New paleomagnetic, rock magnetic, and petrographic results from the Valle Group, Baja California, Mexico: Exploring the causes of anomalously shallow paleomagnetic inclinations

Abstract

[1] A detailed paleomagnetic, rock magnetic, and sedimentologic study was conducted on Cenomanian-age strata of the Valle Group at El Campito, Baja California, to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks. Alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of Dec = 342.6°, Inc = 19.1°, and a 95 = 3.4°. The inclination is 38.7°shallower than that predicted by the reference paleopole for North America. Laboratory experiments indicate that compaction could account for only 7.5°of the inclination shallowing. Fold tests at different scales yield both positive and negative results, making a tight constraint on the remanence's age impossible. A primary postdepositional remanence is supported by bedding-parallel foliations in the anisotropy of anhysteretic remanence fabric. Although primary anomalously shallow inclinations could indicate significant southerly, then northerly, paleolatitudinal offset, a more likely scenario is a late Cenozoic low-temperature remagnetization, which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data. The synthesis of the data indicates that the Vizcaino terrane was contiguous with the Cordillera margin during remagnetization and underwent a subsequent 13.5 ± 5.4°clockwise rotation. We tentatively interpret that the remagnetization resulted from burial and/or Miocene volcanism, which caused an acquisition of thermochemical remanent magnetization (TCRM). This study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks.

Inclination shallowing in Eocene Linzizong sedimentary rocks from Southern Tibet: correction, possible causes and implications for reconstructing the India–Asia collision

“…An important result of our study is the depositional control on inclination shallowing. Overall, we observe a shallowing factor of ∼0.43 [f factor defined using King's (1955) equation], which means that these sedimentary rocks have undergone significantly more flattening than most of the reported magnetite-bearing sedimentary rocks with f factors ranging from 0.56 to 0.79 (Kodama & Davi 1995;Kodama 1997Kodama , 2009Tan & Kodama 1998;Kim & Kodama 2004;Li et al 2004;Vaughn et al 2005;Bilardello & Kodama 2010a). It is becoming increasingly common to assume a flattening factor and apply a blanket (or even 'ad hoc') correction using f factors of ∼0.6 (Kent & Irving 2010;Domeier et al 2011).…”

Sensitive moisture response to Holocene millennial-scale climate variations in the Mid-Atlantic region, USA

“…The IRM of a sample is acquired by exposing the sample to a 1.0 T field using an ASC impulse magnetizer. The detailed magnetic measurements of the lake sediments can be found in Li (2005).…”

Palaeomagnetic assessment of plutons from the southern Peninsular Ranges batholith and the Jurassic Vizcaíno igneous suites, Baja California, México

“…Several workers have noted that sedimentary units may have been affected by inclination shallowing (Butler et al 1991;Kodama and Ward 2001;Li et al 2004;Vaugh et al 2005). Examples include rocks from the Jurassic Eugenia Formation and Upper Cretaceous Valle Formation in the Vizcaíno peninsula that were previously used to support palaeomagnetic reconstructions requiring large northward displacement (Hagstrum et al 1985).…”