A successful rock magnetic technique for correcting paleomagnetic inclination shallowing: Case study of the Nacimiento Formation, New Mexico

The accuracy of an inclination shallowing correction technique was tested on the remanence of the Paleocene Nacimiento Formation. The correction assumes that changes in paleomagnetic inclination during deformation will be directly related to changes in remanence anisotropy. The remanence anisotropy is also a function of the magnetic anisotropy of The individual magnetic grains. The Nacimiento Formation was selected for this test because its inclination is shallow by 7 degrees to 8 degrees and previous paleomagnetic studies indicate that it has a primary remanence with a well-constrained age and that magnetic overprints are minimal. Tectonics are an unlikely explanation for its shallow inclination. These condition's allow the inclination-corrected remanence to be compared to a well-defined paleomagnetic pole to determine the accuracy of the technique. The characteristic remanence from 20 Nacimiento sites was isolated by alternating field and thermal demagnetization and was corrected using the anisotropy of anhysteretic remanence measured for three to five samples per site. Individual particle anisotropy was determined by drying a magnetic separate/epoxy mixture in a 35-50 mT magnetic field. Laboratory compaction of disaggregated Nacimiento material was also used to yield an effective individual magnetic particle anisotropy. The two techniques resulted in similar values for individual particle anisotropy and similar inclination shallowing corrections. Both corrections were successful, and the inclination-corrected formation mean direction was indistinguishable from the direction predicted by North America's Paleocene paleopole. The inclination shallowing correction technique reported here should be applied routinely in paleomagnetic studies, provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes (e.g., compaction).