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GEOS
GEOS, Vol. 25, No. 1, Noviembre, 2005
SESION
ESPECIAL
GEODYNAMICS OF SUBDUCTION
ZONES: FROM NUMERICAL MODELS
TO SEISMOLOGY AND POTENCIAL
FIELD METHODS A SESSION IN
HONOR OF HARTMUT JODICKE
MARTES
SALON
1
MISMALOYA
GEODYNAMICS OF SUBDUCTION ZONES: FROM NUMERICAL
MODELS TO SEISMOLOGY AND POTENCIAL FIELD METHODS A
SESSION IN HONOR OF HARTMUT JODICKE
238
GEOS
GEOS, Vol. 25, No. 1, Noviembre, 2005
GEOS
GEOS, Vol. 25, No. 1, Noviembre, 2005
SE03-1
GEODYNAMICS OF SUBDUCTION ZONES: FROM NUMERICAL
MODELS TO SEISMOLOGY AND POTENCIAL FIELD METHODS A
SESSION IN HONOR OF HARTMUT JODICKE
SE03-2
GEOPHYSICAL MODELING OF VALLE DE
BANDERAS GRABEN
Arzate Flores Jorge (Centro de Geociencias,
Campus UNAM-Juriquilla), Alvarez Béjar Román
(Instituto de Investigaciones en Matemáticas
Aplicadas y en Sistemas, UNAM, CU, México DF,
04510), Yutsis Vsevolod (Facultad de Ciencias de
la Tierra, UANL, Linares N.L.), Pacheco Martínez
Jesús (Centro de Geociencias, Campus UNAMJuriquilla) y López Loera Héctor (IPICYT, San Luis
Potosí, S.L.P.)
[email protected]
A gravimetric survey consisting of five lines and 483
stations, as well as a magnetotelluric (MT) survey
consisting of 17 obser vation sites, were made in the Valle
de Banderas region for the determination of the structural
characteristics of the valley. Additionally, an aeromagnetic
survey previously made was analyzed to correlate with the
above geophysical measurements. Gravimetric and MT
models were derived from those determinations, which
confirm that the valley corresponds to a general graben
structure with slumped blocks that vary from deep
emplacements (2000 m) close to Banderas Bay to
shallow ones (100 m) toward the NE end of the valley.
Faults flanking the valley, infer red from the gravity and
magnetic models, can be connected with offshore faults
in Bahía de Banderas, indicating a structural connection
between Banderas Bay and Banderas Valley. From the MT
measurements we conclude that a 2-D resistivity behavior
is observed within the graben structure whereas outside
of the graben limits the behavior is 1-D, in spite of the
mountainous character of the region. Gravimetric models
suggest the occurrence of basin-like structures within the
valley’s
graben,
coinciding
with
similar
structures
reported elsewhere within Banderas Bay, indicating that
this may be a typical erosional feature of the graben
structure. The aeromagnetic data correlates with the
gravimetric and MT models, and suggests that the graben
structure is an extensional zone separating granite blocks
with similar magnetic signatures; it also indicates that the
extensional zone continues NE beyond the limit of
Banderas Valley. These results tend to confirm that
Banderas Bay and Banderas Valley belong to the same
tectonic structure in spite of an approximate change in
orientation of 30° between them, and strengthen the idea
that these structures constitute part of the NW limit of the
Jalisco Block.
CONDUCTIVIDAD ELÉCTRICA Y MICROSISMICIDAD EN LA REGIÓN DE OJOS NEGROS,
ENSENADA B. C., MÉXICO.
Antonio Carpio Ricardo, Romo Jones José Manuel,
Frez Cárdenas José y Suárez Vidal Francisco
Centro de Investigación Científica y Educación Superior
de Ensenada
[email protected]
La región de Ojos Negros, ~50 km al Este de la
ciudad de Ensenada, es una zona con gran actividad
sísmica, ubicada en la Sier ra Peninsular, en el norte de
Baja California. La mayor actividad es de sismos con
ML<4.0, aunque en su cercanía se han registrado eventos
de magnitud > 6.0 (1954, 1956), asociados al
rompimiento del sector sur de la falla de San Miguel
(FSM), una de las estructuras más activas de la zona. Los
epicentros de la región de Ojos Negros se distribuyen
cubriendo un área de unos 50 km2 entre la FSM (al NE)
y las fallas Ojos Negros y Tres Hermanas (al SO), y sólo
alguna poca actividad puede asociarse con el trazo de las
fallas; los mecanismos focales indican un régimen
extensional con el acimut promedio del eje P en dirección
N-S. En este trabajo utilizamos datos magne-to-telúricos
medidos en 17 sitios a lo largo de un perfil que cruza la
región, para investigar la distribución de la conductividad
eléctrica, tanto en la zona sismogénica como fuera de
ella. Los datos, observados en una banda de frecuencia
entre 0.001 y 100 Hz, se interpretaron utilizando un
algoritmo de inversión en 2-D, para obtener como
resultado una imagen de la resistividad eléctrica del
subsuelo en la sección transversal. Los resultados
muestran una anomalía conductora (~10 Ohm-m) a unos
15 ± 3 km de profundidad, bajo el valle de Ojos Negros.
