Key Paleomagnetic Poles and Their Importance for Precambrian APWPs and Paleocontinental Reconstructions
Paleomagnetic poles are important for understanding and quantifying geodynamic processes. They provide the only quantitative method, using apparent polar wander paths (APWPs), to track the drift of continents in the Precambrian. They can be used to identify periods of rapid continental drift or true polar wander (TPW). They are critical for establishing paleolatitude and testing models of snowball Earth. In addition, they are particularly important for determining paleocontinental reconstructions, and testing reconstructions based on other constraints, such as geological piercing points and bar-codes of the ages of magmatic events. Unfortunately, paleomagnetic poles have proven difficult to use for such studies in the Precambrian, because of poorly-dated rock units and magnetic overprinting. These problems can only be addressed by the use of key paleopoles. Key poles, which currently comprise only a tiny percentage of the Precambrian database, are those that are both well defined and precisely dated. To be considered precisely dated, the rock units from which they are derived must have a precise age (usually based on U-Pb or Ar-Ar methods), and the remanence must be established as primary. There have been a number of recent developments in the application of key Precambrian poles. Rudimentary APWPs based on key Precambrian poles have been developed, including one for the Superior craton in the 2217-1998 Ma interval and another for Laurentia in the 1740-1085 Ma period. They are proving useful as reference APWPs against which key paleopoles (or APWP segments) of similar ages from other cratonic blocks can be directly compared. For example, new key poles from the Slave craton establish with certainty that it drifted relative to the Superior craton during at least a portion of the Paleoproterozoic; those from Baltica demonstrate that Baltica and Laurentia drifted together for several hundred million years in the latest Paleoproterozoic and early Mesoproterozoic. Episodes of TPW or very rapid APW have been proposed at several times in the Neoproterozoic and earliest Cambrian, between ca. 800 and 500 Ma, based on discordant paleopoles that are interpreted to be only slightly different in age. However, a review of the paleomagnetic database indicates that there are as yet an insufficient number of key poles to adequately test these interpretations. Many different reconstructions of Precambrian supercontinents, especially Rodinia, have been proposed using geological constraints and, most recently, by matching bar-codes of ages for magmatic events from different cratonic blocks. The key pole record is still far too limited to test overall Rodinia reconstructions, but does now offer some insights into the relative positions of a few specific cratons.
Critical Baked Contact Tests to Establish a Well-Sampled Early Paleoproterzoic Apparent Polar Wander Path for the Slave Craton
The tectonic prehistory to the 'United Plates' of America, our ancient continental ancestry, is unknown - how were Laurentia's six or seven cratons related to each other (or not) before their dramatic assembly between ca. 2.0 and 1.8 Ga? Paleomagnetism is currently the only available method to quantitatively reconstruct ancient paleogeography. Yet only one of the six or seven United Plates, the Superior craton, presently has a robust apparent polar wander (APW) path spanning Laurentia's prehistory. Considering the first key pole for the Slave craton from the 2.025 Ga Lac de Gras diabase dyke swarm and several heretofore untested poles, the skeleton of an APW path seems to be emerging for Slave - a natural second target after Superior since both cratons are postulated centroids of ancestral supercratons, Sclavia and Superia, respectively, which rifted to spawn many of the Archean cratons dispersed around the globe today. By comparing the Slave and Superior APW paths, we will explore whether these two cratonic clans were indeed independent drifters or part of a larger, globe-girdling supercontient, Kenorland. But the untested remanences from Slave limit such an analysis at present. Here we report results for detailed baked contact tests for the ca. 2.2 Dogrib, ca. 2.1 Indin, and ca. 1.88 Ghost diabase dyke swarms and a ca. 1.85 Ga lamprophyre dyke swarm. Where possible, baked contact tests were conducted using older diabase dykes as host rocks, because diabases are known to carry stable remanences. In each case sampling was carried out along a continuous profile into the host rock at an approximate right angle to the younger dyke contact in an attempt to identify the baked, hybrid, and host magnetizations predicted by conduction theory. Within three years, concerted and coordinated paleomagnetism and U-Pb geochronology should yield a reasonably well-sampled and reliable APW path for the Slave craton. Already multiple pole comparisons to Superior can be made: the APW paths share in common a large and rapid ca. 2170 Ma rotation (Dogrib-Indin and Nipissing-Biscotasing-Marathon swaths) and the 1800 amalgamation, but otherwise the paths imply convergent independent drift histories.
