GEOS, Unión Geofísica Mexicana, Reunión Anual 1998, Puerto Vallarta, Jalisco, México, Resúmenes y Programa, v. 18, no. 4, p. 306, 1998.

Uplifted Holocene terraces and tectonic deformation of the Michoacán coast, southern México

María-Teresa Ramírez*, Jeffrey S. Marshall**, and José J. Zamorano*

*Departamento de Geografía Física, Instituto de Geografía, UNAM, Ciudad Universitaria, Coyoacán, México D.F., 04510

**Geosciences Department, Franklin & Marshall College, Lancaster PA 17604, USA

The Pacific coast of Michoacán, southern México, extends across an active convergent margin characterized by northeast subduction of the Cocos plate at 5-6 cm/yr beneath the North American plate. The coastal morphology of this region reflects relative sea-level changes produced by active tectonic uplift during the Holocene. This section of the subduction zone produced one of the largest (Ms = 8.1, September 19, 1985) and most damaging earthquakes in modern Mexican history. Coseismic coastal uplift of up to 1.0 m caused by the 1985 earthquake (Bodin and Klinger, 1986; Corona-Esquivel et al, 1988) occurred in the Michoacán area.

Uplifted marine strandlines (former shorelines) are represented by emerged marine terraces, beach-ridges, and river terraces. Geomorphic evidence of uplift extends particularly south of Titzupán to about Lagunillas (southeast of the Balsas river delta), with terraces ranging in height from 2.5 up to 8 m above sea level. Radiocarbon dating of marine shells, found on terraces and beach-ridges, and archaeological data (pottery shards) for the region support sudden uplift caused by major earthquakes during the Holocene along the Michoacán coast. Preliminary C14 ages for uplifted Holocene beach ridges near the southern edge of the Michoacán segment (Boca La Saladita and Chucutitán) suggest recent short-term uplift at 1.0-3.0 m/ka.

At least two well-preserved Pleistocene marine and fluvial terraces are also prominent along this coastline at elevations ranging from 10 to 150 m, indicating longer-term uplift rates within the central Michoacán coast (eg. Bahía Bufadero). Further radiocarbon dating of higher marine terraces and correlations with sea level curves will give insight into the rates of tectonic uplift during the Quaternary in this region. In addition, comparison of terrace uplift patterns with that of the 1985 coseismic uplift may help constrain earthquake recurrence intervals.