Thermal Step Demagnetization Research Articles

Paleomagnetism of the Callander Complex and the Cambrian apparent polar wander path for North America

The 7 km2 circular Callander alkaline complex was emplaced into anorthositic and granitic gneisses of the Grenville Province in the Canadian Shield about 575 ± 5 Ma ago at the start of the Cambrian. The complex has not been subsequently metamorphosed or tilted. Detailed alternating-field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic A magnetization component with a direction of D = 82.2°, I = 82.7° (α95 = 3.1°, k = 83, N = 26 sites) in 5 sites of mesocratic to leucocratic syenite from the core of the complex, in 5 sites of fenitized host rock from its aureole, and in 16 sites of lamprophyre from radiating dikes. Isothermal remanent-magnetization tests show that the A component is retained by both magnetite and hematite in a complete spectrum of domain sizes. A reversals test suggests and a contact test shows the A component to be primary. Its pole position at 46.3°S, 121.4°E(dp = 5.9°, dm = 6.1°) does not fall on published but poorly defined Cambrian apparent polar wander paths, leading to speculation on an alternative Cambrian path.

Canadian Cordilleran displacement: palaeomagnetic results from the Early Jurassic Hazelton Group, Terrane I, British Columbia, Canada

SUMMARY The Early Jurassic Telkwa Formation, comprising the base of the Hazelton Group, was sampled at three areas in the central part of the Stikine Terrane, British Columbia. Detailed alternating field and thermal step demagnetization on samples from nine sites from the Red Canyon area and 10 sites from the Zymoetz River area, both of which are in the Bulkley Ranges, and eight sites from the Telkwa Range area isolate stable remanence magnetization components. The components are interpreted to be primary magnetizations because of the presence of two polarities and the much improved agreement of their inclinations after full tilt correction of the sites from the Bulkley Ranges. Within-locality declinations agree well; between-locality declinations are discordant. The expected cratonic declination is ˜340° whereas declinations from this study are 265°, 227° and 328° for the Red Canyon, Zymoetz River and Telkwa Range areas respectively. These results are similar to those of Monger & Irving (1980) and we concur that the discordant declinations are the result of differential rotation about vertical axes between sample localities. Our observed mean inclination of 52° is in good agreement with Monger & Irving's (1980) result of 54°. They interpreted this to indicate ˜1300 km of northward displacement since the Early Jurassic and their interpretation was substantiated by several later studies. However, with revisions to the geologic time-scale and the Early Jurassic reference pole for North America, the Early Jurassic Canadian Cordillera palaeomagnetic data base appears latitudinally concordant. This indicates that Terrane I and southern Wrangellia were in much the same latitudinal position relative to the North American craton as they are now. Tectonic displacement models developed in view of discordant Cretaceous palaeomagnetic data from the Canadian Cordillera must now consider latitudinally concordant Permian to Early Jurassic data and Middle Eocene to Recent data. This tectonic senario appears more reconcilable with the regional tilt hypothesis. However, this hypothesis cannot account for discordant Cretaceous data from bedded volcanic rocks in Terrane I. It is therefore probable that Cretaceous Cordilleran tectonics were characterized by both regional tilting and moderate latitudinal displacement.

Paleomagnetism of Archean granites and Matachewan dikes in the Wawa Subprovince, Ontario: reevaluation of the Archean apparent polar wander path