La mayor parte de los hipocentros se localizan bordeando
a este conductor, en la zona en donde el gradiente de la
resistividad es mayor. L a zona sismogénica de la FSM se
muestra como una región medianamente conductora
(~300 Ohm-m), mientras que las rocas graníticas del
batolito peninsular (corteza superior) tienen altas
resistividades (>3000 Ohm-m) y la corteza inferior,
asísmica, presenta la resistividad más baja (< 30 Ohmm). La anomalía conductora puede asociarse a una
corteza con mayor contenido de fluidos, lo que está de
acuerdo con la idea de una zona de transición o sutura
entre dos terrenos de origen y composiciones distintas.
La distribución observada de microsismos puede estar
asociada a fracturas provocadas por el ascenso de
fluidos desde la corteza profunda.
239
GEODYNAMICS OF SUBDUCTION ZONES: FROM NUMERICAL
MODELS TO SEISMOLOGY AND POTENCIAL FIELD METHODS A
SESSION IN HONOR OF HARTMUT JODICKE
SE03-3
P R O PPA
AGA
TION OF THE 2001-2002 SILENT
ATION
EARTHQUAKE IN THE MEXICAN SUBDUCTION
ZONE
Kostoglodov Vladimir (Instituto de Geofísica,
UNAM), Franco Sánchez Sara Ivonne (Instituto de
Geofísica, UNAM, Mexico City, Mexico), L arson
Kristine M. (Department of Aerospace Engineering
Science, University of Colorado, Boulder, CO, USA),
Manea
Vlad C. (Caltech, Pasadena, CA, USA),
Manea
Marina (Caltech, Pasadena, CA, USA) y
Santiago Santiago Jose Antonio (Instituto de
Geofísica, UNAM, Mexico City, Mexico)
v l a d i @ s e r v i d o r. u n a m . m x
Among a number of silent earthquakes (SQ) recently
recorded by GPS in different subduction zones (Japan,
Alaska, Cascadia, New Zealand) the aseismic slow slip
event of 2001-2002 in Guerrero, Mexico is the largest one
with the equivalent magnitude Mw ~7.5. Sub-horizontal
and shallow plate interface in the Central Mexico
produces specific conditions for the ~100 km extended
zone of slow transient where the SQs develop from ~80 to
~190 km inland from the trench. This wide transient zone
and large slow slips of 10-20 cm on the subduction fault
result in the noticeable surface displacements up to 5 cm
during the SQs. Continuous GPS stations provide reliable
data to trace the propagation of SQs, and to estimate the
arrival time, duration and geometric attenuation. The
knowledge of these propagation parameters is important
to understand the origin of slow slip events and their
triggering effect on large subduction earthquakes. We use
the long-base tiltmeter data to define new time limits for
the SQs and continuous records at 8 GPS stations to
determine the propagation of the 2001-2002 SQ in Mexico.
It occurs that the surface deformation from this SQ
commenced almost instantly at the C AYA and IGUA GPS
stations separated by ~170 km and located along the
profile perpendicular to the trench. The SQ then
propagated laterally parallel to the coast at ~2 km/day with
an
exponential
attenuation
of
horizontal
surface
displacement and a linear decrease of the duration with
the distance. Campaign data measured every year from
2001 to 2005 at the Oaxaca GPS network are modeled by
a propagation of the 2001-2002 SQ displacement pulse.
This modeling shows that the SQ ceased gradually in the
central part of the Oaxaca subduction zone segment
(Puerto Angel) and then apparently triggered another SQ
in the SE Oaxaca (between Puerto Angel and Salina Cruz).
