Slow NE-SW to NNE-SSW extension in the Pasto Ventura region of the southern Puna Plateau

Recent extension on the Puna Plateau of NW Argentina has been linked to lithospheric foundering, gravitational spreading, and edge effects. However, the timing, kinematics and rate of extension are poorly constrained. In the Pasto Ventura region, along the southern margin of the plateau, we map out two different groups of faults: (1) recently formed Quaternary normal faults and strike-slip faults; and (2) pre-Quaternary reverse faults reactivated in the Quaternary. The faults in Group (1) are relatively short (~1-2 km) normal and strike-slip faults that offset Quaternary geomorphic features. The orientation of these faults indicates NE-SW to NNE-SSW extension. The faults in Group (2) bound exposures of basement rock and are associated with basaltic cinder cones and lava flows. Previous studies indicate they were reverse faults which have been reactivated as normal faults. We applied kinematic GPS surveying and 40Ar/39Ar dating of three cinder cones displaced by two of Group-(2) faults. Kinematic analysis on vertical and horizontal offsets obtained by GPS survey shows that the one fault is now undergoing NE-SW to NNE-SSW extension, consistent with Group (1) fault kinematics. A cinder cone has been displaced 34-40 meters horizontally along this fault, yielding a slow extension rate of 0.02-0.04 mm/yr since 0.8-0.5 Ma. The shift from contraction to extension in the Pasto Ventura region is estimated to be between 7.8 and 0.5 Ma, but more likely between 7.8 and 4 Ma. A regional compilation of kinematics on the southern plateau from this study and existing data, although sparse, shows two spatial groups: the extension directions are N-S to NE-SW south of 26°S latitude, while they are NW-SE to NNW-SSW north of 26°S latitude. Mafic volcanism, thought to indicate the timing of the onset of extension in the Puna, shows a similar pattern, with the oldest ages (up to 7.3 Ma) clustered near 26°S latitude, becoming younger to both the north and the south. Kinematic and geochronologic data from the Pasto Ventura region are consistent with this trend. The pattern of ages of mafic volcanism and the fault kinematics imply that the removal of the lower lithosphere beneath the Puna Plateau occurred through the formation of a Rayleigh-Taylor type instability, or "driplet," located around 26°S at about 7.3 Ma. This driplet is probably relatively small since the extension rate observed on the surface is very slow. However, the pattern of extension directions indicates that the "driplet" located around 26°S was probably not perfectly cylindrical and/or the surficial extension pattern is also affected by other drivers, such as gravitational collapse, back-arc extension or other "driplets" located in the other regions.