A Review of Studies of the Holocene Paleomagnetic Record of Canada
The first study of the paleomagnetic record of lacustrine sedimentary sequences was published by Mackereth (1971) for Lake Windemere, England. Subsequently, lacustrine sedimentary sequence studies were carried out for Lake Erie (Creer et al., 1976) and Lake Michigan (Creer et al., 1976). This was followed by studies of Lake Superior (Mothersill, 1979) and Lake Huron (Mothersill, 1981). Based on the records from these four lakes Creer and Tucholka (1982) compiled "type" paleomagnetic records for east-central North America utilizing C14 for age control. Sprowl and Banerjee (1985) provided a comparable record utilizing varved sediments for dating purposes from Elk Lake, Minnesota. Over the past three decades a number of researchers have utilized paleomagnetic studies of the sediments of the Great Lakes Basin to date post- glacial lake stages as well as other events. Similar paleomagnetic studies of lakes in British Columbia were carried out by Turner (1987) and Mothersill (1991, 1994) utilizing Mackereth compressed air corers. In parts of British Columbia the Mt. St. Helens (3300 yrs.BP) and the Mt. Mazama (6845 yrs,BP) tephras provide time control for these records. These records tie in well with the work of Verosub et al. (1986) from Fish Lake, Oregon.
Paleomagnetism and Thermochronometry of the Grenville Province, Canada
The Grenville Province, a marginal orogen of the Canadian Precambrian Shield, extends through southern Ontario, Québec and Labrador, with equivalents in Fennoscandia. Before the Iapetus Ocean opened, the Canadian and Fennoscandian orogens were contiguous. Age-equivalent rocks in small exposures in the Appalachians, Texas and Mexico suggest that the subsurface extent of Grenvillia is comparable to that of the Superior Province, the Archean core of the Canadian Shield. It was therefore a substantial continent in its own right at the time it collided with the Superior, Southern and Nain Provinces. However, most of our paleomagnetic knowledge comes from a narrow zone on the (present-day) northern margin of Grenvillia, comprising the Grenville Front Tectonic Zone of reactivated rocks as old as Archean, the Central Gneiss Belt (CGB) with deeply buried and exhumed Proterozoic rocks, and the Central Medisedimentary Belt (CMB), containing a core of less deeply buried rocks where some pre-collisional paleomagnetic information has survived. 40Ar/39Ar thermochronometry has been vital in determining the age relationships of paleomagnetic overprints (often two, occasionally even three, in individual formations) and establishing the Grenville Track of the Laurentian APWP. Most 40Ar/39Ar mineral ages are in the 850 to 1000 Ma range but there is enough separation between key tiepoint poles to permit clockwise and counterclockwise alternatives for the Grenville "Loop". Of great interest are the short track segments defined by mean poles for different domains of the CGB and terranes of the CMB because these may record the sequence of assemblage and/or burial and uplift. Unfortunately the profound contrast between uplift ages of different CMB terranes, as indicated by their hornblende, mica and feldspar 40Ar/39Ar data, is not borne out by the relatively minor differences in mean poles for the same terranes. The "Grenville problem" is therefore still not laid to rest, and furthermore raises questions about the assumed parallelism between resetting of magnetic remanences and resetting of 40Ar/39Ar mineral ages. One or the other (or both!) seems to be responsive to chemical / thermochemical resetting as well as purely thermal resetting.
Paleomagnetism and geodynamics of the onshore High Arctic Large Igneous Province and its connection to the offshore Alpha Ridge
Here I summarize findings from the University of Rochester paleomagnetic expeditions to the High Canadian Arctic dedicated to understanding the tectonic and magmatic evolution of the region, together with recovering data useful for defining the history of the geomagnetic field. Our work has focused on what we now recognize as two distinct episodes of volcanism: i. massive flood basalt volcanism at ca. 92 Ma and ii. smaller volume, spatially restricted volcanism at ca 77 Ma. Lava flows from the older event have provided key paleomagnetic data that exclude significant latitudinal motion of the Canadian Arctic Islands relative to North America. When combined with other North American sites, these data define a time-averaged dipolar magnetic field. The younger volcanics, preserved in small basins on remote northern Ellesmere Island, may define a failed rifting event; their extent in the Arctic Ocean is uncertain, but they may correlate with dikes reported from northern Greenland. The older flood basalts thicken to the north in the Canadian Arctic (Axel Heiberg Island), near the region where Alpha Ridge of the Arctic Ocean abuts the coastline. Following results of seismic analyses, I interpret this, together with Mendeleev Ridge as a volcanic oceanic plateau, potentially with continental blocks similar in structure to Kerguelen Plateau of the Indian Ocean. Simple hotspot track models fail to predict a tie of this feature to an extant hotspot (although a tie to the Iceland hotspot has been suggested). A new model of plume tilt at mid-mantle depths toward an upper mantle upwelling may better explain the observations.
