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doi:10.2204/iodp.proc.344.101.2013

Scientific objectives

The CRISP program was designed to understand seismogenesis at erosional convergent margins. CRISP Program A is the first step toward deep riser drilling through the seismogenic zone. The principal objectives of CRISP Program A were to characterize the shallow lithologic, hydrologic, stress, deformation, physical property, and thermal conditions that lead to unstable slip in the seismogenic zone. The focus of this expedition is to document the lithology, deformation, and fluid system of the upper plate and to evaluate the subduction channel thickness. This information is necessary to better understand the structural environment and crustal stress that will be encountered during proposed deep riser drilling. Expedition 344 is a continuation of Expedition 334; they are Stages 1 and 2, respectively, of CRISP Program A. The primary goals for Expedition 344 were as follows.

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

At erosive margins, the upper plate material is thought to constitute a primary input into the plate boundary. As the plate boundary migrates upward, upper plate material is incorporated into the subduction channel that feeds the seismogenic zone. The onset of seismogenic behavior along the subduction thrust is influenced by physical properties of the overriding plate material. Characterization of the décollement zone and upper plate basement will provide structural and mechanical constraints on the possible mechanical changes occurring at seismogenic depths. Sampling rocks of the upper plate basement beneath the upper slope is necessary to define drilling conditions for deep holes and to better understand this margin prior to CRISP Program B drilling.

2. Assess rates of sediment accumulation and margin subsidence/uplift in slope sediment.

The subduction of bathymetric features leading to both frontal and basal erosion of the upper plate are expected to be reflected in forearc tectonic processes. Rapid changes in sediment accumulation rates and margin subsidence are one manifestation of these processes. Rates of sediment accumulation will be documented with biostratigraphic and paleomagnetic methods. Margin uplift or subsidence can be documented by facies changes and careful studies of benthic foraminifers. Subsidence can also be indicated by progressive seaward tilting of sedimentary beds. The landward migration of the coastline and the arc magmatic front over time also reflect these processes.

3. Evaluate fluid/rock interaction, the hydrologic system, and the geochemical processes (indicated by composition and volume of fluids) active within the upper plate.

We expect that subduction of the Cocos Ridge caused fracturing of the upper plate, which modified the hydrological system and is now manifested in flow paths and fluid velocities, heat flow, and mass transport. Landward-dipping reflectors cutting through the upper plate have been interpreted to extend to the plate boundary. Geochemistry can open a window directly to the seismogenic zone through the analysis of solutes that can be related to chemical reactions occurring at the depth of seismogenesis.

Important feedback mechanisms exist between fluid pressure, temperature, and the strength of rocks. Temperature and fluid pressure exert important controls on the state of stress and friction. In turn, stress and deformation processes influence porosity and permeability, which have a fundamental influence on fluid pressure. Measuring the thermal and hydrologic regime is critical for understanding these feedback mechanisms. Fluid pressure and temperature may be measured in situ until a depth where the material is semiconsolidated. Laboratory analysis, as consolidation tests, can give indirect but realistic values of pore pressure.

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

The stress field may be inferred from borehole breakouts, anelastic strain recovery analysis (ASR), and sense of deformation. Borehole breakouts and ASR show a present stress state that can be compared with GPS crustal strain. The paleostress field from deposition to present is recorded in deformation of the rock. Structural analysis reveals the sense of shear, and the anisotropy of magnetic susceptibility can quantify the rock strain. Changes of stress field with Cocos Ridge subduction will be evaluated using structural analysis.

GPS data indicate high rates of strain in the vicinity of Osa Peninsula. Patterns of microearthquake epicenters suggest that relative plate motion in the seismogenic zone is accommodated by coseismic slip. CRISP Program A drilling will better define the orientation of the horizontal compressive stresses in the area. Downhole in situ heat flow measurements will improve our understanding of the thermal regime, allowing us to better estimate the temperature associated with the onset of seismicity as well as to develop viscoelastic models of deformation.

5. Study Cocos Ridge subduction and evolution of the Central American volcanic arc.

CRISP Program A is also a standalone project that provides data to solve long-standing problems related to tectonics of the region. These primary objectives included a better understanding of relationships between Cocos Ridge subduction to the tectonics of the upper plate and of the evolution of the Central American volcanic arc. These goals will be achieved through dating the impingement of the Cocos Ridge on the margin, recovery, dating and compositional characterization of tephra layers, and identifying potential late products of Central American arc volcanism.

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