Paleomagnetic study from radiometrically dated lavas associated to the Tepic-Zacoalco Rift (western Mexico): Possible evidence of a new geomagnetic excursion within Gilbert chron

摘要

This work presents the results of a detailed paleomagnetic and rock-magnetic study carried out in the igneous units associated with the Tepic-Zacoalco Rift (western Trans-Mexican Volcanic Belt). Standard paleomagnetic drilled cores were collected from 21 sites distributed over the Rift extension, all of them with available radiometric ages between 3 and 5 Ma. Rock magnetic experiments showed that, in most cases, the minerals carrying the magnetic remanence are titanomagnetites with variable titanium content. Stepwise magnetic treatment allowed to determine the paleomagnetic directions for 17 sites, of which 4 are of normal polarity, 8 reverse, and 5 sites have apparently intermediate polarities (based on paleolatitude less than 45 degrees as a cut-off angle to separate stable and transitional geomagnetic regimes). The mean paleodirection is Dec. = 347.6 degrees, Inc = 29.4 degrees, N = 12, alpha(95) = 9.5 degrees, corresponding to a paleomagnetic pole position Plat = 77.2 degrees N, Plong = 144.3 degrees E, A(95) = 8.8 degrees, N = 12. These results agree, within uncertainties, with the expected directions retrieved from the stable North America reference poles and to the previously reported values for the same region and age. Thus, major tectonic deformation or vertical axis rotation is discarded at least since Early Pliocene. An interesting feature of this study is the occurrence of intermediate paleodirections around 3.86 Ma which probably correspond to post-Cochiti geomagnetic excursion within the Upper Gilbert chron.