Summary and conclusions

Expedition 334 incorporated both shallow-water drilling on the Caribbean plate continental upper slope (Site U1379) and deepwater drilling on the Cocos plate (Site U1381). This latter site also represented the first ever penetration of the Cocos Ridge igneous basement. A third target area was the Caribbean middle slope where we drilled Sites U1378 and U1380. Acquisition of LWD logs prior to coring was planned and successfully implemented at Sites U1378 and U1379.

Slope sediments at Sites U1378–U1380 had 100% recovery. Site U1379 had ~20% recovery in the hard formation forming the basement of the Caribbean plate. Unfortunately both LWD and coring were interrupted without reaching the upper plate basement at Sites U1378 and U1380 because of deteriorated hole conditions.

Before entering into the scientific facts regarding the cruise, we want to thank the drillers and all JOIDES Resolution staff for their collaboration and optimistic vision of the challenges that they always took with no hesitation. All major objectives of the expedition, both in the view of the entire Costa Rica Seismogenesis Project and as stand-alone goals, were fulfilled, and below we will provide major highlights.

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

The basement was successfully drilled at Site U1379. This site is on the upper slope at 127 m water depth. At Site U1379, both cores and LWD logs penetrated below the slope sediment and upper plate basement interface for 50 and 70 m, respectively. LWD logs showed a marked increase in density and resistivity at ~892 mbsf. This change in physical properties was interpreted as penetration into the upper plate basement after drilling a continuous section of slope sediments. Indeed, successive coring at Site U1379 showed a complex change in lithostratigraphy, going from sandstones with lithic fragments of igneous origin to bioclastic sandstones to a poorly sorted, well-lithified breccia with sandy matrix and igneous, calcareous, and mudstone clasts to mudstone. It remains unclear if we entered a transition zone with clasts of basement, for example an erosional surface, or the basement itself. The difficulties are due to the fact that the expected basement at Site U1379 was a mélange; the mudstone recovered from the bottom of the hole may or may not be the matrix of the mélange. We need further analysis to completely answer this question.

A second penetration of the basement was attempted at Site U1378. Both LWD logs and coring were not successful in reaching the basement at this site. Hole conditions did not allow penetration deeper than 457 mbsf during logging and 524 mbsf during coring, whereas the expected depth of top of basement was ~750 mbsf. Successive core description revealed the presence of highly fractured and brecciated zones interpreted as fault zones, which may have caused the impossible advance of drilling.

A final attempt to reach the basement was conducted at Site U1380, where the basement was expected at ~550 mbsf. Site U1380 is ~1 km upslope from Site U1378, but we encountered the same problems, here at 482 mbsf.

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

In an erosive subduction margin, the upper plate material is incorporated into the subduction channel because of basal erosion, resulting in subsidence of the upper plate. Estimation of mass removal from the upper plate in the CRISP area based on subsidence profiles of slope sediments is crucial to assess the thickness of the subduction channel. Expedition 334 successfully recovered slope sediments offshore Osa Peninsula where the Cocos Ridge is subducting beneath the Caribbean plate. The preliminary results of biostratigraphic ages obtained from two slope sites indicate high sediment accumulation rates in the terrestrially sourced slope sequence, ranging from 516–236 m/m.y. at Site U1378 in the middle slope to 1035–160 m/m.y. at Site U1379 in the upper slope. In particular, the accumulation rate of the slope sediments, mainly composed of clayey silt/silty clay, at Site U1379 is 1035 m/m.y., much higher than that of slope sediments offshore Nicoya Peninsula (38–99 m/m.y.) (Kimura, Silver, Blum, et al., 1997). The remarkable high accumulation rate offshore Osa Peninsula could be derived from onland uplift triggered by the subduction of the Cocos Ridge. On the other hand, the subduction of such topographic high likely accelerates the basal erosion of the upper plate. The subsidence/uplift profiles of the slope sediments offshore Osa Peninsula are controlled by the high sediment accumulation rate and the basal erosion; the subsidence of the upper plate likely occurred when the rate of subduction erosion was higher than the slope sedimentation rate. The detailed research of sedimentary facies and benthic foraminifer faunal in slope sediments at Sites U1378 and U1379 are keys to estimate the mass removal associated with basal erosion and the thickness of the subduction channel.

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

Fluid flow and fluid pressure in subduction zones can have a profound impact on the shallow thermal structure and fluid content of the subducting and upper plates; fault zone stability and seismogenesis; and the transfer of elements and isotopes to the ocean, volcanic arc, and mantle. Changes in physical and mineralogical properties with depth and the associated evolution of fluids in subduction zones may be intimately linked to the transition from aseismic to seismic behavior along the plate boundary. Fluids advected along fault zones and other permeable horizons in the upper plate record reactions occurring at greater depths in the subduction zone and can be used to constrain reactions occurring within the seismogenic zone. The shipboard geochemistry program during Expedition 334 identified two zones in the upper plate with fluid compositions indicative of the transport of fluids from greater depths and one zone in the reference site where there is lateral flow of modified seawater in the igneous basement.

Pore fluids in the uppermost ~50 m at all sites drilled during Expedition 334 are dominated by reactions associated with the cycling of organic carbon. Within the uppermost 50 m, active sulfate reduction, biogenic methane production, and precipitation of authigenic carbonates are present. The depth of the sulfate methane transition (SMT) varies from site to site, with the shallowest SMT occurring at a depth of ~13 mbsf at Site U1378. The sulfate reduction profile at Site U1379 has a steep gradient to ~15 mbsf and then becomes less steep, reaching depletion at ~30 mbsf. The change in the gradient at the slope site (U1379) may reflect changes in the type of organic matter in the sediments or a change in sedimentation rates. At Site U1381, the diffusive flux of sulfate from the overlying water column and from the basement aquifer below the sediment section is faster than microbial sulfate reduction rates in the sediment column; thus, sulfate does not reach depletion in the reference site.

