MARINE TERRACES IN PUNTA DE MITA, JALISCO, SOUTHWEST MEXICO - EVIDENCE OF HOLOCENE COASTAL UPLIFT OR SEA LEVEL CHANGES?

Marine terraces and emerged beachrock, formed at sea level and since uplifted, are widely used as indicators of rates and patterns of tectonic activity. Detailed surveying and radiocarbon dating of marine terraces in the complex plate boundary setting of southwestern Mexico reveal the need for considerable care in their interpretation, especially in areas where coastal tectonics and sea level changes have been little studied. The Pacific margin of Mexico, Jalisco, is an active tectonic coastline. It parallels the Middle American Trench where the Rivera plate subducts northeastward at approximately 2.0-2.3 cm/yr beneath the North American plate. This coast has experienced two of the largest earthquakes to occur in Mexico this century: the 1932 (Mw 8.2) and the more recent 1995 (Mw 8.0) earthquakes. Field observations at Punta Mita reveal the presence of beachrock and marine terraces that are elevated 4 to 19 m above mean sea level. Preliminary C14 ages of marine shells collected from beach deposits at the marine terrace platforms provided dates of progressive emergence of these Holocene features. The ages ranging from 1 to 3 ka BP, for the lower terraces, indicate that the elevated landforms developed when sea level had apparently reached its present level. Subsequent tectonic uplift must have raised the terraces above sea level.