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doi:10.2204/iodp.pr.344.2013

Preliminary scientific assessment

Expedition 344 was superlative, and with the exception of not reaching the décollement and the underthrust sediment at the toe site (U1412), exceeded expectations. We recovered material across the Costa Rica erosive convergent margin offshore the Osa Peninsula. Material was recovered from the incoming Cocos plate (Sites U1381 and U1414), the toe of the margin (Site U1412), the midslope region (Site U1380), and the upper slope region (Site U1413).

Recovery was very good at the incoming plate sites, good at the midslope and upper slope sites, and poor at the toe site. At Sites U1381 and U1414 we collected 76 cores with a recovery of 86%, at Sites U1380 and U1413 we collected 122 cores with a recovery of 75%, and at Site U1412 we collected 55 cores with a recovery of 54% (see “Expedition 344 principal results” for details).

The overarching goal of Expedition 344 was a better understanding of seismogenesis at erosive convergent margins through shallow drilling. A second objective was to better understand erosive convergent margins. In this respect, Expedition 344 has given us an increased understanding of subduction erosion that has led to important new insights. Most of the science objectives were achieved. Below we assess these objectives and highlight major results.

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

Slope sediment was recovered at Sites U1380 and U1413. Seismic reflection data over both sites show that the slope sediment unconformably overlies a lower unit. The lower unit at Site U1380 was identified as upper plate basement prior to drilling. Both of these slope sites are divided into three units. Unit I at Sites U1380 and U1413 consists of slope sediment and is characterized silty clay and fine sandstones. Unit II at both sites is characterized by clayey siltstone lithified to varying degrees with calcite cement. Sands are present in these units and tend to be less well lithified. At Site U1380, Unit III consists of silty claystone with intercalated massive coarse sandstones that are well lithified with calcite cement. At Site U1413, Unit III consists of fine- to medium-grained sandstone and siltstone. Units I and III of Site U1380 and all units of Site U1413 represent terrestrially sourced upper slope sequences. Unit II of Site U1380 represents a terrestrially sourced shelf sequence. High sedimentation rates throughout the depositional interval were observed at both sites. Observed organic debris and thin normally graded sands with sharp erosional bases indicate deposition within the distal facies of a clastic turbidite sequence. Carbonate cements are ubiquitous at these sites.

The upper part of Hole U1380C (438–551 mbsf) is characterized by abundant faults and fractures and steeply dipping bedding planes, whereas the lower part of this hole (551–797 mbsf) is characterized by less steeply dipping beds. Two structural domains are also identified at Site U1413. The upper domain (0–180 mbsf) is characterized by subhorizontal to gently dipping bedding planes, and the lower domain (180–582.2 mbsf) is characterized by a downhole increase in bedding dips and an abundance of fault structures.

Physical properties show a strong, sharp change at the boundary between upper slope sediment and upper plate basement. In the upper plate basement, magnetic susceptibility becomes more variable; NGR counts decrease; bulk density increases; porosity decreases; and thermal conductivity, P-wave velocity, and sediment strength increase.

A major result is that the upper plate material drilled at Site U1380 is not a mélange of oceanic material or the offshore extension of the Caribbean large igneous complex, but forearc basin material consisting of lithic sedimentary units.

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

Expedition 344 successfully recovered slope sediment at Sites U1380 and U1413. Preliminary biostratigraphic and paleomagnetic ages from Sites U1380 and U1413 indicate high sediment accumulation rates in the terrestrially sourced slope sequence. Estimates of slope accumulation rates at Site U1380 vary between ~290 and 590 m/m.y. At Site U1413, mass transport deposits make accumulation rates more complicated, but rates vary between 380 and 432 m/m.y. These accumulation rates are remarkably high and could be due to enhanced erosion rates caused by the subduction of the Cocos Ridge or alternatively by subducting bathymetry that may erode the margin, creating lows that are then filled. These rates are the same order of magnitude as those determined during Expedition 334 at Sites U1378 and U1379 (Expedition 334 Scientists, 2012a, 2012b) and an order of magnitude larger than those determined offshore the Nicoya Peninsula (Kimura, Silver, Blum, et al., 1997).

Onshore research of sedimentary facies and benthic foraminifers in slope sediment at Sites U1380 and U1413 will be used to assess margin subsidence and evolution. Expedition 344 was successful at obtaining the material necessary for these onshore studies. An important preliminary implication of the high accumulation rates is that this margin has undergone high rates of vertical motion.

3. Evaluate fluid/​rock interaction, the hydrologic system, and geochemical processes as indicated by the composition and pore fluids within the upper plate.

