Earth Planets Space, Vol. 61 (No. 1), pp. 173-182, 2009
C. I. Caballero-Miranda1, J. R. Torres-Hernández2, and L. M. Alva-Valdivia1
1Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica, UNAM, Ciudad Universitaria, 04510 México DF, Mexico
2Instituto de Geología, Universidad Autónoma de San Luis Potosí, San Luis Potosí, Mexico
(Received November 23, 2007; Revised April 16, 2008; Accepted May 27, 2008; Online published January 23, 2009)
Anisotropy of magnetic susceptibility (AMS) was selected as the key technique to find the source of the widespread Cantera Ignimbrite and to seek its possible relationship with the San Luis Potosí Caldera. Eighteen sites (372 specimens from 155 cores) from the Oligocene Cantera Ignimbrite were sampled. AMS was measured on a KLY2 Kappabridge. AMS data were processed with Anisoft 3 software using Jelinek statistics as well as 'SpheriStat' principal components and density distribution. Mean susceptibilities range from 290 to 5026 × 10-6 SI (average = 2526 × 10-6 SI). The anisotropy degree (Pj) ranges from 1.005 to 1.055, with only one site displaying a value of 1.134 (Pj average = 1.031). AMS ellipsoid shapes are mostly oblate, with the T-factor ranging from 0.843 to 0.144 (T average = 0.529), although one site is mainly prolate (T = -0.005), and three additional sites have an important proportion of prolate specimens. Magnetic fabrics of most sites shows k axes around nearly circular distributions and k1-k2 axes around elongated-girdle distributions defining sub-horizontal foliation planes; exceptions to this are related to sites with a significant percentage of prolate specimens. Flow directions inferred from AMS analysis indicate several ignimbrite sources located along selected NW-SE linear features (faults and fractures such as El Potosino Fault) as well as along the rim of the caldera structure. The geometry of volcanic outcrops, the NW-SE faulting-fracture system, as well as the AMS results suggest that this is a caldera structure resembling the trapdoor-type (Lipman, 1997).
Key words: Anisotropy of magnetic susceptibility (AMS), magnetic fabric, inference of flow source, Cantera Ignimbrite, San Luis Potosi Volcanic Field (SLPVF), Tertiary, Mexico.