ANOMALOUS ACCRETIONARY MARGIN TOPOGRAPHY FORMED BY REPEATED EARTHQUAKES

Along most subduction zones globally, it is difficult to make direct observations of the plate boundary megathrust in the near-field since, in most cases, the area above the locked or coupled plate interface sits offshore. One place where that problem is overcome is along the Pacific margin of Costa Rica, where two terranes - the Nicoya and Osa Peninsulas - are exposed in the near-trench region overtop of the locked plate interface. This fortuitous positioning has been exploited to provide a range of observations that have led to improved estimates of the locked patches and detailed imaging of the relationships between interface seismicity and seismogenic asperities. Although these terranes are well-located for such observations, our uncertainties in how the anomalous topography that exposes these peninsulas developed raises question about how applicable the observations made there are to other subduction margins. Current models for their development tend to differ among the various exposed terranes, although similarities in their morphology and behavior suggest that a common mechanism may be responsible for their formation. here we propose to investigate a new possible mechanism that would be responsible for the formation of both the Nicoya and Osa Penisnulas in the past, and is currently producing a third peninsula - the Burica Peninsula at the intersection of the Panama fracture zone and the margin. In this research we explore an alternative view of the process by which such terranes develop and become sub-aerially exposed. Specifically we will the premise that the anomalous topography along the Pacific coast of Costa Rica has been produced by repeated, great subduction earthquakes that have ruptured across the boundary separating the Cocos and Nazca plates - the Panama fracture zone. The pattern of upper-plate shortening generated by such a process (documented in the 2007 Mw 8.1 Solomon Islands earthquake, and seen in the present-day deformation of the Burica Peninsula - the current location of the Panama fracture zone) convolved with the migration history of the Panama triple junction (PTJ) is proposed as the mechanism to produce substantial along-margin, long-lived accretionary margin topography.