Rapid tectonic and paleogeographic evolution associated with the development of the Chucal anticline and the Chucal-Lauca Basin in the Altiplano of Arica, northern Chile

Abstract

The east-vergent Chucal thrust system, off the east side of the Chapiquina-Belen ridge in the Western Cordillera, was continuously or almost continuously active for - 18 m.y. (< 21 to > 2.7 Ma). Contractional activity deformed late Oligocene tuffaceous, fluvial, or distal alluvial deposits of the uppermost Lupica Formation; fluvial and lacustrine deposits of the Miocene Chucal Formation; tuffaceous and coarse fluvial deposits of the Quebrada Macusa Formation; and the lower part of the westernmost, latest Miocene? -Pliocene, essentially lacustrine Lauca Formation. It controlled the paleogeographic and paleoenvironmental conditions in which these units were deposited. More humid conditions oil the east side of the Chapiquina-Belen ridge favored the development of all abundant mammal fauna and flora. The deformation is characterized b the Jaropilla thrust fault and the Chucal anticline, which is east of the fault. Deformation oil the Chucal anticline began before the deposition of the Chucal Formation and was controlled by a blind thrust fault. The west flank has a nearly constant dip (45-50 degrees) to the west and nearly continuous stratigraphic units, whereas on the east flank, the dip angle is variable, diminishing away front the axis, and the stratigraphic units are discontinuous. The anticline growth on this flank Caused the development of three observable progressive Unconformities. Deformation was particularly rapid during the deposition of the - 600 m thick Chucal Formation (between the 21.7 +/- 0.8 Ma old uppermost Lupica Formation and the 17.5 +/- 0.4 Ma old base of the Quebrada Macusa Formation, a 4 m.y. period). The deformation rate decreased during the deposition of both (1) the similar to 200 m thick Quebrada Macusa Formation (between the 17.5 +/- 0.4 Ma age of its basal deposits and the similar to 11 Ma age Of its Uppermost levels, a 7 m.y. period) and (2) the lower Lauca Formation (between the similar to 11 Ma alle Of the Upper Quebrada Macusa Formation and the 2.3 +/- 0.7 Ma old Lauca ignimbrite, which is intercalated within its middle part). We interpret the contractional deformation to be associated with tectonic activity that led to the uplift of the Altiplano however, paleobotanical evidence does not indicate any major altitude changes during the time period considered here but rather Suggests that rapid uplift took place after the deposition of the Quebrada Macusa Formation.