Scientific objectives

The CRISP program is designed to help us understand seismogenesis along erosional convergent margins. CRISP Program A is the first step toward deep riser drilling through the seismogenic zone. The program focuses on the characterization of the upper plate, lithology, deformation, and fluid system. An evaluation of the subduction channel thickness, necessary to constrain the structural environment that will be drilled during the deep riser drilling, is also a priority of CRISP Program A.

Expedition 344 is a continuation of Expedition 334; together these expeditions comprise Stages 1 and 2 of CRISP Program A. The principal objective of CRISP Program A is to establish the boundary conditions of the Costa Rica erosive subduction system. Proposed work includes the following primary goals.

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

The upper plate material constitutes a primary input into the erosive plate boundary. As the plate boundary migrates upward, upper plate material is dragged into the subduction channel, which then becomes the input to the seismogenic zone. The onset of seismogenic behavior along the subduction thrust is influenced by physical properties of the overriding plate material. Geologic characterization of the 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 better constrain hypotheses for testing during CRISP Program B.

Seismic velocities and structure indicate that upper plate basement could correlate with outcrops of mélange on the Osa Peninsula. Onland exposures of the Osa Mélange indicate the accretion of at least two seamounts occurring as events scattered in time—early Eocene/​middle Oligocene and middle Miocene—that supply rather different rocks to the margin. The mélange of a third seamount edifice could characterize the upper plate basement of the drilling area. Furthermore, mélanges carry implicit reference to heterogeneity with implications for permeability and fluid pressure.

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

The actively slipping plate boundary interface is located within the subduction channel. Estimating the subduction channel thickness is critical for preparatory structural geology work and describing the active slip surface and the damage zone for CRISP Program B. Assessing the subduction channel thickness, the zone of broken upper-plate material currently subducting, will be based on the quantification of mass removal in the CRISP study area. A two-point recovery of fossiliferous sediment across the margin will allow 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. Results from Expedition 334 indicate large rates of subduction erosion and sediment accumulation (Expedition 334 Scientists, 2011).

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 the Cocos Ridge subduction caused extended fracturing of the upper plate that modified the hydrological system (e.g., flow paths, velocities, 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 the depth of seismogenesis.

Fluids are also a key control factor on seismicity because fluid pressure is a physical variable defining the stress state and it is a parameter of the friction laws. Fluid pressure and temperature control the strength 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 semi-consolidated. 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. 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. CRISP Program A 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.

CRISP Program A is also considered a standalone project providing data to solve longstanding problems related to tectonics of the region. These primary objectives are as follows.

1. Cocos Ridge subduction.

Determining the Cocos Ridge subduction arrival time and its effects on the margin tectonics (e.g., acceleration of tectonic erosion processes).

2. Evolution of the Central America volcanic arc.

The most relevant effects would be the timing of the progressive shutoff of the volcanic arc and the uplift of the Talamanca Cordillera.

3. Death of a volcanic arc.

Determining its time progression and the identification of potential late products from the death of a volcanic arc. 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.