Abstract
In the reconstruction of past movements of tectonic plates, the determination of reliable paleomagnetic poles is of utmost importance. To achieve accurate results, a full knowledge of the rock magnetic properties of the samples is required particularly for Curie point, for grain-size analyses in addition to thermal and alternating field (a.f.) demagnetization experiments. We present the comparative results of 20 sites drilled at 3 different Paleozoic areas: The Permian rocks of the Juchatengo area in Oaxaca, Mexico; the Late Silurian (~418 Ma) Mountain Pine Ridge Granite, the Hummingbird Granite in Belize, and the Early Leonardian Chochal Limestone in Guatemala. The samples of all 20 sites were subjected to a.f. and thermal demagnetization in 16 steps from NRM to 100 mT, the thermally demagnetized samples were cleaned in 15 increasing temperature steps from NRM up to 675℃. Principal component analysis was applied to the samples in order to obtain their respective mean directions. Saturation Isothermal Remanent Magnetization (SIRM), hysteresis loops, and coercivity experiments performed indicate that about 90% of the samples were characterized by Multi-Domain (MD) grain sizes and the rest were in the Pseudo-Single Domain (PSD) range of the Day diagram. Curie point determinations results ranged from 190℃ to 660℃, indicating the presence of titanomagnetites as well as hematite. In the Juchatengo area reliable poles were obtained from 3 sites, in Belize 3 sites and only 2 sites of the Permian Chochal Formation, Guatemala yielded coherent and useful results. All the “reliable” paleopoles obtained do not agree with the APWP of North America.
Highlights
In the quest for precise and reliable results to be applied in the reconstruction of past movements of tectonic plates, we have performed a number of rock magnetic tests to discriminate between suitable and unsuitable rocks ranging in age between Carboniferous to Triassic units from Guatemala, Oaxaca and Belize
We contend that several indicators such as the conventional and routinely demagnetization experiments (i.e. a.f. and temperature) as well as magnetic granulometry and Curie point determinations yielded very important information to asses the possibility of obtaining valuable and meaningful results to conduct further paleotectonic reconstruction of the sites in question based on the experimental paleomagnetic and rock magnetic tests
3) The rock magnetic experiments performed allowed us to characterize the magnetic mineralogy of the intrusive rocks under study and we found out that the 8 “successful” sites contain almost pure magnetite, of Pseudo-Single Domain (PSD) and MD magnetic grain sizes
Summary
In the quest for precise and reliable results to be applied in the reconstruction of past movements of tectonic plates, we have performed a number of rock magnetic tests to discriminate between suitable and unsuitable rocks ranging in age between Carboniferous to Triassic units from Guatemala, Oaxaca and Belize. In order to discriminate the “good” versus the “bad” specimens is necessary to conduct more detailed rock magnetic tests in addition to the common alternating field (a.f.) and thermal demagnetization experiments. In our present study we have analyzed 20 sites from the geographic areas mentioned above that after the experiments performed on them we ended up with only 8 sites that yielded reliable, coherent and meaningful results. HERRERO-BERVERA the most reliable and the only sites useful for further tectonic interpretations
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