Erosional control on the geometry and kinematics of thrust belt development in the central Andes

Long-term erosion rates in the central Andes may have influenced not only mean elevation and relief but also the regional geometry and kinematic history of the orogenic belt. A drastic along-strike erosional gradient exists in the modern central Andes, from a high-erosion region directly north of the 17.5 degrees S bend in the Andes to a low-erosion region south of the bend. This gradient has existed since similar to 10-15 Ma based on fission track analyses of middle Miocene to Holocene denudation and qualitative evaluations of the preservation potential of middle-late Tertiary volcanic edifices and synorogenic sediment. Global positioning system velocity data indicate conflicting patterns of active surface shortening north and south of the 17.5 degrees S bend. Whereas present-day shortening in the thrust belt north of the bend is distributed over much of the width of the belt, south of the bend it is concentrated neat the eastern frontal margin. Structural data suggest a similar kinematic situation during late Miocene to Holocene shortening: an out-of-sequence chronology of thrusting in the narrow (200 km wide) thrust belt to the north versus a forward-breaking sequence of thrusting in the relatively wide (350 km wide) thrust belt to the south. The long-term internal deformation and limited width of the thrust belt north of the bend are attributed to prolonged subcritical thrust-wedge conditions, induced by rapid erosion rates since similar to 10-15 Ma. Such conditions inhibit thrust-front advance and favor distributed deformation within the thrust-belt inferior. In the thrust belt south of the bend, a progressive eastward migration of thrusting is interpreted to be the result of long-term critical thrust-wedge conditions promoted by extremely low rates of denudation since middle Miocene time.