JAMSTEC > Institute
For Research on Earth Evolution > Seminar Schedule > Past seminar information (IFREE HQ)
2010
● IFREE HQ seminar
Place: Seminar Room at 1F, YOKOSUKA HQ
Time: 16:00-17:30 on 22th, December (Wednesday)
Speaker: Saneatsu Saito (Nankai Trough Seismogenic Zone Research Team, IFREE)
Title: From subduction inputs to seismogenesis in Nankai Trough
Abstract:
The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE)
is a multiexpedition drilling project, started from 2007,
designed to investigate seismogenesis along subduction
megathrusts through direct sampling, in situ measurements,
and long-term monitoring. Since 2009, drilling expeditions
at input area have been conducted to investigate initial
composition and state to be delivered to the seismogenic zone
and material/fluid budget in whole subduction system.
During IODP Expedition 322 coring was conducted at two sites
in the Shikoku Basin on the subducting Philippine Sea plate,
then followed by Expedition 333 at this moment.
Site C0011 is located on the northwest flank of the Kashinosaki Knoll,
whereas Site C0012 is located near the crest of the knoll.
Coring at Site C0012 penetrated 38 m into igneous basement
and recovered the sediment/basalt interface intact at 540 mbsf.
The petrography and petrochemistry characteristics of the recovered
basalt are similar to those of typical Shikoku Basin basalt.
Clay mineralogical results show that the altered basalts contain 30% of
saponite (10 vol.% of water) before the subduction due to
low-temperature metamorphism.
Saponite eventually releases water at deeper part (100-250oC)
due to saponite-chlorite phasetransition, that yields valuable clues
to estimate fluid rock interactions in seismogenic zone.
Two distinct sandy units were recovered from the sedimentary
sequences in both Sites C0011 and C0012.
An older (early-middle Miocene) turbidite sandstone facies with mixed
detrital provenance occurs in the lower Shikoku Basin; this unit is
broadly correlative with superficially similar Miocene turbidites
on the western side of the basin.
Fission-track and U-Pb ages of zircon and mineral assemblage/chemistry
in volcaniclastics are possibly correlated to the forearc volcanism
of the Southeast Japan arc, including Kumano Acidic Rocks.
An interval of late Miocene tuffaceous and volcaniclastic sandstone
has been discovered and designated as the middle Shikoku Basin facies.
The closest volcanic source of this interval was probably located along
the northeast margin of the Shikoku Basin.
The chemistry of the glass of all tuffaceous sandstone packages is
very uniform and the potassium content is correlative to the
rear arc values.
Chemical composition of hornblende and pyroxene minerals composing
the pumice fragments will be presented to pinpoint the detrital
provenance at the back arc side of Izu-Bonin arc.
● A special 1B's seminar information
Subseafloor Dynamics Research Team
Date：12/21(Tue)9：30-17：00
Place： Guest House 2F, Yokosuka HQ
Schedule:
9:30～10:00 Deep Sea Research Team: Achievements and scope (Kinoshita, Masataka)
10:00～10:30 (Kimura, Toshinori)
10:30～11:00 (Kitada, Kazuya)
11:00～11:30 (Kumagai, Hidenori)
11:30～12:45 lunch
12:45～13:15 (Tamura, Hajimu)
13:15～13:45 (Kawada, Yoshifumi)
13:45～14:15 (Fujiwara, Toshiya)
14:15～14:45 (Ichihara, Hiroshi)
14:45～15:00 Break
15:00～15:30 (Saito, Saneatsu)
15:30～16:00 SSS, MBES, Ground References, and understanding geology of divergent margin (Asada, Miho)
16:00～16:30 (Kasaya, Takafumi)
16:30～17:00 (Abe, Natsue)
● IFREE1B & 2 seminar
Place: International Exchanging Room at 5th Floor, YOKOSUKA HQ
Date: 2010/11/24(Wednesday) 16:00-17:30
Speaker: Toshiya Kanamatsu (Subseafloor Dynamics Research Team)
Title: Magnetic fabric analysis to detect the shortening direction of Nankai Trough off Kumano southwest Japan, Sites IODP C0001 and C0002, Expedition 315
Abstract:
Because a mineral grain alignment in rock reflects its formation
environments (e.g. stress), it is informative for understanding
its genesis. For example we can estimate a strain of rock in
a compressional environment from its grain fabric. Anisotropy of
magnetic susceptibility (AMS) can detect a net grain fabric in rock
and sediment samples quickly without destruction.
A case study to detect deformed sedimentary fabrics by this method
in the Nankai subduction zone will be introduced. Samples were
collected from IODP Exp.315 from C0001, and C0002 sites during
NanTroSEIZE stage1. AMS of the late Pliocene to late Miocene
accretionary prism reveals a tectonically deformed magnetic ellipsoid
(prolate type), and a restored direction of the AMS orientation
indicates northwest-southeast shortening which is same to the
present-day stress field measured by borehole breakout. The older
Miocene accretionary prism cored beneath the forearc basin does not
show a typical prolate type but more compacted feature in parallel to
plunged bedding plane. A restored AMS orientation of the accretionary
prism beneath the forearc basin also indicates the northwest-southeast
shortening which is different from the present-day principal horizontal
stress orientations measured by borehole breakout and anelastic strain
recovery at the location. It is supposed that the shortening direction
was locked in Pliocene time probably when the accretionary prism was
sited in the outer wedge setting. Northwest-southeast directions of AMS
in the Pliocene time agree with the past subducting direction since Pliocene.
