Rex J. E. Johnson

Paleomagnetism and late diagenesis of Jurassic carbonates from the Jura Mountains, Switzerland and France

A crucial fold test for magnetizations measured in Bajocian through Kimmeridgian limestones from the Jura Mountain region shows that rare blue-gray limestones found in this sequence preserve a prefolding magnetization. In contrast, tan limestones, which predominate in this sequence, contain secondary magnetizations acquired after folding in the late Miocene. The postfolding magnetizations in tan limestones appear to be carried in goethite and, because reversed as well as normal polarity directions are present, the magnetization in these limestones is probably a post-Miocene, pre-Brunhes (>700,000 yr) chemical remanent magnetization (CRM). Field relations between tan and blue-gray limestones, which are present in some cases in the same rock unit (for example, the Hauptrogenstein), suggest that the tan limestones may have formed from alteration of blue limestones. Whereas goethite pseudomorphs after pyrite are found in tan limestones, the occurrence of pyrite in the blue limestones supports this model, indicating that goethite could have formed from oxidation of pyrite. Abundant meteoric water probably was available for such an oxidation process during uplift of these strata subsequent to deformation. Natural remanent magnetization in blue limestones appears to be carried in magnetite. The positive fold test for the characteristic magnetization in these rocks is significant at the 99% confidence level (k values for 24 site-mean directions before and after tilt correction are 9.3 and 43.2, respectively). The simplest interpretation for the origin of observed remanence in magnetite in the blue limestones is a primary magnetization of depositional or biogenic origin. Characteristic directions in these rocks yield a mean paleopole position of 77.7N, 148.4E (K = 26.0, A95 = 5.9). This pole is in reasonable agreement with one other Upper Jurassic pole from western Europe and with other Middle to Upper Jurassic poles from the Atlantic-bordering continents after rotation of these continents to a reconstructed configuration for Middle to Late Jurassic time.

Early Paleozoic paleogeography and accretionary history of the Newfoundland Appalachians

New paleomagnetic results were obtained from several units in the Central Mobile belt of the north-central Newfoundland Appalachians. Combined with recently published data, the results place important limits on interpretations of early Paleozoic paleogeography and evolution of the Appalachian-Caledonian orogen. The Ordovician Appalachian margin of North America, located at low paleolatitudes and facing roughly south, was separated from the southern margin of Iapetus (Avalon-Armorica) by ∼3500 km. Primary directions in pillowed volcanic rocks of the Moreton9s Harbour Group that are part of an Early Ordovician island arc show that these rocks were formed near the North American margin, at a paleolatitude of 12°S. Early to Middle Ordovician volcanic rocks of the Roberts Arm, Chanceport, and Summerford groups, on the other hand, were extruded at paleolatitudes of 30°-40°S. Unconformably overlying volcanics of the Silurian Botwood Group were extruded subaerially at a paleolatitude of 24°S. Combined with tectonostratigraphic data, these results suggest the following tectonic history. In Early Ordovician time, northerly subduction of the main Iapetus basin was accompanied by the formation of a volcanic arc and a back-arc basin near the North American margin. Middle Ordovician emplacement of ophiolites on the North American margin reflects the closure of this back-arc basin (Iapetus I), and continued northerly subduction in Silurian time closed the main basin (Iapetus II).

Pre-folding magnetization reconfirmed for the Late Ordovician-Early Silurian Dunn Point volcanics, Nova Scotia

