Detecting and Correcting for Paleomagnetic Inclination Shallowing of Sedimentary Rocks: A Review

Abstract

Magnetic anisotropy and the elongation/inclination (E-I) approaches have been increasingly employed as two important means for detecting and correcting the paleomagnetic inclination shallowing in sedimentary rocks that was first recognized 60 years ago. Both approaches are based on certain assumptions, and thus have advantages and intrinsic limitations in investigating shallow inclinations in sedimentary rocks. The E-I approach is relatively easy to use, but it needs a large dataset to adequately sample paleomagnetic directions due to paleosecular variation (PSV) of the geomagnetic field. Also, slow sediment accumulation rates (SARs) and local tectonics could lead to under-or over-corrections using the E-I approach. For the magnetic anisotropy technique, labor-intensive, sophisticated laboratory rock magnetic experiments are required in order to accurately determine both bulk magnetic anisotropy of remanence-carrying grains and magnetic anisotropy of an individual particle, i.e., "a" factor, of samples. Our review shows that, despite the intensive laboratory work necessary for applying anisotropy-based inclination corrections, it is worth investing the effort. In addition, the joint use of magnetic susceptibility and remanence anisotropy measurements as well as detailed rock magnetic measurements for determining the particle anisotropy "a" factor have the advantage of retrieving direct evidence of inclination shallowing and correcting for it with high confidence. We caution against use of either of the two approaches without full appreciation of the underlying assumptions and intrinsic limitations of each technique. The use and comparison of both techniques could provide the most robust inclination shallowing correction for sedimentary rocks.

Clockwise rotations recorded in redbeds from the Jinggu Basin of northwestern Indochina

“…If the dating of tuff beds at 110-100 Ma by Wang et al (2015) from the Mengyejing Formation is representative of a Cretaceous age for the entire redbed sequence, the prefolding ChRMs could be primary magnetizations acquired during sediment deposition in the Cretaceous normal superchron (121-83 Ma). A primary NRM would explain the inclination bias with respect to Eurasia (Table 3), because inclination shallowing is frequently observed in redbeds (e.g., Li and Kodama, 2016). However, it would also mean that not only the Mengyejing Formation, but also the Denghei Formation and Mengla Group, would have been deposited in the mid-Cretaceous, which is in serious contradiction with the fossil content of these two formations, because fossil assemblages robustly indicate Eocene to Oligocene ages ( YBGMR, 1990).…”

“…A northward motion of the Indochina block since the Cretaceous is, however, not likely given the tectonic history of the region. Plausible reasons for this "anomaly" include widespread inclination shallowing of sediments (e.g., Li and Kodama, 2016), or inaccuracies in the relocation of the Eurasian APWP to sites in East Asia (Dupont-Nivet et al, 2010b).…”

Permian Magnetostratigraphy and End of the Kiaman Reverse Polarity Superchron From the Southeast Karoo Basin, South Africa

“…Using our B+ data, the E-I method yield and f value of 0.62 (E-I corrected inclination of 74.9° with bootstrapped 95% confidence interval of 66.2°-83.7°). However, this method is described by Li and Kodama (2016) as not well suited for the successions such as the Karoo successions that are deposited at slow sediment accumulation rates and affected by local tectonics (Catuneanu, 2004;Tankard et al, 2009). This limitation is based on the relative vertical-axis rotations between these sampling sites which leads to the development of an apparent E-W elongation that is unrelated to inclination shallowing, and a resultant under-or over-corrections for inclination shallowing (Li & Kodama, 2016).…”

Inversion tectonics and magnetic fabrics in Mesozoic basins of the Western Tethys: A review

“…Parés et al, 1999;Gil-Imaz et al, 2000;Hirt et al, 2000;Robion et al, 2007;Oliva-Urcia et al, 2009;Pueyo Anchuela et al, 2012), basin evolution (Mattei et al, 1997(Mattei et al, , 1999Cifelli et al, 2005), fold geometry and internal deformation (e.g. Aubourg et al, 1999), estimation of shallowing effect in sedimentary rocks sampled for paleomagnetic purposes (see Li and Kodama, 2016), fault rocks at shallow crustal levels (Solum and van der Pluijm, 2009;Casas-Sainz et al, 2017, 2018, paleocurrents orientation in sedimentary contexts (e.g. Rees, 1965;Hamilton and Rees, 1970;Tarling and Hrouda, 1993;Piper et al, 1996;Pueyo Anchuela et al, 2013 and references therein), etc.…”