Palaeomagnetism of middle Proterozoic ( c . 1.25 Ga) dykes from central North Greenland

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71617/1/j.1365-246X.1987.tb01660.x.pd

SEM/STEM observations of magnetite in carbonates of eastern North America: Evidence for chemical remagnettzation during the Alleghenian Orogeny

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95085/1/grl5458.pd

Late Devonian to Early Carboniferous palaeomagnetic poles from the Armorican Massif, France

In order to test plate tectonic hypotheses for the Hercynian orogeny in western Europe, Late Devonian and Cambro-Ordovician redbeds and volcanics have been palaeomagnetically studied. The Late Devonian redbeds show nearly univectorial remanent magnetizations during stepwise thermal, chemical and alternating field demagnetization and yield a pole position at 19.8° N, 144.2° E. All three Cambro-Ordovician units studied yielded characteristic directions of magnetization that are interpreted as remagnetizations of Late Devonian-Early Carboniferous age on the basis of negative fold tests, similarities with the directions of the Late Devonian redbeds, reset K/Ar ages of 345 MA, or the occurrence of significant high- temperature magnetizations. A comparison of a mean Late Devonian-Early Carboniferous pole for all four formations (28.1° N, 146.4° E, dp = 3.87°, dm = 7.50°) from the Armorican Massif with contemporaneous poles from stable (‘extra-Hercynian’) Europe indicates that there was little or no separation between Hercynian and stable Europe in that time. A significant separation between the Armorican Massif and Gondwanaland, on the other hand, suggests that an intervening middle Palaeozoic ocean existed which subsequently was consumed by subduction somewhere to the south of the Armorican Massif. Those high-temperature directions from the Cambro- Ordovician redbeds and volcanics that are relatively well grouped are interpreted as original Cambro-Ordovician magnetizations. They show shallow inclinations and north-westerly declinations, but are not sufficiently substantiated to give more than the tentative interpretation that their palaeolatitudes also are roughly in agreement with the data from stable Europe for that time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75659/1/j.1365-246X.1979.tb01026.x.pd

Revision of the age of magnetization of the Montmartin red beds, Normandy, France

A new roadcut has enabled us to sample the south-dipping limb of the Montmartin syncline for a palaeomagnetic reevaluation of an earlier result published by Jones, Van der Voo & Bonhommet. In combination with the results previously published in 1979 for the north-dipping beds of the syncline, a conclusively negative fold test is obtained. The resulting magnetization (declination/inclination =206°/-3°, Α 95 = 12°, palaeopole at 38°S, 325°E) is interpreted to be of Late Carboniferous age, not Late Devonian as thought earlier. Simultaneously, we have re-evaluated the age of the rocks, previously thought to be Late Devonian on the basis of Acritarchs, Chitinozoans and spores. It has not been possible to reconfirm these fossils, not even in the same samples as studied originally; in contrast, the regional presence of Early Palaeozoic fossils suggests to us an age similar to that of other red beds in the Arrnorican Massif, which have been dated as Early Ordovician. The geodynamic implications of our finding that the Montmartin rocks are completely remagnetized, however, are of no great consequence for the geodynamics of the Hercynian belt. Pre-folding magnetization obtained from Silurian and Devonian rocks in Spain and Germany argue for the same conclusion as reached erroneously in our earlier study, namely that the Armorican Massif and adjacent parts of Hercynian Europe were adjointed to North America, Great Britain, the Baltic Shield and the Russian Platform since at least Late Devonian time. If a Medio-European ocean existed during the Palaeozoic, it was virtually closed before the mid-Devonian and of insignificant width during Culm deposition in Early Carboniferoirs time.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/74719/1/j.1365-246X.1985.tb05108.x.pd

Modified lattice parameter/Curie temperature diagrams for Titanomagnetite/titanomaghemite within the quadrilateral Fe 3 O 4 ‐ Fe 2 TiO 4 ‐Fe 2 O 3 ‐Fe 2 TiO 5

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94971/1/grl9245.pd

The ∼270 Ma palaeolatitude of Baltica and its significance for Pangea models

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86901/1/j.1365-246X.2011.05061.x.pd

