Volcano-estratigrafía y tectónica del Valle Central Occidentaly las estribaciones de los Montes del Aguacate a lo largo de la ruta 27, Costa Rica

route 27, known as ciudad colón-caldera highway, offers excellent geological sections showing much of the volcanic and sub-volcanic stratigraphy of the western part of the central valley and the aguacate mountains foothills. here a volcaniclastic succession correlated with the grifo alto formation (pliocene) is exposed. it consists of lavas, breccias and epivolcanic deposits (hyperconcetrated fluvial and debris avalanches deposits) that locally show strong propilitic alteration. in some places the succession is slightly to moderately tilted, faulted and cut by dykes. a series of old debris avalanche deposits is also present, related to the volcanic activity in the aguacate mountains. locally, a series of monomictic mesobreccias units is exposed consisting of glassy andesitic blocks in a vitro-crystalline matrix ash interpreted as block and ash-flow deposits, which is probably coeval with the volcanic monteverde formation (lower pleistocene). at least, 4 units of debris flow/debris avalanche deposits are associated with nuestro amo formation. at the upper part of the succession, several porphyrytic andesitic lava-flows, correlated to the lower colima formation are followed by ignimbrites of the puente de mulas and tiribí formations, of middle pleistocene age, containing hyperconcentrated fluvial-ignimbrite deposits with decreasing thicknesses. a propagation fold deforms these ignimbrites, indicating the existence of a thin-skinned type overthrust with a sw vergency, which was active since, at least, the middle pleistocene. the tilting of the rocks exposed along route 27, the type of fracturing/faulting and folding, and the existence of some structural highs, is well explained with the thrust faulting model proposed in this article.