Neogene tectonic evolution and exhumation of the southern Ecuadorian Andes: a combined stratigraphy and fission-track approach

Coastal marine and continental sedimentary facies of Middle to Late Miocene age are exposed in the Andes of southern Ecuador (Cuenca, Giron-Santa Isab el, Loja, Malacatos-Vilcabamba and Catamayo-Gonzanama Basins). The chronost ratigraphy of the basin series was established by zircon fission-track dati ng on a total of 120 tephra layers. Subsequently, the timing of tectonic ev ents was estimated through the well-dated stratigraphic sequences and inter vening unconformities. Sedimentation from approximate to 15 to 9 Ma (termed Pacific Coastal Stage) was dominantly of coastal marine type, extending ov er an area far greater than the present basin perimeters. It ended when a p eriod of east-west-oriented compression at approximate to 9.5-8 Ma exhumed the region, and sedimentation was then restricted to smaller basins (termed Intermontane Stage). These Late Miocene continental sediments were for the first time sourced from the west in the rising Western Cordillera. Apatite fission-track analysis was applied to some of the tephras in the Cuenca Ba sin and also to the older (Eocene, 42-35 Ma) Quingeo Basin series in order to quantify the basin histories with respect to timing and amount of burial and later exhumation. In the Quingeo Basin burial of the oldest sediments reached temperatures of similar to 100 degrees C at 18 Ma, when they starte d to cool down during a period of exhumation. This process preceded the Pac ific Coastal Stage development of the other basins. In the Cuenca Basin, th e oldest sediments were buried to temperatures of ca. 120 degrees C by 9 Ma , when a period of inversion began and a phase of erosion was dominant. Thi s timing correlates well with that estimated from structural evidence. At c a. 6 Ma the cooling rate slowed down and maybe even reverted to a small inc rease in temperature until 3 Ma, when the final stages of exhumation took p lace. Assuming a geothermal gradient of 35 degrees C/km, total uplift for t his part for Ecuador is about 6100 m over the last 9 million years. Assumin g a steady state continuous movement, this means a mean rock uplift rate of similar to 0.7 mm/yr and a surface uplift of 0.3 mm/yr to the Present. (C) 1999 Elsevier Science B.V. All rights reserved.