Fluid flow was detected at each of the sites cored. The flow at each site overprints the general geochemical profiles that are influenced by in situ diagenetic reactions such as ion exchange, ongoing microbial metabolic reactions, volcanic ash alteration, and carbonate precipitation/dissolution. At Site U1379, a broad zone from ~600 to 800 mbsf contains a fluid with low Cl concentrations and peaks in the concentrations of thermogenic hydrocarbons (ethane, propane, n-butane, iso-butane). The geothermal gradient at Site U1379 is too low to support the in situ production of thermogenic hydrocarbons or for extensive clay dehydration, suggesting a deeper source for the fluid and migration along the permeable sand horizons and fault zones within this depth interval. At Site U1378, there is a monotonic decrease in Cl, Mg, and K concentrations and increase in Ca concentrations with depth, suggesting diffusional communication with fluids below the base of the hole. It is likely that this fluid resides in the fault zone in the basement imaged in the seismic reflection profiles at this site. The fluid at the base of the hole is more dilute (altered) than the fluids sampled at Site U1379. The ratio of methane to the thermogenic hydrocarbons also decreases with depth at this site. The in situ temperature at the sediment/basement contact is too low to support in situ clay dehydration or thermogenic hydrocarbon formation, thus this fluid must also be transported from greater depths. At Site U1381, the geochemical profiles below ~50 mbsf reflect diffusional communication with a fluid with seawater-like chemistry in the igneous basement.

4. Measure the stress field along the CRISP transect.

We estimated present-day in situ stress orientation from borehole breakouts at Site U1378 in the middle slope and Site U1379 in the upper slope. Borehole breakouts in a vertical hole form in a direction perpendicular to the maximum horizontal principal stress (σHmax) (Zoback et al., 2003). During Expedition 334, borehole breakouts were identified from LWD images of borehole radius and density. In addition, we determined types, orientations, and kinematics of faults from cores.

Breakout orientation of slope sediments at Site U1378 indicates that σHmax is oriented north–northwest, south–southeast. Both normal and reverse faults are observed in cores from slope sediments at Site U1378. The σHmax orientation at Site U1378 is consistent with the north–northwest-directed plate motion vector detected by the GPS measurement northwest of Osa Peninsula (LaFemina et al., 2009) and is oblique to the convergence direction (north–northeast) between the Cocos and Caribbean plates (DeMets, 2001). The Cocos Ridge is considered to act as a rigid indenter to the Caribbean plate, resulting in change in plate motion from north–northeast-directed trench-normal motion in southern Costa Rica to trench-parallel motion in central Costa Rica where plate convergence is normal to the trench (LaFemina et al., 2009). North–northwest-oriented σHmax at Site U1378 may represent the shortening of the middle slope opposite the northwestern flank of the Cocos Ridge (thrust faulting stress regime) where plate motion likely deviates from trench-normal motion, although strike-slip and/or normal stress states are also possible.

The σHmax orientation of slope sediments at Site U1379 is east–northeast, west–southwest, perpendicular to σHmax orientation at Site U1378. No obvious borehole breakouts appear in the underlying basement at Site U1379. Normal faults dominate in cores from slope sediments at Site U1379. Kinematic analyses of normal faults indicate a vertical maximum principal stress (σ1), an intermediate principal stress (σ2) oriented west–northwest, and a minimum principal stress (σ3) oriented north–northeast. The σHmax orientation at Site U1379 is consistent with north–northwest-directed horizontal extension parallel to the plate motion vector detected by the GPS measurement northwest of Osa Peninsula (LaFemina et al., 2009) and thus appears to represent a present stress state. On the other hand, the σ3 orientation determined from kinematic analyses of core-scale normal faults is parallel to the convergence direction between the Cocos and Caribbean plates (DeMets, 2001). Core-scale normal faults record instantaneous stress conditions when the faults are active. Although stress data sets are derived from slope sediments overlying the upper plate basement, trench-normal extension at Site U1379 is consistent with extensional faulting in the submarine wedge associated with basal erosion (Ranero and von Huene, 2000).

In summary, stress at the middle slope site is compressional, whereas that at the upper slope site is extensional. This marked change in stress state occurs within ~12 km along the CRISP transect in the northwestern flank of the Cocos Ridge and may correspond to a change from compression (middle slope) to extension (upper slope), marking the onset of subduction erosion between Sites U1378 and U1379.

Cocos Ridge subduction: evolution of the Central America volcanic arc and development of the volcanic arc gap inboard Cocos Ridge

Expedition 334 is the first research endeavor to drill into the sediments and basalts of Cocos Ridge, which is the trace of the Galapagos hotspot on the Cocos plate. We successfully recovered ~80 m of Cocos Ridge basalts, underlying ~100 m of biogenic pelagic sediments, mainly siliceous to calcareous oozes.

Approximately 170 tephra layers were described and sampled from the cored holes at four different sites with ages ranging from middle Miocene to the present. Postcruise analyses of the tephra will shed light on the evolution of the magmatic arc, including the deactivation of the volcanoes located on the present-day location of the Talamanca Cordillera.