Testing models for lithospheric thinning in the Puna Plateau, NW Argentina using alignment and morphology of young, mafic, monogenetic cinder cones

New methods for measuring morphology and alignment of monogenetic cinder cones as a proxy for lithospheric stresses and the location and trend of feeder dikes have been applied to 47 late Miocene (~7Ma) to present mafic cones found on the world’s second highest continental plateau, the Puna plateau of northwest Argentina. Lithospheric thinning is believed to have occurred beneath the plateau; the analysis of spatial and temporal stress patterns recorded by the cinder cones will allow for testing among the proposed models which include delamination, full convective removal, partial convective removal, piecemeal removal, and back-arc extension. These methods were applied to cinder cones in the southern Puna (24°S -27°S, 69°W-66°W) in an attempt to better understand the state of the crust and the lithospheric mantle beneath the plateau. Linear arrays of monogenetic cinder cones can reflect the orientations of subsurface feeder dikes which form parallel to the maximum horizontal stress (SH) and thus lend insight into the state of stress in the lithosphere (Paulsen and Wilson, 2009, in press). Other cone features such as elongation, can also reflect lithospheric stress, and breaching direction can reflect the strike, dip, and sense of displacement of underlying faults (e.g. Tibaldi, JGR, 1995). Mapping of these features was carried out in ArcGIS using ASTER imagery, SRTM DEMs, Google Earth imagery, and was supplemented by field observations. Ellipses were systematically created in ArcGIS using the “Directional Distribution: Standard Deviational Ellipse” tool. A series of measurements and data on each cone ellipse were collected, including: axial ratios, length of long and short axis, elongation and breaching directions, and feature coordinates. Cone fields, defined as localized clusters of similarly aged cones, were analyzed for the number of vents, alignment lengths, standard deviation of vent centers from a best fit line, number of elongate vents, standard deviation of trend of elongate vent axes from a best fit trend, and the average difference of vent spacings. These data demonstrate that between 70-80% of the cones are slightly elongated to elongated, with corresponding axial ratios ≥1.2 and ≥1.4, and a common elongation direction to the northwest/southeast. Many of the cones also show a preferred breaching direction to the west. These preliminary results indicate that generally uniform NW-SE extension across the southern Puna, with no apparent temporal variation since Late Miocene time. The lack of age variation argues against piecemeal foundering of the lithosphere and delamination, both of which would predict complicated temporal extension patterns. Partial or full convective removal would predict a radial or concentric stress field; however, this effect may be overwhelmed by extensional strain related to gravitational extensional collapse or back-arc extension within the plateau, which could produce more uniform NW-SE extension. Additional field and remote observations and Ar/Ar geochronology of cones will help further distinguish these models.