Geomagnetic Paleosecular Variation and Tectonic Correction for the Past 12 Ma in Baja California, Mexico

Abstract:

During the last 30 years, study of Paleosecular Variation (PSV) has been analyzed within specific timescales (e.g. 0-5 Ma; 5-22.5 Ma; etc.), considering the main criteria variation in directions of paleomagnetic data due to displacement of tectonic plates, and the scarcity of spatial and temporal distribution of rock outcrops, we propose using the previous geological and geophysical studies comprising the tectonic displacement of Baja California for the last 12 Ma, a method (working in space and time) to precise the PSV of our paleomagnetic sites (assuming a thermoremanent magnetization).

We completed two paleomagnetic rock field sampling works: Loreto-Santa Rosalia-Punta Abreojos (between 26° and 27.4° N), with 156 cores from 16 sites; and San Borja and Jaraguay volcanic fields (between 28° and 30° N), with 230 cores from 27 sites. These sites are mainly volcanic rocks (andesite-basalt) with radiometric ages previously reported. We selected those younger than 12 Ma age without effect of rotation or flattening within the peninsula. Rock magnetic experiments were done to characterize the magnetic carrier of the remanence, such as susceptibility vs. low and high temperature, hysteresis, FORC, Koenigsberger ratio determination and directional analysis. Our results suggest that in most of the cases (80%) we have a contribution of TM0 to TM20, and magnetic domain of PSD + SP.

In addition to our data, we gathered all paleomagnetic works in volcanic rocks younger than 12 Ma along Baja California. For the PSV, we calculated the angular standard deviation (S_B) of the VGPs with respect to its mean average and also to the geographic axis. Both results were compared with the calculation of the S_B once the correction of paleomagnetic sites based on the tectonic displacement was applied. Finally, all results (with and without correction) were compared with Model G and TK03.GAD, finding that data of the S_B with tectonic correction are the best fit models, suggesting a reduction in uncertainty of the analysis of the PSV with a larger timescale: 0-12 Ma. This is a wider range than previously proposed, allowing the increase the sites number for the study of PSV in time and space, at least for the Baja California rocks.