheet. elsewhere in Avalon terrane, this boundary The along-strike continuity of the western edge anomaly has its best continuity with a zone of of the orogen and the easternmost Appalachian high gravity and magnetic anomalies running terranes as proposed by Williams & Hatcher almost along the S coast of Ireland and cutting its (1982) is therefore supported on geophysical southeastern corner correlative with the Rosslare grounds; indeed, geophysical data were the prime complex. However, the presence here of Mesozoic reason for their making the hypothesis. Further basic intrusions and volcanics (Roberts et al. extension of those terranes and mapping of the Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

x o R.T. Haworth et al.

ii!iiii¸ !i~!i!i!iiii!ili!ii~i~i~ ~ ~i~ ~

O01i, O0000 TE: ~NJ~~ iBP•

} / / / / / /

~J

PROMONTORIESAND EMBAYMENTS .... North American Margin eooooooo Baltic Margin AP ...... Alabama Promontory BP ...... Brendan Promontory HE ...... Hebrides Embayment LE ...... Lofoten Ernbayment LPP ...... Labrador-Porcupine Promontory NE ...... Newfoundland Embayrnent NYP ...... New York Promontory OE ...... Ouachita Ernboyrnent PE ...... Pennsylvania Embayment QE ...... Quebec Ernbayrnent SLP...... St.Lawrence Promontory SP ...... Shetland Promontory TeE ...... Tenessee Ernbayrnent I TE ...... Trondbeim Ernbayrnent TP ...... Texas Promontory ,4 VaP ...... Vi~jinia Promontory VP ...... Vofino Promontory

FIG. 3. Postulated promontories and embayments of the late Precambrian-Palaeozoic continental margins of the N Atlantic borderlands (see Thomas (1983) and text for details). Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

The Appalachian-Caledonide connection I I central Appalachian terranes is possible on the The Labrador-Porcupine promontory (new basis of compilations of gravity and magnetic name) is the largest promontory N of Texas, with data made on pre-drift reconstructions of the N an offset of approximately 600 km (Haworth 1980, Atlantic borderland. Such discussion has been Williams & Max 1980). In much the same way published elsewhere (see the introductory section that N of the Alabama promontory there are no for references) and cannot be adequately de- more major promontories for approximately scribed within the scope of this paper; we shall 1000 km, there are similarly few offsets within instead concentrate on the interruptions to the the Caledonides of the British Isles. Interruptions continuity of the terranes which are perhaps more in continuity may have occurred close to the important in deducing the tectonic evolution of southern margin of the Rockall trough but these the orogen. must be relatively minor. It might therefore be inferred that the style of Caledonide tectonism within Ireland and Scotland might be as uniform as that observed in the southern Appalachians. Segmentation of the Appalachian- Immediately N of Scotland there is a distinct Caledonide orogen break in the gravity and magnetic lineations and a corresponding change in trend of the shelf edge The curvilinear continuity of the Appalachian that argues for a Hebrides embayment and a orogen is believed to be the result of oceanic Shetland promontory. Transforms are also sug- closure and convergence against a continental gested by the interruption in trends in the vicinity margin whose edge was irregular in plan, either of the Brendan seamount and NE from Trond- because it developed along valleys between triple heim. These would appear to be promontory and junctions (Rankin 1976) or because of its separa- embayment respectively, although of the Baltic tion into rift and transform segments (Thomas rather than the N American craton. The existence 1977). The promontories and embayments (con- of a Voring promontory and Lofoten embayment vex and concave respectively towards the ocean) might further be suggested on geophysical on the ancient continental margin result in the grounds. development of recesses and salients (concave and convex respectively towards the craton) in the orogenic belt (Thomas 1980, 1983). In general, Inheritance and rejuvenation we prefer the precursive rift and transform 'interpretation' of evolution (Thomas 1977) but In N America the promontories and embayments recognize, by analogy with the modern margin of on the ancient (Precambrian) margin controlled eastern N America, that the ancient margin is the development of the (Palaeozoic) Appalachian unlikely to have been rectilinear in outline. Just structure. This structure then provided foci for as the present continental margin has geophysical the subsequent development of the present anomalies associated with it which change char- (Mesozoic) continental margin. For example, the acter abruptly at each promontory or embayment, New York promontory on the ancient margin so we might expect to see these in the now produced, as a result of collision, the Appalachian cratonized margin. What we need not see, New York recess. Avalonian trends also follow however, W of that ancient margin is offsets or that recess pattern suggesting that the Avalon recesses and salients unless the craton had been terrane was deformed during collision (Lefort et subject to much earlier faulting or continental al. 1988). The southern extent of Long Island collision/agglomeration respectively. Avalonia is indicated by an E-W magnetic high The major promontories on the ancient margin, (here called the (American) Assembler anomaly, now part of N America and each identified by a equivalent to the (Canadian) Collector anomaly prominent change in trend of the Appalachian on the southern Grand Banks) that subsequently geophysical anomalies, are the Alabama, New acted as a locus for the development of a transform York and St Lawrence promontories (Thomas fault. 1983). The change in trend of the anomalies at This mimicry of ancient margins by younger the Virginia promontory is far less prominent. margins was discussed and modelled for the St Between each of these promontories the style of Lawrence promontory (Haworth 1974, 1975, Appalachian tectonic development was quite Haworth & Keen 1979), but in its case the modern different, and the degree of deformation varies as transform margin leading into the Newfoundland does the width of the orogen along its length. fracture zone does not follow the Collector Recognition of other promontories by geophysi- anomaly at the southern edge of the Newfound- cal means will therefore indicate where changes land section of the Avalon terrane, possibly in tectonic style might be anticipated. because of geometrical constraints on the Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