New baddeleyite U-Pb ages for dolerite dykes and sills in the Zimbabwe craton, and implications for a late Archean - early Paleoproterozoic reconstruction of Superior, Zimbabwe and Yilgarn cratons
We present baddeleyite U-Pb ages of Neoarchaean to Palaeoproterozoic dike swarms and the Mashonaland sill province in Zimbabwe. The 2575.0 ± 1.5 Ma age of the Umvimeela dike is indistinguishable from the 2575.4 ± 0.7 Ma result [Oberthür et al., 2002] for a pyroxenite layer of the Great Dike and testifies to synchronous emplacement of the Great Dike and its satellites. Three samples of WNW- to NNW-trending dikes of the Sebanga swarm yielded ages of 2512.3 ± 1.8 Ma, 2470.0 ± 1.2 Ma and 2408.3 ± 2.0 Ma, the latter of which dates the Sebanga Poort Dike of this swarm. These results record at least three separate generations of dikes within the swarm and, more importantly, invalidate previous inferences of a genetic link between the Sebanga swarm and the Mashonaland sills. Crystallisation ages of 1877 ± 2.2 Ma, 1885.9 ± 2.4 Ma and 1875.6 ± 1.6 Ma for three dolerite samples of the extensive Mashonaland sills from different parts of the Zimbabwe craton were obtained. This is the oldest common igneous event that is recorded in the Zimbabwe and Kaapvaal cratons. Collectively with previous published geochronological and petrological evidence in favour of a major 2.0 Ga event within the Limpopo Belt, these results suggest that the Zimbabwe and Kaapvaal cratons did not form a coherent unit (Kalahari) until ca. 2.0 Ga. In the global barcode record for Archaean cratons, all the three generations of dikes within the Sebanga swarm match exactly events of dike intrusion and LIPs in the Superior, Hearne and Kola-Karelia craton, and corroborates the recognition of Superia as a supercontinent in the Neoarchaean. Specifically we propose attaching Zimbabwe on the eastern side of Superior craton, with predictions for tracing late-Archean greenstone belts and the Great Dyke of Zimbabwe event between the cratons. The 2408.3 ± 2.0 Ma result of the Sebanga Poort Dike also provides a link to Western Australia, i.e. to the giant Widgiemooltha swarm of the Yilgarn craton. We conclude that in the context of the global barcode record, both the Zimbabwe and Yilgarn cratons fit into a Superia reconstruction.
Emplacement and Subsequent Displacement of the Sudbury Olivine Dike Swarm, Sudbury, Ontario, Canada
Emplacement of igneous dykes occurs in a brittle stress regime where dyke propagated faults dilate to accommodate the intruded magma. Dykes are emplaced in a plane which is oriented perpendicular to the axis of least principle stress. Also related to the stress regime at the time of emplacement are parameters such as dyke width, and dyke distribution clustering. This paper presents an analysis of the geometric properties of the 1235 Ma Sudbury olivine diabase dyke swarm in the Southern Province; Sudbury, Ontario. Results are based on the application of statistical and interpretive techniques applied to published geological maps. Data derived in this study provides insight into: 1) the paleo-stress environment at the time of dike emplacement, 2) the type of magmatic system present, 3) determine if the rifting model originally proposed by Fahrig (1987) is a valid hypothesis, and 4) mapping any significant disparities that may require classification as sub-swarms. Once a dyke swarm has been emplaced they provide ideal marker horizons for detecting any subsequent tectonic displacement. In the Grenville Province the Sudbury dikes are obviously highly deformed. In the adjacent Southern Province the dikes superficially do not appear have been deformed, suggesting that this area was not affected by the terminal Grenvillian orogeny. In this study we use published paleomagnetic data to demonstrate that while the Grenville orogeny did not produce any folding of the Southern Province it did produce significant vertical displacements on pre-existing regional scale faults.
