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Abstract

Borehole image and physical properties data from two sites (U1378, U1379) drilled during the Integrated Ocean Drilling Program (IODP) Expedition 334 were analyzed to study the effect of erosional subduction at the Costa Rica convergent margin. The various laboratory and log data were collected from Logging-While-Drilling (LWD), moisture and density analysis (MAD), and Multisensor Core Logger tools (MSCL). Borehole breakouts interpreted from image data acquired from LWD were identified and analyzed using GMI Imager software. In this study, borehole breakouts from holes U1378A and U1379A showed different borehole shape and stress directions, indicating different present-day in situ stress orientation at the two sites. In U1378A, the azimuths of breakouts are from 52.21 to 58.14 degrees, and the widths of breakout are variable (52.46–70.55 degree) with depth. Conversely, the azimuths of breakouts in U1379A are from 106.79 to 116.53 degrees and the widths are variable (51.43–57.89 degrees) with depth. This suggests that present-day upper plate movement in these two sites may be in different directions, related to transition between a compressional regime and extension and subsidence. Generally, the physical properties measured from core samples were in agreement with the log data, and reflected well both lithology and burial effect.

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