Harald Walderhaug

Continental break-up and collision in the Neoproterozoic and Palaeozoic — A tale of Baltica and Laurentia

During the Neoproterozoic and Palaeozoic the two continents of Baltica and Laurentia witnessed the break-up of one supercontinent, Rodinia, and the formation of another, but less long-lived, Pangea. Baltica and Laurentia played central roles in a tectonic menage a trois that included major orogenic events, a redistribution of palaeogeography and a brief involvement of both with Gondwana. Many of these plate re-organisations took place over a short time interval and invite a re-evaluation of earlier geodynamic models which limited the speeds at which large continental plates could move to an arbitrarily low value. Baltica and Laurentia probably shared a common drift history for the time interval 750 – 600 Ma as they rotated clockwise and drifted southward from an equatorial position during the opening of the Proto-Pacific between Laurentia and East Gondwana (initial break-up of Rodinia). On their combined approach toward the south pole, Baltica and Laurentia were glaciated during the Varanger glaciations. Although the two continents drifted toward the south pole during the Late Proterozoic, they began to separate at around 600 Ma (rift to drift) to form the Iapetus Ocean through asymmetric rifting and relative rotations of up to 180°. Initiation of rifting on the Baltic margin is marked by the 650 Ma Egersund tholeiitic dykes (SW Norway) which contain abundant lower crustal zenoliths, and the tholeiitic magma was probably derived from a mantle plume. In latest Precambrian time, the final redistribution of Rodinia is characterised by high plate velocities. In particular, Laurentia began a rapid, up to 20 cm/yr, ascent to equatorial latitudes and essentially stayed in low latitudes throughout most of the Palaeozoic. The high velocities suggest either that Laurentia was pushed off a lower mantle heat anomaly originating from supercontinental mantle insulation or that Laurentia was pulled toward a subduction-generated cold spot in the proto-Pacific. Baltica, except for a short and rapid excursion to lower latitudes in the Late Vendian, remained mostly in intermediate to high southerly latitudes and closer to the Gondwana margin until Early Ordovician times. In Early Ordovician times, Arenig-Llanvirn platform trilobites show a broad distinction between the continents of Laurentia/Siberia/North China Block (Bathyurid), Baltica (Ptychopygine/ Megalaspid) and the areas of NW Gondwana/Avalonia/Armorica (Calymenacean-Dalmanitacean). During the Ordovician, Baltica rotated and moved northward, approaching close enough to Laurentia by the late Caradoc for trilobite and brachiopod spat to cross the intervening Iapetus Ocean. Docking appears to have been irregular both in time and manner: the collision between Scotland/Greenland and western Norway resulted in the early Scandian Orogeny in the Silurian (c. 425 Ma), but further south, there is evidence of late Silurian impingement with subduction of Avalonian continental crust (in England and Ireland) below the eastern edge of Laurentia until the Emsian. In the northern Appalachians the main time of collision appears to have been during the Emsian/Eifellian Acadian Orogeny. Recent analyses invalidates the traditional concept of a sustained orthogonal relationship between Baltica and Laurentia across a single Iapetus Ocean throughout the Caledonide evolution. The active margin of Baltica (Scandinavian Caledonides) faced Siberia during the Late Cambrian and Early Ordovician with oceanic separation between these landmasses in the order of 1200–1500 km. This may explain the local occureences of Siberia-Laurentian type Bathyarid tribobite faunas in Central Norwegian Caledonian nappes, earlier interpreted as Laurentia-Baltica trilobite mixing. Subsequent counterclockwise rotation of Baltica transferred the Caledonian margin in the direction of Laurentia by Silurian times, when the two continents once again started to collide to form Euramerica. This rotation, along with the strongly asymmetric opening of the Iapetus at around 600 Ma, demonstrates a complexity in Precambrian-Palaeozoic plate tectonics, i.e. a collage of metastable plate boundaries which have perhaps too often been simplified to an orthagonal Wilson cycle tectonic scenario.

Reconstructions of the continents around the North Atlantic at about the 60th parallel

Abstract Late Carboniferous–Early Tertiary apparent polar wander (APW) paths (300–40 Ma) for North America and Europe have been tested in various reconstructions. These paths demonstrate that the 500 fathom Bullard et al. fit is excellent from Late Carboniferous to Late Triassic times, but the continental configuration in northern Pangea changed systematically between the Late Triassic (ca. 214 Ma) and the Mid-Jurassic (ca. 170 Ma) due to pre-drift extension. Best fit North Atlantic reconstructions minimize differences in the Late Carboniferous–Early Jurassic and Late Cretaceous–Tertiary segments of the APW paths, but an enigmatic difference exists in the paths for most of the Jurassic, whereas for the Early Cretaceous the data from Europe are nearly non-existent. Greenland’s position is problematic in a Bullard et al. fit, because of a Late Triassic–Early Jurassic regime of compression (>300 km) that would be inherently required for the Norwegian Shelf and the Barents Sea, but which is geologically not defensible. We suggest a radically new fit for Greenland in between Europe and North America in the Early Mesozoic. This fit keeps Greenland ‘locked’ to Europe for the Late Paleozoic–Early Mesozoic and maintains a reconstruction that better complies with the offshore geological history of the Norwegian Shelf and the Barents Sea. Pre-drift (A24) extension amounted to approximately 450 km on the Mid-Norwegian Shelf but with peak extension in the Late Cretaceous.

