HIGH BEDROCK INCISION RATES IN THE ATENGUILLO RIVER VALLEY, JALISCO, WESTERN MEXICO

Lava flows from three basaltic shield volcanoes preserve Pliocene-Plei stocene river levels in the Atenguillo River basin, western Mexico. K- Ar dates of these basalt flows, together with present and palaeoriver levels, allow calculation of bedrock incision rates at three points al ong the length of the Atenguillo River: at Volcan La Laja, dated at 0. 65 Ma, incision rates are 25 cm ka(-1); at Volcan La Cienega (2.2 Ma) incision rates are 23 cm ka(-1) and at Volcan El Vigia (2.7 Ma) incisi on rates are 23 cm ka(-1). These high incision rates, as well as two d istinct knickpoints along the profile of the Atenguillo River, are rel ated to a base level change at the northern end of the basin. The dyna mics of this river basin are controlled by the ongoing process of cont inental fragmentation associated with the opening of the Gulf of Calif ornia. (C) 1997 by John Wiley & Sons, Ltd.