● IFREE HQ special seminar
Place : Seminar Room, the Annex 1F, Yokosuka HQ
Date ：2010/10/21(Thersday) 17:00-18:00
Speaker：Thorsten Becker (University of Southern California)
Title：Seismological constraints on mantle dynamics
Abstruct：
I review recent progress in using a range of constraints from structural
seismology for understanding the mantle system. Such constraints can come
from heterogeneity power spectra, correlation analysis between global
tomography and geodynamic forward models, and comparisons of flow model
predictions with seismic anisotropy, for example. Comparisons can yield
information on the mode of mantle convection, and in particular the
study of seismic anisotropy is promising to be a high resolution tool
to analyze lithosphere- asthenosphere interactions. It may be possible
to construct geodynamic reference models and to probe these models
quantitatively via their deviations from the seismological images,
learning about mantle rheology, intra-plate deformation and asthenospheric
hydration state in that process.
● IFREE HQ seminar
Place : Seminar Room, the Annex 1F, Yokosuka HQ
Date : 2010/10/06 (Wednesday) 16:00-17:00
Speaker : Masaki Yoshida
Title: Supercontinent cycle and mantle convection
Abstract:
The previous numerical studies of mantle convection suggested that
there is a significant feedback between mantle convection and
continental drift. The process of assembly of supercontinents induces
a temperature increase due to the thermal insulating effect. Such
thermal insulation leads to a planetary-scale reorganization of mantle
flow and results in longest-wavelength thermal heterogeneity in the
mantle, i.e., degree-one convection in three-dimensional spherical
geometry. The formation of degree-one convection seems to be integral
to the emergence of periodic supercontinent cycles. The rifting and
breakup of supercontinental assemblies may be caused by either
tensional stress due to the thermal insulating effect, or large-scale
partial melting resulting from the flow reorganization and consequent
temperature increase beneath the supercontinent. Supercontinent
breakup has also been correlated with the temperature increase due to
upwelling plumes originating from the deeper lower mantle or CMB as a
return flow of plate subduction occurring at supercontinental margins.
The active mantle plumes from the CMB may disrupt the regularity of
supercontinent cycles.
Most of the numerical studies presented so far have assumed the
continent/supercontinent to be rigid or nondeformable body mainly
because of numerical limitations as well as a simplification of
models. Here, a new numerical simulation model of mantle convection
with a compositionally and rheologically heterogeneous, deformable,
mobile continental lithosphere is presented for the first time by
using three-dimensional regional spherical-shell geometry. A
compositionally buoyant and highly viscous continental assemblage with
pre-existing weak continental margins, analogous to the past
supercontinent, is modeled and imposed on well-developed mantle
convection whose vigor of convection, internal heating rate, and
rheological parameters are appropriate for the Earth's mantle. The
visco-plastic oceanic lithosphere and the associated subduction of
oceanic plates are incorporated. Earth-like continental drift is
successfully reproduced, and the characteristic thermal interaction
between the mantle and the continent/supercontinent is observed in my
new numerical model. The preliminary model presented here should
represent an important step toward realizing a more realistic model
that could be used to address many outstanding geodynamic problems
about the thermal and mechanical feedbacks between the mantle and
continents and the temporal evolution of the Earth's mantle structure.
● IFREE HQ seminar
Place : Seminar Room, the Annex 1F, Yokosuka HQ
Date : 2010/09/29 (Wednesday) 16:00-17:00
Speaker : Keiko Sato
Title: Two topics from noble gas applications: 1) friction experiment under the controlled gas environment, and 2) source of volcanic rock
Abstract:
Noble gases very limitedly react other elements, recognized as inert nature.
Therefore, isotope ratios of noble gases in certain minerals are believed to be
easily modified by their ambient reservoirs when the minerals reach
temperatures that are higher than their closure temperatures. In this talk,
the following two topics selected to discuss.
1) Frictional degassing and Fault related rocks
We reported the resetting of isotope signatures of noble gases under frictional
experiment. It was only observed in completely melted glass limitedly found at
the surface of frictional plane on friction experiment. However, the most part
of the sample were little or not melted possibly due to very short duration
of the experiment. Instead, the samples were heated and deformed
at a moment. The condition of the friction experiment is similar to the natural
faults. Although the samples shaped as cylinders constantly rotated under
constant pressure, the friction reached its maxima at around 5 seconds from
the beginning of the rotation. This first fuse is limitedly occurred at the
temperature maxima. Then, friction suddenly dropped down, and decreases
gradually further in several seconds associated with moderate melting
(second fuse). Changing the atmospheres of the experiment from dry-air to He
and Ar, the timings of melting were different, which suggests that the
transportation of friction-derived heat depends on the mass numbers of the
gases in the environment.
2) Source of volcanic rock
Heavier noble gases do not easily degas from rocks or magmas than lighter
noble gases, which provides some potentials as the geochemical tracer.