240
GEOS
GEOS, Vol. 25, No. 1, Noviembre, 2005
SE03-4
LOW TEMPERA
TURE AND HIGH AMPLITUDE
TEMPERATURE
MAGNETIC ANOMAL
Y BENEA
TH CHIAP
AS:
ANOMALY
BENEATH
CHIAPAS:
EVIDENCE FOR A HIGHL
Y SERPENTINIZED
HIGHLY
MANTLE WEDGE
Manea
C ALTECH,
Marina y Manea
Vlad Constantin
Seismological L aborator y, Pasadena,
[email protected]
USA
Southern Mexico is an interesting area where the
subducting Cocos slab drastically changes its geometry:
from a flat slab in Central Mexico to a ~ 45º dip angle
beneath Chiapas. Also, the cur rently active volcanic arc,
the modern Chiapanecan volcanic arc, is oblique and
situated far inland from the Middle America trench, where
the slab depth is ~ 200 km. In contrast, the Central
America volcanic arc is parallel to the Middle America
trench and the slab depth is ~ 100 km. A 2D steady state
thermo-mechanical
model
explains
the
calc-alkaline
volcanism by high temperature (~ 1300º C) in the mantle
wedge just beneath the Central America volcanic arc and
strong dehydration (~ 5 wt.%) of the Cocos slab. In
contrast, the thermal model for the modern Chiapanecan
volcanic arc shows high P-T conditions beneath the coast
where the Miocene Chiapanecan extinct arc is present,
and is therefore unable to offer a reasonable explanation
for the origin of the modern Chiapanecan volcanic arc. We
propose a model in which the origin of the modern
Chiapanecan volcanic arc is related to the space-time
evolution of the Cocos slab in Central Mexico. The
initiation of flat subduction in Central Mexico in the middle
Miocene would have generated a hot mantle wedge inflow
from NW to SE, generating the new modern Chiapanecan
volcanic arc. Because of the contact between the hot
mantle wedge beneath Chiapas and the proximity of a
newly formed cold flat slab, the previous hot mantle
wedge in Chiapas became colder in time, finally leading
to the extinction of the Miocene Chiapanecan volcanic arc.
The position and the distinct K-alkaline volcanism at El
Chichón volcano are proposed to be related to the arrival
of the highly serpentinized Tehuantepec Ridge beneath
modern Chiapanecan volcanic arc. The deserpentinization
of Tehuantepec Ridge would have released significant
amounts of water into the overlying mantle, therefore
favoring vigorous melting of the mantle wedge and
probably of the slab.
GEOS
GEOS, Vol. 25, No. 1, Noviembre, 2005
GEODYNAMICS OF SUBDUCTION ZONES: FROM NUMERICAL
MODELS TO SEISMOLOGY AND POTENCIAL FIELD METHODS A
SESSION IN HONOR OF HARTMUT JODICKE
SE03-5
PACIFIC PL ATE REJUVENA
TIO N FR O M PL
REJUVENATIO
PLUME
UME
ACT IN FRONT OF THE KAMCHA
TK A TRENCH:
IMPACT
KAMCHATK
IMP
A MECHANISM TO PRODUCE ADAKITIC MAGMAS
FOR OLD AND FFAST
AST SUBDUCTION ZONES
Manea
C ALTECH,
Vlad Constantin y Manea
Marina
Seismological L aborator y, Pasadena,
[email protected]
USA
The Kamchatka subduction zone is one of the most
active seismic and volcanic regions in the world and
located in the proximity of the Meiji Guyot mantle plume.
We propose a convection model which shows the a hot
plume rising from depths grater that 1000 km would bend
toward the trench, being deflected near surface by the
Pacific plate movement.
Geochemical studies of volcanic rocks in Central
Kamchatka show a complex pattern, from basalts of
intermediate composition to alkaline basalts of plume
type and adakites. Our models suggest that the buoyant
plume cannot penetrate the cold subducting slab in order
to enrich the mantle wedge and to produce the alkaline
plume type basalts. Instead, a gap in the subduction
process, likely created by accretion of new terrains, would
create an easy way for the hot plume material to enrich the
mantle wedge.
The contact between the hot plume and the oceanic
plate offshore Kamchatka produces a rejuvenation of the
~100 Ma old Pacific plate and lowering the thermal age to
less than 26 Ma. 2D steady state thermal models with
such hot incoming slab show that the oceanic crust
beneath the active volcanic arc has undergone melting
and therefore adakitic volcanism.
