Preliminary results from Expedition 334 (CRISP-A1)

Because of the abbreviated operation schedule of Expedition 334, the first objective was addressed at only three locations on the incoming plate (Site U1381), the middle slope (Sites U1378 and U1380), and the upper slope (Site U1379). The two primary slope sites (Sites U1378 and U1379) were characterized with logging-while-drilling (LWD) data. Subsequently, fluids, sediment, and the underlying upper plate basement at these sites were sampled. A third slope location (Site 1380) was attempted but abandoned because of poor drilling conditions. Finally, the incoming Cocos plate was drilled at Site U1381.

Mid-slope drilling at Site U1378, above the unlocked portion of the plate interface (LaFemina et al., 2009), penetrated 523.9 m below seafloor (mbsf) of slope sediments before drilling was terminated because of poor hole conditions. As a result of poor drilling conditions, basement was not recovered at this site and the high-amplitude reflector interpreted as a displacement surface was not intersected. Recovering mid-slope sediments and basement remains a high priority for Expedition 344. The recovered sediments have a terrigenous origin and consist of an upper unit of silty clay with a series of fining-upward sequences of lithic sands and tephras (Expedition 334 Scientists, 2011) (Fig. F8). The lower unit consists of massive, well-consolidated terrigenous clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silty clay, clay, and clayey silt(stone). Throughout this lower unit, fining- and coarsening-upward decimeter-scale sequences of sand are present. In the coarser sand layers, rip-up clasts, rounded clay lenses, and abundant shell fragments are common. Low-angle bedding (<30°) planes are observed, as well as healed and open faults with relatively steep dips. Sediment-filled vein structures are also observed. Physical properties data display patterns consistent with normal compaction patterns, including an increase in wet bulk density values and decreasing porosity. Heat flow is 44 mW/m2, consistent with a forearc setting. In general, observations are consistent with a terrestrially sourced slope sequence that reflects a downslope environment.

Upper slope drilling at Site U1379, above the locked portion of the plate boundary (LaFemina et al., 2009), penetrated 949 mbsf. Basement penetration was first inferred from LWD measurements that indicated a major change in bulk physical properties (density and resistivity). Somewhat surprising, though, was that the rate of LWD penetration slowed only a little relative to that expected for hard rock. Basement material was drilled at Site U1379 in an effort to characterize the lithological, physical, and frictional properties of upper plate material; however, it remains unclear whether the deepest 70 m of this site represents a transition zone with basement clasts or the basement itself. Presumed basement material consists of breccia with softer matrix (sandstone and siltstone) surrounding blocks of basalt and chert. It is still not known with confidence whether these rocks represent true framework rock or a “transition zone”/​“erosional” layer above framework rock.

Four lithologic units within the sediments are recognized at Site U1379 (Fig. F9). The fifth unit is the breccia. Unit I consists of medium- to coarse-grained sand with abundant shell fragments. Unit II consists of clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silt(stone) and silty clay(stone), clay, clayey silt(stone), and tephra. Unit III consists of fining- and coarsening-upward sequences of silty sands and sandstone. Unit IV consists of carbonate-cemented medium- to coarse-grained sand with well-rounded, lithic pebble-sized clasts and thick-walled shell shards. Unit V is composed of matrix-supported breccia with clasts of limestone, basalt, and mudstone in a fine sandy matrix intercalated with basalt in the upper part and a sequence of variably sandy and clayey silt in the lower part. Structurally, the upper sediments have gently dipping bedding planes with few faults. These upper sediments overlie a zone of steep bedding dips with greater numbers of faults that overlies the breccia. The lithology and physical properties are consistent with changing depositional conditions in a forearc basin that may range from a near-shore environment to shelf sediments to upper slope sediments (turbidites) interrupted by calcareous mud debris from close fluid venting areas.

