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

摘要

Coastal marine and continental sedimentary facies of Middle to Late Miocene age are exposed in the Andes of southern Ecuador (Cuenca, Ciron-Santa Isabel, Loja, Malacatos-Vilcabamba and Catamayo-Gonzanama Basins). The chronostratigraphy of the basin series was established by zircon fission-track dating on a total of 120 tephra layers. Subsequently, the timing of tectonic events was estimated through the well-dated stratigraphic sequences and intervening unconformities. Sedimentation from approx= 15 to 9. Ma (termed Pacific Coastal Stage) was dominantly of coastal marine type, extending over an area far greater than the present basin perimeters. It ended when a period of east-west-oriented compression at approx= 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 Basin 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 approx 100 deg C at 18 Ma, when they started to cool down during a period of exhumation. This process preceded the Pacific Coastal Stage development of the other basins. In the Cuenca Basin, the oldest sediments were buried to temperatures of ca. 120 deg C by 9 Ma, when a period of inversion began and a phase of erosion was dominant. This timing correlates well with that estimated from structural evidence. At ca. 6 Ma the cooling rate slowed down and maybe even reverted to a small increase in temperature until 3 Ma, when the final stages of exhumation took place. Assuming a geothermal gradient of 35 deg C/km, total uplift for this part for Ecuador is about 6200 m over the last 9 million years. Assuming a steady state continuous movement, this means a mean rock uplift rate of approx 0.7 mm/yr and a surface uplift of 0.3 mm/yr to the Present.