Microplates and block rotations in High Arctic Canada determined by paleomagnetism of Proterozoic mafic intrusions
The Franklin Large Igneous Province, dominated by a giant radiating dyke swarm in the Canadian Arctic Archipelago, has been dated by the U-Pb method on baddeleyite to 723-712 Ma, and its paleomagnetism has been extensively studied without evidence for any age progression of paleomagnetic directional data. Paleomagnetic poles from Devon and Ellesmere Islands, Nunavut, and from northwest Greenland are significantly different at the 95% confidence level from those reported for the Franklin dykes from elsewhere in the Arctic and are offset from each other by a degree consistent with the tectonic offset suggested by the distribution of dykes in the area, though uncertainties in the poles are too great to define this offset conclusively. The difference in the pole locations can be accounted for, to first approximation, by a simple model of early Cenozoic block rotations between the North American plate, Greenland, and a hypothesized ancient microplate comprising Ellesmere, Devon, and Cornwallis Islands. To produce an optimum fit of the data from both the 'Ellesmere microplate' and Greenland to those of the other Franklin studies, the Ellesmere plate is rotated ca. 20° counterclockwise about an Euler pole nearby and to the south; to satisfy the well-defined spreading history of Baffin Bay and Labrador Sea, Greenland is moved along a different route, implying a plate boundary between Ellesmere and Greenland: i.e., in Nares Strait. A new grand-mean paleopole for the Franklin event, including restoration of Greenland and the proposed "Ellesmere microplate" to North America, is located at (8.4 ° N, 163.8 ° E, A95 =2.8 ° , N=78 sites) and is a key pole for Neoproterozoic supercontinent reconstructions.
Importance of Mafic Dyke Swarms in Tracing the Tectonic Histories of Continents and Their Building Blocks: a Tribute to Henry Campbell Halls
Twenty-five years ago Henry Halls recognized that mafic dyke swarms are unique time markers and are also one of the most valuable rock types in tracing the deformation histories of Precambrian cratons. To understand how dykes can provide deformation histories, his research included such topics as emplacement mechanisms of dykes, the way magma flows in fractures, and how remanent magnetization is blocked in the dyke when it is cooling. Dykes have several advantages over other rocks. First, they occur in every craton and can be traced over long distances using airborne magnetic and satellite image data. Second, they can be dated accurately with U-Pb and 40Ar-39Ar techniques. Taking into account the advantage of dyke cross-cuttings and the baked contact test, the primary nature of Natural Remanent Magnetization (NRM) can be verified. The NRMs of dated dyke swarms are a major source in establishing the past positions of continents. Moreover, dyke swarms often appear to abruptly end at craton margins: the broken arms may be used in assembling the continental pieces back to the once-existing assembly. Using sample profiles across dykes into the country rock and modern microwave heating techniques, Halls and co-workers were able to measure the Earth's magnetic field back to Paleoproterozoic times; a task which markedly contributes to our understanding of the evolution of the Earth's core and dynamo processes therein. Halls gathered the applications of mafic dykes into the first of the series of international dyke conferences which he launched in 1985, and which continue today (the next is in India in 2010). The results of the first conference were published in a volume, "Mafic Dyke Swarms," which he edited with Walter Fahrig in 1987. The first dyke conference evolved into an IGCP project (1987-1992) headed by Halls, that looked at all aspects of dykes on a world-wide basis. Halls was the first to use paleomagnetic and petrographic data of dykes in delineating the tilting and rotations of the Superior and Dharwar cratons. In addition to studying Precambrian deformations using mafic dykes, Halls has developed new techniques for paleomagnetism and rock magnetism. The most valuable is his innovation of using great circles to isolate secondary remanent magnetizations with high precision in cases where standard paleomagnetic methods have failed. The great circle technique was a side product of his study of the origin of curious trends in paleomagnetic directions observed in the Keweenawan Slate Island rocks, which, based on his new data, turned out to be caused by a meteorite impact. This study led Halls to investigate other meteorite impact structures with paleomagnetism, including Gosses Bluff (Australia), Lonar Lake (India) and Lake Lappajärvi (Finland). Since porosity is a key parameter in impactite rocks, Halls and his graduate student M. Pfleiderer developed a novel method which uses ferro-fluid techniques to measure permeability anisotropy. Aside from his scientific innovations, Halls has been a member in the Canadian geoscience community. From his contributions in earth sciences, the Institute of the Lake Superior Geology awarded him the Goldich Medal in 1987. In this presentation, I will outline the use of mafic dyke swarms in tracing the drift and deformation histories of Precambrian crustal blocks as a tribute to Halls, an innovative scientist, a great teacher and mentor for younger generations.