Site U1378 | Site U1379 | Site U1381
IODP Expedition 334:
Costa Rica Seismogenesis Project (CRISP)
Site U1380 Summary
PDF file is available for download.
Background and Objectives
Site U1380 (CMP 2350, Lat. 08° 35.9980'N Long. 84° 4.4037' W, water depth 515 m) was drilled into the middle slope of the Costa Rica margin, 40 km offshore Osa Peninsula and Caño Island along BGR99
Line 7. It located above the unlocked portion of the plate boundary according
to interplate earthquake relocation and geodetic measurements (Bilek, 2003;
LaFemina et al., 2009). The margin here consists of an upper plate basement
underlying about 550 m thick slope sediments. The seismic sections show that
this site is located above the seaward edge of one of the high amplitude
reflectors interpreted as displacement surfaces, which. Site U1380 was designed
to penetrate. The primary purpose of drilling Site U1380 was to determine the
nature, composition and physical properties of the upper plate basement and to
understand the nature of the landward dipping seismic reflectors. Additional
objectives included (1) a determination of the stress and strain regime of the
unlocked portion of the margin (2) an understanding of the fluid-flow regime
above the high amplitude reflector interpreted as a fault, and (3) an
estimation of the quantity of tectonically eroded upper plate material.
The seismic interpretation of Site U1380
is based on the pre-stack depth migrated section BGR99 Line 7 processed by C.R.
Ranero). The seismic stratigraphy of the sedimentary slope at Site U1380 shows
a good continuity of reflection events. The upper 100 m of the slope sediment
sequence shows rather clear horizontal reflections. The next 300 m of sediments
below grade downward into a more highly reflective zone. The reflectors in this
interval are gently dipping seaward, and the sequence is clearly cut upslope
(toward the NE) by a younger sequence forming an angular unconformity. The
bottom part of the sedimentary sequence is sharply marked by a irregular, high
amplitude, seaward dipping reflector that is interpreted as the top of the
upper plate basement. The surface marking the basement top is cut by several
landward dipping, high amplitude reflectors that are interpreted as faults. At
Site U1380 one of this high amplitude reflectors interpreted as faults is
present at about 800 mbsf. This reflector does not clearly displace the
basement top. The velocity in the basement increases from <2m/s in the
sediments to 2.8 m/s in the fault zone to >3.3 m/s.
At Site U1380, the temperature at the
plate boundary was interpreted to be at least 140íC (Grevemeyer et al., 2004).
Recent new modeling of intraplate temperatures lowers the temperature to
between 60íC and 90íC (Harris et al., 2010). Visual observations and the towed
ocean bottom instrument (TOBI) detected several tens of mud mounds and vent
communities within few kilometers from the site (McAdoo et al., 1996; Weinrebe
et al., 2003). Many of these structures seem to be related to active faults
(Hensen et al., 2004; Ranero et al., 2008). Judging from the continuity of the
landward dipping reflectors interpreted as faults, any fluid advection along
these structures will originate in areas of at least 4 km depth.
Site U1380 (CRIS-10A) was drilled as the
alternate for Site U1378 (CRIS-3B) with the objective to core enough sediment
to stratigraphically correlate Sites U1378 and U1380 and to core as much
basement in the remaining time window as possible. Site U1380 is situated on
the middle slope of the Costa Rica margin above the unlocked portion of the
plate boundary according to interplate earthquake relocation and geodetic
measurements. The margin here consists of an upper plate basement underlying
about 550 m thick slope sediments. Operations at this site consisted of
drilling down to a depth of 397 mbsf followed by coring to a total depth into
the formation of about 480 mbsf, at which depth the condition of Hole U1380A
was deteriorating so that it was necessary to terminate and plug and abandon
it. Overall we retrieved 52.37 m of sediment at this site.
Generally, the sediments cored from Hole
U1380A were assigned to a single lithostratigraphic unit, Unit I. It consists
olive green, terrigenous, well-consolidated clayey silt(stone) and silty
clay(stone) with minor layers of sand(stone), sandy silty clay(stone), clay,
clayey silt(stone). Intercalated in the main background sedimentation are
dm-scaled fining- and coarsening- upward sequences of sands as well as several
tephra layers. The tephra layers range in thickness from 0.5 to 35 cm.
