Fabrics and mechanics of forearc deformation at the Costa Rican erosive convergent margin and implications for seismogenic fault zone behavior - Follow-up of IODP expedition 334 (Costa Rica Seismogenesis Project, CRISP)

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Subduction zones at active continental margins, where 90% of the worldwide earthquakes occur, are either accretionary or erosive. At accretionary margins, material from the downgoing oceanic plate, mostly marine sediments, is tectonically detached during subduction and accreted to the upper continental plate. At erosive margins, material from the overriding continental plate, mostly continental basement and slope sediments, is tectonically eroded and attached to the lower plate. Although these two different types of subduction have approximately the same worldwide frequency of occurrence, the Megathrust earthquakes with a magnitude > 8.5 have so far almost exclusively been recorded at accretionary margins (e.g., 8.8 Maule, Chile, 2010; 9.0 Sumatra, Indonesia, 2004; 9.5 Valdivia, Chile, 1960). To better understand the related earthquake mechanisms two major projects of the International Ocean Drilling Program (IODP) endeavor to drill for the first time into the seismogenic zone of an accretionary (Nankai trench, Japan; project NanTroSEIZE) and an erosive margin (Costa Rica; project CRISP) at approximately 5000-6000 meters below sea floor. The subduction channel between the continental upper and the oceanic lower plate is mainly filled with upper plate material at the erosive margin and lower plate material at the accretionary margin. The properties of this infill are among the most important factors controlling the deformation behavior. If deformation is distributed it is usually continuous, whereas if deformation localizes it can be discontinuous. At major thrust faults characteristic of localized deformation, high friction and asperities might lock the fault and hence prevent continuous sliding and strain energy release. When a critical threshold in stress build-up is reached, friction and asperities are overcome and earthquakes nucleate. It is assumed, however, that large and very large earthquakes can only develop if the fault material is prone to velocity weakening, i.e. it becomes weaker with faster sliding rate. The erosive subduction channel material is lithologically rather variable and characterized by a long history of compaction and deformation in the continental forearc wedge, whereas the accretionary subduction channel material mainly consists of more homogeneous pelagic and trench fill deposits which is continuously transported down into the seismogenic zone. In our study we will investigate this input material from the forearc wedge offshore Costa Rica and from the Nankai trench and incoming plate offshore Japan by microstructural analysis and rock mechanial testing. The results should help to clarify if these differences of the material input are significant and if they are crucial for differences in deformation, earthquake nucleation and propagation.