12 R. T. Haworth et aI. early opening history (all transforms must be lineaments, which are seen by Landsat (Lindh parallel to each other but not necessarily perpen- 1980) and which parallel the Klaralven line, part dicular to the rift zone). However, seamount of the regionally defined Protogine zone (Gor- chains did develop at the oceanward limits of the batschev 1980). The left lateral offset in the zones Collector and Assembler anomalies; these were of crustal ages (Stromberg 1976), which is also the Newfoundland and Kelvin seamounts respec- inferred from an offset in the aeromagnetic tively. The northwestern edge of the Avalon anomaly pattern (Riddihough 1972), is entirely terrane has clearly acted as the locus for the consistent with the angularity of the present development of the Charlie fracture zone (Ha- continental margin along the Jan Mayen fracture worth 1977). It is probable that the terrane zone and apparently reflects the latter's inheri- boundary on the European margin is likewise the tance of cratonic discontinuities. Riddihough reason for the marked dissection of the craton (1972) similarly notes the coincidence in trend into the Rockall and Porcupine Banks. The and continuity in the boundary between the Labrador-Porcupine promontory on the Gren- Karalian and Svecofennian fold belts and the ville margin occurs where a major Precambrian Senja fracture zone offnorthern Norway (Talwani gabbroic body mapped by Eade (1962) and & Eldholm 1977), implying similar tectonic bounded on its southern margin by the Gilbert inheritance. River fault intersects the Labrador margin (Ha- Few Caledonide trends are highlighted by plots worth et al. 1976). This might be an example of of the seismicity of northern New England (Foley the situation proposed by Rankin (1976) in which et al. 1984) or of the British Isles (Turbitt 1984, embayments are created on the margin along the 1985). The most obvious of all the seismicity two active arms of a triple junction, with the trends in the vicinity of Britain is associated with gabbroic body in Labrador representing the failed the graben structures of the northern North Sea, arm (aulacogen). whose activity might be a consequence of Just as the modern margin of N America has relaxation along boundary faults following glacial developed in mimicry of the early Palaeozoic loading (Browitt, personal communication). The margin, so offsets in earlier margins might be general quiescence of the Caledonide orogen recognizable within the craton. The St Lawrence invites a sense of security that is periodically Promontory, having developed within the Gren- dispelled by events such as those in New ville Province along a line that is the projection Brunswick in 1982 and in Wales in 1984. of the trend of the boundary between the Superior Considerably more effort is needed to monitor and Churchill Provinces, is the most likely the low-level earthquake activity of the orogen in candidate. However, the Grenville Province, order to locate those zones that have the potential which has been inferred to have collided with the for high-energy release and to develop seismo- Superior-Churchill craton (Dewey & Burke 1973) tectonic models from which practical risk can be shows little offset at that margin and no internal determined. Such models would demonstrate the structure that might provide a locus for the extent to which rejuvenation of Caledonide subsequent development of the St Lawrence structures is of modern practical concern. Promontory. The pattern of offsets in the Scandinavian margin suggests several inherent boundaries Deep structure within the craton. The most obvious of these follows the projection across Scandinavia of the The elements of the orogen discussed so far have trend of the Jan Mayen fracture zone (Riddi- primarily been deduced from potential field data hough 1972). This line follows a prominent which refer either to relatively shallow structures boundary in the regional gravity field of Scandi- (less than 10 km) or to the characteristics of an navia (Wolff 1983) coincident with a prominent entire cratonic block. Deep seismic reflection and boundary in the magnetic map of Scandinavia refraction data (Cook et al. 1979, Brewer et al. (Eleman et al. 1969, Geological Survey of Sweden 1983, Keen et al. 1986) which have had a 1983). The lineament approximately coincides revolutionary effect on the interpretation of with an age boundary within the craton (Strom- Appalachian and Caledonide geology are dealt berg 1976) and a change in crustal thickness with elsewhere (Cook et al. 1988), but other (Husebye & Bungum 1981). It also appears to techniques are available to provide clues to the coincide with the location of earthquakes re- deeper structure of these cratonic blocks. corded during the 1970s and has the same trend Analysis of the inductive response to geomag- as the direction of their maximum horizontal netic variations of different frequencies provides compression (Slunga 1981). The trend is 10°-20 ° a means of deducing conductivity variations with counterclockwise of the trend of topographic depth (Gough 1983). For example, controlled- Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