At least, we found some variation of 38Ar/36Ar ratios different from
atmospheric value for some volcanic rocks to date, though their original
Ar isotope composition should be that of mantle. After the Ar measurement
for K-Ar dating, the residual gas fraction includes heavy noble gases,
such as Kr and Xe. Some of the samples contained slightly enriched heavy
noble gases as their source nature.
● IFREE HQ seminar
Place : Seminar room, the Annex 1F, Yokosuka HQ
Date : 2010/09/15 (Wednesday) 16:00-17:00
Speaker : Hidenori Kumagai
Title: How heterogeneous the uppermost mantle is?
- What will be observed in MOHOLE?
Abstract:
MOHOLE, to drill through the Moho-discontinuity, is also a long standing
dream to a geochemist, because the Moho-discontinuity is one of the first
order discontinuity in the solid earth. Currently, the community tries to
find the most suitable site to drill in order to obtain the maximum scientific achievements.
However, even once the project achieved, the hole is just a point to penetrate
into the uppermost mantle. Thus, we have to find the way to expand the
observation by the Mohole to the understanding to the mantle or whole Earth scale.
Here I present that the helium data so far obtained from abyssal peridotites
and related samples, and discuss the possible heterogeneities in the uppermost mantle.
● IFREE HQ seminar
Place : Seminar Room, the Annex 1F, Yokosuka HQ
Date : 2010/08/25 (Wednesday) 16:00-17:00
Speaker : Hiroko Sugioka
Title: Wave-tide resonance in deep oceans
Abstract:
Ocean tide refers to the oscillatory motion of seawater forced by the
gravitational attraction of the Moon and Sun with periods of a half to
a day and wavelengths of the semi-Pacific to Pacific scale. Ocean
infragravity waves are sea-surface gravity waves with periods of
several minutes and wavelengths of several dozen kilometres. Here we
report the first evidence of the resonance between these two
ubiquitous phenomena in deep oceans mutually very different in period
and wavelength. This evidence is based on the results of long-term,
large-scale observations made using arrays of broadband ocean-bottom
seismometers located at depths of more than 4000 m in the Pacific
Ocean. A simple theory is proposed to show that resonance occurs over
unexpectedly wide areas of the Pacific Ocean, in accordance with our
observations. Through this resonance with infragravity waves, some of
ocean tidal energy is dissipated in deep oceans.
● IFREE HQ seminar
Place : Seminar room, the Annex 1F, Yokosuka HQ
Date : 2010/08/04 (Wednesday) 16:00-17:00
Speaker : Masahiro Kawano
Title : Sorry, no english title.
Abstract: Sorry, no english abstract.
● IFREE HQ seminar
Place : Seminar Room, the Annex 1F, Yokosuka HQ
Date : 2010/07/21 (Wednesday) 16:00-17:00
Speaker : Takafumi Kasaya
Title : Resistivity structure around San-in region and some topics about EM survey technique
Abstract:
The eastern part of San-in region in southwestern Japan is classified
as an area of high seismicity. Hypocenter distribution extends in a
east-west direction, and large earthquakes such as the Western Tottori
earthquake and Northern Hyogo earthquake occasionally occurred in this region.
It is noteworthy that almost all earthquakes occurred at the shallower
than 15 kilometers.
Many types of magma is known to have extrude in this region, and the
andecite-dacite magma in particular was caused by dehydration from the
subducting Philippine Sea Plate (PHP) or slab melting. Therefore, volcanic
activity may be related to seismic activity via subsurface fluid dynamics.
However, not only the shape of PHP but also the dynamics in the crust and
mantle are not clear beneath the San-in region.
Low resistivity zones in the lower crust were detected below
the San-in region as a result of the wide band MT survey.
However, these resitvitity studies based on land
MT survey were not sufficient for imaging the deep structure which
may be related to various magmatism. The objective of this study is
to obtain the crust and mantle resisitivity image using both marine
and land MT surveys that enable deep and regional electromagnetic (EM) imaging.
Deeper images can clarify the relationship between the magmatism and seismicity.
Marine EM surveys were carried out off Tottori/Hyogo Prefecture in 2006 and 2007,
and EM data were obtained at seven sites. After the clock and tilt correction, we
calculated MT responses using the robust remote reference method code (rrrmt ver.8)
for the period range from 100 to 10000 seconds.
To evaluate the static shift of the land data, we improved the 2D inversion code
developed by Ogawa and Uchida (1996) for the marine data. Using improved 2D code,
we carried out the 2D analysis of the marine and land data. As a result of this analysis,
the uppermost mantle below the ocean is shown as a high resitive region.
On the other hand, the deeper part of the mantle is portrayed as a conductive body.
The conductive zone below the land area was detected at the depth of 10 km.
I will also present some topics of EM survey technique that I developed for some project.
● IFREE HQ special seminar
Place : XBR meeting room (306), the Annex 3F, Yokosuka HQ
Date : 2010/07/16 (Friday) 14:00-15:30
Speaker : Takashi Tonegawa (ERI, the University of Tokyo)
Title: Inter-source body wave propagations derived from seismic interferometry
Abstract:
Seismic interferometry has become established as a powerful tool in
estimating the relatively shallow velocity structure of the medium
between two receivers. In addition to seismic interferometry based on
a pair of receivers, seismic interferometry combined with source-
receiver reciprocity has recently been employed to extract inter-
source wave propagations.
