The concept of steady-state growth of compressive orogens is one of the most accepted ideas in continental tectonics developed in the last two decades, yet surprisingly few direct data exist to test the models that have been advanced. Testing and refining the concept of steady-state mountain building requires identification of cases of transient response and dynamic feedback in compressive orogens, and documentation of the spatial and temporal scales at which crustal deformation responds to erosion. This project takes advantage of a unique natural experiment in the active fold-thrust belt of west-central Taiwan where enough modern seismologic, geodetic, structural and geochronologic data exist to assess how an orogen responds to rapid and abrupt changes in wedge volume at scales of tens of kilometers. The Puli embayment in the west-central Taiwan lies approximately 1.5 km lower than neighboring parts of the fold-thrust belt. Geologic and geodetic evidence suggests that thrusting occurs over the width of the topographic embayment, rather than being concentrated at the front of the thrust belt, as is the case in the adjacent part of the thrust belt to the south where the topography maintains a classic wedge shape. The hypothesized connection between strain and topographic gradients is tested by (a) development of a database of late Quaternary slip rates and deformation patterns using a combination of structural, geochronologic and geomorphic methods, and (b) use of a 3D finite element model to determine the necessary and sufficient conditions required to produce the observed topography and deformation patterns. Fault-slip rates are obtained via dating of deformed stream terrace and basin fill deposits. Age dating also enables testing and calibration of long-term stream incision laws. The calibrated laws provide a quantitative basis for topographically derived estimates of strain patterns in inaccessible or poorly constrained areas of the belt such as the Alishan Range.
