Eustacy Feedback Coupling to Seismogenic Behavior of the Northern Peru Subduction Zone

We present a combined analysis of geological and geophysical data collected both onshore and offshore along the northwestern Peru forearc area (3°30'-7°30'S), from the coastal plain to the trench axis. Onshore, geomorphic analysis places constraints on the relative importance of eustatic versus tectonic factors in preserving and modifying the uplifted coastal landforms along the coastal plain. Breaking-wave morphologic markers were dated using the in situ-produced 10Be cosmonuclide. The data document a tectonic segmentation, allowing us to differentiate two areas as regard their evolution through time: the northern Cabo Blanco and the southern Paita-Illesca segments. For the past 200 kyr, both segments uplifted at high rates of 10 to 20 mm.yr-1 through tectonic pulses coeval with the eustatic deglacial sea level rises of isotope stage 1 and warm isotope substage 5e, respectively. Offshore, industry-acquired reflection seismic lines combined with EM12 bathymetric data allow us to investigate the tectonic regime and deformation of the continental margin and shelf. Major dipping-seaward detachments control the long-term subsidence of this area. These main tectonic features define a tectonic segmentation. No clear tectonic correlation in time exists between the onshore and the continental margin segmentations, or in space either. The long-term subsidence of the offshore, indicative of subduction- erosion working at depth requires low coupling along the subduction channel at depth. The distribution of permanent deformation along the northern Peru forearc area includes long-term uplift along the coastal plain and long-term subsidence along the continental margin. The neutral line is located within the 10 km seaward from the Present coastline. An extensive sequence of raised marine cliffs and associated notches evidences that the most recent uplift step (20-23 ka to Present) along the Cabo Blanco segment is related to a sequence of major earthquakes. We infer that eustacy exerts important feedback coupling to seismogenic behavior of the North Peru subduction zone. We speculate that during sea level fall, pore-fluid pressure diminishes along the subduction channel inducing a possible seaward migration of the locked zone (i.e. migration of the updip limit) reaching a maximum by the end of the eustatic low stand. During eustatic sea level rise pore-fluid pressure increases along the subduction channel. This in turn is capable of weakening the previously locked zone along the plate interface beginning an earthquake sequence. Earth's orbital variations are a potential external cause that may control the physical processes at work along plate interface. Reference: Bourgois, J., F. Bigot-Cormier, D. Bourles, R. Braucher, O. Dauteuil, C. Witt, F. Michaud, 2007, Tectonic record of strain buildup and abrupt co-seismic stress release across the northwestern Peru coastal plain, shelf, and continental slope during the past 200 kyr, J. Geophys. Res., in press.