In this study, we applied the technique to seismograms of earthquakes
occurring at depths of 300-500 km beneath the Bonin Islands. The
seismic records were observed by short-period sensors of Hi-net, which
is operated by the National Research Institute for Earth Science and
Disaster Prevention (NIED). The wavelets of direct P and S waves
generated by the deep earthquakes were examined to compute cross-
correlation functions (CCFs). We stacked the CCFs observed at
receivers around the stationary point whose location can be estimated
by ray tracing technique, because the phases in such CCFs potentially
emerge at similar lag times, i.e., the stationary phases.
The results demonstrate the retrieval of direct P and S waves
propagating between pairs of deep earthquakes; in particular, direct P
waves are reconstructed for most event pairs. The following points
should be considered to effectively retrieve the propagation of body
waves: (1) it is important to design a geometry in which the
stationary phase derived from two earthquakes can be observed at the
employed stations, (2) the lag time difference of the stationary
phases that emerge in the employed cross-correlation functions should
be as small as possible, and (3) the direct waves associated with deep
earthquakes have a large amplitude at stations employed for cross-
correlation, which is accomplished by both low noise level at the
stations and radiation pattern of the sources.
● IFREE HQ seminar
Place : Seminar room, the Annex 1F, Yokosuka HQ
Date : 2010/07/07 (Wednesday) 16:00-17:00
Speaker : Yozo Hamano
Title: Effect of an electrically heterogeneous lower mantle on the geomagnetic jerks observed at the Earth's surface.
Abstract:
Most of the geomagnetic field observed at the earth's surface
is generated in the fluid outer core. The main field changes with
time, which is denoted as "secular variation".
Geomagnetic jerks are abrupt changes in the linear trend of this
secular variation ( i.e. secular acceleration jump) , and are surface
observable shortest-period components of the geomagnetic field
variation of core origin. Hence, the jerks have been studied by many
authors to infer the dynamics of the earth's core and the
electrical conductivity of the lower mantle in the last few decades.
Geomagnetic jerks are traditionally studied using time series of the
magnetic field recorded in geomagnetic observatories. The very uneven
distribution of the observatory network precludes the investigation of
global distribution of the geomagnetic jerks, and internal origin of
the jerks have been questioned by a number of authors. Recently, as a
result of continuous satellite measurements since 1999, the magnetic
fields and their variations can now be described with high resolution
in space and time, and the internal origin of most of the known jerks
and their global nature are now firmly established. However, even now,
little is understood of their physical origin.
Since jerks are generated in the core, they will pass through the
electrical conducting mantle, before arriving at the surface.
Consequently, the geomagnetic field observed at the surface will
correspond to a filtered version of the original field generated in
the core. Even an 1-D electrical conductivity distribution in the
mantle exerts screening effects such as delaying and smoothing of
signals on the surface observed geomagnetic field of core origin.
Moreover, a laterally heterogeneous electrical conductivity structure
causes more dramatic changes in time and space on the magnetic fields
come through the heterogeneous layer. In this case, an uniform change
of the magnetic field over the core surface may generate small-scale
complex variations of the surface observed geomagnetic field. Changes
of the toroidal magnetic field in the core, which is not detectable at
the insulating surface of a 1-D earth, generates poloidal magnetic
field in the laterally heterogeneous layer, and can be detected at the
earth's surface as temporal and spatial variations of the
poloidal geomagnetic field. Recent discovery of the post-perovskite
phase change in the D" layer, and measurements of the electrical
conductivity of the natural pyrolitic mantle and MORB materials in the
high P and high T condition expected at the bottom of the mantle,
predict very high and highly heterogeneous electrical conductivity
structure in the lowermost mantle adjacent to the core surface. Hence,
understanding of the filtering effect of an electrical heterogeneous
layer on the geomagnetic field is indispensable to investigate the
origin of the geomagnetic jerks and other geomagnetic phenomena of
core origin.
In the present study, modification effect of the geomagnetic field
due to an electrically heterogeneous layer is examined by using newly
formulated induction equations. Assuming an electrical heterogeneous
D" layer, results of the numerical calculation indicate that
uniform changes of the geomagnetic field at the core surface causes
very complicated variation of the surface geomagnetic field in space
and time. As a source field at the core surface, T20 component of the
toroidal field, and P10 poloidal field are provided. Corresponding to
the abrupt change of these source fields at the core surface, surface
observed magnetic field varies with time in very similar manner to the
observed geomagnetic jerks. Time scale of the change of the secular
variation and the surface morphology of the secular acceleration jump
of the calculated geomagnetic variation suggest that the change of the
toroidal T20 field at the core surface is more plausible as a source
of the actual geomagnetic jerks. The origin of the toroidal field
change can be attributed to the torsional oscillation in the
earth's core, that is expected by theoretical grounds and
observed in MHD dynamo simulation models.
