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

Abstract

Abstract. 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 Nacimiemo Formation was selected for this test because its inclination is shallow by 7 ø to 8 ø 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 conditions allow the inclination-corrected remanence to be compared to a well-defined paleomagneti½ 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 shotfid be applied routinely in paleomagneti½ studies, provided that the remanence is a primary depositional remanence that has been affected only by syndepositional or early postdepositional processes (e.g., compaction).

Geodynamic implications of paleomagnetic data from Tertiary sediments in Sakhalin, Russia (NW Pacific)

“…In samples where the magnetic fabric is oblate, and the minimum susceptibility ( k min ) is perpendicular to the bedding plane, an inclination correction can be applied if the susceptibility is dominated by ferrimagnetic minerals [ Hodych et al , 1999]. Alternatively, the anisotropy of ARM (AARM) can be measured to make such corrections [ Jackson et al , 1991; Kodama , 1997; Tan and Kodama , 1998]. In both cases, successful correction requires the average anisotropy of magnetic particles to be constant and small [ Hodych and Bijaksana , 1993].…”

Theoretical considerations on the anhysteretic remanent magnetization of interacting particles with uniaxial anisotropy

“…At the same time, the ARM has been used as a grain size indicator in rock magnetism [e.g., Johnson et al, 1975;King et al, 1982], and as a normalization factor in paleointensity studies [e.g., Shaw, 1974;Rolph and Shaw, 1985]. The anisotropy of ARM (AARM) has found application in sediments for correction of inclination shallowing of detrital remanent magnetization [e.g., Jackson et al, 1991;Kodama and Sun, 1992;Kodama, 1997]. More recently, ARM magnetization curves have been used to quantify magnetofossils in freshwater and marine sediments [Egli, 2004].…”

Tropical northeast Africa in the middle–late Eocene: Paleomagnetism of the marine-mammals sites and basalts in the Fayum province, Egypt

“…The observed shallow inclination of the demagnetized rocks raises a concern about possible inclination shallowing, which is often recorded in high-clay content sediments as a result of syn-depositional dewatering and/or post-depositional burial compaction (Anson and Kodama, 1987;Deamer and Kodama, 1990;Jackson et al, 1991;Kodama, 1997;Tan and Kodama, 1998;Kim and Kodama, 2004). The sites of this study, however, were mostly in massive limestone and fine-grained sandstones.…”