Paleomagnetic measurements have been completed on 400 specimens from dated Archean granites and Matachewan dikes in the Michipicoten and Gamitagama greenstone belts in the western Wawa Subprovince of the Superior Province, Ontario. Detailed alternating-field and thermal step demagnetization analyses were used to isolate stable remanence directions. A single-component remanence was isolated within three adjacent dated granitic plutons on the eastern margin of the Michipicoten belt, including the Hawk Lake trondhjemite, the Southern external granite, and the Eastern external granite (HSE). The maximum possible age for this remanence is constrained by the intrusion of the last pluton at 2694 Ma. The corresponding HSE paleopole is located at 10°W, 41°S (dp = 8°, dm = 13°). A second paleopole, NB, is derived from the Northern external granite and the Baldhead River quartz monzonite, which give U–Pb zircon ages of 2662 and 2668 Ma, respectively. Their single-component remanence defines a paleopole on the Archean apparent polar wander path (APWP) at 15°E, 27°S (dp = 8°, dm = 13°), with a maximum possible age of 2.66 Ga. A third paleopole, GD, is derived from the north-northwest-trending Gamitagama diabase dikes and yields a position of 57°E, 41°N (dp = 7°, dm = 14°), which agrees with poles determined by other workers from the 2454 Ma Matachewan dike swarm. The GD pole, along with previously determined Matachewan dike poles, demonstrates that a tectonically stable craton has existed since intrusion of this extensive dike swarm, and it improves the precision of the 2454 Ma Matachewan pole on the APWP. These poles, when compared with coeval poles from the eastern side of the Kapuskasing Structural Zone in the Superior Province, imply no tectonic rotation or translation between the Wawa and Abitibi subprovinces along this Early Proterozoic structure.

Early Permian pole: Evidence from the Pictou red beds, Prince Edward Island, Canada

The mid-Permian red beds of the upper Pictou Group retain a primary remanence in hematite that has been isolated by thermal step demagnetization above 600 °C. Conglomerate, contact, and reversals tests show that the remanence is primary. The pole position of let 42.1°N, long 125.5°E (dp = 1.8°, dm = 3.6°) falls on current apparent polar wander paths at older ages than expected. There is also a significant difference between Early Permian poles from the northeastern United States-Canada and the southwestern United States. The results suggest that postdepositional Permian-Triassic remagnetization may have affected some units in the southwestern United States or that there has been about 9° of relative tectonic rotation between the two regions caused by the Laramide orogeny.

Age of the Firesand River carbonatite complex from paleomagnetism

The 2.3 km diameter Firesand River complex intrudes Archean volcanics and granites of the Wawa Subprovince in the Superior Province about 8 km east of Wawa, Ontario. It has given differing Middle Proterozoic K–Ar biotite ages of 1018 ± 50 and 1097 Ma. Alternating-field and thermal step demagnetization of specimens from three calcific carbonatite sites, five ferruginous dolomitic carbonatite sites, and one lamprophyre dike site isolated a stable mean direction of 290°, 33 °(α95 = 12°). Isothermal remanent magnetization tests indicate the remanence is held by single-to pseudosingle-domain magnetite and hematite in the carbonatite. The dike remanence is Keweenawan in age, thereby confirming its genetic relationship to the complex, and it gives a positive partial contact test with its host rock, indicating no postintrusive remagnetization. The blocking-temperature spectra indicate that postintrusive uplift has not exceeded about 4 km. The pole position for the complex is 183°E, 27°N (dp = 8°, dm = 13°). This pole lies directly on the well-dated Keweenawan apparent polar wander path, giving an age of 1090 ± 10 Ma, in agreement with the older K–Ar age. It also confirms geologic and aeromagnetic evidence that the complex has not been tectonically tilted since emplacement.

Paleomagnetism of the Triassic red beds of the lower Fundy Group and Mesozoic tectonism of the Nova Scotia platform, Canada

The lower Fundy Group red beds of the Wolfville and lower Blomidon formations in Nova Scotia and of Wolfville-equivalent units in New Brunswick are exposed around the Bay of Fundy. These red beds were sampled at twenty-six shoreline sites for paleomagnetic analysis, primarily by thermal step demagnetization methods. Samples from four sites proved too fragile to core; however, coating many of the rest of these poorly-indurated samples and cores with waterglass enabled them to withstand the rigours of coring, measurement and thermal cleaning. A stable hematite remanence was found in all twenty-one coherent sites with normal, reversed and mixed polarities. Conglomerate and fold tests indicate the remanence is either primary or very early diagenetic. Its mean direction is D = 196.5°, I = −3.8° ( α 95 = 7.2°) which converts to a pole position of 45.3°N, 97.1°E ( dp = 3.6°, dm = 7.2°). This pole indicates that the lower Fundy Group was likely deposited in the Early Triassic, rather than in the Middle and Late Triassic, as currently thought. It also indicates that the Nova Scotia platform was rotated about 8° clockwise after deposition of the lower Fundy Group and before extrusion of the North Mountain basalts. The consequences are briefly discussed for two alternative tectonic models of Mesozoic motion on the Minas Geofracture that led to formation of the Bay of Fundy rift as the Atlantic Ocean basin started to open in Early rather than Middle Triassic time. The Fundy Group pole is similar to those from granitic plutons in Maine, thereby increasing the probability that there is a significant error in the mid-Triassic portion of the apparent polar wander path for North America. Finally, magnetostratigraphic methods appear to be a promising way to subdivide and correlate between these red beds.

