IODP Expedition 334: An Investigation of the Sedimentary Record, Fluid Flow and State of Stress on Top of the Seismogenic Zone of an Erosive Subduction Margin

Abstract

The Costa Rica Seismogenesis Project (CRISP) is an experiment to understand the processes that control nucleation and seismic rupture of large earthquakes at erosional subduction zones. Integrated Ocean Drililng Program (IODP) Expedition 334 by R/V JOIDES Resolution is the first step toward deep drilling through the aseismic and seismic plate boundary at the Costa Rica subduction zone offshore the Osa Peninsula where the Cocos Ridge is subducting beneath the Caribbean plate. Drilling operations included logging while drilling (LWD) at two slope sites (Sites U1378 and U1379) and coring at three slope sites (Sites U1378-1380) and at one site on the Cocos plate (Site U1381). For the first time the lithology, stratigraphy, and age of the slope and incoming sediments as well as the petrology of the subducting Cocos Ridge have been characterized at this margin. The slope sites recorded a high sediment accumulation rate of 160-1035m m.y. -1 possibly caused by on-land uplift triggered by the subduction of the Cocos Ridge. The geochemical data as well as the in situ temperature data obtained at the slope sites suggest that fluids are transported from greater depths. The geochemical profiles at Site U1381 reflect diffusional communication of a fluid with seawater-like chemistry and the igneous basement of the Cocos plate (Solomon et al., 2011; Vannucchi et al., 2012a). The present-day in situ stress orientation determined by borehole breakouts at Site U1378 in the middle slope and Site U1379 in the upper slope shows a marked change in stress state within ~12 km along the CRISP transect; that may correspond to a change from compression (middle slope) to extension (upper slope).

Horizontal principal stress orientation in the Costa Rica Seismogenesis Project (CRISP) transect from borehole breakouts

“…The change at 865 mbsf does not coincide with the bottom of the Recent to Pleistocene sedimentary apron, which was detected at 895 mbsf. The Pliocene fore‐arc basin sediments below this depth have markedly greater bulk density and resistivity than those in the overlying younger sediment cover [ Vannucchi et al ., ]. The approximately 50 m thick interval logged in the Pliocene sediments does not display any breakouts, likely because the formation is stronger than the overlying sediments and the hoop stress is insufficient to fracture it.…”

“…Therefore, the frontal prism is an accretionary structure formed by sediments transferred from the incoming plate. Moving inboard across the margin, an apron of Recent to Pleistocene sediments up to ∼1 km thick covers unconformably sediments of an older, Pliocene, fore‐arc basin [ Vannucchi et al ., ; Harris et al ., ]. The three CRISP sites discussed here sampled the Recent to Pleistocene sediment cover; the older Pliocene sediments were reached by drilling only in the deepest portion of Site U1379 (below 895 m below seafloor, or mbsf).…”

“…334 measured borehole radius in 16 sectors, and further data processing was necessary to obtain reliable estimates of breakout azimuth, as described in the next section. More details on downhole log measurements acquired during IODP Expeditions 334 and 344 are in the IODP Proceedings [ Vannucchi et al ., ; Harris et al ., ].…”

“…The results presented here were obtained during Integrated Ocean Drilling Program Expeditions 334 and 344 of the Costa Rica Seismogenesis Project [ Vannucchi et al ., ; Harris et al ., ]. The CRISP transect is located at the southern end of the Middle America Trench where the Cocos plate is being subducted beneath the Caribbean plate just north of the Cocos Ridge axis (Figure ).…”

“…An additional set of E‐W striking thrust faults, which tend to form along the crest of anticlinal ridges, is also present. Analyses of the cores obtained in CRISP drilling showed a variety of structures with predominant normal faults and strike‐slip faults, typically concentrated in discrete horizons, and subordinate high‐angle (dipping 70°–80°) reverse faults [ Vannucchi et al ., ; Harris et al ., ].…”