1. Laboratorio de Paleomagnetismo; Instituto de Geofisica; Universidad Nacional Autónoma de México; Mexico DF 04510 Mexico
2. Division of Structural Geology and Tectonophysics; Institute of Applied Geosciences; Karlsruhe Institute of Technology; Karlsruhe 76131 Germany
3. Department of Earth Sciences; Indian Institute of Technology Roorkee; Roorkee Uttar Pradesh India