● IFREE MC & HQ joint special seminar
Place: International Exchanging Room at 5th Floor, YOKOSUKA HQ
Time: 16:00-17:30 on 1st, July (Thursday)
Speaker: David Selby (Senior Lecturer, Durham University, UK)
Title: Successes, Systematics, possible limitations, & unknowns: Insights into the world of Re-Os isotope petroleum geosciences
Abstract:
How can we date the deposition of sedimentary rocks, and the time when oil
is generated from an oil shale? This talk will explore the background, application,
systematics and implications of the rhenium-osmium isotope system to sedimentary
and petroleum geosciences. Using case studies I will show how accurate and
precise dates can be obtained from organic-rich rocks and petroleum. I will also
explore and demonstrate the behavior of the rhenium and osmium systematics
in hydrocarbons. Additionally I will discuss how osmium isotope stratigraphy is
a powerful tool to evaluate paleo- oceanography, climate and the onset and
duration of Large Igneous Provinces.
● IFREE HQ seminar
Place : Seminar Room, the Annex 1F, Yokosuka HQ
Date : 2010/06/09 (Wednesday) 16:00-17:00
Speaker : Satoru Tanaka
Title : The core-mantle boundary topography from novel seismic data sets
Abstruct:
The determination of the core?mantle boundary (CMB) topography has
long been an interesting but difficult subject to address geomagnetic,
Earth rotation and mantle dynamic issues. In previous studies, the
travel times of PKP, PcP, and PKKP reported in the seismic bulletins
are used. However, we have no consensus result owing to low data
quality and the effect of mantle heterogeneity.
In this study, P4KP?PcP differential travel times are examined to
infer the CMB topography. To obtain high quality data, 362 P4KP?PcP
times are measured on seismograms with a reading error of 0.5 s not
retrieving from bulletin data. The travel times are corrected for the
Earth's hydrostatic ellipticity and mantle heterogeneity using a
P-wave tomographic model. Spherical harmonic expansion up to degree 4
is adopted for model parameterization. Then the P-wave velocity
heterogeneity in the lowermost 150 km of the mantle is estimated to
overcome the problem of underestimation of the velocity perturbation
at the base of the mantle in the global 3D P-wave mantle model.
Subsequently, the CMB topography is inferred using the residues of the
above processes. Since the odd-degree components of the CMB topography
are insensitive to the P4KP?PcP times, I solve for only the components
of degrees 2 and 4. The resultant features indicate that the maximum
amplitude of the CMB topography does not exceed ±2 km, with an
uncertainty of less than 0.5 km. A numerical test confirms that the
pattern of degree 4 is more reliable with less amplitude recovery. The
obtained degree 4 pattern shows an amplitude of less than ±1 km
and indicates the presence of depressions under the circum-Pacific,
the Central Pacific, and South Africa.
● IFREE HQ seminar
Place: Lecture room at 5F, YOKOSUKA HQ
Time: 16:30-18:00 on 20th, May (Thursday)
Speaker: Maria Luisa G. Tejada (Associate Professor, the Univ. of the Philippines)
Title: The Platinum Group Element Signature of Oceanic Anoxic Event 1a:
No Evidence of a Bolide Trigger for Ontong Java Plateau Volcanism
Abstruct:
The large-volume eruption and the short-period of emplacement shown by the geochronological,
geological, and geophysical studies of the Ontong Java Plateau (OJP) require some explanation
beyond what is already known from plate tectonic processes. Plume impact, bolide impact, and their
variants have been proposed to account for the origin of these features. The two end member models
(bolide vs. plume) cannot be distinguished from one another based solely on the Os isotopic
compositions of the OJP lavas because both meteorites and mantle have unradiogenic Os isotope signature.
Additional evidence to test the models can be derived from the platinum group element (PGE) abundances
in sedimentary rocks deposited before and during the LIP emplacement.
We analyzed the PGE contents of the Selli Level horizon at Gorgo a Cerbara, central Italy,
which was previously analyzed for Os isotope composition, to further test the mantle plume vs. bolide
impact models for the origin of the OJP and the Early Cretaceous oceanic anoxic event,OAE1a. We found
that Ir concentrations are low compared to other known large impact horizons. Only one black shale bed
yielded high Ir concentration of 544 ppt but it is not at the base of the Selli Level interval.
In addition chondrite-normalized Os/Ir and Pt/Ir values of 1.4-21 and 7.7-31, respectively,
within the Selli Level horizon are nonchondritic, including the bed that yielded the highest concentration
of Re and other PGEs. Previous analysis of the Os isotopic compositions of the organic fractions of
these sedimentary units showed a major interval of unradiogenic Os isotopic compositions coinciding
with the Selli Level (Tejada et al., 2009).The absence of positive correlation between enrichment
of PGE and the unradiogenic Os isotopic compositions suggests that the OAE1a event represented
by the Selli Level horizon was triggered by a significant amount of mantle input into the marine environment
and not by a bolide impact. Highly variable Os/Ir ratios that are uncorrelated with initial 187Os/188Os
are very difficult to reconcile with the impact hypothesis. Our PGE data further corroborate
our previous results that favor a mantle plume origin for the OJP and that the main phase of
plateau volcanism triggered the OAE1a event.
Reference:
Tejada, M. L. G., Suzuki, K., Kuroda, J. ,Coccioni, R., Mahoney, J. J., Ohkouchi, N., Sakamoto, T.
and Tatsumi, Y., 2009. Ontong Java Plateau as a trigger for the Early Aptian oceanic anoxic event.
Geology, 37, 855-858.
