Fault-related folding, drainage network evolution and valley incision during the Neogene in the Andean Precordillera of Northern Chile

Abstract

We describe Neogene drainage network development before, during and after regional folding in the Precordillera of the Andes of Northern Chile (18degrees-19degreesS). The drainage network developed on an extensive regular erosional surface formed on Oligocene-Miocene ignimbrites (Oxaya Formation). The folding affecting the rocks of the Precordillera is represented by the Oxaya Anticline (OA),which trends N to NW, is 50-km long and 30-km wide. The fold is gentle, west-vergent and has resulted in an uplift of the hinge zone by up to 850 m. It formed by the propagation, on its western border, of a subvertical east-dipping blind reverse fault (Ausipar Fault [AF]). The folding occurred in the middle late Miocene, essentially between 11.7+/-0.7 and 10.7+/-0.3 Ma: its maximum duration was 2 My. Two phases of drainage development are recognised in the Precordillera. An initial lower middle Miocene (19-12 Ma) parallel drainage network is well preserved and synchronous with fluvial and lacustrine sedimentation in the Central Depression. The network was poorly structured and attained maximum incision of to 250+/-50 m. The minimum long-term rate of incision was 36+/-7 m/My. Folding generated the progressive abandonment and fossilisation of the initial parallel drainage network, which became trellissed and concentrated in a few deeply incised valleys. In the anticline hinge, the Lluta and Azapa main valleys present a total incision of 1650+/-50 and 1605+/-50 m, respectively, whereas the Cardones valley presents 595+/-50 m. During folding, river incision at the fold hinge was equal to or greater than the amplitude of the folding, which is 790+/-50 m in the Lluta valley and 715+/-50 m in the Azapa valley. Consequently, the long term rate of synfolding incision was much greater than 395+/-25 (Lluta valley) and 358+/-25 m/My (Azapa valley). The Cardones valley was fossilised approximately in the middle of the folding. After folding (11 to 0 Ma), the incision in the Lluta and Azapa rivers was 610+/-100 and 640+/-100 m and with a rate of 56+/-9 and 58+/-9 m/My, respectively. The postfolding incision rate is approximately twice than of the prefolding incision rate. This implies an acceleration of valley incision since the late Miocene, which is considered as consequence of regional uplift of the Andean forearc.