Geometry and State of Stress of the Slab Beneath the North Central Andes

A. Kumar and L. S. Wagner
Geological Sciences, University of North Carolina at Chapel Hill

The central Andean plateau of southern Peru and Bolivia is one of the largest topographic features on Earth. There are two end member models proposed for this uplift: (1) Slow and steady rise since the late Eocene (~40 Ma) with maximum upper crustal shortening between 30 and 10 Ma or (2) rapid surface uplift of ~2.5 km in the late Miocene between 10.3 and 6.7 Ma. We present earthquake locations and focal mechanisms using data from a network of 90. Our new earthquake locations provide an improved insight about the variation in the geometry of subducting Nazca slab within the latitude range of 13°S to 18°S. We found seven different clusters of intermediate depth events in the depth range of 70 to 300 km. The seismic sections drawn parallel and perpendicular to the strike of the trench provide a better idea about the lateral variations of the slab geometry. Apparent width of the flat slab seems to be highest at ~15°S and is almost 20 km shallower than the previous estimates. We found a pronounced seismic gap in a circular region in southern Peru, supported by historical seismicity. Focal mechanism and stress axis orientation of slab events at ~15.5oS indicate down-dip extension. Continuity in the trend of stress perhaps indicates slab continuity and absence of slab tearing in southern Peru. Data from local slab events will eventually be incorporated into a local tomographic body wave inversion to better constrain the velocity structure of the mantle lithosphere. This in turn will provide the valuable information on the process involved in plateau development.

AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013