THE FATE OF OROGENS: HOW DO EXHUMATION PROCESSES SCALE ACROSS TIME?

At the scale of kilometers of exhumation, and from the perspective of thermochronology, we now know a fair bit about the range of process rates than afflict continental rocks and landscapes. At the one extreme, as in the indentor corners of active orogens, we know that sustained erosion of tens of kilometers can occur at rates approaching 10 km/m.y., and that this erosion can lead to the surface acting as an important and impactful boundary condition for geodyanmics. At the other extreme, data from cratons suggest extreme post-orogenic stability, with a kilometer of erosion spread on net over nearly a billion years, and rates of 1 meter per million years or perhaps even less - in these settings it is much hard to imagine erosion as other than a passive response to deeper processes. Both extremes represent remarkable features of planet Earth, but where things are still confusing and complex is how the one extreme evolves into the other. Older orogens like the Appalachians present us with odd juxtapostions of significant relief yet low exhumation rates. In understanding the longer-term evolution of orogens and continents, and implicitly sediment and nutrient supply with their impacts on the surface environment, it is interesting to ask whether a simple suite of processes scales across long time spans, or whether complexes of processes hand off from one another to shape landscapes and their evolution.