THE ORIGINS OF CORAL GEODESY AND PALEOGEODESY AND INVESTIGATION OF CONVERGENT MARGIN TECTONIC PROCESSES

This presentation will comment on the earliest examples of coral geodesy and paleogeodesy, provide some examples of how coral evidence might enhance understanding of relationships between megathrust rupture and arc tectonic evolution, and consider some potential improvements for the method. Probably the first use of individual coral colonies as indicators of coseismic vertical tectonic displacement was in the New Hebrides in 1976. Since then the coral geodesy method has contributed significantly to new perspectives regarding both interseismic and coseismic vertical motions at convergent plate margins and their relationship to the earthquake cycle. However, the method has promise for deeper understanding of longer-term net tectonic deformation of convergent margins and how it relates to the earthquake cycle and other parameters affecting interplate coupling and upper plate deformation.

Despite the challenges for coral paleogeodesy where there is net vertical tectonic deformation, the information from such settings may be the key to understanding new aspects of convergent margin tectonic evolution. Application of coral paleogeodesy is somewhat different in areas where net vertical deformation is negligible compared to areas where net uplift rates are on the order of 0.5 to 8 mm/yr. Very slow to negligible net uplift rates tend to produce microatolls that remain near the same levels as living and dead microatolls that record older events. These reefs and any emerged parts are typically wide, flat, and shallow. In contrast, rapid uplift produces a succession of corals recording emergence events that are vertically separated along a profile sloping uphill from the shoreline. These tend to be narrow fringing reefs that slope steeply offshore. The vertical succession of emerged corals potentially provides a better chance to sort out the sequence of events, but supratidal erosion can be a problem. Subsiding shorelines produce very narrow fringing reefs, ragged shorelines in map view, and evidence of older vertical motions is likely to be buried beneath subsequent coral growth so that it is very difficult to access. However, it is possible that drilling and geochemical techniques will allow vertical tectonic motions on subsiding coasts to be investigated as well.