Paleomagnetism and age of the ignimbrite Panalillo Superior volcanic field of San Luis Potosi, Mexico

Abstract

The Oligocene Upper Panalillo Ignimbrite is widely distributed in the San Luis Potosí Volcanic Field, in the southeastern portion of the Mesa Central province of Mexico. It crops out sporadically over a ~15,000 km2 area. The best exposures of the ignimbrite were sampled for paleomagnetic and geochronological studies, with sites grouped in five zones according to their geographic distribution and structural setting. The five zones are: northwest or La Pendencia (1); north or Valle Umbroso (2); center-north or Mexquitic (3); east or Panalillo (4); and, south or Bledos Graben (5). We determined virtual paleomagnetic poles (VGPs) and mean directions for each sampling site and for each one of the five zones. VGPs for the Upper Panalillo Ignimbrite were combined with published paleomagnetic data for the San Luis Potosí volcanic field in order to determine an Oligocene paleomagnetic pole located at 72.1°N -165.5°E (N = 17, K = 228.2, A95 = 6.8°), which was in turn compared with the North America reference pole. We determined rotation (R) and flattening (F) parameters for the region. The study was supplemented by seven radiometric ages for the ignimbrite, three are single crystal total fusion LASER 40Ar/39Ar ages and four are K/Ar for saninide separates. The combination of paleomagnetic data and isotopic age determinations suggests that the Upper Panalillo Ignimbrite is composed of at least two flow units with different paleomagnetic and radiometric ages. One unit is approximately 26.5 Ma and it is mostly present in La Pendencia area; the other is ca. 28 Ma, and crops out around the city of San Luis Potosí. The overall mean for individual units of the volcanic field suggests that the region has experienced a counter-clockwise rotation of 7.9° ± 9.6° with respect to the North America craton since the Oligocene. However, the estimate of rotation is not statistically significant, and it can be interpreted as an apparent tectonic rotation consequence of complex faulting that affected the region during Paleogene time.