CANYON INCISION: TECTONICS, CLIMATE AND DRAINAGE BASIN CAPTURE IN THE PERUVIAN ANDES

The drivers behind the dramatic 2-3 km relief canyons carved into the ca. 3.5 km high Central Andean plateau are still highly debated, with end member arguments focusing on plateau surface uplift and climate enhanced erosion. To assess canyon incision along the eastern margin of the Peruvian Andean plateau, we use a landscape evolution model (CASCADE), integrated with cross-section derived estimates of structural geometry, kinematics and shortening rates, and global circulation model (GCM) derived estimates of precipitation change. We evaluate simulations of both constant shortening rates and rates that best replicate measured cooling ages and support a significant slowing of shortening (from ~ 4 mm/yr to 0.4 mm/yr) at 10 Ma. Magnitude and location of precipitation is based on GCMs that simulate change in precipitation at select times between the Miocene and present that shift in space as a response to northeastward plateau growth. Simulated topography is compared to modern topographic metrics such as drainage divide location, relief, channel shape and eastward extent of high elevation interfluves.

The geometry and sequential motion of faults and the resulting locations of surface uplift create a series of internally drained basins as the plateau propagates northeast. The location and magnitude of canyon incision is determined by the occurrence and timing of fluvial captures of these basins. Constant shortening models do not allow for the capture of internally drained basins because the rate of uplift defining the basin edge outpaces the stream’s ability to incise. Shortening rates that decrease to ~0.4 mm/yr at 10 Ma allow streams to compete with uplift and begin incision into the plateau edge. Incision magnitude and location are best replicated when drainage basin capture occurs between 3.5 to 0.5 Ma. The resulting increase in drainage area from capture, coupled with discharge fluctuations from precipitation shifts, facilitates canyon incision into the plateau by increasing stream power. The timing and ultimate capture of these drainage basins provides the impetus for streams to carve the impressive canyons observed today.