Unconformable and/or inclined bedding of the tephra was observed throughout the
entire cored material. In general, the tephra layers have a sharp basal
sediment contact and a gradational top contact, and are mainly normally graded
and well sorted. The tephra layers are mainly composed of felsic (>90 vol%),
transparent, colorless, and very fine to coarse ash sized glass shards having
an angular blocky, cuspate, flat or Y-shaped morphology. The majority of the
glasses are altered, as shown by the undulous extinction under crossed
polarized light. In the coarser sand layers rip-up clasts, rounded clay lenses
and abundant shell fragments are commonly observed. Framboidal pyrite was
observed macroscopically and in many of the smear slides throughout Unit I.
The physical property data obtained on
the cored material display an expected behavior with depth and reflect that
only a single sediment unit as well as a limited depth interval has been cored
at this site. Wet bulk densities determined from whole round gamma-ray
attenuation (GRA) measurements are relatively constant throughout the cored
interval, with a mean density value of 1.87 ▒ 0.05 g/cm3. Grain
densities determined by mass/volume measurements on discrete samples are also
relatively constant with depth with an average value of 2.67 ▒ 0.08 g/cm3,
however the variability increases with depth. These values suggest a
terrigenous origin of the deposited material. Porosities, obtained by
mass/volume measurements on discrete samples, are also relatively constant
through the cored interval with a value of 49%. The magnetic susceptibility
measured in the sedimentary sequence is also relatively homogenous and low
indicating an abundance of non-iron bearing clays. The thermal conductivity is
relatively constant throughout the cored interval with a mean of 1.05 ▒ 0.22
The demagnetization experiments made on
the sediments cored at Site U1381 are in general agreement with the physical
property data. The mean NRM (natural remanent magnetization) intensity is about
10–3 A/m and exhibits no significant variations down hole. The
mean carriers of the magnetic signal in the retrieved sediments seem to be
titanomagnetite or maghemite as deduced from thermal demagnetization
experiments on discrete samples. Generally, the number of discrete samples
analyzed up to now from the recovered cores is too limited to confidently
define the magnetic polarity for the cores from Site U1380. Based on the
characteristics of the physical and magnetic properties and based on
lithostratigraphic characteristics the sediments deposited at Site 1380 are
interpreted as a terrestrially sourced upper slope sequence that is consistent
with high sediment accumulation rates throughout the depositional interval. The
clay-rich deposits of Unit I at Site U1380 are similar to those observed in
Unit II at Sites U1378 and U1379.
This is consistent with the biogenic
components distributed within the cored sediments. The biogenic components
observed at this site are similar to the ones observed in Unit II at Site
U1378. Biogenic components have a bimodal distribution in Unit I: shell
fragments, diatoms and nannofossils are sparse but ubiquitous throughout the
unit, whereas foraminifers are partly enriched within the sediments and are a
major constituent of the sand-sized fraction of the sediment. Preservation
ranges from poor to moderate. Based on nannofossil biostratigraphy (Zone NN19)
the sediments retrieved from the basal core were deposited in the early
Pleistocene, thus the sediments cored at Site U1380 are younger than 2 Ma.
However, the top and bottom boundaries of Zone NN19, defined by the last
occurrences (LOs) of Pseudoemiliania lacunosa and Discoaster brouweri, respectively, could not be constrained.
The dominant species observed at this site include Helicosphaera carteri, H. sellii, H. neogranulata/hyalina, Coccolithus pelagicus, and Calcidiscus leptoporus.
Planktonic foraminiferal zones could not
be established in the sediments cored at this site, because of their rare
occurrences. The benthic foraminifers, on the other hand, reflect continuous
paleoenvironmental changes. Planktonic foraminifers were observed in limited
horizons of 6 core catcher samples and they are characteristic of tropical
fauna (Globigerinoides quadrilobatus (Globigerinoides sacculifer),
Globigerinoides ruber, Orbulina universa, Globorotalia menardii, and
The benthic foraminifers observed at this site are similar to modern
assemblages that are characteristic of the oxygen minimum zone in this region.
Thus, benthic foraminiferal faunas mainly represent bathyal species. The faunas
include many species similar to those of the nearby Site U1378. In general, the
following species have been observed: Bolivina argentea, Epistominella
smithi, Uvigerina peregrina, Cibicidoides mckannai, and Gyroidina. Those species are characteristic of an
upper to middle bathyal palaeoenvironments.