Site U1381 on the incoming plate consists of ~99 m of sediments (Fig. F10). An additional 72 m of igneous basement was cored. Downhole equilibrium temperatures acquired using the Sediment Temperature Tool (SET) increase linearly with depth and give a least-squares geothermal gradient of 222°C/km. The heat flow calculated using the mean thermal conductivity of 0.8 W/(m·K) is 178 mW/m2. This value is significantly larger than the half-space prediction for 15 Ma crust (130 mW/m2) and larger than the observed global average heat flow for crust of this age (77 mW/m2) (Stein and Stein, 1994). This high heat flow value suggests significant fluid flow within the underlying crust. Geochemical profiles below ~50 mbsf reflect diffusional communication with a fluid with seawater-like chemistry in the igneous basement. These data suggest the lateral flow of modified seawater. A diffusive flux of sulfate from the overlying water column and from the basement aquifer below the sediment section prevents sulfate depletion in this site.

The second objective, to estimate the subduction channel thickness and the rate of subsidence, is being met by analyses of sedimentary phases and benthic foraminiferal fauna. Preliminary biostratigraphic ages obtained from the two slope sites indicate high sediment accumulation rates in the terrestrially sourced slope sequence. Accumulation rates vary between 516 and 236 m/m.y. for the middle slope site (U1378) and between 1035 and 160 m/m.y. at the upper slope site (U1379). These rates are an order of magnitude higher than that of slope sediments offshore Nicoya (38–99 m/m.y.) (Kimura, Silver, Blum, et al., 1997). Detailed research of sedimentary facies and benthic foraminiferal 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.

The third objective of Expedition 334, to characterize the stress field across the updip limit of the seismogenic zone, is being achieved with the analysis of LWD data. The principal objectives of the LWD program are to document in situ physical properties (natural gamma ray, density, neutron porosity, and resistivity), stratigraphic and structural features, compaction state, and hydrological parameters. Density and radius images were used to determine stress orientations from borehole breakouts. The adnVISION tool collected oriented images of bulk density and borehole radius. Despite its limited azimuthal resolution, the images clearly display vertical bands of large borehole radius in the interval 110–438 mbsf at Hole U1378A and in the interval 292–885 mbsf at Hole U1379A (Expedition 334 Scientists, 2011). These bands are interpreted as borehole breakouts caused by differences in principal horizontal stresses. The average azimuth of the breakouts at Site U1379 is roughly northeast–southwest to east-northeast–west-southwest, indicating that the maximum horizontal stress is oriented northwest–southeast to north-northwest–south-southeast. The average azimuth of the breakouts at Site U1378 is roughly north–south to north-northwest–south-southeast, indicating that the maximum horizontal stress is oriented east–west to east-northeast–west-southwest. These azimuths reveal an in situ stress change from compression within the middle slope to extension at the upper slope, addressing the third objective of the expedition.

The fourth objective involves characterizing the fluid flow system and thermal structure of the erosive margin. These properties affect diagenesis and hydrological parameters (e.g., permeability and pore pressure), which are implicated in regulating the mechanical state of the plate interface at depth. Shipboard geochemical data indicate fluid flow occurring both in slope sediments and on the incoming plate (Expedition 334 Scientists, 2011). In slope sediments, fluid composition indicates transport from deep sources. The flow at each site overprints the general geochemical profiles that are influenced by in situ diagenetic reactions. At Site U1379, a broad brecciated zone cored from ~600 to 800 mbsf contains a fluid with low Cl concentrations and peaks in the concentrations of thermogenic hydrocarbons. At Site U1378, there is a monotonic decrease in Cl, Mg, and K concentrations and an 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 imaged in seismic reflection profiles at this site. The geothermal gradient at these two sites is too low to support the in situ production of thermogenic hydrocarbons or extensive clay dehydration, suggesting a deeper source for the fluid and hence its migration from greater depths. On the incoming plate at Site U1381, lateral flow of modified seawater through the upper oceanic crust was identified. Here the geochemical profiles below ~50 mbsf reflect diffusional communication with a fluid with seawater-like chemistry in the igneous basement. A diffusive flux of sulfate from the overlying water column and from the basement aquifer below the sediment section prevents sulfate depletion in this site.