UTIG RESEARCH PROJECTS ARCHIVE

There is a growing consensus among earth scientists that for some problems focused, interdisciplinary projects are needed to make advances of fundamental scientific, societal, and economic importance. Among these problems is the study of convergent margins, where the deadliest and most damaging earthquakes occur and where a tremendous flux of mass and fluids profoundly affects the land surface and the earth’s climate. One of the geographical areas identified for focused research by geochemical, seismological, and tectonics scientific communities is the southern Middle America Trench (MAT) off Nicaragua and Costa Rica. We propose to participate in a USA-German project that is aimed at characterizing the subducting Cocos Plate. The funded German project "PAGANINI" will dedicate three legs of the R/V Sonne to geophysical data acquisition and extensive geologic sampling in key areas of the Cocos Plate and adjacent Nazca Plate in the Panama Basin. The results of this project will include crustal velocity models across the smooth, seamount dominated, and Cocos Ridge domains of the Cocos Plate that will bear directly on crustal mass flux estimates into the subduction system and on the nature and morphology of the seismogenic plate boundary in these diverse sections of the same convergent margin. Geologic sampling will concentrate on both geochemical characterization of the Cocos Plate and on geochronology to help decipher the complex interaction between the two spreading ridges, East Pacific Rise and Galapagos Rift system, that create the Cocos Plate and the Galapagos hotspot which modifies it.

Our proposed participation will be in the initial geophysics leg of the PAGANINI project, which is planned as a 38 day cruise to collect ocean bottom seismometer/hydrophone (OBS/OBH) data, swath bathymetry, and magnetic data. We (UTIG) will focus our efforts on the OBS/OBH data set, but we will have full access to the various other data sets and will be involved in the overall integrated interpretation of these data. OBS/OBH will be recorded on eight 200+ km long profiles using 20 to 30 instruments each for a total of 180 deployments and resulting instrument spacing of 5-10 km. UTIG will have primary responsibility for analysis and interpretation of two of the eight profiles. UTIG will work closely with the GEOMAR scientists to ensure a consistent interpretation.

We are particularly interested in working with planned OBS/OBH profiles 5 and 6. Profile 5 is outboard of the Nicoya Peninsula and it will sample both the smooth and seamount dominated crust similar to what is now subducting. This profile will allow detailed characterization of the subducting crust that we can use as a comparison to recent crustal seismic investigations (TICOSECT) of the forearc. This profile also likely spans the boundary between crust created at the East Pacific Rise and the Galapagos Rift System; this transition was previously thought to lie at the "rough/smooth" boundary that projects toward the southeast coast of the Nicoya Peninsula, but recent magnetic mapping suggests that it may lie farther north--near the ODP Leg 170 transect. This profile plus the planned magnetic and swath mapping is likely to identify this key boundary. We are also interested in whether seamounts are likely to subduct intact or break apart during the subduction process. Velocity structure of the larger seamounts off central Costa Rica may indicate a relatively homogeneous structure, that would promote intact subduction, or suggest possible zones of weakness, likely to result in partial seamount accretion.

Profile 6 is an onshore/offshore profile that will extend from the Sandino forearc basin of Nicaragua to the land area of northwestern Costa Rica. This is a departure from the Cocos Plate emphasis, but it will provide primary data to begin to analyze the remarkable geochemical changes that occur in arc volcanics from Nicaragua to Costa Rica. In particular it should indicate changes in the upper plate crustal structure and slab dip that must be considered when analyzing factors that could affect the arc volcanism. This project represents an important step between previous work that has been concentrated on the forearc regions and expected future work which will examine the entire subduction system.