The Appalachian-Caledonide connection 13 source induction experiments (Connerney et al. understanding of the evolution of the Appalachi- 1980) and gradient analysis of geomagnetic ans by pointing out the physical consequences for fluctuations (Connerney & Kuckes 1980) dem- the loading of the craton by the extensive onstrate the existence beneath the Adirondacks thrusting proposed, especially as interpreted for of a highly conductive layer whose upper surface deep seismic reflection profiling. Cook (1984a) lies at a depth of 22 km. The distribution of analysed the gravity data to indicate the location geomagnetic-variation stations throughout the of crustal discontinuities beneath these thrust eastern U.S.A. and Canada is now such that sheets in the southern Appalachians. Quinlan & regional interpretations of these data are possible Beaumont (1984) have instead deduced the load (Greenhouse & Bailey 1981). In the S central that must have been applied to the margin in U.S.A. the coastal effect that dominates induction order to produce the foreland basins W of the vectors elsewhere along the eastern seaboard is Appalachians. The stratigraphy of these basins somewhat subdued, probably as a result of the is sufficiently well known that the magnitude of extensive over-thrusting seen on deep seismic the thrust sheets and the time of their emplace- reflection profiles. A highly conductive zone in ment can be deduced, giving an independent the middle to lower crust underlies Pennsylvania check of the hypothetical geological models. and southern New York. This zone is roughly Quinlan & Beaumont (1984) conclude that in coincident with the great thickness of Palaeozoic middle Ordovician time the thrusting was wide- platform rocks, and the zone is relatively free of spread between the New York promontory and seismicity (Greenhouse & Bailey 1981). Its north- Georgia. By late Ordovician time the thrusting ern and western edges are best defined as the was confined to the Pennsylvania embayment zone of highest conductivity, and its shape shows and, following an early Silurian phase of wide- excellent parallelism with the Pennsylvania em- spread thrusting, the loads in this northern bayment. The existence of such a crustal block segment continued to increase, reaching a peak with significant geophysical properties is substan- in the middle and late Devonian. During the tiated only by combining interpretations of early Carboniferous no over-thrust loads were diverse data, each of which may be quite imposed on the foreland, but the loading returned tentative. That, however, is the normal situation during the middle to late Carboniferous, particu- when deducing deeper Earth structure. larly in the vicinity of the Virginia and Alabama Farther N in Atlantic Canada the complica- promontories. The cumulative over-thrust load tions of tectonics and the present coast line make calculated using this method has a maximum interpretation of the induction vectors difficult. thickness of 18 km in Virginia, Maryland and SE However, Cochrane & Wright (1977) tentatively Pennsylvania. identify a zone of enhanced deep conductivity in Such quantification of loading is a powerful eastern Newfoundland. This corresponds to the tool and is complementary to palaeomagnetic location of an inferred subduction zone, for which analyses which have been somewhat rare in these there may be even deeper evidence on the basis areas. The method can also be applied to marine of travel-time residuals for teleseismic PP waves basins such as the lower Palaeozoic basin of the (Stewart 1978). Neither of these techniques can Gulf of St Lawrence where Quinlan (personal be conclusive about the polarity of any residual communication) has shown that the subsidence effect of that subduction zone but each, together of the basin is episodic, with the most significant with other geophysical data, suggests that it dips of these episodes coming in the middle Ordovi- to the SE (Haworth et al. 1978). A similar exercise cian at the time of emplacement of the ophiolite conducted in the Scottish Caledonides indicated suites in western Newfoundland. the presence of deeper conducting zones that correlate with ancient subduction zones deduced on geological grounds (Hutton et al. 1977). Travel- Transcurrent movement along the time delay analysis with which this might be orogen correlated is only at a preliminary stage (E1- Haddadeh & Fairhead 1984). Similar analyses of Although details of the palaeomagnetically de- geomagnetic variation data for Scandinavia are rived movements of crustal blocks within the not known to us, but interpretation is likely to be Appalachian-Caledonide orogen are presented extremely difficult with the major conductivity elsewhere (Briden et al. 1988), a brief mention contrast being close to the coast except in the S cannot be avoided in this introduction or its where further evidence for the offset on the companion paper (Lefort et al. 1987). ancient eastern margin of the Palaeozoic ocean Major movement of the eastern flank of the might be discovered. Appalachians has long been proposed (Roy & Geophysicists have also contributed to an Robertson 1968) and has recently undergone Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