Fluids play a large role in subduction zone processes and dynamics. Thus, understanding their pathways as they enter and exit subduction zones is of fundamental importance and was an important goal of this expedition. The flow of fluids influences the thermal regime, as well as solute budgets and their transfer between the ocean, volcanic arc, and mantle. The distribution of fluid pressure within subduction zones influences flow velocities, effective stress, and fault strength. The chemistry of fluids advected toward the surface can be used to infer reactions occurring at depth and their potential role in seismogenesis.

Solute concentrations indicative of fluid flow were observed in three different environments. These environments include the upper basement of the incoming Cocos plate at Site U1381, within high-permeability sediment above basement at Site U1414, and associated with a shear zone within the lower slope at Site U1380.

At midslope Site U1380, the presence of fluid flow through a shear zone that extends from ~480 to 550 mbsf, at the boundary between lithologic Units I and II, is indicated by anomalous values of Cl, Li, and hydrocarbon data. These tracers indicate that this fluid originated from a source depth where temperatures are ~80°–90°C. At upper slope Site U1413, no unequivocal evidence for fluid flow was observed.

At input Site U1381, concentration-depth profiles of Ca, SO₄, Li, Mn, and possibly Si solutes indicate the diffusion of fluids with seawater-like chemistry in the igneous basement. In fact, vigorous flow in the subducting oceanic basement is suggested by anomalously high heat flow measured at these sites. In contrast, at Site U1414, the cemented sandstone unit at the base of the sediment inhibits the diffusional signal of fluids within the subducting basement. Here, the lateral flow of fluids just above the basement is indicated by a sulfate and calcium minimum at ~330 mbsf. This pattern reflects a lateral flow of sulfate-depleted fluids that likely originated from oxidation of methane and/or other organic carbon sources landward of Site U1414 and migrated updip through more permeable sediment. This flow overprints any potential diffusive signature of flow in the basement. High heat suggests the presence of advective fluid flow.

At toe Site U1412, borehole stability was an issue that precluded crossing the décollement and recovering the underthrust sediment and igneous basement. The geochemistry of pore fluids largely indicates in situ diagenetic reactions. Lithium concentration values indicate diffusive communication with a fluid originating from a source depth where temperatures are ~80°–90°C. The direct lack of evidence for fluid flow, however, is likely due to the lack of recovered sediment.

Pore fluids in the uppermost ~50 m at all margin sites drilled during Expedition 344 are dominated by reactions associated with the cycling of organic carbon. Within the uppermost 50 m of the sites drilled on the slope, active sulfate reduction, biogenic methane production, and precipitation of authigenic carbonates was observed. The depth of the SMT varies from site to site, with the shallowest SMT occurring at a depth of ~14 mbsf at Site U1412. At input Sites U1381 and U1414, there is no methane production and sulfate is controlled by organic matter catabolic reactions.

4. Estimate the stress field along the CRISP transect.

In the CRISP study area, paleostress is assessed through the structural analysis of faults, present-day in situ stress is estimated from borehole breakout data, and anelastic strain recovery measurements are estimated on recovered whole rounds.

At Site U1380, kinematic fault analysis indicates a paleoextensional stress regime with maximum and minimum compressive stresses oriented to the vertical and northeast–southwest, respectively. Both normal and reverse faults were observed at Site U1413, indicating multiple generations of faulting.

Preliminary analysis of borehole breakouts at Site U1413 revealed borehole breakouts between 148 and 169 mbsf, indicating a minimum compressive stress oriented north–south. This stress orientation corresponds to an upper slope extensional stress regime and is consistent with upper slope extension at Site U1379 found during Expedition 334.

5. Better understand the impact of Cocos Ridge subduction, the evolution of the Central American volcanic arc, and the development of the volcanic gap inboard of the Cocos Ridge.

A fundamental question is whether Central American volcanism ceased with the arrival of the Cocos Ridge and how volcanism evolved subsequent to the arrival of the ridge. Expeditions 334 and 344 drilled into the sediment and basalt of the Cocos Ridge for the first time. At Site U1381, 109 m of sediment and 81 tephras were recovered. At Site U1414, 375 m of sediment and 36 tephras were recovered, as well as 96 m of basaltic crust. At Site U1414, a gap in the tephra record between mid- to late Pleistocene and middle Miocene has likely been recovered. Biostratigraphic analysis shows that sediment containing ~20 felsic tephra layers during this time period was recovered. Analyses of these tephra will yield new insights into the evolution of Central American volcanism.

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