The geochemical trends displayed by the
analyzed pore water and gas samples (10 whole rounds, 9 headspace samples, 8
void gas samples) follow mainly the same general trend with depth as the fluids
observed at Site U1378. Salinity, Cl, Na, H4SiO4, and B
concentrations show a monotonic decrease with depth. This indicates a
diffusional communication with fluids at depth. The low chloride concentrations
(380 mM) and low salinity (20 mM), ~60-67% of seawater respectively, reflect
significant freshening of the fluid. Methane (C1) concentrations
range between 3346 and 11925 parts per million per volume (ppmv) in
interstitial waters from sediments sampled between 398.1 to 475.1 mbsf at Site
U1380. The gas at these depths is thermogenic in origin as indicated by the low
ratios of methane to heavier homologues (ethane and propane), with C1/(C2+C3)
values from 459 to 552. The C2 concentrations range between 3.89 to
15.69 ppmv, with the highest concentration at 475.1 mbsf. Propane is also present in the cores with concentrations ranging from 1.39 to 7.33 ppmv at depth. Iso- and normal-butane are detected from 398.1 to 475.1 mbsf with concentrations between 0.55 to 3.45 ppmv. Iso-pentane
concentrations ranged from 0.64 to 2.34 ppmv. Propane, butane, and pentane concentrations were all highest at 475.1 mbsf.
There is a clear offset in the Ca, Mg,
Sr, K, NH4, and alkalinity profiles from ~400-500 mbsf between Site
U1380 and U1378. Ca and NH4 concentrations at Site U1380 are lower
than they are at Site U1378, whereas K, Mg, Sr, and alkalinity concentrations
are higher at Site U1380. The Ca, Mg, and K concentrations were determined
twice via ICP-AES at both Site U1380 and U1378, and the NH4 and
alkalinity concentrations are determined by different methods, thus the offset
is not an analytical artifact. Site U1380 is located ~1 km northeast of Site
U1378, and the lithostratigraphy at Site U1378 is only partially represented at
Site U1380. The depth interval between ~400 and 500 mbsf at Site U1378 contains
more and thicker sections of coarser-grained sediments than at Site U1380,
indicating that these horizons are either not present at Site U1380 or were not
recovered and sampled. It is likely that these coarser-grained horizons are the
conduits for the laterally-migrating fluids observed at Site U1378, which may
explain why the geochemical profiles at Site U1380 show a steady increase (Ca,
NH4) or decrease (Mg, K) through the depth cored and do not display
the marked anomalies observed at Site U1378.
Arroyo, I.G., 2008, Personal communication.
Bilek, S.L., 2003, Personal communication.
Grevemeyer, I., Kopf, A.J., Fekete, N.,
Kaul, N., Villinger, H.W., Heesemann, M., Wallmann, K., Spiess, V., Gennerich,
H.H., Muller, M., and Weinrebe, W., 2004, Fluid flow through active mud Dome
Mound Culebra offshore Nicoya Peninsula, Costa Rica: evidence from heat flow
surveying: Marine Geology, v. 207, p. 145-157.
Harris, R.N., Spinelli, G., Ranero, C.R.,
Grevemeyer, I., Villinger, H., and Barckhausen, U., 2010, The thermal regime of
the Costa Rican convergent margin 2: Thermal models of the shallow Middle
America Subduction Zone offshore Costa Rica: Geochemistry Geophysics
Hensen, C., Wallmann, K., Schmidt, M.,
Ranero, C.R., and Suess, E., 2004, Fluid expulsion related to mud extrusion off
Costa Rica - A window to the subducting slab: Geology, v. 32, p. 201-204.
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Norabuena, E., Turner, H., Saballos, A., Mattioli, G., Protti, M., and Strauch,
W., 2009, Fore-arc motion and Cocos Ridge collision in Central America:
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McIntosh, K., Abbott, L., Galewsky, J., Kahn, L., and Protti, M., 1996,
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Ranero, C.R., Grevemeyer, I., Sahling,
U., Barckhausen, U., Hensen, C., Wallmann, K., Weinrebe, W., Vannucchi, P., Von
Huene, R., and McIntosh, K., 2008, The hydrogeological system of erosional
convergent margins and its influence on tectonics and interplate
seismogenesis.: Geochemistry, Geophysics, Geosystems, v. 9, p. Q03,
Weinrebe, W., Ranero, C.R., von Huene,
R., Phipps Morgan, J., Grevemeyer, I., Fekete, N., Vannucchi, P., Wagner, H.J.,
Douglas, R.H., Partridge, J.C., Frank, T., White, E., Jennings, K., Xiaohong,
D., Marchand, C., Paasch, F., Sweeney, A., Gonzalez Solis, C., and Bonilla
Gerardo, S., 2003, SONNE 173-2 cruise report. Seismogenesis and Tectonic
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