I4 R. T. Haworth et al. more intensive scrutiny including interpretation plains transcurrent movement along the trans- in terms of transcurrent movement along the axis form segment as a result of convergence between of the Appalachians (Kent & Opdyke 1978). the opposing plates. The transcurrent movement Although details of timing and the boundaries is considerably less than has been postulated and the extent of the crustal blocks involved in palaeomagnetically and is localized. Similar the movement are still subject to debate (Roy et analyses elsewhere in the orogen, where the al. 1983), movements of several tens to hundreds geology is better exposed and where palaeomag- of kilometres in Devonian or Carboniferous time netic data are plentiful, must be a necessary are credible. Several thousand kilometres of precursor to any well-substantiated reconstruc- movement have been hypothesized. Independent tion for the early to middle Palaeozoic that evidence for the degree of offset along this fault incorporates pre-Jurassic transcurrent move- zone by means of matching geophysical markers ment. would be highly desirable, but as seen earlier most of the trends are parallel to the early Palaeozoic collision zone which has subsequently been the locus for much of the transcurrent Conclusions movement. Indeed, where geological markers have been found to be offset, there seems to be no Geophysical data demonstrate the serrated con- continuity of the faults or consistency between tinuity of the eastern margin of the Appalachian- movement along them within any zone (e.g. the Caledonide orogen. The western margin of the Great Glen debate (Kennedy 1946)). Whatever orogen does not display such continuity, but movement has taken place will undoubtedly elements of Avalon-like terrane have been iden- remove any correlation between the structure tified throughout the eastern flank of the orogen, observed within the continental blocks on each S of the British Isles. N of the British Isles the side of the transcurrent zone when the orogen is western margin of the orogen shows more restored to its pre-Mesozoic form. It seems continuity, despite offsets that are apparently surprising that there is such a good correlation related to discontinuities in the bordering craton. between structure on opposite sides of the orogen; Geophysical characteristics of the Appalachian in Newfoundland, for example, the Avalonian and Caledonide terranes permit their extrapola- lineations, which have an oroclinal form, parallel tion across continental shelves and beneath the orthogonal form of the Grenville margin, yet coastal plain cover, but work describing this has it is hypothesized that the transcurrent fault zone only been referenced in the paper. Investigation passes between these two structures. Ziegler of conductivity variations in the crust coupled (1984) hypothesizes a reconstruction of the orogen with teleseismic data shows evidence for south- in late Grenvillian time (1000 Ma?) in which the eastward-dipping structures that may be the fossil portion of the Porcupine promontory comprising remnants of the Canadian Appalachian subduc- northwestern Scotland and Ireland is mated with tion zone. Similar evidence supported by deep the St Lawrence promontory. Since the difference seismic reflection profiling indicates southeast- in scale of these promontories is not too great and ward subduction in the southern USA and the New York promontory would match in northwestward subduction on the southern flank location with subsequently developed bending of of the orogen in the British Isles. Gravity and the Avalonian lineations, this juxtaposition might magnetic data are now available for almost the be considered acceptable. However, evidence for entire orogen. Other data such as sedimentary the lineations along which the hypothesized isopachs, seismicity and heat flow collected for transcurrent movement subsequently took place practical programmes (hydrocarbon exploration, is not apparent. Unless such linear transcurrent seismic hazard and geothermal energy respec- faults were present, major 'rhombachasms', for tively) are becoming available and can form the which there is little evidence, would have devel- basis for developing quantitative models for the oped as a result of movement transverse to the evolution of the orogen. Models for the develop- general trend of the highly serrated margin. It is ment of foreland basins and the pressure- therefore difficult to reconcile the postulated temperature histories of their basement as a major transcurrent movement with the geology consequence of thrust loading and subsequent of the orogen. uplift/erosion are already being produced. Phillips et al. (1976) have used the hypothesized ACKNOWLEDGEMENTS : In this paper we have attempted location of Ireland along a transform segment of to provide an overview of the activities of numerous the margin to explain the differences between the earth scientists who have contributed to the geophysical tectonic setting of Ireland and that of Scotland aspects of Project 27. It is as impossible to acknowledge and Newfoundland. Their model, however, ex- the contribution of each as it has been to review their Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021