Global plate motion frames: Toward a unified model

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94772/1/rog1664.pd

Paleomagnetism of the Upper Silurian and Lower Devonian carbonates of New York State: evidence for secondary magnetizations residing in magnetite

Paleomagnetic directions for the Upper Silurian and Lower Devonian carbonates of the Helderberg escarpment (New York State) differ from expected Late Silurian and Early Devonian directions for cratonic North America. The mean direction (D = 165[deg], I = -10[deg]; paleopole at 50[deg]N 129[deg]E) is similar to Late Carboniferous and Early Permian results. Negative fold tests, and a lack of reversals, suggest that the magnetization is secondary. However, low coercivities, low blocking temperatures, the thermomagnetic curves (TC near 570[deg]C) and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite. If a detrital origin of these magnetites is assumed, the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks. However, no evidence for such thermal resetting is seen in the alteration of conodonts. More likely perhaps is a chemical or thermochemical origin of the remanence; this would require the magnetites to be authigenic.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/23790/1/0000028.pd

SEM/STEM observation of magnetic minerals in presumably unremagnetized Paleozoic carbonates from Indiana and Alabama

The Silurian Wabash Formation in Indiana and the Mississippian Pride Mountain Formation in Alabama appear not to have been affected by a late Paleozoic remagnetization event. In an attempt to characterize the magnetic mineralogy in these (presumably) unremagnetized carbonates and in order to compare their magnetic mineralogy to that of remagnetized carbonates, scanning and scanning transmission microscope (SEM/STEM) observations and rock magnetic investigations were carried out.It is possible to recognize differences in magnetic mineralogy in the unremagnetized carbonate from that in remagnetized carbonates: 1. (1) iron oxides associated with iron sulfides are hematite (in this study) as a result of replacement of pyrite (instead of magnetite as was found elsewhere);2. (2) occurrences of large euhedral pure-iron oxides of secondary origin are common in the unremagnetized carbonates3. (3) a rare occurrence of fine-grained single-crystal magnetite capable of carrying a remanence in the unremagnetized carbonates is noticeable as compared to the abundance of such grains in the remagnetized carbonates. Although the abundance of the fine-grained magnetite grains in remagnetized carbonates is inferred to be a diagnostic factor to distinguish the remagnetized from the unremagnetized carbonates, this clarifies only the carriers in the remagnetized rocks and leaves the question of the carriers in unremagnetized limestones unresolved to a large extent.The lack of remagnetization is commonly attributed to a restricted amount of fluid influx into the rocks. For the Wabash and the Pride Mountain Formations this may also be true; early cementation has significantly reduced the porosity and permeability in the Wabash Formation in Indiana, whereas the presence of the impermeable Chattanooga Shale may have `protected' the Mississippian Pride Mountain Formation in Alabama.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29677/1/0000004.pd

A case for a comet impact trigger for the Paleocene/Eocene thermal maximum and carbon isotope excursion

We hypothesize that the rapid onset of the carbon isotope excursion (CIE) at the Paleocene/Eocene boundary (V55 Ma) may have resulted from the accretion of a significant amount of 12C-enriched carbon from the impact of a V10 km comet, an event that would also trigger greenhouse warming leading to the Paleocene/Eocene thermal maximum and, possibly, thermal dissociation of seafloor methane hydrate. Indirect evidence of an impact is the unusual abundance of magnetic nanoparticles in kaolinite-rich shelf sediments that closely coincide with the onset and nadir of the CIE at three drill sites on the Atlantic Coastal Plain. After considering various alternative mechanisms that could have produced the magnetic nanoparticle assemblage and by analogy with the reported detection of iron-rich nanophase material at the Cretaceous/Tertiary boundary, we suggest that the CIE occurrence was derived from an impact plume condensate. The sudden increase in kaolinite is thus thought to represent the redeposition on the marine shelf of a rapidly weathered impact ejecta dust blanket. Published reports of a small but significant iridium anomaly at or close to the Paleocene/Eocene boundary provide supportive evidence for an impact