Although it has been known for 25 years that some Paleozoic sedimentary rock units in Europe and North America were remagnetized during Pennsylvanian or Permian time, it is only very recently that the complex and widespread nature of the late Paleozoic remagnetization phenomenon has been generally acknowledged. It is now recognized that many Paleozoic paleomagnetic poles for North America that were once considered reliable are in fact the result of remagnetization, and as a consequence the paleomagnetic data base for the Paleozoic is undergoing rapid and drastic revision. The causes of late Paleozoic remagnetization in North America are currently the focus of much interest and active research. The remagnetization can reside in either hematite or magnetite, and different remagnetization mechanisms have been important in dif-ferent settings. Chemical remagnetization processes, some related to specific diagenetic events, are dominant in hematite-bearing sandstones and carbonates. In magnetitebeating carbonates both chemical and thermoviscous remagnetization processes appear to have been important, but it is difficult to determine which process is the dominant one in some settings. Some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountainbuilding events that affected much of the continent during the late Paleozoic. Paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids. Paleozoic data base for North America by , 1989 indicates the magnitude of this revolution: many of the paleopoles from pre-Pennsylvanian rocks cited by Van der Voo [1981] as "reliable" had been shown to be late Paleozoic remagnetizations by the time of his 1989 review. The recognition of the prevalence of remagnetizapages 471-494 Paper number 89RG02844 472 ß McCabe and Elmore: LATE PALEOZOIC REMAGNETIZATION 27, 4/REVIEWS OF GEOPHYSICS tion has stimulated new interest in Paleozoic apparent polar wander for North America and has underscored the necessity for detailed laboratory experiments and field tests (e.g., fold test and conglomerate test) to bracket magnetization acquisition ages. Recent investigations have produced new poles of demonstrated early and middle Paleozoic age, thus defining previously unrecognized drift of North America with respect to the pole (see Van der Voo [1989]for a review).The causes for late Paleozoic remagnetization have become the focus of much interest and controversy. From a paleomagnetic point of view, it is obviously essential that the causes of remagnetization be fully understood. If reliable petrologic or geological criteria can be established for predicting the presence or absence of remagnetization, then our effectiveness in finding primary magnetizations will be increased significantly. In addition, a full understanding of the remagnetization phenomenon will result in new applications of t...

Abstract. Results of a paleomagnetic, rock magnetic, geochemical, and petrographic study on Jurassic and Cretaceous carbonates in the Vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization (CRM) carried by magnetite. Where smectite has altered to other clay minerals, limestones are characterized by a prefolding, secondary, normal polarity magnetization throughout the basin. The magnetization is interpreted to be a CRM based on low burial depths which cannot cause thermoviscous resetting. Where significant smectite is still present, the CRM is absent/weakly developed, and where the clays show no evidence for burial alteration, the units are characterized by a primary magnetization. CRM intensity also varies with the amount of smectite and burial. Isothermal, anhysteretic, and natural remanent magnetization intensifies increase where smectite has altered, both stratigraphically and geographically. This is interpreted to indicate magnetite authigenesis associated with clay aliagenesis. Superparamagnetic magnetite is more dominant in highly altered units based on the results of low-temperature experiments. All sections away from the Alps have 87 Sr/86Sr values that are similar to coeval seawater, and stable isotopes of carbon and oxygen show no sign of alteration. Omgenic-type fluids therefore are not a likely agent of remagnetization. Near the Alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the CRM through a reversal. A postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic-type fluids. We conclude that burial diagenesis of smectite is the likely cause for the development of the widespread CRM in the Vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere.

I would like to thank my advisor. Dr. R. Douglas Elmore for his support. He has been of great help during the past years and he has been instrumental in assisting me to leam how to speak, read, and write the second time around. I wish to thank all of my committee members, Drs. Tom Dewers, Mike Engel, Roger Young, and Richard Henry for feedback and for reviewing the manuscript. Dr. Mike Engel is also acknowledged for his collaboration on the geochemical aspects of the dissertation. I especially thank my "better half (more like 60%), Monika Cogoini, for so many things that I would need a separate chapter in this dissertation to mention them all. She is the best friend, colleague, reviewer, orient express, critic, and moral supporter one can imagine, among all the other things. A tremendous "merci beaucoup" goes to Dr. Serge Ferry from the University of Lyon, France, for his great support in the field and for inspiring conversations. I also want to thank the people at the Institute for Rock Magnetism, Minnesota, especially Dr. Mike Jackson, for help and advice during my visit. I thank Dr. James Lee Wilson for being a great inspiration and Dr. Sue Halgedahl for her help during my visit to her Micromag at the University of Utah. Thanks to Dr. Sanjay Banetjee for being a great colleague and for all the inspiring conversations. I am grateful to Jamie Egger for the many hours she dedicated to my samples in the lab. I am also grateful to all my colleagues in the lab, in the School of Geology and Geophysics, and to all the new friends that I made in Oklahoma who made my stay here enjoyable. Last but not least, I thank my parents for making all this possible.

Chemical remagnetization is a very common phenomenon in sedimentary rocks and developing a greater understanding of the mechanisms has several benefits. Acquisition of a secondary magnetization is usually tangible evidence of a diagenetic event that can be dated by isolation of the chemical remanent magnetization and comparison of the pole position to the apparent polar wander path. This can be important because diagenetic investigations are frequently limited by the difficulty in constraining the time frames in which most past events have occurred. Remagnetization can commonly obscure a primary magnetization; developing a better understanding of remagnetization could improve our ability to uncover primary magnetizations. Many chemical remagnetization mechanisms have been proposed, including those associated with chemical alteration by a number of different fluids (orogenic, basinal and hydrocarbons), burial diagenetic processes (clay diagenesis and maturation of organic matter) or other processes. This paper summarizes our current knowledge of these chemical remagnetization mechanisms, with a focus on examples where there is a connection with chemical alteration.

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