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

Abstract

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 α 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. Copyright 2004 by the American Geophysical Union.