Scientific objectives

Overall, the CRISP program is designed to understand seismogenesis along erosional margins. CRISP Program A is the first step toward deep riser drilling through the seismogenic zone. CRISP Program A 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, will be also a priority of CRISP Program A.

CRISP Program A involves drilling along a transect offshore the Osa Peninsula in Costa Rica. Proposed middle slope Site CRIS-3B (or alternate Site CRIS-10A) and upper slope Site CRIS-4A (or alternate Site CRIS-11A) are the primary proposed sites for this expedition (Fig. F3A). These two sites are also proposed to be eventually deepened to reach the aseismic/seismic plate boundary. In our contingency plan, we include two additional sites (Fig. F3A): proposed base of the slope Site CRIS-2B (or alternate Site CRIS-9A) and incoming plate Site CRIS-1A (or alternate Site CRIS-7A), to consider in the event we are unable to perform operations as planned at the primary sites (see "Risks and contingency").

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 the input into the erosive plate boundary. The plate boundary migrates upward and upper plate material is dragged into the subduction channel, which comprises 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 is needed to 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. From what is exposed on land, the Osa Mélange is the result of 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. Indications on 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 CRISP Program B. 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 would 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. Along the CRISP transect we expect the process to be accelerated.

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 stand-alone project providing data to solve long-standing problems related to tectonics of the region. These primary objectives are

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 shut off 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.

Objectives of the contingency plan (Sites CRIS-2B and CRIS-1A)

Proposed Site CRIS-2B is located at the frontal sedimentary prism representing the base of the slope. This site provides sampling of the plate boundary at a shallow depth through the sedimentary frontal prism to define fault state, composition, and fluid system. Proposed Site CRIS-2B will be drilled through a 320 m thick sedimentary pile, penetrating the plate boundary, and continue for 280 m into the underthrust sediment and 150 m into the oceanic basement. Drilling at proposed Site CRIS-2B will allow us to constrain the décollement geometry and deformation at a shallow depth, define fluid pathways, and link it to the seismic cycle.

Proposed Site CRIS-1A represents an oceanic reference site on the Cocos plate. Drilling at this site will define the sedimentary and upper part of the igneous section entering the subduction zone and the hydrologic system. Proposed Site CRIS-1A will penetrate ~150 m into the oceanic basement, allowing description of the igneous mineralogy, petrology, geochemistry, and hydrological input to the subduction zone. Fluids from the Cocos Ridge oceanic basement are expected to reveal low to moderate temperature alteration from near-trench fluid flow along ridge-parallel faults. Faults expose the oceanic basement and could be fluid pathways for recharge and discharge of seawater.