THE LITHOSPHERIC STRUCTURE OF THE SIERRAS PAMPEANAS REGION OF ARGENTINA

One of the intriguing aspects of the subduction of the Nazca plate beneath western South America is the along strike segmentation of the dip of the descending plate as defined by the slab earthquake distribution. At approximately 31°S the Nazca plate has a subhorizontal geometry and extends inland over 300 km beneath the Sierra Pampeanas of west-central Argentina. Understanding what causes the flat slab geometry, and its influence on the overlying lithosphere remains a fundamental goal. To this end, we deployed 22 PASSCAL broadband seismic stations with a station spacing of approximately 60 km across the Andes along two transects over the flat slab (30.5°S) and the steeper subduction zone (36°S) as part of the CHile ARgentina Geophysical Experiment (CHARGE). We find that most of the backarc region above the flat slab is in compression with thrust earthquakes in the depth range of 5-25 km. The earthquakes in the flat segment of the down-going slab are predominantly normal fault events with extension in the direction of slab dip. Receiver function and regional surface wave analysis indicates that the eastern Sierras Pampeanas has a crustal thickness of 35 km and a Vp/Vs ratio of less than 1.75 while the western Sierras Pampeanas crustal thickness increases to 55 km with a Vp/Vs ratio greater than 1.8. The western Sierras Pampeanas has a high velocity lower crust that we suggest represents higher density material that accounts for the similar elevation between the western and eastern Sierras Pampeanas despite the different crustal thickness. The change in crustal character (both thickness and seismic velocity structure) corresponds to a suture between accreted Precambrian terranes. Local seismic tomography indicates that the mantle between the crust and the flat slab has low P-wave velocities and high S-wave velocities that we interpret to be cold lithospheric mantle. It is not consistent with hydrated mantle and our study suggests that the Nazca slab does not dewater until further east after it descends into the mantle. Taken together the CHARGE results suggest that lithospheric structure still reflects Precambrian terrane boundaries and that the present day mantle under the Sierra Pampeanas is dry and strong and able to transmit basal shear from the underlying flat slab.