Morphing Blocks and the Search for Secular Variation in Slip

Understanding continental tectonics has remained a fundamental challenge for over 40 years. Although debate has long focused on block vs. continuum models, a more complex view is required to account for both the spatial and temporal variations in deformation that are observed along plate boundaries. In the morphing block model, fault systems produce behavior roughly analogous to agglomeration within a granular material. In particular, although deformation is block-like, the block geometries are transient because different faults define the block boundaries at different times. This frequent reorganization of the active fault network also leads to secular variation in slip along faults at timescales of 10-100 kyr. Secular variation in slip refers to changes in slip rate that occur at timescales 2-3 orders of magnitude longer than the time between individual rupture events. Such variations are most clearly indicated by statistically significant changes in slope on plots of age vs. displacement for a suite of displaced markers with a range of ages along a given fault reach. But is this approach tractable? Two questions are particularly important. First, what are current detection limits for observing secular variation in slip? In other words, considering typical errors in offset and age data, how much must rates change before they can be detected? Second, how do we identify and quantify uncertainties introduced when linking age and offset measurements? Strike-slip faults provide the best opportunity to search for secular variation in slip and address these questions because burial and erosion obscures slip histories along dip-slip faults. Changes in slip rate are expected in complex strike-slip networks due to kinematic incompatibilities that arise during simultaneous slip along multiple interacting fault strands. Examples from major strike slip systems in both Tibet (e.g., Altyn Tagh) and the western US (e.g., northern San Andreas) help illustrate these ideas.