Evidence of Late Cenozoic transpressive reactivation of an inherited strike-slip fault system and its influence in drainage reorganization in the Longitudinal Valley of Northernmost Chile

The Longitudinal Valley in Northernmost Chile was the main depocenter of widespread fluvial-alluvial systems active through the Neogene. These formed extensive lacustrine systems located at the eastern slope of the Coastal Cordillera until exorheic drainages developed between Arica and Pisagua (~18°30’S – 19°30’S) ca. 3 Ma ago. The top surfaces of the continental deposits form a regional scale pediplain (Pacific Paleosurface), where run-off is focused in present-day perennial streams draining to the Pacific Ocean through deeply incised quebradas. Some geomorphic and climatic constraints and suggestions exist regarding how the uplift of the Coastal Cordillera and the western Andes influence the shift in drainage regimes. However, little is known about how tectonic activity across this region affected landscape evolution since the structural architecture is difficult to unravel in this area of high sedimentation but low displacement rates.

We performed an exhaustive regional mapping of structural and geomorphic evidences of fault activity and drainage patterns based on high-resolution DEMs, satellite, and UAV imagery data, as the long-term hyperaridity of this area leads to well-preserved landforms and lack of vegetation cover. Our investigations reveal evidence of a reactivated complex inherited strike-slip system across the Longitudinal Valley, deforming Miocene to Quaternary surfaces. Local growth strata, angular unconformities, and flower structures within the Late Miocene to Early Pliocene lacustrine deposits suggest syn-sedimentary and dextral transpressional faulting near the boundary between the Longitudinal Valley and Coastal Cordillera. Importantly, we observe that large drainage reorganization patterns can be triggered by only little displacement along often blind fault structures, creating sufficient topography that cannot be surpassed by drainage incision in this hyperarid setting.

The interpretation of a reprocessed ENAP seismic section at ~19°20’S, suggests that this fault system consists of inherited Mesozoic inverted structures deforming Oligocene to Late Miocene strata. Furthermore, progressive abandonment and deformed terraces of low-incised rivers crossing compressive structures suggest that these were active during the Quaternary, most probably ongoing until the recent past. New dating of deformed marker horizons will bring further insights regarding key parameters of fault activity for the Late Cenozoic and Quaternary.