THE ARCELIA GRABEN - NEW EVIDENCE FOR OLIGOCENE BASIN AND RANGE EXTENSION IN SOUTHERN MEXICO

Basin and Range extension, which began in the Tertiary and continues t oday, is well documented in Mexico north of the Trans-Mexican volcanic belt. In contrast, evidence for Basin and Range extension in southern Mexico is largely limited to the Oaxaca basin, a north-northwest-tren ding Miocene graben. We discovered another north-northwest-trending Te rtiary basin, the Arcelia graben, approximately 200 km west of the Oax aca basin and 50 km south of the Trans-Mexican volcanic belt. Arcelia graben subsidence began in the early Tertiary and mostly ended prior t o accumulation of upper Oligocene volcanic rocks, indicating Basin and Range extension in this area was limited to a <32 m.y. interval in th e Tertiary. Extension near Arcelia is among the oldest Basin and Range style deformation documented in Mexico. Mast subsidence in the Arceli a graben preceded onset of Oaxaca basin subsidence by at least 8 m.y. This suggests the intriguing possibility of eastward migration of Basi n and Range extension in southern Mexico during the middle Tertiary.