Geomagnetic Reversal Asymmetry at ∼1.1 Ga: New Tests from Old Rocks
One of the most puzzling and still not fully understood phenomena of the Precambrian field is geomagnetic reversal asymmetry, which is manifested in Keweenawan rocks (∼1.1 Ga) that crop out around the Lake Superior. This reversal asymmetry is also observed in rocks from central Arizona and at the Grand Canyon from Unkar Group rocks. Most but not all of the reversely magnetized lava flows and dikes of this age consistently have characteristic directions of magnetization that are about 40 degrees steeper in inclination than their normally magnetized equivalents, while declinations show the expected 180 degree relationship. If true, this asymmetry may be a sign of a significant non-dipole field at ∼1.0-1.1 Ga. However, other explanations for this phenomenon have also been suggested. Any theoretical developments on the origin of this phenomenon should be based on and preceded by establishing a robust observational basis. Mafic dike swarms associated with the ∼1.1 Ga Mid-Continent Rift System provide an excellent opportunity to study this problem. In particular, mafic dikes of Baraga County in Michigan (the Baraga-Marquette dike swarm) contain a stable remanent magnetization record of both normal and reversed polarities which are asymmetric. During our initial studies we identified several locations which may provide critical data to resolve current controversies regarding the origin of asymmetric reversals. At one location, in particular, a unique diabase dike appears to have cooled during an asymmetrical polarity reversal. At another location, a reversely magnetized Keweenawan dike appears to cross cut a normally magnetized dike, also suggesting an asymmetrical (normal-to-reversed) geomagnetic reversal. We will discuss geological and geodynamical implications of these results.
On the Paleotectonic Evolution of the Pacific Margin of Southern Mexico, the Maya and Juchatengo Terranes and Chochal Formation Guatemala:Insights from Paleomagnetic and Isotopic Studies
In the paleogeographic reconstruction of Mexico and northern Central America, evidence shows that the entire region is a collage of suspect terranes transported from abroad, whose timing and sense of motion are now beginning to be understood. Among these, the Chortis block and the Baja California Peninsula have been proposed as pieces of continent separated from the Pacific coast of southwestern Mexico, that have moved either southeastward by the Farallon plate or northwestward by the Kula plate. Isotopic mineral ages from coastal granites along the coast from Puerto Vallarta, Jalisco (80 Ma) to Puerto Angel, Oaxaca (11 Ma) record systematic decrease of cooling ages from NW to SE. These results also constrain the position of the Kula- Farallon spreading axis north of Puerto Vallarta. Previous studies mainly confined to the northern margin of the Chortis block, confirmed a left-lateral displacement of 130 km in Neogene time. Further studies suggested times of detachment increased to 30 Ma, 40 Ma, and 66 Ma. We conclude that several indicators, namely: (a) the truncated nature of the Pacific coast of SW Mexico; (b) the genesis of the Kula-Farallon ridge at 85 Ma; (c) the 2,600 km of northward transport of Baja British Columbia from the present-day latitude of the Baja California Peninsula, beginning at 85 Ma; (d) the paleomagnetic counterclockwise rotations of areas both in the Chortis block and along the Mexican coast, during Late Cretaceous-Paleogene time, and (e) the systematic NW-SE decrease of radiometric dates beginning at 85 Ma in Puerto Vallarta and ending at approximately 11 Ma in Puerto Angel, Oaxaca , point to this time and region for the onset of strike-slip drifting of the Chortis block toward its current position. On the other hand, in the reconstruction of past movements of tectonic plates, the determination of reliable paleomagnetic poles is of utmost importance. To achieve accurate results, a full knowledge of the rock magnetic properties of the samples is required particularly for Curie points and for grain-size analyses in addition to thermal and af demagnetization experiments. We present the comparative results of 20 sites drilled at 3 different Paleozoic areas: The Permian rocks of the Juchatengo area in Oaxaca, Mexico; the Late Silurian (~418 Ma) Mountain Pine Ridge Granite, the Hummingbird Granite in Belize, and the Early Leonardian Chochal Limestone in Guatemala. The samples of all 20 sites were subjected