Scientific objectives

The target of Expedition 334 was the slope sediments and the shallow portion of the upper plate basement in the Costa Rica erosive subduction system. The scientific objectives of LWD and coring at the two slope sites are the following:

1. Estimate the composition, texture, and physical and frictional properties of the upper plate material.

The upper plate material at erosive convergent margins is transported into the subduction channel, and thus into the seismogenic zone, by upward migration of the plate boundary. The onset of seismogenic behavior along the subduction thrust is influenced by the physical and frictional properties of the overriding plate material. Geologic and experimental characterization of the upper plate basement is needed to provide structural and mechanical constraints on the possible changes in frictional behavior across the updip limit of the seismogenic zone. Sampling rocks of the upper plate basement beneath the upper slope is also useful to define drilling conditions for deep holes.

2. Quantify subduction channel thickness and the rate of subduction erosion.

The actively slipping plate boundary interface is located within the subduction channel. Determination of the thickness of the subduction channel is critical for preparatory structural geology work and the concept of describing the active slip surface and the damage zone for the deep riser drilling. To estimate the thickness of the subduction channel, namely the zone of broken upper plate material currently subducting, we need quantification of mass removal in the CRISP study area. A two-point recovery of fossiliferous sediment across the margin allows the crustal loss rate to be determined through the evaluation of a subsidence profile. Offshore Nicoya, the estimated volume of eroded upper plate rock carried down the subduction zone is essentially four times the volume of subducted trench sediment. Along the CRISP transect we expect the process to be accelerated, possibly as a result of the subduction of the Cocos Ridge.

3. Characterize fluid/rock interaction, the hydrologic system, and the geochemical processes active within the upper plate.

We expect that the Cocos Ridge subduction caused extensive fracturing of the upper plate that modified the hydrological system (e.g., flow paths, flow rates, heat flow, and mass transport). Landward-dipping reflectors cutting through the upper plate have been interpreted to connect all the way to the plate boundary. Geochemistry can open a window directly to the seismogenic zone through the analysis of parameters that can be related to chemical reactions or mineral precipitation occurring at seismogenic depths. Fluids are also a key control factor on seismicity because fluid pressure is a physical variable defining the stress state and is a parameter of the effective stress law. Fluid pressure and temperature control the strength and frictional behavior of the rocks. Stress state and deformation processes, in turn, influence porosity and permeability and, consequently, fluid pressure. Hence, measuring the thermal and hydrologic regime is critical. Fluid pressure and temperature may be measured in situ until a depth where the material is semiconsolidated. Laboratory analysis, such as consolidation tests, can give indirect but realistic values of pore pressure.

4. Measure the stress field along the updip limit of the seismogenic zone.

The stress field may be inferred from borehole breakouts obtained by LWD. Both GPS investigations and the pattern of microearthquake epicenters indicate a highly stressed area in the vicinity of the Osa Peninsula, implying that relative plate motion in the seismogenic zone is primarily accommodated by coseismic frictional slip. Drilling will contribute to a better definition of the orientation of the horizontal compressive stress in the area. Downhole in situ heat flow measurements will improve our understanding of the thermal regime, allowing better temperature estimates associated with the onset of seismicity as well as allowing us to develop viscoelastic models of deformation.

Expedition 334 is also considered a stand-alone project providing data to solve long-standing problems related to the tectonics of the region. These primary objectives are

  1. Determining the Cocos Ridge subduction arrival time and its effects on the margin tectonics (e.g., acceleration of tectonic erosion processes);
  2. Examining the evolution of the Central America volcanic arc, of which the most relevant effects would be the timing of the progressive shut off of the volcanic arc and the uplift of the Talamanca Cordillera; and
  3. Determining the time progression of the death of a volcanic arc and the identification of potential late products. This subject can be explored in detail because we would have at least two sedimentary columns to correlate events and thereby explore the consequences of the time-progressive subduction of the Cocos Ridge.