The Appalachian-Caledonide connection 15 work adequately within the scope of this paper. We paper, to have claimed sole authorship would have hope, however, that sufficient references are provided been misrepresentation and to have ensured that every to allow the interested reader to pursue individual possible interpretation of every observation was cov- aspects further. RTH absolves his co-authors com- ered would have been impossible. The contribution of pletely from responsibility for the opinions that have RTH is made with permission of the Director, British crept into this review/overview paper. To have deleted Geological Survey (Natural Environment Research those opinions would have been further to weaken the Council).

References

The asterisks indicate publications not referred to in the text but considered to be significant concerning the geophysics of the orogen.

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R. T. HAWORTH, British Geological Survey, Keyworth, Nottingham NG 12 5GG, UK. R. HIPKIN, Department of Geophysics, Edinburgh University, West Mains Road, Edinburgh EH9 3JZ, UK. R. D. JACOB1, Department of Geological Sciences, State University of New York at Buffalo, 4240 Ridge Lea Road, Amherst, NY 14226, USA. M. KANE, United States Geological Survey, Box 25046, Federal Center, Denver, CO 80225, USA. J. P. LEFORT, Institut de G6ologie, Universit6 de Rennes, 35031 Rennes C6dex, France. M. D. MAX, Geological Survey of Ireland, Beggars Bush, Haddington Road, Dublin 4, Eire. Present address: Naval Research Laboratory, Washington, DC 20375-5000, USA. H. G. MILLER, Memorial University of Newfoundland, St John's, Newfoundland A1B 3X7, Canada. F. WOLFF, Geological Survey of Norway, Liev Eriksons vei 39, PO Box 306, N-7007 Trondheim, Norway.