Structural imprints at the front of the Choco-Panama indenter: Field data from the North Cauca Valley Basin, Central Colombia

The northern Andes are a complex area where tectonics is dominated by the interaction between three major plates and accessory blocks, in particular, the Choco-Panama and Northern Andes Blocks. The studied Cauca Valley Basin is located at the front of the Choco-Panama Indenter, where the major Romeral Fault System, active since the Cretaceous, changes its kinematics from right-lateral in the south to left-lateral in the north. Structural studies were performed at various scales: DEM observations in the Central Cordillera between 4 and 5.7 degrees N, aerial photograph analyses, and field work in the folded Oligo-Miocene rocks of the Serrania de Santa Barbara and in the flat-lying, Pleistocene Quindio-Risaralda volcaniclastic sediments interfingering with the lacustrine to fluviatile sediments of the Zarzal Formation. The data acquired allowed the detection of structures with a similar orientation at every scale and in all lithologies. These families of structures are arranged similarly to Riedel shears in a right-lateral shear zone and are superimposed on the Cretaceous Romeral suture. They appear in the Central Cordillera north of 4.5 degrees N, and define a broad zone where 060-oriented right-lateral distributed shear strain affects the continental crust. The Romeral Fault System stays active and strain partitioning occurs among both systems. The southern limit of the distributed shear strain affecting the Central Cordillera corresponds to the E-W trending Garrapatas-Ibague shear zone, constituted by several right-stepping, en-echelon, right-lateral, active faults and some lineaments. North of this shear zone, the Romeral Fault System strike changes from NNE to N. Paleostress calculations gave a WNW-ESE trending, maximum horizontal stress, and 69% of compressive tensors. The orientation of (A is consistent with the orientation of the right-lateral distributed shear strain and the compressive state characterizing the Romeral Fault System in the area: it bisects the synthetic and antithetic Riedels and is (sub)-perpendicular to the active Romeral Fault System. It is proposed that the continued movement of the Choco-Panama Indenter may be responsible for the 060-oriented right-lateral distributed shear strain, and may have closed the northern part of the Cauca Valley, thereby forming the Cauca Valley Basin. Conjugate extensional faults observed at surface in the flat-lying sediments of the Zarzal Formation and Quindio-Risaralda volcaniclastic Fan are associated with soft-sediment deformations. These faults are attributed to lateral spreading of the superficial layers during earthquakes and testify to the continuous tectonic activity from Pleistocene to Present. Finally, results presented here bring newinformation about the understanding of the seismic hazard in this area: whereas the Romeral Fault Systemwas so far thought to be themost likely source of earthquakes, themore recent cross-cutting fault systems described herein are another potential hazard to be considered. (c) 2008 Elsevier B.V. All rights reserved.