● IFREE HQ seminar
Place: Lecture room at 5F, YOKOSUKA HQ
Date : 2010/05/13 (Thu) 16:00-17:00
Speaker: Yasushi Harada (Tokai University)
Title: sorry, no english title
● IFREE HQ special seminarr
Place: Lecture room at 5F, YOKOSUKA HQ
Date : 2010/05/13 (Thu) 14:00-15:00
Speaker: Ryota Hosaka (Kochi University)
Title: sorry, no english title
● IFREE HQ seminar
Place : Seminar Room, the Annex 1F, Yokosuka HQ
Date : 2010/05/12 (Wednesday) 16:00-17:00
Speaker : Yujin Kitamura
Title : A fate of sediments in subduction zone
Abstract:
This talk is to introduce Yujin Kitamura's major achievements. The study we performed so far aims at revealing sediment
transfer cycles in subduction zones quantitatively, sediment deformation resulting from underthrusting and its consequences
for plate boundary processes.
In a subduction zone with well-developed accretionary prism, terrigenous material flows into a trench followed
by sedimentation and subducts with oceanic plate beneath the continents. Some part of sediments is accreted by off-scraping
or underplating, but at the same time basal erosion scrapes material off the continental hanging wall, which implies opposite sense
of mass transfer. These material cycles are never estimated quantitatively despite their importance for every field of research
on subduction zone. A role of underthrusting sediments in the plate boundary is also significant for understanding of subduction
processes. Such behavior of sediments in subduction zone is to be investigated.
We studied (1) deformation of underthrusting sediments, (2) its relationship with plate boundary process and
(3) quantitative estimate of sediment cycle. Our observation that sediments keep deforming in the seismogenic zone differs from
the common idea of that sediment lithification completes before reaching this zone and the rock could produce stick slip accordingly.
Although the deformation of tectonic melange is unclear, slow deformation in and around seismogenic zone is a possible candidate
for its mechanism. We estimated mass balance of sediments in southwest Japan and the Nankai Trough that indicates the amount
of sediments buried deeper is quite small but has an important role as a fault rock in plate boundary.
Consequently, we figured out a fate of sediments that are eroded from mountains, drained into and deposited in the trench,
deformed with underthrusting and finally underplate.
● IFREE HQ seminar
Place : International exchange room, the main building 5F, Yokosuka HQ
Date : 2010/04/28 (Wednesday) 16:00-17:00
Speaker : Takatoshi Yanagisawa
Title : Generation of stagnant slabs in 3-D spherical mantle convection models
Abstract:
Seismic tomography reveals the natural mode of convection in the Earth
is whole mantle as subducted slabs are clearly seen as continuous
features into the lower mantle. However, simultaneously existing alongside
these deep slabs are stagnant slabs which are, if only temporarily, trapped
in the upper mantle. Previous numerical models of mantle convection have
observed a range of behavior for slabs in the transition zone depending on
viscosity stratification and mineral phase transitions, but typically only
exhibit flat-lying slabs when mantle convection is layered or trench migration
is imposed. We use 3-D spherical models of mantle convection which
range up to Earth-like conditions in Rayleigh number to systematically
investigate three effects on mantle dynamics: (1) the mineral phase transitions,
(2) a strongly temperature-dependent viscosity with plastic yielding at shallow
depth, and (3) a viscosity increase in the lower mantle.
First a regime diagram is constructed for isoviscous models over a wide
range of Rayleigh number and Clapeyron slope for which the convective
mode is determined. It agrees well with previous results from 2-D simulations,
suggesting present-day Earth is in the intermittent convection mode
rather than layered or strictly whole mantle. Long-term calculations for
4 billion years at Earth-like Rayleigh number which include effects (2) and (3)
are produced, with and without the effect of the mineral phase transitions.
The calculation without the phase transition produces plate-like behavior
with a long wavelength structure and surface heat flow similar to Earth's value.
While the observed convective flow pattern in the lower mantle is broader
compared to isoviscous models, it basically shows the behavior of
whole mantle convection, and does not exhibit any slab flattening at
the viscosity increase at 660 km depth.
The calculation which includes the phase transitions successfully exhibits the
coexisting state of stagnant and penetrating slabs within the transition zone.
These results indicate the importance of both a viscosity increase and
mineral phase transitions for generating the structure of stagnant slabs
observed by seismic tomography.
● IFREE HQ seminar
Place : nternational exchange room, the main building 5F, Yokosuka HQ
Date : 2010/04/14 (Wednesday) 16:00-17:00
Speaker : Yuka Masaki
Title : Hydroregime of Iheya-North hydrothermal field, Mid-Okinawa trough inferred from heat flow observations
Abstract :
The Okinawa Trough is a back-arc basin in the southwestern part of Japan. It
is considered to be in the initial stage of rifting of continental crust,
and volcanic ourcrops interpreted as intrusion after rifting in the trough
of mid-axis. The Iheya-North hydrothermal field is one of the most active
hydrothermal fields.
A lot of heat flow data gives cogent information to discuss the various
scales of hydrothermal circulation. It is available to presume the patterns
of temperature distribution and stream of fluids.
