Constraints on timing and rates of strath terrace formation on actively uplifting anticlines in the foreland of the Chinese Tien Shan.

The formation of strath surfaces (fluvially created, sub-horizontal erosion surfaces) requires that the rate of lateral erosion outpaces the rate of incision of a river. The change from incision to strath cutting has commonly been linked to a decrease of incision rates due to shielding of the river bed by a thick sediment cover¹. Straths are abandoned when the bed cover is reduced and incision resumes. A more recent study suggests that strath terrace formation might be linked to a change between a braided and a single-thread river². Finally, several models have explored strath formation due to inherent dynamics of meandering systems^3,4. In the foreland of the Tian Shan in northwest China, weakly consolidated Pliocene sand and siltstones are being actively uplifted at rates of 1 – 3 mm/y by a series of detachment anticlines. A number of elevated, several-kilometer-wide planation surfaces bear witness to a history of multiple strath cutting events by braided streams. In contrast, modern rivers incise into the uplifting folds creating 10 – 200 m deep canyons while the up- and downstream alluvial fans remain unincised.

We use GIS analysis, field mapping, and OSL dating to describe incision and beveling of the folds. Our chronologic data reveal at least 2 - 3 beveling events over the last 40 ky on the Mutule fold. We find that lateral erosion of the bedrock during beveling events occurs at rates that are more than an order of magnitude higher than average incision rates. During times of incision (which can be tens of thousands of years long), lateral erosion rates need to be considerably lower in order to explain the formation of narrow canyons. Thus, our observations of scale, rate, and intermittency of strath cutting, seem difficult to reconcile with models that explain strath formation by variations of the incision rate¹ or intrinsic meandering dynamics under steady forcing^3,4. The critical requirement to explain our observations appears to be repeated changes in the rate of lateral erosion, for example, by climatically modulated changes in river lateral mobility.

1 Hancock, G. S. & Anderson, R. S. (2002), GSA Bulletin 114, 1131-1142.

2 Finnegan, N. J. & Balco, G. (2013), GSA Bulletin 125, 1114-1124.

3 Finnegan, N. J. & Dietrich, W. E. (2011), Geology 39, 143-146.

4 Limaye, A. B. S. & Lamb, M. P. (2014), JGR: Earth Surface 119, 927-950.