COASTAL TECTONIC UPLIFT ALONG THE CONVERGING MEXICAN MARGIN, JALISCO

Quantifying tectonic deformation along convergent margins is crucial for understanding megathrust faulting and landscape response to active deformation on convergent margins. Precise measurements of vertical displacement using geomorphic markers, such as marine terraces, wave-cut platforms, and emerged beachrock, allowed us to determine rates of coastal tectonic deformation along 45 km of the convergent Pacific margin in Jalisco, southern Mexico. Subduction of the Rivera plate to the N-NE beneath Mexico, at rates that increase from 2.0 up to 5.0 cm/yr from North to South along the strike, and the migration of the Jalisco block to the SE, have an effect on the landscape development along the Pacific coast. Along the Jalisco coast three earthquakes with Ms> 7.5 have occurred during the past 65 years: the June 3 and 18, 1932, Jalisco earthquakes (Ms 8.1, Ms 7.8, respectively), and the most recent October 9, 1995 Colima-Jalisco (Mw 8.0) earthquake. This seismically active margin has experienced coseismic subsidence during large earthquakes along the southern sector of the Jalisco coast. We performed a high precision survey of the marine terrace elevations and emerged beachrock on the northern sector of the Jalisco coast. Marine terraces extend discontinuously along the coast from Bucerias, north of Puerto Vallarta, to Punta de Mita, and their elevations can be correlated along 35 km of the coast. This stretch of coastline coincides with the northernmost sector of the June 3, 1932 earthquake rupture area. We sampled and dated with ¹⁴C shells from marine terrace deposits and beach rock. The taxonomic analysis of these species (ex. Chione sp., Anomalocardia sp., Trigoniocardia sp., and Trachycardium sp.,) indicate a moderately shallow water environment. Marine terrace elevations and the preliminary cosmogenic ¹⁴C dates indicate an early Holocene age for these marine terraces, and an estimated coastal uplift rate of 3.3 +/- 0.5 m/ka. We suggest that the coast has been rising by interseismic uplift or possibly by several “aseismic events”. In the light of historic coseimic subsidence and postseismic recovery reported for the southern part of Jalisco coast, the aseismic component must be the most important process for tectonic uplift and evolution of the coast in this area.