We obtained 78 heat flow data from 2002 to 2008 in and around the
Iheya-North hydrothermal field. This shows that the heat flow is higher than
10 W/m2 within 0.5 km of the hydrothermal vent complex, that it gradually
decrease eastward to <1 W/m2, and the very low heat flow around 0.01 W/m2 is
observed at ~2 km east from the hydrothermal field. The average heat flow
outside of Iheya-North is ~0.1 W/m2. The low heat flow to the east is most
likely caused by an inward of seawater as recharge of the circulation.
Side-scan sonar images show that the sea floor seems very rough inferred
from the reflection distributions (M. Asada personal communication) in this
area. Also the clay sediments exist between the hydrothermal vent and the
low heat flow zone according to the piston coring and video images. The
sediment layer should work as a hydrological barrier to suppress flow
through the seafloor.
The hydroregme of the circulation is provided by the difference of
permeability, geological setting, heat source distribution and so on. We
performed numerical calculations to consider that the different permeability
contribute to the heat flow. The upper and lower 2 layers were assumed under
the structure of the Iheya-North hydrothermal field. Two-order higher value
of permeability affects no more heat flow value. We will show the
hydroregime of the Iheya-North hydrothermal field inspected by numerical
analysis considered permeability sampled and assume Darcy velocity, the
scale of heat source.
● IFREE HQ seminar
Place: Seminar room, the main building 1F, Yokosuka HQ
Date: 2010/3/17 (Wednesday) 16:00-17:00
Speaker: Toshiya Fujiwara (IFREE-1b)
Title: Quick Report of R/V Kilo Moana KM1003 Cruise
-Mohole Site Selection Studies North of Maui-
Abstract:
In the last February, we conducted the KM1003 cruise with the R/V
Kilo Moana, Univ. of Hawaii, and surveyed swath bathymetry, gravity,
and magnetic anomaly in the northern off Hawaii. The survey area is
considered as one of candidate sites for whole-crust penetration
drilling (MOHOLE). "That's one small step for MOHOLE" by any chance.
We will report outline of the cruise.
● IFREE-KANAME special seminar
Place: IT Build. 5F Meeting Room, Yokohama, JAMSTEC
Date: 2010/3/12 (Friday) 15:00-16:00
Speaker: Dr. Noriaki Watanabe(Tohoku University)
Title: Channelized flow in heterogeneous pore structure within the rock fracture
● IFREE 1B special seminar
Place: Conference Room, XBR 3F, Yokosuka HQ
Date: 2010/3/10 (Wednesday) 14:00 - 17:00
Speaker1: Dr.Tsuneo Ishido (National Institute of Advanced Industrial Science and Technology)
Title1: Sorry, no English title.
Speaker2: Dr.Yuji Nishi (National Institute of Advanced Industrial Science and Technology)
Title2: Sorry, no English title.
● IFREE HQ seminar
Place:Seminar room, the main building 1F, Yokosuka HQ
Date:2010/2/24 (Wednesday) 16:30-17:30
Speaker:Millard F. Coffin (University of Southampton)
Title: Life at the Edge: Super Eruptions cause Earth Crises
Abstract:
The formation of large igneous provinces (LIPs)-continental flood basalts,
'volcanic' margins, and oceanic plateaus-may impact the atmosphere, oceans,
and biosphere by rapidly releasing huge amounts of particulates, magmatic
volatiles (CO2, SO2, Cl, F, etc.), and potentially volatiles (CO2, CH4, SO2, etc.)
from intruded sediments (e.g., carbonates, organic-rich shales, evaporites).
A key factor affecting the magnitude of volatile release is whether eruptions
are subaerial or marine; hydrostatic pressure inhibits vesiculation and degassing
of relatively soluble volatile components (H2O, S, Cl, F) in deep water submarine
eruptions, although low solubility components (CO2, noble gases) are mostly
degassed even at abyssal depths. Directly or indirectly, such injections may
cause changes in the atmosphere/ocean system that can lead to perturbations of
atmosphere/ocean chemistry, circulation, ecology, and biological productivity.
These changes can be global in extent, particularly if environmental conditions
were at or near a threshold state or tipping point. LIPs may have been responsible
for some of the most dramatic and rapid changes in the global environment.
For example, between ~145 and ~50 Ma, the global ocean was characterized by chemical
and isotopic variations (especially in C and Sr isotope ratios, trace metal concentrations,
and biocalcification), relatively high temperatures, high relative sea level, episodic
deposition of black shales (oceanic anoxic events), high production of hydrocarbons,
mass extinctions of marine organisms, and radiations of marine flora and fauna.
Temporal correlations between the intense pulses of igneous activity associated with LIP
formation and environmental changes suggest more than pure coincidence.
The 1783-84 eruption of Laki on Iceland provides the only historical record of the type of
volcanism that constructs transient LIPs. Although Laki produced a basaltic lava flow
representing only ~1% of the volume of a typical transient LIP flow (103 km3), the
eruption's environmental impact resulted in the deaths of 75% of Iceland's livestock
and 25% of its inhabitants.
During Cenozoic time, peak eruption of the North Atlantic LIP at ~56 Ma coincided with the
Paleocene-Eocene thermal maximum, when numerous deep-sea benthic foraminifera
became extinct and there was a major turnover in terrestrial mammals. Late Cretaceous
oceanic anoxic event 2 (OAE-2) coincided with the formation of the Caribbean and possibly
Madagascar flood basalts at ~94 Ma, and in Early Cretaceous time, formation of the Ontong Java,
Manihiki, and Hikurangi plateaus at ~122 Ma in the Pacific coincided with oceanic anoxic event
1a (OAE-1a). Eruption of the Siberian flood basalts at ~250 Ma (Permian-Triassic boundary)
coincided with the largest extinction of plants and animals in the geological record; 90% of all
species became extinct at that time.