to AF demagnetization in 16 steps from NRM to 100 mT and the thermally demagnetized cleaned in 15 increasing temperature steps from NRM up to 675 C. Principal component analysis was applied to the samples in order to obtain their respective mean directions. SIRM, hysteresis loops, and coercivity experiments performed indicate that about 90 percent of the samples were characterized by Multi-Domain (MD) grain sizes and the rest were PSD. Curie point determinations results ranged from 190 to 660 C, indicating the presence of titanomagnetites as well as hematite. In the Juchatengo area reliable poles were obtained from 3 sites, in Belize 3 sites and only 2 sites in Guatemala in the Permian Chochal Formation yielded useful results
Intensity variation of the geomagnetic field in Mesoamerica during the last 3500 years
After Nagata's and Bucha's pioneering works in Mesoamerica in the 60th's and 70th's a gap in archeointensity studies in the region seems to be opened, in spite of the abundant well known archeological vestiges available in Mexico. Aimed to improve the insipient reference curve for Mesoamerica, and to contribute to the global intensity database, we present latest archeointensity determinations obtained from oldest Western Mesoamerican archaeological deposits, as well as from pre-Columbian Central and Eastern Mexican archaeological sites. Although still not enough in number to define a variation curve for the region, they outline a tendency that differs to that of model predictions based on previous data.
Paleomagnetic study of Ar-Ar dated lava flows from Tancitaro Volcano and Tacambaro area, The Michoacan Guanajuato Volcanic Field (Western Mexico)
We sampled eleven fresh, apparently not altered outcrops out of 26 sites reported recently while 27 independent cooling units were collected at Tacambaro area belonging to the Michoacan Guanajuato Volcanic Field. All studied sites were dated by means of state-of-the-art 40Ar-39Ar geochronological method and span from 1.23 ± 0.2 ma to present. Low-field continuous susceptibility measurements performed in air show the presence of a single ferrimagnetic phase with Curie point compatible with Ti-poor titanomagnetite. The cooling and heating curves are reasonably reversible. Polished section observations under microscope also confirmed the presence of a near-magnetite phase associated with exsolved ilmenite of trellis or sometimes sandwich texture. In most samples a single and stable component of magnetization was observed upon thermal, alternating field or combined treatments. A secondary component, probably due to the lightning effects was present but was easily removed at very first steps of demagnetization. The mean direction is in agreement with the expected paleodirections for the late Pliocene, as derived from reference poles for the North American plate. Combining the available geochronologic data with the magnetic polarity, better constraints of the age of emplacement are achieved.
Paleomagnetism of Miocenic Rocks Around Chalcatzingo, Morelos, Mexico: A Revaluation
The Chalcatzingo domes southeast of Mexico City have been recently 40Ar/39Ar dated to be around 20.7 Ma old. These rocks and the surrounding Tepexco Volcanic Group are defined in a previous study as showing strongly discordant paleomagnetic directions, about 50º west from the expected Miocene geomagnetic field direction; which prompted the hypothesis of a counter clockwise crustal block rotation, related to a regional left lateral fault system along the Transmexican Volcanic Belt. This rotation is the largest proposed so far for central Mexico, and it is in contrast to data from the nearby Basin of Mexico, which are concordant with the expected field directions. In view of the limited previous sampling and relatively complex remanence record, we have re-sampled the Chalcatzingo and Tepexco rocks to verify the validity of the proposed block rotation. For that purpose samples were collected from 26 sites. Reflected light and scanning electron microscopy were used to determine composition and texture of magnetic minerals, and IRM acquisition and thermomagnetic experiments to determine the rock magnetic properties. Detailed demagnetization was carried out using mainly the AF method to determine the characteristic remanence directions, to calculate site mean directions and VGPs. These are compared to the Miocene reference direction and pole for stable North America, and interpreted in terms of potential tectonic movements of the study area.