SE03-6
FLUIDS RELEASE
C O C O S P L ATE AND
CRUST DEDUCED
STUDIES IN
FROM THE SUBDUCTED
PPA
AR
T I A L M E LLTING
TING OF THE
RT
FROM MAGNETOTELLURIC
SOUTHERN MEXICO
Jodicke Hartmut (Institut für Geophysik der
Westfälischen W ilhelms-Universität Münster,
Germany), Jording Alexander (Institut für Geophysik
der Westfälischen W ilhelms-Universität Münster,
Germany), Ferrari
Luca (Centro de Geociencias,
Campus UNAM-Juriquilla, Querétaro, Qro., México),
Arzate Flores Jorge (Centro de Geociencias,
Campus UNAM-Juriquilla, Querétaro, Qro., México),
Mezger Klaus () y Rupke Lars (Institut für Marine
Geowissenschaften (GEOMAR), Kiel, Germany)
were carried out in southern Mexico along two coast to
coast profiles. The first line, running from Puerto
Escondido to Tlacotalpan, is void of recent volcanism,
whereas the second line further to the north, running from
Acapulco to Tampico, crosses the Transmexican Volcanic
Belt (TMVB) near the active volcano Popocatépetl. The
conductivity-depth
distribution
was
obtained
by
simultaneous 2D inversion of the TM and TE modes of the
magnetotelluric transfer functions after Groom and
Bailey´s decomposition and static shift corrections for part
of the sites were done.
The MT models demonstrate that the subducting plate
itself is not seen electrically. Instead, the southern profile
shows several zones of enhanced conductivity in the deep
crust clearly separated from each other. In contrast, the
northern profile is clearly dominated by an elongated
conductive zone extending more than 250 km below the
TMVB and beyond. The isolated conductivity anomalies on
the southern profile are interpreted as originating from
chemically bounded aqueous fluids, released from the
basaltic oceanic crust at increasing p,T conditions during
subduction, and trapped in the overlying deep continental
crust. By comparison with the pressure-temperature
diagram of metamorphic facies for a fully hydrated
basaltic bulk composition, and adopting a moderate dip of
the slab of about 13° from refraction seismic results, the
conductivity anomalies may be related to the main
dehydration reactions at the zeolite &#61614; blueschist
&#61614; eclogite facies transitions and the breakdown of
chlorite at increasing depths. This relation allows to
estimate a geothermal gradient of ~8.5 °C/km for the top
of
the
subducting
plate.
Trench-near
conductivity
anomalies are the result of water expelled by pressureinduced closure of water-filled open pores and fractures,
and the decomposition of clay minerals. The same
dehydration reactions may be recognized along the
northern profile at the same position relative to the depth
of the plate, but more inland due to a smaller dip, and
merged together near the volcanic front due to a steep
down-bending of the plate. When the oceanic crust has
reached a depth of 80 – 90 km, ascending fluids produce
basaltic melts in the intervening hot continental mantle
wedge that give rise to the volcanic belt. In part, water-rich
basalts may intrude into the lower continental crust
generating partial melting of granulites. The elongated
high conductive zone below the TMVB may therefore depict
a complex of partial melts and fluids of various origin,
ongoing migmatization, ascending basaltic and granitic
melts, growing plutons as well as residual metamorphic
fluids.
[email protected]
In order to study electrical conductivity phenomena
that are associated with subduction related fluid release
and melt production, magnetotelluric (MT) measurements
241
GEODYNAMICS OF SUBDUCTION ZONES: FROM NUMERICAL
MODELS TO SEISMOLOGY AND POTENCIAL FIELD METHODS A
SESSION IN HONOR OF HARTMUT JODICKE
SE03-7
APLICACIONES DE PDE2D, UN PROGRAMA DE
PROPOSITOS GENERALES QUE RESUEL
VE
RESUELV
ECUACIONES
DIFERENCIALES
PARCIALES
Sewell
Granville
University of Texas at Austin, USA
[email protected]
PDE2D es un programa de elementos finitos, que
resuelve
sistemas
no-lineales
de
ecuaciones
diferenciales parciales, dependiente o independiente del
tiempo, y sistemas lineales de autovalores, en 1D, 2D
(regiones arbitrarias), y 3D (regiones no-rectangulares
sencillas). Tiene un interfacio interactivo, por lo tanto es
muy facil de usar, y usa elementos de hasta cuarto grado,
por lo tanto tiene alta precision.
www.pde2d.com
contiene una lista de mas de 160 publicaciones, muchos
de ellos de geofisicia, donde PDE2D se uso para
producir los resultados numericos.
En este charla, se
presentaran algunos aplicaciones tipicas de PDE2D.
242
GEOS
GEOS, Vol. 25, No. 1, Noviembre, 2005