Paleomagnetic and magnetic fabric data for the 4.8 Ma San Gaspar ignimbrite, one of the largest in western Mexico, are used to investigate the source vents, emplacement mechanism and tectonics. Rock magnetic properties from distant parts of the ignimbrite are similar, suggesting relatively homogeneous mineralogy of the unit. Isothermal remanence and continuous susceptibility-temperature experiments point to low to medium-Ti titanomagnetites as the main magnetic carriers. Hysteresis ratio parameters of most samples fall in the pseudo-single-domain grain size region; wasp-waisted hysteresis loops were identified corresponding to high Hcr / Hc values. Stepwise thermal and alternating field demagnetization shows that secondary components are completely removed below 20 mT or 400 °C. Thereafter, the characteristic component is isolated with small within-site dispersion of mean direction (α95