Johnson, R.J.E. and Van der Voo, R., 1990. Pre-folding magnetization reconfirmed for the Late Ordovician-Early Silurian Dunn Point volcanics, Nova Scotia. Tectonophysics, 178: 193-205. Paleomagnetic results have been obtained for three new sites in steeply inclined subarea1 volcanic flows of the Dunn Point Formation from the Avalon terrane of Nova Scotia. Demagnetization analysis reveals a characteristic magnetization, carried by hematite, which is similar to that reported in previous studies of this unit. A new and improved fold test for the characteristic component, combining paleomagnetic and structural data from the present study with paleomagnetic data for vertically dipping flows (and laterites) studied previously by the authors, is significantly positive. The new fold test reconfirms the prefolding age of the characteristic component. A primary age of magnetization is inferred, based on field evidence for early oxidation of the flows. The overall mean direction for the characteristic component of D = 344O, I = -60°, k = 68.6, ass = 4.1“ (paleopole at 2ON, 130’E) corresponds to a Late Ordovician-earliest Silurian paleolatitude for Avalon of 41°S. The latter is much higher than that predicted for the area on the basis of the cratonic North America apparent polar wander path, implying a substantial post-Ordovician displacement (> 1700 km) of Avalon with respect to the craton. The Dunn Point result supports previous paleogeographic reconstructions that show Avalon together with the continental blocks of Hercynian Europe and northwestern Gondwana.

Paleomagnetism of the Moreton's Harbour Group, northeastern Newfoundland Appalachians: Evidence for an Early Ordovician Island Arc near the Laurentian Margin of Iapetus

Paleomagnetic results have been obtained from mafic volcanic units in the upper part of the Moretons Harbour Group, which is part of an accreted island arc terrane that is preserved in the Notre Dame Bay subzone of the Central Mobile Belt of the Newfoundland Appalachians. Detailed thermal and alternating field demagnetization reveals a stable characteristic component of magnetization, carried by magnetite, at a large number of sites in pillow basalts and coeval basaltic dikes. The intrusives contain both polarities, and a primary age for the characteristic magnetization is indicated by a positive contact test for one of the dikes, and by a positive structural test involving a correction for block rotations (strike-correction). The overall mean direction for the characteristic component after tectonic (tilt and strike) correction (flows and intrusives: D = 171°, I = +22°, k = 22.6, α95 = 6.5°, pole 29°N, 135°E; flows: D = 166°, I = +22°, k = 33.2, α95 = 6.5°) corresponds to an Early Ordovician paleolatitude for the arc of 11°S, which is indistinguishable from the expected paleolatitude of the North American margin. This implies that the arc formed at or near the margin of the craton. In contrast, the Avalon block, which formed the other margin of Iapetus, was widely separated from the arc and the craton at this time. The Early Ordovician paleolatitude of the arc terrane supports a tectonic model in which coeval ophiolitic sequences inboard of the arc were formed in a narrow ocean basin between the arc and the craton. Subsequent convergence between the arc and Laurentia in Middle Ordovician time resulted in closure of this narrow ocean basin and obduction of the back-arc basin oceanic crust onto the ancient margin of the craton, thus giving rise to the Taconic orogenic pulse in the Newfoundland Appalachians.

Paleogeography of some vestiges of Iapetus: Paleomagnetism of the Ordovician Robert's Arm, Summerford, and Chanceport Groups, central Newfoundland

In the Central Mobile Belt of Newfoundland, three volcanic groups of early Middle Ordovician age (Robert9s Arm, Summerford, and Chanceport) are thought to have formed within the Iapetus Ocean. Demagnetization analyses of 328 samples from these three groups has yielded similar magnetic directions, calculated by combining stable end-point directions and great-circle poles. The three groups yield similar poles located at 6°N, 140°E; 8°N, 140°E; and 10°N, 131°E, corresponding to paleolatitudes of about 30°S. The magnetization of the Robert9s Arm Group passes the tilt test, and one site shows normal polarity, antipodal to the directions of the other sites. The paleolatitude of 31°S places the volcanic rocks at the time of their formation in the middle of Iapetus, between the Laurentian margin at about 15°S and the location of the Avalon Terrane at >50°S. Subsequent accretion to Laurentia and the presently adjacent island-arc rocks represented by the Moreton9s Harbour Group occurred after northward drift in the later Ordovician/earliest Silurian and before the Middle Silurian.