● IFREE HQ seminar
Place : Seminar room, the main building 1F, Yokosuka HQ
Date : 2010/2/17 (Wednesday) 16:00-17:00
Speaker : Claudia Adam (Universidade de Evora / JAMSTEC-IFREE)
Title: Mantle flow drives the subsidence of oceanic plates
Abstract:
After its creation at the mid-oceanic ridges, the oceanic lithosphere
becomes denser, thickens, and sinks under its own weight. The
evolution of its structure has generally been considered by assuming
its cooling from an initial perturbed state. The main parameter in
this process is then the lithosphere age. Therefore, the seafloor
depth is considered to be a function of lithospheric age only.
However, since the lithosphere is the thermal boundary layer of
mantle convection, its structure depends on the global convective
system. We state here that the evolution of the lithosphere structure
and depth should be considered along flow lines, representative of
the mantle convection. The thermal boundary conditions of the Pacific
plate changed drastically due to the major rearrangements 50 Myr ago.
Flow lines thus strongly differ from "age trajectories". We show
that along flow lines, the seafloor depth varies as the square root
of the distance from the ridge all along the plate, as predicted by
the half-space model, without any 'flattening'.
● IFREE HQ seminar
Place : International Conference Room, the main building 5F, Yokosuka HQ
Date : 2010/1/29 (Friday) 16:00-17:00
Speaker : Akiko Toh
Title: Structure of the D" layer beneath Hawaii inferred from analyses of anomalous Sdiff waveforms
Abstract :
Global shear velocity tomographic models show two large-scale low velocity
structures (LLSVPs) in the lower mantle beneath the Pacific Ocean and Africa.
Their locations correspond to the regionswhere mantle upwelling flows are expected.
Revealing the detail structures inside the LLSVPs and their surrounding regions should
hold key to understanding the generation of mantle upwelling plumes.
Here, we report that prominent postcursors to S/Sdiff waves are observed
in Northern America for Papua New Guinea events. These waves sample the northern side of
the Pacific LLSVP beneath Hawaii. First, we use ray theory to search for a range of models,
which generate the postrursor and also provide the observed travel time difference between
the postcursor and the main Sdiff phase. Then, for some of the models,we created waveforms
Spectral element method installed in Earth Simulator. We obtained a few models, which provide
synthetic waveforms in a fair agreement with observations.
The result shows that two types of low Vs regions are required to explain the data.
One is a broad and weak anomaly region with Vs reduction of 5% or so. The other is a laterally
localized strong anomaly region with more than 30% of Vs reduction. The newly found strong
anomaly region is located several hundred kilometers southwest of Hawaii on the CMB.
The waveform modeling show that the region should be thicker than 80km. This is 2 to 10 times
thicker than the so-called ultra low velocity zones, which were previously detected at some
other regions on the CMB. The thick ultra low velocity zone should be caused by an existence
of chemical heterogeneities rather than a partial melting.
● IFREE HQ seminar
Place : Conference Room, the main building 1F, Yokosuka HQ
Date : 2010/1/20 (Wednesday) 16:00-17:00
Speaker : Azusa Shito
Title: Upper mantle discontinuities beneath the petit-spot
Abstruct :
The petit-spot volcanoes were first reported by Hirano et al. [2006]. They
are young (0-10 Ma) volcanic micro-knolls on very old (~130 Ma) NW Pacific
plate (about 500 km offshore from the Japan Trench). Although the estimated
activity distributes widely in time (0-10 Ma) and in space (over 600 km),
the volume of each volcanic edifice is small (several orders of magnitude
less than the previously known seamounts and knolls). Hirano et al. [2006]
proposed the hypothesis that small fraction of melt came from the
asthenosphere through fractures in the lithosphere which were induced by
flexure around the outer-rise. However, the mechanisms of the melt
production and magma eruption process of this new type of volcanism are
still unknown.
In order to investigate the melt source of the petit-spot, seismological
observation using Broad-Band Ocean Bottom Seismometers (BBOBSs) was
conducted. We deploy three BBOBSs in the petit-spot region with about 100 km
spacing from May 2007 to July 2008.
Applying the receiver function analysis for the BBOBSs data, 1-D velocity
structure beneath the petit-spot is studied. The events used in this study
are chosen by the magnitude (M > 6.0), the epicentral distances (30-90
degrees), and the signal to noise ratio. We conduct the Velocity Spectrum
Stacking of receiver functions to determine mantle discontinuity depth.
The result at long period (0.02-0.1 Hz) show that the depth of "410" is
consistent with the global model and that of "660" is 20 km deeper than
the global model. This suggests that the mantle transition zone has no high
temperature anomaly beneath the petit-spot. The result at short period
(0.02-0.4 Hz) show the prominent negative phase from the depth of 80 km
corresponding to LAB. These results in this study support that the magma
source of the petit-spot is not a hot plume from deep mantle but a partially
molten layer in the asthenosphere.