The age and tectonic significance of dolerite dykes in western Norway

Coast-parallel dykes in SW Norway, primarily of Permo-Triassic age, have been linked regionally to the early tectonic evolution of the Norwegian continental shelf. We demonstrate from palaeomagnetic data (mean declination = 206.1°, inclination = — 30.1°, a95= 11.8°) that dolerite dykes in the coastal Sunnfjord region of Western Norway and immediately west of the Devonian Basins are also of Permian (c. 250-270 Ma) age, and not lower- or pre-Devonian as previously advocated. The Sunnfjord dykes appear to be contemporaneous with dykes from SW Norway at Sotra (262 ± 6 Ma) and the oldest dykes from Sunnhordland (260–280 Ma), and geochemical data attest to a transition from sub-alkaline to alkaline magmatism at the dawn of the Mesozoic. The Sunnfjord dykes are not simple records of E-W extension and magma intrusion, but instead represent significant mid-late Permian time markers within a complex zone of fault activation and rejuvenation. Late Mesozoic-Cenozoic magnetic overprinting (mean declination = 348.6°, inclination = + 68.9°, a95=12°) and metamorphic alteration documented by these dykes are directly dependent upon proximity to major E-W brittle faults south of the Hornelen Devonian Basin, hence some motion and related fluid activity do post-date dyke intrusion. The E-W high-angle normal or oblique-slip faults can be regionally traced offshore to the Øygarden Fault Zone. Onshore, these faults truncate the Hornelen low-angle detachment, which in turn cuts folded D evonian strata. These observations, along with evidence for Permian and Late Jurassic-Cretaceous extension from the nearby Dalsfjord region, demonstrate important reactivation of a Late to post-Caledonian detachment and high-angle fault system in Western Norway.

Ordovician palaeogeography with new palaeomagnetic data from the Montagne Noire (Southern France)

A joint palaeomagnetic and Ar-40/Ar-39 study has been performed on two olistolithic blocks from the Cabrieres Wildflysch in the Montagne Noire region of the Massif Central in France. There, andesitic volcanic and volcaniclastic rocks of Llanvirn-Early Caradoc age (ca 470-458 Ma) occur. Despite extensive secondary alteration, destruction of the dominant magnetic mineral phase and Ar-40/Ar-39 whole rock experiments that demonstrate that the volcanic rocks suffered significant argon loss, a positive fold test and the presence of dual polarities suggest that a primary, Ordovician magnetisation has mostly survived. This is one of the few documented cases where the argon system was substantially reset whilst a subordinate set of small, relatively unaltered magnetite grains, probably hosted in silicates, still carry the original, in this case Ordovician, remanence. The new data show that the Montagne Noire region was located at high southerly latitudes (68degrees (+19)/(-15)) during the Mid-Ordovician. This latitude represents the location for NW Gondwana of which the Massif Central was a part. Palaeomagnetic data from all the Central European massifs and terranes demonstrate a close link to the Gondwana Margin during the Lower and Middle Ordovician.

Geophysical investigation of the Honningsvåg igneous complex, Scandinavian Caledonides

The Honningsvåg igneous complex within the Mageroy nappe, northern Norway, has marked magnetic and gravimetric signatures. Palaeomagnetic studies reveal a Siluro-Devonian (Scandian) dual-polarity remanence (NE and up—SW and down; mean declination = 045°, inclination = -38° and a95 = 8.4°). Rock-magnetic and petrological studies indicate a pure magnetite host for the remanent magnetization component. Magnetic-fabric ellipsoids delineate Dl and D2 Scandian structures affecting the area, and demonstrate an internal reclined fold-structure of the Honningsvåg igneous complex. Forward-modelling demonstrates that a short-wavelength, poúitive aeromagnetic anomaly (c. 700 nT) over the northwestern part of the complex is associated with an area of reverse remanent polarity (SW and down). Conversely, areas of normal remanence polarity (NE and up most common) are associated with magnetic ‘lows’ or ‘quiet’ zones. The gravity field in the vicinity of the complex displays a symmetrical positive anomaly of approximately 200 g.u. (wavelength 15 km). This can be attributed to a density contrast of approximately 0.20 Mg/m3 between the gabbros of the Honningsvåg complex (2.93) and surrounding metasediments (2.73). An optimal geophysical and geological model demonstrates that the complex has been tilted through 90°, and now has the form of a steeply dipping reclined fold-structure extending to a depth of at least 6 km. Palaeomagnetic remanence acquisition post-dates emplacement of the Magerøy nappe (late Dl), and probably originated during post-Dl uplift. Brittle, upper-crust, D2 deformation which continued into Devonian times, folded the Honningsvåg igneous complex and introduced a small but systematic dispersion in the palaeomagnetic remanence directions. The magnetization is, therefore, post-Dl but pre-D2 in age, and can be termed ‘syn-Scandian’. Palaeomagnetic and isotopic evidence suggests a syn-tectonic magnetization age of c. 410 Ma. The palaeomagnetic pole (N7° and E344°) resembles that from the underlying Kalak nappe complex. It is therefore inferred that the Scandian emplacement of the Magerøy nappe was synchronous with a thermal event which affected the older and underlying Finnmarkian-defor-med Kalak nappe complex.

Palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks: a case study from the Palaeozoic Ballantrae Ophiolite, Scotland

Abstract Palaeomagnetic studies of the Ballantrae Ophiolite, southwest Scotland, are assessed in the light of “palaeomagnetic” and “rock magnetic” reliability criteria. The remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction. “Rock magnetic” reliability criteria are found to be ambiguous when treated in isolation. Original grain size effects are detected in magnetic properties across three sections through pillow lavas. We propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability. Thermomagnetic analyses and microscopic observations suggest three remanence-carrying magnetic phases exist within the extrusive layer of the ophiolite. These are almost pure magnetite, hematite and titanomaghemite. Palaeomagnetic, rock magnetic and geological observations are best satisfied by a pre-obduction magnetisation age. This study links the remagnetisation of an ultra-oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested.