Congratulations to Samantha Nemkin who successfully defended her dissertation on April 25, 2017.
Advisors: Rob van der Voo, Ben van der Pluijm
Abstract: Carbonate remagnetizations are globally widespread and typically the result of secondary magnetite growth. Prior to the 1980’s, this posed a problem as such secondary remanences were erroneously interpreted as primary magnetization directions. Remagnetizations were eventually recognized in carbonates, but the directions remained mostly dated by qualitative comparison to an apparent polar wander path (APWP) after paleomagnetic field tests. This thesis demonstrates that quantitative ages can be assigned to remagnetizations by correlating synfolding remagnetization directions with ages from 40Ar/39Ar dating of individual folds in a region.
Central to this study is sampling local scale carbonate folds in order to allow multiple individual fold tests instead of one regional fold test application in a field area. Results from the North American Cordillera in Montana, Idaho/Wyoming, the Monterrey Salient in northern Mexico, and central Mexico are reported. Remagnetization ages are determined for each field area by connecting synfolding remagnetizations with fold ages, which span the Late Cretaceous to Eocene. Mississippian limestones from Montana (Chpt. 2) and the Lower Cretaceous carbonates from the Monterrey Salient (Chpt. 3) have remagnetization ages of 54 Ma and 48-52 Ma, respectively. Results from Cretaceous carbonates in central Mexico (Chpt. 4) preserve two distinct remagnetization events at 77 Ma and 44 Ma. A study of folded Mississippian limestones in Idaho and Wyoming indicate the presence of a remagnetization, but remain inconclusive for lack of suitable sampling sites (Appendix A).
Remagnetization ages coincide with a period of tectonic activity in the Cordillera and are the result of chemical growth of magnetite. Therefore, it is proposed that the mechanism for the formation of secondary remanences is from the interaction of carbonates with an iron-bearing fluid. Lithology and structural characteristics influence whether or not sufficient magnetite will grow and allow the acquisition of a secondary remanence. Moreover, the local-scale fold sampling scheme provides a new, detailed understanding into the development of local paleomagnetic and deformational histories across a field area.
Combining synfolding remagnetizations and fold ages provides an important method to date the timing of remagnetization acquisition in rock units, contributing significantly to the global paleopole database. Given that many carbonates worldwide are remagnetized, this coupled approach also allows for broader scale use of the method. Additionally, the spatial distribution of syn-, pre-, and/or postfolding remanences in local folds constrains local deformation events and provides insights into the connection of remagnetization mechanism(s) and deformation of carbonates.