Paleomagnetism of the Eocene Kamloops Group and the cratonization of Terrane I of the Canadian Cordillera

The lower Middle Eocene (49.4 ± 2.4 Ma) Kamloops Group is exposed in the middle of the Quesnellia subterrane of Terrane I. The group consists of the siliciclastic Tranquille Beds and the overlying Dewdrop Flats plateau basalts and andesites. Detailed alternating field (AF) and thermal step demagnetization was carried out on 282 specimens from 26 flow sites and one conglomerate site, and saturation isothermal remanent magnetization (SIRM) tests were performed to examine the remanence carriers. The petrology of the gently dipping flows, the presence of antiparallel normal and reverse remanence, the conglomerate test, and the fold test all indicate that a primary remanence has been isolated. It resides in both magnetite and hematite over a broad range of AF coercivities, blocking temperatures, and domain sizes. Its mean direction of 355.0°, 73.4 °(α95 = 6.9°) gives a pole position of 138.4°W, 81.4°N (dp = 11.0°, dm = 12.3°) that is statistically indistinguishable from the 50 Ma reference pole for the North American craton. This indicates that the cratonization of Terrane I was complete by the Middle Eocene after it had undergone ~1300 km of northward translation and ~45 °of clockwise rotation since the mid-Cretaceous.

Isolation of early Paleohelikian remanence in Grenville anorthosites of the French River area, Ontario

Specimens were collected at 19 sites from Precambrian anorthosites in the French River region of the Grenville Province. AF and thermal step demagnetization analyses indicate the presence of four components: A, B, C, and D. After bulk AF and thermal demagnetization at 550, 650, and 670 °C, conventional two-tiered statistical analysis reveals only the presence of the B component for the collection. Point density contour plots isolate objectively all four components. Vector subtraction shows that the A component survives up to ~40 ± 10 mT or 300 ± 100 °C and resides in magnetite. Its pole position of 45°W, 26°N, δp = 9°, δm = 10° is consistent with it being a Grenvillian orogenic metamorphic overprint acquired 975 Ma ago at the same time as the K–Ar dates were reset. The B component survives up to 100 mT and resides both in magnetite up to 585° C and in hematite up to 670 °C. Its pole position of 26°W, 13°S, δp = 1°, δm = 2° is ~30° off the pre-1000 Ma portions of the APW path, but falls directly on the 1725 Ma portion. This fits the known whole-rock Rb–Sr and zircon U–Pb ages of 1700 ± 100 Ma found for these rocks, which date the Hudsonian Orogeny. The C component is found only at 650 °C or above. Its pole of 57°W, 27°S, δp = 4°, δm = 7° is 60° away from 1000–1400 Ma portion of the APW path but falls directly on the 1800 Ma path. The D component is isolated only after the 670 °C treatment. Its pole of 1°W, 6°N, δp = 4°, δm = 7° falls on the 1250 Ma segment of the APW path. It is attributed to alteration associated with the Sudbury olivine diabase dikes of this age, which are adjacent to some sites. Thus the Grenville Orogeny is essentially only a thermal event in the anorthosites, and the B and C components were acquired during cooling after the more intense Hudsonian Orogeny. Also population level screening provides a more efficient and objective way of isolating multicomponents than conventional tiered statistics after bulk AF or thermal cleaning of a collection.